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MuntersTechnical Information associated wtih Dehumidification Systems OperationMunters desiccant dehumidifiers attract moisture from the air by creating an area of low vapour pressure at the surface of the desiccant. The pressure exerted by the water in the air is higher, so the water molecules move from the air to the desiccant and the air is dehumidified. Munters desiccant dehumidifiers make use of changing vapour pressures to dry air continuously in a repeating cycle described by the simple diagram Fig 1. As the desiccant picks up moisture from the surrounding air, dry air is discharged to the process area and the reactivation air stream discharges the wet air outside. One subtle distinction between desiccants is their reaction to moisture. Silica Gel desiccant wheels simply collect it like a sponge collects water - the water is held on the surface of the material and in the narrow passages. Another desiccant is lithium chloride which undergoes a chemical change as it collects moisture. Whether the desiccant functions by absorption or adsorption is not usually important to a system designer, but the distinction exists and engineers should be aware of the difference between the two terms. When moisture is removed from air, the reaction liberates heat. In a desiccant dehumidification system, the heat is transferred to the air and to the desiccant, so the process air generally leaves the dehumidifier warmer than when it entered the desiccant unit. The temperature rise is directly proportional to the amount of moisture removed from the air - the drier the air leaves the dehumidifier, the warmer it will be. In any applications - notably product drying and unheated storage - this temperature rise of the dry air is cooled before being delivered to the point of use. Dehumidification by Desiccant WheelThe desiccant wheel is ideally suited to low temperature dehumidification (-40ºC to +40ºC) and for exceptional low dewpoint conditions (down to -70ºC). At the heart of the unique Munters Dry Air method
is an absorbent wheel rotating about 8 times an hour relative to a
segmented housing. In the smaller segment, or ‘reactivation zone’, heated air passes through the wheel absorbing the moisture and allowing it to be exhausted to the exterior. This operating principle provides continuous
dehumidification of the process air and the heat required to effect
this drying process, since it is strictly localised, needs an
extremely small energy input. Where exhausted or moist air to the exterior is difficult - say from basement levels - the dehumidifier can be fitted with an air-cooled condenser. The wheel technology has established itself in a multitude of applications covering the processing of materials, product drying, storage and preservation. PreservationAll materials can corrode, which is to say every substance can change from one form to another through chemical reaction. Many of these reactions, especially those which depend on oxygen, are catalyzed and accelerated by moisture. Ferrous metals like iron and steel are well known for their corrosion Fig 2. (Corrosion Curve) in the presence of moisture. In the past, thousands of desiccant dehumidifiers have been used to surround machinery and equipment with dry air, preserving ferrous metal parts from heavy rust. Currently, dehumidifiers are working to protect materials from subtle and expensive forms of corrosion. Modern society depends more and more on light equipment like computers, telecommunications gear, lightweight composite materials and high energy batteries. While these are less subject to gross rusting, they are very sensitive to microscopic level corrosion. These circuits simply do not have much material to begin with, so small amounts of corrosion create disproportionately large problems. ProcessingVirtually every substance has some affinity for moisture. Even plastic resins like nylon can collect six to ten percent of their dry weight in water vapour. In many cases, this presents no problem. In others, moisture regain can affect critical dimensions just like thermal expansion, or make products that would otherwise flow freely stick together. Moisture regain on humid days clogs packaging machinery, silos and conveying systems and sticky products have major economic consequences. Hygroscopic products are sensitive to high relative humidities rather than absolute humidities, and relative humidity can be high at any time of year. In fact it is often higher in winter than summer. When products are stored at cool temperatures, problems can be especially acute. Desiccant dehumidifiers are effective in controlling humidity at low temperatures, and have been widely applied to prevent moisture regain. Product DryingMost products are dried using hot air to vaporise
moisture and carry it away. Often however, hot air is either too
slow or results in damage to the product. When there is a benefit to
drying at temperatures below 46oC, there is generally a benefit to
using air which has been dehumidified rather than just heated. The
cooler the temperature, the more the economics favour dehumidifiers,
for example, in a fluidized-bed drier. Because of the temperatures
involved, the drying capacity of the fluid bed is doubled when the
air dew-point drops from 18oC to - 4oC. This means the size of the
fluid bed can be cut in half.
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Munters Ltd
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