Proper selection and understanding of heating elements is a critical factor in any operation that utilizes electric furnaces.
hot zone of a straight SiC element.
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.
SiC and MoSi2
elements are linear-type resistance
heaters that convert electrical energy to heat energy following Joule's Law:
W = I2
= 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.
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.)
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.
heating elements from I Squared R Element
**such as Moly-D®
elements from I Squared R Element Co.
typical MoSi2 element is U-shaped with the hot zone in a
in the furnace.
SiC or MoSi2?
elements are manufactured from silicon carbide, a less-expensive raw material
than molybdenum disilicide, so they tend to cost less than
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.
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.
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.
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.
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
(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.
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.
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.
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).
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
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.
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.
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.
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.
2. The MoSi2 resistance at room temperature is about 10
times lower than at an element temperature of 2732ºF (1500ºC).
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.
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
their resistance is much lower than an SiC element.
resistance at room temperature is about 10 times
lower than at an element temperature of 2732ºF (1500ºC), so the element will
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.
the MoSi2 element reaches the required temperature to heat the chamber, the
resistance of the element will not change with use.
Ask the Experts
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.
additional information regarding heating elements, contact I Squared R Element
Co., Inc., P.O. Box 390, 12600 Clarence Center Rd., Akron, NY
(716) 542-5511; fax (716) 542-2100; e-mail email@example.com; or visit www.isquaredrelement.com.