How timber seasoning kilns work
A conventional timber seasoning kiln is a batch chamber in which stacked, stickered timber is dried slowly by warm air, following a schedule of temperature and humidity steps that suits the species and thickness. Overhead axial fans inside the chamber circulate air across the stack and reverse at intervals. Heat comes from steam, hot water or thermic fluid coils, or from a furnace burning wood waste with an air-to-air heat exchanger. Temperatures run from about 40 to 60°C for hardwoods to 70 to 90°C for softwoods, with water sprays to raise humidity. Moist air is vented through roof vents, either by the internal fan pressure or by powered exhaust fans. A charge takes from several days to several weeks.
Where the ID fan sits
- Timber stack
- Internal circulation fans
- Vent opening
- Exhaust (ID) fan
- Roof outlet
The exhaust fan sits on the roof or end wall of the kiln, at the vent opening, with a fresh air inlet on the opposite side. The internal circulation fans move air across the stack; the exhaust fan only takes the share that must leave to carry away the evaporated water. Pressure is low, typically 15 to 50 mmWC, but the fan runs for days in air that is close to saturation.
Gas at the ID fan
| Air temperature, normal | 40 to 80°C, depending on schedule step |
| Maximum or upset | About 90°C on softwood or high temperature schedules |
| Dust at the fan | Negligible |
| Moisture and dew point | Near saturated; condensate runs in the vent and the fan casing |
| Corrosive species | Acetic and formic acid released by many hardwoods |
| 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 timber seasoning kilns
| Blade form | Axial or backward inclined, for large flow at low pressure |
| Material of construction | Aluminium or SS 304 impeller with stainless or coated casing; SS 316 for acidic species |
| Drive and arrangement | Direct driven with the motor outside the air stream, or a motor rated for hot humid air |
| Bearings and cooling | Sealed bearings; motor insulation suitable for humid heat |
| Flow control | On-off or variable speed from the kiln controller |
| Also specified | Drain, backdraft damper and corrosion-resistant fasteners |
What this ID fan has to cope with
Acid condensate
Many hardwoods release acetic and formic acids during drying, and the vent air condenses on the fan casing. The acidic condensate attacks mild steel and galvanising within a few seasons. Stainless or aluminium wetted parts and a drained casing last much longer.
Motor failure in humid heat
A motor inside the vent air stream works at 60 to 80°C in saturated air, and its windings and bearings fail early. Placing the motor outside the air stream, or using a motor rated for the duty, avoids repeated failures.
Over-venting early in the schedule
Venting too hard at the start dries the surface of the timber too fast, causing checks and case hardening. Speed control from the kiln controller lets the exhaust fan vent only as much as the schedule allows.
Energy and control
Venting is the main heat loss in a timber kiln, because every cubic metre of moist air exhausted must be replaced by cool fresh air that has to be heated. Controlling the exhaust fan from the kiln controller, venting only when the wet bulb is too high, saves fuel. Heat recovery from the vent air to the fresh air is used on larger kilns.
ID fan and FD fan in timber seasoning kilns
| ID fan | FD fan | |
|---|---|---|
| Position | At the kiln roof vent | On the wood waste furnace |
| What it handles | Warm saturated air with wood acids | Ambient air |
| Temperature at the fan | 40 to 80°C | Ambient |
| Main design concern | Acid condensate and motor life | Clean firing at low load |
| Control | Kiln wet bulb | Kiln heat demand |
See also the Forced Draft (FD) Fan for timber seasoning kilns.
Questions buyers ask
Does a timber kiln need an exhaust fan?
Many kilns vent by the pressure of the internal circulation fans through roof vents. Powered exhaust fans give better control of humidity, especially in large or tightly built kilns, and allow venting regardless of the internal fan direction.
What material should a timber kiln exhaust fan be made of?
Stainless steel or aluminium for the wetted parts. Kiln vent air is warm and saturated, and many hardwoods release acids that make the condensate corrosive to mild steel and galvanised sheet.
Is the circulation fan in a timber kiln an ID fan?
No. The circulation fans move air across the stack inside the chamber and reverse at intervals. The exhaust fan at the vent removes moist air from the kiln; it is a separate fan with a different duty.
Information needed to quote an ID fan for timber seasoning kilns
- 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.
- Kiln data. Kiln capacity in m³ of timber, species and the drying schedule temperatures.
- Venting. Present venting method and the number and size of vents.
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.