
(a)
The temperature and relative humidity of air when it leaves the heating section.
(a)

Answer to Problem 74P
The temperature and relative humidity of air when it leaves the heating section is
Explanation of Solution
Express initial partial pressure.
Here, partial pressure at state 2 is
Express initial partial pressure.
Here, initial pressure is
Express initial specific volume.
Here, universal gas constant of air is
Express initial specific humidity.
The amount or quantity of air moisture will remain constant while flowing through heating section,
Here, specific humidity at state 1 and 2 is
Express initial enthalpy.
Here, initial specific enthalpy at saturated vapor is
Express partial pressure at state 3.
Here, relative humidity at state 3 is
Express specific humidity at state 3.
Here, partial pressure at stat3 is
Express enthalpy at state 3.
Here, specific enthalpy saturated vapor at state 3 is
Express the mass flow rate of dry air.
Here, initial volume rate is
Express final specific enthalpy by applying an energy balance on the humidifying section.
Here, the rate of total energy entering the system is
Express temperature leaving the heating section.
Here, temperature at state 2 is
Express relative humidity leaving the heating section.
Here, partial pressure at state 2 is
Conclusion:
Refer Table A-2, “ideal-gas specific heats of various common gases”, and write the gas constant and specific heat at constant pressure of air.
Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure at temperature of
Substitute
Substitute
Substitute
Substitute
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the initial and final specific enthalpy saturated vapor corresponding to temperature of
Substitute
Substitute
Substitute
Substitute
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the specific enthalpy saturation vapor at temperature of
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the specific enthalpy saturation vapor at temperature of
Substitute
Hence, the temperature of air when it leaves the heating section is
Refer Table A-4, “saturated water-temperature table”, and write the saturated vapor at temperature of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is temperature and saturated vapor respectively.
Show the saturated vapor corresponding to temperature as in Table (1).
Temperature |
Saturated vapor |
15 | 1.7057 |
19.5 | |
20 | 2.3392 |
Substitute
Substitute
Hence, the relative humidity of air when it leaves the heating section is
(b)
The rate of heat transfer in the heating section.
(b)

Answer to Problem 74P
The rate of heat transfer in the heating section is
Explanation of Solution
Express the rate of heat transfer in the heating section.
Conclusion:
Substitute
Hence, the rate of heat transfer in the heating section is
(c)
The rate at which water is added to the air in the humidifying section.
(c)

Answer to Problem 74P
The rate at which water is added to the air in the humidifying section is
Explanation of Solution
Express the rate at which water is added to the air in the humidifying section.
Conclusion:
Substitute
Hence, the rate at which water is added to the air in the humidifying section is
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Chapter 14 Solutions
Thermodynamics: An Engineering Approach
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