An evaporator is used to evaporate a volatile solvent, usually water, from a solution. Its purpose is to concentrate non-volatile solutes such as organic compounds, inorganic salts, acids or bases. Typical solutes include phosphoric acid, caustic soda, sodium chloride, sodium sulfate, gelatin, syrups and urea.
In many applications, evaporation results in the precipitation of solutes in the form of crystals, which are usually separated from the solution with cyclones, settlers, wash columns, elutriating legs, filters or centrifuges. Examples of precipitates are sodium chloride, sodium sulfate, sodium carbonate and calcium sulfate. The desired product can be the concentrated solution, the precipitated solids, or both.
In some applications, the evaporator is used primarily to recover a solvent such as potable water from saline water. In any case, the relatively pure condensed water vapor from many evaporators is recovered for boiler feed makeup, salt washing, salt dissolving, pump seals, instrument purges, equipment and line washing and other uses.
Swenson is an acknowledged leader in the field of evaporation. Swenson designs include forced-circulation, long-tube vertical (both rising and falling film), and calandria-type evaporators. The requirements of a particular process dictate the evaporator style and model best suited for the application.
Forced-circulation and calandria evaporators are used for processes where crystals are formed. These evaporators are designed to keep crystals suspended in solution to prevent scaling of the equipment. Long-tube vertical evaporators are used to concentrate a liquid that does not have solids present.
Evaporators require a source of heat to operate. This heat may be supplied from a boiler, gas turbine, vapor compressor, other evaporator, or a combination of sources. Multiple effect evaporators are cost effective when cheap, high pressure steam is available to heat the system. A mechanical vapor recompression evaporator uses electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator.