Burning liquids
The combustion of liquids is characterized by two interrelated phenomena - evaporation and combustion of the steam-air mixture above the surface of the fluid. Consequently, the combustion of liquids is accompanied not only by a chemical reaction (oxidation, turning into fiery burning), but also by physical phenomena (evaporation and formation above the surface of the pair-air mixture), without which the combustion is impossible.
The transition of a substance from a liquid state in vapor-shaped is called vaporization.There are two forms of this process: evaporation and boiling. Evaporation - This is a liquid transition to pairs from a free surface at temperatures below the boiling point of the liquid (see Fig. 4.1). Evaporation occurs as a result of thermal motion of fluid molecules. The speed of movement of molecules varies widely, strongly deviating in both directions from its average value. A part of molecules having a sufficiently large kinetic energy is pulled out of the surface layer of fluid into the gas (air) medium. The excessive energy of the liquid lost molecules is spent on overcoming the interaction forces between molecules and the operation of expansion (increase in volume) during fluid transition to steam. Boiling - This evaporation is not only from the surface, but also from the volume of fluid by forming steam bubbles throughout the volume and the allocation of them. Evaporation is observed at any fluid temperature. Boiling occurs only at a temperature at which the saturated pair pressure will reach the value of an external (atmospheric) pressure.
At the expense of the Brownian movement in the gas zone there is a reverse process - condensation. If the volume above the liquid is closed, then at any temperature of the fluid, a dynamic equilibrium is established between the processes of evaporation and condensation.
Couples, located in equilibrium with liquid, is called a saturated ferry. The equilibrium state corresponds to a pair concentration defined for this temperature. Couple pressure in equilibrium with liquid is called pressure of saturated steam.
Fig. 4.1. Fluid evaporation scheme in: a) open vessel, b) closed vessel
The pressure of a saturated pair (r N.P.) of this fluid at a constant temperature is the magnitude of constant and unchanged for it. The magnitude of the saturated steam pressure is determined by the temperature of the fluid: with increasing temperature, the pressure of the saturated pair increases. This is due to the growth of the kinetic energy of fluid molecules with an increase in temperature. At the same time, an increasing fraction of molecules turns out to have an energy sufficient to go to par.
Thus, over the surface (mirror) of the liquid, there is always a steam-air mixture, which in the equilibrium state is characterized by the pressure of saturated vapor vapor or their concentration. With increasing temperature, the pressure of saturated vapors increases according to the Clayperon-Claziusa equation:
, (4.1)
or in integrated form:
, (4.2)
where r n.p. - the pressure of a saturated pair, Pa;
DN is the heat of evaporation, then the amount of heat that is necessary for transfer to the vapor state of the unit of mass of fluid, KJ / mol;
T - Fluid temperature, K.
The concentration of a saturated pair of the bottom surface of the fluid is associated with its pressure by the ratio:
. (4.3)
From (4.1 and 4.2) it follows that with increasing fluid temperature, the pressure of saturated vapor (or their concentration) increases exponentially. In connection with this, at some temperature above the surface of the liquid, a vapor concentration is created equal to the lower concentration limit of flame propagation. This temperature is called the lower temperature limit of the flame propagation (NTRP).
Therefore, for any liquid, there is always such a temperature range, in which the concentration of saturated vapor over the mirror will be in the field of ignition, that is, HKPrp £ J n £ BCPRP.
Have you ever left a bottle of water for a few hours under the scorching sun and heard the "hissing" sound, opening it? This sound is caused by steam pressure. In chemistry, the pressure of the steam is the pressure rendered by a liquid vapor, which evaporates in hermetically closed vessel. To find the pressure of steam at this temperature, use the Klapairone Clauses equation :.
Record the Klapairone Clausius equation, which is used to calculate the pair pressure when it changes over time. This formula can be used in most physical and chemical problems. The equation is as follows: ln (p1 / p2) \u003d (ΔH VAP / R) ((1 / T2) - (1 / T1))Where:
Submold to the values \u200b\u200bof values \u200b\u200bof values \u200b\u200bin the valuation equation. Most tasks are given two temperature values \u200b\u200band pressure value or two pressure values \u200b\u200band temperature value.
Substitute constants. The Klapairone Clausius equation contains two constants: R and ΔH Vap. R is always equal to 8.314 J / (K × mol). The value of ΔH VAP (evaporation enthalpy) depends on the substance, the pressure of the steam of which you are trying to find; This constant, as a rule, can be found in the table in textbooks in chemistry or on sites (for example,).
Decide the equation with the help of algebraic operations.
Write down the law of Raul. In real life, clean fluids are rarely found; Often we are dealing with solutions. The solution is obtained by adding a small amount of a certain chemical, called "dissolved substance", to a greater number of other chemical substances called "solvent". In cases of solutions, use Raoul's Law:, where:
Determine which substance will be a solvent, and any dissolved substance. Recall that the dissolved substance is a substance dissolved in a solvent, and the solvent is a substance dissolving the dissolved substance.
Find the temperature of the solution, as it will affect the pressure of its pair. The higher the temperature, the higher the pressure of the vapor, since steam formation increases with increasing temperature.
Find the pressure of the solvent vapor. In reference books in chemistry, the pressure of the vapor of many common chemicals is given, but, as a rule, such values \u200b\u200bare given at temperatures of substances at 25 ° C / 298 to or at their boiling temperatures. If you have such temperatures in the task, use values \u200b\u200bfrom reference books; Otherwise, you need to calculate the pressure of the vapors at this temperature of the substance.
Find the molar proportion of the solvent. To do this, find the ratio of the number of substance moles to the total number of moles of all substances available in the solution. In other words, the molar proportion of each substance is equal to (the number of moles of the substance) / (the total number of moles of all substances).
Now substitute the data and the found values \u200b\u200bin the Raoul equation, which is in the beginning of this section ( P Solution \u003d P Solvent X Solvent).
Definition of standard conditions. Often, temperature and pressure values \u200b\u200bare used in chemistry as a kind of default values. Such values \u200b\u200bare called standard temperatures and pressure (or standard conditions). In tasks for the pressure of steam, standard conditions are often mentioned, so it is better to remember the standard values:
Rewrite the Klapairone Clausius equation so as to find other variables. The first section of this article showed how to calculate the pressure of the vapor of clean substances. However, not all tasks are required to find P1 or P2 pressure; Many tasks need to calculate the temperature or value of ΔH Vap. In such cases, rewrite the Klapairone-Clausius equation, which makes the unknown value on one side of the equation.
Taking into account the pressure of the pair of the dissolved substance. In our example, from the second section of this article, a dissolved substance - sugar - does not evaporate, but if the dissolved substance produces pairs (evaporated), the pressure of such a pair should be considered. To do this, use the modified species of the Raoul equation: P solution \u003d σ (p substance X substance), where the symbol σ (sigma) means that it is necessary to add the values \u200b\u200bof the pressures of the vapor of all substances from which the solution consists.
Evaporation
Evaporation over a mug of tea
Evaporation - The process of transition of a substance from a liquid state into a gaseous, occurring on the surface of a substance (pairs). The evaporation process is a condensation inverse process (a transition from a vapor state into liquid). Evaporation (vaporization), transition of a substance from condensed (solid or liquid) phase into gaseous (pairs); Phase transition of the first kind.
There is a more disclosed concept of evaporation in higher physics.
Evaporation - This is a process at which the particles (molecules, atoms) are flying from the surface of the liquid or solid body, and the E K\u003e E p.
The evaporation of the solid is called sublimation (sublimation), and the vaporization in the volume of liquid is boiling. Usually, under evaporation, vaporization on the free surface of the fluid as a result of thermal motion of its molecules at a temperature below the boiling point corresponding to the pressure of the gas medium located above the specified surface. At the same time, molecules with sufficiently large kinetic energy are pulled out of the surface layer of fluid into the gas medium; Some of them are reflected back and captured with liquid, and the rest is irretrievably lost.
Evaporation - an endothermic process, in which the heat of the phase transition is absorbed - the heat of evaporation, spent on overcoming the molecular clutch forces in the liquid phase and to work the expansion when the liquid turns into steam. The specific heat of evaporation refers to 1 praying fluid (molar heat of evaporation, J / mol) or to a unit of its mass (mass heat of evaporation, j / kg). The evaporation rate is determined surface density A pair of jP, penetrating a unit of time into the gas phase from a unit of liquid surface [in mol / (SM 2) or kg / (sm 2)]. The greatest value of the JP is achieved in vacuo. In the presence of a relatively dense gas medium, evaporation slows down due to the fact that the removal rate of steam molecules from the surface of the liquid into the gas medium becomes small compared with the velocity vehicle. At the same time, the surface of the phase section is formed by a layer of a vapor-gas mixture, almost saturated with steam. The partial pressure and the concentration of steam in this layer is higher than in the bulk of the vapor-gas mixture.
The evaporation process depends on the intensity of the heat movement of molecules: the faster the molecule is moving, the faster there is evaporation. In addition, important factors affecting the evaporation process are the rate of outer (with respect to the substance) diffusion, as well as the properties of the substance itself. Simply put, with wind, evaporation is much faster. As for the properties of the substance, then, for example, the alcohol evaporates much faster than water. An important factor is also the surface area of \u200b\u200bthe liquid with which evaporation occurs: from a narrow decline it will occur more slowly than from a wide plate.
Consider this process at the molecular level: molecules with sufficient energy (speed) to overcome the attraction of adjacent molecules, break over the boundaries of the substance (liquid). At the same time, the fluid loses part of its energy (cooled). For example, a very hot liquid: we blow on its surface to cool down, while we accelerate the evaporation process.
The violation of the thermodynamic equilibrium between the liquid and the steam contained in the vapor-gas mixture is explained by the temperature jump on the border of the phase partition. However, usually this jump can be neglected and taking that the partial pressure and the concentration of steam at the phase surface correspond to their values \u200b\u200bfor a saturated pair having a surface temperature of the liquid. If the liquid and the steamed mixture are still and the effect of free convection in them is insignificant, the removal of the vapor formed during the evaporation from the surface of the fluid into the gas medium is mainly due to the molecular diffusion and the appearance of the separated surface of the phase separation (so-called) surface Stephanovsky) The flow of a vapor-gas mixture directed from the surface of the liquid into the gas medium (see diffusion). Distribution of temperatures at various modes of evaporative cooling of the liquid. Heat flows are directed: a - from the liquid phase to the surface of evaporation in the gas phase; b - from the liquid phase only to the surface of evaporation; in - to the surface of evaporation from both phases; G - to the surface of evaporation only by the gas phase.
The effects of baro- and thermal diffusion in engineering calculations are usually not taken into account, but the effect of thermal diffusion can be essential at high heterogeneity of the vapor-gas mixture (with a large difference in the molar masses of its components) and significant temperature gradients. When moving one or both phases relative to the surface of their section, the role of convective transfer of the substance and the energy of the vapor-gas mixture and liquid increases.
In the absence of supply of energy to the liquid-gas system from external. Heat sources Evaporation can be supplied to the surface layer of fluid from one or both phases. In contrast to the resulting flow of a substance, always directed during evaporation from fluid into the gas medium, heat flows may have different areas Depending on the ratio of temperature of the main mass of the liquid TG, the boundaries of the phase section of the TGR and the gas environment Tg. When contacting a certain number of liquids with a semi-infinite volume or wash it with a flow of a gas medium and at a temperature of a liquid, higher than the gas temperature (TG\u003e TGR\u003e TG), the heat of heat from the fluid side to the surface of the phase partition occurs: (qu) Q: Q: Q: Aggregate, quits - the amount of heat transmitted from the liquid of the gas environment. At the same time, the liquid is cooled (the so-called evaporative cooling). If the equality of TGR \u003d TG is achieved, the heat transfer from the liquid to the gas is stopped ( QuI \u003d 0) and all the heat supplied by the liquid to the surface of the section is spent on evaporation (quantity \u003d Q.).
In the case of a gas medium, not saturated with steam, the partial pressure of the latter in the surface of the phase section and when queries \u003d Q and remains higher than in the bulk of the gas, as a result of which the evaporation and evaporative cooling of the liquid are not stopped and the TGR becomes lower than TG and TG. At the same time, the heat is supplied to the surface of the section from both phases until the equality of TGR \u003d TG is achieved and the heat of heat from the fluid side is stopped, and from the gas environment, it becomes equal to Q. Further evaporation of the fluid occurs at a constant temperature TM \u003d TG \u003d TGR, which is called the freight cooling limit during evaporating cooling or temperature of the wet thermometer (since it shows the wet thermometer of the psychoometer). The TM value depends on the parameters of the vapor-gas medium and the conditions of heat and mass transfer between the liquid and gas phases.
If liquid and gas medium having various temperaturesare located in a limited volume that does not receive energy from outside and does not send it outward, evaporation occurs until there is a thermodynamic equilibrium between the two phases, at which the temperatures of both phases are equalized with the system unchanged enthalpy, and the gas phase is saturated with steam at system temperature. Thad. The latter is called the temperature of the adiabatic gas saturation, it is determined only by the initial parameters of both phases and does not depend on the conditions of heat and mass transfer.
The rate of isothermal evaporation [kg / (m 2 (c)] with a unidirectional vapor diffusion into a fixed layer of a binary vapor-gas mixture with a thickness D, [M] can be found according to the Stephen formula:, where D is the mutual diffusion coefficient, [m 2 /from]; - gas constant, [J / (kg to)] or [m 2 / (C 2 K)]; T - the temperature of the mixture, [k]; p - pressure of the vapor-gas mixture, [pa]; - Partial vapor pressure on the surface of the section and on the outer border of the layer of the mixture, [Pa].
In general, (moving fluid and gas, non-erotic conditions) in the phase adjacent to the surface of the phase, the pulse transfer is accompanied by the transfer of heat, and in the border layer of gas (vapor-gas mixture), interconnected heat and mass transfer occur. At the same time, experimental coefficients of heat and mass waste are used to calculate the velocity, and in relatively simpler cases - approximate methods numerical solutions Systems differential equations For conjugate border layers of gas and liquid phases.
The intensity of mass transfer during evaporation depends on the difference in the chemical potentials of steam in the surface of the section and in the bulk of the vapor-gas mixture. However, if the baro- and thermal diffusion can be neglected, the difference in chemical potentials is replaced by the difference in partial pressures or vapor concentrations and accept: JP \u003d BP (RP, GR - RP, OSN) \u003d BPR (UE, GR - UP, OSN) or JP \u003d BC ( SP, GR - SP, OSN), where BP, BC is the mass transfer coefficient, P is the pressure of the mixture, RP is a partial pressure of steam, yp \u003d pp / p - molar concentration of vapor, SP \u003d RP / R is a massive concentration of vapor, RP, R - local vapor densities and mixtures; The indices mean: "GR" - at the border of the phase section, "OSN" - in the OSN. Masse mix. The heat flux density given during the evaporation of the liquid is [in j / (m2 c)]: Q \u003d Age (TG - TGR) \u003d RJP + GG (TGR - TG), where AG, AG - the heat transfer coefficient by fluid and gas , [W / (m 2 k)]; R - Heat evaporation, [J / kg].
With very small radii, the curvature of the evaporation surface (for example, when evaporating small drops of liquid), the effect of the surface tension of the fluid leading to the fact that the equilibrium pressure of steam above the surface of the section above the pressure of saturated vapor of the same fluid above the flat surface is taken into account. If TGR ~ TZ, then only heat and mass exchange in the gas phase may be taken into account when calculating evaporation. With a relatively low intensity of mass transfer approximately, an analogy between the processes of heat and mass transfer, from which it follows: NU / NU0 \u003d SH * / SH0, where Nu \u003d g L / LG is the number of nusselt, L is the characteristic size of the evaporation surface, LG - thermal conductivity coefficient PAROUSE MIXES, SH * \u003d BYYG, GRL / DP \u003d BCCG, GRL / D is the number of sherwood for the diffusion component of the flow of steam, Dp \u003d D / RPT diffusion -efficient, referred to the gradient of the partial pressure of steam. The BP and BC values \u200b\u200bare calculated according to the above relations, the NU0 and SH0 numbers correspond to JP: 0 and can be determined according to data for separately occurring heat and mass transfer processes. The number of SH0 for the total (diffusion and convective) pair flow is found by the division sh * on the molar (yg, gr) or mass (SG, GR) the gas concentration at the surface of the section depending on which driving force Mass transferred coefficient b.
Equations of the similarity for NU and SH * in evaporation include besides conventional criteria (Reynolds Reynolds numbers, Archimede Ar, Prandtl PR or SC SC and GEOM. Parameters) Parameters that take into account the effect of the transverse flux of steam and the degree of heterogeneity of the vapor-gas mixture (the ratio of molar mass or gas constant its components) on profiles, speed, temperature or concentrations in the section of the border layer.
At small JP, which do not disturb the substantially hydrodynamic mode of movement of the vapor-gas mixture (for example, during evaporation of water into atmospheric air) and the similarity of the boundary conditions of temperature fields and concentrations, the influence of additional arguments in the similarity equations is insignificant and they can be neglected, taking that Nu \u003d SH. When evaporation of multicomponent mixtures, these regularities are greatly complicated. In this case, the heat of evaporation of the components of the mixture and the compositions of the liquid and vapor-gas phases, which are among themselves in equilibrium, are different and depend on temperature. When evaporated with a binary liquid mixture, the resulting mixture of vapors in a relatively richer more volatile component, excluding only azeotropic mixtures, evaporate at extremum points (maximum or minimum) of the state curves as clean liquid.
The total amount of evaporating fluid increases with an increase in the surface of the contact of the liquid and gas phases, so the design of the devices in which evaporation occurs, an increase in the surface of the evaporation by creating a large mirror of the liquid, crushing it on the jet and droplets or the formation of thin films flowing along the surface of the nozzles. The increase in the intensity of heat and mass transfer during evaporation is also achieved by an increase in the rate of the gas medium relative to the surface of the fluid. However, the increase in this velocity should not lead to excessive liquid under the gas medium and a significant increase in the hydraulic resistance of the device.
Evaporation is widely used in industrial practices for cleaning substances, drying materials, separation of liquid mixtures, air conditioning. Evaporative cooling Water is used in current water supply systems of enterprises.
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Synonyms:Transition to va from a liquid or solid aggregate state in gaseous (pairs). Usually under I. understand the transition of fluid into steam, occurring on the free surface of the fluid. I. solid bodies called. sublimation or sublimation. Pressure dependence ... ... Physical encyclopedia
Variousness occurring on a free liquid surface. Evaporation from the surface of the solid is called sublimation ... Large encyclopedic Dictionary
There is direct relationship between temperature fluid saturation And others pressure. As noted earlier, the increase in the pressure of the fluid raises the saturation temperature. Conversely, the reduction in the pressure of the fluid lowers the saturation temperature.
Consider a closed vessel with water at a temperature of 22.2 ° C. An throttle valve, pressure gauge and two thermometers for process control are installed on the vessel. The valve regulates the pressure in the vessel. The pressure gauge shows the pressure in the vessel, and the thermometers measure the temperature of steam and liquid water. The atmospheric pressure around the vessel is 101.3 kPa.
In a vessel is formed vacuumand closed valve. With internal pressure in 68.9 kPa water saturation temperature 89.6 ° C. This means that boiling will not occur until the pair pressure reaches 68.9 kPa. As maximum Pressure Para At a fluid temperature of 22.2 ° C 2.7 kPa, boiling will not be if the liquid does not inform a large amount of energy.
Instead of boiling under these conditions, evaporation will begin, since the pressure of the liquid pair is lower than the pressure saturated parawhich depends on the water temperature. So it will continue until the volume above the liquid is saturated aqueous ferry. Upon reaching the state of equilibrium, the temperature of the fluid and the environment will be the same, the heat transfer will stop, the number of steam molecules separated from the water and returning to it will be the same, and the pair pressure will be equal to the pressure of the fluid saturation, which depends on its temperature. Upon reaching the status of equilibrium, the pressure of the steam will reach the maximum value of 2.7 kPa, and the volume of the liquid will remain constant.
If the initial balance of equilibrium is reached, open the valve, the pressure in the vessel will quickly increase to 101.3 kPa. Consequently, the boiling point of water will grow to 100 ° C.. Since the temperature of the water remains 22.2 ° C, the pressure of the water remains 2.7 kPa. The water pair pressure will decrease, since the pair comes out of the vessel through the valve, and the evaporation process begins again.
With an increase in the heat transfer of the vessel as a result of fuel combustion, the water temperature begins to rise to 100 ° C. Increasing the water temperature causes a larger amount of steam molecules as a result of an increase in kinetic energy, which increases the pressure of steam to 101.3 kPa. Increase paragraph pressure - This is a consequence of the temperature of liquid water. With increasing fluid temperature, the pressure of the saturated pair also rises. As soon as the pair pressure reaches atmospheric pressure, begins boiling. Based on potential energy The process of changing the state as a result of boiling occurs at a constant temperature. Water will forcefully change the state to gaseous until the vessel receives enough heat.
When separating the steam molecules from the surface of the liquid and the movement in the vessel, some molecules are losing kinetic energy As a result of collisions and fall into the liquid. Some molecules come out of the vessel through open valve And dissipate in the atmosphere. While the valve releases steam, the pair pressure and the pressure in the vessel will remain 101.3 kPa. At the same time, steam will remain saturated, and its temperature and pressure will be the same as in the liquid: 100 ° С at 101.3 kPa. Steam density at such a temperature and pressure of 0.596 kg / m3, and its specific volume of return density is 1.669 mg / kg.
Evaporation
Evaporation - This is a subtle thermodynamic process caused by slow transmission of heat of fluid from the environment. Process evaporation Produces rapid changes in the volume or mass of the liquid. Evaporation Comes as a result of absorption of liquid molecules thermal energy Out of the environment due to a small temperature difference. This increase in energy accordingly increases the kinetic energy of the liquid. When the kinetic energy is transmitted as a result of collisions, some molecules near the surface reaches speeds that are much higher than the average speed of neighboring molecules. Under the approximation of some molecules with high energy, they violate communication to the surface of the fluid, overcome the strength of attraction and go to the atmosphere as steam molecules.
Vaporization Evaporation occurs if the pressure of steam above the liquid is lower than the saturation pressure, which corresponds to the fluid temperature. In other words, evaporation occurs when the lines of pressure and temperature of the pair of liquid intersect on the saturation temperature line at the point below atmospheric pressure. These conditions are on saturation temperature lines Below the horizontal pair pressure line, which corresponds to fluid temperature.
Volume of evaporating fluid Continuously decreases when separating molecules from the surface and introducing into the surrounding atmosphere. After separation, some pair molecules face others in the atmosphere, transmitting part of the kinetic energy. When the energy reduction reduces the speed of steam molecules below the level of separation from the liquid, they fall back and thus restore part of the lost volume. When the number of molecules separated from the liquid is equal to the number of falling back, arises equilibrium condition. As soon as this state occurs, the volume of the fluid remains unchanged until changes in the pressure of the steam or temperature will not produce corresponding changes in the intensity of evaporation.
Pressure para
The pressure of steam in atmospheric air can be clearly illustrated by the following experience. If a multiple droplets of water that pops up with a pipette of a mercury barometer in the bottom, after a while, the level of mercury in the barometer will decrease due to education in torrycelli void Water vapor. The latter creates its own partial pressure PH, acting evenly in all directions, including on a decrease in the surface of mercury.
When carrying out similar experience under conditions with a larger pa vapor temperature in the barometer tube, the value of P increases (there should be some water on the surface of mercury). Such experiments are shown to increase the pressure of a saturated pair with an increase in its temperature. At a pair temperature in a tube 100 ° C, the level of mercury in it drops to its level in a barometer cup, since pressure para will be equal atmospheric pressure. This method studies the functional dependence between the specified steam parameters.
Couple pressure, like all gas, can be expressed in Pascal. When carrying out measurements and calculations in forest Sewing Technology Schedule the pressure of steam from the zero pressure value. Sometimes it takes excessive against the barometric pressure to begin reference. The first is greater than 0.1 MPa. For example, 0.6 MPa will correspond to 0.5 MPa, counted on the pressure gauge on the steam boiler or steam lines.
Saturation temperature
The temperature in which the liquid moves from the liquid phase into a gaseous or vice versa, is called paste temperature. Liquid for saturation temperature Call saturated liquid, and steam at saturation temperature is called saturated ferry. For any environmental conditions or pressure saturation temperature - This is the maximum temperature at which the substance remains in the liquid phase. It is also the minimum temperature at which the substance exists like steam. The saturation temperature of various liquids is different and depends on fluid pressure. With standard atmospheric pressure, the iron evaporates at approximately at 2454 ° C, copper at 2343 ° C, lead - at 1649 ° C, water - at 100 ° C, and alcohol - at 76.7 ° C. Other liquids evaporate with exclusively low temperatures. Ammonia evaporates at -33 ° C, oxygen - at -182 ° C, and helium at -269 ° C with standard atmospheric pressure.
Intensity evaporation
Atmospheric movement over evaporating fluid directly related to intensity of evaporation. If the speed of the atmosphere above the surface of the fluid increases, intensity evaporation Also grow, since the pair molecules do not accumulate over the surface of the liquid. Consequently, the pressure of steam above the liquid remains lower, which reduces the amount of kinetic energy that requires a molecule for separating from the surface and thus increases intensity evaporation. If you place a fan with water vessel, the evaporation intensity will increase, and the fluid will evaporate in a shorter time.
Another factor that affects the intensity of evaporation is surface surface liquidwhich is open to the atmosphere. With an increase in the surface area, the intensity of evaporation increases, since the mass of the steam molecules is distributed by large Squarethat reduces pressure on liquid. Reducing the pair pressure reduces the number kinetic energynecessary molecules for separating from the surface of the fluid, which increases the intensity of evaporation. Therefore, if the volume of water from the vessel, move to the bottle, the surface area of \u200b\u200bthe liquid will be significantly reduced, and will need more time for evaporation of water.
Use of the fluid cooling phenomenon during evaporation; The dependence of the boiling point of water from pressure.
When steaming, the substance moves from a liquid state into gaseous (pairs). There are two types of vaporization: evaporation and boiling.
Evaporation - This is a vaporization that occurs from the free surface of the fluid.
How is evaporation? We know that the molecules of any liquid are in continuous and disorderly movement, and one of them move faster, others are slower. To fly out to them interfere with the strength of attraction to each other. If, however, the liquid surface will be a molecule with sufficiently large kinetic energy, it will be able to overcome the forces of intermolecular attraction and depart from the liquid. The same thing happens with another fast molecule, with the second, third, etc., the outdoor outward, these molecules form a pair liquid. The formation of this couple is evaporation.
Since the fastest molecules, the average kinetic energy of the molecules remaining in the fluid in the fluid becomes becoming less and less in evaporation of the liquid. As a result the temperature of the evaporating fluid decreases: The liquid is cooled. That is why, in particular, a man in wet clothes feels colder than in dry (especially in the wind).
At the same time, everyone knows that if you pour water into a glass and leave on the table, then, despite evaporation, it will not be continuously cooling, becoming more and colder until it freezes. What prevents this? The answer is very simple: water heat exchange with a glass surrounding with warm air.
Cooling fluid during evaporation is more noticeable in the case when evaporation occurs quite quickly (so the fluid does not have time to restore its temperature due to the heat transfer with the environment). Flushing fluids are evaporated, in which the forces of intermolecular attraction are small, such as ether, alcohol, gasoline. If you drop such a liquid on your hand, we will feel cold. Steaming from the surface of the hand, such a liquid will be cooled and take some heat from it.
Fast applications are widely used in technology. For example, in space techniques with such substances cover the descent devices. When passing through the atmosphere of the planet of the body-apparatus, as a result of friction heats up, and its substance begins to evaporate. After evaporated, it cools the spacecraft, saving it to the overheating.
Cooling water during its evaporation is also used in devices that serve to measure air humidity - psychrometers (from the Greek "Psychos" - cold). The psychrometer consists of two thermometers. One of them (dry) shows the air temperature, and the other (the tank of which is bound by a batter, lowered into water) - more low temperaturecaused by the intensity of evaporation from a wet batista. The land of the air, the humidity of which is measured, the stronger the evaporation and therefore, the lower the indications of the moistened thermometer. And vice versa, the more air humidity, the less intensively evaporating and therefore, especially high temperature Shows this thermometer. Based on the testimony of dry and moisturized thermometers using a special (psychoometric) table, the humidity is determined, expressed as a percentage. The greatest humidity is 100% (with such humidity of air ones, dew appears). For a person, the most favorable is considered humidity ranging from 40 to 60%.
With the help of simple experiments it is easy to establish that the evaporation rate increases with increasing fluid temperature, as well as with increasing area of \u200b\u200bits free surface and in the presence of wind.
Why in the presence of wind the liquid evaporates faster? The fact is that simultaneously with evaporation on the surface of the liquid there is a reverse process - condensation. Condensation occurs due to the fact that part of the steam molecules, randomly moving over the liquid, returns to it again. The wind takes the molecule flying out of the liquid and does not allow them to come back.
Condensation can occur when steam does not come into contact with the liquid. It is condensation that explains the formation of clouds: water vapor molecules, rising above the ground, in the colder layers of the atmosphere are grouped into the smallest droplets of water whose clusters are clouds. The consequence of the condensation of water vapor in the atmosphere is rain and dew.
Dependence of boiling temperature from pressure
Water boiling point is 100 ° C; It might think that this is an integral property of water that water, wherever it is in what conditions it is will always boil at 100 ° C.
But this is not the case, and residents of alpine villages are well aware of this.
Near the top of Elbrus there is a house for tourists and a scientific station. Newbies are sometimes surprised, "how difficult to cook egg in boiling water" or "why boiling water does not burn." Under these conditions, they indicate that water boils on the top of Elbrus already at 82 ° C.
What is the case? What physical factor interferes in the phenomenon of boiling? What value is height above sea level?
This physical factor is the pressure acting on the surface of the fluid. No need to climb the top of the mountain to check the justice of what has been said.
Plying the heated water under the bell and pumped up or pumping out air from there, you can make sure that the boiling point grows with an increase in pressure and falls when it decreases.
Water boils at 100 ° C only at a certain pressure - 760 mm Hg. Art. (or 1 atm).
The boiling point curve is shown in Fig. 4.2. At the top of Elbrus, the pressure is 0.5 atm, this pressure and corresponds to the boiling point of 82 ° C.
Fig. 4.2.
But water boiling at 10-15 mm Hg. Art., It is possible to refresh in hot weather. In this pressure, the boiling point will fall to 10-15 ° C.
You can even get "boiling water" having a freezing water temperature. For this, it will have to reduce the pressure of up to 4.6 mm Hg. Art.
An interesting picture can be observed if you place an open vessel with water under the bell and pump air. Pumping makes the water boil, but boiling requires heat. To take it is nowhere, and the water will have to give your energy. The temperature of the boiling water will begin to fall, but since pumping continues, the pressure drops. Therefore, boiling will not stop, the water will continue to cool and will eventually freeze.
Such boil cold water It happens not only when air pumping. For example, when rotating the propeller ships, the pressure in a fast moving around metal surface The water layer drops greatly and the water in this layer boils, that is, numerous bubbles filled with ferry appear in it. This phenomenon is called cavitation (from the Latin word Cavitas - cavity).
Removing pressure, we lower the boiling point. And increasing it? A schedule similar to ours is responsible for this question. The pressure of 15 atm can delay the boiling of water, it will begin only at 200 ° C, and the pressure of 80 atm will force the water only at 300 ° C.
So, a certain boiling point corresponds to certain external pressure. But this statement can and "turn over", saying this: each water boiling point corresponds to its definite pressure. This pressure is called steam elasticity.
The curve depicting the boiling point depending on the pressure, is simultaneously the steam elastic curve depending on the temperature.
The numbers applied to the chart of the boiling point (or on the vapor elastic schedule) show that the elasticity of the steam changes very sharply with a temperature change. At 0 ° C (i.e. 273 K), the elasticity of the steam is equal to 4.6 mm Hg. Art., at 100 ° C (373 K) it is 760 mm Hg. Art., i.e. increases 165 times. With increasing temperature in half (from 0 ° C, i.e. 273 K, up to 273 ° C, i.e. 546 K) the elasticity of the steam increases with 4.6 mm Hg. Art. almost up to 60 atm, i.e., about 10,000 times.
Therefore, on the contrary, the boiling point changes with pressure rather slowly. When the pressure is changed twice from 0.5 atm to 1 atm, the boiling point increases from 82 ° C (355 K) to 100 ° C (373 K) and with a change in 1 to 2 atm - from 100 ° C (373 K) to 120 ° C (393 K).
The same curve, which we now consider, manages and condensation (thickening) steam into water.
You can turn steam into water or compression or cooling.
Both during boiling and in the process of condensation, the point will not move with the curve until the transformation of steam into water or water in steam will not end completely. This can also be formulated as follows: in the conditions of our curve, and only under these conditions, there may be coexistence of liquid and steam. If at the same time it does not fail and not take heat, the amount of steam and fluid in the closed vessel will remain unchanged. They say about such pairs and liquid that they are in equilibrium, and pairs, in equilibrium with its liquid, are called saturated.
The boiling and condensation curve has, as we see, another meaning: this is a fluid equilibrium curve and steam. The equilibrium curve divides the field of chart, into two parts. Left and up (to high temperatures and less pressures) is the area of \u200b\u200bthe steady state of steam. Right and down - the area of \u200b\u200bthe stable state of the liquid.
The equilibrium curve of steam is liquid, i.e., the curve of the dependence of the boiling point from pressure or, which is the same, steam elasticity on temperature is about the same for all fluids. In some cases, the change may be somewhat sharp, in others - somewhat slower, but always the elasticity of the steam grow rapidly with increasing temperature.
Already many times we used the words "gas" and "couples". These two words are quite equal. You can say: water gas There are steam of water, gas oxygen is a pair of oxygen fluid. Nevertheless, when using these two words, some habit has developed. Since we are accustomed to a specific relatively small temperature range, the word "gas" we usually apply to those substances, the elasticity of the steam of which at normal temperatures above atmospheric pressure. On the contrary, we are talking about a pair when the substance is more stable at room temperature and the atmospheric pressure in the form of fluid.
