Vapour Power Cycles Flashcards
Definition of actual cycle
The real cycle that operates, but is complexed with irreversibilities that make it difficult to analyze
Definition of ideal cycle
Resembles the actual cycle closely but is made up only of internally reversible processes
_________ such as ___________ have the highest thermal efficiency of all heat engines operating between the same temperature levels
Reversible cycles, Carnot cycle
Unlike ideal cycles, Carnot cycle are ____________ and unsuitable as a _________ model.
totally reversible, idealistic
The ideal cycles are ________ reversible, but, unlike the Carnot cycle, they are not necessarily _________ reversible.
internally, externally
The thermal efficiency of an ideal cycle, in general, is _____ than that of a totally reversible cycle operating between the same temperature limits.
less
Rank the cycles from least to most thermal efficiency
Actual cycle < internally reversible cycle < totally reversible cycle
Which cycle is the totally reversible cycle?
Carnot cycle
Which cycle is the internally reversible cycle?
Ideal cycle
State the three idealizations and simplifications in the analysis of power cycles
- The cycle does not involve any friction
- All expansion and compression processes take place in a quasi-equilibrium manner.
- The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible
The Carnot cycle is composed of four totally reversible processes
- isothermal heat addition
- isentropic expansion
- isothermal heat rejection
- isentropic compression
State the one similarity between ideal and actual cycles
Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system
The Carnot cycle is the ____ efficient cycle operating between two specified temperature limits but it is not a suitable model for ____________.
most, power cycles
Many of the impracticalities associated with the Carnot cycle can be eliminated by :
The cycle that results is the _______ cycle.
- Superheating the steam in the boiler
- Condensing it completely in the condenser
Rankine
The ideal Rankine cycle does not involve any
internal irreversibilities
The actual vapor power cycle differs from the ideal Rankine cycle as a result of _______ in various components. List the two commons.
Irreversibilities
1) fluid friction
2) heat loss to the surroundings
What is the process in a pump of an ideal Rankine cycle?
Isentropic compression
What is the process in a boiler of an ideal Rankine cycle?
Constant pressure heat addition
What is the process in a turbine of an ideal Rankine cycle?
Isentropic expansion
What is the process in a condenser of an ideal Rankine cycle?
Constant pressure heat rejection
State 2 ways in which the efficiency of the Rankine cycle can be increased
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)
What is the side effect that comes from lowering the condenser pressure on the ideal Rankin cycle?
Increases the moisture content
What is the side effect that comes from superheating the steam to high temperatures on the ideal Rankin cycle?
Decreases the moisture content of the steam at the turbine exit, which is desirable
What is the side effect that comes from increasing the boiler pressure on the ideal Rankin cycle? How can it be corrected?
Cycle shifts to the left, moisture content of steam at the turbine exit increases
By reheating the steam (reheat Rankine)
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?
1) Superheat the steam to very high temperatures.
2) Expand the steam in the turbine in two stages, and reheat it in between
A single reheat can take up to how many percentages of improvement of cycle efficiency?
4% to 5%
The meaning of reheat temperatures
Very close or equal to the turbine inlet temperature
Meaning of optimum reheat pressure
1/4 of maximum cycle pressure
