Process Dryers

Process dryers are used to remove liquids or moisture from bulk solids, powders, parts, continuous sheets or other liquids by evaporation or sublimation. Dryers can be broken up into two main types: direct and indirect. Direct dryers convectively heat a product through direct contact with heated air, gas or a combusted gas product. Indirect dryers conductively heat a product through contact with a heated wall.  Some of the most common process dryers are:

Air bars, which produce dispersed drying air, and floatation dryers that use a series of air bars or louvers to support and gently dry webs of fragile materials.

Air knives, which use a high-pressure air blast to dry discrete parts or continuous sheets / webs or larger bulk materials, as well as wipe water away.  Hot air impingement dryers use blasts of hot air to convectively dry moving webs, boards or other large bulk materials.

Belt conveyor dryers use a perforated or mesh belt to transfer feed materials through the dryer. Heated air is either passed under and through, or over and through the belt and product bed before being reheated and re-circulated. Units usually have multiple zones, and may have differing airflow or a different temperature profile and control in successive zones. Saturated or close to saturated air is exhausted from the dryer via a central exhaust duct.

A calciner consists of an angled drum tube that passes through a furnace. The tube, which is in direct contact with the tube feed, is heated either by electrical elements or by gas burners. Special lifters or louvers within 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 such as a rotary airlock valve.

Chamber-type dryers consist of a heated chamber that is usually heated to a higher temperature than other dryers.

Disc dryers are drum shaped units filled with a series heated discs.  Disc motion or air flow is used to move powders or bulk material through the dryer.  

In fluidized bed dryers, a bed of material is fluidized through vibration or air flow. The powder is then dried by directly heating the hot air or combusted gas flow (direct) or through contact with heated surfaces (indirect).

Predryers or preheaters provide initial heating or drying of a material before the unit is passed into a larger dryer.  Radiant heating dryer are often used to provide predrying of webs before the material is fed into a hot air drying unit.

Rotary dryers feed material into a tumbling or rotating drum or tumbler.  The drum is then heated or heated air is fed into the unit.  The internal surface of the drum may have baffles or louvers to channel the hot air or cascade the material.

Straight pass or flatbed dryers are used to dry continuous webs or sheets that cannot tolerate wrapping around a cylinder or the formation or loops in festoon racks.

Suction drum dryers consist of a series of perforated cylinders or drums with an internal vacuum.  Web or sheet materials are held against the drum by the vacuum or suction generated as a fan draws air from the interior of the drum.  This suction holds the material to the surface of the drum permitting air to pass through the material being dried.  A portion of the drum at the transfer has no suction and permits the material to transfer to the next drum without interference.

Tunnel dryers consist of an oven-like enclosure.  Material is placed onto trays, racks of trays or trucks and loaded into the furnace.  Air is drawn in through an inlet duct and heated to the required temperature for drying. The heated air is then distributed from side-to-side, or bottom-to-top, in a circular motion. The system can be automated with the trucks on a trolley, being moved in and out continuously.

Turbo tray dryers employ a series of horizontal plates or trays with a single slot. Material is fed onto the top tray.  A series of plows wipes the material off the trays and drops the material onto the next tray.  Hot air is circulated up through the unit to dry the material.

Vacuum dryers use an applied vacuum to accelerate drying.  Vacuums can alter vapor pressure to enhance evaporation rates as well as increase the drawing out of liquids in pastes by capillary pressure.

Vibratory dryers use vibratory action to agitate feed, or fluidize powders or other bulk materials.

• Scale-Up from Laboratory to Production-Scale Spray Dryer
  A pharmaceutical company uses various Spraying Systems Co. air atomizing nozzles in a laboratory-scale spray dryer for a proprietary coating application. The process in the laboratory spray dryer was...
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• Fluidized Bed Dryers
  In fluidized bed dryers, a bed of material is fluidized through vibration or air flow. The powders are then dried by direct heating from hot air or combusted gas flow (direct) or through contact with...
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• Visualizing the Dynamics of Fluid-Bed Dryers
  Operating fluid bed dryers today still involves more art than it does science. “Basically, an operator looks through a sight glass and manually controls the process,” says Todd Pugsley,...
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