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Custom laboratory equipment

  1. Jun 16, 2011 #1
    I am in need of constructing a heater probe for a reaction chamber.

    I need to put out a maxium of 500 Watts continuous from the 5 cm end of a probe of either 1/4" or 5/16" in diameter. I am constrained to either of these standard sizes.

    My first inclinations are to wind nichrome wire around a ceramic mandrel, and sleve it with fibre glass shoe string woven insulation in a probe body of stainless steel. I need to be capable of withstanding 50 Bar of external pressure at the temperatures generated. The exterior temperature could be as high as 600 C.

    Is this design even possible? Will I melt the wire or glass trying to put out this much heat?
     
    Last edited: Jun 16, 2011
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  3. Jun 16, 2011 #2

    SpectraCat

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    I think that design is reasonable, but like you I would worry about the fiberglass melting if you get up to 600 C. I have made similar devices for gas pyrolysis by winding tantalum or tungsten wire around a ceramic or quartz tube. We then cover the windings with a ceramic cement to both hold the wire in place, and make the heating more uniform. I am not sure what the highest temperatures we achieved with such sources were .. I don't think we went up as high as 600 C. The kinds of pyrolysis sources I describe were first demonstrated by Peter Chen, and are sometimes called "Chen sources" or "Chen nozzles". I don't have the references for his papers handy, but I remember a paper in Review of Scientific Instruments from the early 90's from his group that was quite useful.

    I should also point out that these Chen-type sources are meant for use in vacuum chambers ... I couldn't tell from your post if you meant that the hot element will be in contact with vapor at 50 bar. If so, I think you will have to make the source REALLY robust, and I would definitely think you would need to protect your wire from exposure to the atmosphere .. perhaps by encasing it in ceramic cement as I described above. If you leave it unprotected, I would guess you will just ablate the metal wire rather quickly at those pressures.
     
  4. Jun 17, 2011 #3
    Wow. You are a weath of information. I'm happy to have your reply. I have multiple concerns: good thermal conductivity to the stainless steel casing and the ability of the stainless steel to withstand external pressures of up to 50 atmospheres, and just all around material availability in dimensions that will work together.

    The heater itself is at atmospheric pressure, so the forces on the steel tube are compressive.

    It's enlightening to know that I could refer to this as a pyrolysis source to help aid me in internet searches.

    I've been worrying the problem of getting the heat energy through the fiberglass sock without requiring the resistive element to be so hot that it melts.

    So just now I have been searching the web for something that is liquid up to temperatures that would deevelop near the surface of the resistive element, a good dielectric, and also a good thermal conductor---but without success. What is a ceramic cement?
     
    Last edited: Jun 17, 2011
  5. Jun 17, 2011 #4

    SpectraCat

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    Well, as I said, pyrolysis sources are usually designed for use in vacuum .. so I wouldn't expect the same design to work in your application necessarily, but at least it might help you brainstorm.

    Why do you need the sock in the first place? That wasn't clear to me earlier ... I assumed it was to protect the wire and even out the heating, but perhaps that's not correct? Also, what is the purpose of the heater itself? Why does it need to be in a stainless steel housing?

    A ceramic cement is just a ceramic paste ... the one we had came in powder form (sorry by I forget the brand & vendor). We simply made a slurry in water, and applied it with a little metal spatula over the top of the wire windings. It hardened (overnight I think), and was fairly strong, although you could still flake it away if you tried hard enough. It seemed to harden further after we used the source a few times .. which would be logical, since that should drive out all of the additional water.
     
  6. Jun 17, 2011 #5

    AlephZero

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    There are high thermal conductivity ceramic-based pastes avaiable, for example http://www.aremco.com/news-item/pyro-putty%c2%ae-2400-high-temp-metal-sealer/ [Broken]

    But there might be an issue with its electrical conductivity for your application, since I guess your nichrome wire is not insulated.

    Aremco have a large range of products - their technical department might have some good ideas.
     
    Last edited by a moderator: May 5, 2017
  7. Jun 17, 2011 #6

    SpectraCat

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    Yes .. Aremco was the name of the manufacturer of the paste we used, although I cannot remember the brand name. We did not have an issue with conductivity of the paste as far as I know, meaning that the tip of the pyrolysis source was easily hot enough for our needs, which was creation of alkyl radicals.
     
    Last edited by a moderator: May 5, 2017
  8. Jun 20, 2011 #7
    The housing doesn't have to be stainless steel. It just seemed like the right material at the time, though I do recall that stainless steel has superior high temperature strength over other common alloys one can purchase in tube form. The strength of both 304 and 316 stainless steel flatten out at about 100 MPa minimum for temperatures over 400C.

    The glass sock is only my solution to insulate the heating wire from the metal tubing. If there is a better way, I'm all for it. Searching around, I can buy 36" of 1/32" thick glass sock for 32 dollars from McMaster Carr that is rated to one MIL spec or another. That's a bit pricey. I need only about 3 inches of it per heater.

    Thanks to the both of you, AlephZero included, for this little jewel of information. I was unaware there where high temperature inorganic materials I could use that begin life in liquid form, or paste form. Apparently the secret to Armeco's materials is a hydraulic cement that can be used up to 1200 F (650 C). The cement can be purchased separately and mixed with compatible powders of your choice. Even mixed with a small percent of metal powder, together with ZnO it could still remain an electrical insulator and have fairly good thermal conductivity.

    Using this, I may be able to form a heating elment without the metal tubing and differential 50 Bar (38000 Torr.) external pressure concerns.

    My main concern in such experimentation is that thermal expantion of the heating element will fracture the ceramic. Any ideas or experience in this?

    Internal to the pressure vessel is a highly reducing atmosphere. Would this interact with adversely with ceramic materials?
     
    Last edited: Jun 20, 2011
  9. Jun 20, 2011 #8

    SpectraCat

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    I think I would probably try to form a ceramic layer (using the paste) over your windings, rather than using a fiberglass sleeve. That will give you better robustness with respect to temperature, and should still be electrically insulating.

    Well, all I can say is that we did not have any issues with fracturing with our pyrolysis sources. However, we were using very thin wire (0.5 mm I think), so that may have minimized the effect.

    That is a possibility, since the ceramics are going to have a high proportion of metal oxides. I don't have much experience with such situations ... the strongest reducing agent I have used is H2 .. I know that only reacts with the surface layers of typical oxides (TiO2, SiO2, MgO, ZnO) at temperatures up to 500ºC. If your reducing agent is strong enough, it may have a more extensive reaction with the material, especially at high temperature. I would probably contact Aremco and ask if they have any recommendations for high-T use in reducing atmospheres.
     
  10. Jun 24, 2011 #9
    Thank you again for your critical help. I got around to looking up Chen sources. This lead to a supplier, Cotronkics.com, of both two part ceramic cements and one part hydrolic cements good at temperatures exceeding 600 C. As often happens, I'm in danger of over-engineering a problem, though mismatched thermal expansion coefficients may still be a problem.

    I can experiment to see if thermal expansion of the metalic elements are a problem. If so, I am considering that alternately winding glass fiber thread whether with a tungsten or NiCr heating element could solve the problem.


    My reducing environment is H2 at temperatures as high as 600C at pressures that could be as high as 30 atm.
     
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