Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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temperatures, and it is enhanced when operating at relatively low temperatures (Fig. 7). The optimum return-air temperature is in the 100 F to 130 F range, where the dryer's desiccant is capable of adsorbing approxi- mately 15% of its own weight in water. RETURN-AIR FILTER (Fig. 6): A dehumidifying dryer's desiccant is sensitive to contamination. The purpose of the return-air flter is to protect the dryer's desiccant. If PET fnes are allowed to bypass the dryer's return-air flters and enter dryer's desiccant, the fnes will build up and restrict air fow through the desiccant. Depending on the regenera- tion temperature, the fnes may begin to melt or release volatiles and perma- nently damage the desiccant. Therefore it is critical that any fnes present in the return air be fltered out before the air reaches the desiccant. The dryer's return-air filter is the second filter in a PET dryer's return-air circuit, the first being the dust collector. As a general statement, the return-air filters are the most service- intensive components of a PET dryer. Depending on the amount of fines in the material being dried, the return-air filter may require cleaning or replacement as frequently as every shift or as little as every several months. Maintaining the dryer's unrestricted airfow is critical to its performance. The only prudent way to determine a reason- able schedule for flter maintenance is to monitor the condition of the return- air flter on a daily basis over a period of time. Then develop a flter maintenance schedule that fts your application. AIR BLOWER (fg. 6): The air blower moves the drying air through the drying-air circuit. The dryer's blower must move a large enough volume of air through the drying-air circuit to over- come the heat losses in the dryer, air- delivery tubing, and the sidewalls of the drying hopper. The dryer's blower must also move a large enough volume of air to heat the incoming PET material from its initial temperature (usually ambient) to the desired drying temperature of 300 F to 350 F. The blower must also be able develop enough pressure to force the DUST COLLECTOR (FIG. 6): The Dust Collector is critical to long-term performance of the heat exchanger. Incoming con- tainers of virgin PET pellets will commonly contain fines, and reground scrap PET often contains large quantities of fines. If these fines are allowed to enter the return-air heat exchanger, many of the fines will adhere to the cooling coils of the heat exchanger. Over a period of time the fines will build up and insulate the cooling coils from the air passing through the heat exchanger, reducing the device's efficiency. Therefore, it is a good decision to include a dust collector in the system to keep the cooling coils clean and free of fines. RETURN AIR HEAT EXCHANGER (Fig. 6): Typical PET drying tempera- tures range from 300 to 350 F. When the drying air exits the top of the drying hopper, it has already given up a substantial amount of its heat content to the PET pellets. The air temperature exiting the top of a typical PET drying hopper will normally be in the range of 200 to 250 F. Even at that reduced temperature, the air is still too hot to return it directly to the dehumidifying dryer. Dehumidifying dryers are sensitive to high return-air temperatures. The reason is the desiccant's moisture-adsorbing performance is greatly compromised when operating at high Molecular-sieve desiccants adsorb moisture most effciently at 100 F to 130 F. Return air leaving the PET dryer at 200 F to 250 F must be cooled before returning to the dryer. To regenerate the desiccant, regeneration heaters heat the air to a higher temperature to drive off the adsorbed moisture. The desiccant is the heart of the dehumidifying dryer. It must be protected against fnes, other contaminants, and excessive drying-air temperatures. % Moisture Loading 100°F 200°F 300°F 400°F 500°F Desiccant Temperature Regeneration Temperature 20 18 16 14 12 10 8 6 4 2 0 FIG 7 Dessicant Moisture Absorption Chart Absorption Temperature 54 DECEMBER 2014 Plastics Technology PTonline.com T ips & Technique s

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