ASHRAE Journal - April 2022 - 12
TECHNICAL FEATURE
Water-Cooled Chillers
Chilled-Water VAV
System Configuration
And Design
BY NABIL NASSIF, PH.D., P.E., MEMBER ASHRAE
The air-handling unit (AHU) in a typical chilled water variable air volume (VAV)
system includes two separate chilled and hot water loops. This configuration eliminates
the opportunity to use both loops for cooling during the hot summer and for
heating during the cold winter. In an article1 in the February issue of ASHRAE Journal,
two possible improvements with several configuration options were introduced for
air-cooled chillers. In this article, various alternative design options and control for
water-cooled chillers are presented.
Typical Chilled-Water VAV System
Figure 1 shows traditional primary/secondary chilled
and hot water configurations. Typically there are three
main loops: a chilled water loop, condenser water loop
and hot water loop. The chillers produce chilled water to
the cooling coils in AHUs for thermal cooling, plus possible
process cooling loads. At relatively low outdoor air
temperatures (OAT), free cooling is typically achieved
through an airside economizer arrangement in each
AHU. However, for the cooling process, cooling may be
achieved through a waterside economizer (ECO) typically
using a plate frame heat exchanger located in the
return side of the chilled water loop.
Office buildings may have only AHU coils and airside
economizers. The boilers serve hot water for the heating
coils located in the AHUs plus VAV box reheat coils
located in zones. In the schematic, as an example, four
boilers (B1, B2, B3 and B4), four chillers (Ch1, Ch2, Ch3
and Ch4) and four cooling towers (CT1, CT2, CT3 and
CT4) are shown. It is assumed one chiller is needed to
serve the process cooling loads. Alternative design configuration
options to replace this base design are introduced
next.
Option 1: One Common Coil in Each AHU
In this option, one coil in each AHU is used instead of
two coils. Four separate water loops are recommended
as shown in Figure 2. The first water loop is the hot water
Nabil Nassif, Ph.D., P.E., is an associate professor of architectural engineering at the University of Cincinnati.
12
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A P R I L 2022
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ASHRAE Journal - April 2022
Table of Contents for the Digital Edition of ASHRAE Journal - April 2022
Contents
ASHRAE Journal - April 2022 - Intro
ASHRAE Journal - April 2022 - Cover1
ASHRAE Journal - April 2022 - Cover2
ASHRAE Journal - April 2022 - 1
ASHRAE Journal - April 2022 - Contents
ASHRAE Journal - April 2022 - 3
ASHRAE Journal - April 2022 - 4
ASHRAE Journal - April 2022 - 5
ASHRAE Journal - April 2022 - 6
ASHRAE Journal - April 2022 - 7
ASHRAE Journal - April 2022 - 8
ASHRAE Journal - April 2022 - 9
ASHRAE Journal - April 2022 - 10
ASHRAE Journal - April 2022 - 11
ASHRAE Journal - April 2022 - 12
ASHRAE Journal - April 2022 - 13
ASHRAE Journal - April 2022 - 14
ASHRAE Journal - April 2022 - 15
ASHRAE Journal - April 2022 - 16
ASHRAE Journal - April 2022 - 17
ASHRAE Journal - April 2022 - 18
ASHRAE Journal - April 2022 - 19
ASHRAE Journal - April 2022 - 20
ASHRAE Journal - April 2022 - 21
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ASHRAE Journal - April 2022 - Cover3
ASHRAE Journal - April 2022 - Cover4
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