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badger
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Joined: 03 Sep 2006
Posts: 63
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 Posted: Fri Apr 13, 2007 12:18 am Post subject: Could this be used as a heating element? |
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http://store.barbecue-smoker-grill.com/brheel.html
I'm considering buying one, but not sure if it will put out enough heat?
The thing that keeps me from making my own free standing one (not using the kitchen oven) is the oven wiring. I've read and re-read Jims page, and TJ's book, but I am just too nervous about doing the heating wiring.
I think with two units, if they worked, it could do up to a 20x24 frame easily enough, allowing for 2" on the outside edge of the heating elements placed side by side. Total SWAG math there.
Comments?
badge |
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remarque
Newbie
Joined: 06 Oct 2006
Posts: 22
Location: The Carolinas
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| Posted: Fri Apr 13, 2007 10:39 am Post subject: |
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| Yes, I think someone has made an oven with a similar heating element on this board. |
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jegner
Site Admin
Joined: 30 May 2003
Posts: 2144
Location: Texas, USA
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| Posted: Fri Apr 13, 2007 11:21 am Post subject: |
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Badger,
For my first 2 vac form machines, I used a similar heater, from a Sunbeam indoor electric grill.
Dr. Crash managed to use some turkey rosting pans, and some creative IR deflecting to get decent results, but for me, these are too small for trooper size formers, but perfect for smaller scale projects. 12x18 is about all I could get from my machine.
Jim |
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badger
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Joined: 03 Sep 2006
Posts: 63
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| Posted: Fri Apr 13, 2007 1:37 pm Post subject: |
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| jegner wrote: | Badger,
For my first 2 vac form machines, I used a similar heater, from a Sunbeam indoor electric grill.
Dr. Crash managed to use some turkey rosting pans, and some creative IR deflecting to get decent results, but for me, these are too small for trooper size formers, but perfect for smaller scale projects. 12x18 is about all I could get from my machine.
Jim |
Yeah, I was thinking of making a smaller free standing over under unit from one of these units, and maybe, once I've got all the logistics figured out from that one, making a larger 20x24 one out of two of these units.
thanks for the tips. |
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cod
Master
Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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| Posted: Wed Apr 18, 2007 5:02 pm Post subject: Re: Could this be used as a heating element? |
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| badger wrote: |
The thing that keeps me from making my own free standing one (not using the kitchen oven) is the oven wiring. I've read and re-read Jims page, and TJ's book, but I am just too nervous about doing the heating wiring.
badge |
I was incredibly intimidated by the electric when building my nichrome wire oven, but in retrospect it's not hard at all ... nor do I shake in fear when using it. I did everything I could to reduce shock risk: 2 circuits on GFCI outlets, every piece of metal not in the circuit is earth grounded.
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jzawacki
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Joined: 12 Jan 2007
Posts: 70
Location: Illinois
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| Posted: Mon Apr 23, 2007 10:28 am Post subject: |
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| badger wrote: | | making a larger 20x24 one out of two of these units. |
I thought the same thing, which is why I have two sunbeam grills in my garage right now, but I don't think I will use the second because you need two dedicated circuits to run both. I believe one alone pulls 12 amps. |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Mon Apr 23, 2007 10:40 am Post subject: |
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| jzawacki wrote: | | badger wrote: | | making a larger 20x24 one out of two of these units. |
I thought the same thing, which is why I have two sunbeam grills in my garage right now, but I don't think I will use the second because you need two dedicated circuits to run both. I believe one alone pulls 12 amps. |
On the other hand, you can run a relatively cheap mass-market extension cord from an outlet on another circuit, so that's really not so bad. (You need a "heavy duty" extension cord, but not an industrial one.)
The big drawback of the sunbeam grill type elements is that they're too small and too hot, and the heat controls on them suck. (That's why got a diode to make a off/half/full heat control for my little Sunbeam grill over-and-under; I got tired of switching the heat on and off to keep it from burning my thicker plastics.)
It's a shame to use 2 elements and not get variable and/or zoned heat control in the bargain. The nice thing about nichrome is that you can have inner and outer zones to fit any size and aspect ratio of oven, because you can route the coils any way you want.
Another possibility for off-the-shelf calrod heating elements is 220 V kitchen oven elements, but running them at 110 instead of 220. That way they'd dissipate 1/4 the heat, over a bigger area. Some combination of things like that would likely work fine, but it's a PITA to try to figure out the combinations to make the oven you want.
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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jegner
Site Admin
Joined: 30 May 2003
Posts: 2144
Location: Texas, USA
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| Posted: Mon Apr 23, 2007 1:34 pm Post subject: |
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Working on the 'I am Legend' fan film, we used some dimmers on an extension chord, worked like a charm! I'm going to make some for various uses, lights, fans, etc. But the same idea should work on the oven control. IF you can find a 20Amp verison.
Jim |
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Mon Apr 23, 2007 2:06 pm Post subject: |
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| jegner wrote: | Working on the 'I am Legend' fan film, we used some dimmers on an extension chord, worked like a charm! I'm going to make some for various uses, lights, fans, etc. But the same idea should work on the oven control. IF you can find a 20Amp verison.
Jim |
20A dimmers are VERY expensive. A 600-watt (5 amp) dimmer is around $10, but that's not quite enough for even one segment of a TJ oven, because you should have about 20 percent extra amp capacity. A mere 1000-watt (8 amp) dimmer costs about $40, 4 times as much as a 600-watt dimmer.
That's why I'm going with using simple power diodes to cut the power in half, and arranging the coils in a double spiral pattern so that I can have two coils on all the time, and two coils variable heat, and either set of coils will heat evenly. (And I get inner and outer zones in the bargain.)
That will give me the ability to cut the power by a quarter or a half, which I think will be fine, and it's a whole lot cheaper than high-amp dimmers.
Also be wary of using dimmers as motor speed controls. It can be bad for the dimmer and/or the motor. I'd use a fan control or router speed control for a motor, and only if it's the right type of motor. (Some motors depend on having a good approximation of sine wave power, and dimmers and speed controls mess up the waveform pretty brutally. The part of the waveform that deviates from a sine wave can't be used efficiently, and ends up being converted to heat that may overheat the motor.)
I wish I understood this stuff better. I particular I wish I knew if I could use a router speed control at anywhere near it's rated amps for a resistance heater. If so, that would be cool because you can regularly get 15-amp RSC's from Harbor Freight on sale for $12.50 or even $10.
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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cod
Master
Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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| Posted: Mon Apr 23, 2007 2:28 pm Post subject: |
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I've seen those router controls before. I would think the only way you could do this with an AC motor is to keep within the 60hz supply cycle somehow, perhaps by doubling, and/or cutting the power in half , or something..... I dont fully understand it either .... would like to see if they work with my cheapo black and decker router.... thy claim to be a 'universal' controls often..
for heater control you could theoretically use big power resistors in a network instead of a single continuous dimmer control, I think, but this might not necessarily be cheaper.
why not just a network of switchable nichrome lengths? Say, 5-10 toggle switches that allow you to turn off segments ? A dimmer is just going to be a resistor anyway, and the heat/power is wasted, adding to costs. if you had multiple circuits you could use just as much power as you need without wasting it thru dimmer heat.
I was going to have dimmers on my circuit, but it seems like an added expense that isnt needed atthis juncture. It might be easier to contro the distance of the plastic from the oven, instead of adding dimmers/resistance.
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drcrash
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Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Mon Apr 23, 2007 7:09 pm Post subject: |
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[quote="cod"]I've seen those router controls before. I would think the only way you could do this with an AC motor is to keep within the 60hz supply cycle somehow, perhaps by doubling, and/or cutting the power in half , or something..... I dont fully understand it either .... would like to see if they work with my cheapo black and decker router.... thy claim to be a 'universal' controls often..
I think a normal dimmer or router speed control mainly just turns the power off at a certain point in each AC half-cycle, chopping the waveform, using a "Triac". The Triac's resistance is very high when the voltage is above a certain threshold, and very low when the voltage is below that threshold.
| Quote: |
for heater control you could theoretically use big power resistors in a network instead of a single continuous dimmer control, I think, but this might not necessarily be cheaper.
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Right. Power resistors do cost money. I've used power transistors to tune the heat of a hot wire in lieu of a voltage-controllable power supply, but it's probably too expensive for something with really high watts like an oven. (And less efficient than a dimmer or a diode, which doesn't dissipate much heat.)
| Quote: |
why not just a network of switchable nichrome lengths? Say, 5-10 toggle switches that allow you to turn off segments ? A dimmer is just going to be a resistor anyway, and the heat/power is wasted, adding to costs. if you had multiple circuits you could use just as much power as you need without wasting it thru dimmer heat.
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A dimmer doesn't waste much power, because its resistance is either very low or very high. (When it's low, it doesn't heat up much because it doesn't resist the current going through it much. When it's high, it doesn't heat up much because there isn't much current through it.) It's like an on-off switch that cycles on and off 120 times a second, rather than a resistor. Unfortunately, high-amp dimmers are not cheap.
One issue in designing something like this is that for a given gauge of nichrome wire, there's only a fairly narrow range of segment lengths that makes sense, amp/watt/wire temp-wise. If you want to have a bunch of low-watt segments, you probably need to go with finer wire than the commonly available 22-gauge-and-larger coils. But if you do that, you can put each segment on a cheap dimmer and get finer control.
Another issue is the layout. If you are trying to control overall heat level by turning some segments off, without affecting heat distribution, you need a lot of closely-spaced coils, or you need to put the plastic far from the heater.
My two-spiral design is a compromise. It reqires more coil than the TJ design, but not a whole lot more. And because I'll only turn off (or down) half the coils, I can leave every other coil on, so the heat distribution is even overall, and you just need to put the plastic an inch or two further from the coils to get good overlap, even with half the coils completely off. If was going to turn off one out of three coils, or one out of four, to get finer-grained heat control, I'd need to put the plastic further from the coils. I figure if I can get half power by using half the coils, and three quarters power by using diodes on half them, that's good enough, and pretty cheap.
| Quote: | | I was going to have dimmers on my circuit, but it seems like an added expense that isnt needed atthis juncture. It might be easier to contro the distance of the plastic from the oven, instead of adding dimmers/resistance. |
Changing the distance to the plastic does not affect the overall heat much if you have a deep oven with reflective sidewalls (as I think you usually should). And when it does, because the sidewalls doen't go most of the way to the plastic, that messes up the heat distribution, and doesn't give you good heat around the edges. (The sidewalls are what let the oven heat the edges comparably to the center, by reflecting IR that would escape out the sides back to the plastic. Most of that IR hits nearer the edge than the center. Without that effect, the center gets more IR than the edges because it's being irradiated from all directions, where the edges are only being irradiated from one side.)
You might want to check out this old posting on IR oven design:
http://www.tk560.com/phpBB2/viewtopic.php?t=391&highlight=conventional++oven+design
The standard TJ oven doesn't have straight reflective sidewalls like that, but it does have extra coils out past the edge of the plastic, to boost the heat to the edges. If you put the plastic too far away, that will work less well, too. (For it to work as well, you'd need to make the oven base that much wider, so that the angle from the edge of the plastic to the outer coils was the same.)
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Paul (a.k.a. Dr. Crash)
Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com |
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