The ISO 50001 international standard for energy performance was introduced in part 1 of this article. In this concluding installment, the individual stages of an induction heating line are examined for individual energy efficiencies. An actual energy audit is also summarized.
ISO 50001 is primarily about lowering energy consumption, reducing the reactive power and increasing the efficiency of a production plant – in this case a forge. To understand the individual energy-efficiency drivers, an overview of the relevant principles is provided.
Induction Heating of Forging Billets and Bars
With induction heating for hot and warm forging, the metal blank (bar, billet or block) is placed in an alternating electromagnetic field generated by a coil (Figure 1). Eddy currents are thereby induced inside the material to generate heat. Consequently, induction heating occurs rapidly, and the resulting workpiece temperature can be very precisely adjusted.
The overall efficiency of an induction furnace is the product of the individual efficiency levels of the various single components – namely, the medium-voltage transformer, frequency converter, bus bars and inductor (the coil), as well as the thermal degree of efficiency.
The individual efficiencies are not equal. Rather, there are components in which an improvement has a significantly positive influence on the system’s overall efficiency. Whereas the medium-voltage transformer, for example, has a high operating efficiency of around 99%, the induction coil – with an individual operating efficiency of close to 75% – has considerable impact on the system’s overall efficiency.
The example given in Figure 2 shows a real situation in a forge shop. Of the 1,253-kW grid energy consumption, only 812 kW actually heat the workpiece to 1250°C (2282°F), corresponding to an efficiency of about 65%. The energy losses of the individual components of the heating plants total 441 kW, of which the coil makes up for more than 200 kW of power loss. It is worth taking a close look at this to identify the factors that have a positive effect on the overall result.
The Induction Coil
The nature of induction heating is such that the smaller the difference between the coil diameter and heating material diameter, the greater the energy efficiency. Forgers should be mindful of this.
The influence of the electromagnetic penetration depth is also of interest. According to Lenz’s law, the eddy current builds up a field that opposes the inductor current. The net result of both fields is in the diminishment of the magnetic field in a radial direction inward. The associated eddy current intensity also drops. The depth at which the current intensity has fallen to 37% of its maximum value is known as the penetration depth (d), as given in the accompanying equation:
The formula shows that the penetration depth depends substantially on the frequency of the induced current. As the frequency increases, the current penetration depth is reduced. In the case of heating operations where optimum through-heating of the material cross section is required, a low frequency should be chosen. As shown in Figure 1, very rapid, homogeneous temperature distribution is achieved over the cross section when the cylindrical workpiece diameter is around 3.5 times greater than the penetration depth. These conditions are the result of a trade-off between direct, consistent heating over the cross section with a correspondingly low frequency and increasing energy efficiency at high frequency.
Clear evidence of this elementary connection was demonstrated in trials performed decades ago in which cylinders of different sizes were heated simultaneously in a common coil. With the varying coloration it was easy to see that both excessively small and excessively large diameters did not produce optimum through-heating. Interestingly, this effect easily overrides the influence of the position of the material being heated within the coil. The optimum-diameter sample, shown at the bottom left of Figure 3, is not centered in the middle of the coil yet still produces the best result in terms of through-heating.
Ideally, the induction coil would be perfectly matched to each material diameter. However, it is usually not practical to have a custom heating coil for each and every billet diameter. The forging shop product spectrum needs to be analyzed and grouped into diameter ranges to be heated by a finite set of coils. The design of the coil, therefore, always represents a compromise between being a geometrically perfect match and production flexibility.
Copper-Coil Alloy Selection
In addition to the operating frequency and coupling distance (i.e. the ratio of coil and workpiece diameters), another efficiency driver is the material quality of the coil. The specific electrical resistance of the heating-coil copper alloy is also an important consideration.
Table 1 shows the key differences for the two copper grades commonly used in electro-technical components. They differ in terms of copper content and machinability, which is particularly important for coil manufacturers. Cu-DHP can be more easily worked, both mechanically and with regard to welding and soldering. Therefore, some coil manufacturers choose this material grade, which has a slightly less copper content.
Coils made from Cu-DHP and Cu-HCP appear to be identical. If we compare the specific electrical resistance of both grades, however, it can be seen that the Cu-HCP material with the higher copper content (greater than 99.95%) shows a lower (i.e. better) resistance value. Over the temperature trajectory, which is of particular interest for copper coils when used for induction heating, the specific electrical resistance of Cu-HCP is about 30% better than that of Cu-DHP at every temperature point. The energy-efficient properties of the higher-grade Cu-HCP represent an interesting option for saving energy and money over the working life of the coil.
Converter (Power Supply) Technology
In the example given in Figure 2, the operating efficiency of the converter (power supply) is the third-largest influence on total heating efficiency – after the inductor and thermal efficiencies.
As shown in the example, the new generation of converter with an efficiency level of 97% (hconverter multiplier of 0.97) and with an L-LC oscillating circuit is already in use. Conventional converter topologies characteristically have far lower efficiencies. With an uncontrolled rectifier, intermediate circuit capacitor, IGBT (insulated-gate bipolar transistor) inverter and output choke, this converter features a constant cos f(power factor) of greater than 0.95, regardless of the output power level.
The L-LC circuit features two points of resonance: one with parallel and one with series resonance. Depending on the desired circuit properties and application, both may be used. To control the inverter, special algorithms have to be used to find the desired point of resonance (parallel or serial) and clearly establish the working point. For this, the L-LC circuit has the advantage that both the frequency and power can be controlled via the inverter.
This article has dealt with the reduction in energy consumption and the improvement in the overall efficiency and power factor of an induction heating line. For these plants in particular, manufacturers offer a variety of possibilities for increasing part efficiency levels through intelligent plant design.
In addition, the specific energy-audit calculation (sidebar) shows that long-term energy cost savings can be made by optimizing just one system component – in this case the coordinated coil set – and that significant reductions in emissions can also be achieved.
In the short term, energy-efficiency audits can be used to work out and implement practical solutions that directly improve the energy efficiency of induction heating lines upstream of forming equipment and, thereby, reduce energy consumption.
Over the long term, the costs of implementing a DIN ISO 50001 energy management system are worthwhile given the continual increase in the company’s overall energy efficiency.
Co-author Dirk M. Schibisch is vice president of sales for SMS Elotherm GmbH, Germany. He can be contacted at firstname.lastname@example.org. Co-author Loïc de Vathaire heads the service spare-parts department for SMS Elotherm, Germany. He may be reached at email@example.com. This article was adapted for publication in FORGE by George Burnet, SMS Elotherm North America. He may be reached at firstname.lastname@example.org or 724-591-0252.
Induction-Line Energy Audit
This is a specific example of how a sustainable reduction in grid power consumption – and thus increased energy efficiency – can be achieved with the optimal design of an induction heating line.
In this example, a forge shop has an induction furnace with a nominal power of 1,500 kW in use upstream of a horizontal multistage press. Bars 25-40 mm in diameter are heated to 1250°C (2282°F) through a series of five induction coils. A single set of coils is used for the entire product range.
The data in Tables A and B were gathered for this induction audit.
This data clearly shows that grid consumption increases substantially if the coil and material diameters are not ideally coordinated. Given the annual tonnage, optimized heating of 28-mm bars, in particular, is desirable. The calculation of the induction coil designed for bar diameters of 28 mm and 32 mm shows that the grid consumption is as shown in Table C.
This results in two optimization strategies as shown in Table D.
In this example, optimization strategy 1 proves to be the better result of the induction-related audit.
Nearly $15,000 can be saved every year with just one additional set of induction coils. The investment in an additional set of coils would increase the savings made by a few percent and would therefore not be cost-effective. Since bars in the greater-than 32-mm-diameter range make up just a small proportion of the annual output (~3%), they should be produced wherever possible using intelligent production schemes to keep changeovers to a different set of coils to a minimum.
As far as the aims of ISO 50001 are concerned, this result – in real terms – means annual savings of around 166 tons of CO2 using a conversion factor of 1 KWh of electricity to 0.566 kg CO2.