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Controls for Air Compressors

Air compressor controls are designed to match the delivery of compress 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. There are six basic types of air compressor controls:

 

Start/Stop

This control is the simplest one available. This turns the motor driving the air compressor on or off in response to a pressure signal (for reciprocating and rotary screw compressors). Typically, a simple pressure switch provides the motor start/stop signal. This type of control should not be used in an application that has frequent cycling because repeated starts will cause the motor to overheat and other compressor components to require more frequent maintenance.

 

Load/Unload

This control allows the motor to run continuously, but unloads the air compressor once 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 (for reciprocating, centrifugal and rotary screw compressors). Most compressor manufacturers use different strategies for unloading a compressor, but in most cases, an unloaded rotary screw compressor will consume 15-33% of full-load horsepower while delivering useless work. As a result, some load/unload control schemes can be efficient.

 

Modulating

This control restricts inlet air to the compressor to progressively reduce the output to a specified minimum, at which point the compressor is unloaded. This is also known as throttling or capacity control (for centrifugal and rotary screw compressors).

 

Dual/Auto Dual

For small reciprocating compressors, this control 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

Some compressors are designed to operate in two or more partially-loaded conditions. With such a control scheme, output pressure can be closely controlled without requiring the compressor to start/stop or load/unload. This control allows progressive reduction of the air compressor displacement without reducing the inlet pressure (for reciprocating – multi-step –  and rotary screw – turn, spiral or poppet valves – compressors).

 

Variable Speed

Variable speed is accepted as an efficient means of rotary compressor capacity control. This control adjusts the air compressor capacity by varying the speed of the electric motor driving the compressor in response to system signals.

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