The drum dryer has been
used for a long time for drying sheets of paper or cloth and, more recently,
for drying liquids and pastes. Drum dryers in general and the various types of
drum dryers (atmospheric single drum, vacuum single-drum, and double-drum
dryers) in particular are selected or rejected for any given drying requirement
based on their individual operating characteristics and costs.
Principle of Drum Dryer
Drum dryers operate by
applying a thin layer of the product to be dried to the outside of a rotating
drum. The drum is internally heated by steam which quickly evaporates any liquid
from the product. After almost one full revolution the remaining dried material
is removed from the drum by a knife as a film or powder.
Construction of Drum Dryer
Drum dryer is a moving
bed dryer. The unit is constructed from cast iron and stainless steel, providing
cleanliness and resistance to chemical attacks. The drums are engineered with
hard chrome plating or chrome-plated over nickel to ensure maximum heat
transfer and to accommodate high steam temperatures and pressures, precision
machined to provide maximum and reproducible heat transfer throughout its life.
The drums are hollow horizontally mounted 0.6 to 3.0 meters in diameter and 0.6
to 4.0 meters in length, whose external surface is smoothly polished. Steam or
heating coils can be used inside the drum for heat generation. Heat is
transferred by conduction to the material that can be controlled with a
thermostat. The drum is rotated with a motor device at 1-10 r.p.m.
Fig.1: Single Drum Dryer with Dip Roller Feed
Double drum dryer:
The extra capacity can be provided by using a double drum dryer. A double drum
dryer is also called a dual drum and is often used for products with low to
medium viscosity. In this system, the product is fed into a pool between the
two drums which always turn in opposite directions. The small space between the
drums can be set accurately so that the desired film layer can be obtained.
Other combinations are possible depending on the product and the desired result.
Fig.2: Double Drum Dryer with Nip Feed.
Vacuum drum dryer:
A drum dryer that operates under vacuum allows the product to be dried at lower
temperatures. These drum dryers provide a higher capacity and less product loss
under vacuum. Two steam-heated drums turn in an air-tight casing under a high
vacuum. The liquid is fed between the drums from the top, dried, and scraped off
before the drums have fully rotated. There is no accumulation of product
residues and no recirculation of the material. The drum dryer is easily
accessed for inspection and maintenance. There is a wide range of applications,
including damage prevention to heat-sensitive product components or evaporation
of solvents at low temperatures. The vapors collected are condensed elsewhere. The
absence of atmospheric pollution and climatic conditions gives reproducible
product quality. Feed materials can benefit from such careful treatment.
Evaporation takes place within a few seconds without the risk of oxidation.
Thermolabile materials such as enzymes and proteins are preserved and blockage
of proteins is prevented. The end product can be dissolved easily into a
liquid. Chemical products, too, can be processed in the dryer by using a vacuum
process and undergo drying treatment. It can use a gas such as nitrogen to keep
the environment inert.
Accessories: Dryers
for pharmaceutical applications are generally provided with a vapor canopy and
extract system to remove the vapors from the operating area. Dryers for
hazardous materials may be provided with complete dust and fume-tight
enclosures.
Working of Drum Dryer
Single drum dryer:
The robust construction of drum dryers made them be used efficiently over the last
60 years and still are in use today. The drum body of the drum dryer is heated
on the inside by steam. Steam heating gives a uniform temperature distribution
over the drum surface to provide consistent product quality. The steam
condenses on the inside of the drying drum. The condensate is continuously
removed from the drum so that the largest possible surface area remains
available on the inside of the drum for condensation of the steam. The steam
system is a closed system, which means that the product cannot come into
contact with the steam or condensate. Depending on the design of the drum
dryer, the product is applied continuously as a thin film on the underside or
top of the main drum. As the drum turns it is heated on the inside and the
product dries on the outside of the drum dryer. Short exposure to a high
temperature reduces the risk of damage to the product. The water or solvent
evaporates and leaves from the surface of the drum. If necessary, the vapor can
also be suctioned-off locally around the drum. The dried product layer finally
reaches the knife and is scraped off.
Double drum dryer:
The liquid to be dried is fed into the valley created by the two drums, where
it is applied to the drums as they rotate together. The dry product is removed
by a knife on each drum. Wet materials are completely dried during slightly
less than one rotation from one side to another side of the drum. Dried
materials are scrapped by a knife, which then falls into a product receiver.
Contact time of the material with hot metal is 6 to 15 seconds only. Therefore,
processing conditions such as film thickness and drum temperature are closely
monitored and controlled. The shaft-mounted main drive speed is adjusted by an electronic
inverter in the control panel. The liquid or paste material present in the feed
pan adheres as a thin layer to the external surface of the drum during its
rotation.
Product Removal: The
product is removed from the drum dryer surface using an alloy steel doctor
blade or knife rigidly clamped in a cast iron knife bar assembly. The knife is
applied to the surface of the drum by pneumatic cylinders located at each end
of the dryer. For improved knife life and reduced drum maintenance, an
oscillating knife system is fitted with either conventional or disposable
knives. After removal from the drum, the product is collected by a transverse
screw conveyor specially designed to break up the product film or flakes into
easy-to-handle particles. For products that may require cooling after removal
from the drum air.
Advantages of Drum Dryer
- Drum dryer gives a rapid (few seconds) drying and its mass transfer rate is higher.
- The entire material is continuously exposed to uniform heat. This process is characterized by a short drying time and minimum product heating.
- The equipment is compact 100% closed system and tailored models are available.
- Their simple operation makes them run with a minimum of training and requires no specialist maintenance.
- Drum dryers are economical with cheap installation and 24 hours a day of continuous production.
- It can have vacuum facilities to prevent dust emission thereby giving a guarantee of optimal hygiene during the complete production process.
- This dryer works with minimum energy consumption and does not require large dust recovery systems such as filters.
- The heat is transferred by steam to the (metal) wall, which in turn transfers it to the product on the other side of the wall. All the heat transferred by the wall is used to dry the product and does not leave the machine or chimney unused making this dryer a much more efficient process.
- Using these dryers product supply can be controlled. Furthermore, drums create a kneading effect that prevents the formation of lumps in sticky products.
- A perfect distribution over the entire length of the drum makes the system ideal for processing doughy or pasty products.
- The temperature of the drum can be controlled independently for the required drying task by setting the steam pressure and thus are best suited for processing certain heat-sensitive products.
Disadvantages of Drum Dryer
- The operating conditions are critical and need to be monitored.
- Skilled operators are needed to control feed rate, film thickness, speed of rotation, and temperature.
- It is not suitable for low concentration solutions or suspensions of low viscosity.
Applications of Drum Dryer
- Drum dryer is used in the drying of liquids and pastes of sticky and highly viscous products.
- It is used under a vacuum for drying temperature-sensitive products such as vitamins, proteins, yeasts, pigments, malt extracts, hormones, and antibiotics.
- It has a closed process area that protects the product and/or the environment.
- Drum dryer is used for drying solutions, slurries, suspensions, etc.
- Used to dry products such as starch products, ferrous salts, suspensions of zinc oxide, suspension of kaolin, calcium, insecticides, barium carbonates, etc.
- Vacuum drum drying is used for instant beverages, chocolate products, chemicals, and recovery of solvents, foods and beauty soaps, etc.
Make sure you also check our other amazing article on : Tray Dryer