Jawaharlal Nehru Technological University Hyderabad B.Tech III THERMAL ENGINEERING II Previous Question Paper (August - 2022 - R13)

The thermal efficiency of a Rankine cycle is defined by the ratio of net work output to heat supplied. In a simple Rankine cycle, the entire mass of water is heated from the condenser temperature to the saturation temperature using high-grade heat from the boiler. In a regenerative cycle, a portion of the steam is extracted (tapped off) after partial expansion to preheat the feedwater before it enters the boiler.

By preheating the feedwater, the average temperature at which heat is added to the cycle is increased. According to the Carnot principle, the efficiency of a cycle is given by 1 - (T_low / T_high_avg). Since the regenerative cycle raises the mean temperature of heat addition, the efficiency must increase compared to a simple Rankine cycle where the feedwater enters at the lower condenser temperature. Even though the work output per unit mass of steam decreases due to extraction, the reduction in heat input required from the boiler is proportionally greater, resulting in a net increase in thermal efficiency.

Reheating involves expanding steam in two or more stages, with the steam being returned to the boiler to be reheated between stages. The primary advantages include:

• Increased Thermal Efficiency: Reheating increases the mean temperature of heat addition, which improves the overall thermal efficiency of the cycle.
• Improved Quality of Exhaust Steam: By reheating the steam, the moisture content at the exit of the low-pressure turbine is significantly reduced. This prevents erosion of the turbine blades caused by high-velocity water droplets.
• Increased Work Output: Reheating increases the total work output of the turbine per kilogram of steam, as the steam maintains a higher average pressure and temperature throughout the expansion process.
• Reduced Condenser Load: Since more energy is converted into work, the amount of heat rejected to the condenser per unit of power produced is decreased, leading to a smaller required condenser size.