COOLING TOWERS AND CONDENSER WATER SYSTEMS: DESIGN AND OPERATION

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Cooling Towers and

Condenser Water Systems

Design and Operation

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Cooling Towers and

Condenser Water Systems

Design and Operation

Cooling tower fundamentals

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5 psia

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evaporatoreconomizerP

c

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water temperature

rejection rate of tower

Hermetic motor:100% of electricalinput

Open motor:electrical input× motor efficiency

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Heat Rejection Example

For 1 ton of evaporator load:

Q = 12,000 Btu/h × (1 + 1/6.10)

= 12,000 Btu/h × (1 + 0.16)= 13,967 Btu/h

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to heat rejection and condenser water flow rate

condenser temperature

condenser pressure rises

ASHRAE Terminology of HVAC&R

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hot waterfill

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Performance Factors

hot watertemperature

cold watertemperature

ambient wet bulb

towerflow rate

cooling tower certification

CTI Performance Limitscooling tower certification

CTI Performance Limits

hot water< 125°F

cold watertemperature

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96°Fhot water

82°Fcold water

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99°Fhot water

85°Fcold water

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ambient wet bulb, °F0.0

4.08.012.016.0

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100% load

50% loadcooling tower performance factorsApproach and Wet Bulbcooling tower performance factorsApproach and Wet Bulb

approach = 7approach = 16

cooling tower performance factors

Approach and Wet Bulb

cooling tower performance factors

Approach and Wet Bulb

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Cooling Towers and

Condenser Water Systems

Design and Operation

10° F ΔT for older, less efficient chillers

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Industry advisorPacific Gas andElectric CoolTools™

10°–15°F T single stage12°–18°F T multistage or positive displacementKelly and Chan14.2°F T for 3.6–8.3%

energy savings in various climates

ASHRAE Green Guide12°–18°F T

Chiller Performance8.0

ASHRAE Standard 90

(1977)(1980)90-7590.1-89 90.1-99

centrifugal>600 tons

screw150-300 tons

scroll<100 tons

reciprocating<150 tons

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Condensing Components

Condenser waterpump

Certified selections help assure expectedchiller performance

Full-load and part-load conditions

Air-Conditioning & RefrigerationInstitute (ARI)

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chilled water plant design

Cooling Tower Selectionchilled water plant design

Cooling Tower Selection

Cooling Technology Institute (CTI)rates tower performance

3960 × pump efficiency

kW =

motor efficiency0.746 × hp

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… in the condenser water loop:

Pipes, valves, fittings

chilled water plant design

Base Design: 500 Tonschilled water plant design

Base Design: 500 Tons

Assumptions for our example …

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0.572/tonCondenser P, ft25.7

Condenser flow rate, gpm/ton:3.0

Flow rate, gpm1500Power rating, bhp40

kW31.2Static head, ft13

Condenser flow rate, gpm/ton:3.0

Basedesign

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Condenser flow rate, gpm/ton:3.0

chilled water plant design: examplePump Selection

Condenser flow rate, gpm/ton:3.0

Basedesign

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Condenser flow rate, gpm/ton:

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Flow rate, gpm15001000Power rating, bhp4025

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tower fancondenser pumpchiller

gpmgpm3.0gpm2.0gpm3.0gpm2.0gpm3.0gpm2.0

50% load75% load100% load

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full energy cost savings

Reduce pipe size to cut installed cost

Condenser water pump must overcome pressure drops

Tower static lift

Pipes, valves, fittings

What if it was0 ft of head?

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tower fancondenser pumpchiller

gpmgpm3.0gpm2.0gpm3.0gpm2.0gpm3.0gpm2.0

50% load75% load100% load

*piping PD = 0 ft

Cost savings: $426,000 construction7.3% operation

Cost savings: $45,000 excavation, concrete6.5% operation

Cost savings: Piping

2% operation (existing tower)

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Cost savings: $426,000 construction7.3% operation

Cost savings: $45,000 excavation, concrete6.5% operation

Cost savings: Piping

2% operation (existing tower)

two years ago

in good shape

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Chiller PlantAnalyzer

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Start at 2 gpm/ton and adjust

Reinvest part of first-cost savings in more efficient chillers

chilled water plant designGuidance

The meter is on

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Saving Energy–And More

“In addition to the electric energy savings, this chiller plant will have prevented the

situation where the first cost is reduced, operating cost is minimized, plus significant environmental benefits are realized as an additional benefit.”

from “A Chiller Challenge” by T. Chan

Cooling Towers and

Condenser Water Systems

Design and Operation

Cooling-towercontrol options

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chiller–tower optimization

Control

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Capacity Control

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cooling tower

Capacity vs. Airflowcooling tower

Capacity vs. Airflow

806040200

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one tower fan

Perfect Capacity Controlone tower fan

Perfect Capacity Control

capacity vs. airflowairflow vs. fan power

speed drive

variable-speed drivesingle-speed fan

one tower fan

Capacity Controlone tower fan

Capacity Control

two-speed fan:100% and 50%two-speed fan:100% and 67%

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2 fans with VSDs2 single-speed fans1 single-speed fan,1 two-speed fan

two tower fans

Capacity Controltwo tower fans

2 fans with VSDs,modulated together2 fans with VSDs,1 to high speed first

two tower fans

Capacity Controltwo tower fans

Capacity Control

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Invest in VFDs on all tower fans

Operating multiple fans at part speed saves energy

full speed and the other off

Sequence of Operation

1. When a chiller is operating and the cooling tower basin temperature rises to two (2) degrees F above the current tower leaving water setpoint, the lead cooling tower fan shall be turned on at minimum speed and the DDC control loop enabled.

a. When the operating fan(s) are operating at 50 percent speed, an additional fan shall be enabled and controlled at the same speed as the operating fans until all active cooling tower cell fans are enabled.

b. When operating fans are running at minimum speed and the tower supply water temperature is five (5) degrees below the current tower leaving water setpoint, the most lag tower fan shall be turned off.

c. Cooling tower fans shall have five (5) minute minimum on and off time delays.

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chiller–tower optimizationControl

Evaporator pressure setby leaving-evaporator watertemperature

41.8°F

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Evaporator pressure setby leaving-evaporator watertemperature

41.8°F

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Reduce water flow over tower—

e.g., 3-way bypass into sump (not preferred)

Reduce flow through condenser

Tower bypass with 3-wayor 2-way valves

Constant flow throughchiller

NOT PREFERREDLonger lag time makescontrol difficult

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Throttling Valve

Throttling valvemodulates based onrefrigerant P

Constant flow over tower

More precise control

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Enables more efficientwater balancing

Short periods withlow flow over tower

Most efficient methodof controlling chillerhead pressureVFD

Sequence of Operation

… for chiller head pressure:

1. The BAS shall monitor the refrigerant pressure in each chiller’s evaporator and condenser. The BAS shall use PID-based

DDC control of the [chiller-condenser-pump

VFD][cooling-tower bypass condenser bypass valve] to maintain no

valve][chiller-less than the minimum pressure differential specified by the chiller manufacturer.

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Water-Cooled Series R Chiller Models RTHB & RTHD Condenser Water Control

Tower flow range can be muchnarrower than that of chiller

500-toncooling towerFlow

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Consult tower manufacturer … Specify limits

Cooling Towers and

Condenser Water Systems

Design and Operation

System

optimization

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Tower design

Condenser watertemperature

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optimalcontrol point

setpoint equals design …85°F for humid climates,80°F for dry climates

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Mexico City OrlandoSan DiegoToronto

55°F lvg toweroptimal controldesign ECWTcontrol strategy:

DubaiParisSao Paulo Singapore

55°F lvg tower

optimal controldesign ECWTcontrol strategy:

chiller–tower control strategiesGlobal Locations

chiller–tower control strategies

Global Locations

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Perspective on SavingsEquivalent

Savings, %chiller efficiency

Documented Savings

PG&E CoolTools™ program (2000)

(July 2004)

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Chillers cannot meet load above this condenser water temperature

Optimal operation1,550 tons, 65°F Wet-bulb

1,160 tons, 59°F Wet-bulb

730 tons, 54°F Wet-bulb Temperature

SOURCE: Hydeman, et al. Pacific Gas and Electric. Used with permission.

The chiller works harder

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Best Sustainable Practice

presented to Trane for“Near optimal

chiller–tower operation”by the Sustainable Buildings Industry Council (SBIC)

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chiller–tower optimizationNecessities

Variable-frequency driveon tower fans

relative humidity sensor

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Calculating the Savings

Available tools include

TRACE™ 700, Chiller Plant Analyzer,System Analyzer™

Full loadWet bulb(deg F)setpointTowertower kWChiller +

7871607160

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chiller–tower optimizationSummary

Proven control

strategy that’s more than 10 years old

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Variable-Speed Pump?

Tower static lift

Proper water distribution throughout tower fill (nozzles)

Required flow through condenser

Variable-Speed Pump?

Increases chiller power (warmer water leaving condenser)

Alters heat-transfer effectiveness of tower

Reduced speed/flow:

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Variable-Speed Pump?

Control strategy varies with each installation based on chiller and tower selections

No definitive answer… yet

Cooling Towers and

Condenser Water Systems

Design and Operation

Answers toyour questions

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tower size or approach

Head pressure control is criticalto reliability

Offers significant operating-cost savings

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COOLING TOWERS AND CONDENSER WATER SYSTEMS: DESIGN AND OPERATION

<b>Cooling Towers and </b>

<b>Condenser Water Systems</b>

Design and Operation

<b>Cooling Towers and </b>

<b>Condenser Water Systems</b>

Design and Operation

<b>Cooling tower fundamentals</b>

<b>5 psia</b>

<b>evaporatoreconomizerP</b>

<b>water temperature</b>

<b>rejection rate of tower</b>

<b>Hermetic motor:100% of electricalinput</b>

<b>Open motor:electrical input× motor efficiency</b>

<b>Heat Rejection Example</b>

<b>For 1 ton of evaporator load:</b>

<i><b>Q = 12,000 Btu/h × (1 + 1/6.10)</b></i>

<b>= 12,000 Btu/h × (1 + 0.16)= 13,967 Btu/h</b>

<b>to heat rejection and condenser water flow rate</b>

<b>condenser temperature</b>

<b>condenser pressure rises</b>

<i><b>ASHRAE Terminology of HVAC&R</b></i>

<b>hot waterfill</b>

<b>Performance Factors</b>

<b>hot watertemperature</b>

<b>cold watertemperature</b>

<b>ambient wet bulb</b>

<b>towerflow rate</b>

<b>cooling tower certification</b>

<b>CTI Performance Limitscooling tower certification</b>

<b>CTI Performance Limits</b>

<b>hot water< 125°F</b>

<b>cold watertemperature</b>

<b>96°Fhot water</b>

<b>82°Fcold water</b>

<b>99°Fhot water</b>

<b>85°Fcold water</b>

<b>ambient wet bulb, °F0.0</b>

<b>4.08.012.016.0</b>

<b>100% load</b>

<b>50% loadcooling tower performance factorsApproach and Wet Bulbcooling tower performance factorsApproach and Wet Bulb</b>

<b>approach = 7approach = 16</b>

<b>cooling tower performance factors</b>

<b>Approach and Wet Bulb</b>

<b>cooling tower performance factors</b>

<b>Approach and Wet Bulb</b>

<b>Cooling Towers and </b>

<b>Condenser Water Systems</b>

Design and Operation

10° F ΔT for older, less efficient chillers

<b>Industry advisorPacific Gas andElectric CoolTools™</b>

<b>10°–15°F T single stage12°–18°F T multistage or positive displacementKelly and Chan14.2°F T for 3.6–8.3% </b>

<b>energy savings in various climates</b>

<b>ASHRAE Green Guide12°–18°F T</b>

<b>Chiller Performance8.0</b>

<b>ASHRAE Standard 90</b>

<b>Condensing Components</b>

<b>Condenser waterpump</b>

<b>Certified selections help assure expectedchiller performance</b>

Full-load and part-load conditions

Air-Conditioning & RefrigerationInstitute (ARI)

<b>chilled water plant design</b>

<b>Cooling Tower Selectionchilled water plant design</b>

<b>Cooling Tower Selection</b>

<b>Cooling Technology Institute (CTI)rates tower performance</b>

<b>3960 × pump efficiency</b>

<b>kW =</b>

<b>motor efficiency0.746 × hp</b>

<b>… in the condenser water loop:</b>

Pipes, valves, fittings

<b>chilled water plant design</b>

<b>Base Design: 500 Tonschilled water plant design</b>

<b>Base Design: 500 Tons</b>

<b>Assumptions for our example …</b>

<b>0.572/tonCondenser P, ft25.7</b>

<b>Condenser flow rate, gpm/ton:3.0</b>

<b>Flow rate, gpm1500Power rating, bhp40</b>

<b>kW31.2Static head, ft13</b>

<b>Condenser flow rate, gpm/ton:3.0</b>

<b>Basedesign</b>