2. Drying Process Drying or Dehydration (term used for food) is the process of thermal removal of water to about 15-20% moisture (dry basis), at ambient air condition. Dryer is a device that removes moisture by heating the mass to a level where it becomes safe to store. Batch dryers are used for solid throughputs (flows) below 50 kg/hr. Continuous dryers is preferred above 1000 kg/hr.
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4. Process Diagram of Dehydration of Diced Potatoes Stored Potatoes Washing Peeling Dicing Blanching Inspection Bin Drying Drying Screening Packing
6. Rotary-Drum Dryer Consists of a rotating drum 2 to 3 m in diameter and 15 to 20 m long. The dryer is slightly inclined toward the outlet. The drum have flights used for lifting the substance. Air temperatures are usually 120-175oC Air velocity ranges from 2000-25,000 kg/m2h for coarse particles
8. Rotary-Louver Dryer Air passes a heater to give the optimum conditions for drying and then is forced by a fan into the dryer. The air moves upward at about 0.5 m/s through the screen and then through the material, leaving at the discharge end.
9. Fluidized-Bed Dryer Fluidized-bed dryer are best and fastest drying systems for drying wet granular solids which do not tend to stick or agglomerate. Incipient fluidization is the point at which fluidization starts. At this stage the solids are loosened, and they will not contact each other. At this stage, the pressured drop slightly falls. The residence time of the material in the dryer is 30 – 120 sec for removing surface mixture. 15 – 30 min for removing moisture by internal diffusion.
12. Cabinet Tray Dryers Cabinet Tray Dryers are relatively small batch units for drying small quantities of product. The product in the form of pieces, particles, or paste is placed in metallic trays, which are re-used for drying operations.
15. Screw Conveyor and Mixer Dryers Screw conveyors with jacketed troughs and hollow augers are used for drying slurries and wet particulate solids. Screw conveyors are steam heated, heat transfer is mainly by conduction. These dryers are also known as hollow flight dryers, frequently used for drying waste materials and biomass (sludge) from wastewater treatment processes.
16. Pneumatic (flash) Dryer The hot gas is conveying medium rapidly with velocity of order 25 m/s in which granular free flowing solid materials are dispersed. Finely powdered wet material is introduced into hot gas (air steam) with the help of a screw conveyor. The dried material is separated from air steam in a cyclone and bag filter. The product from the bottom of the collecting equipments (cyclone and bag filter) is transported to a silo via screw conveyor.
19. Spray Dryer Spray drying is an example of powder production from solution. Fine droplets produced by an atomizer are sprayed into a drying chamber and the powder is collected. Droplet sizes in the range of 10μm to over 100μm can be produced.
24. Drying Selection Location Material to be dried Energy source Initial cost per tonne of dryer capacity Energy consumption per kilogram of water removed Maintenance cost Ease of operation
