This chapter describes the strategies available to increase the safety and energy efficiency in control systems. The volumetric efficiency of a compressor is the ratio between the mass of the gas discharged and the mass of the gas filling the piston prior to compression. The axial compressor drives the gas parallel to the shaft; in the centrifugal compressor, the gas receives a radial thrust toward the wall of the casing where it is discharged. The chapter describes the basic controls for rotary, reciprocating, and centrifugal compressors together with installation and safety considerations, such as surge protection. An important application of the liquid ring rotary compressor is in vacuum service. The suction pressure is often controlled by bleeding gas into the suction on pressure control. The compressor continues to load and unload automatically as a function of plant demand. The characteristic curves of the axial compressor are steeper, which makes it better for constant-flow services.