# Basic compressed air auditing techniques

May 11, 2014

Use data collected by your compressor to verify its efficiency.

You don't have to break the bank to get a head start on auditing your compressed air system for efficiency.  For some systems you can get a good idea of how well your system is running by using hour meter data that is automatically collected by the air compressor controls.
A typical unloaded rotary screw compressor consumes between 20% and 40% of its full load power while running unloaded, even though it is producing no air. For the conditions just described about 45% of the total compressor energy consumption would be consumed in the unloaded condition.  Applying common efficiency measures, such as auto shutoff, start/stop, or VSD compressor control can correct this situation by reducing or eliminating unloaded run time.
But simply taking the ratio of the hours may not give you current conditions.  If you take the readings again  at the same time of day one week later, and subtract the previous numbers, you will end up with the average weekly duty cycle. If you multiply the ratio you calculate by the compressor rated cfm you will come out with a rough average flow from that compressor for the week. And if you know the full load and unloaded kW you can even approximate a specific power number, like a compressor gas mileage reading.  Compare this to the compressor's CAGI specification and you can see how far away you are from the ideal.
For example, if the compressor calculated above consumed 88 kW fully loaded and 31 kW unloaded at 30% duty cycle the average power consumed loaded would be 88 kW x .3 =  26.4 kW .  Unloaded power would consume 31 kW x .7 = 21.7 kW.  Total theroetical power would then be 26.4 + 21.7 = 48.1 kW.  If this compressor is rated at 440 cfm its average output would be 440 x .3 = 132 cfm.  Calculating the specific power would yield 48.1 / 132  x 100 = 36.4 kW per 100 cfm.  But this compressor at full load is rated at 88kW / 440 cfm x 100 = 20 kW/100 cfm.  The numbers suggest the compressor is consuming 80% more power than rated, an indication of inefficiency.  Of course if this compressor is a lubricated screw compressor installed with small storage capacity the real numbers might be significantly worse.
If you have multiple compressors this calculation can be repeated for each compressor and combined in to a complete system number.  These numbers can be tracked week by week by your maintenance staff.  Accuracy can vary, but this is a quick and dirty way to keep track of your basic system efficiency with simple tools like pencil, paper and a calculator.

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