Liquefaction of Gases

A simple example of the second method for liquefying gases is the steam engine. The principle on which a steam engine operates is that water is boiled and the steam produced is introduced into a cylinder. Inside the cylinder, the steam pushes on a piston, which drives some kind of machinery. As the steam pushes against the piston, it loses energy. That loss of energy is reflected in a lowering of the temperature of the steam. The lowered temperature may be sufficient to cause the steam to change back to water.

In practice, the liquefaction of a gas by this method takes place in two steps. First, the gas is cooled, then it is forced to do work against some external system. For example, it might be driven through a small turbine, where it causes a set of blades to rotate. The energy loss resulting from driving the turbine may then be sufficient to cause the gas to change to a liquid.

The process described so far is similar to the principle on which refrigeration systems work. The coolant in a refrigerator is first converted from a gas to a liquid by one of the methods described above. It then absorbs heat from the refrigerator box, changing back into a gas in the process. The difference between liquefaction and refrigeration, however, is that in the former process, the liquefied gas is constantly removed from the system for use in some other process, while in the latter process, the liquefied gas is constantly recycled within the refrigeration system.