How gypsum calcining plants work
Gypsum calcining plants convert natural, FGD or phosphogypsum dihydrate into hemihydrate plaster by heating it to about 140 to 180°C, driving off three-quarters of its combined water, about 15% of the feed weight, plus any free moisture. The main types are kettle calciners, heated indirectly with a burner below the kettle; rotary calciners with direct or indirect firing; and flash or mill calciners, where gypsum is ground and calcined at once in hot gas at 400 to 650°C from a hot gas generator. In direct-fired systems, the exhaust carries the water vapour from the gypsum, the combustion products and fine plaster dust to a bag filter and the ID fan. Kettles also have a separate flue gas fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner or hot gas generator
- Dilution air mixing
- Calciner
The fan sits next to the burner or hot gas generator. On a flash calciner it delivers combustion air, and a separate dilution air fan or opening brings the hot gas to its set temperature before the mill. On kettles and rotary calciners it feeds the burner directly. Outlet pressure is typically 200 to 600 mmWC, set by the burner and the hot gas generator losses.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Gypsum and plaster dust in the plant; filtered intake needed |
| Pressure at the fan outlet | Typically 200 to 600 mmWC |
| Turndown | Follows calciner output and feed moisture |
| Air after the generator | 400 to 650°C hot gas, downstream of the fan |
| 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 calcining plants
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or inlet damper linked to the burner |
| Also specified | Inlet filter and low air pressure interlock |
What this FD fan has to cope with
Plaster dust at the intake
Calcining plants are dusty, and plaster drawn into the combustion air fan builds up on the impeller and disturbs the flame. A filtered intake from a clean area avoids it.
Hot gas temperature control
On flash calciners the product quality depends on hot gas temperature. If combustion and dilution air are not controlled together, the temperature swings and the plaster over or under calcines. Coordinated control of the fans and burner keeps it steady.
Turndown with feed moisture
Wet FGD gypsum needs more heat than dry natural gypsum. The fan must cover the maximum firing for the wettest feed and still control well at low firing.
Energy and control
The combustion air fan is small compared with the ID fan but runs whenever the calciner fires. Speed control linked to the burner saves power at reduced output. Correct air-fuel ratio, and using dilution air only as needed, saves fuel and reduces the exhaust volume the ID fan must handle.
ID fan and FD fan in gypsum calcining plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the burner or hot gas generator |
| What it handles | Moist exhaust with plaster dust | Ambient air |
| Temperature at the fan | 130 to 170°C | Ambient |
| Main design concern | Condensation and plaster setting | Firing and hot gas temperature control |
| Control | Calciner outlet pressure | Burner firing rate |
See also the Induced Draft (ID) Fan for gypsum calcining plants.
Questions buyers ask
What is the FD fan on a gypsum calcining plant?
It is the combustion air fan for the burner or hot gas generator of the gypsum calcining plant. On flash calciners a separate dilution air fan or inlet sets the hot gas temperature.
Does the FD fan see hot gas in a gypsum plant?
No. It handles ambient air and pushes it into the burner. The hot gas from the generator is drawn through the calciner by the ID fan.
How much pressure does the combustion air fan need?
Typically 200 to 600 mmWC, depending on the burner type and the hot gas generator losses. Solid fuel hot gas generators with grates and a deep fuel bed may need more pressure.
Information needed to quote an FD fan for gypsum calcining plants
- 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. Fuel, burner type and maximum firing rate.
- Dilution air. Whether dilution air is supplied by the same fan or separately.
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.