UNIT-2

VAPOUR POWER CYCLES

A.V.N.S. KIRAN
ACADEMIC CONSULTANT
DEPARTMENT OF MECHANICAL ENGINEERING
SV UNIVERSITY, TIRUPATI
 Introduction

Definition of cycle:
• Cycle is defined as a repeated series of operations occurring in a certain order
• It may be repeated by repeating the process in the same order
• The cycle may be of imaginary perfect engine or actual engine
• The former is called ideal engine and later called actual engine
• In ideal cycle all accidental heat losses are prevented and the working substance is
assumed to behave like a perfect working substance
 Air Standard Cycle

• The efficiency of the engine using air as the working medium is known as air
standard efficiency
• To compare the effects of different cycles it is of paramount that the that the effect
of calorific value of the fuel
• The actual efficiency of the cycle is always less than the air standard efficiency of
the cycle under ideal conditions
 Assumptions

• The gas in the engine cylinder is a perfect gas ie.., it obeys the gas laws and ands
has constant specific heats
• The physical constants of the gas in the cylinder are the same as those of air at
moderate temperatures
• The compression and expansion processes are adiabatic and they take place with
out internal friction ie, these process are isentropic
• No chemical reaction takes place in cylinder
• Heat is supplied or rejected by bringing a hot body or a cold body in contact with
cylinder at appropriate points during the process
• The cycle is considered closed with the same air always remaining in the cylinder
to repeat the cycle
 Vapor Power Cycle

• Vapor power cycles are used in steam power plants.

• In a power cycle heat energy (released by the burning of fuel) is converted into
work (shaft work), in which a working fluid repeatedly performs a succession of
processes.
• In a vapor power cycle, the working fluid is water, which undergoes a change of
phase.
Simple Steam Power Plant
• Figure shows a simple steam power plant working on the vapour power cycle.
• Heat is transferred to the water in the boiler (QH) from an external source.
(Furnace, where fuel is continuously burnt)
• to raise steam, the high pressure high temperature steam leaving the boiler expands
in the turbine to produce shaft work (WT),
• the steam leaving the turbine condenses into water in the condenser (where cooling
water circulates), rejecting heat (QL), and
• then the water is pumped back (WP) to the boiler.
• Since the fluid is undergoing a cyclic process, the net energy transferred as heat
during the cycle must equal the net energy transfer as work from the fluid.
Carnot Cycle
Carnot cycle on T-s and p-V diagrams. It consists of (i) two constant pressure operations (4-1) and (2-3) and (ii)
two frictionless adiabatics (1-2) and (3-4). These operations are discussed below :
1. Operation (4-1). 1 kg of boiling water at temperature T1 is heated to form wet steam of dryness fraction x1.
Thus heat is absorbed at constant temperature T1 and pressure p1 during this operation.
2. 2. Operation (1-2). During this operation steam is expanded isentropically to temperature T2 and pressure p2.
The point ‘2’ represents the condition of steam after expansion.
3. 3. Operation (2-3). During this operation heat is rejected at constant pressure p2 and temperature T2. As the
steam is exhausted it becomes wetter and cooled from 2 to 3.
4. 4. Operation (3-4). In this operation the wet steam at ‘3’ is compressed isentropically till the steam regains its
original state of temperature T1 and pressure p1. Thus cycle is completed.
Heat supplied at constant temperature T1 [operation (4-1)] = area 4-1-b-a = T1 (s1 – s4) or T1 (s2 – s3).
Heat rejected at constant temperature T2 (operation 2-3) = area 2-3-a-b = T2 (s2 – s3).
Since there is no exchange of heat during isentropic operations (1-2) and (3-4)
Net work done = Heat supplied – heat rejected
= T1 (s2 – s3) – T2 (s2 – s3)
= (T1 – T2) (s2 – s3).
Carnot cycle
 Limitations

Though Carnot cycle is simple (thermodynamically) and has the highest thermal efficiency for given
values of T1 and T2, yet it is extremely difficult to operate in practice because of the following
reasons :
• 1. It is difficult to compress a wet vapour isentropically to the saturated state as required by the
process 3-4.
• 2. It is difficult to control the quality of the condensate coming out of the condenser so that the state
‘3’ is exactly obtained.
• 3. The efficiency of the Carnot cycle is greatly affected by the temperature T1 at which heat is
transferred to the working fluid. Since the critical temperature for steam is only 374°C, therefore, if
the cycle is to be operated in the wet region, the maximum possible temperature is severely limited.
• 4. The cycle is still more difficult to operate in practice with superheated steam due to the necessity
of supplying the superheat at constant temperature instead of constant pressure (as it is customary).
In a practical cycle, limits of pressure and volume are far more easily realized than limits of
temperature so that at present no practical engine operates on the Carnot cycle, although all modern
cycles aspire to achieve it.
RANKINE CYCLE
Define
The rankine cycle is modification of carnot
cycle.
When the carnot cycle had been issued it had
some mistakes so Mr.Rankine get the solution
and give the updated cycle.
So, this cycle is called the rankine cycle.
Principle
• It works on the principle of heat
engines which converts chemical
energy of fuel in thermal energy
for the generation of steam.
Construction
There are main 4 components of this cycle:
1) Steam boiler
2) Steam turbine
3) Steam condenser
4) Feed pump
Rankine Cycle
The Rankine cycle is shown in Fig.
It comprises of the following processes :
Process 1-2 : Reversible adiabatic expansion in the turbine (or steam engine).
Process 2-3 : Constant-pressure transfer of heat in the condenser.
Process 3-4 : Reversible adiabatic pumping process in the feed pump.
Process 4-1 : Constant-pressure transfer of heat in the boiler.
Fig. 12.3 shows the Rankine cycle on p-v, T-s and h-s diagrams (when the saturated steam enters
the turbine, the steam can be wet or superheated also).

ηRankine =
Working
This process is starts with feed pump. Feed pump supplies
the water in necessary amount to the steam boiler.
In this device, heat is supplied for the generation of steam
from supplied water.
When the steam is generated, it is transferred to the steam
turbine and turbine starts to rotate and give the work done.
After this the steam is transferred to the steam condenser,
where the heat is rejected and steam is converted into hot
water and it is converted into cool water which is supplied
to the pump.
And cycle repeats again………
Energy Analysis
h1=hf low pressure (saturated liquid)
Wpump (ideal)=h2-h1=vf(Phigh-Plow)
vf=specific volume of saturated liquid at low pressure
Qin=h3-h2 heat added in boiler (positive value)
Rate of heat transfer = Q*mass flow rate
Usually either Qin will be specified or else the high
temperature and pressure (so you can find h3)
Qout=h4-h1 heat removed from condenser (here h4 and h1
signs have been switched to keep this a positive value)
Wturbine=h3-h4 turbine work
Power = work * mass flow rate
h4@ low pressure and s4=s3
Efficiency
Wnet=Wturbine-Wpump
Heat supplied = Qin-Qout
Thermal efficiency,
thermal efficiency

hth
 Comparison

The following points are worth noting :

(i) Between the same temperature limits Rankine cycle provides a higher specific work output
than a Carnot cycle, consequently Rankine cycle requires a smaller steam flow rate resulting in
smaller size plant for a given power output. However, Rankine cycle calls for higher rates of
heat transfer in boiler and condenser.
(ii) Since in Rankine cycle only part of the heat is supplied isothermally at constant higher
temperature T1, therefore, its efficiency is lower than that of Carnot cycle. The efficiency of
the Rankine cycle will approach that of the Carnot cycle more nearly if the superheat
temperature rise is reduced.
(iii) The advantage of using pump to feed liquid to the boiler instead to compressing a wet vapour
is obvious that the work for compression is very large compared to the pump.
 Concept of MEAN Temperature of Heat Addition

The Rankine cycle efficiency can be increased by

• Increasing the mean temperature at which heat is supplied
• Decreasing the temperature at which heat is rejected
The increasing of mean temperature at heat addition or decreasing the
mean temperature at heat rejection can be performed by
1. Increasing boiler pressure
2. Super heating
3. Reducing condenser pressure
• Increasing Boiler Pressure:
It is found that by increasing the boiler operating pressure the cycle
efficiency increases and obtains a maximum value at a boiler pressure
of about 166 bar.
• Super Heating
It is found that the cycle efficiency is increased if the quality of the
steam entering the turbine is superheated condition.
• Reducing condenser Pressure
The cycle efficiency is found to be increase when the condenser
pressure is reduced.
Methods to improve the cycle
performance
• By Re-heating of steam
• By Re- generative feed heating
• By water extraction
• By using binary vapour
 Thermal Efficiency – How to enhance
it?
Thermal efficiency can be improved by manipulating the temperatures and/or pressures in various
components
(a) Lowering the condensing pressure (lowersTL, but decreases quality, x4 )
(b) Superheating the steam to a higher temperature (increases TH but requires higher temp
materials)
(c) Increasing the boiler pressure (increases TH but requires higher temp/press materials)

T 3 (c) increase pressure

T 3

2 (b) Superheating
2 1 4 2

1 4 T 1 4
s
(a) lower pressure(temp) Low quality
2 high moisture content s
Red area = increase in W net
1 Blue area = decrease in W net

s
 Reheating
• The optimal way of increasing the boiler pressure without increasing the
moisture content in the exiting vapor is to reheat the vapor after it exits from a
first-stage turbine and redirect this reheated vapor into a second turbine.

T high-P 5
3 turbine 3
high-P Low-P
low-P
turbine turbine
turbine

boiler 4
4
4

5 2
6

1
6
2 pump
1
condenser s
Reheat Rankine Cycle

• Reheating allows one to increase the boiler pressure without increasing the
moisture content in the vapor exiting from the turbine.
• By reheating, the average temperature of the vapor entering the turbine is
increased, thus, it increases the thermal efficiency of the cycle.
• Multistage reheating is possible but not practical. One major reason is because
the vapor exiting will be superheated vapor at higher temperature, thus,
decrease the thermal efficiency. Why?
• Energy analysis: Heat transfer and work output both change
qin = qprimary + qreheat = (h3-h2) + (h5-h4)
Wout = Wturbine1 + Wturbine2 = (h3-h4) + (h5-h6)
 Regeneration
• From 2-2’, the average temperature is very low, therefore, the heat addition process
is at a lower temperature and therefore, the thermal efficiency is lower. Why?
• Use a regenerator to heat the liquid (feedwater) leaving the pump before sending it
to the boiler. This increases the average temperature during heat addition in the
boiler, hence it increases efficiency.

Extract steam @ 6
From turbine to provide
Lower temp higher temp heat source in the
heat addition heat addition regenerator
3 5
T T
2’ 4
6
2
3
2

1 7
1 4 s
s
Use regenerator to heat up the feedwater
 Regenerative Cycle
• Improve efficiency by increasing feedwater temperature before it enters the
boiler.
• Two Options:
• Open feedwater : Mix steam with the feedwater in a mixing chamber.
• Closed feedwater: No mixing.
Open FWH
5
T 5

boiler 4
6 (y) 6
Open
(y) 7 (1-y)
FWH 2 (1-y)
3

3 2
4 Pump 2 7
1
s
Pump 1
1 condenser
 Regenerative Cycle - Analysis
• Assume y percent of steam is extracted from the turbine and is directed into open
feedwater heater.

• Energy analysis:
qin = h5-h4, qout = (1-y)(h7-h1),
Wturbine, out = (h5-h6) + (1-y)(h6-h7)
Wpump, in = (1-y)Wpump1 + Wpump2
= (1-y)(h2-h1) + (h4-h3)
= (1-y)v1(P2-P1) + v3(P4-P3)

• In general, more feedwater heaters result in higher cycle efficiencies.

BINARY VAPOUR CYCLE
Binary Vapour Cycle