Compressors open inlet-valve unloaders

Constant-speed control should be used when gas demand is fairly constant. With this type of control, the compressor runs continuously but compresses only when gas is needed. Tiiree methods of unloading the compressor with this type of control are in common use (1) closed suction unloaders, (2) open inlet-valve unloaders, and (3) clearance unloaders. The closed suc tion unloader consists of a pressure-actuated valve which shuts off the compressor intake. Open inlet-valve unloaders (see Fig. 10-89) operate to nold the compressor inlet valves open and thereby prevent compression. Clearance unloaders (see Fig. 10-90) consist of pockets or small reservoirs which are opened when unloading is desired. The gas is compressed into... 

See Prob. 9j29. An alternative procedure is to use an inlet or outlet valve which can be stopp ed in the open position by some remote controller. For a 100 percent efficient, adiabatic compressor, what is the effect of a stuck-open inlet valve on the pressure I ratio, flow rate, and power requirement Such devices are normally called valve unloaders. 

For a constant speed driver such as motor, capacity of a reciprocating compressor can be controlled by Unloading inlet valve and/or unloaditig pockets or reservoirs at the cylinder. Lift suction valve (suction valve unloading) will prevent compression and open pocket to cylinder (clearance unloading) will increase cylinder clearance volume. Either action will reduce compressor capacity. If gas demand is intermittent, on-ofT control of motor can be used to control capacity. 

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