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TRG-TRC016-EN 71
period three
System Variations
notes
The control of a heat-recovery centrifugal chiller, although seemingly simple,
is critical to reliable chiller operation. Typically, either the temperature or the
flow of the water entering the standard condenser is modulated to meet the
capacity required by the heat-recovery condenser.
Controlling heat-recovery capacity based on the temperature of the hot water
leaving the heat-recovery condenser can cause operational problems for a
centrifugal chiller. This is explained best by using a map of centrifugal-
compressor operation (see Figure 79). Control based on the temperature of the
water leaving the heat-recovery condenser causes the condenser-to-evaporator
pressure differential to remain relatively high at all loads (line A to B). High
pressure differentials at low cooling loads increases the risk of a centrifugal
compressor operating in its unstable region, commonly known as surge.
The preferred method is to control heat-recovery capacity based on the
temperature of the hot water entering the heat-recovery condenser. This allows
the condenser-to-evaporator pressure differential to decrease as the chiller
unloads (line A to C), thereby keeping the centrifugal chiller from surging and
resulting in more stable operation. If high leaving-hot-water temperatures are
required at low-cooling-load conditions, another method to prevent surge is
to use hot gas bypass on the centrifugal chiller.
For other types of chillers that are not prone to surge, operating at these high
pressure differentials at low cooling loads may cause the chiller to consume
more energy than it recovers in the form of heat.
Control of a Heat-Recovery Chiller
percent maximum pressure differential
percent maximum pressure differential
percent load
percent load
u
n
s
t
a
b
l
e
u
n
s
t
a
b
l
e
o
p
e
r
a
t
i
o
n
o
p
e
r
a
t
i
o
n
(
s
u
r
g
e
)
(
s
u
r
g
e
)
B
A
unloading with
unloading with
constant
constant
entering
entering
hot
hot
-
-
water temperature
water temperature
100
100
80
80
60
60
120
120
100
100
50
50
0
0
C
unloading with
unloading with
constant
constant
leaving
leaving
hot
hot
-
-
water temperature
water temperature
Figure 79