Principle and Construction

A rotary dryer is a continuous drying equipment used for reducing the moisture content of bulk solids such as minerals, fertilizers, sand, biomass, chemicals, and food products. The principle of operation is based on direct or indirect heat transfer between a stream of hot gas and the wet material while it is continuously tumbled and agitated inside a rotating cylindrical shell. The combined effect of heat transfer, mass transfer, and mechanical movement ensures efficient and uniform drying of large volumes of material.

In a rotary dryer, the wet material is fed into a rotating inclined drum, through which hot air or gas flows either co-currently (same direction) or counter-currently (opposite direction) to the movement of the material. As the drum rotates, lifting flights (internal fins) mounted on the inside of the shell lift and shower the material through the hot air stream, maximizing surface contact and improving heat transfer. The moisture evaporates rapidly, and the dried material is discharged continuously from the lower end of the drum.

Principle

The working principle of a rotary dryer is the transfer of heat and mass between the hot air (or gas) and the wet material through convection and conduction.

• Convection: The hot drying gas transfers heat to the solid particles as it passes over and around them, evaporating surface moisture.
• Conduction: Heat is also transferred through direct contact between the hot drum surface and the material as it tumbles.
• Diffusion: The internal moisture within the material migrates to the surface and evaporates into the surrounding air stream.

The drum rotation ensures continuous mixing and exposure of new surface areas, promoting uniform drying. Depending on the process requirements, rotary dryers can operate with co-current airflow (hot air and material move in the same direction, reducing thermal stress) or counter-current airflow (hot air and material move in opposite directions, allowing higher thermal efficiency and lower final moisture).

Construction 

A rotary dryer consists of the following main components:

• Rotating Shell (Drum):
  The main body of the dryer is a large, cylindrical shell, slightly inclined (1–5°) from the horizontal axis. It is typically fabricated from carbon steel or stainless steel, depending on the product and temperature. The shell length-to-diameter ratio generally ranges from 4:1 to 10:1. The inner surface may be lined with refractory bricks or stainless steel for high-temperature or corrosive applications. The drum rotates slowly, typically between 2–10 rpm, and is supported on tyres (riding rings) and support rollers.
• Flights (Lifters):
  Inside the drum, lifting flights or fins are welded along the interior circumference. These flights lift the material from the bottom and shower it through the hot gas stream as the drum rotates, increasing the contact area for efficient heat and mass transfer. The flight design (radial, spiral, or cascade type) varies depending on the material’s flow characteristics and moisture content.
• Feed System:
  The wet material is introduced into the upper end of the drum through a feed hopper or screw conveyor. A rotary valve is often used to regulate feed rate and maintain a seal between the feed system and the hot drying chamber.
• Heating System:
  The drying air or gas is heated using steam coils, gas burners, oil-fired heaters, or hot flue gases from a furnace. The air is directed into the drum through a hot air inlet duct equipped with a distribution cone or manifold to ensure even airflow.
  • In direct-type dryers, the hot gases contact the material directly.
  • In indirect-type dryers, the drum is heated externally, and the material does not contact the gas directly (used for dusty or toxic products).
• Drive Assembly:
  The drum is rotated by an electric motor and gearbox through a chain, gear, or friction drive system. Large units may include a girth gear and pinion drive for smooth rotation. The drive assembly is supported on trunnion rollers with adjustable alignment to maintain balance.
• Air Exhaust and Dust Collection System:
  The moist exhaust air exits from the opposite end of the dryer through an exhaust duct. A cyclone separator, bag filter, or scrubber removes entrained fine particles from the air stream before venting. Some systems include induced draft (ID) fans to maintain the desired airflow and pressure balance.
• Discharge System:
  The dried product leaves the lower end of the drum through a discharge chute or screw conveyor. Depending on the product, it may then pass through a cooler, classifier, or conveyor for further processing or packaging.
• Support Structure and Seals:
  The drum rests on support rollers mounted on a rigid steel base frame. Sealing rings or flexible seals are provided at both feed and discharge ends to prevent air leakage and maintain pressure control.
• Instrumentation and Controls:
  The dryer is equipped with temperature sensors (inlet/outlet air), pressure gauges, speed indicators, and flow controls. A PLC–HMI control panel may be used to monitor and automate drying parameters for consistent performance.