# Joule-Thomson Coeff. For Air at High dP

1. Aug 28, 2009

### Tom C

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.

Thanks

2. Aug 28, 2009

### stewartcs

Hi Tom,

Welcome to PF.

Have a look here:

http://webbook.nist.gov/chemistry/fluid/

Hope this helps.

CS

3. Aug 28, 2009

### Tom C

Thankyou for the quick reply Stewart.
Unfortunately for me, the "species" pulldown list does not include air.
Still searching the net though.

4. Aug 28, 2009

### stewartcs

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

CS

EDIT:

What range of pressure/temp are you looking for?

5. Aug 28, 2009

### stewartcs

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.

CS

#### Attached Files:

• ###### JT_Air.zip
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6. Aug 28, 2009

### Tom C

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.

7. Sep 1, 2009

### stewartcs

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.

CS