How tobacco redrying plants work
In a green leaf threshing and redrying plant, cured tobacco is conditioned with steam and water, threshed to separate lamina from stem, and then redried to a uniform moisture for storage and ageing. Lamina and stems pass on separate lines through apron redryers: long, multi-zone conveyor dryers in which zone fans blow heated air up through the bed. The drying zones run at about 60 to 100°C, heated by steam coils from the plant boiler. Cooling zones then bring the leaf down with ambient air, and a reordering zone adds steam and water spray to bring it back to about 11 to 13% moisture before packing. Each zone has its circulation fan, and exhaust fans remove the moist air from the drying and reordering zones.
Where the FD fan sits
- Zone return air
- Fresh air make-up damper
- FD (circulation) fan
- Steam coils
- Plenum under apron
- Leaf bed
Each drying zone has its own fan, mounted beside or above the zone, which draws air from above the bed, mixes in fresh make-up air, and pushes it through the steam coils into the plenum beneath the perforated apron. Most of the air is recirculated; the exhaust fan removes only part of it. The fan pressure covers the coils, the plenum, the apron and the leaf bed, typically 80 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | 50 to 90°C, recirculated zone air |
| Air after the coils | 60 to 100°C, downstream of the fan |
| Dust | Tobacco dust and fibre carried in the recirculated air |
| Humidity | High; set by the exhaust rate of the zone |
| Cooling zones | Separate fans on ambient air |
| Density at the design temperature | About 1.00 kg/m³ at 80°C, 83% 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 redrying plants
| Blade form | Backward curved or backward inclined, for a flat flow across the bed |
| Material of construction | Mild steel with heat-resistant coating; galvanised or stainless in humid zones |
| Drive and arrangement | Belt driven or direct coupled with a variable speed drive |
| Bearings and cooling | Bearings outside the warm air stream, with high temperature grease |
| Flow control | Variable speed drive per zone |
| Also specified | Inspection door and access to the coil face for cleaning |
What this FD fan has to cope with
Coil fouling reduces flow
Tobacco dust and fibre load the face of the steam coils, raising resistance so that a fixed-speed fan delivers less air and the zone dries less. Coil cleaning schedules and a speed-controlled fan that can hold the air flow keep drying even.
Uneven drying across the apron
Poor inlet conditions or a fan oversized and throttled can give uneven air distribution in the plenum, with wet and dry streaks in the lamina. Good plenum design and a fan selected near its best efficiency point give uniform flow.
Warm air on the bearings
The fan handles air at up to 90°C, so bearings close to the casing run warm and grease degrades. Bearings mounted clear of the casing, with a shaft guard and suitable grease, last the season.
Energy and control
Circulation fans run all the time the redryer runs, and a large line may have twenty or more of them, so they make up most of the redryer's electrical load. High efficiency wheels and variable speed drives, set by zone, pay back quickly over a long season. Running each zone only as hard as the leaf needs also saves steam.
ID fan and FD fan in tobacco redrying plants
| ID fan | FD fan | |
|---|---|---|
| Position | On the zone exhaust hoods | Beside each zone, before the steam coils |
| What it handles | Humid air with tobacco dust | Recirculated warm zone air |
| Temperature at the fan | 50 to 80°C | 50 to 90°C |
| Main design concern | Deposits, condensation and fire risk | Even flow through the leaf bed |
| Control | Zone humidity | Zone air flow |
See also the Induced Draft (ID) Fan for tobacco redrying plants.
Questions buyers ask
Is the zone fan in a tobacco redryer an FD fan?
In practice it does the job of a forced draft fan for each zone: it pushes air through the heaters and the leaf bed. Unlike a boiler FD fan, it handles mostly recirculated warm air, at 50 to 90°C, rather than ambient air.
What pressure does a redryer zone fan need?
Usually 80 to 200 mmWC, to overcome the steam coils, the plenum, the perforated apron and the leaf bed. The value depends on bed depth and coil design, and allowance must be made for coil fouling.
Why do redryer zone fans need speed control?
Different zones need different air flows, and coil fouling and bed depth change during the season. A variable speed drive on each zone fan lets the operator set the flow for each zone without throttling.
Information needed to quote an FD fan for tobacco redrying plants
- 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.
- Zone data. Zone air flow, temperature and the bed depth on the apron.
- Heater. Steam coil pressure drop clean and fouled.
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.