Doubling the Efficiency with Superconductivity
Conventional AC induction heating has been used in industry since the 1920s. In 1990, a new concept emerged for DC induction heating using strong electromagnets. The magnet-wire and motor-drive technologies available at the time, however, did not permit an economical embodiment of the concept. With the emergence of both high-temperature superconductors (HTS) as a commercially available wire and advances in solid-state electric-motor drive equipment, this almost 20-year-old concept is now a commercially viable product.
Principles of OperationInduction heating relies on induced eddy currents to heat a conductive object. When a conductive material is exposed to a magnetic field changing in time, electric currents – eddy currents – are induced in the material. In a conventional induction billet heater, an electromagnetic coil made from copper surrounds a metallic billet (Fig. 1). When an alternating current is applied to the copper coil, an alternating magnetic field is generated. This in turn induces eddy currents in the billet, which then heat it due to its resistivity – a phenomenon called Joule Heating.
The electromagnetic coil is generally made from water-cooled copper tubing since the high current in the copper excitation coils creates ohmic losses and thus needs to be cooled to prevent melting. The heating of the copper coil is the main source of energy loss of this approach. This energy loss is given by the ratio of the resistances of the copper coil and the metal billet. Since a copper coil and a nonferrous metal have very similar resistivity, the energy is divided approximately evenly between them. This effect is amplified by the fact that an induction coil should be as close as possible to the billet, thus the conductor is additionally heated by the hot billet. Hence, the efficiency of the conventional AC induction heater for heating aluminum or copper is typically only 50% or less. In addition to the low efficiency of conventional AC induction heaters, considerable VAR compensation for the oscillating circuit is required by the user in order to increase the power factor and reduce the utility charges. Finally, these circuits require adjustments on changing billet dimensions, alloy composition or heating power.
All of these shortcomings do not apply to a superconducting arrangement as described in Figure 1. In a superconducting DC induction heater, very electrically efficient superconductors are used to create a large DC magnetic field. Superconductivity is a phenomenon that occurs when certain materials are cooled to low temperatures, resulting in large electric currents flowing with virtually no resistance. Hence, less than 200 W of energy are required to create a magnetic field for the DC induction heater. Since the field is DC and not varying, however, the workpiece must move in order to create eddy currents. Hence, the billet is rotated. The rotation induces eddy currents in the billet, which work to oppose the rotation. This is also the principle of the magnetic induction brake. This large braking torque is overcome by using large (e.g., 200kW-400kW size), efficient industrial motors. As the billet is rotated, the energy used by the motors is transferred to the billet, which is heated by eddy currents. Therefore, the source of energy is not the coil producing the magnetic field but the efficient motors, and all of the energy used to rotate the billet is transferred to heating the billet. There are some small losses to the power electronics and drive systems for the motors as well as some losses to the magnet’s cooling system, but the net result of the complete machinery is energy efficiency that is 80% or higher. Typical consumptions for heating aluminum are 150 kWh per metric ton of heated billets in such a configuration, which is even better than gas usage in highly efficient gas furnaces.
Advantages of HTS Induction HeaterIn addition to the obvious advantage of the significant energy efficiency compared to conventional induction heating, there are other advantages to this innovative approach. These are categorized as product quality, process reproducibility and manufacturing operations advantages.
Product Quality Advantages
In conventional 50-60Hz induction heating, the eddy currents are located primarily at the surface of the billet. This is because of a phenomenon called the “skin effect,” which is a strong function of the frequency. The eddy-current penetration increases as the frequency is decreased or magnetic field is increased, resulting in more uniform heating. Conventional AC induction heating usually relies on line frequency of 50-60 Hz, whereas the rotating-billet approach described here uses a rotational speed of about 240-600 RPM, which corresponds to 4-10 Hz. The benefits of the deeper heating are illustrated in the experimental results shown in Figure 2.
In addition to much more uniform heating, the rotating-billet approach described here results in a very reproducible temperature from billet to billet. This is due to the fact that radial and axial temperature uniformity is established immediately during the heating process and not after the billet is removed from the furnace. Hence, as shown in Figure 4, the billet temperature profile is reproducible to +/-4°C along the length of the billet and from billet to billet. This figure also illustrates the ability of the heater to create two temperature zones within a billet – a hotter zone for the front of the billet and a colder zone for the rear. In addition, although not shown in this figure, a linear temperature taper of about 1°C/cm is also available.
Manufacturing Operations Advantages
The machine is mechanically very straightforward, with simple installation procedures and minimal maintenance requirements when compared to conventional AC induction heaters. In addition to electrical power, the DC machine uses only a hydraulic system to supply the clamping pressure of the motors to the billet and a small simple water chiller for the heat exchangers on the refrigeration units. Very importantly, there is no reactive power compensation required for power-factor management as with conventional induction machines. Also, billets of different lengths may be heated without any further adjustments of coils or power factor or any loss of efficiency. The heating chamber contains no complex moving parts for billet transport, and the drives as well as the coil are completely thermally shielded from the heated billet. Finally, the superconducting magnet is durable and is not expected to be replaced during the lifetime of the machine since it is not exposed to heat or vibrations. Hence, no coil maintenance is required, which is considered a major maintenance issue for conventional machines.
Featured MachineThe machine pictured in Figure 5 is being readied for delivery in July 2008 to a commercial aluminum extrusion company in Minden, Germany. The broad operating characteristics of this machine are as follows:
- Capacity: 2.2 tons/hour (48 billets/hour) aluminum
- Billet size: 7 inches (178 mm) x 27 inches (690 mm)
- Max. temperature: 520°C (968°F)
- Power of drives: 360 kW
- Power of coil: <200 W
- Energy consumption <150 kWh/t
The motors on either side of the billet provide the rotational energy. These motors can slide in and out to accommodate different billet lengths. They have flanges that hold the billet during rotation without producing any damage or deformation. One of the key points about this machine is its simplicity. The only item that heats up is the billet. No critical component is exposed to major heat loads, vibrations or other potential damaging influences. The main components are the motors, which are a very mature technology; the refrigerators, which are also very mature; and the superconductor magnet, which is secured in a very robust steel enclosure. Site requirements and maintenance are easy and very minimal.
SummarySuperconductor technology has been applied to the production of a new generation of nonferrous induction heaters with shorter heating times and nearly double the efficiency of conventional induction heaters. A key element of these unique machines is the rotation of the workpiece. Superconducting Induction Heaters, available in sizes from 0.25 MW of thermal rating, revolutionize aluminum, copper and brass billet heating prior to extrusion – cutting energy demand and operating costs to almost half. The induction coils are manufactured from advanced superconductor material, chilled with compact machine-mounted chillers to 30 Kelvin and carry high DC current with virtually no losses. To create the induction-heating effect, the billet is rotated in a powerful electromagnetic field – the speed profile being determined by the size of billet and type of material. As well as doubling operating efficiency, the Superconducting Induction Heater requires less maintenance and is expected to have a longer working life because of no conventional thermal loads. For the same reason, tool changing is faster and safer. So, the bottom line is improvement in productivity, flexibility and operating costs.
For more information: Contact Larry Masur, Ph.D. at Zenergy Power Inc., 379 Oyster Point Blvd., Suite 1, South San Francisco, CA; tel: 781-738-8501; e-mail: Larry.Masur@zenergypower.com; web:www.zenergypower.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: eddy currents, electromagnetic coil, induction heater, thermal equilibrium, extrusion, temperature uniformity