How leather drying tunnels work
After tanning, dyeing and fatliquoring, and again after each finishing coat, leather is dried under controlled conditions. Tunnel dryers carry hides and skins on overhead conveyors, toggle frames or flat belts through an insulated chamber where recirculating fans blow warm air across them. Air is heated by steam, hot water or thermic fluid coils, typically to 40 to 70°C for crust drying and up to 80 to 90°C in finishing tunnels where water-based or solvent coats are cured. Temperatures are kept low because wet leather shrinks and hardens if overheated. A supply fan brings in filtered fresh air, and an exhaust fan removes the humid air, with solvent and finishing chemical vapours, to a stack or scrubber.
Where the FD fan sits
- Outdoor air louvre
- Filter
- FD (supply) fan
- Heating coils
- Supply duct
- Drying tunnel
The supply fan sits at the dry end of the tunnel or on a platform above it, drawing outdoor air through louvres and filters and pushing it through steam, hot water or thermic fluid coils into the tunnel. Placing it before the coils keeps it on cool air. It works against the filter, coils and supply ducts, typically 40 to 120 mmWC, and its flow is matched with the exhaust fan to hold the tunnel close to neutral pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the coils | 40 to 90°C, downstream of the fan |
| Dust | Outdoor dust and tannery fibre; filtered |
| Pressure at the fan outlet | Typically 40 to 120 mmWC |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 leather drying tunnels
| Blade form | Backward curved, quiet and efficient on clean air |
| Material of construction | Mild steel, coated for a tannery atmosphere |
| Drive and arrangement | Belt driven or direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive tracking the exhaust fan |
| Also specified | Filter with pressure gauge, inlet louvre, and flexible connections |
What this FD fan has to cope with
Air imbalance with the exhaust
If the supply fan delivers more than the exhaust fan removes, the tunnel goes positive and vapour leaks into the tannery; if it delivers too little, cold air is drawn in at the openings. Tracking supply flow to the exhaust fan with speed control keeps the tunnel close to neutral.
Clogged filters and coils
Tannery air carries fibre and dust that load the filter and the face of the heating coils. Air flow falls, coil output drops and the leather dries slowly. A filter pressure gauge, regular cleaning, and a fan selected with pressure margin for a loaded filter keep the flow steady.
Corrosion from the tannery atmosphere
Ammonia and acid fumes in tannery air corrode painted steel from the outside, even on a clean-air fan. Coating systems suited to tannery atmospheres, stainless fasteners, and an intake placed away from the wet processing sections extend fan life. A yearly check of the coating at the base and flanges catches damage early.
Energy and control
Supply air has to be heated, so the supply fan sets heating energy as well as its own power. Matching supply to the drying load, instead of running a fixed flow, reduces both. Recirculating a larger share of tunnel air and bringing in only enough fresh air to control humidity is the usual saving, with the supply fan turned down on its variable speed drive.
ID fan and FD fan in leather drying tunnels
| ID fan | FD fan | |
|---|---|---|
| Position | On exhaust hoods at the wet end | At the dry end, before the heating coils |
| What it handles | Humid air with tannery chemicals and solvent vapour | Filtered fresh air |
| Temperature at the fan | 40 to 60°C | Ambient, 20 to 45°C |
| Main design concern | Corrosion and solvent safety | Air balance with the exhaust |
| Control | Tunnel humidity | Tracks exhaust flow |
See also the Induced Draft (ID) Fan for leather drying tunnels.
Questions buyers ask
What does the supply fan do on a leather drying tunnel?
It brings filtered fresh air into the leather drying tunnel through the heating coils, replacing the humid air removed by the exhaust fan. Recirculation fans inside the tunnel move the air across the leather. The supply fan handles clean air, so it is a simple backward curved forced draft fan.
Should a leather drying tunnel be under suction or pressure?
Close to neutral or slightly negative, so that vapour does not leak into the tannery. The exhaust fan should remove slightly more air than the supply fan delivers. Too much suction draws cold air in at the conveyor openings, which wastes heat and leaves cold spots near the ends.
Why is leather drying unevenly in the tunnel?
Uneven air distribution is the common cause: blocked supply ducts or filters, a fouled supply fan, or a failed recirculation fan in one section. Measuring supply flow and air temperature at several points along the tunnel shows where the problem lies, and uneven loading of hides has a similar effect.
Information needed to quote an FD fan for leather drying tunnels
- 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.
- Heating medium. Steam, hot water or thermic fluid, coil resistance, and the required air temperature.
- Air balance. Exhaust flow, and whether supply should track it automatically.
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.