Multiple Chiller System Control Applications - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
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 kW per ton 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.
DESIGN
A
HEAD
1.1
43°F∆T
MAXIMUM
1.0
CAPACITY
HEAD
C
31°F∆T
0.9
0.8
E
MINIMUM
CAPACITY
HEAD
19°F∆T
0.7
0.6
0.5
12.5
ENGINEERING MANUAL OF AUTOMATIC CONTROL
ONE
CHILLER
B
D
F
37.5
25 50
LOAD IN PERCENT OF CHILLER SYSTEM CAPACITY
Fig. 13. Efficiency–Two Equal Size Chillers.
Curves A and B in Figure 13 illustrate that for the chillers
operating at design condition with a 43F 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 (31F) 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 19F ∆T.
ADD ONE
CHILLER
A 1 CHILLER - 43°F ∆T(85-42°F)
B 2 CHILLERS - 43°F ∆T
C 1 CHILLER - 31°F ∆T(75-44°F)
D 2 CHILLERS - 31°F ∆T
E 1 CHILLER - 19°F ∆T(65-46°F)
F 2 CHILLERS - 19°F ∆T
62.5 75
87.5
308
TWO
CHILLERS
100 112.5
C1048