How fluidised bed thermic fluid heaters work
In a fluidised bed thermic fluid heater, crushed coal, lignite or biomass is burnt in a bed of sand or ash held at about 800 to 900°C by air blown through a distributor plate with nozzles. Heat reaches the thermic fluid through coils in the freeboard and convective section, and in some designs through tubes immersed in the bed. Because the fuel is crushed to below 6 to 10 mm, much of the ash and some bed material leave with the gas. The flue gas passes the convective coils at 300 to 380°C, an air preheater or waste heat economiser, a cyclone or multicyclone and a bag filter or ESP before the ID fan. The FD fan supplies fluidising air at high pressure.
Where the ID fan sits
- Fluidised bed
- Freeboard and coils
- Air preheater
- Cyclone
- Bag filter or ESP
- ID fan
- Chimney
The fan follows the final collector, a bag filter or ESP on most modern units, with a cyclone ahead to take the coarse bed fines. It holds the freeboard at a small suction while the FD fan pressurises the bed, so the balance point sits just above the bed surface. Suction at the fan inlet includes the coil banks, air preheater, cyclone and filter, typically 150 to 250 mmWC.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the air preheater or economiser |
| Maximum or upset | Up to 300°C with fouled heat surfaces or at overload |
| Dust at the fan | Fine fly ash and bed fines; 50 to 150 mg/Nm³ after a bag filter or ESP, several g/Nm³ after a cyclone alone |
| Corrosive species | SO₂ from coal or lignite; limestone in the bed reduces it |
| Moisture and dew point | Lignite and wet biomass raise the water dew point; keep the gas well above the acid dew point at low load |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 fluidised bed thermic fluid heaters
| Blade form | Backward curved with wear liners behind a bag filter; radial or radial tipped behind a cyclone only |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with hard-faced blade edges |
| Drive and arrangement | Direct coupled arrangement 8 at moderate speed to limit erosion |
| Bearings and cooling | Pedestal bearings with a cooling disc; temperature monitoring on larger units |
| Flow control | Variable speed drive on freeboard draft |
| Also specified | Replaceable liners at the scroll cut-off and inlet cone, and an inspection door |
What this ID fan has to cope with
Erosion from bed fines
Sand and ash elutriated from the bed are coarse and angular. If the cyclone or filter is bypassed, or bag failures go unnoticed, the fan sees a heavy abrasive load and the blade tips wear quickly. Liners, moderate tip speed and a dust monitor after the filter protect the impeller.
Draft swings during bed upsets
Bed slumping, fuel feed interruptions and ash drain operations cause sudden changes in freeboard pressure. A fan with a stable curve and a speed drive tuned for fast response holds the freeboard negative so that hot gas and sand do not blow back through the feeders.
Collector resistance growing
Bag filter resistance rises as the dust cake builds between cleaning pulses, and a cyclone partly choked with ash adds more. The fan duty includes the filter at its highest operating pressure drop and a margin for the cyclone, so that the heater does not lose freeboard draft and puff hot gas at the feeders before the cleaning cycle catches up.
Energy and control
FBC heaters run efficiently over a narrower range than grate units, and at part load the bed temperature is controlled by reducing air and fuel together. The ID fan follows that turndown, and a variable speed drive avoids the large throttling losses a damper would cause. Reducing air ingress at the collector hoppers and expansion joints lowers the gas volume the fan must handle at every load.
ID fan and FD fan in fluidised bed thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Before the air box |
| What it handles | Flue gas with fly ash and bed fines | Clean ambient air |
| Temperature at the fan | 170 to 220°C | Ambient |
| Static pressure | 150 to 250 mmWC suction | 600 to 1,000 mmWC |
| Main design concern | Erosion and collector resistance | Stable high pressure and fluidisation |
| Control | Freeboard draft | Bed air flow with a minimum limit |
See also the Forced Draft (FD) Fan for fluidised bed thermic fluid heaters.
Questions buyers ask
Why does an FBC thermic fluid heater need a bigger ID fan than a grate fired heater?
The FBC heater carries far more ash and bed fines in the gas, so it needs a cyclone and a bag filter or ESP in series, and their resistance adds to the fan pressure. The fan also has to balance a pressurised bed. Both raise the static pressure compared with a grate heater of the same output.
Which blade type suits the ID fan on a fluidised bed thermic fluid heater?
Behind a well-maintained bag filter or ESP, a backward curved impeller with wear liners gives the best efficiency and the lowest running cost. Where the fan follows only a cyclone, or bag failures are frequent, a radial or radial tipped impeller tolerates the heavier and coarser dust far better, at some loss of efficiency.
How is the furnace kept under suction on an FBC heater?
The FD fan pressurises the bed and the ID fan pulls from the freeboard above it. The ID fan speed is controlled from a freeboard pressure transmitter, usually set a few mmWC below atmosphere, so that hot gas and sand do not escape at the fuel feeders, inspection doors or ash drain points when the bed is disturbed.
Information needed to quote an ID fan for fluidised bed 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.
- Collector train. Cyclone, bag filter or ESP, and the design and maximum pressure drop of each.
- Fuel and bed. Fuel type and size, and whether limestone is added to the bed.
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.