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