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 FD fan sits
- Air intake
- FD fan
- Grate
- Wood waste furnace
- Heat exchanger
- Flue
The fan is mounted on the furnace, outside the kiln, and blows air under the grate and sometimes above the bed as overfire air. The furnace gas passes through a tubular heat exchanger in the kiln air path and then to the chimney, so no smoke enters the kiln. The pressure needed is typically 40 to 100 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 40°C |
| Dust | Sawmill yard air with sawdust nearby |
| Pressure at the fan outlet | Typically 40 to 100 mmWC |
| Fuel | Offcuts, sawdust, bark and shavings with varying moisture |
| Turndown | High early in the schedule, low in the final steps |
| 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 timber seasoning kilns
| Blade form | Backward curved or radial, small and direct driven |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed or on-off from the kiln controller |
| Also specified | Inlet screen against sawdust and a backflow flap |
What this FD fan has to cope with
Low load firing
Late in the schedule the kiln needs little heat, and a furnace damped down for days smokes and fouls the heat exchanger. A fan with speed control that keeps a small, clean fire, or cycles the fire, avoids the soot.
Sawdust in the intake
A fan drawing from the sawmill yard ingests sawdust that clogs the screen and coats the blades. An intake away from the dust and a simple screen keep the air clean.
Heat exchanger leaks
If the furnace runs under pressure and the exchanger cracks, smoke and sparks can enter the kiln and stain or ignite the timber. Keeping the flue side under suction, with a chimney or an ID fan, and inspecting the exchanger regularly prevent this.
Energy and control
The FD fan on a kiln furnace is small, but it sets how well the wood waste is burnt over a long charge. Matching air to heat demand from the kiln controller cuts excess air, which is the main heat loss from the furnace, and keeps the exchanger clean. Dry fuel stored under cover also makes the furnace easier to control.
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 Induced Draft (ID) Fan for timber seasoning kilns.
Questions buyers ask
Does a timber kiln heated by a boiler need an FD fan?
Not on the kiln. If the kiln is heated by steam, hot water or thermic fluid, the forced draft fan is on the boiler or heater. The kiln itself has only circulation fans and vent fans.
What fuel does a timber kiln furnace burn?
Usually the sawmill's own waste: offcuts, sawdust, bark and shavings. A grate furnace with forced draft combustion air burns these well if the fuel moisture is not too high.
How is smoke kept out of the kiln?
The furnace gas passes through a sealed heat exchanger and the flue side is kept under suction by the chimney or an ID fan, so any leak draws kiln air into the flue rather than smoke into the kiln.
Information needed to quote an FD fan for timber seasoning kilns
- 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.
- Furnace. Heat output, grate area and the fuel mix.
- Control. How the kiln controller commands the furnace: on-off, modulating or staged.
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.