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LPG Oxygen Ratio


#1

Does anyone know the LPG to oxygen ration in goldsmithing? Please
help, I am trying to get the right size cylinders based on the ration
we use as goldsmiths.

Thanks
Raakhi


#2

Raakhi-

I assume that you are referring to stoichiometric combustion of
propane and pure oxygen. Stoichiometric means that all of the fuel
is burned and there is no excess of either oxygen or fuel in the
products of combustion. (Perfect combustion also requires perfect
mixing within the burner, not necessarily always achieved in
practice.)

In terms of moles, it takes 5 moles of oxygen to burn 1 mole of
propane. Since at standard conditions a mole of gas occupies 22.8
liters, one can make the inference that at equal conditions of
pressure and temperature, it takes 5 volumes of oxygen to completely
burn 1 volume of propane gas at whatever the ambient pressure and
temperature may be. (This is part of the reason it takes a big
bottle of oxygen to burn a much smaller bottle of propane.) The
products of combustion are 3 volumes of carbon dioxide and 4 volumes
of water vapor, also measured at standard conditions.

In the case of butane, it takes 13 volumes of oxygen to burn 2
volumes of vaporized butane, again at equal conditions of
temperature and pressure. The one additional carbon and two
additional hydrogen atoms in the butane molecule need a lot more
oxygen than is needed for propane.

In goldsmithing it may be necessary to adjust the flame to something
other than stoichiometric conditions. If less oxygen is supplied,
the flame will be reducing because the hot hydrogen and carbon tend
to remove oxygen from anything in the flame. If excess oxygen is
supplied, the flame will be oxidizing.

Hope this helps.
Dick Davies