i would like to build a furnace w sic elements...

[Icarus]

I just want to pop in to say that this thread is awesome! It's threads like this that Talkglass is here for. Thank you all of rsharing your knowledge ideas and opinions!

[nicko0]

im reading and rereading joppa, hotgassbits, and all the pdfs isquared has to offer.

natural gas is cheap here in southern la, but so is electricity. i like the high quality of glass an electric produces.

if gas is so good why dont we see boro furnaces on gas?

i feel like alot more temp controllers can tell a scr to voltage and or current regulate/limit?

[nicko0]

i cant quite figure how to ask the internet the right questions about transformers

[nicko0]

how much of the cold zone of an sic has to be inside the heating space? i figure the hot zone could be as close as an inch from the refractory. ? how off am i?

[nicko0]

back to using just ssr.

lets say ive got about 120v per leg on my 2 leg 240v house

i could use two 14mm 63v 2112watt sic rods on one leg without a transformer or an scr. 33amps isnt too bad. put this on the other leg too and ive got 4000w+ inside a 1.3 sqft space. should be plenty?

[nicko0]

for future reference, when i ask for good operating temps for a type of element i always mean the furnace temperature. if i ever need to know the actual element temp ill specify that.

furnace temperatures is what im concerned with and if an element can work 'reliably" relative to something.

max temp of the actual element or its max operating temp, i dont have enough knowledge to use that info to figure out what temp the furnace would have to be set at.

furnace temp is the temp we control and monitor, not the element.

im sorry if i dont know enough to use the info provided the way it is.

[brads]

Well, that's a bunch of questions. I'll answer them in reverse order.

Regarding element temperature vs. furnace temperature, you missed the point. Without knowing exactly what furnace the elements are going into and how it's built and insulated, there's no way to determine what the maximum furnace temperature might be before the elements reach their limit. For example, if you used exactly the same elements in one furnace with 2 1/2" walls and no insulation and then in another with the same walls and 10 inches of insulation, and set both furnaces to hold 2500F, the elements in the one with no insulation would have to run hotter to make up for the extra heat being lost. So while the maximum element temperature is a solid number, the max furnace temperature is not, and anyone who claims to be able to tell you what it is without knowing the furnace the elements will be going into is full of shit - which is why none of us have given you a number for that. Like you, WE don't know enough to use the info the way it's been provided. I.e., we don't know how you're planning to construct your proposed furnace, so can't tell you what the maximum operating temperature might be.

Next post - Where did you get those figures for SiC elements? Specifically "14mm 63v 2112watt" elements? SiC isn't normally quoted like that. Hot zone length, cold zone length, and diameter, are the normal parameters. The rest, such as resistance, watt loading, power output, etc. can be figured from those numbers by using manufacturer charts. Regardless, without even doing the math it looks like your watt loading would be too high. AFAIK, 14mm is not a standard SiC diameter. Even if it is available, it's a very small diameter with a very small surface area which would lead to high watt loading - not good.

Here are the basic electrical equations you need to know - Watts = Amps x Volts; Amps = Volts / Ohms; running two elements in parallel doubles the power output and power draw; running two elements in series cuts both of those in half.

Next post - NONE of the cold zone of an SiC element needs to be in the hot zone of the furnace. And none of the hot zone should be in the wall. If you have to break one of those rules, break the first and not the second one. Better yet, don't break either of them.

Next post - "i cant quite figure how to ask the internet the right questions about transformers". Don't feel too bad, neither can I.

Next - Joppa and hotgassbits are likely to be useless for this. I squared R will be better.
"i like the high quality of glass an electric produces." So do I. That's a large part of why I continue to use one. My original reason for going electric was that electricity was cheap where I lived. Unless I had no choice, these days I'd still use it even if gas was cheaper.
"if gas is so good why dont we see boro furnaces on gas?" I dunno. Because people inexperienced with glass furnaces are more comfortable building electric than gas? <---- Pure speculation.

And last, "i feel like alot more temp controllers can tell a scr to voltage and or current regulate/limit?" You pretty much lost me on that statement/question. It's not the temperature controller that does the current limiting, it's the SCR power controller. You'll need a temperature controller with an analog output to drive a phase angle fired SCR. Fortunately, most controllers have that available as an option. If you can reword what you posted so I can understand it, I will try to elaborate.

Not to be discouraging, but based on these questions I would say you need to do a lot more background research before thinking about building a boro furnace of any type.

[nicko0]

lets say i went with 4.5" brick w two inches of insulation.

crap, i hought i put a link into where i found those elements. im on a different pc now but they were a shenjen company at sicrod.com ill use the charts a isquared and get back to you.

ohms law. yes in series

so the wall should b precisely at the intersection no the cold zone and hot zone optimally?

joppa hotglass and mikegigi were just the most inforatv on any furnace construction seemed like a good place to start

i understand the scr does the limiting. the controller tell it what to limit it to. thought mos controllers woul have that option, only th pricey, urpose built ones mention the option tho.

this i my backgroud research. you all are helping a great deal in helping me ask the questions that i don know to ask.
and fin it invaluable the direct info you all give to my questions and the info you provide based on my obvious lack o nowin a certain aspec even exists.

im anything but discouraged thx

[nicko0]

what are the standard sizes of sic rods. just plainjane straight ones. thats whats frustrating about the suppliers, as ive said before,

tell me what ya got an ill build around that.

(whats standard szes for moly while were at it)

[nicko0]

http://www.sicrod.com/productshow.asp?id=1

here is where i saw some actual product with lots of info.

they have a pretty good breakdown of how to select the right sic for glass furnace

[Draconis]

Before you make up your mind about what type of element to use, download the Kanthal Super Handbook free in Pet from Kanthal.
It will tell you everything you need to know about moly elements, plus some furnace designs, and math about refractories. If you're
only batching soft glass, you could get away with a max furnace temp of around 2700 F, which means an element temp of around
2900 F. Sic can achieve that temp, but not for very long. That is their upper limit (2920), if you run them all out you will only get a maximum
life of about 6 months. If you melt glass at that temp with them, 3-4 months is about all you can expect. The alkaline atmosphere
will severely degrade the elements and shorten their life drastically. I used Kanthal Global Sic elements. I had 8, 4 parallel series of 2. They need to
be matched in resistance or they won't work right. When one dies, you can't simply replace it, you need to replace the series. You also need to shut the furnace down to replace them, with molys you can replace one if you need to, and you can do it without shutting the furnace off. Also, you do need a transformer for SiC. The initial resistance is huge until 1600 F, then the resistance drops to a minimum, then as they age the resistance
grows higher until they die. When I started my furnaces up, they drew over 200 amps with very little voltage. When they got hot, they only drew 30 - 40 amps with much higher voltage. As they aged, the amps went up, and the voltage went down. Running off a two pole single phase power supply rated at 208 volts and 50 amps, you definitely need a transformer. I also used Spang 751 SCRs to soft start the transformers, and to use phase angle firing, which is better than on/off control. Molys, on the other hand, have almost no resistance to start, and gain resistance as they get hotter. It's actually possible to use line voltage with them, however, the larger the mass you want to heat equates to larger power requirements and line voltage falls short, hence the need for a transformer for them as well. Another benefit to molys is that when they get to a certain temp, they bleed a layer of SiO2 that protects them from alkaline fluxing. If they get fluxed, it drips off the bottom, and they just bleed a new layer. Both types of elements would rather never get turned off(especially SiC), but molys(if you don't touch them or otherwise disturb them) can be shut off and started for many years.

[Draconis]

As for crucibles and insulation. I used a zirconia crucible sitting inside another zirconia crucible,
with crushed zirconia filling the void between the two. The liners of my furnaces were 2 inches of zirconia,
backed by 3000 F hard bricks backed by 2800 F soft bricks backed by 2600 F fiber blanket. My furnaces
measured only 3 X 3 X 3, with a charge weight of only 40 pounds. At 2900 F you could lean on the furnace
and not get burned. The outside temp was 115 F. Zirconia may have been overkill, but it can take the heat
and is very strong and handles thermal shock well. For crucibles, it is unparalleled. No wetting, no
leached impurities getting into your glass, and once it gets hot, it doesn't give up the heat readily. When I
shut a furnace down, it was a week and a half until it cooled to 700 F, when I could open it without
worrying about breaking anything from thermal shock.

[nicko0]

no wetting! very interesting!

[nicko0]

im reading the handbook.

[nicko0]

what would nikola tesla do?

[nicko0]

ooo nikola tesla milli

[nicko0]

how much power did u use to get that up to temp and keep it there?

i figure not much, just a good bit more time, which is fine for me.

[Draconis]

It's been 8 years, so bear with me. If I remember right, it required about 10,000 watts for maybe two days to bring an empty furnace up to
temp. But it took another couple days to invest that temp into the mass of the furnace. Once there, I had to add my charge slowly,
so it wouldn't act as a heat sink to the furnace. Every time you open the top, or add something cold, it drops the ambient temp requiring
time or more power to overcome.

[Draconis]

Regarding transformers. They need to be wound according to your power requirements. I got mine from Spangler Power Electronics.
Talk to an engineer about what you are doing and need, and they will tell you what you need. You can't just buy a transformer
for general use, they need to be exactly what you are using it for.

[Greymatter Glass]

ooo nikola tesla milli

You rang?



Artist: Chris Juedemann / Glasskitchen

Also... this is an awesome thread.
Check out Glass Notes by Henry Halem if you don't already have it - lots of good general information about furnace design.