View source for Physics:Vapor quality

{{short description|Mass fraction of a saturated mixture which is vapor}}

{{Thermodynamics|cTopic=[[Physics:List of thermodynamic properties|System properties]]}}

In [[Physics:Thermodynamics|thermodynamics]], '''vapor quality''' is the [[Chemistry:Mass fraction|mass fraction]] in a [[Physics:Vapor–liquid equilibrium|saturated mixture]] that is [[Chemistry:Vapor|vapor]];<ref name=boles>{{Cite book| last1 = Cengel | first1 = Yunus A. | last2 = Boles | first2 = Michael A. | title = Thermodynamics: an engineering approach | year = 2002 | publisher = McGraw-Hill | location = Boston, Massachusetts  | isbn = 0-07-121688-X | pages = 79}}</ref> in other words, saturated vapor has a "quality" of 100%, and saturated [[Physics:Liquid|liquid]] has a "quality" of 0%.  Vapor quality is an intensive property which can be used in conjunction with other independent intensive properties to specify the [[Physics:Thermodynamic state|thermodynamic state]] of the [[Physics:Working fluid|working fluid]] of a [[Physics:Thermodynamic system|thermodynamic system]]. It has no meaning for substances which are not saturated mixtures (for example, compressed liquids or [[Physics:Superheating|superheated]] fluids).
Vapor quality is an important quantity during the [[Physics:Adiabatic|adiabatic]] expansion step in various thermodynamic cycles (like Organic Rankine cycle, [[Physics:Rankine cycle|Rankine cycle]], etc.). Working fluids can be classified by using the appearance of droplets in the vapor during the expansion step.

Quality {{mvar|χ}} can be calculated by dividing the mass of the vapor by the mass of the total mixture:
:<math>\chi = \frac{m_\text{vapor}}{m_\text{total}}</math>
where {{mvar|m}} indicates mass.

Another definition used in [[Chemistry:Chemical engineering|chemical engineering]] defines quality ({{mvar|q}}) of a fluid as the fraction that is saturated liquid.<ref>{{Cite book|last=Wankat|first=Philip C.|title=Equilibrium Staged Separations|publisher=Prentice Hall|place=Upper Saddle River, New Jersey|year=1988|pages=[https://archive.org/details/equilibriumstage00phil_0/page/119 119–121]|isbn=0-13-500968-5|url-access=registration|url=https://archive.org/details/equilibriumstage00phil_0/page/119}}</ref> By this definition, a saturated liquid has {{math|1=''q'' = 0}}. A saturated vapor has {{math|1=''q'' = 1}}.<ref>Perry's Chemical Engineers' Handbook (7th Edition), p 13-29</ref>

An alternative definition is the 'equilibrium thermodynamic quality'. It can be used only for single-component mixtures (e.g. water with steam), and can take values <&nbsp;0 (for sub-cooled fluids) and >&nbsp;1 (for super-heated vapors):

:<math>\chi_{eq} = \frac{h-h_{f}}{h_{fg}}</math>
where {{mvar|h}} is the mixture specific enthalpy, defined as:
:<math>h = \frac{m_f \cdot h_f + m_g \cdot h_g}{m_f+m_g}.</math>

Subscripts {{mvar|f}} and {{mvar|g}} refer to saturated liquid and saturated gas respectively, and {{mvar|fg}} refers to [[Physics:Vaporization|vaporization]].<ref>{{cite book|last1=Ghiaasiaan|first1=S. Mostafa|title=Two-phase flow, boiling and condensation in conventional and miniature systems|date=2008|publisher=Cambridge University Press|location=New York|isbn=978-0-521-88276-7|page=96}}</ref>

==Calculation==
The above expression for vapor quality can be expressed as:
:<math>\chi = \frac{y - y_f}{y_g - y_f}</math>
where <math>y</math> is equal to either specific enthalpy, specific [[Physics:Entropy|entropy]], [[Specific volume|specific volume]] or specific internal energy, <math>y_f</math> is the value of the specific property of saturated liquid state and <math>y_g - y_f</math> is the value of the specific property of the substance in dome zone, which we can find both liquid <math>y_f</math> and vapor <math>y_g</math>.

Another expression of the same concept is:
:<math>\chi=\frac{m_v}{m_l + m_v}</math>
where <math>m_v</math> is the vapor mass and <math>m_l</math> is the liquid mass.

==Steam quality and work==
{{Further|Work energy|Work (physics)|Outline of energy|
Index of energy articles}}
The origin of the idea of vapor quality was derived from the origins of [[Physics:Thermodynamics|thermodynamics]], where an important application was the [[Physics:Steam engine|steam engine]]. Low quality steam would contain a high [[Earth:Moisture|moisture]] percentage and therefore damage components more easily.{{Citation needed|date=September 2010}} High quality steam would not corrode the steam engine. [[Physics:Steam engine|Steam engine]]s use water vapor ([[Physics:Steam|steam]]) to push pistons or turbines, and that movement creates [[Physics:Work (thermodynamics)|work]]. The quantitatively described ''steam quality'' (steam dryness) is the proportion of saturated steam in a saturated water/steam mixture. In other words, a steam quality of 0 indicates 100% liquid, while a steam quality of 1 (or 100%) indicates 100% steam.

The quality of steam on which steam whistles are blown is variable and may affect [[Physics:Frequency|frequency]]. Steam quality determines the velocity of sound, which declines with decreasing dryness due to the [[Physics:Inertia|inertia]] of the liquid phase. Also, the [[Specific volume|specific volume]] of steam for a given temperature decreases with decreasing dryness.<ref>Soo, Shao L. (1989). Particulates and Continuum: A Multiphase Fluid Dynamics. CRC Press.</ref><ref>Menon, E. Sashi. (2005). Piping Calculations Manual. New York: McGraw-Hill.</ref>

Steam quality is very useful in determining [[Physics:Enthalpy|enthalpy]] of saturated water/steam mixtures, since the enthalpy of steam (gaseous state) is many orders of magnitude higher than the enthalpy of water (liquid state).

==References==
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{{DEFAULTSORT:Vapour Quality}}
[[Category:Water]]
[[Category:Physical quantities]]

{{Sourceattribution|Vapor quality}}