Adjustable unloading pocket

The proper way to reduce the volumetric efficiency is to increase the starting volumetric clearance. This is done with an adjustable unloading pocket, as shown in Fig. 29.4. This device, also called the head-end unloader, works by increasing the starting volumetric clearance. This is known as the volume of gas trapped between the cylinder head and the piston, when the piston position is at bottom dead center, as explained in Fig. 29.2. 

The second way to unload a reciprocating compressor is with valve disablers. Most of the unloaders you have on your compressors are likely of this inferior type. They typically consist of steel fingers, which are pressed down through the valve-cap assembly. The fingers prevent the valve plate from moving. These valve disablers are far less costly than an adjustable clearance pocket. They may be used on both the crank and head ends of the cylinder. They can reduce the gas flow through a cylinder to zero. 

Many reciprocating compressors come equipped with unloading pockets. This is an open area at the end of the cylinder whose length is adjustable. The compressor piston cannot compress the gas in the unloading pockets. The purpose of the pockets is to allow the operator to decrease compressor capacity without wasting horsepower. If you are looking for maximum capacity, make sure these adjustable pockets are closed. 

The head-end unloader has a great advantage in that it unloads the driver without increasing the amount of work required per mole of gas compressed. The disadvantages of the adjustable clearance pocket or head-end unloader, shown in Fig. 42.4, are... 

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