A coal-fired power plant had been operating using a standard Rankine cycle to produce power. The operating conditions are as given in Illustration 5.2-1. However, the boiler is aging and will need to be replaced. While waiting for the replacement, it has been suggested that for safety the operating temperature be reduced from 600°C to 400°C. The plant operates with steam, with a condenser temperature of 100°C, and in this emergency mode the boiler would operate at 3.0 MPa and 400°C.

a. Can the plant function in this mode? Why or why not? 

b. A clever operator suggests that a Joule-Thompson valve could be placed after the boiler and before the turbine. This valve is to be designed such that the exhaust from the turbine is at the same conditions as in Illustration 5.2-1, 0.10135 MPa and approximately 126°C. Assuming that the pump and turbine operate adiabatically and reversibly, fill in the missing thermodynamic properties in the table below. [Values that are unchanged from Illustration 5.2-1 are given in bold.] 

c. Determine the heat and work flows per kg of steam in the pump, boiler, turbine and condenser. 

d. What is the efficiency of this new cycle? How does it compare with the efficiency of the cycle in Illustration 5.2-1?

Illustration 5.2-1.

Calculating the Efficiency of a Steam Power Cycle 

A Rankine power generation cycle using steam operates at a temperature of 100°C in the condenser, a pressure of 3.0 MPa in the evaporator, and a maximum temperature of 600°C. Assuming the pump and turbine operate reversibly, plot the cycle on a T-S^ˆ diagram for steam and compute the efficiency of the cycle.