How zinc oxide furnaces work
Zinc oxide is made mainly by the indirect or French process. Zinc metal or zinc scrap is melted and boiled in a fired retort, crucible or muffle at about 950 to 1,050°C, heated by oil or gas burners in a separate combustion chamber. The zinc vapour flows into an oxidation chamber, where it burns with air drawn in by the product fan to form zinc oxide. The air and oxide pass long cooling ducts or hairpin coolers to the bag filter, which collects the zinc oxide as product. The burner flue gas leaves through its own stack or recuperator. The direct or American process instead reduces zinc ores or residues with coal on a grate, giving a less pure oxide with combustion gas in the product line.
Where the ID fan sits
- Oxidation chamber
- Cooling ducts or hairpin coolers
- Product bag filter
- ID (product) fan
- Stack
The fan sits after the product bag filter and draws air in at the oxidation chamber, past the zinc vapour outlet. In the indirect process this air is not mixed with burner flue gas, so the fan handles hot air with a trace of oxide. Suction covers the oxidation chamber, the cooling ducts and the bag filter, typically 200 to 350 mmWC. A separate small flue fan or a natural draft stack serves the retort burners.
Gas at the ID fan
| Air temperature, normal | 90 to 150°C after the coolers and bag filter |
| Maximum or upset | Up to 200°C if the coolers are fouled or zinc throughput is high |
| Dust at the fan | Traces of submicron zinc oxide; any loss is lost product |
| Moisture and dew point | Ambient moisture only in the indirect process; not limiting |
| Other relevant species | Lead or cadmium oxide traces when impure zinc or scrap is used |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 zinc oxide furnaces
| Blade form | Backward curved on the clean side, for steady and efficient flow |
| Material of construction | Mild steel; stainless steel for pharmaceutical or food grade oxide |
| Drive and arrangement | Direct coupled, arrangement 8 or 4 |
| Bearings and cooling | Standard pedestal bearings; cooling disc above about 120°C |
| Flow control | Variable speed drive on measured air flow or oxidation chamber suction |
| Also specified | Smooth, crevice-free internals, inspection door and casing drain |
What this ID fan has to cope with
Particle size drift
The ratio of air to zinc vapour and the cooling rate decide the particle size and surface area of the oxide. If the fan flow drifts as the bags load, the product goes out of specification. A speed-controlled fan holding measured air flow, not damper position, keeps the product consistent.
Product loss and contamination
Zinc oxide escaping through a failed bag is lost product and coats the fan. For high-purity grades, rust or paint from a mild steel fan casing can also contaminate the product if it is returned. Broken bag detection and stainless steel where purity matters protect the product.
Cooler fouling and fan temperature
Oxide builds up in the cooling ducts and reduces heat transfer, so the air reaches the filter and fan hotter. The fan must be rated for this, and cooler cleaning or rapping keeps the temperature within the bag and fan limits. Fouling builds gradually over a campaign, so trending fan inlet temperature is a simple guide to when the coolers need cleaning.
Energy and control
Zinc oxide furnaces run steadily at a production rate set by the zinc feed. The product fan is sized for the air required by the oxidation and cooling, and running it at matched speed rather than throttled saves power. Excess air raises fan power and lowers the temperature in the oxidation chamber, so the air flow should be held at the minimum that gives the product grade.
ID fan and FD fan in zinc oxide furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the product bag filter | Beside the furnace, feeding the retort burners |
| What it handles | Oxidation air with zinc oxide traces | Ambient combustion air |
| Temperature at the fan | 90 to 150°C | Ambient |
| Main design concern | Steady flow for product quality | Even retort heating |
| Control | Measured air flow | Burner ratio and retort temperature |
See also the Forced Draft (FD) Fan for zinc oxide furnaces.
Questions buyers ask
What does the ID fan do on a zinc oxide plant?
It draws air into the oxidation chamber where zinc vapour burns to zinc oxide, and carries the oxide through the cooling ducts to the bag filter. In the indirect process it handles air and product, not burner flue gas, and its flow setting affects the quality of the zinc oxide.
How does fan air flow affect zinc oxide particle size?
More air relative to the zinc vapour dilutes and cools the oxide faster, generally giving finer particles with higher surface area, while less air gives coarser particles. Holding a measured air flow with a speed-controlled fan keeps the product grade steady. Cooling duct length and temperature also matter, so the fan is chosen to hold flow steady as bag resistance changes.
Should a zinc oxide product fan be stainless steel?
For rubber, ceramic and general industrial grades, a mild steel fan after the bag filter is normal. For pharmaceutical, cosmetic and food grades, where any returned material must stay pure, stainless steel internals are often specified to avoid rust contamination. The decision depends on the grade made and on whether fines are recovered from the fan.
Information needed to quote an ID fan for zinc oxide furnaces
- 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.
- Production rate and grade. Zinc oxide output, process route and the product grade or surface area required.
- Cooling. Length and type of cooling ducts and the bag filter design 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.