They do have water and steam. Water, water vapor and their properties
Topic 2. Fundamentals of heat engineering.
Thermal engineering is a science that studies methods of obtaining, converting, transferring and using heat. Thermal energy is obtained by burning organic substances called fuel.
The basics of heat engineering are:
1. Thermodynamics is a science that studies the transformation of heat energy into other types of energy (for example: thermal energy into mechanical, chemical, etc.)
2. Heat transfer - studies the heat exchange between two coolants through the heating surface.
The working fluid is a coolant (water vapor or hot water), which is capable of transferring heat.
In the boiler room, the coolant (working fluid) is hot water and water steam with a temperature of 150°C or steam With temperatures up to 250°C. Hot water is used for heating residential and public buildings; this is due to sanitary and hygienic conditions and the ability to easily change its temperature depending on the outside temperature. Water has a significant density compared to steam, which allows it to transfer a significant amount of heat over long distances with a small volume of coolant. Water is supplied to the heating system of buildings at a temperature no higher than 95°C to avoid dust burning on heating devices and burns from heating systems. Steam is used for heating industrial buildings and in production and technological systems.
Working fluid parameters
The coolant, receiving or giving away thermal energy, changes its state.
For example: The water in the steam boiler is heated and turns into steam, which has a certain temperature and pressure. The steam enters the steam-water heater, cools itself, and turns into condensate. The temperature of the heated water increases, the temperature of the steam and condensate decreases.
The main parameters of the working fluid are temperature, pressure, specific volume, density.
t, P- is determined by instruments: pressure gauges, thermometers.
Specific volume and density are calculated values.
1. Specific volume- volume occupied by a unit of mass of a substance at
0°C and atmospheric pressure 760 mmHg. (under normal conditions)
where: V- volume (m 3); m is the mass of the substance (kg); standard condition: P=760mm h.st. t=20 o C
2. Density- the ratio of the mass of a substance to its volume. 
Each substance has its own density:
In practice, relative density is used - the ratio of the density of a given gas to the density of a standard substance (air) under normal conditions (t° = 0°C: 760 mm Hg)
By comparing the density of air with the density of methane, we can determine from which places to sample for the presence of methane.
we get,
gas is lighter than air, which means it fills the upper part of any volume; the sample is taken from the upper part of the boiler furnace, well, chambers, room. Gas analyzers are installed in the upper part of the premises.
(fuel oil is lighter, occupies the upper part)
The density of carbon monoxide is almost the same as that of air, so a sample for carbon monoxide is taken 1.5 meters from the floor.
3. Pressure- this force acting per unit surface area.
Pressure force equal to 1 N, uniformly distributed on a surface of 1 m 2 is taken as a unit of pressure and is equal to 1 Pa (N/m 2) in the SI system (now in schools, in books everything is in Pa, instruments are also in Pa).
The value of Pa is small in value, for example: if you take 1 kg of water and pour it over 1 meter, you get 1 mm.in.st. Therefore, multipliers and prefixes are introduced - MPa, KPa...
Larger units of measurement are used in technology
1kPa=10 3 Pa; 1MPa=10 b Pa; 1 GPa = 10 9 Pa.
Non-system pressure units kgf/m2; kgf/cm 2 ; mm.h.st.; mm.h.st.
1 kgf/m 2 = 1 mm.in st =9.8 Pa
1 kgf/cm 2 = 9.8. 10 4 Pa ~ 10 5 Pa = 10 4 kgf/m 2
Pressure is often measured in physical and technical atmospheres.
Physical atmosphere- average atmospheric air pressure at sea level at sea level.
1 atm = 1.01325. 10 5 Pa = 760 mm Hg. = 10.33 m water. st = 1.0330 mm h. Art. = 1.033 kgf/cm2.
Technical atmosphere- the pressure caused by a force of 1 kgf is uniformly distributed over a surface normal to it with an area of 1 cm 2.
1at = 735 mm Hg. Art. = 10 m.v. Art. = 10,000 mm h. Art. = =0.1 MPa= 1 kgf/cm 2
1 mm V. Art. - a force equal to the hydrostatic pressure of a water column with a height of 1 mm on a flat base 1 mm V. st = 9.8 Pa.
1 mm. rt. st - force equal to the hydrostatic pressure of a column of mercury with a height of 1 mm on a flat base. 1 mm rt. Art. = 13.6 mm. V. Art.
In the technical characteristics of pumps, the term pressure is used instead of pressure. The unit of measurement for pressure is mW.O. Art. For example: The pressure created by the pump is 50 m water Art. this means it can raise water to a height of 50 m.
Types of pressure: excess, vacuum (vacuum, draft), absolute, atmospheric .
If the needle deviates in a direction greater than zero, this is excess pressure; if it deviates below zero, this is a vacuum.
Absolute pressure:
P abs = P ex + P atm
P abs = P vac + P atm
P abs = P atm - P dissolved
where: P atm = 1 kgf/cm 2
Atmosphere pressure- average atmospheric air pressure at sea level at t° = 0°C and normal atmospheric R=760 mm. rt. Art.
Overpressure- pressure above atmospheric (in a closed volume). In boiler rooms there is water under excess pressure, steam in boilers and pipelines. R izb. measured by pressure gauges.
Vacuum (Vacuum)- pressure in closed volumes is less than atmospheric (vacuum). The furnaces and chimneys of boilers are under vacuum. The vacuum is measured by draft gauges.
Absolute pressure- excess pressure or vacuum taking into account atmospheric pressure.
According to the intended purpose, the pressure is:
1). Channel - highest pressure at t=20 o C
2). Working – the maximum excess pressure in the boiler, which ensures long-term operation of the boiler under normal operating conditions (indicated in the production instructions).
3). Permitted - the maximum permissible pressure established based on the results of a technical examination or a control strength calculation.
4). Design – the maximum excess pressure at which the strength of the boiler elements is calculated.
5). Rtest - excess pressure at which hydraulic tests of boiler elements are carried out for strength and density (one of the types of technical examination).
4. Temperature- this is the degree of heating of the body, measured in degrees. Determines the direction of spontaneous heat transfer from a more heated to a less heated body.
Heat transfer will take place until the temperatures become equal, i.e., temperature equilibrium occurs.
Two scales are used: international - Kelvin and practical Celsius t ° C.
In this scale, zero is the melting point of ice, and one hundred degrees is the boiling point of water at atm. pressure (760 mm rt. Art.).
Absolute zero (the lowest theoretically possible temperature at which there is no molecular movement) is used as the reference point in the Kelvin thermodynamic temperature scale. Designated T.
1 Kelvin is equal to 1° Celsius
The melting temperature of ice is 273K. The boiling point of water is 373K
T=t + 273; t = T-273
Boiling point depends on pressure.
For example, At R ab c = 1,7 kgf/cm2. Water boils at t = 115°C.
5. Warmth - energy that can be transferred from a more heated body to a less heated one.
The SI unit of heat and energy is the Joule (J). The non-systemic unit of heat measurement is the calorie ( cal).
1 cal.- the amount of heat required to heat 1 g of H 2 O by 1 ° C at
P = 760 mm. Hg
1 cal.=4.19J
6. Heat capacity – the body's ability to absorb heat . In order to heat two different substances with the same mass to the same temperature, different amounts of heat must be expended.
The specific heat capacity of water is the amount of heat that must be supplied per unit of a substance to increase its temperature by 1°C, equal to 1 kcal/kg deg.
Methods of heat transfer.
There are three methods of heat transfer:
1.thermal conductivity;
2. radiation (radiation);
3. convection.
Thermal conductivity-
Heat transfer due to thermal movement of molecules, atoms and free electrons.
Each substance has its own thermal conductivity, it depends on the chemical composition, structure, and moisture content of the material.
A quantitative characteristic of thermal conductivity is the thermal conductivity coefficient, which is the amount of heat transferred through a unit of heating surface per unit of time with a difference t in about C and a wall thickness of 1 meter.
Coefficient of thermal conductivity ( ):
Copper = 330 kcal . mm 2. h . hail
Cast iron = 5 4 kcal . mm 2. h . hail
Steel =39 kcal . mm 2. h . hail
It can be seen that metals have good thermal conductivity, copper is best.
Asbestos =0.15 kcal . mm 2. h . hail
Soot =0.05-0, kcal . mm 2. h . hail
Scale =0.07-2 kcal . mm 2. h . hail
Air =0.02 kcal . mm 2. h . hail
Porous bodies (asbestos, soot, scale) conduct heat poorly.
Soot makes it difficult to transfer heat from the flue gases to the boiler wall (it conducts heat 100 times worse than steel), which leads to excessive fuel consumption and reduced steam or hot water production. The presence of soot increases the temperature of the flue gases. All this leads to a decrease in the efficiency of the boiler. When boilers are operating hourly using instruments (logometer), the t of carbon gases is monitored, the values of which are indicated in regime map boiler If the temperature of carbon gases has increased, then the heating surface is blown.
Scale is formed inside the pipes (it conducts heat 30-50 times worse than steel), thereby reducing heat transfer from the boiler wall to the water, as a result the walls overheat, deform, and rupture (rupture of boiler pipes). Scale conducts heat 30-50 times worse than steel
Convection -
Heat transfer by mixing or moving particles among themselves (typical only for liquids and gases). There are natural and forced convection.
Natural convection- free movement of liquid or gases due to the difference in densities of unevenly heated layers.
Forced convection- forced movement of liquid or gases due to pressure or vacuum created by pumps, smoke exhausters and fans.
Ways to increase convective heat transfer:
§ Increasing flow speed;
§ Turbulization (vortex);
§ Increasing the heating surface (due to the installation of fins);
§ Increasing the temperature difference between the heating and heated media;
§ Countercurrent movement of media (countercurrent).
Radiation (radiation) -
Heat exchange between bodies located at a distance from each other due to radiant energy, the carriers of which are electromagnetic vibrations: thermal energy is converted into radiant energy and vice versa, from radiant to thermal.
Radiation is the most effective way to transfer heat, especially if the body being studied has a high temperature and the rays are directed perpendicular to the heated surface.
To improve heat transfer by radiation in boiler furnaces, special slots are made of refractory materials, which at the same time act as heat emitters and combustion stabilizers.
The heating surface of the boiler is a surface from which on one side it is washed by gases and on the other side by water.
Discussed above 3 types of heat exchange are rarely found in their pure form. Almost one type of heat exchange is accompanied by another. All three types of heat exchange are present in the boiler, which is called complex heat exchange.
In the boiler furnace:
A) from the burner to the outer surface of the boiler pipes - radiation.
B) from the generated flue gases to the wall - by convection
B) from the outer surface of the pipe wall to the inner surface - thermal conductivity.
D) from the inner surface of the pipe wall to the water, by circulation along the surface - convection.
The transfer of heat from one medium to another through a dividing wall is called heat transfer.
Water, water vapor and its properties
Water is the simplest chemical compound of hydrogen with oxygen, stable under normal conditions, the highest density of water is 1000 kg/m 3 at t = 4 o C.
Water, like any liquid, obeys hydraulic laws. It almost does not compress, therefore it has the ability to transfer pressure exerted on it in all directions with the same force. If several vessels of different shapes are connected to each other, then the water level will be the same everywhere (the law of communicating vessels).
Related information.
water vapor
Water vapor
water contained in the atmosphere in a gaseous state. The amount of water vapor in the air varies greatly; its highest content is up to 4%. Water vapor is invisible; what is called steam in everyday life (steam from breathing in cold air, steam from boiling water, etc.) is the result of condensation of water vapor, just like fog. The amount of water vapor determines the most important characteristic for the state of the atmosphere - air humidity.
Geography. Modern illustrated encyclopedia. - M.: Rosman. Edited by prof. A. P. Gorkina. 2006 .
See what “water vapor” is in other dictionaries:
Water vapor is the gaseous state of water. It has no color, taste or smell. Contained in the troposphere. Formed by water molecules during its evaporation. When water vapor enters the air, it, like all other gases, creates a certain pressure,... ... Wikipedia
water vapor- steam Water in gaseous state. [RMG 75 2004] Topics of measuring the humidity of substances Synonyms of steam EN water steam DE Wasserdampf FR vapeur d eau ... Technical Translator's Guide
water vapor- Water found in the earth's atmosphere in the vapor phase and below the critical temperature for water... Dictionary of Geography
WATER VAPOR- water in a gaseous state. It enters the atmosphere as a result of evaporation from the surfaces of water basins and soil. Condenses into (see) in the form of fogs, clouds and clouds and again returns to the surface of the Earth in the form of various precipitation... Big Polytechnic Encyclopedia
water vapor- gaseous state of water. If at 101.3 kPa (760 mm Hg) water is heated to 100 ° C, then it boils and water vapor begins to form, having the same temperature, but a much larger volume. A state in which water and steam... ... Encyclopedic Dictionary of Metallurgy
WATER VAPOR. Vapor is a gaseous body obtained from a liquid at the appropriate temperature and pressure. All gases may be converted into a liquid state, and therefore it is difficult to draw the line between gases and vapors. In technology, steam is considered a gaseous body whose state is not far from turning into a liquid. Since there are significant differences in the properties of gases and vapors, this difference in terms is quite appropriate. Water vapor is the most important of the vapors used in technology. They are used as a working fluid in steam engines (steam engines and steam turbines) and for heating and heating purposes. The properties of steam are extremely different, depending on whether the steam is in a mixture with the liquid from which it is obtained, or whether it is separated from it. In the first case, the steam is called saturated, in the second case - superheated. In technology, initially almost exclusively saturated steam was used; at present, superheated steam is most widely used in steam engines, the properties of which are therefore carefully studied.
I. Saturated steam. The evaporation process is better understood by graphic images, for example, a diagram in p, v coordinates (specific pressure in kg/cm2 and specific volume in m3/kg). In fig. 1 shows a schematic diagram of the evaporation process for 1 kg of water. Point a 2 depicts the state of 1 kg of water at 0° and pressure p 2, and the abscissa of this point depicts the volume of this amount, the ordinate - the pressure under which the water is located.
Curve a 2 aa 1 shows the change in the volume of 1 kg of water with increasing pressure. The pressures at points a 2, a, a 1 are respectively equal to p 2, p, p 1 kg 1cm 2. In fact, this change is extremely small, and in technical matters the specific volume of water can be considered independent of pressure (that is, the line a 2 aa 1 can be taken as a straight line parallel to the ordinate axis). If you heat a taken amount of water, keeping the pressure constant, the temperature of the water rises, and at a certain value the water begins to evaporate. When water is heated, its specific volume, theoretically speaking, increases slightly (at least starting from 4°, i.e., from the temperature of the highest density of water). Therefore, the points at which evaporation begins at different pressures (p 2, p, p 1) will lie on some other curve b 2 bb 1. In fact, this increase in the volume of water with increasing temperature is insignificant, and therefore, at low pressures and temperatures, the specific volume of water can be taken as a constant value. The specific volumes of water at points b 2, b, b 1 are denoted respectively by v" 2, v", v" 1; the curve b 2 bb 1 is called the lower limit curve. The temperature at which evaporation begins is determined by the pressure under which heated water. During the entire period of evaporation, this temperature does not change if the pressure remains constant. It follows that the temperature of the saturated steam is a function only of the pressure p. Considering any line depicting the evaporation process, for example bcd, we see that the volume of the mixture of steam and. liquid during the evaporation process increases as the amount of evaporated water increases. At a certain point d, all water disappears, and pure steam is obtained; points d for different pressures form a certain curve d 1 dd 2, which is called. upper limit curve, or dry saturated steam curve; steam in this state (when the evaporation of water has just finished) is called dry saturated steam. If you continue heating after point d (towards some point e), leaving the pressure constant, then the temperature of the steam begins to increase. In this state, the steam is called superheated. Thus, three regions are obtained: to the right of the line d 1 dd 2 - the region of superheated steam, between the lines b 1 bb 2 and d 1 dd 2 - the region of saturated steam and to the left of the line b 1 bb 2 - the region of liquid water. At some intermediate point c there is a mixture of steam and water. To characterize the state of this mixture, the amount x of steam contained in it is used; with a mixture weighing 1 kg (equal to the weight of water taken), this value x is called proportion of steam in the mixture, or vapor content of the mixture; the amount of water in the mixture will be (1-x) kg. If v" m 3 / kg is the specific volume of dry saturated steam at temperature t and pressure p kg/cm 2, and the volume of water under the same conditions v", then the volume of the mixture v can be found by the formula:
The volumes v" and v", and therefore their difference v"-v" are functions of pressure p (or temperature t).
The form of the function that determines the dependence of p on t for water vapor is very complex; There are many empirical expressions for this dependence, which, however, are all suitable only for certain limited intervals of the independent variable t. Regnault for temperatures from 20 to 230° gives the formula:
Currently, the Dupre-Hertz formula is often used:
where k, m and n are constants.
Schüle gives this formula as follows:
and for temperature:
a) between 20 and 100°
(p - in kg/cm 2, T - absolute steam temperature);
b) between 100 and 200°
c) between 200 and 350°
The nature of the steam pressure p curve as a function of temperature is visible in Fig. 2.
In practice, they directly use tables that give the relationship between p and t. These tables are compiled on the basis of accurate experiments. To find the specific volumes of dry saturated steam, there is a theoretically derived Clapeyron-Clausius formula. You can also use Mollier’s empirical formula:
The amount of heat q required to heat 1 kg of water from 0 to t° (beginning of evaporation) is expressed as follows:
where c is the heat capacity of water, which differs little from unity over a wide range; Therefore, we use an approximate formula:
However, Regnault was already convinced of a noticeable increase in c at high temperatures and gave the expression for q:
In modern times, the following data is given for s (Diterichi formula):
For the average heat capacity with m in the range from 0 to t° the expression is given:
The experimental data of the German Institute of Physics and Technology deviate somewhat from this formula, whose observations give the following values of c:
To turn water heated to a temperature into steam, you still need to expend a certain amount of heat r, which is called latent heat of vaporization. Currently, this heat expenditure is divided into 2 parts: 1) heat Ψ, which goes to the external work of increasing volume when water turns into steam (external latent heat of evaporation), and 2) heat ϱ, which goes to the internal work of separation of molecules that occurs during evaporation water (internal latent heat of evaporation). External latent heat of evaporation
where A = 1/427 is the thermal equivalent of mechanical work.
Thus
For r the following formula is given (based on experiments at the German Institute of Physics and Technology):
The total heat of evaporation λ, i.e., the amount of heat required to convert water taken at 0° into steam at temperature t, is obviously equal to q + r. Regnault gave the following formula for λ:
this formula gives results close to the latest experimental data. Shule gives:
Internal energy u of water at 0° is assumed to be zero. To find its increment when heating water, it is necessary to find out the nature of the change in the specific volume of water with changes in pressure and temperature, i.e., the type of curves a 2 aa 1 and b 2 bb 1 (Fig. 1). The simplest assumption would be to take these lines as straight lines, and, moreover, coinciding with each other, i.e., taking the specific volume of water v" as a constant value that does not depend on either pressure or temperature (v" = 0.001 m 3 /kg). Under this assumption, all the heat used to heat the liquid, i.e. q, goes to increase the internal energy (since no external work is performed during this heating). This assumption is valid, however, only for relatively low pressures (Zeiner’s tables are given up to pressures of 20 kg/cm2). Modern tables (Mollier et al.), reaching critical pressure (225 kg/cm2) and temperature (374°), cannot, of course, ignore changes in the volume of water (the specific volume of water at critical pressure and critical temperature is 0.0031 m 2 /kg, i.e. more than three times more than at 0°). But Stodola and Knoblauch showed that the Diterici formula given above for the value of q gives precisely the value of the change in internal energy (and not the value of q); however, the difference between these values up to a pressure of 80 kg/cm2 is insignificant. Therefore, we assume that for water the internal energy is equal to the heat of the liquid: u" = q. During the period of evaporation, the internal energy increases by the amount of the internal latent heat of evaporation ϱ, i.e., the energy of dry saturated steam will be: 
(Fig. 3).
For a mixture with the proportion of steam x we obtain the following expression:
The dependence of the heat of evaporation and pressure on temperature is shown graphically in Fig. 3.
Mollier introduced into technical thermodynamics the thermodynamic function i, defined by the equation and called heat content. For a mixture with steam proportion x this will give:
or, after the cast:
for water (x = 0) it turns out:
for dry saturated steam:
The value of the product APv" is very small compared even to the value q (and even more so compared to the value q + r = λ); therefore, we can accept
In Mollier’s tables, therefore, it is not the values of q and λ that are given, but the values of i" and i" as a function of p or t°. The entropy of saturated steam is found by its differential expression dQ for all bodies has the form:
For saturated water vapor
The first term represents the increase in the entropy of water when it is heated, the second term is the increase in the entropy of the mixture during evaporation. Believing
we get 
or, integrating:
Note that when calculating s" the change in specific volume v" is usually also neglected and it is assumed that tables are used to solve all questions relating to saturated vapors. In the past, Zeiner tables were used in technology, but they are now obsolete; you can use the tables of Schüle, Knoblauch or Mollier. In all these tables, pressures and temperatures are brought to a critical state. The tables include the following data: temperature and pressure of saturated steam, specific volume of water and steam and specific gravity of steam, entropy of liquid and steam, heat content of water and steam, total latent heat of evaporation, internal energy, internal and external latent heat. For some issues (relating, for example, to capacitors), special tables are compiled with small pressure or temperature intervals.
Of all the changes in steam, the adiabatic change is of particular interest; it might. studied point by point. Let the starting point 1 of the adiabatic be given (Fig. 4), determined by the pressure p 1 and the proportion of steam x 1; it is required to determine the state of the steam at point 2, lying on the adiabatic path passing through point 1 and determined by pressure p 2. To find x2, the condition for equality of entropies at points 1 and 2 is expressed:
In this equation, the quantities s" 1, r 1 /T 1, s" 2 and r 2 /T 2 are found from the given pressures p 1 and p 2, the proportion of steam x 1 is given, and only x 2 is unknown. The specific volume v -2 at point 2 is determined by the formula:
The quantities v"" 2 and v" 2 are found from the tables. The external work of the adiabatic change under consideration is found from the difference in internal energies at the beginning and end of the change:
To simplify calculations, the empirical Zeiner equation, which expresses the adiabatic as a polytrope, is often used when studying adiabatic change:
The exponent μ is expressed through the initial proportion of steam x 1 as follows:
This formula is applicable in the range from x 1 = 0.7 to x 1 = 1. Adiabatic expansion with an initial high proportion of steam, above 0.5, is accompanied by the conversion of part of the steam into water (a decrease in x); at initial steam proportions less than 0.5, adiabatic expansion is accompanied, on the contrary, by the evaporation of part of the water. Formulas for other cases of changes in saturated steam are found in all textbooks of technical thermodynamics.
II. Superheated steam. Attention to superheated steam was attracted back in the 60s of the last century as a result of Girn's experiments, which showed significant benefits when using superheated steam in steam engines. But superheated steam became particularly widespread after V. Schmit created special designs of superheaters specifically for producing high-superheat steam (300-350°). These superheaters found wide application first (1894-95) in stationary steam engines, then in locomotive engines and in the 20th century in steam turbines. Currently, almost no installation can do without the use of superheated steam, and the superheat is brought to 400-420°. To make possible the rational use of such high superheat, the very properties of superheated steam were carefully studied. The original theory of superheated steam was given by Zeiner; she relied on the few experiments of Regnault. Its main provisions: 1) a special type of equation of state, which differs from the equation for ideal gases by an additional term, which is a function of pressure only; 2) adoption of a constant value for heat capacity c p at constant pressure: c p = 0.48. Both of these assumptions were not confirmed in experiments on the properties of superheated steam carried out over a wider range. Of particular importance were the extensive experiments of the Munich Laboratory of Technical Physics, which began around 1900 and continue to this day. A new theory of superheated steam was given in 1900-1903. Callender in England and Mollier in Germany, but it was not final, since the expression for heat capacity at constant pressure obtained from this theory is not entirely consistent with the latest experimental data. Therefore, a number of new attempts have appeared to construct an equation of state for superheated steam, which would be more consistent with the experimental results. From these attempts, the Eichelberg equation became famous. These attempts found their final completion in the new theory of Mollier (1925-1927), which led to the compilation of his last tables. Mollier adopts a very consistent notation system, which we partially used above. Mollier designations: P - pressure in kg/m 2 abs., p - pressure in kg/cm 2 abs., v - specific volume in m 3 /kg, γ = 1/v specific gravity in kg/m 3, t - temperature from 0°, T = t° + 273° - absolute temperature, A = 1/427 - thermal equivalent of mechanical work, R = 47.1 - gas constant (for water vapor), s - entropy, i - heat content in Cal /kg, u = i–APv - internal energy in Cal/kg, ϕ = s – i/T, c p - heat capacity at constant pressure, c ii p = 0.47 – limiting value of c p at p = 0.
The " and " icons refer to water itself and dry saturated steam. From Mollier's equation
Using formulas arising from the I and II laws of thermodynamics, all the most important quantities characterizing superheated steam are obtained, i.e., s, i, u and c p. Mollier introduces the following temperature auxiliary functions:
Using these functions, the following expressions are obtained:
Formulas for finding the specific volume and other quantities for superheated steam are quite complex and inconvenient for calculations. Therefore, the latest Mollier tables contain calculated values of the most important quantities characterizing superheated steam as a function of pressure and temperature. With the help of Mollier tables, all problems relating to superheated steam are solved quite simply and with sufficient accuracy. It should also be noted that for an adiabatic change in superheated steam within certain limits (up to 20-25 kg/cm 3), the polytropic equation retains its value: pv 1.3 = Const. Finally, many questions regarding superheated steam may solved using graphical techniques, especially the IS Mollier diagram. This diagram shows curves of constant pressures, constant temperatures and constant volumes. That. you can directly obtain the values of v, s, i as a function of pressure and temperature from the diagram. Adiabats are depicted on this diagram by straight lines parallel to the ordinate axis. It is especially easy to find the differences in heat content values corresponding to the beginning and end of the adiabatic expansion; these differences are necessary to find the steam outflow rates.
In this material we will look at water vapor, which is the gaseous state of water.
The gaseous state refers to the three main physical states of water found in nature under natural conditions. This issue is discussed in detail in the material.
water vapor
Clean water vapor has no color or taste. The greatest accumulation of steam is observed in the troposphere.
Water vapor is water contained in the atmosphere in a gaseous state. The amount of water vapor in the air varies greatly; its highest content is up to 4%. Water vapor is invisible; what is called steam in everyday life (steam from breathing in cold air, steam from boiling water, etc.) is the result of condensation of water vapor, like fog. The amount of water vapor determines the most important characteristic for the state of the atmosphere - air humidity.
Geography. Modern illustrated encyclopedia. - M.: Rosman. Edited by prof. A. P. Gorkina. 2006.
How is water vapor formed?
Water steam is formed as a result of “vaporization”. Vaporization occurs as a result of two processes - evaporation or boiling. During evaporation, steam is formed only on the surface of the substance, while during boiling, steam is formed throughout the entire volume of the liquid, as evidenced by bubbles actively rising during the boiling process. Boiling of water occurs at temperatures that depend on the chemical composition of the aqueous solution and atmospheric pressure; the boiling point remains unchanged throughout the entire process. Steam, formed as a result of boiling, is called saturated. Saturated steam in turn, it is divided into saturated dry and saturated wet steam. Saturated wet steam consists of suspended droplets of water, the temperature of which is at the boiling level, and, accordingly, the steam itself, and the saturated dry steam does not contain water droplets.
There is also “superheated steam”, which is formed when wet steam is further heated; this type of steam has a higher temperature and lower density.
Water vapor is an indispensable element of such an important process for our planet as.
We constantly encounter steam in everyday life, it appears above the spout of the kettle when boiling water, while ironing, when visiting a bathhouse... However, do not forget that, as we noted above, clean water vapor has no color or taste. Due to its physical properties and qualities, steam has long ago found its practical application in human economic activity. And not only in everyday life, but also when solving large global problems. For a long time, steam was the main driving force of progress, both in the literal and figurative sense of the expression. It was used as a working fluid for steam engines, the most famous of which is the STEAM LOGO.
Human use of steam
Steam is still widely used in economic and industrial needs today:
- for hygiene purposes;
- for medicinal purposes;
- for extinguishing fires;
- the thermal properties of steam are used (steam as a coolant) - steam boilers; steam jackets (autoclaves and reactors); heating of “freezing” materials; heat exchangers; heating systems; steaming of concrete products; in a special kind of heat exchangers...;
- use the transformation of steam energy into movement - steam engines...;
- sterilization and disinfection – food industry, agriculture, medicine...;
- steam as a humidifier - in the production of reinforced concrete products; plywood; in the food industry; in the chemical and perfume industry; in woodworking industries; in agricultural production...;
To summarize, we note that, despite all its “invisibility,” water vapor is not only an important element of the Earth’s global eco-system, but also a very useful substance for human economic activity.
WATER VAPOR IN THE ATMOSPHERE
AIR HUMIDITY. CHARACTERISTICS OF WATER VAPOR CONTENT IN THE ATMOSPHERE
Humidity is the content of water vapor in the atmosphere. Water vapor is one of the most important components of the earth's atmosphere.
Water vapor continuously enters the atmosphere due to the evaporation of water from the surface of reservoirs, soil, snow, ice and vegetation, which consumes an average of 23% of solar radiation arriving at the earth's surface.
The atmosphere contains on average 1.29 1013 tons of moisture (water vapor and liquid water), which is equivalent to a layer of water of 25.5 mm.
Air humidity is characterized by the following quantities: absolute humidity, partial pressure of water vapor, saturated vapor pressure, relative humidity, saturation deficit of water vapor, dew point temperature and specific humidity.
Absolute humidity a (g/m3) - the amount of water vapor, expressed in grams, contained in 1 m3 of air.
Partial pressure (elasticity) of water vapor e - the actual pressure of water vapor in the air, measured in millimeters of mercury (mmHg), millibars (mb) and hectopascals (hPa). Water vapor pressure is often called absolute humidity. However, these different concepts cannot be mixed, since they reflect different physical quantities of atmospheric air.
Saturated water vapor pressure, or saturation elasticity, E - the maximum possible value of partial pressure at a given temperature; measured in the same units as e. Saturation elasticity increases with increasing temperature. This means that at a higher temperature, air is able to hold more water vapor than at a lower temperature.
Relative humidity f is the ratio of the partial pressure of water vapor contained in the air to the pressure of saturated water vapor at a given temperature. It is usually expressed as a percentage accurate to whole numbers:
Relative humidity expresses the degree of saturation of air with water vapor.
Saturation deficit of water vapor (lack of saturation) d - the difference between the saturation elasticity and the actual elasticity of water vapor:
= E- e.
The saturation deficit is expressed in the same units and with the same accuracy as the values of e and E. With increasing relative humidity, the saturation deficit decreases and at / = 100% it becomes equal to zero.
Since E depends on the air temperature, and e - on the water vapor content in it, the saturation deficit is a complex value that reflects the heat and moisture content of the air. This allows the saturation deficit to be used more widely than other moisture characteristics to assess the growing conditions of agricultural plants.
Dew point td (°C) is the temperature at which water vapor contained in the air at a given pressure reaches a state of saturation relative to a chemically pure flat surface of water. At / = 100%, the actual air temperature coincides with the dew point. At temperatures below the dew point, condensation of water vapor begins with the formation of fogs, clouds, and dew, frost, and frost form on the surface of the earth and objects.
Specific humidity q (g/kg) - the amount of water vapor in grams contained in 1 kg of humid air:
q= 622 e/R,
where e is the water vapor pressure, hPa; P - atmospheric pressure, hPa.
Specific humidity is taken into account in zoometeorological calculations, for example, when determining evaporation from the surface of the respiratory organs of farm animals and when determining the corresponding energy costs.
CHANGES IN CHARACTERISTICS OF AIR HUMIDITY IN THE ATMOSPHERE WITH ALTITUDE
The greatest amount of water vapor is contained in the lower layers of air directly adjacent to the evaporating surface. Water vapor penetrates into the overlying layers as a result of turbulent diffusion
The penetration of water vapor into the overlying layers is facilitated by the fact that it is 1.6 times lighter than air (the density of water vapor relative to dry air at 0 °C is 0.622), therefore air enriched with water vapor, being less dense, tends to rise upward.
The vertical distribution of water vapor pressure depends on changes in pressure and temperature with height, on the processes of condensation and cloud formation. Therefore, it is difficult to theoretically establish the exact pattern of changes in the elasticity of water vapor with height.
The partial pressure of water vapor decreases with height 4...5 times faster than atmospheric pressure. Already at an altitude of 6 km, the partial pressure of water vapor is 9 times less than at sea level. This is explained by the fact that water vapor continuously enters the surface layer of the atmosphere as a result of evaporation from the active surface and its diffusion due to turbulence. In addition, the air temperature decreases with height, and the possible content of water vapor is limited by temperature, since its decrease promotes saturation of the vapor and its condensation.
A decrease in vapor pressure with height can alternate with its increase. For example, in an inversion layer, vapor pressure usually increases with height.
Relative humidity is distributed unevenly vertically, but with average height it decreases. In the surface layer of the atmosphere on summer days it increases slightly with height due to a rapid decrease in air temperature, then begins to decrease due to a decrease in the supply of water vapor and again increases to 100% in the cloud formation layer. In inversion layers it sharply decreases with height as a result of increasing temperature. Relative humidity changes especially unevenly up to a height of 2...3 km.
DAILY AND ANNUAL VARIATION OF AIR HUMIDITY
In the surface layer of the atmosphere there is a well-defined daily and annual variation in moisture content associated with corresponding periodic changes in temperature.
The daily variation of water vapor pressure and absolute humidity over the oceans, seas and coastal areas of land is similar to the daily variation of water and air temperature: minimum before sunrise and maximum at 14...15 hours. The minimum is due to very weak evaporation (or its absence at all) at this time of day. During the day, as the temperature increases and, accordingly, evaporation, the moisture content in the air increases. The diurnal variation of water vapor pressure over the continents in winter is the same.
In the warm season, in the interior of the continents, the daily variation of moisture content takes the form of a double wave (Fig. 5.1). The first minimum occurs early in the morning along with the temperature minimum. After sunrise, the temperature of the active surface rises, the rate of evaporation increases, and the amount of water vapor in the lower layer of the atmosphere increases rapidly. This growth continues for up to 8...10 hours, until evaporation prevails over the transfer of vapor from below to higher layers. After 8...10 hours, the intensity of turbulent mixing increases, and therefore water vapor is quickly transferred upward. This outflow of water vapor no longer has time to be compensated by evaporation, as a result of which the moisture content and, consequently, the elasticity of water vapor in the surface layer decreases and reaches a second minimum at 15...16 hours. In the pre-evening hours, turbulence weakens, while a fairly intense supply of water vapor into the atmosphere by evaporation is still ongoing. Vapor pressure and absolute humidity in the air begin to increase and at 20...22 hours they reach a second maximum. At night, evaporation almost stops, resulting in a decrease in water vapor content.
The annual variation of water vapor pressure and absolute humidity coincides with the annual variation of air temperature both over the ocean and over land. In the Northern Hemisphere, the maximum air moisture content is observed in July, the minimum in January. For example, in St. Petersburg, the average monthly vapor pressure in July is 14.3 hPa, and in January - 3.3 hPa.
The daily variation of relative humidity depends on vapor pressure and saturation pressure. With increasing temperature of the evaporating surface, the rate of evaporation increases and, therefore, e increases. But E increases much faster than e, therefore, with increasing surface temperature, and with it the air temperature, the relative humidity decreases [see. formula (5.1)]. As a result, its course near the earth’s surface turns out to be the opposite of the course of surface and air temperature: the maximum relative humidity occurs before sunrise, and the minimum at 15:00 (Fig. 5.2). Its daily decrease is especially pronounced over the continents in the summer, when, as a result of turbulent diffusion of vapor upward, E at the surface decreases, and due to an increase in air temperature, E increases. Therefore, the amplitude of daily fluctuations in relative humidity on continents is much greater than over water surfaces.
In the annual cycle, relative air humidity, as a rule, also changes inversely to the temperature trend. For example, in St. Petersburg, relative humidity in May averages 65%, and in December - 88% (Fig. 5.3). In areas with a monsoon climate, the minimum relative humidity occurs in winter, and the maximum in summer due to the summer transfer of moist sea air masses to land: for example, in Vladivostok in summer / = 89%, in winter / = 68%.
The course of the saturation deficit of water vapor is parallel to the course of air temperature. During the day, the deficit is greatest at 14...15 hours, and the smallest - before sunrise. During the year, the saturation deficit of water vapor has a maximum in the hottest month and a minimum in the coldest month. In the arid steppe regions of Russia in the summer at 13:00, a saturation deficit exceeding 40 hPa is observed annually. In St. Petersburg, the water vapor saturation deficit in June averages 6.7 hPa, and in January - only 0.5 hPa
AIR HUMIDITY IN PLANT COVER
Vegetation cover has a great influence on air humidity. Plants evaporate a large amount of water and thereby enrich the ground layer of the atmosphere with water vapor; there is an increased moisture content in the air compared to the bare surface. This is also facilitated by the reduction of wind speed by the vegetation cover, and, consequently, the turbulent diffusion of vapor. This is especially pronounced during the daytime. The vapor pressure inside tree crowns on clear summer days can be 2...4 hPa greater than in an open area, in some cases even 6...8 hPa. Inside agrophytocenoses, it is possible to increase the vapor pressure by 6...11 hPa compared to the steam field. In the evening and night hours, the influence of vegetation on moisture content is less.
Vegetation cover also has a great influence on relative humidity. So, on clear summer days, inside the crops of rye and wheat, the relative humidity is 15...30% higher than above the open area, and in the crops of tall crops (corn, sunflower, hemp) - 20...30% higher than over bare soil. In crops, the highest relative humidity is observed at the soil surface shaded by plants, and the lowest is in the upper tier of leaves (Table 5.1). Vertical distribution of relative humidity and saturation deficit
Accordingly, the saturation deficit of water vapor in crops is significantly less than over bare soil. Its distribution is characterized by a decrease from the upper tier of leaves to the lower (see Table 5.1).
It was previously noted that vegetation cover significantly influences the radiation regime (see Chapter 2), soil and air temperature (see Chapters 3 and 4), significantly changing them compared to an open place, i.e., a its own special meteorological regime - phytoclimate. How strongly it is expressed depends on the type, habit and age of the plants, the density of the planting, and the method of sowing (planting).
Weather conditions also influence the phytoclimate - in partly cloudy and clear weather, phytoclimatic features are more pronounced.
IMPORTANCE OF AIR HUMIDITY FOR AGRICULTURAL PRODUCTION
Water vapor contained in the atmosphere, as noted in Chapter 2, is of great importance in maintaining heat on the earth's surface, since it absorbs the heat emitted by it. Air humidity is one of the weather elements that is essential for agricultural production.
Air humidity has a great influence on the plant. It largely determines the intensity of transpiration. At high temperatures and low humidity (/"< 30 %) транспирация резко увеличивается и у растений возникает большой недостаток воды, что отражается на их росте и развитии. Например, отмечается недоразвитие генеративных органов, задерживается цветение.
Low humidity during the flowering period causes pollen to dry out and, consequently, incomplete fertilization, which in cereals, for example, causes transgrain. During the period of grain filling, excessive dryness of the air leads to the fact that the grain turns out puny and the yield is reduced.
Low moisture content in the air leads to small-fruited fruit, berry crops, grapes, poor bud formation for next year's harvest and, consequently, a decrease in yield.
Air humidity also affects the quality of the crop. It has been noted that low humidity reduces the quality of flax fiber, but increases the baking qualities of wheat, the technical properties of linseed oil, the sugar content in fruits, etc.
A decrease in relative air humidity with a lack of soil moisture is especially unfavorable. If hot and dry weather lasts for a long time, the plants may dry out.
A prolonged increase in moisture content (> 80%) also has a negative effect on the growth and development of plants. Excessively high air humidity causes the large-cell structure of plant tissue, which subsequently leads to lodging of grain crops. During the flowering period, such air humidity prevents normal pollination of plants and reduces the yield, since the anthers open less and the flight of insects decreases.
Increased air humidity delays the onset of full ripeness of grain, increases the moisture content in grain and straw, which, firstly, adversely affects the operation of harvesting machines, and secondly, requires additional costs for drying grain (Table 5.2).
A decrease in the saturation deficit to 3 hPa or more leads to virtually the cessation of harvesting work due to poor conditions.
In the warm season, increased air humidity contributes to the development and spread of a number of fungal diseases of agricultural crops (late blight of potatoes and tomatoes, mildew of grapes, white rot of sunflower, various types of rust of grain crops, etc.). The influence of this factor especially increases with increasing temperature (Table 5.3).
5.3. The number of plants of spring wheat Cesium 111 affected by smut, depending on humidity and air temperature (by, The timing of a number of agricultural works also depends on air humidity: weed control, laying feed for silage, ventilation of warehouses, grain drying, etc.
In the thermal balance of farm animals and humans, heat exchange is associated with air humidity. At air temperatures below 10 °C, increased humidity increases heat transfer from organisms, and at high temperatures it slows it down.