Comparison Of Dryers

Vijay Ratna

Compartment Tray Dryer :

1. Rectangular Chamber, whose walls contain a suitable
   heat insulating material.
2. Material kept on Trays.
3. Dryer is loaded, doors are closed and hot air is
   circulated over trays
4. Direction vanes for uniform velocity distribution of
   air on all the trays.
5. Air is recirculated for economic reasons.
6. Dampers, with the help of automatic instruments,
   control the percentage of fresh air introduced and the moist air
   discharged.
7. Wet scrubbers used for recovery of valuable solvents.
8. If different products are to be dried, thorough
   cleaning is necessary.

Problems:

1. Non-Uniform air flow

2.Not suitable for oxidisable or thermolabile material.

3. Labour requirements are high

4.Takes a long time for drying.

Applications:

1. Useful for sticky or plastic substances, granular
   masses, pastes and precipitates.
2. Material loss during loading and unloading is minimal
   – so useful for valuable materials.
3. Batch operation – so useful when material is
   available in small quantities.
4. When we want to economise on investments we use this.

Vacuum Compartment Dryer

1. Indirectly heated batch dryer.
2. Rectangular
3. Steam is used for heating
4. Vacuum pump or steam jet ejector is used to create
   vacuum.
5. Heating medium admitted into the shelves only after
   the chamber is evacuated.

Applications:

1. Useful for drying thermolabile or easily oxidisable
   substances.
2. Suitable for dusty or hygroscopic materials.
3. Suitable when materials being dried are expensive and
   also when varied types of materials are to be dried from time to time.

Problems:

1. Much labour involved in putting in and removing the
   trays.
2. Expensive in establishment costs.

Continuous Tunnel Dryer

1. Trays or trucks containing wet material move on tracks
   through a tunnel which is heated.
2. The material may also move on a conveyor belt.
3. Air flow may be parallel current or countercurrent.
4. Circulating fans will be there at different places in
   the tunnel.
5. Consists of a number of individual sections; each
   section complete with fan and heating coils.

Applications:

1. Useful for handling granular or particulate solids.
2. Useful for large solid objects which require slow
   drying.
3. Useful for fibrous, flaky and coarse granular
   materials.

Advantages:

1. Labour involved in loading and unloading insignificant
2. We can use different temperatures at different places
   in the tunnel. High temperature at
   wet end, followed by lower temperatures down the tunnel.

Rotary Dryer

1. This dryer is a cylindrical drum, set with its axis at
   an angle to the horizontal, for ease of movement of material.
2. The drum is insulated to prevent heat loss.
3. It is mounted on rollers and rotates slowly.
4. Because of the slow movement of the drum, the wet
   material does not build up on the walls, but moves slowly down the dryer.
5. Internal shelves or flights help in the movement.
6. A fan helps in countercurrent movement of air.
7. Air outlet is connected to a dust separator.
8. Longitudinal flights lift the material and shower it
   down through hot air.
9. Final product is taken out by conveyors
10. Dust is removed by cyclone separators and bag filters.

Points to remember:

1. Countercurrent flow of hot air and solids gives
   better heat transfer efficiency.
2. Co-current flow is used to dry heat sensitive
   materials.
3. Feed is 10% to 15% of the dryer volume.
4. Gas flow rate is 400 to 4000 lb/hr.
   Applications:
   Useful for a material which after drying can freely flow down the shell.

This type of equipment operating at higher
temperatures is called rotary kiln or calciner.

6. In a steam tube dryer, steam heated tubes run the
   full length of the cylinder. They
   are fastened to the shell symmetrically in concentric rows.
7. Whereas in a Rotary dryer flights are used, in a
   Roto-Louvre dryer ‘Louvres” are used.
8. In Roto – Louvre dryer there will be a tapered
   assembly of overlapping plates, which will be connected longitudinally.
   Thus channels will be formed.
9. Material to be dried will be held in these channels
   and hot air will be blown through these channels.
10. Flow of air and material is in parallel current
    manner.
11. Material is not lifted and dropped as in a rotary
    dryer. Material simply rolls on the louvers. So there is no size reduction
    of fragile material.
12. Material and hot air are in intimate contact. So
    there is better heat transfer and better mass transfer. High pressure drop
    will be there.
13. Useful for coarse granular solids (a) that do not
    offer high resistance to air flow, (b) that require good contact with hot
    air and which (c) do not produce
    much dust.
14. Efficiency is higher than ordinary Rotary drier, size
    is smaller but investment cost is higher, because of complicated
    construction.

Agitated Dryers

1. Consists of a rectangular housing which contains
   mechanical agitators.
2. Agitators break the lumps and agglomerates into small
   masses.
3. Dried material is free
   flowing and granular.
4. Vacuum may be created in this.
5. The machine is a jacketed stationery horizontal
   cylindrical shell containing agitator blades that turn on a central shaft.
6. Two blades will be there which will be moving the
   material in opposite directions.
7. The shaft is heated by hot water or stream
8. Material too sticky for rotary driers but not
   valuable enough for tray driers are dried by agitated dryers.
9. Double cone rotating vacuum dryer is an agitator
   dryer.

Turbo Tray Dryer

1. It consists of a central shaft to which several
   shelves or trays are arranged. As the shaft is rotating the wet material
   which enters at the top will be falling from top to bottom, i.e. from one
   shelf to the next and so on.
2. Air will enter the equipment at the bottom and will
   be moving upwards helped by fans.
3. Suitable for fragile material.
4. Low power consumption.
5. Suitable for a variety of materials from thick
   slurries to fine powders.
6. Not suitable for fibrous or too doughy or tacky
   materials.
7. Material is constantly turned over, so drying rate is
   much better than in a compartment dryer.

Drum Dryers

1. When a colloidal solution is to be dried to obtain a
   dried powdery product, then drum driers are employed. They are used in
   textiles and paper industry and also in pharmaceuticals.
2. Drums are heated from inside. On the top of the drum colloidal
   solution will be flowing or rolling as the drum is rotating.
3. Doctor knifes, placed on the top of the rolls on the
   outside are used for scraping off the dried material.
4. Vacuum drum drier may be used when the material is thermolabile.
   Only one drum is used which is heated by steam, and is placed in a large
   cast iron casing which can withstand atmospheric pressure.
5. Useful for thermolabile materials in colloidal solution.

Spray Dryer

1. Consists of a vertical chamber into which feed enters
   from the top, sprayed from nozzles. Hot air enters from one side
   tangentially and with force. Intimate contact between hot air and sprayed
   material. Air will be in spiral motion. By the time the droplets of the
   sprayed solution touch the internal surface of the dryer, they are dried.
2. Coarse particles collect at the bottom and fine
   particles are separated in external cyclone separators.
3. Inlet gas temperatures vary from 300
   ^O to 1500 ^O F.
4. Atomising devices are high pressure nozzles, two
   fluid nozzles and high speed centrifugal discs.
5. Discs are used for atomizing viscous solutions, thick
   suspensions and pastes.
6. The physical properties of spray dried materials
   depend on the design of the equipment and the entry mode of hot air and
   wet material.
7. Large drying surface – so rapid drying
8. Produces uniform hollow spheres as products.
9. (a) First surface of the particle is dried, (b) then heat is transmitted to the droplet faster than
   diffusion of the moisture from inside to outside. (c) steam
   is generated in the centre of the drop. (d) steam
   pressure blows the drop into a shell.
10. Temperature of drying is low, a little above wet bulb
    temperature.
11. Low drying time.
12. Useful for thermolabile materials and for foods where
    taste is to be preserved.
13. Costly
14. Spherical products of high cost.
15. For drying aqueous solutions and suspensions
    containing ultrafine particles – spray drying is used.
16. For concentrated food products and fruit juices.
17. Flavour, colour retained.
18. Examples : animal blood,
    yeast aluminium hydroxide, coffee extracts, clays.

Fluidised Bed Dryer

1. A bed of granular, nonsticky, free-flowing particles
   is placed in the bottom of the equipment. Stream of hot air enters from
   the bottom of the bed and moves through the bed of granules. The velocity
   of the hot air is gradually increased. At a certain velocity the material
   is fluidized, and with large drying surface, fast drying takes place.
2. Intimate contact between material and air. Good
   mixing and heat transfer, rapid drying
3. Useful when large tonnages of solids are to be
   handled.
4. Can be used as a dryer and classifier
5. Solutions and slurries can be dried,
   they are taken in through spray nozzles.
6. Used extensively in the Pharmaceutical industry to
   dry granules.