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 ID fan sits
- Wood furnace
- Spark arrestor
- Tube heat exchanger
- Cyclone
- ID fan
- Chimney
On an indirect generator the fan sits after the heat exchanger and a small cyclone, at the chimney base. It pulls the furnace, the exchanger tubes and the cyclone, typically 60 to 120 mmWC. Fly ash from wood is light and low in quantity. The fan is on the flue side only, so a leak in the exchanger draws clean air into the flue rather than smoke into the product air, provided the clean air side is at higher pressure.
Gas at the ID fan
| Gas temperature, normal | 180 to 280°C after the heat exchanger |
| Maximum or upset | Up to 400°C with fouled tubes or overfiring |
| Dust at the fan | Light wood ash and char, low in quantity |
| Moisture and dew point | Wet wood gives a dew point of 60 to 70°C; tar condenses below about 150°C |
| Deposits | Tar and creosote at light-up and low fire |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 wood fired hot air generators
| Blade form | Radial tipped or backward curved with smooth blades that shed tar deposits |
| Material of construction | Mild steel for normal duty; heat-resisting steel impeller where gas regularly exceeds 350°C |
| Drive and arrangement | Belt driven on smaller units, direct coupled arrangement 8 on larger |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive or inlet damper on furnace draft |
| Also specified | Large inspection door for cleaning, casing drain and casing insulation |
What this ID fan has to cope with
Tar and creosote at light-up
Every cold start fills the furnace with smoke while the wood dries and the refractory heats up. The fan casing and impeller are cold, and tar and creosote condense on them. Over weeks this builds a hard, uneven coating that unbalances the wheel. Insulating the casing, lighting up on dry wood and cleaning through a large access door control it.
Wet wood lowers the capacity
Fresh chips at 40% moisture produce much more gas for the same heat than seasoned logs, and the fan runs out of capacity on wet wood. Sizing on the wettest fuel the plant will actually burn, with speed control to trim back on dry wood, avoids losing drying capacity in the monsoon.
Hot gas from fouled tubes
Soot and tar fouling in the exchanger tubes reduce heat transfer, and the gas reaches the fan hotter. A fan selected for 250°C on clean tubes may run at 350°C or more before cleaning. The bearings, shaft and impeller are rated for the fouled condition, and a temperature indication prompts tube cleaning.
Energy and control
Wood fired generators on crop dryers run through a season, often around the clock, at a firing rate set by the batch or the dryer feed. A speed controlled ID fan holding furnace draft avoids the losses of a throttled damper. Using seasoned wood rather than fresh chips reduces gas volume and fan power, and regular tube cleaning keeps the gas temperature and fan volume at design.
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 Forced Draft (FD) Fan for wood fired hot air generators.
Questions buyers ask
Why does tar build up on the ID fan of a wood fired hot air generator?
Wood releases tar and creosote while it dries and devolatilises, especially at light-up and low fire when the furnace is cool. These condense on cold metal, including the fan. Insulating the casing, avoiding long smouldering periods and cleaning the impeller regularly keep the deposit from unbalancing the wheel.
Is a wood fired hot air generator direct or indirect fired?
For food products such as spices, cardamom, cashew and copra it is normally indirect, through a tube heat exchanger, so smoke does not taint the product. For minerals, bricks and sand, direct firing is common. The ID fan duty differs: flue gas on an indirect unit, dryer exhaust on a direct one.
What temperature should the ID fan on a wood fired hot air generator be designed for?
Usually 180 to 280°C after the heat exchanger, with the fan rated for about 350 to 400°C to cover fouled tubes and overfiring on dry wood. Above about 350°C for long periods a heat-resisting steel impeller is used, and the bearings are protected with a cooling disc and, on larger fans, temperature monitoring.
Information needed to quote an ID fan for wood fired hot air generators
- 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.
- Wood. Type and moisture of the wood, and whether logs, chips or offcuts are fired.
- Heat exchanger. Gas temperature at the exchanger outlet when clean and when fouled.
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.