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

Answer to Problem 137RP
The temperature 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
The amount of moisture in the air remains constant as it flows through the heating section as process involves no dehumidification or humidification.
Here, specific humidity at state 1 and 2 is
Express initial pressure of vapor.
Here, relative humidity at state 1 is
Express initial humidity ratio.
Here, pressure at state 1 is
Express initial enthalpy.
Here, specific heat at constant pressure is
Express specific volume at state 1.
Here, gas constant of air is
Express pressure vapor at state 3.
Here, relative humidity at state 3 is
Express humidity ratio at state 3.
Here, pressure at state 3 is
Express enthalpy at state 3.
Here, specific heat at constant pressure is
Express enthalpy at state 2.
Express enthalpy at state 2 to obtain the temperature at heating section.
Here, temperature at heating section is
Express pressure of water vapor at state 2.
Here, saturation pressure at temperature at leaving section is
Express relative humidity at heating section.
Here, pressure at state 2 is
Conclusion:
Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure and initial specific enthalpy saturated vapor at temperature of
Substitute
Substitute
Substitute
Refer Table A-2, “ideal-gas specific heats of various common gases”, and write the properties of air.
Substitute
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure and initial specific enthalpy saturated vapor at temperature of
Substitute
Substitute
Substitute
Substitute
Substitute
Hence, the temperature of the air when it leaves the heating section is
Substitute
Refer Table A-4, “saturated water-temperature table”, and write the saturation temperature or exit temperature at temperature of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is temperature and exit or saturation temperature respectively.
Show the saturation pressure corresponding to temperature as in Table (1).
Exit temperature |
Saturation pressure |
30 | 4.2469 |
33.1 | |
35 | 5.6291 |
Substitute
Substitute
Substitute
Hence, the relative humidity of the air when it leaves the heating section is
(b)
The rate of heat transfer in the heating section.
(b)

Answer to Problem 137RP
The rate of heat transfer in the heating section is
Explanation of Solution
Express the rate of heat transfer in the heating section.
Here, mass flow rate of air is
Express mass flow rate of air.
Here, volume flow rate at inlet is
Conclusion:
Substitute
Substitute
Hence, the rate of heat transfer in the heating section is
(c)
The rate of water added to air in the evaporative cooler.
(c)

Answer to Problem 137RP
The rate of water added to air in the evaporative cooler is
Explanation of Solution
Express the rate of water added to air in the evaporative cooler.
Conclusion:
Substitute
Hence, the rate of water added to air in the evaporative cooler is
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Chapter 14 Solutions
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