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 ID fan sits
- Heat exchanger
- Circulation fans
- Tray, bin or belt bed
- Exhaust duct
- Dust separator
- Exhaust (ID) fan
- Outlet
The exhaust fan sits on the dryer roof or end wall and draws moist air from the chamber through dampers. On chilli dryers a small cyclone or bag filter is often added ahead of the fan to catch skin flakes and capsaicin-laden dust before discharge. Without it the fan handles that dust directly. Suction is modest, typically 30 to 100 mmWC including the separator.
Gas at the ID fan
| Air temperature, normal | 40 to 55°C at the exhaust fan |
| Maximum or upset | Up to 75°C at the end of a batch with the burner on high fire |
| Dust at the fan | Chilli skin flakes, seed fragments and fine capsaicin-bearing dust; turmeric and pepper dust on other products |
| Moisture and dew point | Humid at the start of a batch, dew point 35 to 45°C, falling as the batch dries |
| Irritant and combustible | Chilli dust is a strong respiratory irritant; dry spice dust is combustible |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 chilli and spice dryers
| Blade form | Backward curved or open radial that sheds flakes and fibre |
| Material of construction | SS 304 for food contact and humid air |
| Drive and arrangement | Direct mounted or belt driven with variable speed drive |
| Bearings and cooling | Bearings outside the air stream with a shaft seal |
| Flow control | Variable speed drive on chamber humidity |
| Also specified | Cyclone or bag filter upstream, access door for cleaning, earthing |
What this ID fan has to cope with
Pungent dust emission
Chilli dust discharged without separation causes coughing and complaints downwind. A cyclone or bag filter ahead of the exhaust fan captures most of it and protects the fan from build-up. The separator also recovers saleable chilli fines. A bag filter gives better capture than a cyclone on fine dust.
Exhaust rate through the batch
Early in the batch the chilli releases water quickly and needs high exhaust; later it needs little. A fixed exhaust rate either slows the start or wastes heat at the end. Humidity-based speed control solves both. Different spices need different exhaust curves: pepper and turmeric release water more slowly than chilli. Storing a profile per product keeps operation simple.
Turmeric staining and build-up
Turmeric dust stains and sticks in humid air, coating the impeller. Stainless steel, smooth surfaces and periodic washing keep the fan balanced. Yellow turmeric deposits also contaminate the next product, such as white pepper or chilli, if the fan is not cleaned between batches. An access door and smooth surfaces make cleaning quick.
Energy and control
Spice dryers run batches of 10 to 24 hours through the harvest season. Controlling exhaust on chamber humidity typically saves a large share of fuel by recirculating air that is still able to carry moisture. Heat recovery from the exhaust to the fresh air further reduces burner use. The fan itself is small, but its control decides the fuel bill.
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 Forced Draft (FD) Fan for chilli and spice dryers.
Questions buyers ask
How do I control chilli dust from my dryer exhaust?
Fit a cyclone or bag filter between the dryer and the exhaust fan. It captures skin flakes and fine dust before they leave the plant and keeps the fan cleaner. The fan then handles cleaner air and stays balanced. Workers near the dryer should still use masks when handling dried chilli.
Why must a chilli dryer be indirectly heated?
Direct smoke contaminates the chilli and can raise polycyclic aromatic hydrocarbon levels above export limits. A heat exchanger keeps flue gas away from the product. The exhaust fan also keeps the chamber at slight suction, so if any smoke enters through a leak, it is drawn out with the exhaust rather than settling on the product. Regular exchanger inspection remains essential.
What exhaust fan material is used for spice dryers?
Stainless steel SS 304 is usual, because the air is humid and the dust is food product. It also cleans more easily after turmeric or chilli batches. For non-food duties or where budgets are tight, coated mild steel works if it is kept clean and dry between batches.
Information needed to quote an ID fan for chilli and spice 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.
- Products. Spices dried, batch size and moisture in and out.
- Dust handling. Whether a cyclone or bag filter is to be supplied with the exhaust fan.
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.