Published 12/13/2010

Dryers Use Less Energy to Dry PET

Novatec Inc., Baltimore (novatec.com), is introducing two patented dryer control technologies intended to cut energy costs for processors of PET.

Jim Callari

Editorial Director, Plastics Technology

ADAPTIVE CONTROL OF HEATING

“If the air-flow rate and temperature rise are minimized, the process heating energy can be reduced to little more than what’s necessary to heat the resin,” Haynie states. “One technology, which we call adaptive control, minimizes the air required so that the minimum energy is used for process heating. This means the temperature of the resin is raised to the appropriate temperature for drying, but all of the energy remains in the resin, in the drying hopper, and a minimum of the generated heat is returned to the dryer.”

The system achieves this by measuring the temperatures of the incoming resin and of the return air leaving the drying hopper, and adjusting the air flow on the fly so that the return-air temperature (returning to the dryer from the drying hopper) is only slightly higher than the temperature of the resin entering the drying hopper. This is controlled by varying the speed of the blower with a variable-frequency drive (VFD) that changes the air-flow rate. By minimizing the air flow while maintaining the temperature of the resin, the process heat is maintained at the lowest possible level.

The temperature of the return air from the hopper typically rises if the material throughput rate is less than the design rate for the dryer, or if the resin moisture is lowered or the resin inlet temperature increases, Haynie explains. By measuring the return-air temperature and holding it at the incoming resin temperature, adaptive control ensures that the energy used can raise the resin temperature to the appropriate level for processing, vaporize all moisture retained in the resin and adjust for variations in resin temperature, he adds.

Haynie says recent tests revealed that IntelliPET can cut power costs up to 30% vs. conventional technologies; savings can exceed 50% when IntelliPET is teamed with a high-efficiency gas heater.

REGENERATION OPTIMIZATION

Novatec refers to its second approach to reduce dryer energy consumption as regeneration optimization. The idea here, Haynie says, is to minimize the power used in regenerating the desiccant, which can account for as much as 35% of a PET dryer’s total power usage. The system has two parts: First, the rotational speed of the desiccant wheel is minimized, reducing the amount desiccant media to be heated on a lb/min basis. Wheel speed is controlled by a VFD to no more than is required to adsorb the moisture from the return air. By controlling the wheel speed, the desiccant media is loaded to its maximum while maintaining a consistent dewpoint below -40 F.

The second part of the feature involves the use of a VFD on the regeneration air blower. The VFD minimizes the air flow to the point where water is desorbed from the desiccant, but only minimal heat leaves the wheel during the regeneration process. As the ambient air is heated and passes through the wheel, the discharge temperature is constantly monitored and the VFD adjusts the air flow to ensure that minimal air flow is used to remove all of the moisture gained in drying but no excess air is used or heat removed.

“The temperature of the air exiting the top of the wheel is just enough to remove the moisture and carry it away but is kept to a constant temperature,” says Haynie. “This ensures that the wheel will remove all of the moisture from the resin regardless of its moisture level and will automatically adjust as the moisture changes due to seasonal variations or changes in virgin/flake ratios.” Haynie says the technology cuts regeneration energy by as much as 30%.