The efficiency of a boiler increases with an increase in boiler pressure primarily due to the following reasons:
Increased Carnot Efficiency: The efficiency of a heat engine, including a boiler, is governed by the Carnot efficiency, which depends on the temperature difference between the heat source and the heat sink. In a boiler, the heat source is the combustion of fuel, and the heat sink is the environment or the heat transfer medium (e.g., water or steam). By increasing the boiler pressure, the temperature difference between the combustion gases and the heat transfer medium increases. This increase in temperature difference results in a higher Carnot efficiency, leading to improved overall boiler efficiency.
Reduced Stack Heat Losses: Stack heat losses occur when the flue gases exit the boiler at a high temperature. These losses represent unused energy that is discharged to the environment. When the boiler pressure is increased, the flue gases can be cooled to a lower temperature before they are discharged. This reduction in stack heat losses improves the overall efficiency of the boiler.
Enhanced Heat Transfer: Increasing the boiler pressure allows for higher temperature differentials across heat transfer surfaces. This leads to improved heat transfer rates between the combustion gases and the heat transfer medium (e.g., water or steam) within the boiler. The enhanced heat transfer ensures that more heat is efficiently transferred to the working fluid, increasing the boiler efficiency.
Reduced Boiler Blowdown: Boiler blowdown is the process of removing concentrated water (containing dissolved solids) from the boiler to prevent the build-up of impurities. Higher boiler pressure enables a lower blowdown rate, reducing the amount of water and heat wasted during the blowdown process. Consequently, the boiler operates with a higher percentage of feedwater, which improves its efficiency.
Increased Steam Cycle Efficiency: In many applications, the steam generated by the boiler is utilized in a steam turbine to produce electricity or perform mechanical work. Higher boiler pressures result in higher steam temperatures and pressures, which can increase the efficiency of the steam turbine. This overall improvement in the steam cycle efficiency contributes to the higher efficiency of the boiler system.
However, it's important to note that increasing the boiler pressure beyond certain limits may lead to diminishing returns or may require additional measures to ensure safe operation and prevent equipment damage. Design considerations, material limitations, and operational factors need to be carefully evaluated when determining the optimal boiler pressure for a given application.