Understand Compressor Control Methods

Plant professionals know that compressed-air demand rarely matches the supply from the compressor system.

That’s where compressor controls come in. They’re used to maximize system performance, i.e., keep generation costs down and efficiency as high as possible, taking system dynamics and storage capabilities into account

Brian Bergmann of EXAIR Corp., Cincinnati (exair.com), explained several control methods in a recent post on the company’s blog. He also noted specific types of systems for which some of these methods are best suited.

Control Functions

The primary functions of compressor controls are to match supply to demand, save energy, and protect the compressor from overheating, over-pressure situations, and excessive amperage draw. Other functions include safety, i.e., protecting personnel, equipment, and processes, and providing diagnostic information related to maintenance and operation warnings.

START/STOP

This is the most basic control method for turning a compressor motor on and off in response to a pressure signal (appropriate for reciprocating- and rotary-type compressors).

LOAD/UNLOAD

This control method keeps the motor turning continuously, but unloads the compressor when a pre-determined pressure level is achieved. When the pressure drops to a set level, the compressor reloads (appropriate for reciprocating-, rotary-screw, and centrifugal-type units).

MODULATING

This compressor-control method restricts air entering the compressor to reduce output to a specified minimum, at which point the compressor is unloaded (appropriate for lubricant-injected rotary-screw, and centrifugal-type units).

DUAL/AUTO DUAL

The dual-control method provides the ability to select between start/stop and load/unload modes. Automatic (auto) dual control goes a step further by adding an over-run timer feature. When activated, the compressor motor is stopped after a certain period of time without a demand.

VARIABLE DISPLACEMENT (Slide, Spiral, or Turn Valves)

This version of variable-displacement control allows gradual reduction of the compressor displacement while keeping the inlet pressure constant (appropriate for rotary-screw-type units).

VARIABLE DISPLACEMENT (Step-Control or Poppet Valves)

This version of variable-displacement control has a similar effect as that for slide, spiral, or turn valves, but instead of a gradual reduction, the change is step-like (appropriate for lubricant-injected-rotary-type units).

VARIABLE SPEED

This compressor-control method works by way of a variable-frequency AC drive or a switched-reluctance DC drive to vary the speed of the motor turning the compressor. The speed at which the motor turns affects the output of the system. Variable-speed technology can be applied to most types of compressors. EP

Brian Bergmann is an application engineer with EXAIR Corp., Cincinnati. Learn more about compressed-air efficiency at exair.com.