16-05-2013, 12:42 PM
INTRODUCTION TO DESSICANT COOLING TECHNOLOGY
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DESSICANT
A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container. Commonly encountered pre-packaged desiccants are solids, and work through absorption or adsorption of water, or a combination of the two. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules.
A desiccant is a substance with very hygroscopic properties, meaning it will soak up water vapor from the air surrounding it. A number of different substances are capable of doing this, but only a relative few of them are of practical use and fewer still are going to be readily available to the average person. Before elaborating on the different types that might be useful for our purposes it's necessary to explain how to choose a desiccant.
Desiccant cooling technology provides a tool for controlling humidity (moisture)
levels for conditioned air spaces. Desiccant systems work in conjunction with conventional air conditioning systems to dehumidify the air. Desiccant materials are
those that attract moisture due to differences in vapor pressure. Most people are familiar with desiccants such as silica gel packages that are included with new electronics or textile products. Desiccants can be in the form of a solid or a liquid. People have identified types of desiccants that are appropriate as a component of commercial heating, ventilation and air conditioning (HVAC) systems. These desiccants have been selected based on their ability to hold large quantities of water, their ability to be reactivated, and cost.
In order to be effective, the desiccant must be capable of addressing the latent cooling load in a continuous process. In order to accomplish this, commercial desiccant systems consist of a process air path and a reactivation air path.
The desiccant that is in the process air path has been prepared to have a lower vapor pressure than the air passing over it. Thus, the moisture in the air is transferred onto the desiccant material.
As the desiccant vapor pressure increases due to the presence of the moisture
that it has attracted, the desiccant material is transferred to a reactivation process. In the reactivation process, hot air is passed over the desiccant. The vapor pressure of the hot air is lower than the desiccant surface which forces the moisture to transfer from the desiccant surface into the hot air stream. The moist hot air is then exhausted from the system into the outdoor air. The desiccant material that has had the trapped moisture removed is now prepared to attract moisture as it is transferred back into the process air path. The dry process air leaving the desiccant is then passed over a conventional cooling coil which addresses the sensible cooling work required to meet the air specification of the conditioned space.