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Propane Tank Freezing Up


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Hey all, I'd been running my forge for about 6 hours when I noticed that the bottom of the 20lb tank was starting freeze a little bit. About an hour later, the whole thing had frost on it, and when I was working in the forge, I would heat, hammer, shake the tank, heat, etc... When I would shake the tank it would go back to normal, but if i left it, the psi would start to drop. How do I prevent this, and should I just buy a bigger tank like 40 or 60lb tank??

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It's a problem with the BBQ sized tanks. If you put two or three of them on a common manifold, that fixes it, or you can try the float the tank in a tub of water trick (which is good for keeping your...ah...beverages cold). The best way is to go to a bigger tank.

 

Geoff

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John,

 

As you probably know the propane stored in the cylinder is actually a liquid (due to the pressure in the tank), there is open volume left in the cylinder (they only fill to about 80 %) and in this space, some of the liquid propane is allowed to enter its gas phase. As you draw off this propane gas, the pressure in the cylinder drops slightly allowing more of the liquid propane to enter the gas phase. This conversion from a liquid to a gas is an endothermic process (meaning it requires heat energy to be supplied to the system in order for this to occur). As a result, the propane absorbs heat from the cylinder walls in order to complete the process. Usually this is a slow enough process that the cylinder walls can be heated by the surroundings at a quick enough rate to not cause much of a pressure fluxuaiton. However, if you start drawing out propane at a faster rate than the rate at which the cylinder walls can pull heat from the tank's surroundings the overall temperature of the tank and lp inside will drop, and as temperatures drop, pressure drops as well. You probably noticed that the tank began to ice over when the liquid level had droped, this is because there was tank surface in contact with the liquid to supply heat to cause the evapporation process. When you shook the tank it brought the lp in contact with more and warmer parts of the cylinder walls and allowed more heat to enter the system.

 

Sorry to get all technical on you, it just happens sometimes...

 

Anyway, in order to prevent tanks from icing over there are a few options. Firstly you can decrease the rate at which you draw gas out of the cylinder, you can increase the volume of lp and tank size, or you can "add" heat to the system (I don't mean sticking a burner or something on the tank, so don't do that). Its not practical to decrease the rate of gas flow out of a single cylinder since you are running a forge, which requires high gas volume flow rates, however you can take two 20lb tanks and build a manifold to hook them together (this way you still get the same gas flow, but you are not drawing it out of a single cylinder as quickly). I think Larry Zoeller has one pre-made for sale if you don't want to fool with it on your own. Second option is to just buy a larger tank. This was my choice, I was blasting through 2-3 those little 20lbers in a weekend, and having to make a bunch of trips to refill got old, fast, so I found a used 100lber for a decent price ($50, and its in quite good condition). Also as some aside info, if you buy a tank smaller than 100lb it is required to have a OPD (overflow protection device) valve, so if you get a used tank be sure that it has this valve or you won't be able to get it filled (some places like propane companies might still let you, but your standard rental shop won't. This is not required on 100lb tanks. Larger tank means more tank surface area to provide heat to the system, and more lp volume means it will take a lot more energy removal (through evaporation) to change the temperature. Third option is you can make some sort of a water bath to throw the tank in (this is what I mean by "add heat to the system"). A large volume of water around the tank has a lot of heat energy and as a result it will take a lot of energy removal to lower its temperature, so it will keep the tank walls warmer and prevent the temperature from lowering to the point it decreases pressure. Those are your options, hope it helps.

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i have been trying the bigger tank route. i had a 20# it froze up in less than a hour at welding temps so i got a 40# it takes about 3 hours or so (but losses welding heat around 2 or so). So now i am leasing a 100# tank (which may i add cost less than buying my 40# tank and 30$ more to fill while you get more than double the propane ) from Amerigas, and building a smaller forge (my current is a 16" fog style forge so the heat chamber is huge its great for big forging work but i keep welding in this thing i am gonna go broke).

 

So the motto of this story.....

save your self the time and buy a big tank they are cheaper to fill and when you pay for fill up you get the tank for free as long as you go to Amerigas to fill it.

 

hope this helps

 

~~DJ

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Okay, well I have a two burner diamondback ironworks bladesmith forge, i'll use the 20lb for now in a bucket of water, but when I have the money, I'll buy a 100lb tank. I am also going to put a firebrick in the other opening when welding and doing other work that doesn't require both ends to be open, should save a lot of heat, and ill end using less propane cuz I can work faster, and turn it off faster cuz I have the idler valve.

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