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Designing Molybdenum Heating Element for a Vacuum Furnace

09/11/2017 1:44 PM

I have to design a 8 kW heating element for a Vacuum furnace of size 1Meter X 0.6 meter X 0.6 meter working at 850'C

As no data sheets are available for current carrying capacity of molybdenum wires or strips under vacuum or its resistivity like we get for Nichrome or Kanthal wires it becomes difficult to arrive at a perfect solution without trial and error method . Could any body throw some light on how to proceed on the project and wherefrom can I get relevant data ? Any suggestion or guidance will be welcome

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#1

Re: Designing Molybdenum heating element for a vacuum furnace

09/11/2017 2:17 PM

Have you checked Mat Web? searchable database of material properties

Or, Engineering ToolBox?

Perhaps you can find a friendly engineer who works for an oven or heating element supplier to help you?

This is completely out of my field.

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#2

Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/11/2017 3:36 PM

..."Another electric element type used for high temperature ceramics and glass melting is molybdenum disilicide (MoSi2). These are also rigid elements, typically fabricated in a "U" shape, which is suspended vertically. These elements have a maximum temperature of 3360oF/1850oC, and do not suffer from the aging associated with SiC elements. Their resistance does vary with temperature, however, and SCR power control systems are required. MoSi2 elements are popular in commercial applications, due to high power output and reduced maintenance costs over SiC, but they do represent a higher initial cost. Again, it is recommended that an experienced vendor be involved in the design process."...

http://www.kanthal.com/en/products/furnace-products-and-heating-systems/electric-heating-elements/molybdenum-disilicide-heating-elements/

http://www.kanthal.com/globalassets/kanthal-global/downloads/furnace-products-and-heating-systems/heating-elements/mosi2-heating-elements/s-ka058-b-eng-2012-01.pdf

http://www.kanthal.com/globalassets/kanthal-global/downloads/furnace-products-and-heating-systems/heating-elements/mosi2-heating-elements/s-ka059-b-eng-2012-01.pdf

http://www.kanthal.com/globalassets/kanthal-global/downloads/furnace-products-and-heating-systems/heating-elements/mosi2-heating-elements/s-ka060-b-eng-2012-01.pdf

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#3

Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/11/2017 4:02 PM

1) Are you certain that pure Molybdenum is the material of choice for the heating element? If so, then use the properties available in the CRC Handbook of Chemistry and Physics. I do not see why this material is being used, as there are other choices.

2) Any material you select with exhibit a changing resistivity with temperature, some more than others. Are you seeking a heating element with the lowest possible vapor pressure? Pure metal will not be a good choice, I suspect.

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#4
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/11/2017 10:42 PM

We have several vacuum furnaces of similar and larger sizes. Some of them use carbon blocks as heating elements; the rest (I've been told) use Molybdenum elements. I do NOT know if they are pure Molybdenum or some alloy. Ipsen (http://www.ipsenusa.com/aftermarket-support/parts) should be able to verify.

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#5

Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/11/2017 10:54 PM

Do you know the voltage and current capabilities of the power supply that will control these heaters? These values will determine the total required resistance. If all the heating elements are identical, then simply divide the total resistance by the number of elements (and subtract a fudge factor for the resistance of the leads and contacts), to determine the resistance of each element.

Don't forget that the heaters will be operating at a temperature well above the desired 850° C.

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#10
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 11:15 AM

After doing a bit more research, it has become apparent that the critical design item is indeed the electrical unit that supplies power to the heating elements. It must be capable of slowly bringing the heating elements up to temperature, while being able to provide sufficient voltage and current to continue the heating when the element resistance has increased by a factor of 3-5. Our furnaces all use saturable reactor transformers to produce the high current at low voltage required by the heating elements.

Fortunately for you, your target temperature is relatively low compared to the limits of molybdenum, and you are operating in a vacuum. This gives you flexibility in the design of the heating elements.

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#12
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 11:37 AM

It was my understanding that the resistance decreases with temperature....? So that the current must decrease as the element heats up...

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#13
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 11:56 AM

That would be very unusual for a metal!

From https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/temperature-coefficient-resistance/

Only the bottom two may have negative values.

The resistance of a tungsten lamp filament when operating is around 10 times its cold resistance. That's why incandescent lamps nearly always burn out at turn-on, due to the large initial current causing the magnetic field of the coil to collapse itself.

According to this table, the alpha for Molybdenum is higher than that of tungsten, although of course the molybdenum won't be getting as hot as a lamp filament.

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#19
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/13/2017 2:08 AM

Very great help indeed

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#15
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 1:59 PM

That only takes place in a higher dimension!

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#6

Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 9:02 AM

http://nationalelement.com/wp-content/uploads/2017/03/NEI-MolybdenumTechnicalData-DL-170220-1.pdf

This may give you a place to start.

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#7
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 10:18 AM

national element molybdenum tech data pdf

Here is your link re-done as an actual navigable link.

This literature is extremely clear as to what environment is suitable for working with molybdenum (pure or the suitable alloys) in elemental form, not as compound.

Only works under vacuum conditions. Loss of vacuum while the element is still very hot will result in rapid oxidation. Other gases than oxygen can react vigorously if not violently with molybdenum at very high temperatures. Oxidizing molten salt contact is forbidden, etc.

Good information at the bottom of your .pdf about resistivity (electrical "sensitivity").

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#9
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 10:57 AM

Thanks and welcome to CR4! Thanks also to JS for creating the link.

Another source is https://www.eaglealloys.com/molytechdata/

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#8

Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 10:30 AM

DON'T DO IT!

Molly will form a toxic gas oxide and it will kill you if you use it for an open-air heating element. You can only use molly in a vacuum chamber as a heating element, usually inductively heated.

Stick to Nichrome wire. Lots of specs and easy to use and SAFE.

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#11
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 11:25 AM

The OP did say he's designing a VACUUM furnace. We've been using Molybdenum heating elements in some of our vacuum furnaces for years, with no health problems that I'm aware of.

Clearly, the heaters must not be turned on until the vacuum has reached an appropriate value, and the furnace must not be opened to air until after the heaters have cooled below the reaction temperature threshold for air.

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#14
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 12:08 PM

Oh, please. Only if you're welding it, maybe. I have a chunk of pure molybdenum - have even carried it around in my pocket for days.

It is not toxic unless you get it hot enough to oxidize and give off MO3 fumes.

Even then, it's not that bad. Here is a toxicity study of MO3:

https://ntp.niehs.nih.gov/ntp/htdocs/lt_rpts/tr462.pdf

In the study, death rates for male rats were:

Dose, mg/m3 Death Ratio

======== ========

------ 0 -------- 17/50

----- 10 -------- 10/50

----- 30 -------- 16/50

---- 100 ------- 17/50

In this 2-year study, the death rate was the same for rats exposed to nothing and those exposed to the highest dose.

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#16
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 2:01 PM

"the death rate was the same for rats exposed to nothing..." - so they were starving these poor rats?

I suppose the dead ones allowed the remaining 33 to survive?

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#18
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 7:30 PM

Ha ha! The life expectancy of rats (2 years) could have something to do with it.

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#17
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Re: Designing Molybdenum Heating Element for a Vacuum Furnace

09/12/2017 5:09 PM

You can't weld it anyway. MoO3 as a gas will condense into a solid crystaline fibre. NOT something you want happening in your lungs. I have worked with Molly extensively in HiVac applications and it is not the best material to be dealing with if it is hot and exposed to oxygen.

Furthermore, the use of it as a heating element is limited because of it's tendancy to crack and form hot-spots. When it melts, it does not like to combine with itself to form a puddle like iron and other metals do. It tends to expand the crack and pool away from the break. You cannot make a long lasting heater out of it.

The metal itself is not toxic. It's the trioxide in the gaseous state that is dangerous. Once MoO3 gas enters the lungs it condenses and it will never come back out. The rat study you sited was for only a three years. The problems with MoO3 in the lungs won't surface for many years or even decades, but the effects are the same as miners lung.

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