[Jewelry making - Article 00293] - Re: [Orchid] Ozone Friendly Torches?

[Peter W . Rowe]

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>        Okay, now I'm really confused. I keep being told that propane
>     burns hotter than butane--Peter just reiterated that. But, in the
>     chart just posted by Randy: http://www.flamex.com/gasprop.htm the
>     relative 'values" are all over the place. I do see that, for 
>     "total heating value after oxidation," propane is 19768 BTU/lb.,
>     and  butane is 19494 BTU/lb., but, in every other measure, butane
>     actually  seems considerably hotter than propane. Is "after
>     oxidation" the only  one that counts? 

    Rather than worrying about charts and numbers, which can, like
    scripture, seem to support almost any position depending on just what
    is being reported and how it was measured, do this the old fashion
    way.  Take a small air/propane torch, like the typical plumbers
    torches, and one of the typical little butane fired torches. Set them
    to a similar flame, and see how fast you can melt a piece of silver
    on a charcoal block.  You'll find the propane out performs the
    butane.  Perhaps not by a tremendous amount, but it will be faster.  
    There are a number of factors that make this all hard to judge.  The
    main figure you want is how fast a flame can transfer BTUs to a metal
    being heated.  It's not just the BTU output, but the transfer rate,
    which can be tricky to measure.  It involves BTU generation in the
    flame, as well as flame velocity, and no doubt other things too. 

    In the chart you mention, look specifically at these:  Propane has a
    higher burning velocity.  That means that even if the energy stored
    in a given volume of butane were slightly higher (it's not), you'd
    get the energy out of the propane faster.  Even more significant, I
    think (I might be wrong here.  not sure) is the figure for combustion
    ratio.  There you'll note that it takes less butane per volume of
    oxygen or air to burn.  What that means is that for the energy in a
    given volume of butane, more oxygen is required to get it out, or
    when burning in air, more oxygen and a LOT more nitrogen.  Guess
    what.  That means that those other gasses, nitrogen and oxygen, are
    also at room temperatue when mixed with the fuel, and share in the
    energy release by the fuel to heat it to flame temp. That results
    perhaps in the stated amount of BTUs being released by that volume of
    fuel gas, but it's diluted through a much larger total volume of
    flame gas, thus giving a lower actual flame temperature, especially
    when burned in air (remember all that excess nitrogen)   It's the
    combination of higher flame temperature, and higher flame velocity,
    that allows propane or natural gas to perform better in our
    applications.  In a commercial heating fixture like a furnace, the
    appliance is designed to extract as much of the total energy produced
    by the fuel, as possible, so then the actual flame temp makes little
    difference, only the total BTUs per volume of fuel.  For us, most of
    the flame goes right past the metal, and what matters is what portion
    of those BTUs manage to transfer to the metal, and how fast do they
    transfer.   Differences in BTU transfer rate are more important than
    actual flame temperature, since remember that the metal as it heats
    is constantly trying to radiate it's heat away again, and the flame
    must add heat faster than the metal gets rid of it.   It's a
    different requirement than what a home heating furnace would present 

    Butane is not a BAD fuel, just a less practical one in real life for
    many of us. Among other things, it's often more costly, not as easily
    or widely available in larger sized tanks, and the torches we
    generally use tend to be optimized, design wise, for natural gas or
    propane, and Butane, though it may work in them, may not work quite as
    well.  The differences here may not be strong, though, and if you
    have Butane available, and can use it, no doubt you can make it work
    just fine, if you wish.  It may actually be a slightly cleaner burning
    fuel, though I'm not sure of that... 

Peter

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