170-pound Silicon Carbide Electric Glass Furnace
$45 for the 2 dvd set.
Following are photos from
the 4-hour instructional video for the design and
Click on photos to see enlargements
The base is arranged to offer support directly under the side walls which support much of the weight of the crown. Tabs welded on the corners are for fastening the side panels. The expanded mesh offers support and ventilation. The efficiency of the outer, lighter insulations depends on their ability to ventilate and dissipate accumulated energies.
The side panels are 6 feet high and are welded frames with expanded mesh. Six inches of low-density fiber board is the outer insulation. This type of furnace is constructed from the outside inwards. On the base is one inch of light fiber which will compress slightly. Clamps hold the side panels in position while the insulations are being placed. This work is done outdoors because of the insulation dust health concerns.
The base bricks are K-23 soft firebrick with one layer of dense 1" 2300 fiberboard between two layers of brick. The top layer is oriented to offer 4-1/2" of thickness. Bricks with damage are placed facing the outsides. Two layers of K-23 bricks are also placed on the sides of the investment cavity. Wax paper or aluminum foil is placed on the absorbent brick surfaces for proper curing of the investment casting. As a liquid cement, it is important that the liquid content be consistent throughout the hardening process. Soft bricks and the spaces between them can absorb some of this liquid before the casting has set. This "skin" burns off during the "burn-in" the first time the furnace is heated.
The crucible in the investment cavity offers a good cut-away look at the lower furnace construction. The crucible is elevated, sitting on a hard firebrick "split" (half-height brick). This ensures that the investment can be vibrated right down under the bottom, and the bottom of the pot is not sitting on soft firebrick. The 6" of fiber and the 2 layers of K-23 bricks are also added to the front.. The shorter front "cage" is clamped on. These lighter density insulations are appropriate below the glass line when an investment is used around the glass crucible. The density of the crucible, it's contents, and the investment will be nearly 500 pounds. The corners can be filled with soft brick to reduce the investment volume a little bit, but the function of the investment is to support the weight of the glass. In applications like this, I consider the crucible to be the container for the liquid glass, but I do not expect it to support the weight of the glass. This is the role of the investment. This brings up the debate of "free-standing vs. invested". Free-standing pots have a much shorter life and must be heated and charged carefully. Invested furnaces have more density and therefore absorb more heat during warm-up, but offer the mechanical support and "peace of mind" that cannot be had with a free-standing pot.
The crucible is filled with bricks to prevent it from floating up once the refractory cement is placed underneath it. The sides of the casting are trowelled down on a slight angle to the pot edge. This allows sloppy charging bits to flow down into the pot. If glass cannot sit on a horizontal surface, it is less likely to cause erosion. The angled sill trough is cast in place as well. It is a separate casting from the pot casting, isolated by a piece of wax paper. This sill is angled for the same reason, so sloppy gathers can flow back down into the pot, instead of accumulating on the sill and gluing the door shut. It is removable, so can be removed hot, then re-cast into position in a cold furnace. The following photo illustrates the positioning of the upper, heating chamber bricks. The outer row is K-23 bricks, and the inner row is K-26 bricks. Notice the orientation which ensures that all joints on the outer layer are overlapped by the inner row. These bricks are not cemented in place, just "placed" in position. The crown with it's little "keeper" channel will prevent side-wall movement.
The Silicon Carbide rods have a 23" heating zone which is the width of the heating chamber. The feed-through holes in the bricks are slightly larger than the rods. The rods are centered to have no contact with the brick sides, and tightly packed in the cooler outer fiber insulations. This is another consideration in an effort to be "super energy efficient". All minerals are metal oxides and conduct electricity at elevated temperatures. The bricks used here begin conducting electricity at a mere 1200 f. At operating temperature, resistive paths are created everywhere that the elements contact the hot bricks. (Imagine how efficient a pottery kiln is where the coils are sitting in troughs... I wonder if the manufacturers of those things are familiar with the thermal electrical properties of their bricks...) Having said that, check out my 40-pound furnace, which is actually quite efficient for a (wire melter" http://www.mayneislandglass.com/40poundfurnace.htm)
The crown form has to be strong enough to support 150 pounds of wet refractory cement. Foil and wax paper is again used as a mold release and to prevent moisture from wicking into the bricks. Arches don't need to be curved unless they are made up of several individual pieces, but the arch shape does provide strength. The front of this crown is spanning 23". The back and sides are supported, and the front will be bricked in a little bit. Note the 6 feed-through holes for the element rods. They are 4 inches apart and the same below the crown to ensure good heat dissipation. They are located near the crown to be safely away from the charging and gathering region. They're also far enough away to avoid any snapping bits of charging cullet. The elements may seem too far away from the glass pot, they're not. One of the comments I get is, "why are the elements up there when heat rises?". Radiant optical heat doesn't rise, it radiates out in all directions.
Here there are 2 layers of face bricks in front of the crown. The outer layer is K-23, and the inner layers and entrance are K-26. These are cemented into place against a thick piece of plate glass, to ensure flat contact with the door. This lintel is 5-6" below the crown and elements. This helps to prevent convection heat loss across the elements and crown when the door is open. (During the taking of a gather, the temperature drops 3-5 degrees.) Above right is the furnace with the upper fiber insulation and side panels clamped on.
The crown insulation is 6" of 2300f 6-pound density refractory blanket, with 6" of fiber above that. The cross-bars bolted to the top keep the cage square and secure. Clamps are used to squeeze the cage sides tightly against the fiber insulation before bolting together.
The door, as the only moving part, can be a bit tricky to design. "V"-groove rollers and angle iron tracks are mounted on an angle so the door moves away from the face as it moves up. This prevents erosion as there are no bricks rubbing.
The door is one layer of K-23 bricks oriented to offer 4-1/2" of insulation. Small threaded rods run through the bricks to pin them to the roller frame, and slide out if the bricks need changing. The foot-pedal is a door-closer from CRLaurence. They can be adjusted for speed and delay so are very good for closing furnace doors gently. The cable runs up the side to the top of the door, through a counterweight.
The counterweight is a piece of pipe filled with lead. I fill it until it pulls the doors open, then I remove about 4 pounds. There is only this 4 pounds displacement when the foot pedal is depressed, although the door weighs about 60 pounds. It closes gently and smoothly with the aid of the door closer. Here is a look into the pot with the door open all the way. The yoke bar is also in place.
Stainless spring-steel clamps are provided by Kanthal for securing the braided jumpers to the ends of the element rods. The controller contains a 2-pole mercury relay and a small control relay which the temperature controller and door switch use.
The front panel contains an accumulated hours meter for the calculation of energy consumption, and a programmable temperature controller module. Silicon Carbide has a strange resistance temperature curve. As the furnace heats up, it runs as high as 14,000 watts, but as it reaches operating temperature, this drops to around 11,000. Using the hour meter and the price of electricity where I am located, I calculate this furnace works out to $13/day to operate. That's 7.6 cents per melted pound of glass, as I empty the furnace out fully and charge & cook a full pot every day.
The temperature is sensed using an R-type Platinum-Rhodium thermocouple junction. Rhodium and Platinum are very expensive metals, so even the teeny wire used here costs around $200. The welded junction is shown here next to a ball-point pen. The alumna protection tube provides a sheath around the temperature sensing junction. It is placed near the top of the crucible, near the entrance to the furnace.
Click on photos to see enlargements
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- 2010 Mark Lauckner
last update November 26, 2013