Q: Looking on the charts, at “total heating value after oxidation,” propane is 19768 BTU/lb. and butane is 19494 BTU/lb., but in every other category butane looks hotter. Which is really going to heat my jewelry items faster?
A: 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, try doing this the old-fashioned 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 (mostly oxygen plus nitrogen) to burn. What that means is that for the energy in a given volume of butane, more oxygen is required to get it out. That means that those other gases, 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 they transfer. Differences in BTU transfer rate are more important than actual flame temperature, since the metal as it heats is constantly trying to radiate its 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 have.
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 a torch designed for 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 about that.
by Peter W. Rowe M.F.A., G.G.