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