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

[Gene]

Good day, all.

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

THANKS,
Gene

 

[Tim Wescott]

Good day, all.

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

THANKS,
Gene

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?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Gene]

Thanks, Tim.

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

Yes, the entire sphere will simply be the heating element. That is,
you connect one wire to one side of the sphere and the other wire
to the opposite side of the sphere & turn on the juice.

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

Inductive (this would be a candidate)?

How even do you need the temperature?

We want the entire sphere to have a skin temperature of 300F.
That is, we want VERY even heat.

How fast to get up to temperature?

Five minutes would be ideal - just not in hours.

Any current/power limitations?

Yes, we want to use the normally available 220 or 440VAC as
a source to the transformer(s) or whatever.

Can we specify that the sphere has to move?

The sphere will be suspended and insulated.
Think of the sphere as you would the heating element in
a common home electric stove.

We simply want to heat the sphere without having to construct an
oven or other device to supply the heat.

 

[Nobody]

Yes, the entire sphere will simply be the heating element. That is,
you connect one wire to one side of the sphere and the other wire
to the opposite side of the sphere & turn on the juice.


We want the entire sphere to have a skin temperature of 300F.
That is, we want VERY even heat.

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.

 

[Jon Slaughter]

Good day, all.

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

THANKS,
Gene

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.

 

[John Fields]

Thanks, Tim.

Yes, the entire sphere will simply be the heating element. That is,
you connect one wire to one side of the sphere and the other wire
to the opposite side of the sphere & turn on the juice.

We want the entire sphere to have a skin temperature of 300F.
That is, we want VERY even heat.

---
_How_ even?

What's your spec of delta t over the surface?

 

[Gene]

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?

 

[Gene]

Thanks.

_How_ even?

+ or - 5% would be OK. (~15deg+-)

OK, but since you say it's insulated, How good is the insulation?

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

 

[Nobody]

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?

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.

 

[Tim Wescott]

Gene wrote:
(top posting fixed)

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?

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[BobG]

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.

 

[Tolstoy]

Place a heating element inside the sphere?

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.

 

[john jardine]

Good day, all.

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

THANKS,
Gene

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.

 

[Ross Herbert]

We simply want to heat the sphere without having to construct an
oven or other device to supply the heat.

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.

 

[Gregg]

Good day, all.

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

THANKS,
Gene

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