
The exergy destruction per unit mass of dry air.

Answer to Problem 112P
The exergy destruction per unit mass of dry air is
Explanation of Solution
Apply the dry air mass balance on the cooling tower.
Here, mass flow rate of air at inlet and outlet is
Apply the water mass balance on the cooling tower.
Here, mass flow rate of water at state 3 and 4 is
Apply the energy balance on the cooling tower.
Write the expression to obtain the mass of water stream at state 3
Write the expression to obtain the mass of water stream at state 4
Write the expression to obtain the entropy change of water stream
Write the expression to obtain the entropy change of water vapor in the air stream
Write the expression to obtain the vapor pressure at inlet conditions
Here, saturation pressure of water at
Write the expression of the atmospheric pressure of an ideal gas mixture
Here, dry air partial pressure at state 1 is
Write the expression to obtain the vapor pressure at second inlet conditions
Here, saturation pressure of water at
Write the expression of the atmospheric pressure of an ideal gas mixture
Here, dry air partial pressure at state 2 is
Write the expression to obtain the entropy change of dry air
Here, temperature at state 2 is
Write the expression for entropy generation in the cooling tower
Write the exergy destruction per unit mass of dry air
Conclusion:
Refer Table A-4E, “Saturated water – Temperature table”, obtain the enthalpy
Refer Table A-4E, “Saturated water – Temperature table”, obtain the enthalpy
Refer Fig A-31E, “Psychrometric chart at 1 atm total pressure”, at inlet temperature
Refer Fig A-31E, “Psychrometric chart at 1 atm total pressure”, at outlet temperature
Substitute
Substitute
Substitute
Substitute
Refer Table A-4E, “Saturated water – Temperature table”, obtain the entropy
Refer Table A-4E, “Saturated water – Temperature table”, obtain the entropy
Substitute
Refer Table A-4E, “Saturated water – Temperature table”, obtain the properties of water at a temperature of
Substitute
Rewrite Equation (VIII) and substitute 14.696 psia for
Refer Table A-4E, “Saturated water – Temperature table”, obtain the properties of water at a temperature of
Substitute
Rewrite Equation (X) and substitute 14.696 psia for
Refer Table A-2E, “Ideal gas specific heats of various common gases”, obtain the value of
Substitute
Substitute
Substitute
Thus the exergy destruction per unit mass of dry air is
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Chapter 14 Solutions
Thermodynamics: An Engineering Approach
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