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56 ASHR AE Journa l ash rae .o rg Februar y 2010

By James B. (Burt) Rishel, P.E., Fellow/Life Member ASHRAE

 ASHRAE Guideline 22-2008, Instrumentation for Monitoring Central Chilled-

 Water Plant Efficiency, is the result of years of work to come up with a

meaningful procedure for determining the overall energy rate for the total electric

motor-driven, chilled water plant. The overall efficiency of central chiller plants is

important, as they consume many megawatts; it is estimated that their energy

use is around 30% of the entire HVAC industry.

This guideline was developed for con-

ducting a test on a specific chiller plant to

determine its coef ficient of performance

(COP). The data collected included many

values that would be needed for such a test.

This article’s goal is to assist designers and

operators in using this guideline in the con-

tinuous measurement and recording of the

energy rate of an operating chilled water

 plant. It is limited to electric motor-driven

 plants (as is the guideline), but includes

chiller plants with heat recovery chillers

and water-side economizers.

Measured kW/ton can be trended

against plant load in tons of cooling,

outdoor temperatures and condenser

water temperature. The resultant informa-

tion can be a useful tool in maintenance

 programming; an increase in kW/ton may

signal a need for maintenance or an im-

minent equipment failure. For example,

an increase in kW/ton may indicate dirty

condenser tubes in the chillers, a partially

clogged strainer for tower pumps, or a

leaking coil valve as theT  for the chilled

water drops below design. 

Harvard University was an early pro-

 ponent of using kW/ton as a means of

measuring the improvement of chiller

 plant ef ficiency. For years, they displayed

it in large numerals on the wall of one of its

chiller plants. It proved to be an effective

tool in the operation and maintenance of

that plant. Today, they show it prominently

on their chiller plant computer screens as

shown in Figure 1.

Most of the central chiller plants in the

United Sates use I-P instrumentation and

measure the energy rate in kW/ton. Sincethis is a practical document rather than a

theoretical one, COP and SI procedures

will not be covered. Likewise, uncertainty

of measurement will not be addressed.

Anyone who wishes to determine this can

use the guideline for assistance in mak-

ing this evaluation. The instrument error

expressed in this article should result in

an uncertainty of around 2.2%. This is

derived from Table 5-2 of the guideline.

Derivation of the Basic kW/ton EquationThe basic kW/ton equation is derived

in Paragraph B1.3 of the guideline. It is

Equation B-3 in the guideline; it is re-

 peated here with a different designation

for flow as Equation 1.

kW = kW× 24 (1)

Q×(T 2 – T 1)

 About the Author 

 James B. (Burt) Rishel, P.E.,  is director of me-

chanical systems for tekWorx, LLC.

Guideline for Instrumentation for Monitoring Central Chilled-Water Plant Efficiency

This article was published in ASHRAE Journal, February 2010. Copyright 2010 American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permissionof ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.