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.
Need more info on using compressed air? Here’s the spot.
Kindly give me the formulas for calculating cfm of all air compressor & related details of air compressor it may be in form of a catlogue or an article
Thanking you
vasim
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Vassim, all the information you ask for is available on my compressed air information website http://www.about-air-compressors.com.
Regards,
Bill
I need to put a air dryer/ filter on my air line to keep the moisture out. The dryer I am presently interested in has 1/4 inch inlet and outlet nipples. My compressor puts out 12 cfm at 90psi with 3/8 inch inlet and outlets and my equipment needs 10 cfm at a minimum to perform properly. My question is: Does the decreased nipple size lower my cfm and if so by how much? Lastly what formula is used to determine this?
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You can learn about flow through air line hole size and pressure drop at http://www.about-air-compressors.com/estimating-pressure-drop.html.
Yes, the decreased size of the filter port from the main line will reduce the amount of air flow, since the smaller ports and filter create an air dam. However, I don’t believe that the flow to your equipment will be negatively affected since the flow capacity of a standard 1/4″ filter (as long as the element is not plugged up with gunk) is higher than the flow you require.
MY COMPRESSORS OUTPUTS 82 CFM BUT MY EQUIPMENT REQUIRED
78 CFM NOW EQUIPMENT NOT PROPRLY RUN PIPELINE 1/4 WHY.
If your equipment needs 78 CFM, and your compressor outputs 82 CFM, then I would surmise that your air line from your compressor to your application is too small to allow an adequate flow of air to reach your equipment. Go to http://www.about-air-compressors.com, look at the sitemap page, and click the link for Pressure Drop. These pages will explain further.
Thanks for posting this great, easy to understand information. I do have 1 question for you. I was basically wondering if CFM increases as PSI does. For example, if I had a blowgun attached to a (fixed diameter) outlet port on a compressor tank, If I squeezed the trigger while tank pressure was @ 100PSI would I get more CFM flowing through the blowgun than when I did that when tank pressure was at 50PSI? If so, would you happen to know if there’s a formula out there that might quantify this? Many thanks.
Ken
Hi Ken…
I am not an engineer, however yes, I do believe the higher the starting pressure over the destination pressure the higher the flow of compressed air from one to the other. A tank with 100 PSI in it will flow faster to an opening to atmospheric pressure (14.7 PSI) versus that same tank flowing to another tank with 90 PSI in it, for example.
Therefore, I do believe that if you are using a blowgun blowing air from a tank at 100 PSI to atmosphere, air will flow faster (with more volume) than that same tank with just 50 PSI left in it.
I am sorry, I don’t know of a formula, though I have no doubt that others know of one. Consider posting your question on my compressed air website http://www.about-air-compressors.com. It “sees” more than 1200 visitors a day, and I have no doubt that someone with the info you seek will see your post.
No problem not knowing. I prefer honesty over a BS answer any day! Just posted it on your site and keeping my fingers crossed =).
Hi Bill
I have CFM at ANR (Normal Atmospheric conditions) from which i need to calculate CFM at 6 Bar, 40 Degrees Centigrade. Can you help me?
Thanks in advance, cheerio!
Harsha, while at first the request seems straightforward, it is not. As air is compressed it’s flow characteristics change. Temperature affects the flow, and the medium through which the air is flowing (pressure drop) also affects the flow. I have never, despite years of searching, been able to find a formula that allows one to plug in different variables to determines how those changes affect the flow, and the pressure. It will require engineering input I would suspect, and I’m just a lowly air tool type guy. Good luck on the search. If you have success, do please let me know. Many people are interested in this subject. I get questions all the time on my ASK page on the air site.
Thanks bill, I’ll, make sure to let you know if i find any correlation.
HI
I NEED cfm for compressed air of 3 bar 3/4 inch pipe diameter at a temperature of 30 deg celcius and 75 % humidity
Naresh, visit http://www.about-air-compressors.com for information about compressed air flow.