Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Looking for input of other peoples opinions on the relationship of CFM and PSI?

  1. Feb 21, 2008 #1
    Hey. As the title dipicts, is there any relationship between CFM and PSI, linear or non-linear?

    Any mathamatical relationship either?

    CFM (cubic feet per minute) is a volume over time, and PSI (lbs per square inch) is a weight essentially over an area. So I take it, theres no real way to linearize it?

    For instance, whats the compression ratio of a typical 12V compressor (high end)?

    Also, if one compressor can produce 100 psi, and another 300 psi. Does the 300 psi unit have a higher compression ratio? I know the compression is usually pistons that very quickly pump in tinny volumes of air (30mm - 60+mm pistons) so I'm sure the size of the piston's determine how fast and how much max pressure they can compress (I think would be inversly perportional ( bigger pistons faster compression, but lower Max pressure?)would it not)?

    Also, if a compressor and tank combo store lets say at 200PSI. If you tap a valve directly off the Tank, the second you open the valve air comes out at a Descending PSI rate would it not? So all that being said, the larger diameter the valve, the higher the CFM, but the lower the PSI? Or is PSI the same, but the tank just drains faster, and the PSI drops faster proportionatly?

    (sorry for some of my spelling errors, I'm trying to type fast B4 I go to work)

    I'm doing research on compressors and how the sizes of the chambers/ports/wheels all play a part in balancing CFM and PSI, depending on the application. So in order to work with them I would like to understand if there is any relationship between the two or not.
    So from a design perspective I would like to learn how to design compressors for the variet of applications (from low cfm (high psi), to High CFM(high PSI) and the few combinations between.

    I've read a crap load of formulas, and engineering documentation, so now I would like to understand/re cap with what I've learned and hopefully develop upon that.

    The designing is for different methods of forced induction on automobiles. So even if you have automotive experiance I would appreciate If you chimmed in. thanks in advance.
    Last edited: Feb 21, 2008
  2. jcsd
  3. Feb 21, 2008 #2


    User Avatar
    Science Advisor

    There is no unit conversion between the two; however, there is a relationship between Volume and Pressure.

    Depending on the process by which you went from the initial state to the final state of the gas, you can use the Gas Laws.

    If the tank has a fixed amount of air in it, and the compressor doesn't make up any that is lost, then the pressure will drop as the gas escapes. Generally, depending on the initial pressure, the flow rate will be relatively constant up until the very end of the discharge.

  4. Feb 21, 2008 #3


    User Avatar
    Science Advisor

    As has been mentioned, there is no direct relationship except those in the gas laws.

    What do you mean by linearize it? I am going to assume that you want a relationship between the two. Again, see the gas laws.

    It will be different for every compressor.

    It has a higher, overall pressure ratio.

    In the end, if you look at any compressor data, you will see stated by the manufacturer the design flowrate at a specified pressure.

    There are a lot of styles of compressors other than piston type. You can not infer information about the output based on just the size of piston. In general, the larger the piston, the greater the flow rate. Whether it can generate a higher pressure depends on a lot of other factors including the entire driving engine.

    The outlet condition is dictated by the pressure that the valve opens up to. If it opens to atmosphere then that will be the outlet pressure.

    The tank drains faster.
  5. Feb 21, 2008 #4
    One more Question. If I had a 1 gallon tank with a compressor that can compress at 250 PSI, and released it with a regulator at 10 psi, does that mean it could continue to push out 10PSI for (X) amount of time (the gallon worth) if it wasn't "chargeing/compressing"? So in comparison, if with a regulator I was pushing 50 psi out does that mean it would drain 5 times faster than if I only allowed 10 psi to release? (compare by blowing into open air at atmospheric pressure) In other words, if I regulate the psi down, does that extend the amount of time I could continue running at the selected PSI? ie. 250 psi max, but only letting out 10 psi for a duration?

    Sorry for any incoherance, I'm having trouble putting my questions into words.
  6. Feb 21, 2008 #5


    User Avatar

    Staff: Mentor

    You are correct.
  7. Feb 22, 2008 #6
    Cool. You guys have been a huge help. Ill post my design in a couple weeks with pictures so you guys see what I'm doing. I have all the info I need, I just have to finish putting the setup together, and flow bench it.

    Take it easy, and have an e-beer on me :P
  8. Feb 22, 2008 #7


    User Avatar

    Staff: Mentor

    Just make sure you pay attention to the thermodynamics. When a compressor compresses a gas, it gets hot. This is a potential source of energy loss and how a compressor does the compression and how a regulator does the expansion both make a difference in how much energy you lose.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Looking for input of other peoples opinions on the relationship of CFM and PSI?
  1. Looking for input (Replies: 4)