Compressor Control – A Way to Match Supply to Demand

Rarely does the compressed air demand match the supply of the compressor system. To keep the generation costs down and the system efficiency as high as possible Compressor Controls are utilized to maximize the system performance, taking into account system dynamics and storage. I will touch on several methods briefly, and leave the reader to delve deeper into any type of interest.

air compressor

  • Start/Stop – Most basic control –  to turn the compressor motor on and off, in response to a pressure signal (for reciprocating and rotary type compressors)
  • Load/Unload – Keeps the motor turning continuously, but unloads the compressor when a pressure level is achieved.  When the pressure drops to a set level, the compressor reloads (for reciprocating, rotary screw, and centrifugal type)
  • Modulating – Restricts the air coming into the compressor, as a way to reduce the compressor output to a specified minimum, at which point the compressor is unloaded (for lubricant-injected rotary screw and centrifugal)
  • Dual/Auto Dual – Dual Control has the ability to select between Start/Stop and Load /Unload control modes.  Automatic Dual Control adds the feature of an over-run timer, so that the motor is stopped after a certain period of time without a demand.
  • Variable Displacement (Slide Valve, Spiral Valve or Turn Valve) – Allows for gradual reduction of the compressor displacement while keeping the inlet pressure constant (for rotary screw)
  • Variable Displacement (Step Control Valves or Poppet Valves) – Similar effect as above, but instead of a gradual reduction, the change is step like (for lubricant injected rotary types)
  • Variable Speed – Use of a variable frequency AC drive or by switched reluctance DC drive to vary the speed of the motor turning the compressor. The speed at which the motor turns effects the output of the system.

In summary – the primary functions of the 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 (protecting the plant and personnel), and provide diagnostic information, related to maintenance and operation warnings.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Air Compressor Throughput Control

Throughput Control

At the end of my last blog, I mentioned the slide valve operation on a screw compressor.  A slide valve is the basis of throughput control for a screw compressor.  Throughput control is a term used to describe the process of controlling the energy input to the compressor in order to reach the control objective (output pressure and/or flow).  No matter the type of compressor, throughput control is achieved by using speed control, suction throttling, discharge throttling, or recycle control.  There are a few other methods of controlling throughput, but these four are the most common, and throughput control is a common practice used to dial in the needs of a compressed air system/application.

The first, speed control, is the most common and most efficient method.  Essentially, the output flow and pressure are regulated by adjusting the speed of the motor driving the compressor unit.  Increasing the speed of the motor driving the compressor will result in an increased output flow at a constant pressure, or an increased output pressure at a constant flow.  Speed control can also be coupled with other control methods to fine tune the throughput of the compressor.

Suction valve throttling is exactly what it sounds like.  The incoming air flow and pressure are restricted by installing a control valve immediately upstream of the compressor inlet, and the valve’s position is controlled as a function of the exhaust discharge pressure and/or flow.  When the valve is activated and the suction is “throttled” or restricted, the output flow will decrease (because there is less air taken in by the compressor), and the output pressure will subsequently increase.

Discharge valve throttling restricts the pressure from the compressor to match the process requirements at a constant flow.  As a result of this setup, the compressor must work harder than the process requires and this control scheme is extremely inefficient.

Recycle control uses a valve to return compressor discharge flow back to the suction port of the compressor.  As many people know, compressing a gas can generate a good amount of heat, and this heat is often transferred into the compressed air.  Because of this, a cooler is usually (and should be) installed in the line between the recycle control and the suction valves.  The recycle valve can modulate from fully open to fully closed, which gives a full range of control over the discharge flow and can help with loading/unloading of the compressor.

These control methods are all fairly straightforward and on their surface aren’t too intimidating.  They remind me of rudimentary PID controllers, which can be dialed in to a tee.   Think of the way an elevator car reaches the intended floor without slamming to a stop or jolting when it starts moving.  That’s achieved though PID control, and similar methodology is applied to compressor load and unload as well as operation.  But if I get under the surface of compressor control and see PID diagrams, I’m getting the professor!

Lee Evans
Application Engineer