Q=Cp*(T2-T1)

[dpolet]

In which temperature Cp should be used, T1 or T2 or average?

I found the NCEES 2001 sample exam #538 (T&F) use a air Cp of 0.24 btu/lbm-F. It seems like low temperature (T1) was used to find Cp.

Any comments? Thanks.

 

[ongreystreet]

I think for most problems you can just use 0.24 btu/lbm-F as a constant, and disregard temperature unless it is an extreme temperature or they are looking for a really specific answer.

 

[MapuaTech]

Q=Cp(T2-T1) is always used in heat absorbed by water. Avoid using the "constant 0.24". Use the initial temp as reference to find the correct Cp since this is the property of the water entering the heat exchanger.

 

[ongreystreet]

But the specific question was about the specific heat of air, which varies very little. I'm not saying it's correct to always use Cp = 0.24, I just found doing the practice exams that it worked well enough not to worry about it. It's nice to plug a constant in without looking it up, and get on to the next question.

 

[MapuaTech]

Sorry, I thought we're talking about water. For hvac and aerodyn calcs, use the initial temp as reference to find the Cp of air since this will greatly influence ur answer. Same as when the working substance is water.

Avoid assuming it's always constant. Getting the correct Cp will not take 1 minute of ur time.

 

[dpolet]

Thank you all.