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Unformatted text preview: Chapter 9 Gas Mixtures and Psychrometrics Dry and Atmospheric Air, Specific and Relative Humidity 9-53C Yes; by cooling the air at constant pressure. 9-54C Yes. 9-55C Specific humidity will decrease but relative humidity will increase. 9-56C Dry air does not contain any water vapor, but atmospheric air does. 9-57C Yes, the water vapor in the air can be treated as an ideal gas because of its very low partial pressure. 9-58C The partial pressure of the water vapor in atmospheric air is called vapor pressure. 9-59C The same. This is because water vapor behaves as an ideal gas at low pressures, and the enthalpy of an ideal gas depends on temperature only. 9-60C Specific humidity is the amount of water vapor present in a unit mass of dry air. Relative humidity is the ratio of the actual amount of vapor in the air at a given temperature to the maximum amount of vapor air can hold at that temperature. 9-61C The specific humidity will remain constant, but the relative humidity will decrease as the temperature rises in a well-sealed room. 9-62C The specific humidity will remain constant, but the relative humidity will decrease as the temperature drops in a well-sealed room. 9-63C A tank that contains moist air at 3 atm is located in moist air that is at 1 atm. The driving force for moisture transfer is the vapor pressure difference, and thus it is possible for the water vapor to flow into the tank from surroundings if the vapor pressure in the surroundings is greater than the vapor pressure in the tank. 9-64C Insulations on chilled water lines are always wrapped with vapor barrier jackets to eliminate the possibility of vapor entering the insulation. This is because moisture that migrates through the insulation to the cold surface will condense and remain there indefinitely with no possibility of vaporizing and moving back to the outside. 9-65C When the temperature, total pressure, and the relative humidity are given, the vapor pressure can be determined from the psychrometric chart or the relation sat P P v = where P sat is the saturation (or boiling) pressure of water at the specified temperature and is the relative humidity. 9-25 Chapter 9 Gas Mixtures and Psychrometrics 9-66 A tank contains dry air and water vapor at specified conditions. The specific humidity, the relative humidity, and the volume of the tank are to be determined. Assumptions The air and the water vapor are ideal gases. Analysis ( a ) The specific humidity can be determined form its definition, = = = m m v a 0 3 . kg 21 kg 0.0143 kg H O / kg dry air 2 ( b ) The saturation pressure of water at 30 C is kPa 246 . 4 C 30 @ sat = = P P g Then the relative humidity can be determined from = + = + = P P g ( . ) ( . )( ( . . ) . 0 622 00143 100 0622 0 0143 4 246 kPa) kPa 52.9% ( c ) The volume of the tank can be determined from the ideal gas relation for the dry air, P P P P P V m R T P v g a v a a a = = =- =- = = = ( . )( ....
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- Spring '08