Sintering/Powder Metallurgy

Energy-Saving Silicon Carbide Heating Element

October 1, 2014
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In Japan, there is a governmental regulation called “Law concerning the Rationalization of the Energy Use.” All industries are required to spend sufficient efforts within specific guidelines to conserve energy in areas of plant operation such as transportation, buildings, etc.



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Fig. 5. Lab kiln


 

We believe this trend is basically the same worldwide. “Energy saving” and “sustainability” are globally becoming very important in today’s business. All of us are expected to take concrete actions to reduce energy consumption in every part of our business activities.

With this background, industries such as automotive and home electronics are able to utilize “energy use” as a market opportunity for new products. Most business entities, however, do not have these new market opportunities and must make themselves more energy efficient in all aspects of their business.

Our company, Tokai Konetsu Kogyo Co., Ltd. (TKK) in Japan, has been manufacturing and selling silicon carbide (SiC) heating elements under the “EREMA” brand since 1936. There are several types of heating elements available for industrial heating. Generally speaking, metallic heating elements such as Nichrome wire (Ni-Cr) are commonly used in lower temperature ranges (up to 1100°C), and molybdenum disilicide (MoSi2) elements are often used in high-temperature ranges (1200-1900°C). The EREMA SiC heating elements cover between the two at temperatures up to 1600°C (2912°F).

The SiC material is unique in that it exhibits a high resistance to acid and corrosion and can therefore be used in severe environments. Consequently, SiC heating elements are often found in the glass, powder-metallurgy and ceramics industries, where the products off-gas and create these severe atmospheres.



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Fig. 1. Heat usage/loss; Fig. 2. Heat flow of a typical SiC element

Heat Balance and Heat Loss

In our study, most atmospheric continuous furnaces lose heat to four components: the furnace walls, products being treated in furnace, slabs and crucibles, and the cold ends of SiC heating elements.

Figure 1 shows the proportion of heat loss by the four main causes. Approximately 23% of the entire heat loss is from the cold end of the SiC elements, according to our study. This means SiC elements themselves contribute significantly to the loss of heat from the furnace. SiC elements make the heat, but at the same time they lose the heat and, therefore, should play an important role in energy-saving considerations.

Figure 2 shows the parts of a typical SiC heating element and its heat flow. Typically, we think that all the heat generated by the element flows from the hot zone to the products under heat treatment. In the heat balance of the real furnace environment, however, heat loss from the cold ends exists and is an unavoidable waste of heat in this type of furnace today.

We focused on this point and developed a new product (named EREMA EH) based on the idea that reducing the heat released from the cold ends should save electric consumption of the heating elements and the furnace operation as a whole.



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Special Material for Cold End

The cold end of the element actually gets hot due to significant electrical resistance in this area. To reduce the heat loss from the cold end, the electrical resistance of the cold end needs to be lowered. In order to make the resistance of the cold end lower, we re-examined the raw materials and the sintering process to make SiC heating elements. The result was finding a method to reduce the resistance of the cold end to one-third of our conventional products.

We cannot specify the extent of savings exactly, but the ratio of anticipated energy savings of EREMA EH is calculated to be 4-5%. The exact value depends on the diameter and the proportion of hot-zone length to cold-end length.



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Fig. 3. Energy-savings rate for different hot-zone and cold-end lengths; Fig. 4. Comparison of power consumption

Energy Consumption



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Fig. 6. Comparison of cold-end temperature of a furnace at 1200°C (2192°F

Temperature at Cold Ends

Another benefit of EREMA EH is that the temperature of the cold ends will be lowered as a result of less heat generation there.

The verification experiment using the lab kiln (Fig. 5) in our factory showed a significant reduction of the temperature of the cold ends from the conventional product’s 276°C to EREMA EH’s 208°C. Figure 6 shows a thermal-image comparison of the cold ends of the elements protruding from the hot furnace.

Although the extent of reduction of temperature depends on many other factors (such as temperature in the furnace, the kind of insulation material, its thickness, atmosphere, etc.), EREMA EH will reduce the temperature of cold ends to some extent. This may provide an additional benefit when considering the total furnace environment and design.

In some operations, the furnace atmosphere must be carefully controlled. For this kind of operation, furnaces are usually built with a gas-tight enclosure, so the temperature in the terminal box at the end of the heating elements tends to go up higher. It is common to install a cooling device to prevent the degradation of electrode components at the terminals under such environments. With EREMA EH heating elements, however, it is not always necessary to install such cooling devices because its cold-end temperature is less than the conventional SiC heating elements. This is an additional, unquantified benefit that contributes more energy savings when considering total furnace operation.



Conclusion

A new product – EREMA EH heating elements – can save approximately 4-5% of its heating costs in the temperature range of 800-1600°C (1472-2912°F). This savings has been achieved by reducing the heat loss out of the cold ends of the element, making EREMA EH a direct replacement item for current products. Tokai Konetsu is excited to introduce this product in the spirit of world energy savings and sustainability, and we look forward to providing these savings to our customers. IH

For more information: Contact Toshio Nakai, Tokai Carbon USA, Inc., 4495 NW 235th Ave., Hillsboro, OR 97124; tel: 503-640-2039 x302; e-mail: tnakai@tokaicarbon.com; web: www.tokaicarbon.com  

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