The data provided in the steam table can also be expressed in graph form. Figure 1 illustrates the relationship between enthalpy and temperature at various pressures, and known as the phase steam diagram.
|Figure 1: Phase Steam Diagram|
When water is heated from 0° C to saturation temperature, its condition follow the line of saturated liquid until receives all of liquid enthalpy, hf, (A - B). If heat is added further, it will change the phase steam diagram to the saturated steam enthalpy and continually improving while remaining at saturation temperature, hfg, (B - C).
If the dryness of mixture of steam and water increase, the condition moves from the saturated liquid line to the saturated steam line. Therefore at the middle point between both conditions, the dryness fraction (x) is 0.5. The same condition occurs on the saturated steam line which has steam 100% dry.
Upon receiving enthalpy of evaporation it will reach the saturated steam line. If heating is continued after this point, the temperature of steam will begin to rise to supersaturated (C - D). The lines of saturated liquid and saturated steam cover areas where there is mixture of steam/water-wet steam. In the area of left side of saturated liquid line, there is only water, and on the right side of the saturated steam line there is only supersaturated steam. The point where the saturated liquid line and saturated steam meet is known as the critical point.
If the pressure raises towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This indicates that the water turns directly into saturated steam at the critical point. Gas may just exist above the critical point. Gaseous state is the most diffused situation where the molecule is almost has unlimited movement and the volume increases without limit when the pressure is reduced.
The critical point is the highest temperature where fluid is in liquid form. Giving the pressure at constant temperature below the critical point will not cause phase steam diagram change. Even so, giving the pressure at constant temperature below the critical point will result in melting of steam when passing superheated area to wet steam. Critical point occurs at temperature of 374.15 ° C and 221.2 bars steam pressure. Above this pressure are called supercritical steam and there is not boiling point can be applied.