DRYING Classification of dryers Based on heat transfer PDF/PPT

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Presented by:


Asst. Professor

Faculty of Pharmaceutical Sciences, Rama University Kanpur


Classification of dryers

Based on heat transfer

 Drying of wet solids

-Convective drying :e.g. tray dryer, tunnel dryer, rotary dryer.

-Conductive drying: e.g. vacuum dryer

-Radiation drying: e.g. microwave dryer.

 Drying of dilute solutions & suspensions -Drum dryer.

 Freeze drying


Based on methods of solids handling


Factors affecting selecting of dryers

1. Properties of material being handled

Physical characteristics when dry

Physical characteristics when wet




Particle size



2. Drying characteristics of material:

Type of moisture (bound/unbound/both)

Initial moisture content

Final moisture content

Permissible drying temperature

Probable drying time for different dryer

3. Flow of material to and from dryer:

Quantity to be handled per hour

Type of operation (batch/continuous)

Process prior to drying

Process subsequent to drying


4. Product qualities



Uniformity of final moisture content

Decomposition of product

Rate of subdivision

Product temperature

Bulk density

5. Recovery problems:

Dust recovery

Solvent recovery


6. Facilities available at site of installation




Cleanliness of air

Available fuels

Available electric power

Source of wet feed

Permissible noise, vibration, dust or heat losses

Exhaust-gas outlets


List of equipment used for drying

 Drum dryer,

 Spray dryer,

 Fluidised bed dryer,

 Tray dryer,

 Tunnel dryer,

 Vacuum dryer,

 Microwave,

 Radiant heat dryer (infra red)

 Rotary dryer

 Freeze dryer




In tray dryer hot air is continuously passed over wet mass. Heat transfer takes place by

forced convection.


It consists of a rectangular chamber whose walls are insulated. Trays are placed inside the

heating chamber. The number of trays may vary with the size of the dryer. Each tray is

rectangular or square and about 1.2 to 2.4 metres square in area . Trays are usually loaded

from 10.0 to 100 millimetres deep. Dryer is fitted with a fan for circulating air over the

trays. Electrically heated elements are provided inside (rather than outside) to heat the air.

In the corner of the chamber, direction vanes are placed to direct air in the expected path.



 Wet solid is loaded into trays. Trays are placed in the chamber.

 Fresh air is introduced through inlet which passes through the heaters and heated up.

 The hot air is circulated by means of fans at 2 to 5 meter per second.

 The water is picked up by air and moist air is removed from outlet.

 During the cycle of drying only 10 to 20 % of fresh air is introduced and 80 to 90% air

is circulated back.



 The method is operated batch wise.

 Attrition is not observed.

 Loading and unloading can be done without losses.


 Only a fraction of the solid particles is directly exposed.

 The method is costly and time consuming.


 Sticky materials, plastic substances, granular mass or crystalline materials, precipitates
and paste can be dried in a tray dryer.


Fig. Tray dryer




In drum dryer heated hollow cylindrical drum is rotate on longitudinal axis, which is dipped

into the solution to be dried.

The solution is carried as a film on the surface of the dryer and dried to form a layer. Dried

material is removed with the help of the knife.



The drum dryer consists of a horizontally mounted hollow steel drum of 0.6 to 3.0 metres

diameter and 0.6 to 4.0 metres length, whose external surface is smoothly polished.

Below the drum, feed pan is placed in such a way that the drum dips partially into the feed.

On one side of the drum a spreader is placed and on the other side a doctor’s knife is placed

to scrap the dried material.

A storage bin (or a conveyor ) is placed connecting the knife to collect the material.



Steam is passed inside the drum. Heat transfer coefficient of the drum metal is high. Heat is

transferred by conduction to the material.

Simultaneously drum is rotated at the rate of 1-10 revolutions per minute. The liquid material

present in the feed pan adheres as a thin layer to the external surface of the drum during its


The materials are completely dried during its journey in slightly less than one rotation (from

one side to another side of the drum). The dried materials are scrapped by the doctor’s knife,

which than falls into a storage bin.

The time of contact of the material with hot metal is 6 to 15 seconds only. Therefore

processing conditions such as film thickness, steam temperature are closely controlled.



The method gives rapid heat drying and mass transfer are higher

The entire material is continuously exposed to heat source.

The equipment is compact.

Heating time is short being only a few seconds.

The product obtained is completely dried and is in the final form.


Operating conditions are critical.

Attrition is not possible.

Skilled operators are essential to control feed rate ,film thickness, speed of rotation and

Maintenance cost of a drum dryer is higher than spray dryer.

It is not suitable for solutions of salts with less solubility.



Drum dryer is used for drying solution, slurries, suspensions etc.

The products dried are milk products, starch products, ferrous salts, suspensions of zinc

oxide, suspension of kaolin, yeasts, pigments, malt extracts, antibiotics, DDT, calcium,

insecticides and barium carbonates.


Fig. drum dryer