Compressed air - Applying an automotive analogy

Oct. 10, 2014

Some compressor control is like putting the pedal-to-the-metal and using the brake to control the speed!

In my last blog I related compressed air system efficiency to “gas mileage”.  The specific power of a system, or how many kilowatts (kW) it is consuming per every 100 cubic feet of flow, is somewhat like the gas mileage rating of an automobile.  There are also other automotive analogies that can be applied to help us understand compressed air systems.

The first one is the way we pay for the electricity for our system.  Most industrial sites pay a certain number of cents per kilowatt hours they consume.  The power meter tracks this energy consumption on a kWh meter.  This meter is somewhat like an automotive odometer, tracking the accumulation of miles.  A compressed air system consumes a certain power (kW) over time (hours) so you could relate the power meter to a car odometer.

But there is usually another component of a power bill, peak demand.  The calculation for this is somewhat like an automotive speedometer.  It relates to how fast you use the energy, or the rate of energy consumption for the system.  The higher your peaks, the more you pay.  Sometimes the operation of an extra air compressor for only 15 minutes in a month will set a monthly peak demand that you have to pay for the whole month, or even the whole year if your utility has a ratchet clause in their billing structure.

Compressor control is another area where an automotive analogy can be used.  There are various ways to change the output of a lubricated screw compressor to match the actual demand.  These are:

  • Inlet modulation
  • Displacement control
  • Load/unload
  • Start/stop
  • Variable speed

Inlet modulation chokes off the inlet of the air compressor to control the flow, this is the worst way to control an air compressor in terms of energy efficiency.  Inlet modulation is like putting the pedal to the metal in your car and using the brake to control the speed.

Displacement control uses special valves to effectively reduce the volume of the compression element in an air compressor, increasing the efficiency at part load compared to modulation.  This control somewhat like those vehicles that select the number of cylinders to fire when highway driving, eight cylinders when under load, only 4 when cruising, this increases the gas mileage.

Load/unload alternately loads and unloads an air compressor to match the load.  If a compressor is only 50% loaded then it is loaded half the time and unloaded the other half.  Load/unload can be equated to stop and go driving, but where the car engine is kept running at full speed when stopped and in neutral.  Imagine running at 120 mph in your car 50% of the time when you wanted an average speed of 60 mph.

Start/stop control is where the compressor motor is turned off when unloaded, reducing the average power consumption.  This control is only suitable for small compressors where the number of starts is limited to under 8 per hour.  This is like stop-and-go driving, where the car motor is turned off when waiting at a light.  This strategy is used effectively for hybrid gas/electric vehicles….and air compressors.

Variable speed control varies the compressor motor speed up and down to match the air demand.  This is like putting your air system on cruise control.  A constant pressure is maintained, with minimal wasteful unloaded run time.  This is typically the most efficient way to control a screw compressor at part loads.

Learn more about compressor control by attending a Compressed Air Challenge Fundamentals of Compressed Air Systems Seminar.  To find the nearest location visit the CAC website. Our calendar of trainings is here.

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