Today the requirement of annealing furnaces is increasing for the heat treatment processes of various mechanical products such as sheet metal, wires, gears, etc. Heat treatment process is very important step in manufacturing industries for which furnaces are developed over a period of time with continuous innovations. Over the years variety annealing furnaces have been developed working on different principles. Out of these, the furnace operating on electric resistance heating element is found to be most efficient and cost effective.
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. The SiC material is unique in that it exhibits a high resistance to acid and corrosion and can therefore be used in severe environment.
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 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.
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.
Energy saving varies by the size and the ratio between the lengths of hot zone and cold ends of the heating element. Figure 3 shows a simulation between such hot zone and cold-end lengths. More heat will be lost if the cold-end length is longer and if the proportion of cold-end versus hot zone lengths is higher.
Table 1 shows energy saving in typical products. This value may not be huge reduction of the energy used. However, it is significant enough to be considered, and it can be easily realized using this product. So, 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.
Mitsuaki Tada, Energy-Saving Silicon Carbide Heating Element, Industrial Heating, USA, pp 71-74, 2012.
M. Hasanuzzaman, R. Saidur and N. A. Rahim, Energy, exergy and economic analysis of an annealing Furnace, International Journal of the Physical Sciences, vol. 6(6), pp. 1257-1266, 2011.
Siddharaj V. Kumbhar and Mahesh M. Salotagi, Technological Developments in Annealing Furnaces for Performance Improvement, IJARIIE, Vol-3, Issue-3, 2017.