It is Ideal in operating Design. The Slopping Walls of the cones permit more rapid emptying of solids when the dryer is in a stationary position. The dryer required continuous rotation during emptying to convey product to the discharge nozzles. As a result, a circular dust hood is frequently necessary to enclose the discharge nozzle turning circle and prevent serious dust losses to the atmosphere during unloading. Several new designs of the double cone type employ internal additional heating surface.
On all Rotating Dryers, The Vapour-Outlet tube is stationary. It enters the shell through a rotating gland and is fitted with an elbow and upward extension so that the vapour inlet, usually protected by a felt dust filter, will be at all times near the top of the shell.
Vacuum is used in conjuction with drying or other chemical operations when low solids temperatures must be maintained because heat will cause damage to the product or change its nature, where air combines with the product when it is heated causing oxidation or an explosive condition, where solvent recovery is required and when materials must be dryed to extremely low moisture levels.
In vacuum processing and drying the objective is to create a large temperature driving force between the jacket and the product to accomplish this purpose at fairly low jacket temperatures, it is necessary to reduce the internal process pressure so that the liquid being removed will boil at a lower vapour pressure. It is not always economical however to reduce the internal pressure to extremely low levels because of the large vapour volumes thereby created it is necessary to compromise on operating pressure considering leakage problems and the size of the vapour lines and pumping system very few vacuum dryers operate below 5 MM Hg. Pressure on a commercial scale. Air in leakage through gasket surfaces will be in the range of 0.15 lb (hr.) [Lin ft. gasketed surfaces] under this condition.