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 ID fan sits
- Furnace
- Flue pipes or tube heat exchanger
- Flue outlet
- ID fan
- Chimney
The fan sits outside the barn at the end of the heat exchanger or flue pipe run, at the base of the chimney. Upstream are the furnace and the heat exchanger, often with several passes and bends to extract as much heat as possible inside the barn. The suction needed is modest, typically 20 to 60 mmWC, but it must be held through a long cure in which the firing rate changes several times over.
Gas at the ID fan
| Gas temperature, normal | 120 to 250°C, rising through the cure |
| Maximum or upset | 350 to 400°C during hard firing in stem drying or if the exchanger is fouled |
| Dust at the fan | Light fly ash and soot; heavier with coal or loose biomass |
| Moisture and dew point | Wood smoke with tar and water vapour; condenses during low firing in yellowing |
| Corrosive species | Wood acids and tar; SO₂ where coal is burnt |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 tobacco curing barns
| Blade form | Radial or open paddle, to tolerate soot and tar |
| Material of construction | Mild steel; heat-resisting steel only where the gas can exceed about 350°C |
| Drive and arrangement | Direct coupled with a cooling disc, or belt driven, arrangement 1 or 3 |
| Bearings and cooling | Bearings away from the casing with a shaft cooling disc |
| Flow control | Speed control or damper linked to the barn temperature controller |
| Also specified | Access door for cleaning tar and soot; spark arrestor with loose biomass |
What this ID fan has to cope with
Tar and creosote during yellowing
In the first two days the barn needs only gentle heat, so the furnace burns slowly and the flue gas leaves cool and smoky. Tar condenses in the exchanger and the fan and builds up into a sticky layer. Keeping the gas above about 120°C at the fan and cleaning between cures stop the layer growing into an unbalance problem.
Smoke leakage into the barn
A heat exchanger that runs under pressure leaks smoke through any crack or loose joint and taints the leaf. With the ID fan holding the flue side under suction, a leak draws clean barn air in. An alarm on loss of suction gives warning before the leaf is affected.
Overfiring in stem drying
In stem drying the furnace is fired hard and the gas reaching the fan can rise quickly, especially when the exchanger is sooted and transfers less heat. Designing for the upset temperature and cleaning the tubes before each cure protect the fan.
Energy and control
A bulk curing barn swings from very low firing in yellowing to full firing in stem drying, and stands idle between cures. An ID fan with speed control tied to the barn controller uses far less power at low fire than a fan run flat out against a closed damper, and also avoids drawing excess cold air through the furnace, which wastes fuel. Clean heat exchanger tubes are the biggest fuel saving.
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 Forced Draft (FD) Fan for tobacco curing barns.
Questions buyers ask
Does a tobacco curing barn need an ID fan?
Traditional barns with a tall chimney use natural draft. Bulk curing barns with a compact tubular heat exchanger have more resistance on the flue side and often use an ID fan, which also keeps the exchanger under suction so that smoke cannot leak onto the leaf.
What temperature does the flue gas reach in a tobacco barn furnace?
After the heat exchanger the flue gas is typically 120 to 250°C, rising as the cure moves into leaf and stem drying. Hard firing with a fouled exchanger can push it to 350°C or more, which sets the fan design temperature.
Is the barn circulation fan an ID fan?
No. The circulation fan moves warm air through the leaf inside the barn and handles clean, humid air. The ID fan is on the furnace flue and handles smoke. They are separate fans with very different duties.
Information needed to quote an ID fan for tobacco curing barns
- 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.
- Furnace and fuel. Fuel type, firing rate at stem drying and the furnace and heat exchanger arrangement.
- Barn size. Barn capacity in kg of green leaf and the chimney height.
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.