A source of microwave energy is introduced into a pressure vessel in which a vacuum dryer is drawn
. Powdered materials to be dried are introduced and removed from the vessel by screw conveyors which are long enough so that in combination with the material, they prevent the loss of the vacuum dryer in the vessel. The materials are transported through the vessel as the moisture is removed therefrom at essentially ambient temperatures. A water vapor condenser is also included within the pressure vessel to reduce the vacuum pumping requirements thereof.
The vacuum dryer as defined in claim 16 wherein said belt has a conveyor surface with opposite sides, said vessel including a microwave chamber into which said microwaves are introduced and through which the material is transported by said belt,said chamber being defined by said adjustable baffle wall, a similar back wall adjacent said outlet conveyor means and a pair of side shields on the opposite sides of said belt.
Another object is to provide means for reducing the energy requirements to maintain a desired vacuum dryer in a microwave vacuum drying apparatus.
The outlet of the inlet screw conveyor dumps the material into a bin fromwhich it is metered onto a moving conveyor belt which passes the material into a chamber where the water in the material is heated to its vapor state by the microwave energy. For lossy or thick materials, means may be provided along the belt to stir thematerial as it is subjected to the microwave energy and the vacuum dryer simultaneously. The speed of the belt is controlled so that the material is at its desired moisture content when its passes out of the microwave chamber and into a second bin where it isconveyed out of the pressure vessel by an outlet screw conveyor similar to the inlet screw conveyor. Since the process is conducted at reduced pressure, the temperature necessary to vaporize the moisture in the material need not be higher than theambient temperature thereby greatly reducing heat loss and enhancing efficiency of the overall apparatus. In addition, the present dryer does not needlessly pollute the air in the vicinity of the dryer since no blown air is circulated.