Author Topic: Induction heater for graphite  (Read 16387 times)

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Offline AF6LJ

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Re: Induction heater for graphite
« Reply #25 on: November 25, 2015, 03:30:20 pm »
The one problem I see with melting graphite is...
Won't it combine with the oxygen in the air and turn to a gas???
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Online mikeselectricstuff

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Re: Induction heater for graphite
« Reply #26 on: November 25, 2015, 04:27:55 pm »
As graphite is very black, how about a laser ?
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Offline calexanian

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Re: Induction heater for graphite
« Reply #27 on: November 25, 2015, 06:23:50 pm »
As graphite is very black, how about a laser ?

Another much easier idea.
Charles Alexanian
Alex-Tronix Control Systems
 

Offline Rachie5272Topic starter

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Re: Induction heater for graphite
« Reply #28 on: November 25, 2015, 08:54:15 pm »
I tested a propane torch last night.  It sort of works, but it's hard to control.  If it stays in one place for too long (half a second is too much), the graphite will get too hot and ruin the reaction.  Oxidation isn't really a problem, since I'm only trying to heat it to 200-300 °C.

I also thought of a laser.  It certainly could work, but there's a safety issue.  Hmm, maybe I can use a CNC laser cutter for testing.
 

Offline T3sl4co1l

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Re: Induction heater for graphite
« Reply #29 on: November 25, 2015, 10:39:39 pm »
Heat lamp and reflectors would work.  Cheaper than lasers.

tim
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Offline calexanian

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Re: Induction heater for graphite
« Reply #30 on: November 26, 2015, 04:20:30 am »
I still like my loop of nichrome wire idea. So easy to control the temp.
Charles Alexanian
Alex-Tronix Control Systems
 

Offline Wolfram

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Re: Induction heater for graphite
« Reply #31 on: November 26, 2015, 09:44:52 am »
The resistivity of graphite is roughly three orders of magnitude as high as that of copper, so the skin depth for any frequency is going to be around 30 times that of copper. The efficiency of induction heating starts to decline rapidly when the workpiece diameter is smaller than four skin depths (due to eddy current cancellation within the workpiece). This suggests that the operating frequency for induction heating should be in the 10 MHz range or above. Since graphite is so much more resistive than copper, going below this frequency won't be disastrous for the electrical coil efficiency (the ratio of the reflected workpiece resistance to the total AC resistance of the coil + workpiece), but the amount of reactive power needed in the tank circuit will go drastically up. Personally I wouldn't go below a couple of MHz in this application.

This suggests that repurposing a metcal soldering station might not be such a bad idea afterall, if it provides enough power for you. I would calculate how much power you need given the thermal mass of graphite at the speed you want to move it, while pessimistically assuming that 50% of your RF power ends up in the workpiece.

For example, assuming you use a 3mm diameter rod and move it at 5mm/sec, you need to heat up pi * ((3 mm)/2)^2 * 5 mm = 35 mm^3 ~ 80 mg of graphite a second. The specific heat of graphite is around 0.72 J/gK, so if you want to raise the temperature by 300 K then you will need 0.72 J/gK * 300 K * 0.08 g/s = 17 J/s = 17 W of power delivered to the graphite. This suggests that a common 50 W metcal station would have enough power, as long as you can get the impedance matching reasonably good. If you need more power, you could use a higher powered RF amplifier to drive it, like those used by radio amateurs.

If you want to go in this direction, I would suggest repeating my calculations as they were done quickly and may contain errors.
 

Offline GNU_Ninja

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Re: Induction heater for graphite
« Reply #32 on: November 26, 2015, 10:09:56 am »
Graphite has quite high resistivity, relativivly seen and  therefore it will be difficult to induce sufficient currents for heating an object made from graphite.

Actually, Graphite is an excellent material for induction heating because of its high resistivity and is often used as a susceptor material to transfer heat (by conduction) to other non conducting materials. You can heat Graphite to temperatures of the order of 3000 C and it's easily machinable. The stuff is widely used in the induction heating industry.
 

Offline T3sl4co1l

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Re: Induction heater for graphite
« Reply #33 on: November 26, 2015, 10:29:19 am »
Actually, Graphite is an excellent material for induction heating because of its high resistivity and is often used as a susceptor material to transfer heat (by conduction) to other non conducting materials. You can heat Graphite to temperatures of the order of 3000 C and it's easily machinable. The stuff is widely used in the induction heating industry.

Yabbut...

Ain't no body makes their susceptors all of 3mm in diameter. ;)

3mm might be a typical wall thickness, but the diameter might be 25 or 50 or 500 mm, which makes things quite a lot easier to heat!

Tim
Seven Transistor Labs, LLC
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Offline GNU_Ninja

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Re: Induction heater for graphite
« Reply #34 on: November 26, 2015, 11:07:56 am »
Actually, Graphite is an excellent material for induction heating because of its high resistivity and is often used as a susceptor material to transfer heat (by conduction) to other non conducting materials. You can heat Graphite to temperatures of the order of 3000 C and it's easily machinable. The stuff is widely used in the induction heating industry.

Yabbut...

Ain't no body makes their susceptors all of 3mm in diameter. ;)

3mm might be a typical wall thickness, but the diameter might be 25 or 50 or 500 mm, which makes things quite a lot easier to heat!

Tim

Depends on the frequency. At 1 MHz the penetration depth for Graphite is on the order of 1 to 2 mm.
 


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