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Relationship Between PSI & CFM

Air Compressor PSI and CFM

In addition to HP/kW, air compressors are also rated based on their air flow (CFM) as well as pressure (PSI). CFM and PSI is extremely important in any manufacturing environment, so understanding how adjusting the PSI setting on your air compressor will affect your CFM is vital.

Rule of Thumb: As Pressure Increases, Flow (CFM) Decreases

Why? Two reasons.

(1) When air is pressurized, you are taking a given volume of air at atmospheric pressure, and compressing it into a smaller space. The higher the pressure, the more air that is required. Since there is only so much space between the rotors, only so much air can be drawn in per minute to be pressurized. And you can’t solve this issue by increasing the speed of the rotor due to …

(2) In order to reach a higher pressure at a given horsepower and amp draw, the air-end will need to spin slower, thus giving mechanical advantage to the drive end or motor to reach the higher desired pressure.  The slower speed means lower volumetric output as rpm is directly proportional to cfm or volume/time in a specific air-end.

The Numbers

A common mistake we see is that a facility does not have enough air, so to compensate they increase the pressure setting on their air compressor thinking it will raise it’s production. However, they would be taking a step backwards here. Let’s look at an Engineering Data Sheet to put the above reasoning into action.

Above we have an L160, with a max working pressure of 190PSI. If this air compressor was running at 75 PSI, the output would be 865.9 CFM. However, if the pressure were to be increased to 190PSI, the output would decrease down to 819.5 CFM. This is exactly because of the principles discussed above.

It is also worth noting that increasing the pressure to 190PSI also resulted in an increase in energy consumption, from 127.69 kW to 197.45 kW. On average, every 2PSI increase will result in energy cost increase of 1%.

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