- Rankine Cycle Definition: The Rankine cycle is defined as a steam power cycle used in power plants to convert heat into mechanical work.
- Lowering Condenser Pressure: Decreasing condenser pressure increases net work output but can raise moisture content and boiler heat input.
- Superheating the Steam: Raising steam temperature increases efficiency and reduces moisture in the turbine’s final stages, despite metallurgical limits.
- Re-Heat Rankine Cycle: Reheat cycles enhance efficiency by expanding steam in two stages and reheating it, reducing moisture content.
- Modified Rankine Cycle: The modified Rankine cycle with reheat cycles effectively reduces moisture in large turbines, improving overall efficiency.
Steam power plants use the Rankine cycle for power generation. Increasing thermal efficiency reduces the heat input and fuel required for each unit of electricity.
Rankine cycle efficiency can rise through lower condenser pressure, higher turbine-inlet temperature, higher boiler pressure and reheat. Each method has material, moisture, cooling and cost limits.
The two temperature-based changes listed below raise the net work or the average temperature of heat addition in the power plant:
- Decrease the average temperature of heat rejection by lowering condenser pressure within cooling and moisture limits.
- Increase the steam temperature entering the turbine within material limits.
Lowering The Condenser Pressure
Steam commonly leaves the low-pressure turbine as a wet mixture and enters the condenser. Lower condenser pressure increases the turbine expansion ratio and can increase net cycle work when cooling conditions permit it. Air leakage and exhaust moisture must also remain acceptable.
The T-s diagram shows the idealized change in cycle area when condenser pressure falls.
Positive Effects of Lowering the Condenser Pressure
A condensing Rankine Cycle normally operates below atmospheric pressure to reduce its heat-rejection temperature. The practical floor depends on cooling-water or ambient temperature, condenser approach temperature, air ingress and equipment size.
In the idealized T-s diagram, the coloured area represents the increase in net work when condenser pressure falls from P4 to P4’.
Negative Effects of Lowering the Condenser Pressure
Lower pressure also changes boiler duty, turbine-exhaust moisture and condenser operating requirements:
- The boiler may require more heat per unit mass because condensate returns at a lower saturation temperature.
- Lower exhaust pressure can increase moisture in the final turbine stages. Wetness reduces turbine efficiency and liquid droplets can erode blades.
Net Effects of Lowering the Condenser Pressure
Within the plant’s design range, lower condenser pressure usually raises ideal cycle efficiency because the increase in turbine work exceeds the added heat input. The allowable pressure is set by the turbine’s exhaust-quality limit, not by a universal 10 to 12% dryness fraction.
Super Heating The Steam to Higher Temperature
Superheating adds heat to saturated steam at nearly constant boiler pressure and raises the turbine-inlet temperature.
The shaded T-s area (3-3’-4’-4) represents the idealized increase in net work from raising the superheat temperature of steam.
Higher turbine-inlet temperature raises the average temperature of heat addition. In the ideal model, turbine work grows enough to increase thermal efficiency.
Positive Effects of Increasing the Steam Temperature
Higher inlet temperature also increases steam quality in the final turbine stages, reducing moisture during expansion. Figure 2 shows this shift on the T-s diagram.
Negative Effects of Increasing the Steam Temperature
Superheating requires more boiler heat and exposes pressure parts and turbine components to higher temperature. Creep strength, oxidation, component life and alloy cost set the practical limit.
Published ultra-supercritical reference conditions use main and reheat steam temperatures in the range of 600 to 620oC. The approved limit for a plant depends on its alloys, welds, expected service life and cost.
Net Effects of Increasing the Steam Temperature
Figure 2 shows why higher steam temperature can improve ideal cycle efficiency and reduce exhaust moisture. The operating temperature should increase only when component life, reliability and project economics support it.
Increasing Boiler Pressure With Sub Critical Parameters
Increasing boiler pressure can raise Rankine cycle efficiency by raising the average temperature at which heat is added.
The T-s diagram shows the idealized effect at a fixed maximum steam temperature.
At higher pressure the expansion path shifts left on the T-s diagram, which introduces the following trade-offs:
- The average heat-addition temperature rises, which tends to increase thermal efficiency.
- At the same maximum temperature, the turbine exhaust moves deeper into the wet region and its quality falls.
- Heat rejected in the condenser changes with the selected pressure and state points; it is not a fixed reduction.
The ideal thermal efficiency normally increases, but moisture and material limits determine the usable pressure.
Increasing the Boiler Pressure with Super Critical Parameters
A water-steam Rankine plant is supercritical when its boiler operates above water’s critical pressure of 22.064 MPa. There is no liquid-vapor phase boundary above that pressure. Higher pressure can improve cycle efficiency, while the actual result also depends on steam temperature, component efficiency and auxiliary loads.
Re-Heat Rankine Cycle
A Re-heat Rankine cycle expands steam in a high-pressure turbine, returns it to the boiler for heating and then expands it through a lower-pressure turbine.
Reheat raises the average heat-addition temperature and increases exhaust quality. Its main practical benefit is lower moisture in the last turbine stages; the efficiency gain depends on pressures, temperatures and component performance.
Theoretical Way of Reducing the Last Stage Moisture
Superheating before the turbine also reduces final-stage moisture, but material temperature limits still apply. Published ultra-supercritical examples use about 600oC main steam and 620oC reheat; advanced nickel-alloy programmes target higher conditions.
Modified Rankine Cycle
Reheat is used where the selected pressure ratio would otherwise produce excessive last-stage moisture. Figure 5 shows a simple cycle modified with one reheat stage.
Re-Heat Cycle Differs from Rankine Cycle in Following Aspects
Steam first expands in the high-pressure turbine. Its exhaust returns to the steam generator for reheat, then enters the low-pressure turbine for final expansion at a higher quality than it would have without reheat. The exhaust finally enters the condenser.
Analysis of Re-Heat Cycle is as Follows
Heat input during the cycle (2-3-4-5) is
Turbine work output for the cycle is
A single reheat stage in a thermal power plant may improve efficiency, but no universal four-to-five percentage-point gain applies to every design.
What is the Practical Limit of Re-Heating?
Additional reheat stages divide turbine expansion into more pressure ranges and can raise the average temperature of heat addition.
Each extra stage adds reheater surface, piping, pressure loss, controls and capital cost. The incremental efficiency gain is plant-specific and usually diminishes.
Double reheat must be evaluated with the selected main pressure, reheat pressures and exhaust state. A subcritical plant may reject more superheat in the condenser, but that does not create a universal ban on double reheat.
Designers stop adding reheat stages when the small thermodynamic gain no longer justifies the added pressure loss, cost and complexity.
