This document discusses various types of drying equipment and processes used to remove moisture from foods and other materials. It describes batch and continuous dryers and provides details on rotary drum, rotary louver, fluidized bed, cabinet tray, tunnel, screw conveyor, spray, and pneumatic dryers. Characteristics such as operating temperatures and air velocities are outlined. Diagrams illustrate the set ups and working of different dryer types.

1. Drying Equipments

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.

4. Process Diagram of Dehydration of Diced PotatoesStored PotatoesWashingPeelingDicingBlanchingInspectionBinDryingDryingScreeningPacking

5. Types of DryersRotary-Drum DryerRotary-Louver DryerFluidized-Bed DryerCabinet Tray DryerTunnel (truck) Dryer and Conveyor DryerPneumatic (Flash) DryerScrew Conveyor DryerSpray Dryer

6. Rotary-Drum DryerConsists 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

7. Heat Transfer Rate

8. Rotary-Louver DryerAir 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.

10. Fluidized-Bed Dryer

11. Schematic Representation of Moisture and Temperature Profiles in Fluidized-Bed Dryer

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.

13. Cabinet Tray Dryers

14. Tunnel (truck) Dryer and Conveyor Dryer

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.

17. Pneumatic (flash) Dryer

18. Pneumatic Ring Dryer

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.

20. Spray Dryer

21. Classification of Various Types of Dryers

22. Characteristics of Common Food Dryers

23. Operating Conditions of Various Types of Dryers

24. Drying SelectionLocationMaterial to be driedEnergy sourceInitial cost per tonne of dryer capacityEnergy consumption per kilogram of water removedMaintenance cost Ease of operation

25. Reference: Grain Drying: Theory and Practice by Stanislaw Pabis, Digvir S. Jayas, and Stefan Cenkowski © 1998 by John Wiley & Sons, Inc.Fundamentals of Food Engineering by D.G. Rao © 2010 by PHI Learning Private Limited, New Delhi Physical Principles of Food Preservation, 2nd Edition, Revised and Expanded by Marcus Karel and Daryl B. Lund © 2003 by Marcel Dekker, Inc. Food Process Design by Zacharias B. Maroulis and George D. Saravacos © 2003 by CRC Press Ceramic Materials: Science and Engineering by C. Barry Carter and M. Grant Norton © 2007 Springer Science + Business Media, LLC Food Process Engineering and Technology by ZekiBerk © 2009 Elsevier Inc.