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Element Density Reduction and Decreaser

edited October 2016 in General

I have been thinking about this for a while so I thought I would just throw it out there. As Cartman says,"feel free to kick me in the nuts" if you think it wont work. Please don't say it wont work without some back up.

I have some solid 2400W heating elements. They are about 200mm long and 12mm in diameter. A solid little stainless steel rod that gets bloody hot.

They get so hot they are not really usable for anything but heating water. But then I thought the heat is distributed over that tiny area. What if I put that element inside a stainless steel pipe of either 51mm or 38mm or 25mm sealed on one end and filled it with a suitable medium to get the heat to the surface of the pipe and evenly distributed.

Here are the calculations.

Surface area of a cylinder is A=2πrh+2πr2

Element = 77cm2 = 2400/77 = 31W/cm2

51mm pipe X 270mm = 473cm2 surface area of pipe = 2400W/473cm2 = 5.07W/cm2 @ 2400W.

38mm pipe x 270mm = 345cm2 = 6.95W/cm2 @ 2400W.

25mm pipe x 270mm = 221cm2 = 10.86W/cm2 @ 2400W.

By rights a value of 5.07watts per square cm2 is not really that hot but immersed in a wash would still deliver the full 2400 watts to the wash but not burn anything/grain. Am I right? Or have I missed something here?

Comments

  • An inside-out bain marie? Just be careful it isn't sealed, obviously, or it might pop with expansion.
    I didn't look at the math but if you have a heap why don't you just use a few and turn them down.
    Just don't run them over 50%.

    You could just wire them up in series too?
    Two of them would make 1200W and double the surface area.
    1200/(77*2)=7.8W/cm²

    Assuming this is right??

    @Mickiboi said: Surface area of a cylinder is A=2πrh+2πr2

    Is it a fold back? Is h the total length? The '+2πr2' probably isn't there their either, certainly not 2 of them.

  • edited October 2016

    What would be ideal would be to find a suitable packing material that didn't require a liquid as the medium.

    With that small area, a liquid doesn't make sense at all (while it makes perfect sense in a tank jacket).

    Why not small stainless ball bearings? Stainless outer tube, pack it full of BBs - no worries about expansion (some worries), but no pressure worries, or coking, etc. Maybe a mix of BBs and sand?

    Just throwing out crazy ideas. The trick, I suppose, is getting the heat transfer good enough so you aren't just vaporizing the element.

  • I can't use a heap of them I only have 2 element holes in my little milk can. So I need to use 2 at almost full power.

    The elements are out of an outside switchboard. They were used to stop condensation forming inside the board. They are fully sealed and 100% water proof. h is the total length.

    I double checked my math, I'm getting old and occasionally miss things but it does seem right to me. I understand what you mean by +2πr2. I did the first calculation using +πr2 but the way I was going to make it would put most of the mounting end of the element in the wash anyway so I did the calculation using a full cylinder.

    I'm going to try it anyway using various mediums inside. Dry fine powdered sand, extremely high temperature silicone based oil and whatever else I can think of. If it explodes its going to explode in the wash. Nothing ventured nothing gained. If it doesn't work, it doesn't work.

  • edited October 2016

    Thanks @grim. The elements wont vaporise. They are designed to run in air at full power. Never thought of the ball bearing/sand mix,excellent idea. That's on the list to try now.

  • You say they are solid? You might be able to TIG some fins on too but I think sand is probably the way forward.
    It sounds like a lot of stuffing around when elements a relatively cheap. You could just use them in a HLT.

  • The elements weren't cheap. They were left over from a switchboard build and from memory they were around $300 each. I have no idea of the construction inside so I'm a bit unsure of the TIG welding idea. Good idea none the less so I will find out what goes on inside them.

    I can see what you are saying about the stuffing around but I'm sure there isn't an element suitable for al grain mash. Is there?

  • While you're welding, why not wild them directly to the tank?

  • I've seen flange mount tubular tank heaters that have a free-air heating assembly that slides into the body. I think these were for oil heating, very low density and surface temp.

  • @grim said: I've seen flange mount tubular tank heaters that have a free-air heating assembly that slides into the body. I think these were for oil heating, very low density and surface temp.

    Brilliant. Thank you.

  • edited October 2016

    I'll tell you, if you get this right, it's a game changer.

    I think you still need to push watt density lower though. I don't know what the magic number is, but 5 seems closer to what a typical immersion heater would do, versus a bain marie or steam jacket, which is likely lower than 1w/cm2.

    Realistically, still geometry would likely need to change to accomodate, tall narrow stills will likely be difficult.

  • You have given me another idea @grim. Thanks mate.

  • Take a google @

    Thermal properties of Perlite (hydroponic media)

    Thermal properties of a salt slurry/ molten battery/ heat sink - couple of youtube clips

  • That's a good point. What temp range is going to be in there?
    Molten salt is probably out but you might be able to use something like Cerrosafe.

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