Condensation examples from life. Ecological dictionary: what is condensation, what does it mean and how to spell it correctly

). Condensation occurs at isothermal conditions. compression, adiabatic expansion and cooling or simultaneously. lowering it and so on, leading to the fact that condensation. the phase becomes thermodynamically more stable than the gaseous phase. If at the same time the temperature is higher than in for a given substance, (liquefaction) is formed, if lower, the substance passes into the solid state, bypassing the liquid state (desublimation). TO condensation is widely used in chemistry. technologies for separating mixtures through, with and cleaning materials etc., in, for example. in condensers of steam turbines, in refrigeration equipment for condensation of the working fluid, in desalination. installations, etc. During condensation in narrow pores, the latter can absorb, which means. quantity of substances from the gas phase (see). The consequence of water condensation is rain, snow, dew, frost. Condensation in liquid state. In the case of condensation in a volume or a vapor-gas mixture (homogeneous condensation), condenser. phase is formed in the form of small drops (fog) or small . This requires the presence of condensation centers, which can be very small droplets (nuclei) formed as a result of fluctuations in the density of the gas phase, dust particles and particles carrying electricity. charge(). In the absence of condensation centers, it can last for a long time. time to be in the so-called. metastable (supersaturated) state. Stable homog. condensation begins at the so-called. critical supersaturation P kp =p k /p n where p k is the equilibrium corresponding to the critical. diameter of nuclei, pH - sat. over a flat surface (for example, for water, cleared of solid particles or, P cr = 5-8). The formation of fog is observed both in nature and in technology. devices, for example when cooling a vapor-gas mixture due to radiation emission, wet. Condensation on a saturated or overheated surface occurs at a surface temperature that is less than the saturation temperature at its equilibrium above it. Observed in many industries. devices, which serve for condensation of target products, heating decomposition. environments, separation of steam and vapor-gas mixtures, cooling of wet, etc. During liquefaction, a continuous film is formed on a surface that is well wetted by condensate (film condensation); on a surface not wetted by condensate or partially wetted - individual drops (drip condensation); on surfaces with inhomogeneous properties (for example, on polished metal with oxidized contaminated areas) - areas covered with a film of condensation and drops (mixed condensation). With film condensation of pure coefficients. heat transfer is determined mainly. thermal resistance of the condensate film, which depends on its flow regime. The latter, in the case of practically motionless, is determined by the Reynolds number of the film: Re pl =w d /v to, where w, d - resp. cross-sectional average speed and thickness of the condensate film, vk - kinematic. condensate For condensation on a vertical or pipe at Repl less than 5-8, the film flow is purely laminar, when Repl exceeds these values, it is laminar-wave, and at Repl >>350-400, it is turbulent. That means on vertical surfaces. altitudes, areas with different heights may be observed. condensate film flow regimes. In laminar flow, an increase in Repl with increasing film thickness leads to a decrease in the coefficient. heat transfer, in turbulent flow - to its increase. If overheated, condensation is accompanied by convective heat transfer from to condensate, the surface temperature of which is practically equal to the saturation temperature at . For substances with a large heat of condensation (for example, ) the heat of superheating is usually insignificant compared to the heat of condensation and can be neglected. In the case of film condensation, the moving tangential stress at the interface, caused by the interphase and transfer of momentum by condensed particles, which are attached to the condensate film, causes an increase in speed and a decrease in film thickness during the downward flow, resulting in a coefficient. heat transfer increases. With more high speeds steam flow, its impact on the condensate film can lead not only to a change in its speed and thickness, but also to a flow disturbance (wave formation, turbulization), which intensifies heat transfer in the film. If the flow is directed upward, the movement of the laminar film of condensate is slowed down, its thickness increases and the coefficient. heat transfer decreases as the speed increases until the interfacial action causes the so-called. reversed (upward) flow of the condensate film. When condensation moves inside the pipe (channel), the flow regimes and the nature of the interaction. vapor and liquid phases can change significantly as a result of changes in the rate of condensate formation, tangential stress on the interphase surface and Re pl. At high speeds (when the effect of gravity on the condensate film is negligible and its flow is determined mainly by the force), local and average coefficients along the length of the pipe. heat transfer does not depend on spaces. pipe orientation. If the forces of gravity are comparable, the condensation conditions are determined by the angle of inclination of the pipe and the mutual direction of movement of the phases. In the case of condensation inside a horizontal pipe and at low speed, an annular film of condensate forms only on the top, part of the internal surface of the pipe. On the bottom parts, a “stream” arises, in the zone of which, as a result, relatively large thickness layer, heat transfer is much less intense than in the rest of the surface. In the case of condensation on a bundle of horizontal pipes, the flow rate of flowing condensate increases from top to bottom due to the flow of condensate from the overlying pipes to the underlying ones, and the flow rate along the path of its movement decreases. In a bundle with a constant or relatively slightly decreasing height of the open cross-section between the pipes, the speed of the downward flow gradually decreases, and condensate flows from the top of the pipes to the bottom. Initially, this leads to a decrease in local coefficients. heat transfer (averaged along the perimeter of the pipes) with an increase in the number of the horizontal row of pipes, measured from above. However, starting from a certain series, as a result of condensate leakage, the flow of the film is disturbed and its thermal temperature is disturbed. resistance decreases. Thanks to this coefficient. heat transfer can stabilize, and with increasing impact of film flow disturbances on the bottom. tubes - increase with increasing row number. Intensification of heat transfer during film condensation can be achieved by profiling its surface (for example, using what is called a finely wavy surface), which helps to reduce the average thickness of the condensate film, creating art, roughness on the surface, leading to turbidity. bulization of the film, exposure to dielectric. liquid phase (for example, during condensation) electrostatic. field, suction of condensate through a porous surface, etc. During condensation of liquids, the liquid phase is very high. Therefore, the share of thermal The resistance of the condensate film in the total resistance to heat transfer is insignificant, and the interfacial thermal temperature is decisive. resistance due to molecular kinetics. effects at the interface. Sometimes film condensation on the surface is accompanied by homog. condensation in the layer adjacent to the phase interface. If the formation of fog in this case is undesirable (for example, in the production of H 2 SO 4 nitrous method or when capturing volatile solutions), the process is carried out at max. supersaturation below P cr. During drip condensation, primary small drops formed on a dry vertical or inclined surface grow as a result of the continuation of the process, the merging of closely spaced and touching drops and the pull-up of condensate that appears between the drops and quickly breaks to them. Drops that have reached the “separation” diameter flow down, combining (coalescing) with underlying small drops, after which small drops again form on the freed surface, and the cycle repeats. The conditions that determine the spontaneous occurrence of droplet condensation are rarely observed. Usually, to carry out dropwise condensation, they apply thin layer lyophobizer - a substance that has low and non-wettable condensate (for example,). In the case of drip condensation, coefficient. heat transfer is much higher (5-10 times or more) than with film. However, maintaining industrial operating conditions. It is difficult to use stable drip condensation devices. Therefore, condensation chemical devices industries, as a rule, operate in film condensation mode. Condensation on the surface of the same substance occurs in technology. devices on the surface of dispersed jets supplied into the volume (for example, using spray nozzles) or flowing down. or the distribution does not allow the phase contact surface to be greatly developed. In some cases, condensation is observed when entering the volume in the form of jets or bubbles (bubbling), as well as when steam bubbles form in the volume, for example. during cavitation. TO condensation from a mixture of it with non-condensing (or non-condensing at a given temperature) on the surface

Condensation is the transition of a substance from a gaseous to a liquid state.
The liquid molecules that left it during the evaporation process are in the air in a state of continuous thermal motion. Since the movement of molecules is chaotic, some part of the molecules again enters the liquid. The greater the vapor pressure above the liquid, the greater the number of such molecules. The steam condenses.

The process of turning steam into liquid occurs with the release of some heat.

The amount of heat released during condensation is determined by the formula:

where L - specific heat vaporization.
The above formula is suitable both for calculating the amount of heat required to convert a liquid into steam (during boiling) and for the amount of heat released during condensation.

The rate of condensation depends on the type of liquid, the presence of condensation centers and temperature.

The temperature of the substance does not change during the condensation process.
The condensation temperature of the vapor of a substance is equal to the boiling point of this substance.

HOW FOG AND DEW APPEAR

There is always water vapor in the air, although their density is hundreds of times less density air. The amount of water vapor in the air cannot be infinitely large. There is a maximum mass of water that can be contained in 1 cubic meter of air at a given temperature. The higher the air temperature, the more water vapor may be present in the air.
As the air temperature decreases, water vapor at some point becomes saturated.
With further cooling, they begin to condense and appear in the form of tiny droplets on condensation centers - dust particles, smoke particles, gas ions.
The droplets that appear in the air are called fog.
And drops on the surface of the earth, on leaves and grass are called dew.
Fogs don't last long. Drops in the air can merge, and then rain falls, or evaporate, and then the fog dissipates.

DID YOU KNOW

What, white trail in the sky from a flying airplane there is fog formed from water vapor, the supplier of which is burning fuel. Hot exhaust gas, saturated with water vapor, enters the cold atmosphere and forms fog.

INTERESTING

If there is an open pan of water close to boiling on a gas stove with an extremely high burner flame, then as soon as you turn off the gas, abundant steam appears above the pan. It turns out that when the burner was operating, steam condensation occurred on long distance from the pan, the condensate was carried away by convection air currents, so the condensed particles of steam were not visible. When the burner was turned off, the steam began to condense above the pan and therefore became visible.

Why does the refrigerator have to be turned off and defrosted from time to time? Most foods contain water. Evaporating, it then freezes on the coldest part of the refrigerator - the evaporator, and it is covered with a thick snow coat, which has low thermal conductivity. This leads to a decrease in heat removal from the chamber, and the temperature in the refrigerator does not decrease enough.

C'mon

Why a glass with cold water does the outside become covered with drops of water when we bring it into a warm room?
Why do these drops disappear after a while?

Types of condensation

Condensation of saturated vapors

In the presence of a liquid phase of a substance, condensation occurs at arbitrarily low supersaturations and very quickly. In this case, a mobile equilibrium arises between the evaporating liquid and condensing vapor. The Clapeyron-Clausius equation determines the parameters of this equilibrium - in particular, the release of heat during condensation and cooling during evaporation.

Condensation of supersaturated steam

Availability of re saturated steam possible in the following cases:

the absence of a liquid or solid phase of the same substance.
absence condensation nuclei- solid particles or liquid droplets suspended in the atmosphere, as well as ions (the most active condensation nuclei).
condensation in the atmosphere of another gas - in this case, the rate of condensation is limited by the rate of diffusion of vapors from the gas to the surface of the liquid.

Condensation into solid phase

Condensation, bypassing liquid phase, occurs through education small crystals(desublimation). This is possible in the case of vapor pressure below the pressure in triple point at low temperatures.

Condensation on windows

The formation of condensation on glass occurs during the cold season - either in winter or late autumn. From a physics point of view, the formation of condensation on windows occurs due to the temperature difference between the contacting surfaces, especially at the junction of the frame and the glass itself. The greater this difference, the greater the amount of moisture deposited on a unit surface per unit of time. If the temperature difference exceeds 55-60°, then the settled condensate can turn into a thin crust of ice or frost. The reason for the formation of condensation on glass is the slow circulation of air in the room, as well as excessive humidity.

Literature

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Synonyms:

Antonyms:

Condensation (heat engineering)
Condenser (heat engineering)

See what “Condensation” is in other dictionaries:

CONDENSATION- (lat. condensatio). Thickening, compaction. Dictionary foreign words, included in the Russian language. Chudinov A.N., 1910. CONDENSATION is generally condensation: condensation of electricity, condensation of vapors of any substance into liquid (using pressure and ... ... Dictionary of foreign words of the Russian language

condensation- and, f. condensation f. condensatio 1. special Thickening, compaction. BAS 1. Steam condensation. Condensation of electricity. Ush. 1934. 2. Transition of gas or vapor into a liquid state. SIS 1954. Condensation aya, oe. Condensation water. BAS 1.… … Historical Dictionary Gallicisms of the Russian language

CONDENSATION- (from Late Latin condensatio compaction, thickening), the transition of matter from gaseous state into liquid or solid. Condensation is a first-order phase transition. Condensation is only possible at temperatures below critical point … Modern encyclopedia

CONDENSATION- CONDENSATION, condensation, women. (specialist.). Action under Ch. condense and condense. Condensation of electricity. Condensation of steam (turning it into liquid). Dictionary Ushakova. D.N. Ushakov. 1935 1940 ... Ushakov's Explanatory Dictionary

CONDENSATION- (from Late Lat. condensatio compaction, thickening), the transition to water due to its cooling or compression from a gaseous state to a condensed state (liquid or solid). K. steam is possible only at a temp pax below critical for a given VA (see... ... Physical encyclopedia

Condensation- – transition of a substance from a gaseous state to a liquid or solid. [ Terminological dictionary on concrete and reinforced concrete. FSUE "National Research Center "Construction" NIIZHB and m. A. A. Gvozdev, Moscow, 2007 110 pages] Condensation - formation ... ... Encyclopedia of terms, definitions and explanations of building materials

Condensation- (from Late Latin condensatio compaction, thickening), the transition of a substance from a gaseous state to a liquid or solid. Condensation is a first-order phase transition. Condensation is only possible at temperatures below the critical point. ... Illustrated Encyclopedic Dictionary

CONDENSATION- (from Late Lat. condensatio condensation, thickening), the transition of a substance from a gaseous state to a liquid or solid. Condensation is only possible at temperatures below critical temperature … Big Encyclopedic Dictionary

condensation- accumulation, thickening, compaction. Ant. rarefaction Dictionary of Russian synonyms. condensation noun, number of synonyms: 7 homopolycondensation (2) ... Dictionary of synonyms

Condensation- (from the Latin condense I thicken) the transition of atmospheric water vapor into a liquid state. Plays an important role in water exchange, in particular in desert ecosystems, where night condensation of moisture on the surface of plants (dew) and soil particles is very important, and... ... Ecological dictionary

condensation- – first-order phase transition from a gaseous state to a liquid or solid. Dictionary by analytical chemistry capillary condensation... Chemical terms

Condensation of water vapor in the air over a cup of hot water

Condensation occurs in many heat exchangers (for example, in fuel oil heaters at thermal power plants), in desalination plants, and technological devices (distillation units). The most important application in thermal power plants is steam turbine condensers. In them, condensation occurs on water-cooled pipes. For increasing efficiency In the thermodynamic cycle of thermal power plants, it is important to reduce the condensation temperature (by lowering the pressure), and usually it is close to the temperature of the cooling water (up to 25÷30°C).

Condensation is a process, in a certain sense, the reverse of boiling. But upon condensation more important than the problem increasing heat transfer in order to ensure quick selection warmth.

Types of condensation

Condensation can occur in the volume (fog, rain) and on the cooled surface. In heat exchangers - condensation on the cooled surface. We will consider it further. Of course, with such condensation, the wall surface temperature Tw must be less than the saturation temperature Ts, that is, Tw< Ts. В свою очередь, конденсация на охлаждаемой поверхности может быть двух видов:

Film condensation– occurs when a liquid wets a surface (liquid is wetting, surface is wettable, these properties are studied in the Physics course), then the condensate forms a continuous film.

Drip condensation– when condensate is a non-wetting liquid and collects on the surface into drops that quickly drain, leaving almost the entire surface clean.

With film condensation, heat transfer is much less because of thermal resistance films (the film interferes with the removal of heat from the steam to the wall). Unfortunately, Implementing droplet condensation is difficult– non-wettable materials and coatings (for example, such as fluoroplastic) themselves conduct heat poorly. And the use of additives - water repellents (for water such as oil, kerosene) turned out to be ineffective. Therefore usually film condensation occurs in heat exchangers . Hydrophobizer, hydrophobicity - from the Greek “hydör” - “water” and “phóbos” - fear. That is, hydrophobic is the same as water-repellent, non-wettable. Such additives for arbitrary liquids are called lyophobizers.

The term “stationary steam” in in this case implies the absence of significant forced movement (of course, free-convective movement will take place).

A film of condensation forms on the wall surface. It flows down, while its thickness increases due to ongoing condensation (Fig. ...). Due to the thermal resistance of the film, the wall temperature is noticeably lower than the temperature of the film surface, and on this surface there is a small jump in the temperatures of condensate and steam (for water the jump is usually on the order of 0.02–0.04 K). The temperature of the vapor in the volume is slightly higher than the saturation temperature.

At first, the film moves stably laminarly - this laminar mode. Then waves appear on it (with a relatively large step, running through the film and collecting the accumulating condensate, since in a thicker layer in the wave the speed of movement is greater, and such a flow mode is energetically more favorable than the established one). This laminar wave mode. Next, when large quantities condensation mode may become turbulent.

On vertical pipes the picture is similar to the case of a vertical wall.

On a horizontal pipe, the condensation heat transfer is higher than on a vertical pipe (due to the smaller average film thickness). When steam moves, heat transfer increases, especially when the film is blown away.

In the case of pipe bundles (in particular, in condensers), the following features occur:

1) The velocities of steam as it passes through the beam decrease due to its condensation.

2) In horizontal bundles, condensate flows from pipe to pipe, on the one hand, increasing the thickness of the film on the lower pipes, which reduces heat transfer, on the other hand, the fall of condensate drops disturbs the film on the lower pipes, increasing heat transfer.

Intensification of heat transfer in condensers

The main way of intensification is to reduce the thickness of the film, removing it from the heat exchange surface. For this purpose, condensate drain caps or twisted ribs are installed on vertical pipes. For example, caps installed in 10 cm increments increase heat transfer by 2–3 times. Low ribs are installed on horizontal pipes, along which condensate quickly drains. It is effective to supply steam in thin streams that destroy the film (heat transfer increases 3÷10 times).

Effect of gas impurities on condensation

When steam moves, this influence is much less, but still in industrial installations the air has to be pumped out of the condensers (otherwise it takes up the volume of the apparatus). And they try to exclude his presence in the couple altogether.

Since condensation is the reverse process to boiling, the main calculation formula essentially the same as during boiling:

G = Q / γ (\displaystyle G=Q/\gamma )

where G is the amount of condensate formed (condensing steam), kg/s;

Q – heat flux removed from the wall, W;

γ – heat of phase transition, J/kg.

This formula does not take into account the heat of cooling steam to saturation temperature ts and subsequent cooling of the condensate. They are easy to take into account at known temperatures of steam at the inlet and condensate at the outlet. But, unlike the case of boiling, it is difficult to estimate even approximately the value of Q due to the small temperature difference of heat transfer (from steam to the coolant cooling the wall). Formulas for various cases of condensation are available in textbooks and reference books.

Condensation of saturated vapors

Condensation of supersaturated steam

The presence of supersaturated steam is possible in the following cases:

the absence of a liquid or solid phase of the same substance.
absence condensation nuclei- solid particles or liquid droplets suspended in the atmosphere, as well as ions (the most active condensation nuclei).
condensation in the atmosphere of another gas - in this case, the rate of condensation is limited by the rate of diffusion of vapors from the gas to the surface of the liquid.

Condensation into solid phase

Condensation, bypassing the liquid phase, occurs through the formation of small crystals (desublimation). This is possible in the case of vapor pressure below the pressure at the triple point at a low temperature.

Condensation on windows

Condensation on glass occurs during the cold season. Condensation on windows occurs when the surface temperature drops below the dew point temperature. The dew point temperature depends on the temperature and humidity of the air in the room. The reason for the formation of condensation on windows can be either an excessive increase in indoor humidity caused by poor ventilation, or the low thermal insulation properties of a double-glazed window, metal-plastic frame, window frame, incorrect installation depth of a window in a homogeneous wall, incorrect installation depth relative to the wall insulation layer, V complete absence, or poor quality insulation of window slopes.

Steam condensation in pipes

As the steam passes through the pipe, it gradually condenses and a film of condensate forms on the walls. In this case, the steam flow rate G" and its speed, due to a decrease in the steam mass, decrease along the length of the pipe, and the condensate flow rate G increases. The main feature of the condensation process in pipes is the presence of a dynamic interaction between the steam flow and the film. The force of gravity also acts on the condensate film. As a result, depending on the orientation of the pipe in space and the speed of the steam, the nature of the movement of condensate can be different. In vertical pipes, when steam moves from top to bottom, the forces of gravity and the dynamic effect of the steam flow coincide in direction and the condensate film flows down in short pipes at a low steam speed. flow, the flow of the film is mainly determined by the force of gravity, similar to the case of condensation of stationary steam on a vertical wall. The intensity of heat transfer also increases. This is explained by a decrease in the thickness of the condensate film, which flows faster under the influence of the steam flow. In long pipes at high steam velocities, the picture of the process becomes more complicated. Under these conditions, partial separation of liquid from the surface of the film and the formation of a vapor-liquid mixture in the core of the flow are observed. In this case, the influence of gravity is gradually lost, and the laws of the process cease to depend on the orientation of the pipe in space. In horizontal pipes at not very high steam flow velocities, the interaction of gravity and friction of steam on the film leads to a different flow pattern. Under the influence of gravity, the condensate film flows down the inner surface pipes down. Here condensation accumulates and forms a stream. This movement is superimposed by the movement of condensate in the longitudinal direction under the influence of steam flow. As a result, the intensity of heat transfer turns out to be variable around the circumference of the pipe: in the upper part it is higher than in the lower part. Due to the flooding of the lower section of a horizontal pipe with condensate, the average intensity of heat transfer at low steam velocities may be even lower than when static steam condenses outside a horizontal pipe of the same diameter.

Condensation(Late Latin condensatio- condensation, from Latin condenso compact, thicken) - the transition of a substance from a gaseous state to a liquid or solid due to its cooling or compression. Steam condensation is possible only at temperatures below the critical temperature for a given substance. Condensation, like the reverse process - evaporation, is an example of phase transformations of a substance ( phase transitions 1st kind). During condensation, the same amount of heat is released that was expended on the evaporation of the condensed substance. Rain, snow, dew, frost - all these natural phenomena are a consequence of the condensation of water vapor in the atmosphere.

Types of condensation

There are two known modes of surface condensation: film and droplet. The first is observed during condensation on a wetted surface; it is characterized by the formation of a continuous film of condensate. On non-wetted surfaces, condensation forms in the form of individual droplets. With droplet condensation, the intensity of heat transfer is much higher than with film condensation, since a continuous film of condensate makes heat transfer difficult.

The rate of surface condensation is higher, the lower the surface temperature is compared to the saturation temperature of the steam at a given pressure. The presence of another gas reduces the rate of surface condensation, since the gas impedes the flow of steam to the cooling surface. In the presence of non-condensable gases, condensation begins when the steam at the cooling surface reaches a partial pressure and temperature corresponding to the saturation state (dew point).

Condensation can also occur inside the volume of steam (steam-gas mixture). For volumetric condensation to begin, the steam must be noticeably supersaturated. A measure of supersaturation is the vapor pressure ratio p to saturated steam pressure ps , which is in equilibrium with a liquid or solid phase having flat surface. Steam is supersaturated if p/ps > 1 , at p/ps = 1 steam saturated. Degree of supersaturation p/ps needed to get started. Condensation depends on the content of tiny dust particles (aerosols) in the vapor, which are ready-made centers, or nuclei, of condensation. The purer the steam, the higher it should be initial degree supersaturation. Electrically charged particles, in particular ionized atoms, can also serve as condensation centers. This is the basis, for example, of the operation of a number of nuclear physics instruments.

Application

Condensation is widely used in technology: in energy (for example, in steam turbine condensers), in chemical technology(for example, when separating substances by fractionated condensation), in refrigeration and cryogenic technology, in desalination plants, etc. The liquid formed during condensation is called