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TRG-TRC016-EN 67
period three
System Variations
notes
The second preferential-loading configuration, shown in Figure 75, ensures
that the chiller in the sidestream position receives the warmest entering-water
temperature, and that it can be fully loaded whenever the system load is high
enough. This arrangement is unique because it not only allows preferential
loading, but it also permits the chiller (or other cooling device) in the sidestream
position to operate at any temperature difference. In other words, it does not
need to supply water at the same temperature that the other operating chillers
do. The chiller in this position precools the system return water, reducing the
load on the downstream chillers. In this example, a heat-recovery chiller is
located in the sidestream position so that it can be preferentially loaded to
maximize the amount of heat recovered, thus reducing the overall building
energy consumption. Because a heat-recovery chiller is typically less efficient
than a standard cooling-only chiller, the heat-recovery chiller only has to
provide as much cooling as is required to meet the heat-recovery load, letting
the more efficient cooling-only chillers meet the rest of the cooling load.
One drawback of the sidestream arrangement is that it does not add water flow
capability to the system, it simply reduces the load on other chillers. Therefore,
the other system pumps must ensure that the system flow requirements are
met. For this reason, the capacity of the sidestream chiller is often smaller than
the other chillers in the plant.
Either of these arrangements allows a chiller to be preferentially loaded.
Preferential loading is typically most beneficial in the following applications:
n In a system that has a high-efficiency chiller along with several standard-
efficiency chillers, the high-efficiency chiller can be preferentially loaded to
reduce system energy consumption.
n In a system with a heat-recovery chiller, preferentially loading the heat-
recovery chiller maximizes the amount of heat recovered, thus reducing
the overall building energy consumption.
Sidestream Configuration
b
y
p
a
s
s
p
i
p
e
b
y
p
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s
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distribution
distribution
pump
pump
heat
heat
-
-
recovery
recovery
chiller
chiller
Figure 75