How tobacco curing barns work
Flue-cured Virginia tobacco is cured in barns where leaf hangs on sticks or sits in racks while warm air passes over it. The heat comes from a furnace burning wood, coal, briquettes or other biomass, whose flue gas runs through pipes or a tubular heat exchanger inside the barn and then up a chimney, so the smoke never touches the leaf. A cure lasts five to seven days through three stages: yellowing at about 35 to 38°C with high humidity, leaf drying at 40 to 55°C, and stem drying at up to 70 to 74°C. Traditional barns rely on natural draft. Bulk curing barns add an air circulation fan, controlled fresh air and humid air dampers, and on the furnace side an ID fan and a combustion air fan.
Where the FD fan sits
- Air inlet
- FD fan
- Undergrate air box
- Grate
- Furnace
- Flue pipes or heat exchanger
The fan is mounted on the furnace front or side, outside the curing chamber, and delivers into the ash pit or an air box under the grate. On automatically fed biomass or briquette furnaces it is switched or speed-controlled by the barn temperature controller. The pressure needed is low, typically 20 to 60 mmWC, set by the grate and the depth of the fuel bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 40°C depending on season and region |
| Dust | Clean outdoor air; ash pit dust near the inlet |
| Pressure at the fan outlet | Typically 20 to 60 mmWC |
| Fuel | Wood, coal, briquettes or other agricultural residue |
| Turndown | Very wide; near idle in yellowing, full in stem drying |
| Density at the design temperature | About 1.15 kg/m³ at 35°C, 96% 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 tobacco curing barns
| Blade form | Backward curved or forward curved, small and direct driven |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | On-off or variable speed from the barn temperature controller |
| Also specified | Non-return flap so that hot gas does not back into the fan when it stops |
What this FD fan has to cope with
Hunting between on and off
With simple on-off control the fire flares when the fan starts and dies when it stops, and the barn temperature cycles around the set point. Variable speed or a low and high speed setting give steadier heat, which matters during leaf drying when colour is fixed.
Backfire when the fan stops
If the fan stops on a hot fire, smoke and hot gas can flow back through the fan casing and burn the motor or the impeller. A gravity flap or damper at the fan outlet and a power failure alarm prevent this.
Power failure during a cure
A barn that loses its fans in the middle of a cure can lose the batch. A small standby generator or a natural draft bypass that keeps the fire alive at low rate is worth planning with the fan.
Energy and control
The FD fan on a curing barn is small, but it runs for days per cure. Its value is in control: matching air to the heat demand avoids the excess air that carries heat up the chimney, which is where most of the fuel in a barn is lost. Speed control from the barn controller gives better curing and lower fuel use than a fan that is on or off.
ID fan and FD fan in tobacco curing barns
| ID fan | FD fan | |
|---|---|---|
| Position | At the chimney base, after the heat exchanger | On the furnace front, under the grate |
| What it handles | Smoky flue gas with tar and soot | Ambient air |
| Temperature at the fan | 120 to 250°C | Ambient |
| Main design concern | Flue side suction and tar deposits | Control across a very wide turndown |
| Control | Flue suction | Barn temperature |
See also the Induced Draft (ID) Fan for tobacco curing barns.
Questions buyers ask
What size of FD fan does a bulk tobacco barn furnace need?
Bulk curing barn furnaces are small, and the combustion air fan is typically a few hundred to about 1,500 m³/h at 20 to 60 mmWC, depending on barn capacity, fuel and grate design. The fan must have enough margin for the full firing rate in stem drying.
Is the forced draft fan controlled by the barn temperature?
Usually yes. The barn controller switches or varies the speed of the combustion air fan to raise or lower the heat, and the circulation fan inside the barn runs continuously to move air through the leaf.
Can briquettes be burnt in a tobacco barn with a forced draft fan?
Yes. Briquettes and other dense biomass burn well on a grate with forced draft air. The fan must give enough pressure for the bed depth used, and the flue side should have enough draft, from a chimney or an ID fan, to keep the furnace under suction.
Information needed to quote an FD fan for tobacco curing barns
- 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.
- Control. How the barn controller will command the fan: on-off, two speed or variable speed.
- Power supply. Single or three phase supply available at the barn and any standby power.
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.