Humidity control

Humidity control

Regulation of the degree of saturation (relative humidity) or quantity (absolute humidity) of water vapor in a mixture of air and water vapor. Humidity is commonly mistaken as a quality of air.

When the mixture of air and water vapor is heated at constant pressure, not in the presence of water or ice, the ratio of vapor pressure to saturation pressure decreases; that is, the relative humidity falls, but absolute humidity remains the same. If the warm mixture is brought in contact with water in an insulated system, adiabatic humidification takes place; the warm gases and the bulk of the water are cooled as heat is transferred to that portion of the water which evaporates, until the water vapor reaches its saturation pressure corresponding to the resultant water-air-vapor mixture temperature. Relative humidity is then 100% and absolute humidity has increased. Heating of the mixture and use of the heated mixture to evaporate water is typical of many industrial drying processes, as well as such common domestic applications as hair drying. This same sequence occurs when warm furnace air is passed over wetted, porous surfaces to humidify air for comfort conditioning. See Air conditioning

To remove moisture from the air-vapor mixture, the mixture is commonly cooled to the required dew point temperature (corresponding to the absolute humidity to be achieved) by passage over refrigerated coils or through an air washer where the mixture is brought in contact with chilled water. The result is a nearly saturated mixture which can be reheated, if required, to achieve the desired relative humidity. See Dehumidifier

Moisture is also removed without refrigeration by absorption, a process in which the mixture passes through a spray of liquid sorbent that undergoes physical or chemical change as it becomes more dilute. Typical sorbents include lithium and calcium chloride solutions and ethylene glycol. See Absorption

Another means of dehumidification, by adsorption, uses silica gel or activated bauxite which, through capillary action, reduces the vapor pressure on its surface so that the water vapor in its vicinity, being supersaturated, condenses.