Wet Steam vs. Dry Steam: The Importance of the Steam Dryness Fraction

Did you know that boilers do not generate 100% saturated steam (dry steam)? When a steam boiler heats up water, bubbles breaking through the water surface will pull tiny water droplets in with the steam. Unless a superheater is used, this will cause the steam supply to become partially wet (wet steam) from the added liquid.

Steam Dryness Fraction

The steam dryness fraction is used to quantify the amount of water within steam. If steam contains 10% water by mass, it's said to be 90% dry, or have a dryness fraction of 0.9.

Steam dryness is important because it has a direct effect on the total amount of transferable energy contained within the steam (usually just latent heat), which affects heating efficiency and quality.

For example, saturated steam (100% dry) contains 100% of the latent heat available at that pressure. Saturated water, which has no latent heat and therefore 0% dryness, will only contain sensible heat.

Steam Dryness = 100% - [% Entrained Water] (by mass)

Calculating the Total Heat of Wet Steam

Steam tables contain values such as enthalpy (h), specific volume (ν), entropy (s), etc. for saturated steam (100% dry) and for saturated water (0% dryness), but typically not for wet steam.

These can be calculated by simply considering the ratio of steam to water, as described in the equations below:

Specific Volume (ν) of Wet Steam

• where:
• X = Dryness (% / 100)
• ν_f = Specific Volume of Saturated Water
• ν_g = Specific Volume of Saturated Steam

Specific Enthalpy (h) of Wet Steam

• where:
• X = Dryness (% / 100)
• h_f = Specific Enthalpy of Saturated Water
• h_fg = Specific Enthalpy of Saturated Steam - Specific Enthalpy of Saturated Water

Specific Entropy (s) of Wet Steam

• where:
• X = Dryness (% / 100)
• s_f = Specific Entropy of Saturated Water
• s_fg = Specific Entropy of Saturated Steam - Specific Entropy of Saturated Water

The wetter the steam, the lower the specific volume, enthalpy, and entropy will be because the dryness percentage is a factor of the 100% condition. Since steam dryness has a significant effect on all these values, to enable greater heating efficiency it is crucial to supply steam that is as close to being 100% dry as possible.

The Relationship Between Steam Dryness and Enthalpy

As the amount of water in steam increases, the latent heat decreases, providing less heat to transfer from the steam to the process / product being heated.

Steam Dryness Decreases During Transport

During transport, radiant heat loss from piping causes part of the steam to lose some of its latent heat and revert back to water, thereby decreasing steam dryness.

Water Droplets Entrained in Steam

Proper measures should be taken to discharge all condensate within steam piping, including water droplets entrained within the flow of steam.

Since wet steam not only affects heat transfer efficiency, but can also cause erosion of piping and critical equipment such as turbine blades, it is highly recommended to take preventative measures such as using a steam separator to remove the entrained condensate and by following the advice written in these articles:

• Best Practices for Condensate Removal on Steam Lines
• Erosion in Steam and Condensate Piping