Conventional air conditioning units use cooling coil both for temperature and humidity controls. When this coil lowers temperature of air to bring it to comfortable conditions, moisture in air condenses. This maintains humidity level close to fifty percent, which is a comfortable condition. This is true only for specific range of ambient conditions and internal loads. When outside humidity level is high, ventilation air which is treated in air conditioning units is laden with more moisture. In other conditions, internal materials or activities increase latent loads.
When latent loads are greater then thirty percent, comfortable conditions with dual purpose cooling coil alone are difficult to achieve. For this purpose temperature off coil is reduced to get extra dehumidification. In this case humidity controller overrides temperature controller. But supplying this low temperature air to space creates uncomfortably low temperatures in the space.
There are different techniques to reheat air before supplying to space in order to bring temperature back to comfortable range. All these techniques use additional energy. Air is overcooled and then reheated. This is highly inefficient operation. This reheat can be achieved either by putting heating coil or electric heating element downstream. Hot gas bypass can be used in case of direct expansion units. Two coil systems are also available, one downstream and one upstream of cooling coil, connected with heat pipe system. All these technologies are energy intensive type.
A very energy efficient solution to get low humidity level is to use desiccant wheel. High temperature return air from space is passed to one side of coil. Dehumidified supply air out of coil passes over other half of wheel. Supply air is dehumidified further by return air with this technology. Cooling coil performance enhances as it takes humidity loads of supply air. Initial cost of this system increases due to additional components, but energy savings are achieved in operation. Additional cost is recoverable in one year.
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