Silicon Carbide (SiC) Heating Elements from Silcarb

High-Performance SIC Heaters from Silcarb

Due to their high density of 2.4gms/cc (or more), the Alpha Rod Elements are designed to give superior performance. This gives the element exceptional characteristics and additional strength.

Special Surface Coatings

Various kinds of special surface coatings are provided onto the Alpha SiC rods. These vary from glass coatings to Chemical Vapor Deposition (CVD) coatings depending on the area of application. We provide different coatings for assaying furnaces, coatings to be employed in very high vacuum applications and coating to be employed in reducing atmospheres.

Do get in touch with us and let us know about your applications and atmospheres in which you wish to use the heaters and we will suggest the right surface coating for the application.

Very High Operating Temperatures

The operating temperature of the Alpha Rod Elements can go up to 1550°C in an oxidizing atmosphere, or inert gas atmospheres of argon or helium. in a reducing atmosphere, the maximum obtainable operating temperature is 1350°C.

Characteristics of SIC – Alpha Rod Elements

• The Alpha Rod Heating element is a linear type resistance heater that converts electrical energy to heat energy (Joule’s Law W=I2R). W=power in watts, I=current in amperes, R=resistance in Ω.
• The heating elements are self-bonded, for the lattice structure or bonds that hold the elements together that are formed by recrystallizing the silicon carbide at high temperatures.
• The elements are made from green(beta) silicon carbide which is an excess electron type semiconductor.
• It is difficult to measure the resistance of a silicon carbide element at room temperature. This is due to the presence of minor impurities, contact resistance, and self-healing.
• Silicon carbide also has a characteristic negative resistance temperature from room temperature to approximately 800°C. It turns positive at this point and remains so throughout its operating temperature range.
• A typical resistance-temperature characteristic curve of a silicon carbide heating element Nominal element resistance is measured at a standard calibrating temperature of 1050°C.

Service Life

The Alpha Rod elements increase in resistance gradually, with use. This characteristic of increasing resistance is called Aging. Aging is a function of the following:-

• Operating temperature.
• Electrical Loading – Usually Expressed in watts per square centimeter of element radiating surface.
• The type of Atmosphere.
• Type of operation
• Operating and Maintenance Techniques.

Mounting of alpha rod heaters

There are no restrictions on the mounting positions of Silicon Carbide Heating Elements. Horizontal and vertical positions are the most common positions. The Heating elements must be mounted with extreme caution to ensure that the elements are not mounted in tension. There should be adequate allowances provided for the furnace and elements to expand and contract without any hinderance. When the elements are mounted vertically, it must be made sure that they are supported on the lower end by electrically insulated supports. It is ideal for the Silicon Carbide Elements to have their heating sections centered in the furnace chamber so that no portion of the heating section extends into the furnace wall. For heat to be radiated properly and the temperature to be maintained, a conical or truncated cone-shaped recess inch deep is provided on each interior wall where the element passes through.

Technical Characteristics

Electrical Characteristics of ‘Ultra-Spiral /Alpha Rod Elements’

The resistance of Ultra-Spiral/Alpha SiC rod Element varies with temperature as detailed in the below table. From relatively high value at room temperature, it falls to a minimum at about 800℃ and then gradually rises to its maximum operating temperature.

The resistance value at 1400℃ will approximately be 10% more than its value at 800℃. The voltage across the element and the current passing through it should be measured at about 800℃ to determine the resistance (V=IR)

Operating Temperatures

Alpha Ultra Spiral/Alpha Rod elements can operate in air(oxidizing) at the furnace temperature of up to 1550℃. Elements can also be used in a reducing or neutral atmosphere but at lower temperatures. The recommended operating temperatures for Ultra Spiral/Alpha Rod Elements in different process atmospheres are as follows.

(Note that element temperatures are indicated and that these may be Considerably higher than the furnace temperature)

Element Loading

The operating temperature of the elements is mainly dependent on two things-(a)the furnace temperature and (b)the specific element loading, normally expressed in watts per square centimeter(inches) over the hot zone length.

Resistance/Temperature Characteristics

Effect of Atmospheres

Ultra-Spiral/ Alpha Rod elements are preferably operated in air or other oxidizing atmospheres, as other gases tend to react with the element material or the protective glaze, therefore reducing the life of the element. It is recommended to consult the manufacturer if the use of Ultra-Spiral heaters in corrosive atmospheres

Water vapor too has an adverse effect on ‘Ultra-Spiral / Alpha Rod heaters, it increases the oxidization of the material and hence the aging rate. The furnace should be completely dried before the elements are installed but if the requirement is to use the elements for drying, then it should be made sure that the furnace is well ventilated, and no build-up of steam should be allowed.

Other process vapors can also have an adverse effect on the element of life. It can either chemically attack the silicon carbide and the protective glaze or condense in the element support holes causing restriction and eventually breakage. Most alkali vapors will have a detrimental effect, halogen gases, metal halides (e.g. Fluxes in aluminum furnaces) and also most of the metallic oxides.

To minimize any attack an efficient extraction system should be incorporated to reduce the volatile concentration in the chamber to an acceptable level. This will also encourage an inflow of air over the element cold ends and minimize any condensation at these points.

Installation Methods

The elements must be accurately positioned in the furnace to ensure that the hot zone does not enter the element support holes. It is important to ensure that elements are free to move in all directions and mounting holes must be aligned and sufficiently large to prevent restriction, for expansion during the hot condition.

Type double-ended alpha ultra spiral

Special lead-in sleeves are available for each size of the element. They should be fitted from the outside of the furnace in holes bored to a diameter which will ensure a loose fit. Sleeves should never be cemented into position. If sleeves are not used then the element support holes should be about 3-6mm large than the element diameter for sizes up to 18mm about 8-10mm for larger sizes. For exceptionally thick furnace lining and where there is a possibility of volatiles condensing on the cold ends, a fairly large hole diameter should be used and the elements centralized by supporting them on a small ceramic fiber pad under cold end.

Type single-ended alpha ultra spiral

Special lead-in sleeves are recommended for use with Alpha ultra single-ended elements to provide the correct alignment and support required. These sleeves should be of a loose fit and not cemented into positions. Type Alpha ultra single-ended Elements can be mounted horizontally or vertically, projecting up or hanging down. The terminal assembly should be outside the furnace structure to keep it as cool as possible. No Portion of the spiral heating section should extend into the refractory wall. The element should be mounted in a high Die-electric lead-in sleeve to prevent possible shorting of the terminal ends because full element voltage exists along the entire length of the Terminal.

Installation and Maintenance

• The correct size of elements should be chosen at the design stage of the furnace. This is to ensure that the wattage loading does not exceed that shown in the Element Loading graph.
• The element support holes must be of the right size, i.e. 3 to 6 mm larger than the element diameter up to 18 mm and 8-10 mm for larger sizes and should be installed along with ceramic lead-in sleeves for both type DE & SE Elements.
• The aluminum braid connections should be flexible and long enough to avoid any stress on the elements.
• Connecting spring steel clips tend to lose their tension over a period of use. Therefore, new clips should be used whenever an element is replaced.
• When elements are connected in series it must be ensured that they are well-matched in resistance. Generally, not more than 2 elements should be connected in series.
• If an element breaks prematurely due to mishandling it may be replaced with a new one. However, if the furnace has been run for a long period, it is preferable to replace the full set of elements owing to the incompatibility of resistance due to aging. Partially aged elements may be stored for later use with others of the same resistance.
• When starting with new elements always select the minimum voltage tapping available.
• When an element is used continuously as in the case of a tunnel Kiln, special glaze treatment is required. Please specify use while ordering. it’s counseled that a chart of the resistances of the weather be maintained and therefore the resistance values of the weather in operation be entered each 15days. This helps to monitor the life of the elements.
• As these elements are fragile, handle with care, particularly whilst fitting the braids and clamps.
• Block off the annular space between the element and the surrounding refractory with the ceramic fiber of glass wool to prevent heat loss at the cold ends.

Accessories

Accessories Silcarb manufactures a range of element connections To suit its Ultra-Spiral /Alpha Rod Elements

• Connecting Braids: Aluminum Braid is recommended for making all element connections because of its high conductivity, flexibility, and resistance to oxidation at high temperatures.
• Terminal Clips: Alpha Rod / Ultra-Spiral type DE & SE elements are supplied with complete terminal Accessories including lengths of aluminum braid for connection to the power supply. In a majority of applications where the terminal ends are well ventilated and unlikely to exceed temperatures of about 250^oC, connections are made by wrapping the aluminum braid around the terminal end and securing with Type H spring clips and are fitted manually. Silcarb recommends that terminal clips should be changed with the element as the clips tend to lose their tension over a period of time.
• Special lead: in sleeves can also be provided along with the elements. The inner and outer diameters of the lead – in the sleeve are fixed depending upon the diameter of the element. However, the length of the lead-in sleeve, depending upon the insulation thickness of the furnace. While ordering please mention the length of the lead- in sleeve necessary for your requirements.

Power Input

A variable voltage source is necessary to provide power input to the heating elements to offset the drop in rating due to the aging of the elements with use. A transformer (tapping or continuously variable) or a thyristor drive would serve the purpose. However, it is recommended that a 100% voltage reserve be provided to ensure the maximum life from the heating elements. In the case of a stepped output tapping transformer, it should be ensured that adequate taps are provided. It is recommended that a minimum of 8 Stepped outputs or taps be provided to ensure that the elements are not drastically overloaded While changing from one tap to the next higher one.

Element Spacing

Alpha Rod/alpha ultra spiral should be spaced at a minimum of 1 diameter between element centers and the refractory lining. A clearance of at least 2.5 diameters, should be allowed between the element centers and the wall, but it may be necessary to increase this if uniformity of heating is required, especially if the distance between adjacent elements is large.

A = ( 1.5 x D) = Minimum spacing between component centre & adjacent refractory

B = ( 2.5 x D) = Minimum spacing between adjacent element centres

C = ( 1.5 x D) = Minimum spacing between component centers & hearth plates or work

D = component Diameter

Note: if under heating is to be used, then the hearth plates should have good thermal conductivity. Suggested material being Silicon Carbide.

Silcarb today is one of India’s leading manufacturer of Silicon Carbide Elements. Silcarb has been manufacturing heaters now in Bangalore, India for the last four decades. Silcarb is known for its high quality and fast response time to customer needs. Due to high volumes of production, Silcarb is also able to price its products competitively. Silcarb is constantly innovating to reduce energy consumption for both its customers & its own in-house production facilities.