Trane TRG-TRC004-EN manual RPSUHVVRU8QORDGLQJ

period two

Compressor Capacity Control

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VXFWLRQ￿WHPSHUDWXUH Figure 32

At design conditions, the capacities of the evaporator coil and compressor balance (A) at a suction temperature of 45°F [7.2°C] and a capacity of 31 tons [109 kW]. As the cooling load decreases below this balance point, assuming a constant condensing pressure, the compressor pumping capacity decreases with the falling suction temperature along the six-cylinder curve until it reaches B. Here, the compressor unloads the first set of two cylinders.

When the first set of two cylinders is unloaded, the compressor operates with only four active cylinders and the compressor capacity falls immediately to

19 tons [66.8 kW] along the four-cylinder curve (C). As the load continues to decrease, the capacity and suction temperature follow the four-cylinder curve until it reaches D. Here, the second set of two cylinders is unloaded, decreasing the compressor capacity to 9.5 tons [33.4 kW] along the two-cylinder curve (E).

As the load continues to decrease, the suction temperature reaches the minimum set point, 28°F [-2.2°C] in this example (F), and the two remaining cylinders are deactivated by shutting off the compressor. The minimum capacity of the compressor in this example is 7 tons [24.6 kW].

This illustrates how cylinder unloading extends the stable part-load range of a reciprocating compressor. The example compressor is able to perform over 77% of its capacity range (31 tons to 7 tons [109 kW to 24.6 kW]). An increasing load reverses the sequence.

22 TRG-TRC004-EN