What is Transverse flux induction heating?
||Principle of transverse flux:
The magnetic Field created by the coils is transverse to the strip flow and creates the eddy current heating up the strip by the Joule effect.
Transverse flux is one of the oldest electrical induction heating technique. During the heating process, the steel strip passes through induction coil pairs located on both sides of the strip.
Induction heating with transverse flux
Induction heating with longitudinal flux
The State of Art
Since the 1980s-90s, some work has been achieved in transverse flux induction heating of non magnetic metal strips. We can mention Ajax Tocco (USA) for example with a double J inductor or a Davy MacKee (UK) prototype heating up aluminum strip. They managed to reach an acceptable strip temperature transversal homogeneity. Nevertheless the outcome of the projects was often limited by the low density of power the inductor was able to inject into the strip and by the small distance available for the strip travel (<40mm), making applications outside laboratories impossible.
In the 1990s, no solution for heating non magnetic metal strip by transverse flux induction could result in an industrial application in process lines.
In 2000, Fives Celes partnered with Usinor and EDF to develop a new generation of machines.
Thanks to its high flux technology (patent EP 93401582.7 for high power inductors); the equipment designed by Fives Celes could break the technological deadlocks in the application of the transverse flux induction dedicated to strips heating in process lines.
The inductor could then transfer power densities of 3MW on less than 2m of pass height. The transverse flux inductor was then capable of heating magnetic or non magnetic metals with heating slopes in the range of 200 to 300 °C / second and a window for strip passage of about 200mm, making all industrial applications possible for the continuous treatment of the magnetic or non magnetic metals.
||First Prototype of High Power Transverse Flux.
EDF – Usinor - Celes.
Challenge and innovation
This technology has been developed for some specific fields and needs in the steel industry:
- Induction heating of non magnetic materials,
- Heating metals to high temperature above “Curie point” (750°C)
- Reach of extremely fast temperature ramps with high density heating units
- Providing fast response heating system with very low thermal inertia
Transverse flux inductor
In the heating process of continuous annealing of steel strip, gas or electric resistances are currently used. These technologies have low heating slopes (50°C / second approx.), low thermal efficiencies (about 50%) and generate NOX or CO2 gas emissions.
Years of research and joint development with EDF and USINOR, thanks to a support of ADEME and the French Ministry of Industry, made it possible to come up with a high density inductor.
- Injected power more than 3Mw per inductor, into a steel strip,
- Dimension in the longitudinal direction of the strip less than 2 meters
- Thermal efficiency 75%
- Temperature homogeneity of +/-3% thanks to patented electromagnetic devices and coils