Compressor Capacity control of a refrigeration plant, also known as refrigeration compressor unloader valve, can be defined as a system which monitors and controls the output of the plant as per the load on demand. Similarly, the compressor shaft seal ensures the crankcase lube and the refrigerant does not leak out of the compressor from the shaft connected to the driving motor.

The efficiency of the reefer plant depends on its ability to keep different rooms or holds at a different temperature. Each room will have its own load demand hence the refrigeration plant’s output has to be controlled to achieve required cooling.
As the load (temperature) of one room is reached to its required temperature, there will be no more need of the refrigerant for cooling. Hence, the solenoid valve supplying refrigerant to that room will shut.

The simplest forms of controlling the room temperature are:

– Using Variable Speed Motor (Normally used on small AC units)-
– Controlling on-off cycling of the compressor
– Using the Cylinder unloading method of keeping the suction valve in open position. This is accomplished by introducing a capacity controller valve in the compressor which is operated by lube oil pressure (hydraulic type) or by using a solenoid operated control valves

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