Theory

Recall from the First and Second Law of Thermodynamics that the adiabatic process where entropy remains constant provides the maximum energy for work. As shown on the H-S coordinates, the difference in enthalpy, (H₁-H₂), is maximum when the lowest enthalpy (H₂) is reached at the exit conditions. The ideal expansion is, therefore, a vertical line.

On the diagram above, T₁, P₁ and P₂ are known process variables, for example, H₁ is determined by using T₁ and P₁. H₂ then can be found drawing a vertical line from P₁ to P₂ by following adiabatic isentropic expansion (expansion at constant entropy).

Non-ideal processes or real processes, however, do not present straight lines as shown on the Mollier diagram due to such factors as friction. If the expansion is not isentropic (i.e. entropy is not constant but it increases), the lowest enthalpy (H₂) cannot be reached at the exit conditions, in other words, H_2’ > H₂. This means that ΔH for the ideal expansion is greater than ΔH for the non-ideal expansion between the same pressure boundaries. The internal turbine efficiency is therefore given by:

The difference in enthalpy H_2’-H₂ is called the reheat factor and is the basis for multi-stage turbines. As can be seen on the Mollier diagram, the pressure curves are divergent. This means that the higher the pressure drop in a single stage turbine the greater the reheat factor and in turn the lower the turbine efficiency. However, if the steam is expanded through multiple stages and between each stage the steam is reheated, higher turbine efficiencies can be achieved. We will see this effect later in the Power Plant Efficiency lab.

Hints & Tips

In addition to various pressure and temperature values; log the following tags in your trends:

• Z03020
• E03018

To calculate the enthalpy values, you may use an app or online tool such as the Superheated Steam Table: https://goo.gl/GdVM4U