A typical thermodynamics question

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Homework Help Overview

The discussion revolves around a thermodynamics problem involving an evaporator and the application of the first law of thermodynamics. Participants are exploring methods to determine heat transfer and exit temperature in a system involving air and refrigerant R12.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to apply the first law of thermodynamics to analyze the evaporator and the air, questioning the appropriateness of their method and the sequence of solving parts (a) and (b). Other participants suggest reviewing the original poster's work and considering additional factors such as fluid quality and mass flow rate calculations.

Discussion Status

Participants are actively engaging with the original poster's approach, providing feedback on their method and suggesting areas for improvement. There is a recognition of the need to consider specific conditions, such as the quality of the refrigerant and the pressure of the air.

Contextual Notes

Participants note the importance of specific conditions, such as the 30% quality of the refrigerant and the pressure being lower than standard air, which may affect the calculations and assumptions made in the problem.

tony_engin
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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?
 

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Please kindly help...
 
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).
 
Last edited:
yes..attached is my "solution" to this problem..
please point out my faults..
 

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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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