Dehumidifying dryer

Note 1 An inexpensive pressure-differential switch, common option for almost every brand of dehumidifying dryer, will signal when a filter is restricting air flow. 

Time is critical to the drying process because plastic pellets do not dry instantaneously. Once the pellets are surrounded with heated, low-dewpoint air in a dehumidifying dryer, sufficient time must be provided to allow the heat from the surroimding air to migrate to the center of the pellets. As the temperature inside the pellet increases, the moisture diffuses and migrates toward the hot, dry air surrounding the surface of the pellet. When the water molecule reaches the pellet s surface, the hot, dry air will carry it away. 

A dehumidifying dryer is used to remove absorbed moisture from hygroscopic polymers. While a hot-air dryer heats the drying air to lower its relative humidity, a typical dehumidifying dryer first removes as much moisture as possible from the drying air by means of a desiccant, and then heats the air to obtain an extremely low relative humidity. 

Most dehumidifying dryers used in the plastics industry today employ a molecular sieve desiccant. A molecular sieve is a synthetic zeolite, which has a very strong attraction for water across a certain temperature range. When the temperature of the molecular sieve is maintained at a reasonably low level (under 150°F), such as when it is in the on-stream drying position, it will adsorb moisture readily. When the molecular sieve temperature is increased above 400°F, as is the case during regeneration, it will release its adsorbed moisture. Figure 7.39 shows how temperature dramatically affects desiccant s ability to hold moisture. 

A hot-air dryer uses a single-pass drying air circuit, exhausting the spent drying air and removed moisture to the atmosphere, while a dehumidifying dryer employs a closed-loop air circuit, returning the moisture-laden, spent air to the dehumidifier, where the moisture is removed. 

Figure 7.40 Carousel dehumidifying dryer airflow diagram.

In a dehumidifying dryer, unlike a hot-air dryer, airflow is a closed-loop process (see Fig. 7.40). Hot, dry air is delivered to the drying hopper where it picks up moisture from the resin before returning to the desiccant bed where the moisture is removed. Air is then reheated on its way back to the hopper for another trip through the system. 

Desiccant bed. The heart of a dehumidifying dryer, the desiccant, removes moisture from the drying air. Without the desiccant, the dehumidifying dryer would be no more than a hot-air dryer. 

These filters are the most maintenance- and service-intensive components of a dehumidifying dryer. They must be kept relatively clean so they do not restrict and reduce air volume, and they must be maintained in good condition since any ruptures will allow fines to pass into the dehumidifier, risking desiccant contamination. Depending on the amount of fines in the material being dried, the return air filter may require service as frequently as every shift or as little as every several months. The only prudent way to determine a reasonable filter service schedule is to monitor the filter condition on a daily basis for a period of time. If the amount of fines in a particular application is extreme, it may be advisable to add a secondary return air filter, such as a cyclone separator or free-standing bag house type filter. 

Many of today s dehumidifying dryers have dewpoint instruments integrated into the dryers control, but it is also a good idea to have a... 

An excellent alternative to machine-moimted drying, mobile systems (Fig. 7.47) are comprised of a dedicated dehumidifying dryer and hopper, mounted on a sturdy frame with casters for mobility. This system has... 

Modem desiccant, dehumidifying dryers offer excellent drying capabilities with many advanced featm-es to assist the molder in obtaining a consistent drying process. However, measuring the moistiure content will ensure the resin is in the 0.02 to 0.07% range. In addition the moistiure content should be measimed periodically to ensure that it has not drifted out of the 0.02 to 0.07% range. 

FIGURE 36.49 A solar-dehumidifier dryer. (Adapted from Chen, P.Y.S., Helmer, W.A., and Rosen, H.N., Experimental solar-dehumidifier kiln for drying lumber, Forest Prod. 32(9) 35 1, 1982.)... 

Mixed types include a solar-dehumidifier dryer with forced-air recirculation, as shown in Figure 36.49. 

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