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Anevaporator is a type ofheat exchanger device that facilitatesevaporation by utilizing conductive and convective heat transfer, which provides the necessary thermal energy for phase transition from liquid to vapour. 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 anevaporator assembly are:
Heat is transferred to the liquid inside the tube walls via conduction providing the thermal energy needed for evaporation.Convective currents inside it also contribute to heat transfer efficiency.
There are various evaporator designs suitable for different applications including shell and tube, plate, and flooded evaporators, commonly used in industrial processes such as desalination, power generation and air conditioning. Plate-type evaporators offer compactness while multi-stage designs enable enhanced evaporation rates at lower heat duties. The overall performance of evaporators depends on factors such as the heat transfer coefficient, tube/plate material properties, flow regime, and achieved vapor quality.
Advanced control techniques, such as online fouling detection, help maintain evaporator thermal performance over time. Additionally, computational fluid dynamics (CFD) modeling and advancements in surface coating technologies continue to enhance heat and mass transfer capabilities, leading to more energy-efficient vapor generation. Evaporators are essential to many industries because of their ability to separate phases through a controlled phase change process.
Someair conditioners andrefrigerators use compressed liquids with a low boiling point that vaporizes within the system to cool it, whilst emitting the thermal energy into its surroundings.[1][2]
Evaporators are often used to concentrate a solution. An example is theclimbing/falling film plate evaporator, which is used to makecondensed 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 behinddistillation, which is used toconcentrate alcohol, isolateliquid chemical products, or recoversolvents in chemical reactions. The fragrance andessential oil industry uses distillation to purify compounds. Each application uses specialized devices.
In the case ofdesalination of seawater or inZero Liquid Discharge plants, the reverse purpose applies; evaporation removes the desirabledrinking water from the undesired solute/product, salt.[3]
Chemical engineering uses evaporation in many processes. For example, themultiple-effect evaporator is used in Kraft pulping,[4] the process of producing wood pulp from wood.
Largeships usually carry evaporating plants to produce fresh water, reducing their reliance on shore-based supplies. Steamships must produce high-quality distillate to maintainboiler-water levels.Diesel engine ships often utilizewaste 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 70–80 °C (158–176 °F), it would not be possible to flash off any water vapor unless the pressure in theheat exchanger vessel is dropped.
A brine-air ejectorventuri pump is then used to create avacuum inside the vessel, achieving partial evaporation. The vapor then passes through ademister before reaching thecondenser 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. Asalinometer 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.
Evaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the plate type (such as the type designed byAlfa Laval). Temperature, production and vacuum are controlled by regulating the system valves. Seawater temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes and shuts down altogether when the ship is maneuvering. An alternative in some vessels, such as naval ships and passenger ships, is the use of thereverse osmosis principle for fresh-water production instead of using evaporators.
Evaporation, orvaporization, is anendothermic phasetransition process that is thoroughly understood in the field ofthermodynamics. It is intimately related to thevapor pressure of the liquid and surrounding pressure, in addition to theenthalpy of vaporization.
Evaporators work using the same principle design. A heat source is in contact with the liquid causing it to evaporate. The vapor is removed entirely (like in cooking), or it is stored for reuse (like in a refrigerator) or a product for isolation (essential oil).
Rotary evaporators use a vacuum pump to create a low pressure over a solvent while simultaneously rotating the liquid flask to increase surface area and decrease bubble size. Typically, the vapor is passed over a cold finger or coil so that the vaporized material does not damage the pump. The rotary evaporator is best used for removing solvent from solutions containing the desired product that will not vaporize at the operating pressure to separate thevolatile components of a mixture from non-volatile materials.
Natural circulation evaporators are based on the natural circulation of the product caused by thedensity differences that arise from heating (convection). A chamber containing a solution is heated, and the vaporized liquid is collected in a receiving flask.
This type of evaporator is generally made of 4–8 m (13–26 ft) tubes enclosed by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters the evaporator and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which also flows downward. This evaporator is usually applied to highlyviscous solutions, so it is frequently used in the chemical, sugar, food, and fermentation industries.
This type of evaporator is useful inconcentrating solutions.[5] The operation is very similar to that of acalandria where the liquid isboiled inside vertical tubes by applyingheat to the outside of the tubes. The produced solvent vapor presses the liquid against the walls of the tubes forming a thin film that moves upwards with the vapor. The vapor may be released from the system while the liquid may be recirculated through the evaporator to further concentrate thesolute. In many cases, the tubes of a rising film evaporator are usually between 3–10 metres (9.8–32.8 ft) in height with a diameter of between 25–50 millimetres (0.98–1.97 in). Sizing this type of evaporator requires a precise evaluation of the actual level of the liquid inside the tubes and the flow rates of the vapor and film.
Climbing and falling-film plate evaporators have a relatively large surface area. The plates are usuallycorrugated and are supported by the frame. During evaporation, steam flows through the channels formed by the free spaces between the plates. The steam alternately climbs and falls parallel to the concentrated liquid. The steam follows a co-current, counter-current path with the liquid. The concentrate and the vapor are fed into the separation stage, where the vapor is sent to a condenser. This type of plate evaporator is frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is its limited ability to treat viscous or solid-containing products. There are other types of plate evaporators that work with only climbing film.
Unlike single-stage evaporators, these evaporators can be composed of up to seven evaporator stages (effects). The energy consumption for single-effect evaporators is very high and is most of the cost for an evaporation system. Putting together evaporators saves heat and thus requires less energy. Adding one evaporator to the original decreases energy consumption by 50%. Adding another effect reduces it to 33% and so on. A heat-saving-percent equation can estimate how much one will save by adding a certain number of effects.
The number of effects in a multiple-effect evaporator is usually restricted to seven because, after that, the equipment cost approaches the cost savings of the energy-requirement drop.
Two types of feeding can be used when dealing with multiple-effect evaporators:
In recent years, multiple-effect vacuum evaporator (withheat pump) systems have come into use. These are well known to be energetically and technically more effective than systems withmechanical vapor recompression (MVR). Due to the lower boiling temperature, they can handle highly corrosive liquids or liquids which are prone to forming incrustations.[6]
Agitated thin-film evaporation has been very successful with difficult-to-handle products. Simply stated, the method quickly separates the volatile from the less volatile components using indirect heat transfer and mechanical agitation of the flowing product film under controlled conditions. The separation is normally made under vacuum conditions to maximize ∆T while maintaining the most favorable product temperature so that the product only sees equilibrium conditions inside the evaporator and can maximize volatile stripping and recovery.[7]
Technical problems can arise during evaporation, especially when the process is used in the food industry. Some evaporators are sensitive to differences in viscosity and consistency of the dilute solution. These evaporators could work inefficiently because of a loss of circulation. The pump of an evaporator may need to be changed if the evaporator needs to be used to concentrate a highly viscous solution.
Fouling also occurs when hard deposits form on the surfaces of the heating mediums in the evaporators. In foods, proteins and polysaccharides can create such deposits that reduce the efficiency of heat transfer. Foaming can also create a problem since dealing with excess foam can be costly in time and efficiency.Antifoam agents are used, but only a few can be used when food is being processed.
Corrosion can also occur whenacidic solutions such ascitrus juices are concentrated. The surface damage caused can shorten the long life of evaporators. The quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution must be taken into careful consideration.
