How cardamom curing dryers work
Freshly harvested green cardamom capsules, at about 80% moisture, are cured down to 10 to 12% moisture over 18 to 30 hours. Traditional curing houses heat the chamber with a firewood furnace and flue pipes running under the trays, so the smoke never touches the capsules, and the flue gas leaves through a chimney by natural draft. Modern electric or fuel-fired dryers use a heat exchanger and fans. The process runs in stages: an initial sweating stage at about 40 to 50°C with high humidity, then drying at about 45 to 55°C, and a final stage at 55 to 60°C. Keeping the temperature low and the air moving holds the green colour that sets the market price.
Where the ID fan sits
- Firewood furnace
- Flue pipes under trays
- Curing chamber
- Exhaust damper
- Exhaust (ID) fan
- Outlet
The exhaust fan sits in the chamber wall or roof, drawing humid air from above the trays. In the sweating stage it runs little or not at all so the capsules stay moist on the surface; in the drying stages it removes the moisture released. Traditional curing houses have no ID fan on the flue, relying on chimney draft; improved units add a small flue fan to steady the draft and allow a shorter chimney. Suction is low, typically 10 to 40 mmWC.
Gas at the ID fan
| Air temperature, normal | 40 to 55°C at the exhaust fan |
| Maximum or upset | Up to 70°C if the furnace is overfired |
| Dust at the fan | Negligible; occasional capsule stalks and fragments |
| Moisture and dew point | Very humid, especially during sweating; condensation on cold surfaces |
| Product sensitivity | Above about 60°C the capsules lose green colour and aroma |
| 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 cardamom curing dryers
| Blade form | Axial or backward curved for low pressure and high volume |
| Material of construction | SS 304 or coated aluminium against humid air |
| Drive and arrangement | Direct mounted on the motor |
| Bearings and cooling | Sealed bearings with motor rated for humid, warm air |
| Flow control | Variable speed drive or timer by curing stage |
| Also specified | Back-draft damper to hold heat when the fan stops |
What this ID fan has to cope with
Losing colour by over-heating
If the exhaust fan removes too little air, the chamber heats and humidifies, and the capsules turn yellow-brown. Matching exhaust to the stage keeps both temperature and humidity in range. Holding the chamber below about 55 to 60°C through the main drying stage, with exhaust matched to the moisture being released, protects the colour that sets the auction price.
Wasting heat in sweating stage
Running the exhaust fan at full speed during sweating removes the humid air the capsules need and wastes firewood. Stage-wise control, by timer or humidity sensor, saves fuel. A simple humidity sensor in the chamber, linked to the fan speed, is enough to switch between the sweating and drying stages without relying on the operator's judgement at night.
Condensation and corrosion
Humid exhaust condenses on the fan and duct, and mild steel parts rust within a season. Stainless or coated parts and a drain reduce this. Mild steel parts rust within a season in this saturated air. Stainless or coated parts, a sloped duct and a drain reduce this, and the motor is chosen for warm, humid surroundings.
Energy and control
Curing runs for a day or more per batch through the harvest months. A small variable speed exhaust fan controlled by chamber humidity cuts firewood use compared with fixed ventilation. Insulating the curing chamber and recovering heat from the exhaust air to the fresh air reduce fuel further. A back-draft damper also stops heat loss through the fan when it is off.
ID fan and FD fan in cardamom curing dryers
| ID fan | FD fan | |
|---|---|---|
| Position | Chamber wall or roof | End of chamber or above the heater |
| What it handles | Humid exhaust air | Warm recirculated air |
| Temperature at the fan | 40 to 55°C | 40 to 60°C |
| Main design concern | Stage-wise humidity control | Even air across trays |
| Control | Chamber humidity | Curing stage |
See also the Forced Draft (FD) Fan for cardamom curing dryers.
Questions buyers ask
Does a cardamom curing house need an exhaust fan?
Traditional houses use natural ventilation. An exhaust fan with stage-wise control gives more consistent colour and shorter curing time, and is standard on modern cardamom dryers. Natural ventilation depends on the weather and the operator, and the cure varies from batch to batch. A fan also makes it practical to recover heat from the exhaust.
What temperature should a cardamom dryer run at?
Typically 40 to 50°C during sweating, 45 to 55°C during drying, and up to 55 to 60°C at the end. Higher temperatures lose green colour. The exhaust fan is the main tool for holding these: run lightly during sweating, harder during drying, and reduced again at the end. Curing times vary with capsule size and moisture.
Why is my cardamom turning brown in the dryer?
Usually the chamber runs too hot or too humid. Poor exhaust control, an overfired furnace or leaking flue pipes are common causes. A cracked flue pipe lets smoke into the chamber. Checking chamber temperature and humidity at several tray levels shows whether the exhaust fan and air movement are adequate.
Information needed to quote an ID fan for cardamom curing 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.
- Batch size. Kilograms of green capsules per batch and curing time.
- Heating. Firewood, electric or fuel heater, and whether a flue 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.