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High Temperature Water Heating Systems Course No: M03-006

Credit: 3 PDH

Steven Liescheidt, P.E., CCS, CCPR

Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 [email protected]

Information Handling Services, 2000

ARMY TM 5-810-2AIR FORCE JOINTMANUAL 32-1057

(FORMERLYAFR 88-28)

HIGH TEMPERATURE WATER HEATINGSYSTEMS

THIS COVER PAGE OFFICIALLY CHANGES THEAIR FORCE PUBLICATIONNUMBER FROMAFR 88-28

TO AFJMAN 32-1057

(Affix to thefront ofthe publication)

31 DECEMBER 1991

DEPARTMENTS OF THE ARMYAND THE AIR FORCE

ARMY TM 5-810-2AIR FORCE AFR 88--28

HIGH TEMPERATURE WATER HEATING SYSTEMS


APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

DEPARTMENTS OF THE ARMY AND THE AIR FORCEDECEMBER 1991

REPRODUCTION AUTHORIZATION/RESTRICTIONSThis manual has been prepared by or for the Government and, except to the extent indicated below, ispublic property and not subject to copyright.

Copyrighted material included in the manual has been used with the knowledge and permission of theproprietors and is acknowledged as such at point of use. Anyone wishing to make further use of anycopyrighted material, by itself and apart from this text, should seek necessary permission directlyfrom the proprietors .

Reprint or republications of this manual should include a credit substantially as follows: "JointDepartments of the Army and Air Force, TM b-810-2/AFR 88-28".

If the reprint or republication includes copyrighted material, the credit should also state: "Anyonewishing to make further use of copyrighted material, by itself and apart from this text, should seeknecessary permission directly from the proprietor ." .


TECHNICAL MA,, uALNo. 5-810-2AIR FORCE REGULATIONNo. 88-28

'This manual supersedes TM 5-810-2/AFM 88-28, dated 12 September 1984


HIGH TEMPERATURE WATER HEATING SYSTEMSI Approved for public re lease; distribution is unlimited

TM 5-810-2/AFR 88-28

HEADQUARTERSDEPARTMENTS OF THE ARMY

AND THE AIR FORCEWashington, DC, 31 December 1991

CHAPTER 1 . DESIGN CONSIDERATIONS

A��~hPurpose . . . . . .. ... . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . .... . . . . . . .. .. . . . . . . . . . . . .. .. . . . . . . . . . .. .. . . . . . . . . ... . . . . . . . . . .. ... . . . . . . . . .. . . .. . . . . . . . . . ... ..... . .. . . . . . . . . . . . . .. 1-1 1-1Scope . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . ... . . . . . . . . .... . . . . . . . . . .. .. .. . . . . . . ... .. . . . . . . . . . . . .. .. . . . . . .. . .. . .. . . . . . . . . .... . . . . . . . . . . . ..... .. . . . . . . . . . ... . .. . . . 1-2 1-1References . . . . . . . . . . .. .. . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . .. .. . . . . . . . . . . . .. .. . . . . . . . . ..... . . . . . . . . . . .. . . . . . . . . ... .. . . . . . . . . . . . ... . . . . . . . . . . . . ...... . . . . . . . . . . ... .. .. . . 1-3 1-1General . . .. . . . . . . . . . .... . . . . . . . . . . ... .. . . . . . . . . .... . . . . . . . . .. ... . . . . . . . . .. . . . . . . . . . . . . . ... . . . . . . . . . . . . .. . . . . . . . . .... . . . . . . . . . . .. .. . . . . . . . . . . . . . ..... . . . . . . . . . . . .. .. .. . . . 1-4 1-1Advantages of HTW systems . . . .. .. . . . . . . . . . . . .. . . . . . . . . ... . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . ... .. . . . . . . . . . . .. .. . . . . . . . . . . . . .. ... . . . . . . . . . . . .... . . . . . 1-5 1-1Properties of high temperature water. . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . ... . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . .. . . . . . . . . .. . . ... . . . . . 1-6 1-2Pressurization . . . . . . . . . . . .. .. . . . . . . . . . . . . ... . . . . . . . .... . . . . . . . . . . . .. . . . . . . . . . .. . .. . . . . . . . . . . . .. . . . . . . . . . . .. . .. . . . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . . . . . . . .. .. . . . . . . . . 1-7 1-3Water circulation . . . . .... . . . . . . . . . .. . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . ... .. . . . . . . . . . . .. . . . . . . . . ... .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . ... ... . . . . . . . . 1-8 1-4HTW generators . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . ..... . . . . . . . . .... . . . . . . . . . .... . . . . . . . . . . . .. . . . . . . . . . . ... .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . 1-9 1-12Design and selection procedure . . . . .. ... . . . . . . . . .... . . . . . . . . .. .... . . . . . . . .. . . . . . . . . . . . . . .... . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . 1-10 1-12Economic justification . . . . . . . ... . . . . . . . . .. . . . . .. . . . . . . . .. . . . . . . . . . . . .. ... . . . . . . . . ... . . . . . . . . . .. .... . . . . . . . .. . . . . . . . . . . . . . . .... . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . . . 1-11 1-18

CHAPTER 2 . LOAD CHARACTERISTICS AND CALCULATIONSHTW requirements. . . . . . . . . .. . . . . . . . . . ..... . . . . . . . . . . . . ... . . . . . . . . ..... . . . . . . . . . .... .. . . . . . .. . ... .. . . . . . . . . .... . . . . . . . . .. .... . . . . . . . . . . . . . . ... . . . . . . . . . . . ..... . 2-1 2-1Space heating . ..... . . . . . . . . .. . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . . . ... . . . . . . . . ..... . . . . . . . . . . ..... . . . . . . . . ...... . . . . . . . .. . .. . . . .. . . . . . . . . . .. ... .. . 2-2 2-1Process heating .. . . . . . . ....... . . . . . . . . .. ... . . . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . .. .... . . . . . . . . . . . ... ... . . . . . .. . ..... . . . . . . . . ... .... . . . . . . . . ... ... .. .. . . 2-3 2-1Diversity factors . . . . . . . .. . . ... . . . . . . . . .. ... . . . . . . . . .... . . . . . . . . .. ... . . . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . .... .. . . . . . . . . . ..... . . . . . . . . . . . .. .. .. . . .. . 2-4 2-2Operating temperatures and pressures . . . . . . . . .. .. .. . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . .. . . .. . . . . . . . . . ...... .. . . . . . . . . ..... . . . . . . . . . . . . ... . .. . . . . 2-5 2-2Maximum initial load .. . . . . . . . ... .. . . . . . . . . .... .. . . . . . . .. . .. ... . . . . . . . . .... . . . . . . . . . . . .. ... . . . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . ..... . . . . . . . 2-6 2-3Maximum ultimate load . . . . . ..... . . . . . . . . ..... . . . . . . . ... .. . . . . . . . . . . . .... . . . . . . . . .. . .. ... . . . . . . . .... . . . . . . . . . . . . ..... . . . . . . . . . . . . ... . . . . . . . . . . . ....... . . . . . 2-7 2-3Essential load . .. . . . . . . . . .... . . . . . . . . ..... . . . . . . . . . ... . . . . . . . . ..... . . . . . . . . ...... . . . . . . . .. .. ... . .. . . . . . .. . . ... . . . . . . . . .. . .. .. . . . . . . .. . . .. . . .. . . . . . . . . .. . ..... . . . . . . . 2-8 2-8Matching plant capacity to load . . . . .... . . . . . . . . ..... . . . . . . . . .. ... . . . . . . . . . . ... .. . . . . . . . . . . . .. .. . . . . ... . . .. .. . . . . . . . . .. . .. .. . .. . . . . . . . . .. ... . .. . . . . . . . 2-9 2-4System heat loss . . . . ... . . . . . . . . . . .... . . . . . . . . . ... . . . . . . . . . . . .. ... . . . . . . . . .... . . . . .... .. . . . . . . . . .. .. . . . . . . . . ... .. . . . . . . . . . . . ... . . . . . . . . . . . . . ....... . . . . . . . . . . 2-10 2-4Flywheel factor . . . . .. .. . . . . . . . . . ..... . . . . . . . . .... . . . . . . . . . .... ... . . . . . . . . .... . . . . . . . . ..... . . . . . . . . . . . .. .. . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . . .... . . . . . . . . . . . .. 2-11 2-4Calculations . . . . . . . . . . .. .. . . . . . . . . . .. ... . . . . . . . . ... . . . . . . . . . . ..... . . . . . . . . .. . .. . . . . . . . . . ... .. . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . .. .. . . . . . . . . . . . ..... . . . . . . . . . . . .. 2-12 2-4

CHAPTER 3 . DISTRIBUTION PIPING AND EQUIPMENTDesign of system . . .. . . . . . . . . ... .. . . . . . . . . . ... . . . . . . . . .. . .. . . . . . . . . . . . .... . . . . . . . . ... .. .. . . . . . . . ...... . . . . . . . . . . ... . . . . . . . . . . . .. . . . . . . . . . . . . . .. ..... . . . . . . . . . . . ... 3-1 3-1Pipe sizing . . . . . . . . .. . . . . . . . . . . .. .... . . . . . . . . . . . ... . . . . . . . . ... .. . . . . . . . . . .. . . . . . . . . . . . . .. . .. . . . . . . . . .... . . . . . . . . . . . .. ... . . . . . . . . .. . . . . . . . . . . . . . .. . .. . .. . . . . . . . . . . . . . .. . 3-2 3-1Distribution piping . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . .. .. . . . . . . . . . . ... . . . . . . . . .. . .. . . . . . . . . . . . .... . . . . . . . . .. ... . . . . . . . . . ..... . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . .. . 3-3 3-1Underground and aboveground systems . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . ... . . . . . . . . .. . . . . . . . . . . . .. .. . . . . . . . . . . . . .. ... .. . . . . . . . . . . . ... . . . . . 3-4 3-4

CHAPTER 4 . HEATING PLANTIntroduction . . . ... . . . . . . . . .. ... . . . . . . . . .. .. . . . . . . . . .. . .. . .. . . . . . . .. . . . . . . . . . . . . .. ... . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . .. .. . . . . . . . . . . . . . . . . . .. . . . . . . . 4-1 4-1HTW generators . . . . . . ..... . . . . . . . . .... . . . . . . .. .. ... . . . . . . . . .. .. . . . . . . . . .. . .. . .. . . . . . . . . . .. . . . . . . . . . .. ... . . . . . . . . .. . . . . . . . . . . . . . ..... . . . . . . . . . . . . .. .. .. . . . . . . . . .. 4-2 4-1Combustion equipment and controls .. . . . . . . . . . .. . . . . . . . . . .. . .. . . . . . . . . . . . .. .. . . . . . . . . .. . .. . . . . . . . . .. . . . . . . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . . . 4-3 4-3Pressurization system . . . . . . . . . . . ... . . . . . . . ..... . . . . . . . . . .... . . . . . . . . ... .. . . . . . . . . .... . . . . . . . . .. . .. . . . . . . . . . . .. .. . . . . . . . . . .. . ..... . . . . . . . . . . . .... . . . . . . . . . . . .. 4-4 4-5Pumps . . . . . . . . .... . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . ...... . . . . . . . . . . .... . . . . . . . .... .. . . . . . . . . . ... . . . . . . . . ..... . . . . . . . . .... . . . . . . . . . . . .. ... . . . . . . . . . . . . .. .. .. . . . . . . . . . . ... 4-5 4-6Makeup water treatment . .. . ... . . . . . . . . . ... . . . . . . . . . . ... . . . . . . . . . .... . . . . . . . . . . ... . . . . . . . . ... .. . . . . . . . . .... . . . . . . . . . . . .. ... . . . . . . . . . . . .. . . . . . . . . . . . . . . .... 4-6 4-7Instrumentation . . . . ..... . . . . . . . . .... . . . . . . . . .. ... . . . . . . . . .. . . . . . . . . . . . ..... . . . . . . . . . . . .. . . . . . . ...... . . . . . . . . .. .... . . . . . . . . ... .. . .. . . . . . . . . .. . . . .. . . . . . . . . . . . .. .. . 4-7 4-8Pollution control . . . ..... . . . . . . . . ... . . . . . . . . . .. .... . . . . . . . .. . ... . . . . . . . ..... . . . . . . . .. .. .. . . . . . . ...... . . . . . . . . . . . ... . . . . . . .. ..... . . . . . . . . . . . .. .. . . . . . . . . . .. .. ... ... 4-8 4-9

CHAPTER 5. CONVERSION AND U71LIZATIONPotential users of the system . . . . . . ..... .. . . . . . . ... . . . . . . . . . ..... . . . . . . . . .. . .. . . . . . . . . ..... . . . . . . . .. . . .. . . . . . . ...... .. . . . . . . . . .. .. ... . . . . . . . . . . . .. . .. . .. 5-1 5-1Building service . . . .. ... .. . . . . . . . . .... . . . . . . . . ..... . . . . . . . ..... . . . . . . . . ..... . . . . . . . ..... . . . . . . . . ..... . . . . . . . . ... . . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . .. ...... . . . 5-2 5-1Location of equipment . . . . . . .... . . . . . . .. ... .. . . . . . . .. .... . . . . . . . . ..... . . . . . . . ..... . . . . . . . . ..... . . . . . . . ..... . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . ..... . . . . . . 5-3 5-1Design ofheat exchangers ... . . . . . ... ... .. . . . . . . ...... . . . . . . .. ... .. . . . . . . . . . ... . . . . . . . . .. ... . . . . . . . .. . .. . . . . . . . . ..... . . . . . . . . . . . .... . . . . . . . . . . . ..... . . . . . . 5-4 5-5Controls . . ..... . . . . . . . . ...... . . . . . . . ... ... . . . . . . ..... . . . . . . . . ..... .. . . . . . ... .. . .. . . . . . .... .. . . . . . . . . ... . .. . . . . . .... . . . . . . . . . . ...... . . . . . . . . . ... ... . . . . . . . . . .... ... . . . . . . 5-5 5-6

APPRNDCx A. REFERENCES . . . . . .. . . . . . . . . . .. .. . . . . . . . . . .. .. . . . . . . . . ... . . . . . . . . . . . ... . . . . . . . . .. . . . . . . . . . . . ... . . . . . . . . . ... . . . . . . . . . . ... . . . . . . . . . . . . ..... . . . . . . . . . . . . .. . . . . . . . . . . . . .. . .. . .. . . . . A-1APPENDIX B. SAMPLE CALCULATIONS FOR DATA GIVEN IN CHAPTER 2 . .. . .. . . . . . . . . . ... .. . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . . . . . . . ... .. . .. . . . . B-1APPENDIX C. EXAMPLE DISTRIBUTION LAYOUTS . . . . . . . . . . . .. . . . . . ...... . . . . . . . . . . . ... . . . . . . .. ... . . . . . . . . . . .... . . . . . . . . ..... . . . . . . . . . . . . .. . . . . . . . . . . . . .. .... .. . . . . . . . C-1

*TM 5-810-2/AFR 88-28

LIST OF FIGURESAgue 7Ytk POO


LIST OF TABLES

Tabk 1Ttk

Pao

2-1.

Influence of Temperature Differentials on Selection of Pump Sizes for HTWSystems . . . ... . . . . . . . . . . . ... ... . . . . . . . . . .. . . . . . . . . . . . . .. . . .

2-8

1-1. Inert Gas-Pressurized Single Circulation Method . . . . . . . . . . . . . .. . .. . . . . . . . . .... . . . . . . . . .. ... . . . . . . . .. . . . . . . . . . .. .. . . . . . . . . . . . . .. .. . . . . . . . . . . ... ... . . . . . . . . . . ... . .. . . 1-51-2. Inert Gas-Pressurized Dual Circulation Method . . . . .... . . . . . . . . .. .. . . . . . . . . . .... . . . . . . . . .. . .. . . . . . . ... . . . . . . . . ..... . . . . . . . . . . . . ... . . . . . . . . . . . ..... . . . . . . . . . . . . .. . . . . . 1-71-3 . Steam-Pressurized Single Circulation Method . . . . . . . . .. . . . . . . ..... . . . . . . . . .. . ... . . . . . . . . . .. . .. . . . . . . ... . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . .... . . . . . . . . . . . . .. . . . . . . 1-91-4 . Steam-Pressurized Dual Circulation Method . . . . . . . .... . . . . . . . . ..... . . . . . . . . . . .. . . . . . . . . ..... . . . . . . . . .... . . . . . . . . . . . . . . . . . . .. . . .. . . .. . . . . . .. . . . . .. .. . . . . . . . . . . ... .. . . . . . 1-111-5 . Flow Diagram-Steam-Pressurized Single Circulation System . . .. .. . . . . . . . . ..... . . . . . . . . .... . . . . . .. ... . .. . . . . . ... . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . ..... . . . . . . 1-141-6. Flow Diagram-Steam-Pressurized Dual Circulation System . . . . . . . .. . . . . . . . ... . . . . . . . . . . . . .. . . . . . . .. . .. . . . . . . . . ... . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . .. . . . . . . . 1-151-7 . Flow Diagram Inert Gas Pressurized Single Circulation System . . . . . . . . . .... . . . . . . . .. . . .. . . . . . . ... .. . .. . . . . . ... . . . . . . . . . . . . ... .. .. . . . . . . . . . . . . . . .. . . . . . . . 1-161-8 . Flow Diagram Inert Gas Pressurized Dual Circulation System . . . . . . . . . ..... . . . . . . . . .. .. . . . . . . . . ... .. . . . . . . . . .... . . . . . . . . . . . ..... . . . . . . . . . . . .. . . . .. . . . . . . . 1-171-9 . Inert Gas Pressurization Using Variable Gas Quantity with Gas Recovery . . . .. . .. . . . . . ..... . . . . . . . . .... . . . . . . . . . . . ..... . . . . . . . . . . . .. . . . .. . . . . . . . 1-182-1.3-1.

Expansion Tank Volumes . .. . . . . . . . . . .... . . . . . .. . ... .. . . . . . . . . ... . . . . . . . . . .. ... . . . . . . .. .. . . . . . . . .. ... . . . . . . . . .. .. . . . . . . . . ..... . . . . . . . . .. .... . . . . . . . .. ..... . . . . . . . . . . . .. .. . . . . . . . . . . . ..Typical Vent and Typical Drain

2-6

4-1.. .... . . . . . . . . ..... . . . . . . . . .... . . . . . . . . .. ... . . . . . . .... . . . . . . . .. .... . . . . . . . .. .. . . . . . . . . ..... . . . . . . . . .. .... . . . . . . .. . .... . . . . . . . . . . . . . . .. . . . . . . . . . . . . .

Cascade HTWSystem in Process Steam System . . . . . . . . . . .. . .. . . . . . .. .. . . . . . . . . . . . .... . . . . . . ... . . . . . . . . . . .. .. .. . . . . . . .. . . . .. . . . . . . . . ..... . . . . . . . . . . . . ..... . . . . . . . . . . .8-84-2

4-2. Typical Combustion Control Systems. . ... .. . . . . . . . . ..... . . . . . . . .. ... . .. . . . . . .. .. . . . . . . . . . .. .. .. . . . . . . ... . . . . . . . . . ..... . . . . . . . . . . .. . . . . . . . . .. . ..... . . . . . . . . . . . . ... . . . . . . . . . . . .. 4-45-1. Various Heat Converters . . . . . . . . . . . . . .. . . . . . . . . ... .. . . . . . . . . .... . . . . . . . . .. . . . . . . . . . . . .... . . . . . . . . ... .. .. . . . . . . ... . . . . . . . . . ..... . . . . . .. . . .. . . .. . . . . . .. . .. . .. . . . . . .. . .... ... . . . . . . . . . . . .. 5-25-2. Heat Exchangers and Control Valves . ... .. . . . . . . . . .... . . . . . . . . .. .. . . .. . . . . . .... . . . . . . . . ..... .. . . . . . . ... . . . . . . . . . ..... . . . . . . . . . ... . . . . . . . . .. . ... .. . . . . . . . . . .. . . .. . . . . . . . . . . . .. 5-4C-1. Direct Supply Reverse-Return .. . . . . . . . . ... .. . . . . . . . . . ... . . . . . . . . .. . . . . . . . . . . . .... . . . . . . . . ... .. . . . . . . . . ... . . . . . . . . . ..... . . . . . .. . . .. . . . . . . . . . ... ... .. . . . . . . . . . .. . . . . . . . . . . . . . . . .. C-1C-2. Direct Supply Radial . ..... . . . . . . . . . . . . . .. . . . . . . . .... .. . . . . . . . . . ... . . . . . . .. .. . . . . . . . . . . . .... . . . . . .. . .. . .. . . . . . . . ... . . . . . . . . .. .... . . . . . . .. . .... . . . . . . . . ... .. . .. . . . . . . . . . .. . . .. . . . . . . . . ..... C-2C-3. One-Pipe Loop Main ..... . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . ... . . . . . . .. .. . . . . . . . . . . . ... . . . . . . .. . .. . . . . . . . . . . . .. . . . . . . . . .. .... . . . . . . ... .... . . . . . . . . ... .. . . . . . . . . . . . . . . . . .. . . . . . . . . ..... C-8C-4. Primary and Secondary Systems . . . . . . .. ... . . . . . . . . . . . . .. . . . . . . . .. .. . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . .. .. . . . . . . . . .... . . . . . . .. . .... . . . . . . . . . .. .. . . . . . . . . . . . . . . . . .. . . . . . . . . .. ... C-4

1-1 . PURPOSEThis manual provides guidance for the design ofhigh temperature water (HTW) heating systemsclassified as operating with supply water tempera-ture above 240 degrees F. and designed to a pres-sure rating of 300 psi.

1-2. SCOPEThis manual presents the unique features of HTWsystems, factors for comparison with other heatdistribution mediums, and criteria to design ormodify HTWsystems.

1-3. REFERENCESAppendix A contains a list of references used inthis manual.

1-4. GENERALIn district and area heating systems, water is gen-erally circulated at temperatures from 320 to 440degrees F., corresponding to a saturated pressurerange from 75 to 367 psig. The usual practical tern-perature limit is 440 degrees F. because of pres-sure limitations on pipe and fittings, equipment,and accessories . HTW systems are similar to themore familiar low temperature hot water systemsbut must be carefully designed because of therapid rate of pressure rise occurs in hot water over440 degrees F. Higher pressures increase systemcosts as higher pressure rated components are re-quired. Heat generation equipment will be de-signed in accordance with ASME Boiler Codes.Compared to a boiler which will generate steam orhot water, the high temperature water generatoris specifically designed to keep water in the liquidstate at high temperatures. The system must bemaintained at a positive pressure to do this and auniform flow through the generator must be main-tained at all operating conditions.

1-5. ADVANTAGES OF HTW SYSTEMSHTW systems have numerous advantages oversteam heat distribution systems. The inherentlosses of a steam system may be saved resulting inpossible fuel savings in a HTW system over theequivalent capacity steam system. The amount ofblowdown required by a boiler depends on theamount and nature of the makeup water supplied .HTW systems have closed circuits, require littlemakeup, therefore, practically never require blow-down whereas steam systems commonly lose about


CHAPTER 1DESIGN CONSIDERATIONS

TM 5-810-2/AFR 88"-28

1% to 3 percent of the total boiler output becausefrequent blowdowns are required. Uneven firing insteam systems results in excessively high stacklosses because of excessive boiler heat transfer andfrequent maladjustments in the combustion airsupply . Due to the heat storage capacity of HTWsystems, short peak loads may be absorbed fromthe accumulated heat in the system and unevenfiring is substantially reduced, keeping these lossesto a minimum. This results in higher generator ef-ficiencies than the equivalent steam boiler wouldhave since the pressure in a steam boiler drops di-rectly after a change in load requiring an adjustedfiring rate . HTW has many characteristics whichmake substantial savings possible in the operationand installation costs of properly designed heatdistribution systems.a. Design. The closed recirculation system re-

duces transmission and thermal losses to a mini-mum while practically eliminating corrosion andscaling of generators, heat transfer equipment,and piping. Makeup requirements of HTW systemsare almost nonexistent, less than '/z of one percentwater loss per day of the total contents of thesystem . Operation within closed circuits permitsreducing the size of water treating systems to aminimum. Both supply and return high tempera-ture water piping can be run up or down and atvarious levels to suit the physical conditions ofstructures and contours of the ground betweenbuildings without the problems of trapping andpumping condensate . Traps and pressure reducingvalves, which require substantial maintenance andwhich are the causes of substantial losses in steamsystems, are eliminated. These features simplifyboth new design and subsequent extensions to ex-isting systems. Transmission distances do not offerunacceptable constraints . Steam suffers rather ex-treme pressure and temperature drops duringtransmission ; high temperature water is much lessaffected by such pressure drops for a given pipesize. A circulating pump head takes care of piperesistances . Since the requirement of several tem-perature levels can be met with HTW systemswithout reducing the pressure of the heatingmedium, pressure reducing valves are not needed.Since the water is circulated at generator pressureor slightly higher, single-stage low head circula-tion pumps take the place of the high-pressureboiler feed pumps required in steam systems. The

TM 5-810-2/AFR 88-28

heat storage capacity of HTW systems evens outthe heating load on the generator which giveshigher generator efficiencies because of the elimi-nation of overfiring and sudden changes in firingwhich result in poor combustion and high stackgas temperatures. Small generator installationsare possible due to the more uniform firing rateand the capability of the heat accumulated in thesystem to take peak loads. Precise modulated tem-perature control may be obtained by hand or auto-matic means. Because the heat transfer character-istics of high temperature water can be more ex-actly calculated and predicted than those of steam,if correctly designed, high temperature water pro-duces more dependable and uniform surface tem-peratures of the heat transfer equipment thansteam can achieve. ASHRAE "HVAC Systems andApplications Handbook", chapter 15, "Medium andHigh Temperature Water Heating Systems" willbe consulted for design guidance as well as for ref-erences for specialized applications .

b. Capital Investment. Smaller pipe sizes areused with HTW systems than with steam systems.This, together with the 15 to 20 percent smallergenerator requirements because of elimination ofcondensate return losses, the reduction in size ofthe feedwater treatment plant, and the long life ofthe installation, results in a lower capital invest-ment. Even though the generator may be smallerthan a corresponding steam boiler, the HTW gen-erator may typically cost more. The analysis willbe based on overall system costs. The cost of heatexchangers to convert the heat to lower tempera-ture and pressure mediums is usually more thanjustified, based on an overall system cost analysis,by the elimination of traps, return condensatepumps, pressure reducing stations at the heatusing device, as well as the reduction in fuel costof HTW systems as compared to steam systems ofcomparable size .c. Operation and Operating Costs. Savings in

operating costs are possible in the closed circula-tion high temperature water system which returnsall heat unused by the users or not lost throughpipe radiation to the heating plant. This elimi-nates the losses of condensate and the heat in thecondensate due to faulty operation of traps andleakage in a steam system . Simple methods can beused to determine the heat produced and deliveredto the various buildings and heat users since onlythe temperatures and flow rates are required tocompute these quantities. Reduction in distribu-tion temperatures to correspond with heat de-mands and seasonal variations makes possible ad-ditional operational savings. Steady firing of gen-erators results in higher efficiency and fuel sav-


ings. Leakage in HTW systems is limited toamounts lost from pump glands and valve packing.d. Repairs and Maintenance. Maintenance of

traps and reducing valves, a substantial expense insteam systems, is eliminated with HTW systemswhich require only the maintenance of valve andpump packing to eliminate leakage. As an exam-ple, one HTW system, in operation for 15 yearswithout water treatment, was found to be free ofscale and corrosion, proving substantial reductionin maintenance of this type . Steam condensatepiping, on the other hand, must commonly be re-newed every five to ten years due to high corro-sion caused by oxygen in the condensate .e. Safety of Operation.

Breaks or leaks in HTWlines are not nearly as dangerous as they are insteam lines. One reason for this is the refrigerat-ing action accompanying release of the water as itexpands, which makes it possible to hold the handwithin a foot or two of the rupture without beingburned. The water is cooled further by evaporationin the air. Another reason is that the amount ofsaturated water which can pass through an open-ing is about one half the amount of cold waterwhich would pass through, and less than theamount of steam which would pass through.Therefore, combining these two effects, theamount of heat, in Btu's, which would passthrough an opening is from 5 to 10 times as greatwith saturated steam than with saturated water,depending upon the pressures involved, the size ofthe break, and the length and size of the pipe. Anyhigh-pressure system, however, whether steam orwater, requires experienced operation as well asgood design . Operational risks such as those due towater hammer must be avoided in the design andoperation of both types of systems.f. Provision for Future Expansion. Future ex-

pansion should be considered in the initial designof any system so that the system can be expandedat any time up to the design capacity of the plantand the distribution piping . Heating plant and dis-tribution system capacity may be equally expanda-ble in either system .

1-6. PROPERTIES OF HIGH TEMPERATUREWATERThe properties of low temperature water are fa-miliar to most engineers. It is a fluid with a highdensity, high specific heat, low viscosity, and lowthermal conductivity, and requires high pressureto be maintained at high temperature. Becausewater is inexpensive and readily available, the un-favorable high pressure requirements are counter-balanced economically . It is also known that vari-

ations in density, specific heat, viscosity, and con-ductivity with changes in pressure are negligible.It is not well known that the properties of waterat high temperatures are even more favorablethan those at low temperatures with the main dis-advantage being high pressure . As an example, thespecific heat of water is as high . as 2.0 Btu/lb/degree F. at pressures of about 160 atmospheresand temperature of 660 degrees F. Of greater im-portance, however, are the properties of waterwithin the temperature range of 300 to 400 de-grees F. as applied to process and district heatingsystems. The influence of pressure on the proper-ties of high temperature water within this operat-ing range has negligible effect upon its properties .The influence of temperature, however, is consid-erable and deserves closer study. Refer toASHRAE "HVAC Systems and Applications Hand-book", chapter 15 for tables of water properties fortemperatures up to 400 degrees F. and other pub-lished handbooks for the thermal properties ofwater from 400 degrees F. to 700 degrees F. Notethe rapid rise in pressure as the temperature risesabove 400 degrees F. and the increase in specificheat above 240 degrees F.a. Pressure/Temperature Relations. As temper-

ature rises, the pressure rises rapidly causing theeconomic pressure limit to be reached at 450 de-grees F. or below for most applications. Beyondthis point the cost of equipment and piping is pro-hibitive thus eliminating the savings in usingHTW.b. Density. This property is very important

since it reflects the expansion and contraction ofwater in a system with temperature changes andthereby determines the size of the expansion ves-sels required in hot water systems. Between 340and 450 degrees F., the volume of water in theHTW system increases from 10 to 18 percent abovethat at 70 degrees F. An expansion vessel able tostore this additional volume is required when thesystem is brought up to maximum temperature.Two expansion tanks are recommended, each sizedfor 50 percent of the total capacity of the operat-ing system plus the additional expansion volume .This will facilitate required periodic inspection ofeither tank without causing whole system shut-down.c. Relative Heat ofSteam and High Temperature

Water. A comparison of the heat contained in acubic foot of water going through a 150 degree F.drop, or other value used in design, with the latentheat of steam at utilization temperature showsthat the HTW contains much more heat per cubicfoot than does the steam . This property of high


1-7 . PRESSURIZATION

TM 5-810-2/AFR 88-28

temperature water accounts for the large heat ac-cumulation capacity of HTW systems in relativelysmall pipelines.d. Piping Pressure Drops. Comparisons of

steam and HTW piping pressure drops cannot bemade without reference to assumed comparableconditions. Pressure drops in high temperaturewater circuits have only a very minute effect uponthe water temperature and are important only forselecting pumping power and pipe sizing . Steamsuffers, in practice, many times the pressure dropssuffered by water, causing a substantial energyloss as well as a temperature drop. This is due tothe large volume of steam and consequent high ve-locities commonly used to transmit heat with mod-erate pipe sizes.

The maintenance of the proper temperature in thedistribution system is a function of the pressuremaintained on the system. There are two basicpressurization methods employed and an alternate(hydraulic pressurization) method for standby serv-ice.a. Steam Pressurized-The steam pressurization

method utilizes an expansion vessel separate fromand downstream of the HTW generators. In thisvessel HTW is allowed to flash into steam to pro-vide a cushion to take care of expansion of waterin the piping system . The selected steam pressuredetermines the temperature of water in the expan-sion vessel which is then utilized to supply the dis-tribution system pumps. The selected saturationpressure is chosen with due consideration of re-quired HTW delivery temperature and with properallowances for system heat losses . The expansionvessel must be located above the HTW generatoroutlets. This method should not be used for a newsystem or for system upgrades because of the lackof sufficient extra pressure (above the saturationpressure of the liquid) needed to prevent flashingunder all operating conditions . This method mayonly be used for a base loaded plant where thesteam demands are relatively constant.

b. Inert Gas Pressurization. This method alsoutilizes an expansion vessel separate from and con-nected to the system return water header . Aninert gas cushion is maintained within the expan-sion vessel . Nitrogen is usually utilized as theinert gas because it is inexpensive and widelyavailable. Air must not be used as it contributes tocorrosion of the system . Pressure is usually main-tained at 40 to 60 psi above saturation tempera-ture of the distribution system. In determining the

TM 5-810-2/AFR 88-28

minimum starting pressure, a 10 degrees F. safetyfactor should be added to the HTW delivery tem-perature which is the governing factor . Pressure ismaintained by automatic control independently ofthe heating load. The expansion vessel can be lo-cated on the operating floor of the heating plant.c. Hydraulic Pressurization Method. The hy-

draulic system consists of a pressurizing pumpwith a regulator valve which continuously by-passes pump discharge water to a makeup storagetank and injects water into the system to maintainthe desired pressure . This system may be utilizedfor smaller systems only or included as a standbysystem to keep the HTW system operational whenthe system expansion tank is out of service for in-spection and maintenance.


1-8. WATER CIRCULATIONWater is circulated by either a single or dual cy-cling method. The single cycling method uses oneset of pumps to circulate water through both theHTW generator and the distribution system usinga bypass control valve to regulate flow through theHTW generator. The dual cycling method uses twosets of pumps, one to circulate water through theHTW generators and a separate set of pumps tocirculate water through the distribution system .Combinations of these methods result in four basictypes of water circulation :

a. Inert Gas-Pressurized Single CirculationMethod. A single circulation system, also called aone-pump system, is shown in figure 1-1 .

HTW

Supp

ly

Expa

nsio

nVessel

i rW1

ControlLine

i t t t

HTW

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ain

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ingleC

ircu

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r

Makeup

Feed

Water

TM 5-810-2/AFR 88-28

b. Inert Gas-Pressurized, Dual CirculationMethod. A dual circulation system, also calledtwo-pump system, is shown in figure 1-2.


Drain

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TM 5-810-2/AFR 88-28

c. Steam-Pressurized, Single Circulation Method.A single circulation system, also called one-pumpsystem, is shown in figure 1-S.


U.S

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TM 5-810-2/AFR 88-28

d. Steam-Pressurized, Dual Circulation Method.A dual circulation system, also called two-pumpsystem, is shown in figure 1-4.

1-10


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TM 5-810-2/AFR 88-28

1-9. HTW GENERATORSThere are three basic types of HTW generators .However, only the water tube controlled forced cir-culation once through type HTW generator is suit-able for all fuels and is recommended for all sys-tems . Generators must be specifically designed forHTW service and no attempt should be made tofield-adapt a steam boiler for HTW service. Thenatural circulation HTW generator and the firetube Scotch marine generator should not be usedfor a new system for the reasons cited in the fol-lowing paragraphs. Use of the water tube naturalcirculation HTW generator or the fire tube Scotchmarine generator must be approved by the usingagency.a. Water Tube Controlled Forced Circulation

HTW Generator. The water tube controlled forcedcirculation type is primarily designed for hightemperatures and high rate of heat transfer .Water is strained and metered by orifice/strainersto each tube circuit to accommodate the heat ab-sorbing capacity of the circuit. The generator is de-signed for low waterside pressure drop and vaporbinding will not occur under any operating condi-tion since all tube circuits are vented to the outletheaders. All circuits are drainable. The counterflow of gas and water assures maximum efficiency(better heat transfer for a set of given conditions).The undesirable features are: external pumping isrequired and chemical control of the water in thesystem must be closely maintained . However, goodwater treatment and proper operation and mainte-nance usually avoid tube failure. In small sizes upto 10 mega Btu/hr output, a single-pass continuouswater tube type is available.b. Water Tube Natural Circulation HTW Gener-

ator. This type is available from most generatormanufacturers in a wide range of sizes; there areproven designs for steam operation; and a largegroup of operators experienced in their use. Theundesirable features are: the unequal heat distri-bution which is overcome by water mass whichpermits no real control of circulation; the largedrums and headers provided for steaming result inareas in which water is not exposed to high tem-perature and, therefore, internal turbulence exists ;the large size of the unit ; and the fact that the useof steaming boilers is, at best, a compromisewhether water is taken from below the water lineor obtained from cascades .c. Fire Tube and Scotch Marine.

This generatoris low in cost and the complete packaged unit isavailable as a shelf item in smaller sizes butshould not be used because of the following unde-sirable features :

1-12


(1) Unequal heat distribution with no realcontrol of water circulation .

(2) Tendency to make steam.(3) Thermal shock is encountered even with

internal distribution tubes.(9) Unequal expansion of tubes results in ex-

cessive maintenance when operated at HTW tem-peratures.

1-10. DESIGN AND SELECTION PROCEDUREPlanning for a new heating system should be asystematic method that in addition to HTW alsoincludes consideration of alternative heating sys-tems such as steam as the heating medium .a. System Design.

Loads for various heat usingdevices at the facility will be determined takinginto consideration future expansion or a possiblefuture change in the mission of the facility. Thiscan be done by various approximating or estimat-ing procedures . In some instances the actual loadsfor existing facilities are available. On a copy ofthe master plan, indicate demands of various heatusers. The thermal center of demand will be deter-mined to see if it is compatible with the plan forthe location of the central heating plant. If not,select a site that is compatible with the masterplan. Zones of distribution from the heating plantwill be developed. Make every attempt to have bal-anced loads in each zone if possible . Using pipesizing tables suitable for water above 300 degreesF., approximate main and branch sizes.b. System Selection. Review chapter 2 of this

manual and also TM 5-810-1 and approximate theequipment and the heating plant configuration forHTW and for steam. At this point it is not neces-sary to develop highly refined calculations or se-lections . Define selections to the extent necessaryto meet basic requirements for developing thesystem. Fuel for generators will be selected in ac-cordance with current DOD policy and agency orservice directives and criteria .

(1) Small Systems.

HTW central heating sys-tems with an estimated total capacity rangingfrom 10,000,000 to 50,000,000 Btu/hr will be designed as a single circulation system also knownas a one-pump system, with combined pumps asshown in figure 1-5 if system is an existing baseloaded, steam pressurized system, or figure 1-7 ifan inert gas pressurized system . In these systemspumps take suction from the expansion vessel(also called expansion drum) and deliver water tothe system and through the generators to the ex-pansion drum . These plants are relatively small insize and will be designed as simply and as trouble-free as possible.

(2) Medium Size Systems. HTW central heat-ing systems larger than 50,000,000 Btu/hr but notover 120,000,000 Btu/hr will be designed as dualcirculation systems with separate generator recir-culating and system circulating pumps. Thissystem is also known as a two-pump system . Pres-surization will be inert gas pressurized.

(8) Large Size Systems.

HTW central heatingsystems larger than 120,000,000 Btu/hr will be de-signed the same as those over 50,000,000 and willbe inert gas pressurized .

(4) Final Selection. The engineer designing


the system will investigate current trends indesign and equipment before selecting the system .Typical basic flow diagrams for the systems areshown on figures 1-5, 1-6, 1-7, and 1-8. Figure 1-9illustrates inert gas-pressurization using variablegas quantity with gas recovery. The total heatingsystem plan must be reviewed to determine anddevelop costs for incremental development basedupon proposed development phases of the masterplan . Tabulate the system plan by developmentphase and determine costs using current costingprocedures .

TM 5-810-2/AFR 88-28

1-13

TM 5-810-2/AFB 88-28

1-14

Reprinted with permission from1987 ASHRAE Handbook HVACSystems and Applications


Figure 1-5. Flow Diagram Steam Pnmsurized Single Circulation System.



Figure 1-t".

Flow Diagram Steam Pressurized Dual Circulation System .

TM 5-810-2/AFR 88-28

1-15

TM 5-810-2/AFR 88-28

1-16

e HTW GENERATOR

MAKE-UPTANK

DUPLEXMAKE-UPPUMPS

DUMP VALVEACTIVATEDBY HIGH LEVELSWITCH

SYSTEMEXPANSIONLINE

-1rmloNo. .

HTW GENERATOR

SYSTEM FLOWBALANCEVALVE

ORIFICE PLATE

SYSTEM BY-PASSOPERATED BYLOW FLOW SIGNAL

INDICATING FLOWMETER WITH LAWCUT-OUT SWITCH

BURNER CUT-OFFSWITCH INSERIES WITHLOW FLAWSWITCH

WATERSUPPLYMETER

DISCHARGETO WASTE

VENTEXPANSIONTANK

VENT

RETURN

U.S . Army Corps of Engineers


SUPPLY

SAFETYVALVE

IGH LEVEL ALARM

EXPANSION RANGE

LOW LEV PUMP STARTLOW LEV CUT OFF

SYSTEMPUMPS

THERMALISOLATIONLOOP

N2 PRESSREGULATOR

N2STORAGE

Figure 1-7.

Flow Diagram Inert Gas Pressurized Single Circulation System.



SYSTEM CIRC PUMPS

Figure 1-8. Flow Diagram Inert Gas Pressurized Dual Circulation System.

TM 5-810-2/AFR 88-28

1-17

TM 5-810-2/AFR 88-28

1-18

1-11 . ECONOMIC JUSTIFICATION


SUCTION SIDE Of MAIN

RELIEF VALVEMESSUREGAUGEPRESSURE CONTROLLERLOW PRESSURE


CIRCULATING PUMPS

OS -NIGH AND LOWPRESSURE ALARM

6O- WATER LEVELCONTROL

Figure I-9.

Inert Gas-Pressurization Using Variable Gas quantity with Gas Recovery.

Heating systems for all installations will be de-signed for lowest overall initial and operating costsfor the life of the facility.a. Economic Analysis.

The selection of one par-ticular type of design for a heating plant, whentwo or more types of design are known to be feasi-ble, must be based on the results of an economicstudy in accordance with the requirements of ap-plicable criteria . The results of all studies are tobe included in the design analysis documentationfor the project. Clarification of the basic criteriafor a particular design application in the MilitaryConstruction Program may be obtained by requestto HQ USACE (CEMP-ET), Washington, DC20332-1000 .

b. Central Heating Plants . Central heatingplants are justified when the total life cycle costsof central heating plants with connecting distribu-tion systems for groups of two or more independ-ent buildings (to be built simultaneously or withina period of years) are less than totals for individ-ual heating plants which would provide the sameservice. However, this comparison does not applywhen steam-electric power plants are involved andthe overall cost of providing heat from extractionsteam would be less than either of the above meth-ods. Central plants will have their own enclosures,but when economically justified, small plants maybe located in one of the buildings of the facilitiesthey serve.

c. Individual Heating Plants . These plantsmay be in the buildings they serve or in separatebuildings when economically justified . Such plantsshall be considered in preference to central heat-ing plants under the following conditions :

(1) When total life cycle costs of individualheating plants are less than the costs of a centralheating plant with connecting distribution pipingto buildings receiving heat .

(2) When installation and maintenance costsfor constructing an extension of an existing distri-bution system from a central plant to an isolatedbuilding are not economically justified .

(3) When dispersal of facilities and continuityof services are so essential that disruption of serv-ice to a central heating plant or its distributionpiping cannot be tolerated .

(4) When only a single building is involvedwithout prospects of adding buildings in thefuture.d. HTW Versus Steam Heating Plants.

(1) A HTW central heating plant will be se-lected in preference to a steam plant under the fol-lowing conditions :

(a) When the thermal storage capacity of anentire HTW system results in less cost in equip-ment such as boilers, pumps, and piping.


TM 5-810-2/AFR 88-28

(b) When operating and maintenance costsof an entire HTW system are less than those of asteam system .

(c) When costs of excavation, makeup water,and heat losses are appreciably reduced by using aHTW distribution system .

(d) When the pressure and temperature re-quirements of heat using equipment may be satis-fied more economically by HTW distribution .

(e) When needed expansion of existingsteam or low temperature water systems are morecostly than installing a new HTW system .

(2) A steam central heating plant will be se-lected under the following conditions :

(a) When a HTW system cannot be justifiedon the basis of the analysis above.

(b) When fluid pressures and temperaturesrequired by equipment cannot be provided by aHTW system .

(c) When the engineering design requiredfor a HTW system and its equipment is not avail-able .

(d) When rehabilitation of an existing steamheating plant and distribution system is more eco-nomical overall.

1-19

2-1. HTW REQUIREMENTSThis chapter deals generally with the load charac-teristics and calculations for load summaries forHTW systems for area or district heating. Area ordistrict heating involves the distribution of heat tospace heaters and various equipment for processheating. The various types of heat users may beseparated into these two categories: space heatingloads which are subject to variations in weatherconditions and process heating loads which areusually steady. The general features of these appli-cations are discussed in this chapter. Equipmentcommonly used is described in a later chapter.

2-2. SPACE HEATING

CHAPTER 2LOAD CHARACTERISTICS AND CALCULATIONS

Space heating is usually provided by indirect heat-ing using secondary heating medium such assteam or hot water. High temperature may beused as a direct heating medium only in limitedsituations and only where close temperature con-trol is not a requirement.

2-3. PROCESS HEATINGThis term applies to all forms of heating otherthan space heating including domestic hot water;steam and hot water for kitchens, laundries, andhospitals; and steam cleaning and snow meltingequipment.a. Direct Process Heating. Direct use of high

temperature water is the most economical use ofheat transmission for air and roller driers andwashing equipment in laundries; for washingequipment in kitchens ; and for sterilizers andother equipment in hospitals. Equipment for directapplication will be of special designs and materialsand not standard designs of manufacturers,

b. Indirect Process Heating.(1) Domestic Hot Water. Domestic hot water

is required for showers, lavatories, bathrooms, andkitchens . It can be produced by high temperaturewater coils inserted in the lower part of the do-mestic water storage tank . Generally domestic hotwater temperatures up to 140 degrees F. are rec-ommended . Control of the quantity of high temper-ature water flowing through the coils is main-tained by an element sensitive to the temperatureof the water in the storage tank.

(2) Laundries.

To keep the high temperaturewater return temperature at a minimum, it is sug-gested that the heat requirements for laundries be


TM 5-810-2/AFR 88-28

separated as (a) hot water requirements, (b) low-pressure steam requirements, and (c) high-pressuresteam requirements . Laundries are generally con-sidered to require high-pressure steam at 75 to 100psig and low-pressure steam at 5 to 15 psig. Beforethe pressure ordinarily prescribed for steam laun-dry equipment is accepted as the basis for designsin high temperature water systems, careful consid-eration should be given to the actual equipmentneeds. Frequently, if the steam is generated inhigh temperature water converters, somewhatlower pressures can be used without sacrifice ofperformance. It is not economical to produce all ofthe steam needed in a laundry at the highest pres-sure in one central steam generating converterusing high temperature water as this will result inunnecessarily high return temperatures in thehigh temperature water distribution system .Therefore, separate steam generating convertersare used for high and low pressure steam. Low-pressure steam from 5 to 15 psig can be producedin a low-pressure steam generating converter. Ifspace heating steam at 5 psig is also required, acombination may be made producing 10 prig steamfor both purposes . However, such a combinationwith 30 prig steam would certainly be impractical.High-pressure steam requirements can be meteither by using high temperature water directly,provided that the equipment is designed for it, orby producing 75 to 100 psig steam in a separatehigh-pressure steam generating converter. Thehigh-pressure steam requirements are never alarge fraction of the total heat load for a laundry.The steam generator must be equipped with anautomatic condensate return system and makeupwater system as well as adequate controls to limitthe pressure of the steam when the maximumsteam pressure is reached. Hot water require-ments will be met, ordinarily, by a storage waterheater similar to a domestic water heater.

(3) Kitchens. Kitchens require hot water at140 degrees F. for dishwashing which may be ob-tained from a domestic water heater describedabove. Water at 180 degrees F. required for messand diet kitchen areas for final rinsing will be ob-tained from booster heaters located in the serviceareas. Steam at 40 to 80 prig is required in thekitchen for cooking. The high temperature wateris used, in this case, to generate steam at the re-

TM 5-810-2/AFR 88-28

quired pressures. Steam production will be similarto that discussed above.

(4) Hospitals.

Hospitals require domestic hotwater for baths, lavatories, showers, sinks, and forother uses . In addition, steam is needed for sterilizers and autoclaves at 40 to 80 psig . A high temper-ature water system has the advantage that heatcan be taken to any number of heat exchangers lo-cated adjacent to the sterilizers in different partsof several hospital buildings without needingsteam traps and condensate return .

(5) Steam Cleaning. Steam cleaning equip-ment, sometimes required for cleaning airplanesor machinery, operates by ejecting water usinghigh-pressure steam. In an HTW system thismethod would require production of high-pressuresteam by heat exchangers . It is advisable toemploy a high-pressure circulation pump whichcan supply water heated to the desired tempera-ture in a heat exchanger, thereby accomplishingthe same purpose as conventional steam cleaningequipment utilizing high-pressure water.

(6) Snow Melting. Snow melting may usehigh temperature water as the primary heat carri-er . The secondary heat carrier can receive its heatthrough a converter and may be conventional lowtemperature water of 150 to 200 degrees F. or,preferably, a suitable heat transferring fluid suchas Glycol or high temperature water. Both low andhigh temperature water should contain antifreezesuch as Glycol when used for this duty. If hightemperature water is used as the secondary heatcarrier, it should be heated to temperatures of 300to 350 degrees F. These elevated temperaturespermit spacing of the lines in snow melting coilsat 3 to 5 feet on centers, a design especially adapt-able for large aprons, runways, and other areaswhere an expansive surface justifies application ofhigh temperatures .

2-4. DIVERSITY FACTORSOn any system serving more than one point of use,the possibility of all use points requiring maxi-mum heat input is almost nonexistent. Therefore,diversity factors are applied to the demand loads.Each individual use point load is not diversified,but total system loads are. When a system is beingdesigned with plans for large numbers of futurebuildings, it is advisable to use diversity factorsonly for equipment sizing. Piping in the distribu-tion system should be designed for undiversifiedconditions to allow for unscheduled future addi-tions. This allows greatest flexibility in the piping.When a system is initially fixed with minimumfuture modifications or additions anticipated, thenthe distribution system piping will be designed

2-2


fully diversified to get the most economical instal-lation.a. Heating Loads.

A diversity factor of 80 per-cent is usually used for heating loads. This factormay be lowered to 70 to 75 percent in systems uti-lizing all automatic control for heating. The factormay also be adjusted after a review of the systemindicates need for both automatically controlledheating and nonautomatically controlled heating.

b. Process Loads. For process loads encoun-tered for only brief periods of time, such as for hotwater and process steam, a diversity factor of 65percent is usually used . The use point must beanalyzed for demand characteristics, and the di-versity factor adjusted upward with longer ormore continuous demands.

2-5. OPERATING

TEMPERATURES

ANDPRESSURESTemperatures and pressures used in HTW systemsdepend upon the nature of the application . Thefactors determining the flow temperatures are thehighest temperature needed in the system forheating or process requirements and the length ofthe distribution system . If the length of the distri-bution system exceeds six miles, it is frequentlyadvisable to maintain a flow temperature substan-tially greater than that needed so that the pipesize may be reduced. Pressures in the systemdepend upon the temperatures required and at alltimes are maintained higher than the saturationpressure corresponding to the water temperature.

a. Supply and Return Temperature. A supplytemperature limit of 440 degrees F. is generallyfound to be the economic limit for space and proc-ess heating because of the high pressures (400 psig)required . Higher supply temperatures require rap-idly increasing pressures throughout the system,and while high pressures result in higher heat car-rying capacities in smaller pipe sizes, the saving inpipe size is partially or fully offset by the expenseof generators, fittings, pipelines and heat exchang-ers strong enough to withstand higher pressures.Return water temperatures should be between 250degrees F. to 275 degrees F. minimum. Primaryfuels or alternate fuels with high sulfur contentcan cause generator tubes to corrode. The selectionof temperatures in the hot water generator duringall operating conditions should be given specialconsiderations with high sulfur content fuels.When flue gas is cooled below the dew point, thegas side of boiler tubes is subjected to corrosion.Corrosion is caused by moisture in the flue gasand acids which result from the combination ofcondensed moisture and sulfur compounds in the

flue gas. The higher the percent by weight ofsulfur the higher the minimum metal temperaturemust be to prevent this type of corrosion. When itis desired to transfer very large heat quantitiesover unusually long distances, an increase in thegenerator pressure to 600 psig corresponding to asupply temperature of about 480 degrees F. mightbe justified. For distances up to six miles, the high-est supply temperature will be selected below 440degrees F. The generator supply temperature isalways maintained somewhat above the tempera-ture of the water distributed to the system . Insteam pressurized systems this is necessary to pre-vent vaporization from taking place if the pumpsshould suddenly be stopped and where the heatfusers are located at elevations higher than thegenerator. The HTW system supply temperature ismaintained by blending the generator outputwater with the system return water at the pumpsuction header . The HTW generator water is pro-portioned through an automatic mixing valve. Dueto heat losses from the distribution system piping,the supply temperature at the use points will bereduced by approximately three to five degrees permile, depending upon the insulation efficiency andthe amount of water being circulated . The fasterthe water is circulated, the less the temperaturedrops. Process equipment using either direct hightemperature water or steam produced by convert-ers generally requires temperatures at the usepoints ranging from 250 to 400 degrees F. For ex-ample, some laundry drying equipment requiressteam pressures of 100 psig which necessitateshigh temperature water of more than 355 degreesF. Therefore a heating installation which includessuch laundry equipment will need a supply tem-perature of at least 375 degrees F. to provide fortemperature losses and use point requirementsand a generator pressure of no less than 175 to 200psig. It often happens that while temperatures ashigh as these are not needed for space heating,they are economical supply temperatures to usefor lower temperature applications because small-er pipe sizes are required. All HTW systems willbe designed for a minimum of 150 degrees F. dif-ferential between supply and return water tem-peratures. See table 2-1 for influence of varioustemperature differentials on system components ;compare 100 degrees F. with 150 degrees F. andnote the difference in pump horsepower and pipesizes as an example.b. Operating Pressure.

This pressure is directlyrelated to the operating temperature which is es-sentially equal to saturation temperature in theexpansion drum in a steam-pressurized system andslightly higher in an inert gas-pressurized system .


TM 5-810-2/AFR 88-28

Pressure head to overcome frictional and otherlosses must be added to the operating pressurewhen selecting pumps, piping, fittings, and othersystem components . The pumps should never beused as part of the pressurization system .

Table 2-l. Influence ofTemperature Differentials on SelectionofPump Sizes for HTWSystems.

2-6. MAXIMUM INITIAL LOADThe maximum initial load is the sum of heating,process, distribution loss, and plant auxiliaryloads, all suitably diversified . Maximum summerload is also determined so equipment may be se-lected for ultimate loads which are as compatiblewith this load as possible . If not compatible, thenspecial equipment for summer load only may beselected .

2-7. MAXIMUM ULTIMATE LOADMaximum ultimate load is the total of estimatedfuture loads added to the maximum initial load .

2-8. ESSENTIAL LOADThe essential load is the diversified load on allbuildings where no cutback can be tolerated plusthe minimum permissible loads where cutback canbe tolerated plus additional minimum heatingloads to avoid freezing.

2-3

TemperatureDifference (deg .F .) . . . . .. . . . . . . . . . . . ... . . . . . . . . 20 50 100 150 200

Dischargetemperature(deg . F .) . . . . . . ... .. . . . . . . . . 270 300 350 400 450

Returntemperature(deg . F .) . . . . . . ..... . . . . . . . . 250 250 250 250 250

Mean Temperature(deg. F.) . . . . . . ..... . . . . . . . . 260 275 300 375 350

Flow rate per 20Mega Btu/Hr (Mlbs/hr) . . . . . . . ... ... . . . . . . . . 1,000 400 200 133 100

Density ofReturning Water(lbs/gal) . . ... .. ... . . . . . . . . 7.86 7.86 7 .86 7 .86 7 .86

Pump Capacity(GPM) . . . . . ..... . . . . . . . . ... 2,091 840 421 283 213

Assumed pumphead (ft) . . .... . . . . . . . . . . .. 100 100 100 100 100

Pump HP required(HP) . . . . . . ..... . . . . . . . . . . . . . . 82.7 32.9 16.3 10.8 7 .9

Pump efficiencyW. . . . . . . . .. .. . . . . . . . . . .. .. . . 60 60 60 60 60

Pump suction size(not pipe size) ... . . . . . 8' 6' 4' 3`/a' 3'

TM 5-810-2/AFR 88-28

2-9. MATCHING PLANT CAPACITY TO LOADa. Maximum Initial Load Generation . Where

possible, size the initial heating plant for at leastthree generators of equal size. The maximum con-tinuous capacity of the plant with one generatordown will not be less than the essential plant load,and the total maximum continuous capacity of allgenerators will not be less than the maximum ini-tial plant load . Where the ultimate plant load isknown or may be estimated accurately, and wherethe construction program indicates plant expan-sion will be required within three years of thestartup of the initial plant, then the ultimate loadshall be weighed carefully as the basis for select-ing the capacities of the initial generators. Underno condition shall the flow through any one gener-ator be less than 100 percent of its designed capac-ity.

(1) Avoid installation of initial main genera-tors of capacities radically smaller than those tobe added when the plant is expanded to the ultimate size. Also avoid an unnecessarily largenumber of generators of size equal to the size ofthe initial generators .

(2) Include one small "summer load" genera-tor where increased efficiency at low loads eco-nomically justifies its installation .

(3) The selection of oversize generators for ini-tial installations will be submitted to HQ USACE(CEMP-ET), Washington, DC 20314-1000 or HQUSAF/CECE, Washington, DC 20332-5000, withsupporting data, for approval before proceedingwith final design .

(4) For plants over 50,000,000 Btu/hr, size theultimate heating plant for three or more genera-tors such that when one generator is down, the remaining generators shall carry the essential load .b. Minimum Load Generation.

Choose a meansof bridging the gap between minimum and maxi-mum loads to suit job conditions from the follow-ing design possibilities.

(1) Establish operating ranges of combustioncontrol.

(2) Provide manual operation at minimumload . The turndown range of burners, or one of anumber of burners on a generator, must includethe minimum load. By changing burner tips in oilfiring, very low minimum loads may be obtained.

(3) Establish

intermittent

operations

forplants 30,000,000 Btu/hr or smaller, which willpreclude a continuous watch or allow a single op-erator to leave a generator room for trouble callson other matters by providing an auxiliary, fullyautomatic packaged generator which will shut offon satisfaction of heat demand and restart auto-

2-4


matically when the demand increases. Avoid fullyautomatic generator operation of the entire plant.Plants over 30,000,000 Btu/hr will have a continu-ous watch.

(4) Use more but smaller generators to lowerthe plant's minimum capacity.

(5) Provide minimum load generators. Wherethe gap between a plant's minimum load and aplant's minimum capacity when using the maingenerators is very large, a small, fully automatic,packaged generator unit with its own circulatingpumps may be used to fill the gap.

(6) Consideration should be given to the factthat the peak operating efficiency of a HTW unitis at approximately 80 percent of design capacity.At 80 percent load, the operators should be start-ing to think about bringing an additional unit online and splitting the load . Typical operating effi-ciency curves are fairly flat from 50 percent to fullloads.

(7) The best control philosophy is to employ afully modulating burner with a turndown capabil-ity of at least 8 to 1. The unit can modulate overthis range and be set up for automatic recycleafter load decays below the maximum turndown.The simplest control utilizes single point position-ing jack shaft for fuel and air. This method pre-dominates the industry and eliminates many ofthe operational problems associated with more so-phisticated systems as well as the requirement'formore experienced operators.

2-10 . SYSTEM HEAT LOSSSystem heat loss is the heat loss from the distribu-tion system and is dependent upon the length oflines, ground water conditions, and type of conduitand insulation . It is recommended that a factor of5 percent be applied to the diversified peak load .

2-11 . FLYWHEEL FACTORFlywheel factor or storage effect of the system isanother consideration applied when selectingequipment for system sizing . Because of thevolume of heated water, there is a considerableheat storage which can be considered available atpeak design conditions . The accepted factor ap-plied to the peak load is 85 percent.

2-12 . CALCULATIONSFor the purpose of illustration of typical calcula-tions for a high temperature water system, an ex-ample is given in appendix B.a. Calculations of Temperatures and Flow Quan-

tities . The heat required by the using equipmentis usually the starting point in calculating the

return temperatures and the required flow quanti-ties in the system . It may be expressed using equa-tions which give results in pound per hour as fol-lows:Qi=w, (h.-h,)=w, cp (T.-T,).

(eq 2-1)WHERE: Q, is the heat used by equipment X, ex-

pressed in Btu/hr ; undiversified .w, is the flow rate, expressed in lb/hr, passingthrough equipmentX.h. is the enthalpy of the supply to equipmentX; expressed in Btu/lb .h, is the enthalpy of the return from equip-ment X; expressed in Btu/lb .T, is the supply temperature; expressed in de-grees F.T, is the return temperature from equipmentX.cp is the specific heat at constant pressure inBtu/1b.F

T, is determined by using equipment require-ments, i.e., assume heating with 220 degrees F. lowtemperature water, then with 20 degrees F. ap-proach on convertor 220+20=240 degreesF.=220+20=240 degrees F.=T, . Considering asystem with three heat using devices, use the fol-lowing equations: (the subscripts identify the con-sumer under consideration).Q=w, cp (T.-T,)

(eq-2-2)Q,=w2 cp (T,-T2)

(eq-2-3)Q,=wa cp (T.-Ta)

(eq-2-4)Qt=cp [(w,+w2+waXTd-w1T,-w2T2-wsT3]Qt=cP[(w.XTd-w,T,-w2T2-w2T2] (eq 2-5)

Since w.=(w,+w2+w2), the sum of the flowsthrough all three heat users equals the total flow,through the system . Qt is the total amount of undi-versified heat supplied to the system by the gener-ator and is equal to the sum of the heat suppliedto all consumers, or :Qt=Q,+Q+Q9

(eq 2-6)The return temperature, Tr, may be found fromthe above equations and the expression :


TM 5-810-2/AFR 88-28

Qt=w. cp (T.-T,.)

(eq 2-7)Combining, we get:Tr =(w,T,+w2T2+w2T3)/w, .

(eq 2-8)The calculations above will supply an average tem-perature condition for a system with a variety ofloads, i.e ., heating, domestic, and process. If thesystem is essentially heating (80 percent or great-er), then T.-Tr equal to 150 degrees F. design dropmay be used to simplify calculations without intro-ducing an unacceptable amount of overdesign.

b. Pump Selection. For a single circulationsystem, (or a one-pump system), minimum flow ca-pacity of any pump is based on the essential load,and pump head requirement is the resistancethrough the HTW circulation system added to theflow resistance through the HTW piping system .Total pumping capacity is determined by combin-ing the maximum initial load with the HTW circu-lation load . For a dual circulation system, (or atwo-pump system), minimum pump capacity isbased on the future anticipated load added to themaximum initial load. Multiple pumps are used toprovide the flow needed for the essential load oneach boiler. If there is no substantial anticipatedload, the essential load can be utilized for sizingthe generator circulation circuit. In any event, theminimum flow in this circuit must always be atleast equal to the essential requirements.c. Expansion Vessel Design . Graphical repre-

sentation of volumes required for calculations areillustrated in figure 2-1. Figure 2-1 illustrates acircular tank section as fora horizontal tank, how-ever, the expansion tank may be horizontal or ver-tical . For small systems between 1,000,000 and10,000,000 Btu/hr, it is practical to size the expan-sion vessel for the total water expansion from theinitial fill temperature. For larger systems waterexpansion is based upon operating conditions andnot on startup condition. When placing the systemin operation, it is necessary to bleed off thevolume of water because of expansion from the ini-tial starting temperature to the lowest operatingtemperature.

2-5

TM 5-810-2/AFR 88-28

2-6


(1) System Expansion Volume.

Water temper-ature is assumed to vary approximately 10 percentunder normal operating conditions. The temperature changes in supply and return lines are gener-ally computed separately and the results combinedto obtain the total system expansion volume.Where: Vf,=specific volume of supply piping

water at supply temperature.Vf2 =specific volume of supply piping water attemperature of 10 percent less .AVs=percent volume change in supply piping=(Vf,=Vf2) / Vf2X 100

(eq. 2-9)


Figure 2-1. Expansion Tank Volumes.

VIr,=specific volume at maximum return tem-perature .Vfr2=specific volume at minimum return tem-perature .

(This temperature is Tr less the 10 percent ATfound for Vf1 and Vf2) .

Ar=percent change in return system piping .=(Vfrt-Ir2) / VIr2Xl00

(eq 2-10)Then: Total expansion volume= V,

=A.+AVr/zooxsystem volume .

(eq 2-11)(2) Steam Expansion Vessel Design Data. The

minimum volume to be provided in a steam-pres-

surized expansion vessel is the sum of the systemliquid expansion volume, a reserve volume for 1/z-minute pumping capacity, and a pressurization al-lowance of 20 percent of the sum of the two vol-umes above, or as shown on figure 2-1 for a hori-zontal tank:

V, =system expansion.V2= 1/2-minute reserve for pumps based on thepump capacities outlined previously.V9=pressurization

space=0.2

(VI+Vg).(ASHRAE states that 20 percent of the sum ofV, +V2 is a reasonable allowance.)

Then : Minimum expansion tank volume=V, +V,+Vs+allowance for level controls +sludge allowance.

(eq 2-12)The allowance for level controls should = (tank di-ameter) (length) (1-ft. depth), or 150 cubic feetnominal allowance should be used to determinetank volume and diameter. Then this volumeshould be checked against tank diameter selected .The sludge allowance should be at least equal tothe bottom 6 inches of the tank, but the require-ment can vary depending on the size of the systemand HTW generator capacities . ASHRAE statesthat 40 percent of V, is a reasonable value. Tankdiameters over 8 feet are uneconomical because ofthe shell thickness required to withstand the oper-ating pressures and they are difficult and costly tofabricate. Bottom of tanks must be located abovethe high point of the HTW generators. System willbe designed that a single expansion vessel willmeet the operational requirements of the system .An exception may be a system which includes alarge process load requiring continuous operation.In such a situation two tanks might be needed toallow isolation of one tank to periodically inspectfor development of stress cracks without totalsystem shutdown . Steam-pressurized systems usu-ally require horizontal tanks since it is necessarythat the tank bottom be higher than the hot watergenerator outlet .

(3) Inert Gas Pressurized Expansion VesselDesign. A single expansion tank is preferred asthe most economical installation . A system withlarge process heating, which requires some contin-ous operation, might require two tanks each sizedfor 50 percent of total system expansion to allowfor one tank to be periodically inspected withouttotal system shutdown . A two-tank arrangement isshown in figure 1-8. When multiple tanks areused, nitrogen and water equalization lines are re-quired between the two tanks. The tank will con-tain a gas pressurization space and should be avertical tank to reduce the contact area betweenthe gas and water and thereby minimize gas ab-


TM 5-810-2/AFB 88-28

sorption by water. Gas wastage will affect operat-ing costs. The gas recovery system as shown infigure 1-9 should be analyzed on the economics ofeach application. It is generally more applicable tolarger systems, as shown in figure 1-8. The sim-plest gas pressurization system has a fixed quanti-ty of gas in the expansion tank to accommodatethe change in water volume within the tank . Asthe water temperature increases the expansion ofwater into the tank raises the pressure of thesystem and the system pressure decreases as thewater temperature of the system drops. The pres-sure is allowed to vary a minimum value abovesaturation pressure to a maximum determined tobe within pressure range of the piping equipmentand HTW generators. Referring to figure 2-1 andassuming a vertical tank, the minimum totalvolume required in the expansion tank isV, + Vs + V,9 + sludge allowance

(eq 2-13)Where: V, = a volume required for water expan-sion .

Vs= a volume for a 1/2-minute pumping re-serve.V9 = the pressurization space. Select an oper-ating range to keep system pressures withinpressure rating of valves and fittings and com-pute V,. The nominal operation pressureshould be a minimum of the saturation pres-sure for the HTW generator plus a differentialpressure (AP,). This differential pressure isusually taken as 40 psi. Since the returnwater temperature will vary, some pressureswill be lower and some higher than this as-sumed nominal operating pressure. Assume adifferential operating pressure range (AP2).Add 0.75 APQ to the nominal operation pres-sure to obtain the maximum operating pres-sure P,9 and subtract 0.25 AP, from the nomi-nal operation pressure to obtain a minimumoperating pressure, P, .

To size the expansion tank, select a convenientdiameter and compute the required tankheight as follows:

Let: Vu = volume in cubic feet per foot of tankheight = wD'/4where D = diameter of tank in feet.Vertical displacement for V, - V,/VuExpansion displacement: V., can be deter-mined from the relationship V, minimum =P,V,/(Pp-P,)Displacement = V,/Vy.

Total tank height is the sum of the above comput-ed vertical displacements plus an allowance for

2-7

TM 5-810-2/AFR 88-28

sludge equivalent to 10 percent of V,+Vs.For the smaller systems it is possible to accommo-date total system expansion volume in the expan-

2-8


lion vessel . The makeup water should be handledin a separate tank and not be part of the expan-sion vessel circuit.

3-1. DESIGN OF SYSTEMThe distribution system for a HTW heating systemincludes the supply and return piping, conduit (ifburied), and related equipment extending from theheating plant to the buildings to be heated. Themaster or site plan of the facility must be studiedto plan the distribution lines. The heat usingequipment (consumers) of a facility may be servedby a number of different arrangements, so variousdistribution plans should be studied before choos-ing sizes and the number of distribution zones.Ideally, zones should be drawn to segregate thoseconsumers having high return temperatures fromthose having low return temperatures ; thosewhich require constant high supply temperatures,such as those serving processes, from those whichmight have viable supply temperatures; and, final-ly, separating consumers which must be operatedthroughout the year from those which operateintermittently . These separations of loads helpmake the system as flexible and economical as pos-sible. One factor which tempers these ideal ap-proaches is the selection of distribution lines basedon maximum capacity of lines of practical dimen-sions. A layout will be prepared following the pro-cedures outlined above showing the distributionsystem and the flow required in each zone, mainsupply, and return line and branch line . In con-trast to steam systems where buildings can be con-nected to the steam mains which form an openloop around the site, high temperature water willhave a circulating system with supply and returnmains. However, it is possible to connect designat-ed buildings, such as hospitals, to two different dis-tribution zones so that either of the zones mayserve these buildings at any time. Switching overfrom one zone to the other must be done very care-fully, however, to avoid shocks . For this reason, allvalves used for changing zones must have by-passes . Example distribution system layouts areshown in appendix C.

3-2. PIPE SIZINGa. General.

In a HTW district heating system,pipe sizing is determined largely by the allowablevelocities used for design and resultant pressuredrops. Minimum allowable velocity should be 2feet per second (fps) to avoid stratification exceptthat minimum pipe size shall be lV2 inches, maxi-mum allowable velocity should be 7 fps. Five fps isa good nominal design velocity . The maximum vo-


CHAPTER 3DISTRIBUTION PIPING AND EQUIPMENT

TM 5-810-2/AFR 88-28

locity of 7 fps may be used for long delivery lineswith pipe sizes 6 inches and larger which have fewbranch takeoffs, provided protection is incorporat-ed in the design to take care of surges caused bypower failure at the system circulating pumps.Lower velocities result in lower transient surges inlong lines.b. Calculation of Pressure Drop.

The pressuredrop due to the friction of water flowing throughpipe and fittings may be calculated by formulae ascovered in ASHRAE "Fundamentals Handbook",or may be selected from charts which have beendeveloped and published in many handbooks.Since selection of pipe sizes is limited to sizes com-mercially available, using extensively refined for-mulae for extreme accuracy prove time-consumingand impractical for the average system .

3-3. DISTRIBUTION PIPINGa General. Standard weight steel pipe, Sched-

ule 40, is generally satisfactory for most HTW sys-tems. Seamless is the preferred type fabrication;however, it is more expensive than welded pipingand this should be considered in designing thesystem . Extra strong weight (Schedule 80) pipewill be used on sizes 2 inches and smaller. Alljoints will be welded and designed in accordancewith ASME 1331 .1, however, Class 300 insulatingflanges will be used for dielectric connections atevery pipe connection from a trench system oraboveground system to an underground systemand at dissimilar metals. Gaskets for flanged con-nections will be of material designed for dielectricservice for pressure/temperature at each point ofapplication in the system . Flanged connectionsmay be allowed at connections to converters andequipment. Great care must be exercised in thedesign and installation of piping and vessels toensure enough flexibility to permit thermal expan-sion to take place without creating stresses greaterthan those allowable for the pipe, fittings, or ves-sels . Distribution lines are installed with properlydesigned U-bends, L-bends, or z-bends to permit ex-pansion and with anchor points and guides whereneeded . Provision must be made for venting anddraining all lines. Branch takeoffs must be de-signed properly to prevent interference with flowthrough both the main distribution lines and thebranch circuits . Serious problems have occurredwhere specifications depend on ASME B31.1 for

TM 5-810-2/AFR 88-28

nondestructive testing. ASME does not requiresuch unless the temperature is above 350 degreesF. at 1025 psi. Radiographic inspection of all weldsin the distribution systems may be highly benefi-cial at little increase in cost. Proper pipe andvessel design stress limits require a workingknowledge of the provisions of ASME B31.1, para-graph 102.3 .2 relating to thermal stress.b. Expansion Loops andAnchors.

Steel pipe ex-pands approximately 3 inches per 100 feet whensubjected to a temperature change of 400 degreesF. Refer to table Cl in ASME B31 .3 for unit ex-pansions for steel pipe . Expansion of steel pipewith temperatures as follows:

3-2

Pipe Temperature


Expansion,in ./100 ft,from 0

degrees F. topipe

temperature

The flexibility of piping systems must be adequateto prevent thermal expansion from causing unsafestresses in the pipe and fittings, excessive bendingmoments at the joints, or excessive thrusts onequipment or at the anchorage points . Credit forcold springing will not be used in calculations fordetermining amount of expansion to be incorporat-ed . Methods for calculating expansion bends areavailable in handbooks and in pipe manufacturers'literature . Expansion loops or elbows will be usedas the most practical means of accommodating ex-

pansion of distribution piping located either aboveor below the ground . Loops and elbows are prefer-able to expansion joints because they are not sub-ject to the hazards of misalignment which cancause line breaks . Expansion U-bends are general-ly located at the midpoint between two anchorpoints with guides at the loop and a vertical re-straint at the midpoint of the loop, if aboveground.It is preferable to keep the axes of the long legs ofthese bends in a common plane. Anchors, solidlyconnected to a concrete base, must be sufficientlystrong to withstand the full unbalanced pressureof the water and the stresses of expansion as wellas the weight of the line filled with water. An-chors are commonly located between expansionbends both in underground and aboveground in-stallations . Pipe guides of the type used in steamline construction must also be used . In general,the procedures used in the design of steam linesshould be followed taking into account the addi-tional weight of the water carrying conduit.c. Expansion Joints . Slip, bellows, ball, and

corrugated type expansion joints are not practicalwith HTW systems and will not be used .d Air Bottles, Drains and Vents.

Air bottles ofadequate size must be provided at all the highpoints of HTW systems. Automatic drain valvesare not practical. The vents on air bottles veryseldom have to be used since air enters the systemonly when filling the system with water or whenstarting up after a long stoppage . Drain connec-tions will be installed at the low points of the dis-tribution piping. When drains cannot be run tosewers, draining may be accomplished by portableself-priming pumps. Figure 3-1 shows a typicaldrain and a typical vent with air bottle . When dis-tribution piping is run underground, it is neces-sary to locate drains and vents in suitably locatedmanholes with connections to the outside. Drainsand vents are required on both sides of sectioningvalves .

100. ... . .. . . . . . .... . . . . . . . . . . ... . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . ... . . . . . . . . ... . . . . . . . . .. 0.6150 . . . . . . . . . . . . .. . . . . . . . . ... . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . ... . . . . . . .. . . . . . . . . .. . . . 0.9200 .. . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . .. . . . . . . . . . ... . . . . . . . . . . . . . . . . ... . . 1 .3250 ... . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . .... . . . . . . ... . . . . . . . . . .... . . . . . . .... . . . . . ... . . 1 .7800 .... . . . . . ... .. . . . . . . . . .... . . . . . . . . .. ... . . . . . .... . . . . . . . . .. .. . . . . . . . . . . .. . . . . . . .. . . . 2.2850 .... . . . . . ... .. . . . . . . . . . ... . . . . . . . . .... . . . . . . . . . .. .. . . . . . ... .. . . . . . . .. . . .. . . . . . ... .. 2.6400 . ..... . . . . . . ... . . . . . . . . . . ... . . . . . . ... .. . . . . . . . . .... . . . . . ... .. . . . . . . . . . ... . . . . . ..... 3.0450 .... . . . . . . .. .. . . . . . . . . . . .. . .. . . . . . ... . . . . . . . . .. . . .. . . . . . ... .. . . . . . ... . . . . . . . . . .. . . . 3.5500 . . .. . . . . . . . . ..... . . . . . . . . . . . . . . . . . .... . . . . . . . . . . ... . . . . . . . . ... . . . . . .. . .. . . . . . . .. . .. 4.0

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TM 5-810-2/AFR 88-28

branch valves at junction with main piping on ac-count of inaccessibility in the case of fire or otherhazards which might make the building inaccessi-ble. Valves such as sectioning valves in the mainswhich are used only in the fully open or fullyclosed positions can be gate valves . All othervalves which must be opened gradually or whichare used to make adjustments preferably should beglobe types. Branch valving is commonly locatedin manholes when the piping is run undergroundor, when lines are not too far below grade, in valveboxes. In either case branch valves must be acces-sible for maintenance.f. Sectioning Valves. Sectioning valves on the

supply and return lines must be provided at anumber of locations to isolate sections of themains for repairs and emergencies. Sectioningvalves are operated only at infrequent intervalsand, therefore, should be valves especially de-signed for easy operation, tight seating, and resist-ance to corrosion. Lever-operated rising stemvalves would be suitable sectioning valves for thesmaller pipe sizes. Gate valves designed for thepressures and temperatures expected are generallyused for section valves .g. Bypass Values. Bypass valves may be in-

stalled at the end of zones which are planned forextension to assure circulation of hot water andprevent stagnation . Normally a 1 1/2-inch globevalve is satisfactory for this service.h. Insulation.

All parts of the plant supply andreturn lines operating above 140 degrees F. will beinsulated with insulation suitable for the operat-ing temperature. Flanges, valves, and pumps shallalso be insulated.

i. Relief Valves . Relief valves should be highquality carbon steel with stainless steel disc andnozzle rated at 750 psi and 800 degrees F. Valvesshould be of the type which can be repacked with-out removing them from the line.

3-4. UNDERGROUND AND ABOVEGROUNDSYSTEMS.a. General. High temperature water distribu-

tion lines may be run either above or below theground . Underground lines may follow the con-tours of the ground. Lines aboveground may berun on short concrete supports, or 10 to 12 feetabove the ground on wooden, concrete, or steelsupports . They may be run over or under obstruc-tions without difficulties. While concrete supportsmay be best suited to many applications, they arenot necessarily the least expensive. It is essentialthat the lines be protected by metal covers calledjackets. Underground piping may be located 2 feetbelow the ground level. Although it is not essential

3-4


that the lines be located below the frost line, hightemperature water lines have not been found tofreeze when located only 2 feet underground evenwhen they have been out of operation for manyhours. One of the greatest concerns in the designof underground piping is the elimination of mois-ture from conduits and manholes. Sumps will beprovided in the manholes with facilities for pump-ing out the water. Proper sealing of conduit en-trances and exists in manholes is of very great im-portance . The arrangement of manholes for hightemperature water applications is similar to thatused for underground steam lines except that noprovision need be made for the disposal of dripline condensate.b. Basic Criteria for All Types of Conduit Sys-

tems. Since all underground systems eventuallybecome wet and water is the major adverse factorencountered, all systems will be of a type that canbe drained and dried. The insulation will be of atype that can be drained and dried. The insulationwill be of a type that can withstand repeated orextended boiling and drying without physicaldamage and/or loss of insulating characteristics.The project designer will include in contract docu-ments the following information regarding the siteand where conditions vary along the proposedpath of the system and will define separately theconditions for the various segments of the system .If at all practicable, a geotechnical engineer famil-iar with underground water conditions at the siteshall be employed to establish the following classi-fications. If the system to be installed is expectedto be used for less than 10 years, considerationshould be given to classifying the site one classlower than it would ordinarily be classified (e .g .,bad rather than severe).

(1) Severe.

The water table is expected to risefrequently above the bottom of the system ; or thewater table is expected to occasionally rise abovethe bottom of the system and surface water is ex-pected to accumulate and remain for long periodsin the soil surrounding the system .

(2) Bad. The water table is expected to riseoccasionally above the bottom of the system andsurface water is expected to accumulate andremain for short periods (or not at all) in the soilsurrounding the system ; or the water table isnever expected to rise above the bottom of thesystem but surface water is expected to rise abovethe bottom of the system but surface water is ex-pected to accumulate and remain for long periodsin the soil surrounding the system.

(3) Moderate. The water table is never ex-pected to rise above the bottom of the system but

surface water is expected to accumulate andremain for short periods in the soil surroundingthe system.

(4) Mild. The water table is never expectedto rise above the bottom of the system and surfacewater is not expected to accumulate in the soilsurrounding the system .c. Corrosive Classification.

The soil of each siteshould be classified as corrosive or noncorrosive onthe basis of the following criteria:

(1) Corrosive. The soil resistivity is less than30,000 ohms per centimeter (ohm/cm) or straydirect currents can be detected underground; allsites classified as having severe water conditionsshould be classified as corrosive.

(2) Noncorrosive.

The soil resistivity is 30,000ohm/cm or greater and no stray direct currentscan be detected underground. The classificationshall be made by an experienced corrosion engi-neer after a field survey of the site carried out inaccordance with recognized guidelines for conduct-ing such surveys. The results of the field survey

information Handling Services, 2000

TM 5-810-2/AFR 88-28

should be summarized in a report and submittedby the design organization to the Contracting Offi-cer with the contract documents.d Soil pH. If there is any reason to suspect

that the soil pH will be less than 5.0 anywherealong the proposed path of the system, pH meas-urements should be made at close intervals alongthe proposed route, and all locations in which thepH is less than 5.0 should be indicated in the con-tract documents. Soil pH should be determined byan experienced geotechnical engineer, preferablythe same engineer responsible for other soils engi-neering work . The load bearing qualities of thesoil in which the system will be installed should beinvestigated by an experienced geotechnical engi-neer, again preferably the same engineer responsi-ble for other soils engineering work, and the loca-tion and nature of potential soils problems shouldbe identified.

e. Cathodic Protection.

Cathodic protection willbe designed in accordance with TM 5-811-7 whenrequired for metallic casing systems.

3-5

4-1. INTRODUCTIONThis chapter describes the elements that go intothe central heating plant of a high temperaturewater system . The information is of a generalnature because of the many equipment manufac-turers involved in the special specification require-ments of each system .a. Capacity. The capacity of the central heat-

ing plant must be large enough to handle thedesign loads of the connected system . The capac-ities of installed generators must be able to pro-vide the essentfal plant load with one generatorout of operation. Both winter and summer loadsand day and night loads will be considered whenthese loads are highly variable . In addition, plan-ning plant size must phase heating loads for theinitial installation and for the future extensions sothat the heating plant will operate efficientlyduring each stage of expansion. The individualgenerators selected should not be too small andthe larger the number of generators the more eco-nomical will it be to adapt to the variations in theheating load and to operate the plant continuouslyat maximum efficiency . Usually three or moregenerators are required to permit installation ofthe plant in increments.

6. Savings.

If the heating plant can be de-signed initially for the final maximum size, carefulanalysis will be undertaken to see whether subdi-vision of generator units greater than three orfour is justified . The savings obtainable by reduc-ing the size of the spare unit may be overbalancedby the additional costs created by the subdivision.In this evaluation the possible increase in operat-ing efficiency should not be overlooked .

4-2. HTW GENERATORSMost HTW generators are the fuel-fired type gen-erating HTW directly. There are other installa-tions where HTW can be generated from steam.One such condition would be obtaining HTW froma steam-powered turbine generating electricpower. Boiler operating temperatures and pressurewould fall within the range of heat requirements


CHAPTER 4HEATING PLANT

TM 5-810-2/AFB 88-28

for generating HTW. There may also be instancesof process steam systems being available toproduce HTW.a . Steam-Activated HTW Generators.

(1) Heat-Exchanger HTW Generator. A basic(shell-and-tube) heat exchanger could be utilized togenerate HTW, and the HTW system from theoutlet of the HTW generating heat exchangerwould be a standard system requiring expansiontank, pressurization, and circulating pumps withthe return water directed to the heat exchanger.

(2) Cascade HTW Generator. There is anothertype of HTW source which utilizes a cascadeheater . This is a direct contact vertical vesselwhere system return water is cascaded over traysin the upper part of the vessel and makes directcontact with the steam supply. The lower part ofthe cascade heater serves as the system expansiontank and the upper part serves as the steam-pres-surization chamber. Surplus water generated bythe water absorption of steam is usually returneddirectly to the boiler through a pipe from the cir-culating pump discharge header . This unit re-quires little equipment and lends itself to being lo-cated in any location convenient to the steam dis-tribution system . Different zones can be handledby providing multiple units matching zone require-ments. A typical cascade HTW system is shown onfigure 4-1. Cascade heaters are especially applica-ble where high-pressure process steam is available.An ideal situation would be a boiler plant supply-ing cogeneration steam turbines for local powerand also supplying high-pressure process steam.Since the cascade heater raises the water tempera-tures by direct absorption of steam by the HTWcircuit, the practically continuous excess liquidcontent is bled off and becomes part of the boilercondensate return system . This is an ideal condi-tion both for heating HTW and obtaining goodquality condensate return . If high-pressure steamis distributed to distant buildings, then a cascadetype heater fits into the remote location and sim-plifies zoning and running of extra water linesfrom a central plant.

TM 5-810-2/AFR 88-28

4-2


b. Direct-Fined HTW Generators. Most manu-facturers offer water tube generators specificallydesigned for high temperature water applicationusing controlled forced circulation. The majority ofgenerators installed are of this type . Natural cir-culation generators using larger-sized water tubeswithout orificing should not be used . Forced circu-

Figure 4-J. Cascade HTW System in Process Steam System.

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aw

lation generators for high temperature water ap-plications use the recirculation principle. Evapora-tion is limited to the small amount necessary toraise the return water from the system to thesaturation temperature. The forced circulation se-cures positive flow in one direction through each ..~tube circuit at all times regardless of the rate of

heat transfer. Orifices with strainers placed at theentrance of the tube circuits, proportion, equalize,and direct the required amount of water flow ineach circuit. The degree and location of the restric-tion may be varied to suit the length, arrange-ment, and heat absorption of the circuit. Forcedcirculation generators are usually designed withsmaller overall heating surface than natural circu-lation generators of the same output . This does notmean that the critically loaded parts of the gener-ator surface such as the water walls are morehighly loaded than they would be with natural cir-culation generators . The higher overall heat trans-fer per unit surface area results largely from thefact that a larger portion of the generator surfaceis operating under higher loading than would bepossible with natural circulation generators. Innatural circulation generators, the heat absorbedby the water produces the buoyancy which startsand maintains the circulation. The circulationtherefore varies with the rate of firing or the localrate of heat transfer . In order not to restrict thiscirculation, comparatively large tubes are favored.The effectiveness of the heating surface of the gen-erator is reduced by the fact that the necessarydowncomers must not be heated as intensely asthe risers so as to assist circulation . This increasesthe size of natural circulation generators in com-parison with forced circulation generators . Greatcare must be exercised in the application of natu-ral circulation generators to HTW systems so thatthe forced circulation in the external circuit doesnot interfere with the natural circulation insidethe generator.c. Generator Configuration. Two basic outlines

have evolved and are in general use today: thehorizontal unit, in which gases travel horizontallyout of the furnace and through the convector sec-tion, has a large base area and a relative lowheight ; and the vertical unit, in which the convec-tor section sets above the furnace and which has arelatively small base area and a high profile. Thisvariance in shape affects the space design in theheating plant so that early selection of the genera-tor is desirable to obtain an economical structure.If early selection is not possible, the buildingshould be designed to house either configuration.d. Package Unit. The term "package unit" is

used rather loosely, and manufacturers ship pack-aged units in more than one piece. However, thetendency is to prefabricate as much as is economi-cally feasible and to field-assemble as little as pos-sible. Railway and road clearances are the factorslimiting size. Most manufacturers supply units upto 75 Mega Btu/hr in a single factory-assembledpackage, and sizes up to 160 Mega Btu/hr in two


TM 5-810-2/AFR 88-28

pieces: the furnace section couples with the con-vector section on site . Above the 150 Mega Btu/hrsize, the units are generally "knocked-down" andbuilt up in panels : four walls, base, and top forfield assembly . Through all the size ranges, burn-ers and accessories may be shipped separately ifmounted dimensions exceed route clearances . Stok-ers for coal firing are generally installed at thesite .

4-3. COMBUSTION EQUIPMENT AND CON-TROLSFuel for generators will be selected in accordancewith the current DOD policy and agency or servicedirectives and criteria. Select fuels which producethe required plant performance at lowest overallproduction cost for an entire plant, including am-ortization and operating and maintenance costs forall elements.a. Combustion Equipment.

Oil and gas burnersshould be UL 733 and UL 795 approved units.Stokers for coal will be selected on the basis ofgenerator size, the type of coal used, and the char-acteristics of the load . In general, however, spread-er stokers are recommended as most suitable forHTW generators . They will be of the continuousash discharge type . Oil storage and handlingequipment and coal and ash storage and handlingequipment should be selected and sized based uponthe size of burners or stokers selected and agencyor service directives and criteria.b. Control Systems.

Combustion controls usual-ly are set to regulate the firing rate to maintain apreset water temperature at the discharge nozzleof the generator. For closer control and morestable operation, anticipating compensation is fre-quently used . Since flow through the generator isvery rapid, the sensing device must respond rapid-ly to temperature changes. One arrangement, de-veloped specifically for high temperature water ap-plication and successfully applied on several in-stallations, make use of two small and extremelysensitive and highly reliable solid-state sensorscalled thermistors. One is located in the generatordischarge water and the other in the systemreturn water flowing to the generator. These sen-sors are electronically balanced in the bridge cir-cuit which adjusts the firing rate of each generatorin direct proportion to the temperature spread be-tween inlet and outlet water and maintains theoutlet water temperature at the controllers set-ting. Manual potentiometers are provided to adjustsettings and throttling range. The two commontypical control systems are shown schematicallyon figure 4-2.

4-3

TM 5-810-2/AFR 88-28

4-4

Fuel/Air RatioController

U .S . Army Corps of Engineers


Master Controller

Furnace Draft Signal

METERING TYPE

Water Temperature Signal

POSITIONING TYPE

Rgure 4-2.

Typical Combustion Control Systems .

(a) Positioning controls .

(e) Metering controls.

Actuator ForGenerator Outlet

Damper

Actuator ForFuel Control

Valve

(1) For systems having generators of 20 Mega

(d) For systems having generators of moreBtu/hr capacity each, or less, the following is rec-

than 20 Mega Btu/hr capacity, the followingommended :

should be used :

(b) Separate actuators on fuel and air.

(f) Fuel and air actuators linked for tandem

(c) Manual fuel-to-air ratio controllers .

operation, where physical location permits.

(g) Automatic fuel-to-air ratio controllers.Safety devices and limiting switches will be inter-locked for low water flow, high temperature, andhigh pressure at each generator. Good practice dic-tates a redundancy of sensing elements; that is,even though a sensor is signalling a generator'sdischarge water temperature to a controller, a dif-ferent sensor will be used to indicate high limitingtemperature.

(2) To aid pollution control and energy conser-vation, closer control of fuel-air ratio is evolving .Research on current methods will be made prior tofinal selection of controls .

4-4. PRESSURIZATION SYSTEMa. Criteria for Selection of Pressurization

Method. The following criteria for selection willbe used :

(1) Collapse of pressurization must be avoidedwhen system is in operation.

(2) Water must be kept free of oxygen.(3) The control of pressurization must be

simple .(4) Waste of compressed inert gas must be

avoided.(5) System should minimize fluctuations in

HTW system pressure and outgoing temperatureof water from generator.

(6) System should provide for a modulatingcontrol of firing rates.

(7) Maintenance of pressurization should beeasy and minimal.

(8) Installation and operating costs should below.

(9) Proper utilization of safety devices shouldbe assured.b. Steam-Pressurization. One type of steam

pressurization is acceptable, that is a separate ex-pansion vessel . This system has the forced circula-tion hot water generators connected to a separateexpansion vessel . System water is drawn from theexpansion vessel for supply to the pumps. Waterin the expansion vessel is allowed to flash intosteam to maintain sufficient pressure on the entiresystem . Sufficient net positive suction head mustbe maintained to prevent flashing at the eye of thepump.c. Inert Gas-Pressurization . Two methods of

inert pressurization are acceptable.(1) Separate expansion vessel, variable gas

quantity, constant water quantity . This system in-cludes a high and low pressure gas receiver, compressor, and necessary control valves. As water ex-pands, the control valves open at a preset point al-


TM 5-810-2/AFR 88-28

lowing the inert gas to relieve to a low-pressure re-ceiver. A compressor picks up the gas in the low-pressure receiver and compresses it into a high-pressure receiver for storage. As water contracts,the control valve closes and the gas supply valveopens to permit the required increase in gas quan-tity .

(2) Separate expansion vessel, fixed gas quan-tity, fixed water quantity, variable pressure . Aswater expands, inert gas is compressed, increasingsystem pressure . As water contracts, system pres-sure is decreased. Although system pressure is al-lowed to fluctuate, the pressure is never allowed todrop below saturation pressure. The need for keep-ing the expansion vessel within reasonable sizeand for avoiding pressures in excess of the ratingof fittings, piping, and heat exchangers usuallylimits the size of such systems.d. Sizing the Expansion System.

The expansionsystem is sized to take the volume fluctuations atoperating conditions. It is impractical to try to sizean expansion system for HTW based upon coldconditions, as too large a volume change takesplace. The expansion system must, however, in-clude reserve capacity for some sludge buildup andpump supply of 30 seconds for each circulatingpump (both boiler and system on separate pumpsystems) .

(1) Steam-pressurization makes use of a large,horizontal expansion drum with a steam chest inthe upper void to impose saturation pressure onthe system . Generator discharge water is taken tothe drum supply water to the distribution systemis taken from the drum so that drum water is thehottest in the system with the highest vapor pres-sure . The saturation pressure corresponding to thehighest drum water temperature is the systempressure . Consequently, system operating pres-sures are the very lowest possible . Inert gas pres-surization of the void space of the expansion drumat about 40 prig above saturation pressure willassure stable operation and prevent flashing withchanges in system volume demand. With inert gaspressurization the expansion drum can be smallerand set vertically at grade level. In some cases theuse of inert gas pressurization will result in in-creasing the system pressure so that valves andfittings would be heavier and costlier and requireadded surveillance and maintenance of the pres-sure control system by the operating personnel.

(2) Draining and filling connections must beprovided so that the expansion vessel and the con-necting lines can be completely drained and completely filled with water after a shutdown. Ventsare necessary at all the high points of both the

4-5

TM 5-810-2/AFR 88-28

tank and piping to permit purging of any trappedair. A single tank is preferred on the basis that itis the most economical installation . If the volumevariation cannot be practically handled in onetank, two, or a maximum of three tanks may beused . These tanks must be equipped with equaliza-tion nitrogen and water lines to permit adjustmentof water level and pressure differences betweentanks. Provision must be made for filling anddraining the expansion drum . Suitable water levelcontrols may be installed to provide for overflow ifthe water level gets too high and for manual orautomatic supply if it should get too low. The over-flow connections must be equipped with a single ordouble manual shutoff valve and may be equippedwith a cooler or a cold water mixer to eliminateflashing and increase the capacity of the line .Large quantities of water should be drained onlythrough double sets of blowoff valves in case oneshould fail to seat. Makeup and emergency feedconnections to the system will have nonreturntype valves such as check valves to prevent systempressures from being imposed on equipment whennot in service. Safety valves must be installed inthe steam space for the full generator capacityconnected to the expansion drum . These should beset according to the practice of setting boilersafety valves. Purge or vent valves need not belarger than 1 1/z inches. Saddles and supports forhorizontal expansion tank must be designed topermit movement due to thermal expansion of thetank and to support a weight of 1 1/a to 2 times theweight of the tank and its contents . Pressuregauges are required at the top of the tank, andthermometers should be located at several levelsof the expansion tank, usually at 1/a points ofvessel . A long gage glass is needed to follow thewater level. Leaving a nitrogen cushioned expan-sion tank uninsulated should be considered so thetank will radiate and operate at a lower tempera-ture. The inclusion of a heat trap loop in the ex-pansion line is always desirable if space permits asthis will aid in preventing heat migration from thereturn piping into the water in the tank .

4-5. PUMPSSingle-stage centrifugal pumps are used to circu-late high temperature water through the distribu-tion system and through the generator or throughboth the distribution system and the generator, de-pending upon the type of circuit selected for theHTW system . Pumps selected for this service mustbe designed especially for high temperature waterto secure efficient, reliable operation with a mini-mum of maintenance. Standard pumps for this ap-plication have capacities ranging from 100 to 2000

4-6


gpm, with heads up to 250 feet of water. The headcharacteristic of circulating pumps should be flatin order to deliver nearly constant head through-out the range of operating capacities . At the sametime the maximum head should occur at shutoffand should fall off gradually up to maximum gal-lonage, with a decrease in pressure no more thanabout 15 percent below the shutoff pressure at themaximum operating gallonage. Pumps operatingin parallel will carry more or less their proper por-tion of the capacity when the pump characteristichas a continuous drop of 10 to 15 percent. Circula-tion pumps are located in the supply line of thesystem to maintain the highest possible positivesuction head. To further improve efficient oper-ation of these pumps, a mixing connection is pro-vided so that a portion of the system return watermixes with the water from the generator thus low-ering its temperature to avoid the danger of flash-ing at the pump suction. The suction intake ofhigh temperature water pumps must be carefullydesigned to avoid sudden changes in velocity or di-rection which might contribute to flashing and in-efficient operation. With nitrogen cushioned sys-tems any pump suction rumbling (flashing) can bequieted simply by raising the pressure slightly (in-suring that the expansion line connection is asclose to the pump suction as possible also helps) .This cannot be done with a steam cushionedsystem as no means to increase the pressure otherthan raising the water temperature exists . Pumpswhich have split casing with an axial suction andupward discharge are preferred. Adequate watercooling must be provided for all pumps. Drip waterand cooling water should have drains near thepump and the drains should be visible. Mechanicalseals are recommended after the pump has had a"run-in" period of a few months . This gives thesystem water time to become stabilized and mostdirt will be out of the system. Mechanical sealswill be selected to suit each pump manufacturer'srecommendation and installed accordingly. Me-chanical seals provide for less maintenance andelimination of packing leakage. Each pump shouldbe direct-connected to its motor on a common castiron or structural steel base . When individual gen-erator circulation pumps are used, one pump mustbe provided for each generator. The head underwhich these pumps must operate, as well as thecapacity, is determined by the generator require-ments. These pumps take their suction from thereturn water line . Systems having a number ofzones should have the total capacity of the zonessubdivided for several pumps of equal capacity toavoid operation of pumps at low efficiencies ratherthan having one pump for each zone . A common

pump discharge header then supplies the zones.The operating head of the system circulationpumps must be sufficient to handle the calculatedpressure drop through the complete circuit whichthe pump serves. This includes the pressure dropsthrough valves and piping in the heating plant,the distribution system, and the heat-consumingequipment. In case the system circulation pumpsalso serve to circulate the water through the gen-erators, the pump head must also be sufficient forthe generator circuit. Provide multiple pumps inparallel so that when one pump is inoperative theremaining pumps will have capacity to provide 100percent flow. Yoke mounted pumps should beavoided. Use of centerline mounted pumps exclu-sively will avoid associated expansion problems .Variable speed pumps should be considered forsystem pumps in a dual pumping arrangement.Constant speed (volume) pumps will be used withforced circulation HTW generators as they re-quired a constant flow, minimum 90 percent ofdesign or 100 percent of the flow required by themanufacturer of the boiler installed, whichever isgreater, (only return water temperature will varywith load). The water flow element must be locat-ed in the return line to the generator and inpiping straight runs to insure an accurate reading.Flow switches will be included to prevent startupif flow has not been established and will take theunit off line should the flow drop below the mini-mum rate . The pressure loss of the bypass valveand associated piping should be as close to that ofthe HTW generator loop to insure that the circu-lating pump operates on its curve under any condi-tion.a. Dual-Pump Systems. Dual-pump systems

have generator circulating pumps and system cir-culating pumps. The generator circulation pumpdraws water from the return system and deliversit directly to the generator inlet header . Thesepumps must be designed to circulate the quantityof water specified by the manufacturer against thehead required to overcome the resistance of thegenerator plus the connecting piping and fittings.The quantity of water circulated through eachgenerator is therefore kept more or less constantregardless of the generator firing rate. The systemcirculation pumps are designed to circulate thequantity of water determined by the heat load ofthe heat users and the design temperature dropbetween the supply and return lines against thetotal resistance of this circuit. There need be norelationship during operation between the quanti-ties of water circulated by the system pumps andthe generator circulation pumps in this arrange-ment. With this dual pump arrangement no in-


TM 5-810-2/AFR 88-28

struments and other equipment are needed to con-trol the flow through the generator because thecirculation pumps have a constant capacity andare entirely independent of the system circulation.The only safety devices required to protect thegenerators of this system are the generator pres-sure differential control and a thermocouple con-trol which safeguards the tubes against excessivetemperatures .b. Single-Pump Systems. Single-pump systems

have one set of pumps which circulate waterthrough both the system and the generators. Thecirculation pumps must have sufficient head to cir-culate water first through the system and subse-quently through the generators. The water volumeof either the generators or the system will deter-mine the capacity of the pumps, depending uponwhich is greater. The water volume of the systemwill vary with the head load and the temperaturedrop between the supply and return temperatures,since the heat users throttle the flow of high tem-perature water to adjust to changing heat require-ments. The volume of water circulated throughthe generators may be allowed to vary, but to pro-tect the generator tubes from overheating it mustnever be allowed to fall below the minimumamount required to guarantee proper distributionof flow through all the water circuits of the gener-ator. A generator flow meter or other device whichindirectly indicates the flow is required to operateand automatic bypass valve which assures ade-quate volume of water to the generators at alltimes. This bypass valve is sized for the minimumflow required at the generators .

4-6 . MAKEUP WATER TREATMENTHTW systems are closed systems and, therefore,makeup is limited to the extremely small amountof leakage which occurs at pump glands and valvestems. Additional makeup is required for fillinglines or equipment which are drained. Whenmakeup is small, accumulation of salts and otherimpurities in the generator is so slow that genera-tor blowdown, another cause of losses, is hardlyever needed . The makeup water requirementseven in the largest systems should not exceed 200to 1000 gal. per day.a. External Treatment. All makeup water in-

troduced into the HTW system must be filtered toremove suspended matter and treated to removehard elements of calcium and magnesium, andmust be oxygen free. A demineralized unit is usu-ally not required . The calcium and magnesiumwill be removed by a water softener . The softeningoperation is performed by filtering makeup waterthrough a bed of ion-exchanger material common-

4-7

TM 5-810-2/AFR 88-28

ly called "zeolite." At intervals determined by theamount of water used for makeup, the zeolite ma-terial must be regenerated by backwashing withconcentrated salt brine solution. Capacity of thesystem is normally in the range of 15 to 20 gal. permin. The water softening system should consist ofdual zeolite tanks, a brine solution tank, manuallyoperated multiport control valve, bell alarm watermeter, and water distribution manifold . This watersoftener system may need to be supplemented forthe initial fill requirements when raw water condi-tions are poor . The initial fill should be relativelyfree of oxygen . Any remaining oxygen should bescavenged with sodium sulfite or similar treat-ment.b. Analysis of Water. Analysis for control of

water is essential in HTW systems to prevent theformation of insoluble deposits of scale within thegenerator tubes and other parts of the system andto prevent corrosion and deterioration . To providea system with the greatest resistance to corrosionand chemical attack economically possible, a rela-tively high level of alkalinity is maintained in thesystem water. Tests have shown that iron is leastsoluble at a pH of 9.3 and that corrosion of iron in-creases rapidly as the pH falls below 9.3 . To assuremaximum protection it is recommended that a pHof 9.3 to 9.9 with zero hardness and zero oxygen bemaintained. The services of a competent waterspecialist should be used for the primary and sec-ondary water circuits on a regular schedule tosample, analyze system water, and make recom-mendations for corrections, if necessary.c. Storage Tank.

A treated water storage tankwill be provided for emergency pumping require-ments. This tank is usually sized for approximate-ly 20 minutes pump demand. It is advisable also toprovide a means of heating the treated water to atleast 210 degrees F. to avoid cold shock to thesystem .

4-7. INSTRUMENTATIONInstrumentation serves the following necessaryfunctions: records and indicates vital factors suchas water flow, temperatures, and pressures whichare essential to the operators; provides supervisorycheck readings and information for determiningefficiency of operation; assures maximum utiliza-tion of total plant; and provides monitoring ofemission controls. The minimum instrumentationrequirements will be reviewed with current emis-sion monitoring requirements of EPA or state orlocal codes.a. Generator Panel. The minimum instrumen-

tation for each HTW generator should be mounted

4-8


on a generator panel and, where applicable, shouldinclude the following:

(1) Multipoint draft gage, 4 points with me-chanical ash collector, 3 points without mechanicalash collector (varies with fuel).

(2) Water flowmeter and recorder with 3-pencircular chart:

(a) Water flow .(b) Outlet water temperature gage.(c) Return water temperature gage .

(3) Recorder with 2-pen circular chart:(a) Combustion air flow .(b) Flue gas temperature.

(4) COs meter and recorder with 1-pen circularchart.

(5) Smoke density indicator and alarm.(6) Individual inlet and outlet water pressure

gages.(7) Stoker grate temperature multipoint indi-

cator (solid fuel-fired generators only).(8) Opacity monitor.

b. Master Control Board. A master controlboard with annunciating alarm panel should beprovided to include the following:

(1) Expansion drum pressure gage .(2) Zone distribution meters and recorders

with 3- or 4-pen circular chart:(a) Water flow rate.(b) Water temperature in.(c) Water temperature out.(d) Expansion drum pressure (on first zone

only).(3) Expansion drum water level indicator.(4) Annunciator with illuminated windows

and horn to indicate the following:(a) Overflow .(b) Low level.(c) Low level cutoff.(d) Generator low flow (one for each unit).

(e) Burner low pressure (each oil burnerwhen used).

(5) Indoor and outdoor thermometer.(6) Electric clock.(7) Circulating pumps status lights .(8) Excess water temperature annunciator

point.(9) how expansion tank pressure (inert gas

pressurized systems).(10) Makeup pump status lights.

c. Additional Features. Fault finders and an-nunciators are available which will aid the opera-tor in determining which limit has caused a "failto operate on demand" condition or an inadvertentshutdown . Because of the high costs of installationand maintenance during operation, the benefit to


TM 5-810-2/AFR 88-28

this installation should be analyzed before includ-ing in the design .

4-8 . POLLUTION CONTROLPollution control will conform to the latest re-quirements of EPA or the state or local codes,whichever is more stringent.

4-9

5-1. POTENTIAL USERS OF THE SYSTEMThe design of building distribution circuits in-cludes the arrangements for bringing high temper-ature water supply and return lines into eachbuilding, locating equipment in the equipmentroams, and designing suitable heat exchangers,control systems, and auxiliary piping such as cir-culation systems, drains, vents, and bypasses. Ingeneral, branches are taken off the high tempera-ture distribution lines to serve one or more equip-ment rooms in each building. For space heatingand process applications, heat exchangers or con-verters are required to transfer the heat from thehigh thermal potential of the high temperaturesupply water lines to the lower temperature levelsof the spaces and equipment requiring heat. Heatwill be transferred directly by radiation and con-vection only with special approval otherwise maybe indirectly by radiation and convection, or indi-rectly by the circulation of secondary heatingmedia such as air, water, or steam. Suitable con-trol equipment is required so that the desired tem-perature levels can be maintained under varyingheat loads.

5-2. BUILDING SERVICEAt the point where the HTW supply and returnlines enter a building, shutoff valves are required.For maximum accessibility and convenience,locate them inside the building in the mechanicalequipment room nearest to where the service lines


CHAPTER 5CONVERSION AND UTILIZATION

enter the building . The lines beyond the shutoffvalves must be drained if there is danger of freez-ing due to long periods of shutdown . A minimumcirculation must continue when none of the heatexchangers require heat. For this reason, a bypassline equipped with a %-inch gate valve should beinstalled at the entrance to each equipment roomahead of the shutoff valves.

5-3. LOCATION OF EQUIPMENTHeat exchangers or converters may be locatedeither in an equipment room or in an open base-ment since a special room is not essential. If theindividual buildings are very large or extensive,such as hospitals and research laboratories, it maycost less to distribute the heat exchangersthroughout the building serving them from mainHTW headers. In laying out the equipment spaces,sufficient space must be provided for the removalof HTW coils, especially if the low temperatureheat carrier is expected to form scale or other de-posits . Since the HTW will not foul, scale, or cor-rode the piping through which it passes, no provi-sions are needed for cleaning this circuit.a. Heat Exchangers.

Figure 5-1 shows a simpli-fied subcircuit including the various types of con-verters commonly used in district heating applica-tions. Steam generator, domestic water converter,and hot water converter use steam or water as sec-ondary heat carriers. All of the converters are ar-ranged in parallel between the supply and returnlines of the HTW system.

TM 5-810-21AFR 88-28

TM 5-810-2/AFR 88-28

5-2



Figure 5-1.

b. Piping.

It is most convenient to locate mostof the piping connecting converters and heat ex-changers above rather than below the equipmentwith risers and downcomers serving the variouspieces of apparatus. This arrangement keeps thepiping up and out of the way making it easier tofulfill the venting and draining requirements .Vents and air bottles must be located at all thehigh points in the piping, and drains and fillingconnections must be provided at the low points tofacilitate maintenance of the equipment. Nopiping should be located in the space reserved forremoving the tube bundles. The disconnection of a

Various Heat Converters.

Domestic Water Converter

Hot Water Converter

pair of flanges in the HTW lines must permit re-moval of the tube bundle . If walls or windows arelocated close to the tube bundle, removable sec-tions must be provided to permit removal of thebundle . Piping distributing the secondary heat car-riers should be designed in the conventionalmanner as recommended by ASHRAE Handbook .c. Valves.

Each heat exchanger using HTW re-quires at least one supply and one return shutoffvalve to permit maintenance. In addition, an auto-matic control valve in the return is usually re-quired with provision for its removal or replace-ment . Figure 5-2 shows three types of heat ex-

changers commonly used . HTW connections andcontrols are basically the same for all three types.Shutoff valves may be either gate or plug valves .A balancing valve, either a globe or plug, also isinstalled ahead of the return shutoff valve so thesubsystem may be balanced to avoid short circuit-ing or excessive flow of supply water when controlvalve drives to full open position . The controlvalves are operated by thermostats or pressure-stats which actuate the valves through a position-er using electric, electronic, or pneumatic modula-tion . Control devices sensing secondary media tem-peratures must be selected to provide flow andpositive shutoffwhen required under all conditionsof operation. Controls will be located in returnsince this reduces or eliminates flashing of thewater flowing through the valve, provides bettercontrol characteristics, and prevents plug erosioncaused by high temperature water flashing tosteam at lower discharge pressure . Control valvesin HTW supply are not recommended. Controlvalves of the three-way type or three-way pressurecontrols which bypass hot water around the con-


TM 5-810-2/AFR 88-28

verter should not be used . They can easily deprivethe other circuits of flow and produce high returntemperatures and excessive pump loads therebylowering the overall efficiency of the system.Bypass globe valves of the same size as the controlvalves are provided around the control valves . Theshutoff valves must be located so as to permitmaintenance of the control valves as well as of theheat exchanger. To assure no short circuiting andloss of heating capacity, great care should betaken that flow is assured to all heat exchangersat all times: for this reason, piping and valuingmust be sized carefully and branch lines leading toindividual heat exchangers should be taken offcommon headers. In the case of air heaters used toheat outside air, if the control valve is tight-clos-ing a bypass with a V2-inch globe valve must beprovided to maintain enough circulation at alltimes to prevent freezing of the water in the coils.The construction of valve seats and plug contourswill be carefully selected to minimize erosion inHTW piping .

b-3

TM 5-810-2/AFR 88-28

5-4

PressureInstrument

Flow ControlBalancing Valve_

InstrumentGlobe Valve

Flow Control

Hot Water SupplyBalancing Valy e~7

I ~------


Control

\1E

Compressed Air Supply

High Temp. HotWater SupplyAnd Return

Temperature Control

High Temp. Hot WaterSupply And Return

Temperature Control

Compressed Air SupplyInstrument

--------------------\E

High Temp. Hot WaterSupply And Return

U .S . Army Corps of Engineers

d. Flow Control. Provisions shall be made inthe HTW subsystem so accurate balancing can be

achieved. Control of flow to individual systems to

avoid excessive flow and short circuiting is very

important. The continual checking of the flowscannot be overstressed to the designer. Flow must

l_.-... .~

m~ High Water Linei

..l

team Generator

Figure 5-2.

Heat Exchangers and Control Valves.

DFeedwater

STEAM GENERATOR

Compressed Air Supply

HOT WATER CONVERTER

Domestic Hot Water

'Cold Water SupplyDOMESTIC HOT WATER CONVERTER

be controlled by the plant operators and must bekept in their jurisdiction.

e. Temperature Gages. Gages will be providedon the supply and return lines to each piece ofHTW equipment. Gages allow a quick visual checkof the temperature differential and an indicationof unbalance in the system if differential is below

design . Design differential will be posted near theequipment or kept on file at plant operations.

5-4. DESIGN OF HEAT EXCHANGERSa. Standards.

Heat exchangers using HTW arebest classified according to their secondary modeof heat transfer since they all circulate high tem-perature water in the primary circuit. Heat maybe transferred in the secondary circuit by directradiation, by radiation and convection to air, byconvection to water or to steam. Each heat ex-changer must be designed to deliver a specifiedamount of heat per hour which represents themaximum design heating load to which it is con-nected. It must have sufficient area to deliver thisamount of heat with specified velocity and enter-ing and leaving temperatures of the primary HTWto the secondary heat carrier under specified con-ditions and in the case of convection, with speci-fied circulation. The film coefficients of heat trans-fer by convection or radiation may be determinedby suitable formulae available in the standards ofequipment manufacturers. Fouling factors may beincluded when fouling or scaling of the secondaryheat transfer surface will occur over a period oftime. No fouling factor need be applied to theHTW side since this closed circuit is not subject tofouling, scaling, or corrosion. The specified supplytemperature of the HTW circuit is determinedfrom the design supply temperature leaving thegenerator plant less a proper allowance for heatlosses in transmission, usually about 4 to 8 degreesper mile . The return HTW temperature for eachpiece of heat transfer equipment may generally bespecified as between 10 to 20 degrees higher thanthe outlet temperature of the secondary heat carri-er for peak load . For very large heat exchangers,temperature crossing is practical. The flow rate ofHTW through the heat exchangers is determinedby the heat load and the design supply and returntemperatures of the primary circuit. The physicalarrangement of HTW heat exchangers must becarefully considered to be certain that it complieswith the special requirements of this heat carrier.In general, the coils must be horizontal to permitdraining and venting, and must not permit bypass-ing or short circuiting of the water from inlet tooutlet or permit stratification to occur. The coilsor tubes of HTW heat exchangers should be madefrom cupro-nickel, stainless steel, or Admiraltymetal. Most available material is 90-10 cupro-nickel which is good for normally expected pres-sure and temperatures. Other cupro-nickel such as70-30 or 80-20 is available on special order. Stain-less steels are used for higher pressures. Bronzeand brass are not suitable for this service. In all


TM 5-810-2/AFB 88-28

shell and tube heat exchangers, the high tempera-ture water will be in the tubes and lower tempera-tures and pressure medium in the shell. The thick-ness of the shell and the gage of the tubes must besufficient to carry the required pressures; extrathickness is not required to allow for corrosion.The tubes or coils will always be arranged as mul-tipass so that nearly equal velocity occurs throughall the tubes of all passes . The overall pressuredrop on the shell or coil side should generally notexceed 7 feet of water. Coils must be removablethrough a flanged opening and be accessible forrerolling of the tubes in the tubesheets. The coilsshould be capable of being cleaned on the outsidemechanically or chemically and on the inside bychemical means only since accumulations arehardly ever found on the surfaces contacted byhigh temperature water. It is very important to re-alize that the economy of the entire installationdepends to a great extent upon the design of theheat exchangers . It is generally cheaper to in-crease the heat exchanger surface area than to in-crease pipe sizes and pump capacity. It is impor-tant that a serious effort be made to reduce returntemperatures of the HTW as much as feasiblesince this determines the pump capacity and pipesizing as well as transmission losses . High pres-sure drops through the exchangers and their valu-ing and piping are also uneconomical although acertain minimum must be allowed for good balanc-ing and operation. A 40-foot total drop is consid-ered a good design parameter. Water coil velocitiesshould be at least 4 feet per second (fps) and notexceed 8 fps.b. Radiant Coils and Panels. HTW may be

used directly with special approval in radiant coilsand panels when they can be mounted more than15 feet above the floor. Radiant coils and panelsare especially desirable since comfort conditionscan be obtained at lower room temperatures thanthose required for convection heating. This is aparticularly advantageous method for heatingwarehouses, factories, hangars, and other areaswhich have high ceilings or roofs. The disadvan-tage is that close temperature control is difficult.The panels or coils must be carefully distributed toproduce uniform radiation throughout the spacesto be heated. The return water temperatures fromradiant coils and panels depend upon the heatload, the effective radiant area, the room tempera-tures, and the mean coil temperature; they shouldbe kept as low as possible, as pointed out above,without requiring excessive surface area. Whencoils are located in floors and walls, lower surfacetemperatures are required and HTW cannot beused directly. In this case, heat exchangers may be

5-5

*rm 5-8102/AFR 88-28

used to transform the heat to lower temperaturelevels . The heat radiated by HTW coils may beregulated to suit demand by throttling the flowthrough the coil which reduces the mean tempera-tures of the coils.c. Forced Circulation Hot Water Converters.

Forced circulation hot water converters are re-quired to produce lower temperature hot water forspace and process heating. The use of circulatinghot water for building heating is preferable to theuse of steam when HTW is used as the primaryheat carrier. Not only is greater comfort andeasier control achieved, but also a more economi-cal arrangement results since lower return HTWtemperatures are produced . The distribution of thelow temperature water may be designed in thesame way as conventional forced circulation hotwater systems. Such a design would require a cir-culating pump, a mixing valve controlled by insideor outside temperatures, zone control valves, con-ventional convectors or radiant panels, and an ex-pansion tank to take care of variations in thevolume of water in the system with temperature.The heat exchanger required for this applicationshould be the multipass type on the HTW side andshould be well baffled on the secondary circuit toprevent short circuiting of the lower temperaturewater. Relief valve or valves installed on the shellto relieve pressure should be sized with added ca-pacity should the control valves on the HTW sidefail .d. Large Domestic Hot Water Converters. Stor-

age type domestic hot water converters are usedfor heating cold water to about 140 to 180 degreesF. Converters for this application are similar tothose used with forced hot water circulation . Baf-fles may be installed in the shell to prevent by-passing of the entering cold water to the outletconnection and stratification of water in the tank.Throttle controls are required to vary the flow ofHTW to maintain a constant temperature of thedomestic water in the upper portion of the tank.e. Steam Generators. Steam generators can be

used to raise steam at any desired pressure, limit-ed only by the temperature of the HTW availableto the converter. Slightly wet steam is producedunless provision is made to eliminate the en-trained water. A steam separator built into thesteam space of the converter and suitable waterlevel control are necessary for these converters.Automatic condensate return to the steam genera-tor is practical, but care must be taken that thecontrol should not deliver more than the necessaryamount of fresh makeup water to the generator. Acondensate pump and condensate tank are re-quired when a two-pipe steam distribution system

5-6


is used, but may be eliminated, of course, in single-pipe systems of the gravity type or in systemswhich waste the steam. The control valve throttlesthe flow of HTW in the primary circuit and is op-erated by a pressurestat connected to the steamchamber of the converter. A safety valve is re-quired to relieve pressure in the shell should thecontrols fail to operate properly .

5-5. CONTROLSa. General.

Control of temperature within closelimits is an important factor in all heating instal-lations, comfort and process, both for proper func-tioning of the equipment and for best economy.Considerable savings in fuel can be achieved bycontrols which adjust the consumption of the heat-ing medium closely to the heat demand. For thisreason, the cost of proper controls is usually easilyjustified from the fuel savings alone. Automaticcontrols must be selected carefully to suit the ap-plication. This is particularly true in the case ofHTW controls due to the great heat potential ofthis heat medium . Controls for HTW equipmentgenerally receive their control impulses from athermostat or pressurestat located where they cor-rectly indicate the temperature or pressure of thesecondary heating medium . Great care must betaken not to locate thermostats in stagnant re-gions where they could give false readings. Controlvalves should be the two-way single seated type .There are two types of controls which can be oper-ated either electrically or pneumatically; on-offcontrols and modulating controls .b. On-Off Controls,

Quick-Opening

Valves .When the load is fairly constant, and when widetemperature differentials may be permitted, on-offcontrols are often satisfactory . On-off controls maycause serious water hammer, so their use shouldbe restricted to small units and to short runs ofpipe . Quick-opening valves are not suitable for theclose temperature control required for hot waterheaters or domestic hot water converters or for theclose pressure control required for most steam pro-ducers. Regulating valves suitable for use with thistype of equipment must be of the modulating type .c. Modulating Controls .

Modulating controls ingeneral are far more satisfactory than on-off con-trols, but they cost considerably more . Their use isalways justified, however, by the better controland higher economy which they produce. A modu-lating control assembly will consist of a thermo-stat or a pressurestat, a control instrument, and acontrol valve. The thermostat or pressurestat maytransmit its impulse to the instrument by gas,vapor, or liquid pressure or by electrical impulse.The impulse from the instrument to the control

valve also may be transmitted by gas, liquid, orelectricity. The control valve itself in generalshould be designed with an equal percentage flowcharacteristic . Valve positioners are preferable forall valves 2 inches and larger if the distributionpump pressure head is in excess of 100 psig. Thevalves must always be arranged to be closedagainst the impulse of the instrument so that thecontrols will close automatically should the im-pulse from the instrument fail . Control applica-tions where the pump pressure is more or less con-


TM 5-810-2/AFR 88-28

stant may use controls with narrow ranges of only10 percent band widths and without any reset.Based on present experience applications wherethe inlet pressure of the valve varies widely due tothe fluctuations in load, instruments with bandwidths up to 150 percent and with automatic resetwill provide a safe control. Exact sizing of thevalves in accordance with available pressure dropis essential and it is recommended that a pressuredifferential of 20 feet of water should not be ex-ceeded .

5-7

Government Publications

Non-Government Publications


APPENDIX A

REFERENCES

Departments of the Army and the Air ForceTM 5-810-1

Mechanical Design-Heating, Ventilating and Air ConditioningTM 5-811-7

Electrical Design, Cathodic Protection

American Society ofHeating, Refrigerating, and Air Conditioning Engineers (ASHRAE), 1791 Tullie CircleNE, Atlanta, GA 30329Handbook, Equipment, 1988Handbook, Fundamentals, 1989Handbook, HVAC Systems and Applications, Chapter 15, 1987Handbook, Refrigeration Systems and Applications, 1990

American Society ofMechanical Engineers (ASME), 22 Law Drive, Box 2300, Fairl"ield, NJ 07007-2300Boiler Pressure Vessel Code (1989)

B31.1-1989

Power PipingB31.3-1990

Chemical Plant and Petroleum Refinery Piping

TM 5-810-2/AFR 88-28

Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook, IL 60062773-75

Oil-Fired Air Heaters and Direct-Fired Heaters, Third Edition, 14 August 1985795-73

Commercial-Industrial Gas Heating Equipment, Third Edition, 13 July 1989

APPENDIX BSAMPLE CALCULATIONS FOR DATA GIVEN IN CHAPTER 2

B-1. Heating load and distribution systemFirst the heat requirements throughout the distri-bution system are tabulated; then system pres-sures and temperatures are determined . In this ex-ample system the heating load is 80 percent of thetotal load. T,-Tr is then selected at 150 degrees F.Tr averaged 240 degrees F. following the procedureset forth in subparagraph 2.12.a. The supply tem-perature T, isTr = 240 F.

+ 150 F.T, = 390 F. + allowance for loss (40 degrees F.) _430 degrees F. The system supply is selected at 440degrees F. at the generators.Tr = 440 - 150 = 290 degrees F.B-2. HTW generator selectionUsing the following requirements the basic calcu-lations are performed .Maximum Initial Load (Qd=120,000,000 Btu/hrFuture Anticipated Load=40,000,000 Btu/hrMaximum Ultimate Load

=160,000,000 Btu/hrEssential Load

=55,000,000 Btu/hrFlywheel Factor

=0.85Temperature Supply (Ts)=440 F.Temperature Return (Tr)=290 F.Average Temperature Drop=150 F.Volume in Ultimate System=4,000 cubic feet

Use two 55,000,000 Btu/hr units initially; onedown will provide essential load . Add one55,000,000 Btu/hr unit in future . Generator circu-lation requirements are based upon averagesystem temperature differential.

T, = 440 F. = 418.9 Btu/lb (enthalpy)Tr = 290 F. = ,259.31 Btu/lb

Ah = 159.59 Btu/lb55,000,000/159.59 = 344,630 lb/hr, based on essen-tial load.B-3. Circulating pump selectionTwo generator circulation pumps are selected forthis flow requirement with head based upon resist-ance through generator and associated piping fortwo-pump systems . For single-pump systems,pumps must provide this flow as a minimum con-dition ; pump head requirements would be theabove resistance added to system resistance. Distri-bution system circulation, pumps, and pipe size,

information Handling Services, 2000

TM 5-810-2/AFB 88-28

requirements are based upon average system tem-perature differential.120,000,000/159.59 =751,927 lb/hr, based on maxi-mum initial load.40,000,000/159 .59 = 250,642 lb/hr, based on futureanticipated load .If the distribution system design is for a two-pumpsystem the best selection would be three pumpsplus one spare based upon the 250,000 lb/hr. If thedesign is for a single pump system, then the bestselection would be two pumps plus one spare basedupon the 344,630 lb/hr for the boiler . Pipe sizes forthe mains in the circulating loop and at the mani-folds for the pumps and generators should be sizedfor maximum future load. This type of analysisshould be made for every system to assure thatadequate pumping capacity is provided. For thisexample a single pump system is selected.B-4. Expansion vessel designThe first step is to determine the system expan-sion volume as outlined in subparagraph 2.12.c(1):

T, = 440 F.Tr = 290 F.

Vt, = specific volume at 440 F. = 0.01926Vt 2 = specific volume atT, - 10 percent = 400 F. = 0.0186.¢

VfI - Vf, = 0.00062Utilizing Equation (2-9) :AV, = (Vt i -

Vf z)/V f , ] 100 = (0.00062X100)/0.01864 = 3.3 percent change in supply systemwater.This establishes the operating range between 400degrees F. and 440 degrees F. This 40-degree differ-ential is applied to Tr to obtain the minimumreturn temperature.Vt,, = specific volume at Tr = 290 F. --- 0.01733

Vni = specific volume at minimum return = Tr -40 F. = 250 F. = 0.01700

Vf, - Vr ra = 0.00033Utilizing Equation (2-10):AVr = [Vtrl - Vha]100/Vt2 = 0 .00033/0.01700(100) = 1.9 percent change in return system water.From Equation (2-11):V, =total system expansion volume = AV, +

AV,/100 (system volume)

TM 5-810-2/AFR 88-28

V, = 3.3 + 1.9/100 (4000) = (0.052x4000) = 208cubic feet .Steam expansion vessel design follows data in sub-paragraph 2.12 .c.(2) :V, = 208 cubic feet per calculations above .

= '/z-minute reserve for pumps based on themaximum initial load plus the essential loadwhich is the circulating load through theboiler in this single-pump system := 344,630 + 751,927 = 1,096,557 lb/hr or :

V2 = (1,096,557X0.0183 ft3/lbXV2 minute)/60 min-utes = 167.2 cubic feet where specific volumeis selected for average of T, and Tr=440 + 290/2 = 365 F.

V, = 0.2(V, + V2)= 0.2(208 + 167.2) = 75.04 cubic feet .

V2

Allowance for level controls = (tankdiameter)(lengthXl ft) . Assume largest practical di-ameter = 8 ft and use 150 cubic feet nominal al-lowance.

B-2


From Equation (2-12) :

Required nominal volume = V, + V2 + Va +control allowance = 208 + 167.2 + 75.04 + 150 =600.24 cubic feet .Length required = 600.24/7r Dz/4 = 600.24/(0.7854X64) = 1.94 tank length in feet .These proportions are not good, therefore assumetank length of 25 feet . Then 600.24 = 0.7854D2(25) : D2 = 30.57, D = 5.53 feet . Use tank with 6-foot inside diameter . Volume for controls =(6)(25X1) = 150 cubic feet, which checks with theminimum recommended.

Total volume of tank = (6) 2 yr (25)/4 = 706 .86

Required computed volume = 600.24

Net volume available for sludge = 106.62 cubicfeet . ASHRAE recommendation = 40 percent (V,)= 0.4(208) = .83.2 cubic feet. Therefore, tanksludge capacity is adequate.

Selection of a distribution layout will depend upon the particular terrain and the need, if any, for separa-tion of consumer loads. Several example layouts are illustrated.Figure C-1. In this arrangement balancing of the system is easily accomplished and pressure differentialsat all building connections are nearly equal.

BUILDING SERVICECONNECTIONS (TYPICAL)


APPENDIX CEXAMPLE DISTRIBUTION LAYOUTS

TM 5-810-2/AFR 88-28

Figure C-1 . Direct Supply, Reverse-Return.

TM 5-810-2/AFR 88-28

Figure C-2. In this arrangement a number of individual circuits are used . Control valve sizing is not asdifficult as with direct supply, single circuit (not shown) where pressure at each connection is different .

C-2

0 0


BUILDING SERVICECONNECTION (TYPICAL)

Figure C-2.

Direct Supply, Radial.

s

-yo--~1 CENTRAL II HEATINGi PLANT -L-fl4--

I

Figure C-3. Return from each connection is fed back into the loop main. The effect of lower temperaturesat connections farthest from the central plant must be considered .

CENTRALHEATINGPLANT

BUILDING SERVICE CONNECTION (TYPICAL)

Figure C-9. One-Pipe Loop Main.


TM 5-810-1/AFR 88-28

INDIVIDUALBUILDING(EXAMPLE)

t-j14

C-3

TM 5-810-2/AFR 88-28

Figure C-4. This is a type of distribution that can be used with any layout, where high temperature wateris converted for distribution of lower temperature water.

C-4

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W Nw zN O_t9 1-z W- WO 2J Z

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f

By Order of the Secretaries of the Army and the Air Force:

GORDON R. SULLIVAN

4w ,

General, ief of States ArmyOfficial :

Chief Staff

MILTON H. HAMILTONAdministrative Assistant to the

Secretary of the Army

MERRILL A. McPEAKGeneral, USAF

Official :

ChiefofStaff

EDWARD A. PARDINIColonel, USAF

Director ofInformation Management

Distribution:Army: To be distributed in accordance with DA Form 12-34B, requirements forTM 5-810-2.Air Force: F


The proponent agency of this publication is the Office of the Chiefof Engineers, United States Army . Users are invited to send com-ments and suggested improvements on DA Form 2028 (Recom-mended Changes to Publications and Blank Forms) to HQUSACE(CEMP-ET), WASH DC 20314-1000.

TM 5-810-2/AFR 88-28

*U .S . GOVERNMENT PRINTING OFFICE : 1992 - 309-808

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