What is Saturated Steam?

What is Saturated Steam? – Saturated steam is a condition where water vapor is at the equilibrium of pressure and temperature equal to liquid phase water. Saturated steam becomes a phase transition between liquid phase of water to its pure gas phase, or commonly known as superheated steam. When the water is in this phase of transition, there is mixing between the liquid phase of water with gas phase of water (saturated steam) in proportion to the amount of latent heat absorbed by the fluid.

Saturated steam begins to form just as the water reaches its boiling point, until all the energy from latent heat is absorbed by water. While all latent heat has been absorbed by water, and the amount of vapor phase has reached almost 100% compared to its liquid phase, that is the end of the phase of saturated steam. The process of reaching almost 100% of the vapor phase occurs at a constant pressure and temperature. Furthermore, if thermal energy continues to be fed to saturated steam, there will be an increase in fluid temperature and encourage steam to turn the phase into superheated steam.

According to the water phase diagram above, the phase of saturated steam can only form along the saturated curve. The lower limit of the saturated curve is the triple point, while the upper limit of the curve is the critical point. Water in more than triple-point conditions will not experience a phase of saturated steam. Water that has a pressure above 22.1 kPa, if it continues to be heated will immediately turn the phase into supercritical steam.

The mixture between water vapor and liquid water in the saturated steam can be determined in amount by using a saturated steam diagram. This diagram uses pressure as the Y axis and the enthalpy as the X axis. This saturated steam diagram is made of a curve. Half of the curve from the lowest point to the top is called the saturated water curve. This part curve becomes the boundary between liquid water with the saturated steam phase. For the right curve from the top of the curve to the lowest point is called the saturated steam curve. This curve becomes the boundary between the phase of saturated steam and superheated steam phase. Right at the vertex of the curve is a critical point, the same point as the critical point in the phase diagram of water.

Since the saturated steam is in constant pressure, a certain amount of saturated steam is represented by a horizontal straight line connecting a point on the saturated water curve to another point on the saturated steam curve. The point on the saturated water curve (hf) shows the enthalpy value of saturated water, ie how much heat energy required for water at pressure P per one unit of mass can reach saturated water. While the point on the saturated steam curve (hg) is the total enthalpy value required so that the water reaches 100% of steam.

The simple relationship is:
hg – hf = hfg

Where:
hf = enthalpy saturated water
hg = enthalpy saturated steam
hfg = difference of enthalpy required saturated water to achieve saturated steam

In other cases, the enthalpy value given to water is not as large as hg, ie only by hmix. The hmix point is anywhere along the horizontal line. In this case the saturation vapor is a mixture of vapor with water whose ratio can be easily determined using the following equation:
$x=\dfrac{h_{mix}}{h_{fg}}$

So:
hmix = hf + x . hfg

Where:
x = comparison of the amount of water in the overall vapor mixture of saturation
hmix = enthalpy mixture