38
CHAPTER FOUR
4 REVIEW OF THE VAPOUR POWER CYCLES
4.1 Introduction
All vapour power cycles have the following two characteristics in common;
1. The working fluid is a condensable vapour, which is in the liquid phase during part of
the cycle
2. The cycle consists of a succession of steady-flow processes with each process carried
out in a separate component specially designed for the purpose.
In the simplest form a steam power plant may be considered to consist of a heater or a boiler
and a prime mover (engine or turbine). In the heater, heat energy produced by the combustion
of fuel is transferred to the working medium (water, steam) whereas the purpose of the prime
mover is to enable the working medium to convert part of its heat energy into work.
QMechanical
Fuel QUseful
QLosses
Air
QLosses
The working medium
A fluid is capable of absorbing and discharging heat energy during one of the following
processes
1. A change in temperature (t)
2. A charge in chemical composition
3. A change in physical state
An example of energy transfer through a change in temperature is the operation of a gas
turbine where air (or gas) is compressed and then heated to a high temperature, then flowing to
a prime mover, it expands, does work and is cooled. In the case of change of physical state,
MPE 571E: POWER PLANT ENGINEERING
, 39
energy is transferred to or from the vapour prime mover during evaporation or condensation.
The advantages of using vapour as a working medium are two folds namely; less pumping
work than compression work and ease of condensing the exhaust steam
Requirements of vapours for vapour cycles.
1. Ample amounts should be available at low cost.
2. It should be non-corrosive and nontoxic
3. The pressure corresponding to maximum and minimum possible
temperatures should not be so high as to be commercially impracticable nor
so low as to render maintenance of vacuum difficult.
4. It's sensitive heat should be minor part of the total heat since the available
heat is contained in latent form.
5. There should be considerable reduction in volume upon condensation
Criteria for the comparison of cycles.
The choice of a power plant for a given purpose is determined largely by consideration of:-
1. Operating cost - which is a function of the overall efficiency of the plant.
2. Capital cost - which depends mainly on the size and complexity of the plant.
In general the efficiency can always be improved by adding to the complexity of the plant; so
that a suitable comprise between low operating and capital cost must be reached.
Plant Efficiency
Overall efficiency is expressed as a product of two efficiencies:-
(a) the combustion efficiency which is the part of energy in the fuel
transferred as heat energy to the working fluid
(b) cycle efficiency
w Q1 Q2
Q1 Q1 eq. 4.1
According to second law of Thermodynamic, the cycle which gives the highest efficiency is
the Carnot cycle in which
MPE 571E: POWER PLANT ENGINEERING
CHAPTER FOUR
4 REVIEW OF THE VAPOUR POWER CYCLES
4.1 Introduction
All vapour power cycles have the following two characteristics in common;
1. The working fluid is a condensable vapour, which is in the liquid phase during part of
the cycle
2. The cycle consists of a succession of steady-flow processes with each process carried
out in a separate component specially designed for the purpose.
In the simplest form a steam power plant may be considered to consist of a heater or a boiler
and a prime mover (engine or turbine). In the heater, heat energy produced by the combustion
of fuel is transferred to the working medium (water, steam) whereas the purpose of the prime
mover is to enable the working medium to convert part of its heat energy into work.
QMechanical
Fuel QUseful
QLosses
Air
QLosses
The working medium
A fluid is capable of absorbing and discharging heat energy during one of the following
processes
1. A change in temperature (t)
2. A charge in chemical composition
3. A change in physical state
An example of energy transfer through a change in temperature is the operation of a gas
turbine where air (or gas) is compressed and then heated to a high temperature, then flowing to
a prime mover, it expands, does work and is cooled. In the case of change of physical state,
MPE 571E: POWER PLANT ENGINEERING
, 39
energy is transferred to or from the vapour prime mover during evaporation or condensation.
The advantages of using vapour as a working medium are two folds namely; less pumping
work than compression work and ease of condensing the exhaust steam
Requirements of vapours for vapour cycles.
1. Ample amounts should be available at low cost.
2. It should be non-corrosive and nontoxic
3. The pressure corresponding to maximum and minimum possible
temperatures should not be so high as to be commercially impracticable nor
so low as to render maintenance of vacuum difficult.
4. It's sensitive heat should be minor part of the total heat since the available
heat is contained in latent form.
5. There should be considerable reduction in volume upon condensation
Criteria for the comparison of cycles.
The choice of a power plant for a given purpose is determined largely by consideration of:-
1. Operating cost - which is a function of the overall efficiency of the plant.
2. Capital cost - which depends mainly on the size and complexity of the plant.
In general the efficiency can always be improved by adding to the complexity of the plant; so
that a suitable comprise between low operating and capital cost must be reached.
Plant Efficiency
Overall efficiency is expressed as a product of two efficiencies:-
(a) the combustion efficiency which is the part of energy in the fuel
transferred as heat energy to the working fluid
(b) cycle efficiency
w Q1 Q2
Q1 Q1 eq. 4.1
According to second law of Thermodynamic, the cycle which gives the highest efficiency is
the Carnot cycle in which
MPE 571E: POWER PLANT ENGINEERING
