Publisher Summary A compressor is a machine that is used to increase the pressure of a gas or vapor. They can be grouped into two major classifications: centrifugal and positive displacement. This chapter provides a general discussion of these types of compressors. The centrifugal designation is used when the gas flow is radial and the energy transfer is predominantly due to a change in the centrifugal forces acting on the gas. The force used by the centrifugal compressor is the same as that used by centrifugal pumps. In a centrifugal compressor, air or gas at atmospheric pressure enters the eye of the impeller. As the impeller rotates, the gas is accelerated by the rotating element within the confined space that is created by the volute of the compressor's casing. Positive-displacement compressors can be divided into two major classifications: rotary and reciprocating. The rotary compressor is adaptable to direct drive by the use of induction motors or multi- cylinder gasoline or diesel engines. These compressors are compact, relatively inexpensive, and require minimum operating attention and maintenance. They occupy a fraction of the space and weight of a reciprocating machine having equivalent capacity. Reciprocating compressors are widely used by industry and are offered in a wide range of sizes and types. They vary from units requiring less than 1 hp to more than 12,000 hp. Pressure capabilities range from low vacuums at intake to special compressors capable of 60,000 psig or higher.