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

Pressure, diameter and flow in a pneumatic system

  1. Mar 8, 2014 #1
    Hi everyone,

    I have a problem (which is likely to be fairly simple), which I need a hand to resolve. I'm trying to determine the impact of changing the internal diameter of the hose/fitting system in a compressed air system (in this case a portable compressor, with an inbuilt air receiver).

    The starting information is as follows...
    Compressed air tank outlet diameter: 3/8" ID (9.525mm)
    Compressed air tank pressure: 160 psi (1,103.16 kPa)
    Compressed air tank size: 1.94231 cubic feet (55L)
    Piping and fitting size: 1/4" ID (6.35mm)
    Hose length: 20m

    The system is fitted with a 1/4" ID pressure regulator, straight after the outlet from the air reciever, and this regulator is set to 90 PSI.

    The question is, since the receiver has a 3/8 outlet but is being constricted by the air hose and fittings 1/4 ID, how much extra air flow could I expect to gain by moving to a 3/8 ID hose and fitting system?

    Somewhat more specifically, I'm looking at this from the point of the peak torque output from an impact wrench. I'm not certain what factors influence the torque output of these tools, but most of them list a peak air requirement (in L/min or cubic feet per minute) which I assume would correlate with the published peak torque (up to the devices engineered safety limits etc). Most have a maximum operating pressure of 90 PSI, hence the regulator setting.

    All other things being equal (and assuming i don't exceed any engineered limits on the pneumatic tool), I would anticipate better performance from the tool if the air hose and fitting system all the way from the receiver outlet was of a larger ID (3/8 vs 1/4), the real question is, how much better...?
  2. jcsd
  3. Mar 12, 2014 #2
    So, should I take it from the lack of response that it's not possible to calculate what I'm seeking with the information provided, or that it is unclear what I'm asking?
  4. Mar 14, 2014 #3
    To calculate the pressure drop vs flow you need the friction factor for the hose. This is something I don't know. Have you looked at the compressor / hose / tool manufacturer's websites? They should have charts or tables or at least "thumb rules" for system design. These might be in the form of recommended hose diameter vs hose length. I think a 20 meter hose is on the long side, if you really need hose that long it would be better to use a larger diameter. Another option is to use hard pipe for most of the length, and put the regulator and hose connection closer to the work. This allows higher pressure closer to the work and shorter hose.
  5. Mar 14, 2014 #4
    Hi, thanks for the advice. Unfortunately this system is a portable one so fixed tubing isn't an option, otherwise I would be doing it :)

    I'm going to get a small pressure gauge that I can fit inline at the end of the current system so I can measure pressure drop across the current setup. If it seems to be a significant drop, then I'll just suck up the cost and upgrade the whole show to 3/8" or maybe even 1/2" all the way through. Then I know that the air delivery system is as good as it can be (within practical limits for portability etc) and if more air is needed, I can just swap the whole show on to a larger compressor.

    I'll do a rudimentary calculation based on this data as well:

    Last edited: Mar 14, 2014
  6. Mar 14, 2014 #5
    I just googled
    Air tool hose size
    Got lots of hits. I'd say some of the advise is good and some is misguided. But if you read thru you can make up your own mind. Me, I'd get some 3/8 hose. You don't necessarily have to change all the fittings since a lot of the losses are in the hose.
  7. Mar 15, 2014 #6
  8. Mar 15, 2014 #7
    Similar question, no real answers, more focused on the air tool than the flow/pressure, despite what the OP asked. I'm going to stick with the obvious theory that larger diameter equals more goodness. How much extra goodness I still haven't determined; but definitely extra goodness.

    I'll also cut the existing 20m hose in half and add another quick connect, at least that way I'm only incurring the extra pressure drop when it's necessary with that hose.

    This link has some handy tables relating to hose diameters, flow and pressure drop:

    Looks like 1/2 ID would always work fine for my purposes. I'll save some pennies and then go for that in the longer run.
    Last edited: Mar 15, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook