temperature increase with compressing air? 2

[spdu4ea]

Forgive me for my ignorance, but I havent had a formal thermodynamics class. I believe that the temperature of air increases as it is compressed, but I would like to know by how much. Is there a formula based on volume, intake air temp, and compression amount? Basically I'd like to know how hot the air is when compressed to 120psi. If my calculations are correct, it would need to compress about 6 cubic feet of air per second.

 

[buzz41]

Reducing it to a pressure ratio, the prob you cite is 14.7 psi(abs) to 120 psi (gauge)+ 14.7 psi or (120+14.7)/14.7 = 9.16 pressure ratio.

For air, the absolute temp ratio will be the power ~.4/1.4 or .285 of the pressure ratio.

So, using a calc, the absolute temp ratio:
9.16^.285 = 1.88

If you start out with a temp of 70F, the abs temp is 70+459=529R. the abs temp after compression will be 529 X 1.88 = 994R

Bringing it back to F, 994R - 459 = 535F

Of course, much of the heat will be lossed in the compressor, so the temp will actually be lower.

A more exact exponent for air is .283, reducing the temp a little.

 

[TD2K]

The rise in temperature depends mostly on the pressure change and to a lesser extent, the inlet temperature. The volume has nothing to do with the temperature rise.

For air, you can estimate it as

dT = T1 * [(P2/P1)^0.286 - 1]/eff

dT is the temperature rise
T1 is the inlet temperature, degrees absolute
P2 is the final pressure, absolute, not gauge
P1 is the inlet pressure, absolute, not gauge
eff is the efficiency of compression.

 

[energysystems]

give me exact definitions of Entropy.

 

[TBP]

Entropy - In thermodynamics, a certain property of a body, expressed as a measureable quantity, which remains constant if no heat enters or leaves the body, whilst it does work or alters its volume, but which increases or diminishes should a small amount of heat enter or leave.

Audels Mechanical Dictionary