The basic principle of a food oven dryer can be summarized as follows: hot air is generated through electric heating tubes or steam heat exchangers, driven by a fan, and forced and evenly circulated inside the box, constantly in contact with the surface of the food. Hot air serves as a "carrier" and completes two tasks simultaneously: 1) transferring heat to food; 2) Take away the water vapor evaporated from food. The entire process is a continuous process of heat and mass transfer.
Detailed workflow and key steps
It can be decomposed into the following core components:
1. Hot air generation and circulation (core power system)
Heating: The electric heating tube or steam finned heat exchanger installed in the oven duct is activated to heat the air passing through it.
Drive and circulation: High power centrifugal fans or axial fans are started to suck in air from the working chamber of the box, forcing it to pass through the heater and become dry hot air.
Uniform distribution: The heated hot air is evenly blown horizontally or vertically towards the food on the tray through specially designed air ducts (usually located at the top, bottom, or sides of the oven) and uniform air plates (porous plates). This is the key to ensuring temperature uniformity throughout the box.
2. Heat and mass exchange (occurring on the surface and inside of food)
Heat transfer: The flowing hot air comes into contact with the surface of food at a lower temperature, and heat is transferred to the food through convection and conduction.
Water evaporation: After food obtains heat, the internal water (free water and partially bound water) obtains energy and transforms from liquid to gas (water vapor), escaping from the surface of the food.
Mass transfer: A moist saturated air layer (also known as the "boundary layer") is formed on the surface of the food. The flowing hot air will quickly blow away this layer of moist air, replacing it with dry hot air, thereby maintaining the humidity difference (i.e. "driving force") between the surface of the food and the air, allowing moisture to continue evaporating.
3. Moisture removal and temperature control (maintaining drying power)
Dehumidification: The hot air that has absorbed water vapor (which has now become humid and hot air) will be partially discharged outside the box through an adjustable opening of the dehumidification port under the action of the fan. Dehumidification is to continuously reduce the relative humidity of the air inside the box, preventing saturation and stopping evaporation.
Ventilation: At the same time, fresh air outside the box is sucked in through the ventilation port (usually through coarse filtration), mixed with the circulating air inside the box, and heated and utilized again. This' dehumidification air replenishment 'process is dynamically balanced to ensure efficient drying process.
Temperature control: Temperature sensors (such as PT100) monitor the temperature inside the box in real time and transmit signals to intelligent temperature controllers (PLC or PID controllers). The temperature controller precisely maintains the set drying temperature by controlling the on/off of the heater or adjusting the opening of the steam valve, preventing the temperature from being too high or too low.
4. Moisture migration inside food
For thicker foods, the drying process occurs not only on the surface. The moisture inside the food will gradually diffuse towards the surface due to concentration gradients (wet inside, dry outside) and temperature gradients (hot outside, cold inside), and then be carried away by hot air on the surface. This process is usually slower than surface evaporation, so it is the limiting step of the entire drying process.