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'''Steam''' is the [[vapor]] (gaseous) phase of [[water]] (H<sub>2</sub>O). When the steam does not contain any [[liquid]] water, it is known as ''dry steam'' and it is completely colorless. However, when the steam contains tiny droplets of [[Condensation|condensed]] liquid water, it appears to the eye as a white cloud (see the steam being vented from a [[geothermal power plant]] in the adjacent photograph).
What is very often referred to as ''smoke'' from [[cooling tower]]s and other vents in industrial facilities is water vapor which has partially condensed and is mistaken to be white ''smoke''.
Steam is manufactured in industrial processes by the [[Boiling point|boiling]] and [[vaporization]] of liquid water. It also occurs naturally by being vented from [[volcano]]es, [[fumarole]]s, [[geyser]]s and other [[geothermal]] sources.
Steam has a great many industrial and domestic uses. Probably the most important and by far the largest use of steam is in [[Power plant|nuclear, fossil fuel and geothermal power plants]].
==Types of steam==
{{Image|Steam Temperature-Enthalpy Diagram.png|right|310px}}
As shown in the adjacent diagram, there are three types of steam:
* '''Wet steam''': A mixture of water plus steam (liquid plus vapor) at the [[boiling point|boiling point temperature]] of water at a given [[pressure]].
* '''Dry steam''': Steam, at the given pressure, that contains no water (also referred to as '''saturated steam''').
* '''Superheated steam''': Dry steam, at the given pressure, that has been heated to a [[temperature]] higher than the [[boiling point]] of water at that pressure.
Referring to the adjacent drawing again, water is converted into '''wet''', '''dry saturated''' or '''superheated steam''' in three steps:
* Water at point 1 is heated to its boiling point at the given pressure of point 2 (the dark blue line). At that point the water is then referred to as '''saturated water'''. The amount of heat added between points 1 and 2 is called '''sensible heat'''.
* The water is further heated at constant pressure (the red isobar from point 2 to point 3) to form '''wet steam'''. When it is completely vaporized (at point 3), it is then '''dry saturated steam'''. The amount of [[heat]] required to completely vaporize the water is called the [[heat of vaporization]] and denoted as '''''H'''''<sub>'''''v'''''</sub> or '''''H'''''<sub>'''''vap'''''</sub>.
* The dry saturated steam is yet further heated at constant pressure (the red isobar from point 3 to point 4). The steam is then referred to as '''superheated steam'''. The amount of heat added to superheat the dry saturated steam is also called '''sensible heat'''.
* It should be noted that the points 2 and 3 are at the same boiling point temperature and pressure and also that, at those conditions, the liquid and the steam (whether wet or dry) are in equilibrium with each other.
==Uses==
===Electricity generation===
{{main|Conventional coal-fired power plant|Nuclear power plant}}
The worldwide capacity of electrical power generation by [[conventional coal-fired power plant]]s currently amounts to about 800,000 M[[Watt (unit)|W]]<ref>A megawatt (MW) of electrical power is often denoted as MW<sub>e</sub> to differentiate it from other forms of power.</ref><ref name=IEA>[http://www.iea.org/textbase/nppdf/free/2006/key2006.pdf International Energy Agency, 2006, Key Energy Statistics] ([[International Energy Agency]])</ref><ref name=EIAHighlights>[http://www.eia.doe.gov/oiaf/ieo/highlights.html International Energy Outlook 2008; Highlights] ([[Energy Information Administration]], [[U.S. DOE]])</ref><ref name=EIACh.5>[http://www.eia.doe.gov/oiaf/ieo/electricity.html International Energy Outlook 2008: Chapter 5] (Energy Information Administration, U.S. DOE)</ref> and the worldwide capacity of [[Nuclear power plant|nuclear power generation]] amounts to about 370,000 MW.<ref>[http://www-pub.iaea.org/MTCD/publications/PDF/RDS1-29_web.pdf Energy, Electricity and Nuclear Power Estimates for the Period up to 2030] 2009 Edition, [[International Atomic Energy Agency]]</ref><ref>[http://www-pub.iaea.org/MTCD/publications/PDF/RDS1-29_web.pdf Nuclear Power Plants, Worldwide] [[European Nuclear Society]]</ref> That amounts to a total of 1,170,000 MW of worldwide generation, most of which involves the use of superheated steam to drive the [[Steam turbine|turbines]] that spin the [[electrical generator]]s (see the adjacent schematic diagram).<ref>The amount would be even larger if power plants using other fuels  were included (i.e., fuel oil, natural gas and biomass, wood, etc).</ref>
Assuming an overall thermal efficiency of 34%, a [[steam generator]] (boiler) efficiency of 75 to 85% and an [[electrical generator]] efficiency of 98.5%, a conventional coal-fired power plant would use superheated steam at a rate of 3.47 to 3.93 ([[tonne|t]]/h) per MW of power output. Thus, a 1000 MW power plant would use 3,470 to 3,930 metric tons (tonnes) of steam per hour and the steam used by the 1,170,000 MW of worldwide power generation by coal-fired and nuclear power plants might be as much as 4,000,000 to 4,600,000 metric tons of steam per hour.
The diagrams below schematically depict the equipment used in a conventional fuel-fired steam to electric power plant as well as the temperature-entropy (T-S) diagram of the corresponding [[Rankine cycle]]. A nuclear power plant differs only to the extent that the heat required by the boiler is provided by heat derived from a [[nuclear reactor]].
{|border="0" align="center" cellpadding="0" cellspacing="0"
|-
|{{Image|Rankine Cycle.png|left|300px}}
|{{Image|Rankine Cycle T-S Diagram.png|right|300px}} 
|}
===Cogeneration of heat and power]]===
[[Cogeneration]] is also referred to as ''combined heat and power'' or CHP. In electrical power generation, the turbine exhaust steam is typically condensed and returned to the boiler for re-use.  However, in one form of [[cogeneration]], all or part of the turbine exhaust steam is distributed through a [[district heating]] system to heat buildings rather than being condensed.  The world's biggest steam cogeneration system is the district heating system which distributes steam from seven cogeneration plants to provide heating for 100,000 buildings in the city of [[New York]].<ref> [http://www.gothamgazette.com/article/issueoftheweek/20031110/200/674 Steam] Carl Bevelhymer, ''[[Gotham Gazette]]'', November 10, 2003</ref><ref>[http://www.coned.com/newsroom/energysystems_steam.asp Newsroom: Steam] From the website of the [[Con Edison]] company in New York.</ref>
===Steam engines===
As an overall definition, a [[steam engine]] is an engine that uses steam to perform mechanical [[work]]. By that definition, steam turbines are steam engines. However, this section describes steam engines that use the expansion of steam to move a piston that performs work. Such steam engines were the driving force behind the [[Industrial Revolution]] of the 18th century and gained widespread commercial use for driving machinery in factories, powering water pumping stations and transport application such as [[railway locomotive]]s, [[steamship]]s and road vehicles. The use of [[tractor]]s driven by steam engines led to an increase in the land available for agricultural cultivation.
Although largely replaced by steam turbines, stationary [[reciprocating steam engine]]s are still used in industry and elsewhere for driving [[pump]]s, [[gas compressor]]s and other types of machinery.
===Uses in industrial process facilities===
A great many processes in [[Petroleum refining processes|petroleum refineries]], [[Natural gas processing|natural gas processing plants]] and [[Petrochemicals|petrochemical plants]] use steam as:
* A heat source in [[heat exchangers]] designed to increase the temperature of [[Continuous distillation|distillation column]] feedstocks.
* A heat source for distillation column [[reboiler]]s.
* A heat source injected directly into various distillation columns known as ''steam strippers'' and ''side-cut strippers'' to provide the required distillation heat.
* The motive force for [[Injector|injectors and ejectors]].
* A heat source for stripping the spent [[catalyst]] in [[fluid catalytic cracking]] processes of volatile [[hydrocarbon]]s.
* A medium for purging of process vessels such as [[petroleum coke]] drums in [[delayed coking]] units.
* A reactant in catalytic [[steam-methane reformer]]s for the industrial production of [[hydrogen]].
* A diluent in [[steam cracking]] units which produce [[Petrochemicals|petrochemical]] feedstocks such as [[ethylene]] and other [[hydrocarbons|olefins]] from [[natural gas]] and [[Petroleum crude oil|petroleum]] hydrocarbons such as [[methane]], [[ethane]], [[propane]], [[butane]]s, [[petroleum naphtha]] and [[petroleum gas oil]].
* The heat source for [[evaporation]] processes in [[refineries|sugar refining]] and in [[water desalination]] processes.
* A heat source flowing through tubular coils installed inside storage tanks to heat certain stored liquids.
===Other uses===
:;Sterilization:An [[autoclave]], which uses steam under pressure, is used in microbiology laboratories and similar environments for [[Sterilization (microbiology)|sterilization]].
:;Agricultural:In [[agriculture]], steam is used for [[soil steam sterilization|soil sterilization]] to avoid the use of harmful chemical agents and increase soil health.
:;Commercial and domestic uses:Steam's capability to transfer heat is also used in the home as well as commercial facilities for [[pressure cooker]]s, [[steam cleaning]] of fabrics and carpets, [[steam iron]]ing and for heating of buildings.
==Steam tables and Mollier diagrams==
'''Steam tables''' provide tabulated thermodynamic data for the liquid and vapor phases of water. The table below is only an example of the type of data provided. In a complete set of steam tables, the table would provide data in temperature increments of 1 °C. The complete steam tables would also provide a similar "Saturated Steam:Pressure Table" in which the first column would have pressure values and the second column would be temperature values. Most steam tables include tabulated thermodynamic data for superheated steam as well.
{|class="wikitable" align="center"
|+ Saturated Steam:Temperature Table<ref>[http://www.nist.gov/srd/WebGuide/nist10v2.2/NISTIR5078.htm NISTIR 5078, Thermodynamic Properties of Water] Tabulation from the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use (1998), Allan H. Harvey, National Institute of Standards and Technology</ref>
! T<br/>°C!!P<br/>bar!!P<br/>kPa!!ρ<sub>L</sub><br/>kg/m<sup>3</sup>!!ρ<sub>V</sub><br/>kg/m<sup>3</sup>
!H<sub>L</sub><br/>J/g!!H<sub>Vap</sub><br/>J/g!!H<sub>V</sub><br/>J/g!!S<sub>L</sub><br/>J/(g·K)
!S<sub>Vap</sub><br/>J/(g·K)!!S<sub>V</sub><br/>J/(g·K)
|-
|10||0.012282||1.2282||999.65||0.00941||42.02||2477.2||2519.2||0.15109||8.7487||8.8998
|-
|20||0.023393||2.3393||998.19||0.01731||83.91||2453.5||2537.4||0.29648||8.3695||8.6660
|-
|30||0.042470||4.2470||995.61||0.03042||125.73||2429.8||2555.5||0.43675||8.0152||8.4520
|-
|40||0.073849||7.3849||992.18||0.05124||167.53||2406.0||2573.5||0.57240||7.6831||8.2555
|-
|50||0.12352||12.352||988.00||0.08315||209.34||2381.9||2591.3||0.70381||7.3710||8.0748
|-
|100||1.0142||101.42||958.35||0.5982||419.17||2256.4||2675.6||1.3072||6.0469||7.3541
|-
|150||4.7616||476.16||917.01||2.5481||632.18||2113.7||2745.9||1.8418||4.9953||6.8371
|-
|200||15.549||1554.9||864.66||7.8610||852.27||1939.7||2792.0||2.3305||4.0996||6.4302
|-
|250||39.762||3976.2||798.89||19.967||1085.8||1715.2||2800.9||2.7935||3.2785||6.0721
|-
|300||85.879||8587.9||712.14||46.168||1345.0||1404.6||2749.6||3.2552||2.4507||5.7059
|-
|colspan=11|Symbols:<br/>
'''P''' = absolute pressure &nbsp; '''ρ<sub>L</sub>''' = liquid [[Density (chemistry)|density]]  &nbsp; '''ρ<sub>V</sub>''' = vapor density<br/>
'''H<sub>L</sub>''' = liquid specific [[enthalpy]] &nbsp; '''H'''<sub>Vap</sub> = [[Heat of vaporization|enthalpy of vaporization]] &nbsp; '''H<sub>V</sub>''' = vapor specific enthalpy<br/>
'''S<sub>L</sub>''' = liquid specific [[Entropy (thermodynamics)|entropy]] &nbsp; '''S'''<sub>Vap</sub> = [[entropy of vaporization]] &nbsp; '''S<sub>V</sub>''' = vapor specific entropy
|}
'''Mollier diagrams''' are graphical representations of the thermodynamic properties of materials involving "Enthalpy" as one of the coordinates. Mollier diagrams are named after [[Richard Mollier]] (1863 - 1935), a professor at [[Dresden University]] in [[Germany]], who pioneered the graphical display of the relationship of temperature, pressure, enthalpy, entropy and volume of steam (as well as for moist air) that has aided in the teaching of thermodynamics to many generations of engineers. His enthalpy-entropy (H-S) diagram for steam was first published in 1904.<ref>[http://www.chemicalogic.com/mollier/default.htm Mollier Charts] From the website of the ChemicaLogic Corporation</ref><ref>{{cite book|author= R. K. Rajput|title=Engineering Thermodynamics|edition=3rd Edition|publisher=Jones & Bartlett|year=2009|id=ISBN 1-934015-14-8}} [http://books.google.com/books?id=YnXSHFmPdzMC&pg=PA77&dq=%22Mollier+diagram%22&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=1990&as_maxm_is=0&as_maxy_is=2010&num=30&as_brr=3#v=onepage&q=%22Mollier%20diagram%22&f=false Google Books] Use the search function for "Mollier diagram" and select page 77.</ref>
Mollier diagrams are routinely used to visualize the working cycles of thermodynamic systems involved with the [[chemical engineering]] process design of power plants (fossil or nuclear), gas compressors, steam turbines, refrigeration systems and air conditioning.
A sample Mollier diagram for steam is presented below and others are available online:<ref>[http://www.engineeringtoolbox.com/docs/documents/308/mollier-diagram-water_2.png Mollier diagram of water-steam] From the website of Engineering ToolBox</ref><ref>[http://www.ent.ohiou.edu/~thermo/property_tables/H2O/hs_water.gif H-S Diagram for Water] Associate Professor Israel Urieli, Department of Mechanical Engineering, [[Ohio University]].</ref><ref>[http://www.ecourses.ou.edu/cgi-bin/eBook.cgi?doc=&topic=th&chap_sec=mollier&page=SI&appendix=thermotables Mollier Diagram for Water] and the excellent animation video at [http://www.ecourses.ou.edu/cgi-bin/view_anime.cgi?file=th060202f.swf&course=th&chap_sec=06.2 Mollier Diagram for Water]</ref>
{{Image|Mollier Diagram.png|center|925px|Mollier H-S Diagram for Water-Steam}}
==References==
{{reflist}}

Revision as of 23:00, 13 November 2009

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