Lesson 2: CFM vs PSI vs HP

A customer calls and says, “I need a 10 horse compressor.” That is not enough to quote. Horsepower is the size of the motor. It does not tell you if the machine will run their tools. You need all three numbers, and you need to know what each one means.

PSI: how hard the air pushes

PSI is pressure, pounds per square inch.

• psig is gauge pressure. A gauge reads zero at sea level, open to the air. This is what customers mean when they say “we run 100 psi.”
• psia is absolute pressure. It adds atmosphere, about 14.7 psi at sea level. Use psia in any formula that involves compressing air.

Pressure decides force. A cylinder pushes harder at 90 psig than at 60 psig.

CFM: how much air flows

CFM is cubic feet per minute. It is volume, not force. Flow decides how many tools run at once and how fast a cylinder cycles.

The catch is that air squeezes. One cubic foot at 100 psig holds far more air than one cubic foot in the room. So you will see:

• SCFM: flow corrected to a set of “standard” conditions. Different makers use different standard conditions, so ask which one.
• ACFM: actual flow at the real inlet conditions.

CAGI performance data sheets report a compressor’s rated capacity in acfm at its full-load operating pressure, tested to ISO 1217. That makes them a fair way to compare two machines.

HP: the motor

HP is the motor rating. One horsepower equals 0.746 kW. Horsepower is what the customer pays to run (Lesson 1). It is not the output.

Rule of thumb: an industrial rotary screw near 100 psig delivers roughly 4 to 5 cfm per horsepower. Small piston units often deliver less. Always confirm on the data sheet.

How the three connect

For a given motor, higher pressure means less flow. The same compressor rated at 125 psig delivers fewer cfm than its 100 psig version. Squeezing air harder takes more of the motor’s work per cubic foot.

The best single number for comparing efficiency is specific power, shown on CAGI data sheets in kW per 100 cfm. Lower is better. Two 25 hp compressors can differ in specific power, and the difference shows up on the power bill every month.

Shop example

Customer wants “a 10 horse.” Ask:

1. What tools or machines run off it? (Get cfm for each.)
2. What pressure do they need at the tool?
3. How many run at the same time?

Now you are sizing to the work, not to a motor rating. Lesson 6 covers the math.

Sources

1. Compressed Air and Gas Institute (CAGI), Performance Verification Program data sheets for rotary compressors (rated capacity in acfm at full-load operating pressure; specific package input power in kW/100 cfm).
2. ISO 1217, Displacement compressors: Acceptance tests (test basis referenced by CAGI data sheets).
3. DOE/Compressed Air Challenge, Improving Compressed Air System Performance, 3rd ed. (pressure, flow, and horsepower relationships).
4. 1 hp = 0.746 kW and 14.7 psia sea-level atmosphere are standard engineering constants.
5. The 4 to 5 cfm per hp figure is a widely published industry rule of thumb, not a rating. Use the CAGI sheet for the real number.