chilled water system.ppt

8/10/2019 CHILLED WATER SYSTEM.ppt

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Chilled Water Piping Systems (VPF Focus)


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AgendaChilled Water Distribution Systems

Chilled Water Distribution Systems

Primary (Constant) / Secondary (Variable2W Valves)

Low Delta T

Primary Only (Variable Flow - 2W Valves)

VPF Design/Control Considerations


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Primary (Constant Flow) / Secondary (Variable Flow)


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Primary/Secondary System

Primary Pumps

Secondary Pumps

Common Pipe

Typical

load

with

two

way

valve


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Primary (Constant Flow) / Secondary (Variable Flow)

2 Way Valves

Higher Capital Cost Installed (vs Constant Flow 3W Valve system)

Lower CHW Pumping Energy (vs Constant Flow 3W Valve system)

Well Understood & Easy to Control


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Primary/Secondary System at Design

56.0 F

56.0 F

56.0 F

44.0 F

44.0 F

44.0 F

Primary Pumps

1000 GPM Each

3000 GPM @ 56.0 F

Secondary Pumps

3000 GPM @ 44.0 F

Typical

Coil

No flow

44.0 F

56.0 F

500 ton chillers

1000 GPM Each

56.0-44.0F


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Primary/Secondary System at Part Load

53.0 F

53.0 F

53.0 F

44.0 F

44.0 F

44.0 F

Primary Pumps

1000 GPM Each

3000 GPM @ 53.0 F

Secondary Pumps

2250 GPM @ 44.0 F

Typical

Coil

44.0 F

56.0 F

750 GPM @ 44.0 F

2250 GPM @ 56.0 F

75% System Load


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53.0 F

53.0 F

44.0 F

44.0 F

Primary Pumps

1000 GPM Each

2000 GPM @ 53.0 F

Secondary Pumps

1500 GPM @ 44.0 F

Typical

Coil

44.0 F

56.0 F

500 GPM @ 44.0 F

1500 GPM @ 56.0 F

OFF 50% System Load


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Low Delta T Syndrome


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Dirty Coils

Major Causes of Low Delta T


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Chilled Water Coil


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Dirty Coils

Controls Calibration

Leaky 2 Way Valves

3 Way Valves at end of Index circuit



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Primary Pumps

Secondary Pumps

Common Pipe


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Dirty Coils

Controls Calibration

Leaky 2 Way Valves

3 Way Valves at end of Index circuit

Coils piped up backwards



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Chilled Water Coil


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P Load= Flow X Delta T S Load= FlowX Delta T

Primary (Constant) / Secondary (Variable)

SecondaryPumps

PrimaryPumps

Typical load with 2 way valve

Decoupler/Bypass


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100% Load = 100% Sec Flow


Ideal Operation

SecondaryPumps

PrimaryPumpsDecoupler

/Bypass

100% Flow = 3000 gpm

100% Flow = 3000 gpm

0 gpm

1

2


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SecondaryPumps


/Bypass

67% Flow = 2000 gpm

67% Flow = 2000 gpm

0 gpm

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Ideal Operation

1

2


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Primary / Secondary Rule of Flow

Primary flow must always be equal

to or greater than Secondary flow.


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SecondaryPumps


/Bypass

100% Flow = 3000 gpm

80% Flow = 2400 gpm (400 gpm over-pumped)

600 gpm

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Low Delta T Operation

1

0


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Higher Secondary Pump Energy

Higher CHW Plant Chiller/Auxiliary Energy

Major Effects of Low Delta T


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Solution to (or reduce effects of) Low Delta T

Address the causes

Clean Coils

Calibrate controls occasionally

Select proper 2W valves (dynamic/close-off ratings) and maintain them

no 3W valves in design

find and correct piping installation errors

Over pump chillers at ratio of Design Delta T / Actual Delta T

Increase Delta T across chillers with CHW Re-set (down).

Use Variable Speed Chillers & sequence to operate from 30 to 70% Load

Use VPF Systems (mitigates energy waste in plant)

Header pumps & operate more pumps than chillers

If dedicated pumping, over-size (design at 80% speed).


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Primary Pumps

Secondary Pumps

Common Pipe

P


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Primary Only (Variable Flow)


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Variable Primary System

SecondaryPumps

PrimaryPumps

Primary Pumps

Flow Meter

BypassValve

Automatic Isolation Valve




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2 Way Valves

Lower Capital Cost Installed (vs Primary/Secondary)

No secondary pumps/piping/valves/electrical to buy and install

No large Common pipe, but smaller Bypass pipe/valve/flow meter/controls

Lower CHW Pumping Energy

Smaller Footprint (vs Primary/Secondary)

Relatively New & More Complex Controls

Reduces Negative Impacts from Low Delta T

Chillers are not staged on by flow requirements

Chillers can load up and are staged on load


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Disadvantages

Higher (potentially) PSID rated 2-Way valves in system

Requires more robust (complex and calibrated) control system

Requires coordinated control of chillers, isolation valves, and pumps in sequencing

Longer (potentially) Commissioning time

Requires greater operator sophistication


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Variable-Primary-Flow System

Primary Pumps

Flow Meter

Typical

load

with

twoway

valve

AutomaticIsolation Valve

Bypass


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Variable Primary System at Design

56.0 F

56.0 F

56.0 F

44.0 F

44.0 F

44.0 F

Primary Pumps

1000 GPM Each

3000 GPM @ 56.0 F

3000 GPM @ 44.0 F


Bypass

Closed

500 ton chillers

1000 GPM Each56.0-44.0F

Typical

load

with

two

wayvalve


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Variable Primary SystemPart Load

56.0 F

56.0 F

56.0 F

44.0 F

44.0 F

44.0 F

Primary Pumps

750 GPM Each

2250 GPM @ 56.0 F

2250 GPM @ 44.0 F


Bypass

Closed

Typical

load

with

twoway

valve

75% System Load


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56.0 F

56.0 F

44.0 F

44.0 F

Primary Pumps

750 GPM Each

1500 GPM @ 56.0 F

1500 GPM @ 44.0 F


Bypass

Closed

Typical

load

with

twoway

valve

50% System LoadChiller off

Pump off


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52.0 F

52.0 F

44.0 F

44.0 F

Primary Pumps

750 GPM Each

2250 GPM @ 52.0 F

2250 GPM @ 44.0 F


Bypass

Closed

Typical

load

with

twoway

valve

50% System Load

Low T

Chiller off

Pump on


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Variable Primary SystemMin Flow (400 gpm each)

50.0 F

44.0 F

Primary Pumps

400 GPM (oneoperating)

400 GPM @ 50.0 F

200 GPM @ 44.0 F


Bypass

Open

Typical

load

with

twoway

valve

System flow below chiller

minimum flow

Chiller off

Chiller off

Closed

Closed

200 GPM @ 44.0

200 GPM @ 56.0 FFlowmeter

Pumps

off


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Chiller Design Considerations

Flow rate changesStaging on additional chillers


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Variable Primary System (1 chiller running)

56.0 F

44.0 F

Primary Pumps

333 GPM Each

1000 GPM @ 56.0 F

1000 GPM @ 44.0 F


Bypass

Closed

Typical

load

with

two

wayvalve1000 GPM


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Variable Primary System (Staging on second chiller)

57.0 F

45.0 F

Primary Pumps

333 GPM Each

1100 GPM @ 57.0 F

1100 GPM @ 45.0 F


Bypass

Closed

Typical

load

with

two

wayvalve1100 GPM

Need to add chiller


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Variable Primary System (Open isolation valve)

57.0 F

45.0 F

Primary Pumps

333 GPM Each

1100 GPM @ 57.0 F

1100 GPM @ 45.0 F


Bypass

Closed

Typical

load

with

two

wayvalve550 GPM

550 GPM

Load = F X DT DT = 12 = 57- 45

24Load = 1/2F X 2DT DT = 24

24 LCHWT = 35!


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Variable Primary System (Open isolation valve)

57.0 F

45.0 F

Primary Pumps

333 GPM Each

1100 GPM @ 57.0 F

1100 GPM @ 45.0 F


Bypass

Closed

Typical

load

with

two

wayvalve550 GPM

550 GPM

LCHWT approaches 35

LWT Cutout at 4 deg below

44 set-point or 40

Off goes chiller 1


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Variable Primary System (Open isolation valve slowly)

57.0 F

45.0 F

Primary Pumps

333 GPM Each

1100 GPM @ 57.0 F

1100 GPM @ 45.0 F


Bypass

Closed

Typical

load

with

two

wayvalve1100 GPM

Open over 1.5 to 2 min


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VPF Systems Design/Control Considerations Summary

Chillers

Equal Sized Chillers preferred, but not required

Maintain Min flow rates with Bypass control (1.5 fps)

Maintain Max flow rates (11.0 to 12.0 fps)

Isolation Valves (Modulating or Stroke-able to 1.5 to 2 min)

Dont vary flow too quickly through chillers (VSD Ramp function typical setting of 10%/min)

Chiller Type

System Water Volume

Chiller Load

Active Loads

Sequence

If Constant Speedrun chiller to max load (Supply Temp rise). Do not run more chillers than

needed (water-cooled) If Variable Speedrun chillers between 30% and 70% load (depending on ECWT). Run more

chillers than load requires.

Add Chiller - CHW Supply Temp or Load (Adjusted* Flow X Delta T) or amps (if CSD)

Subtract Chiller - Load (Adjusted* Flow X Delta T) or Amps (if CSD)


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Pumps

Variable Speed Driven

Headered arrangement preferred

Sequence

with chillers (run more pumps than chillers for over-pumping capability)

on flow (add pump when existing inadequate, subtract when can)

optimized algorithm (total kW of more pumps, lower than less pumps)

Stay within pump/motor limits (25% to 100% speed)

Subtract a Pump at 25 to 30% speed

Add a pump back when speed of operating pumps high enough

Speed controlled by pressure sensors at endof index circuit


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Bypass Valve

Maintain a minimum chilled water flow rate through the chillers

Differential pressure measurement across each chiller evaporator

Flow meter preferred

Modulates open to maintain the minimum flow through operating chiller(s).

Bypass valve is normally open, but closed unless Min flow breeched

Pipe and valve sized for Min flow of operating chillers

High Rangeability (100:1 preferred)

PSID Ratings for Static, Dynamic, And Close Off = Shut Off Head of Pumps

Linear Proportion (Flow to Valve Position) Characteristic preferred

Fast Acting Actuator

Locate in Plant around chillers/pumps (preferrred)

Energy

Avoid Network traffic


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Summary on VPF Design

Chillers

Size equally with same WPDs (best)

Respect Min/Max Flows through chillers

Set Pump VSD Ramp function to about 10%/min (600 sec 0 to Max Speed)

Use Modulating or Strokeable Valves (preferred) on chiller evaps, headered pumping

Use 2 Position Valves (1 min stroke) on chiller evaps, dedicated pumping

Pumps

VSD Controllers

Headered Pumping Arrangement (preferred)

Dedicated Pumping OK (over-size pumps)

2 Way Valves

Select for Static, Dynamic, Close-off ratings (PSID) equal to pump SOH (plus fill pressure)

Range-ability 100 to 200:1

If Bypassfast acting, linear proportion

If Coilsslow acting, equal percentage, On-Off stagger air units (10-15 min intervals)

Controls

Set-point far out in index circuit (lower the value, the better the pump energy)

Set Ramp function in VSD Controller (10%/min average)

Run 1 more pump than chillers (when headered)

Chillers On by common Supply Temp, Load, Amps, Adj Flow (Adj for Low Delta T)

Chillers Off by Amps, Load, Adj Flow (Adj for Low Delta T)

Over-pump Chillers to combat Low Delta T and get Max Cap out of chillers

Bypass controlled by Min flow (preferred) or Min WPD of largest chiller (locate in plant for best energy, but can go anywhere in system)


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Chilled Water Piping Systems (VPF Focus)

Questions?


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Supply

Terminal

Balance

and

Service

Valve

2-Way

Control

Valve

Return

Service Valve

Air

2 Way Valve/Coil Detail


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Electric Energy Cost Equations

Chiller

Energy Cost

Lbs Refrig/hr X Head

33,015 X Comp Eff

X Hours X Cost/Unit Energy=

Pump

Energy Cost

GPM X Head

3960 X Pump Eff


Fan

Energy Cost

CFM X TSP

6356 X Fan Eff


0.7459

Mot Eff

0.7459

Mot Eff

0.7459

Mot Eff

X

X

X

Energy Cost

Mass Flow/t X Lift

33,015 X EfficiencyX Hours X Cost/Unit Energy=

0.7459

Mot EffX

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