Compare the results of Problems 4-9 and 4-10.

A. Without the heat exchanger, the pair of turbines in Problem 4-10 would function identically to the turbine in Problem 4-9A. Comment on the effects the heat exchanger has on the process.

B. If the heat exchanger in Problem 4-10 was moved in front of the first turbine, and the two turbines were combined, the result would essentially be the process examined in Problem 4-9B. The process in Problem 4-9B looks simpler: the same heat is added, it’s just being added sooner, so any benefits you noted in part A of this problem are still obtained, and there’s no need for two separate turbines. Can you see any rationale for using the apparently more complicated process described in Problem 4-10?

Problem 4-9

Steam enters a turbine at 10 bar. The effluent pressure is 1 bar and the efficiency of the turbine is 80%. Determine the state of the turbine effluent (if pure liquid or vapor, find the temperature, and if a mixture, find the quality) and compute the work produced per kg of entering steam, when

A. The entering steam has T = 250°C.

B. The entering steam has T = 325°C.

Problem 4-10

A series of two turbines and a heat exchanger are used to obtain shaft work from steam in a steadystate process. The steam enters the first turbine at 10 bar and 250°C, and exits the turbine at 3 bar. In the heat exchanger, the steam is heated back up to 250°C, while the pressure remains 3 bar. This steam enters the second turbine in which it is expanded to 1 bar. Each turbine has an efficiency of 80%. For each kilogram of steam entering the process, find the amount of work produced in each turbine, and the amount of heat added in the heat exchanger