Optimizing Chilled Water Plant Performance

David Herman, PE, LEED APPrincipal

EnerG Associates, LLC

Optimizing Chilled Water Plant Performance

Learning Objectives• Review the design and operating characteristics of each 

equipment component of a chilled water plant• Understand how the design characteristics and 

operation of each equipment component effects overall chiller plant efficiency

• Learn how to evaluate strategies to optimize chilled water plant performance based on existing chilled water plant design and operation

• Review actual operating data from central chilled water plants before and after implementation of optimization strategies

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Optimizing Chilled Water Plant Performance

• Benchmark the existing equipment• Ensure proper preventive maintenance• Optimize operation of existing equipment

• Evaluate possible capital retrofits

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A Typical Chiller Plant

What is the average total kW/ton for an existing chiller plant including chillers, 

cooling towers and pumps?Approx. 1.0 kW/ton

What is an optimized total kW/ton for a chiller plant?

Approx. 0.8‐1.0 kW/ton(lower with capital investment)

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A Typical Chiller Plant

What is one of the most useful diagnostic tools for benchmarking pumps?

The Pump Curve

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Benchmarking Pumps

Example:TACO Model #TA1538HP 75GPM 1400HD 140’IMP 12.5RPM 1750

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Benchmarking Pumps

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Benchmarking Pumps

How can pump impeller size be confirmed?

By deadheading the pump

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Benchmarking Pumps

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Benchmarking Pumps

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Benchmarking Pumps

Why is it important to use the same gauge to measure suction and discharge 

pressures?

To avoid gauge error since the differential pressure is needed

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Benchmarking Pumps

Example:Measured Suction P: 9 psigMeasured Discharge P: 58 psigMeasured ΔP: 58 psigMeasured ΔP: 134’ head

To convert psi to head, multiply by 2.31

Deadhead: 163’

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Benchmarking Pumps

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Benchmarking Pumps

Basic Formulas

• 1 ton = 12,000 Btuh

• GPM = Q / (500 x ΔT)

• Tons = GPM x ΔT / 24

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Benchmarking Chillers

What ΔT is the basis of the rule of thumb of 2.4 GPM/ton for evaporator flow?

ΔT = 10 degrees 

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Benchmarking Chillers

Tag # Manuf Model # Nom Tons

#1 Trane CVHE090R00 925#2 Trane CVHE090R00 925#3 Trane CVHE‐050‐F‐BC 515

Evaporator

GPM Ent Temp Lvg Temp Temp Diff

1,350 58.4 42.0 16.41,350 58.4 42.0 16.4750 58.5 42.0 16.5

Condenser

GPM Lvg Temp Ent Temp Temp Diff

1,800 99.9 85.0 14.91,800 99.9 85.0 14.91,000 100.0 85.0 15.0

FLA kW kW/ton Refrig Charge Starts Hours927 666 0.72R‐123 1,111 34,236927 666 0.72R‐123 1,131 25,575526 382 0.74R‐11 1170 2,276 > 21935

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Benchmarking Chillers

Barrel Nom Tons Rated Flow Rated PD FT Rated Temp Diff

Evap 515 750 8.1 16.5

Cond 1,000 9.7 15.0

Meas Ent P Meas Lvg P Meas Diff P Meas PD FT Meas Flow

84 80 4 9 801

78 68 10 23 1,543

Meas Ent T Meas Lvg T Meas Temp Diff Tons Amps kW kW/ton

54.0 40.0 14.0 467 440 315 0.67

80.0 89.0 9.0

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Benchmarking Chillers

Measured Flow =Rated Flow * (Rated PD/Measured PD)^0.5

kW = (3)^0.5 *V * A * PF / 1000

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Benchmarking Chillers

• Pumps• Chillers• Cooling Towers

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Ensure Proper Maintenance

• Over‐pumping results in:Flow problemsEnergy penalty

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Optimize Pump Operation

• Lower condenser water temperature set point

• For every one degree drop in condenser water temperature, chiller efficiency will increase 2%

• Consider the trade‐off between lower compressor energy and increased cooling tower fan energy

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Optimize Chiller Operation

What is the typical location of the differential pressure sensor controlling the 

pump variable speed drive?

Somewhere along the chilled water supply and return piping

What is the optimum location?

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Optimize Chiller Operation

• Relocate differential pressure sensor controlling pump variable speed drive

• Optimum location is across the chilled water supply and return piping at the farthest chilled water coil

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Optimize Chiller Operation

• Install variable speed drive on existing chiller(s)

• Significant energy savings in chiller energy use is possible (up to 30% in some applications)

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Evaluate Capital Retrofits

Optimizing Chilled Water Plant Performance

• Benchmark the existing equipment• Ensure proper preventive maintenance• Optimize operation of existing equipment

• Evaluate possible capital retrofits

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To download a copy of this presentation, go to 

www.energengineers.com

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