Boil problems - Propane pressure?

[eko]

Is there something about propane tanks that they can't output as much propane as they get emptier? Or perhaps the longer the tanks been open & burning?

I planned for a two hour boil yesterday, and adjusted my volume up to compensate for expected burn off. I had almost 10 gallons of liquid that I got up to boiling, but had a real hard time keeping the boil going. After an hour and a half I had lost about 1 gallon, when I should have lost closer to 3.

My tank was not empty, but I switched it for a new one. The new one immediately produced a much bigger flame. And gave me a good two hour boil. Both were the same quick exchange tanks, maybe 20 lbs.

Both tanks' valves were open all the way. And I played around with different levels of opening the regulator. The first before I gave up on it was open all the way and producing only a medium amount of flame. It may be worth noting the first I had used for about 30 minutes to boil water to do some cleaning, and then for about 45 minutes to get the sparged wort up to the initial boil.

So, why the difference between the two tanks? Why did the first not produce enough output, first noticed about 30 minutes into the boil.

 

[acidrain]

As you burn through propane, the temperature of the propane in the bottle drops because of the high to low pressure use of the tank. It's how ac units work.
Often times (especially if it's cold outside), as you use propane, the bottle will actually freeze.
Either warming the bottle as it's being used, or switching to a warmer bottle are your only options.

 

[day_trippr]

Right - the propane phase change from liquid to vapor consumes thermal energy, dropping the tank temperature in the process.

Get a plastic tub, set the tank in it, and fill the space with warm water...

Cheers!

 

[JohnSand]

I have found that I get less pressure in a near empty tank, both for the burner and the grill.

 

[david_42]

There is also the problem of heat transfer through the wall/bottom; as the tank empties very little liquid propane is in contact with the wall. Typically the bottle is sitting on the ground or concrete, which tend to be colder than the air; that means little heat is entering the bottle through the bottom. Net result, slower evaporation of propane and lower pressure.

 

[eko]

Thanks for confirming, and the explanations. I'll put some of these tips to use next time. Saturday I was just glad I had a second tank on hand.