Evaporator (process engineering)

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Cross section through a boiler evaporator

In process engineering, an evaporator is an apparatus for converting a liquid into its vaporous state. A wide variety of evaporators are used industrially. For example the Robert evaporator in the sugar industry or the thin-film evaporator for evaporating food concentrates or pharmaceutical solutions. To reduce costs, it is common to connect several evaporators in series. For this purpose, the first evaporator is heated with heating steam (possibly with overpressure). The following evaporators are heated with the generated vapor (= solvent vapor ) from the previous evaporator. From the first to the last evaporator, the pressure must be reduced step by step in order to gain energy from the vapor.


The supply of thermal energy is necessary for the evaporation of the liquid. Evaporators therefore usually consist of a surface through which heat is transferred from a heating medium to the liquid to be evaporated. In contrast, with direct evaporators, the heating medium can be brought into direct contact with the liquid. The heating medium can consist of a hot liquid that is cooled or of a vapor that is condensed.

Boiling point curve = vapor pressure curve

Physical basis

A liquid evaporates when the vapor pressure exceeds the prevailing pressure. The pressure here can be different from the ambient pressure, i. H. higher or lower. The vapor pressure in turn depends on the temperature and increases with it. In order to evaporate the liquid, energy is required which corresponds to the enthalpy of evaporation . The same amount of energy is released when the vapor formed is condensed again.

When a liquid is in an inert, overflowing gas such. B. air evaporates, one speaks of evaporation. The energy for evaporation comes from the liquid and the air itself, so that cooling takes place. This is used with the compression refrigeration machine (refrigerator, freezer) to cool. The evaporator is located in the cold room for this purpose.

Types of evaporators

There are different types of vaporizers. The most common types are:


  • W. Matz, G. Matz: The thermodynamics of heat and material exchange in process engineering. Volume 1, Springer Verlag Berlin Heidelberg GmbH, Berlin Heidelberg 1979.
  • Karl Schwister , Volker Leven: Process technology for engineers. 2nd updated edition, Carl Hanser Verlag, Munich 2014, ISBN 978-3-446-44214-6 .

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