Vacuum Dryers do not utilize desiccant and there is no dew point reference for the process air. Instead, they use a very strong vacuum to draw moisture away from warm pellets.
Vacuum dryers use a multi-stage process. In the small dryers (up to about 100 lb./hr.) Plastic pellets are contained in a stainless steel sealed vessel that rotates between three stations, staying in each position for about 20 minutes. At the first station, the vessel is filled with pellets and they are heated to allow the molecular release of moisture. In the second station, they are repeatedly subjected to a strong vacuum, to draw away the moisture.
The dried pellets are discharged at the third station. This simplified process affords users fast drying with less energy consumption than most alternatives. The process is, by nature, a batch process so the material is loaded, processed, then dumped, and the cycle repeats.
There are larger Vacuum Dryers with drying throughput capacities of up to 200 lb./hr. and up to 1000 lb./hr. These models use a vertical configuration where the resin is loaded into an insulated hopper (at the top of the unit) where it is heated. Then the resin drops into a vacuum vessel where the moisture is removed. After the resin is dry, it drops int a retention hopper and transferred to the process machine. The whole process takes less than an hour and uses about 1/6 the energy of conventional doal bed desiccant dryers.