Screw-type roasting machines for dried fruits and nuts (also known as Spiral Roasters or Screw Conveyor Roasters) are designed for continuous, high-volume production. Unlike drum roasters that tumble the product in a batch, screw roasters move the product through a heated tube or trough using a large internal screw (auger), ensuring that every single piece of fruit or nut receives identical heat exposure.

1. How the Screw-Type System Works

1. Infeed: Raw dried fruits (like raisins or dates) or nuts (peanuts, cashews) enter a hopper.
2. Conveyance: A motor-driven screw shaft rotates inside a stationary tube or "U" trough.
3. Heating: Heating elements (electric, gas, or thermic oil) are wrapped around the tube or embedded below the trough. As the screw turns, it gently pushes the product forward through several heat zones.
4. Uniformity: The constant turning of the screw ensures that the product is continuously "turned over," preventing one side from scorching.
5. Discharge: The roasted product exits the other end, usually onto a cooling conveyor.

2. Technical Specifications for Industrial Models

3. Advantages Over Drum Roasters

• Continuous Output: No downtime for loading and unloading batches.
• Precise Residence Time: You can precisely control how long the product stays in the heat by adjusting the screw's rotation speed.
• Space Efficient: Because the roasting happens inside a narrow tube, these machines take up less floor space than massive rotating drums.
• Minimal Breakage: The gentle sliding motion of the screw is often safer for fragile dried fruits than the "tumbling" of a drum.

4. Key Components for "Automatic" Operation

• Variable Speed Drive: Essential for switching between different products (e.g., a 10-minute roast for cashews vs. a 3-minute "flash roast" for raisins).
• Thermal Insulation: High-quality glass wool or rock wool insulation keeps the heat inside the screw chamber, reducing energy costs.
• Automatic Feeding: Integrated with a vibratory feeder to ensure a steady, even stream of product enters the screw.
• Inbuilt Cooling: The final section of some screws is unheated or fan-cooled to bring the product temperature down before packaging.