How full is your tank?

As you lower the refrigerant cylinder’s temperature, some of the vapour that is above the liquid refrigerant in the cylinder will condense. With this in mind, one would think that the liquid refrigerant level in the cylinder would rise. There is one more very important factor that must be considered, and that is the volume of the refrigerant liquid and vapour that is in the cylinder at the various ambient temperatures. To clarify these two points, let’s select a refrigerant and specify example temperatures. The question mentions a partially full cylinder of refrigerant being placed into a freezer overnight. In our example we will use a new, full 30 pounds DOT 39 cylinder of R-134a (the theory will be the same for a partially full cylinder) that is in a room having an ambient temperature of 75F. We will then place this same container of refrigerant into a freezer that has an ambient temperature of -25F. The average internal volume of a DOT 39 refrigerant cylinder is 829 cubic inches, or 0.48 cubic feet. Using these facts, and a thermodynamic saturated property table for R-134a, you can determine the refrigerant’s volume of liquid and vapour at both of the above stated temperatures:

At 75F, R-134a liquid has a volume of 0.0132 cubic feet per pound.

At 75F, R-134a vapour has a volume of 0.5119 cubic feet per pound.

At minus 25F, R-134a liquid has a volume of 0.0115 cubic feet per pound.

At minus 25F, R-134a vapour has a volume of 3.9032 cubic feet per pound.

By using the thermodynamic information above you can determine that a cylinder at 75F will have 0.396 cubic feet of liquid (0.0132 x 30) and 0.084 cubic feet of vapour (0.48 – 0.396). The cylinder will have 29 pounds 13.38 oz. of liquid and 2.62 oz. of vapour at this condition of 75F. If this same cylinder was to be cooled to an ambient temperature of -25F, we again can calculate the liquid and vapour weights as above using the thermodynamic saturated properties stated at -25F. We will now have 0.345 cubic feet of liquid (0.0115 x 30) and 0.135 cubic feet of vapour (0.48 – 0.345). This same cylinder will now have 29 pounds 15.45 oz of liquid and 0.55 oz. of vapour at this revised ambient temperature of -25F.

In conclusion: The refrigerant cylinder has the same amount (30 pounds) of R-134a Refrigerant in it at both the 75 and -25 ambient temperatures but the liquid to vapour saturated ratio is quite different. As you can see, as you lower the ambient temperature around a refrigerant cylinder, the weight of liquid refrigerant in the cylinder will increase, while the liquid volume will decrease, and the vapour weight of the refrigerant will decrease while the volume of the refrigerant vapour will increase.

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