A coal-fired power plant had been operating using a standard Rankine Cycle to produce power. The operating conditions are 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 conditions be reduced from 600 ^oC to 400 ^oC. The plant operates with steam, with a condenser temperature of 100 ^oC, and in this emergency mode, the boiler would operate at 3.0 MPa and 400 ^oC.

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

b) A clever operator suggests that a Joule-Thompson value 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 ^oC. Assuming that the pump and turbine operate adiabatically and reversibly, fill in the missing values in the table associated with the problem.

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

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

 

Transcribed Image Text:

Q2-3 = T ds This heat flow is given by the area shown in Fig. 5.2-2e. Similarly, the heat flow from point 4 to 1 is equal in magnitude to the area shown in Fig 5.2-2f, but negative in value (since Są is larger than S). Finally, since from the energy balance, the net work flow is equal to the difference of the two heat flows into the cycle, this work flow is given by the area in Fig. 5.2-2g. Similar graphical analyses can be done for the other cycles considered in this section, though we will not do so. 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 con- denser, a pressure of 3.0 MPa in the evaporator, and a maximum temperature of 600°C. Assum- ing the pump and turbine operate reversibly, plot the cycle on a T-S diagram for steam, and compute the efficiency of the cycle. SOLUTION We start by plotting the cycle on the temperature-entropy diagram for steam (Fig. 3.3-1b, re- peated below). Then we construct the table of thermodynamic properties (which follows) for each point in the cycle using the known operating conditions and the path from each point in the cycle to the next point. Based on this information, we can then compute the efficiency of the cycle -(Wr + Wp) -(-947.6 + 3.03) 944.3 = 0.290 or 29.0 percent 3260.2 3260.2 Q2-3 = T ds This heat flow is given by the area shown in Fig. 5.2-2e. Similarly, the heat flow from point 4 to 1 is equal in magnitude to the area shown in Fig 5.2-2f, but negative in value (since Są is larger than S). Finally, since from the energy balance, the net work flow is equal to the difference of the two heat flows into the cycle, this work flow is given by the area in Fig. 5.2-2g. Similar graphical analyses can be done for the other cycles considered in this section, though we will not do so. 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 con- denser, a pressure of 3.0 MPa in the evaporator, and a maximum temperature of 600°C. Assum- ing the pump and turbine operate reversibly, plot the cycle on a T-S diagram for steam, and compute the efficiency of the cycle. SOLUTION We start by plotting the cycle on the temperature-entropy diagram for steam (Fig. 3.3-1b, re- peated below). Then we construct the table of thermodynamic properties (which follows) for each point in the cycle using the known operating conditions and the path from each point in the cycle to the next point. Based on this information, we can then compute the efficiency of the cycle -(Wr + Wp) -(-947.6 + 3.03) 944.3 = 0.290 or 29.0 percent 3260.2 3260.2