Many liquid propane torches (including burners for hot-air balloons) have coils placed in the line of fire heating the liquid before it reaches the nozzle. I am very puzzled by the exact function of this coil.

The liquid propane is heated by it's own flame in the coils:

  • Does this vaporize the liquid in the coil, or does the propane remain liqud until the nozzle?
  • E.g. does the propane vaporize inside the coils and the heat is just to counter the heat of vaporization?
  • How is the pressure profile from the tank to the nozzle through the coil? It seems that the temperature increase would mean pressure increase, but that would mean that part of the flow works against the pressure gradient. Or does the temperature increase result in vapor expansion that only accelerates the flow (no pressure change)?

By looking at the output jet from the nozzle it seems that (at least part of) the propane is liquid as it exits but rapidly vaporizes:

  • Is the function of the coil to increase the output liquid temperature, increasing the speed of vaporization, to start the flame earlier and shorten the flame length/make it more intensive?
  • If so, is the coil only strictly necessary in cold weather?

UPDATE: The setup is equivalent to the image below.

Diagram of system. The fuel output heats itself.

Minimum visual example of a liquid propane torch. *Liquid* fuel enters at the valve


When a compressed gas is throttled down through an orifice nozzle to atmospheric pressure, it behaves like a refrigerant: its temperature drops suddenly at the nozzle, and it may exit the orifice as a liquid instead of as a gas. If the gas is combustible and the nozzle is part of a burner assembly, the presence of liquid fuel in the burner might cause it to malfunction. So you are right, the purpose of the coil is to preheat the propane a bit so that it vaporizes promptly upon exiting the nozzle, even in cold weather. If there is any moisture in the gas, the preheat also helps prevent it from freezing in the nozzle and blocking the flow of gas with an ice plug.

This effect can be demonstrated with a handheld propane torch but I will not explain how in detail here because it is sufficiently dangerous that you can set your clothes or hair on fire. Just take my word for it!

  • $\begingroup$ Thanks for the answer - I have added an image to clarify that the fuel is a liquid as it enters the system. The first liquid to exit will have not flame heating it. How can moisture cause ice plugs if there is a constant flux of 25°C propane? $\endgroup$ – Paamand Dec 5 '17 at 8:14
  • $\begingroup$ the ice will slowly build up right at the nozzle exit and can restrict the gas flow, at least until the nozzle gets warm enough from the flame to keep it melted. this is not a common condition so not to worry. $\endgroup$ – niels nielsen Dec 5 '17 at 18:02
  • $\begingroup$ @nielsnielsen, it sounds like you have personal experience with the handheld propane torch. $\endgroup$ – David White Jan 15 at 19:29
  • $\begingroup$ yes, and propane-fired space heaters and whatnot. some of the experience was good, some of it just... interesting. $\endgroup$ – niels nielsen Jan 15 at 22:37

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