Trane TRG-TRC004-EN manual FOLQJ6FUROO&RPSUHVVRUV

period two

Compressor Capacity Control

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VXFWLRQ￿WHPSHUDWXUH Figure 36

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.

TRG-TRC004-EN 25