Handmade-Glass.com - Electric Glass Furnace

After looking at the diagram above, the more electrically astute will probably be saying to themselves, "That 's pretty much the same way to wire a 3 phase annealer", and they'd be right. That is the nice thing about this control system - simple and cheap, especially relative to other types of electric furnace control systems requiring a transformer. There's no big mystery involved in doing this, but done this way you DO need to choose your elements wisely. Out of the many sizes and resistances available in SiC elements, only a small percentage will work well (i.e., last a long time) when using this system. This DOESN'T mean you need to buy the fancy-schmancy, high priced elements to make this work. My elements are the standard type and composition - the cheapest model available. You just need to choose the right element size (which translates to the right resistance and surface area). Something else to keep in mind about the diagram above is that a 240V, 3 phase unit connected this way actually delivers only 140 Volts to the elements. I won't try to explain why here - it's one of those weird electrical things.

This isn't the only way to do this and I won't claim it is the best. It just happens to be the way I did it when I was short of cash. I wanted to build an electric glass furnace that worked well and was relatively cheap to build. I did have some doubts when I built it - due primarily to the electric furnace manufacturer who told me, "It will NEVER work. You should buy one of my high-priced units instead!" (Okay, not in those EXACT words - but pretty close.) In spite of his warning, it works great and didn't cost an arm and a leg to build. Not including the temperature controller (which you'd probably want no matter what type of furnace you built), or the manual disconnect switch (which was given to me), the original control system on this furnace cost me less than $500, at retail prices. That doesn't count the elements, or the heat sinks that I scrounged for the SSR's . If you are a good scrounger it could cost even less.

As of this update of this page, this control system worked virtually* (see below) trouble-free for more than 13 years, so it certainly met that part of my goals. The only reason it didn't last longer is because I finally closed my studio and dismantled the furnace due to sale of the building in which my studio was located. I am confident that otherwise both the furnace and control system could have gone years longer without any trouble at all. *(The "see below" part): The only trouble I encountered with the control system was with the original mechanical door switch, which had been scrounged from a surplus yard. It only lasted about 6 or 8 months before dying, at which point I replaced it with an Omron photo-electric door switch. The Omron was still working perfectly when I dismantled the furnace. Because I used an old mechanical shut-off switch on the door when the furnace was built, instead of the Omron photoelectric switch that replaced it, this control system actually cost me less than $350 when I built it. But as mentioned above, the mechanical switch proved to be unreliable (my only problem with the system over the years) so I was forced to spend $165 for the Omron (it was the only thing immediately available when the mechanical switch failed). While the Omron is a very nice switch, less expensive photoswitches are available that will still do the job. Decent ones usually cost in the $100 - $120 neighborhood. Based on my experience, they are well worth it. I strongly recommend a photoelectric door switch instead of a mechanical one.

**My approximate parts cost**
(2) 125 Amp Solid State Relays	$65 each - Allied Electronics
(2) 75 Amp definite purpose contactors	$100 each - Grainger
(1) Omron E3K-R10K4 Photoelectric switch	$165 - Grainger

There are some modifications that could be made to the system (and might well be advisable) like fuses and such. Wiring a controller alarm output or a limit controller into the circuit to shut down power in case an SSR fails and locks on is also a good idea, and something I eventually did on my own furnace. My disconnect switch is fused, but there are no fuses protecting the SSR's, which require a special (expensive) type of fusing. The reasoning here is that this type of fuse may or may not protect the SSR, and they cost just as much as a new SSR anyway. However, proper fusing to protect personnel is a must.

Since building the furnace, I added the overtemp alarm shut-off wiring to my system, but it isn't shown on this drawing. An overtemp alarm and cut-off switch is a very good idea no matter what, but especially if you forego the semiconductor fuses for the SSR's. For maximum protection a separate overtemp controller is best, but that's an extra expense and I haven't had any problems using the alarm circuit of my controller. Either one is far better than nothing at all (this is true on annealers too). All in all, this has proven to be a very reliable system which was reasonably cheap and easy to assemble. My elements typically last 2+ years in nearly continuous use.

SiC elements do have some drawbacks. The main problem with them is an increase in resistance over time (aging) which either requires an increase in voltage to maintain furnace power, or slowly decreases the maximum available power. My elements - which are standard Norton Hot Rods (Kanthal bought out Norton's element business quite a while back) - are exposed to the furnace atmosphere and do not seem be adversely affected by batch dust (I have used Spruce Pine, Glasma, and a couple of different mixes from a friend). The set I recently replaced was in use for over two years before I changed the wiring to give them more power. They were over 4 years old when they were finally replaced.

One VERY IMPORTANT thing about electric furnaces - most refractories leak electricity at glass melting temperatures and molten glass itself is a good conductor. If you build an electric furnace, you MUST have a door switch to shut off the power when you gather. This also makes the choice of materials for your thermocouple tube important. Mullite cannot be used because of electrical leakage. At best, this leakage would cause faulty temperature readings. At worst, it would toast your temperature controller and possibly you as well. Use a pure alumina protective tube for your thermocouple instead of mullite and if it ever cracks, replace it immediately.

One other thing - Solid State Relays (and SCR's) leak a small amount of electricity even when they are turned off. It isn't much, but depending on the situation could be enough to put you in a world of hurt. So just turning off the SSR's isn't good enough, you need to disconnect the power to the elements with a contactor, either electro-mechanical or mercury. Otherwise, you are tempting fate in a BIG way. YOU WERE WARNED!

My electric furnace design recommendations based on personal experience are:

1) Don’t underpower your furnace. One of the most common complaints I have heard about electric furnaces is that they take too long to heat. Mine heats just fine. And not just with new elements, but after they have been in use for one or two years as well. The initial element sizing should provide at least 50 percent more element output at full power than necessary. This will allow a power "cushion" for the elements as they age and the resistance changes. Keeping the watt loading as low as possible will also help them last. I recommend a watt loading of 35 Watts per square inch, or less. Some "store bought" electric furnaces use Watt loadings of 60 Watts per square inch, which I believe is a large part of why their users complain of short element life. That, and the fact they were underpowered to begin with.

2) Don’t necessarily believe the “conventional wisdom”. The higher priced spiral cut, single ended SiC elements may be better for some applications, but they have some fatal flaws in a studio glass furnace, one of which is their tendency to "short out" between the element windings. Their main advantage is that the power connection is on one end only, making them easier to install and the furnace easier to insulate. An expensive, phase angle fired SCR control system isn't necessarily the best way to go for power control either, judged on a cost effectiveness basis. As shown above, mine uses solid state relays and works well. While SCR's can have some advantages, unless one finds a great deal on surplus equipment, they are quite a bit more expensive than the control system above.

3) Listen to the people who use electric furnaces (or, possibly better yet, the people who have STOPPED using them). If you hear the same complaints over and over, you had better pay attention. I listened and was able to design around some common problems in my furnace. It not only saved me the aggravation of having the same problems as many others, but in addition to that the furnace design I came up with was cheaper to build. I consider that a win-win situation.
Also, be aware that someone who has a stake in selling a particular brand or type of furnace MAY not be an entirely reliable source of information. (Trust me, I know.) Act accordingly.

Along those same lines, someone who has recently bought an electric furnace is often caught up in the joy of having a new piece of equipment and/or the manufacturer's hype. Due to this he/she will often praise it no matter what. My advice on this is to discount the opinion of anyone who hasn't had their furnace for at least 5 years. Over that time period any hidden problems are likely to surface and what may have been a glowing opinion initially will tend to get a lot more realistic.

4) Be prepared if some of your ideas don’t work. Not everything I tried worked the way I hoped it would. I had to make some changes to the materials I used when I changed my first crucible. Replacing ALL of my homemade insulation WASN’T fun!

5) You don’t need to be an electrical engineer to build or use an electric furnace, but the more you know, the better off you are when something goes wrong. Learn about electricity and power control BEFORE trying to build one. It will save headaches later. If you don’t know Watts from Volts, either learn beforehand or stick to a gas furnace. A glassmaking furnace is NOT the equivalent of a kitchen appliance. Don't make the mistake of thinking it is.

My feelings after more than 13 years of use? I love my electric furnace! It melts good glass, is totally silent, easy to control, and won’t blow up if things go wrong (my landlord is quite fond of that feature). I won’t go back to a gas furnace if I have any choice at all.

So that's it, for what it's worth. Although my furnace runs on 3 phase power, the general principles are similar for single phase. Anyone with a bit of experience and common sense should be able to adapt this information to a single phase furnace or annealer. If this information helps you out and you save a pile of money, feel free to send me a small chunk of the pile, or even a piece of your glass. :-)