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 FD fan sits
- Inlet filter
- FD fan
- Recuperator
- Air manifold
- Burner ratio valves
- Radiant tube burners
The fan sits beside the furnace at floor level and delivers ambient air to the recuperator, where the air picks up heat from the outgoing flue gas, and then to the burner air manifold. Each burner draws from the manifold through its own ratio or flow valve. The fan works against the recuperator air side, the manifold and the burner pressure requirement, typically 300 to 600 mmWC for nozzle-mix radiant tube burners. A filtered inlet is usual because enamel shops handle frit powder.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 250 to 450°C, downstream of the fan |
| Dust | Enamel frit and slip overspray in the shop air; filtered inlet recommended |
| Pressure at the fan outlet | Typically 300 to 600 mmWC depending on burner type |
| Back-heating risk | Hot air can flow back to the fan if it stops while the recuperator is hot |
| 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 vitreous enamel firing furnaces
| Blade form | Backward curved or high-pressure radial, chosen for a steep, stable pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft for most furnace sizes |
| Bearings and cooling | Motor bearings or standard pedestal bearings; no cooling needed |
| Flow control | Constant manifold pressure, with variable speed drive or inlet damper as burners turn down |
| Also specified | Inlet filter, low air pressure switch interlocked with the burners, and a non-return damper |
What this FD fan has to cope with
Frit dust on the impeller
Fine glass frit drawn from the shop air builds on blades and inside the recuperator air passages, reducing flow and causing unbalance. A filtered inlet taken from a clean area and periodic filter changes prevent this.
Manifold pressure swings
When many burners switch between high and low fire, manifold pressure moves and burners drift off ratio, which affects firing zone temperature and enamel colour. A fan with a steep curve and speed control on manifold pressure holds the ratio.
Hot air backflow on trip
If the fan stops while the recuperator is hot, air in the recuperator can heat the fan outlet duct and the fan itself. A non-return damper and a run-on period after burner shutdown protect the fan and the recuperator.
Energy and control
The combustion air fan runs whenever the furnace is at temperature, including idle periods between batches or shifts. A variable speed drive holding manifold pressure follows the burner demand and cuts power at low fire. Checking excess oxygen in the radiant tube flue is the simplest way to see whether the fan is supplying more air than the burners need, which also wastes fuel.
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 Induced Draft (ID) Fan for vitreous enamel firing furnaces.
Questions buyers ask
What pressure does a combustion air fan for radiant tube burners need?
Typically 300 to 600 mmWC at the fan outlet, depending on the burner design, the recuperator air side and the manifold. Burner makers state the air pressure needed at the burner, and the fan is sized to that plus the losses upstream.
Does the FD fan on an enamel furnace handle hot air?
No. It takes ambient air and the air is heated in the recuperator downstream. The fan is selected at ambient density, but its outlet duct and damper must tolerate some backflow heat after a trip.
Is one combustion air fan enough for a whole enamel firing furnace?
Usually yes, with a common manifold feeding all burners. Larger furnaces with separate preheat and firing zone controls sometimes use one fan per zone so that each zone can be controlled and maintained independently.
Information needed to quote an FD fan for vitreous enamel firing 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.
- Burner air pressure. Required air pressure at the burner inlet and the number of burners on the manifold.
- Recuperator air side. Air side pressure drop of the recuperator at full flow.
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.