Understanding Air Compressor CFM, PSI and Pressure Ratings
Reading the data plate
An air compressor data plate carries several numbers, and they are not interchangeable. The two that decide whether a machine suits your job are delivered volume and working pressure. Everything else on the plate supports them.
Definitions you need
- CFM (cubic feet per minute) and m3/min (cubic metres per minute) both measure volume flow. 1 m3/min equals roughly 35.3 cfm.
- Free air delivery (FAD) is the volume of air actually delivered at the outlet, measured to ISO 1217 at a stated pressure. It is the only figure you can compare between machines.
- Displacement is the swept volume of the compressor element. It is always higher than FAD because of leakage, re-expansion and internal losses, and it is not a delivered-air figure.
- PSI (pounds per square inch) and bar are pressure units. 1 bar equals 14.5 psi, so 7 bar is about 102 psi and 10 bar about 145 psi.
- Working pressure is the pressure at which the machine is rated to deliver its quoted FAD continuously.
- Maximum or rated pressure is the highest pressure the machine may be set to. It is a limit, not a duty point.
Why cfm falls as pressure rises
A rotary screw element moves a fixed swept volume per revolution. As working pressure rises, more air leaks back past the rotors and more is lost to re-expansion, so delivered volume drops. The same machine might deliver 5.0 m3/min at 7 bar but only 4.2 m3/min at 10 bar. A single cfm figure without a pressure is therefore meaningless. Always ask for FAD at the pressure you will actually run.
Power: nameplate versus package input
Motor nameplate power is the motor's rated shaft output. Package input power is what the complete unit draws at the terminals, including motor efficiency, fan, controls and transmission losses. Package input is what you pay for on the electricity bill, and it is typically 10 to 20 percent above motor nameplate power. Compare specific power, kW per m3/min, when judging efficiency.
Worked sizing example
A small workshop runs a 1.0 m3/min impact wrench, a 0.6 m3/min spray gun and a 0.4 m3/min grinder, but rarely all at once. Total simultaneous demand is about 2.0 m3/min at 7 bar. Adding a 20 to 30 percent margin for leakage and future tools gives roughly 2.5 m3/min at 7 bar. Specify a machine with FAD of at least 2.5 m3/min at 7 bar, not a 3.0 m3/min figure quoted at 5 bar.
Common comparison mistakes
- Comparing cfm figures quoted at different pressures. This is the most frequent error and it can make a weaker machine look stronger.
- Treating displacement as delivered volume.
- Using maximum pressure as the duty point when quoting output.
- Comparing motor nameplate power instead of package input power.
Send your tool list and required pressure to Kaishan engineers, and we will match a compressor to your real demand.
Frequently Asked Questions
What does CFM mean on an air compressor?
CFM stands for cubic feet per minute and measures the volume of air a compressor delivers. On a data plate it should be stated as free air delivery (FAD) at a specific working pressure, measured to ISO 1217. A cfm figure without a pressure is not comparable. In metric markets the same value is given in m3/min, where 1 m3/min equals about 35.3 cfm.
What is the difference between working pressure and maximum pressure?
Working pressure is the pressure at which the compressor is rated to deliver its quoted free air delivery continuously. Maximum or rated pressure is the highest pressure the machine can be set to, and it is a limit rather than a duty point. Delivered volume falls as pressure rises, so a machine quoted at 7 bar will give less air if you run it at 10 bar.
How do I convert psi to bar?
One bar equals 14.5 psi, so divide psi by 14.5 to get bar, or multiply bar by 14.5 to get psi. Common values: 7 bar is about 102 psi, 8 bar about 116 psi, 10 bar about 145 psi and 12 bar about 174 psi. Most industrial compressed air systems in Europe and Asia are specified in bar, while North American equipment often uses psi.
Why is the cfm lower at higher pressure?
A rotary screw element displaces a fixed swept volume per revolution. As working pressure increases, more air leaks back past the rotors and more is lost to re-expansion, so the delivered volume drops. A machine might deliver 5.0 m3/min at 7 bar but only 4.2 m3/min at 10 bar. This is why you must compare cfm figures at the same pressure.
What size air compressor do I need for a small workshop?
Add the air demand of the tools you expect to run at the same time, then add 20 to 30 percent for leakage and future tools. For example, a 1.0 m3/min impact wrench, 0.6 m3/min spray gun and 0.4 m3/min grinder give about 2.0 m3/min, so specify at least 2.5 m3/min FAD at your working pressure, typically 7 bar.