Principle

A Rotary Louvre Dryer (also known as a Louver Dryer) is a direct-contact type rotary dryer designed for drying free-flowing, granular, and crystalline materials such as minerals, fertilizers, grains, and chemicals. It is similar in appearance to a rotary cascade dryer, but differs in its internal construction — instead of lifting flights, it contains stationary or semi-stationary angled louvres (or louvers) fitted along the periphery inside the drum.

It operates on the principle of direct heat transfer by convection, where hot air or gas passes through the louvres and the bed of material as it moves through the rotating drum. The louvre arrangement ensures that the material is continuously turned, mixed, and aerated, promoting uniform heat transfer and efficient drying.

Principle of Operation

The principle of a rotary louvre dryer is based on convective heat transfer between hot air (or flue gas) and moist solids that are continuously agitated inside a rotating, inclined cylinder fitted with louvres.

• The wet feed material enters one end of the rotating drum, while hot gases flow through it, either in a co-current or counter-current direction.
• The louvres, which are fixed plates or angled vanes mounted on the inner wall of the drum, lift, shower, and turn the material through the hot gas stream.
• As the drum rotates, the material cascades through the heated air repeatedly, exposing fresh surface area for drying.
• Heat transfer occurs primarily by convection, while some conduction also takes place through contact with the heated metal surfaces.
• Moisture from the material evaporates, and the vapor-laden air is carried away by the exhaust system. The dried product is discharged continuously from the opposite end.

This louvre design provides controlled agitation and uniform air distribution, resulting in more efficient drying than a plain rotary drum and gentler handling than a flighted cascade dryer.

Construction 

A Rotary Louvre Dryer consists of the following main components:

1. Rotating Shell (Drum):

The dryer consists of a cylindrical shell that is slightly inclined (1°–5°) from the horizontal, allowing the material to move gradually from the feed end to the discharge end by gravity.

• The drum is fabricated from mild steel or stainless steel depending on the product and process conditions.
• The length-to-diameter ratio typically ranges from 4:1 to 10:1.
• The drum rotates at a slow speed of 2–10 rpm, ensuring adequate mixing and residence time.
• The drum is supported on rollers or trunnions and driven by an electric motor and reduction gearbox through a chain or girth gear drive.

2. Louvres (Internal Fittings):

The most distinctive feature of the rotary louvre dryer is the series of fixed or adjustable louvres fitted inside the drum.

• These angled plates are arranged in rows along the inner circumference of the shell.
• The louvres are designed to lift and gently shower the material through the hot gas stream while allowing air to pass freely through the solid bed.
• The angle and spacing of the louvres can be varied to control the residence time, mixing intensity, and drying efficiency.
• Some designs use adjustable or removable louvre sections for different materials or maintenance convenience.

This louvre arrangement ensures that the material is uniformly distributed and continuously agitated, avoiding dead zones and overheating.

3. Feed System:

The wet material is introduced at the higher end of the drum through a feed chute or screw conveyor. The feed rate can be controlled using a rotary valve or variable-speed feeder to maintain a steady material flow.

4. Hot Air or Gas Supply System:

The drying medium (hot air or flue gas) is generated by a furnace, gas burner, or hot air generator and introduced into the drum through an air distributor or duct.

• The air may flow in either co-current or counter-current configuration:
  • Co-current: Hot air enters from the feed end and flows in the same direction as the material — gentler drying suitable for heat-sensitive materials.
  • Counter-current: Hot air enters from the discharge end, providing higher thermal efficiency and lower final moisture.

Typical inlet air temperatures range from 150°C to 600°C, depending on the product.

5. Exhaust System:

The moisture-laden exhaust air exits from the opposite end of the drum and passes through a dust collection system, such as a cyclone separator, bag filter, or scrubber, to remove entrained particles before being released to the atmosphere.

6. Discharge System:

The dried material exits through a discharge chute or screw conveyor at the lower end of the drum. In some cases, a rotary airlock valve is installed to prevent air leakage while allowing continuous discharge.

7. Drive Mechanism:

The drum is rotated by an electric motor with a reduction gearbox, using a chain drive, belt drive, or girth gear. A variable frequency drive (VFD) is often provided to control drum speed and optimize drying time.

8. Insulation and Enclosure:

The outer surface of the drum is insulated with mineral wool or glass wool and covered with a sheet metal cladding to minimize heat loss and ensure operator safety.

9. Instrumentation and Controls:

A rotary louvre dryer is equipped with temperature sensors, airflow meters, and pressure gauges at both ends.
Modern systems are automated with PLC–HMI control panels that regulate inlet temperature, drum speed, and air velocity for consistent drying and energy efficiency.