Jawaharlal Nehru Technological University Hyderabad B.Tech III THERMAL ENGINEERING – II Previous Question Paper (October - 2020 - R13)

The primary importance of Reheating in a Rankine cycle is to increase the dryness fraction of steam at the turbine exhaust, thereby preventing erosion of the turbine blades caused by moisture droplets. Additionally, it improves the thermal efficiency of the cycle by allowing for a higher mean temperature of heat addition.

For a cycle with reheat, the efficiency is derived as the ratio of net work done to the total heat supplied:

Where W_T1 is the work from the high-pressure turbine, W_T2 is the work from the low-pressure turbine, W_P is the pump work, Q_s1 is the heat added in the boiler, and Q_s2 is the heat added during the reheating process.

Given: P1 = 2 MPa (20 bar), T1 = 300°C. P_condenser = 3.5 kPa (0.035 bar). Reheat pressure (P_reheat) is the pressure where steam becomes saturated vapor during expansion from 2 MPa, 300°C.

1. From steam tables at 2 MPa, 300°C: h1 = 3023.5 kJ/kg, s1 = 6.7664 kJ/kgK.
2. Expansion to saturation: Saturated vapor entropy at 2 MPa is 6.339 kJ/kgK. Since s1 > sg, the steam is superheated. Find P_reheat where s = sg. By interpolation, P_reheat is approximately 0.5 MPa (5 bar).
3. At 5 bar, h_g = 2747.5 kJ/kg.
4. Work of HP turbine: W_T1 = h1 - h_g = 3023.5 - 2747.5 = 276 kJ/kg.
5. Reheat to 300°C at 5 bar: h3 = 3064.2 kJ/kg, s3 = 7.547 kJ/kgK.
6. Expansion to 0.035 bar: s3 = sf + x3(sfg). 7.547 = 0.391 + x3(8.13), giving x3 = 0.879.
7. h4 = hf + x3(hfg) = 112 + 0.879(2437) = 2254 kJ/kg.
8. Work of LP turbine: W_T2 = h3 - h4 = 3064.2 - 2254 = 810.2 kJ/kg.
9. Total Work = 276 + 810.2 = 1086.2 kJ/kg.
10. Heat Supplied = (h1 - hf_pump) + (h3 - h_g) = (3023.5 - 112) + (3064.2 - 2747.5) = 3228.2 kJ/kg.
11. Efficiency = (1086.2 / 3228.2) * 100 = 33.65%.