Drying takes place when heat flows from outer cone through a jacket, and vacuum is applied in the inner cone. The drying operation is based on the principle that water or other volatile product moves from a zone of low pressure. This is accomplished by warming the product to be dried to raise the vapour pressure of the zone of low pressure. This is accomplished by warming the product to be dried to raise the vapour pressure of the absorbed or free liquid, by vacuum, or by sweeping it away with an air current. Thus the essential conditions for effective drying are efficient and uniform heat transfer throughout the batch and rapid removal of the vapour.