How vitreous enamel firing furnaces work
Vitreous enamel is fused onto pressed steel or cast iron ware such as cookware, water heater tanks, bathtubs, signage and panels. Coated parts hang on a conveyor that carries them through a preheat zone, a firing zone at typically 780 to 860°C for sheet steel, and a cooling zone, usually in a U-shaped or straight-through continuous furnace; smaller shops use box or bell furnaces. Most gas or LPG fired enamel furnaces heat the work indirectly through radiant tubes or a muffle, because sulphur and water vapour in combustion products cause bubbling and fishscale defects in the enamel. The radiant tube flue gas is collected in a header, passes a recuperator that preheats the burner air, and is drawn out by the exhaust (ID) fan. A combustion air fan feeds the burners.
Where the ID fan sits
- Radiant tube burners
- Flue gas header
- Recuperator
- Dilution air inlet
- ID fan
- Stack
The fan sits at the end of the flue gas header, after the recuperator and usually on the furnace roof or a platform beside it. It works against the radiant tube or muffle resistance, the header and the recuperator, typically 60 to 150 mmWC in total. Because each radiant tube needs the same draft to fire evenly, the fan must hold header suction steady across the burner turndown. A small dilution air inlet ahead of the fan is often used to cap the temperature during high fire or when the recuperator is bypassed.
Gas at the ID fan
| Gas temperature, normal | 300 to 400°C after the recuperator |
| Maximum or upset | Up to 550°C with the recuperator bypassed or at high fire without dilution air |
| Dust at the fan | Essentially clean combustion products from gas or LPG; trace scale from radiant tubes |
| Moisture and dew point | About 15 to 18% water vapour from natural gas or LPG; dew point well below the gas temperature |
| Corrosive species | Low sulphur fuel, so acid attack is minor; fluoride and boron vapour from the frit stays mostly in the furnace atmosphere, not the tube flue |
| Density at the design temperature | About 0.57 kg/m³ at 350°C, 47% 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 vitreous enamel firing furnaces
| Blade form | Backward curved or radial, depending on size; small furnaces often use a radial bladed hot gas fan |
| Material of construction | Heat-resisting steel impeller; mild steel casing with external insulation and allowance for thermal expansion |
| Drive and arrangement | Belt or direct drive, arrangement with the motor outside the hot zone and a long overhung shaft |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and heat shield between casing and bearing housing |
| Flow control | Variable speed drive on header suction, or an inlet damper on smaller furnaces |
| Also specified | Dilution air damper with temperature interlock, casing drain, and a balance check after the first heat cycle |
What this ID fan has to cope with
Uneven radiant tube firing
If header suction drifts, tubes near the fan get more draft than those at the far end, and the firing zone develops cold spots that show as underfired enamel. A fan with a flat, stable characteristic and speed control from header pressure keeps every tube at the same draft through the burner turndown.
Impeller creep at high temperature
Recuperator bypass or failed dilution air can put gas above 500°C into the fan. A mild steel wheel loses strength quickly at that temperature and can stretch or distort. A heat-resisting steel impeller, a temperature trip and a dilution damper that fails open protect the fan.
Bearing overheating
Heat conducted along the shaft and radiated from the casing raises bearing temperature, especially where the fan sits on the furnace roof. A cooling disc, heat shield, high-temperature grease and an adequate gap between casing and pedestal keep bearings within limits.
Energy and control
Enamel furnaces run on a fixed temperature profile with production rate changing the burner firing rate. On continuous lines that idle at temperature during breaks or with empty hooks, a variable speed ID fan drops to a fraction of full speed while still holding tube draft. Recovering more heat in the recuperator lowers the gas temperature at the fan, which raises gas density and reduces the volume the fan has to move for the same firing rate.
ID fan and FD fan in vitreous enamel firing furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator, at the end of the flue header | Ahead of the recuperator, feeding the burner manifold |
| What it handles | Radiant tube flue gas | Ambient combustion air |
| Temperature at the fan | 300 to 400°C, up to 550°C upset | Ambient, 25 to 45°C |
| Main design concern | Heat resistance and even tube draft | Steady manifold pressure |
| Control | Header suction | Burner manifold pressure |
See also the Forced Draft (FD) Fan for vitreous enamel firing furnaces.
Questions buyers ask
What temperature does the ID fan see on an enamel firing furnace?
After a recuperator the flue gas from radiant tubes is usually 300 to 400°C. Without a recuperator, or with it bypassed, it can be well above 500°C. The fan is designed for the upset temperature, not only the normal one, with dilution air used to cap the peak.
Why do enamel furnaces use radiant tubes instead of direct firing?
Combustion products contain water vapour and sulphur compounds that react with the molten enamel and cause bubbles, pinholes and fishscale. Radiant tubes or a muffle keep the burner gas separate from the ware, which is why the ID fan handles the tube flue gas and not the furnace atmosphere.
Can a mild steel ID fan be used on an enamel firing furnace?
Only if the gas at the fan is reliably below about 350°C, with a recuperator in service and dilution air under control. Most enamel furnace exhaust fans use a heat-resisting steel impeller because recuperator bypass and high fire push the temperature higher.
Information needed to quote an ID fan for vitreous enamel firing 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.
- Burners. Number of radiant tubes or muffle burners, fuel, and maximum firing rate per burner.
- Recuperator. Whether a recuperator is fitted, its gas outlet temperature and whether it can be bypassed.
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.