744 Fermentation and Biochemical Engineering Handbook pressure nozzle which have narrow paths, The product must also be able to withstand high inlet temperatures for very short times. Usually the product will reach the outlet temperature 5.9 Flash Dryers Flash dryers have many variations in design. The most basic is a long pipe with a fan, using a high velocity air stream to fluidize and move the wet product in the pipe while concurrently drying the product. Wet feed consistency is extremely important here because if the feed is too sticky or tacky, it will tend to lump together in the feed inlet. The use of paddle type backmixers is common to combine dry material with wet cake to provide a consistent friable feed to the dryer. Backmixing 50% of the dry material with the wet cake is not uncommon. The inlet of the flash dryer is sometimes referred to as the throat---it is of the venturi design so that product is whisked nto the chamber and exposed to a high velocity and high temperature as quickly as possible. most products being flash dried are inorganic salts because the process lends itself to materials which are not temperature sensitive and their final particle shape is not critical. The larger chunks of material will abrade quickly in the long pipe as the material traverses the entire distance of the dryer 5.10 Ring Dryers A second flash dryer design is a ring dryer. The product may be introduced into this type of dryer in a similar fashion as the standard flash Dryer. However, instead of the long pipe as the drying chamber, the product is brought into a ring of pipe where the high velocity air stream and centrifugal forces keep the larger particles in the loop, while smaller particles leave via a contoured inner discharge port. The inlet and outlet port are tangential to the ring and two fans are utilized; one as an exhaust fan for providing the motive air and the second for providing the closed loop recirculation and itroducing the product into the ring 5.11 Mechanically agitated Flash Dryers a third design is a flash dryer design(combined fluid bed and flash) with a small drying chamber using high speed agitators and complicated swirling arrangements. In this design the cake can be in the pasty or even runny form. Material enters a feed vat where pitched turbine-type blades744 Fermentation and Biochemical Engineering Handbook pressure nozzle which have narrow paths. The product must also be able to withstand high inlet temperatures for very short times. Usually the product will reach the outlet temperature. 5.9 Flash Dryers Flash dryers have many variations in design. The most basic is a long pipe with a fan, using a high velocity air stream to fluidize and move the wet product in the pipe while concurrently drying the product. Wet feed consistency is extremely important here because if the feed is too sticky or tacky, it will tend to lump together in the feed inlet. The use of paddle type backmixers is common to combine dry material with wet cake to provide a consistent friable feed tothe dryer. Backmixing 50% ofthe dry material with the wet cake is not uncommon. The inlet of the flash dryer is sometimes referred to as the throat-it is ofthe venturi design so that product is whisked into the chamber and exposed to a high velocity and high temperature as quickly as possible. Most products being flash dried are inorganic salts because the process lends itself to materials which are not temperature sensitive and their final particle shape is not critical. The larger chunks of material will abrade quickly in the long pipe as the material traverses the entire distance of the dryer. 5.10 Ring Dryers A second flash dryer design is a ring dryer. The product may be introduced into this type of dryer in a similar fashion as the standard flash dryer. However, instead ofthe long pipe as the drying chamber, the product is brought into a ring of pipe where the high velocity air stream and centrifugal forces keep the larger particles in the loop, while smaller particles leave via a contoured inner discharge port. The inlet and outlet port are tangential to the ring and two fans are utilized; one as an exhaust fan for providing the motive air and the second for providing the closed loop recirculation and introducing the product into the ring. 5.11 Mechanically Agitated Flash Dryers A third design is a flash dryer design (combined fluid bed and flash) with a small drying chamber using high speed agitators and complicated swirling arrangements. In this design the cake can be in the pasty or even runny form. Material enters a feed vat where pitched turbine-type blades