How gypsum kettle calciners work
A gypsum kettle is a vertical steel vessel, typically 3 to 5 m in diameter, with an agitator, set over a refractory firebox. A gas or oil burner fires into the firebox, and the hot gas flows around the kettle bottom and through horizontal flue tubes passing through the gypsum, heating it indirectly to about 150 to 165°C. The gypsum boils as it gives off its combined water, and the steam with fine plaster leaves through a separate vapour duct to a dust collector. Kettles run batch or continuously; in batch operation the firing rate changes through heat-up, first boil and finish. The combustion gas, kept separate from the gypsum, leaves the flue tubes at about 250 to 350°C, and part of it is often recirculated to temper the firebox.
Where the FD fan sits
- Inlet filter
- Combustion air FD fan
- Air control valve
- Burner
- Firebox
The fan is mounted on or beside the burner at the firebox front. It delivers air through a control valve to the burner, typically at 200 to 500 mmWC. On recirculating kettles the combustion air and recirculated flue gas meet in the firebox; the combustion air fan handles only the fresh air. The firebox suction is set by the flue gas fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Plaster dust in the building; filtered intake needed |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Turndown | High during heat-up and boil, low during finishing |
| Fuel | Natural gas, LPG or fuel oil |
| 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 gypsum kettle calciners
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, burner-mounted or skid |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or modulating damper with ratio control |
| Also specified | Inlet filter, air pressure switch and flow measurement |
What this FD fan has to cope with
Turndown through the batch
The burner fires hard at heat-up and boil, then drops to low fire. A fan throttled to low flow by a damper becomes unstable. Speed control and a fan selected near the middle of its range give steady air at both ends.
Plaster dust intake
Plaster dust drawn into the fan coats the impeller and the burner air passages, changing the air-fuel ratio. A filtered intake from outside the calcining hall prevents it.
Air-fuel ratio and the kettle bottom
Excess air cools the flame but wastes fuel; too little gives long, smoky flames that can touch the kettle bottom. Measured air flow with ratio control keeps the flame correct through the batch.
Energy and control
The combustion air fan is small, but correct air-fuel ratio through the batch cycle has a large effect on fuel use per tonne. Speed control on the fan, linked to the burner firing rate, saves fan power at low fire and gives better ratio control than a damper. Keeping the intake filter clean also keeps the air delivery steady through each batch.
ID fan and FD fan in gypsum kettle calciners
| ID fan | FD fan | |
|---|---|---|
| Position | At the outlet of the kettle flue tubes | At the burner on the firebox |
| What it handles | Clean combustion gas | Ambient air |
| Temperature at the fan | 250 to 350°C | Ambient |
| Main design concern | Hot gas and firebox temperature | Turndown and air-fuel ratio |
| Control | Firebox pressure and recirculation | Burner firing rate |
See also the Induced Draft (ID) Fan for gypsum kettle calciners.
Questions buyers ask
Is the FD fan on a gypsum kettle the burner fan?
Yes. On a gypsum kettle calciner the forced draft fan supplies combustion air to the burner in the firebox. It handles ambient air and is often supplied as part of the burner package.
What pressure does a kettle burner fan need?
Typically 200 to 500 mmWC, depending on the burner type and firing rate. Packaged burners specify the pressure needed at the burner inlet, and the fan must add the losses of the air line and control valve.
Why does the air flow need to change during a gypsum batch?
The heat demand is highest at heat-up and boil and falls during finishing. The air must follow the fuel to keep the flame correct and protect the kettle bottom.
Information needed to quote an FD fan for gypsum kettle calciners
- 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.
- Burner. Burner type, fuel and maximum firing rate.
- Cycle. Firing rates through the batch and the turndown needed.
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.