Steam with a temperature equal to the boiling point at that pressure is known as dry saturated steam. However, to produce 100% dry steam in an industrial boiler designed to produce saturated steam is rarely possible, and the steam will usually contain droplets of water.
In practice, because of turbulence and splashing, as bubbles of steam break through the water surface, the steam space contains a mixture of water droplets and steam.
Steam produced in any shell-type boiler (see Block 3), where the heat is supplied only to the water and where the steam remains in contact with the water surface, may typically contain around 5% water by mass.
If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.
The actual enthalpy of evaporation of wet steam is the product of the dryness fraction () and the specific enthalpy (hfg) from the steam tables. Wet steam will have lower usable heat energy than dry saturated steam.
Because the specfic volume of water is several orders of magnitude lower than that of steam, the droplets of water in wet steam will occupy negligible space. Therefore the specific volume of wet steam will be less than dry steam:
Where vg is the specific volume of dry saturated steam.
Steam at a pressure of 6 bar g having a dryness fraction of 0.94 will only contain 94% of the enthalpy of evaporation of dry saturated steam at 6 bar g. The following calculations use figures from steam tables:
The steam phase diagram
The data provided in the steam tables can also be expressed in a graphical form. Figure 2.2.3 illustrates the relationship between the enthalpy and temperature of the various states of water and steam; this is known as a phase diagram.
Fig. 2.2.3 Temperature enthalpy phase diagram
As water is heated from 0°C to its saturation temperature, its condition follows the saturated water line until it has received all of its liquid enthalpy, hf, (A - B).
If further heat continues to be added, the water changes phase to a water/vapour mixture and continues to increase in enthalpy while remaining at saturation temperature ,hfg, (B - C).
As the water/vapour mixture increases in dryness, its condition moves from the saturated liquid line to the saturated vapour line. Therefore at a point exactly halfway between these two states, the dryness fraction () is 0.5. Similarly, on the saturated steam line the steam is 100% dry.
Once it has received all of its enthalpy of evaporation, it reaches the saturated steam line. If it continues to be heated after this point, the pressure remains constant but the temperature of the steam will begin to rise as superheat is imparted (C - D).
The saturated water and saturated steam lines enclose a region in which a water/vapour mixture exists - wet steam. In the region to the left of the saturated water line only water exists, and in the region to the right of the saturated steam line only superheated steam exists.
The point at which the saturated water and saturated steam lines meet is known as the critical point. As the pressure increases towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This suggests that water changes directly into saturated steam at the critical point.
Above the critical point the steam may be considered as a gas. The gaseous state is the most diffuse state in which the molecules have an almost unrestricted motion, and the volume increases without limit as the pressure is reduced.
The critical point is the highest temperature at which water can exist. Any compression at constant temperature above the critical point will not produce a phase change.
Compression at constant temperature below the critical point however, will result in liquefaction of the vapour as it passes from the superheated region into the wet steam region.
The critical point occurs at 374.15°C and 221.2 bar a for steam. Above this pressure the steam is termed supercritical and no well-defined boiling point applies.