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TRG-TRC004-EN 25
period two
Compressor Capacity Control
notes
At design conditions, the capacities of the evaporator and this four-compressor
unit balance at a suction temperature of 43°F [6.1°C] and a capacity of 44 tons
[154.7 kW] (A). As the cooling load decreases below this balance point,
assuming a constant condensing pressure, the capacity of the unit decreases
with the falling suction temperature along the four-compressor curve until it
reaches B. Here, the first scroll compressor is shut off and the capacity of the
unit decreases immediately to 30 tons [105.5 kW] (C) along the three-
compressor curve.
As the load continues to decrease, the individual compressors shut off in a
similar manner until the suction temperature reaches a minimum set point and
the final compressor is shut off. The minimum capacity of the four-compressor
unit in this example is 8 tons [28.1 kW].
Excessive starting and stopping of scroll compressors is not a concern. The
reciprocating compressor system on Figure 35 includes a single large
compressor with a single large motor. In contrast, the scroll compressor system
has four small compressors, each with its own small motor. These small motors
are designed to cycle, just like those used with small reciprocating
compressors.
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Figure 36