The most common type of dehumidifier is the refrigerative or mechanical dehumidifier. In the former dehumidifier, the moist is removed by a refrigerated coil with a small fan in it. The mechanism of this dehumidifier is when temperature decreases, the vapour pressure also decreases which results in condensation of water in the air. The refrigerated coil is then used to reheat the air which is effective in higher temperature regions.
In regions where there is low temperature and low humidity level, a special type of dehumidifier is used namely “Desiccant Dehumidifier” where there is a desiccant material to reduce the humidity and it has got high affinity towards water vapour. This desiccant material is 抽濕系統 exposed to high humidity air stream to collect water vapour which in turn is exposed to low humidity air stream where the moisture retained is also collected. The stream used in the former is dehumidified and latter stream is used to regenerate the desiccant. These dehumidifiers consume heat energy to produce the effect of dehumidification. There are 2 kinds of desiccants namely absorbent and adsorbent. In the former, there is a chemical change when they retain water vapour. Adsorbent attract greater water molecules through the air pores present in it and there is no chemical change. The adsorbent material often used is the silica gel. This material can be reactivated by heat at a temperature of 160 – 170 oC and the heat required is 4800 KJ/Kg water removed.
The very small dehumidifier called “Electronic Dehumidifier” consists of low cost parts. There is a heat pump to produce a cold surrounding and there is no noise produced as there are no mechanical parts. Spray dehumidifier is used, when the temperature of water is lower than the dew point of the air flow. This spray dehumidifier resembles an air washer. When the moist in the air is saturated, the air is dehumidified when water is sprayed on the surface.
An example of a refrigerative dehumidifier is the “Air Conditioner”. They produce a cool surface. There is a refrigerated coil along with a fan which removes moisture. These thermally activated technologies employ heat energy to heat or cool the refrigerative dehumidifier. The energy from the thermal source can be in the form of heat. Thermally activated technology in dehumidifier is also used as a natural gas burner.
Dehumidifiers are used in a variety of applications. It is used in food & beverage industry as an absorption dehumidifier. This dehumidifier prevents the product from coagulating, corroding, breaking and also from bacterial growth. In baking industry, enrobing is done by these dehumidifiers to control humidity, product cooling is done for frost free cooling and when the product is stored, dehumidifier prevents from bacterial growth.