Which steam is called saturated. Saturated steam - cheat sheet. Saturated and unsaturated steam
Steam is called rich, if the number of molecules of a substance passing into the gaseous state is equal to the number of molecules returning to the liquid or solid phase. This is a state of dynamic equilibrium.
If the number of evaporating molecules is greater than condensing, then the vapor unsaturated. The evaporation process continues until a state of dynamic equilibrium is reached, or until all the substance has evaporated.
Vapor pressure - what is it?
Rich and not saturated steam primarily associated with water vapor. The vapor content in the air is an important characteristic of the climate, weather conditions, and sanitary and hygienic conditions of the room. At the same time, steam pressure is technical specifications state of a thermodynamic system.
When evaporating in a hermetically sealed container, steam exerts pressure on the surface of the liquid. The higher the temperature, the higher the vapor pressure. It can even rupture an iron boiler if the temperature is raised too much.
In a closed vessel, dynamic equilibrium is quickly achieved between water and steam, and the steam becomes saturated. In the open air, unsaturated steam is more often observed. Important characteristic weather conditions - relative air humidity, which is calculated as the ratio of the vapor pressure in the air to the saturated vapor pressure at a given temperature.
Water vapor: the colder the weather, the drier the air
Absolute humidity is the amount of water vapor contained in a unit volume of air. At low temperatures there is little water vapor in the air in absolute terms, but the vapor may be saturated or have a relative humidity of more than 90%. When the air is heated to 20° C, the absolute content remains the same, but the relative humidity decreases sharply, the air becomes dry, the absolute humidity can be 15-20%.
Therefore, in winter, the air in the heated room is too dry, and this is not related to the type of heater, but is related only to the difference in temperatures outside and indoors.
Saturated steam in thermodynamics
Saturated vapor pressure increases with increasing temperature much faster than pressure ideal gas when the temperature in a closed volume increases. That is why the first heat engines used water steam, or rather the water-steam system. When such a system is heated, not only the vapor pressure increases, but also the number of vapor molecules and their concentration.
After boiling, the temperature of the water stops rising and remains unchanged until it evaporates completely. Vaporization is the process of transition from a liquid state to vapor, which has the same temperature as a boiling liquid. This evaporation is called saturated steam. Once all the water has evaporated, any subsequent addition of heat raises the temperature. Heated steam beyond the saturated level is called superheated. Industrial applications typically use saturated steam for heating, cooking, drying, or other procedures. Superheated is used exclusively for turbines. Various types pairs have different exchange potential energies and this justifies their use for completely different purposes.
Steam as one of the three physical states
A better understanding of the properties of steam can be achieved by understanding the general molecular and atomic structure of the substance, and by applying this knowledge to ice, water and steam. A molecule is the smallest unit of any element or compound. This in turn is made up of even smaller particles called atoms, which make up the basic elements such as hydrogen and oxygen. Specific combinations of these atomic elements provide the connection of substances. One of these compounds is presented chemical formula H 2 O, whose molecules consist of 2 hydrogen atoms and 1 oxygen atom. Carbon is also abundant and is a key component of all organic matter. Most minerals can exist in three physical states ( solid, liquid and vapor), which are called phases.
Steam generation process
As the temperature of water approaches its boiling point, some molecules gain enough kinetic energy to achieve speeds that allow them to momentarily separate from the liquid in the space above the surface before returning. Further heating causes greater excitation and the number of molecules wanting to leave the liquid increases. At atmospheric pressure the saturation temperature is 100 °C. Steam with a boiling point at this pressure is called dry saturated steam. Like the phase change from ice to water, the evaporation process is also reversible (condensation). The critical point is the highest temperature at which water can remain liquid. Above this point, steam can be considered as a gas. The gaseous state is similar to the diffuse state, in which molecules have an almost unlimited possibility of movement.
Relationship of Variables
At a given temperature, there is a certain vapor pressure that exists in equilibrium with liquid water. If this indicator increases, the steam overheats and is called dry. There is a relationship between pressure and temperature: knowing one value, you can determine the other. The state of steam is determined by three variables: pressure, temperature and volume. Dry saturated steam is a condition where steam and water can be present simultaneously. In other words, this occurs when the rate of vaporization is equal to the rate of condensation.
Saturated steam and its properties
When discussing the properties of saturated steam, it is often compared to an ideal gas. Do they have anything in common or is this just a misconception? Firstly, at a constant temperature level, density does not depend on volume. Visually, this can be imagined as follows: you need to visually reduce the volume of the container with steam without changing the temperature indicators. The number of condensed molecules will exceed the number of evaporated ones, and the steam will return to a state of balance. As a result, density will be a constant parameter. Secondly, characteristics such as pressure and volume do not depend on each other. Thirdly, taking into account the invariance of volumetric characteristics, the density of molecules increases when the temperature increases, and becomes smaller when it decreases. In fact, when heated, water begins to evaporate faster. In this case, the balance will be disrupted and will not be restored until the vapor density returns to its previous positions. During condensation, on the contrary, the density of saturated vapor will decrease. Unlike an ideal gas, saturated steam cannot be called a closed system, since it is constantly in contact with water.
Heating benefits
Saturated steam is pure steam in direct contact with liquid water. It has many characteristics that make it an excellent source of thermal energy, especially at high temperatures (above 100 °C). Some of them:
Various types of steam
Steam is the gaseous phase of water. It uses heat during its formation and releases large amounts of heat afterwards. Therefore he
can be used as a working substance for heat engines. The following states are known: wet saturated, dry saturated and overheated. Saturated steam is preferable to superheated steam as a coolant in heat exchangers. When it is released into the atmosphere from pipes, part of it condenses, forming clouds of white, moist evaporation containing tiny droplets of water. Superheated steam will not be subject to condensation, even when coming into direct contact with the atmosphere. In an overheated state, it will have greater heat transfer due to the acceleration of molecular movement and lower density. The presence of moisture causes sedimentation, corrosion and reduced service life of boilers or other heat exchange equipment. Therefore, dry steam is preferred as it produces more energy and is non-corrosive.
Dry and rich: what is the contradiction?
Many people get confused with the terms "dry" and "rich". How can something be both at the same time? The answer lies in the terminology we use. The term “dry” is associated with the absence of moisture, that is, “not wet.” “Saturated” means “soaked,” “soaked,” “flooded,” “overwhelmed,” and so on. All this would seem to confirm the contradiction. However, in steam engineering, the term "saturated" has a different meaning and in this context means the state at which boiling occurs. Thus, the temperature at which boiling occurs is technically known as saturation temperature. Dry steam in this context does not contain moisture. If you watch a boiling kettle, you can see white steam coming out of the kettle's spout. In fact, it is a mixture of dry, colorless steam and wet steam containing water droplets that reflect light and become colored. White color. Therefore, the term "dry saturated steam" means that the steam is dehydrated and not superheated. Free from liquid particles, it is a substance in a gaseous state that does not follow the general gas laws.
Before answering the question posed in the title of the article, let’s figure out what steam is. The images that come to mind for most people 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 precise 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, more precisely by the ratio 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 significantly greater – solid state, if the energy of thermal motion is significantly greater - gaseous, if the energies are comparable - liquid.
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? The average speed of thermal movement 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 pass from vapor to 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.
Steam at certain conditions can become a liquid, but for this its temperature should not exceed a certain value. This value is called the 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 larger number reverse transitions of molecules into 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. It turns out that such a 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
- 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 comparison temperature must be constant.
- 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 proportional. 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.
- 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. Dependency in in this case straight. If the temperature increases, the density also increases, if the temperature decreases, the density also changes.
- If the volume is constant, unsaturated steam behaves in accordance with Charles' law: as the temperature increases, the pressure 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.
DEFINITION
Evaporation is the process of converting liquid into vapor.
In a liquid (or solid) at any temperature there is a certain number of “fast” molecules, kinetic energy of which there are more potential energy their interactions with other particles of matter. If such molecules find themselves near the surface, they can overcome the attraction of other molecules and fly out of the liquid, forming vapor above it. Evaporation of solids is also often called sublimation or sublimation.
Evaporation occurs at any temperature at which a given substance can be in a liquid or solid state. However, the rate of evaporation depends on temperature. As the temperature rises, the number of “fast” molecules increases, and, consequently, the intensity of evaporation increases. The rate of evaporation also depends on the free surface area of the liquid and the type of substance. For example, water poured into a saucer will evaporate faster than water poured into a glass. Alcohol evaporates faster than water, etc.
Condensation
The amount of liquid in an open container continuously decreases due to evaporation. But this does not happen in a tightly closed vessel. This is explained by the fact that simultaneously with evaporation in a liquid (or solid), the reverse process occurs. Vapor molecules move chaotically over the liquid, so some of them, under the influence of the attraction of free surface molecules, fall back into the liquid. The process of turning steam into liquid is called condensation. The process of turning steam into a solid is usually called crystallization from steam.
After we pour the liquid into the vessel and close it tightly, the liquid will begin to evaporate and the vapor density above the free surface of the liquid will increase. However, at the same time, the number of molecules returning back to the liquid will increase. In an open vessel the situation is different: the molecules that have left the liquid may not return to the liquid. In a closed vessel, an equilibrium state is established over time: the number of molecules leaving the surface of the liquid becomes equal to the number vapor molecules returning to the liquid. This condition is called state of dynamic equilibrium(Fig. 1). In a state of dynamic equilibrium between liquid and vapor, evaporation and condensation occur simultaneously, and both processes compensate each other.
Fig.1. Liquid in a state of dynamic equilibrium
Saturated and unsaturated steam
DEFINITION
Saturated steam is steam in a state of dynamic equilibrium with its liquid.
The name “saturated” emphasizes that no more vapor can be present in a given volume at a given temperature. Saturated steam has a maximum density at a given temperature, and, therefore, exerts maximum pressure on the walls of the vessel.
DEFINITION
Unsaturated steam- steam that has not reached a state of dynamic equilibrium.
For different liquids, vapor saturation occurs at different densities, which is due to differences in molecular structure, i.e. differences in the forces of intermolecular interaction. In liquids in which the molecular interaction forces are strong (for example, in mercury), a state of dynamic equilibrium is achieved at low vapor densities, since the number of molecules capable of leaving the surface of the liquid is small. On the contrary, in volatile liquids with low molecular attractive forces, at the same temperatures a significant number of molecules fly out of the liquid and vapor saturation is achieved at high density. Examples of such liquids are ethanol, ether, etc.
Since the intensity of the steam condensation process is proportional to the concentration of steam molecules, and the intensity of the evaporation process depends only on temperature and increases sharply with its growth, the concentration of molecules in saturated steam depends only on the temperature of the liquid. That's why Saturated vapor pressure depends only on temperature and does not depend on volume. Moreover, with increasing temperature, the concentration of saturated vapor molecules and, consequently, the density and pressure of saturated vapor rapidly increase. The specific dependences of pressure and density of saturated vapor on temperature are different for different substances and can be found from reference tables. It turns out that saturated steam, as a rule, is well described by the Clayperon-Mendeleev equation. However, when compressed or heated, the mass of saturated steam changes.
Unsaturated steam obeys the ideal gas laws with a sufficient degree of accuracy.
Examples of problem solving
EXAMPLE 1
| Exercise | In a closed vessel with a capacity of 0.5 liters at temperature, water vapor and a drop of water are in equilibrium. Determine the mass of water vapor in the vessel. |
| Solution | At temperature, the saturated vapor pressure is equal to atmospheric pressure, so Pa. Let's write the Mendeleev-Clapeyron equation:
where do we find the mass of water vapor: The molar mass of water vapor is determined in the same way as the molar mass of water. Let's convert the units to the SI system: volume of the vessel steam temperature . Let's calculate: |
| Answer | The mass of water vapor in the vessel is 0.3 g. |
EXAMPLE 2
| Exercise | In a vessel with a volume of 1 liter at temperature, water, water vapor and nitrogen are in equilibrium. The volume of liquid water is much less than the volume of the vessel. The pressure in the vessel is 300 kPa, Atmosphere pressure 100 kPa. Find the total amount of the substance in the gaseous state. What is the partial pressure of nitrogen in the system? What is the mass of water vapor? What is the mass of nitrogen? |
| Solution | Let us write the Mendeleev-Clapeyron equation for the gas mixture water vapor + nitrogen: from where we find the total amount of substance in the gaseous state: Universal gas constant. Let's convert the units to the SI system: volume of the vessel pressure in the vessel temperature . Let's calculate:
According to Dalton's law, the pressure in the vessel is equal to the sum of the partial pressures of water vapor and nitrogen: where does the partial pressure of nitrogen come from: At temperature, the saturated vapor pressure is equal to atmospheric pressure, therefore . |
During evaporation, simultaneously with the transition of molecules from liquid to vapor, the reverse process also occurs. Moving randomly over the surface of the liquid, some of the molecules that left it return to the liquid again.
Saturated vapor pressure.
When saturated vapor is compressed, the temperature of which is maintained constant, the equilibrium will first begin to be disturbed: the density of the vapor will increase, and as a result, more molecules will pass from gas to liquid than from liquid to gas; this will continue until the vapor concentration in the new volume becomes the same, corresponding to the concentration of saturated vapor at a given temperature (and equilibrium is restored). This is explained by the fact that the number of molecules leaving the liquid per unit time depends only on temperature.
So, the concentration of molecules of saturated steam at a constant temperature does not depend on its volume.
Since the pressure of a gas is proportional to the concentration of its molecules, the pressure of saturated vapor does not depend on the volume it occupies. Pressure p 0, at which the liquid is in equilibrium with its vapor is called saturated steam pressure.
When saturated steam is compressed, most of it goes into liquid state. Liquid occupies less volume than vapor of the same mass. As a result, the volume of steam, while its density remains unchanged, decreases.
Dependence of saturated vapor pressure on temperature.
For an ideal gas it is true linear dependence pressure versus temperature at constant volume. As applied to saturated steam with pressure p 0 this dependence is expressed by the equality:
p 0 =nkT.
Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature.
Experimentally determined dependence p0(T) differs from dependence ( p 0 =nkT) for an ideal gas.
With increasing temperature, the pressure of saturated vapor increases faster than the pressure of an ideal gas (section of the curve AB on the image). This becomes especially obvious if we draw an isochore through the point A(dashed line). This happens because when a liquid is heated, part of it turns into steam, and the density of the steam increases. Therefore, according to the formula ( p 0 =nkT), the saturated vapor pressure increases not only as a result of an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. The main difference in the behavior of an ideal gas and saturated vapor is the change in the mass of vapor with a change in temperature at a constant volume (in a closed vessel) or with a change in volume at a constant temperature. Nothing like this can happen with an ideal gas (the molecular kinetic theory of an ideal gas does not provide for the phase transition of gas into liquid).
After all the liquid has evaporated, the behavior of the vapor will correspond to the behavior of an ideal gas (section Sun curve in the figure above).
Unsaturated steam.
If in a space containing vapor of a liquid, further evaporation of this liquid can occur, then the vapor located in this space is unsaturated.
Vapor that is not in equilibrium with its liquid is called unsaturated.
Unsaturated vapor can be converted into liquid by simple compression. Once this transformation has begun, the vapor in equilibrium with the liquid becomes saturated.