Induction heating
Induction heating is used in many industrial processes, such as heat treatment in metallurgy, Czochralski crystal growth and zone refining used in the semiconductor industry, and to melt refractory metals that require very high temperatures.
In ferromagnetic and ferrimagnetic materials, such as iron, heat also is generated by magnetic hysteresis losses.
Utility frequency (50/60 Hz) induction heating is used for many lower-cost industrial applications as inverters are not required.
Once molten, the high-frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that alloying additions are fully mixed into the melt.
Most induction furnaces consist of a tube of water-cooled copper rings surrounding a container of refractory material.
Induction furnaces often emit a high-pitched whine or hum when they are running, depending on their operating frequency.
Currents induced in a tube run along the open seam and heat the edges resulting in a temperature high enough for welding.
In the Rapid Induction Printing metal additive printing process, a conductive wire feedstock and shielding gas is fed through a coiled nozzle, subjecting the feedstock to induction heating and ejection from the nozzle as a liquid, in order to refuse under shielding to form three-dimensional metal structures.
The core benefit of the use of induction heating in this process is significantly greater energy and material efficiency as well as a higher degree of safety when compared with other additive manufacturing methods, such as selective laser sintering, which deliver heat to the material using a powerful laser or electron beam.
Induction heating is used in cap sealing of containers in the food and pharmaceutical industries.
A layer of aluminum foil is placed over the bottle or jar opening and heated by induction to fuse it to the container.
Induction heating can produce high-power densities which allow short interaction times to reach the required temperature.
Limits to the flexibility of the process arise from the need to produce dedicated inductors for many applications.
Induction heating improves energy efficiency for injection and extrusion processes.
Heat is directly generated in the barrel of the machine, reducing warm-up time and energy consumption.
The induction coil can be placed outside thermal insulation, so it operates at low temperatures and has a long life.
The reduction in the cost of inverter equipment has made induction heating increasingly popular.
Heat is directly generated into shaker reactor walls, enabling the pyrolysis of the biomass with good mixing and temperature control.
In the simplest case of a solid round bar, the induced current decreases exponentially from the surface.
If operated below the critical frequency, heating efficiency is reduced because eddy currents from either side of the workpiece impinge upon one another and cancel out.
Thus the reference depth can vary with temperature by a factor of 2–3 for nonmagnetic conductors and by as much as 20 for magnetic steels.
Hysteresis heating occurs below the Curie temperature, where materials retain their magnetic properties.
The energy transfer of induction heating is affected by the distance between the coil and the workpiece.
Energy losses occur through heat conduction from workpiece to fixture, natural convection, and thermal radiation.
Diameter, shape, and number of turns influence the efficiency and field pattern.
The furnace consists of a circular hearth that contains the charge to be melted in the form of a ring.
The metal ring is large in diameter and is magnetically interlinked with an electrical winding energized by an AC source.
It is essentially a transformer where the charge to be heated forms a single-turn short circuit secondary and is magnetically coupled to the primary by an iron core.
