Concept explainers
Saturated water vapor at 300°C is expanded while its pressure is kept constant until its temperature is 700°C. Calculate the change in the specific enthalpy and entropy using (a) the departure charts and (b) the property tables.
(a)
The change in specific enthalpy and entropy of saturated water per unit mass using departure charts.
Answer to Problem 72P
The change in specific enthalpy and entropy of saturated water per unit mass using departure charts is
Explanation of Solution
At ideal gas state, the enthalpy is the function of temperature only.
Write the formula for difference in molar specific enthalpy of water vapor at ideal gas state.
Here, the molar enthalpy at ideal gas state corresponding to the temperature is
Write the formula for change in molar specific entropy.
Here, the molar specific entropy at reference sate is
Write formula for enthalpy departure factor
Here, the molar enthalpy at ideal gas state is
Rearrange the Equation (III) to obtain
Refer Equation (IV) express as two states of enthalpy difference (final – initial).
Write formula for entropy departure factor
Here, the molar entropy at ideal gas state is
Rearrange the Equation (VI) to obtain
Refer Equation (VII) express as two states of entropy difference (final – initial).
Write the formula for enthalpy
Here, the molar enthalpy is
Refer Table A-1, “Molar mass, gas constant, and critical-point properties”.
The critical temperature and pressure of water vapor gas is as follows.
Refer Table A-1, “Molar mass, gas constant, and critical-point properties”.
The molar mass
The pressure is kept constant until its temperature reaches to
Refer Table A-4, “Saturated water-Temperature table”.
The saturation pressure corresponding to the temperature of
The reduced pressure
The reduced pressure
At initial:
Refer Figure A-29, “Generalized enthalpy departure chart”.
The enthalpy departure factor
Refer Figure A-30, “Generalized entropy departure chart”.
The entropy departure factor
At final:
Refer Figure A-29, “Generalized enthalpy departure chart”.
The enthalpy departure factor
Refer Figure A-30, “Generalized entropy departure chart”.
The entropy departure factor
Refer Table A-19, “Ideal-gas properties of water vapor,
Obtain the initial properties corresponding to the temperature of
Obtain the final properties corresponding to the temperature of
The universal gas constant
Conclusion:
Substitute
Equation (I).
Substitute
Substitute
Substitute
Substitute
Substitute
Thus, the change in specific enthalpy and entropy of saturated water per unit mass using departure charts is
(a)
The change in specific enthalpy and entropy of water vapor per unit mass using property tables.
Answer to Problem 72P
The change in specific enthalpy and entropy of water vapor per unit mass using property tables is
Explanation of Solution
At state 1:
The steam is at state of saturated vapor at the temperature of
Refer Table A-4, “Saturated water-Temperature table”.
The enthalpy
At state 2:
The water vapor is expanded to the temperature of
The pressure is kept constant until its temperature reaches to
Refer Table A-4, “Saturated water-Temperature table”.
The saturation pressure corresponding to the temperature of
Refer Table A-6, “Superheated water”.
Obtain the enthalpy
Conclusion:
The enthalpy changes are expressed as follows.
The entropy changes are expressed as follows.
Thus, the change in specific enthalpy and entropy of water vapor per unit mass using property tables is
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Chapter 12 Solutions
Thermodynamics: An Engineering Approach
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