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70 Chiller System Design and Control SYS-APM001-EN
Chilled-Water System Variations
A number of chilled-water system variations can and should be used when
appropriate. Each configuration offers specific advantages to solve problems
and add value to the system.
Heat Recovery
ASHRAE/IESNA Standard 90.1–2007
20
requires heat recovery in specific
applications. Indoor air quality concerns have spurred the use of systems that
subcool supply air to dehumidify, then temper, the air to satisfy space
conditions. ASHRAE/IESNA Standard 90.1–2007 limits the amount of new
reheat energy used in these applications. With these drivers, and energy costs,
there has been a resurgence of heat recovery chillers. The example on page 75
describes a cost-effective recovered-heat strategy. This scheme is commonly
used for service-water heating in resort hotels and for certain process loads.
A separate application manual
21
discusses heat recovery, as does an Engineers
Newsletter
22
, so it is not discussed in depth in this manual. However,
considerations for heat recovery on chilled-water system design are discussed.
Condenser “Free Cooling” or Water
Economizer
There are several ways to accomplish free cooling through the use of a water
economizer circuit. Three common techniques for chilled water systems are
using a plate-and-frame heat exchanger, refrigerant migration, or well, river, or
lake water. Each technique is discussed in more detail below and in an
Engineers Newsletter
23
.
Plate-and-frame heat exchanger
A plate-and-frame heat exchanger may be used in conjunction with a cooling
tower to provide cooling during very low wet-bulb temperature conditions.
When it is to be used for these purposes, designers often specify a cooling
tower larger than necessary for design conditions so that it can be used for
many hours with the plate-and-frame heat exchanger.
In this type of water economizer, the water from the cooling tower is kept
separate from the chilled-water loop by a plate-and-frame heat exchanger. This
is a popular configuration because it can achieve high heat-transfer efficiency
without cross-contamination. With the addition of a second condenser-water
pump and proper piping modifications, this heat exchanger can operate
simultaneously with the chiller, provided the chiller is placed downstream of the
heat exchanger (as shown in Figure 40). As much heat as possible is rejected
through the heat exchanger while the chiller handles the rest of the cooling
Chiller 2
Chiller 1
Plate-and-Frame
Bypass Line
Heat Exchanger
Distribution
Production
Figure 40. Sidestream plate-and-frame
heat exchanger