a fundamental perspective on chilled water systems

1

A FUNDAMENTAL PERSPECTIVEon

CHILLED WATER SYSTEMS

Wm J. CoadCoad Engineering Enterprises

2

Fundamental Parameter

Performance

3

Examples of Other Parameters

FinancialCost (Investment)Operating CostMaintenance & RepairEnergy

ReliabilityServiceabilityEnergy ConsumptionPower consumption

EnvironmentalRefrigerantsWater UseChemicals

FlexibilityExpandabilityAdaptability

4

1. Single Cooling Coil for Human Comfort (HC)

2. Multiple Coils (HC), Simultaneous Loading

3. Multiple Coils (HC), Non-simultaneous Loading

4. Multiple Coils, Some HC, Some Process Loads

5. Process Loads, Similar Requirements

6. Process Loads, Dissimilar Requirements

7. Any Combination of the Above

Nature of Loads

5

First Law

Heat Capacity Equationq = mC Δt Btu/hr

For Chilled WaterQ = GPM (500)(tr-ts) Btu/hr

6

Energy Flow DiagramFor Load System

Load

Water Flow InX gpm @ t

Water Flow OutX gpm @ t

Load Heat Inq

BoundaryS

R

L

7

Independent Variable qL

Dependent VariablesGPM

tStR

qL = GPM (500) (tR-tS)

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To Obtain Maximum Humidity Control~ Constant tS

To Obtain Maximum Chiller Benefit~ Constant tR

9

Energy Flow DiagramFor Source System

Source

Water Flow OutX gpm @ t

Water Flow InX gpm @ t

Plant Energy Inq

Boundary S

R

P

RejectedHeat Out

q = q + qL PR

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LoadSource(Plant)

CustomerSupplier

Flow: X gpm @ t

Flow: X gpm @ tR

S

tt

tR

R

S

is the highest temperature in the system

PlantEnergy In

q

LoadHeat In

qP L

RejectedHeat Out

q = q +qR P L

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qL = GPM (500) (tR-tS)

If Load wants constant tS

Plant wants constant tR

Only option is to vary the GPM

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Load Control Options

COIL1

COIL2

X gpm @ tX gpm @ t

X gpm @ t

X gpm @ t

X gpm @ t

X gpm @ t

S S

R R

X < X

q = GPM 500 (t -t ) q = GPM 500 (t -t )

Variable Flow in LoadVariable Flow in SystemConstant t and t

Variable Flow in LoadConstant Flow in SystemConstant tVariable t

variable variable

R

R

1

R SR S

1 R

1

11

SS

S

S R S

R

13

COIL1

COIL2

S S

R RVariable Flow, Constant t

Variable Flow, Constant t

Constant Flow LoadsVariable Flow System

SmallBypass

Pumped Tertiary System

ConstantFlowLoads(Variable t )

S

S

R

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COIL1

COIL2

S S

R RVariable Flow, Constant t

Variable Flow, Constant t

Multiple Variable Flow Loads

SmallBypass

Pumped Tertiary System

VariableFlowLoads(Constant t )

S

S

R

15

Distributed Pumping

Variable GPM, Constant t

Variable GPM, Constant t

Constant orVariable GPM

Multiple LoadOptions

R

SS S

RR

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1. The water entering the chillers can never be warmer than the water entering the plant.

2. The water entering the plant can never be warmer than the weighted average leaving the loads.

3. The load on the plant is equal to the product of the flow leaving the plant, the Δt and the appropriate constant.

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Single Chiller Plant

Chiller

S

R

Constant Flow, Constant t

Variable tR

S

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Chiller1

Chiller2

Load

Multiple Chiller PlantParallel Chillers

Constant Flow, Variable t

Constant Flow, Variable t

ConstantFlow

S

R

S

R

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Chiller2

Chiller1

Load

Multiple Chiller PlantSeries Chillers

Constant Flow, Constant t

Constant Flow, Variable t

ConstantFlow

R

S

R

S

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Chiller1

Chiller2

Load2

Load1

Other Multiple Chiller Plants (1)Constant or Variable GPMConstant or Variable t

Variable t Constant t

BypassValve

R R

S

Y

S

R

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Chiller1

Chiller2

Load2

Load1

Other Multiple Chiller Plants (2)Constant or Variable GPMConstant or Variable t

Variable t Constant t

BypassValve2 Position

ChillerValves

S

R RYR

S

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Chiller1

Chiller2

Load2

Load1

Multiple Chiller PlantCompound Piping Equal Unloading

Variable GPMConstant t

Variable GPMVariable t

Variable GPMConstant t

Constant GPMConstant t

Constant GPMVariable t

S

R

S

S

R

R R

B

A

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Chiller1

Chiller2

Load2

Load1

Multiple Chiller PlantCompound Piping Sequence Unloading

Variable GPMConstant t

Variable GPMConstant t

S

R

R

S

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1. Identify the types of loads to be served.

2. Design the load connections to receive water at the temperature(s) supplied from the plant and return water at the temperature required by the plant.

3. Design the plant to operate in harmony with the load requirements.

4. Keep all design concepts and algorithms as simple and understandable as possible.

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The dynamics of the load and the source are intrinsically interdependent, thermally and hydraulically, and the failure of any component to perform as designed cannot be accommodated by adding complexity to the other components.