Condensation is the transition of a substance from a gaseous to a liquid aggregate state . The product of condensation is called condensate . Condensed matter , on the other hand, generally refers to matter in liquid or solid form. The values for pressure and temperature that prevail during condensation characterize the condensation point . During condensation, thermal energy is released from the condensate to the environment. This heat of condensation has the same value as the heat of evaporation .
Condensation is an important process related to weather. Clouds , fog , dew and hoarfrost are caused by the condensation of water from the air. In steam power plants , water is first evaporated and then, after passing through the turbine, returned to its liquid state in a condenser . Commercially available refrigerators use a cycle of evaporation and condensation to transport heat.
The opposite of condensation is evaporation , boiling or evaporation . The transition from solid substances to the gaseous state is called sublimation , its opposite being called resublimation . In physical parlance, the term condensation is used more generally, see e.g. B. Condensed matter or Bose-Einstein condensate .
Depending on the type of nucleation process, two basic types of condensation can be distinguished. In any case, the condition is that the gas phase is supersaturated with respect to the condensing component . If individual gas particles unite when they meet within the gas, one speaks of homogeneous condensation . For this it is necessary that sufficiently slow particles come together to form larger structures without the participation of interfaces. This process is only possible with a high level of oversaturation, usually several hundred percent. In contrast, with heterogeneous condensation, only very low supersaturations of often even less than one percent are required . This form of condensation takes place on already existing surfaces, i.e. usually on solid particles suspended in the gas phase, the condensation nuclei or aerosol particles. These act as a kind of particle catcher in relation to the respective gas, with the radius and chemical properties of the particle essentially determining how well the gas particles adhere to it. This also applies analogously to surfaces of non-particulate bodies, which is then referred to as fitting.
Condensation processes in the atmosphere
In the case of water in the earth's atmosphere , condensation and evaporation are of special importance, since the phase transition between water vapor and liquid water is a fundamental process of the natural water cycle and of the weather in general. At the macrophysical level, the turnover quantities are enormous, since the atmospheric water with around 13 · 10 15 kg has an average residence time of only around 10 days, i.e. within this period it is mainly converted via condensation. The condensation is the basic process of every formation of liquid precipitation from water vapor as well as the formation of fog and cloud . Using the latent heat and the case is heat budget of the Earth significantly shaped.
At the microphysical level, however, the condensation processes are, as shown, very complex and cannot be precisely predicted. In the atmosphere there is practically only heterogeneous nucleation , i.e. in this case the formation of water droplets from the air. The supersaturation of the air required for this must vary in strength depending on the prevailing conditions in order to cause condensation. It can be achieved on the one hand by increasing the absolute air humidity in the course of evaporation or sublimation and on the other hand by reducing the air temperature . Cooling dominates, especially adiabatic , i.e. a reduction in the maximum humidity that the air is able to absorb. If the diameter of the aerosol particles is roughly 1 μm, then oversaturation of a few tenths of a percent is often sufficient. It is also important whether the surface of the particles has hydrophilic or hydrophobic properties that facilitate or make the accumulation of water vapor particles more difficult. Of course, the concentration of the aerosol particles in the gas phase is just as important.
Meteorological and climatological aerosol research must therefore take into account a whole range of influencing factors, with other factors such as the spatial and temporal occurrence of the aerosol particles added to those already considered. All these factors have to be related to each other in order to get a correct understanding of the processes of precipitation and cloud formation, which in turn have an influence on the water and radiation balance. Although this is quite possible on a qualitative level, the quantitative influence of these parameters, especially on a global level, is difficult to determine and forms an uncertainty factor in all climate models .
Steam power plant
In steam power plants , the exhaust steam from the steam turbine is cooled further at the condenser and condensed into water. This water is used again as feed water for the steam generator . This results in a closed cycle.
In chemical plants , the condensation of water vapor is an economically important variable, since the energy supply for chemical processes is done with water vapor. After the thermal energy has been released, there is condensed water, which is collected via ring lines . This normally "pure" water is fed back to the steam generator as so-called feed water for generating steam after quality controls and possible treatment. Such a condensate return allows massive savings to be made.
Heating networks in railroad trains or buildings also use (et) some wet steam to transport heat. The radiator temperature adjusts itself to max. approx. 100 ° C (condensation temperature of the water at the low overpressures used).
Flue gas condensation
In modern Feuerungs systems the exhaust gas is in a condenser cooled. The focus is on the use of the latent heat of the steam portion. In addition to water, the separated condensate contains other accompanying substances, the release of which into the environment is reduced by the flue gas condensation.
In winter gardens, overheating or draft problems can be avoided by a so-called "hypo exchanger" system, in which the warm air in the winter garden evaporates and the moist air that rises is sucked off at the highest point of the winter garden and is passed through hypocaust pipes on the colder ground, whereupon the water vapor condenses there and the released heat of condensation is given off to the ground. Air conditioning systems that use the heat of evaporation from water also work in a similar way .
Dehumidifiers are often used to dry the building moisture in new buildings and to dry out walls and for wet rooms in which high amounts of water vapor occur (swimming pools) . These condense the water vapor from the room air sucked in, and the dried air leaving the device can then absorb moisture again. Condensation dryers that are used to dry laundry work similarly .
Exhaust gas cleaning
In exhaust gas cleaning , condensation processes are used, in particular, to separate and recover solvents . Since these often have very low dew point temperatures, condensation coolers are usually used as pre-separators before a further cleaning stage. As the sole separation stage, condensation processes are usually not able to comply with emission limit values .
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