How chilli and spice dryers work
Red chilli, turmeric, black pepper, ginger and coriander are traditionally sun-dried, but mechanical dryers give faster, cleaner and more consistent product. Chilli at 65 to 80% moisture is dried to about 10% in batch bin or tray dryers, tunnel trolley dryers or multi-stage belt dryers. Boiled turmeric fingers and pepper berries follow similar routes. Hot air at typically 55 to 70°C is made by a diesel or LPG burner, a biomass furnace or a thermic fluid heater, nearly always through a heat exchanger so that smoke does not contaminate the spice or raise polycyclic aromatic hydrocarbons. Circulation fans move the air through the bed, and exhaust fans remove moist air carrying pungent dust from chilli skins and seeds.
Where the FD fan sits
- Fresh air damper
- Burner combustion air fan
- Heat exchanger
- FD (circulation) fan
- Plenum
- Chilli bed
The circulation fan sits beside the heat exchanger and pushes heated air into the plenum under the bin floor, or across the trays in a tunnel. It handles a mix of recirculated and fresh air at about 55 to 70°C. Pressure covers the exchanger and bed, typically 30 to 100 mmWC; deep bin dryers need the higher end. The burner's own combustion air fan is small and handles ambient air.
Air at the FD fan
| Air temperature at the fan | 55 to 70°C, recirculated air |
| Air after the fan | Same, into the plenum |
| Dust | Light chilli dust in recirculated air |
| Humidity | Moderate to high early in a batch |
| Pressure at the fan outlet | Typically 30 to 100 mmWC |
| Density at the design temperature | About 1.04 kg/m³ at 65°C, 87% 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 chilli and spice dryers
| Blade form | Backward curved for bin dryers; axial for tray tunnels |
| Material of construction | SS 304 or coated steel |
| Drive and arrangement | Belt driven or direct, motor outside the hot air |
| Bearings and cooling | Cooling disc where the fan handles 60°C and above |
| Flow control | Variable speed drive |
| Also specified | Screens against chilli stalks |
What this FD fan has to cope with
Uneven drying in deep beds
Chillies pack unevenly in bins, and air takes the easiest path, leaving wet pockets that grow mould. A fan with enough pressure for the full bed depth, and a plenum that spreads air evenly under the floor, give even flow. Reversing the airflow or turning the bed midway also helps.
Colour loss from over-heating
The red colour of chilli, measured as ASTA value, falls if the air is too hot or the product sits at high temperature at the end of the batch. Controlling air temperature and reducing fan flow once most of the water is gone protects colour and pungency, which set the price.
Stalks and seeds into the fan
Loose stalks, calyxes and seeds are drawn into the recirculation path and wrap or clog the impeller, causing vibration and loss of flow. A screen on the recirculation inlet that the operator can reach and clean between batches keeps them out of the fan. Cleaning takes minutes.
Energy and control
The circulation fan runs throughout the batch and moves far more air than the exhaust fan. Speed control that reduces flow as the product dries saves power and avoids over-heating the surface of the chillies. Reversing the airflow halfway through the batch improves uniformity without extra fan power, and recirculating air that can still carry moisture saves burner fuel.
ID fan and FD fan in chilli and spice dryers
| ID fan | FD fan | |
|---|---|---|
| Position | Dryer roof or wall, after a dust separator | Beside the heat exchanger |
| What it handles | Humid air with chilli dust | Warm recirculated drying air |
| Temperature at the fan | 40 to 55°C | 55 to 70°C |
| Main design concern | Dust capture and humidity control | Even flow through deep beds |
| Control | Chamber humidity | Batch stage |
See also the Induced Draft (ID) Fan for chilli and spice dryers.
Questions buyers ask
What fan is used in a chilli bin dryer?
A backward curved fan with enough pressure to push air through a deep bed of chillies, built for warm humid air and placed so its motor is out of the hot air stream. Tray tunnels use axial fans instead, because they move air across the trays at low pressure.
What temperature is used for drying red chilli?
Typically 55 to 70°C. Higher temperatures dry faster but reduce colour and pungency and can scorch the skin. Turmeric and pepper are dried at similar or slightly higher temperatures depending on the product and the buyer's specification. The air temperature is reduced towards the end of each batch to protect colour.
Is the combustion air fan the same as the circulation fan on a spice dryer?
No. The burner or biomass furnace has its own small combustion air fan on ambient air. The circulation fan moves the drying air through the heat exchanger and product, and the exhaust fan removes moist air. Each is sized separately. The burner fan is matched to the burner maker's data.
Information needed to quote an FD fan for chilli and spice 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.
- Dryer type. Bin, tray or belt, and bed depth.
- Air flow. Air flow and pressure at the plenum.
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.