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 ID fan sits
- Fresh air supply
- Heating coils
- Drying tunnel and recirculation fans
- Exhaust hoods
- Exhaust (ID) fan
- Scrubber or stack
The exhaust fan draws from one or more hoods along the tunnel, mostly at the wet entry end where humidity is highest, and discharges to a stack or to a wet scrubber for solvent and odour. There is seldom any collector upstream, so lint, leather dust and finishing overspray reach the fan directly. Suction is low, typically 30 to 100 mmWC, and exhaust is balanced against the supply fan to keep the tunnel near neutral so vapour does not escape into the tannery.
Gas at the ID fan
| Air temperature, normal | 40 to 60°C at the exhaust hood |
| Maximum or upset | Up to about 90°C in finishing tunnels |
| Dust at the fan | Leather fibre, buffing dust and finishing overspray |
| Moisture and dew point | Humid; condensation on cold ducts and casing in winter and at shutdown |
| Corrosive species | Ammonia, formic acid, chromium salts and solvent vapour from finishing |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 leather drying tunnels
| Blade form | Backward curved, with smooth blades that shed overspray |
| Material of construction | Mild steel with epoxy coating, or SS 304 or SS 316 where condensate persists |
| Drive and arrangement | Belt driven arrangement 9, motor outside the air stream |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive on tunnel humidity |
| Also specified | Spark-resistant construction and earthing on solvent lines, casing drain and cleaning door |
What this ID fan has to cope with
Finishing overspray on the blades
Pigment and binder overspray carried into the exhaust dries as a layer on the blades, causing unbalance and loss of flow. Smooth blades, a cleaning door, a regular cleaning schedule and simple filters at the exhaust hoods on spray finishing lines keep the wheel clean and the tunnel humidity steady.
Corrosion from tannery chemicals
Ammonia, formic acid and chromium salts in the exhaust condense in cold ducts and on the fan casing. Ordinary painted mild steel corrodes quickly, especially at the casing floor where condensate lies. Epoxy coatings or stainless steel for wetted parts, and a drain at the lowest point, extend life.
Solvent vapour and ignition
Solvent-based finishes release flammable vapour, and the exhaust must keep the concentration well below the lower explosive limit at the highest coating rate. The fan must not be an ignition source: spark-resistant construction, earthing, a motor outside the air stream and a flow switch interlocked with the spray line are specified.
Energy and control
Exhausting warm air takes heat with it, and many tannery tunnels run exhaust dampers fully open whatever the load. A variable speed drive controlled from tunnel humidity reduces exhaust when the line carries fewer hides, cutting fan power and steam or thermic fluid use. Heat recovery from exhaust to fresh air is practical on large tunnels where the exhaust is clean enough not to foul an exchanger.
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 Forced Draft (FD) Fan for leather drying tunnels.
Questions buyers ask
What type of exhaust fan is used on a leather drying tunnel?
A backward curved centrifugal fan, belt driven with the motor outside the air stream, is usual. Construction is coated mild steel or stainless steel because the exhaust carries humid air with tannery chemicals. On solvent finishing tunnels the fan is also spark-resistant and earthed, with a flow switch proving exhaust before the line starts.
Why is the leather drying tunnel too humid?
Usually the exhaust fan is not removing enough air, because of a fouled impeller, slipping belts, a closed damper or blocked hoods. A humid tunnel dries leather slowly and unevenly and can mark the finish. Measuring exhaust flow and checking the fan against its curve shows whether the fan or the system is at fault.
Does a leather finishing tunnel need an explosion-proof fan?
If solvent-based finishes are dried, the exhaust may carry flammable vapour, and the fan should be spark-resistant with the motor outside the air stream and suited to the area classification. Water-based finishes reduce the hazard but may still contain some solvent, so check the finish data before deciding.
Information needed to quote an ID fan for leather drying tunnels
- 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.
- Finish type. Whether water-based or solvent-based finishes are dried, and the solvent load per hour.
- Exhaust treatment. Stack, scrubber or solvent recovery downstream of the fan.
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.