When it comes to applications for compressed air, a user must consider both CFM and PSI when they are determining if they have sufficient force their application. In every application for the use of compressed air, that air has to be able to provide a certain force at a certain flow rate to do the work expected.
When it’s a simple use, such as a blow gun in a blow off application, if the air didn’t have sufficient force, it wouldn’t blow anything away. In a spray painting application, if the compressed air lacks sufficient force, the paint won’t spray. Similarly in an air actuator. The air has to deliver force to move the piston.
Force is calculated as Pressure Times Area. In all three examples above, there is an “area” involved. In the blow gun and spray application, it’s the area of the nozzle or the surface area of the paint in the spray can. In an actuator, the “area” is the surface area of the piston.
The pressure part of F=PxA (force equals pressure times area) is measured in PSI. PSI is an acronym for Pounds per Square Inch.
The discharge port on your compressor should have an air regulator on which you can select the pressure level reaching your downstream application. That pressure setting is independent of the pressure that’s actually inside the compressor tank, unless the regulator is set higher than the pressure in the tank. In this case, the downstream pressure will match the tank pressure.
If you set the compressor regulator to 5 PSI, then air will reach your application with 5 PSI of pressure, and if the area of your application (eg: the surface area of the paint in the spray reservoir) was, for example, 10 square inches in size, then that 5 PSI would generate 50 LBS. of force on the surface of the paint.
The specifications for the air using device (air tool, cylinder, spray gun etc.) will tell you what pressure that the device requires, and it should also tell you the flow that that device will require to operate properly.
That’s where CFM comes in.
CFM is an acrynym for Cubic Feet per Minute.
Some folks measure flow from the air compressor in SCFM, but in my opinion, that’s incorrect. SCFM refers to “Standard” Cubic Feet per Minute of air, and a “Standard” Cubic Foot of air is at 68 deg. F, at sea level, with a specific humidity level, circumstances far removed from the condition of the air discharging from your compressor. For measuring the pressure coming out of the compressor, I use CFM.
You can have compressed air delivered to your application from your compressor at the correct PSI level, and the device may not work properly.
In order for blow guns, spray guns, air cylinders etc. to work satisfactorily, the compressed air has to be delivered to these devices at the correct PSI (that to generate the force required) and also at the correct flow rate ( so that your force is delivered within the acceptable time frame of the air using device).
If the air brush requires 4 PSI of air to work properly, and you supply that air through a pin hole sized tube, then the air brush is getting the correct pressure, but not enough flow to make the paint spray properly. Similarly, 30 PSI delivered to your car’s tire through that same pin hole tube will eventually fill the tire, but it will take unacceptably long time to do so.
So, when you are looking at supplying compressed air for your application remember that you have to consider the compressed air flow (CFM) and that you must have that flow at the specified pressure (PSI) to be sure to generate enough force for your device to work properly.
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