Define saturated steam. Saturated steam, boiling, air humidity

Steam that is in contact with water and has the same temperature as it, equal to the boiling point at a given pressure, is called saturated steam. Saturated steam can be wet or dry. Wet saturated steam is saturated steam containing the smallest particles of water, i.e., it is a mixture of steam and water. The steam produced in a steam boiler usually contains 2-5% water (i.e., the degree of steam dryness is correspondingly 98-95%). Dry saturated steam is saturated steam that is completely free of water impurities. Superheated steam is steam that has more high temperature than saturated steam of the same pressure.

Purpose of the smoke exhauster and fan. The procedure for starting and stopping the smoke exhauster and fan

Blower fans serve to supply air to the boiler furnace. Chimneys and smoke exhausters create draft (vacuum), which is necessary for continuous supply fresh air into the furnace and removing fuel combustion products from it. Smoke exhausters are installed in cases where chimney cannot provide the necessary traction. The design of the smoke exhauster is similar to that of a fan (but has a number of features: the body is made of heat-resistant steel, a coil with a water supply for cooling the oil is placed in the oil bath, the body is covered with thermal insulation).

Starting the smoke exhauster: Completely close the damper on the suction pipe (in front of the smoke exhauster) and turn on the electric motor. Check for the absence of extraneous noise, interference of moving parts with the housing, vibration of bearings, and correct rotation of the impeller. Next, slowly open the gate (so that the electric motor current under load does not exceed permissible value). First, turn on the smoke exhauster, and then the fan.

Stop: First stop the fan by closing the fan damper, and then the smoke exhauster by closing the exhaust damper damper.

If an open glass of water is left on for a long time, then eventually the water will completely evaporate. More precisely, it will evaporate. What is evaporation and why does it happen?

2.7.1 Evaporation and condensation

At a given temperature, liquid molecules have at different speeds. The velocities of most molecules are close to a certain average value (characteristic of this temperature). But there are molecules whose speeds differ significantly from the average, both smaller and larger.

In Fig. Figure 2.16 shows an approximate graph of the velocity distribution of liquid molecules. The blue background shows the majority of molecules whose velocities are grouped around the average value. The red “tail” of the graph is a small number of “fast” molecules, the speeds of which significantly exceed the average speed of the bulk of liquid molecules.

Number of molecules

Fast molecules

Speed ​​of molecules

Rice. 2.16. Distribution of molecules by speed

When such a very fast molecule finds itself on the free surface of the liquid (i.e., at the interface between liquid and air), the kinetic energy of this molecule may be enough to overcome the attractive forces of other molecules and fly out of the liquid. This process and there is evaporation, and the molecules leaving the liquid form vapor.

So, evaporation is the process of converting a liquid into vapor, occurring on the free surface of the liquid7.

It may happen that after some time the vapor molecule returns back to the liquid.

The process of vapor molecules changing into liquid is called condensation. Vapor condensation is the reverse process of liquid evaporation.

2.7.2 Dynamic balance

What happens if a vessel with liquid is hermetically sealed? The vapor density above the liquid surface will begin to increase; vapor particles will increasingly interfere with other liquid molecules flying out, and the evaporation rate will begin to decrease. At the same time it will start

7 When special conditions The transformation of liquid into vapor can occur throughout the entire volume of the liquid. This process is well known to you - boiling.

p n = n RT:

the condensation rate will increase, since with increasing vapor concentration the number of molecules returning to the liquid will increase.

Finally, at some point the rate of condensation will be equal to the rate of evaporation. A dynamic equilibrium will occur between liquid and vapor: per unit time, the same number of molecules will fly out of the liquid as return to it from the vapor. From this moment on, the amount of liquid will cease to decrease, and the amount of vapor will cease to increase; steam will reach ¾saturation¿.

Saturated vapor is vapor that is in a state of dynamic equilibrium with its liquid. Vapor that has not reached a state of dynamic equilibrium with the liquid is called unsaturated.

The pressure and density of saturated steam are designated pн in. Obviously, this is maximum pressure and the density that steam can have at a given temperature. In other words, the pressure and density of saturated steam always exceeds the pressure and density of unsaturated steam.

2.7.3 Properties of saturated steam

It turns out that the state of saturated steam (and even more so of unsaturated steam) can be approximately described by the equation of state of an ideal gas (Mendeleev-Clapeyron equation). In particular, we have an approximate relationship between saturated vapor pressure and its density:

This is very amazing fact, confirmed by experiment. Indeed, in its properties, saturated steam differs significantly from an ideal gas. Let us list the most important of these differences.

1. At a constant temperature, the density of saturated vapor does not depend on its volume.

If, for example, saturated steam is compressed isothermally, then its density will increase at the first moment, the condensation rate will exceed the evaporation rate, and part of the vapor will condense into liquid until dynamic equilibrium occurs again, in which the vapor density will return to its previous value.

Similarly, during isothermal expansion of saturated steam, its density will decrease at the first moment (the steam will become unsaturated), the rate of evaporation will exceed the rate of condensation, and the liquid will further evaporate until dynamic equilibrium is established again, i.e., until the steam again becomes saturated with the same density.

2. The pressure of saturated steam does not depend on its volume.

This follows from the fact that the density of saturated vapor does not depend on volume, and pressure is uniquely related to density by equation (2.6).

As we see, Boyle-Mariotte's law, which is valid for ideal gases, does not hold true for saturated steam. This is not surprising, since it was obtained from the Mendeleev-Clapeyron equation under the assumption that the mass of the gas remains constant.

3. At a constant volume, the density of saturated vapor increases with increasing temperature and decreases with decreasing temperature.

Indeed, as the temperature increases, the rate of liquid evaporation increases. The dynamic equilibrium is disrupted at the first moment, and additional

evaporation of some liquid. The pair will be added until dynamic equilibrium is restored again.

In the same way, as the temperature decreases, the rate of liquid evaporation becomes slower, and part of the vapor condenses until dynamic equilibrium is restored, but with less vapor.

Thus, when saturated steam is heated or cooled isochorically, its mass changes, so Charles’s law does not work in this case. The dependence of saturated vapor pressure on temperature will no longer be a linear function.

4. Saturated vapor pressure increases with temperature faster than according to a linear law.

In fact, with increasing temperature, the density of saturated vapor increases, and according to equation (2.6), the pressure is proportional to the product of density and temperature.

The dependence of saturated vapor pressure on temperature is exponential (Fig. 2.17). It is represented by section 1–2 of the graph. This dependence cannot be derived from the ideal gas laws.

isochore pair

Rice. 2.17. Dependence of steam pressure on temperature

At point 2 all liquid evaporates; with a further increase in temperature, the steam becomes unsaturated, and its pressure increases linearly according to Charles’s law (section 2–3).

Let us recall that the linear increase in pressure of an ideal gas is caused by an increase in the intensity of impacts of molecules on the walls of the vessel. When saturated steam is heated, the molecules begin to beat not only harder, but also more often because the steam becomes larger. The simultaneous action of these two factors causes an exponential increase in saturated vapor pressure.

2.7.4 Air humidity

Absolute humidity is the partial pressure of water vapor in the air (i.e., the pressure that water vapor would exert on its own, in the absence of other gases). Sometimes absolute humidity is also called the density of water vapor in the air.

Relative air humidity "is the ratio of the partial pressure of water vapor in it to the pressure of saturated water vapor at the same temperature. As a rule, this is

the ratio is expressed as a percentage:

" = p 100%: pн

From the Mendeleev-Clapeyron equation (2.6) it follows that the ratio of vapor pressures is equal to the ratio of densities. Since equation (2.6) itself, recall, describes saturated steam only approximately, we have an approximate relation:

" = 100%:n

One of the devices that measures air humidity is a psychrometer. It includes two thermometers, the reservoir of one of which is wrapped in a wet cloth. The lower the humidity, the more intense the evaporation of water from the fabric, the more the reservoir of the wet thermometer cools, and the greater the difference between its readings and the readings of the dry thermometer. From this difference, air humidity is determined using a special psychrometric table.

The molecular kinetic theory allows us not only to understand why a substance can be in gaseous, liquid and solid states, but also to explain the process of transition of a substance from one state to another.

Evaporation and condensation. The amount of water or any other liquid in an open container gradually decreases. Evaporation of the liquid occurs, the mechanism of which was described in the VII class physics course. During chaotic motion, some molecules acquire so much kinetic energy that they leave the liquid, overcoming the attractive forces of other molecules.

Simultaneously with evaporation, the reverse process occurs - the transition of part of the chaotically moving vapor molecules into liquid. This process is called condensation. If the vessel is open, then the molecules that have left the liquid may not return to the

liquid. In these cases, evaporation is not compensated by condensation and the amount of liquid decreases. When the air flow over the vessel carries away the resulting vapor, the liquid evaporates faster, since the vapor molecule has less opportunity to return to the liquid.

Saturated steam. If the vessel with liquid is tightly closed, its loss will soon stop. At a constant temperature, the liquid-vapor system will reach a state of thermal equilibrium and will remain in it for as long as desired.

At the first moment, after the liquid is poured into the vessel and closed, it will evaporate and the vapor density above the liquid will increase. However, at the same time, the number of molecules returning to the liquid will increase. The higher the vapor density, the larger number vapor molecules return to the liquid. As a result, in a closed vessel at constant temperature Eventually, a dynamic (mobile) equilibrium will be established between liquid and vapor. The number of molecules leaving the surface of the liquid will be equal to the number of vapor molecules returning to the liquid during the same time. Condensation occurs simultaneously with the evaporation process, and both processes, on average, compensate each other.

Vapor that is in dynamic equilibrium with its liquid is called saturated vapor. This name emphasizes that a larger amount of steam cannot be present in a given volume at a given temperature.

If the air from a vessel with a liquid is previously pumped out, then only saturated vapor will be above the surface of the liquid.

Saturated vapor pressure. What will happen to saturated steam if the volume it occupies is reduced, for example, by compressing the steam in equilibrium with the liquid in the cylinder under the piston, maintaining the temperature of the contents of the cylinder constant?

When the steam is compressed, the equilibrium will begin to be disturbed. At first, the vapor density increases slightly, and a larger number of molecules begin to move from gas to liquid than from liquid to gas. This continues until equilibrium and density are established again, and therefore the concentration of molecules takes on its previous value. The concentration of saturated vapor molecules is therefore independent of volume at constant temperature.

Since pressure is proportional to concentration in accordance with the formula, from the independence of the concentration (or density) of saturated vapor from volume, it follows that the pressure of saturated vapor is independent of the volume it occupies.

The volume-independent vapor pressure at which a liquid is in equilibrium with its vapor is called saturated vapor pressure.

When saturated steam is compressed, everything most of it turns into a liquid state. A liquid of a given mass occupies less volume than vapor of the same mass. As a result, the volume of steam, while its density remains unchanged, decreases.

We have used the words "gas" and "steam" many times. There is no fundamental difference between gas and steam, and these words are generally equivalent. But we are accustomed to a certain, relatively small temperature range environment. The word "gas" is usually applied to those substances whose saturated vapor pressure at ordinary temperatures is higher than atmospheric (for example, carbon dioxide). On the contrary, we talk about a couple when room temperature the saturated vapor pressure is less than atmospheric and the substance is more stable in the liquid state (for example, water vapor).

The independence of saturated vapor pressure from volume has been established in numerous experiments on the isothermal compression of vapor in equilibrium with its liquid. Let the substance in large volumes be in a gaseous state. As isothermal compression proceeds, its density and pressure increase (section of the AB isotherm in Figure 51). When pressure is reached, steam condensation begins. Subsequently, when saturated steam is compressed, the pressure does not change until all the steam turns into liquid (straight line BC in Figure 51). After this, the pressure during compression begins to increase sharply (segment of the curve since liquids are slightly compressible.

The curve shown in Figure 51 is called the isotherm of a real gas.

Before answering the question posed in the title of the article, let’s figure out what steam is. The images that most people have when hearing this word are: a boiling kettle or pan, a steam room, a hot drink and many more similar pictures. One way or another, in our ideas there is a liquid and a gaseous substance rising above its surface. If you are asked to give an example of steam, you will immediately remember water vapor, alcohol, ether, gasoline, acetone.

There is another word for gaseous states - gas. Here we usually remember oxygen, hydrogen, nitrogen and other gases, without associating them with the corresponding liquids. Moreover, it is well known that they exist in a liquid state. At first glance, the differences are that steam corresponds to natural liquids, and gases must be specially liquefied. However, this is not entirely true. Moreover, the images that arise from the word steam are not steam. To give a more accurate answer, let’s look at how steam arises.

How is steam different from gas?

The state of aggregation of a substance is determined by temperature, or more precisely by the relationship between the energy with which its molecules interact and the energy of their thermal chaotic motion. Approximately, we can assume that if the interaction energy is much greater, it is a solid state; if the energy of thermal motion is much greater, it is a gaseous state; if the energies are comparable, it is a liquid state.

It turns out that in order for a molecule to break away from the liquid and participate in the formation of vapor, the amount of thermal energy must be greater than the interaction energy. How can this happen? average speed thermal motion of molecules is equal to a certain value depending on temperature. However, the individual speeds of molecules are different: most of them have speeds close to the average value, but some have speeds greater than the average, some less.

Faster molecules can have thermal energy greater than the interaction energy, which means that, once on the surface of a liquid, they are able to break away from it, forming vapor. This method of vaporization is called evaporation. Due to the same distribution of speeds, the opposite process also exists - condensation: molecules from vapor pass into liquid. By the way, the images that usually arise when hearing the word steam are not steam, but the result of the opposite process - condensation. The steam cannot be seen.

Under certain conditions, steam can become a liquid, but for this its temperature should not exceed certain value. This value is called critical temperature. Steam and gas are gaseous states that differ in the temperature at which they exist. If the temperature does not exceed the critical temperature, it is steam; if it exceeds it, it is gas. If you keep the temperature constant and reduce the volume, the steam liquefies, but the gas does not liquefy.

What is saturated and unsaturated steam

The word “saturated” itself carries certain information; it is difficult to saturate a large area of ​​​​space. This means that in order to obtain saturated steam, you need limit the space in which the liquid is located. The temperature must be less than the critical temperature for a given substance. Now the evaporated molecules remain in the space where the liquid is located. At first, most of the molecular transitions will occur from the liquid, and the vapor density will increase. This in turn will cause a greater number of reverse transitions of molecules into the liquid, which will increase the speed of the condensation process.

Finally, a state is established for which the average number of molecules passing from one phase to another will be equal. This condition is called dynamic equilibrium. This state is characterized by the same change in the magnitude and direction of the rates of evaporation and condensation. This state corresponds to saturated steam. If the state of dynamic equilibrium is not achieved, this corresponds to unsaturated steam.

They begin the study of an object, always with its simplest model. In molecular kinetic theory, this is an ideal gas. The main simplifications here are the neglect of the molecules’ own volume and the energy of their interaction. Turns out, similar model describes unsaturated steam quite satisfactorily. Moreover, the less saturated it is, the more legitimate its use. An ideal gas is a gas; it cannot become either vapor or liquid. Consequently, for saturated steam such a model is not adequate.

The main differences between saturated and unsaturated steam

1. Saturated means that this object has the largest possible value of some parameters. For a couple this is density and pressure. These parameters for unsaturated steam have lower values. The further the steam is from saturation, the smaller these values ​​are. One clarification: the reference temperature must be constant.
2. For unsaturated steam: Boyle-Mariotte law: if the temperature and mass of the gas are constant, an increase or decrease in volume causes a decrease or increase in pressure by the same amount, pressure and volume are inversely related proportional dependence. From the maximum density and pressure at a constant temperature, it follows that they are independent of the volume of saturated steam; it turns out that for saturated steam, pressure and volume are independent of each other.
3. For unsaturated steam density does not depend on temperature, and if the volume is maintained, the density value does not change. For saturated steam, while maintaining volume, the density changes if the temperature changes. The dependence in this case is direct. If the temperature increases, the density also increases, if the temperature decreases, the density also changes.
4. If the volume is constant, unsaturated steam behaves in accordance with Charles' law: as the temperature increases, the pressure also increases by the same factor. This dependence is called linear. For saturated steam, as the temperature increases, the pressure increases faster than for unsaturated steam. The dependence is exponential.

To summarize, we can note significant differences in the properties of the compared objects. The main difference is that steam, in a state of saturation, cannot be considered in isolation from its liquid. This is a two-part system to which most gas laws cannot be applied.

Saturated steam.

If a vessel with close the liquid tightly, the amount of liquid will first decrease and then remain constant. When not Menn At this temperature, the liquid-vapor system will reach a state of thermal equilibrium and will remain in it for as long as desired. Simultaneously with the evaporation process, condensation also occurs, both processes on average compencourage each other. At the first moment, after the liquid is poured into the vessel and closed, the liquid willevaporate and the vapor density above it will increase. However, at the same time, the number of molecules returning to the liquid will increase. The greater the density of the vapor, the greater the number of its molecules returning to the liquid. As a result, in a closed vessel at a constant temperature, a dynamic (mobile) equilibrium will be established between liquid and vapor, i.e., the number of molecules leaving the surface of the liquid after a certain R th time period will be equal on average to the number of vapor molecules returning to the liquid during the same time b. Steam, nah floating in dynamic equilibrium with its liquid is called saturated vapor. This is the definition of underscoreIt means that in a given volume at a given temperature there cannot be a larger amount of steam.

Saturated steam pressure .

What will happen to saturated steam if the volume it occupies is reduced? For example, if you compress steam that is in equilibrium with liquid in a cylinder under a piston, maintaining the temperature of the contents of the cylinder constant. When the steam is compressed, the equilibrium will begin to be disturbed. At first, the vapor density will increase slightly, and a larger number of molecules will begin to move from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on the temperature, and compression of the vapor does not change this number. The process continues until dynamic equilibrium and vapor density are established again, and therefore the concentration of its molecules takes on its previous values. Consequently, the concentration of saturated vapor molecules at a constant temperature does not depend on its volume. Since pressure is proportional to the concentration of molecules (p=nkT), it follows from this definition that the pressure of saturated vapor does not depend on the volume it occupies. Pressure p n.p. vapor pressure at which a liquid is in equilibrium with its vapor is called saturated vapor pressure.

Dependence of saturated vapor pressure on temperature.

The state of saturated steam, as experience shows, is approximately described by the equation of state of an ideal gas, and its pressure is determined by the formula P = nkT As the temperature increases, the pressure increases. Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature. However, the dependence of p.n. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With increasing temperature, the pressure of real saturated vapor increases faster than the pressure of an ideal gas (Fig.drain curve 12). Why is this happening? When a liquid is heated in a closed container, some of the liquid turns into steam. As a result, according to the formula P = nkT, the saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration central ii. (The main difference in behavior andideal gas and saturated steam is that when the temperature of the steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the steam changes. The liquid partially turns into vapor, or, on the contrary, the vapor partially condensestsya. Nothing like this happens with an ideal gas.) When all the liquid has evaporated, the steam will cease to be saturated upon further heating and its pressure at constant volume will increaseis directly proportional to the absolute temperature (see Fig., curve section 23).

Boiling.

Boiling is an intense transition of a substance from a liquid to a gaseous state, occurring throughout the entire volume of the liquid (and not just from its surface). (Condensation is the reverse process.) As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent converting it into vapor. Under what conditions does boiling begin?

A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of liquid have more low temperature, then steam condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before it boils. The dependence of saturated vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column. Boiling begins at the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid. The greater the external pressure, the higher the boiling point. And vice versa, by reducing external pressure, we thereby lower the boiling point. By pumping air and water vapor out of the flask, you can make the water boil at room temperature. Each liquid has its own boiling point (which remains constant until all the liquid has boiled away), which depends on its saturated vapor pressure. The higher the saturated vapor pressure, the lower the boiling point of the liquid.

Air humidity and its measurement.

There is almost always some amount of water vapor in the air around us. Air humidity depends on the amount of water vapor contained in it. Damp air contains a higher percentage of water molecules than dry air. Pain Of great importance is the relative humidity of the air, messages about which are heard every day in weather forecast reports.

Dew point

The dryness or humidity of the air depends on how close its water vapor is to saturation. If wet air cool, then the steam in it can be brought to saturation, and then it will condense. A sign that the steam has become saturated is the appearance of the first drops of condensed liquid - dew. The temperature at which vapor in the air becomes saturated is called the dew point. Dew point also characterizes air humidity. Examples: dew falling in the morning, fogging up of cold glass if you breathe on it, the formation of a drop of water on a cold water pipe, dampness in the basements of houses. To measure air humidity, measuring instruments - hygrometers - are used. There are several types of hygrometers, but the main ones are hair and psychrometric.