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 ID fan sits
- Drying zones
- Zone exhaust hoods
- Exhaust duct
- Dust separator or scrubber
- Exhaust (ID) fan
- Stack
Exhaust fans sit on the roof or beside the redryer and pull from hoods over the drying and reordering zones, often through a cyclone, a dust separator or a wet scrubber. The amount exhausted is set by the humidity the zones must hold: too little and the leaf leaves wet, too much and heat is wasted. The suction is low, typically 50 to 150 mmWC including any separator.
Gas at the ID fan
| Air temperature, normal | 50 to 80°C from the drying zones |
| Maximum or upset | About 100°C if the line stops with the steam coils on |
| Dust at the fan | Fine tobacco dust and fibre; heavier where no separator is fitted |
| Moisture and dew point | Humid air; saturated from the reordering zone, with condensate in the ducts |
| Other | Tobacco dust is combustible and deposits can smoulder on hot surfaces |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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 redrying plants
| Blade form | Backward inclined flat blade, plain and easy to clean |
| Material of construction | Mild steel with a coating for drying zones; SS 304 for the reordering zone exhaust |
| Drive and arrangement | Belt driven or direct coupled with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream; no cooling needed |
| Flow control | Variable speed drive or damper on zone humidity |
| Also specified | Casing drain, cleaning door and earthing |
What this ID fan has to cope with
Dust and nicotine deposits
Fine tobacco dust in warm humid air sticks to blades and the casing, and with time forms a brown, tarry layer that throws the wheel out of balance. A simple blade shape, a separator ahead of the fan and cleaning during the off-season overhaul are the remedy.
Wet exhaust from reordering
The reordering zone exhaust is near saturation and condenses in the duct and the fan, washing dust into a wet paste and corroding mild steel. A stainless fan or casing, a falling duct with drains and a separate fan for this zone keep the problem away from the drying exhausts.
Fire risk
Tobacco dust deposits on steam coils and in exhaust ducts can smoulder. The exhaust fan is earthed, has no rubbing contact, and is included in the plant's fire detection and cleaning routine.
Energy and control
Redrying plants run hard through the buying season, usually two or three shifts, and then stand idle. Exhaust air carries most of the heat lost from the redryer, so controlling the exhaust fan on zone humidity instead of a fixed damper saves steam as well as fan power. Heat recovery from the exhaust to the incoming fresh air is worth assessing on large lines.
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 Forced Draft (FD) Fan for tobacco redrying plants.
Questions buyers ask
How many exhaust fans does a tobacco redryer have?
Usually one exhaust fan for a group of drying zones and a separate one for the reordering zone, though some lines exhaust each zone separately. The number depends on the redryer length, zone layout and how independently the zones are controlled.
Why is the reordering exhaust fan made of stainless steel?
The reordering zone adds steam and water to the leaf, so its exhaust is almost saturated and condenses in the duct and fan. Stainless steel resists the wet corrosion and the dust paste that forms, which would quickly attack mild steel.
Does the exhaust fan control the redryer moisture?
Indirectly. The exhaust rate sets how much moist air leaves each zone and so the humidity of the drying air. Controlling the exhaust fan on zone humidity helps hold the leaf moisture at the redryer exit.
Information needed to quote an ID fan for tobacco redrying plants
- 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.
- Zone layout. Number of drying, cooling and reordering zones and which are served by this fan.
- Separator. Cyclone, dust separator or scrubber before the fan and its pressure drop.
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.