The withdrawal of propane vapour from a vessel, lowers the contained pressure. This causes the liquid to "boil" in an effort to restore the pressure by generating vapour to replace that which was withdrawn. The required "latent heat of vapourization" is surrendered by the liquid air and causes the temperature of the liquid to drop as a result of the heat so expanded.

The heat lost due to the vapourization of the liquid is replaced by the heat in the air surrounding the container. This heat is transferred from the air through the metal surface of the vessel into the liquid. The area of the vessel in contact with vapour is not considered because the heat absorbed by the vapour is negligible. The surface area of the vessel that is bathed in liquid is known as the "wetted surface". The greater this wetted surface, or in other words the greater the amount of liquid in the vessel, the greater the vapourization capacity of the system. A larger container would have a larger wetted surface area and therefore would have greater vapourization capacity. If the liquid in the vessel receives heat for vapourization from the outside air, the higher the outside air temperature, the higher the vapourization rate of the system. How all this affects the vapourization rate of a 100 lb cylinder is shown in another section (return to contents and see 100# DOT cylinders). You will note that the worst conditions for vapourization rates are when the container has a small amount of liquid in it and the outside air temperature is low.

With the principles stated above in mind, simple formulae for determining the proper number of DOT cylinders and proper size of ASME storage containers for various loads where temperatures may reach 0 degrees F. will be found in the sections on 100# DOT Cylinders and ASME Storage tanks respectively