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 ID fan sits
- Shell or husk furnace
- Heat exchanger
- Drying chamber
- Exhaust duct
- Exhaust (ID) fan
- Outlet
The exhaust fan sits on the chamber roof or end, drawing moist air above the kernel bed. On improved designs, a furnace ID fan sits after the heat exchanger flue outlet, pulling flue gas at 150 to 250°C to the chimney and keeping the flue side under suction so that smoke cannot leak into the drying air. Suction at the exhaust fan is typically 20 to 60 mmWC.
Gas at the ID fan
| Air temperature, normal | 45 to 60°C at the exhaust fan |
| Maximum or upset | Up to 85°C if the furnace is overfired |
| Dust at the fan | Negligible; some shell ash on direct systems |
| Moisture and dew point | Very humid early in the batch; oily vapour from the kernel |
| Oily deposits | Coconut oil condenses on cool surfaces and collects dust |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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. |
ID Fan selection for copra dryers
| Blade form | Backward curved or axial for low pressure |
| Material of construction | SS 304 or coated steel for humid, oily air; mild steel for the furnace ID fan |
| Drive and arrangement | Direct mounted or belt driven |
| Bearings and cooling | Cooling disc on the furnace ID fan |
| Flow control | Variable speed drive by batch stage |
| Also specified | Drain for oily condensate and access door |
What this ID fan has to cope with
Mould from slow early drying
Kernel at 50% moisture is an ideal medium for mould in the first 24 hours, and slow drying raises the risk of aflatoxin. If the exhaust fan cannot remove moisture fast enough at the start of the batch, the chamber stays humid. The fan is sized for the peak evaporation rate early in the batch, not the average.
Oily film on the fan
Coconut oil vapour and fine kernel particles condense on the impeller and casing, building a sticky film that collects dust and goes rancid between batches. Smooth stainless surfaces, an access door and periodic cleaning with hot water and detergent keep the fan balanced and free of odour.
Smoke leaking into the chamber
A cracked heat exchanger or pipe grid lets shell smoke into the drying air, giving dark, smoky copra that sells at a lower price. Holding the flue side under suction with a furnace ID fan makes any leak run from the clean air into the flue gas instead, and regular inspection of the exchanger finds cracks early.
Energy and control
Copra dryers burn coconut shell and husk that the processor often has on hand, but fuel still costs labour and the drying time limits throughput. Speed control on the exhaust fan, high in the first day when the kernel releases most of its water and lower later, cuts fuel and shortens the batch. Recirculating air that is not yet saturated keeps heat inside the dryer.
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 Forced Draft (FD) Fan for copra dryers.
Questions buyers ask
Why is my copra dark and smoky?
Usually smoke is reaching the kernel, either because the dryer is a direct kiln or because the heat exchanger in an indirect dryer is leaking. An indirect copra dryer with a sound exchanger and a furnace ID fan holding the flue side under suction gives white, smoke-free copra.
What temperature should a copra dryer run at?
Typically 60 to 80°C hot air, highest at the start when the kernel is wet and cooled by evaporation, and reduced later. Higher temperatures can scorch the kernel and darken the oil. The exhaust fan rate is set to match each stage. Kernel surface temperature is the real limit.
Does a copra dryer need an exhaust fan?
Yes, for fast moisture removal early in the batch, which is what prevents mould. Natural ventilation works on small kilns but depends on the weather. A speed-controlled exhaust fan gives consistent drying times and copra quality across the season. It also keeps the chamber at slight suction, which helps keep any smoke out.
Information needed to quote an ID fan for copra dryers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Dryer capacity. Number of nuts per batch and drying time.
- Furnace. Fuel and whether a furnace ID fan is needed.
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.