# Constant Cp vs. Average Cp



## SK82 P.E. (Oct 13, 2016)

Sorry if this is a repeat question but I didn't see an answer that convinced me one way or another.  

So I'm ok with the principle of Qdot = m*cp*delta T.  So far I've been assuming constant specific heat regardless if it's air or water entering a heat exchanger.  If I understand the MERM correctly, you can take an average cp when the temp range of the inlet and outlet is significant or if both temps are high?  There is a post that said to don't assume constant cp and just use the cp for the temp of the inlet since this will have more impact on the system. 

There are problems in the NCEES exams where they give you the specific heat and It looks like the other problems where they don't give you this info, they assumed constant specific heat.  So is it good assumption to assume constant specific heat when they don't give you the value in the problem? or what?

The solution for problem #514 uses the constant cp of 1 for water.  Their answer is roughly 32,000.  If use the cp for the average, 1.19, my answer is roughly 38,000.  If i use a cp at the inlet of 460 of about 1.12, I get 35,996.  The answer choice given is 35,000.

Right not, I will assume a constant cp unless someone more informed tells me otherwise.


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## Audi Driver P.E. (Oct 13, 2016)

In most cases it's correct to assume a constant specific heat.  And if it is given in the problem it's pretty safe to assume that is the value they want you to use.  For the most part specific heat doesn't change a lot.  Even if you were to use the Cp of 1.19, the closest answer in this problem is still B.  The test writers take that into consideration when giving answer choices.


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