Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
306
When two chillers of equal size and similar characteristics are
used, the point at which the second chiller is activated is usually
when the first chiller reaches 100 percent load. This is
demonstrated in Figure 13 which plots kilowatt per kilowatt versus
percent load for one chiller and two chillers at various temperature
differences between condenser and chilled water temperatures.
Curves vary slightly for different temperature differences so a
microprocessor-based control system is used for maximum
efficiency. The microprocessor checks chilled and condenser water
temperature, looks up chiller efficiency at those temperatures, and
calculates the optimum changeover point.
Curves A and B in Figure 13 illustrate that for the chillers
operating at design condition with a 24 kelvins temperature
differential (
∆
T) between chilled and condenser water the
second chiller must be added when the first chiller reaches 100
percent load (50 percent of chiller system capacity). The next
set of curves (C and D) show the chiller is more efficient because
of the smaller
∆
T (17K) and that the first chiller can be loaded
to 110 percent (55 percent of system load) before the second
chiller is added. The third set of curves shows the extremely
efficient operation with a 10.5 kelvins
∆
T.
Fig. 13. Efficiency–Two Equal Size Chillers.
0.14
0.17
0.20
0.23
0.26
0.32
kW/kW INPUT
LOAD IN PERCENT OF CHILLER SYSTEM CAPACITY
C4084
12.5 25 37.5 50 62.5 75 87.5 100 112.5
DESIGN
24 KELVINS
MAXIMUM
CAPACITY
17 KELVINS
ADD ONE
CHILLER
A
B
C
D
F
A 1 CHILLER - 24 KELVINS (29.5
°
C - 5.5
°
C)
B 2 CHILLERS - 24 KELVINS
C 1 CHILLER - 17 KELVINS (24
°
C - 7
°
C)
D 2 CHILLERS - 17 KELVINS
E 1 CHILLER - 10.5 KELVINS (18.5
°
C - 8
°
C)
F 2 CHILLERS - 10.5 KELVINS
MINIMUM
CAPACITY
10.5 KELVINS
0.29
ONE CHILLER
E
TWO CHILLERS