# Pressure with Cylinders

1. Mar 7, 2017

### don_greene

I have been wondering about how Propane is stored in a liquid state at 12 bar in cylinders. If a cylinder of propane was connected to a cylinder of the same size that was not pressurised would the pressure then across the two cylinders be reduced to 6 bar and the propane possibly expand? If this is the case then are the cylinders pressurised with normal air before propane is added? Or does the temperature of the propane predetermine the pressure of the cylinders in which it is stored and so the second cylinder would be raised to 12 bar also, this seems unlikely to me however.

2. Mar 7, 2017

### Averagesupernova

The propane compresses itself by boiling. As it boils the pressure in the tank increases until an equilibrium is reached. Temperature definitely affects tank pressure. Propane tanks are often filled to a certain percent based on ambient temp. Connecting an empty bottle a full will not halve the pressure as there will be liquid propane that boils off.

3. Mar 8, 2017

### Baluncore

So long as liquid propane is present, it will boil off to raise the pressure in the container to the vapour pressure of propane at that temperature. The vp of propane will be about 25 psi at 0°C and 40 psi at 20°C. At about 100°C the vp will be close to the 12 Bar design pressure.
The specified 12 Bar pressure is probably the maximum cylinder pressure expected when being sprayed with water during a fire.

Connecting a liquid propane cylinder to an evacuated cylinder will not change the propane pressure once the temperature stabilises.

4. Mar 8, 2017

### don_greene

Ok thanks for the explanations, so the propane expanding in container one will bring up the pressure of container two to roughly the same pressure. Does this mean that if container one were sitting in a bath of warm water and container two in a bath of cold water that gradually after some time all the gas would evaporate and condense into container two?

5. Mar 8, 2017

### Baluncore

All the liquid in container 1 would evaporate. Liquid would boil in hot container 1 and move to condense in cold container 2. The pressure in the system would be the vp at the temperature of container 2, where the liquid reservoir forms.

6. Mar 8, 2017

### don_greene

So in this instance the pressure in container one wouldn't necessarily rise to the vp of the temperature of the warm bath because the propane can escape, instead the propane boils and moves to container two where it condenses. Thanks a million, for the reply by the way.