Vapor quality

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In thermodynamics, vapor quality is the mass fraction in a saturated mixture that is vapor;^[1] in other words, saturated vapor has a "quality" of 100%, and saturated 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 thermodynamic state of the working fluid of a thermodynamic system. It has no meaning for substances which are not saturated mixtures (for example, compressed liquids or superheated fluids). Vapor quality is an important quantity during the adiabatic expansion step in various thermodynamic cycles (like Organic Rankine cycle, Rankine cycle, etc.). Working fluids can be classified by using the appearance of droplets in the vapor during the expansion step.

Quality χ can be calculated by dividing the mass of the vapor by the mass of the total mixture:

${\displaystyle \chi ={\frac {m_{\text{vapor}}}{m_{\text{total}}}}}$

where m indicates mass.

Another definition used in chemical engineering defines quality (q) of a fluid as the fraction that is saturated liquid.^[2] By this definition, a saturated liquid has q = 0. A saturated vapor has q = 1.^[3]

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 < 0 (for sub-cooled fluids) and > 1 (for super-heated vapors):

${\displaystyle \chi _{eq}={\frac {h-h_{f}}{h_{fg}}}}$

where h is the mixture specific enthalpy, defined as:

${\displaystyle h={\frac {m_{f}\cdot h_{f}+m_{g}\cdot h_{g}}{m_{f}+m_{g}}}.}$

Subscripts f and g refer to saturated liquid and saturated gas respectively, and fg refers to vaporization.^[4]