Carnot Cycle Limitations: What are the Limitations of Carnot Cycle?

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Questions and Answers

What is the primary limitation of the Carnot cycle in practical applications?

It is impractical to design a compressor that handles two phases (correct)

It is not efficient

It is not ideal for vapor power cycles

It is not suitable for steam power cycles

What is the primary purpose of the Rankine cycle?

To provide a model for vapor power cycles

To generate power using biomass

To convert heat into mechanical work (correct)

To operate a steam turbine system

What is the percentage of electric power generated by the Rankine cycle?

90% (correct)

80%

95%

100%

What is the typical working fluid used in the Rankine cycle?

Water Signup and view all the answers

What is the name of the Scottish polymath and Glasgow University professor who the Rankine cycle is named after?

William John Macquorn Rankine Signup and view all the answers

Why is it difficult to approximate the Carnot cycle in actual devices?

All of the above Signup and view all the answers

What is the primary purpose of a boiler in a Rankine cycle?

To generate steam from water Signup and view all the answers

What is the temperature of the steam turbine entry in a Rankine cycle?

565°C Signup and view all the answers

What is the efficiency of a modern coal-fired power station?

42% Signup and view all the answers

What is the primary difference between a water-tube boiler and a fire-tube boiler?

Direction of hot gases Signup and view all the answers

What is the name of the cycle that is limited by the high heat of vaporization of the working fluid?

Rankine cycle Signup and view all the answers

What is the origin of the word 'turbine'?

Greek Signup and view all the answers

What is the process that occurs in a steam turbine?

Isentropic expansion Signup and view all the answers

What is the result of the isobaric heat rejection process?

Heat is rejected from the system Signup and view all the answers

What is the work done by the turbine equal to?

H4 – H3 Signup and view all the answers

What is the net effect of the entire cycle?

Energy is conserved Signup and view all the answers

What happens to the entropy during the isentropic expansion process?

It remains unchanged Signup and view all the answers

What is the primary function of a condenser?

To condense a substance from its gaseous to its liquid state Signup and view all the answers

How are pumps classified?

According to the method they use to move the fluid Signup and view all the answers

What occurs during isentropic compression in a Rankine cycle?

The surroundings do work on the fluid, increasing its enthalpy and compressing it Signup and view all the answers

What is the purpose of a feedwater pump in a Rankine cycle?

To pump the liquid condensate from the condenser into the boiler Signup and view all the answers

What is the process called when the liquid condensate is heated from state 2 to state 3a?

Isobaric heat addition Signup and view all the answers

What is the change in entropy during isentropic compression in a Rankine cycle?

Entropy remains unchanged Signup and view all the answers

Study Notes

Heat Sources and Efficiency

Heat sources used in power plants include nuclear fission and combustion of fossil fuels like coal, natural gas, and oil.
The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid.
Steam turbine entry temperatures are around 565°C, while steam condenser temperatures are around 30°C.
Theoretical maximum Carnot efficiency for the steam turbine is 63%, while actual overall thermal efficiency for a modern coal-fired power station is around 42%.

Parts of a Rankine Cycle

Boiler

A closed vessel or arrangement of vessels and tubes, along with a furnace or heat source, to generate steam or vapor.
Types of boilers include:
- Water-tube boiler: water circulates in tubes exposed to fire and enveloped by hot gases.
- Fire-tube boiler: hot gases from a fire pass through one or more tubes running through a sealed container of water.

Turbine

A rotary mechanical device that extracts energy from fluid flow and converts it into useful work.
Has at least one moving part called a rotor assembly, with blades attached to a shaft or drum.

Condenser

A device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it.
Latent heat is given up by the substance, transferring to the condenser coolant.

Pump

A device that moves fluids (liquids or gases) or slurries by mechanical action.
Classified into three major groups: direct lift, displacement, and gravity pumps.

Rankine Cycle Process

Isentropic compression: liquid condensate is compressed adiabatically from state 1 to state 2 by centrifugal pumps.
Isobaric heat addition: constant-pressure heat transfer to the liquid condensate from an external source.
Isentropic expansion: steam from the boiler expands adiabatically from state 3 to state 4 in a steam turbine to produce work.
Isobaric heat rejection: constant-pressure process in which heat is rejected from the partially condensed steam.

Carnot Cycle vs. Rankine Cycle

Carnot cycle is the most efficient cycle operating between two specified temperature limits.
However, it is not suitable for steam power cycles due to turbine erosion, compressor design, and condensation control.
Rankine cycle is a more realistic model for vapor power cycles, describing the process of steam-operated heat engines in thermal power generation plants.

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Description

Learn about the reasons why the Carnot cycle is not a suitable model for steam power cycles, including issues with turbine erosion, compressor design, and condensation control. Understand the limitations of this efficient cycle operating between two specified temperature limits.

Carnot Cycle Limitations