Why Does Ice Form on Gas Bottles and Regulators?

Under the right circumstances, condensation or ice can form on gas bottles and/or gas regulators.  But why does this happen?

Initially, condensation forms when the temperature of the gas bottle or regulator drops below the dew point.  This is exactly the same as the condensation you get on a humid day with a glass of ice water.  As the temperature continues to drop, the condensation may turn to ice. But why do the gas bottle and regulator get cold in the first place?

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Ice on a gas bottleTo make sense, it helps to first explain how LPG works. LPG is stored under pressure, as a liquid, in a gas bottle.  It turns back into a gas by ‘boiling’ into gas vapour.  This happens at the very low temperature of -42°C.  To boil, the liquid LPG draws heat from the steel walls of the gas bottle.  This, in turn, makes the gas bottle feel colder than the ambient temperature.  The gas bottle gets even colder when you are actually using the gas.  So, with sufficient humidity and when you are using gas very rapidly, condensation or ice can form on the gas bottle.  The visible condensation or frost line indicates the level of the liquid gas remaining in the gas bottle.  The picture to the right shows this on a small dark coloured gas bottle, so it is easier to see.

LPG cyliner diagramThe gas vapour that forms in the gas bottle moves downstream to the point at which it is used.  But before making its way to the LPG appliances in your home, it passes through your gas regulator, where the pressure is reduced to the appropriate level.  The regulator delivers a constant safe pressure while the gas bottle pressure can significantly vary, depending on the ambient temperature and the amount of liquid gas remaining in the bottle.  As the LPG passes through the regulator, it expands, resulting in very cold gas vapour temperatures.  This causes the regulator to also reach extremely cold temperatures, as the cold gas vapour passes through it.  Depending on the humidity of the surrounding air and the rate at which the gas is being used, condensation or even ice will form on the regulator.  The faster the gas is used, the colder the regulator will get.  This is why, under normal operation in warm and humid climates, the outside of a gas regulator will feel cold to the touch and may also be wet, frozen or frosted.

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