4.2 Continuous Dryer

4.2.1 Rotary Dryer

The rotary drier is basically a cylinder, inclined slightly to the horizontal, which may be rotated, or the shell may be stationary, and an agitator inside may revolve slowly. In either case, the wet material is fed in at the upper end, and the rotation, or agitation, advances the material progressively to the lower end, where it is discharged. Figure (2.2) shows a direct heat rotary drier. Typical dimensions for a unit like this are 9 ft diameter and 45 ft length. In direct-heat revolving rotary driers, hot air or a mixture of flue gases and air travels through the cylinder. The feed rate, the speed of rotation or agitation, the volume of heated air or gases, and their temperature are so regulated that the solid is dried just before discharge.

Figure 2.2: Counter current direct heat rotary dryer

The shell fits loosely into a stationary housing at each end. The material is brought to a chute that runs through the housing; the latter also carries the exhaust pipe. The revolving shell runs on two circular tracks and is turned by a girth gear that meshes with a driven pinion. The inclination is one in sixteen for high capacities and one in thirty for low ones. As the shell revolves, the solid is carried upward one-fourth of the circumference; it then rolls back to a lower level, exposing fresh surfaces to the action of the heat as it does so. Simple rotary driers serve well enough when fuel is cheap. The efficiency is greatly improved by placing longitudinal plates 3 or 4 in. wide on the inside of the cylinder. These are called lifting flights. These carry part of the solid half-way around the circumference and drop it through the whole of a diameter in the central part of the cylinder where the air is hottest and least laden with moisture. By bending the edge of the lifter slightly inward, some of the material is delivered only in the third quarter of the circle, producing a nearly uniform fall of the material throughout the cross section of the cylinder. The heated air streams through a rain of particles. This is the most common form of revolving rotary cylinder. It has high capacity, is simple in operation, and is continuous.

Table 2.1: Rotary dryers practical ranges of dimension and operating parameters


Shell i.d. : D = 1 to 10 ft	Length, L = 4 D to 15 D
Radial flight height: D/12 to D/8; shell rpm: 4 to 5	Pripheral shell speed: 50 – 100 ft/min
The flight count per circle: 2.4D to 3 D
Inclination of the shell to the horizontal: up to 8cm/m	Avg. solid retention time: 5 min to 2h
Mass flow rate of the drying gas: 300 to 5000 lb/h.ft²	Drying capacity: 0.4 to 2.5 lb moisture/(h) (ft³ dryer volume)
Number of heat transfer units in the dryer (NT): 1.5 to 2	Solid hold up m(i.e. fraction of the shell volume occupied by the solid at any time): 5-15%

Courtesy: Principle of Mass Transfer and Separation Processes, B.K. Dutta, 2007.