Two Wheel Desiccant Dehumidification Systems   The schematic above shows a Two-Wheel Desiccant Dehumidification System (TWDDS). A typical TWDDS consists of a desiccant wheel, heat exchanger wheel, supply fan, exhaust fan, a heat source for regenerating the desiccant and an optional cooling coil. The TWDDS is divided into two sides, the process side (blue) where air is dehumidified and cooled, and the regeneration side (orange) where the desiccant wheel is regenerated. Both the desiccant wheel and the heat exchanger wheel are constantly rotating so that half of each wheel is always in the process side and half is in the regeneration side of the TWDDS. As it rotates, the desiccant wheel continuously picks up moisture in the process side and releases that moisture in the regeneration side. Similarly, the heat exchanger wheel continuously picks up heat on the process side and releases that heat in the regeneration side. The desiccant is a finely divided material, usually silica gel, titanium silicates, or some type of zeolite. The desiccant material is impregnated into a fibrous support structure that has been rolled into the shape of a wheel or into a wheel-shaped rotor with a lightweight structural honeycomb core of man-made, fire-retardant material. The heat exchanger wheel resembles the desiccant wheel in appearance and design. 1. Dehumidifying the Process Air The process airstream is made up of either outside air, return air from the conditioned space or a mixture of both, depending on the specific application. As the process air passes through the desiccant wheel, the desiccant adsorbs the moisture it holds. As the moisture is adsorbed it condenses and gives off heat. The heat remains in the process air stream, increasing the air stream's temperature. The process air is now dry and hot. It must be cooled before it can be supplied to the conditioned space. 2. Cooling The Dehumidified Air In a system with a supplemental cooling coil, as shown, cooling the dehumidified air takes place in two stages. First the dehumidified air passes through the heat exchanger wheel where much of the heat is removed. After passing through the heat exchanger wheel, the dehumidified air passes through the cooling coil to further lower the air temperature, making it cool enough to be introduced into the conditioned space. 3. Regenerating the Desiccant The saturated desiccant must be dried (regenerated) before it can pick up more moisture. The desiccant is regenerated by the regeneration air stream. The regeneration air stream is made up of outdoor air, return air or a mixture of both and it must be at 180º F or higher to regenerate the desiccant. The regeneration air stream is heated in two stages. First, the regeneration air stream passes through the heat exchanger wheel where it picks up the heat that the heat exchanger wheel recovered from the dehumidified air. Next the regeneration air is further heated by a direct-fired natural gas burner or other heat source. A TWDDS can be configured in several modes for different applications. In Recirculation Mode the TWDDS is used to dehumidify and recirculate conditioned inside air. In Ventilation Mode the TWDDS can be used to introduce cool, dry outdoor air into a building space. In Makeup Air Mode it supplies cool, dry air to replace air that is exhausted from a process application. back to "Desiccant Dehumidification Systems" article back to PilotLight online