Fig. 1. Silicon carbide heating elements in operation[2] (Photograph Courtesy of Keith Company)
Electric heating elements are a popular choice of many heat treaters. They come in a variety of shapes, sizes and materials. One of the most common types are silicon carbide heating elements, known by several tradenames including Globar® and StarBar®. They are used extensively throughout the heat-treating industry when high temperatures, maximum power and heavy-duty cycles are required. Let’s learn more.
A silicon carbide (SiC) heating element (Fig. 1) is typically an extruded tubular rod or cylinder made from high-purity grains of silicon carbide that are fused together by either a reaction-bonding process or a recrystallization process at temperatures in excess of 3900°F (2150°C). The result is a chemically stable material with a low thermal-expansion coefficient and little tendency to deform.
Recrystallization forms fine grains of silicon carbide that act as “bridges” or connection points between larger grains, thus forming conductive pathways. The number of bridges formed dictates the material’s resistance – the greater the number, the lower the resistance. The secret to the creation of a good heating element is controlling this formation process within the material so as to develop a consistent electrical resistance.
The factors that influence the life of a silicon carbide heating element include the type of furnace atmosphere, watt density, operating temperature, type of service (continuous or intermittent) and maintenance. Furnace type, design and loading play an important role as well. Silicon carbide heating elements are extremely versatile, operating, for example, in air up to 3000°F (1650°C).
Transformers used for silicon carbide heating elements have multiple secondary taps in anticipation of a change in resistance of the elements over time. Silicon carbide heating elements, being 20–30% porous, oxidize or otherwise react with the furnace atmosphere and increase in resistance during their operational life. Oxidation causes a reduction in the cross-sectional area of the bridges, resulting in greater resistance to electrical flow. The oxygen in the air reacts with the silicon carbide grain, reducing it to silica (SiO2) as shown by the equation below:
SiC + 2O2®SiO2+ CO2
It is estimated that new silicon carbide bars will increase in resistance 10–15% on startup, which should be taken into consideration when considering replacing the bars. In most cases, silicon carbide heating elements fail mechanically long before they fail due to aging.
Tips for Extended Service Life
To maximize element life, be sure to do the following:1. Handle the elements with care – Silicon carbide heating elements have low tensile strength and, therefore, are sensitive to mechanical damage from rough handling, dropping (even in the packaging) or forced bending that can occur during storage, unpacking or installation.
2. Match resistance – The purpose of matching resistance of elements is to improve their life and to improve temperature uniformity in the furnace. Silicon carbide heating elements are typically factory tested with the test amperage marked on the shipping box and/or the element. Elements can be connected in parallel (preferred since they tend to come into balance in use), series or series-parallel. Elements connected in parallel should be matched in resistance within ±20%, while elements connected in series should be matched within ±5%.
3. Choose the proper size element for the equipment – If there are any doubts about the size to use, check the design parameters with the original furnace equipment manufacturer.
4. Install carefully – Check that the terminal holes through the insulation are in alignment so that the elements slide in without striking the opposite side or are put under tension due to forcing them into position. Be sure to center the elements in the furnace chamber so that no portion of the heating section of the element is in the brickwork.
5. Pack the element with ceramic fiber to a depth of about 1 inch (25 mm) so as to avoid heat loss, but be sure that the terminal ends of horizontally mounted elements lie flat in the terminal holes and are supported by the furnace walls.
6. Use the lowest voltage that will maintain the desired furnace operating temperature. This will ensure the lowest possible surface temperature of the element and lengthen its service life.
7. Run the correct silicon carbide element watt density for the required furnace atmosphere (Table 1).
8. Perform in-service inspections – Check the amperage as an indication that the elements are operating correctly.
9. Maintain matched-resistance circuits at all times. Don’t mix old and new elements in the same circuit.
10. Be sure that the elements are loose in the terminal holes not only when the furnace is cold but also hot.
Fig. 2. Spiral-cut silicon carbide heating-element design provides increased resistance for applications up to 3000°F (1650°C)
How Do You Know It's Time to Change Heating Elements?
Furnace type, design and cycling make a difference. Let’s answer this question by considering a mesh-belt copper-brazing furnace with a muffle operating at 2050°F (1120°C) in a hydrogen/nitrogen (75%/25%) atmosphere. The furnace is run six days a week with a typical belt loading of 10–12 lbs/linear foot (15–18 kg/linear meter). Typical element life is expected to be in the range of 12–24+ months. Here’s how to determine when its time to change elements:1. Beginning with a change of elements, measure the amperage and voltage to the individual elements on a quarterly basis. Calculate their resistance and watt input to determine whether they are balanced or not. If a particular element shows erratic readings or large changes in resistance from one set of readings to the next, it is time to replace it. Note: Silicon carbide heating elements should be changed in sets depending on how many elements are in series with one another (sometimes in pairs, sometimes more).
2. Also, there is a relatively simple procedure that should be done to take the guesswork out of knowing when to change elements. With all new elements, lower the furnace temperature (and monitor it during this procedure) and place the power controller (SCR) in manual mode calling for 100% power. Take a digital meter with extra long leads and, CAREFULLY touching the element on either end, measure the voltage drop across the element. Note: Do not take this reading by touching the straps as they run slightly cooler and your reading won’t be as accurate. Measure the current using a clamp-on ammeter around the straps and calculate resistance and watt input. Record this information for future reference. When the elements have doubled in resistance, it is time to change elements. Repeat these measurements every quarter.
3. When you see that the elements have degenerated to about 2/3 of their original resistance value, it is time to change to the next highest tap on the transformer.
4. If you are fully “tapped up” – on the highest tap setting on the transformer – and a particular zone begins to “struggle” (when it has difficulty maintaining your temperature set point), then you know it’s time to change all of the elements in that zone.
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