Compressors may be driven by electric motors, steam turbines, or gas turbines. Aboard ship, most low-, medium-,and high-pressure air compressors that supply the ships service air are driven by electric motors.

The driving unit may be coupled to the com-pressor by one of several methods. When the com-pressor and the driving unit are mounted on the

Figure 14-1.-A simple two-stage reciprocating low-pressure air compressor (vertical V configuration).

same shaft, they are close-coupled. This method is usually restricted to small-capacity air com-pressors driven by electric motors. When the speed of the compressor and the speed of the driving unit are the same, flexible couplings are used to mount the driving unit to the compressor. This is called a direct-coupled drive.

V-belt drives are commonly used with small, low-pressure, motor-driven compressors and with some medium-pressure and high-pressure com-pressors. In a few installations, a rigid coupling is used between the compressor and the electric motor of a motor-driven compressor. In a steam-turbine drive, the compressors are usually (but not always) driven through reduction gears. Cen-trifugal high-speed compressors are usually driven through speed increasing gears.