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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Fri Oct 05, 2012 7:03 pm 
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You could try hiding it in the garage, maybe behind some other tool.

My favorite pitch is to tell her that I'm getting the new equipment for her, so that I can make things for her. She must believe me since she invariably says, "Right". I'm kind of stupid about facial expressions, so I'm assuming that this means that she approves. But then again maybe not.


No more room in the garages. :D


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Ted Themelis .Have you met the man??

Only online, briefly, years ago.

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I am NOT referring to "Radio frequency Generating Induction Heating" so this does NOT apply:

"All induction heaters are essentially very powerful radio transmitters which are then connected to a coil."
This will give you a very simple understanding:
http://en.wikipedia.org/wiki/Induction_heating
http://www.youtube.com/watch?v=I_gFJ71Kp8M


This is exactly what I was referring to. The copper water cooled coils (the water cooling to prevent the coils themselves from overheating from ohmic losses) are attached to what amounts to a radio transmitter. If you attached them to an antenna they would transmit perfectly. The item put in the coil essentially short circuits the RF energy and converts it to heat.

The second link calls those units portable. As you say portable is relative. They refer to IGBT transistors which means they don't have water cooled metal transmitting tubes like these sorts of machinery used to have though it is quite likely that the transistors themselves are mounted to a water cooled heat sink,

One method that could be used would be to encase a steel pipe inside a ceramic coating and putting your crucible inside that. The ceramic coating would prevent the steel pipe from burning up. The steel would also hog all the RF energy.

The smallest unit mentioned in the chinese video is a 12,000 watter. It is going to draw more than that from its power cord. You will need a big generator. It may be more portable than a big industrial unit but you are going to need a truck. It is not going to go into your carry on.

Notice how fast it heats the gear and the golf club head to brilliant yellow heat. That's a lot of watts.

I think the advice to get some engineering support is good advice. All those coils need to be designed for the particular job they are not very generic.

I am responsible for the care and feeding of a couple of ICP instruments. All but one are under software control and they are all set up so that if there is the slightest glitch they won't operate. This is because all that kilowattage can melt that coil very close to instantly. The voltages involved can also be rather high. You need to make sure that all the safety aspects are adequately covered so nobody gets burned or electrocuted.

Using this kind of heating it should be quite possible to have very fine control of the power input and heating rate. You have not really said what it is you are trying to do. You may need more than one temp measurement instrument to cover the range you want.


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Sat Oct 06, 2012 12:18 am 
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earthling..

Thanks for the input..

Once the power supply is defined and operative the need for future electrical/electronic engineering is limited to repairs or changes in operational parameters...This is not something I will have to study in depth and understand fully prior to operating such equipment successfully...This is why we specialize...None of us can know everything in our relatively short lifetimes...

In any engineering project there are always hurdles to overcome...The inability to heat Corundum threw induction heating is a very simple hurdle but I'm thankful that this was brought to my attention...I was not aware...So this posting has already worked to some degree and I'm thankful for that...

The question is are other ceramics responsive to induction heating and if so which ones??...Graphite is used quite heavily in this process but due to it's Oxygen scavenging at high heat is not applicable...Does the Graphite itself heat from the Induction process or just the metals the that crucible contains??

The necessary equipment is most likely already on the market...It's than just a mater of matching my process with the right equipment and the project is on to it's next real hurdle...Accurate temperature control -w- slow heating/cooling...


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Sat Oct 06, 2012 1:32 am 
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G4Lab...

Maybe I'm confused...I was not aware that High Frequency AC was also Radio Frequency??...But I'm not very well versed in the electrical/electronic sciences...

No the generator will NOT fit in my luggage...Even a 100# generator head would have to be shipped and there are other items that need to be shipped as well such as phosphoric acid and other hazardous chemicals used in the gem treating processes...

"One method that could be used would be to encase a steel pipe inside a ceramic coating and putting your crucible inside that."

Heating at 1600 C would be 2,611 F and almost to the melting point of steel...1800 C would well beyond it's capacity for heat...Other metals such as tungsten can take much higher heat but will powder if exposed to Oxygen so need to be within a protective ceramic coating...This is why I'm targeting a ceramic that's responsive to Induction heating...Such ceramic coatings on metals are prone to failures at high temperatures...A difference in Coefficient of Thermal Linear Expansion may cause separation/failure...

"The smallest unit mentioned in the chinese video is a 12,000 watter."

My Inverter Welding machine was only 10,000 watts...LoL!!...I said 10,000 amps...My question is are the higher watts used to make the heating process faster or to reach a specific temperature???...Could the same temperatures be reached with lower wattage over time??

"You have not really said what it is you are trying to do."

Parameters: Heat treating Ruby/Sapphire..

Basic heating ranges depending upon the process..
1250/1600/1800 C and variables there of depending upon results...

Time: 20-150 hrs dependent upon temp -n- process...

Temp control: Accurate slow heating threw specific ranges of 150-250 C/hr and accurate cooling at the rate of 2-3 C/hr to a specific end process temp...

Internal atmosphere: Oxygen usually but also Nitrogen/CO/Argon and others depending upon the process...

Pressure: The ability to establish a relatively short duration vacuum followed by Oxygen pressure of perhaps 3-4 Bar at lower temps...This is to facilitate the penetration of fluxes into surface reaching fractures and increase the Oxygen diffusion rate as per speculations...

"You may need more than one temp measurement instrument to cover the range you want."

By burring the gem rough in Alumina powder I can minimize the thermal shock...This would allow for quicker external temp fluctuation without adversely affecting the gems being treated as the heat will migrate slowly...I suspect I will need both internal heat sensing as well as external with some yet unknown relationship between the two...


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Sun Oct 07, 2012 1:11 am 
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I have managed to find a answer to one of my questions:

"Does the Graphite itself heat from the Induction process or just the metals the that crucible contains??"

http://www.youtube.com/watch?v=fXsIbJG- ... playnext=2

Here a Graphite Rod is heated white hot...

So if other ceramics such as Chromite...Chromium Oxide don't respond to the induction process than I can use a Graphite tube as a casing which than heats everything contained within...The Graphite can withstand any potential temperature required with repeated applications and is readily available for replacement...

I have another question I would like answered if possible:

"My question is are the higher watts used to make the heating process faster or to reach specific temperatures???...Could the same temperatures be reached with lower wattage over time??"

Slower heating is a desirable application and lower wattage would be more cost effective/smaller more portable equipment...


But I also stumbled upon this:

http://www.youtube.com/watch?v=aLwaPP9cxT4

Frozen H2O??...

Water does conduct electricity...That is a known...But I have to admit to being rather surprised by this and wonder why??


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Sun Oct 07, 2012 11:36 pm 
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Quote:
"Does the Graphite itself heat from the Induction process or just the metals the that crucible contains??"
http://www.youtube.com/watch?v=fXsIbJG- ... playnext=2
Here a Graphite Rod is heated white hot...

But graphite requires a reducing atmosphere. If you flood it with Oxygen it will simply burn off as CO2 and probably instantaneously. It may even reduce the sapphire to metallic aluminum. I don't know offhand which is more electronegative. You may be able to look that up in a chart of electronegativity.
Your link did not transcribe correctly and does not work. But if there is metal you can be sure it and not the crucible is grabbing the energy!

Edit: Yes if you heat the corundum in a graphite crucible the oxygen will transfer to the graphite making CO2 and leaving metallic aluminum.
See the table of electronegativites
http://www.green-planet-solar-energy.co ... alues.html


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So if other ceramics such as Chromite...Chromium Oxide don't respond to the induction process than I can use a Graphite tube as a casing which than heats everything contained within...The Graphite can withstand any potential temperature required with repeated applications and is readily available for replacement...

Graphite is used for all kinds of crucibles and hight temp furnace furniture. Other ceramics and refractories are available too. I think Judith Osmer used to use platinum crucibles when she did flux grown rubies. And Kanthal heaters too I think.

I think a silicon carbide crucible might be better. It probably will catch the RF (High frequency AC and RF are synonymous) and can resist very high temps and maybe oxygen too


Quote:
I have another question I would like answered if possible:
"My question is are the higher watts used to make the heating process faster or to reach specific temperatures???...Could the same temperatures be reached with lower wattage over time??"

When you heat things they lose heat. the more watts you put in the faster they get hot. The hotter they get the more watts they need to maintain thermal equilibrium with the environment. So more watts equals faster heating and also higher ultimate temp.
(say you had a one thousand watt heater. think of a 1200 watt electric heater you can buy for nothing these days. The Nichrome wire heating element or quartz heating element is connected directly across the AC line and is engineered to draw the correct number of watts. The all stop at dull orange red heat because the rate of energy input equals the rate of heat energy loss (ie heating the room which is what they are designed to do)
The better you can insulate your chamber the higher temp you can achieve with a given number of watts. That is why the furnaces the aforementioned Judith Osmer used had very thick walls of foamed ceramic insulation and probably most of the systems that Ted Themelis engineers probably do too.(haven't been to his website in a long time)

Quote:
Slower heating is a desirable application and lower wattage would be more cost effective/smaller more portable equipment...

There is some minimum wattage you will have to exceed to reach the temp require in the volume required. Each different type of solid matter in addition to whether or not it can snag the energy from the coils, also has a specific heat which is how many calories it takes to raise it one degree. (choose your units)


Quote:
But I also stumbled upon this:
http://www.youtube.com/watch?v=aLwaPP9cxT4
Frozen H2O??...
Water does conduct electricity...That is a known...But I have to admit to being rather surprised by this and wonder why??

This strikes me at best as a cool parlor trick and at worst an advertising fraud. Perhaps the ice had been cooled down to liquid nitrogen temperature first so that the interior could get white. Maybe the part actually white was a small part in the center maybe with some chemical additives. I want to know why the ice was not melting faster , nor cracking nor exploding. It may be they have been working on this and tuning the parameters of the system to get it just right. You have to do that with all induction heating applications. Notice the specialized coil.

I am not at all an expert on this but I am sure Ted Themelis has looked at whether or not induction methods would work for gem heating. It is after all the exact subject to which he has devoted his life. What do you suppose you will discover that he does not already know???

There is nothing whatsoever magic about induction heating. It can do some tricks that are harder to do with regular heating because it does allow very high energy densities.
So in the analytical instruments that I take care of a sprayer makes a mist of the solutions to be analyzed. These are carried in a stream of Argon gas into a set of ICP coils where the gas turns into a plasma of such high temperature that all the molecules are broken down into their component atoms.(this alone is like something out of a sci fi movie. Straight up Matter Disruption. I love it.!!!) The plasma then makes these atoms emit their characteristic light wavelength (ICP OES Inductively coupled plasma optical emission spectrosopy) to then be analysed OR sends the plasma into a mass spectrometer where the atoms can be sorted and detected according to atomic weight (ICP MS Inductively couple plasma Mass Spectroscopy) So it can get real hot like the surface of the sun, but very low mass and density. Still surrounded by very dark glass because there is hellacious UV radiation emitted that would give you sunburn FAST. The chambers are quartz glass and synthetic sapphire. There is laminar flow gas to keep the plasma well away from the chamber wall. Sort of a gaseous skull.

Similar techniques are also used in CVD diamond growth. They are also related to microwave oven technology too but microwave don't use coils. They are higher frequency (shorter wavelength why they are called microwaves) and heat the water in food directly. But they can be used to run temps way up if there is a suitable containment vessel. Have you considered a microwave powered furnace? Might be do-able.


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Mon Oct 08, 2012 2:48 am 
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G4Lab..

Thanks for the input..

I was not very clear on this:

" I can use a Graphite tube as a casing which than heats everything contained within"

I was referring to the graphite as a "Casing Pipe" while a ceramic tube containing the Oxygen atmosphere and corundum was inserted inside and vented above...After further review graphite would not be a suitable material...

I had stated that graphite is an Oxygen scavenger and than failed to apply that to the graphite casing in air (20% Oxygen)...Yes it would form CO2 while decomposing...Prof that 2 heads are better than one (*_~)

"Yes if you heat the corundum in a graphite crucible the oxygen will transfer to the graphite making CO2 and leaving metallic aluminum."

We use Graphite but not with the Oxygen atmosphere for pale Sapphire in attempts to darken them as the Oxygen is removed from the rough...The Coke ovens form reducing atmospheres as well...Reportedly the Oxygen/LPG Furnace is also a reducing atmosphere even though excess Oxygen is being fed into it???

"There is some minimum wattage you will have to exceed to reach the temp require in the volume required."

I have found that the diameter of the coils as well as the number of loops and distance between the coils and work all have an affect upon the efficiency of the induction process...

Most often it appears only a few loops are employed and the heat is concentrated in a narrow area of the work which heats very quickly...My application would require a few more loops so as to heat a much broader area of the casing...Somewhere in the realm of 8-10" so I could heat 2 stacked 4" tall crucibles at once...The Diameter will be in the realm of 4-5" OD leaving me 3.125-3.25" ID for standard sized corundum crucibles...

Having an outer casing which is heated by the induction process actually serves well in this application as the heat will migrate more slowly and prolley more evenly...

The exterior insulation is also needed to protect the equipment form scorching...Most of what I have been able to view is configured in this manner and would be subject to severe heat damage me thinks!!

http://www.amazon.com/dp/B0085QIHFQ/ref ... nkCode=asn

http://www.alibaba.com/product-gs/32555 ... chine.html

http://sell.lulusoso.com/selling-leads/ ... chine.html

"What do you suppose you will discover that he does not already know???"

Nobody knows everything and there's always new ideas/realms/aplications left yet undiscovered...

Speculation: Prior to investigating the Induction Process I was working on the concept of "Iron Precipitation" and how to influence it in super heated electromagnetic fields...Problems arise in the application of the process while using conventional convection ovens due to the high heat with high Oxygen atmospheres and metal windings that are prone to damage...Than even if the windings are protected by ceramic glazes the leads coming in or out of the heat chamber are possibly exposed...Another problem is that the ceramic coatings may dissolve the windings during application due to the fluxes being used??...Attempting to place windings outside the heating chamber would subject the interior heating elements to these magnetic fields as well and possibly have adverse effects upon the elements...Also attempting to sustain vacuum/pressure atmospheres becomes inhibited as well...

The tube furnace used in class weighs several hundred pounds/consumes energy in excess of what the Thai power company was supplying/Requires it's own room with water cooling/Periodic replacement of very expensive Mo heating elements/$30K...NOT at all potable!!

The exterior heating magnetic fields produced in the induction heating process solve many of these previous issues...Than too a system of windings could be installed outside the copper heating coils without adverse effects on nonmagnetic copper induction tubes???...The induced polar magnificent filed could be used intermittently between Induction heating cycles during soak/cooling periods???

The Induction generated magnetic fields can stir molten metals and even cause levitation...Will this have a magnetic affect upon (Fe2O3) Iron Oxide held in solution or in latices structure when the bonds become weakened threw heat???

Looking at a Fe2O3-Al2O3 Phase Diagram the lower transformation temperature for Iron in Corundum is around 1325 C while the upper transformation temperature is around 1425 C...The liquid line is at 1700 C...

http://www.springerlink.com/content/g63 ... .html#Fig1

Within the rage of Iron Transformation in Corundum the Iron Bonds are weakened while (Ti) Titanium/(Cr) Chromium transformation occurs at higher levels...Within these ranges are where (Fe) iron is most soluble in corundum but also most easily removed...

Will (Ti) Titanium/(Cr) Chromium held in solution migrate to voids (Holes) in latices structure vacated by (Fe) iron???...If so modification of color and clarity may occur...

We all have our pet projects that we are willing to waste tons of time and our $$$$ on while trying to reinvent the wheel as my friends say...But I can't envision myself sitting on a porch with a dog watching the children play...@ 62 there's so much yet to accomplish and new places to explore...

I'm also researching/experimenting with fluxes and a modified application thereof...High relief Flux fingerprints are partially dissolved fluxes and gases trapped during the healing process...How do I achieve a more natural appearing fingerprint???


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Mon Oct 08, 2012 6:36 am 
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G4Lab..

" Have you considered a microwave powered furnace? Might be do-able."

Not really because of the fits they throw when metals are encountered...My oven burned up...Thus haven't even looked into it...Don't know how you would focus the energy over a wide enough area??...Seems rather pin-pointed??

Than the fact that the Induction Field Generators are so wide spread now and relatively low priced...I projected a cost of around $4K...Not sure what microwave equipment can be had besides small ovens...

$4K is also the price for a small chamber (4" Cube) 1600 C 220V Convection Furnace -w- Gas Injection Ports weighing around 120#s...Must be shipped...

The electric costs would double due to the small heat cavity because 2 successive operations would have to be done over time instead of one to process the same Kilo...I have 5.5 Kilo at present to process...Prolley 1 week per process??

The Oxygen used would more than double with continuous flow rather than maintaining 3-4 Bar Oxygen pressure while being vented periodically to purge contaminated gases...With continuous flow the corundum will scavenge what it wants at ambient pressure but under 3-4 Bar Oxygen should become forced into solution under heat at some unknown accelerated rate??

There's a German manufactured oven on the market that was discussed in heat treating class that stirred our interest and can maintain 150 Bar or a Vacuum @ 1800 C...$30K as of 2010...If/When my induction process proves worthy that will be my next application...Pressure Vessels and Heat Exchangers are were a large part of my former life prior to retirement...


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Mon Oct 08, 2012 8:20 pm 
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I wish you good luck on your project. Let us know how it turns out. I don't think I have anything else to add. I am not an expert on this just someone who has encountered it a few times along the way. You need an applications engineer from the company that builds the Induction gear to assist you.


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Tue Oct 09, 2012 9:31 am 
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That's where I'm at now and still a problem until I'm actually back in the States...They seem hesitant to reply to someone outside their area of operations...That or they just get way too much inquiry traffic and don't bother to reply to it all???..Maybe no tech personnel involved with these inquiries??...Just sales people??

But I will keep trying...

One note: If a Graphite Casing Pipe is viewed as a consumable just as the crucibles are than it may be applicable depending upon how many cycles of life it has in the application...

But thanks for the input..It helps...

Also this is interesting:

Cromox Ceramics Al2O3 + 0.5% Cr2O3:

http://www.industrycortex.com/datasheet ... mium-oxide

It's dealing with electron beam not induction but the melting point should be elevated and this may effect changes in how the Al2O3 responds to an induction field??...At present there's allot of research involved with this ceramic and with additions of Zirconium/Silver oxides as well...

http://www.bnl.gov/isd/documents/19353.pdf


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Tue Oct 09, 2012 3:58 pm 
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That's where I'm at now and still a problem until I'm actually back in the States...They seem hesitant to reply to someone outside their area of operations...That or they just get way too much inquiry traffic and don't bother to reply to it all???..Maybe no tech personnel involved with these inquiries??...Just sales people??


the reason they are not replying is that they dont want do to consulting work for free.

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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Wed Oct 10, 2012 3:26 am 
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earthling...

Good point...

A manufacture/dealer will prolley be willing if tailor fit their equipment for my application on the basis of a confirmed sale...That communication will most likely have to take place in their office...

I'm very hesitant to go threw any design applications here in Asia regardless of how inexpensive their Chinese manufactured equipment is on the market...In general some will say anything as long as they make a sell...That general philosophy is way too widespread here...

I may be willing to purchase a 2nd unit if needed from them after having this operation already developed but not willing to throw $$$$ -n- Hours away after bad!!!


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Wed Oct 10, 2012 9:41 am 
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As I continue to research and learn more: Found this...

Quote:

Although induction heating is normally used with metals or other conductive materials, plastics and other non-conductive materials can often be heated very effectively by using a conductive metal susceptora material which is heated as a result of its presence in the induction field, then passes its heat to the workpiece. The susceptor is used to transfer heat to the target part through conduction or radiation.

Graphite is frequently used as a susceptor because it offers machinability, high resistivity (ideal for induction), and a temperature range up to 3000°C (5430°F). Susceptors can also be made from Molybdenum, Silicon Carbide, Stainless Steel, Niobium, Aluminum, and other conductive materials. The susceptor can be made in the form of a crucible, disk, tube, a layer in the material, or whatever form best suites the application. Some example applications include: crucible melting of non conductive materials like glass, heating fluids in a pipe, plastic sealing, thermoplastic composite manufacture, cap sealing, and CVD processes in semiconductor manufacturing.


Quote:
Modern induction heating provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time. Solid state systems are capable of heating very small areas within precise production tolerances, without disturbing individual metallurgical characteristics.

When heating a susceptor, solid state RF induction power supplies provide accuracy and speed. During heating, temperature ramping can be controlled by using optical pyrometers or thermocouples to form a closed loop system. Uniform surface temperatures can be achieved with careful coil design.

Typical RF power supplies for susceptor heating range from 1 to 20kW, depending on the parts and application requirements.


Exactly what I need to do...Graphite is a good "Suscetor" well above the 1800 C that I need to achieve...An "optical pyrometer" can be used to control the surface heating and cooling cycles accurately along with a "Eurotherm Controller" for programed repeatable cycles...A secondary Pyrometer can be used to track the internal temps...

Now I just need someone who can put it all together in a workable package for me and take my $$$$ doing so!!!

Quote:
The supply frequency and the skin effect which characterize the dispersion of the induced current throughout the part: the higher the frequency, the closer to the surface the induced currents will concentrate. This fundamental notion is determined by the penetration depth, also referred to as skin thickness. - simpfied formulae do = 503.3 (ρ/μr f)1/2

Maxwell Equation Electro magnetism fundamental Laws

rot H =j +∂D/∂t
rot E= -∂B/∂t ;
D=εE
B=μH


Yikes!!...Looks like I have to go back to school...A little (*_~)


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Wed Oct 10, 2012 10:17 am 
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Susceptor is the correct term for the steel tube encased in ceramic which I referred too.

An optical pyrometer will only tell you the temp on a radiating surface and there is nothing that I know of that can measure at 1800 C otherwise.


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Wed Oct 10, 2012 12:32 pm 
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G4Lab...

The oven used in our heat treating class uses an analog Pyrometer located inside the Tube Oven and hooked to a Eurothem Controller...I will have to ask about the make...Not sure the location either...Low or high in the tube...

1800 C is a frequently used temp for diffusion processes but this can also take place at lower temps over longer periods of time...It basically represent the Max Temp for gem heating equipment designs but encounters far more problems and cost in a convection oven above 1600 C...

1600 C is a temp suited for a wider range of heat treatments and more frequently used...Most of my procedures will likely be 1625 C or below for both the Borax Flux (750 C Melt) and my new flux (1575 C Melt)....


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 Post subject: Re: Need Technical Data For Induction Heating Process
PostPosted: Wed Oct 10, 2012 1:26 pm 
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http://www.inductionatmospheres.com/app ... phite.html

This site gives me some basic parameters:

7.5 kW induction power supply

Operating Frequency 123 kHz

7 turn 6" tall helical inductor with a minimum inside diameter of 4”

1200 C achieved in 7 min
-----------------------------------------------------------

Now I prolley need a 12 turn 10" tall helical inductor with an outside diameter of 4.5" and ID 3.25" -w-.625" wall thickness..

Min 1650 C operating Temp with heat up over a 6-7 hr span...

If this can be maintained with a 6 KW Induction Power Supply than I'm in business...

But this needs to be evaluated: Synthetic Graphite crucible 4.2" D X 6.5" tall -w- .4" wall thickness

"heated to a surface temperature of 1500°C, while at the same time the interior temperature rose to 2700°C in equilibrium"...Yikes!!!...My Ruby has melted!!!

Quote:
Abstract

It is shown that in the induction heating of a long cylinder of sufficient wall thickness, the temperature difference between the inside and outside wall, for a given power input, is inversely proportional to the square root of the frequency. This can be most easily observed in materials of low thermal conductivity, and is here applied to pyrolytic graphite, which is a good thermal insulator normal to the surface on which it is deposited. At outer temperatures of 1000–1500°C, the temperature difference across a pyrolytic graphite wall is negligible at 500 kc/s, but is remarkably large at frequencies below 10 kc/s. For example, using about 25 kW of power at 3·6 kc/s, a pyrolytic graphite crucible 105 mm dia. and 160 mm long, with about 10 mm total wall thickness, was heated to a surface temperature of 1500°C, while at the same time the interior temperature rose to 2700°C in equilibrium. Using a pyrolytic graphite susceptor in this way, sizable systems can be heated to high temperatures without any additional thermal insulation. The larger the diameter of the susceptor, the lower the frequency which can be used for efficient coupling, and thus the larger the temperature difference which can be established across the susceptor wall. Furthermore, when used as radiation screening, thermal shields of slotted pyrolytic graphite are up to 100 per cent more effective at frequencies below 10 kc/s than at radio frequencies.


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