How induction furnace melt shops work
An induction furnace melts steel scrap, sponge iron and pig iron in a refractory crucible heated by a water-cooled induction coil. There is no fuel and no combustion air, so the furnace has no flue in the usual sense. Fume rises from the crucible during charging and melting, heaviest when oily, painted or galvanised scrap goes in, and again at tapping and slag removal. A swivel or side-suction hood on each furnace draws this fume with a large quantity of ambient air through a spark arrestor and cyclone or directly into a bag filter, and an exhaust (ID) fan delivers the cleaned air to a stack. Melt shops typically run two or more crucibles from one extraction system.
Where the ID fan sits
- Furnace hood
- Spark arrestor
- Cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan is placed after the bag filter, so it handles cleaned air and the fine fume that passes the bags. Suction at the fan covers hood entry losses, long ducts from each furnace, the spark arrestor, cyclone and filter, typically 200 to 350 mmWC. Positioning it after the filter keeps sparks and coarse particles away from the impeller and makes a simple mild steel fan practical. The ductwork usually has a motorised damper at each furnace branch.
Gas at the ID fan
| Gas temperature, normal | 50 to 90°C, mostly diluted ambient air |
| Maximum or upset | 120 to 150°C briefly if a hood is lowered close to the bath |
| Dust at the fan | Fine iron and zinc oxide after the filter, usually below 30 mg/Nm³ |
| Other contaminants | Oil vapour and smoke from contaminated scrap; sparks stopped upstream |
| Moisture | Low, ambient level |
| Density at the design temperature | About 1.00 kg/m³ at 80°C, 83% 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 induction furnace melt shops
| Blade form | Backward curved, for efficiency on filtered air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Belt driven on small shops; direct coupled with VFD on larger multi-furnace systems |
| Bearings and cooling | Standard pedestal bearings, no cooling |
| Flow control | Variable speed drive with motorised dampers at each furnace hood |
| Also specified | Inlet box with drain, access door, and a margin for future furnaces on the same system |
What this ID fan has to cope with
Poor capture at the hood
Fume escapes into the shop when the hood is swung away for charging, exactly when emission is worst. The fan must be sized for the capture velocity at the actual hood opening, not for an idealised closed hood, and the dampers must send most of the suction to the furnace being charged while the others are only melting.
Spark and fire risk in the filter
Sparks and burning oil from scrap can reach the bags. A spark arrestor or cyclone upstream, and a fan placed after the filter, protect both the bags and the impeller. The fan control should allow a quick reduction in flow, and the dampers a quick isolation, if the filter inlet temperature rises.
Sticky deposits from oily scrap
Oil mist condenses with fine fume into a tacky layer on the impeller and in the ducts. Regular inspection, a blade form that is easy to clean, and avoiding cold spots in the ducting reduce the buildup, and with it the unbalance that otherwise appears a few months after commissioning.
Energy and control
Induction melting is a batch operation, with charging, melting, sampling and tapping in sequence, and the furnaces in a shop are rarely at the same stage. A variable speed fan, with each hood damper opened according to its furnace status, supplies suction where it is needed and cuts power when only one furnace is producing fume. Constant speed fans throttled by a single damper waste much of their power.
Questions buyers ask
Does an induction furnace need an ID fan or FD fan?
An induction furnace needs no FD fan because there is no combustion. It does need a fume extraction fan, which works as an exhaust or ID fan, drawing fume from the hood through a spark arrestor and bag filter to the stack. FD fans and combustion air blowers apply only to fuel-fired furnaces such as reheating furnaces.
What capacity fan is needed for induction furnace fume extraction?
The volume is set by the hood opening and the capture velocity needed to keep fume out of the shop, not by the furnace power alone. A shop with several crucibles usually sizes the fan for the furnaces likely to be charging at the same time, with dampers that shift suction between them as the melt cycle moves on.
Why does the fume extraction fan in our melt shop vibrate?
The most common cause is buildup of oily or painted-scrap residue on the impeller, followed by wear where the filter has been bypassed or bags have failed. Cleaning the impeller, rebalancing it to ISO 21940-11 and correcting the filter bypass usually solve the problem. Fitting an inspection door makes later cleaning quicker.
Information needed to quote an ID fan for induction furnace melt shops
- 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.
- Furnace and hood details. Number and size of crucibles, hood type and opening dimensions.
- Charge mix. Typical proportion of oily, painted or galvanised scrap and any sponge iron.
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.