This document discusses vapor power cycles. It provides details on the classification and features of vapor power cycles. Specifically, it notes that vapor power cycles use a working substance that does not come into contact with fuel, allowing for easier achievement of isothermal processes. It then discusses the Rankine cycle in detail, outlining the key processes and assumptions made in analyzing vapor power cycles. The document also summarizes the effects of various parameters like condenser pressure and boiler pressure on cycle efficiency. Finally, it briefly introduces regenerative and reheat cycles which aim to improve upon the basic Rankine cycle.

1. Vapor Power cycles
Power cycles are used for generating power for
either for producing electricity or in automobiles
• Classification of power cycles
1. Gas power cycles:
 IC engines
 Gas turbine cycles.
2. Vapor power cycles

2. Features of Vapor power cycles
• Vapor power cycles uses the working substance
which does not comes in contact the fuel. So,
impurities in the fuel will not affect the working
substance or the machine through which the hot
fluid has to pass in doing work.
• Secondly, in gas power cycles it is extremely difficult
to achieve an isothermal process where as this can
be easily achieved in a vapor power cycle using
constant pressure phase change process.
• Vapor power cycle has the further advantage that it
can use high speed, light weight turbines to produce
work output instead of the bulky reciprocating piston
engines that are used in IC engines.

3. • Another advantage of vapor power cycle over
the Brayton cycle is that compression work is
very small which can be neglected in
comparison with the net work output.
• But vapor power cycles suffer from poor
thermal efficiencies as compared to gas power
cycles. High efficiencies in vapor power cycles
can be achieved only by using very high
pressure or super-critical pressure system with
multi stage feed water heating and reheating.

4.  Cycle efficiency : Work output/heat supplied
Relative efficiency: ratio of actual to ideal efficiency
 Work ratio and back work ratio: - In every cycle
there are processes involving both positive and
negative work. The net work in the cycle,
W net = W positive –W negative.
 The ratio of network to the positive work of
the cycle is called as work ratio.
rw = Work ratio = Wnet / Wpositive
The ratio of W negative to W positive is called as back
work ratio

5. Specific steam consumption or steam rate: - It is
defined as the mass flow rate of steam per unit
power developed (kWh). It may be expressed as
reciprocal of network
 SSC = 1/Wnet = 3600/Wnet (kg/kWh)
 Higher SSC, greater the size of the plant, thus it
governs the capital cost
 Lower specific steam consumption is always
desired.

6. Carnot vapor power cycle
The ideal vapor cycle will be the Carnot cycle
comprising of two reversible isothermal and
two reversible adiabatic processes.
As the working substance changes its phase
the two isothermal processes are easily
attainable by boiling the liquid and
condensing the vapor.

7. T
s
TL
P = C
2
3
1 4
P = C
TH

8. Draw backs of the Carnot vapor power cycle: -
It is found that there are many difficulties in the
application of Carnot vapor power cycle. The major
difficulties are;
(1) The design and control of a partial condenser, that
would terminate condensation at state-1 is difficult.
(2) It is also difficult to design a compressor to handle a
mixture of largely liquid and partly vapor at state-1 and
discharge it as saturated liquid at state 2.
(3) Work of compression is large compared to the work of
turbine; hence back work ratio is large, and low work
ratio. Hence actual  is low.
(4) The turbine that takes in saturated steam at state 3
produces exhaust steam at state 4 with low quality.
This causes pitting and hence corrosion of turbine .

9. Assumptions made in the analysis of Vapor power cycles
(i) The expansion process in the turbine and the
compression process in the pump are isentropic.
(ii) There are no pressure losses in the piping
connecting various components as well as in the heat
exchangers like boiler, condenser, re-heaters and
feed water heaters.
(iii) Changes in kinetic and potential energies of the
working fluid as it flows through the various
components are negligible.
(iv) Fluid flow is steady and one-dimensional.

10. • In a simple Rankine cycle the steam is completely
condensed in a condenser and then pumped to the boiler
in the liquid state.
• The steam can be super-heated to obtain a better quality
at the end of expansion. This results in increased life of
the turbine blade and SSC decreases.
• Instead of isothermal heat addition, constant pressure
heat addition is being done.
• For the process 1-2:
Wp=(h2 – h1)=vf1(P2–P1)x100 kJ/kg if P2 and P1are in bars.
• For the process 2-3 : - HS = (h3 – h2) kJ/kg
• For the process 3-4: - WT = (h3 – h4 ) kJ/kg
• For the process 4—1: - HR = (h4 – h1) kJ/kg

11. Rankine cycle
• It consists of following processes:
• 1– 2 Reversible adiabatic
pumping of condensed steam
• 2 – 3 Constant pressure heat
addition in the boiler.
• 3 – 4 Reversible adiabatic
expansion process in the turbine
• 4-1 Constant pressure
condensation in the condenser

12. • For the process 1-2: - Wp = (h2– h1) = vf1 (P2– P1) x 100 kJ/kg
if P2 and P1 are in bars.
• For the process 2-3 : - HS = (h3 – h2) kJ/kg
• For the process 3-4: - WT = (h3 – h4) kJ/kg
• For the process 4—1: - HR = (h4 – h1) kJ/kg

13. Effect of different parameters on Rankine cycle Efficiency
(i) Condenser pressure
(ii)Boiler pressure
(iii)Superheating of steam

14. Decreasing the Condenser pressure
 Heat rejection rate decreases
 Heat added slightly increases
 Hence efficiency increases
 On decreasing the condenser
pressure specific volume
increases which increases the size
of the condenser
 If the pressure is less than
atmospheric, air the leaks into
the condenser. Hence
continuously air has to be
removed which increases the cost
 Quality of the steam decreases
 Hence optimum condenser
pressure will be maintained

15. Boiler pressure
When boiler pressure is increased
average temperature of heat addition
increases,
Heat supplied may be equal in both
cases.
Heat rejection rate is reduced for
increased boiler pressures.
hence efficiency increases.
The disadvantage in increasing the
boiler pressure is the quality of
steam, which is poor resulting in
increased corrosion of turbine blades.
Optimum pressure depends on the
metallurgical conditions

16. Superheating the steam
If the steam is heated above the
saturation temperature then it is
called superheating
The super heating results in
increase in average temperature of
heat addition and temperature at
which heat is rejected remains
constant. Thus, the efficiency with
super heating increases.
 Heat added will be increased and
Heat rejected also will slightly
increase.
Quality of the steam increases
Specific steam consumption
decreases or size of the turbine
decreases.
More dangerous and costlier.

17. Reheat cycle
• The reheat cycle aims at
 Attaining high thermal efficiency by utilizing
high boiler pressures and super heating,
 Eliminating the problem of excessive moisture
content in the exhaust steam by reheating the
steam
• In a reheat cycle the expansion of the steam
takes place in two stages:
 The high-pressure stage and low- pressure
stage.
 The steam expands in the high-pressure stage
to some intermediate pressure and then
reheated in a separate reheat coil
approximately to the original temperature.
 It then enters the low - pressure stage turbine
and expands to the condenser pressure as
usual.

18. Reheat Cycle
Advantages: - Efficiency increases by a small margin but quality of
steam improves considerably and network output increases. The
specific steam consumption decreases, and hence the smaller plant.

19. Regenerative cycle
• In a simple Rankine cycle significant amount of heat is
added for sensible heating of water, which results in
lower thermal  compared to Carnot cycle.
• The average temperature of heat addition can be
maintained at a higher level by eliminating or reducing
the heat added at lower temperatures. This could be
possible by making use of regenerative cycle.
• The regenerative principle involves taking heat from one
part of the cycle and adding the same in another part.
• It means the working substance is heated in one part of
the cycle by exchanging heat with the same substance,
which gets cooled in another part of the cycle.

20. Ideal regenerative cycle: -
 In an ideal regenerative cycle feed
water leaving the pump is circulated
around the turbine casing in counter
flow directions compared to expanding
steam.
 Then heat exchange takes place
between water and steam.
 Water gets heated from 2 to 3 and
steam gets cooled along 4 – 5’.
Heat gained by water is equal to heat
lost of steam. Area 1 – 2 – 3 – 1’ is equal
to 5 –5’ – 4 – 4’. The thermal efficiency
of an ideal regenerative cycle is equal to
Carnot cycle efficiency.

21. Ideal Regenerative cycle
Problems with ideal regenerative cycle: -
The ideal regenerative cycle is impractical because
 It is not physically possible to arrange heat transfer between
water flowing around the turbine casing and steam expanding
internally.
 Even if it is possible, heat transfer could never be reversible.
Due to such a regenerative cycle the exhaust steam quality
will be very poor which is most undesirable

22. Practical regenerative cycle
• In a practical regenerative cycle nearly the same objective
is achieved by heating feed water with the help of steam
extracted or bled from the various intermediate stages of
the turbine called as regenerative feed water heating.
•There are two types of regenerative feed water heating.
(i) Open feed water heating.
(ii) Closed feed water heating.
• In an open feed water heating the extracted steam is
mixed with feed water, both are at same pressure.
• In a closed feed water heater, there is no mixing and heat
exchange takes place between the two fluids, which can be
at different pressures.
• Regenerative heating helps in improving thermal efficiency.

23. 1. Open feed water Heater

25. 2. Closed feed water heater:
(a) Condensate back to high pressure line
(b) Condensate back to low pressure line

26. Properties of ideal liquid
1. The specific heat capacity of liquid (CPL) should be small
or saturation line should be steep. The heat required to
bring the liquid to boiling point will be small.
2. Enthalpy of vaporization should be large so that specific
steam consumption (SSC) is less and hence smaller plant
size for a given power output. The specific volume
should be small or density should be large.
3. The saturated vapor line should be steep so that dryness
fraction after expansion can be maintained above 0.9
without going for superheating.
4. The saturation pressure at condenser temperature
should be slightly more than atmospheric, so that no
vacuum is necessary in the condenser. This reduces the
leakage of air into condenser.
5. The fluid should be cheap, chemically stable, non-toxic,
non-corrosive, non-inflammable and non-explosive.

27. Binary vapor cycle
• Not a single substance posses all the desired properties
required for working substance in vapor power cycles.
• Thermal benefits can be obtained by using different
fluids at different temperature range of the cycle.
• The cycles with two fluids are referred as binary cycles.
• The primary fluid is usually steam used at low
temperature end hence it is called as bottomer fluid
• The secondary fluid used at the high temperature end of
the cycle is called as topper fluid.
• Possible plant using mercury as a topper fluid and
steam as bottomer fluid is called Hg -steam binary
vapor power cycle.

28. • The Hg has a critical temperature
(14600C,Pcri =1080 bar) well above the
metallurgical limit of about 6000C.
• The most of the heat from the external
source can be transferred at maximum
temperature of the cycle.
• The boiler pressure would be about 23 bar
at 6000C.
• The Hg condenser acts as the steam boiler.
• In the binary Hg-Steam cycle, topper cycle
consists of an Hg boiler, a Hg turbine and a
Hg condenser.
• The Hg condenser acts as a steam boiler.
• The fluids flow separately in its own circuit.

29. • The heat rejected by the Hg is used to boil the water and
resulting steam may be super heated and then expanded
in the steam turbine
• The ideal  will be comparatively high.
• The addition of hg cycle to the steam cycle results in a
marked increase in mean effective temperature and thus
the efficiency increases.
• The maximum pressure in the cycle remains at relatively
low value.

30. Flow diagram and T-s plot for Hg- steam Binary cycle
6’

31. Thermodynamic analysis of binary cycle
• Let m represents the mass of HG circulated for every one kg of
steam circulated in the steam circuit.