Vapour Power Cycles

Question 1

Definition of actual cycle

Answer 1

The real cycle that operates, but is complexed with irreversibilities that make it difficult to analyze

Question 2

Definition of ideal cycle

Answer 2

Resembles the actual cycle closely but is made up only of internally reversible processes

Question 3

_________ such as ___________ have the highest thermal efficiency of all heat engines operating between the same temperature levels

Answer 3

Reversible cycles, Carnot cycle

Question 4

Unlike ideal cycles, Carnot cycle are ____________ and unsuitable as a _________ model.

Answer 4

totally reversible, idealistic

Question 5

The ideal cycles are ________ reversible, but, unlike the Carnot cycle, they are not necessarily _________ reversible.

Answer 5

internally, externally

Question 6

The thermal efficiency of an ideal cycle, in general, is _____ than that of a totally reversible cycle operating between the same temperature limits.

Answer 6

less

Question 7

Rank the cycles from least to most thermal efficiency

Answer 7

Actual cycle < internally reversible cycle < totally reversible cycle

Question 8

Which cycle is the totally reversible cycle?

Answer 8

Carnot cycle

Question 9

Which cycle is the internally reversible cycle?

Answer 9

Ideal cycle

Question 10

State the three idealizations and simplifications in the analysis of power cycles

Answer 10

1. The cycle does not involve any friction
2. All expansion and compression processes take place in a quasi-equilibrium manner.
3. The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible

Question 11

The Carnot cycle is composed of four totally reversible processes

Answer 11

• isothermal heat addition
• isentropic expansion
• isothermal heat rejection
• isentropic compression

Question 12

State the one similarity between ideal and actual cycles

Answer 12

Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system

Question 13

The Carnot cycle is the ____ efficient cycle operating between two specified temperature limits but it is not a suitable model for ____________.

Answer 13

most, power cycles

Question 14

Many of the impracticalities associated with the Carnot cycle can be eliminated by :
The cycle that results is the _______ cycle.

Answer 14

1. Superheating the steam in the boiler
2. Condensing it completely in the condenser

Rankine

Question 15

The ideal Rankine cycle does not involve any

Answer 15

internal irreversibilities

Question 16

The actual vapor power cycle differs from the ideal Rankine cycle as a result of _______ in various components. List the two commons.

Answer 16

Irreversibilities

1) fluid friction
2) heat loss to the surroundings

Question 17

What is the process in a pump of an ideal Rankine cycle?

Answer 17

Isentropic compression

Question 18

What is the process in a boiler of an ideal Rankine cycle?

Answer 18

Constant pressure heat addition

Question 19

What is the process in a turbine of an ideal Rankine cycle?

Answer 19

Isentropic expansion

Question 20

What is the process in a condenser of an ideal Rankine cycle?

Answer 20

Constant pressure heat rejection

Question 21

State 2 ways in which the efficiency of the Rankine cycle can be increased

Answer 21

1) Increase the average temperature at which heat is transferred to the working fluid in the boiler (TH)
2) Decreasing the average temperature at which heat is rejected from the working fluid in the condenser (TL)

Question 22

What is the side effect that comes from lowering the condenser pressure on the ideal Rankin cycle?

Answer 22

Increases the moisture content

Question 23

What is the side effect that comes from superheating the steam to high temperatures on the ideal Rankin cycle?

Answer 23

Decreases the moisture content of the steam at the turbine exit, which is desirable

Question 24

What is the side effect that comes from increasing the boiler pressure on the ideal Rankin cycle? How can it be corrected?

Answer 24

Cycle shifts to the left, moisture content of steam at the turbine exit increases
By reheating the steam (reheat Rankine)

Question 25

How can we take advantage of the increased efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stages of the turbine?

Answer 25

1) Superheat the steam to very high temperatures.

2) Expand the steam in the turbine in two stages, and reheat it in between

Question 26

A single reheat can take up to how many percentages of improvement of cycle efficiency?

Answer 26

4% to 5%

Question 27

The meaning of reheat temperatures

Answer 27

Very close or equal to the turbine inlet temperature

Question 28

Meaning of optimum reheat pressure

Answer 28

1/4 of maximum cycle pressure