Concept explainers
10 kg of R-134a at 300 kPa fills a rigid container whose volume is 14 L. Determine the temperature and total enthalpy in the container. The container is now heated until the pressure is 600 kPa. Determine the temperature and total enthalpy when the heating is completed.
FIGURE P3–42
The temperature and total enthalpy at initial and final states in the container.
The temperature and total enthalpy at initial and final states when the heating is completed.
Answer to Problem 42P
The temperature and total enthalpy at initial and final states in the container are
The temperature and total enthalpy at initial and final states when the heating is completed are are
Explanation of Solution
Since the process is a constant volume, calculate the specific volume.
Here, specific volume at states 1 and 2 are
The initial state represents the mixture, so temperature to be considered as the saturation temperature at given pressure.
Calculate the dryness fraction at state 1.
Here, specific volume at state 1 is
Calculate the specific enthalpy at state 1.
Here, specific enthalpy at saturated liquid is
Calculate the total enthalpy at state 1.
Here, mass of the refrigerant R-134a is m.
Similarly, calculate for
Here, dryness fraction at state 2 is
Calculate the specific enthalpy at state 2.
Calculate the total enthalpy at state 2.
Conclusion:
Substitute 14 L for
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y are pressure and temperature.
Refer to Table A-12, obtain the values of below variables as in Table (I).
Pressure, kPa | Temperature, |
280 | –1.25 |
300 | ? |
320 | 2.46 |
Substitute 280 for
Thus, the value obtained for saturation temperature at the given pressure is
Similarly, calculate the values of
Substitute
Substitute
Substitute 10 kg for m and 54.56 kJ/kg for
Thus, the temperature and total enthalpy at initial and final states in the container are
Similarly, calculate the values of
Repeat the above steps for
Thus, the temperature and total enthalpy at initial and final states in the container are
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