How gypsum flash dryers work
Natural gypsum, phosphogypsum from phosphoric acid plants or FGD gypsum is dried and often calcined to plaster of Paris in a flash dryer or flash calciner. A hot gas generator burning coal, oil, gas or biomass supplies gas at typically 300 to 600°C. Wet gypsum, often 10 to 25% free moisture for phosphogypsum, is milled or dispersed into the gas, dried in the flash duct and, for calcination, heated further so that the outlet runs at about 140 to 170°C. Drying alone leaves the gas at about 90 to 120°C. The product is collected in cyclones and a bag filter, and the ID fan after the bag filter sends the gas to the stack. Combustion air for the hot gas generator comes from an FD fan.
Where the FD fan sits
- Inlet filter
- FD (combustion air) fan
- Burner or grate
- Hot gas generator
- Dilution air
- Flash duct
The fan sits beside the hot gas generator and supplies air to the burner or under the grate. The hot gas leaves the generator and is mixed with dilution air or recirculated gas to set the flash duct inlet temperature. The fan pressure covers the burner or fuel bed and the generator, typically 100 to 300 mmWC. On recirculating designs, part of the exhaust gas is returned to the mixing chamber instead of fresh dilution air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Unheated |
| Dust | Gypsum and plaster dust in the plant air; filtered intake needed |
| Humidity | No significant effect |
| Pressure at the fan outlet | Typically 100 to 300 mmWC |
| 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 flash dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on burner firing rate |
| Also specified | Inlet filter against plaster dust |
What this FD fan has to cope with
Plaster dust on the fan
Gypsum plants are dusty, and plaster dust drawn into an unfiltered fan intake sets on the blades in humid air. A filtered intake from a clean area avoids this. The intake is placed upwind of the calciner and product handling, and the filter is checked often in dry weather.
Inlet temperature control
Gypsum calcination is sensitive to temperature. Poor combustion air control causes swings in flash duct inlet temperature and uneven product. Speed-controlled combustion and dilution air give steadier temperatures. Precise temperature control also limits overheating, which forms anhydrite and changes the setting time of the plaster.
Fuel changes
Switching between coal, oil and biomass changes the air needed. The fan is sized for the fuel needing the most air at full load. Coal and biomass on a grate also need undergrate and overfire air split, which affects the fan pressure. These are settled with the hot gas generator supplier.
Energy and control
The combustion air fan follows the hot gas generator load. Speed control holds the air-fuel ratio and limits excess air, which reduces both fuel and the gas volume that the ID fan must handle. Using recirculated exhaust gas instead of fresh dilution air further reduces fuel on some designs. Matching the dilution air to the actual flash duct temperature avoids overheating the product.
ID fan and FD fan in gypsum flash dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the hot gas generator |
| What it handles | Humid gas with fine gypsum | Ambient combustion air |
| Temperature at the fan | 100 to 160°C | Ambient |
| Main design concern | Plaster setting and condensation | Inlet temperature control |
| Control | System suction or outlet temperature | Burner firing rate |
See also the Induced Draft (ID) Fan for gypsum flash dryers.
Questions buyers ask
What does the FD fan do on a gypsum flash dryer?
It supplies combustion air to the hot gas generator, and in some designs dilution air that cools the hot gas to the flash duct inlet temperature. It handles clean ambient air, so it is mild steel and selected at ambient density. The intake is filtered against plaster dust.
Is a separate dilution air fan needed?
Not always. Some hot gas generators take dilution air from the same fan through a split duct; others use a separate fan or recirculated gas for finer temperature control. Recirculated gas saves fuel but adds moisture to the flash duct, which the ID fan and bag filter must then handle at a higher dew point.
Why does flash duct inlet temperature swing?
Often because combustion air and fuel are not well matched. Speed-controlled combustion air, tied to the burner and to the duct temperature, steadies it. Fouling in the hot gas generator, uneven fuel feed and a sticking dilution damper are other common causes worth checking during a site visit.
Information needed to quote an FD fan for gypsum flash dryers
- 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.
- Hot gas generator. Fuel, firing rate and air pressure required.
- Dilution. Whether dilution air comes from the same fan or from recirculated exhaust.
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.