How foundry thermal sand reclamation units work
Used foundry sand bonded with organic chemical binders, such as furan, phenolic urethane or alkaline phenolic, is crushed, screened and fed to a thermal reclaimer, usually a fluidised bed held at about 650 to 800°C by gas or oil burners. The binder burns off the grains, and the clean sand passes to a fluidised bed cooler before reuse. A fluidising air blower supplies air under the bed through a distributor plate; in many units this air is preheated in a recuperator after the blower. Flue gas leaves the bed at 650 to 800°C carrying fine silica dust, is cooled by the recuperator, a gas cooler or dilution air to below about 200°C, and passes a bag filter before the ID fan. Some plants use rotary kilns instead of fluidised beds.
Where the ID fan sits
- Fluidised bed reclaimer
- Freeboard and cyclone
- Recuperator or gas cooler
- Bag filter
- ID fan
- Stack
The ID fan is after the bag filter, so in normal running it handles filtered gas with little dust. The gas has already been cooled from 650 to 800°C to below about 200°C to protect the bags. If bags fail or the filter is bypassed at start-up, the fan sees fine, highly abrasive silica. It works against the freeboard suction, cyclone, recuperator or cooler and bag filter, typically 250 to 450 mmWC in total.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the bag filter |
| Maximum or upset | Up to about 250°C if the gas cooler fails, limited by a dilution air damper |
| Dust at the fan | Fine silica; low after the bags, heavy and very abrasive if bags fail |
| Moisture and dew point | Low; combustion water only, well above the dew point |
| Corrosive species | Sulphur compounds from furan binder catalysts and nitrogen compounds from urethane binders, in small amounts |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 thermal sand reclamation units
| Blade form | Backward curved with wear liners, or radial tipped where bag failures are likely |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with hard-faced leading edges |
| Drive and arrangement | Direct coupled arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings outside the gas, with a shaft cooling disc |
| Flow control | Variable speed drive on freeboard pressure |
| Also specified | Bolted liners at the scroll cut-off, inspection door, and an inlet temperature alarm |
What this ID fan has to cope with
Silica erosion after bag failure
Silica fines from sand attrition are harder than steel. A single burst bag sends grams per cubic metre to the fan, and the blade leading edges and scroll cut-off erode quickly. Wear-resistant plate, bolted liners, a moderate tip speed and a broken-bag detector on the filter outlet keep the damage small.
Overheating when the gas cooler fails
If the recuperator bypass sticks or cooling water or dilution air fails, gas well above the bag rating heads for the filter and fan. A temperature-controlled dilution damper before the filter, and a fan designed for the upset temperature, protect both until the burners are cut back.
Freeboard pressure swings
The fluidised bed surges as sand feed and burner firing change, and the freeboard pressure swings with it. If the fan does not follow, dusty gas puffs out at the feed chute and inspection doors. Speed control on freeboard pressure with a fast loop keeps the reclaimer under suction.
Thermal cycling on daily starts
Many reclaimers run one or two shifts and cool down overnight. Repeated heating and cooling loosens liners and fatigues welds at the impeller. Liners designed for thermal movement and inspection at each planned stop catch cracks before they spread. Holding the fan at low speed during cool-down also limits the temperature shock to the wheel.
Energy and control
Reclaimers often run in shifts that follow the moulding line, with a heat-up period each day. Variable speed on the ID fan, holding freeboard pressure, cuts power during heat-up, idling and reduced sand feed. Recovering heat from the flue gas to preheat fluidising air, as many units do, lowers burner fuel and also reduces the gas volume the fan must handle.
ID fan and FD fan in foundry thermal sand reclamation units
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Beside the reclaimer, before the recuperator |
| What it handles | Cooled flue gas with residual silica | Ambient air |
| Temperature at the fan | 120 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Silica erosion and cooler failure | High, steady fluidising pressure |
| Control | Freeboard pressure | Fluidising air flow |
See also the Forced Draft (FD) Fan for foundry thermal sand reclamation units.
Questions buyers ask
Why does the ID fan wear on a thermal sand reclaimer?
The flue gas carries fine silica worn from the sand grains. Behind a healthy bag filter very little reaches the fan, but a burst bag or a filter bypass sends a heavy load of abrasive dust straight to the impeller. Wear-resistant plate, liners and broken-bag detection protect the induced draft fan.
What temperature does the ID fan on a sand reclamation unit see?
After the bag filter, typically 120 to 180°C. The flue gas leaves the bed at 650 to 800°C and is cooled by a recuperator, gas cooler or dilution air. If the cooling fails, temperature can rise quickly, so the fan is designed for the upset and protected by a dilution damper.
Where should the ID fan be placed on a thermal sand reclaimer?
After the bag filter, so that it handles filtered, cooled gas. Placing it before the filter exposes it to the full silica dust load and much higher temperatures, and the impeller would erode in weeks. The fan then holds the freeboard at slight suction through the whole gas path.
Information needed to quote an ID fan for foundry thermal sand reclamation units
- 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.
- Reclaimer. Fluidised bed or rotary kiln, sand throughput, bed temperature and binder type.
- Gas cooling. Recuperator, gas cooler or dilution air, and the bag filter temperature limit.
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.