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 ID fan sits
- Hot gas generator
- Mill or disperser
- Flash duct
- Cyclones
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, which captures fine plaster or gypsum as product, so the fan handles relatively clean but very humid gas. The suction at the fan covers the flash duct, any mill, the cyclones and the bag filter, typically 350 to 550 mmWC. Because the bag filter must stay above the dew point to avoid blinding, the fan inlet temperature is also held above it, usually by 20°C or more.
Gas at the ID fan
| Gas temperature, normal | 100 to 160°C after the bag filter, higher on calcination duty |
| Maximum or upset | Up to 250°C if the gypsum feed stops |
| Dust at the fan | Fine gypsum or plaster, low after a bag filter |
| Moisture and dew point | Water vapour from free moisture and, on calciners, combined water; dew point typically 60 to 80°C |
| Corrosive species | Fluorides and P₂O₅ from phosphogypsum; SO₂ from sulphur-bearing fuels |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% 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 flash dryers
| Blade form | Backward curved behind a bag filter, with wear protection on the leading edges |
| Material of construction | Mild steel for natural gypsum; SS 316 or corrosion-resistant coatings for phosphogypsum |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on system suction or outlet temperature |
| Also specified | Casing insulation and drain, inspection door, and heat tracing on some installations |
What this ID fan has to cope with
Plaster setting on the impeller
Calcined gypsum that meets condensate sets hard, as plaster does. A layer on the blades grows and breaks away unevenly, causing sudden vibration. Insulating the fan, holding the gas above dew point and purging the system with hot dry gas before shutdown prevent setting. Start-up sequences that warm the system before feed is introduced also help.
Phosphogypsum corrosion
Residual phosphoric acid and fluorides in phosphogypsum make the condensate acidic. Mild steel fans corrode at the casing bottom and blade roots. SS 316 or a suitable lining, plus a drain, extends life. Where the plant switches between natural gypsum and phosphogypsum, the fan is built for the phosphogypsum condition.
Bag filter leaks
A torn bag passes plaster fines to the fan, which then erodes and builds deposit quickly. Monitoring the bag filter differential pressure and dust at the stack protects the fan. A broken bag also shows up at the fan as rising current and vibration, so those trends are useful early warnings alongside the filter instruments.
Energy and control
Gypsum plants usually run continuously, and the ID fan carries the whole system resistance. A variable speed drive tuned to outlet temperature and suction saves power at partial throughput. Holding the hot gas generator excess air low reduces the gas volume. On calciners, controlling the outlet temperature precisely also improves product quality and limits overheating to anhydrite.
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 Forced Draft (FD) Fan for gypsum flash dryers.
Questions buyers ask
Why does plaster build up on my gypsum flash dryer fan?
Fine calcined gypsum reacts with condensate and sets on the blades. This happens when the fan or bag filter runs near the dew point, especially at start-up and shutdown. Insulation, hot purging and keeping the outlet temperature up prevent it. Short stops are the most risky, because the system cools just enough to condense.
What material is used for a phosphogypsum dryer ID fan?
Phosphogypsum carries residual phosphoric acid and fluorides, so stainless steel SS 316 or a suitable lining is used where condensate can form. Natural gypsum dryers usually use mild steel. The fan casing is also insulated and drained, because corrosion is worst where condensate collects at the bottom of the scroll during shutdowns.
How much suction does a gypsum flash dryer ID fan need?
Typically 350 to 550 mmWC, covering the flash duct, cyclones and bag filter. Plants with a mill in the circuit need more. The fan pressure must also cover the bag filter at its dirty condition. Measuring the suction at the present fan inlet gives a reliable basis for a replacement.
Information needed to quote an ID fan for gypsum flash dryers
- 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.
- Gypsum source. Natural, phosphogypsum or FGD gypsum, its free moisture, and whether the plant dries or calcines.
- Collector. Bag filter type and its operating temperature.
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.