Experiments have shown that manufacturers can often increase their productivity and quickly recoup their initial furnace investment by choosing higher temperature heating elements.

Several types of heating element materials are commonly used in electric furnaces, including metallic, silicon carbide and molybdenum disilicide (MoSi₂). The maximum temperatures to which each element can be used are shown in Table 1.

The prices of electric elements vary based on temperature and durability. For elements that can operate in oxidizing atmospheres, the price of the element typically increases with the maximum rated element temperature. However, the true price of operation is not necessarily directly related to the price of the element.

A comprehensive study was undertaken by Evans et. al.¹ to determine the lowest cost per part for various heating elements and heating configurations. The study considered the cost of the furnaces, energy, consumables, replacements and several other factors specific to ceramic processing. One of the key results of the study was that furnaces using molybdenum disilicide heating elements offered the lowest cost/kg for processing a part. Typical comparative numbers for the cost of manufacturing with different elements are shown in Table 2. Since molybdenum disilicide elements can be used with high surface loads, the overall cost of use and operation is reduced.

But does a correlation exist between the temperature of heating and the overall productivity? (For the purposes of this article, productivity is very simply calculated as Kg/s of the material processed, as this reflects on the efficiency of operation, energy cost and labor hours.) Conversely, does the added investment in heating elements that can reach a higher temperature offer a better return when compared to more economical but lower-temperature-capable heating elements? And is the magnitude of the benefit substantial enough to recommend the use of a higher-temperature-capable heating element for a given process?

These are important questions for the processing of ceramics or metals. In lower-temperature operations (below 1000?C), the presence of convection dramatically improves process control and efficiency,² making element selection less critical. In temperatures above 1000?C, however, a decision must often be made, for example, between using silicon carbide heating elements (maximum temperature 1550?C) and molybdenum disilicide heating elements (maximum temperature 1900?C). The plant/design engineer typically must choose based on the initial cost of the element or by using the complex calculations reported by Evans et. al.¹ In many cases, the higher-temperature heating element will more than pay for the higher initial cost through increased productivity.

Calculating Productivity

To prove this theory, several calculations were carried out using aluminum oxide parts and the lumped parameter approach, in which only the overall heat balance is considered.³ This approach is considered ideal for furnace power and cost calculations as it not affected by part geometry. After extensive numerical simulations, the results were all condensed into plots, which illustrate the relative productivity comparison for each heating situation, uniquely identified by the wall temperature. (Furnaces use electric heaters to transfer energy from the electric heating element to the furnace wall, which then radiates the heat to the parts being processed. During use, the furnace wall is at a temperature slightly lower than the electric element being used.)

For the purpose of this article, an example was chosen to highlight the issues related to making the heating configuration decision. In the example, all of the heating elements mentioned in Table 1 could be used to heat a part until the part temperature reached 1100?C (the preset temperature). In this manner, the efficiency/productivity of heating could be compared, and the cost of using a particular type of heating element could be calculated.

The calculations to determine the amount of material processed per hour in a typical furnace configuration yielded equations like those shown in Table 3, which indicate that the productivity increases exponentially with an increasing wall temperature.

The results pertaining to the example chosen for this article are shown in Figure 1. In the figure, plots of the productivity as a function of the temperature for various part sizes are shown. Here, productivity is defined as the amount of material per second that can be heated to the preset temperature, which in the example is 1100?C. Note that the productivity increases in correlation with the wall temperature. Productivity also increases with the furnace size, which simply reflects the fact that a large furnace will produce more in a given time. Also note that the slope of the curve is not influenced by part size, thus indicating that the relative gain on account of using a higher temperature furnace is independent of the part size.

Recouping the Investment

The price of a typical furnace is influenced by the heating elements, refractories and systems (controls and casings). A higher-temperature furnace is more expensive, and the higher price reflects the increased price of the heating elements and furnace refractories. The question is, can the higher price be recouped quickly?

Figure 1 can be used when making a decision on the right furnace to purchase. Table 4 shows an example of the relative time to recoup the investment. Even though the investment in a lower-temperature-rated furnace is considerably smaller when compared to the higher-temperature furnace, the time to recoup this investment more than doubles when comparing an 1800?C furnace and a 1450?C furnace. Note also that the 1450?C furnace’s price is half that of the 1800?C furnace’s price.

The limitations of these calculations and predictions should be understood. The calculations assume that the time to reach the temperature in a furnace is rapid. Although this is true for the silicon carbide, molybdenum disilicide and forced air convection furnaces, furnaces made of zirconia are designed to be very slow for applications in which slow heating is desirable. Additionally, furnace heat up rates might be deliberately set to a low value in some cases to prevent excessively fast heating of the part’s surface, or to ensure that the binder burns out completely. The relative payback periods will be affected by such manipulations of the heat-up rate.

The Bottom Line

Electric heating elements become more expensive as their maximum rated temperature increases. Similarly, furnaces become more expensive with an increase in their rated temperature because of the added cost of the more expensive heating elements, refractories and other systems. However, the efficiency and returns from a production process also increase considerably when using a higher-temperature-rated furnace. Companies that are looking to increase their productivity should therefore look beyond the initial purchase price of the furnace and instead choose the design that best meets their production goals. Choosing the right furnace will yield benefits in both productivity and long-term cost savings.

For More Information

For more information about heating elements, see “An Elemental Analysis,” CI February 2002, pp. 33-35; or online at http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,71514,00.html.

For more information about choosing a furnace, see "Choosing the Right Furnace," CI May 2002, p. 38, or online at http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,78567,00.html.

For more information about heat transfer calculations and products, contact MHI at 613 Redna Ter., Cincinnati, OH 45215; (513) 772-0404; fax (513) 672-3333; e-mail sales@mhi-inc.com; or visit http://www.mhi-inc.com.