Mass Transfer Operation Humidification


Cooling tower working model, water and air flow
Cooling Tower

The transfer of material between a pure liquid phase and a fixed gas that is insoluble in the liquid delt as humidification. Adiabatic saturation temperature is attained by the gas. Dry bulb temperature of a gas is more than the wet bulb temperature. Relative humidity is the ratio of partial pressure of the vapor to the vapor pressure of the liquid at the gas  temperature.

Operation of cooling tower which cools the hot water to wet bulb temperature
Cooling Tower
At all humidities other than 0 and 100 percent, the percentage humidity is less than the relative humidity. Heat in BTU necessary to increase the temperature of 1 lb of gas and its accompanying vapor by 1oF is called humid heat. Cooling tower is the example for the humidification operation. In a cooling tower water is cooled from 95 oF to 80 oF by exposure to air with a wet-bulb temperature of 70 oF.

Heat required to vaporize 1 lb of water is roughly 1000 BTU. The lowest temperature to which water can be cooled in a cooling tower through out the year depends on the maximum wet bulb temperature. Adiabatic saturation line and wet bulb temperature line are the same for the system air – water. If a warm vapor-gas mixture is contacted with cold liquid so that the humidity of the gas is sensible heat transfer will occur from the gas to the liquid. Purge of a small amount of water from a cooling tower is necessary in order to keep the load on the cooling tower at a specified level keep the dissolved salt concentration at a specified level. Dew point of a gas-vapor mixture (for a fixed vapor mole – fraction in gas) increases with an increase in pressure.

During evaporative cooling process with recirculated water, wet bulb temperature remains constant while partial pressure of vapor and relative humidity increases. When a gas-vapor mixture is cooled by sensible heat transfer relative humidity decreases & wet bulb temperature decreases. If PA is the partial pressure of vapor A in a vapor – gas mixture and PS is the saturation vapor pressure of liquid A, condensation of vapor-gas mixture just begins when P A = PS. During adiabatic saturation of unsaturated air the parameter that remains substantially constant is wet bulb temperature. For example if an unsaturated air-water vapor mixture at a dry bulb temperature of the 40oC.The dew point for the mixture is 20 oC. The mixture is passed through a water spray chamber maintained at 17oC. The air, on passage through the chamber, will be cooled and dehumidified with an decrease in wet bulb temperature.