Problem: Cause a sphere that is 6 feet in diameter and made of
0.05 inch thick plate steel (or other metal?) to reach a uniform skin temperature of 300F degrees by using 220 or 440VAC as a source?
First, is it possible? (I'm sure there are better ways of doing this than making the entire sphere a shorted heating element - but this question relates only to whether it's POSSIBLE, not practical.)
"IF" it's possible, what type of transformer would you use? That is, what would the OUTPUT in volts, amps, watts, etc be - assuming that you wanted the sphere to reach a uniform skin temperature of 300F? Basically, we just want to "short-out" the sphere & cause it to heat to 300F with AC or DC current - so what's the transformer design :-)
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Thanks.
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Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
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First of all, you'd have to define the ambient environment of the sphere. You'll need to calculate the heat loss through convection (affected by ambient air temp and velocity) and possibly consider emissivity as well.
That will tell you how much power you'll need.
As far as delivering the power to the surface, how uniform is uniform? Are there any restrictions on internal equipment or life safety considerations (running 220V on the surface isn't exactly safe).
If temperature uniformity isn't critical, you could mount some heating elements on the surface, electrically insulated from it. The number and layout would depend on the maximum allowable difference between max and min temp spots and a little heat flow calculation.
Alternately, if the sphere is hollow and available, you could mount a single isotropic IR radiator in the center and paint the inner surface with an appropriately absorptive treatment.
Except for schemes where the sphere itself is energized, voltage isn't really an issue, as sources should readily be available for the range you mentioned.
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Paul Hovnanian paul@hovnanian.com
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Place an orb in the center of your sphere. Heat it red hot, or to whatever temp. works, with an electric arc in it's center. Hopefully the radiated heat would uniformly heat the inside of your sphere, except, of course at the support structure of your interior orb.
Sure. Just suspend it in a 10-foot cube oven with several thermostats controlling different parts of the walls. Put a 9-foot diameter shroud around the sphere, and run a bunch of fans to stir the air both inside and outside the shroud.
Or wrap it on heating tapes, as we do for vacuum chambers, and insulate it VERY WELL. Gene can try using layers of tinfoil, as we do to bake out our chambers.
Gene needs to specify the enviroment this sphere is going to be in. This is going to require much much more energy in free air than if the sphere is heavily insulated.
The heat radiated/convected/conducted from the sphere is going to significantly raise the ambient temperature if it is uninsulated in a room.
For a ballpark guess at the amount of energy required for free air I roughly estimate that my soldering iron has a surface area of 0.0025 m^2 and requires 10 watts to reach 149degC (300F)
A 1,83m (6ft) diamter sphere has a surface area of 168 square meters so scaling up the watt per foot eastimate for my soldering iron gives
67000 watts. This is a very very rough estimate.
That's enough power to make a large room very very hot inside so that estimate can be reduced significantly for an enclosed space. It might be higher if there is airflow over the sphere. Anyone who has tried to use a little soldering iron at the top of a ladder in a cold wind will appriciate that.
After convection inside and outside the sphere, the main problem I can see with heating by passing a current through the metal skin is that if you have two electrical connections, one on either side of the sphere, the current density and heat will be higher close to the connections. Perhaps the skin could be made thicker near the connections. Perhaps the electrical connections could be made lower than the mid point of the sphere to balance the convection. Perhaps the current could be periodically switched between different connection points to give even heating. I'm assuming the sphere is stationary.
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Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com
Please visit my GURU\'s LAIR web site at http://www.tinaja.com
The next usefull number to have is an estimate of the resistance from one side of the sphere to the other.
I'm assuming wires attached on opposite side of the sphere.
Since i have mostly forgotten calculus I made a spreadsheet that used a bit of geometry and trig to calculate the resistance of zones of the sphere. (imagine slicing into coin shaped pieces, the outer edge of each coin is a zone). Wasting my time with this question has reminded me of some of the basics of maths. I'v assumed that quadrangles with roughly equal length edges are rectangles so these numbers are estimates.
If we ignore the resistance of a contact patch for both wires about 30cm in diameter the resistance is 0.0000811
The resistance of the 16cm wide zone around the contact patch is 12 times larger than the resistance of the zone around the equator of the sphere (north and south being the wires) but it has a twelfth of the amount of metal. That means 144 times more heat per unit area.
If we calculate for a smaller contact patch the numbers get more extreme. With 20cm contact patches about 20% of the heat will be concentrated within 1m diameter circles around the wires. That's 20% of the heat in 2.4% of the surface area.
Assuming I have got the maths correct the resistance of the sphere is far too low to practically use as a resistive heater. Maybe it is possible with a transformer the size of a house.
I doubt that cutting slots in the sphere or making it out of thin nichrome would make this a practical idea.
Of course, something that big, made out of something that floppy, is quite unlikely to survive either of our solutions. ;) It'd need to be pressurized just to support its own weight.
Um, wouldn't the current density melt the ends of the sphere while doing no practical heating in the middle?
--
Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com
Please visit my GURU\'s LAIR web site at http://www.tinaja.com
Nope: any direct electrical heating method requires at least a dipole (either current source and sink electrodes in direct contact for Joule heating, or an alternating magnetic field for hysteresis and eddy current heating), which leads directly to uneven heating, even with a rather complicated array of points.
Tim
-- Deep Fryer: A very philosophical monk. Website @
You should use self regulating heat cables. They exist in commerce and will vary the power output to maintain the constant temperature of
300F on your sphere.Shorting the sphere will not get you anyway near your required temperature. If you would get a current that could get you 300F you'd need a superconductor to bring it in otherwise normally wires with all their insulation will burn down to maintain the current you need.
To have an idea of the power you'd need to use the heat diffusion equation . For a sphere it is relatively easy to apply as the form factor is 4*Pi*R.
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