West Union, Illinois | FFJR - how does the smaller exposed surface area of a 80% full tank end up with more total surface area for vapor to boil off of then a 50% full horizontal tank which has a larger "layer" of exposed surface area at the top of the liquid level. Simple, you're thinking backwards. You are thinking of the vapor space. We are concerned with the wetted area.
Propane boils at about -44° F That's why it is stored in a closed container. A propane tank is basically a big canner or pressure cooker. When you are canning do you heat the part of the canner with water in it or the part with air in it? Same idea with propane. We are wanting to produce vapor which requires heating the liquid. But since propane boils at -44° and water boils at 212° they are different animals.
The wetted area (the part of the tank in contact with liquid propane) is what transfers heat to the liquid which in turn causes the propane to boil giving us propane vapor.
When you crank up the burner under the pressure cooker the little weight on top rattles faster and faster. The liquid inside is getting hot and making more steam (vapor) which increases the pressure. What the weight does is relieves the pressure to keep it constant. LP tanks have a relief valve as well, but when they start rattling it becomes what professionals refer to as an OH CRAP situation (that may be a slight paraphrase).
A little winter day shop experiment. Get an empty aluminum can and fill it half full with water. Have someone hold it with a longer pair of Channelocks® then use a torch with a brazing tip to heat it (You can use a cutting tip, just don't get silly with it). As long as you heat the wetted portion of the can it just boils. But boil it dry, or heat the area that is not wetted, and see what happens.
You are heating the non-wetted area and the heat is not transferring to the liquid. Undesirable result occur
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? on 500 gal l.p. tank
