Many industrial applications require tight moisture control, but high drying temperatures would ruin the product. Desiccant dehumidification solves the problem. Here's how the technique works.

The dictionary defines dehumidification as the process of removing atmospheric moisture, and there are three ways to accomplish the task: by increasing the total pressure or compressing the air, by cooling the air to condense the water vapor, and by introducing a desiccant material into the airstream. Each of these techniques has advantages and disadvantages, and each is based on the properties and behavior of moist air. Because the amount of water vapor air can hold is a function of the temperature and pressure on that air, I'll look at ways to remove moisture by changing the temperature or pressure.

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 40^oF (4.44^oC) dewpoint.

Adsorbent Materials

Usually, 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:

Product Drying. In manufacturing processes for product such as gelatin capsules, candy, sausages and investment casting, elevated temperatures during the product's drying process will cause deleterious quality problems. Desiccant dehumidifiers can remove moisture at lower temperatures, resulting in uniform quality and efficient operation.

Corrosion Prevention. Corrosion can be defined as a change from one form to another through chemical reaction. In many cases, the presence of moisture in the air accelerates the reaction. Desiccant dehumidification has been used effectively in electronics manufacturing, chemical plants, pipe galleries, and storage of idle equipment at industrial sites.

Moisture-Regain Prevention. Hygro-scopic materials have an affinity for water and are sensitive to high relative humidity. Moisture regain by some of these materials causes them to become sticky and restricts free flow, causing product problems. Desiccant dehumidifiers are used to control the moisture content of the air in the manufacturing process to ensure reliable operation and product quality.

Condensation Prevention. When cold surfaces are surrounded by moist air, water vapor will condense on the surface if the temperature of the surface is lower than the dewpoint of the surrounding air. Desiccant dehumidifiers are used to blanket the surface with dry air, resulting in condensation prevention and economic benefits to the end user.

Mold and Fungus Prevention. Mold and fungus can lay dormant for years without moisture, remaining harmless. But when moisture is added, fungi begin to grow. Desiccant dehumidifiers can control the moisture content of the air that surrounds the material or object to be protected, keeping the microorganisms dormant.

Plastic-Resin Drying. Many plastic resins are hygroscopic. If moisture is not properly removed, it will boil off when heated during the molding or extrusion process. Desiccant dehumidifiers dry plastic resins to very low moisture levels prior to the molding process.

Dehumidification, no matter the technique used, plays an essential part in the efficient operation of various industries, ensuring product quality. PCE

Links

• Bry-Air Inc.