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 ID fan sits
- Burner and firebox
- Kettle bottom
- Flue tubes through the gypsum
- Flue gas (ID) fan
- Recirculation or stack
The fan sits beside the kettle at the outlet of the flue tubes. It draws against the firebox, the kettle bottom passes and the flue tubes, typically 100 to 200 mmWC, and keeps the firebox at slight suction. On recirculating kettles the fan outlet splits, with part of the gas returned to the firebox to lower the gas temperature at the kettle bottom and the rest going to the stack. The kettle vapour exhaust with plaster dust is a separate fan and collector.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after the flue tubes |
| Maximum or upset | Up to 450°C during heat-up of an empty or partly filled kettle |
| Dust at the fan | Very low; clean combustion gas from gas or oil firing |
| Moisture and dew point | Combustion water only; well above dew point at the fan |
| Corrosive species | SO₂ with oil firing; negligible with natural gas |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 gypsum kettle calciners
| Blade form | Backward curved, for clean hot gas |
| Material of construction | Boiler quality plate stress-checked at the upset temperature; heat-resisting steel impeller where gas exceeds about 400°C |
| Drive and arrangement | Direct coupled, arrangement 8, casing free to expand |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive on firebox pressure, recirculation damper on firebox temperature |
| Also specified | Insulated casing and slow running after shutdown |
What this ID fan has to cope with
Hot gas during heat-up
At the start of a batch, or with a low charge, the gypsum absorbs less heat and the flue gas leaves hotter. The fan is designed for this upset temperature, and the recirculation damper opens to hold the firebox below the limit of the kettle bottom.
Kettle bottom burn-out
The kettle bottom is the most stressed part of the calciner. Too little recirculation or poor firebox draft lets hot flame touch it. Steady fan control and enough recirculated gas to temper the firebox protect the kettle.
Mixing up the two exhausts
The flue gas and the vapour from the gypsum have very different duties. The vapour carries plaster dust and steam near its dew point; the flue gas is clean and hot. Each needs its own fan, and a fan specified for one will not suit the other.
Energy and control
Batch kettles fire hard during heat-up and boil and less during finishing, and continuous kettles follow production. A variable speed drive on the flue gas fan follows these changes on firebox pressure and saves power over a fixed damper. Recirculating flue gas recovers heat that would otherwise go up the stack, which lowers fuel use per tonne of plaster.
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 Forced Draft (FD) Fan for gypsum kettle calciners.
Questions buyers ask
What does the ID fan do on a gypsum kettle?
On a gypsum kettle calciner it is the flue gas fan. It draws combustion gas from the firebox around the kettle and through the flue tubes, and on recirculating kettles returns part of it to the firebox to control temperature.
What temperature does a kettle flue gas fan handle?
Typically 250 to 350°C after the flue tubes, with higher temperatures during heat-up or with a low charge. The fan is designed for the upset temperature with a cooling disc to protect the bearings.
Is the vapour exhaust fan the same as the ID fan on a kettle?
No. The vapour exhaust handles steam and plaster dust from inside the kettle, near its dew point, through a dust collector. The ID or flue gas fan handles clean combustion gas outside the kettle. They are separate fans with different designs.
Information needed to quote an ID fan for gypsum kettle calciners
- 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.
- Recirculation. Whether flue gas is recirculated, and the split between recirculation and stack.
- Batch cycle. Batch or continuous operation and the firing rate through the cycle.
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.