Heat rejection from air compressor

[slav72]

Hello everyone,
How much heat does air compressor produce with inter and after coolers per 1 HP.
Any rules? Formulas? Help please.
Thanks.

 

[dcasto]

Heat capacity of air is about .25 BTU/lb-F. Air has a density of .0765 lb/cf at std conditions. If you cool 100 cfm (at std conditions) air from 250 F to 100 F then 100 cfm * .0765 lb/cf * 150 F * .25 BTU/lb-F = 287 BTU/min =17200 BTU/hr

 

[dcasto]

I forgot about water. You may have to condense water if you have high humidity. This could add another 10% duty or more.

 

[zdas04]

dcasto,
That is an interesting calculation. 17,200 BTU/hr is 6.76 hp. To get a temperature rise from 80F (assuming the cooler above has a 20F approach to ambient) to 250F in air at sea level requires 2.6 compression ratios.

To raise 100 CFM (0.144 MMCF/d) 2.6 ratios requires 7.1 hp.

Based on this arithmetic (that looks fine to me), it takes almost as much energy to cool the air as it takes to compress the gas in the first place.

Does this say that only 5% of the energy input to a compressor is useful for raising pressure and 95% is just waste heat? What does that say about thermodynamic efficiency?


David

 

[crjones]

Doesn't dcasto's calculation simply show recoverable heat, say that would be input to cooling water? So between the two calc's there is a 5% irreversible system loss?

 

[sailoday28]

zdas04 (Mechanical)
Consider isothermal compression and neglecting KE and elevation ---- energy equation is

WHin + energy of compression - heat transfer = WHout

But for Hin=Hout
enery per unit mass compression = heat transfer

Doesn't this agree with your conclusion?

 

[zdas04]

Sailoday28,
Probably, it was just so foreign to my expectations that I was having a problem visualizing it. I didn't have any basis for my expectations, but the heat transfer number just looked BIG.

David

 

[dcasto]

OK guys, I ran a Peng-Robinson EOS of an air compressor goin from 60F 14.7 psia saturated with water and 100CFM inlet to 34.5 psia at 78$ polytropic efficency to get a 251 F discharge temperature. The compressor used 8.35 HP and the after cooler rejected 16,867 BTU/hr at a 100F temperature.