Wet Steam Quality and Dryness Fraction

Introduction and definition of steam quality and dryness fraction including calculating wet steam enthalpy and specific volume

To produce 100% dry steam in an boiler and keep the steam dry throughout the piping system is in general not possible. Droplets of water will escape from the boiler surface due to turbulence and splashing when bubbles of steam break through the water surface. The steam leaving the boiler space will contain a mixture of water droplets and steam.

In addition heat loss in the pipe lines condensates parts of the steam to droplets of water.

Steam - produced in a boiler where the heat is supplied to the water and where the steam is in contact with the water surface of the boiler - contains approximately 5% water by mass.

Dryness fraction of Wet Steam

If the water content in the steam is 5% by mass, then the steam is said to be 95% dry with a dryness fraction 0.95.

Dryness fraction can be expressed:

ζ = ws / (ww + ws)         (1)


ζ = dryness fraction

ww = mass of water (kg, lb)

ws = mass of steam (kg, lb)

Enthalpy of Wet Steam

Actual enthalpy of wet steam can be calculated with the dryness fraction - ζ - and the specific enthalpy - hs - of "dry" steam picked from steam tables. Wet steam will always have lower usable heat energy than "dry" steam.

ht = hs ζ + (1 - ζ ) hw         (2)


ht = enthalpy of wet steam (kJ/kg, Btu/lb)

hs = enthalpy of "dry" steam (kJ/kg, Btu/lb)

hw = enthalpy of saturated water or condensate (kJ/kg, Btu/lb)

Specific Volume of Wet Steam

The droplets of water in wet steam occupies a negligible space in the steam and the specific volume of wet steam will be reduced by the dryness fraction.

vt = vs ζ         (3)


vt = specific volume of wet steam (m3/kg, ft3/lb)

vs = specific volume of the dry steam (m3/kg, ft3/lb)

Example - Enthalpy and Specific Volume of Wet Steam

Steam with pressure 5 bar gauge (6 bar abs) has a dryness fraction of 0.95. From the steam table 

hs = 2755.46 (kJ/kg)

hw = 670.43 (kJ/kg)

The total enthalpy can be calculated:

ht = (2755.46 kJ/kg) 0.95 + (1 - 0.95) (670.43 kJ/kg)

    = 2651 kJ/kg

Specific volume can be calculated:

v = (0.315 m3/kg) 0.95

    = 0.299 m3/kg

Related Topics

Related Documents

Tag Search

  • en: vapor steam quality dryness water
  • es: agua sequedad calidad del vapor de vapor
  • de: Dampf Dampfqualit├Ąt Trockenheit Wasser

Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

Support the ToolBox

If you find the Engineering ToolBox useful and want this website to be available tomorrow - and the days after tomorrow - enable ads or make a contribution!

Contribute to the Engineering ToolBox