How wood fired hot air generators work
A wood fired hot air generator burns logs, chips, sawmill offcuts or plantation wood on a fixed or inclined grate in a refractory furnace. Wood typically carries 2,500 to 3,800 kcal/kg depending on moisture, which ranges from 15% for seasoned wood to 45% for fresh chips, and has low ash, usually 1 to 3%. For food products such as cardamom, spices, cashew and copra, wood generators are normally indirect, heating clean air through a tube heat exchanger, because wood smoke taints the product. For minerals, bricks, sand and coir pith, direct firing through a spark arrestor and mixing chamber is common. Flue gas leaves an indirect exchanger at 180 to 280°C.
Where the FD fan sits
- Inlet screen
- FD (combustion air) fan
- Undergrate and overfire air
- Wood furnace
- Heat exchanger or mixing chamber
The fan sits beside the furnace and blows ambient air under the grate and through secondary air ports above the bed. Many smaller wood generators run on natural draft or on the ID fan alone, with air drawn through the fire door and grate, and a combustion air fan is added for larger units, chip firing or steadier control. Pressure is low, typically 40 to 100 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 40 to 100 mmWC |
| Dust | Clean air; sawdust and bark dust from the fuel yard |
| Air split | Logs need more air above the bed; chips more under it |
| Duty | Follows the hot air temperature at the dryer |
| 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 wood fired hot air generators
| Blade form | Backward curved, on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive or damper from the dryer air temperature |
| Also specified | Inlet screen and dampers on undergrate and secondary air branches |
What this FD fan has to cope with
Smoke from too little secondary air
Wood releases most of its energy as volatile gas that must burn above the bed. If the fan supplies only undergrate air, the volatiles leave unburnt as smoke and tar, which fouls the exchanger and the ID fan. A secondary air branch with its own damper is part of the fan system.
Overfiring the furnace
A combustion air fan running at full speed on dry wood drives the furnace hotter than the refractory and exchanger tubes are designed for. Linking the fan speed to the hot air temperature, with a high temperature limit at the exchanger inlet, protects the tubes.
Low pressure fans and dirty screens
With only 40 to 100 mmWC available, a partly blocked inlet screen cuts the air flow noticeably, and the furnace starts to smoke. Placing the inlet away from the wood pile and the chipper, and checking and clearing the screen each shift, keep the fan delivering its rated air through the season.
Energy and control
The combustion air fan on a wood generator is small, and its main effect on energy is through the fuel. Supplying the right amount of air, split correctly between undergrate and secondary air, reduces excess air and unburnt volatiles, saving wood. A speed controlled fan that follows the hot air temperature also avoids overfiring and the extra heat lost up the chimney.
ID fan and FD fan in wood fired hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the heat exchanger and cyclone | Beside the furnace, under the grate |
| What it handles | Flue gas with tar, light ash and moisture | Clean ambient air |
| Temperature at the fan | 180 to 280°C, up to 400°C fouled | Ambient |
| Main design concern | Tar deposits and hot gas from fouled tubes | Undergrate and secondary air split |
| Control | Furnace draft | Hot air temperature |
See also the Induced Draft (ID) Fan for wood fired hot air generators.
Questions buyers ask
Does a wood fired hot air generator need an FD fan?
Small units often run with air drawn through the grate by the ID fan or by natural draft. Larger units, chip fired generators and those needing a steady hot air temperature use a combustion air fan, which gives better control of the fire, less smoke and tar, and a more even drying temperature for the product.
What is the difference between the combustion air fan and the process air fan on an indirect wood fired generator?
The combustion air fan supplies air to burn the wood. The process air fan blows clean air through the heat exchanger to the dryer. They are separate fans with very different volumes and pressures; the process air fan is usually several times larger and must be at higher pressure than the flue side so smoke cannot leak into the drying air.
Why does a wood fired hot air generator smoke even with the FD fan running?
Usually because the fan supplies undergrate air but too little secondary air above the bed, where the wood volatiles burn. Wet wood and a cold furnace make it worse. Adding or opening secondary air, firing seasoned wood and keeping the furnace hot reduce smoke and tar on the heat exchanger and the ID fan.
Information needed to quote an FD fan for wood fired hot air generators
- 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.
- Fuel form. Logs, chips or offcuts, and the grate type.
- Air system. Whether secondary air is supplied and whether the process air fan is part of the scope.
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.