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QUESTIONS ABOUT DRYING? Visit the Drying Zone and short.ptonline.com/drytips for more advice. Nearly all dehumidifying resin dryers today used a closed-loop drying-air circuit. The example shown here includes a desiccant wheel, but the principle is the same for dryers with multiple desiccant beds. Desiccant Wheel Dryer Air Flow Circuits Closed-Loop Dry-Air Circuit Dust Collector Return Air Cooler Return Air Filter Desiccant Cooling Circuit Desiccant Wheel Regeneration Circuit FIG 6 Guidelines for PE T drying hopper with excessive airfow is heating all of the material within the hopper to the setpoint drying temperature and wasting energy by passing excessive heat back to the dryer. The drying hopper with optimum airfow is developing a vertical temperature profle that is providing the desired amount of drying time and using the fresh, cool incoming material as a heat sink to cool the return air leaving the drying hopper. OPERATING A DEHUMIDIFYING DRYER There are three air circuits in a well-designed dehumidifying dryer: • Closed-loop drying-air circuit; • Single-pass regeneration-air circuit; • Desiccant cooling circuit. CLOSED-LOOP DRY-AIR CIRCUIT: The drying air entering the drying hopper has been dehumidifed to a low dewpoint and heated to the selected drying temperature prior to entering the drying hopper (see Fig. 6). The hot, dry air enters the drying hopper and is dispersed throughout, enveloping each and every PET pellet within the hopper. As the hot, dry air travels upward through the hopper, the drying air gives up its heat to the PET pellets. As the wet incoming PET pellets begin to absorb the heat from the drying air, they will begin to give up their moisture to the hot, low-dewpoint drying air. When the drying air exits the top of the hopper, it has given up a substantial amount of its heat to the PET pellets, and in exchange the drying air has absorbed the moisture given up by the resin. Nearly all of the dehumidifying polymer dryers used in the plastics industry use a closed-loop drying-air circuit. The reason for this is that the moisture content of the air exiting the top of the drying hopper will always be drier than the ambient air. The dewpoint levels in the return-air circuit of a typical PET drying system are usually in the range of zero to -20 F, while ambient dewpoint levels usually run much higher. Therefore it makes economic sense to employ a closed-loop drying-air circuit to salvage and reuse the moderately dry return air exiting the drying hopper rather than using much wetter ambient air in a single-pass drying-air circuit. @plastechmag 53 Plastics Technology G u i d e l i n e s f o r P E T D R Y I N G P E T