Principle 

A tunnel oven is a type of continuous heating and drying system used extensively in the food, chemical, ceramic, and metallurgical industries for baking, curing, or drying products. The principle of operation of a tunnel oven is based on heat transfer by conduction, convection, and radiation. The product is placed on a moving conveyor belt or trays, which travel slowly through a long, enclosed heating chamber (the “tunnel”). As it moves through different temperature zones, it is heated uniformly by hot air, steam, or radiant energy until it reaches the desired temperature or moisture content.

The process relies on the controlled circulation of heated air or gases to ensure efficient and uniform heat transfer to the product. The temperature, air velocity, and residence time are carefully regulated to achieve consistent quality. In essence, a tunnel oven functions as a continuous process oven, providing steady throughput with minimal manual handling and high thermal efficiency.

Principle of Operation

The working principle of a tunnel oven is straightforward:
Products to be baked, cured, or dried are loaded onto a conveyor belt, mesh band, or trolleys, which move at a controlled speed through a thermally insulated tunnel divided into several heating zones. Each zone maintains a specific temperature and airflow pattern depending on the stage of the process.

Heat is transferred to the product primarily through convection (hot air flow), conduction (contact with trays or belt), and radiation (infrared or gas-fired burners). As the product progresses through the oven, it absorbs heat gradually and uniformly. The moisture or volatile components evaporate and are removed by the exhaust air system. By adjusting conveyor speed and zone temperatures, the desired baking, drying, or curing profile can be achieved.

Construction

A tunnel oven consists of several key components designed to ensure uniform heating, efficient operation, and continuous processing:

• Oven Tunnel / Chamber:
  The main body of the oven is a long, thermally insulated enclosure made of mild steel or stainless steel panels, often lined with high-temperature insulation material such as mineral wool or ceramic fiber to prevent heat loss. The tunnel length typically ranges from 5 to 60 meters, depending on the production capacity and product type. The chamber is divided into multiple zones—pre-heating, baking/heating, and cooling—each equipped with independent temperature and airflow control.
• Conveyor System:
  The conveyor transports the product through the oven. It may be a steel mesh belt, chain conveyor with trays, or a wire mesh band, depending on the product and temperature. The conveyor runs the full length of the oven and is driven by an electric motor with variable-speed control (VFD), allowing adjustment of residence time. In bakery ovens, the conveyor is often perforated to allow hot air to circulate around the products evenly.
• Heating System:
  Heat energy is supplied by electric heaters, gas burners, oil-fired heaters, or steam coils. The heating system may be direct-fired (combustion gases contact the product chamber) or indirect-fired (heat exchangers transfer heat from combustion gases to circulating air). The air inside the oven is heated to temperatures typically between 100°C and 300°C, depending on the product requirements.
• Air Circulation System:
  One of the most critical features is the air circulation system, which ensures uniform temperature and drying throughout the tunnel. Centrifugal or axial flow fans circulate hot air across the conveyor and around the product. The air can flow horizontally, vertically, or crosswise, depending on the oven design. Baffles and ducts are used to control the airflow direction and velocity. In modern tunnel ovens, recirculation of hot air is employed to increase energy efficiency.
• Exhaust and Ventilation System:
  As moisture or gases are released from the product, they are removed by an exhaust system that maintains proper humidity and air balance inside the oven. The exhaust air passes through filters or heat recovery systems before being vented to the atmosphere.
• Temperature Control and Zoning:
  The tunnel oven is divided into multiple temperature zones, each with independent temperature sensors, thermocouples, and control loops. This zoning allows different heating profiles along the oven length — for example, preheating, drying, and final curing zones. A PLC–HMI control panel enables precise control of temperature, airflow, and conveyor speed.
• Cooling Section (Optional):
  After heating or drying, the product may pass through a cooling section, where ambient or chilled air is blown across it to bring it down to a safe handling temperature before discharge.
• Insulation and Outer Shell:
  The oven’s external surface is insulated to reduce heat loss and ensure operator safety. The outer casing is usually made of painted mild steel or stainless steel panels for durability and easy cleaning.
• Feed and Discharge Systems:
  The feed end of the oven includes loading conveyors or feeders for uniform product feeding. The discharge end may include a cooling conveyor, stacking system, or packaging line, allowing continuous downstream operations.