Chapter 10

Question 1

The Carnot cycle is the most efficient cycle…

Answer 1

Operating between two specified temperature limits

Question 2

Carnot cycles are not suitable models for power cycles because…

Answer 2

1. Limiting the heat transfer processes to 2-phase systems severely limits the maximum temperature that can be used in the cycle (374C for water)
2. The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear
3. It it not practical to design a compressor that handles 2 phases

Question 3

The ideal cycle for vapor power cycles

Answer 3

Rankine cycle

Question 4

Ranking cycle creation

Answer 4

Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser

Question 5

The ideal Rankine cycle does not involve any ____

Answer 5

Internal irreversibilities

Question 6

The efficiency of power plants in the U.S. if often expressed in terms of ____

Answer 6

Heat rate

Question 7

Heat rate

Answer 7

The amount of heat supplied to generate 1kWh of electricity

Question 8

The actual vapor power cycle differs from the ideal Rankine cycle as a result of…

Answer 8

Irreversibilities in various components

Question 9

Actual vapor power cycle most common sources of irreversibilities

Answer 9

Fluid friction and heat loss

Question 10

The basic idea behind all the modifications to increase the thermal efficiency of a power cycle is the same:

Answer 10

Increase the average temperature at which heat is transferred to the working fluid in the boiler or decrease the average temperature at which heat is rejected from the working fluid in the condenser

Question 11

How can we increase the efficiency of the Rankine cycle

Answer 11

Lowering the Condenser Pressure (Lowers Tlow,avg)

Superheating the Steam to High Temperatures (Increases Thigh,avg)

Increasing the Boiler Pressure (Increases Thigh,avg)

Question 12

Lowering the condenser pressure _____ the moisture content of the steam at the final stages of the turbine

Answer 12

Increases

Question 13

To take advantage of the increased efficiencies at low pressures, the condensers of steam power plants usually operate…

Answer 13

Below the atmospheric pressure

Question 14

Superheating the steam to high temperatures

Answer 14

Both the net work and heat input increase as a result of superheating the steam to a higher temperature

The temperature is limited by metallurgical considerations. Presently the highest steam temperature allowed at the turbine inlet is about 620C

Question 15

Superheating to higher temperatures ____ the moisture content of the steam at the turbine exit, which is desirable

Answer 15

Decreases

Question 16

Increasing the boiler pressure

Answer 16

For a fixed turbine inlet temperature, the cycle shifts to the left and the moisture content of steam at the turbine exit increases. This side effect can be corrected by reheating the steam

Question 17

Today many modern steam power plants operators at…

Answer 17

Supercritical pressures (P > 22.06 MPa)

Thermal efficiencies of about 40% for fossil-fuel plants and 34% for nuclear plants