Types of Air Compressor Controls

Air Compressor controls are designed to match air compressor delivery with compressed air demand by maintaining the compressor discharge pressure within a specified range. This discharge pressure should be set as low as possible to minimize energy use. Control strategies need to be developed using a systems approach, taking into account system dynamics and storage. There are six basic types of individual compressor controls:

• Start/Stop: Turns the motor driving the air compressor on or off in response to a pressure signal (reciprocating and rotary screw compressors).

• Load/Unload: Allows the motor to run continuously, but unloads the air compressor when a predetermined pressure is reached. The air compressor reloads at a predetermined lower discharge pressure. This type of air compressor control is also known as constant speed or constant run control (reciprocating, centrifugal, and rotary screw compressors).

• Modulating: Restricts inlet air to the compressor to progressively reduce air compressor output to a specified minimum, at which point the compressor is unloaded. Also known as throttling or capacity control (centrifugal and rotary screw compressors).

• Dual/Auto Dual: For small reciprocating compressors, allows the selection of either start/stop or load/unload. Dual/auto dual provides modulation for lubricant-injected rotary screw compressors to a pre-set reduced capacity followed by unloading with the addition of an overrun timer to stop the air compressor after running unloaded for a pre-set time.

• Variable Displacement: Allows progressive reduction of the air compressor displacement without reducing the inlet pressure (Reciprocating (multi-step) and rotary screw compressors (turn, spiral, or poppet valves)).

• Variable Speed: Adjusts the air compressor capacity by varying the speed of the electric motor driving the compressor in response to system signals.