CHEE2325 – Lecture 8
Vapour power cycles:
- These are cycles in which the working fluid is alternatively vapourised and
condensed.
- The most common working fluid for vapour power cycles is steam, due to its
low cost, availability, and high enthalpy of vapourisation.
- Depending on the type of fuel used to supply heat to the steam, steam power
plants are commonly referred to as coal plants, nuclear plants, or natural gas
plants. The steam goes through the same basic cycle in all of them and they
all can be analysed in the same way.
Rankine cycle:
- The Rankine cycle is the ideal cycle for vapour power plants and involves four
steps in reversible steady state processes:
o 1-2: Isentropic compression in a pump.
o 2-3: Constant pressure heat addition in a boiler.
o 3-4: Isentropic expansion in a turbine.
o 4-1: Constant pressure heat rejection in a condenser.
W net Q
ηth = =1− out W net =Q¿ −Qout Q¿ = ^ ^ ¿= H
H out − H ^ −^
3
^ ¿− H
H 2 , Qout = H ^ =^
out H4−^
H1
Q¿ Q¿
W ¿= ^
H ¿− ^ ^ 2− ^
H out = H H 1 , W out = ^ ^¿= H
H out − H ^3− ^
H 4W ¿ = V ^1 ( P2−P1 )
- Steam power plants are globally responsible for the production of most
electric power. Therefore, small improvements in thermal efficiency can lead
to significant savings from the fuel/energy requirements.
- Thermal efficiency can be improved by maximising T ¿, avg ., and minimizing
T out .avg . of the working fluid.
- This can be achieved by:
o Lowering the condenser pressure to decrease T out .avg ..
o Superheating the steam to high temperatures to increase T ¿, avg ..
o Increasing the boiler pressure to increase T ¿, avg ..
- The area enclosed within a process curve is the net heat transfer.
Vapour power cycles:
- These are cycles in which the working fluid is alternatively vapourised and
condensed.
- The most common working fluid for vapour power cycles is steam, due to its
low cost, availability, and high enthalpy of vapourisation.
- Depending on the type of fuel used to supply heat to the steam, steam power
plants are commonly referred to as coal plants, nuclear plants, or natural gas
plants. The steam goes through the same basic cycle in all of them and they
all can be analysed in the same way.
Rankine cycle:
- The Rankine cycle is the ideal cycle for vapour power plants and involves four
steps in reversible steady state processes:
o 1-2: Isentropic compression in a pump.
o 2-3: Constant pressure heat addition in a boiler.
o 3-4: Isentropic expansion in a turbine.
o 4-1: Constant pressure heat rejection in a condenser.
W net Q
ηth = =1− out W net =Q¿ −Qout Q¿ = ^ ^ ¿= H
H out − H ^ −^
3
^ ¿− H
H 2 , Qout = H ^ =^
out H4−^
H1
Q¿ Q¿
W ¿= ^
H ¿− ^ ^ 2− ^
H out = H H 1 , W out = ^ ^¿= H
H out − H ^3− ^
H 4W ¿ = V ^1 ( P2−P1 )
- Steam power plants are globally responsible for the production of most
electric power. Therefore, small improvements in thermal efficiency can lead
to significant savings from the fuel/energy requirements.
- Thermal efficiency can be improved by maximising T ¿, avg ., and minimizing
T out .avg . of the working fluid.
- This can be achieved by:
o Lowering the condenser pressure to decrease T out .avg ..
o Superheating the steam to high temperatures to increase T ¿, avg ..
o Increasing the boiler pressure to increase T ¿, avg ..
- The area enclosed within a process curve is the net heat transfer.
