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Sure.

The law is pV=nRT.

Basically, if the amount of gas (n) is constant (i.e. a closed tank), and the pressure (p) goes down, the temperature of the gas (T) goes down as well.

Conversely, if the pressure goes up (i.e. you squeeze more gas into a space), the temperature goes up.

And yes, you’re using up propane more of the liquid propane is converting to gaseous propane due to the lower pressure, so the (n) is basically constant. Therefore the pressure change causes more cooling than the loss of gas causes heating.

The reason the ice forms on the bottom is because of one more thing: some of the liquid propane at the bottom turns into gas and this boiling takes energy, so the liquid gets even more cooled than the gas does.

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Energy (heat) is released when you compress a gas. When the gas expands, it absorbs heat.

Same principle as why when you get any of the sprays (Insect Spray, Oil Spray, Deoderant, Canned Air, etc) and spray it, the can gets cold.

Also works from the other side - Cool a gas (i.e remove heat), and it shrinks, heat it and it expands.

You can test this by blowing up two balloons about half way - put one in the freezer, the other in warm water.

The one in the freezer will shrink, the one in the warm water will expand.

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Following up on the first post: if you’re wondering about the mechanics behind this process, consider an “ideal gas” to be a collection of particles that fly around, bouncing off one another and the walls of the container in which they are stored.

If you add energy to the system in the form of heat, the little particles move around faster, which means that more collisions occur between the walls and the particles themselves. This causes the pressure inside the tank to increase.

Conversely, taking energy away by dropping the temperature means that the particles move more slowly, and the pressure drops because there are fewer collisions between the particles and the container walls.

As the first poster indicated, this works from a pressure point of view as well:

Stuff more particles into a given space (increase the pressure), and there will be more collisions and therefore temperature will increase as each collision between wall/particle imparts a bit of energy to the container. Take particles out (decrease pressure) and temperature drops.

Stuff like this (well, only bigger, with weird particles and radiation) is what made me decide to do a Physics degree.

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This phenomenon is exploited by the, world famous in New Zealand at least, Jet Engine Beer Cooler;

http://www.asciimation.co.nz/beer/

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Thermodynamics 101

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Physical property of propane:
When you draw the gaseous fuel out of the container and lower the pressure the liquid will boil into a gas. This boiling point or ‘change of state’ from a liquid to a gas takes place at -44 degrees Fahrenheit.

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Paul Reply:
March 17th, 2009 at 4:58 am

As EB stated, propane boils at -44 degrees F. The heat needed to boil the liquid is drawn from the air surrounding the cylinder and transferred to the liquid through the metal wall of the container.

The area where the liquid is in contact with the cylinder is called the ‘wetted area’. When the cylinder is full there is a lot of wetted area to transfer the heat needed to vaporize the propane.

As the propane is used, and the liquid level drops, there is less and less wetted area to provide heat transfer. As this happens, the part of the cylinder that is still in contact with the liquid becomes very cold and moisture from the air condenses on the metal. When the temperature of the metal drops below 32 degrees F. the moisture begins to freeze and frost or ice is formed, usually at the bottom, where the remaining propane liquid is in contact with the container.

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