Concept explainers
The exergy lost in the cooling tower.

Answer to Problem 113P
The exergy lost in the cooling tower is
Explanation of Solution
As the process is a steady flow and thus the mass flow rate of dry air remains constant during the entire process.
Here, the mass flow rate of air at inlet is
Express the water mass balance:
Here, mass flow rate of water at inlet and exit is
Express the energy balance.
Here, the rate of total energy entering the system is
Determine the mass flow rate of steam at state 3 per unit mass of dry air.
Determine the mass flow rate of steam at state 4 per unit mass of dry air.
Determine the change in entropy of water steam.
Determine the change in entropy of water vapour in the air stream.
Determine the partial pressure of water vapour at state 1 for air steam.
Determine the partial pressure of dry air at state 1 for air steam.
Here, the pressure at the state 1 is
Determine the partial pressure of water vapour at state 2 for air steam.
Determine the partial pressure of dry air at state 2 for air steam.
Here, the pressure at the state 1 is
Determine the entropy change of dry air.
Here, the temperature at the state 1 is
Determine the entropy generation in the cooling tower is the total entropy change.
Determine the exergy destruction per unit mass of dry air.
Conclusion:
Refer Figure A-31, “psychometric chart at
Refer Figure A-31, “psychometric chart at
Refer Table A-4, “saturated water-temperature table”, and write the enthalpy and entropy at state 3 at temperature of
Here, enthalpy of saturation liquid is
Refer Table A-4, “saturated water-temperature table”, and write the enthalpy at state 4 at temperature of
Here, enthalpy of saturation liquid at temperature of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is temperature and specific enthalpy at state 4 respectively.
Show the specific enthalpy at state 4 corresponding to temperature as in Table (1).
Temperature |
Specific enthalpy at state 4 |
20 | 83.915 |
22 | |
25 | 104.83 |
Substitute
Substitute
Repeat the Equation (XIV), obtain the value of entropy for water streams at
Substitute
Substitute
Substitute
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the entropy at state 1 at temperature of
Repeat the Equation (XIV), obtain the value of entropy for water vapour at
Substitute
Substitute
Substitute 1 atm for
Substitute
Substitute 1 atm for
Substitute
Substitute
Substitute
Thus, the exergy lost in the cooling tower is
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