In colloquial language, water vapor is usually understood as the visible vapor plumes of condensed water vapor ( wet steam ). Steam clouds are visible, because microscopic droplets have formed, as well as clouds and fog that due to the Mie scattering sunlight scatter , causing the Tyndall effect occurs and the actually colorless droplets are visible.
Origin and states
In a normal ambient pressure of 1,013 bar (101,325 kPa ) of water boils at 100 ° C . If energy (heat) is added to the remaining water, it evaporates without causing any further temperature rise. 1 liter (corresponding to 1 kg ) of water produces 1673 liters of water vapor (under normal conditions), for which an energy supply of 2257 kJ is required.
The energy supplied increases the internal energy of the steam by 2088 kJ and does a volume change work W compared to the ambient pressure .
The two contributions add up to give the enthalpy of evaporation H, which can be read as a specific variable in an enthalpy-entropy diagram (hs diagram) in the form of a difference on the y-axis. The one shown here, the Ts diagram for the evaporation necessary (at 100 ° C) heat is in the form of dotted blue surface.
The increase in evaporation entropy (Delta S) can also be determined:
- = Heat of evaporation or enthalpy of evaporation
- = Boiling temperature in K
As can be seen from the phase diagram , water boils at an air pressure of 0.4 bar at around 75 ° C (for example on Mount Everest ). The heat of evaporation to be expended is correspondingly greater, as is the increase in volume of the steam. With increasing pressure, the heat of evaporation of the water decreases until it is zero at the critical point . The resulting areas in the Ts diagram are becoming smaller.
The vapor pressure of the water is temperature dependent. At temperatures below the boiling point one speaks of evaporation . In saturated ambient air, an equilibrium is established between evaporating water and condensing water vapor. The transition conditions between liquid water and water vapor are shown in the boiling point curve of the state diagram.
When steam flows into a colder environment, parts of the gaseous water condense into very fine droplets. Such a mixture is called wet steam, which can be observed, for example, when boiling water. In the Ts diagram, the wet steam range extends to the critical point at 374 ° C and 221.2 bar .
The pure steam content of the wet steam is characterized by the mass fraction x , which can be calculated using the following formula
This definition limits the steam content between 0 ≤ x ≤ 1.
Equivalent definitions can be derived from the ideal gas equation , which do not limit the range of the steam content:
The state of saturated liquid is indicated by , that of saturated vapor by .
Superheated steam is steam with a temperature above the boiling point . The steam is "dry" and does not contain any droplets. The steam content is according to the above formula . In steam boilers , the generated steam is brought into this state by means of the superheater.
If the temperature and pressure of water in a suitable pressure vessel are greatly increased, the so-called critical point will be exceeded at some point and a “supercritical” state will be reached. In the supercritical state, the distinction between water vapor and liquid water is meaningless, because they cannot be distinguished from each other either in terms of their density or other properties. There is then no longer any phase boundary . Below the critical point, the water vapor is consequently “subcritical” and can be in equilibrium with liquid water. If the liquid water is completely evaporated in this area and the temperature is then increased further, "superheated steam" is created. This form of steam no longer contains any water droplets and its physical behavior is also a gas.
Supercritical water has particularly aggressive properties. Attempts have therefore been made to hydrolytically break down organic pollutants such as dioxins and PCBs that are difficult to biodegrade .
The transition to the supercritical state requires a special design for the steam boiler . Because of the small difference in density between water and steam, there is no buoyancy and therefore no stable natural circulation . Boilers that are operated above or close to the critical point are therefore always forced-flow boilers . Since the separation of steam and water phases is no longer necessary or possible with supercritical boilers, the drum is no longer necessary and the design is a once- through boiler , often of the Benson type .
Saturated steam or dry saturated steam
The border area between wet and superheated steam is called “saturated steam”, also saturated steam or dry saturated steam, occasionally also “dry steam” to distinguish it from wet steam. Most of the table values for water vapor states are based on this.
Dry steam is produced at a temperature of 170 degrees. The quality of the steam depends on the temperature and the pressure build-up. Dry steam is free of minerals and therefore cleans without streaks.
In the Ts diagram, the two limit curves x = 0 and x = 1 , which meet at the critical point , are of particular importance .
- The curve x = 0 , also boiling line or lower limit line, delimits the area of the liquid from the wet steam, while
- the curve x = 1 , also dew line, saturated steam curve or upper limit line, separates the wet steam from the superheated steam and at the same time marks the state of the saturated steam.
The notation with x for the mass fraction is not uniformly defined here, since the mass fraction is specified with w , especially in chemistry , and x here mostly stands for the mole fraction . Both variables can be converted into each other and are the same in the limit values 0 and 1.
Gaseous or superheated water vapor is colorless and actually invisible, like most gases. Wet steam, on the other hand, is visible through the entrained water droplets. Contact with sufficiently cool ambient air results in falling below the dew point and consequently condensation of further fine water droplets. The existence of the water vapor in the air becomes visible through the light scattered on the droplets .
Water vapor can also arise directly from the solid phase of water: ice or snow are "licked away by the sun". This phenomenon is particularly observed in dry air in high mountains, when snow-covered slopes become snow-free over time at temperatures well below 0 ° C. The ice, i.e. the solid water, sublimes into water vapor. The humidity increases due to evaporation from the snow, and previously snow-covered areas apper , a phenomenon for example in the Himalayas . For the same reasons, laundry hung outside also dries at temperatures below zero as soon as the relative humidity is low enough.
Water vapor that is invisible in the air condenses under special conditions (due to crystallization nuclei) and becomes visible, for example when an aircraft flies close to the ground at high speed. This effect, which is clearly visible in the picture , is often wrongly referred to as "the sound barrier ", but this effect is not Over or under sonic effect. Due to the high flow velocity of the air, for fluid-mechanical reasons, for example high pressure fluctuations, the temperature of the inflowing air can drop sharply and thus below the dew point, which leads to condensation. The water vapor in the hot exhaust gas, however, is absorbed by the warming air.
If the temperature of the heating surface is a few degrees above the boiling point, bubble germs form on unevenness . Bubbles form up to heat flux densities of 2 kW / m², which condense again when ascending. This form of boiling is called still boiling .
As the heat flux increases, the bubble formation increases and the bubbles reach the surface. The bubbles tearing off the heating surfaces lead to a high heat transfer coefficient . The wall temperatures do not rise significantly above the boiling point (up to about 30 K). With nucleate boiling , heat flux densities of up to 1000 kW / m² can be achieved.
If the heat flow density is increased even further, the film boils suddenly : A continuous vapor film is formed. This acts like an insulating layer and the heat transfer coefficient is drastically reduced. If the heat flow is not reduced, a state of equilibrium will only be reached again when the heat can be given off by means of sufficiently high thermal radiation . However, this condition is only reached when the heating surface overheats by around 1000 K. As a rule, the heating surface is destroyed during this transition from nucleate boiling to film boiling.
In order to prevent the destruction of heating surfaces on steam boilers , the maximum heat flow density is limited to 300 kW / m². In smaller cases there is an overshoot due to delayed boiling.
Tables, charts and formulas
Because of its enormous importance for the energy industry, water vapor is one of the best-researched substances in thermodynamics . Its physical properties were determined by extensive and frequent measurements and calculations and recorded in extensive tables , the so-called water vapor tables .
The Ts diagram shows that the entropy increases with the transition from liquid to vapor . This corresponds to the view that the particles in a liquid are much more ordered 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 property of showing the amount of heat required for evaporation of the water as an area. The relationship: ΔH = T · ΔS results in a rectangular area for the enthalpy of vaporization, which is spanned between T = 0 K and the respective straight line of vaporization.
In a Mollier diagram, the entropy of the steam is plotted on the abscissa and the associated enthalpy on the ordinate. The basic physical properties of water vapor are not easy to interpret, but the amount of heat required to change the state of the vapor, for example the enthalpy of vaporization, can be read directly from the ordinate.
An approximation formula for calculating the saturation vapor pressure as a function of the temperature is the Magnus formula :
Temperature θ in ° C, coefficient
This formula is very accurate (below 0.22%) in the range between 0 and 100 ° C and still good (below 4.3%) between −20 and 374 ° C, the maximum error is 290 ° C. Because of its simple structure and high accuracy, it is used to determine the dew point , especially in meteorology and building physics .
With slightly different coefficients
the result is values that correspond to 0.1% with the table for building physics calculations printed in DIN 4108.
The Magnus formula was determined empirically by Heinrich Gustav Magnus and since then has only been supplemented by more precise values of the coefficients. A law for phase diagrams derived from thermodynamics is represented by the Clapeyron equation and the Clausius-Clapeyron equation . However, due to many practical problems relating to these rather theoretical equations, the Magnus formula represents the best and most practicable approximation.
A useful rule of thumb for calculating the saturated steam temperature from the saturated steam pressure and vice versa is
if the pressure p is used in bar (absolute). 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) accurate to about 3%.
Water vapor plays a decisive role in terrestrial weather . At 30 ° C and 1 bar pressure, one kilogram of air can absorb around 26 grams of water vapor as humidity . This amount drops to about 7.5 g / kg at 10 ° C. The excess amount is excreted from the air as precipitation in the form of rain, snow, hail, fog, dew , frost or hoar frost , depending on the weather .
Clouds send some of the incoming solar radiation back into space and in this way reduce the amount of energy arriving on the ground. They do the same with the thermal radiation coming from below and thus increase the atmospheric counter-radiation . Whether clouds warm or cool the earth's surface depends on the altitude at which they are: Low-lying clouds cool the earth, high-lying clouds have a warming effect.
Traces of water vapor in the stratosphere are considered to be particularly climate-relevant. The 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 the average global temperature. The origin of the water vapor at these altitudes is still unclear, but there is suspicion of a connection with the methane output from industrial agriculture, which has risen sharply in recent decades . Methane is oxidized to carbon dioxide and water vapor at these high altitudes , which only explains half of the increase.
Water vapor present in the earth's atmosphere is the main source of atmospheric counter-radiation and the carrier of the “natural” greenhouse effect , with a share of 36% to 70% . The wide range (36% to 70%) is not due to the fact that the effect cannot be measured precisely, but rather because the atmospheric humidity is subject to strong natural fluctuations in terms of time and location. The greenhouse effect is an important effect on the earth's radiation budget and increases the global average temperature to a level of 15 ° C. This made life on earth possible in the first place. The average temperature without greenhouse effect is usually given as a temperature of around −18 ° C.
Water vapor feedback
A rising average temperature of the earth leads to a rising average water vapor content in the atmosphere. According to the Clausius-Clapeyron equation , the atmosphere can contain 7% more water vapor with every degree of temperature increase.
In the context of global warming , the so-called "water vapor feedback" is the strongest positive feedback known to date alongside the ice-albedo feedback : With an assumed climate sensitivity of 2.8 ° C and a doubling of the atmospheric carbon dioxide concentration, this is 1.2 ° C due to the direct warming effect of the CO 2 , one degree is due to the water vapor feedback and the rest to the other feedbacks. Over the past 35 years, the humidity at the top of the weather layer has increased by an average of around ten percent.
Scientists assume that water vapor feedback can also occur on other planets; The planet Venus is said to have had oceans from four and a half billion years ago, and water vapor feedback is also said to have come into play in its development history.
Pure water vapor occurs naturally on earth in volcanoes , fumaroles and geysers . It is the most important parameter in volcanic eruptions and determines their character. It is crucial that many minerals or rocks bind water or other volatile substances in their crystal lattice , especially under the effect of high pressures. Since magma experiences a pressure relief as it rises in the crust , the water vapor drives out of the magma together with other fluids and forms bubbles, which initially do not expand freely due to the pressure. If the pressure falls below a certain value, these fluid bubbles combine and lead to a kind of enormous delay in boiling , i.e. are released like an explosion. In doing so, they also carry away larger amounts of magma and cause the comparatively rare explosive volcanic eruptions. Since the proportion of fluids in the rocks is particularly large at converging plate boundaries , the clearest tendency for this type of volcano is also shown here.
Human water vapor
Water vapor is an important aid for the human heat balance. At high ambient temperatures, excess body heat ( evaporative cooling ) is released into the environment through sweating for thermoregulation . The amount of heat converted is considerable; to evaporate one gram of sweat, 2.43 kJ of heat are required. At normal ambient temperatures, healthy people generate around 500 g of water vapor daily through sweating, and twice as much with the exhaled air . Also characterized is the body temperature at 37 ° C regulated .
Water vapor entry
When petroleum products are burned, the hydrocarbons in the petroleum fractions are essentially converted into carbon dioxide and water vapor. In car traffic the sources are petrol and diesel , in air traffic kerosene , in house heating heating oil and in industry heavy oil . The condensing water vapor contained in the exhaust gas is noticeable in aircraft through contrails in the sky. When burning natural gas , which is now used to heat buildings, twice as much water vapor as carbon dioxide is produced because of the four hydrogen atoms per carbon atom in the methane molecule. This is the reason why condensing boilers work more effectively for natural gas than for heating oil. In many large-scale processes, water vapor is released into the atmosphere as a waste product.
Water vapor in air conditioning
An air conditioner is a building equipment that guarantees a defined water vapor content of the air. Warehouses are air-conditioned to protect finished products made of iron and steel materials from corrosion , stocks such as books from weathering and food from drying out. In air conditioning in living spaces, the water vapor content makes a significant contribution to human wellbeing. When assessing room air, the concept of comfort plays a central role; one aspect is the relationship between room air temperature and relative humidity, which is perceived as pleasant. This is ensured by an air conditioning system and is usually between 30% and 70% relative humidity.
Quantification of water vapor
Since water vapor plays a major role in a wide variety of conditions and processes, it is recorded using a wide variety of measuring methods and devices and specified in a large number of sizes.
The relative humidity φ is often used for meteorological purposes in relation to the moist air . This can be measured with a hair hygrometer , among other things . In engineering, the absolute humidity x is generally used. This is measured with a LiCl transmitter or a coulometric humidity sensor , in which (based on strongly hygroscopic diphosphorus pentoxide ) the water vapor content of the air is deduced. Another possibility for determining the water vapor content of the air is to measure its temperature on a dry and a damp thermometer , the measuring point of the second thermometer being wrapped in a water-soaked fabric and blown with a small fan to promote evaporation. With the help of the two values read off, the associated humidity can be read off immediately from the Mollier-hx diagram. The psychrometer is the practical result of the further development of this measuring method.
Water vapor in history
The sight of water vapor has been known to man since the fire was harnessed; it was more or less unintentional when cooking or when putting out the fireplace with water. The first considerations on the technical use of steam are attributed to Archimedes , who constructed a steam cannon . Leonardo da Vinci made initial calculations on this subject, according to which an eight-kilogram bullet fired from such a cannon would fly about 1250 meters.
The practical design of the pressure cooker goes back to Denis Papin . This first pressure vessel was from the beginning with a safety valve fitted after it a with a prototype during the first attempts to bursting came.
The invention and use of the steam engine made it necessary to examine the working medium water vapor theoretically and practically. Practitioners include James Watt and Carl Gustav Patrik de Laval , who became wealthy men by marketing their machines. Nicolas Léonard Sadi Carnot , on the other hand, was one of the theorists , who thought about steam and the steam engine. Rudolf Julius Emanuel Clausius and Ludwig Boltzmann also belong to the ranks of researchers who have dealt in depth with the properties of water vapor .
Use in technology
- as work equipment in steam engines, steam locomotives and steam turbines ,
- in the extraction of crude oil and as an aid in steam cracking for the production of gasoline,
- as an intermediate product in seawater desalination ,
- as a raw material for the production of water and generator gas through steam reforming ,
- in steam heating and evaporative cooling as a carrier of thermal energy.
- for pumping liquid water with a steam jet pump ,
- in steam distillation as a blowing agent ,
- Creating a vacuum by displacing the air from a closed pressure vessel with subsequent condensation.
When using steam for technical purposes, it should be noted that, unlike most other liquids and gases, wet steam can not be pumped . The water hammer that occurs when compressing the steam would destroy the hoisting machine within a very short time.
- for soil sterilization and soil sanitation by steaming (soil disinfection) with superheated steam
- for cleaning with steam cleaners ,
- in the kitchen for gentle preparation of food through steaming ,
- in the flour production, especially in whole grain flour , to stabilize the grain germ,
- for processing of wood in boats, furniture and musical instruments,
- to create a vacuum in closed pressure vessels by displacing the air and subsequent condensation,
- for the sterilization of medical and microbiological instruments by so-called autoclaving ,
- the ironing of laundry .
- In medicine and therapeutics , water vapor is used for heat transfer and as a carrier of therapeutic substances:
Hazards from water vapor
Small amounts of water vapor can transport large amounts of heat and thus energy . For this reason, the destructive potential of steam-carrying equipment such as steam generators and pipelines is considerable. The cracks of steam boilers were among the worst accidents in the history of technology ; In the past, such events have destroyed industrial plants in one fell swoop .
The danger arises from the “invisible” water vapor that escapes freely from a defective steam boiler at high temperature and high pressure in a jet of considerable length. If you look at the hs diagram above, the release of saturated steam first means an adiabatic change in state, during which the pressure is reduced. The starting point is the saturated steam curve to the right of the critical point (= saturated steam state in the boiler). The pressure reduction runs parallel to the x-axis (the enthalpy remains the same). The emerging free jet mixes with the ambient air and cools down. If the temperature falls below 100 ° C (= saturated steam temperature at ambient pressure), the steam begins to condense and become visible.
On the other hand, a danger with large steam escapes is the formation of fog, which makes orientation difficult for refugees. And finally, overheated steam escaping can even cause fires . The re-evaporation of liquid water occurs through the pressure reduction occurring in the vicinity of the defective point.
Extensive contact with a jet of steam or hot water is fatal because of the instant scalding . Lately fewer accidents have occurred in connection with water vapor because the state of the art in this field has constantly developed towards greater safety .
Due to the large volume difference between water and water vapor (1: 1700), it is dangerous to extinguish certain fires with water. In the event of a chimney fire , the extinguishing water can tear the chimney and thus endanger the fire fighting forces and cause property damage. Even a fat fire must not be extinguished with water, because water gets under the burning fat because of its higher density, evaporates on the hot surface and expands and burns fat with it, so it comes to a fat explosion .
Terms and material values
- Other names
- the diagram opposite
- Molecular formula
- H 2 O
- Density at 100 ° C and 1.01325 bar
- 0.598 kg / m³
- spec. Heat capacity
- 1,864 kJ / (kg K)
- Thermal conductivity
- 0.0248 W / (m K)
- Triple point
- 273.160 K corresponds to 0.01 ° C at 0.00612 bar
- critical point
- 374.150 ° C at: 221.20 bar
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