Compressor flow rate must be regulated to match the system demand. Usually the discharge pressure is the controlled variable. The type of control depends upon the characteristics of the compressor, the prime mover and the system. The control may be manual or automatic; it may be discontinuous (on-off) or continuous through speed variation of the motor and/or inlet throttling. Unloaded start of the compressor must be provided for if the torque of the prime mover is not sufficient to turn the compressor under load.
Smaller stationary compressors may be run continuously with only a blow-off (pressure relief valve) on the delivery side which opens when a specified maximum pressure is reached to allow compressed air to be blown off to atmosphere instead of being delivered to the system (Figure 1). This is a wasteful way of operation, since the prime mover is always working at its full power even though there may be no demand e,’~m the system. If a receiver is included, a check valve may be used to prevent high pressure being blown back into the compressor (Figure 2). This is always necessary with valveless compressors (eg vane or screw compressors) which might otherwise act as motors, driven backwards by the air in the system, but may not be necessary in piston or diaphragm types which have their own delivery valves. A check valve may still be used with the latter as a backflow protective device. A safety valve should also be incorporated in the system to protect against over-pressure. In this case, the safety valve would be set rather higher than the system operating pressure (about + 10%).
For continuous duty it may be desirable to maintain a higher pressure in the receiver than is required in the supply line. In this case, the receiver is followed by a pressure regulator (Figure 3).
An alternative method of pressure regulation for constant speed motor driven compressors is by on-off cycling using an electrical pressure switch on the receiver to cut the motor supply at a predetermined ‘high’ pressure level, and back on again at a predetermined ‘low’ level (Figure 4). The receiver pressure is held between these pressures provided that the demand does not exceed the recharging capacity of the compressor.