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 FD fan sits
- Inlet screen
- FD (combustion air) fan
- Recuperator (if fitted)
- Air control valve
- Burners
- Retort or muffle combustion chamber
The fan sits beside the furnace and supplies air to the burners in the combustion chamber around the retort or muffle. Where a recuperator on the flue gas preheats the air, the fan pushes cold air through it first. Pressure required is typically 200 to 500 mmWC, depending on burners and any recuperator. The burner flue gas leaves separately from the product stream.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 250 to 400°C where fitted, downstream of the fan |
| Dust | Clean air; zinc oxide dust in the building should be kept out of the inlet |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Turndown | Firing follows the zinc vaporisation rate |
| 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 zinc oxide furnaces
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or modulating valve with burner ratio control |
| Also specified | Inlet screen and ducting from a clean area |
What this FD fan has to cope with
Zinc oxide dust at the inlet
Zinc oxide plants have fine white dust on every surface from bagging and handling. A fan inlet in the building draws this in, and it builds up on the impeller. A duct to a clean air source keeps the fan and burners clean. Cleaner air also keeps burner nozzles and flame sensors working reliably.
Retort life and firing
Retorts and muffles fail early if overheated locally. Steady burner air and correct ratio, without large swings from an oversized fan on a damper, help keep the retort temperature even and extend its life. Uneven heating also changes the vaporisation rate, which in turn upsets the oxidation side and the product grade, so stable combustion air helps both.
Matching firing to production
When production is reduced, the burners fire less but an oversized fan runs heavily throttled. A variable speed drive keeps control stable over the production range. At low fire a throttled damper gives coarse control and unstable flames, which shows up as uneven retort temperature. Speed control holds the air pressure steady at every production rate, and the burner ratio valves then trim each burner accurately.
Energy and control
The retort or muffle burners consume the fuel in a zinc oxide plant, so combustion efficiency matters more than fan power. A recuperator that preheats combustion air from the flue gas saves fuel, and its pressure drop is part of the fan duty. Speed control on the fan suits plants that change production with orders.
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 Induced Draft (ID) Fan for zinc oxide furnaces.
Questions buyers ask
Does the FD fan supply the air that oxidises the zinc?
No. In the indirect process the FD fan supplies combustion air to the burners heating the retort. The oxidation air is drawn into the oxidation chamber by the product fan after the bag filter, and is kept separate from the burner flue gas. This separation is what keeps indirect process oxide free of combustion products.
What pressure does a zinc oxide furnace combustion air fan need?
Typically 200 to 500 mmWC, depending on the burner type, number of burners and whether the air passes a recuperator. We size the fan from the burner data and air line layout and confirm it against the existing fan where one is being replaced. A small margin covers burner wear.
Can a recuperator be added to a zinc oxide furnace?
Often yes, using the hot burner flue gas leaving the combustion chamber. The FD fan must then supply the extra pressure drop of the recuperator air side, so an existing fan may need a new impeller or a higher speed. The motor rating should be checked at the same time, as the fan absorbs more power at the higher pressure.
Information needed to quote an FD fan for zinc oxide furnaces
- 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.
- Burners. Number of burners, fuel, firing range and required air pressure.
- Recuperator. Whether combustion air is preheated, and its air side pressure drop.
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.