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22 TRG-TRC004-EN
notes
period two
Compressor Capacity Control
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.
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Figure 32