Vapor compression evaporation process is generally used for small scale desalting units. The heat for evaporating the water comes from the heat resulting from compressing the vapor rather than the direct exchange of heat from steam. The vapor compression evaporation process is also designed to take advantage of the principle of reducing the boiling point temperature by reducing the process pressure. The primary method used to produce heat to evaporate incoming feed water is a mechanical compressor. The mechanical compressor is usually electrically driven, which eliminates the need to obtain or generate steam. The mechanical compressor creates a vacuum in the vessel and compresses vapor removed from the vessel. As vapor leaves the vessel, it condenses on the inside of the heated tube bundle, where it boils and partially evaporates, producing more vapor.