Question: Is it possible to heat a LARGE metal object with 220 or 440VAC?

Your question lacks detail. With your wording, one answer is "Sure! Just build an oven that runs on 220, takes a 6' diameter sphere, and will heat to 300F in a few days with the door closed!"

So:

Direct heating (and the inevitable contact grunge on your sphere)?

Indirect (i.e. can we specify an oven)?

Inductive (this would be a candidate)?

How even do you need the temperature?

How fast to get up to temperature?

Any current/power limitations?

Can we specify that the sphere has to move?

You aren't going to get that using a uniform sphere as a resistive heating element. The contact points will get hotter than the rest of it. If you want even heat, you would need to make the metal thinner as you get further from the contacts.

Place a heating element inside the sphere? The heat will move to the outside of the sphere. It is not possible to use the sphere itself as a heating element because there is very little resistance so you would need extremely large currents which means your power source must be very large.

The main issue seems to the uniformity as any air convection out side would result in local cool spots(not sure how much though and I imagine it depends on the ability of steel to hold its heat). Air convection inside the sphere is a good thing though as it will help keep it uniform... although I'm not sure if it would be needed.

Maybe you can pump gas into the sphere and ignite it/burn it and have some method to circulate the air. Or better yet, use some liquid with a high boiling point(to reduce pressure)?

I'm sure there are many ways to do it. Maybe using a liquid would help in the uniformity(specially if it too is circulated). Not sure if this is practical for your situation though as you haven't given many details.

Thanks.

That was my gut feeling. However, is it possible to "bring it up slowly"? That is, use a microprocessor or a timer to apply the current VERY slowly?

Thanks.

• or - 5% would be OK. (~15deg+-)

At these volts, it's ~ 100% - NP.

Sure, but it won't help.

The stable state will have the regions near the connections at a higher temperature than the regions further away.

The heating will be proportional to the square of the current density, while the cooling will be proportional to temperature. The stable state occurs when heating is equal to cooling, so regions with a higher current density will have a higher stable temperature than regions with a lower current density.

Gene wrote: (top posting fixed)

As "nobody" pointed out, you can't bring it up slowly because no matter how slow you go your source will still be uneven.

You may be able to heat it evenly by spinning it while using an inductive heater, but you'd either have to use a heater that'll heat up one patch and spin the sphere while precessing it (and suffer heat irregularities), or use a bunch of heaters strung along a 180 degree arc that are sized to make the latitudinal bars receive even heating.

Ultimately if you want even heat I think you need to think about using an oven and stirring the air.

Have hexagonal or dodecahedral heating pads with heat sink thermal compound pressed on the inside. IR pyrometers read the temp in ea section and control the juice to ea heater.

Jon makes the best suggestion so far. Use a fan inside it as well. Otherwise your heat may float to the top and settle. Some quality cookware uses aluminum bonded to the underside of stainless steel to conduct heat faster, and thus more evenly. What kinda bucks you got? I'd like to see you pay to fab a six foot steel sphere with a layer of aluminum bonded to the inside. As long as yore blowin hot air here, anything's possible! I assume you will have gas fill, not liquid. A six foot sphere full of liquid would get mighty heavy. So, is this for real, why on earth do you need a metallic six foot sphere to get so hot? Sounds almost like some doofy art project. I can't imagine a practical use.

whether

You've about 250lbs of steel there, so in theory 6.8MJ will get it up to temperature. This is about 2kW electrical heat input for an hour. Springs to mind therefore that an arc welding transformer would be ideal. But ... from theory into the real world and you move into the tricky subject of thermodynamics, which needs account taking of many other environmental and material radiative/diffusion characteristics. A finite element, 3D heat flow solver program, seems the only way to point to some kind of real answer. Or maybe if it's a class project, program something based on Fouriers law of heat conduction but make a lot of simplifying assumptions.

Without some form of enclosure for the sphere, maintaining a fixed and uniform temperature distribution over the entire surface would be almost impossible for any length of time.

Very tough problem, I would suggest porcelain enamel on the inside or outside. Then apply thick film resistor elements on the enamel. The configuration of the resistor pattern could be applied in any pattern desired. saying that - I would suggest modeling the whole thing first (FEA) to design a pattern that would minimize hot spots.

The only issue would be to find an oven large enough to hold a 6' diameter sphere to apply the PE and resistor coating. The ovens are out there - getting someone to let you use it is another thing..... Gregg