Learn everything about drum dryers, including their principle, construction, types, working process, applications, advantages, and limitations. Ideal for drying viscous and heat-sensitive materials.
Drum Dryer
A drum dryer is a continuous drying system widely used in pharmaceutical, chemical, food, and allied industries for drying liquids, slurries, suspensions, and pastes. Due to its simple design, fast drying rate, and ability to handle viscous materials, the drum dryer remains a reliable drying solution even after decades of industrial use.
Principle of Drum Dryer
The working principle of a drum dryer is based on conduction heat transfer. During operation, a thin layer of the product is applied to the outer surface of a rotating, steam-heated drum. Steam supplied to the inside of the drum rapidly evaporates the moisture present in the product. After nearly one full rotation, the dried film or powder is scraped off using a sharp knife or doctor blade.
Construction of Drum Dryer
A drum dryer is designed for durability, hygiene, and efficient heat transfer. The unit is typically constructed using cast iron and stainless steel, making it resistant to corrosion and chemical attack.
Key construction features include:
- Horizontally mounted hollow drums
- Drum diameter: 0.6 to 3.0 meters
- Drum length: 0.6 to 4.0 meters
- Smooth, polished drum surfaces for uniform drying
- Internal heating using steam or heating coils
- Drum rotation speed: 1–10 rpm, controlled by an electric motor
- Heat transfer regulated through thermostatic control
The precise machining of the drums ensures consistent and efficient heat transfer throughout the equipment’s lifespan.
Types of Drum Dryers
Single Drum Dryer
A single drum dryer consists of one steam-heated rotating drum. The product is applied either to the top or underside of the drum surface, where it dries rapidly and is scraped off after one rotation.
Double Drum Dryer
A double drum dryer uses two parallel drums rotating in opposite directions. The product is fed into the narrow gap (pool) between the drums, forming a uniform film on both surfaces.
Advantages of double drum dryers:
- Higher drying capacity
- Suitable for low-to-medium viscosity products
- Adjustable gap allows precise film thickness control
- Consistent and uniform drying
Vacuum Drum Dryer
A vacuum drum dryer operates under reduced pressure inside an airtight enclosure. Two steam-heated drums rotate under high vacuum, allowing drying at lower temperatures.
Key benefits include:
- Reduced oxidation
- Preservation of heat-sensitive materials
- No product recirculation or residue buildup
- Easy inspection and maintenance
- Capability to operate under inert gas (e.g., nitrogen)
This type of drum dryer is ideal for drying enzymes, proteins, vitamins, and solvents.
Working of Drum Dryer
Working of Single Drum Dryer
In a single drum dryer, steam enters the drum and condenses on the internal surface, providing uniform heat. Condensate is continuously removed to maintain effective heat transfer. The product forms a thin film on the outer surface and dries within seconds. The dried layer is then removed using a knife, while vapors are extracted if required.
Working of Double Drum Dryer
In a double drum dryer, liquid material is fed between the rotating drums. As the drums rotate, the material spreads evenly and dries rapidly due to direct contact with the heated metal surfaces. Each drum is equipped with a knife to scrape off the dried product, which is collected in a receiver.
Product Removal System
The dried product is removed using a doctor blade or alloy steel knife, mounted on a cast-iron knife bar assembly. Pneumatic cylinders ensure consistent knife pressure against the drum surface.
Additional features include:
- Oscillating knife systems for longer blade life
- Screw conveyors to break flakes into manageable pieces
- Optional air cooling for heat-sensitive products
Uses of Drum Dryer
Drum dryers are widely used for:
- Drying sticky, viscous liquids and pastes
- Drying heat-sensitive materials under vacuum
- Processing solutions, slurries, and suspensions
- Drying starch products, zinc oxide, calcium suspensions, ferrous salts, and insecticides
- Applications in food, pharmaceuticals, chemicals, cosmetics, and solvent recovery
Industries such as instant beverage production, chocolate manufacturing, and specialty chemicals rely heavily on vacuum drum drying.
Merits of Drum Dryer
- Very fast drying rate (only a few seconds)
- Continuous operation, suitable for 24/7 production
- Uniform heat application with minimal product damage
- Energy-efficient due to direct steam-to-wall heat transfer
- Simple operation with low maintenance requirements
- Compact, enclosed design improves hygiene
- Excellent control over feed rate and film thickness
- Ideal for doughy and pasty materials
Demerits of Drum Dryer
- Operating conditions must be closely monitored
- Requires skilled operators to control temperature, speed, and feed rate
- Not suitable for low-viscosity or dilute solutions
- Precise control of film thickness is critical for consistent quality
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