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 ID fan sits
- Burner and recirculation fan
- Core oven chamber
- Exhaust duct
- Exhaust (ID) fan
- Scrubber or stack
The exhaust fan sits on the oven roof or beside it and draws from the oven chamber, usually from the return air duct of the recirculation system. There is seldom a collector ahead of it, so it handles the full oven fume. Suction is low, typically 20 to 60 mmWC, enough to hold the oven slightly negative so that smoke does not leak from the doors into the core shop. Some foundries add a scrubber after the fan for odour, which raises the fan pressure.
Gas at the ID fan
| Air temperature, normal | 150 to 220°C at the exhaust |
| Maximum or upset | Up to about 250°C during an overbake |
| Dust at the fan | Sand fines and binder smoke |
| Moisture and dew point | Heavy steam in the first part of the bake; condensation in cold ducts |
| Fumes | Oil binder decomposition products that condense to tar below about 150°C |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 62% 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 foundry core baking ovens
| Blade form | Radial or radial tipped, open blades that can be scraped clean |
| Material of construction | Mild steel, with an insulated casing |
| Drive and arrangement | Belt driven arrangement 9, with the motor away from the hot casing |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed or two-speed drive following the bake cycle |
| Also specified | Large cleaning door, tar drain at the casing bottom, and a fire damper in the duct |
What this ID fan has to cope with
Tar build-up
Oil binder vapour condenses as the exhaust cools and leaves a sticky tar that hardens on the blades, scroll and duct. A backward curved wheel with closed passages clogs and goes out of balance quickly. Open radial blades that can be scraped, an insulated casing and a large cleaning door make the build-up manageable.
Steam first, smoke later
A bake starts with heavy steam from the core moisture and ends with oily fume. A fixed exhaust set for the steam peak wastes heat for the rest of the bake, while one set for the fume leaves the oven humid early on. A drive that follows the bake cycle gives the right flow at each stage.
Duct and fan fires
Tar deposits in the exhaust duct and fan can ignite during an overbake or from burner flame. A fire damper at the oven connection, a temperature alarm in the duct, a cleaning schedule and a fan casing without pockets where tar collects reduce the risk of a fire travelling to the roof.
Energy and control
Core baking is a batch process with a clear shape: high exhaust for the first part of the bake to carry off steam, then a lower flow for the curing fume, then little or none during cooling and unloading. A variable speed drive following that profile, in place of a damper left wide open, saves fan power and a good share of the burner fuel.
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 Forced Draft (FD) Fan for foundry core baking ovens.
Questions buyers ask
Why does the core oven exhaust fan get covered in tar?
Oil binders give off vapour that condenses into tar as the exhaust cools in the duct and fan. The tar hardens and unbalances the impeller. An insulated casing and duct keep the exhaust hotter, and open radial blades with a large cleaning door let the tar be scraped off at regular stops.
How much exhaust does a foundry core baking oven need?
Enough to carry away the water evaporated from the cores in the first part of the bake, which is the peak, and later the binder fume. The flow depends on core weight per batch and moisture. A variable speed exhaust fan lets the flow follow the bake rather than running at the steam peak throughout.
What temperature does a core oven exhaust fan handle?
Typically 150 to 220°C, following the oven temperature, with peaks up to about 250°C during an overbake. Mild steel suits this duty, with a shaft cooling disc, a heat shield and an insulated casing. The insulation also keeps the casing above the point where binder tar condenses.
Information needed to quote an ID fan for foundry core baking ovens
- 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.
- Oven and load. Batch or continuous oven, core weight per batch, core moisture and bake cycle.
- Binder. Oil sand, coating drying or other binder systems baked in the oven.
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.