Question

1. The thermal efficiency of a Rankine power cycle may be improved by i. Superheating the steam ii. Reheating the steam between high and lower pressure sections of the turbine iii. Regenerative Feedwater Heating iv. Insulating the turbine and decreasing the entropy production during the expansion process v. Incorporating a Rankine cycle power system as part of a cogeneration system a. Items i), ii), and iv) only b. Items i), iii), and v) only c. Items ii), iv) and v) only d. All of these e. None of these

2. The back work ratio is … a. The ratio of the compressor outlet to inlet pressure in a vapor compression power system. b. The ratio of the pump work (power) input divided by the turbine work (power) output in a Rankine cycle power system. c. The ratio of the inlet pressure to the outlet pressure in a steam turbine. d. The ratio of the inlet pressure to the outlet pressure in the feedwater pump system of a Rankine cycle power system.

3. For the Rankine cycle process where steam expands through the turbine, in a realistic process, the entropy at the exit is … a. Greater than the entropy at the inlet. b. Equal to the entropy at the inlet. c. Less than the entropy at the inlet. d. Has no relationship to the entropy at the inlet.   e. None of these answers is correct.

4. Regenerative feedwater heaters may be … a. Devices where natural gas is used to heat feedwater to prevent freezing under cold conditions.   b. Open devices where the steam and the water being heated are at the same pressure, c. Closed devices where the steam and the water being heated may be at different pressures and do not mix, d. Open or closed devices, where both have their advantages and applications. e. Devices where steam is diverted, passed back into the steam generator, and then sent back into the turbine.   f. None of the above.

5. With regenerative feedwater heating, a powerplant will not have more than one stage of feedwater pump (i.e., each feedwater pump will take in water at condenser pressure and deliver water at steam generator/turbine inlet pressure). a. True b. False

6. In a power plant, the heat rejected from the condenser … a. Is not a significant amount of heat, is rejected into the surroundings, and is not a concern.   b. Is a significant amount of heat and is captured to turn the main turbine.   c. Is a significant amount of heat, is rejected into the surroundings, and can change the local environment. d. Heat is not rejected in the condenser.

7. Deaeration is needed in systems using water as a working fluid to remove air from the water and to minimize corrosion.   a. True b. False

8. A closed feedwater heater may be used for deaeration. a. True b. False
9. In a reciprocating power system,   i. Material flows at a constant rate through the device and passes through a turbine to produce shaft power output, ii. Material does not flow at a constant rate through every section of the device.   iii. Power is produced at all times. iv. Power is produced only during part of the cycle in each section of the device and is not produced uniformly at every instant.   v. The power unit consists of one or more piston and cylinder sections with intake and exhaust valves and where fresh fuel and air are taken in during one part of the process, exhaust gases are ejected during another part of the process, and at other times the cylinder is closed off from the intake and exhaust sections (manifolds).   vi. The power unit consists of a compressor, a burner section, and a turbine.   a. Items i), iii), and vi) are correct. b. Items ii) and iii) are correct. c. Items ii), iv) and v) are correct. d. Items ii), iv), and vi) are correct. e. None of these combinations are correct.

10. The Otto cycle model is used with … a. Reciprocating internal combustion engines where the fuel-air mixture is ignited by a spark. b. Reciprocating internal combustion engines where the fuel-air mixture is ignited by high pressures in the cylinders. c. Internal combustion engines with continuous flow of fuel and air (i.e., gas turbine engines). d. External combustion “hot air” engines. e. Vapor compression refrigeration.

11. In an air standard analysis, we pretend that the substance in an engine is pure air, and we analyze this as if energy is put into the air from the outside and, later, waste heat is removed from the air. a. True b. False

Answer 1

Option D is the correct choice among all. Thermal efficiency of a Rankine cycle can be improved in the all ways given. Superheating the steam increases the area under the process curve in Ts diagram. Hence total workdone is increased, thereby increasing thermal efficiency.

Reheat Rankine cycle is a practical solution to moisture problem. Thereby increasing efficiency.

In regeneration, steam bleed from the turbine is used to heat feedwater. It increases cycle efficiency.

Decreasing the entropy generation always increases efficiency.

The main purpose of cogeneration is to increase efficiency.

2. Back work ratio is the ratio of compressor work to the turbine work in the gas turbine power plant.

This also holds good for steam power plant where an ideal cycle is Rankine cycle.

Option b is the correct choice.

3. a. Entropy at exit is greater than the entropy at the entry.

This is due to irreversibilities in the turbine.

4. d. Both open and closed devices.