Short Answer:

The effect of boiler pressure on cycle efficiency is very significant in thermal power plants. When the boiler pressure increases, the average temperature of heat addition also increases, which improves the thermal efficiency of the Rankine cycle. Higher boiler pressure allows more energy to be extracted from the steam, resulting in better performance.

In simple words, increasing boiler pressure makes the Rankine cycle more efficient because the steam enters the turbine with higher enthalpy. However, very high pressure can cause material and operational problems, so an optimum pressure is maintained for the best efficiency and reliability.

Detailed Explanation :

Effect of Boiler Pressure on Cycle Efficiency

The boiler pressure plays a key role in determining the performance and efficiency of the Rankine cycle, which is the fundamental thermodynamic cycle used in steam power plants. The thermal efficiency of the cycle depends on the average temperature at which heat is added in the boiler and rejected in the condenser. By increasing the boiler pressure, the temperature at which heat is added rises, thereby improving the overall thermal efficiency.

However, this increase in efficiency is limited by the practical constraints of materials, equipment design, and condensation problems at low pressures. To understand this effect clearly, we must examine how boiler pressure influences the various stages of the Rankine cycle.

1. Rankine Cycle and Its Efficiency

The Rankine cycle consists of four main processes:

1. Pumping: Water is pumped from low pressure (condenser pressure) to high boiler pressure.
2. Heating: Heat is added in the boiler at constant pressure to produce high-temperature steam.
3. Expansion: The steam expands in the turbine, converting thermal energy into mechanical work.
4. Condensation: The exhaust steam is condensed into water at constant pressure in the condenser.

The thermal efficiency (η) of the Rankine cycle is given by:

Where:

•  = Work done by turbine
•  = Work done by pump
•  = Heat supplied in the boiler

The efficiency mainly depends on the difference between the high and low temperatures in the cycle. By increasing the boiler pressure, the average temperature of heat addition increases, which improves efficiency.

2. Effect of Increasing Boiler Pressure

When the boiler pressure is increased, several changes occur in the Rankine cycle, which affect its efficiency and performance.

1. Increase in Mean Temperature of Heat Addition

• As the boiler pressure increases, the saturation temperature of water and steam also increases.
• This means that the average temperature at which heat is added in the boiler becomes higher.
• Since the thermal efficiency of a heat engine increases with the mean temperature of heat addition, the cycle efficiency increases.

1. Decrease in Condenser Pressure Difference

• The turbine operates between boiler pressure (high) and condenser pressure (low).
• Increasing the boiler pressure increases the pressure ratio across the turbine, resulting in greater expansion and higher turbine work.

1. Increase in Work Output of Turbine

• With higher boiler pressure, the steam entering the turbine has a higher enthalpy and energy content.
• The steam expands more through the turbine, leading to greater mechanical work output per kilogram of steam.

1. Increase in Pump Work

• When boiler pressure increases, the feedwater pump must work against higher pressure, increasing pump work slightly.
• However, the pump work is very small compared to turbine work, so its effect on efficiency is negligible.

1. Increase in Moisture Content in Steam

• One disadvantage of increasing boiler pressure is that the steam at the turbine outlet becomes wetter.
• High moisture content can cause erosion and damage to turbine blades.
• To overcome this, superheating or reheating of steam is usually employed.

3. T–S (Temperature–Entropy) Diagram Explanation

In the Temperature–Entropy (T–S) diagram, when the boiler pressure increases:

• The boiling point of water shifts upward.
• The area under the curve representing the heat addition process increases.
• The average temperature of heat addition rises, which directly enhances the cycle’s efficiency.
  However, if pressure is increased too much without superheating, the moisture content at the end of expansion rises, reducing the turbine’s life and performance.

4. Mathematical Relationship

The efficiency improvement with increased boiler pressure can be understood through the Carnot efficiency relationship:

Where:

•  = Temperature at which heat is rejected (condenser temperature)
•  = Temperature at which heat is added (boiler temperature)

When boiler pressure increases,  increases because the saturation and superheat temperatures rise. Thus, the efficiency (η) of the Rankine cycle increases.

5. Practical Considerations

While increasing boiler pressure improves efficiency, it also introduces some practical limitations:

1. Material Strength Limitations:
   • At very high pressures and temperatures, materials used in boilers, pipes, and turbines must withstand high stresses and corrosion.
   • This requires special alloys, which are expensive.
2. Increase in Moisture at Turbine Exit:
   • As the boiler pressure increases, the dryness fraction of steam at the turbine exit decreases.
   • Wet steam can cause blade erosion and reduce the turbine’s life.
3. Complex System Design:
   • High-pressure systems require thicker boiler shells, stronger components, and better safety controls.
4. Economic Limit:
   • The cost of maintaining very high pressure systems may outweigh the gain in efficiency beyond a certain point.

Therefore, an optimum boiler pressure is selected to balance efficiency improvement with practical and economic considerations.

6. Use of High Boiler Pressure in Modern Power Plants

Modern power plants use supercritical and ultra-supercritical boilers to achieve high pressures and temperatures for maximum efficiency.

• Subcritical boilers: Operate below 221 bar.
• Supercritical boilers: Operate above 221 bar, where there is no distinct phase change.
• Ultra-supercritical boilers: Operate at pressures above 250 bar and very high temperatures.

These systems achieve efficiencies as high as 45–50% due to increased boiler pressure and temperature.

7. Summary of Effects

• Increase in boiler pressure → Increase in mean heat addition temperature → Increase in efficiency.
• Increase in boiler pressure → Increase in turbine work.
• Increase in boiler pressure → Increase in pump work (slight).
• Increase in boiler pressure → Increase in outlet steam moisture (disadvantage).

Thus, boiler pressure directly affects the efficiency and performance of the cycle.

Conclusion

The effect of boiler pressure on cycle efficiency is that higher boiler pressure increases the average temperature of heat addition, thereby improving the thermal efficiency of the Rankine cycle. However, it also increases the moisture content of exhaust steam and requires stronger, costlier materials. Therefore, while increasing boiler pressure enhances efficiency, it must be optimized for safe, economical, and durable operation. Modern power plants achieve this balance through the use of superheating, reheating, and advanced materials.