How wood fired thermic fluid heaters work
Wood fired thermic fluid heaters burn logs, chips, offcuts or plywood waste on a fixed or step grate, fed manually through a firing door or by a screw or ram feeder. Wood typically has 20 to 45% moisture, a calorific value of about 2,500 to 3,500 kcal/kg as fired and only 1 to 3% ash. The volatiles burn above the grate with secondary air. The flue gas leaves the coil at around 330 to 380°C, passes an air preheater, and goes through a cyclone or multicyclone to the ID fan and chimney. Manual firing gives large swings in combustion as each charge of wood is added and the door is opened.
Where the ID fan sits
- Grate and firing door
- Radiant coil
- Convective coil
- Air preheater
- Cyclone
- ID fan
- Chimney
The fan sits after the cyclone. It sees gas at about 190 to 230°C with fine char and fly ash that the cyclone missed. Suction is typically 80 to 150 mmWC on heaters of 1 to 4 million kcal/h. When the firing door opens, cold air rushes in and the furnace draft drops; the fan must hold enough suction to stop flame and smoke coming out of the door.
Gas at the ID fan
| Gas temperature, normal | 190 to 230°C after the air preheater |
| Maximum or upset | Up to 350°C with preheater bypass or dry, fast-burning wood |
| Dust at the fan | Light; fine char, ash and sparks after the cyclone |
| Moisture and dew point | Wet gas from fuel moisture; tar condenses on cold surfaces at start-up |
| Corrosive species | Organic acids in tar condensate; low sulphur |
| Density at the design temperature | About 0.73 kg/m³ at 210°C, 61% 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 thermic fluid heaters
| Blade form | Backward curved or radial tipped, open blades that shed char |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door, casing drain and spark arrestor at the cyclone |
What this ID fan has to cope with
Firing door draft swings
Opening the firing door to load wood drops furnace draft and lets a large slug of cold air in. If the fan has no reserve, flame and smoke come out of the door and the operator is at risk. A fan with pressure reserve and fast draft control keeps the furnace under suction through every charge.
Tar and creosote deposits
At start-up and low load, wet wood smoke condenses tar and creosote on the cold casing and blades. Deposits harden and shed unevenly, unbalancing the rotor and raising bearing temperatures. Warming up on a small, dry fire and keeping the gas temperature above about 150°C at the fan reduce this, with an inspection door for cleaning.
Sparks and char
Burning char carried from the grate can reach the cyclone, the fan and the chimney, especially when dry offcuts flare up. Sparks from the chimney are a fire risk in a timber or textile plant. A spark arrestor at the cyclone and a fan with open blades that shed char protect the plant.
Energy and control
Manual firing creates large swings in heat release through each charge, and a variable speed drive on furnace draft follows them better than a fixed damper. Even, regular firing of smaller charges of drier wood reduces the swings, improves heater efficiency and lowers fan power. Storing wood under cover for a few weeks before use often cuts moisture, and fan volume, noticeably.
ID fan and FD fan in wood fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | Before the air preheater, under the grate |
| What it handles | Flue gas with char, sparks and tar | Ambient air |
| Temperature at the fan | 190 to 230°C | Ambient |
| Main design concern | Draft at the open firing door, tar deposits | Door interlock and burning rate |
| Control | Furnace draft | Oil outlet temperature |
See also the Forced Draft (FD) Fan for wood fired thermic fluid heaters.
Questions buyers ask
Why does smoke come out of the firing door on my wood fired heater?
The ID fan is not keeping the furnace under enough suction when the door is opened. The fan may be undersized, its damper or speed may be set too low for the door-open case, or the cyclone and preheater may be choked with ash. Check draft with the door open before changing the fan.
Does wet wood affect the ID fan on a thermic fluid heater?
Yes. Wet wood produces more flue gas volume, because the moisture leaves as vapour, and more tar condensate at low load. That raises the fan duty, lowers the furnace temperature and causes deposits on the impeller. A fan sized on dry wood can run short of draft on monsoon wood.
What blade form suits a wood fired thermic fluid heater ID fan?
Backward curved blades are efficient but can collect tar on their underside. Radial tipped blades shed deposits and char better at some loss of efficiency. The choice depends on the wood, the moisture, and how well the cyclone performs on the wood fired thermic fluid heater in question.
Information needed to quote an ID fan for wood fired thermic fluid heaters
- 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. Type of wood, moisture and how it is fed.
- Firing pattern. Manual or automatic feeding and how often the door is opened.
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.