6.1 Basic concepts
Humidification is the process of constituting the water-vapor content in a gas. The reverse of the operation is called dehumidification. Both are important for many industrial operations such as air conditioning, gas cooling, controlled drying of wet solids, comfort heating etc. When a gas is brought in contact with a pure liquid in which it is essentially insoluble, interphase mass and heat transfer takes place [1-3].
Four major applications of humidification operations are as follows:
a) Humidification of gases for controlled drying of wet solids
b) Dehumidification and cooling of gas in air conditioning
c) Gas cooling with the help of water
d) Cooling of liquid (e.g. water) before reuse
6.1.1 Terminologies and definitions
Three most important quantities, namely, ‘temperature’, ‘humidity’ and ‘enthalpy’, are essential terminologies in dealing humidification.
1) Dry-bulb temperature: It is true temperature of air measured (or, any noncondensable and condensable mixture) by a thermometer whose bulb is dry.
2) Wet-bulb temperature: It is the steady-state temperature attained by a small amount of evaporating water in a manner such that the sensible heat transferred from the air to the liquid is equal to the latent heat required for evaporation.
3) Relative humidity: It is the ratio of partial pressure of water vapor (pA) in air at a given temperature to the vapor pressure of water ( p0A ) at the same temperature.
“Relative humidity does not ‘explicitly’ give the moisture content of a gas, but gives the ‘degree of saturation’ of the gas at a given temperature.
4) Absolute humidity (simply humidity): It is the direct measurement of moisture content in a gas. The mass of water vapor per unit mass of dry gas is called absolute humidity,
It is occasionally called ‘Grosvenor humidity’ after the name of the inventor.
5) Percent humidity or percent saturation: It is the relation between absolute humidity to that of saturation humidity at the same temperature and pressure.
where, is absolute humidity of sample of air and is humidity at same temperature and pressure if saturated with water vapor.
and vapor pressure of water can be calculated by Antoine Equation: where, pressure is in bar and temperature is in K.
6) Dew point: Dew point is a temperature at which a vapor-gas mixture must be cooled (at constant humidity) to become saturated. The dew point of a saturated gas equals the gas temperature. If a vapor-gas mixture is gradually cooled at a constant pressure, the temperature at which it just becomes saturated is also called its dew point.
7) Humid volume: The humid volume, , is defined as the volume of unit mass of dry air with accompanying water vapor at a given temperature and pressure.
assuming ideal gas behavior. TG is gas temperature in ºC.
8) Humid heat: The humid heat, cH, is the heat energy required to raise the temperature of unit mass of dry air with the accompanying water vapor by one
(1) degree. first part of right hand side is heat capacity of dry air in kJ/kg.K and second part is heat capacity of water vapor in kJ/kg.K.
9) Enthalpy: The enthalpy of a vapor-gas mixture is the sum of the relative enthalpies of gas and vapor content.
where λ0 is latent heat of vaporization of water, 2500 kJ/Kg