Silicon Carbide (SiC) Heating Elements from Silcarb
Silcarb manufactures Silicon Carbide Heating Elements that are known under the tradename of “Alpha Rod, Alpha Ultra Rod, Alpha Ultra Spiral”
The Alpha Rod elements are tubes with thick walls and are made from high resistance silicon carbide elements. The element tubes consist of a central heating section referred to as a hot zone and two terminal sections called cold ends. The cold ends are infused with silicon metal to bring down their resistance and operating them at a lower temperature. The extreme ends of the elements are coated with aluminum contact surface with low resistance to which the electrical connections are made with braided aluminum straps. Elements are described by giving the- overall length, the heating section length, the diameter and the resistance.
Silcarb manufactures the following types of SiC heaters
- Three Piece Straight Alpha Silicon Carbide (SIC) Rods
- Alpha Ultra Rod Single Piece silicon carbide Elements
- Reaction Bonded Single Spiraled Heating Elements
- Reaction Bonded Double Spiraled Heating Elements
- U-Shaped Alpha Silicon Carbide Rods
- Dumbell Shaped Silicon Carbide Heating Elements
High-Performance SIC Heaters from Silcarb
Special Surface Coatings
Very High Operating Temperatures
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.
- 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
Ordering with Silcarb
Alpha Rod Elements are specified as follows:-
Diameter / Hot zone length / Overall length / Nominal resistance.
Ordering sort of the alpha ultra elements are specified as follows:
Diameter / Hot zone length / Overall lengths / Nominal resistance.
Electrical Characteristics of ‘Ultra-Spiral /Alpha Rod Elements’
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)
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.
Effect of Atmospheres
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 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 250oC, 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.
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.
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.
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.
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