486 ENGINEERING THERMODYNAMICS
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\M-therm\Th10-2.pm5(i) The heat supplied to the air ;
(ii) The final relative humidity. [Ans. 16.8 kJ/kg, 29.2%]- The air supplied to a room of building in winter is to be at 17ºC and have a relative humidity of 60%. If the
barometric pressure is 1.01325 bar, calculate the specific humidity. What would be the dew point under
these conditions? [Ans. 0.00723 kg/kg of dry air, 9.18ºC] - If air at the condition of example 6, is passed at the rate of 0.5 m^3 /s over a cooling coil which is at a
temperature of 6ºC, calculate the amount of vapour which will be condensed. Assume that the barometric
pressure is the same as in example 6, and that the air leaving the coil is saturated. [Ans. 3.082 kg/h] - An air and water vapour mixture at 1 bar and 26.7ºC has a specific humidity of 0.0085. Determine the
percentage saturation. [Ans. 37.7%] - A mixture of air and water vapour at 1.013 bar and 16ºC has a dew point of 5ºC. Determine the relative and
specific humidities. [Ans. 48%, 0.0054 kg/kg of dry air] - Atmospheric air at a pressure of 760 mm Hg has a temperature of 32ºC and a percentage saturation as
determined from a psychrometric chart of 52%. Calculate
(i) The partial pressure of the vapour and the dry air
(ii) The specific humidity
(iii) The dew point
(iv) The density of the mixture.
[Ans. (i) 0.0247 bar, 0.988 bar, (ii) 0.01556, (iii) 20.9ºC, (iv) 1.147 kg/m^3 ] - In a laboratory test, a psychrometer recorded 36ºC DBT and 30ºC WBT. Calculate :
(i) Vapour pressure (ii) Relative humidity (iii) Specific humidity
(iv) Degree of saturation (v) Dew point temperature (vi) Enthalpy of the mixture.
[Ans. (i) 0.0385 bar, (ii) 64.5%, (iii) 0.025 kg/kg dry air, (iv) 0.63, (v) 28ºC, (vi) 99.2 kJ] - The pressure and temperature of the air in a room is 1 bar and 28ºC. If the relative humidity is found to be
30 per cent, determine :
(i) The partial pressure of the water vapour and dew point,
(ii) The specific volume of each constituent, and
(iii) The specific humidity.
[Ans. (i) 0.0378 bar, 8.8ºC ; (ii) vair = 0.874 m^3 /kg, vvap = 122.7 m^3 /kg ; (iii) 0.00712 kg/kg dry air] - 100 m^3 of air per minute at 35ºC DBT and 60% relative humidity is cooled to 20ºC DBT by passing through
a cooling coil. Find the following :
(i) Capacity of cooling coil in kJ/h
(ii) Amount of water vapour removed per hour, and
(iii) Relative humidity of air coming out and its wet-bulb temperature.
[Ans. (i) 1037088 kJ/h, (ii) 465.36 kg/h, (iii) 100%, 20ºC] - Atmospheric air at 38ºC and 40 per cent relative humidity is to be cooled and dehumidified to a state of
saturated air at 10ºC. The mass rate of flow of atmospheric air entering the dehumidifier is 45.4 kg/h.
Neglecting any pressure drop, determine :
(i) The mass of water removed ; (ii) The quantity of heat removed.
[Ans. (i) 0.397 kg/h, (ii) 2287 kJ/h] - 1 kg of air at 24ºC and a relative humidity of 70% is to be mixed adiabatically in a steady state, steady flow
device with 1 kg of air at 16ºC and a relative humidity of 10%. Assuming that the mixing is to be carried out
at a constant pressure of 1.0 atm, determine the temperature and relative humidity of the stream leaving
the device. [Ans. 19.5ºC, 50%] - An air-water vapour mixture enters an adiabatic saturator at 30ºC and leaves at 20ºC, which is the adiabatic
saturation temperature. The pressure remains constant at 1 bar. Determine the relative humidity and the
humidity ratio of the inlet mixture. [Ans. 39.8%, 0.0107 kg/kg dry air]