How castor shell fired boilers work
Castor shell is the husk left when castor seed is decorticated, produced mostly in north Gujarat and parts of Rajasthan. It fires boilers in castor oil mills and castor derivative chemical plants, often on AFBC units or travelling grates and frequently blended with lignite, coal or other husks. The shell is crumbly and breaks into fines in handling. It carries 8 to 12% moisture, about 6 to 12% ash including sand from threshing floors, and a calorific value of about 3,500 to 4,000 kcal/kg. In an AFBC the shell burns in a bed of sand at about 800 to 900°C, and fines and bed material carried over reach the cyclones. The flue gas passes the boiler bank, economiser and air heater, then cyclones and a bag filter or ESP, before the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Air heater
- Air box and distributor
- AFBC bed
The FD fan sits near the air heater and pushes ambient air through it, heated to about 150 to 200°C, into the air box and the nozzles of the bed distributor. A separate secondary or overbed air fan is common. The FD fan must overcome the air heater, ducts, distributor and bed, typically 800 to 1,100 mmWC on an AFBC, or 150 to 200 mmWC on a grate unit.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 48°C |
| Air after the air heater | 150 to 200°C, downstream of the fan |
| Dust | Shell fines and desert dust in summer |
| Pressure at the fan outlet | Typically 800 to 1,100 mmWC for an AFBC |
| Density | Summer air above 45°C is about 10% lighter than at 15°C |
| Density at the design temperature | About 1.10 kg/m³ at 48°C, 92% 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 castor shell fired boilers
| Blade form | Backward curved, high pressure design with a stable rising curve |
| Material of construction | Mild steel; high strength impeller plate where tip speed is high |
| Drive and arrangement | Direct coupled, arrangement 8, at two-pole or four-pole speed |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow, with minimum fluidising flow interlock |
| Also specified | Inlet filter, inlet silencer and acoustic lagging |
What this FD fan has to cope with
Summer air density
At 45°C and above, the air is markedly lighter than in winter, and a fan selected at standard density delivers less mass and pressure. The bed may defluidise at full load on the hottest days. Selection at maximum ambient temperature, with the motor checked at winter density, covers the whole year.
Bed defluidisation
If fluidising air falls below the minimum, the bed slumps, hot spots form and the ash and sand agglomerate. The fan must hold minimum fluidising flow at every load, with the drive interlocked so speed never falls below it, and pressure margin for a coarsening bed.
Dust at the intake
Summer winds in north Gujarat carry fine dust, and shell handling adds more. Drawn into the FD fan, it erodes the impeller slowly and builds on the distributor nozzles. An inlet filter sized for low velocity, with a pressure drop alarm, protects both. The filter's dirty pressure drop is added to the fan duty so airflow holds as it loads.
Energy and control
The FD fan on an AFBC is a high pressure fan and one of the largest auxiliaries. Keeping the bed height near design reduces the pressure it must develop, and a variable speed drive avoids the damper losses common on older units. Oxygen trim on the secondary air reduces the gas volume the ID fan handles.
ID fan and FD fan in castor shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Before the air heater, feeding the bed |
| What it handles | Flue gas with shell ash and bed sand fines | Hot, dusty ambient air |
| Temperature at the fan | 150 to 180°C, up to 220°C | Ambient, 15 to 48°C |
| Main design concern | Abrasive fly ash | Fluidising pressure at summer density |
| Control | Furnace draft | Air flow with minimum fluidising interlock |
See also the Induced Draft (ID) Fan for castor shell fired boilers.
Questions buyers ask
What FD fan pressure does a castor shell AFBC boiler need?
Typically 800 to 1,100 mmWC, covering the air heater, ducts, distributor and the bed. A grate fired castor shell boiler needs far less, typically 150 to 200 mmWC. We size it from the bed design and the boiler data, then check against site readings, with margin for distributor nozzle wear and a coarsening bed.
Why does my AFBC lose fluidisation on hot days?
Hot air is less dense, so the forced draft fan delivers less mass flow and pressure at the same speed. If the fan was selected at standard density, it may fall below minimum fluidising flow at full load. Selection at the maximum ambient temperature solves it.
Does the FD fan on a castor shell boiler need an inlet filter?
Yes, in most north Gujarat plants. Summer dust and shell fines would otherwise erode the impeller and block distributor nozzles. A filter sized for low velocity, with a pressure drop alarm, is a small cost against these problems. The intake is best taken on the side away from shell unloading and the prevailing summer wind.
Information needed to quote an FD fan for castor shell fired boilers
- 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.
- Site ambient. Minimum and maximum ambient temperature at the site.
- Bed design. Bed area, bed height, distributor pressure drop and minimum fluidising velocity.
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.