A typical thermodynamics question

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SUMMARY

This discussion focuses on solving a thermodynamics problem involving an evaporator and the application of the first law of thermodynamics. The user initially proposed using the first law for a control volume to determine the heat transfer rate from the evaporator to the air, followed by calculating the exit temperature using specific heat. Feedback indicated that the proposed method is valid, but emphasized the importance of accounting for the 30% quality of the refrigerant R12 and suggested using the ideal gas equation to calculate the mass flow rate, especially given the lower pressure of 100 kPa.

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  • Understanding of the first law of thermodynamics
  • Knowledge of control volume analysis
  • Familiarity with specific heat and latent heat concepts
  • Ability to apply the ideal gas equation
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  • Study the calculation of mass flow rate using the ideal gas equation
  • Learn about quality and its impact on phase change in refrigerants
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Students and professionals in mechanical engineering, particularly those studying thermodynamics, HVAC systems, or anyone involved in the analysis of evaporators and heat transfer processes.

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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