An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer to provide the necessary thermal energy for phase transition from liquid to vapor. Within evaporators, a circulating liquid is exposed to an atmospheric or reduced pressure environment, causing it to boil at a lower temperature compared to normal atmospheric boiling.
The four main components of an evaporator assembly are: (1) tubes or channels where the refrigerant liquid is circulated, (2) fins or other enhanced surfaces to increase heat transfer area, (3) a source of heat such as steam or combustion gases directed over the tubes, and (4) distillation of vapor into an outlet piping system. Heat is transferred through the tube walls to the liquid inside via conduction, providing the thermal energy needed for evaporation. Convective currents inside also contribute to heat transfer efficiency.
There are various evaporator designs suitable for different applications. Shell and tube, plate, and flooded evaporators are commonly used in industrial processes like desalination, power generation, and air conditioning. Plate-type evaporators offer compactness while multi-stage designs enable enhanced evaporation rates at lower heat duties. Overall evaporator performance is dependent on factors such as heat transfer coefficient, tube/plate material properties, flow regime, and vapor quality achieved.
Advanced control techniques like online fouling detection are utilized to maintain evaporator thermal performance over time. CFD modeling and new surface coating technologies also continue improving heat and mass transfer capabilities for more energy efficient vapor generation. Evaporators remain an essential unit operation across many industries owing to their ability to separate mixed phases through a controlled phase change process.
Uses
Air conditioning and refrigeration
Some air conditioners and refrigerators use compressed liquids with a low boiling point that vaporizes within the system to cool it, whilst emitting the thermal energy into its surroundings.
Food industry and synthetic chemistry
Evaporators are often used to concentrate a solution. One example is the climbing/falling film plate evaporator, which is used to make condensed milk.
Similarly, reduction (cooking) is a process of evaporating liquids from a solution to produce a "reduced" food product, such as wine reduction.
Evaporation is the main process behind distillation, which is used to concentrate alcohol, isolate liquid chemical products, or recover solvents in chemical reactions. The fragrance and essential oil industry uses distillation to purify compounds. Each application uses specialized devices.
Chemical engineering
In the case of desalination of seawater or in Zero Liquid Discharge plants, the reverse purpose applies; evaporation removes the desirable drinking water from the undesired solute/product, salt.
Chemical engineering uses evaporation in many processes. For example, the multiple-effect evaporator is used in Kraft pulping, the process of producing wood pulp from wood.
Marine
Large ships usually carry evaporating plants to produce fresh water, reducing their reliance on shore-based supplies. Steamships must produce high-quality distillate to maintain boiler-water levels. Diesel-engine ships often utilize waste heat as an energy source for producing fresh water. In this system, the engine-cooling water is passed through a heat exchange, where it is cooled by concentrated seawater. Because the cooling water, which is chemically treated fresh water, is at a temperature of , it would not be possible to flash off any water vapor unless the pressure in the heat exchanger vessel is dropped.
A brine-air ejector venturi pump is then used to create a vacuum inside the vessel, achieving partial evaporation. The vapor then passes through a demister before reaching the condenser section. Seawater is pumped through the condenser section to cool the vapor sufficiently for condensation. The distillate gathers in a tray, from where it is pumped to the storage tanks. A salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilization is carried out after the evaporator.