HEAT TRANSFER DURING BOILING AND CONDENSATION
BOILING HEAT TRANSFER
Boiling called the process of intense vaporization occurring in the entire volume of the liquid, which is at the saturation temperature or somewhat overheated relative to the saturation temperature, with the formation of vapor bubbles. In the process of phase transformation, the heat of vaporization is absorbed. The boiling process is usually associated with the supply of heat to the boiling liquid.
Modes of liquid boiling.
Distinguish between the boiling of liquids on a solid heat exchange surface, to which heat is supplied from the outside, and boiling in the volume of a liquid.
When boiling on a solid surface, the formation of a vapor phase is observed in some places on this surface. During volumetric boiling, the vapor phase arises spontaneously (spontaneously) directly in the bulk of the liquid in the form of individual vapor bubbles. Bulk boiling can only occur when the superheat of the liquid phase is greater relative to the saturation temperature at a given pressure than boiling on a solid surface. Significant overheating can be obtained, for example, by rapidly depressurizing the system. Bulk boiling can take place when there are internal heat sources in the liquid.
In modern power engineering and technology, boiling processes on solid heating surfaces (pipe surfaces, channel walls, etc.) are usually encountered. This type of boiling is mainly discussed below.
The mechanism of heat transfer during nucleate boiling differs from the mechanism of heat transfer during convection of a single-phase liquid by the presence of an additional transfer of the mass of matter and heat by vapor bubbles from the boundary layer into the volume of the boiling liquid. This leads to a high intensity of heat transfer during boiling compared to the convection of a single-phase liquid.
For the boiling process to occur, two conditions must be met: the presence of overheating of the liquid relative to the saturation temperature and the presence of centers of vaporization.
Liquid overheating has a maximum value directly at the heated heat exchange surface. It also contains centers of vaporization in the form of wall irregularities, air bubbles, dust particles, etc. Therefore, the formation of vapor bubbles occurs directly on the heat exchange surface.
Figure 3.1 - liquid boiling modes in an unlimited volume: a) - bubble; b) - transitional; c) - film
On fig. 3.1. schematically shows the modes of boiling of a liquid in an unlimited volume. At bubble mode boiling (Fig. 3.1, a) as the temperature of the heating surface increases tc and according to the temperature difference, the number of active centers of vaporization grows, the boiling process becomes more and more intense. Vapor bubbles periodically break away from the surface and, floating up to the free surface, continue to grow in volume.
With an increase in temperature difference Δ t the heat flux, which is removed from the heating surface to the boiling liquid, increases significantly. All this heat is ultimately spent on the formation of steam. Therefore, the heat balance equation for boiling has the form:
Where Q- heat flow, W; r- heat of liquid phase transition, J/kg; G p- the amount of steam generated per unit time as a result of liquid boiling and removed from its free surface, kg/s.
heat flow Q with an increase in temperature difference Δ t does not grow indefinitely. For some value Δ t it reaches its maximum value (Fig. 3.2), and with a further increase in Δ t starts to decrease.
Figure 3.2 - The dependence of the heat flux density q
from temperature difference Δ t when boiling water in a large volume at atmospheric pressure: 1- heating to saturation temperature; 2 - bubble mode; 3 - transition mode; 4 - film mode.
Give plots 1 2 3 and 4
Bubble boiling occurs in section 2 (Fig. 3.2) until the maximum heat removal is reached at the point q kr1 , called the first critical heat flux density. For water at atmospheric pressure, the first critical heat flux density is ≈ W/m 2 ; the corresponding critical value of the temperature difference W/m 2 . (These values refer to the conditions of water boiling in free movement in a large volume. For other conditions and other liquids, the values \u200b\u200bare different).
For larger Δ t comes transitional regime boiling (Fig. 3.1, b). It is characterized by the fact that, both on the heating surface itself and near it, the bubbles continuously merge with each other, and large vapor cavities are formed. Because of this, the access of liquid to the surface itself is gradually more and more difficult. In some places on the surface, “dry” spots appear; their number and size continuously grow as the surface temperature increases. Such sections are, as it were, excluded from heat exchange, since the removal of heat directly to the steam occurs much less intensively. This determines the sharp decrease in the heat flux (section 3 in Fig. 3.2) and the heat transfer coefficient in the region of the transitional boiling regime.
Finally, at a certain temperature difference, the entire heating surface is covered with a continuous film of vapor, which pushes the liquid away from the surface. From now on there is film mode boiling (Fig. 3.1, V). In this case, the transfer of heat from the heating surface to the liquid is carried out by convective heat transfer and radiation through the vapor film. The intensity of heat transfer in the film boiling mode is rather low (section 4 in Fig. 3.2). The vapor film experiences pulsations; steam, periodically accumulating in it, breaks off in the form of large bubbles. At the moment of onset of film boiling, the heat load removed from the surface and, accordingly, the amount of steam formed are minimal. This corresponds to Fig. 3.2 point q kr2 , called the second critical heat flux density. At atmospheric pressure for water, the moment of the beginning of film boiling is characterized by a temperature difference of ≈150 °C, i.e., the surface temperature tc is approximately 250°C. As the temperature difference increases, an increasing part of the heat is transferred due to heat exchange by radiation.
All three boiling regimes can be observed in reverse order, if, for example, a red-hot massive metal product is lowered into water for quenching. Water boils, at first the cooling of the body proceeds relatively slowly (film boiling), then the cooling rate increases rapidly (transitional mode), the water begins to periodically wet the surface, and the highest rate of decrease in surface temperature is achieved in the final stage of cooling (bubble boiling). In this example, boiling proceeds under non-stationary conditions in time.
On fig. 3.3 shows the visualization of bubble and film boiling modes on an electrically heated wire in water.
rice. 3.3 visualization of bubble and film boiling modes on an electrically heated wire: a) - bubble and b) - film boiling mode.
In practice, conditions are also often encountered when a fixed heat flux is supplied to the surface, i.e. q= const. This is typical, for example, for thermal electric heaters, fuel elements of nuclear reactors and, approximately, in the case of radiant surface heating from sources with a very high temperature. In conditions q= const surface temperature tc and, accordingly, the temperature difference Δ t depend on the boiling state of the liquid. It turns out that under such conditions of heat supply, the transient regime cannot exist stationary. As a result, the boiling process acquires a number of important features. With a gradual increase in heat load q temperature difference Δ t increases in accordance with the bubble boiling line in Fig. 3.2, and the process develops in the same way as described above. New conditions arise when the supplied heat flux density reaches a value that corresponds to the first critical heat flux density q cr1 . Now, for any slight (even accidental) increase in the value q there is an excess between the amount of heat supplied to the surface and that maximum heat load q kr1 , which can be diverted into a boiling liquid. This excess ( q-q kp1) causes an increase in the surface temperature, i.e., non-stationary heating of the wall material begins. The development of the process acquires a crisis character. In a fraction of a second, the temperature of the heating surface material increases by hundreds of degrees, and only if the wall is sufficiently refractory does the crisis end successfully with a new stationary state corresponding to the film boiling region at a very high surface temperature. On fig. 3.2, this crisis transition from nucleate to film boiling is conditionally shown by an arrow as a “jump” from the nucleate boiling curve to the film boiling line at the same heat load. q cr1 . However, this is usually accompanied by melting and destruction of the heating surface (its burnout).
The second feature is that if a crisis has occurred and the film boiling regime has been established (the surface has not collapsed), then with a decrease in the thermal load, film boiling will persist, i.e., the reverse process will now occur along the film boiling line (Fig. 3.2). Only upon reaching q kr2, the liquid begins to periodically reach (wet) the heating surface at separate points. The heat removal increases and exceeds the heat input, as a result of which there is a rapid cooling of the surface, which also has a crisis character. There is a rapid change of regimes, and stationary nucleate boiling is established. This reverse transition (the second crisis) in Fig. 3.2 is also conventionally shown by an arrow as a "jump" from the film boiling curve to the nucleate boiling line at q = q cr2 .
So, under conditions of a fixed value of the heat flux density q supplied to the heating surface, both transitions from bubble to film and vice versa are of a crisis nature. They occur at critical heat flux densities q kr1 and q cr2, respectively. Under these conditions, the transition mode of boiling cannot exist stationary; it is unstable.
In practice, heat removal methods are widely used when boiling liquid moving inside pipes or channels of various shapes. So, steam generation processes are carried out due to the boiling of water moving inside the boiler pipes. Heat is supplied to the surface of the pipes from the hot products of fuel combustion due to radiation and convective heat transfer.
For the boiling process of a liquid moving inside a limited volume of a pipe (channel), the conditions described above remain valid, but along with this, a number of new features appear.
vertical pipe. A pipe or channel is a limited system in which, during the movement of a boiling liquid, a continuous increase in the vapor phase and a decrease in the liquid phase occur. Accordingly, the hydrodynamic structure of the flow changes both along the length and along the cross section of the pipe. Accordingly, the heat transfer also changes.
There are three main areas with different structure of the fluid flow along the length of the vertical pipe when the flow moves from bottom to top (Fig. 3.4): I- heating area (economizer section, up to the pipe section, where T c \u003d T n); II- boiling area (evaporation section, from the section where T c \u003d T n, i f<i n, to the section, where T c \u003d T n, i cm→i n); III- the area of drying of wet steam.
The evaporation section includes areas with surface boiling of a saturated liquid.
On fig. 3.4 schematically shows the structure of such a flow. Section 1 corresponds to the heating of a single-phase liquid to saturation temperature (economizer section). In section 2, surface nucleate boiling occurs, in which heat transfer increases compared to section 2. In section 3, an emulsion regime takes place, in which a two-phase flow consists of a liquid and relatively small bubbles uniformly distributed in it, which subsequently merge, forming large bubbles - plugs commensurate with the diameter of the pipe. In the plug mode (section 4), the steam moves in the form of separate large bubbles-plugs, separated by interlayers of the vapor-liquid emulsion. Further, in section 5, wet steam moves in a continuous mass in the core of the flow, and a thin annular liquid layer near the pipe wall. The thickness of this liquid layer gradually decreases. This section corresponds to the annular boiling regime, which ends when the liquid disappears from the wall. In section 6, steam drying occurs (increase in the degree of steam dryness). Since the boiling process is completed, the heat transfer decreases. In the future, due to an increase in the specific volume of steam, the steam velocity increases, which leads to some increase in heat transfer.
Fig. 3.4 - Structure of the flow when liquid boils inside a vertical pipe
Increasing the circulation rate at given q with, pipe length and inlet temperature leads to a decrease in areas with developed boiling and an increase in the length of the economizer section; with increasing q with at a given speed, on the contrary, the length of the sections with developed boiling increases, and the length of the economizer section decreases.
Horizontal and inclined pipes. When a two-phase flow moves inside pipes located horizontally or with a slight inclination, in addition to changing the structure of the flow along the length, there is a significant change in the structure along the perimeter of the pipe. So, if the circulation rate and the vapor content in the flow are small, there is a stratification of the two-phase flow into the liquid phase moving in the lower part of the pipe, and the steam moving in the upper part of it (Fig. 3.5, A). With a further increase in the vapor content and circulation rate, the interface between the vapor and liquid phases acquires a wave character, and the liquid periodically wets the upper part of the pipe with wave crests. With a further increase in the vapor content and velocity, the wave motion at the phase boundary increases, which leads to partial ejection of the liquid into the vapor region. As a result, the two-phase flow acquires the character of a flow, first close to a slug flow, and then to an annular flow.
Rice. 3.5 - The structure of the flow during the boiling of a liquid inside a horizontal pipe.
A– stratified boiling mode; b– rod mode; 1 - steam; 2 - liquid.
In the annular mode, the movement of a thin layer of liquid is established along the entire perimeter of the pipe, a vapor-liquid mixture moves in the core of the flow (Fig. 3.5, b). However, even in this case no complete axial symmetry is observed in the flow structure.
if the intensity of heat supply to the pipe walls is high enough, then the boiling process can also occur when the flow in the pipe is subcooled to the liquid saturation temperature. Such a process occurs when the wall temperature tc exceeds saturation temperature t s . it encloses the boundary layer of the liquid directly at the wall. Vapor bubbles entering the cold core of the flow quickly condense. This type of boiling is called boiling with subcooling.
Heat removal in bubble boiling mode is one of the most advanced methods of cooling the heating surface. It finds wide application in technical devices.
3.1.2. Heat transfer during nucleate boiling.
Observations show that with an increase in temperature difference Δ t = tc-t s, as well as pressure R on the heating surface, the number of active centers of vaporization increases. As a result, an increasing number of bubbles continuously arise, grow and detach from the heating surface. As a result, turbulence and mixing of the near-wall boundary layer of the liquid increase. During their growth on the heating surface, the bubbles also intensively take heat from the boundary layer. All this contributes to the improvement of heat transfer. In general, the process of nucleate boiling is rather chaotic.
Studies show that the number of vaporization centers on technical heating surfaces depends on the material, structure and microroughness of the surface, the presence of heterogeneity of the surface composition and the gas (air) adsorbed by the surface. A noticeable effect is exerted by various raids, oxide films, as well as any other inclusions.
Observations show that in real conditions, the centers of vaporization usually serve as individual elements of surface roughness and microroughness (preferably various depressions and depressions).
Usually, the number of vaporization centers on new surfaces is higher than on the same surfaces after prolonged boiling. This is mainly due to the presence of gas adsorbed by the surface. Over time, the gas is gradually removed, it mixes with the steam in the growing bubbles, and is carried out into the steam space. The boiling process and heat transfer are stabilized in time and in intensity.
The conditions for the formation of vapor bubbles are greatly influenced by the surface tension at the interface between liquid and vapor.
Due to surface tension, the vapor pressure inside the bubble R n above the pressure of the fluid surrounding it R and. Their difference is determined by the Laplace equation
where σ is the surface tension; R is the radius of the bubble.
Laplace's equation expresses the condition of mechanical equilibrium. It shows that the surface tension, like an elastic shell, "compresses" the vapor in the bubble, and the stronger, the smaller its radius. R.
The dependence of vapor pressure in a bubble on its size imposes features on the condition of thermal or thermodynamic equilibrium of small bubbles. The vapor in the bubble and the liquid on its surface are in equilibrium if the surface of the liquid has a temperature equal to the saturation temperature at the vapor pressure in the bubble, t s( R P). This temperature is higher than the saturation temperature at the external pressure in the liquid t s( R and). Therefore, in order to achieve thermal equilibrium, the liquid around the bubble must be overheated by an amount t s( R P)- t s( R and).
The next feature is that this equilibrium turns out to be unstable. If the temperature of the liquid slightly exceeds the equilibrium value, then some of the liquid will evaporate into the bubbles and its radius will increase. In this case, according to the Laplace equation, the vapor pressure in the bubble will decrease. This will lead to a new deviation from the equilibrium state. The bubble will start to grow indefinitely. Also, with a slight decrease in the temperature of the liquid, part of the vapor will condense, the size of the bubble will decrease, and the vapor pressure in it will increase. This will entail a further deviation from the equilibrium conditions, now in the other direction. As a result, the bubble will completely condense and disappear.
Consequently, in a superheated liquid, not any randomly formed small bubbles are capable of further growth, but only those whose radius exceeds the value corresponding to the conditions of unstable mechanical and thermal equilibrium considered above. This minimum value
where the derivative is a physical characteristic of a given substance, it is determined by the Clapeyron - Clausis equation
i.e., it is expressed in terms of other physical constants: the heat of phase transition r, vapor density ρ p and liquids ρ w and absolute saturation temperature Ts.
Equation (3-2) shows that if steam nuclei appear at certain points of the heating surface, then only those of them whose radius of curvature exceeds the value Rmin. Since with increasing Δ t magnitude Rmin decreases, Equation (3-2) explains
the experimentally observed fact of an increase in the number of vaporization centers with an increase in surface temperature.
An increase in the number of vaporization centers with increasing pressure is also associated with a decrease Rmin, because as the pressure increases, the value p's is growing and σ decreases. Calculations show that for water boiling at atmospheric pressure, at Δ t= 5°С Rmin= 6.7 µm, and at Δ t= 25°C Rmin= 1.3 µm.
Observations made using high-speed filming show that at a fixed boiling regime, the frequency of formation of vapor bubbles is not the same both at different points on the surface and in time. This gives the boiling process a complex statistical character. Accordingly, the growth rates and separation sizes of various bubbles are also characterized by random deviations around some average values.
After the bubble reaches a certain size, it breaks away from the surface. Tear-off size is determined mainly by the interaction of gravity, surface tension and inertia. The latter value is a dynamic reaction that occurs in a liquid due to the rapid growth of bubbles in size. Typically, this force prevents bubbles from breaking off. In addition, the nature of the development and detachment of bubbles to a large extent depends on whether the liquid wets the surface or does not. The wetting ability of a liquid is characterized by the contact angle θ, which is formed between the wall and the free surface of the liquid. The larger θ, the worse the wetting ability of the liquid. It is generally accepted that for θ<90° (рис. 3.6, A), the liquid wets the surface, but at θ >90° it does not. The value of the contact angle depends on the nature of the liquid, the material, the condition and the cleanliness of the surface. If the boiling liquid wets the heating surface, then the steam bubbles have a thin leg and easily come off the surface (Fig. 3.7, A). If the liquid does not wet the surface, then the vapor bubbles have a wide leg (Fig. 3.7, b) and come off along the isthmus, or vaporization occurs over the entire surface.
Boiling- this is vaporization that occurs simultaneously both from the surface and throughout the volume of the liquid. It consists in the fact that numerous bubbles pop up and burst, causing a characteristic seething.
As experience shows, the boiling of a liquid at a given external pressure begins at a quite definite temperature that does not change during the boiling process and can only occur when energy is supplied from the outside as a result of heat transfer (Fig. 1):
where L is the specific heat of vaporization at the boiling point.
Boiling mechanism: there is always a dissolved gas in a liquid, the degree of dissolution of which decreases with increasing temperature. In addition, there is adsorbed gas on the walls of the vessel. When the liquid is heated from below (Fig. 2), the gas begins to evolve in the form of bubbles near the walls of the vessel. The liquid evaporates into these bubbles. Therefore, in addition to air, they contain saturated steam, the pressure of which increases rapidly with increasing temperature, and the bubbles grow in volume, and, consequently, the Archimedes forces acting on them increase. When the buoyant force becomes greater than the gravity of the bubble, it begins to float. But until the liquid is uniformly heated, as it rises, the volume of the bubble decreases (the saturated vapor pressure decreases with decreasing temperature) and, before reaching the free surface, the bubbles disappear (collapse) (Fig. 2, a), which is why we hear a characteristic noise before boiling. When the temperature of the liquid equalizes, the volume of the bubble will increase as it rises, since the saturated vapor pressure does not change, and the external pressure on the bubble, which is the sum of the hydrostatic pressure of the liquid above the bubble and the atmospheric pressure, decreases. The bubble reaches the free surface of the liquid, bursts, and the saturated vapor comes out (Fig. 2, b) - the liquid boils. The saturation vapor pressure in the bubbles is practically equal to the external pressure.
The temperature at which the saturated vapor pressure of a liquid is equal to the external pressure on its free surface is called boiling point liquids.
Since the pressure of saturated vapor increases with increasing temperature, and during boiling it should be equal to the external pressure, the boiling temperature increases with an increase in external pressure.
The boiling point also depends on the presence of impurities, usually increasing with increasing concentration of impurities.
If the liquid is first freed from the gas dissolved in it, then it can be overheated, i.e. heat above boiling point. This is an unstable state of the liquid. Sufficient small shaking and the liquid boils, and its temperature immediately drops to the boiling point.
Everything that surrounds us in everyday life can be represented as physical and chemical processes. We constantly perform a lot of manipulations that are expressed by formulas and equations, without even knowing it. One of these processes is boiling. This is the phenomenon that absolutely all housewives use during cooking. It seems to us absolutely ordinary. But let's look at the boiling process from a scientific point of view.
Boiling - what is it?
Since the school course of physics, it is known that matter can be in a liquid and gaseous state. The process of transforming a liquid into a state of vapor is boiling. This happens only when a certain temperature regime is reached or exceeded. Participates in this process and pressure, it must be taken into account. Each liquid has its own boiling point, which triggers the formation of vapor.
This is the essential difference between boiling and evaporation occurring at any temperature regime of the liquid.
How does boiling happen?
If you have ever boiled water in a glass container, you have observed the formation of bubbles on the walls of the container in the process of heating the liquid. They are formed due to the fact that air accumulates in the microcracks of the dishes, which, when heated, begins to expand. Bubbles are made up of liquid vapor under pressure. These pairs are called saturated. As the liquid heats up, the pressure in the air bubbles increases and they increase in size. Naturally, they begin to rise to the top.
But, if the liquid has not yet reached the boiling point, then in the upper layers the bubbles cool, the pressure decreases and they end up at the bottom of the container, where they heat up again and rise up. This process is familiar to every housewife, as if the water starts to make noise. As soon as the temperature of the liquid in the upper and lower layers is equal, the bubbles begin to rise to the surface and burst - boiling occurs. This is possible only when the pressure inside the bubbles becomes the same as the pressure of the liquid itself.
As we have already mentioned, each liquid has its own temperature regime, at which the boiling process begins. Moreover, during the entire process, the temperature of the substance remains unchanged, all the released energy is spent on vaporization. Therefore, pots burn out at negligent housewives - all their contents boil away and the container itself begins to heat up.
The boiling point is directly proportional to the pressure exerted on the entire liquid, more precisely, on its surface. In the school physics course, it is indicated that water begins to boil at a temperature of one hundred degrees Celsius. But few people remember that this statement is true only under conditions of normal pressure. The norm is considered to be a value of one hundred and one kilopascals. If the pressure is increased, the liquid will boil at a different temperature.
This physical property is used by manufacturers of modern household appliances. An example would be a pressure cooker. All housewives know that in such devices food is cooked much faster than in conventional pans. What is it connected with? With the pressure that is formed in the pressure cooker. It is twice the norm. Therefore, water boils at approximately one hundred and twenty degrees Celsius.
If you have ever been in the mountains, you have seen the reverse process. At a height, water begins to boil at ninety degrees, which greatly complicates the process of cooking. Local residents and climbers who spend all their free time in the mountains are well aware of these difficulties.
A little more about boiling
Many have heard such an expression as "boiling point" and are probably surprised that we did not mention it in the article. In fact, we have already described it. Do not rush to read the text. The fact is that in physics the point and temperature of the boiling process are considered identical.
In the scientific world, separation in this terminology is made only in the case of mixing different liquid substances. In such a situation, it is precisely the boiling point that is determined, and the smallest of all possible. It is she who is taken as the norm for all the constituent parts of the mixture.
Water: interesting facts about physical processes
In laboratory experiments, physicists always take a liquid without impurities and create absolutely ideal external conditions. But in life, everything happens a little differently, because often we add salt to water or add various seasonings to it. What will be the boiling point in this case?
Salt water requires a higher temperature to boil than fresh water. This is due to impurities of sodium and chlorine. Their molecules collide with each other, and their heating requires a much higher temperature. There is a certain formula that allows you to calculate the boiling point of salt water. Keep in mind that sixty grams of salt per liter of water increases the boiling point by ten degrees.
Can water boil in a vacuum? Scientists have proven that it can. That's just the boiling point in this case should reach the limit of three hundred degrees Celsius. After all, in a vacuum, the pressure is only four kilopascals.
We all boil water in a kettle, so we are familiar with such an unpleasant phenomenon as "scale". What is it and why is it formed? In fact, everything is simple: fresh water has a different degree of hardness. It is determined by the amount of impurities in the liquid, most often it contains various salts. In the process of boiling, they are transformed into sediment and in large quantities turn into scale.
Can alcohol boil?
Boiling alcohol is used in the moonshine brewing process and is called distillation. This process directly depends on the amount of water in the alcohol solution. If we take pure ethyl alcohol as a basis, then its boiling point will be close to seventy-eight degrees Celsius.
If you add water to alcohol, the boiling point of the liquid increases. Depending on the concentration of the solution, it will boil in the range from seventy-eight degrees to one hundred degrees Celsius. Naturally, during the boiling process, alcohol will turn into steam in a shorter time interval than water.
Boiling is the process of changing the aggregate state of a substance. When we talk about water, we mean the change from liquid to vapor. It is important to note that boiling is not evaporation, which can occur even at room temperature. Also, do not confuse with boiling, which is the process of heating water to a certain temperature. Now that we have understood the concepts, we can determine at what temperature water boils.
Process
The very process of transforming the state of aggregation from liquid to gaseous is complex. Although people don't see it, there are 4 stages:
- In the first stage, small bubbles form at the bottom of the heated container. They can also be seen on the sides or on the surface of the water. They are formed due to the expansion of air bubbles, which are always present in the cracks of the tank, where the water is heated.
- In the second stage, the volume of the bubbles increases. All of them begin to rush to the surface, as there is saturated steam inside them, which is lighter than water. With an increase in the heating temperature, the pressure of the bubbles increases, and they are pushed to the surface due to the well-known Archimedes force. In this case, you can hear the characteristic sound of boiling, which is formed due to the constant expansion and reduction in the size of the bubbles.
- In the third stage, a large number of bubbles can be seen on the surface. This initially creates cloudiness in the water. This process is popularly called "boiling with a white key", and it lasts a short period of time.
- At the fourth stage, the water boils intensively, large bursting bubbles appear on the surface, and splashes may appear. Most often, splashes mean that the liquid has reached its maximum temperature. Steam will start to come out of the water.
It is known that water boils at a temperature of 100 degrees, which is possible only at the fourth stage.
Steam temperature
Steam is one of the states of water. When it enters the air, then, like other gases, it exerts a certain pressure on it. During vaporization, the temperature of steam and water remains constant until the entire liquid changes its state of aggregation. This phenomenon can be explained by the fact that during boiling all the energy is spent on converting water into steam.
At the very beginning of boiling, moist saturated steam is formed, which, after the evaporation of all the liquid, becomes dry. If its temperature begins to exceed the temperature of water, then such steam is superheated, and in terms of its characteristics it will be closer to gas.
Boiling salt water
It is interesting enough to know at what temperature water with a high salt content boils. It is known that it should be higher due to the content of Na+ and Cl- ions in the composition, which occupy an area between water molecules. This chemical composition of water with salt differs from the usual fresh liquid.
The fact is that in salt water, a hydration reaction takes place - the process of attaching water molecules to salt ions. The bond between fresh water molecules is weaker than those formed during hydration, so boiling liquid with dissolved salt will take longer. As the temperature rises, the molecules in water containing salt move faster, but there are fewer of them, which is why collisions between them occur less often. As a result, less steam is produced and its pressure is therefore lower than the steam head of fresh water. Therefore, more energy (temperature) is required for full vaporization. On average, to boil one liter of water containing 60 grams of salt, it is necessary to raise the boiling point of water by 10% (that is, by 10 C).
Boiling pressure dependences
It is known that in the mountains, regardless of the chemical composition of water, the boiling point will be lower. This is because the atmospheric pressure is lower at altitude. Normal pressure is considered to be 101.325 kPa. With it, the boiling point of water is 100 degrees Celsius. But if you climb a mountain, where the pressure is on average 40 kPa, then the water will boil there at 75.88 C. But this does not mean that cooking in the mountains will take almost half the time. For heat treatment of products, a certain temperature is needed.
It is believed that at an altitude of 500 meters above sea level, water will boil at 98.3 C, and at an altitude of 3000 meters, the boiling point will be 90 C.
Note that this law also works in the opposite direction. If a liquid is placed in a closed flask through which vapor cannot pass, then as the temperature rises and steam is formed, the pressure in this flask will increase, and boiling at elevated pressure will occur at a higher temperature. For example, at a pressure of 490.3 kPa, the boiling point of water will be 151 C.
Boiling distilled water
Distilled water is purified water without any impurities. It is often used for medical or technical purposes. Given that there are no impurities in such water, it is not used for cooking. It is interesting to note that distilled water boils faster than ordinary fresh water, but the boiling point remains the same - 100 degrees. However, the difference in boiling time will be minimal - only a fraction of a second.
in a teapot
Often people are interested in what temperature water boils in a kettle, since it is these devices that they use to boil liquids. Taking into account the fact that the atmospheric pressure in the apartment is equal to the standard one, and the water used does not contain salts and other impurities that should not be there, then the boiling point will also be standard - 100 degrees. But if the water contains salt, then the boiling point, as we already know, will be higher.
Conclusion
Now you know at what temperature water boils, and how atmospheric pressure and the composition of the liquid affect this process. There is nothing complicated in this, and children receive such information at school. The main thing to remember is that with a decrease in pressure, the boiling point of the liquid also decreases, and with its increase, it also increases.
On the Internet, you can find many different tables that indicate the dependence of the boiling point of a liquid on atmospheric pressure. They are available to everyone and are actively used by schoolchildren, students and even teachers in institutes.
The process of boiling water consists of three stages:
- the beginning of the first stage - slipping from the bottom of the kettle or any other vessel in which water is brought to a boil, tiny air bubbles and the appearance of new bubble formations on the surface of the water. Gradually, the number of such bubbles increases.
- On the second stages of water boiling there is a massive rapid rise of the bubbles upward, causing at first a slight turbidity of the water, which then turns into a “whitening”, in which the water looks like a stream of a spring. This phenomenon is called boiling white key and extremely short.
- the third stage is accompanied by intense processes of water seething, the appearance of large bursting bubbles and splashes on the surface. A large amount of splashing means that the water has boiled strongly.
By the way, if you like to drink tea brewed with pure natural water, then you can place an order for this without leaving your home, on the website, for example: http://www.aqualeader.ru/. After that, the water delivery company will bring it to your home.
Simple observers have long paid attention to the fact that all three stages of boiling water are accompanied by different sounds. Water in the first stage makes a subtle subtle sound. In the second stage, the sound turns into noise, reminiscent of the hum of a swarm of bees. In the third stage, the sounds of boiling water lose their uniformity and become sharp and loud, growing chaotically.
All stages of water boiling easily verified by experience. Having started heating water in an open glass container and periodically measuring the temperature, after a short period of time we will begin to observe bubbles covering the bottom and walls of the container.
Let's take a closer look at the bubble that occurs near the bottom. Gradually increasing the volume, the bubble also increases the area of contact with the warming water, which has not yet reached a high temperature. As a result of this, the vapor and air inside the bubble are cooled, as a result of which their pressure decreases, and the gravity of the water bursts the bubble. It is at this moment that the water emits a sound characteristic of boiling, which occurs due to collisions of water with the bottom of the tank in those places where the bubbles burst.
As the temperature in the lower layers of water approaches 100 degrees Celsius, the intrabubble pressure equalizes with the water pressure on them, as a result of which the bubbles gradually expand. An increase in the volume of bubbles also leads to an increase in the action of the buoyancy force on them, under the influence of which the most voluminous bubbles break away from the walls of the container and rapidly rise upwards. In the event that the upper layer of water has not yet reached 100 degrees, then the bubble, falling into colder water, loses part of the water vapor that condenses and goes into the water. In this case, the bubbles again decrease in size and fall down under the influence of gravity. Near the bottom, they again gain volume and rise up, and it is these changes in the size of the bubbles that create the characteristic noise of boiling water.
By the time the entire volume of water reaches 100 degrees, the rising bubbles no longer decrease in size, but burst on the very surface of the water. In this case, steam is released to the outside, accompanied by a characteristic gurgling - this means that water is boiling. The temperature at which a liquid reaches boiling depends on the pressure experienced by its free surface. The higher the pressure, the higher the required temperature, and vice versa.
That water boils at 100 degrees Celsius- a well-known fact. But it is worth considering that such a temperature is valid only under the condition of normal atmospheric pressure (about 101 kilopascals). As pressure increases, the temperature at which a liquid reaches boiling also increases. For example, in pressure cookers, food is cooked under pressure approaching 200 kilopascals, at which the boiling point of water is 120 degrees. In water with this temperature, boiling proceeds much faster than at a normal boiling point - hence the name of the pan.
Accordingly, lowering the pressure lowers the boiling point of water. For example, residents of mountainous regions living at an altitude of 3 kilometers achieve water boiling faster than inhabitants of the plains - all stages of boiling water occur faster, since it requires only 90 degrees at a pressure of 70 kilopascals. But the inhabitants of the mountains cannot boil, for example, a chicken egg, since the minimum temperature at which the protein coagulates is just 100 degrees Celsius.
