Honeywell AUTOMATIC CONTROL User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
308
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.
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.
Fig. 13. Efficiency–Two Equal Size Chillers.
DESIGN
HEAD
43°F∆T
MAXIMUM
CAPACITY
HEAD
31°F∆T
MINIMUM
CAPACITY
HEAD
19°F∆T
ONE
CHILLER
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
LOAD IN PERCENT OF CHILLER SYSTEM CAPACITY
12.5
25
37.5
50
62.5
75
87.5
100
112.5
0.5
0.6
0.7
0.8
0.9
1.0
1.1
A
B
D
C
F
E
TWO
CHILLERS
C1048
KW/TON INPUT

Honeywell AUTOMATIC CONTROL Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL
Category	Controller
Language	English