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The title line says it all. I know um.. empirically that it's ok, but I'd like to hear a confirmation of safety and/or a more extensive empirical experiment.

The tank in question is a BBQ tank measuring about 1 ft in diameter and about 1 1/2 ft in height.

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Let me just add car gasoline tanks as a reference for comparison. The lore from folks who own a car they do not drive in winter is to fill the tank in Fall, then to park the car in the garage with the tank full. (The typical garage is not heated, but it absorbs heat from the adjacent structure, and so as an extreme the temperature can get close to freezing—32F/0C.) If the tank is left half-full, water condenses (and so Harper's description makes sense), but it condenses inside the tank. Needless to say, come Spring the engine is not too happy to be fed gasoline contaminated with water.

Will water condense in a similar way inside a propane tank left outside in freezing temperatures (of as low as 15F/-10C, say)?

  • The freezing point of propane is -306.4°F (-188C), so I'd try to keep it a bit above that temperature... Though if the weather where you live gets that cold, you probably have bigger things to worry about, like how to arrange rocket launches to deliver supplies to your Lunar outpost. – Johnny Feb 17 '17 at 0:20
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Actually, no. You need to keep it exposed to weather to keep it warm. Yes, you heard right.

Or any other heat source would do.

Propane is stored as a liquid, but used as a vapor. As you use it, it must convert from liquid state to vapor state (i.e. Boil). That requires it absorb energy, its latent heat of vaporization, from its surroundings (i.e. the liquid propane in the tank). This will make the liquid even colder than ambient air. You may have seen propane tanks condense water or "ice up" when in heavy use. That's because they are colder than ambient. This is that effect.

If the liquid propane can't absorb its latent heat of vaporization, it won't boil. So when the liquid propane falls below its natural boiling point, then you will have no vapor pressure at all, and your propane accessory won't work. Much like a boiler gives no useful steam if its temperature is below 212F (100C).

You rely on heat transfer from outside into the tank to keep the propane "warm enough" to boil on its own. Trouble is, heat transfer works best when there's a big temperature differential. When the outside is already cold, heat transfer is slower. You want the opposite of insulation, you want a heat exchanger to speed it up.

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    Not exposed to weather, per se, but to a surrounding fluid of some kind. – isherwood Feb 16 '17 at 21:06
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I'm going to be a little bit contrary and say, that depends. If the temperature where you live gets below -44°F, then, yeah, you will need to keep it a few degrees above that. According to articles here and here the boiling point of propane is -44°F (-42.4°C), so in order for your propane to exist as a vapor, the temperature of the propane need to be kept above -44°F. And I'm guessing that to generate at least some usable pressure, it would need to be 10 - 20 or so degrees above that, and that doesn't take into consideration that fact that the temperature of the propane will drop as it turns from liquid to vapor (as Harper points out). Simply moving the tank indoors temporarily away from any ignition sources would probably suffice.

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No. Half-full tanks are used by millions of ice anglers, for example, in absurdly cold temperatures. They do tend to lose pressure, though, especially if being depressurized further by use. The fuel just doesn't vaporize nearly as readily when below zero F.

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I think I see the answer. There is no risk that a propane tank will have water condensation inside the tank because there is no water vapor in the tank. The tank is 100% propane. Whatever gas there is inside, it is also propane.

As with all fluids, there will remain at all time a balance between the pressure inside and the temperature—an increase in temperature will increase pressure (the principle behind pressure cookers, for instance) and, as Harper points out, a decrease in pressure (due to release of the fluid to cook with it) will decrease the temperature of the fluid. Since the boiling point of propane is −42.25 to −42.04 °C; −44.05 to −43.67 °F, and for all of us lucky enough for the temperature never to get anywhere near that low, the pressure inside will exceed atmospheric pressure. Even if the temperature drops below propane's boiling point, the only thing that will happen is that the pressure will be below atmospheric pressure. Assuming that the tank wall is so thick to withstand intense pressure when the tank is full, there will be no difficulty for the same wall to withstand a minor pressure from the outside at extreme cold. The tank wall will not cave in.

To sum up, there is no risk in leaving a propane tank outside in any kind of cold—provided it's properly filled with just propane. If one wants to worry, one can keep in mind that leaving the tank outside in the summer at a temperature of 43 °C; 110 °F, for example, will produce an internal pressure of 197 psi, and so it is a good precaution to shield the tank from direct sunlight in the summer.

  • Good answer with a minor correction - cooling of a propane tank during use is not caused by a drop in pressure, but by evaporation of the liquid. – Mark Feb 17 '17 at 17:27
  • @Mark The release of the gas reduces the pressure, which cools the liquid. Likewise cooling the liquid absorbs the gas, which reduces the pressure. So it's a question of deciding which is the cause and which is the effect. Since the action taken by the user is to open the valve, the action reduces the pressure (by releasing the gas). The temperature change is the effect. But, really, (excuse the pun) it boils down to the same. – Calaf Feb 20 '17 at 0:41
  • I think the pressure change is virtually unnoticeable. But even so, why would reducing the pressure cool the liquid? – Mark Feb 20 '17 at 19:55
  • @Mark This is an excellent hydraulics question. Let me try. Say we have a gas in a sealed syringe. We push the lever, hence pushing the gas, but it's sealed. Nothing gets out. Where did the energy we put in go? The air molecules now have less volume to move around, but they still have the same energy they had a moment ago. The difference manifests as heat. And vice versa. – Calaf Jul 4 '17 at 0:26

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