How zeolite and silica calciners work
Synthetic zeolites, molecular sieves and silica-based adsorbents are formed as beads, extrudates or spray-dried powder, usually with a clay binder, then calcined at typically 500 to 700°C to set the binder and drive water out of the crystal pores. Some grades are ion exchanged with ammonium salts first, so the calciner also releases ammonia. Direct gas or oil fired rotary kilns are common, with the hot gas flowing counter-current to the product, and some plants use indirect kilns or belt calciners. Zeolites hold 20 to 25% water, so the off-gas is very humid. It leaves the feed end at 200 to 350°C carrying fine, abrasive aluminosilicate dust, passes a cyclone and bag filter, and the ID fan delivers it to the stack.
Where the ID fan sits
- Rotary calciner
- Feed end hood
- Cyclone
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter so that it handles only the dust that passes the bags. The gas reaches it at 150 to 220°C. Because the product releases so much water, the dew point is high, often 65 to 80°C, and the filter, ducts and fan casing are insulated. The fan works against the kiln, cyclone and filter, typically 250 to 450 mmWC, and holds the feed end at slight suction.
Gas at the ID fan
| Gas temperature, normal | 150 to 220°C after the bag filter |
| Maximum or upset | Up to 300°C if feed stops while the burner fires |
| Dust at the fan | Fine aluminosilicate dust, abrasive; low after the filter, heavy on a bag failure |
| Moisture and dew point | Very humid gas from product water; dew point about 65 to 80°C |
| Corrosive species | Ammonia on ammonium-exchanged grades; otherwise mildly corrosive only when wet |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 zeolite and silica calciners
| Blade form | Backward inclined, with replaceable wear strips on the leading edges |
| Material of construction | Mild steel with wear-resistant plate on the blades and inlet cone |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Cooling disc for the upset temperature; standard bearings |
| Flow control | Variable speed drive on kiln feed end draft |
| Also specified | Insulated casing, casing drain and inspection door for checking wear |
What this ID fan has to cope with
Condensation and mud on the blades
With a dew point near 75°C, any cool spot collects water, and the fine dust turns to a hard mud on the impeller and casing. Insulating the whole gas path, preheating before feed and purging before stops prevent it.
Erosion of leading edges
Aluminosilicate dust is hard. Even after a bag filter, small amounts erode the blade inlet and the inlet cone over time, and a bag failure speeds this up. Wear strips and liners at those points give a simple repair.
Bag filter blinding affecting the fan
Wet, fine dust can blind the bags, raising filter resistance until the fan cannot hold kiln draft. A fan with margin on pressure, and filter operation kept above the dew point, keep the kiln under suction.
Energy and control
Zeolite calciners run continuously or in grade campaigns. Off-gas volume depends heavily on the product water load, which changes between grades. Speed control on kiln draft follows these changes, and avoiding excess air in the kiln saves both fan power and the fuel needed to heat the extra air. Most of the fan power goes into moving water vapour.
ID fan and FD fan in zeolite and silica calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | At the kiln discharge end, feeding the burner |
| What it handles | Humid kiln gas with fine abrasive dust | Ambient air |
| Temperature at the fan | 150 to 220°C | Ambient |
| Main design concern | Dew point and erosion | Stable air to avoid overheating |
| Control | Kiln feed end draft | Firing rate |
See also the Forced Draft (FD) Fan for zeolite and silica calciners.
Questions buyers ask
Why is the ID fan gas so humid on a zeolite calciner?
Zeolites hold 20 to 25% water in their pores, and calcining drives it off. This water adds to the combustion water vapour, giving a dew point of about 65 to 80°C, much higher than on most kilns.
What causes wear on a zeolite kiln ID fan?
The aluminosilicate dust is hard and fine. Dust passing the bag filter, and especially a bag failure, erodes blade leading edges and the inlet cone. Wear strips and liners are the usual remedy.
Is the ID fan on a silica calciner different from one on a zeolite kiln?
The duty is similar, with fine abrasive dust and humid gas. Silica dusts are often lighter and finer, so filter efficiency matters more, and some silica products are dried rather than calcined, at lower temperature.
Information needed to quote an ID fan for zeolite and silica 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.
- Product water. Water content of the feed and product, and any ammonium exchange.
- Dust. Particle size and loading at the filter outlet, and filter type.
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.