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Mission Critical Cooling

©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 10, October 2017

By Seemant Sharma, Member ASHRAE; Mukul Anand, Member ASHRAE

About the Authors
Seemant Sharma is director–product portfolio management, chiller solutions–Asia, building technologies and solutions for Johnson Controls in New Delhi, India; and Mukul Anand is manager, applications engineering, building technologies and solutions for Johnson Controls in York, Pa.

For a designer or facility owner, chiller selection is based on standard local practices, past experience, resource availability and legislative guidelines. However, deciding between air- or water-cooled chillers for a critical facility such as a data center, hospital or manufacturing facility having 24/7 operation depends on many factors that can impact the cost of ownership and swing the decision.

To understand the sensitivity of different parameters—such as capital cost, power cost, water cost, weather and load profile—consider the model of a typical 1,200 ton plant for a data center application operating 24/7 in four cities, representing different weather profiles:1

  • Beijing (mixed and dry, ASHRAE Climate Zone 4);
  • Singapore (hot and humid, ASHRAE Climate Zone 1);
  • Dubai (hot and humid, ASHRAE Climate Zone 1); and
  • San Francisco (warm and dry, ASHRAE Climate Zone 3).

For each plant location, we compared high-efficiency air-cooled screw and water-cooled centrifugal chillers, both with variable speed drives.


Modeled Chiller Configurations

The centrifugal chiller comparison evaluates both oil-based bearings and high-efficiency magnetic bearing designs. Mission critical applications vary from small capacity (100 ton [352 kW]) to large capacity (over 2,000 ton) plants in which decision making is clearer at both ends of the spectrum. However, there is a gray zone in the mid-capacity plant from 500 to 1,500 tons (1759 to 5276 kW). Consequently, this discussion is based on a 1,200 ton (4220 kW) plant, focusing on the factors that swing the decision towards an air- or a water-cooled chiller in mid-capacity applications.

The model chillers are sized for a building load of 1,200 tons supplied by two 600-ton (2110 kW) water-cooled centrifugal chillers or three 400-ton (1407 kW) air-cooled screw chillers, all with variable-speed drives, a 60°F (16°C) design leaving chilled water temperature (without reset) and a typical 7°F (4°C) cooling tower approach temperature. (Cooling tower approach does not apply to air-cooled chillers.) Design ambient and wet-bulb temperatures for each city are represented in Table 1. Considering this is a mission critical application, one standby chiller of same capacity for N+1 configuration is included.


Modeled Load Profiles

Two load profiles are considered in the comparison:2

Heavy Load Profile: Load variation between 100% to 80%, assuming a consistent high internal load application, such as a data center.

Medium Load Profile: Load variation between 100% to 50%, for manufacturing and health-care facilities.

To arrive at an annual load requirement, the ASHRAE Modified Bin Method is used for each city, giving total annual ton-hours and system part load value (SPLV).3 SPLV is the annual average input kilowatt per ton (IkW/ton) for the plant including chillers, chilled water pumps, condenser water pumps and cooling towers. These figures are used to compare the annual energy cost for each city for both load profiles.


Read the Full Article


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Table 1


  • 05 Oct 2017 | Omar Hawit
  • A well timed article with interesting results. I noticed the SPLV used for the air-cooled chillers in San Francisco appears to be about 0.9 kW/ton, which appears to be around the IPLV for the minimum efficiency listed in ASHRAE 90.1-2013 for air-cooled chillers. Could you confirm the air-cooled and water-cooled chiller EER and IPLV that was modeled to determine the SPLV?
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