Question 12.6: R134a Properties Find the enthalpy change in a process in wh......

The solution to this problem is most quickly done by using the p–h diagram since the process is just a horizontal line at 200 psia with the left endpoint at the 80% quality-line intersection. The right end of the process line is at the intersection of the 200 psia line and the 300°F line. The enthalpy at the left endpoint can be approximated by first finding the enthalpies at saturated liquid (55  Btu/lb_{m}) and saturated vapor (120  Btu/lb_{m}) and then interpolating.
The enthalpy at the right endpoint can be read directly from the p–h diagram.
Thus, the left and right endpoints are
h_{1} = 55 + (120  –  55) \times 0.8 = 107  Btu/lb_{m}
and
h_{2} = 165.0  Btu/lb_{m}
The enthalpy change is the product of the enthalpy change per unit mass and the mass:
\Delta H = 38  lb_{m} \times (165  –  107)  Btu/lb_{m} = 2204  Btu

Comments
For a more accurate calculation, the reader may use the saturated table (Table A6) and the superheated R134a tables (Table A8).