Boiling

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Rolling boil of water in an electric kettle Boilingkettle.jpg
Rolling boil of water in an electric kettle

Boiling or ebullition is the rapid phase transition from liquid to gas or vapor; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Boiling and evaporation are the two main forms of liquid vapourization.

Contents

There are two main types of boiling: nucleate boiling where small bubbles of vapour form at discrete points, and critical heat flux boiling where the boiling surface is heated above a certain critical temperature and a film of vapour forms on the surface. Transition boiling is an intermediate, unstable form of boiling with elements of both types. The boiling point of water is 100 °C or 212 °F but is lower with the decreased atmospheric pressure found at higher altitudes.

Boiling water is used as a method of making it potable by killing microbes and viruses that may be present. The sensitivity of different micro-organisms to heat varies, but if water is held at 100 °C (212 °F) for one minute, most micro-organisms and viruses are inactivated. Ten minutes at a temperature of 70 °C (158 °F) is also sufficient to inactivate most bacteria. [1]

Boiling water is also used in several cooking methods including boiling, steaming, and poaching.

Types

Free convection

The lowest heat flux seen in boiling is only sufficient to cause [natural convection], where the warmer fluid rises due to its slightly lower density. This condition occurs only when the superheat is very low, meaning that the hot surface near the fluid is nearly the same temperature as the boiling point.

Nucleate

A video showing water being boiled. As boiling proceeds, more nucleation sites (where bubbles are formed) can be seen.

Nucleate boiling is characterised by the growth of bubbles or pops on a heated surface (heterogeneous nucleation), which rises from discrete points on a surface, whose temperature is only slightly above the temperature of the liquid. In general, the number of nucleation sites is increased by an increasing surface temperature.

An irregular surface of the boiling vessel (i.e., increased surface roughness) or additives to the fluid (i.e., surfactants and/or nanoparticles) facilitate nucleate boiling over a broader temperature range, [2] [3] [4] while an exceptionally smooth surface, such as plastic, lends itself to superheating. Under these conditions, a heated liquid may show boiling delay and the temperature may go somewhat above the boiling point without boiling.

Homogeneous nucleation, where the bubbles form from the surrounding liquid instead of on a surface, can occur if the liquid is warmer in its center, and cooler at the surfaces of the container. This can be done, for instance, in a microwave oven, which heats the water and not the container.

Critical heat flux

Critical heat flux (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing localised overheating of the heating surface. As the boiling surface is heated above a critical temperature, a film of vapour forms on the surface. Since this vapour film is much less capable of carrying heat away from the surface, the temperature rises very rapidly beyond this point into the transition boiling regime. The point at which this occurs is dependent on the characteristics of boiling fluid and the heating surface in question. [3]

Transition

Transition boiling may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.

The formation of bubbles in a heated liquid is a complex physical process which often involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.

Film

If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapour, which has low thermal conductivity, insulates the surface. This condition of a vapour film insulating the surface from the liquid characterises film boiling.

Influence of Geometry

Pool boiling

"Pool boiling" refers to boiling where there is no forced convective flow. Instead, the flow occurs due to density gradients. It can experience any of the regimes mentioned above.

Flow boiling

"Flow boiling" occurs when the boiling fluid circulates, typically through pipes. [5] Its movement can be powered by pumps, such as in power plants, or by density gradients, such as in a Thermosiphon or a heat pipe. Flows in flow boiling are often characterised by a void fraction parameter, which indicates the fraction of the volume in the system that is vapor. One can use this fraction and the densities to calculate the vapor quality, which refers to the mass fraction that is in the gas phase. Flow boiling can be very complex, with heavy influences of density, flow rates, and heat flux, as well as surface tension. The same system may have regions that are liquid, gas, and two-phase flow. Such two phase regimes can lead to some of the best heat transfer coefficients of any system.

Confined boiling

Confined boiling refers to boiling in confined geometries, typically characterized by a Bond number that compares the gap spacing to the capillary length. Confined boiling regimes begin to play a major role when Bo < 0.5. This boiling regime is dominated by "vapour stem bubbles" left behind after vapour departs. [6] These bubbles act as seeds for vapor growth. Confined boiling typically has higher heat transfer coefficient but a lower CHF than pool boiling. CHF occurs when the vapor momentum force at the two-phase interface balances the combined surface tension and hydrostatic forces, leading to irreversible growth of the dry spot. [7] Confined boiling is particularly promising for electronics cooling.

Physics

The boiling point of an element at a given pressure is a characteristic attribute of the element. This is also true for many simple compounds including water and simple alcohols. Once boiling has started and provided that boiling remains stable and the pressure is constant, the temperature of the boiling liquid remains constant. This attribute led to the adoption of boiling points as the definition of 100 °C.

Distillation

Mixtures of volatile liquids have a boiling point specific to that mixture producing vapour with a constant mix of components - the constant boiling mixture. This attribute allows mixtures of liquids to be separated or partly separated by boiling and is best known as a means of separating ethanol from water.

Uses

Refrigeration and air conditioning

Most types of refrigeration and some type of air-conditioning work by compressing a gas so that it becomes liquid and then allowing it to boil. This adsorbs heat from the surroundings cooling the fridge or freezer or cooling the air entering a building. Typical liquids include propane, ammonia, carbon dioxide or nitrogen.

For making water potable

As a method of disinfecting water, bringing it to its boiling point at 100 °C (212 °F), is the oldest and most effective way since it does not affect the taste, it is effective despite contaminants or particles present in it, and is a single step process which eliminates most microbes responsible for causing intestine related diseases. [8] The boiling point of water is 100 °C (212 °F) at sea level and at normal barometric pressure. [9] In places having a proper water purification system, it is recommended only as an emergency treatment method or for obtaining potable water in the wilderness or in rural areas, as it cannot remove chemical toxins or impurities. [10] [11]

The elimination of micro-organisms by boiling follows first-order kinetics—at high temperatures, it is achieved in less time and at lower temperatures, in more time. The heat sensitivity of micro-organisms varies, at 70 °C (158 °F), Giardia species (which cause giardiasis) can take ten minutes for complete inactivation, most intestine affecting microbes and E. coli (gastroenteritis) take less than a minute; at boiling point, Vibrio cholerae (cholera) takes ten seconds and hepatitis A virus (causes the symptom of jaundice), one minute. Boiling does not ensure the elimination of all micro-organisms; the bacterial spores Clostridium can survive at 100 °C (212 °F) but are not water-borne or intestine affecting. Thus for human health, complete sterilization of water is not required. [8]

The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than 60 °C (140 °F) and bringing it to its boiling point is also a useful indication that can be seen without the help of a thermometer, and by this time, the water is disinfected. Though the boiling point decreases with increasing altitude, it is not enough to affect the disinfecting process. [8] [12]

In cooking

Boiling pasta Flickr - cyclonebill - Pasta (1).jpg
Boiling pasta

Boiling is the method of cooking food in boiling water or other water-based liquids such as stock or milk. [13] Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles. [14]

The boiling point of water is typically considered to be 100 °C (212 °F; 373 K), especially at sea level. Pressure and a change in the composition of the liquid may alter the boiling point of the liquid. High elevation cooking generally takes longer since boiling point is a function of atmospheric pressure. At an elevation of about one mile (1,600 m), water boils at approximately 95 °C (203 °F; 368 K). [15] Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly. [16] Similarly, increasing the pressure as in a pressure cooker raises the temperature of the contents above the open air boiling point.[ citation needed ]

Boil-in-the-bag

Also known as "boil-in-bag", this involves heating or cooking ready-made foods sealed in a thick plastic bag. The bag containing the food, often frozen, is submerged in boiling water for a prescribed time. [17] The resulting dishes can be prepared with greater convenience as no pots or pans are dirtied in the process. Such meals are available for camping as well as home dining.

Contrast with evaporation

At any given temperature, the molecules in a liquid have varying kinetic energies. Some high energy particles on the liquid surface may have enough energy to escape the intermolecular forces of attraction of the liquid and become a gas. This is called evaporation.

Evaporation only happens on the surface while boiling happens throughout the liquid. When a liquid reaches its boiling point bubbles of gas form in it which rise into the surface and burst into the air. This process is called boiling. If the boiling liquid is heated more strongly the temperature does not rise but the liquid boils more quickly.

This distinction is exclusive to the liquid-to-gas transition; any transition directly from solid to gas is always referred to as sublimation regardless of whether it is at its boiling point or not.

See also

Related Research Articles

<span class="mw-page-title-main">Boiling point</span> Temperature at which a substance changes from liquid into vapor

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

<span class="mw-page-title-main">Evaporation</span> Type of vaporization of a liquid that occurs from its surface; surface phenomenon

Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas. When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.

<span class="mw-page-title-main">Vapor</span> Substances in the gas phase at a temperature lower than its critical point

In physics, a vapor or vapour is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature of the vapor. A vapor is different from an aerosol. An aerosol is a suspension of tiny particles of liquid, solid, or both within a gas.

<span class="mw-page-title-main">Vapor pressure</span> Pressure exerted by a vapor in thermodynamic equilibrium

Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's thermodynamic tendency to evaporate. It relates to the balance of particles escaping from the liquid in equilibrium with those in a coexisting vapor phase. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the attractive interactions between liquid molecules become less significant in comparison to the entropy of those molecules in the gas phase, increasing the vapor pressure. Thus, liquids with strong intermolecular interactions are likely to have smaller vapor pressures, with the reverse true for weaker interactions.

<span class="mw-page-title-main">Leidenfrost effect</span> Physical phenomenon

The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this repulsive force, a droplet hovers over the surface, rather than making physical contact with it. The effect is named after the German doctor Johann Gottlob Leidenfrost, who described it in A Tract About Some Qualities of Common Water.

<span class="mw-page-title-main">Latent heat</span> Thermodynamic phase transition energy

Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process—usually a first-order phase transition, like melting or condensation.

<span class="mw-page-title-main">Heat transfer</span> Transport of thermal energy in physical systems

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

<span class="mw-page-title-main">Vaporization</span> Transition of a liquid to vapor

Vaporization of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, where as boiling is a bulk phenomenon.

<span class="mw-page-title-main">Sublimation (phase transition)</span> Transition of a substance directly from the solid to the gas state

Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. The verb form of sublimation is sublime, or less preferably, sublimate. Sublimate also refers to the product obtained by sublimation. The point at which sublimation occurs rapidly is called critical sublimation point, or simply sublimation point. Notable examples include sublimation of dry ice at room temperature and atmospheric pressure, and that of solid iodine with heating.

<span class="mw-page-title-main">Heat pipe</span> Heat-transfer device that employs phase transition

A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces.

<span class="mw-page-title-main">Two-phase flow</span> Flow of gas and liquid in the same conduit

In fluid mechanics, two-phase flow is a flow of gas and liquid — a particular example of multiphase flow. Two-phase flow can occur in various forms, such as flows transitioning from pure liquid to vapor as a result of external heating, separated flows, and dispersed two-phase flows where one phase is present in the form of particles, droplets, or bubbles in a continuous carrier phase.

<span class="mw-page-title-main">Superheated steam</span> Steam whose temperature can be decreased without immediately condensing

Superheated steam is steam at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured.

Thermal hydraulics is the study of hydraulic flow in thermal fluids. The area can be mainly divided into three parts: thermodynamics, fluid mechanics, and heat transfer, but they are often closely linked to each other. A common example is steam generation in power plants and the associated energy transfer to mechanical motion and the change of states of the water while undergoing this process. Thermal-hydraulic analysis can determine important parameters for reactor design such as plant efficiency and coolability of the system.

<span class="mw-page-title-main">Absorption refrigerator</span> Refrigerator that uses a heat source

An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of convenient heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. Absorption refrigerators are commonly used in recreational vehicles (RVs), campers, and caravans because the heat required to power them can be provided by a propane fuel burner, by a low-voltage DC electric heater or by a mains-powered electric heater. Absorption refrigerators can also be used to air-condition buildings using the waste heat from a gas turbine or water heater in the building. Using waste heat from a gas turbine makes the turbine very efficient because it first produces electricity, then hot water, and finally, air-conditioning—trigeneration.

A zeotropicmixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points. For example, nitrogen, methane, ethane, propane, and isobutane constitute a zeotropic mixture. Individual substances within the mixture do not evaporate or condense at the same temperature as one substance. In other words, the mixture has a temperature glide, as the phase change occurs in a temperature range of about four to seven degrees Celsius, rather than at a constant temperature. On temperature-composition graphs, this temperature glide can be seen as the temperature difference between the bubble point and dew point. For zeotropic mixtures, the temperatures on the bubble (boiling) curve are between the individual component's boiling temperatures. When a zeotropic mixture is boiled or condensed, the composition of the liquid and the vapor changes according to the mixtures's temperature-composition diagram.

In the study of heat transfer, critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid.

<span class="mw-page-title-main">Evaporator</span> Machine transforming a liquid into a gas

An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer to provide the necessary thermal energy for phase transition from liquid to vapor. Within evaporators, a circulating liquid is exposed to an atmospheric or reduced pressure environment, causing it to boil at a lower temperature compared to normal atmospheric boiling.

In fluid thermodynamics, nucleate boiling is a type of boiling that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux. For water, as shown in the graph below, nucleate boiling occurs when the surface temperature is higher than the saturation temperature by between 10 and 30 °C. The critical heat flux is the peak on the curve between nucleate boiling and transition boiling. The heat transfer from surface to liquid is greater than that in film boiling.

In thermodynamics, explosive boiling or phase explosion is a method whereby a superheated metastable liquid undergoes an explosive liquid-vapor phase transition into a stable two-phase state because of a massive homogeneous nucleation of vapor bubbles. This concept was pioneered by M. M. Martynyuk in 1976 and then later advanced by Fucke and Seydel.

A climbing/falling film plate evaporator is a specialized type of evaporator in which a thin film of liquid is passed over a rising and falling plate to allow the evaporation process to occur. It is an extension of the falling film evaporator, and has application in any field where the liquid to be evaporated cannot withstand extended exposure to high temperatures, such as the concentration of fruit juices.

References

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  2. Doretti, L.; Longo, G. A.; Mancin, S.; Righetti, G.; Weibel, J. A. (2017). "Nanoparticle Deposition During Cu-Water Nanofluid Pool Boiling". Journal of Physics: Conference Series. 923 (1): 012004. Bibcode:2017JPhCS.923a2004D. doi: 10.1088/1742-6596/923/1/012004 . ISSN   1742-6596.
  3. 1 2 Taylor, Robert A.; Phelan, Patrick E. (2009). "Pool boiling of nanofluids: Comprehensive review of existing data and limited new data". International Journal of Heat and Mass Transfer. 52 (23–24): 5339–5347. doi:10.1016/j.ijheatmasstransfer.2009.06.040.
  4. Robert A Taylor, Patrick E Phelan, Todd Otanicar, Ronald J Adrian, Ravi S Prasher, Vapor generation in a nanoparticle liquid suspension using a focused, continuous laser , Applied Physics Letters, Volume 95, Issue 16, 2009
  5. Holden, James B.; Rowzee, E. Ralph (1931). "Adiabatic flow of boiling water through a horizontal pipe". Massachusetts Institute of Technology. Department of Chemical Engineering.
  6. Alsaati, A.A.; Warsinger, D.M.; Weibel, J.A.; Marconnet, A.M. (2021). "Vapor stem bubbles dominate heat transfer enhancement in extremely confined boiling". International Journal of Heat and Mass Transfer. Elsevier BV. 177: 121520. doi:10.1016/j.ijheatmasstransfer.2021.121520. ISSN   0017-9310.
  7. Alsaati, Albraa A.; Warsinger, David M.; Weibel, Justin A.; Marconnet, Amy M. (2023). "A mechanistic model to predict saturated pool boiling Critical Heat Flux (CHF) in a confined gap". International Journal of Multiphase Flow. Elsevier BV. 167: 104542. doi:10.1016/j.ijmultiphaseflow.2023.104542. ISSN   0301-9322.
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  10. US EPA, OW (18 November 2015). "Emergency Disinfection of Drinking Water". US EPA. Retrieved 11 January 2019.
  11. Curtis, Rick (March 1998). "OA Guide to Water Purification, The Backpacker's Field Manual".
  12. CDC (6 September 2019). "Making Water Safe in an Emergency". Centers for Disease Control and Prevention. Retrieved 7 January 2020.
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  15. IAPWS. "What is the effect of pressure on the boiling of water? Why does water boil at a lower temperature at high altitudes?". FAQs About Water and Steam. Archived from the original on 6 August 2009. Retrieved 5 December 2009.
  16. "High Altitude Cooking and Food Safety". United States Department of Agriculture. 15 June 2013. Retrieved 10 February 2020.
  17. "Boil-in-bag - Define Boil-in-bag at Dictionary.com". Dictionary.com.