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exposed to air at the correct drying temperature and dewpoint while in the drying hopper. Figure 4 is a moisture-loss analysis of a PET drying system. Small samples of the PET being dried were taken from the hopper every hour during static drying and the residual moisture was analyzed. The drying parameters are described on the chart attached to the graph. Note the dramatic drop in residual moisture during the frst hour of drying, from 0.1823% to 0.04%, after which the rate of decline slows signifcantly, requiring another 4 hr to drop below the desired moisture level of 0.005% AIRFLOW: Measured in cubic feet per minute (CFM), the airfow of a dehumidifying dryer is the fourth fundamental drying param- eter. It is absolutely crucial to the drying process. The airfow of a dehumidifying dryer is the medium that transfers the heated, low-dewpoint drying air from the dryer to the PET pellets in the drying hopper. Insufcient airfow will not transfer enough heat from the dryer's heat source to the PET pellets in the drying hopper. With insufcient airfow through the drying hopper, the system will not be able to maintain the desired vertical tempera- ture profle within the hopper to provide the required drying time. Excess airfow will transfer an excessive amount of heat from the dryer's heaters to the bed of material in the drying hopper. The excess heat that is not absorbed by the PET pellets will exit the drying hopper, resulting in high return- air temperatures, poor desiccant performance, wasted heat energy, high dryer operating costs, and the possibility of overheating and damaging the dryer's process-air blower. The volume of airfow needed to adequately dry PET will vary from one drying system to the next, depending upon several factors, including the incoming pellet temper- ature, drying hopper design, heat losses within the drying system, and hourly throughput (lb/hr) through the hopper. Determining optimum airfow is therefore a complex question that will require a separate article of its own. But a simple answer to the question of airfow volume is this: You will need enough airfow through the drying hopper to maintain the PET granules at the desired drying temperature for the desired amount of drying time. Figure 5 is a visual representation of low airfow, exces- sive airfow, and optimum airfow within a drying hopper. The drying hopper with low airfow will not provide enough drying time at the desired drying temperature. The It's important to know that the time pellets spend in the drying hopper is not neces- sarily the same as effective drying time. The importance of proper airfow: Insuffcient airfow (left) will not maintain suffcient heat to dry the resin. Excessive airfow (center) will deliver more heat than the PET pellets can absorb, resulting in high return-air temperatures, poor desiccant performance, and wasted energy and expense. Optimal airfow (right) develops a vertical temperature profle that provides the desired amount of drying time while using the fresh, cool incoming resin as a heat sink to cool the return air leaving the drying hopper. FIG 5 Low Airfow Excessive Airfow Optimum Airfow material. Then, the drying air is heated to the desired drying temperature. Heating the dehumidifed air reduces its relative humidity to an extremely low level and further increases the drying air's moisture-absorption capacity. When PET pellets are surrounded with low-dewpoint air, which has been heated to 300 F or higher, the vapor-pressure diferential between the surface of the pellets and the drying air is greatly increased. Once the polymer has been heated and the attraction between the water molecules and the polymer chains has been sufciently reduced, the freed water molecules will begin to migrate toward the area of low vapor pressure at the surface of the pellets, as illus- trated in Fig. 3. When the water molecules reach the surface of the pellets they will be quickly swept away by the drying air. DRYING TIME: PET pellets do not dry instantaneously; the drying time, which is the third fundamental drying parameter, is critical to the drying process. When the PET pellets in the drying hopper have been surrounded with a steady stream of heated, low-dew- point air, sufcient time must be provided to allow the heat from the surrounding air to migrate to the interior of the pellets. PET pellets, or any type of plastics pellets, do not conduct heat very well. It therefore takes time for the heat from the surrounding air to be absorbed and distributed throughout the pellet's interior. It is important to note that the time the PET pellets spend in the drying hopper is not necessarily efective drying time. The efective drying time is the amount of time the pellets are 52 DECEMBER 2014 Plastics Technology PTonline.com T ips & Technique s