hvac pump handbook, 2nd ed 2006-05-23 0071484264_ar020

Chapter

495

20Low-Temperature Hot

Water Heating Systems

20.1 Introduction

Low-temperature hot water heating systems comprise the majority ofHVAC hot water heating systems. The obvious reason for this is thelack of a need for water temperatures in excess of 250°F on most ofthese systems. Also, the advent of the condensing boiler has drivendown the design water temperature of many systems. As was dis-cussed in Chap. 19, lower operating water temperatures increaseappreciably the boiler efficiency. This fact and the development of thevariable-speed pump constitute two of the greatest improvements inthe efficiency of space heating in commercial, governmental, educa-tional, institutional, and industrial buildings. Residential heating by hotwater will not be included herein, since this is a specific field by itself.Also, the pumps for residential heating consist of small circulatorswhose horsepower is not of major consideration in terms of energy use.

20.2 Classification of Hot Water Systems

Hot water systems in HVAC can be classified as to the source of theirheat as well as by use of the hot water. These systems receive their heatfrom (1) electric or fuel-fired boilers, (2) low- and high-pressure steam,(3) medium- and high-temperature water, (4) district heating, and (5)heat recovery. Hot water is used for (1) space heating, (2) heating of out-side air, (3) reheat in cooling systems, and (4) process heating in indus-try. Heating of domestic water is not considered to be an HVAC hotwater system.

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Source: HVAC Pump Handbook

496 Pumps for HVAC Hot Water Systems

20.3 Sources of Heat for Hot Water Systems

20.3.1 Hot water boilers

Most of the discussion of hot water boilers was included in Chap. 19.From a hot water heating system standpoint, the boiler type shouldbe selected that offers optimal efficiency, simplicity in the design ofthe hot water system, and ease of installation. First cost is, of course,a consideration on many installations.

The static pressure imposed on the hot water system will havesome effect on the type of boiler selected. For example, high-rise build-ings have high static pressures, and steel boilers designed to 100- or160-psig working pressure are suitable for these applications. If theboilers are installed in equipment rooms on top of these buildings,low-pressure boilers can be used.

20.3.2 Low- and high-pressure steam

Steam systems provide an important source of heat for hot water heat-ing systems. Both low- and high-pressure steam are used for heatinghot water. There is very little difference in the configuration of heatexchangers for generating this hot water. The traditional method forconnecting these heat exchangers is shown in Fig. 20.1. The principaldifference between the use of low- and high-pressure steam is in the

Figure 20.1 Traditional method for connecting a steam-heated heat exchanger.




Low-Temperature Hot Water Heating Systems

Low-Temperature Hot Water Heating Systems 497

Figure 20.2 Primary pump with heat exchanger.

selection of the safety relief valves that protect the hot water heatingsystem from the higher pressures or temperatures of the steam. Greatcare should be taken in sizing and configuring these safety valves.Various governing agencies have specific regulations on the selectionof such valves. The designer must review local codes to ensure that theinstallation of the heat exchanger complies with all applicable regula-tions. Obviously, the heat exchanger, piping, and appurtenances mustbe rated to accommodate the temperatures and pressures of the instal-lation. Also, the discharge of these relief valves must comply withthese codes. The steam emitted by them must not be allowed to escapeinto the equipment room. It must be vented into a stack or outdoors atan elevation that will not create harm or destruction. If vented to anarea that can be at temperatures below freezing, the relief valve mustbe equipped with a standard drip-pan elbow.

Most heat exchangers for this service have been of the shell andtube type. Some plate and frame manufacturers are offering theirproducts for use on low-pressure steam. One important factor thatmust be addressed is the possibility that a vacuum may be imposed onthe heat exchanger; the plate and frame heat exchanger must bedesigned with gasketing that will withstand this vacuum. Heatexchangers can withstand variable hot water flow through them(Fig. 20.2), so there is no need for special piping arrangements suchas primary-secondary pumping on most installations.

The traditional method of connecting these heat exchangers isshown in Fig. 20.1. The pumps, expansion tank, and makeup waterequipment must be installed on the cooler return water side of theheat exchanger. This reduces the operating temperature of the pumpsand offers longer mechanical seal life. There is little advantage tolocating the air separator and the pumps on the discharge side of the






heat exchanger; a review of Table 2.5 will reveal that only a smallamount of air is removed with additional heating of the water. Most ofthe air coming into the system is from the makeup water. The useof chemical treatment, as discussed in Chap. 9, is the proper method ofeliminating air and therefore oxygen from these systems.

The traditional arrangement of the heat exchanger and its piping asshown in Fig. 20.1 has had some operational difficulties on variable-volume hot water systems. The use of variable-speed hot water pumpsand the resulting low flow through the heat exchanger have createdoperational problems. These problems are as follows:

1. At very low heating loads, the steam temperature regulatorreduces the steam flow until a vacuum occurs in the heat exchang-er. This causes the heat exchanger to fill with air or condensatethrough the vacuum breaker. If air is injected into the heat exchanger, this increases the possibility of corrosion in the internalparts of the heat exchanger. Control of the temperature of waterrequired by the system is lost.

2. If the heat exchanger is filled with condensate, any increase inload in the water system will cause the steam valve to open widein an attempt to recover the water temperature. Full steam pres-sure may be exerted on the condensate in the heat exchanger todrive the condensate out of the heat exchanger. This can causehammering and pounding in the heat exchanger.

3. The steam trap on the heat exchanger is overloaded and may limitthe flow of condensate out of the heat exchanger.

An attempt has been made to solve these problems by using twosteam temperature regulators, one for one-third of the design loadand the other for two-thirds of the design load. This has solved theseproblems in some installations, but a hunting problem may existwhen the steam control transfers from the one-third control valve tothe two-thirds control valve.

An alternative to the standard method of piping and pumping theseheat exchangers is described in Fig. 20.3. A three-way mixing valve isused to control system temperature, and on most systems, a constantsteam pressure is maintained on the heat exchanger. Following aresolutions to the problems of the standard hookup:

1. Since a constant steam temperature is maintained on the heat exchanger, there is no vacuum produced in it, and it is not filledwith air or condensate. This provides a constant-temperaturesource for the hot water system.

Rishel_CH20.qxd 20/4/06 6:49 PM 98





Figure 20.3 Heat exchanger installation with three-way valve control.

2. The steam valve can consist of one valve in most cases, not themore complex one-third, two-thirds valve arrangement.

3. The steam trap receives a steady flow of condensate that is basedon the load on the hot water system; its overloading is eliminated.

The steam pressure required in the heat exchanger is determinedby the condensate system that exists at the heat exchanger. If thecondensate drops to a condensate pump, a minimum steam pressureof 1 or 2 psig gauge pressure may be adequate. If the condensate issent to a condensate return pipe, the elevation of that pipe withrespect to the elevation of the heat exchanger will determine thesteam pressure. A rule of thumb that is used in the industry is toallow 1 lb of steam pressure per foot of elevation difference. Thisaccounts for the elevation plus pipe friction. A more precise method isto calculate the pipe friction and the elevation change to arrive at theexact steam pressure in the heat exchanger.

There is a requirement for the sizing of the three-way control valveon these systems, and that is minimum flow. Manual control valvesshould be located out in the system at its far ends to (1) maintainwater temperature in the supply mains, (2) provide minimum flowthrough the pumps, and (3) ensure some flow through the three-wayvalve to aid in the control of water temperature. Usually, this flow can






be calculated, and normally, it is a low percentage of the full flow ofthe hot water system.

With the availability of digital control, it may be possible to varythe steam pressure with the load on the hot water system, which willreduce the size and cost of the heat exchanger and steam controlvalve. For example, the minimum steam pressure may be calculatedat 2 psig. If the steam pressure is changed on a schedule with the loadon the heating system so that 10 psig is on the heat exchanger at fullload, the size of the heat exchanger and control valve may be reducedappreciably.

Another value of the three-way valve system is the ability to generatemore than one temperature of water on one heat exchanger system.Figure 20.4 describes the use of two three-way control valves to pro-duce two different temperatures of water.

20.3.3 Medium- and high-temperature water

Low-temperature hot water heating systems can receive their heatfrom medium- and high-temperature water systems that may beavailable on campus-type installations. Heat exchangers of the shelland tube-type are usually the heat transfer medium rather thanblending water. Blending water may create temperature or makeupwater problems.

Figure 20.4 Multiple-temperature water services.






The heat exchangers for this service are much like those discussedearlier for low- and high-pressure steam. The problems with conden-sate and vacuums that were reviewed do not exist on medium- andhigh-temperature water sources. The heat exchangers must complywith any applicable codes for pressure vessels of the size and temper-atures involved.

20.3.4 District heating systems

District heating systems can be a heat source for low-temperaturehot water systems. Chapter 18 provides information on such low-temperature hot water systems.

20.3.5 Heat-recovery processes

Heat recovery from various sources can be applied to low-temperaturehot water systems. Utilization of the heat of rejection from chillers isreviewed in detail in Chap. 16. Heat recovery from exhaust air orother gas streams can be a source of heat for these water systems.Special hot water heating coils are provided for these applications;usually, they are installed upstream from the main hot water heatingcoil. The metallurgy of the heating coil must be designed for the con-ditions of the gas stream. Dirty process gases may require cleaningequipment ahead of the heating coil.

20.4 Uses of Hot Water in HVAC Systems

Low-temperature hot water is used for (1) space heating, (2) heatingof outside air, (3) reheat in cooling systems, and (4) process heating inindustry. All these uses of hot water employ a heating coil when thereceiving fluid is air. Some industrial heating processes may use aheat exchanger to heat other liquids.

The same problems with laminar flow and freezing that occur incooling coils also exist in heating coils. The discussion in Chap. 9 thatcovers most of these problems also should be applied to hot water coils.

The distribution of hot water to all the preceding uses is much thesame. Some industrial processes might require special cleaning orsensors to detect unacceptable material in the return water.

20.5 Distribution of Low-Temperature Hot Water

The distribution of low-temperature hot water is substantially thesame as that for chilled water, as developed in Chap. 15. Greater con-cern must be given to air elimination, thermal expansion, and corrosioncontrol.






20.5.1 Primary systems

There are more primary systems in hot water than in chilled waterowing to the ability of some boilers to accept variable flow throughthem. Most fire-tube boilers are subject to thermal shock without pri-mary pumps of some type.

Some boiler manufacturers provide a circulating pump with theboiler. This gives such boilers the ability to control minimum flow.Other boiler manufacturers require standard primary pumps withconstant flow through the boilers similar to that of chillers. Also, con-trol for limiting return water temperature may be recommended andfurnished by some boiler manufacturers. As discussed in Chap. 19, itis imperative that the boiler manufacturer’s recommendations oninstallation and operation be adhered to without exception.

20.5.2 Primary-secondary systems

As indicated earlier, primary-secondary pumping should be used forboilers that require constant flow. This includes some cast iron andsteel fire-tube boilers. There is very little difference to this methodfor piping low-temperature hot water boilers from that described inChap. 15 on chilled water distribution systems. The principal differencefor hot water is the ability to use much higher differential tempera-tures than is possible for chilled water. Differential temperatures ashigh as 100°F are designed into some hot water installations.

20.5.3 Primary-secondary-tertiary systems

There is even less reason to use primary-secondary-tertiary pumpingon hot water than on chilled water owing to the lower water flowsand pump horsepowers. There are very few hot water systems thatshould utilize this complicated pumping system. Some retrofit pro-jects may justify it, but even in these instances, variable-speed zonepumping or a modification of distributed pumping may offer moreenergy savings.

20.5.4 Distributed pumping

The advantages of distributed pumping as fully developed in Chap. 15on chilled water systems also apply to campus-type hot water sys-tems. The energy savings may not be as dramatic as those for chilledwater because the pumps are usually smaller. There are no specialdesign conditions for hot water over those already described forchilled water.






20.5.5 Special distribution systems

There are some special hot water systems that utilize manometerfittings that divert water into coils from a single pipe loop; with thedevelopment of digital electronics and other contemporary controls,the use of these systems seldom can be justified except, possibly, onsmall residential heating systems.

20.6 Hot Water Design and DifferentialTemperatures

With the development of the condensing boiler, the classic water tem-perature difference of 20°F for low-temperature hot water systemscan be expanded to as much as 100°F. In the past, in an attempt tosecure higher differential temperatures, operating temperatures var-ied from 240°F at design load down to 140°F at minimum load. Thistemperature range reduced greatly the pumping energy, but it ele-vated the boiler’s stack temperature and resulted in a maximum com-bustion efficiency of around 80 percent. It should be pointed out thatthe reset of water temperature with percentage of load is usuallyaccomplished through the use of outdoor temperature as the parame-ter for changing water temperature. With digital electronics, anyrange that is desired can be inserted into the control algorithm. Indays of mechanical control, only two or three ranges were available.

It is now recognized that for most installations it is better to designwith lower water temperatures to secure higher combustion efficien-cies. If noncondensing boilers are utilized, a typical temperature rangewould be 200°F at design and 140°F at minimum load. At a 60°F dif-ferential temperature, efficiency in pumping has been achieved with abetter combustion efficiency for the boiler, smaller pipe, and lower heatlosses from the pipe surface.

With the advent of the condensing boiler, it is more important toevaluate carefully the design temperature difference selected for hotwater heating systems. Figure 20.5 describes the pronounced reductionin hot water flow when higher temperature differences are utilized.This figure also illustrates the reduction in pipe size with comparablefriction losses per 100 ft of piping. The actual pipe selection along withthe temperature difference is determined by the energy and installa-tion costs that apply to each installation.

The condensing boiler goes beyond this overall efficiency and offersa new level of efficiency of operation that was never attained withnoncondensing boilers. For example, low-temperature hot water sys-tems can be operated with a 60°F differential temperature with a maxi-mum temperature of 150°F at full load and 90°F at minimum load on






the hot water system. At 150°F water temperature, the boiler effi-ciency would be around 86 percent and at 90°F water temperature,94 percent.

The use of these lower operating temperatures may require allheating coils to be of fan type; most types of radiation lose much oftheir heating effect below 140°F. It also opens the way for the use ofradiant floors, ceilings, or walls where unusually high boiler efficien-cies can be achieved with their low water temperatures. It is obviousfrom these facts that the selection of operating temperatures for low-temperature hot water systems is much more complex than it waswith just noncondensing boilers.

100

90

80

70

60

50

40

30

20

10

00

Flo

w –

GP

M

10 20 30 40 50 60 70 80 90 100

Temperature difference – °FPipe size: 3" 21/2" 2" 2" 2" 11/2" 11/2" 11/2" 11/4"

Loss/100' 2.4' 3.4 4.7' 3.1 2.2' 5.9 4.4 3.5 6.3'

or 11/2"

2.9

Figure 20.5 Variation in flow and pipe size with temperature difference.





20.6.1 Reset of boiler water temperatureversus zone reset

As demonstrated in Chap. 8 on the use of water in HVAC systems, inmost cases the reset of water temperature at the boiler offers greaterenergy savings than does resetting water temperature in variouszones of a hot water system. The preceding dramatic increases inboiler efficiency with condensing boilers illustrate what can be donewith resetting the boiler temperature. Also, the coil circulating pumpsfor hot water systems are small with low pump efficiency.

On heating coils, a substantial savings in pumping energy can beachieved by using central pumps with their higher pump efficiencies;small circulators for individual coils have much lower efficiencies. Ifthere is a pronounced difference in zones for heating in a hot watersystem, such as a north zone and a south zone on a building, an alter-native could be the use of two sets of central pumps, as described inFig. 15.5 for chilled water.

20.7 Bibliography

James B. Rishel, The Water Management Manual, SYSTECON, Inc., West Chester,Ohio, 1992.






hvac pump handbook, 2nd ed 2006-05-23 0071484264_ar020

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