How copra dryers work
Split coconut kernel at about 50 to 55% moisture is dried to around 6% moisture to make copra for oil milling or edible use. Traditional kilns burn coconut shell and husk directly under the kernel, which gives smoky, dark copra of lower value. Improved indirect dryers burn shell, husk or other biomass in a furnace whose flue gas passes through a heat exchanger or a pipe grid, so only clean hot air at about 60 to 80°C reaches the kernel. Batch platform dryers, tunnel trolley dryers and continuous belt dryers are all used. Mould grows on kernel that dries slowly in the first 24 hours, so fast early moisture removal matters as much as the final temperature.
Where the FD fan sits
- Fresh air inlet
- Heat exchanger
- FD (hot air) fan
- Plenum
- Kernel bed
- Exhaust
The hot air fan draws fresh air, sometimes mixed with recirculated chamber air, over the heat exchanger or pipe grid and pushes it into the plenum under the kernel bed or across the trolleys in a tunnel. It handles clean air at about 60 to 80°C. Pressure is low, typically 20 to 60 mmWC through the plenum and bed. The furnace combustion air fan, where fitted, is a separate small fan on ambient air.
Air at the FD fan
| Air temperature at the fan | 60 to 80°C after the heat exchanger |
| Air after the fan | Same, into the plenum |
| Dust | Clean air |
| Humidity | Ambient; high in coastal areas |
| Pressure at the fan outlet | Typically 20 to 60 mmWC |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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 copra dryers
| Blade form | Backward curved or axial |
| Material of construction | Mild steel or coated steel |
| Drive and arrangement | Belt driven, motor outside the hot air |
| Bearings and cooling | Cooling disc |
| Flow control | Damper or variable speed drive |
| Also specified | Inspection door |
What this FD fan has to cope with
Uneven drying across the bed
A poorly designed plenum leaves wet corners where mould starts and dry spots where kernel scorches. A fan with enough pressure for the full bed, and a plenum that spreads air evenly, give consistent copra. Turning the kernel once during the batch also helps on deep beds.
Scorching near the inlet
Air that is too hot scorches the kernel nearest the air inlet and darkens the copra. Controlling furnace firing and fan speed on a temperature sensor in the plenum keeps the air in range, and a damper on fresh air lets the operator cool it quickly.
Coastal corrosion
Most copra dryers are near the coast, where humid, salty air corrodes mild steel fans, motors and fasteners. Hot-dip galvanised or coated steel, stainless fasteners and motors with suitable enclosure last much longer in this environment. An inlet hood that sheds rain helps too. Inspecting the fan before each season catches corrosion early.
Energy and control
The hot air fan runs throughout the batch. Speed control matched to the drying stage saves power and fuel: high flow while the kernel is wet, lower flow later when the risk is scorching rather than mould. Recirculating part of the warm chamber air in the later stages recovers heat and cuts shell consumption.
ID fan and FD fan in copra dryers
| ID fan | FD fan | |
|---|---|---|
| Position | Chamber roof or end | After the heat exchanger |
| What it handles | Humid oily exhaust | Clean warm air |
| Temperature at the fan | 45 to 60°C | 60 to 80°C |
| Main design concern | Fast early moisture removal | Even airflow through kernel |
| Control | Batch stage | Air temperature |
See also the Induced Draft (ID) Fan for copra dryers.
Questions buyers ask
What is the FD fan on a copra dryer?
On an indirect copra dryer it is the hot air fan that draws air over the heat exchanger and pushes it through the kernel bed. Improved furnaces also have a small combustion air fan, which handles ambient air and is separate from the drying air.
Does the copra dryer fan handle hot air?
Yes. In the usual layout it draws air through the heat exchanger and handles clean air at about 60 to 80°C. A cooling disc on the shaft and a motor mounted outside the hot air stream protect the bearings and windings. Bearings are greased often.
What fan suits a copra dryer?
A backward curved fan for bed dryers, where air must pass through the kernel, or an axial fan for tunnel and tray dryers, where air flows across trolleys at low pressure. Both are selected for warm, humid coastal conditions. The motor is sized with margin for the long batches.
Information needed to quote an FD fan for copra 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.
- Bed area. Kernel bed area and depth.
- Air flow. Air flow and temperature required.
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.