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 FD fan sits
- Fresh air inlet
- FD (circulation) fan
- Heat exchanger or flue pipes
- Air distribution
- Tray stack
- Exhaust
The circulation fan sits at one end of the chamber or above the heater, drawing a mix of fresh and recirculated air across the flue pipes or heat exchanger and pushing it across or through the tray stack. It handles warm, humid air at chamber temperature. Pressure is low, typically 10 to 30 mmWC, because the air only has to pass the heater and the open tray stack. Baffles guide the air so the top and bottom trays see similar flow.
Air at the FD fan
| Air temperature at the fan | 40 to 60°C, recirculated chamber air |
| Air after the fan | Heated to chamber setpoint |
| Dust | Clean air |
| Humidity | High during sweating, falling through the cure |
| Pressure at the fan outlet | Typically 10 to 30 mmWC |
| 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. |
FD Fan selection for cardamom curing dryers
| Blade form | Axial or backward curved |
| Material of construction | Aluminium, SS 304 or coated steel |
| Drive and arrangement | Direct mounted |
| Bearings and cooling | Motor rated for warm humid air |
| Flow control | Variable speed drive by stage |
| Also specified | Guards and easy access for cleaning |
What this FD fan has to cope with
Uneven colour across trays
Without forced air, trays near the flue pipes over-dry and lose colour while trays far from the heat stay wet and need extra hours. A circulation fan with baffles, sized to move air evenly across the full stack, gives consistent colour and moisture so that trays need less rotating by hand.
Too much air velocity
High air speed across the capsules dries the skin too fast, hardens it and can split the capsule, which lowers the grade. Gentle, well-distributed flow over a longer time gives a better product than high velocity, so the fan is selected for volume at low pressure, not for speed.
Motor failure in humid air
Standard motors fail in warm, near-saturated air, especially during the sweating stage. Motors with suitable enclosure and insulation for the duty, or motors mounted outside the chamber driving through an extended shaft, last the season and avoid stopping a batch midway. A drip shield over the motor also helps.
Energy and control
The circulation fan is a small motor, but it shortens the cure and reduces firewood by moving heat from the flue pipes to every tray instead of relying on natural convection. Speed control by stage keeps the air gentle during sweating and stronger during drying. Recirculating most of the chamber air, and exhausting only what the humidity requires, keeps the furnace load down.
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 Induced Draft (ID) Fan for cardamom curing dryers.
Questions buyers ask
Why use a circulation fan in a cardamom curing dryer?
It moves warm air evenly across all trays, so colour and moisture are consistent from top to bottom and the cure is shorter. Traditional houses depend on natural convection, which leaves hot and cold spots and needs frequent tray rotation by hand. It also lets the furnace run lower.
What fan type suits a cardamom dryer?
An axial or backward curved fan running at low pressure, with a motor rated for warm humid air or mounted outside the chamber. The fan moves a large volume gently rather than a small volume at high speed, which protects the capsule skin and colour.
Can the same fan do circulation and exhaust in a cardamom dryer?
On small dryers, one fan with dampers can recirculate most of the air and exhaust a share. Separate circulation and exhaust fans give better control, because circulation can stay constant while the exhaust is adjusted for each curing stage. Both are small fans with modest power.
Information needed to quote an FD fan for cardamom curing 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.
- Chamber layout. Chamber size, tray arrangement and number of trays.
- Control. Whether stage control by timer or humidity sensor is wanted.
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.