The Effect of Lowing the
Condenser Pressure

There are three ways to increase the efficiency of the simple ideal Rankine cycle.

1. Decreasing the condenser pressure

The effect of lowering the condenser pressure on the Rankine cycle efficiency is illustrated on a T-s diagram on the left. Steam exits as a saturated mixture in the condenser at the saturation temperature corresponding to the pressure in the condenser. So lower the pressure in the condenser, lower the temperature of the steam, which is the heat rejection temperature. The blue area is the net work increases due to the decreasing of the condenser pressure.

2. Superheating the steam to a high temperature

The effect of superheating the steam to a high temperature on the Rankine cycle efficiency is illustrated on a T-s diagram on the left. By superheating the stream to a high temperature (from state 3 to state 3'), the average steam temperature during heat addition can be increased. The blue area is the net work increased due to superheating the steam to a high temperature.

3. Increasing the boiler pressure

The effect of increasing the boiler pressure on the Rankine cycle efficiency is illustrated on a T-s diagram on the left. If the operating pressure of the boiler is increased, (process 2-3 to process 2'-3'), then the boiling temperature of the steam raises automatically. For a fixed inlet turbine temperature, the blue area is the net work increased and the gray area is the net work decreased. Also, the moisture content of the steam increases from state 4 to state 4', which is an undesirable side effect. This side effect can be corrected by reheating the steam, and results in the reheat Rankine cycle.

The above section states that increasing the boiler pressure can increase the thermal efficiency of the Rankine cycle, but it also increases the moisture content at the exit of the turbine to an unacceptable level. To correct this side effect, the simple Rankine cycle is modified with a reheat process. The schematic of an ideal reheat Rankine cycle is shown on the left with its T-s diagram. In this reheat cycle, steam is expanded isentropically to an intermediate pressure in a high-pressure turbine (stage I) and sent back to the boiler, where it is reheated at constant pressure to the inlet temperature of the high-pressure turbine. Then the steam is sent to a low-pressure turbine and expands to the condenser pressure (stage II) . The total heat input and total work output is

      q_in = q_primary + q_reheat = (h₃ - h₂) + (h₅ - h₄)

      w_{total, out} = q_{turb, I} + q_{turb, II} = (h₃ - h₄) + (h₅ - h₆)

When the number of the reheat stages increases, the expansion and reheat processes approach an isothermal process at the maximum temperature. But using more than two stages is not practical.