Drivers

A.   Efficiencies:  85-95% for motors, 40-75% for steam turbines, 28-38% for gas engines and turbines.
B.  Electric motors are nearly always used for under 100 HP (75 kW).  They are available up to 20,000 HP (14,915 kW).
C.  Induction motors are most popular.  Synchronous motors have speeds as low as 150 rpm at ratings above 50 HP (37.3 kW) only.  Synchronous motors are good for low speed reciprocating compressors.
D.  Steam turbines are seldom used below 100 HP (75 kW).  Their speeds can be controlled and they make good spares for motors in case of a power failure.
E.  Gas expanders may be justified for recovering several hundred horsepower.  At lower recoveries, pressure let down will most likely be through a throttling valve.