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Air Consumption

  1. Feb 8, 2012 #1
    I want to work out the dynamics of a compressed air system consisting of a filter/regulator unit some spool valves and a rotary actuator with all the associated fittings.

    Essentially I have a 2" nominal bore compressed air line at roughly 7.5 bar, I have a system which feeds off this air main on 12mm nylon tubing to a filter/regulator unit which regulates the air to around 5 bar. This feeds a valve manifold of roughly 8 3/2 spool valves each feeding a spring return rotary actuator on 6mm nylon.

    So I can calculate the total volume of each actuator and associated tubing to be xm3 Each valve has to be actuated in 0.5 seconds i.e. from 1bar (air dumped) xm3 to 5 bar xm3 in 0.5 seconds.

    So by applying Boyle's Law:




    So I need to shift 5x in 0.5 seconds so my flow rate when the valve opens is (5x)/2m3/s is this correct?

    Now here's the thing I haven't got to grips with (I'm electrically biased by trade) In electrical engineering we use ohm's law and from what I am told voltage is analogous to pressure, current is analogous to flow rate and resistance is presumably analogous to air resistance in the components that make up the system and the actuator.

    In an electrical circuit I can calculate the volt drop across a resistor quite easily and I assume the same can be done with pressure drop yet I can't find any decent texts that explain this. I would like to know how to work out the pressure drop of a system and how it affects a compressed air ring main. For example when I open my spool valve it surely must have some effect on the pressure of the system?

    Finally, what does Cv actually tell you? To calculate it you need flow rate and pressure drop etc. but what actually is it?

    If anybody could recommend some decent text to bring me up to speed it would be much appreciated

  2. jcsd
  3. Feb 9, 2012 #2
    Basically what I'm saying is if I know the required flow rate how do I calculate the optimum pipe diameter capable of carrying this flow, increasing the pipe diameter increases the volume so required flow rate will increase but decreasing the pipe diameter must restrict the flow. I'm pretty sure Bernoulli's principle will not apply as the air is compressible but this needs clarification.
  4. Feb 9, 2012 #3
    Your problem may be practically solved most simply by downloading software from Festo or another manufacturer of pneumatic components.
  5. Feb 10, 2012 #4
    But that is too easy!
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