How foundry core baking ovens work
Cores made from sand bonded with drying oils such as linseed oil, with cereal binder and water, are baked at about 200 to 240°C for one to several hours, so that the oil oxidises and polymerises into a hard bond. Ovens are batch car-bottom or rack ovens, or continuous vertical tower and conveyor ovens, and the same ovens dry core and mould coatings. Heating is by oil- or gas-fired burners, firing into the recirculated air directly or through a heat exchanger. Water evaporates first; then the oil binder gives off smoky, acrid vapour as it cures. An exhaust fan removes this vapour-laden air, and a combustion air fan supplies the burner. High-volume foundries have largely moved to cold box and shell cores, but baked oil-sand cores remain common in jobbing and large-casting foundries.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Burner
- Combustion chamber or heater box
- Recirculation fan
- Core oven chamber
The combustion air fan is usually part of a packaged burner mounted on the heater box at the side or top of the oven. On older or larger ovens a separate fan feeds the burner through a duct and damper. The fan draws air from the core shop, where sand dust is common, and delivers it at the burner pressure, typically 100 to 300 mmWC at full fire.
Air at the FD fan
| Air temperature at the fan | Ambient to warm shop air, 25 to 50°C |
| Dust | Silica sand dust in the core shop; inlet filter needed |
| Pressure at the fan outlet | Typically 100 to 300 mmWC at the burner |
| Turndown | Full fire at heat-up, low fire through the soak |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 foundry core baking ovens
| Blade form | Backward curved, for stable delivery on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Damper linked to the fuel valve, or variable speed drive |
| Also specified | Inlet filter against sand dust, and an air pressure switch |
What this FD fan has to cope with
Sand dust wearing the fan
Core shop air carries fine silica from core making and cleaning. Drawn through an unfiltered fan, it slowly abrades the impeller and settles in the burner head. An inlet filter, or an intake ducted from a cleaner area, protects both the fan and the burner over years of running.
Poor combustion at low fire
Through the long soak the burner runs at low fire. A fan and damper that cannot hold the right air-fuel ratio at the bottom of the range let the burner run with far too much air, wasting fuel, or too little, sooting the heater box. A characterised damper or speed control fixes this.
Burner lockouts at heat-up
At the start of each bake the burner goes to full fire. If the fan has lost pressure from a loaded filter or worn impeller, the air pressure switch trips and the bake starts late. A fan with margin over the burner requirement and a filter pressure gauge avoid these delays.
Energy and control
The burner fan follows the bake cycle: full output at heat-up, low output through the soak, and off during loading. Matching its air delivery to the fuel across that range, with a characterised damper or speed control, keeps excess air low. Since excess air is heated and then exhausted with the binder fume, this saves fuel each bake.
ID fan and FD fan in foundry core baking ovens
| ID fan | FD fan | |
|---|---|---|
| Position | Oven roof, on the exhaust duct | On the burner at the heater box |
| What it handles | Steam and oily binder fume | Ambient shop air |
| Temperature at the fan | 150 to 220°C | 25 to 50°C |
| Main design concern | Tar deposits and fire | Turndown through the bake |
| Control | Bake cycle stage | Oven temperature |
See also the Induced Draft (ID) Fan for foundry core baking ovens.
Questions buyers ask
What does the FD fan do on a core baking oven?
It supplies combustion air to the burner that heats the core baking oven. In most ovens it is part of a packaged burner, though larger ovens may have a separate forced draft fan. It handles ambient air and must keep a correct air-fuel ratio from full fire at heat-up to low fire through the soak.
Why does my core oven burner trip at the start of the bake?
At the start of a bake the burner goes to full fire and needs full air pressure. A clogged inlet filter, sand dust on the impeller or a worn fan can leave the pressure just short, so the air pressure switch trips. Cleaning, checking the fan curve and replacing the filter usually solve it.
Does a core oven combustion air fan need a filter?
Yes, in most core shops. The air carries fine silica sand dust that wears the impeller and fouls the burner head. A simple panel filter at the fan inlet, changed on a schedule, or an intake ducted from a clean area protects both the fan and the burner.
Information needed to quote an FD fan for foundry core baking ovens
- 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. Fuel, burner rating and the air pressure it needs at full fire.
- Bake cycle. Heat-up time, soak temperature and the firing range through the cycle.
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.