Publisher Summary Compressors are gas movers and devices where mechanical work is done on the gas, resulting in an increased pressure. Compressors are classified into two basic types: reciprocating and centrifugal. The reciprocating compressor is well suited for high pressures and low volume flow rates. These are furnished as single stage or multistage with ratings from fractional to more than 20,000 horsepower per unit. The pressures range from a low vacuum at suction to 30,000 psi and higher at discharge. The number of stages depends on the overall compression ratio. The compression ratio per stage is limited by the discharge temperature and does not exceed four. A centrifugal compressor uses the relationship between velocity and pressure to increase the gas pressure. Centrifugal compressors are used in a variety of chemical process industries (CPI). In a centrifugal compressor, at least half the driver power is changed into gas velocity energy. The increased velocity can be carried forward to the next impeller for further increase in the velocity. A centrifugal compressor uses the impeller to furnish rotational pressure that squeezes the gas outward and closer together, resulting in some increase in the pressure.