Drying equipments

Equipment for handling drying operation

1. Drying Equipment Prepared by A. Nandakumar, M.tech.

2. Drying Removal of a liquid from a solid/semisolid / liquid to produce solid product by thermal energy input causing phase change (Sometimes converts solid moisture into vapor by sublimation eg . Freeze drying with application of heat) Needed for the purposes of preservation and storage, reduction in cost of transportation, etc.

3. Drying Batch Driers: It is an expensive operation when it is used in batch mode, limited to small scale operations. Direct driers: Tray driers: It is also called cabinet, shelf or compartment driers used for drying solids which must be supported on trays. [ Eg : pasty materials]. These consist of an insulated cabinet fitted with shallow mesh or perforated trays, each of which contains a thin (2-6 cm deep) layer of food. Hot air circulated through the cabinet at 0.5-5 ms-1 per square meter tray area.

4. Drying Fresh air enters the cabinet, is drawn by the fan through the heater coils, and is then blown across the food trays to exhaust. One of the most important problems encountered is not to supply the same drying rate at the all position within the tray dryers. Tray dryers are used for small-scale production (1-20 t-day-1) or for pilot-scale work. They commonly are used to dry fruit and vegetable pieces, and depending upon the food and the desired final moisture, drying time may be of the order of 10 or even 20 hr. They have low capital and maintenance costs but has relatively poor control and produces more variable product quality .

5. TRAY DRIERS

6. Drying Two – Truck Driers: A simple modification of tray driers, where the trays are racked upon trucks which can be rolled into and out of the cabinet. Since the loading and unloading is done outside the cabinet, considerable time can be saved during drying cycles. Nature of the drying substances, the design process also has some modifications. Thus, skiens of fibers can be hung from poles, wood or broad like materials can be stacked in piles, the layers separated from each other by spacer blocks.

7. Two – Truck Driers.

8. Drying Through – Circulation driers: With granular materials, the solids can be arranged in thin beds supported on screens so that air or other gas can be passed through the beds. Results in rapid drying. Crystalline solids and silica gel can be dried with this method. Needs some preliminary treatment (performing). Pastes, eg . Those resulting from precipitation can be performed by Extrusion into short, spaghettilike rods. Granulation. (i.e., forcing through screens) Briquetting.

9. Through – Circulation driers.

10. Drying Indirect driers: Vacuum shelf driers: Batch vacuum dryers are substantially the same as tray dryers, except that they operate under a vacuum, and heat transfer is largely by conduction or by radiation. The trays are enclosed in a large cabinet, which is evacuated. The water vapour produced is generally condensed, so that the vacuum pumps have only to deal with non-condensable gases. Another type consists of an evacuated chamber containing a roller dryer.

11. Drying Indirect driers: Agitated pan driers: Used to dry pates or slurries in small batches, are shallow, circular pans, 1 to 2 m in diameter and 0.3 to 0.6 m deep, with flat bottoms and vertical sides. The pan are jacketed for admission of steam or hot water for heating. The pastes in pan are stirred and scraped by a set of rotating plows, in order to expose new materials to the heated surfaces. Moisture is evaporated into the atmosphere in atmosphere pan driers, or the pan may be covered and operated under vacuum .

12. Drying Indirect driers: Vacuum rotary driers: They are steam jacketed cylindrical shells, arranged horizontally, in which the slurry, or paste can be dried in a vacuum. The slurry is stirred by a set of rotating agitator blades attached to a central shaft which passes through the ends of the cylindrical shells. Vaporized moisture passes through an opening in the top to a condenser, and non-condensable gas is removed by a vacuum pump. The dried solid is discharged through a door in the bottom of the drier.

13. Drying Freeze Driers: The material is held on shelves or belts in a chamber that is under high vacuum. In most cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed. In one process, given the name accelerated freeze drying, heat transfer is by conduction; sheets of expanded metal are inserted between the foodstuffs and heated plates to improve heat transfer to the uneven surfaces, and moisture removal.

14. Drying The pieces of food are shaped so as to present the largest possible flat surface to the expanded metal and the plates to obtain good heat transfer. A refrigerated condenser may be used to condense the water vapour . Freeze Driers

15. Drying Continuous Driers: Turbo – Type (Rotating shelf) driers: Solids which ordinarily can be dried on trays, such as powdery and granular materials, heavy sludges and pastes, beads and crystalline solids, can be continuously dried, a form of direct driers. The drier is fitted with a series of annular trays arranged in a vertical stack and rotate slowly. Solid fed in at the top of the tray and spread uniformly. In this way, with overturning and respreading on each tray, the solid progresses to discharge chute at the bottom of the driers. Drying gas flows upwards and circulated over the trays.

16. Turbo – Type drier.

17. Drying Through – Circulation driers: Same as described in batch driers, but it is operated in continuous mode of operation. The food is spread as a thin layer on a horizontal mesh or solid belt and air passes through or over the material. In most cases the belt is moving, though in some designs the belt is stationary and the material is transported by scrapers .

18. Through – Circulation driers

19. Drying Rotary Driers: The foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.

20. (Rotary) Ruggles – Coles XW hot air driers.

21. Drying Roller or Drum Driers: In these the food is spread over the surface of a heated drum. The drum rotates, with the food being applied to the drum at one part of the cycle. The food remains on the drum surface for the greater part of the rotation, during which time the drying takes place, and is then scraped off. Drum drying may be regarded as conduction drying.

22. Dip – feed single drum drier.

23. Drying Spray Driers: In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counterflow . Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

24. Spray drier

25. Drying Through – Circulation Rotary Driers: Combination of through - circulation and rotary driers. It consists of a slowly revolving tapered drum fitted with louvers to support the drying solid and to permit entrance of the hot gas beneath the solid. The hot gas is admitted only to those louvers which are underneath the bed of solids. No showering of solid through the gas stream and consequently minimum of dusting results . The device is satisfactory for both low and high temperature drying of the same materials ordinarily treated in a rotary drier.

26. Through Circulation rotary drier

27. Drying Fluidized Bed Driers: • In a fluidized bed dryer, the food material is maintained suspended against gravity in an upward flowing air stream. • There may also be a horizontal air flow helping to convey the food through the dryer. • Heat is transferred from the air to the food material, mostly by convection.

28. Fluidized bed drier

29. Drying Pneumatic (Flash) Driers: In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension. Heated air accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying.

30. Pneumatic drier

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