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Joule-Thomson Coeff. For Air at High dP

  1. Aug 28, 2009 #1
    I am studying compressed air flow through a valve at a high pressure differential (3000psig to atmosphere). The air will be saturated at 3000 psig and in other circumstances it will be dried to a dew point of -10 deg F.

    My purpose is to calculate the temperature of the air stream at the valve exit and will assume the flow is isenthalpic. The simple equation I would prefer to start with is :
    µ = (dT/dP) at constant enthalpy

    However, I have not been able to find tabulations of µ for air at at various high and low pressure. Can anyone refer an on-line source for this information?

    I suppose an alternative method would be to use enthalpy tables, but can't find enthalpy tables for air at the high pressure condition either.

    Suggestions and alternate ideas for solution are gladly received.
  2. jcsd
  3. Aug 28, 2009 #2


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    Hi Tom,

    Welcome to PF.

    Have a look here:


    Hope this helps.

  4. Aug 28, 2009 #3
    Thankyou for the quick reply Stewart.
    Unfortunately for me, the "species" pulldown list does not include air.
    Still searching the net though.
  5. Aug 28, 2009 #4


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    Sorry I thought they would surely have air listed! They do on their desktop version of that program (REFPROP 8.0).



    What range of pressure/temp are you looking for?
  6. Aug 28, 2009 #5


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    I ran a range for you using their program. See the attached spreadsheet with the table. It has the J-T coefficient and the Enthalpy.

    Hope it helps.


    Attached Files:

  7. Aug 28, 2009 #6
    Hey Stewart!!
    Thats great and helps me with a good start.
    I just looked at the cost of the REFPROP program, wow! expensive.

    Does REFPROP have a feature to adjust moisture content of air? from saturated conditions to % saturated before it reports the properties? I see you made the run based on -10 deg F, but the -10 deg F was not intended as designating actual temperature, it is a way to express the relative dryness of the air (i.e., that is an alternate way of expressing "grains of moisture" or "% of saturation").

    I would guess that the JT coeff will vary with % of saturation.
  8. Sep 1, 2009 #7


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    No, it does not have a feature to adjust the moisture content of the air.

    I'm not entirely certain of how (if at all) the quality of the air affects the J-T coefficient.

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