Steam Quality Calculator
Find the dryness fraction (steam quality) of a wet steam mixture from its enthalpy. Enter the actual enthalpy, the saturated-liquid enthalpy and the latent heat to get the quality as a fraction and a percentage, plus the moisture content.
Enter Values
How to use this calculator
- Enter the actual specific enthalpy h of the steam in kJ/kg.
- Enter the saturated-liquid enthalpy hf and the latent heat of vaporisation hfg (both from steam tables at your pressure) in kJ/kg.
- Read the dryness fraction x, the equivalent percentage and the moisture (wetness) fraction.
How it works
Wet steam is a mixture of saturated liquid and saturated vapour. Its quality (dryness fraction) x is the mass fraction that is vapour, found from enthalpy as x = (h - hf) / hfg, where hf is the saturated-liquid enthalpy and hfg is the latent heat of vaporisation at the same pressure. The moisture (wetness) fraction is simply 1 - x. A result between 0 and 1 is valid wet steam; below 0 means subcooled liquid and above 1 means superheated steam.
Worked example
Worked example. For h = 2000 kJ/kg, hf = 500 kJ/kg and hfg = 2000 kJ/kg: x = (2000 - 500) / 2000 = 0.75, i.e. 75% quality, with a moisture fraction of 0.25.
Common mistakes
- Reading hf and hfg at the wrong pressure: both properties must come from the steam table row matching the mixture's pressure (or saturation temperature).
- Confusing hg (saturated-vapour enthalpy) with hfg (latent heat); hfg = hg - hf.
- Treating an x above 1 as valid quality: it actually means the steam is superheated and quality no longer applies.
Frequently asked questions
What does a dryness fraction of 0.75 mean?
It means 75% of the mass is dry saturated vapour and 25% is still liquid water droplets. Higher quality steam carries more usable energy and is less erosive to turbine blades.
Why did I get a quality above 1 or below 0?
Values outside 0-1 mean the state is not a wet mixture: below 0 the fluid is a subcooled (compressed) liquid, and above 1 it is superheated steam. Recheck that h, hf and hfg are all at the same pressure.
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Tip: Enter any known values to calculate the remaining results.
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