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A flash dryer is used to dry residual moisture from centrifuged cakes or powders.
Drying process is accomplished by contacting the powder with hot air travelling at high velocity. The heat is used for drying and the air pneumatically conveys the powder for separation in a cyclone or a bag filter. A product classifier ensures only dry powder leaving the drying chamber.
A modified version of a flash dryer is a Ring Dryer and incorporates a centrifugal classifier that uses deflector blades to select and classify air borne particles on the basis of their densities. As a result of centrifugal action, the behavior and wetter particles follow the ring duct periphery more closely and are recycled back, while the fine dried material leaves the dryer with the exhaust air to be collected in the cyclone or bag filter before being scrubbed in a venturi scrubber.
The very word FLASH means instantaneous or rapid drying of products. There are five primary factors which govern the speed of drying. These are:
This is defined as the accessibility of the moist surface to the drying medium (hot air or gas). This is achieved in a flash dryer by use of a product disperser such as mechanical scatterer, double paddle feeder or cage mill etc. By mixing dried product in the feed.
In flash drying it is possible to use high initial temperatures. These are possible due to short contact time with the result that the product temperature never exceeds the wet bulb temperature of the gas unless all surface moisture is removed.
In flash dryer, agitation or turbulence is accomplished by the use of high gas velocities. The vapour film is continuously swept away from moist particles. As a result, moisture removal is practically instantaneous.
Materials having capillaries will be able to evolve moisture to the surface from internal areas more rapidly resulting in swift drying action. Dense particles which have no voids are difficult to dry in a flash drying system.
Drying is accomplished by evaporation from the surface of the particle. Therefore, to remove the internal moisture, it is necessary that the moisture reaches the surface by diffusion. This transfer will obviously take place most rapidly with smaller particles.
