What are the different types of indirect dryers and how do they operate?

1. Conical (Double Cone) Tumble Dryer

Maybe the simplest and probably the most cost-effective dryer is the conical tumble dryer. It gets its name from its basic shape (though several variations are available). The dryer is mounted on a horizontal axis, enabling a full 360 degree rotation during operation to provide good mixing and regular contact of the product with the heated wall. The machine is moved to a vertical position after drying is achieved to discharge solids from a nozzle at the bottom of the cone. Materials of construction include glass-lined steel, which provide excellent corrosion resistance.

2. Paddle Dryer

Operating in the horizontal plane, the paddle dryer features a cylindrical vessel equipped with a multi-paddled agitator supported at both ends. In most designs the heating fluid that circulates through the vessel wall also flows through the agitator. The movement of the agitator in combination with the heating element makes this machine ideally suited for efficient mixing and heat transfer. Furthermore, the agitator’s support structure allows for the handling of high-shear or lumpy solids. Product discharge is achieved through an outlet on the bottom of the cylindrical shell, but due to the shape total discharge is not possible.

3. Disc Dryer

Known to be extremely versatile, the disc dryer can be used in batch or continuous mode and can process a variety of solids including wet cake or slurry. The equipment consists of a vessel shell that encompasses a vertically or horizontally-mounted rotor. A series of discs are set at an angle and attached to the tube-shaped rotor. Heating fluid flows through not only the vessel wall but the rotor and discs as well, creating a high heat transfer area (additional heat via hot-gas blow through is also capable in direct conduction mode). The complexity of this design, however, hinders its cleanability so it is not typically used for high-value solids.

4. Pan Dryer

The pan dryer is a round, pressure-rated vessel that features either a flat or conical-shaped bottom. Like some of the previous dryer types mentioned, the pan dryer’s agitator can be heated to maximize heat surface area (in addition to the vessel walls, base, and even top head). The agitator can be installed from the top or bottom of the vessel, the top-mounted arrangement usually being preferred as it eliminates dead spots where the product can build up and also allows for translational (up and down) movement to help with the wear and tear. The benefit of the bottom-driven agitator is the amount of torque that can be applied due to the proximity of the agitator blades do the gearbox. The discharge of product from the pan dryer is facilitated by the agitator; when the large nozzle on the side wall is opened, the agitator arm works to push the dried solids through the opening as it rotates. This method of discharge leaves a small amount of “heel” in the vessel since metal-to-metal contact between the agitator blades and dryer base must be avoided. To resolve this issue, three options are available: the heel may simply become part of the next batch, it can be manually removed, or the remaining solids can be physically washed away.

5. Filter/Dryer

Identical in appearance to the pan dryer, the filter/dryer is designed with a filter plate on the bottom that contains a filtrate chamber, allowing for filtration and washing to be performed in addition to drying and discharge. This arrangement is beneficial to processes that involve wet cake as it eliminates the hassle of transferring wet cake into the dryer (wet cake is not pumpable!). Conversely, the filter/dryer’s multipurpose design slightly reduces heat-transfer area with the replacement of the solid base with a filter plate.

6. Conical Screw Dryer

The conical screw dryer gets its name from the tapered shape of the vessel and top-driven screw-type agitator. Positioned close to the heated vessel wall via the inverted “L” shaped arm, the agitator acts as a workhorse to provide superior mixing and movement of the product across the vessel walls. Discharge of solids by means of the bottom central outlet is made easy due to the vessel’s conical walls and vertical orientation. Like the filter/dryer, the versatile conical screw dryer is capable of more than just drying. Powder-blending, crystallization, extraction, and alkalization and other reactions can also be accomplished. While the ranges of operations that can be performed make this dryer a favorite in many industries, the screw-type agitator poses clean-in-place and size degradation issues so it is not a design that is recommended for pharmaceutical applications or processes involving crystals.

7. Spherical Dryer

The spherical dryer is constructed from a round vessel fitted with an agitator that can be mounted from the top (to eliminate dead space/product build-up) or bottom (makes for greater torque). Additionally, the bottom-driven unit features a uniquely designed agitator that imitates the curve of the heated wall surface to expedite mixing and drying of the solids. Almost complete solids discharge can be attained through the outlet at the bottom of the vessel, which utilizes a metal-to-metal-sealed full port ball valve. The geometry of the vessel allows for easy drainage of cleaning fluid through the bottom outlet valve, making this dryer ideal for clean-in-place pharmaceutical applications.