ASHRAE 199802 Controlling Chillers in Variable Flow Systems
ASHRAE 199802 Controlling Chillers in Variable Flow Systems
ASHRAE 199802 Controlling Chillers in Variable Flow Systems
20/20
peak efficiency The cooling plant for Allied Signal, The Results
and discusses built in 1983 in Rocky Mount, N.C.,
includes two 700-ton (2462 kW) and two Figure 2 charts the total annual kWh
the changes 350-ton (1231 kW) centrifugal chillers that energy consumption of the individually
serve the HVAC system and meet the cool- metered chillers, for the years 1991 through
made to ing requirements for the industrial produc- 1996. The energy consumed by the chillers
tion heat exchangers. The facility produces for the years 1993 through 1996, after the
improve its per- and refurbishes controls and accessories for valve was installed, decreased an average
formance." aircraft jet engines. The chilled water system of 20% per year. The average annual chiller
utilizes primary chiller pumps and sec- About the Author
ondary load pumps similar to the system
shown in Figure 1. Gil Avery is vice president of Kele and
The author visited the plant in Associates, Memphis, Tenn. He is a mem-
September 1992 to discuss methods of ber of the subcommittee to revise Chapters
improving the system efficiency and opti- 1 through 5 of the ASHRAE Handbook,
mizing chiller performance. The production chair of Guideline Projects Committee
heat exchangers required cooling water at GPC-16, and a member of SSPC-62.
This article appeared in the ASHRAE Journal, February 1998. Used with permission.
utilization (a 350-ton [1231 kW] chiller operating one hour P.P.
I.V.
( 1979)
most primary/secondary systems). The logic behind this 2,000
( 1864)
( 1794)
assumption is that the ∆T (generally 10°F to 16°F [5.55°C
T O TA L A N N UA L
CHILLER MWH
(1640)
1,800
(1394)
system. As long as the ∆T is constant, the chiller load is
(1304)
1,400
directly proportional to flow.
1,200
Factors that may reduce the ∆T and chiller efficiency
include: 1,000
91 92 93 94 95 96
• The coils fouling on the air and water side enough to YEAR
Btu/h [58 620 W]) as the coil fouling factor drops from 1.0 KWH DECREASED 20%
(6,731K)
8,000K
CHILLER UTILIZATION
(5,740K)
7,000K
temperature and increased gallons-per-minute of the "old"
(5,580K)
HOURS*
(4620K)
6,000K
coil. (A new coil operating at an entering water tempera-
(4,324K)
5,000K
ture of 41.2°F (5°C) and with a flow of 26.0 gpm (1.64
L/s), [line 3] would have a capacity of 222,700 Btu/h (65 4,000K
273 W). Note that over 6°F cooler water is needed when 3,000K
91 92 93 94 95 96
the fouling factor is 0.8. This is a large part of the increase YEAR
efficiency at cooler supply water temperatures. YEARS 93-96 (POST-CHECK VALVE) = 5,066,000.00
• The airflow decreases as the filters and other air handling DECREASE = 1,952,000.00
TM