The combination of lower temperatures and higher viscosity in subsequent effects provides good conditions for treating heat-sensitive products, such as enzymes and proteins. The second effect uses the heated vapor created in the first stage as its heat source (hence the saving in energy expenditure). It is then fed into the second effect, which is slightly lower in temperature. The product is then partially concentrated as some water is transformed into vapor and carried away. Forward feeding occurs when the product enters the system through the first effect at the highest temperature. Two types of feeding can be used when dealing with multiple-effect evaporators. 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. A heat-saving-percent equation can estimate how much one will save by adding a certain number of effects. Adding another effect reduces it to 33% and so on. Adding one evaporator to the original decreases energy consumption by 50%. Putting together evaporators saves heat and thus requires less energy. The energy consumption for single-effect evaporators is very high and is most of the cost for an evaporation system. Unlike single-stage evaporators, these evaporators can be composed of up to seven evaporator stages (effects). It is intimately related to the vapor pressure of the liquid and surrounding pressure, in addition to the enthalpy of vaporization. An alternative in some vessels, such as naval ships and passenger ships, is the use of the reverse osmosis principle for fresh-water production instead of using evaporators.Įvaporation, or vaporization, is an endothermic phase transition process that is thoroughly understood in the field of thermodynamics. For this reason, the evaporator is adjusted as seawater temperature changes and shuts down altogether when the ship is maneuvering. Seawater temperature can interfere with production, as can fluctuations in engine load. Temperature, production and vacuum are controlled by regulating the system valves. Sterilization is carried out after the evaporator.Įvaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the plate type (such as the type designed by Alfa Laval). A salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. The distillate gathers in a tray, from where it is pumped to the storage tanks. Seawater is pumped through the condenser section to cool the vapor sufficiently for condensation. The vapor then passes through a demister before reaching the condenser section. Because the cooling water, which is chemically treated fresh water, is at a temperature of 70–80 ☌ (158–176 ☏), it would not be possible to flash off any water vapor unless the pressure in the heat exchanger vessel is dropped.Ī brine-air ejector venturi pump is then used to create a vacuum inside the vessel, achieving partial evaporation. In this system, the engine-cooling water is passed through a heat exchanger, where it is cooled by concentrated seawater. Diesel-engine ships often utilize waste heat as an energy source for producing fresh water. Steamships must produce high-quality distillate to maintain boiler-water levels. Large ships usually carry evaporating plants to produce fresh water, reducing their reliance on shore-based supplies. For example, the multiple-effect evaporator is used in kraft pulping, the process of producing wood pulp from wood. ![]() Ĭhemical engineering uses evaporation in many processes. ![]() ![]() 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. Each application uses specialized devices. The fragrance and essential oil industry uses distillation to purify compounds. Similarly, reduction (cooking) is a process of evaporating liquids from a solution to produce a "reduced" food product, such as wine reduction.Įvaporation is the main process behind distillation, which is used to concentrate alcohol, isolate liquid chemical products, or recover solvents in chemical reactions ( green chemistry). One example is the climbing/falling film plate evaporator, which is used to make condensed milk. Food industry and synthetic chemistry Įvaporators are often used to concentrate a solution. Some air conditioners and refrigerators use a compressed liquid with a low boiling point, such as chlorodifluoromethane or R-410A, that vaporizes within the system and cools it, while emitting the thermal energy into its surroundings. An evaporator is a device used to turn a liquid chemical substance, such as water, into its gaseous form, vapor.
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