Dry Air Calculations for Work of Compression 1

[tc7]

I am sizing up air flasks for a piston accumulator. The purpose of the accumulator is to capture a large volume of liquid (glycol) which is exhausted through an upstream valve in our process. The gas side of the piston will be charged with dry air (I have been told the air quality must be based on a dewpoint of -10 degrees F or lower). The initial charge on the air (and the entire system) will be approximately 400 psig. During the fluid accumulation, the pressure will rise to about 650 psig in a matter of about 3 seconds. I am assuming an adiabatic compression on the airside.

Now to the question – for the purpose of future energy balance calculations, I cannot find any equations which takes air quality (dryness) into account when calculating the work required to compress a volume of air, yet I know it requires more work to compress saturated air vs. dry air. Can anyone advise on this or lead me to a reference for this type of calc? Standard thermo or fluids texts aren't helping me much.

Thanks in advance,
TC7

 

[lpgii]

Should be able to get there using one of the Tds equations, second law of thermo, for adiabatic, dS = dQ / T = 0. Then you have TdS = CvdT + RT / V dV which you should be able to integrate from "uncompressed" to "compressed" state assuming that the air behaves like an ideal gas. Use the universal gas constant and calculate the MW of the moist air for use in the equation.

 

[saxon]

tc7, Ch3eck the following sites for accumulator sizing:

http://www.boschrexroth.com

http://www.fch.co.uk/products/acc_sizing_data.htm

Hope this helps.
saxon