Enthelpy table application in a refrigeration problem

[yecko]

Homework Statement

How to get the enthalpy of point 3?
(see pic attached)

Relevant Equations

(Cengel 8th ed) table A12 & A13 & A14

How to get h3 = 111.37?
According to table a-13, sat temp@1.2MPa = 46.29oC, and T3 is lower than that. So the refrigerant should be totally in liquid state, and heat capacity should be used instead. However, from textbook, no heat capacity is given from table.
If graph of A14 is used, at 1.2MPa and T=42oC, enthalpy should be lying somewhere between 250-300kJ/kg instead.

However, from solution, 111.37 is found, how to obtain this answer?

Thank you.

 

[Lnewqban]

Those values seem to be incorrect.
Compared to a Pressure-Enthalpy diagram for HFC-134a, the four values of enthalpy shown on that table are off by around 150 KJ/Kg.
$h_1$ should be around 376 KJ/Kg.
$h_2$ should be around 460 KJ/Kg.
$h_3$ and $h_4$ should be around 260 KJ/Kg.

Please, see:
https://www.pfri.uniri.hr/web/dokum...4_103058_kralj_R134a.pressure.enthalpy.si.pdf

 

[Chestermiller]

According to this table, https://www.ohio.edu/mechanical/thermo/property_tables/R134a/R134a_TempSat2.html
the enthalpy of the saturated liquid at 42 C is 111.26 kJ/kg, and the corresponding pressure and specific volume are 1072 kPa and 0.000878 m^3/kg. Using the equation $\Delta h=v\Delta P$, we have: $$h=111.26+(0.000878)(1200-1072)=111.37\ kJ/kg$$This neglects the term $\alpha T$ in the equation $$dh=C_pdT+v(1-\alpha T)dP$$where $\alpha$ is the coefficient of volume expansion.

 

[yecko]

According to this table, https://www.ohio.edu/mechanical/thermo/property_tables/R134a/R134a_TempSat2.html
the enthalpy of the saturated liquid at 42 C is 111.26 kJ/kg, and the corresponding pressure and specific volume are 1072 kPa and 0.000878 m^3/kg. Using the equation $\Delta h=v\Delta P$, we have: $$h=111.26+(0.000878)(1200-1072)=111.37\ kJ/kg$$This neglects the term $\alpha T$ in the equation $$dh=C_pdT+v(1-\alpha T)dP$$where $\alpha$ is the coefficient of volume expansion.

But why do we use temperature table instead of pressure table?

 

[Chestermiller]

But why do we use temperature table instead of pressure table?

It can be done using the pressure table too, but not as straightforwardy, and probably not as accurately.

 

[yecko]

how can we decide which of pressure or temperature

It can be done using the pressure table too, but not as straightforwardy, and probably not as accurately.

can be used?

 

[Chestermiller]

how can we decide which of pressure or temperature

Do it both ways and see which requires a larger correction.

can be used?

I'll help you work it out. Start with the equation I gave in post #4, and see if you can figure out how to use it to get the heat capacity (using the data in the pressure table).