How wood fired hot water generators work
A wood fired hot water generator burns logs or chips on a fixed grate in a water-jacketed furnace, and the gas passes through fire tubes or a water tube bank before leaving to the chimney. It supplies water at 80 to 95°C, or pressurised water up to about 120°C, for food processing such as blanching, cooking, washing and CIP, for cashew and seafood units, and for hostels, hospitals and laundries. Because the water is cooler than steam or thermic fluid, the gas can be cooled to 150 to 200°C. The return water may be as low as 40 to 60°C, so the cold end of the heat transfer surface can sit below the dew point of wood flue gas.
Where the FD fan sits
- Inlet screen
- FD fan
- Undergrate air
- Secondary air ports
- Wood furnace
The fan is usually mounted on the generator front or beside it, blowing ambient air into the ash pit under the grate and into secondary air ports above the fire. Pressure is low, typically 30 to 80 mmWC. On units with an ID fan, the FD fan supplies a controlled share of the air and the ID fan holds the furnace draft; on natural draft units, air enters through the door and grate without a fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 30 to 80 mmWC |
| Dust | Clean air; wood dust and ash near the fuel door |
| Air split | Secondary air above the bed is important in a water-cooled furnace |
| Duty | Follows hot water temperature, often on-off or two-step |
| 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 water generators
| Blade form | Backward curved or forward curved for small low pressure duties |
| 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 | Speed control or damper from water temperature; minimum air for smoke control |
| Also specified | Inlet screen and separate dampers on undergrate and secondary air |
What this FD fan has to cope with
Smoke in a water-cooled furnace
Water-jacketed furnace walls are cool, and wood volatiles can be quenched before they burn, leaving smoke and tar. Secondary air supplied by the fan through ports above the bed, mixing well with the volatiles, burns them in the hottest part of the furnace and reduces deposits in the tubes and on the ID fan.
On-off control and tar
Simple controls stop the FD fan when the water reaches temperature. The bed then smoulders without enough air, producing tar, soot and carbon monoxide that foul the tubes and the ID fan. Keeping a small minimum air flow, or using speed control instead of on-off, reduces tar and makes the restart cleaner when demand returns.
Overfiring dry wood
With dry wood and the fan at full speed, the furnace can overfire and boil water locally in the jacket, especially on unpressurised units at 90 to 95°C. A water temperature limit that reduces fan speed, and a fan sized for the realistic firing rate rather than oversized, prevent local boiling and the scaling that follows.
Energy and control
The FD fan on a hot water generator is small, and its effect on energy is mainly through combustion quality and fuel use. A speed controlled fan that follows water temperature, with secondary air set correctly, burns wood more completely and reduces excess air. It also cuts tar deposits, so the tubes stay cleaner and the exit gas temperature stays lower.
ID fan and FD fan in wood fired hot water generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the tubes and cyclone | At the furnace front |
| What it handles | Cool, moist flue gas with tar and light ash | Clean ambient air |
| Temperature at the fan | 150 to 200°C, lower at low load | Ambient |
| Main design concern | Condensation and tar | Secondary air for clean burning |
| Control | Furnace draft | Water temperature |
See also the Induced Draft (ID) Fan for wood fired hot water generators.
Questions buyers ask
Does a wood fired hot water generator need an FD fan?
Not always. Many small units run on natural draft or with only an ID fan. An FD fan with secondary air helps where the furnace smokes, the load changes often, or a steadier water temperature is needed, because it controls combustion air independently of the draft.
Why does my wood fired hot water generator smoke when the water reaches temperature?
Usually the FD fan stops on temperature and the bed smoulders without enough air, giving off smoke and tar. Keeping a small minimum air flow, or using speed control instead of on-off, lets the fire burn down cleanly. Secondary air above the bed also helps burn the volatiles before they reach the cool water-jacketed surfaces.
What size FD fan does a wood fired hot water generator need?
It depends on the heat output, wood moisture and the split between undergrate and secondary air. Pressure is low, usually 30 to 80 mmWC, and the volume follows the firing rate with 50 to 100% excess air typical of hand firing. Send the generator rating and fuel details, and we will work out the air volume and pressure.
Information needed to quote an FD fan for wood fired hot water 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.
- Generator. Heat output, furnace type and whether an ID fan is fitted.
- Control. Present control method: on-off, two-step or modulating.
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.