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 FD fan sits
- Inlet filter
- FD (fluidising air) fan
- Recuperator
- Air box
- Distributor plate
- Fluidised sand bed
The fluidising air blower sits at ground level beside the reclaimer, drawing ambient air through a filter and pushing it through the recuperator, where flue gas preheats it, into the air box under the distributor plate. Placing the fan before the recuperator keeps it on cool air. It works against the recuperator, the distributor and the sand bed, typically 800 to 1,500 mmWC in total, much higher than an ordinary combustion air fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 300 to 500°C, downstream of the fan |
| Dust | Silica dust in the foundry air; inlet filter needed |
| Pressure at the fan outlet | High, typically 800 to 1,500 mmWC |
| Flow stability | Must stay above the minimum fluidising velocity at all loads |
| 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 foundry thermal sand reclamation units
| Blade form | Backward curved high-pressure wheel, with a steep, stable curve |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, with a heavy-duty shaft for high pressure |
| Bearings and cooling | Standard bearings; no cooling needed on ambient air |
| Flow control | Variable speed drive on flow measurement, with a minimum flow limit |
| Also specified | Inlet filter and silencer, non-return damper, and air box pressure measurement |
What this FD fan has to cope with
Bed defluidisation
If air flow drops below the minimum fluidising velocity, the bed slumps, heat transfer collapses and hot spots form at the burners. A fan with a steep curve, so flow changes little as bed pressure rises, and a minimum flow interlock on the burners prevent it.
Sand back-flow into the air box
When the blower stops with a hot, full bed, sand can sift through the distributor nozzles into the air box and recuperator. A non-return damper, distributor nozzles designed against back-flow and a controlled shutdown sequence that empties or cools the bed avoid it. Clearing sand from the air box before restart prevents nozzle damage.
Silica dust drawn in from the foundry
Foundry air is dusty, and at the high speeds of a pressure blower silica abrades the impeller and settles in the recuperator. A good inlet filter, with a pressure gauge, protects the fan and keeps the recuperator clean. Taking the intake from outside the moulding hall reduces the dust load further.
Energy and control
The fluidising blower is the largest motor on most reclaimers and runs against a high pressure all shift. Holding flow at the minimum needed for good fluidisation and binder burnout, rather than running with a generous margin, saves power and fuel, because every extra cubic metre of air is heated to bed temperature. A variable speed drive on flow measurement does this.
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 Induced Draft (ID) Fan for foundry thermal sand reclamation units.
Questions buyers ask
What pressure does a fluidising air blower on a sand reclaimer need?
Typically 800 to 1,500 mmWC at the fan outlet, depending on bed depth, sand density, the distributor design and the recuperator. The fan must hold this pressure across the flow range without dropping below the minimum fluidising flow, so a steep, stable curve is needed.
Is a centrifugal fan or a roots blower used for the fluidising air?
Both are used. A high-pressure centrifugal fan suits most beds and gives smooth flow with a variable speed drive. Positive displacement blowers are used where the bed pressure is very high or the flow must stay fixed regardless of bed resistance. The choice depends on pressure and turndown.
Does the FD fan handle hot air on a thermal sand reclaimer?
Not usually. The forced draft fan takes ambient air and pushes it through the recuperator, where flue gas heats it to about 300 to 500°C before the air box. Keeping the fan on the cold side avoids heat-resisting construction and keeps its bearings and seals simple.
Information needed to quote an FD fan for foundry thermal sand reclamation units
- 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.
- Bed. Bed area, depth, operating temperature and the distributor plate pressure drop.
- Air system. Recuperator resistance and whether burner combustion air is taken from the same fan.
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.