Removing Moisture with Desiccants
As air is compressed, the dewpoint, or temperature at which water condenses, is raised. Therefore, to get dry air, you need to find a way to cool the compressed air. Costs, though, can be prohibitive because of the equipment, space and auxiliary equipment necessary for the process. However, if compressed air already is being used in the primary operation and only small amounts of dry air are needed for humidity control, compression may be a feasible route to dry air.
Air also may be dried through cooling. Lowering air temperature decreases the air's ability to hold moisture; therefore, air can be made drier by cooling it. However, cooling air just to dry it usually is not practical. An exception might be when cool air is needed anyway, when that cool air's dryness satisfies the needed moisture conditions, or when enough conditioned air is available. Normally, this method is reserved for applications in which outdoor air is being dried to levels only slightly lower than the incoming ambient.
To remove large amounts of water with cooling, the air must be over-cooled and then reheated. This approach can cause problems with operation and maintenance as well as cycle and control. Therefore, the method generally is unsuitable for producing large quantities of dry air. Another limitation to this technique is the freezing point of water. When air is dried via refrigeration, the cooling surfaces of the coils may reach subfreezing temperatures. This causes ice to form, which reduces the efficiency of the cooling system, and may require anti-icing devices or dual systems and defrost cycles.
To prevent icing, a brine spray commonly is used, which requires additional equipment, maintenance and operating costs. Although this strategy is workable and often satisfactory, the complexities associated with cycling and controlling may be detracting factors.
The specific dehumidification capabilities of cooling systems become more complex as the temperature is lowered in order to get more moisture removal. Therefore, cooling systems frequently give way to desiccant dehumidification in applications in which it is necessary to dry air below 40oF (4.44oC) dewpoint.
Adsorbent MaterialsUsually, the most simple, straightforward way to obtain dry air is to use desiccants, which are absorbents or materials that have a natural affinity for water. So an airstream can pass through a desiccant and become significantly drier without elaborate cooling, compression, cooling water, or other complex systems or controls. Once the drying task is complete, the desiccant is regenerated via heat, after which it is ready to dry more air.
A dehumidifier utilizes a relatively small amount of desiccant at any given time, constantly regenerating it as part of a continuous cycle. This simple device is manufactured in many designs and sizes, from very small to very large, to meet various dry air requirements. A desiccant dehumidifier can function equally well at extremely low to very high levels of humidity without regeneration problems or changes in cycle control. The following are typical applications for desiccant dehumidifiers:
Dehumidification, no matter the technique used, plays an essential part in the efficient operation of various industries, ensuring product quality. PCE