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 FD fan sits
- Inlet filter or screen
- FD fan
- Air preheater
- Air box
- Distributor nozzles
- Fluidised bed
The FD fan delivers ambient air through the air preheater, where fitted, into the air box below the distributor plate. It works against the distributor nozzles, which need a set pressure drop for even fluidisation, and against the weight of the bed itself. On many units the pressure at the fan outlet is 600 to 1,000 mmWC, several times that of a grate heater of similar output.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to 150 to 200°C where an air preheater is fitted |
| Pressure at the fan outlet | Typically 600 to 1,000 mmWC |
| Dust | Clean air; an inlet filter protects nozzles from blockage |
| Duty range | Must stay stable from minimum fluidisation to full load |
| 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 fluidised bed thermic fluid heaters
| Blade form | Backward curved high pressure wheel, or radial bladed for the highest pressures |
| Material of construction | Mild steel, with the impeller designed for the high tip speed |
| Drive and arrangement | Direct coupled arrangement 4 or 8, often at 2,900 rpm |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet guide vanes, with a minimum flow limit to hold fluidisation |
| Also specified | Inlet silencer and acoustic lagging, since high pressure fans are noisy |
What this FD fan has to cope with
Bed slumping at low load
Below the minimum fluidising velocity the bed collapses and hot spots form. The fan control has a floor, and the fan curve must be stable near that flow without surging. Turndown is taken by bed temperature and fuel, not by cutting air below the minimum.
Unstable operation near shut-off
High pressure fans can run into the unstable part of their curve when the distributor and bed resistance rise. Selecting the operating point well to the right of the peak pressure, with the bed at its heaviest, avoids surging and the pressure pulsation that disturbs fluidisation.
Noise
A fan at several hundred mmWC and high speed is loud at the inlet, the casing and the outlet duct, and it is often placed close to the operator platform. An inlet silencer, acoustic lagging on the casing and outlet duct, and a flexible connection to stop structure-borne noise bring levels down to what the heater house can accept.
Energy and control
The FD fan on an FBC heater is the largest power consumer on the unit because of its high pressure. Variable speed control saves more here than on almost any other heater fan, but only down to the fluidisation limit. Keeping the distributor nozzles clean and the bed height at design avoids running the fan at a higher pressure than the process needs.
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 Induced Draft (ID) Fan for fluidised bed thermic fluid heaters.
Questions buyers ask
What pressure does the FD fan on an FBC thermic fluid heater need?
Usually 600 to 1,000 mmWC, made up of the air preheater, air box, distributor nozzles and the weight of the bed. The exact figure depends on the bed depth, bed material and nozzle design, and should be taken from the heater supplier's data or measured at the air box with the bed at its normal operating height.
Can the FD fan speed be reduced to turn down an FBC heater?
Only to a point. The air flow must stay above the minimum fluidising velocity of the bed, or the bed slumps, the fuel stops mixing and hot spots form on the distributor. Further turndown comes from reducing fuel feed and bed temperature, or on larger units from sectioned beds where one compartment can be slumped deliberately.
Why is the fluidising air fan on a fluidised bed heater so noisy?
It runs at high pressure and often at 2,900 rpm, so blade passing tones and turbulence noise are high, and the inlet is open to the heater house. An inlet silencer, a lagged casing and a flexible connection on the outlet reduce the noise at the operator position without affecting the fan duty.
Information needed to quote an FD fan for fluidised bed thermic fluid heaters
- 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.
- Distributor and bed. Distributor pressure drop, bed depth and the design air box pressure.
- Turndown. Minimum fluidising air flow and the normal operating range.
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.