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 ID fan sits
- Wood furnace
- Fire tubes or water tube bank
- Cyclone or settling box
- ID fan
- Chimney
The fan sits after a small cyclone or settling box at the chimney base, and on compact packaged units directly at the generator outlet. It pulls the furnace, the tube passes and the collector, typically 40 to 100 mmWC. Many small generators run on natural draft; an ID fan is fitted on multi-pass units, where draft must be steady for a fuel door that is opened often, and where the chimney is short.
Gas at the ID fan
| Gas temperature, normal | 150 to 200°C at the generator outlet |
| Low load | Can fall to 110 to 130°C with cool return water |
| Dust at the fan | Light wood ash and char, low in quantity |
| Moisture and dew point | Wood moisture of 20 to 45% gives a water dew point of 55 to 70°C |
| Deposits | Tar, creosote and wet soot when gas or metal falls below about 100 to 150°C |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 water generators
| Blade form | Radial tipped or open paddle wheel that sheds tar and soot |
| Material of construction | Mild steel with corrosion allowance; stainless casing where low return water is common |
| Drive and arrangement | Belt driven or direct mounted on smaller units |
| Bearings and cooling | Bearings outside the gas stream with a cooling disc |
| Flow control | Speed control or damper on furnace draft, or simple on-off with the firing |
| Also specified | Casing drain, insulation and a large access door for cleaning |
What this ID fan has to cope with
Condensation from cool return water
When return water is at 40 to 60°C, the tubes it enters sit below the dew point of wood flue gas. Water and tar condense, form a sticky acidic deposit and carry on to the fan. A return water temperature control, a mixing loop that keeps the return above about 60°C, and a drained, insulated fan casing control this.
Tar build-up on the impeller
Each cold start and each period of smouldering fire puts tar on the fan casing and blades. It hardens unevenly and unbalances the wheel, and on a small belt driven fan the vibration soon damages bearings and belts. An open, smooth blade form that sheds deposits, and regular cleaning through a large access door, keep the fan running without vibration.
Smoke at the fuel door
Hot water generators are often hand fired, and every time the fuel door opens, cold air rushes in and the furnace draft collapses. A fan with enough suction margin, and a door switch that briefly speeds it up while the door is open, stop smoke spilling into the boiler room, kitchen area or laundry where these units are often installed.
Energy and control
Hot water demand in food units, hostels and hospitals peaks at certain times of day and drops at others. A speed controlled ID fan, or at least a fan that follows the firing rate, avoids pulling excess cold air through the furnace during low demand. Keeping return water warm protects the tubes and the fan, and cleaning tar from the tubes keeps the exit gas temperature, and fuel use, down.
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 Forced Draft (FD) Fan for wood fired hot water generators.
Questions buyers ask
Does a wood fired hot water generator need an ID fan?
Small single-pass units often run on natural draft. Multi-pass and packaged units, those with a short chimney and those fired by hand with frequent door opening need an ID fan to hold the furnace under suction and keep smoke out of the working area. An ID fan also gives steadier combustion as the fire and water demand change.
Why is the ID fan on my wood fired hot water generator covered in tar?
Cool return water and smouldering fire let tar and moisture condense in the tubes and on the fan. Raising the return water temperature above about 60°C, firing dry wood, and insulating the fan casing reduce it. An open wheel that sheds deposits and regular cleaning keep the fan balanced.
What temperature does the ID fan on a hot water generator handle?
Usually 150 to 200°C at full load, falling to 110 to 130°C at low load with cool return water. The fan is rated for the higher end, with materials and drainage chosen for the wet, tar-laden gas at the lower end. Where the return water is often cold, a stainless casing and an insulated inlet duct are worth the extra cost.
Information needed to quote an ID fan for wood fired hot water 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.
- Water temperatures. Supply and return water temperatures, and whether there is a return mixing loop.
- Firing. Hand fired or mechanically fed, and the wood type and moisture.
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.