Compressor power - Actual vs nameplate

June 12, 2014

Exactly how many horsepower do you think a 100 HP should consume?  The answer is not that easy.

One question I like to ask when leading Compressed Air Challenge (CAC) seminars is about the horsepower of air compressors.  If you measure the input to a 100 HP (75 kW) compressor, how many horsepower do you think it should consume?  Strangely enough this is a trick question.

The obvious answer is a 100 HP rated compressor should consume 100 HP, but this is rarely the case.

Part of the issue is how compressors are rated.  A 100 HP compressor might be one that is designed for about 100 brake horsepower input to the shaft of the compression element at full pressure.  This shaft is driven by an electric motor, and that motor has losses, therefore, the actual input to the motor is more than 100 HP. For example, if the motor efficiency is 95% then 105 HP must be applied to the motor to overcome the losses to produce 100 HP of shaft input. 

A second reason for higher than nameplate power is that the compressor has ancillary equipment installed on it that consumes additional power, such as cooling fans and controls. These items add to the total input power required.

But a third reason is a typical compressed air industry practice that significantly boosts the input power. Most manufacturers design the compressors so that the motors are pushed into their service factor at full pressure and full flow, that is, they are intentionally loaded to higher than the motor nameplate rating.  Why is this done?  Well, the way people make purchasing decisions might be to blame.

If a typical buyer is faced with a decision about buying a new 100 HP rated air compressor and had to choose between product A that puts out 360 cfm or product B that produces 400 cfm or product C that makes 440 cfm of compressed air which one do you think they would choose?  Most go with the higher flow.  Therefore, it is the best interest of the manufacturer to design their compressors for higher output numbers if they want to enjoy high sales.

What people don’t usually notice is that driver motor of the unit with the higher output numbers might also be also consuming more power.  If you looked up the specifications by finding the Compressed Air and Gas Institute (CAGI) rating of the compressors you might find that product A would have the best efficiency, meaning it would consume less power for every unit of compressed air output.

Before writing this I did a quick check of CAGI ratings of typical 100 HP compressors.  This data is available on manufacturers’ websites.  The data showed total package kW consumption numbers of 91, 89 and 90 kW for typical 125 psi rated air cooled screw compressors from large US manufacturers.  One horsepower is equivalent to 0.746 kW. A power level of 91 kW is equivalent to 122 HP.

Because the CAGI data sheets are increasingly being used to compare compressors before purchase most manufacturers have been optimizing their compressors enhance their efficiency numbers to attract more sales.  Since motors are most efficient when loaded in the 80% of full load range perhaps designing compressors to hit this motor efficiency sweet spot is the next area that will be explored in improving machine efficiency.

I suggest for your next compressor purchase you use these CAGI sheets to help select a more efficient unit to help bring your power costs down.

For more information about compressed air power consumption see the articles on this Plant Services website like the one about Calculating Compressed Air Efficiency.  More information is also available at the Compressed Air Challenge website or can be gained by attending locally scheduled CAC Fundamentals or Advanced Compressed Air Systems seminars.  Our calendar of trainings is here.

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