A genuine Propane Puzzler

MEXICOWANDERER wrote:
with regard to reading vapor pressures = temperatures, the amount of boil off must be taken into consideration. A pot, cylinder or tank with low amount boil off surface area will become COLDER than the liquid was when starting off. Expansion. When the expansion rate is very low the tank temp will remain near ambient temp, when the expansion rate is very high (high amount of boil off) the temperature can *easily* sag -20F.

In extreme areas, 97% propane and 3% methane is the mix to achieve proper vapor pressure. The Soviets transported LNG into the remoter regions of Siberia where -80F was not unheard of.

In the winter in the mountains I always had pots of butane to start charcoal with. Some liquid was allowed to soak into the carbon then when set alight it burned furiously for a few minutes. And not a hint of taste contamination.

I reality, a 7.5 gallon propane tank, filled with propane, not propane butane mix, is marginal for a 30Kish furnace or HWH anyway. It will usually do fine when full, but as it gets depleted the mass of the remaining liquid is low enough to really drop the temp as you mentioned, which reduces vapor pressure and how much you can draw. On my tanks, if the furnace is the main draw in the morning even at 45F or so and is near the bottom, after the furnace runs for 30 minutes or so tanks will switch. But if I switch them back the "empty" tank still has 10-20 percent capacity and will do fine for the fridge or stove till evening.

One chart I saw says that 10lbs of propane in a cylinder will support about a 30KBTU draw at 0F PROPANE temperature. but that 1/3 tank of a 7.5 gallon tank, or 1/2 a 5 gallon tank.