How briquette fired thermic fluid heaters work
Biomass briquettes, pressed from sawdust, groundnut shell, bagasse, coffee husk or mixed agro residue, are fired on a fixed or reciprocating grate in a refractory combustion chamber surrounded by the radiant coil of the thermic fluid heater. Briquettes typically carry 3,600 to 4,300 kcal/kg, 6 to 12% moisture and anywhere from 3 to 15% ash depending on the raw material. The gas leaves the radiant and convective coils at about 330 to 380°C, because it cannot be cooled much below the fluid return temperature, and then passes an air preheater, a multicyclone and sometimes a bag filter before the ID fan and chimney. The FD fan supplies undergrate air through the air preheater.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate plenum
- Briquette bed
The FD fan stands beside the heater and pushes ambient air through the air preheater, where it is warmed to about 120 to 180°C by the outgoing flue gas, and into the undergrate plenum. Some heaters split the air into undergrate and overfire streams with dampers. The fan works against the air preheater, the plenum and grate, and a briquette bed that can be 150 to 300 mm deep, typically 60 to 120 mmWC in all.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 120 to 180°C, downstream of the fan |
| Dust | Clean air; briquette fines in the fuel yard can be drawn in |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Load pattern | Rapid swings between low fire and full fire with fluid temperature |
| 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 briquette fired thermic fluid heaters
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive from the fluid outlet temperature controller |
| Also specified | Inlet screen, and a split outlet with dampers where overfire air is used |
What this FD fan has to cope with
Clinker raises bed resistance
Briquettes with low ash fusion temperature form clinker on the grate, and the bed resistance rises as the shift goes on. A fan with pressure margin and a rising curve keeps air flowing through a partly fused bed; the margin should come from the curve, not from running permanently throttled.
Too much air at low fire
When the fluid reaches set point and the fire is turned down, a fan left at full speed blows excess air through the bed, cooling the furnace and wasting heat up the chimney. Linking the fan speed to firing rate, with an oxygen check, keeps excess air in the usual 40 to 60% band for grate firing.
Air preheater leakage
Cracked tubes or failed seals in the air preheater pass FD air straight into the flue gas. The FD fan then has to deliver more than the furnace receives, and the ID fan is loaded with extra cold air. Measured leakage is added to the fan duty on retrofit.
Energy and control
Briquette fired heaters cycle between high and low fire many times a shift as the fluid temperature controller responds to process demand. A variable speed drive follows these swings without the losses of a throttled damper and keeps the air-fuel ratio steady. On older heaters with on-off control of the FD fan, moving to modulating speed control also reduces thermal shock to the coils.
ID fan and FD fan in briquette fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone or bag filter | Before the air preheater |
| What it handles | Flue gas with fine ash and alkali salts | Clean ambient air |
| Temperature at the fan | 200 to 250°C, up to 380°C upset | Ambient |
| Main design concern | Variable fuel, deposits and hot gas excursions | Bed resistance and fast load response |
| Control | Furnace draft | Fluid outlet temperature |
See also the Induced Draft (ID) Fan for briquette fired thermic fluid heaters.
Questions buyers ask
What pressure does the FD fan on a briquette thermic fluid heater need?
Usually 60 to 120 mmWC, covering the air preheater, the undergrate plenum, the grate and the briquette bed. Heaters with deep beds or heavy clinkering fuel need the upper end. We confirm it from the heater design data or from a site measurement of plenum pressure.
Should the FD fan on a briquette heater have a variable speed drive?
Yes, in most cases. The heater turns up and down with the fluid temperature many times a day as process demand changes. A speed drive follows the load smoothly, saves power at the part loads where the heater spends most of its time, and keeps excess air lower than damper control or switching the fan on and off.
Does the FD fan handle hot air from the air preheater?
No. The FD fan pushes ambient air into the air preheater and the air is heated downstream of the fan, so the fan is selected at ambient density in mild steel. Only in unusual layouts where the fan draws through the preheater does it see hot air, and that arrangement is avoided because the fan becomes larger and needs bearing cooling.
Information needed to quote an FD fan for briquette fired 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.
- Air split. Whether air is supplied only under the grate or split into undergrate and overfire streams.
- Grate type. Fixed, reciprocating or step grate, and the normal bed depth.
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.