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April 1, 2010
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Proper selection and understanding of heating elements is a critical factor in any operation that utilizes electric furnaces.

Glowing hot zone of a straight SiC element.


When building or operating an electric furnace, it is important to keep several guidelines in mind to ensure best heating element selection. Silicon carbide (SiC) elements* work best at temperatures from 1470-2912ºF (800-1600ºC), while molybdenum disilicide (MoSi2) elements** are best-suited to temperatures of 1832-3182ºF (1000-1750ºC). Both types of elements work extremely well in oxidizing atmospheres.

Both SiC and MoSi2 elements are linear-type resistance heaters that convert electrical energy to heat energy following Joule's Law:

W = I2R

where W = watts, I = current in amperes and R = resistance in ohms. A good rule of thumb to determine the amount of energy required for a well-insulated furnace is to measure the square inches or square centimeters of the inside heated chamber, then use 5 watts per square inch(0.77 watts per square centimeter) to determine the power. At this power level, a well-insulated furnace will come up to 2000ºF (1100ºC) in approximately 6-8 hours. A faster heat-up rate will require additional power.

For example, take the total inside surface area of the furnace chamber, including the four walls, the ceiling and the floor. Multiply the surface area by 5 watts per square inch to find the minimum recommended power in watts. (If using the metric system, take the inside surface area in square centimeters and multiply that amount by 0.77 watts to find the minimum power required in watts.)

It is also a good idea to always over-power the furnace. No additional operating costs will be incurred if excess power is available but not used. On the other hand, firing the furnace with too little power can result in problems such as under-fired product or extremely long firing cycles.

*such as Starbar® heating elements from I Squared R Element Co.
**such as Moly-D® heating elements from I Squared R Element Co.

A typical MoSi2 element is U-shaped with the hot zone in a vertical position in the furnace.

SiC or MoSi2?

SiC elements are manufactured from silicon carbide, a less-expensive raw material than molybdenum disilicide, so they tend to cost less than MoSi2 elements. SiC elements have a very low coefficient of thermal expansion and good creep resistance, so they will not sag when they get hot. This makes them ideal for installation over and under a load.

SiC elements are available in essentially three types: straight tubular rods with the electrical connections on opposite ends, U-type elements with the connections on one end, and spiral elements with the connections on either the opposite ends or one end. Spiral elements are of a higher density and are therefore better-suited for severe applications.

SiC and MoSi2 elements are both commonly used in various types of furnaces, from small glass-melting crucible furnaces and ceramic firing furnaces to large float glass furnaces. With today's marketplace demanding larger products, heating element suppliers are keeping up with modern technology by manufacturing longer hot zones and larger-diameter elements. Unheard of 10 years ago, an SiC element can now be manufactured with a hot zone of 200 in. (5100 mm) long and 2¾ in. (70 mm) in diameter.

MoSi2 elements are recommended for high-temperature furnaces, intermittent furnaces or when a fast heat-up is required. These elements can operate to an element temperature of 3272ºF (1800ºC) and can withstand a high watt loading. The elements become pliable over 2000ºF (1100ºC); therefore, the U-shaped elements are typically installed with the cold ends through the roof of the furnace. Elements with a 90º bend are installed from the inside chamber with the cold ends extending through the side walls.

The cold end of the MoSi2 is two times the hot zone diameter and therefore operates cooler. The element comes in standard hot zone/cold end diameters (measured in millimeters) of 3/6, 4/9, 6/12, 9/18 and 12/24. The hot zone length can range from 3 to 55 in. (75 to 1400 mm), and the maximum length is diameter- and temperature-dependent.

Figure 1. Resistance change as an SiC element is powered.

Element Life

Many manufacturers wonder how they can prolong the life of their heating elements. The answer is simple-temperature control. For SiC elements, the lower the temperature, the longer the life. However, many manufacturers do not have the option of running at a lower temperature. Proper maintenance and correct power control can help. For example, power losses through cracks or door openings will cause the elements to run more often and therefore shorten element life.

It is important to acknowledge that SiC heating elements increase in resistance gradually during their service life. The rate at which aging occurs is affected by many factors, including element watt loading, operating temperature, atmosphere, continuous or intermittent operation, and power control methods.

A heating element with a higher watt loading will operate hotter, and this higher temperature is directly related to the rate at which an SiC element increases in resistance. For optimum element life, the lowest possible watt loading should be considered, typically in the range of 20 to 50 watts per square inch (3-8 watts per square centimeter).

SiC elements will last longer when operated continuously. These elements are essentially 100% pure silicon carbide with a very thin surface layer of silicon dioxide. As the element is used, the silicon carbide oxidizes and forms more silicon dioxide.

This is what causes the element to increase in resistance over its life. When the silicon dioxide in the element is cooled or cycled, it goes through several phase transformations. Both the high-low quartz transformation at 1060ºF (571ºC) and the crystobolite transformation at 2420ºF (215ºC) involve substantial volume changes, which can lead to an increase in resistance or the fracture of bodies containing large amounts of silica.

Some manufacturers feel it is more economical to turn the furnace off to conserve energy and suffer a shorter SiC element life, while others prefer the longer element life at the expense of higher energy costs. Each manufacturer must make this decision based on their specific application.

It is a common practice to tuck fiber in the terminal holes to conserve energy and prevent a chimney effect. However, it is very important that the elements are free to move in every direction. Any binding of the SiC element can cause breakage during heat-up or operation due to the expansion or movement of the furnace walls. If the elements are tight in the terminal holes or something bumps or jars the elements, they will break.

The elements should be able to move about 1/8-3/8 in. (4-6 mm) in all directions with a slight force (e.g., a thumb and finger pushing on a pencil). This test should be done both at room temperature and at maximum furnace operating temperature. In addition, electrical connection straps should be long enough so that no stresses are transferred to the elements.

MoSi2 elements can be used continuously or intermittently. Like SiC, MoSi2 forms a protective silicon dioxide layer that can experience the same phase transformations. However, because MoSi2 elements are so dense, the oxide is only on the surface and simply flakes off upon cooling without damaging the element. It is very important not to operate new elements at low temperatures (below 1200ºC) for long periods of time, since the glaze must be grown to protect the element from failure.

Figure 2. The MoSi2 resistance at room temperature is about 10 times lower than at an element temperature of 2732ºF (1500ºC).

Power Supply

The appropriate power supply can be selected when the element size, required power and network resistance are known. Figure 1 shows the resistance change as an SiC element is powered. Silicon-controlled rectifiers (SCRs) are recommended to control the power because they provide an even, oscillating power to the element. The SiC element arrangement is used directly on a common line voltage for most applications.

The heating element suppliers' technical staff can select the best element for the application, and then, depending on the available voltage, determine how the elements should be connected (series or parallel connections). MoSi2 elements will often require a step-down transformer since their resistance is much lower than an SiC element.

The MoSi2 resistance at room temperature is about 10 times lower than at an element temperature of 2732ºF (1500ºC), so the element will draw very high amperage at low temperatures (see Figure 2). As a result, the amp draw should be limited at the start. The maximum recommended amps at start-up are: 3/6 element, 75 amps; 4/9 element, 115 amps; 6/12 element, 200 amps; and 9/18 element, 365 amps. The amp limit for the 12/24 element is 560 amps. When the MoSi2 element reaches the required temperature to heat the chamber, the resistance of the element will not change with use.

Ask the Experts

Today's SiC and MoSi2 heating elements can be used in just about any high-temperature application. Manufacturers are encouraged to contact their heating element supplier to assist with their design or provide advice on improving furnace performance and element life.  

For additional information regarding heating elements, contact I Squared R Element Co., Inc., P.O. Box 390, 12600 Clarence Center Rd., Akron, NY 14001; (716) 542-5511; fax (716) 542-2100; e-mail sales@isquaredrelement.com; or visit www.isquaredrelement.com.

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