What is water vapor


Lexicon> Letter W> Water vapor

Definition: water in the gaseous state, or (colloquially) visible plumes of condensing water vapor in air

More specific terms: saturated steam, superheated steam, wet steam, dry steam, live steam, process steam

Molecular formula: H2O

English: water vapor

Categories: Energy carriers, basic terms, physical principles, heat and cold

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: December 22nd, 2014; last change: 03/14/2020

URL: https://www.energie-lexikon.info/wasserdampf.html

Colloquially, water vapor is often something different than in science and technology.

In science and technology, water vapor is the substance water (H.2O) in the gaseous state. Most of this article also refers to this definition. Colloquially, however, water vapor often refers to visible vapor plumes that arise when fine water droplets are formed in the air through condensation of the water vapor contained. For example, clouds contain a large number of such small water droplets and are therefore only visible because light is scattered on the droplets. By the way, unlike large raindrops, very small water droplets can linger in the air for a long time because their speed of fall is very slow.

Often one speaks only of steamwhen it is clear from the context that water vapor is meant - even if there are of course vapors from many other substances as well.

Water vapor is very important in energy technology:

Outside of energy technology, too, is often so-called Process steam required, d. H. Water vapor used in a process. Often its most important function is the supply of heat; in some cases the water vapor also takes part in a chemical reaction (for example in petroleum refineries and in coal gasification).

In the past, some buildings had steam heating, in which water vapor transported the heat from a boiler (steam boiler) to the radiators. However, such systems are rarely used any more because they have a number of disadvantages compared to today's central heating systems (with liquid water). These include the lower energy efficiency as a result of the high line losses at high temperatures and also safety-related disadvantages.

Water vapor plays an extremely important role in the earth's atmosphere. Among other things, it is the most important greenhouse gas.

Water vapor also plays an extremely important role in the earth's atmosphere. For example, air loaded with a lot of water vapor transports a large amount of latent heat that can later be released when it condenses. (This is the reason why air is warmer after crossing a mountain range, where the clouds will rain down as the clouds rise; this phenomenon is known as a hair dryer.) In addition, water vapor (and not carbon dioxide, for example) is the largest contributor to the so-called greenhouse effect in the earth's atmosphere: Clouds lead on the one hand to a partial reflection of sunlight back into space, but on the other hand also to the reflection of thermal radiation from the earth, with the latter effect predominating on average. The effect of carbon dioxide (CO2) is thereby indirectly strengthened quite significantly (Water vapor feedback): An intensification of the greenhouse effect through CO2 leads to a higher water vapor content in the atmosphere and thus to a further intensification of the greenhouse effect, i. H. to a significantly increased global warming; see also the article on climate hazards.

Since many fuels contain substantial amounts of hydrogen (in chemically bound form), water vapor is produced when they are burned. This applies in particular to natural gas and petroleum products. One consequence of this can be the sooting of chimneys if the exhaust gas temperature is chosen to be quite low and the materials used are not moisture-resistant. On the other hand, additional heat can be obtained in the condensing boiler by condensing the water vapor and draining the condensed water.

Physical basics

Water can evaporate or even boil below 100 ° C and still be liquid well above 100 ° C!

According to a widespread belief, water is liquid at temperatures between 0 ° C and 100 ° C and gaseous above 100 ° C. The reality is, however, much more complicated. First of all, the mentioned temperature limits between the aggregate states only apply to normal pressure (1013 mbar), which roughly corresponds to the atmospheric pressure that usually occurs at locations that are not too high. The boiling temperature, i.e. the boundary between the liquid and gaseous state of aggregation, increases with increasing pressure (see Figure 1). In addition, there is water below the boiling point evaporate can, until the resulting pressure (partial pressure) of the water vapor reaches the so-called vapor pressure, which in turn is temperature-dependent.

Boiling occurs when, at a given pressure, the liquid water dies Boiling temperature has reached and heat continues to be supplied. Steam bubbles are formed which rise in the water. The vapor pressure at the boiling point corresponds to the external pressure. Therefore, in Figure 1, the boiling temperature for a certain pressure can be determined as the temperature for which the pressure reaches the corresponding value.

Figure 2 also shows the boiling curve, but in a higher temperature range, where correspondingly higher pressures are achieved. The curve ends at the so-called critical point at approx. 374 ° C and 221 bar; above this, liquid and gaseous water can no longer be distinguished from one another. This difference is already small just below the critical point, i. H. the volume hardly increases when it boils, and the heat of evaporation is much lower than at normal pressure.

Despite the supply of heat, the temperature of the water does not rise during boiling; the supplied heat is used solely for evaporation (→ latent heat, see Figure 3). The amount of specific heat of vaporization (enthalpy of vaporization) is particularly high with water; At normal pressure it is approx. 2257 kJ / kg, compared to only 420 kJ / kg for heating the water from 0 ° C to 100 ° C.

At very low pressures (below approx. 6 mbar, according to the Triple point of water) there is no longer any liquid water, only solid (ice) and water vapor. When heated, ice then sublimes directly to form water vapor without being melted beforehand. Sublimation can also occur at normal pressure, similar to evaporation.

Water vapor can not only transport heat, it also contains exergy. This is used when operating steam turbines and steam engines.

Wet steam, saturated steam and superheated steam

When boiling, what is known as Saturated steam, which is exactly on the boiling curve in terms of temperature and pressure (as far as one is below the critical point). If this steam loses some heat, for example through contact with cooler air, some of the water vapor condenses into small water droplets, so that visible vapor plumes are formed. Here one speaks of Wet steam. The proportion of the actual steam (gaseous water) is often determined by its mass fraction x (between 0 and 1).

If, on the other hand, the steam is further heated after boiling, so that its combination of pressure and temperature is to the right of the boiling curve in the diagram above, it is called Superheated steam or superheated steam. This no longer contains any liquid water droplets, since condensation can only take place when the boiling curve is reached again by cooling or increasing the pressure.

Caution: With dry steam you don't mean superheated steam, but saturated steam!

The designation Dry steam can easily be misunderstood as it is usually more precise dry saturated steam understands, synonymous with saturated steam, and not about superheated steam. Dry steam in this sense does not contain any water droplets, but unlike superheated steam, these form immediately, even if only minimal cooling takes place.

Above the critical point, one speaks of supercritical steam, which is chemically and physically relatively aggressive, for example strongly degreasing.

Steam states in the steam turbine power plant

Some steam turbines have to be operated in such a way that the water vapor remains in the range of the superheated steam at all points, i. H. that no water droplets appear. This could damage the turbine in the long term. There is also Condensing turbinesthat can tolerate the condensation of a significant portion of the water vapor. In a steam turbine power plant, several turbine stages are usually used, the first being operated with strongly superheated steam (superheated steam), while the last is a condensation turbine. A so-called reheater is often used between the turbine stages, which brings the steam back into the area of ​​the superheated steam. This not only protects the turbines, but also enables the power plant to be more efficient.

The first turbine stage in modern steam turbine power plants works well in the supercritical range, for example at 600 ° C and 285 bar. Even higher values ​​of 700 ° C and 350 bar are targeted for future power plants in order to increase efficiency even further. The limits are essentially set by the resilience of the available materials.

Generation of steam

Steam is often produced in steam boilers as part of steam generators. The core component of a steam boiler is a heat exchanger, in which liquid water is supplied with heat (for example from a combustion process or from a nuclear reactor) so that it boils. This creates saturated steam, the temperature of which is determined by the prevailing pressure.

For some applications (especially for steam turbines) a so-called Superheater inserted, d. H. a second heat exchanger with which the temperature of the steam is increased further while the pressure remains approximately the same in order to obtain superheated steam.

In some nuclear reactors, known as boiling water reactors, water vapor can be generated directly. In the case of pressurized water reactors, on the other hand, evaporation is prevented by a high operating pressure and a separate steam generator is used. It is hardly possible to overheat the steam, as this would require a heat source with a higher temperature. This is an important reason for the usually lower efficiency of nuclear power plants.

If combustion gases are used, their temperature remains above the steam temperature (in front of the superheater), so that high exhaust gas losses would result if the gases were directly emitted uselessly as exhaust gas. For this reason, a so-called economizer is often used, which extracts additional heat from the exhaust gas, which mostly serves to preheat the feed water (i.e. the liquid water fed to the steam boiler). Another possibility of heat recovery is to preheat the combustion air.

In some cases, a power plant not only supplies electrical energy, but also steam as process steam for neighboring industrial companies. If this steam is withdrawn as intermediate steam, because the steam temperature is not required too high, this is more energy-efficient than operating a separate steam boiler for industrial operations. After all, part of the exergy of the originally generated hotter steam is used to generate electricity.

Water vapor in the air

As explained above, water evaporates even at low temperatures until the so-called vapor pressure (a variable that only depends on the temperature, can be read off in Figure 1) is reached. If liquid water is in contact with air, the water vapor content of the air increases as it evaporates until the partial pressure of the water vapor (and not the total air pressure) corresponds to the aforementioned vapor pressure. When this point is reached, water can continue to evaporate from a microscopic point of view, but the same amount condenses at the same time, so that the water vapor content of the air can no longer increase.

The humidity is a measure of the water vapor content of the air, which is used in two variants. The absolute humidity is for example in g / m3 (Grams per cubic meter), while the relative humidity shows what proportion of the vapor pressure of water or what proportion of the maximum water vapor content of the air has been reached. The humidity in buildings plays an important role in human wellbeing. Too high humidity can lead to mold growth.

How much water vapor the air absorbs does not depend on its properties!

It is often said that the air can only absorb a certain amount of water vapor at a certain temperature. This is misleading as this maximum content is not determined by the air molecules (ie their “tolerance” for water), but rather a property of the water itself. If a gas other than air was used (for example argon, with completely different chemical and physical properties than air), there would be no other maximum water vapor content.

Water vapor as a greenhouse gas

Is that CO2 So important at all when the greenhouse effect of the water vapor is stronger?

In certain spectral ranges, water vapor absorbs infrared light (thermal radiation) and therefore acts as a greenhouse gas in the atmosphere. Due to the high content of water vapor in the atmosphere, this effect is even much stronger than that of carbon dioxide (CO2). It does not follow from this, however, that (as claimed in particular by a number of “climate skeptics”) the CO2-Emissions are in reality insignificant. The fact is that the CO2-Content of the atmosphere due to the temperature increase also causes an increased water vapor content in the air, which then increases the temperature even further. This effect does not lead to an ever increasing temperature increase even without additional CO2Emissions, but it increases the effective greenhouse effect of the CO2 considerably beyond the extent that the CO2 alone would have caused. So on the one hand it is true that the majority of the greenhouse effect comes from water vapor; on the other hand, however, exactly this effect is due to the CO2Emissions significantly increased. That's why it comes down to the CO2Emissions.

On the other hand, emissions of water vapor caused by humans - for example from the cooling towers of large power plants - surprisingly do not lead to global climate pollution, but only to local effects through cloud formation. This is because such emissions are offset by increased rainfall. Ultimately, the water vapor content of the atmosphere is limited by its temperature.

Water vapor, which is emitted by planes at high altitudes and forms contrails, has an even stronger greenhouse effect. This effect only has a short-term effect, but it is relatively strong, and is therefore an essential component of the greenhouse effect currently generated by air traffic. Serious supplier of CO2-Compensation take this into account by correspondingly higher CO2Offsetting emissions.

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See also: steam turbine, steam engine, steam boiler, steam generator, humidity
as well as other articles in the categories of energy sources, basic terms, physical principles, heat and cold