Enthelpy change in isentropic condition (air-con)

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Homework Help Overview

The discussion revolves around the enthalpy change in an isentropic condition for refrigerant-134a, specifically in the context of a vapor-compression refrigeration cycle. Participants are examining how to determine enthalpy values from pressure and entropy data using reference tables.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants are attempting to find the enthalpy value (h2) at a specific pressure (0.8 MPa) using tables, while questioning the accuracy of their readings and the consistency of the data across different editions of reference materials.

Discussion Status

The discussion is active, with participants sharing their findings from various tables and questioning the discrepancies in enthalpy values. Some guidance has been provided regarding the use of different editions of reference materials, but no consensus has been reached on the correct enthalpy value.

Contextual Notes

There is mention of potential changes in physical properties across different editions of reference tables, which may affect the results. Participants are also navigating the limitations of the information provided in the original problem statement.

yecko
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Homework Statement
How to get h2?
(see pic)
Relevant Equations
isentropic
dH = dQ + Vdp = Tds + Vdp
H = U + pV
48nCmSj.png


h and s can be obtained from "Saturated refrigerant-134a—Pressure table"

however, how to get h2? it is not on the curve, and neither p or dV is given in the question. Thank you
 
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Ity says P 0.8 MPa on the curve ! You have s,p and can look up T in table A-13
 
1583933825540.png


This is table A13, which at P=0.8MPa, s=0.94456, h should be somewhere between 286.71 and 296.82.
However, from the solution, h= 275.39kJ, am I looking at the table in a correct way?
 
Yes you are. Turns out the phys props have changed slightly.
I have an older edition (2002) of cengel and there it matches better.
40273.660.9374
50284.390.9711

https://www.egr.msu.edu/classes/me417/somerton/R134a Tables.pdf

I googled "and operates on an ideal vapor-compression refrigeration cyle between 0.14 and 0.8 MPa" and did indeed find other results too.
 
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