Silicon Carbide

The heating elements, called Starbars^®, used in Electroglass^® furnaces are made of a special high density, reaction-bonded silicon carbide (SiC). Electroglass^® Starbars^® are manufactured in the United States by I Squared R Element Co. There are two electrical connector straps attached to the back end of each element. Connecting a Starbar® is safe and easy. One simply slides it in, horizontally, through an element port in the back of the furnace. The two aluminum braided straps fit over two 3/8-inch diameter insulated bolts, and are tightened down with only two nuts and two washers using a 9/16-inch socket wrench.

Electroglass^® Starbars^® are made to our exacting specifications from a single tube of silicon carbide. There are no weld joints. There is a cold end that passes through the furnace wall, and a hot zone that extends horizontally into the furnace, up out of the way, above the gathering port. This position provides the best possible heat transfer down to the glass: greater than 80% by direct radiation. In contrast, furnace designs, like Moly furnaces, in which the elements drop down from the crown, sacrifice this fast radiant heat for the slower and less efficient transfer by convection through the air and conduction through the sides of the crucible. In furnaces where the elements are arranged vertically, parallel to the walls, a significant amount of heat is transferred into the walls rather than to the glass. Those furnaces take a big hit on efficiency. Only silicon carbide elements can be mounted horizontally over the glass.

The hot zone in Electroglass^® Starbars^® is formed by cutting a double spiral slot which reduces the cross sectional area through which current flows. By controlling the width of the slot, one can control the ampere or current rating of the element. The cold end has two longitudinal slots along its length. The double spiral begins at the point where the straight slots of the cold end enter the interior chamber of the furnace. An insulated ceramic collar and the two connector straps are pre-assembled onto the end of the element, so the user does not have to put anything together.

Superior Performance

At 2.7 gm / cc, these high-density, low-porosity elements are able to survive severe environments, making them the most demanded, top choice elements in the glass industry. This high density prevents the internal, crystal-lattice structure from becoming oxidized. Electroglass^® Starbars^®, therefore, have an extremely slow aging characteristic. The structure of Electroglass® Starbars® consists of a lattice of closed packed silicon carbide crystals of the polytype, moissanite. In Nature, moissanite is a mineral or crystal that is found in some meteorites. It forms a tetragonal, pyramid–shaped crystal with four silicon atoms at its corners, and one carbon atom at its center. The carbon atom makes a tetragonal bond with all four of the silicon atoms. The pyramid crystals are closed packed within a hexagonal framework, like a honeycomb. Moissanite has a hardness of 9.5, second only to that of diamond. The aging factor for Electroglass® silicon carbide Starbars® represents only about a 10% increase over many years. Electroglass® Starbars® are dimensioned to take that increase in resistance into account, so aging is a non-issue. In modern Electroglass® furnaces silicon carbide Starbars® have lasted six years and longer, with three years being the average.

Operating Temperatures and Atmospheres

In air Electroglass^® Starbars^® can be operated at furnace temperatures up to 3000ºF. In reducing atmospheres, such as Nitrogen used in Electroglass^® glory holes to control reduction, the maximum operating temperature is 2500ºF. Glass batches containing fluorspar (CaF2) give off fluorine gas. Fluorine has little effect on Electroglass® silicon carbide elements at temperatures below 2160ºF, where a popular fluorspar-containing glass is melted and fined. Above that temperature, however, fluorine will damage silicon carbide elements. Because a protective layer of quartz forms on the surface of the elements, most other chemicals that make up glass batches have little or no effect on the Starbars^®.

Power Supply

Power is applied to Electroglass^® Starbars^® by phase-angle fired silicon controlled rectifiers (SCRs). In Electroglass^® furnaces each element is controlled independently by its own SCR, so in the rare case when one element fails, the other elements stay on. Because each element is independent, one does not have to worry about matched sets --- old elements and new elements work fine together. The SCRs are controlled by a Honeywell temperature controller that comes preprogrammed for both the furnace start up and for the fining profile of the glass that you will be melting. Essentially all you have to do is turn on the power, and press the run button. The rest is automatic. Once the furnace reaches charging temperature, you can fill the furnace with either batch or cullet, and press the run button again to fine the glass. The next day you will have perfect glass. Should you have a power failure, when the power comes back on, the furnace will automatically return to where it was when the power went off. This is better than plug and play, because once you've turned the furnace on, it's just play!

Replacing Elements

Another really nice thing about Electroglass^® furnaces is that if you ever have to replace a Starbar^®, it can be hot-swapped with a new one in all of about five minutes. Cold Starbars^® do not thermal shock; in fact, they become stronger as they get hotter. No other type of heating element has such desirable characteristics. The simplicity of design, the ease of operation, and low maintenance make Electroglass^® furnaces a pleasure to use.

Starbar^®	Hot-swapping a Starbar^®