Powder Dryers Information
Powder dryers are devices used to dry granular materials, from free flowing powders to fibers and flakes. Selecting a powder dryer involves consideration of a number of factors, including the dryer type, the heating method, its performance specifications, and the secondary capabilities it possesses.
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 or tumblers are available in rotating and non-rotating styles. The rotating styles have a rotating or tumbling housing (drum) that keeps the media in motion to prevent media buildup. The drum is heated or heated air is fed into the unit. The non-rotating styles have internal moving components that also keep the media in motion to prevent buildup.
Continuous tray or turbo-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.
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.
Flash or pneumatic 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). They offer an effective method of removing surface or unbound moisture from a feed product.
Fluidized bed dryers fluidize beds of solid material through vibration or air flow. The solids are then dried by direct heating form hot air or combusted gas flow (direct) or through contact with heated surfaces (indirect).
Freeze dryers are used for the reliable preservation of a wide variety of heat sensitive products and demands the highest standards of reliability and control.
Paddle agitator dryers operate by breaking up and dispersing the material to improve drying uniformity and prevent agglomeration. They typically consist of a long trough or drum along which runs a shaft holding the paddles.
Spray dryers are the most widely used industrial equipment for 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.
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.
There are a number of ways dryers can dry or apply heat to a powder material.
- Direct heating involves hot air or combusted (or heated) gas being directly circulated through the material being dried to convectively heat the powder or evaporate moisture from its surface.
- Indirect heating involves heating walls, tubes, jackets, or discs by steam, gas, thermal oil, or hot air. These heated elements transfer their heat to the target powders by contact via conduction. Indirect dryers may be useful when direct contact with combustible gas or hot air is not desirable due to product or process requirements.
- Radiant heating involves radiant heat generated by electric or gas-fired infrared heaters. Radiant heat dryers are useful for drying powder materials where a clear line of sight can be provided. Straight pass infrared dryers are used to pre-dry web materials before these materials are fed into a conventional, direct, hot dryer.
It may be important to consider certain performance specifications when sourcing a powder dryer.
Air flow is the flow rate of the air that feeds through the dryer, typically expressed in standard cubic feet per minute (scfm).
Operating temperature defines the temperature or range of temperatures at which the dryer can operate. This is particularly important if the process or powder material has temperature limitations.
Evaporation rate is the rate that water and other fluids are removed from the powder in the dryer, commonly expressed in pounds per hour (lb/hr).
Powders dryers may also be distinguished based on other capabilities which can be incorporated into the equipment.
- Agglomeration and granulation - Dryers can form powders or fibrous feeds into granules or agglomerates.
- Classification screening - Dryers have integral classification or screening capability.
- Coating - Dryers provide the application of a coating on the web material before, during, or after drying.
- Cooling and freezing - Web materials are cooled or frozen in the dryer during or after the drying process.
- Moisturizing and humidifying - Dryers can restore a level of moisture to an over-dried and possibly brittle product.
- Preheating - Dryers have an integral preheater or preheating capability, which reduces the heating demands on the drying unit and increases line speed.