Vapor-compression evaporation

Vapor-compression evaporation (also known as mechanical vapor recompression MVR and also mechanical vapor compression MVC) is the evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the temperature of the vapor produced. In this way, the vapor can serve as the heating medium for its "mother" liquid or solution being concentrated, from which the vapor was generated to begin with. If no compression was provided, the vapor would be at the same temperature as its "mother" liquid/solution, and no heat transfer could take place.

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The efficiency and feasibility of this process lie on the efficiency of the compressing device (e.g., blower, compressor) and the heat transfer coefficient attained in the heat exchanger contacting the condensing vapor and the boiling "mother" solution/liquid. Theoretically, if the resulting condensate is subcooled, this process could allow full recovery of the latent heat of vaporization, which would otherwise be lost if the vapor, rather than the condensate, was the final product; therefore, this method of evaporation is very energy efficient. The evaporation process may be solely driven by the mechanical work provided by the compressing device.

A vapor-compression evaporator can make clean water from any water source. As such it competes in the marketplace with reverse osmosis. Where vapor compression chiefly differs from reverse osmosis is its ability to make clean water from saturated or even crystallizing brines with TDS up to 650,000 mg/L. Reverse osmosis can make clean water from sources no higher in TDS than approximately 35,000 mg/L.

For economic reasons evaporators are seldom operated on low-TDS water sources. Those applications are filled by reverse osmosis. The already brackish water which enters a typical evaporator is concentrated further. The increased dissolved solids act to increase the boiling point well beyond that of pure water. Seawater with a TDS of approximately 30,000 mg/L— exhibits a boiling point elevation of less than 1 degree F. But saturated sodium chloride at 360,000 mg/L has a boiling point elevation of about 13 degrees F. This boiling point elevation represents a challenge for vapor-compression evaporation in that it increases the pressure ratio that the steam compressor must attain to effect vaporization. Since boiling point elevation determines the pressure ratio in the compressor, it is the main overall factor in operating costs.