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Re: [hobbicast] BTU/BRASS?

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  • edi malinaric
    Hello Geoff - first my apologies for not answering your question on first posting. I was hoping that someone else would jump in. To begin with - I m going to
    Message 1 of 4 , Dec 2, 2004
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      Hello Geoff - first my apologies for not answering your question on first
      posting. I was hoping that someone else would jump in.

      To begin with - I'm going to use the figures for 80/20 brass - just to make
      my life easier.

      1.0 Properties of the main actors:

      1.1 Copper

      Melting Point 1981 F
      Specific Heat 0.0951
      Latent heat of fusion 75.6 BTU/lb

      1.2 Tin

      Melting Point 449 F
      Specific Heat (solid) 0.0551
      (482 F) 0.05799
      (2000 F) 0.0758
      Latent heat of fusion 25.2 BTU/lb

      (Notice that the specific heat of tin varies with its state (solid/liquid)
      and its temperature. Odd, but not unusual.) To simplify things, I have
      averaged the Specific Heat from 482 to 2000 and will use 0.0699

      1.3 Propane 20 000 BTU/lb
      Flame temp 3 600 F

      1.4 Brass (80/20) MP 1830 F
      Temp in crucible ~ 2 000 F

      1.5 Shop temperature 100 F

      1.6 Charge weight 20 lbs

      Copper 16 lbs
      Tin 4 lbs



      OK lets see what we can do with all those figures.


      2.0 Copper:

      To warm it from 100 to 2000.

      (2000 - 100) x 0.0951.............................. 180.7
      To change its state from solid to liquid
      (fancy way of saying we melt the stuff!).... 75.6
      --------
      sub-total 256.3 BTU/lb


      3.0 Tin:

      100 to 450: 350 x 0.0551 .................. 19.3
      melt .... 25.2
      450 to 2000: 1550 x 0.0699 .................. 108.3
      --------
      sub-total 152.8 BTU/lb

      4.0 For a charge weight of 20 lbs

      Copper 16 lb x 256.3 BTU/lb ................. 4 100.8
      Tin 4 x 152.8 ............................... 611.2
      ---------
      Total VOOMA required 4 712.0 BTU

      5.0 Propane used (4 712/20 000) ................. 0.24 bs

      Now, I would guess that you used much more Propane than that to melt the
      charge.

      Let's play some more.

      6.0 EXTRAS - (in a former life these bugged me no end)

      6.1 Furnace

      Let's assume that the crucible and the refractory weighs in at around 80
      lbs.
      The specific heat of brick is around 0.22.

      To heat that up to 2000 F will need
      (2000 - 100) x 0.22 x 80 ........................... 3 344 BTU

      Propane needed 3344/20 000 ................... 0.16 lbs

      6.2 Losses

      6.2.1 Useful heat

      The average temperature of the furnace and contents from cold to
      ready-to-pour is (2000 - 100)/2 = 950 F.
      However things are never linear (it takes longer to heat that last 950 than
      the first 950) so we'll fudge it by moving the useless temp up to 3/4 of the
      range, say 1400 F.
      All flame temperature below 1400 F is really only cooling things in the
      furnace.

      We have to correct the amount needed by 1 + 1400/3600 = 1.39.

      So 40% of the gas that we have burnt is doing absolutely nothing. Actually
      that's not true, it is warming itself!

      So our calculated minimum propane requirement is

      (0.24 + 0.16) x 1.4 ..................................... 0.56 lbs

      6.2.2 Insulation Loss

      Assume that the insulation is only 80% efficient so that's another 20%
      extra.

      That makes it 0.56 x 1.2 ........................... 0.67 lbs

      6.2.3 Exhaust loss

      Heaven only knows what the actual retention time of the gas in the kiln
      actually is - I'm not even going to hazard a guess. Tell you what we'll work
      that one backwards - how much propane did you use?

      7.0 Estimated Propane usage

      l'll climb out on a limb - but with, I hope, a well packed parachute.

      I would guess that you have probably used around 6 lbs of propane - an
      overall efficiency of around 10 - 15% seems to me to be all that one could
      expect at such a small scale.


      Enough - I have a gas burner to make and nine 1/2" holes to drill through 8"
      old 20 year old concrete with a not-quite-up-to-it hammer drill. Especially
      as said holes are at ankle level so I can't get any push behind the drill.

      Hope all the above makes sense

      cheers edi


      ----- Original Message -----
      From: "geoffrey roberts" <rbrts1035@...>
      To: <hobbicast@yahoogroups.com>
      Cc: <rbrts344@...>
      Sent: Friday, December 03, 2004 3:16 AM
      Subject: Re: [hobbicast] BTU/BRASS?


      >
      > Hi, i want to know the heat required to melt brass. i can melt 20 pounds
      of Silicon Brass in 35 minutes from a cold start. i'm building a new furnace
      and will be changing from propane to waste oil. The new furnace will have a
      number 60 crucible. If i can find the heat needed i can work backwards and
      determine the relative btu needed to melt the Brass. At this point i will
      see how much waste oil and how many burners i need. i think this method will
      be easier and definitely than building a waste oil burner for the older,
      smaller furnace. Thanks, geoff<>< Jahnke



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