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SS wall heat conduction math

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  • Andrew Forsberg
    Hi all, The internet has provided me with far too many conflicting rates and formulae for calculating whether an external water bath for a stainless steel
    Message 1 of 13 , Jun 1, 2005
      Hi all,

      The internet has provided me with far too many conflicting rates and
      formulae for calculating whether an external water bath for a stainless
      steel boiler will or will not provide sufficient heat.

      I'd very much appreciate it if an engineer on the list would give this a
      once over:

      T1 = 105 degC (water + table salt jacket)
      T2 = 78 degC (mash)
      Heat transfer coefficient = no idea... Ignoring it for the moment.
      K = approx 14 W(m2 . K) (I have seen more figures for SS than I care to
      poke sticks at. This is the most conservative, and is for 270K, so
      anything better would be super).
      Q = Watts
      A = Area = .64m2
      D = thickness of the SS wall = approx 2 mms

      Q/A = (T1 - T2) / (D . 1/K)

      Q/A = (27) / (.002 / 14)
      = 27 . 7000
      = 189,000

      Q = 189,000 . 0.64
      = 120.96 kW

      So, what I'd like to know is -- will a 2mm stainless steel wall for an
      area of .64m2 allow heat to pass through at a rate of up 120kW for that
      temperature differential? Even approximately? As long as it's higher
      than 3 or 4kW I'm happy. Unfortunately some formulae I've found
      suggested the figure would be closer to 800W...

      Cheers and thanks,
      Andrew
    • abbababbaccc
      Andrew, a mash temperature will typically vary inbetween 90C (~17%) - 99C (~1%) during operation. IMO you ll get missleading results by using 78C for mash
      Message 2 of 13 , Jun 1, 2005
        Andrew, a mash temperature will typically vary inbetween 90C (~17%) -
        99C (~1%) during operation. IMO you'll get missleading results by
        using 78C for mash temperature.

        - Riku

        --- In Distillers@yahoogroups.com, Andrew Forsberg <andrew@u...>
        wrote:
        > Hi all,
        >
        > The internet has provided me with far too many conflicting rates and
        > formulae for calculating whether an external water bath for a
        stainless
        > steel boiler will or will not provide sufficient heat.
        >
        > I'd very much appreciate it if an engineer on the list would give
        this a
        > once over:
        >
        > T1 = 105 degC (water + table salt jacket)
        > T2 = 78 degC (mash)
        > Heat transfer coefficient = no idea... Ignoring it for the moment.
        > K = approx 14 W(m2 . K) (I have seen more figures for SS than I
        care to
        > poke sticks at. This is the most conservative, and is for 270K, so
        > anything better would be super).
        > Q = Watts
        > A = Area = .64m2
        > D = thickness of the SS wall = approx 2 mms
        >
        > Q/A = (T1 - T2) / (D . 1/K)
        >
        > Q/A = (27) / (.002 / 14)
        > = 27 . 7000
        > = 189,000
        >
        > Q = 189,000 . 0.64
        > = 120.96 kW
        >
        > So, what I'd like to know is -- will a 2mm stainless steel wall for
        an
        > area of .64m2 allow heat to pass through at a rate of up 120kW for
        that
        > temperature differential? Even approximately? As long as it's higher
        > than 3 or 4kW I'm happy. Unfortunately some formulae I've found
        > suggested the figure would be closer to 800W...
        >
        > Cheers and thanks,
        > Andrew
      • Andrew Forsberg
        That s a very good point. Well, even at only 1 deg C difference the rate is 4.4kW for that area of 2mm SS wall. The other problem will have to be how to
        Message 3 of 13 , Jun 1, 2005
          That's a very good point. Well, even at only 1 deg C difference the rate
          is 4.4kW for that area of 2mm SS wall. The other problem will have to be
          how to insulate the jacket optimally. One thing at a time.

          So, do those calculations look even roughly correct to you?

          Cheers
          Andrew


          abbababbaccc wrote:

          >Andrew, a mash temperature will typically vary inbetween 90C (~17%) -
          >99C (~1%) during operation. IMO you'll get missleading results by
          >using 78C for mash temperature.
          >
          >- Riku
          >
          >
          >
        • dearknarl
          Hate to bring bad news, but I m guessing the conduction of heat through the steel will not be your limiting factor, and your 120kW sounds about right. It will
          Message 4 of 13 , Jun 1, 2005
            Hate to bring bad news, but I'm guessing the conduction of heat
            through the steel will not be your limiting factor, and your 120kW
            sounds about right.

            It will more likely be the convection of the water bath and the wash
            inside the boiler, more so the water bath once it is boiling, that
            will limit heat transfer.

            Agitation of both will provide better heat transfer. Trying to
            calculate heat transfer for such a system would be a nightmare.

            I'd say your best bet is an experiment, unless you're a hardcore nerd
            that wants to create a computer model, or a hardcore physicist/maths
            guru who has way too much spare time =). Let us know how you get on.

            knarl.

            --- In Distillers@yahoogroups.com, Andrew Forsberg <andrew@u...>
            wrote:
            > Hi all,
            >
            > The internet has provided me with far too many conflicting rates and
            > formulae for calculating whether an external water bath for a
            stainless
            > steel boiler will or will not provide sufficient heat.
            >
            > I'd very much appreciate it if an engineer on the list would give
            this a
            > once over:
            >
            > T1 = 105 degC (water + table salt jacket)
            > T2 = 78 degC (mash)
            > Heat transfer coefficient = no idea... Ignoring it for the moment.
            > K = approx 14 W(m2 . K) (I have seen more figures for SS than I
            care to
            > poke sticks at. This is the most conservative, and is for 270K, so
            > anything better would be super).
            > Q = Watts
            > A = Area = .64m2
            > D = thickness of the SS wall = approx 2 mms
            >
            > Q/A = (T1 - T2) / (D . 1/K)
            >
            > Q/A = (27) / (.002 / 14)
            > = 27 . 7000
            > = 189,000
            >
            > Q = 189,000 . 0.64
            > = 120.96 kW
            >
            > So, what I'd like to know is -- will a 2mm stainless steel wall for
            an
            > area of .64m2 allow heat to pass through at a rate of up 120kW for
            that
            > temperature differential? Even approximately? As long as it's higher
            > than 3 or 4kW I'm happy. Unfortunately some formulae I've found
            > suggested the figure would be closer to 800W...
            >
            > Cheers and thanks,
            > Andrew
          • Andrew Forsberg
            ... Hey Knarl, Bad news is still news, and it s far better to be brought it in the form of advance notice than to jump in first, only to swear and curse later.
            Message 5 of 13 , Jun 2, 2005
              On Wed, 2005-06-01 at 22:52 +0000, dearknarl wrote:
              > Hate to bring bad news, but I'm guessing the conduction of heat
              > through the steel will not be your limiting factor, and your 120kW
              > sounds about right.
              >
              > It will more likely be the convection of the water bath and the wash
              > inside the boiler, more so the water bath once it is boiling, that
              > will limit heat transfer.
              >
              > Agitation of both will provide better heat transfer. Trying to
              > calculate heat transfer for such a system would be a nightmare.
              >
              > I'd say your best bet is an experiment, unless you're a hardcore nerd
              > that wants to create a computer model, or a hardcore physicist/maths
              > guru who has way too much spare time =). Let us know how you get on.
              >
              > knarl.

              Hey Knarl,

              Bad news is still news, and it's far better to be brought it in the form
              of advance notice than to jump in first, only to swear and curse later.

              The other problem with the jacket proposal was the thought that it will
              need to be a stainless steel jacket that's welded to the stainless steel
              tank. Through tubes might improve the situation, as might encroaching
              further under the tank... But, tbh, the last thing I want after all that
              is to need to build an automated stirring paddle mechanism.

              On a brighter note: the L-bend at the bottom of the tank, before the
              gate valve, has a couple of small 3/4"-ish OD screw fittings with caps.
              They look decidedly like they were designed for a steam mixer... :->
              Hmmmmmmmm.

              Cheers & thanks,
              Andrew
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