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a device for concentrating solutions of solids in liquid solvents by the partial or complete removal of the solvent in the form of vapor. Evaporators for evaporating the water supplied to boiler plants and steam power plants and those for evaporating the refrigerant in refrigerating plants are called vaporizers.
I. M. PETRENKO
a heat-exchange device for evaporating liquids. In thermal power engineering, the evaporator is used to produce a distillate that compensates for losses of condensate in steam power plants. A tube vertical evaporator is usually heated by the steam coming from the turbine and passing through the space between the tubes. The evaporating water, which is softened in advance, passes within the tubes. There are also evaporators that are heated by flue gases emerging from boiler units. The steam produced in such evaporators may be used both to compensate for losses of condensate and to provide heat. High-capacity evaporators find application in atomic power plants located near seas and oceans for the distillation of seawater. Evaporators, which are sometimes called distillers, are installed on oceangoing vessels. They are the principal components of refrigeration units, in which the refrigerating agent used to provide direct (or using brine) cooling of refrigeration chambers is evaporated. Evaporators are also devices used to increase the concentration of various solutions.
G. E. KHOLODOVSKII
A device used to vaporize part or all of the solvent from a solution. The valuable product is usually either a solid or a concentrated solution of the solute. If a solid, the heat required for evaporation of the solvent must have been supplied to a suspension of the solid in the solution, otherwise the device would be classed as a drier. The vaporized solvent may be made up of several volatile components, but if any separation of these components is effected, the device is properly classed as a still or distillation column. When the valuable product is the vaporized solvent, an evaporator is sometimes mislabeled a still, such as water still, and sometimes is properly labeled, such as boiler-feedwater evaporator. In the great majority of evaporator installations, water is the solvent that is removed.
Evaporators are used primarily in the chemical industry. For example, common salt is made by boiling a saturated brine in an evaporator. The salt precipitates as a solid in suspension in the brine. This slurry is pumped continuously to a filter, from which the solids are recovered and the liquid portion returned for further evaporation. Evaporators are widely used in the food industry, usually as a means of reducing volume to permit easier storage and shipment. Evaporators are also the most commonly used means of producing potable water from sea water or other contaminated sources.
The vaporization of solvent requires large amounts of heat. Provisions for transferring this heat to the solution constitute the largest element of evaporator cost and the principal means of distinguishing between types of evaporators. Practically all evaporators fall into one of the following categories:
1. Submerged-combustion evaporators: those heated by a flame that burns below the liquid surface, and in which the hot combustion gases are bubbled through the liquid.
2. Direct-fired evaporators: those in which the flame and combustion gases are separated from the boiling liquid by a metal wall, or heating surface.
3. Stem-heated evaporators: those in which steam or other condensable vapor is the source of heat, and in which the steam condenses on one side of the heating surface and the heat is transmitted through the wall to the boiling liquid.