How is steam quality (dryness fraction) defined?

• A. Mass fraction of vapor in a steam-water mixture
• B. Mass fraction of liquid in a steam-water mixture
• C. Temperature of the steam
• D. Pressure of the steam

Answer: (B)

Steam quality, or dryness fraction, is the mass fraction of vapor in a saturated steam–water mixture. It is defined as x = m_v / (m_v + m_l), meaning the portion of the total mass that is in the vapor phase. This value ranges from 0 to 1: 0 means all liquid, 1 means all vapor. The mass fraction of liquid would be the complement, 1 − x, not the standard definition of dryness fraction. Temperature and pressure help determine when a mixture is saturated, but they do not by themselves define dryness fraction; dryness fraction applies only to mixtures with both liquid and vapor present (partially saturated states).