Vapor-compression distillation, also known as compression distillation, is a process which uses its own steam after it has been compressed as its primary heat source. In this process, it is necessary to provide a mechanical energy source, specifically a mechanical compressor.
In this process, sea water is preheated in a tubular heat exchanger by the outgoing fresh and salt water streams. The sea water is then boiled in the tubes of the still. The vapors are compressed and led back to the still to condense on the outside of the tubes. It is here that they provide the heat necessary for the boiling process. The non-condensing gases are removed fromthe steam-condensation space by a vent pump or ejector. This process would not function without the compressor. If the vapors were not condensed, they would be unable to condensate on the tubes which carry the boiling sea water because the temperature required to condense a pure liquid is less than the boiling temperature of sea water at the same pressure. This make it necessary to increase the pressure of the vapors in order for them to condense.