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I have a steel vessel placed concentrically within a larger steel vessel, resulting in air-filled annulus (air gap) between the two vessel walls (figure below). Due to the existing vessel construction, the only way to heat the inner vessel is to externally heat the larger outer vessel. The air gap makes this extremely inefficient. I would like to fill the annulus with a liquid to help with the heat transfer. I am needing to heat the inner vessel to 400 degrees Fahrenheit. Since the liquid will be open to atmospheric pressure, a liquid that wont boil at 400degF and 1atm pressure is needed. A non-toxic fluid is needed (we don't want to be breathing any harmful fumes). Is it possible to dissolve a particular kind of salt into water to achieve this goal? Is there any other type of liquid to consider?

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    $\begingroup$ Mineral/silicone oil typically used for oil heated baths is rated for approx. 200 to 250 °C (Sigma). $\endgroup$
    – andselisk ♦
    Commented Jun 27, 2022 at 19:19
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    $\begingroup$ Note that for many mineral oils, the flash point is below the boiling point (as an example, one SDS I checked had boiling point of 260-340C, flash point of 146C, or under 300F). Not good for an open container. $\endgroup$
    – Jon Custer
    Commented Jun 27, 2022 at 19:37
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    $\begingroup$ If a solid is allowed, maybe powered iron. Sand is not as good, but no significant safety issues compared to any liquid. $\endgroup$
    – Ed V
    Commented Jun 27, 2022 at 21:33
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    $\begingroup$ Why dom't you fill the gap with metal beads, or metallic powder ? Iron or aluminum ! Heat capacity of metals is much smaller than water, and heat conductivity is higher than air. $\endgroup$
    – Maurice
    Commented Jun 27, 2022 at 21:36
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    $\begingroup$ @Maurice and Ed V (+1), those are excellent ideas! I guess there will still be some air gaps via the porosity. I’ll have to consider their thermal expansions and melting points (don’t want them to become stuck in the annulus). $\endgroup$
    – Armadillo
    Commented Jun 27, 2022 at 21:40

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Mineral oil should be good (aka white oil). Vary similar to motor oil , flashpoints not easy to find but around 450 F. Or, for a much higher flash point- phosphate ester. I don't know what it is but we used it for lube oil in very high temperature applications.

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You can use a heat transfer oil like Mobiltherm 610. They also have some other oils that you can use, if you are able to shield the surface of the oil from the air. If you use a lower grade oil, it can still work, but the oil may slowly decompose.

Dont just buy some generic mineral oil, without checking if it will decompose or catch fire under the conditions you are expecting.

Heat transfer oils can be poured at room temperature, unlike some molten salt baths.

Phillips66Lubricants makes some heat transfer oils, but afaik none that can handle your temperatures in an open system.

Chevron has this line in the data sheet for HTOs: The oil surface in contact with air in open systems should not exceed 107°C (225°F)

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A good starting suggestion is probably a mineral oil. These are often used in small scale labs for heating baths that need to reach temperatures much higher than boiling water. So they should be easily available and their properties are well known.

A possible alternative would be a molten salt mixture. Some of these are available even for low temperature situations but many are fine at higher temperatures (see this, for some examples, though it isn't a review for this sort of application, just an indication that such salts are available). Many molten salts are non-volatile and non-flammable but whether they are better than mineral oil will need some investigation.

Another alternative (though possibly more appropriate for lab-scale not large scale) is to use Armor metal beads. These are small metal beads that can be used as an alternative to liquids in heating baths in laboratories (either to avoid liquid contamination or to reach higher temperatures). They are quite expensive, but (obviously) are not flammable, can reach higher temperatures than many liquids and can be reused indefinitely.

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    $\begingroup$ Aren't molten salts highly corrosive? $\endgroup$
    – toolforger
    Commented Jun 28, 2022 at 15:53
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    $\begingroup$ @toolforger not universally. There is enough knowledge in the field to choose one that will work with the materials you need. $\endgroup$
    – matt_black
    Commented Jun 30, 2022 at 7:54
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This seems a very bad idea to me. Please tell us how you intend to heat the outer tank and some idea of actual dimensions; then consult a professional chemical engineer with experience in heating. There are industrial grade heating pads or sheets that reach 250C that can fit in very small spaces. Investigate also the possibility of inductive or infrared heating. This will involve some expense but if you can't use good, appropriate, equipment you should reconsider doing this.

One suggestion would be to fill the space with aluminized Mylar helium balloons; that will give more heat conductivity than air and they should not melt [mp~240C].

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    $\begingroup$ "Please tell us how you intend to heat the outer tank and some idea of actual dimensions" — this should be a comment. Please ask OP prior to posting an answer if something is unclear to you. Note that unit symbols should be separated from the numbers with the non-breaking space, and C is the symbol for coulombs, not degrees Celsius. $\endgroup$
    – andselisk ♦
    Commented Jun 28, 2022 at 7:54
  • $\begingroup$ I purposely put the question in an answer because I was getting perturbed by suggestions that could possibly lead to a large scale industrial accident. $\endgroup$
    – jimchmst
    Commented Jun 29, 2022 at 4:17

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