SILICON CARBIDE HEATERS
SIC HEATERS FOR INDUSTRIAL ELECTRIC FURNACES
SIM S.r.l. SiC heating elements are silicon carbide heating elements. They are composed of a central heating section called hot zone and two terminal sections called cold ends. There are two types of SIM S.r.l. SiC:
- one type has the cold ends impregnated with silicon metal — referred to as “one piece”;
- one type has low resistance cold ends which are furnace welded to the hot zone — referred to as a “three piece” or LRE (Low Resistance End) type.
This lower electrical resistance cold end causes them to operate at a lower temperature. The extremities of the elements are metallized with aluminum to provide a low resistance contact surface to which the electrical connections are made using braided aluminum straps.
SIM S.r.l. SiC resistors are described by giving the overall length, the heating section length and the diameter.
SIM S.r.l. SiC resistors will give you superior performance due to their high density — approximately 2.4 gms/cc. This gives to SIM S.r.l. SiC resistors very slow aging characteristics and additional strength.
SIM S.r.l. SiC resistors are interchangeable with all silicon carbide heating elements manufactured in Europe and manufactured for the Asian and United States markets. It is important to provide the nominal electrical resistance when ordering to SIM S.r.l.
SIM S.r.l. SiC resistors can be manufactured in any length up to 228 inches (5800 mm). The maximum hot zone length is 166 inches (4216 mm).
In an air or inert atmosphere of argon or helium the “one piece” can be operated at furnace control temperatures up to 3100°F (1700°C), the “three piece” up to 2600°F (1425°C). There is a protective coating of silicon dioxide on the silicon carbide. Hydrogen reduces this coating and causes the SiC resistor to deteriorate. Very dry or very wet hydrogen is detrimental to long service life.
Nitrogen atmosphere applications are limited to 2500°F (1370°C) and 20 to 30 watts per square inch (3.1 to 4.6 watts per square centimeter) maximum surface watt loading. A Too high surface temperature will result in a silicon nitride reaction. A thermally insulative layer forms around the SiC resistor resulting in very high surface temperatures which damage the resistor.
The silicon carbide resistor of SIM S.r.l. is a linear resistance heater that converts electrical energy to heat energy. Our resistors are manufactured of green silicon carbide that is classed as a conductor with excess of electrons.
The nominal SiC resistor resistance is measured at the calibrating temperature of 1960ºF (1071°C). The nominal resistance values of SiC resistors in ohms per unit of length are shown in Table A.
SiC resistors are not sized to a specific wattage output like metallic heating elements. The amount of energy that a SiC resistor is capable to convert from electrical to heat energy depends on the ambient furnace temperature and atmosphere in which the SiC resistor is operating.
SiC resistors may be connected in parallel, series, or combination thereof. Parallel connections are preferred. In a parallel arrangement, the voltage across all the SiC is the same. The SiC resistors in the parallel circuit with the lowest resistance will supply more heat energy and therefore operate at a higher temperature. This higher SiC resistors temperature will cause a gradual increase in resistance until all resistors have the same resistance. At this time, the SiC resistors should all have approximately the same resistance values and surface temperatures and therefore remain in balance.
The resistance of SiC elements increases gradually during their useful life. Therefore, some means of keeping the power input to the kiln or furnace at a level sufficiently high to maintain the desired temperature.
SiC resistors can be used directly on the line (fixed voltages) at temperatures up to 2400°F (1315°C). To compensate for the reduced output as the SiC resistors gradually age or increase in resistance, the furnace or kiln is initially overpowered by 25% to 50%. This type of arrangement eliminates the expensive voltage varying equipment and it is very satisfactory in many applications. It is not recommended when fine process temperature control is required.
The temperature of the kiln or furnace is controlled by an off-on controller. When the SiC resistors are new, they will only be powered for 24/30 or 24/36 of an hour. As the SiC increase in resistance, they will be on for a greater percentage of the time. When they have increased in resistance to a point at which they supply 24.000 watts, they will be on 100% of the time. A SCR (silicon controlled rectifier) or thyrister can also be used.
For applications where close temperature control is desired and/or for temperatures above 2400ºF (1315ºC), a device for increasing the voltage to the SiC resistors is required. There are several methods of providing this variable voltage source:
1 – The multiple tap transformer is the most common, because it is usually the least expensive. The secondary of the transformer is provided with taps which usually vary in number from 10 to 36. By carefully selecting the voltage taps, the correct voltage output to match the resistance of the SiC resistors over their complete useful life can be made.
2 – Saturable reactors and induction regulators are used to provide a stepless voltage control. They are also sometimes used with multiple tap transformers.
3 – Capacitor controls are used infrequently. Certainly, they will tend to improve a power factor, which makes their use desirable in some areas.
4 – Silicon controlled rectifiers, (SCR) have become quite popular with the advances in solid state devices.
To compensate for the reduced output as the SiC increase in resistance, a voltage range is required to compensate for a 100% increase in the resistance of Sic resistors.
Lower voltage taps are usually provided for idling and slow heatups. To calculate the minimum voltage, take 70% of the nominal voltage. For periodic applications, take 30% of the nominal full load voltage. Auto transformers may be used if primary voltage is 230 volts or less.
EASE OF REPLACEMENT
SiC resistors can be replaced while the furnace is at operating temperature. The power to the SiC resistors being changed should be shut off, the spring clips and aluminum braid released, and the old SiC removed. The new SiC should be inserted smoothly through the hot furnace with sufficient speed to ensure that the aluminum is not melted off the terminal end but not so fast as to cause thermal shock.
SiC resistors resistance increases gradually with use. This characteristic of increasing in resistance is called aging. Aging is a function of the following elements:
1 – Operating temperature
2 – Electrical loading (usually expressed in watts per square inch or watts per square centimeter of SiC radiating surface)
3 – Atmosphere
4 – Type of operation (continuous or intermittent)
5 – Operating and maintenance techniques
There are no restrictions on the mounting positions of SiC resistors, although the horizontal and vertical positions are the more common. Extreme caution should be used when mounting to ensure that the resistors are not placed in tension. There should be adequate freedom to allow for the furnace and SiC resistors expand and contract independently. When mounting SiC resistors vertically, they must be supported on the lower end by electrically insulated supports. SiC resistors should have their heating sections centered in the furnace chamber so that no portion of the heating section extends into the furnace wall. A conical or truncated cone shaped recess 1/2 inch (13mm) deep is sometimes located on each interior wall where the SiC passes through. This allows the hot zone to radiate properly and helps maintain a uniform temperature in the furnace.
FURNACE HEATING CHAMBER
The furnace heated chamber dimension can be the same of the hot zone length resistors. Alternately, the furnace heating chamber dimension can be one inch (25mm) less than the effective heating length of the resistors. In this case, there must be a 45° conical recess in the furnace wall for the SiC resistor above the load. Recommended terminal hole diameters for various refractory walls and SiC resistors sizes are shown in Table B.
Pay attention during resistors placement: between one resistor and one other, it must be a space equal to two resistors or, if close a wall or a reflecting body, the space must be equal to one and half resistor.
If resistor is not able to dissipate heat energy equally in all directions, it may cause local overheating and possible failure.
SPECIFICATIONS AND MATCHING
SiC resistors have a manufactured tolerance of plus or minus 20% on the nominal resistance. All SiC resistors are calibrated at least twice before shipping to ensure the respect of specifications. The calibrated amperage of each SiC resistor is marked on the carton and on the right end of each resistor. When installing, arrange SiC resistors with amperage values as close to each other as available. Longer service life will be obtained when series connected SiC resistors are matched in resistance. SiC resistors are shipped as closely matched as possible.
SiC resistors can be shipped immediately from our stock, or two / three weeks after receipt of the order.
Special sizes and shapes are available. Cold ends can have different lengths (for example, in furnaces with arched roofs that require longer cold ends through the roof and shorter through the floor). Another modification is a two-temperature hot zone, helpful to get additional heat energy into a tunnel kiln, slower and more densely loaded. This modified hot zone will not create a specific temperature differential and it offers a convenient way to get more heat energy into a specific area of a furnace. The cold ends are attached perpendicular to the hot zone. The RA is normally installed with the cold end through the roof of the furnace.
OUR COMPLETE RANGE OF HEATING ELEMENTS IN SILICON CARBIDE
Three Pieces Straight Heating Elements SIC Rods
The three pieces straight heating elements, SIC Rods, have low resistance cold ends (LRE), colder than any other piece. The maximum featured temperature of the rods is 1550°C and, for better energy efficiency, heat is concentrated in the furnace, not on the ends of the rods. With the highest Hot:Cold ratio of 1:40, these rods make one of the most efficient Silicon Carbide Heaters.
“Reaction Bonded” Single Spirale Heating Elements (reaction bonded silicon carbide)
The Reaction Bonded Silicon Carbide is used in the fabrication of SPIRAL Silicon Carbide heating elements. These resistors are available in different sizes ranging from 12 to 50 mm in diameters and 2250 mm in length. They are spiraled heating elements, shaped in a thin wall and finely grained form of reaction bonded silicon carbide. They can withstand high electrical loads, rapid heating & cooling cycles and thermal shocks. To suit various heating processes, the elements are available in different forms.
Reaction Bonded Double Spiraled Heating Elements
The Reaction Bonded Double Spirale Heating Elements possess all the terminal connections at one end. These resistors are ideal in the conditions where the furnace access is limited to only one plane.
U-Shaped Heating Elements SIC Rods
The ‘U’ Shaped heating elements are the SIC rods that are joined from both the terminals in a form of thickened bridge. These rods are ideal for the conditions where one single rod is not able to span the heating chamber. Further, these rods are also good for radiant tube systems.
Drum (or dumbbell) shaped Silicon Carbide Heating Elements
The enlarged cold ends of the heating elements give them the name of Dumbbell Shaped Silicon Carbide Heating Elements. The cold ends of the elements lower down the electrical resistance and helps in increasing the cold end cross section that is turn lowers the cold end operating temperature. In the modern dumbbell alpha rods, the advanced technology is used for keeping the ends of the terminals cool, thus the oversized cold ends are no longer required. Old style resistance ratio was 1:3. New resistance ratio is 1:40 Maximum temperature is 1550°C.