Example 3 (A poorly maintained system) Consider the same turbine in the previous problem. Hence steam enters the turbine at a pressure of 3 MPa, temp. of 400°C and a velocity of 150 m/s. The exit conditions also remain the same;100% saturated vapor exits at 100°C with velocity of 50 m/s. Due to poor maintenance, the turbine now develops less work, 400kJ/kg instead of 500 kJ/kg of steam. There is now more heat transfer between the turbine and its surroundings at an averaged surface temperature of 550 K. The higher temperature is reasonable since more heat is generated due to the less than ideal operating conditions. Determine the new rate at which entropy is produced within the turbine per kg of steam. dQ/dt at 550 K dW/dt 1 2 T s 1 2

Solution

Solution

Example 4 (An ideal turbine) Again considering the same turbine as in the previous problem. Assume that the inlet conditions are the same. However the steam expands in an ideal process in the turbine to the same exit pressure as before and there is no heat transfer. Determine the work produced by the turbine and the rate at which entropy is produced within the turbine per kg of steam. Show this process on a T-s diagram. Solution: To be solved in class dQ/dt at 550 K dW/dt 1 2 T s

Example 5 Determine the work required to compressed steam isentropically from 100 kPa to 1 MPa if the steam is (a) saturated liquid and (b) saturated vapor at the inlet state. Negelct change in KE and PE. T s 100 kPa 1 MPa

Example(cont.)