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 ID fan sits
- Grate and furnace
- Radiant coil
- Convective coil
- Air preheater
- Multicyclone
- ID fan
- Chimney
The fan sits after the multicyclone, or after a bag filter where emission norms require one, at the foot of the chimney. Upstream it sees the furnace draft, the coil bank, the air preheater and the collector, usually 80 to 160 mmWC on heaters of 1 to 6 million kcal/h. It is placed after the air preheater so that it handles gas at about 200 to 250°C rather than the 350°C leaving the coils, which keeps the fan in mild steel and reduces the gas volume.
Gas at the ID fan
| Gas temperature, normal | 200 to 250°C after the air preheater |
| Maximum or upset | 350 to 380°C if the air preheater is bypassed or fouled |
| Dust at the fan | Fine ash and char after a multicyclone, roughly 0.3 to 1 g/Nm³; much lower behind a bag filter |
| Deposits | Sticky potassium and chloride salts from agro residue briquettes condense on cooler surfaces |
| Moisture and dew point | Water dew point about 50 to 60°C; not limiting at normal temperature |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 briquette fired thermic fluid heaters
| Blade form | Backward curved, with radial tipped as the choice behind a multicyclone with high ash briquettes |
| Material of construction | Mild steel casing and impeller, with wear liners on the blade leading edges |
| Drive and arrangement | Direct coupled arrangement 8, or belt driven on smaller heaters for easy speed matching |
| Bearings and cooling | Pedestal bearings with a cooling disc on the shaft; rated for 380°C for short periods |
| Flow control | Variable speed drive on furnace draft, with an inlet damper for isolation |
| Also specified | Inspection door and casing drain for clearing salt deposits |
What this ID fan has to cope with
Fuel quality changes the duty
A load of groundnut shell briquettes behaves differently from sawdust briquettes: more ash, more volatile matter, lower ash fusion. Gas volume and temperature at the fan change with the fuel. The fan is sized on the worst briquette the plant buys, with a speed drive to trim back on better fuel, rather than on the supplier's best analysis.
Alkali deposits on the impeller
Briquettes from agro residue carry potassium and chlorine that leave the furnace as vapour and condense as sticky salts on the coils, air preheater and fan blades. Uneven build-up throws the wheel out of balance. Smooth blades, an access door and washing at each shutdown keep vibration in check.
Hot gas when the air preheater fails
A choked or bypassed air preheater sends gas at 350°C or more to the fan. Bearings, shaft and casing are rated for this as a short-term condition, and a high gas temperature alarm at the fan inlet tells the operator to reduce firing before damage occurs.
Energy and control
Thermic fluid demand in a dyeing or process plant follows the batches, so the heater often fires at 50 to 80% of rating for long periods. A variable speed drive on the ID fan holding furnace draft is the main saving, typically a third or more of the fan power against damper control. Keeping the air preheater clean saves twice: hotter combustion air and a cooler, smaller gas volume at the fan.
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 Forced Draft (FD) Fan for briquette fired thermic fluid heaters.
Questions buyers ask
What temperature should the ID fan on a briquette fired thermic fluid heater be designed for?
Design for the gas leaving the air preheater, usually 200 to 250°C, with the fan rated to survive the coil outlet temperature of about 350 to 380°C for short periods. That covers air preheater fouling or bypass without overspecifying the whole fan in heat-resisting steel.
Why does the ID fan vibrate after a change of briquette supplier?
Briquettes from agro residue such as groundnut shell or mustard husk carry more potassium and chlorine than sawdust briquettes. The salts condense on the impeller as sticky deposits that build up unevenly. Cleaning the wheel usually restores balance; if the new fuel continues, plan regular washing.
Is a multicyclone enough before the ID fan on a briquette heater?
For fan life on good quality briquettes it usually is, with wear liners on the blades. For emission norms in many states a bag filter or wet scrubber is also needed, and the ID fan must then be sized for the added resistance of that equipment.
Information needed to quote an ID fan for briquette 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.
- Briquette types. The raw materials of the briquettes used, with any ash and moisture analysis available.
- Air preheater. Gas temperature before and after the air preheater at full and part load.
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.