Compressed air system gas mileage

Sept. 13, 2014

How efficiently are you producing your compressed air?

One of the secrets to compressed air system optimization mentioned in my last post was to “compress air more efficiently”.  Well, the question is how do you do this and after you implement your efficiency measures how do you know you are achieving this?

First, let’s answer the last question.  Determining how efficiently you are producing compressed air involves taking measurements.  Measuring the power input and the flow out of your compressor room can tell you how well your system is running if you do a bit of simple math, or better yet have a permanent monitoring system do it for you.  The Specific Power of an air compressor, which relates to its efficiency, is simply the kilowatts it consumes divided by the cubic feet per minute of compressed air produced (divided by 100).  For example if an air compressor consumes 125 kW and the measured output of the compressor is 500 cubic feet per minute (cfm) then the specific power is 25 kW per 100 cfm.  I like to call this the compressor “gas mileage”.  I live in Canada and we rate our automobile gas mileage in litres per 100 kilometer so for me this relationship is not a stretch.

So what do we do with this number?  Well, if we look up the compressor Compressed Air and Gas Institute (CAGI) rating and see that it should be running at 17 kW per cfm we know that there may be an efficiency issue.  With Specific Power, like golf, the lower the number the better.

You may not have a flow meter on your system.  If not see the article “Compressed Air Audits on a Budget”.  The article will assist you in finding your approximate specific power if your system uses load/unload compressors by tracking loaded and running hours.

How do you produce compressed air more efficiently?  Well the answer relates to the compressor operating pressure, the characteristics of the compressor itself, and how well it is controlled:

  • Lower the pressure – At reduced discharge compressor an air compressor consumes about 1 percent less power for every 2 psi of pressure reduction (at around 100 psi).  The pressure reduction often causes reduced flow in pressure sensitive air consumers in a plant, which further reduces the compressor power.  A 2 psi reduction might reduce the flow a further 1.5 to 2 percent. In order to reduce the pressure some system components other than the compressor may have to be changed to reduce pressure differentials.
  • Better control the compressor – Inefficient compressor control is the number one reason an air compressor might not be achieving rated Specific Power.  If your compressor is running in modulation mode, or spending a significant period of time running unloaded then this is a sign you have efficiency issues.  Don’t know what modulation is? Then ask, it will be worth your while to do so!
  • Purchase a more efficient compressor – Manufacturers have tweaked the efficiency of air compressors in recent years.  If you need a new air compressor don’t forget to check out its CAGI specific power rating and calculate the potential savings.  Purchasing the lowest price may not be the best decision if the operating costs will be higher.
  • Ensure proper cooling – Hot air in the compressor room will reduce the efficiency of the air compressor and other associated equipment like dryers and drains.  Relocating the compressor intake to draw in outside air, while not restricting the intake flow, can boost the compressor efficiency.
  • Maintain your compressed air system – Clogged filters, separators, coolers and dryers can boost the compressor internal and discharge pressures and cause inefficiency.  Open drains, leaks, drying equipment failures and other waste in the compressor room can steal compressed air flow away from your plant before it ever gets out of the compressor room.
  • Use the heat of compression – If your industrial process needs heat for temperature control, hot water, or in the process itself consider the significant amount of heat the compressor produces as an economical source of energy.

Learn more about produce compressed air more efficiently by attending a Compressed Air Challenge Fundamentals of Compressed Air Systems Seminar.  To find the nearest location visit the CAC website.

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