Why We Don't Make a Moly Furnace -
It's Overly Complicated
Stephen R. Sinotte, M.S., M.S.
President, Electroglass®
The success that Electroglass® furnaces have enjoyed over the last two decades is largely a result of our philosophy: " K.I.S., or Keep it Simple." It's the old saying, "The fewer the parts, the less things there are that can go wrong!" That, in a nutshell, is why we don't make Moly furnaces. If you think a furnace powered by molybdenum disilicide (MoSi2) elements sounds interesting, the following are some points that you might first want to consider. Much of this information is taken from the Moly-D® Technical Handbook, published by I Squared R Element Co., Inc. (Phone: 716-542-5511). Page references are listed below in { }.
Historically, Moly powered furnaces were developed as bench-top, laboratory, and research furnaces that could rapidly cycle from room temperature up to as high as 3182ºF. In recent years several people have successfully adapted that bench-top design to the scale of contemporary studio glass-melting furnaces. Limitations, however, due to the physical properties of Moly elements, create a complexity in design not seen in Electroglass® silicon carbide furnaces. Some of these limitations are:
(1) New Moly elements must be brought to 2200ºF or higher, quickly. If operated within the 900-1300ºF temperature range for any length of time, the elements will be destroyed {2}. Silicon and molybdenum oxidize at approximately 1020ºF. This reaction turns the elements into a yellow powder called "pest" {15}. At 2200ºF a glassy coating forms on the surface of the elements that can protect them from further oxidation. However, this coating continuously drips off into the furnace, and a new coating re-forms when the elements once again reach 2200ºF. This fast heat up recommended for Molys doesn't jibe well with the slow heat up required by most refractory materials that are susceptible to thermal shock.
(2) Molys have almost zero resistance at room temperature: that's a dead short {4}! It's tricky to coax them up with power. A very small amount of power has to be applied in increments. For example, a 9mm/18mm Moly with a 32-inch hot zone and a 17-inch "cold end," will draw 365 amperes! On the other hand, a single silicon carbide element in an Electroglass® furnace will draw only 30 amps. To get around this problem, one normally has to hook up multiple Moly elements in series, so as to add up the individual resistances. The idea being that a higher total resistance allows fewer amperes to flow through the elements. It is not uncommon to have to use nine to twelve Moly elements to achieve the same result that one gets with only two or three silicon carbide elements! As the Moly elements heat up, their resistances increase with temperature, and so they become somewhat more controllable. But unfortunately, because of the Moly's high amperage, one usually gets stuck with buying a relatively expensive transformer and / or a three-phase power supply in order to control them.
(3) At temperatures above 2100ºF, Moly elements soften and therefore will elongate or slump when suspended vertically. For this reason they can not be mounted horizontally, up out of the way, above the gathering port, like Electroglass® silicon carbide Starbars®. Instead, they have to drop down through the crown, and are arranged around the sides of the furnace chamber. Not only do Moly elements slump with temperature, but they also elongate further over time. Moly elements will be destroyed if they come in contact with the furnace floor or glass from a cracked crucible {5,9}.
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Moly Element and Parts |
Elements in a Moly Furnace |
(4) At temperatures over 2000ºF the electromagnetic forces cause the two parallel legs to bow apart. The amount of bowing depends on both the hot zone length and the watt loading. This problem is exacerbated above 2100ºF, as the elements soften. If adjacent elements make contact they will both fail. Elements in line, parallel to the furnace wall will bow toward each other; therefore, adjacent elements must be carefully spaced, and they must not come in contact with the furnace wall, or once again, they will fail {10}.
(5) Two parallel elements when carrying current flowing in the same direction will be attracted to each other. This electromagnetic force must be considered in single phase, two phase, and three phase arrangements. Adjacent elements must never be connected to the same voltage or power lead . Unlike with silicon carbide elements, this is a major drawback, and it means that Moly elements must not be connected in parallel with the power leads; instead, they must be connected in series {12}. Like the lights on an old-time Christmas tree, when one element fails, the whole string turns off! On the other hand, the beauty of the Electroglass® furnace design is that each silicon carbide Starbar® is independently controlled, so that if one element fails, the rest stay on. Not only that, but one can have both old elements and new ones in the furnace at the same time - it doesn't matter.
(6) Unlike silicon carbide Starbars®, if too much power is applied too fast, Moly elements will break. Both silicon carbide Starbars® and Moly elements can be replaced while the furnace is hot, but there are several important differences. First, silicon carbide Starbars® become stronger as they get hotter. Molys, on the other hand, become softer and weaker. Second, connecting a silicon carbide Starbar® is safe and easy. It slides in horizontally through an element port in the back of the furnace, and is connected with only two nuts and two washers. Molys, on the other hand, in addition to connector straps, require ceramic element holders that keep the elements from falling down into the furnace. The connector straps have up to six parts per element, and the holders have up to eighteen parts per element, along with a special brick that goes into the crown. Because heat rises it creates a chimney effect, so it can be rather dangerous replacing a Moly, and rather tedious assembling all of the parts. The element holder must be tight on the element. If even one holder is loose, that end will slowly fall into the furnace. The hot section is soft over 2100ºF and will move towards an adjacent element. As pointed out in paragraph 4, above, this can cause both elements to fail {13}. The element straps have clamps that tighten over the ends of the elements. If a clamp is loose, it causes a poor electrical connection that is high in resistance which causes the element terminal end to overheat. This can cause the element and terminal strap to fail {12}. The connector strap must not cause a bending movement; i.e, side force on the element. This will cause the Moly to bend and possibly break {12}.
(7) A three-phase Moly furnace with nine elements will have eighteen element terminals sticking up through the crown of the furnace. That represents a large amount of heat loss; therefore, each element terminal must be cooled. One Moly furnace design that was recently introduced has an air hose going to each terminal. Think about it: nine elements, each with up to 24 small parts, plus two air hoses. That's 234 parts just to hook up the elements! On the other had, a three-phase Electroglass® furnace with three silicon carbide Starbars® requires only six nuts and six washers. K.I.S: "Keep it Simple."
(8) No one has enough experience yet with Moly furnaces to be able to make a good comparison of the expected life of Molys with that of Electroglass® silicon carbide Starbars®. There are different types of silicon carbide. Eighteen years ago we built furnaces using Kanthal silicon carbide Hot Rods®. Those elements had to be replaced every six months due to their increase in resistance over that short length of time. Recent advances in silicon carbide element technology have all but eliminated the aging problem. In modern Electroglass® furnaces silicon carbide Starbars® have lasted six years and longer, with three years being the average. Electroglass® silicon carbide Starbars® are high density, reaction bonded moissanite. In Nature, moissanite is a mineral or crystal that is found in some meteorites. It has a hardness of 9.5, second only to that of diamond. The aging factor for Electroglass® silicon carbide Starbars® represents about a 10% increase over time. Electroglass® Starbars® are dimensioned to take that increase in resistance into account, so it is a non issue. The aging factor for Moly elements is different. Instead of increasing in resistance, Molys initially undergo about a 5%-reduction which makes them even harder to control on subsequent start ups {14}.
(9) While silicon carbide elements can be used in both a reducing and an oxidizing atmosphere, Molys are designed to be used mostly in oxidizing atmospheres. This means that Molys are not suitable for use in an electric glory hole, where nitrogen gas is used to replace oxygen, and thus control reduction. Nitrogen has little effect on silicon carbide, but will react with MoSi2 on the Moly surface to form silicon nitride (Si3N4), which will scale off and damage the element {15}.
(10) Glass batches containing fluorspar (CaF2) give off fluorine gas. Molys are severely attacked by fluorine, even at low temperatures {15}. 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 as well.
(11) Because of the high watt loading density of Molys, their power regulation requires phase-angle fired SCR's with current limiting circuitry. With silicon carbide elements that's not a problem, but with the Molys and their high current draw, the power has to be drastically phased back. Not only does this create a terrible power factor (expensive to operate), but it also creates electromagnetic and radio frequency interference that can play havoc with radios, TVs, cell phones, computers, and other electronic devices in the neighborhood.
(12) Moly furnaces are physically larger than comparable-sized Electroglass® furnaces. The main reason for this is that the interior space has to be larger in order to accommodate the Moly elements that occupy one or more sides. A larger interior volume translates into added thermal mass that has to be heated up; consequently, the Moly furnace design is not as efficient as the Electroglass® silicon carbide furnace design. The dimensions of a 300-pound capacity Moly furnace being marketed today are: 50"wide x 53"deep x 63.5"high. The exterior radiating surface area is then approximately 128 square feet. The dimensions of the 325-pound capacity Electroglass® silicon carbide furnace are: 40"wide x 45"deep x 49"high, so the exterior radiating surface area is only 44.4 square feet. That is a 65% less radiating surface area, significantly less thermal mass, smaller footprint, and 25 pounds more glass capacity. The bottom line is operating cost savings through efficiency.
(13) The same 300-pound Moly furnace, referenced above, is reported by its manufacturer to cost $450 / month to operate, charging twice per week (2400 pounds total). That turns out to be 18¾ cents per pound of glass per month. Likewise, the 325-pound Electroglass® silicon carbide furnace, with the same electricity rate, costs only $350 / month to melt 2600 pounds, or just 13½ cents per pound. In other words that Moly furnace costs on the average 5½ cents per pound, or 40% more to operate than the comparable-size Electroglass® silicon carbide furnace. The cost savings with Electroglass® adds up fast!
(14) Except for some inferior
Molys being made in
In conclusion, while some people have successfully built Moly glass melting furnaces; in our opinion, the designs of such furnaces are overly complex, not very user-friendly, and too expensive to operate. Glass artists don't want to have to tinker with their equipment to keep it running. They just want to blow glass at the lowest possible cost --- with reliable equipment. The secret to that is to K.I.S.
