A vacuum dryer of fundamentally new design is said to make operation much simpler and more reliable than previous vacuum systems while still providing substantial reductions in drying time and energy consumption relative to desiccant dryers. The new VBD vacuum dryer from Maguire Products, Inc., Aston, Pa., uses vacuum to pull moisture out of resin pellets. However, it differs from previous models through the use of gravity to move material vertically from one drying stage to another, with discharge of material between stages controlled by high-speed slide-gate valves, widely employed on other Maguire equipment.
This eliminates most moving parts, particularly the three-station indexing carousel mechanism of the Maguire LPD vacuum dryer introduced in 2000. The redesign also does away with sealing gaskets, which can become worn and compromised by resin dust, and with perforated screens that require cleaning. The VBD dryer uses ambient air that is circulated by a blower with energy-saving variable-speed drive. It also uses the same controller hardware and software as the LPD dryer.
The first model of the new generation is the VBD-1000, with throughput capacity up to 1000 lb/hr. It is 16 ft tall and has a footprint of only 6.8 x 3.7 ft. It can be mounted on the shop floor or on a mezzanine. Cost is around $60,000. Smaller models starting at 100 lb/hr will be introduced in coming months.
From top to bottom, the three main parts of the VBD unit are:
•Heating hopper: A 10-hp centrifugal blower and 40-kW heater bring material up to setpoint temperature as high as 350 F. The volume of this stainless-steel hopper is three times that of the next (vacuum) vessel, ensuring adequate heating time.
•Vacuum vessel: Vacuum applied by a venturi generator reduces the boiling point of water, causing moisture in the pellets to volatilize and escape the pellet into the low-pressure environment. Moisture is purged from the vessel with -40 dewpoint air generated by a small membrane air dryer mounted on the unit.
•Retention/take-away hopper: The membrane air dryer also continually purges this pressurized vessel, ensuring that material remains dry until discharged.
VBD dryers are sold both by Maguire and its affiliated company, Novatec Inc., Baltimore.