A typical thermodynamics question

tony_engin

Hi all!
For this question's part (a), I initially thought that I should first apply the first law for control volume for the system of the evaporator so that the rate of heat transfer from the evaporator to the air can be found. Then apply first law again on the air with using the specific heat to find out the exit temperature. Does this method sound?
It seems that I'm doing part(b) before part (a)...
What should be the more appropraite method?

tony_engin

Please kindly help...

russ_watters

Yes, your method sounds good and yes, it'll yield the answer to B before A. I wouldn't worry about that. If you post some of your work, we can check it. Do you know how to use the table?

Btw, homework questions belong in homework help (and you'll probably get more responses that way too).

tony_engin

yes..attached is my "solution" to this problem..
please point out my faults..

quark

Procedure is ok. I think you didn't consider the 30% quality. 30% fluid is already in gaseous state before the evaporator, so heat extracted from air is equal to latent heat of 70% of R12.

Secondly, it is better to calculate the mass flowrate based on ideal gas equation. 100kPa is a bit lower pressure than that of standard air.

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russ_watters Mentor	Yes, your method sounds good and yes, it'll yield the answer to B before A. I wouldn't worry about that. If you post some of your work, we can check it. Do you know how to use the table? Btw, homework questions belong in homework help (and you'll probably get more responses that way too).

quark	Procedure is ok. I think you didn't consider the 30% quality. 30% fluid is already in gaseous state before the evaporator, so heat extracted from air is equal to latent heat of 70% of R12. Secondly, it is better to calculate the mass flowrate based on ideal gas equation. 100kPa is a bit lower pressure than that of standard air.