The air in a room has a dry-bulb temperature of 26°C and a wet-bulb temperature of 21°C. Assuming a pressure of 100 kPa, determine (a) the specific humidity, (b) the relative humidity, and (c) the dew-point temperature.
(a)
The specific humidity.
Answer to Problem 29P
The specific humidity is
Explanation of Solution
Express the specific humidity.
Here, specific heat at constant pressure of air is
Express specific humidity at exit.
Here, final pressure is
Conclusion:
Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure at temperature of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is temperature and saturation pressure respectively.
Show thesaturation pressure corresponding to temperature as in Table (1).
Temperature |
Saturation pressure |
20 | 2.3392 |
21 | |
25 | 3.1698 |
Substitute
Thus, the saturation pressure at temperature of
Substitute
Refer Table A-4, “saturated water-temperature table”, and write final specific enthalpy evaporation at temperature of
Show thefinal specific enthalpy evaporationcorresponding to temperature as in Table(2).
Temperature |
Final specific enthalpy evaporation |
20 | 2453.5 |
21 | |
25 | 2441.7 |
Use excels and tabulates the values from Table (2) in Equation (III) to get,
Refer Table A-4, “saturated water-temperature table”, and write initial specific enthalpy saturated vapor at temperature of
Show theinitial specific enthalpy saturated vapor corresponding to temperature as in Table (3).
Temperature |
Initial specific enthalpy saturated vapor |
25 | 2546.5 |
26 | |
30 | 2555.6 |
Use excels and tabulates the values from Table (3) in Equation (III) to get,
Refer Table A-4, “saturated water-temperature table”, and write final specific enthalpy saturated liquid at temperature of
Show thefinal specific enthalpy evaporationcorresponding to temperature as in Table(4).
Temperature |
Final specific enthalpy saturated liquid |
20 | 83.915 |
21 | |
25 | 104.83 |
Use excels and tabulates the values from Table (4) in Equation (III) to get,
Refer Table A-2 (a), “ideal gas specific heats of various common gases”, and write specific heat at constant pressure of dry air.
Substitute
Hence, the specific humidity is
(b)
The relative humidity.
Answer to Problem 29P
The relative humidity is
Explanation of Solution
Express saturation pressure of water at temperature of
Here, initial pressure is
Conclusion:
Refer Table A-4, “saturated water-temperature table”, and write the saturation pressure at temperature of
Show thesaturation pressure corresponding to temperature as in Table (5).
Temperature |
Saturation pressure |
25 | 3.1698 |
26 | |
30 | 4.2469 |
Use excels and tabulates the values from Table (5) in Equation (III) to get,
Substitute
Hence, the relative humidity is
(c)
The dew point temperature.
Answer to Problem 29P
The dew point temperature is
Explanation of Solution
Express initial partial pressure of water vapor.
Express the dew point temperature
Conclusion:
Substitute
Substitute
Here, saturation pressure at pressure of
Refer Table A-4, “saturated water-temperature table”, and write temperature at saturation pressure of
Show thetemperature corresponding to saturation pressure as in Table (6).
Saturation pressure |
Temperature |
1.7057 | 15 |
2.166 | |
2.3392 | 20 |
Use excels and tabulates the values from Table (6) in Equation (III) to get,
Substitute
Hence, the dew point temperature is
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Chapter 14 Solutions
Thermodynamics: An Engineering Approach
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