Deck 10: Vapor and Combined Power Cycles

Question 1

Consider a steady-flow Carnot cycle with water as the working fluid executed under the saturation dome between the pressure limits of 10 MPa and 50 kPa. Water changes from saturated liquid to saturated vapor during the heat addition process. The net work output of for this cycle is

A)973 kJ/kg
B)907 kJ/kg
C)518 kJ/kg
D)799 kJ/kg
E)1072 kJ/kg

Answer 1

518 kJ/kg

Question 2

A simple ideal Rankine cycle operates between the pressure limits of 20 kPa and 3 MPa, with a turbine inlet temperature of 500°C. Disregarding the pump work, the cycle efficiency is

A)26%
B)33%
C)57%
D)67%
E)74%

Answer 2

33%

Question 3

A simple ideal Rankine cycle operates between the pressure limits of 10 kPa and 4 MPa, with a turbine inlet temperature of 500°C. The mass fraction of steam that condenses at the turbine exit is

A)0%
B)8%
C)14%
D)92%
E)86%

Answer 3

14%

Question 4

A steam power plant operates on the simple ideal Rankine cycle between the pressure limits of 10 kPa and 10 MPa, with a turbine inlet temperature of 500°C. The rate of heat transfer in the boiler is 800 kJ/s. Disregarding the pump work, the power output of this plant is

A)199 kW
B)1285 kW
C)470 kW
D)323 kW
E)800 kW

Question 5

Consider a combined gas-steam power plant. Water for the steam cycle is heated in a well-insulated heat exchanger by the exhaust gases that enter at 800 K at a rate of 110 kg/s and leave at 380 K. Water enters the heat exchanger at 200°C and 8 MPa and leaves at 350°C and 8 MPa. If the exhaust gases are treated as air with constant specific heats at room temperature, the mass flow rate of water through the heat exchanger becomes

A)22 kg/s
B)74 kg/s
C)28 kg/s
D)16 kg/s
E)110 kg/s

Question 6

An ideal reheat Rankine cycle operates between the pressure limits of 20 kPa and 10 MPa, with reheat occurring at 3 MPa. The temperature of steam at the inlets of both turbines is 500°C, and the enthalpy of steam is 3027 kJ/kg at the exit of the high-pressure turbine, and 2385 kJ/kg at the exit of the low-pressure turbine. Disregarding the pump work, the cycle efficiency is

A)26%
B)32%
C)57%
D)40%
E)60%

Question 7

Pressurized feedwater in a steam power plant is to be heated in an ideal open feedwater heater that operates at a pressure of 0.5 MPa with steam extracted from the turbine. If the enthalpy of feedwater is 215 kJ/kg and the enthalpy of extracted steam is 2442 kJ/kg, the mass fraction of steam extracted from the turbine is

A)5%
B)10%
C)19%
D)9%
E)29%

Question 8

Consider a steam power plant that operates on the regenerative Rankine cycle with one open feedwater heater. The enthalpy of the steam is 3140 kJ/kg at the turbine inlet, 2655 kJ/kg at the location of bleeding, and 2346 kJ/kg at the turbine exit. The net power output of the plant is 110 MW, and the fraction of steam bled off the turbine for regeneration is 0.172. If the pump work is negligible, the mass flow rate of steam at the turbine inlet is

A)326 kg/s
B)149 kg/s
C)806 kg/s
D)288 kg/s
E)139 kg/s

Question 9

Consider a simple ideal Rankine cycle. If the condenser pressure is raised while keeping turbine inlet state the same, (select the correct statement)

A)the turbine work output will increase.
B)the amount of heat rejected will decrease.
C)the cycle efficiency will decrease.
D)the moisture content at turbine exit will increase.
E)the pump work input will increase.

Question 10

Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures. If the steam is superheated to a higher temperature, (select the correct statement)

A)the turbine work output will decrease.
B)the amount of heat rejected will decrease.
C)the cycle efficiency will decrease.
D)the moisture content at turbine exit will increase.
E)the amount of heat input will increase.

Question 11

Consider a simple ideal Rankine cycle with a fixed condenser pressure. If the boiler pressure is raised while the turbine inlet temperature is held constant, (select the correct statement)

A)the turbine work output will decrease.
B)the amount of heat rejected will increase.
C)the cycle efficiency will decrease.
D)the moisture content at turbine exit will increase.
E)the pump work input will decrease.

Question 12

Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures and turbine inlet temperature. If the cycle is modified with ideal reheating, (select the correct statement)

A)the turbine work output will decrease.
B)the amount of heat rejected will increase.
C)the pump work input will decrease.
D)the moisture content at turbine exit will increase.
E)the amount of heat input will decrease.

Question 13

Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures and turbine inlet temperature. If the cycle is modified with ideal regeneration that involves one closed feed water heater, (select the correct statement per unit mass of steam flowing through the boiler)

A)the turbine work output will increase.
B)the amount of heat rejected will increase.
C)the cycle thermal efficiency will decrease.
D)the quality of steam at turbine exit will decrease.
E)the amount of heat input will decrease.