Process dryers are used to remove liquids or moisture from bulk solids, powders, parts, continuous sheets, gases or other fluids. The introduction of water, solvents and other fluids is critical to processing and manufacturing that spans from food and pharma to paper, construction and even the auto industry. All of this process water and solvents must be removed before the final product is shipped. Some products such as food, paint, cement, plaster, etc. have water removed beyond usable state for transport, storage and weight purposes before water is reintroduced to make it consumable. Dryers are a key component in the manufacture of these goods. Optimizing dryer type and configuration with the materials and processes at hand is critical to optimizing product throughput and quality.

Basic process drying method theory includes evaporation, sublimation or desiccation. Process dryers fall into two main types: direct and indirect. Direct dryers convectively heat a product through direct contact with the drying medium. Indirect dryers conductively transfer drying heat to the product to be dried through contact with a heated wall, pipe, belt, etc.

Industries Using Process Dryers

Food and Pharma – Dryers for these highly-regulated industries must meet higher standards and regulations than dryers for industries discussed below. Dryers are constructed of easily-sterilized material and are designed specifically for hygienic purposes such as medical, pharmaceutical, or food processing applications. Usually, the equipment is certified by a national or international organization, such as USDA or FDA. Equipment is used to dry liquid, solid, and liquid-solid mixtures of food products such as grain, milk, dried potatoes, apples, sugar, spices, vegetables, or other foods. Additional guidelines are followed when systems are designed specifically for use in processing pharmaceuticals.

Material Processing – Dryers are used through all phases of metal and mineral processing. They are used with ores and extraction processing. Drying equipment is used for processing metal sheet, metal powders, carbide powders, metal casting or finished products, slags or other components used in winning metals (smelting), or scrap metal prior to recycling. Other processed materials needing drying include abrasives, green ceramics, refractories, cement, limestone, gravel, sand, brick, grog, clay, shale, or asphalt. Rotary drum and fluidized bed dryers are useful in drying the binders that form and hold together agglomerates.

Paper and Textiles -- Drying equipment is used for drying cellulose feeds, cotton fiber, or pulp for the production of paper. Large heated drier rolls or cylinders remove most of the water content of sheet products. Paperboard, fiber and other paper products are made in this manner. Equipment is similarly used to dry woven cloth webs or non-woven mats of nylon, cotton, fiberglass, or other fibrous materials.

Construction Materials – Dryers are used in all phase of construction. They are used for drying lumber, forest products (logs), wood boards, or wood chips. Post processing of wood products into glue-laminated beams, particle board, and plywood depends on dryers. Project times are reduced on the job site by using heated and unheated dryers to cure paint and to dry joint compound.

Manufacturing -- Equipment is used for drying adhesives or binders. Radiant heat dryers are useful for drying adhesive layers on surfaces, as well as flat products or web materials where a clear line of sight can be provided. They are used for drying metal sheet, metal powders, carbide powders, metal casting or finished products, slags or other components. Drying rooms or booths are used for drying paints or coatings. Radiant heat dryers are useful for drying paint layers or coatings on surfaces, as well as flat products or web materials where a clear line of sight can be provided. Straight pass infrared or forced hot air dryers are also useful in drying thin, painted layers or printed patterns. Drying equipment is suitable for drying plastics, elastomers, rubbers or other polymeric materials in pellet, finish product, or sheet forms.

Semiconductor and Aerospace -- When assembly conditions are hypercritical, dryers are essential to remove all traces of water that may degrade components in a very short time. For instance, precision bearings used in guidance mechanisms must be kept free of any opportunity for corrosion. Equipment is used in semiconductor or electronic manufacturing (e.g., the drying of wafers after etching and rinsing steps).

Types of Process Dryers

Moving Air Dryers work by moving low humidity air over the part or material to be dried.

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

Hot air impingement dryers use blasts of hot air to convectively dry moving webs, boards, or other large bulk materials.

Continuous tray dryers have 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 from the trays and drops it onto the next tray. Hot air is circulated up through the unit to dry the material.

Conveyor belt drying uses a perforated 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 in successive zones. Additionally, each zone may use a different temperature profile and control. Saturated or almost
saturated air is exhausted from the dryer via a central exhaust duct.

Spray drying is the most widely used industrial process involving particle formation and drying. A spray dryer consists of a feed pump, atomizer, air heater, air disperser, drying chamber, and systems for exhaust air cleaning and powder recovery.

Contact Evaporation Dryers use contact with a heated surface to promote evaporation. Either the product is moved or the dryer is moved to complete the process.

Calciners or rotary drums remove bound water from a material. 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 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.

Column dryers consist of tall, vertical vessels often with baffles to cascade the material. Powders or bulk materials are fed down through the column and heated by contact with baffles or drying air.

Disc dryers are drum-shaped units that are filled with a series heated discs. Disc motion or airflow is used to move powders or bulk material through the dryer. Often, steam is used to heat the discs.

Cylinder or roll dryers usually consist of a steam-heated drum with a smooth outer surface around which a moving web or sheet is passed for drying.

Festoon or loop dryers are used to heat webs or continuous sheets with minimal contact. The web is supported on bars or threaded onto movable idler rolls that are driven by a conveyor. Festoon dryers are used to dry or cure coated abrasive webs, flocked wall paper or coated fabrics.

Steam tube dryers are drum-shaped dryers with a series of internally mounted and spaced tubes. Steam passes through the tubes and bulk material or powders are indirectly heated through contact with the heated tubes (tubular rotary).

Heated Space Dryers heat an entire room or compartment to promote evaporation.

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

Freeze drying, or lyophilization, is used for the reliable preservation of a wide variety of heat sensitive products and demands the highest standards of reliability and control.

Pan dryers are specialized units designed to dry sensitive pharmaceutical and chemicals. Pan dryers consist of a heated lower pan in an enclosed vessel under air, vacuum or a pressurized cover gas. The pan may rotate or a rotating agitator bar may be used to prevent sticking and improve drying uniformity. /p

Tunnel dryers consist of an oven-like enclosure. The material is loaded onto trucks or trays that are stacked on a movable truck. The trucks are rolled into the tunnel where the material is dried. 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. 

Mechanical Drying Enhancers

Centrifugal dryers use a spinning action to mechanically enhance the removal of water, and to increase drying line speed. Typically, centrifugal dryers are used for discrete parts or products, or after washing of these components.

Conical dryers are available in rotating and non-rotating styles. The rotating styles have a rotating or tumbling housing that keeps the media in motion to prevent media buildup. The non-rotating styles have internal moving components that also keep the media in motion to prevent buildup.

Evaporators use a mechanically-created vortex to force a liquid sample outward against the tube walls. This increases surface area for faster evaporation and maximum sample recovery. Liquid can also be removed by gentle heating at atmospheric pressure with flowing air, an inert gas, or under vacuum conditions.

Paddle dryers use blades, paddles, breaker bars or other internal agitation devices to break up and disperse the material to improve drying uniformity and prevent agglomeration. Paddle dryers typically consist of a long trough or drum along which runs a shaft holding the paddles.

Screw or auger conveyor dryers use a screw conveyor to agitate and feed a powder or bulk material through the dryer.

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

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

Flash Dryers

Flash Dryers offer an effective method of removing surface or unbound moisture from a feed product. The technology behind flash drying employs a low residence time within the actual dryer; thus, the unbound moisture is "flashed" off the feed. Flash dryers, pneumatic dryers, ring dryers, and flash tube or cyclone dryers use a heated carrier gas (usually, air) to pneumatically convey the feed through the flash dryer (flash tube) and into a primary gas-separation device (most commonly, a cyclone or series of cyclones in series or parallel). The carrier is induced or forced into the feed throat from a hot gas generator that heats the gas to the desired operating inlet temperature. In the feed throat, the gas entrains the feed, and the moisture is evaporated quickly as the product is conveyed through the system to the primary gas / product separation device. Because the entire product is entrained in the gas stream, both primary product collection (typically, cyclones) and secondary and tertiary dust collection (such as multicyclones, bag houses, scrubbers and electrostatic precipitators) usually are installed to meet emission limits. Flash dryers are an efficient method of drying products such as slurries, pastes and sludge (most with back mixing), friable filter cakes, powders and granules.

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 heated surfaces (indirect). In general, fluid bed dryers operate on a through-the-bed flow pattern with the gas passing through the product perpendicular to the direction of travel. In batch operation, a dose of feed is metered into the dryer bowl and processed until the desired final moisture has been attained. For continuous systems, wet feed is introduced by being metered into the drying section (zone) by means of a suitable feeder. The dry product is discharged from the same section. In direct systems, hot gas is introduced into the dryer's wind box or plenum. The plenum is a "pressurized box" that distributes the gas evenly across the distributor or fluidizing plate that separates the product from the wind box. The fluidizing plate is a proprietarily-designed perforated plate that has either non-directional or directional holes (nozzles), with or without caps, through which the gas passes. The process gas passes from the wind box into the drying chamber and interacts with the feed, lifting and maintaining the feed in a fluidized state termed a bed. The bed attains fluid-like properties, flowing and mixing in a similar fashion to a liquid. The fluidization provides intimate contact between each material particle and the gas stream, creating an efficient transfer device.

Rotary Dryers

Rotary dryers feed the material into a tumbling or rotating drum called a tumbler. The drum is 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.

Rotary cascade dryers pass feed materials through a rotating cylinder together with a stream of hot gas. Internal lifters or flights elevate the feed and drop it in a curtain from the top to the bottom cascading along the length of the dryer. Flights need to be carefully designed to prevent asymmetry of the curtain. Material moves from one end of the dryer to the other by the motion of the material falling due to the angle of inclination of the drum. Some rotary dryers are double and triple pass units where each drum is nested inside the previous drum.

Rotary louver dryers have the feed materials supported and moving over a set of louvers mounted to an external rotating drum. The hot gas is introduced into a tapered bustle below the feed ring. The air passes through the louvers and then the product before being exhausted from the dryer. The rotation of the drum causes the material to roll and mix providing intimate contact with the drying gas. Material moves from one end of the dryer to the other by the motion of rolling, some fluidization and due to the angle of inclination of the drum.

Image credits:

Bepex International LLC | Buflovak LLC | Carrier Vibrating Equipment, Inc.