Understanding compressed air CFM, PSI, Force & Flow

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.

About these ads

19 Responses to Understanding compressed air CFM, PSI, Force & Flow

  1. Vasim says:

    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
    ____________________

    Vassim, all the information you ask for is available on my compressed air information website http://www.about-air-compressors.com.

    Regards,

    Bill

  2. WILL HAYNES says:

    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?
    ________________________

    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.

  3. AKASH SHARMA says:

    MY COMPRESSORS OUTPUTS 82 CFM BUT MY EQUIPMENT REQUIRED
    78 CFM NOW EQUIPMENT NOT PROPRLY RUN PIPELINE 1/4 WHY.

    • Bill says:

      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.

  4. Ken says:

    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

    • Bill says:

      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.

  5. Ken says:

    No problem not knowing. I prefer honesty over a BS answer any day! Just posted it on your site and keeping my fingers crossed =).

  6. Harsha Vardhan Sripathi says:

    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!

    • Bill says:

      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.

  7. NARESH says:

    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

  8. erne says:

    ok i need help i just bought a compressor it cycles
    what does that mean the cfm is 28 the air dryer they sold me is 24
    cfm my machine needs 10 cfm at 100 lbs all day long will
    the 24 cfm work with the 28 cfm compressor or will there be less
    air to the mach. i notice the air pipe i put up is 3/4 i d and the air drier is 5/8 why did they sell me this Smaller unit will it be a problem
    or will it work and what will the drawbacks be.

    • Bill says:

      A bit hard to understand … what does what mean, exactly?

      If the compressor is able to output 28 CFM and your dryer is able to handle 24 CFM, and you are using both at capacity, then your dryer will be overtaxed, meaning your air exiting the dryer will not be as dry as if the flow through the dryer was less than its capacity.

      If your machine needs the 10 CFM continuously at 100 PSI, and your compressor is able to output the 28 CFM at 90 PSI all day long, then in terms of air supply, you should be OK. The compressor should be fairly robust, and best, if it had 100% duty cycle as well.

      In my opinion a 5/8″ pipe will handle the flow of 10 CFM quite easily. What is the discharge pipe size of the compressor?

      Wander over to about-air-compressors.com for lots of information about, and about using, air compressors, and all of the questions you have above can be answered by the information at that site.

      • erne says:

        looks like its 1/2 inch outlet its 7.5 horse power
        new i will be useing all 10 cfm all day at 100psi
        and then some will the unit get over taxed. and heat up
        should bigger drier its a D41 ingersal drier.now. still not done building the loop
        yet we are going to have a loop all the way around
        the building using two 80 gallon tanks at both ends of the loop
        as to not have pressure drop at either end of the loop.

      • Bill says:

        OK, if 1/2 a the compressor, then moving up to 3/4 and down to 5/8 shouldn’t create any flow problems.

        What compressor and what’s it’s duty cycle? You need to know this

        As to the drier, an over-flow through the drier means that the outgoing air will have more water vapor in it than if the flow was less than the drier capacity. The air will still be drier, just not as dry. if you are using compressed air filters at each point of use, that will take out any free water condensate that will occur in the lines. How dry does your air have to be? Knowing that will help you decide if the drier is big enough.

        Good having the loop and the tanks at both end. Make sure you install auto drains on the tanks to ensure that they are drained regularly, and that you don’t need a maintenance person to be doing that. If you install the main air ring with an incline, which is recommended, then water condensing in the lines will flow to a spot, or two, where you would have a drop leg with an auto drain installed at the bottom, to help void those lines.

        Except for the drain drop legs, take your drops to the point of use off the top of the main, not the bottom, so free water in the mains will flow around to the drops, and not flow down to your point of use.

      • erne says:

        getting a bran new tank delivered Thursday
        the line will be at the top or ceiling line,and the tanks will
        be at the bottom with down tubes to feed
        the blow hoses and spindle air lines are mid height
        hoping that the drier can keep up with the compressor
        on the cfm, still just not shur as to i need complete dry
        air should I get a bigger dyer say 29 cfm is the next one
        up from the 24cfm i have now. this thing is going to be running
        at 100% allday every day till the cows come home LOL
        will the Compressor overdrive the dryer and produce
        water in the system with this smaller dryer, maybe i should trade
        up while its still new what do you think Bill.

      • Bill says:

        Erne, if your dryer is too small for the compressor, it will not be as effective in drying the air. You need to decide how dry your air needs to be, and then add the drying capability necessary to get the air that dry.

        Even with a dryer, if the ambient temperature in the plant is below the temperature of the air coming along the main, then you will still get some condensation of water, hence the need for a filter (water and debris remover) at each point of use.

        Whoever is supplying your compressor and dryer equipment to you either did not understand your requirements, or made a mistake. Go back to them and ask them to size the system properly, based on your equipment compressed air needs including flow, pressure and dryness. That’s my advice.

      • erne says:

        Hi, Bill See Now your Talking, Im going to do just that Because it is
        very important that i have completely dry air. at 100 duty cycle
        i have a room just for this set up . thats 55 to 65 degrees all the time clean and dry so im hoping this will help and yes i have
        a large glass filter with new element at the sorce and small water
        separators at the lines. Thank you Bill I will get Back to you when im set up
        and running Erne Again thank you for your Addvice.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

%d bloggers like this: