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 FD fan sits
- Inlet filter
- FD fan
- Primary and secondary air ducts
- Burner
- Rotary calciner
The fan sits at the discharge end of the kiln, close to the burner. It delivers air through separate primary and secondary ducts, and sometimes to a tempering air port that keeps the gas below the temperature at which the zeolite structure collapses. Outlet pressure is typically 300 to 700 mmWC for gas or oil burners. A separate cooler fan, where fitted, is not part of this duty.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Filtered air; ambient dust in a powder plant can be heavy |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Firing follows feed rate and grade, about 3:1 |
| Tempering air | Some kilns add ambient air to limit gas temperature at the hot end |
| 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 zeolite and silica calciners
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air main pressure, with control dampers per duct |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Overheating the product
Zeolite crystal structure is damaged above its limit, often 750 to 850°C depending on type. Unstable air and flame can create hot spots. Stable fan pressure and separate tempering air control hold the hot zone within range.
Fine dust at the inlet
Powder handling around the kiln fills the air with fines, which coat the impeller and burner. An inlet filter or a duct from a clean area prevents this.
Poor control at low firing
A fan sized well above duty runs throttled, with unstable air at low firing. Speed control on a correctly sized fan gives smooth turndown. It also reduces wear on the dampers.
Energy and control
The combustion air fan runs whenever the kiln fires. Air demand follows feed and grade. Speed control on air main pressure, with oxygen trim, avoids excess air that adds to the humid off-gas volume, to the bag filter load and to the ID fan power. The fan itself is small, but it runs every hour the kiln fires.
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 Induced Draft (ID) Fan for zeolite and silica calciners.
Questions buyers ask
What does the FD fan supply on a zeolite calciner?
It supplies combustion air to the burner at the discharge end, split into primary and secondary air, and sometimes tempering air to limit the hot zone temperature. The air is ambient and the fan never sees the humid kiln gas.
Why does a zeolite kiln need tempering air?
Zeolite crystals lose their structure if overheated. Tempering air mixed into the hot gas keeps the hot zone below that limit while the burner fires at the rate the throughput needs.
What pressure does a calciner combustion air fan need?
Typically 300 to 700 mmWC for gas or oil burners, enough to give the burner the air velocity it needs for flame shape, plus duct and damper losses.
Information needed to quote an FD fan for zeolite and silica calciners
- 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. Burner type, fuel and air pressure needed at full firing.
- Tempering. Whether the fan also supplies tempering air and the flow required.
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.