Compressed air energy conversion - Wire to work

A big "Wow" moment for me when I first started in the field of compressed air efficiency was when the real energy conversion efficiency of a system explained to me. Before this compressed air seemed like a great medium to use to power industrial machines and tools.  Taking an unlimited free resource like the air we breathe and using it to make stuff, how could it get any better? I never gave it a second thought.

And then a training seminar focused my attention. Compressed air systems, as the instructor explained, have poor energy transfer "wire to work". He explained that in any system very little of the energy flowing through the electrical wires going into an air compressor actually translates to useful work coming out the shaft of a compressed air powered device at the end of the system.

Here is an example:

A one horsepower air motor drives the shaft of a cooling fan in a paint room.  This motor consumes a compressed air flow of 30 cubic feet per minute.  The air compressor and air dryer feeding the motor consumes 20 kW for every 100 cfm produced.  This means that the air motor consumes an equivalent of 6 kW of electrical power.  One kW is the equivalent of 0.746 kW.  This means it takes 8 hp of air compressor power to produce 1 hp of power output to the cooling fan. This means you are getting back only about 12% of the power you put in, a poor energy conversion rate.

But there is more bad news; the figures used in the example are for a lossless system, where the air compressor is perfectly controlled running at full load, there are no pressure restrictions, and there is no system leakage. The reality of it is that the typical compressed air system wastes about 30% of the compressed air produced before it even gets to the final end use. And imperfect compressor control and pressure differential in system components increases the cost to produce compressed air to levels substantially higher than the 17 kW/100 cfm quoted.

What does this all mean?  If you have a typical system you may be receiving the benefit of less than 5% of the energy you put into your air compressor.  The good news is there is something you can do about it.

For more information about compressed air efficiency see the Compressed Air Challenge website or attend a locally scheduled CAC Fundamentals or Advanced Compressed Air Systems seminars  Our calendar of trainings is here.