The Important Of Dryness Of Steam

Do you know that steam generated from boiler are not 100% dry? When steam boiler heats up the water, the foam on the water surface will mix some small water droplets into the steam when it is broken, and then make it becomes wet.

The dryness of steam

Steam dryness is a measure of the water content in the steam. If the steam contains 10% moisture, it means that the steam dryness is 90%, or we can say dryness is 0.9.

Steam dryness is very important because it reflects the heat that can be transferred in the steam (often only latent heat), which directly affects the heating efficiency and quality. For example, saturated steam (100% dry) contains 100% latent heat at the current pressure. For saturated water, there is no latent heat, and the dryness is 0, which includes only sensible heat.

The dryness of steam = 100%-（% water content contained）(count with mass flow)

Calculating the total heat of wet steam

Wet steam specific volume(v)

ν = X • ν_g + (1 – X) • ν_f

here, X means dryness (%/100)

vf means saturated water specific volume

vg means saturated steam specific volume

Wet steam specific enthalpy

h = h_f + X • h_fg

here, X means dryness (%/100)

vf means saturated water specific enthalpy

hfg means saturated steam specific enthalpy – saturated water specific enthalpy

Specific Entropy (s) of Wet Steam

s = s_f + X • s_fg

here, X means dryness (%/100)

sf means saturated water specific enthalpy

sfg means saturated steam specific enthalpy – saturated water specific enthalpy

Steam dryness is an influencing parameter for whether the working condition can be 100%. The damper the steam, the lower its specific volume, specific heat and specific entropy. Now that steam dryness has such a large effect on these, we can understand why it is important to provide nearly 100% dry steam to improve heating efficiency.

The relationship between steam dryness and enthalpy

Saturated water (0% dryness), temperature=100°C,

total heat: hf+0%*hfg=419kJ/kg;

Wet steam (x% dryness), temperature=100°C,

total heat: hf+x%*hfg＜2676kJ/kg;

Saturated steam (100% dryness), temperature=100°C,

total heat: hf+100%*hfg=2676kJ/kg;

As the moisture content of the steam increases, the latent heat decreases, and the amount of heat provided by the steam used in the process / heating product decreases.

Steam dryness reduce during delivery

During the transportation process, the heat radiation loss of the pipeline will cause a part of the steam to lose latent heat and become condensed water, which will reduce the steam dryness.

During the long way delivery process, the heat radiation loss of the pipeline will cause a part of the steam to lose latent heat and become condensed water, which will reduce the steam dryness.

Wet steam not only affects the heat conversion efficiency, but also causes erosion of important equipment such as pipes and turbine blades. Therefore, it is strongly recommended to take some protective measures, such as installing a steam separator to remove the condensed water in the steam. Some suggestions below can also provide some protection

Condensate in steam pipes should be completely drained in the correct way, including water droplets en-trained in the steam flow.