Vapor quality

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. By this definition, a saturated liquid has q = 0. A saturated vapor has q = 1.

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.

Calculation

The above expression for vapor quality can be expressed as:

${\displaystyle \chi =\frac{y-y_f}{y_g-y_f}}$

where ${\displaystyle y}$ is equal to either specific enthalpy, specific entropy, specific volume or specific internal energy, ${\displaystyle y_f}$ is the value of the specific property of saturated liquid state and ${\displaystyle y_g-y_f}$ is the value of the specific property of the substance in dome zone, which we can find both liquid ${\displaystyle y_f}$ and vapor ${\displaystyle y_g}$.

Another expression of the same concept is:

${\displaystyle \chi =\frac{m_v}{m_l+m_v}}$

where ${\displaystyle m_v}$ is the vapor mass and ${\displaystyle m_l}$ is the liquid mass.