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.