How belt and tray dryers work
Belt dryers carry a layer of product, such as sliced onion, garlic, vegetables, fruit, herbs or pelletised feed, through a series of heated zones on a perforated conveyor belt. In each zone a circulation fan blows hot air, typically 60 to 100°C, up or down through the bed, and most of the air is recirculated, with only part exhausted to remove moisture and replaced with fresh air. Tray dryers do the same in batches, with product on trays in a cabinet or truck tunnel. Heat comes from steam coils, thermic fluid or an indirect fired heater. Moist air is withdrawn by one or more exhaust fans, and fresh air enters through dampers or a supply fan.
Where the FD fan sits
- Fresh air damper
- FD (supply) fan
- Zone heater coil
- Circulation fan
- Plenum under belt
- Product bed
Each zone circulation fan sits inside or beside the dryer and handles recirculated air at the zone temperature. It pushes the air through the heater coil and up or down through the belt and product. A separate fresh air fan, where fitted, supplies make-up air to the wet zones. The circulation fans therefore handle hot, humid air, unlike most FD fans.
Air at the FD fan
| Air temperature at the fan | 60 to 100°C for circulation fans; ambient for the fresh air fan |
| Air after the heater | Zone setpoint, typically 60 to 100°C |
| Dust | Light product fines recirculated with the air |
| Humidity | Recirculated air is humid; fresh air ambient |
| Pressure at the fan outlet | Typically 30 to 80 mmWC through heater, belt and bed |
| Density at the design temperature | About 0.99 kg/m³ at 85°C, 82% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for belt and tray dryers
| Blade form | Backward curved or plug fans for circulation; axial fans on some tray dryers |
| Material of construction | SS 304 for food, aluminised or coated mild steel for other products |
| Drive and arrangement | Motor outside the hot zone with an extended shaft |
| Bearings and cooling | Bearings outside the hot air with a cooling disc |
| Flow control | Variable speed drive per zone |
| Also specified | Access door for cleaning and screens against product fines |
What this FD fan has to cope with
Uneven drying across the belt
Poor air distribution leaves wet patches at the edges or centre. Plenum design and fan selection at the right pressure give uniform flow through the bed. Speed control by zone also lets the operator correct a wet edge without changing the whole dryer. A simple anemometer survey across the belt shows where air is short.
Motor heat in the hot zone
Motors inside the dryer overheat. Placing the motor outside on an extended shaft, with a cooling disc, solves this. Bearings mounted outside the casing with a shaft seal stop hot humid air tracking along the shaft. On tray dryers with internal axial fans, motors rated for high ambient temperature and humidity are used instead.
Over-drying at the dry end
Running all zones at full flow over-dries the product at the discharge. Zone speed control lets the operator reduce air and temperature where the product is nearly dry. Lower air at the dry end also improves colour and aroma on vegetables and herbs, which are easily over-heated once most of the water has gone.
Energy and control
Circulation fans run continuously at high flow. Speed control per zone, tuned to product moisture, saves power and improves quality. Efficient plug fans reduce power compared with older forward curved units. Recirculating air within each zone and adjusting only the fresh air share keeps the heater load low. Older forward curved circulation fans are often replaced with more efficient backward curved or plug fans without changing the dryer.
ID fan and FD fan in belt and tray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | Roof or end of dryer, on the exhaust duct | Inside or beside each zone |
| What it handles | Humid exhaust air with fines and odour | Hot recirculated air |
| Temperature at the fan | 40 to 60°C | 60 to 100°C |
| Main design concern | Humidity control and condensate | Uniform air through the bed |
| Control | Exhaust humidity | Zone temperature and flow |
See also the Induced Draft (ID) Fan for belt and tray dryers.
Questions buyers ask
Do belt dryer circulation fans handle hot air?
Yes. Unlike most FD fans, they handle recirculated air at the zone temperature, typically 60 to 100°C, so bearings, motors and materials are selected for that. Motors are placed outside the hot zone wherever possible. Where the circulation fan sits inside the dryer, its motor must be rated for the zone temperature and humidity, or cooled with fresh air.
What is the difference between the circulation and exhaust fans on a belt dryer?
The circulation fans move large volumes of hot air through the product in each zone. The exhaust fan removes a small share of moist air to carry the water out of the dryer. The circulation fans therefore set uniformity and drying rate, and the exhaust fan sets humidity and heat loss.
Can tray dryers use axial fans?
Yes. Many cabinet and tunnel tray dryers use axial fans for the high-volume, low-pressure circulation across the trays. Axial fans suit the high flow and low pressure across trays. Reversing the airflow halfway through the batch, with reversible axial fans, evens out drying between the front and back of each tray.
Information needed to quote an FD fan for belt and tray dryers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Zone conditions. Temperature and air flow in each zone.
- Fan location. Whether fans are inside or outside the dryer, and space for the motor.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.