Homework Help: What is an Adiabatic Expansion Coefficient?

1. Nov 16, 2008

Natla

Hi,

Can anyone tell me what this means?

I'm trying to find the adiabatic expansion coefficient in order to use this air compressor calculator.

http://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.html

These are the variables I have at the moment.....

Number of stages- 1

absolute initial pressure (psi)- 14.5

volume of compressed air at atmospheric pressure (cfm)- 20

absolute final pressure (psi)- 100.

So what is this adiabatic expansion coefficient and how can I calculate for it?

Thanks.

2. Nov 16, 2008

Q_Goest

I'm not absolutely sure, but it looks like they're using the ratio of specific heats. Actually, it's the exponent in the polytropic equation, but for a perfectly isentropic process, the exponent becomes the ratio of specific heats. For air, use 1.4

3. Nov 17, 2008

Natla

Hi,

Thanks so much for your help.

4. Nov 17, 2008

Natla

On second thought. Perhaps 1.41 is not correct. Becuse then the calculation just doesnt seem correct, it does'nt take not nearly enough horsepower.

I don't think that this calculator considers the rise in temperature of the compressed gas. I mean I made it calculate for 7cfm at 14.5 psi to compress to 1,450 psi. Thats a compression in a single stage to a factor of 100. So, the rise in temperature for this compressed gas would be about 1,500 degress celcius.

This would bring the actual final pressure to about 14,500 psi. And it's trying to say that the compressor would only be rated at 4.29 HP. That isnt right.

So what else could it be?

Thanks.

5. Nov 17, 2008