Direct Heat Rotary Dryer

Introduction

Direct heat rotary driers are used to lower the moisture content of materials passing through the dryer.

Materials are directly heated using heat from a combustion process; the fuel used is typically gas although pulverised coal, light fuel oil or waste process gasses could also be used. As the material comes into direct contact with the hot gas path, direct heat dryers are not favoured in the edible food and drink industries.

Materials entering a direct heated dryer should not be overly heat sensitive because an even heat distribution throughout the dryer cannot be assumed. It's important that the temperature across the dryer length is as even as possible, as large temperature differences could cause distortion of the dryer shell. Where greater temperature control and a more even heat distribution is required, an indirect dryer should be used (usually utilizing steam).

Direct heat rotary driers are utilized in many industries, including mining, agriculture and fertilizer (to name a few). Smelting plants use driers in order to ensure all moisture content is removed prior to smelting as any entrained moisture could lead to a molten explosion.

Indirect rotary driers often use steam as a heating medium. These types of dryer are prevalent in the oil seed, chocolate and pulp industries (to name a few).

Process

Material is gravity fed into the dryer at a constant rate. Flights mounted to the interior of the dryer shell have two purposes. The first purpose is to lift the material for a quarter or half a turn, then allow it to fall back to the base due to gravity; this process is repeated several times as the material travels through the dryer. The second purpose is to convey the material towards the discharge end. Both purposes are achieved due to the shape of the flights.

There are two ports at the discharge end of the dryer. Dried material exits the dryer through the lower discharge port whilst hot gasses and contaminants exit through the upper port. Contaminants are usually in the form of light density particles such as dust. The hot gas is often passed through a cyclone to extract these contaminants, then recirculated to the dryer to save energy and thus cost.

Design and Features

An electric three phase motor is used to rotate the dryer shell. Notice the motor is attached to the shell via gears; this allows the motor to rotate the dryer slowly without drawing a large current during operation and start-up. It is not unusual for the motor speed to be controlled by a frequency drive, this allows the material’s time within the dryer to be increased or decreased. The time the material is within the dryer is known as the ‘retention time’ or ‘contact time’.

Rotational force applied by the electric motor is transferred to the conditioner via spur gears. Gear faces should be inspected periodically for cracks and wear, as this could be a cause of conditioner failure and associated business interruption.

Support rollers and riding rings are used to support the weight of the conditioner along its entire length; these parts should also be inspected periodically.

3D Model Details

This 3D model shows all major components associated with a typical direct heated rotary drier, these include:

  • Wet Feed Port
  • Hot Gas Inlet
  • Dryer Shell
  • Riding Ring
  • Support Roller
  • Three Phase Motor
  • Drive Train Assembly
  • Discharge Hood
  • Product Discharge
  • Material Flights
  • Hot Gas Discharge
Encyclopedia - saVRee