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Indirect Dryers

Indirect Dryers

For heat sensitive, toxic or solvent based products.

Feeding / discharge:
Manual, batch, automatic and continuous. Vibrators, screw feeders, rotary airlocks, double flap valves, gravity chutes

Feed Characteristics:
liquid, slurries, sludges, pastes, filter cakes, powders, granules, agglomerates. Low to medium feed rates

Heat Source:
Steam, hot water, thermic oil, or gasses

Exposure:
Product contacts the heated surface of the equipment. Devices within the unit agitate, mix, scrape, or turn the product exposing new product to the heated surface

Residence time:
Controllable. Residence time can be altered by varying the movement of the equipment, speed of rotation or appropriate mechanical aspect.

Extraction:
Normally induced, only vapors are extracted. There is little air introduced into the system.

Dust Collection:
Systems can be low dust with minimal separation requirements. If required cyclones, bag houses, scrubbers, Electrostatic precipitators.

Control:
PLC or solid state controllers. System controls on discharge temperature. Can modulate energy and feed.

Benefits:
  • Processes a wide variety of feed materials
  • Can be designed for vacuum operation
  • Good for toxic and noxious products
  • Excellent for solvent recovery applications
  • Relatively small units
  • Can have zoned temperature profile
  • Gentle handling
  • Intimate product contact
  • Unit can have integral cooling section.

Limitations:
  • Limited to low capacities
  • Expensive construction
  • Require code compliance for jackets
  • Requires expensive ancillaries such as steam boilers, vacuum pumps, etc.

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 by 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 'doctor' 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 is 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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