Water vapor
In everyday language we mean by water vapor usually the visible steam plume of condensing steam ( wet steam). Steam clouds are visible, because microscopic droplets have formed, as well as in clouds and fog.
In technology and science water vapor is the name for water in the gaseous state. This is invisible as air, but is not referred to as water gas, as this term has a different meaning.
- 3.1 T-S diagram
- 3.2 H- s diagram
- 3.3 Magnus formula
- 3.4 approximation formula
- 4.1 Water vapor feedback
- 5.1 Human water vapor
Development and states
In a normal ambient pressure of 1.013 bar ( 101.325 kPa ) Water boils at 100 ° C. If the remaining water beyond energy (heat) supplied, it evaporates, without leading to a further increase in temperature. From 1 liter (equivalent to 1 kg) of water created in 1673 liters of water vapor ( under normal conditions ), for which an energy intake of 2257 kJ is required.
The supplied energy increases the internal energy of the steam for 2088 kJ and provides with respect to the ambient pressure, a volume change of work W.
Both contributions summed result the enthalpy of vaporization, H, which is in a enthalpy -entropy diagram ( hs diagram) can be read as a specific quantity in the form of a difference in the y-axis. The diagram illustrated here represents Ts for the necessary evaporation (at 100 ° C) heat to form the dotted blue surface
Likewise, it can be the increase in entropy of vaporization ( Delta S ) can be determined:
- = Heat of vaporization or enthalpy of vaporization
- = Boiling temperature in K
Is seen from the phase diagram, water boils at an air pressure of 0.4 bar already at about 75 ° C ( such as on Mount Everest ). The expended heat of vaporization is correspondingly greater, as the increase in volume of the vapor. With increasing pressure, the heat of vaporization of the water decreases until it is equal to zero at the critical point. Consequently, the shrinking space in the Ts diagram
Manifestations
The vapor pressure of water is temperature dependent. At temperatures below the boiling point is referred to as evaporation. In saturated surroundings, an equilibrium between the misting water and condensing steam sets. The transition conditions between liquid water and water vapor are shown in the boiling point curve of the state diagram.
Wet steam
When steam flows into a cooler environment, parts of the gaseous water condense to fine droplets. Such a mixture is referred to as wet steam, which can be observed for example in boiling water. In the Ts diagram, the area of the wet steam extends to the critical point bar at 374 ° C and 221.2.
The content of the wet vapor on liquid water x is characterized by the mass fraction, which can be calculated using the following formula
This definition limits the vapor content between 0 ≤ x ≤ 1
About the ideal gas equation equivalent definitions can be derived that do not limit the scope of the vapor content:
Is referred to the specific volume, enthalpy and entropy.
The state of the saturated liquid is characterized by the saturated vapor through.
Superheated steam
Superheated steam
Superheated steam is steam at a temperature above the boiling point. The steam is "dry " and does not contain droplets. In steam boilers, the steam generated is brought in this condition by means of the superheater.
Supercritical steam
Above the critical point of water vapor and liquid water in density are indistinguishable from each other, which is why this condition is referred to as " critical ". Below the critical point of water vapor is thus " subcritical ", where it is in equilibrium with the liquid water. If it is heated in this area, after the complete evaporation of the liquid on the associated evaporation temperature, thus producing " superheated steam ". This form of the vapor contains no more water droplets and is in their physical behavior is also a gas.
Supercritical water has particularly aggressive characteristics. There have therefore been attempts, by means of which readily biodegradable organic pollutants such as dioxins, PCBs to hydrolytically cleave.
For the steam boiler of the supercritical state requires a special design. Because of the small density difference between water and steam does not constitute buoyancy and thus no stable natural circulation. Boilers that are operated on or close below the critical point, so are always forced circulation boiler. Since supercritical boilers no separation of steam and water phase is more necessary or possible, eliminating the drum and the design is a once-through boiler, often of the type Benson.
Saturated steam or dry saturated steam
The boundary between wet and superheated steam is called " saturated steam ", also saturated steam or dry saturated steam, occasionally in contrast to the wet steam also " dry steam ". Most table values to water vapor states are related to it.
Limit curves
In the Ts diagram the two boundary curves x = 0 and x = 1 is of particular importance, who meet at the critical point.
- The curve x = 0, and boiling or lower boundary line delimits the area of the liquid from the wet steam as
- The curve x = 1, and dew line, saturated steam curve or upper boundary line, the wet steam is separated from the superheated steam and at the same time marks the state of saturated steam.
The notation x is the mass fraction here is not uniformly defined, as is indicated especially in the chemistry of the mass fraction w and the majority is the mole fraction x here. Both quantities can convert into each other and the same in the limits 0 and 1
Appearance
Gaseous or superheated water vapor is colorless and actually invisible, like most gases. Wet steam is visible through the entrained water droplets against it. Contact with sufficient cool air, it comes to falling below the dew point and hence to a further condensation of fine water droplets. The existence of the water vapor in the air is visualized by the scattered light to the droplets when these droplets are large compared to the wavelength of the radiation.
Water vapor can also arise directly from the solid phase of water: ice or snow is " licked by the sun ." This phenomenon is observed especially in dry air at high altitudes, when snow-covered slopes at temperatures well below 0 ° C are free of snow with time. The ice, so the solid water sublimes to water vapor. The humidity increases by evaporation from the snow, and snow-covered surfaces previously apern of a phenomenon, for example, in the Himalayas. For the same reasons dries outdoor hung laundry at temperatures below zero when the relative humidity is low enough.
Air invisible water vapor present condenses under special conditions ( by crystallization nuclei ) and is visible, such as when an aircraft is flying near the ground at high speed, this in the picture clearly visible effect is often mistakenly referred to as " the sound barrier ", but this effect is not higher or lower sound effect. Due to the high flow velocity of the air can flow from mechanical reasons, such as high pressure fluctuations, the temperature of the air flowing strongly and thus fall below the dew point, which leads to a condensing. The water vapor in the hot exhaust gas is, however, taken up by the warming air.
Boil
As a function of the heat flux, which is fed to the boiling liquid over a heating surface, to form different shapes of boiling.
If the temperature of the heating surface a few degrees above the boiling point of water to form on uneven bubble nuclei. Up to heat flow densities of 2 kW / m² bubbles that condense again when ascending form. This Siedeform is called silent boiling.
With increasing heat flux density, the formation of bubbles increases, and the bubbles reach the surface. The incessant on the heating surfaces bubbles lead to a high heat transfer coefficient. The wall temperatures do not rise substantially above the boiling temperature (up to about 30 K). In the nucleate boiling heat flux / m can be achieved up to 1000 kW.
If the heat current density can then further increased by leaps and bounds is the film boiling one: It forms a continuous film of vapor. It acts as an insulating layer, and the heat transfer coefficient is significantly reduced. The heat flow is not reduced, so an equilibrium state will be only be achieved if the heat can be released by a sufficiently high heat radiation. This condition is not achieved until at a superheat of the heating surface of about 1000 K. In general, the heating surface is destroyed by this transition from nucleate boiling to film boiling.
To prevent the deterioration of heating surfaces of steam boilers, the maximum heat flux density is limited to 300 kW / m². In smaller cases, there is the overshoot by boiling.
Tables, charts and formulas
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Because of its enormous importance for the energy industry is one of water vapor to the most researched substances in thermodynamics. Its physical properties were determined by extensive and frequent measurements and calculations and comprehensive tables, the so-called steam tables, covered.
T- s-diagram
In the TS diagram it can be seen that the transition from liquid to vapor, the entropy increases. This corresponds to the intuition that the particles of a fluid are substantially minor than the chaotic mixing of the particles in a gas. The entropy is plotted on the abscissa. Another special feature of the diagram is its ability to represent the time required to evaporate the water amount of heat as a surface. With the relationship: AH = T ·? S is obtained for the evaporation enthalpy of a rectangular area, which is spanned between T = 0 K, and the respective evaporation line.
H- s diagram
In a Mollier diagram of the entropy of the vapor is plotted on the abscissa and the associated enthalpy on the ordinate. The basic physical properties of the water vapor can not be true simply interpret, however, the necessary condition for the change of the vapor amounts of heat, so for example, the enthalpy of vaporization, be read directly from the ordinate.
Magnus formula
An approximate formula for the calculation of the saturation vapor pressure depending on temperature is the Magnus formula:
Temperature θ in ° C, coefficient
This formula is very accurate ( less than 0.22 %) in the range between 0 and 100 ° C and still good (below 4.3%) -20 to 374 ° C, the maximum error is at 290 ° C. Because of the simple structure and high accuracy it is used for dew point, especially in meteorology and in building physics.
With slightly different coefficients
Resulting values , which agrees to 0.1% with the format specified in DIN 4108 Table for building physics calculations.
The Magnus formula was determined empirically by Heinrich Gustav Magnus and since then only supplemented by more precise values of the coefficients. One derived from the thermodynamics laws of phase diagrams, the Clausius-Clapeyron equation, and the Clausius- Clapeyron equation dar. Due to many practical problems with regard to these more theoretical equations, the Magnus formula nevertheless the best and most practical approximation dar.
Approximate formula
A useful rule of thumb for the calculation of the saturated steam temperature in the saturated steam pressure and vice versa,
When the pressure p in bar (absolute) is employed. The associated temperature θ results in degrees Celsius. This formula is in the range p kr. > P> p = 3 bar (200 ° C> θ > 100 ° C) to about 3%.
Climate effects
In terrestrial weather patterns water vapor plays a crucial role. One kilogram of air can hold about 26 grams of water vapor and humidity at 30 ° C and 1 bar pressure. This amount drops off at 10 ° C to about 7.5 g / kg. The excess amount is excreted depending on weather conditions as precipitation in the form of rain, snow, hail, fog, dew, frost or frost from the air. Clouds reflect incoming solar radiation back into space and thus reduce the amount of energy arriving at the ground. They reflect on the other hand also coming from the ground thermal radiation and thus increase the atmospheric counter-radiation. Whether clouds warm the earth's surface or cool depends on the level at which they are located: Low standing clouds cool the Earth, High Clouds act warming.
In the stratosphere, any traces of water vapor are considered particularly harmful to the climate. Climate researchers observed over the last 40 years, an increase of water vapor in the stratosphere by 75 % (see polar stratospheric clouds) and make this partly responsible for the increase in mean global temperature. The origin of the water vapor at these altitudes is still unclear, but one suspects a connection with the sharp rise in recent decades methane output by industrial agriculture. Methane is oxidized in these high altitudes to carbon dioxide and water vapor, which, however, only half of the increase can be explained.
In the atmosphere water vapor present is 36% up to 70% share of the main source of atmospheric counter-radiation and winner of the "natural" greenhouse effect. The wide range (36 % to 70 %) does not come from the fact that one could not accurately measure the effect, but by the fact that the atmospheric humidity in time and place strong natural fluctuations. The greenhouse effect is an important effect on the radiation balance of the Earth and has an increase in the average global temperature to a level of 15 ° C result. Life on earth was this fact that possible. As the average temperature without the greenhouse at a temperature of about -18 ° C and is usually given.
Water vapor feedback
A rising average temperature of the earth leads to an increasing mean water vapor content of the atmosphere. According to the Clausius -Clapeyron equation, the atmosphere can with any degree temperature rise include 7 % more water vapor. In the context of global warming is this so-called " water vapor feedback " next to the ice - albedo feedback, the strongest positive feedback. Any heating or cooling is enhanced by it.
Natural Occurrence
Pure water vapor is formed in nature on earth in volcanoes, fumaroles and geysers at. It is the most important parameter in volcanic eruptions and determines their character. It is decisive that many minerals and rocks integrate water or other volatile substances in their crystal lattice, especially under the effect of high pressures. As magma undergoes a pressure relief during ascent in the crust, pushing the water vapor along with other fluids from the magma and forms bubbles, which however do not expand due to the pressure initially free. The pressure falls below a certain value, so they connect to fluid bubbles and lead to a kind of enormous boiling retardation, are so explosively free. They also tear with large amounts magma and cause comparatively rare explosive volcanic eruptions. Since the proportion of fluids in the rocks at convergent plate boundaries is especially great shows at this, the clearest trend for this volcano type.
Human water vapor
Water vapor is an important tool for human heat balance. At high ambient temperatures for thermoregulation by sweating the excess body heat ( evaporative cooling ) is discharged to the environment. The case quantities of heat are significant, for evaporation of one gram of sweat 2.43 kJ of heat are required. The healthy person produces at normal ambient temperatures daily 500 g of water vapor from perspiration, added again with double the amount of exhaled air. This also makes the body temperature is regulated to 37 ° C.
Water vapor entry
In the combustion of petroleum products, the hydrocarbons of petroleum fractions are reacted in substantially carbon dioxide and water vapor. In traffic, the sources petrol and diesel, in aviation kerosene in the home heating fuel oil and heavy fuel oil in industry. The condensing water vapor contained in the exhaust makes the aircraft vapor trails in the sky noticeable. The combustion of natural gas, which is now used for heating buildings, falls due to the four hydrogen atoms per carbon atom in the methane molecule to twice as much water vapor as carbon dioxide. This is the reason that condensing boilers for gas work more effectively than for fuel oil. Water vapor is registered in many industrial processes as waste into the atmosphere.
Water vapor in air conditioning technology
Air conditioning is a building amenities that guarantees a defined water vapor content of the air. To protect finished products of iron and steel materials against corrosion, stocks such as books against weathering and food from drying out, warehouses are air-conditioned. In the living room air conditioning of the water vapor content contributes significantly to the well -being of the people. In assessing the ambient air, the concept of comfort plays a central role; one aspect is the perceived as pleasant relationship between air temperature and relative humidity. This is ensured by an air conditioning system and is usually between 30% and 70 % relative humidity.
Quantification of water vapor
Since the water vapor plays an important role in a wide variety of conditions and processes, it is covered with a variety of measurement methods and equipment and stated in a variety of sizes.
For meteorological purposes in relation to the humid air often the relative humidity is used φ. This can be measured, among other things with a hair hygrometer. In the art, the absolute humidity x is used generally. This is measured with a LiCl - donor or coulometric moisture sensor, in which (starting from strongly hygroscopic phosphorus pentoxide ) is closed to the water vapor content of the air. Another possibility for determining the water vapor content of the air is to measure its temperature at a respective dry and damp thermometer, wherein the measurement location of the second thermometer is wrapped with a water- soaked fabric, and blown to promote evaporation of a small fan. Using the two readings can be read from the Mollier hx diagram immediately the associated humidity. The psychrometer is the practical result of further development of this method of measurement.
In steam generators used in addition thermometer and pressure gauge for easy measurement of the steam parameters.
Water vapor in the history
The sight of water vapor is man since the discovery of fire known; he was more or less accidentally while cooking or when deleting the fire area with water. First thoughts on the technical use of water vapor can be attributed to Archimedes, who constructed a steam cannon. Leonardo da Vinci created on this topic at first been calculated that an eight pound ball from such a gun fired would fly about 1250 meters.
Heron of Alexandria invented the Heronsball, a first steam engine. His invention had in ancient times no practical value, but they showed the technical possibilities of the use of water vapor.
On Denis Papin the practice of the pressure cooker goes back. Said first pressure vessel was equipped from the outset with a safety valve, after it has been a to bursting with a prototype in the first experiments.
The invention and use of the steam engine made it necessary to examine the working medium water vapor theoretically and practically. The practitioners include James Watt and Carl Gustav Patrik de Laval, who were by marketing their machines to wealthy men. Among the theorists, however, belonged to Nicolas Léonard Sadi Carnot, hired the considerations to steam and the steam engine. In the series of researchers who thoroughly dealt with the properties of water vapor, Rudolf Julius Emanuel Clausius include and Ludwig Boltzmann.
Use in the art
Water vapor is generated in the art and is used in steam boilers, for example, for the following purposes:
- As the working fluid in steam engines, steam engines and steam turbines,
- In the production of oil and as an aid in the steam cracking for the production of gasoline,
- As an intermediate in the desalination of sea water,
- As a raw material for the production of water and generator gas by steam reforming,
- In steam heating systems and evaporative cooling in a carrier of heat energy.
- For conveying liquid water with a steam jet pump,
- Wherein the steam distillation as a blowing agent,
- Creating a vacuum by displacement of air from a closed pressure vessel with subsequent condensation.
The current largest power plant steam generators have a capacity of up to 3600 tons of steam per hour. Such amounts are provided for example with a water -tube boiler.
The industrial use of water vapor is to be noted that the wet steam in contrast to most other gases and liquids can not be pumped. The water hammer occurring during compression of the steam would destroy the carrier within a very short time.
Other applications
- For soil sterilization and Bodenhygienisierung by steaming (soil disinfection) with superheated steam
- For cleaning with steam cleaners,
- In the kitchen for gentle cooking food by steaming,
- In the production of flour, especially in whole grain flour, for the stabilization of the grain germ,
- For working with wood in boat, furniture and musical instruments,
- For creating a vacuum in the closed pressure vessel by displacing the air and subsequent condensation
- For the sterilization of medical and microbiological instruments by so-called autoclave,
- For ironing.
- In medicine and therapeutics, water vapor is used for the heat transfer and as a carrier of therapeutic agents: Inhalation for healing, such as cough, or for the relief of colds, with inhalers or a facial sauna,
- The spa area with steam baths.
Dangers by water vapor
Small amounts of water vapor can transport heat energy and thus large amounts. For this reason, the destructive potential of steam-carrying equipment such as steam generator and pipe is considerable. Kesselzerknalle of steam boilers were among the worst accidents in the history of technology; such events in the past have destroyed at a stroke industries.
The danger caused by the "invisible" water vapor emerges without high temperature and high pressure into a jet of considerable length of a defective steam boiler. Considering the above HS- diagram represents the release of saturated steam at first an adiabatic change in state in which the pressure is reduced. The starting point is the saturated steam curve to the right of the critical point ( = saturated steam condition in the boiler ). The pressure reduction is parallel to the x-axis ( the enthalpy remains constant ). The emerging free jet mixes with the ambient air and cools. When below 100 ° C (= saturated steam temperature at ambient pressure ) of the steam begins to condense and become visible.
One danger of large steam exits the other hand, the formation of fog, which makes orientation difficult for fugitives. And finally, can even trigger fires outflowing superheated steam. Nachverdampfen the liquid water is done by entering the vicinity of the defect site pressure reduction.
A large-area contact with a jet of steam or hot water is deadly because of the instantly entering scalding. In recent times fewer accidents have happened, because the state of the art in this field has permanently developed into major collateral out in connection with steam.
Due to the large volume difference between water and water vapor ( 1:1700 ) it is dangerous to delete certain fires with water. When a chimney fire the extinguishing water can lead to tearing of the chimney and thus endanger the firefighters and cause property damage. A grease fire can not be extinguished with water, since water enters because of the higher density under the burning fat, evaporated on the hot surface and expands it and burning fat with tears, so it comes to fat explosion.