Yep I know about black body radiation and knew about STEFAN'S LAW ect.


Not really a problem if you want to rapidly heat something but if you want to maintain a temperature then you would lose a lot of power due to the power of 4 relationship.

Actually, I disagree. It is precisely the problem if you want to heat something rapidly. The blackbody radiation power is an instantaneous power loss! You must supply that power level to reach the temperature you want. If you want to maintain that temperature then you would lose a lot of ENERGY. Note that Energy is power times time.

 

Hi,

I think the original idea of just blasting a massive current through the thing isn't feasible especially considering I would need a BIGG transformer to ramp the 440 volts on site down to some 10 volts.

Did you consider using the rod/tube as part of a single secondary winding in a transformer (where the rest of the winding was copper bar)?
Sure, it would still need to be a bit larger than a cell phone charger, but any control should go on the primary side (and gut-feeling tells me it won't get up to 10V on the graphite, although I didn't calculate it).

It could be combined with propane for a faster heat up, but anyway, it will be more efficient than nichrome heaters, since all available power will go into the "pencil" - unlike with external heaters.

 

I would need a BIGG transformer to ramp the 440 volts on site down to some 10 volts.

I like Soren's idea of wrapping 44 or 45 turns of water-cooled copper tubing around the graphite!

Its a tube about 700mm high and 200mm diameter. Inside is hollow about 100mm so the wall of this graphite tube is some 50mm thick.

This combination of diameter and wall thickness should lend itself quite nicely to line frequency (50 or 60Hz) induction heating.

I am also working on a easy robust igniter and I think nichrome would work with that too. Currently our guys leave “pilot” flames on all the time which they use to light large “hand scarfers” which are hand held propane burners. The problem with the “pilot flame” is that they are HUGEEE. Think of a flame about 30-50cm high and 10-20cm wide, probably costing more than £10k ($15k) per year per one and there are probably a dozen of them.

Make each of them carry these: http://www.plumbersurplus.com/Prod/Turbo-Torch-L-1-Single-Flint-Striker-(1423-0023)/22807/Cat/436

 

I like Soren's idea of wrapping 44 or 45 turns
Make each of them carry these: http://www.plumbersurplus.com/Prod/Turbo-Torch-L-1-Single-Flint-Striker-(1423-0023)/22807/Cat/436

Would be great if it was 50cm longer. The propane torches they use are huge and you really want to be at least 50cm away when lighting the thing.


Not sure about induction heating, seem a little complex for me. A simple furnace would probably be more realistic for my skill level.

 

Actually, I disagree. It is precisely the problem if you want to heat something rapidly. The blackbody radiation power is an instantaneous power loss! You must supply that power level to reach the temperature you want. If you want to maintain that temperature then you would lose a lot of ENERGY. Note that Energy is power times time.

Negligible if the peak blackbody radiation (for the temperature you want) is 10kw and your pumping in 100kw until you reach that temperature.

 

Negligible if the peak blackbody radiation (for the temperature you want) is 10kw and your pumping in 100kw until you reach that temperature.

Ah, OK. I see why we are mis-communicating. You should recalculate the blackbody radiation. I calculated 90kW.

The thing that was concerning me was that you are neglecting the dominant power loss.

Actually, I just saw that you posted the dimensions of the graphite bar. If I use this geometry, I get 75kW.

By the way, you calculate the outer surface area of the bar, and treat it as a solid bar and include the area of the ends. The internal area will result in reabsorption, but the open ends will emit radiation.

 

Not sure about induction heating, seem a little complex for me. A simple furnace would probably be more realistic for my skill level.

Induction heating has been used for nearly a century in industry. But if you prefer a furnace instead, I suggest the following:

- Minimum 50mm insulation (Something like Kaowool or Inswool would be suitable.)

- Gap of 50mm between workpiece and furnace walls

- Exhaust cross sectional area not less than five times the burner inlet total area (if burners are used)

- Total burner (or heating element) output not less than 29 kJ per cc of furnace interior

Whether propane or resistance heating would be cheaper in the years ahead will depend on the projections for your local economy.

 

Ah, OK. I see why we are mis-communicating. You should recalculate the blackbody radiation. I calculated 90kW.

The thing that was concerning me was that you are neglecting the dominant power loss.

Actually, I just saw that you posted the dimensions of the graphite bar. If I use this geometry, I get 75kW.

By the way, you calculate the outer surface area of the bar, and treat it as a solid bar and include the area of the ends. The internal area will result in reabsorption, but the open ends will emit radiation.

you could always insulate the rod, then zap it with lots of current hence little blackbody radiation.

 

Induction heating has been used for nearly a century in industry. But if you prefer a furnace instead, I suggest the following:

- Minimum 50mm insulation (Something like Kaowool or Inswool would be suitable.)

- Gap of 50mm between workpiece and furnace walls

- Exhaust cross sectional area not less than five times the burner inlet total area (if burners are used)

- Total burner (or heating element) output not less than 29 kJ per cc of furnace interior

Whether propane or resistance heating would be cheaper in the years ahead will depend on the projections for your local economy.

thanks


I don't see any way propane heating could be cheaper than electrical heating without a costly heat exchanger. Even then it is probably likely that it would not be cheaper than electric heating. The problem with propane is that you use cold propane and oxygen, burn it, and hope some of that heat goes into the rod. In reality a LOT of that energy is wasted in the expelled gas. The higher the temperature the worse the efficiency of propane heating. Electrical heating doesn't have that problem as you don't transfer any gas.

 

you could always insulate the rod, then zap it with lots of current hence little blackbody radiation.

What do you mean by insulation? Do you mean a thermal insulator? If so, this is what I said in my earlier post. This is the principle of the kiln; to insulate. So, you have to use enough insulation so that the outer skin temperature is low enough to not emit significant blackbody radiation. But, this will be a slower process since you are heating the object and large insulator.

Or, do you mean to use a reflector to reflect the blackbody radiation back. If so, that may work. Graphite is a very good blackbody, which means it is very efficient at emitting and absorbing radiation. However, if you surround the object with a material that is not a good blackbody (for example a polished metal like Al, or Cu) then the radiation emitted from the graphite should reflect back and be reabsorbed by the graphite. That seems reasonable, but I'm not an expert in this area. I would think that you need the reflector to be thermally isolated from the graphite with sufficient air, or even vacuum if possible.

 

Or, do you mean to use a reflector to reflect the blackbody radiation back. If so, that may work. Graphite is a very good blackbody, which means it is very efficient at emitting and absorbing radiation. However, if you surround the object with a material that is not a good blackbody (for example a polished metal like Al, or Cu) then the radiation emitted from the graphite should reflect back and be reabsorbed by the graphite. That seems reasonable, but I'm not an expert in this area. I would think that you need the reflector to be thermally isolated from the graphite with sufficient air, or even vacuum if possible.

Steve is correct. There are IR reflective kiln linings available. They are used by painting them onto the refractory material. One brand name is "ITC-100."

 

What do you mean by insulation? Do you mean a thermal insulator? If so, this is what I said in my earlier post. This is the principle of the kiln; to insulate. So, you have to use enough insulation so that the outer skin temperature is low enough to not emit significant blackbody radiation. But, this will be a slower process since you are heating the object and large insulator.

Or, do you mean to use a reflector to reflect the blackbody radiation back. If so, that may work. Graphite is a very good blackbody, which means it is very efficient at emitting and absorbing radiation. However, if you surround the object with a material that is not a good blackbody (for example a polished metal like Al, or Cu) then the radiation emitted from the graphite should reflect back and be reabsorbed by the graphite. That seems reasonable, but I'm not an expert in this area. I would think that you need the reflector to be thermally isolated from the graphite with sufficient air, or even vacuum if possible.

I didn't actually know what a kiln was.
I googled it and found a website that sell some kilns perfect for what I require. Basically high temperature electric ovens, often used in making pottery but ideal for what I require.


Thanks for the advise, truly appreciated.