208 vs 240

Some local fellow is trying to replace his 240 volt ceramics kiln with essentially the same thing wired for 208 volt. I don't know if he has an
electronic timer or just a simple kilnsitter that uses pyrometric bars to shut off. If electronic then I understand but if not what difference would it make. Firing times might differ but hot is hot and the cones know when it is just right. The resistor wire shouldn't be that different for 30 nominal volts. What am I missing? or is he just wrong.
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daniel peterman wrote:

What's the question?
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daniel peterman wrote:

The only reason I can think of is that when a 240 volt kiln was run on 208 volts it would get up to temperature slower and (possibly) it might never reach some high temperature he needs to accomplish something he wants to do.
Watts dissipated in resistance heating is E^2 divided by R, so for a given value of heater resistance the heat produced at 240 volts would be about 1/3 more than at 208 volts. That could be enough to make a difference in the temperature he needs.
Jeff
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On Wed, 6 Sep 2006 14:38:40 -0700, the renowned snipped-for-privacy@webtv.net (daniel peterman) wrote:

Power will be about 25% lower for that 32V difference in voltage. It will heat more slowly and won't reach as high a maximum temperature. He can always buy new elements for the lower voltage. They wear out anyway, fairly quickly if he's operating at high enough temperatures to care about the power.
Best regards, Spehro Pefhany
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The question was, wouldn't the kiln wired for 240 still reach the desired temperatue even at 208? I have used kilns alot mostly to fire overglazes and even at cone 018 they still take many hours to ramp up and ramp down. Once they shut off you still keep the lid closed for about 5 hours unless you want the ware to crack. I know where he can get a 208 kiln but for about 30 dollars he could just buy new kanthal coils for some sections and not worry about it. Thanks
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daniel peterman writes:

The *ultimate* temperature (running full duty cycle for a long time) will be lower at 208 vs 240, but if the *desired* temperature is lower than the ultimate, then yes, it will still reach the desired temperature, but taking a longer time to do so.
Do you know the duty cycle needed to maintain the desired temperature at 240 volts? If that was more than about 60 percent, then you'll likely never get there with 208 volts at 100 percent duty cycle.
Resistance wire is cheap. Consider winding your own elements.
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Thanks guys. I don't think he knows what he is doing as regards electricity. he runs one of those "you paint it, I fire it ceramics shops so I doubt the kilns are ever full and there is really no rush. The kilns are rated cone 10 but I don't think he ever goes that hot. I might be missing something so I'll talk to him in the next few days. I thought I could save him some money. Secondhand kilns are often kinda beat up and his is fine.
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Is he still planning to run the "wired for 208 kiln" on 240 volts? What actual voltage was he inputting to the 240 volt kiln?
Sounds to me that he'll get lots of heat!
Pete Stanaitis --------------------
daniel peterman wrote:

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That's the way I read it, too, Pete. He said "Some local fellow is trying to replace his 240 volt ceramics kiln with essentially the same thing wired for 208 volt"
What he can expect is faster heating, and higher overall temps, assuming he cares to achieve them.
I own a heat treat furnace made by McEnglevan. My particular furnace was marketed for 208 volts, three phase (Wye) wiring. I have three phase delta (240 volts). A phone call to McEnglevan yielded the advice to rewire the control circuit to address the different voltage (there's taps on the transformer for various voltages)----and use the furnace. I was informed that they use 240 volt coils in all their furnaces, so the 208 volt furnaces heat somewhat slower.
Harold
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I have a 4000 Watt 208 Volt 2000 degree furnace. 240 Volts will work if fed with the right gauge and length of wire. Don't coil up the wire and put a blanket over it:)
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On Wed, 6 Sep 2006 14:38:40 -0700, snipped-for-privacy@webtv.net (daniel peterman) wrote:

The control circuit is the only really voltage sensitive item, you might have to change connections on the pyrometer or contactor coil.
The heater is just coiled Nichrome wire in a slot and more than likely dual rated for both voltages, it just gets a bit hotter on 240.
Rather than re-stringing the elements in the furnace (unless it is due anyway...) install a Buck-Boost transformer (or three for 3-phase) to drop the 240 to 208 or vice versa.
The Buck-Boost transformer only has to be rated for the percentage of the voltage it's dropping or raising. If you are boosting 32 Volts you can operate a 10 KVA 1Ph load with a roughly 1.5 KVA transformer.
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On 9 Sep 2006 08:21:46 -0700, "Too_Many_Tools"

Furnaces are a simple resistance device, unless it has a 3-phase fan motor all it takes is a rewire. And a roughly 33% larger power feed, since you are going from three hot lines to two.
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Bruce that's a great idea about the transformer. I'll suggest it to him. Thanks
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daniel peterman wrote:

First I would try it as is. It will not heat as fast, but may be good enough. If it does not heat fast enough ( or Hot enough ), it can easily have the elements replaced with 208 volt elements. But before you do that, just shorten up the elements. Should work good, but maybe not last a long time. But so what. Elements are a consumable and burn out even if you start with heavier gauge wire.
New elements are likely to be cheaper than a buck/boost transformer.
Dan
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wrote:

Absolutely - but you have to consider how long the heater elements are going to live to decide whether to Buck-Boost or restring.
If you are trying to do a Boost from 208 feed to 240 elements, do nothing - it will take longer to warm up, but that will be solved the first time you need to change the elements.
Shortening the elements sounds nice - but the ones I've seen have the wire welded to the feed screw heads for a long lasting connection, you stretch the element wire coil out to the length needed. It gets too hot there to do a field bolted or lug connection, you will have failures at the feed points.
Besides, the wire gets brittle after it's been cycled through normal white-hot operating temperatures once or twice - try stretching the element out longer to cut the end off, and it's going to snap at multiple points.
If it's got Calrod elements that tend to live forever and cost more to replace when they die, then the Buck/Boost may make more sense.
--<< Bruce >>--
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