Indirect dryers employ similar principles of operation to direct dryers. However, the fundamental difference is that the method of heat transfer is predominantly by conduction as opposed to convection. These units are sometimes referred to as contact dryers.
Tube Furnace Dryers.Rotary tube furnace dryers transport the feed from the inlet to the discharge of the dryer by the rotation of the drum and the angle of inclination of the drum. The drum tube passes through a furnace that is heated either by electrical elements or gas burners. This energy heats the tube, which is in direct contact with the feed internally. Special lifters in the drum promote the exposure of the feed to the heated surfaces by a rolling motion of the material. The product is discharged through a suitable airlock device. The evolved moisture is exhausted from the system through an induced-draft fan.
Drum Dryers.Drum dryers consist of a relatively large-diameter machined drum that is heated internally. Feed is metered, using different techniques, onto the outer surface of the drum while the drum rotates. The feed dries as the unit rotates, and the product is scraped off the drum surface by means of a blade. The evolved moisture is exhausted from the system through an exhaust hood and an induced-draft fan. Machines may have a second drum (double-drum dryers) and/or may have the drum installed inside a vacuum chamber for vacuum drying operation.
Pan Dryers.Pan dryers utilize a heated surface upon which the feed is placed. The pan may rotate, and the product is discharged off the tray by means of a sweep or blade mechanism. These units may have multiple pans arranged so that the material from one pan discharges onto the next successive pan. The evolved moisture is exhausted from the system through an induced-draft fan. The pan may be installed inside a vacuum chamber for vacuum drying operation.
Paddle Dryers.Paddle dryers consist of either a single- or twin-paddle or screw arrangement (which may or may not be internally heated) running through a trough, which may or may not be jacketed. The feed is introduced at the feed end and is conveyed through the length of the dryer, being brought into contact with the heated surfaces. The constant motion and agitation of the screw or paddle exposes new surfaces and intimately mixes the product. Dry product is discharged from the opposite end of the unit. The evolved moisture is exhausted from the system through an induced-draft fan. The unit (trough) may be designed for vacuum drying operation.
Double-Cone Dryers.Double-cone dryers consist of a double-cone mixer that uses an external jacket to heat the internal surface of the unit. As the cone rotates, the material is tumbled and new surfaces are brought into contact with the cone surface. These operations are batch type and are most commonly evacuated for full vacuum operation. Vacuum is pulled by means of liquid-ring vacuum pumps. The evolved moisture is exhausted from the system through an induced-draft fan or the vacuum pump. Other variations of this technology include single cones with internal scrapers or screws to further agitate the material.
Steam-Tube Dryers.A steam tube dryer consists of a number of tubes symmetrically arranged that run the length of the dryer. The tubes are connected to a header into which steam or other heat transfer fluid is introduced to provide the energy for the drying process. The feed is introduced into the unit and brought into contact with the steam tube bundle. In stationary units the motive force is air and will create a fluid bed around the tube bundle. Rotary units will lift the material and cascade it over the steam tubes multiple times while the feed dries. The dry product is discharged from the unit through an airlock device. The evolved moisture is exhausted from the system through an induced-draft fan. The unit may be designed for vacuum operation or installed inside a vacuum chamber for vacuum drying operation.
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