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Old 07-21-2002, 01:38 AM
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Jon Myers Jon Myers is offline
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Join Date: Apr 2002
Location: Portland OR
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Hi Mark, Sorry not to chime in sooner but I've been away from the computer for a while. I origionaly was going to run my furnace direct from the wall as I have 3 Phase 200 amp of power to my shop (208v). To do this you would need around 50% more elements (either size or number) than you would to run them 120V. For instance I2R quoted me a furnace when I was just starting to think about the moly's that used 6 28" elements(6 28" elements need at least 156v to fire correctly.) run in series to provide 37.5 Kva. In fact I built my furnace to accomadate just this setup. (I have run 6 28" on 120v and they are pokey.) I've also run them as two sets of three elements and (two parallel sets of three)(3 element(28") set needs @ 80v) it ran great. Problem was you had to limit the power. I ran it this way (2 sets of three) until I needed the controller I was using for a high limit for an annealer.(and I found out about the Peak demand charge)(see below) I then put it into series (needing 156v) and ran it on 120v. It was pokey. (It was what I was running when Pete was here.) I've since bought another Hi Limit controller and am now running 4 28" elements in series, essentially the same as 6 18") (the extra 4" is to make up for the radius in each of the 18" elements) and they draw 85-90 amps (170-180 amp 120v) max. I like the longer elements because you can tuck them into the corners of a square furnace and add several inches between the elements and the pot. (It probably makes no difference but it helps me rationalize why I built the obviously less efficent square format furnace instead of round ) Here in OR we have a demand charge on our electricity bill that takes your Peak Demand and multiplys it by a factor (which depends on your electrical contract). We built this furnace (and signed our commercial power contract) after the Cali power problems so our peak demand charge is high, often $250-300. That is with the shop pulling an avg of 19kva and a high of 29 kva. Before I found out about peak charges we would run peaks of 35, 40 kva (charging, running the annealer and drying clothes at the same time etc) We would have $500 peak fees. You would have this level of fee every month if you were running the straight out of the wall variety furnace. Sure you could limit the power with the scr but what would be the point. I've been unable to find a 200 amp contactor and a 200 amp SCR for less than I could find a 150 amp contactor and a 150 amp SCR AND a transformer (New no less). And that would be running the SCR at its rating...bad idea. With a transformer all the components are 2/3 their rating (assuming we're talking about 100 amp 220v and 150 amp components) Now I'm sure that someone has figured a way to heat these things with $15 worth of parts and some tape but I'll bet it took them a while to figure it out($$$) and I'll bet it isn't very reliable (also$$$). The neat thing about these elements is that they work well for a long period of time. It's like having a gas furnace. You go to the shop every day knowing there'll be hot glass. With the wire furnace (at least the one I had) you went to the shop hoping there would be glass. Speaking of which I need to get back to work. I've spent my whole Internet time for today on this one thread/post...jeeze I need to learn to type faster.
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