Atd ppt

Pure Substance
• In Chemistry you defined a pure substance as an
element or a compound
• Something that can not be separated
• In Thermodynamics, we’ll define it as something
that has a fixed chemical composition throughout

Examples
• Ice in equilibrium with water
• Air
• Air in equilibrium with liquid air is not a pure
substance – Why?

Phases of Pure Substances
• We all have a pretty good idea of what the three
phases of matter are, but a quick review will help us
understand the phase change process

Solid
• Long range order
– Three dimensional pattern
– Large attractive forces between atoms or molecules
– The atoms or molecules are in constant motion – they
vibrate in place
– The higher the temperature – the more vibration

Liquid
• When a solid reaches a high enough temperature
the vibrations are strong enough that chunks of the
solid break of and move past each other
• Short range order
– Inside the chunks the atoms or molecules look a lot like
a solid
– Ex. You only break 5% to 15% of the water hydrogen
bonds to go from solid to liquid

Gas
• Molecules are far apart
• No long or short range order
• High kinetic energy
• In order to liquefy, lots of that kinetic energy must
be released

Solid to Liquid to Gas
• On a molecular level, the difference between the
phases is really a matter of degree
• We identify melting points and vaporization points
based on changes in properties
– Ex – big change in specific volume

Consider what happens when we
heat water at constant pressure
Piston
cylinder
device –
maintains
constant
pressure
Liquid
Water

Two Phase
Region
Compressed
Liquid
Superheated
Gas

Critical Point
• Above the critical point there is no sharp difference
between liquid and gas!!

P-v Diagram of a Substance that
Expands on Freezing

P-v Diagram of a Substance that
Contracts on Freezing

Property Diagrams
• T – v diagram
• P – v diagram
• P – T diagram

P-V-T surface for pure substances
that contracts on freezing

P-V-T surface for pure substances
that expands on freezing

Property Tables
• P - pressure
• T - temperature
• v – specific volume
• u – specific internal energy
• h – specific enthalpy (h = u + Pv)
• s – specific entropy

Saturated Liquid and Saturated Vapor States

Saturation Properties
• Saturation Pressure is the pressure at which the
liquid and vapor phases are in equilibrium at a
given temperature.

• Saturation Temperature is the temperature at
which the liquid and vapor phases are in
equilibrium at a given pressure.

u u u
h h h
s s s
fg g f
fg g f
fg g f
= −
= −
= −
g - stands for gas
f - stands for fluid
fg - stands for the difference between gas and fluid

Quality
x
mass
mass
m
m m
saturated vapor
total
g
f g
= =
+
Fraction of the material that is gas
x = 0 the material is all saturated liquid
x = 1 the material is all saturated gas
x is not meaningful when you are out of
the saturation region

Average Properties
f g f
f fg
y y x( y y )
y x y
= + −
= +
When x = 0 we have all liquid, and y = yf
1
When x = 1 we have all gas, and y = yf + yfg = yg
= yg

Introduction
Vapour is the substance that can
change its phase during a course of
cycle.
In the gas power cycles, the working
fluid remains gas throughout the
entire cycle.
But vapour power cycles are
external combustion systems in which
the working fluid is alternately
vaporized and condensed.

Why steam?
Steam is the most common working
fluid in vapour power cycles since it
has several desirable characteristics,
such as:
 low cost
 easy availability
 chemically stability
 physiologically harmless, and
 high enthalpy of vaporization

Introduction
 Steam power plants are referred to as coal plants,
nuclear plants, or natural gas plants depending on the
type of fuel used to supply heat to the steam. But steam
goes through the same basic cycle in all of them.
 In steam power plants water changes into steam and
again steam changes into water in different process.
Similarly in refrigeration process, refrigerant changes its
phase from liquid to vapour and vice versa.
 If such a substance undergoes cyclic process and
generates power, it is known as vapour power cycle.
The most famous vapour power cycles are Carnot cycle
and Rankine cycle.

Carnot cycle
( )
1
2
1
21
sup
1
suppliedHeat
rejectedHeat-suppliedHeat
suppliedHeat
doneNet work
T
T
dST
dSTT
Q
W
plied
net
Carnot −=
−
====η

Drawbacks of Carnot cycle
1) Steam condensation is not allowed to proceed to
completion. The condensation process is controlled one
and to be stopped at point C.
2) The working fluid at point C is both in the liquid and
vapour phase, and these do not form a homogeneous
mixture which cannot be compressed isentropically.
3) The vapour has a large specific volume and to
accommodate greater volumes, the size of the
compressor becomes quite big.
4) More power is required for running larger compressors
and hence poor plant efficiency is achieved.
5) The cycle has high specific steam consumption, large
back work ratio and low work ratio.
6) The steam at exhaust from the turbine is of low quality,
i.e. high moisture content. The liquid water droplets
causes pitting and hence erosion of the turbine blades.

Rankine cycle
 Prof. Rankine modified Carnot cycle and presented a
technically feasible cycle, called Rankine cycle.
 It is also a reversible cycle but differs from Carnot cycle
in the following respects.
1) The condensation process is allowed to proceed to
completion; the exhaust steam from the engine/turbine
is completely condensed. At the end of the condensation
process, the working fluid is only fluid and not a mixture
of liquid and vapour.
2) The pressure of liquid water can be easily raised to the
boiler pressure (pressure at which steam is being
generated in the boiler) by employing a small sized
pump.
3) The steam may be superheated in the boiler so as to
obtain exhaust steam of higher quality that can prevent
pitting and erosion of turbine blades.

Processes in Rankine cycle
1) Process 3–3′: (Reversible adiabatic
pumping / compression - PUMP)
2) Process 3′–4-1: (Reversible isobaric
heating / vaporization – BOILER)
3) Process 1–2: (Reversible adiabatic
expansion - TURBINE)
4) Process 2–3: (Reversible isobaric
cooling / condensation - CONDENSER)

P-V and T-s diagrams (Rankine cycle)

Simple steam power plant(Rankine cycle)

Elements of Steam power plant
working on Rankine cycle
 A boiler which generates steam at constant
pressure.
 An engine (or) turbine in which steam
expands isentropically and work is
developed.
 A condenser in which heat is removed from
the exhaust steam and it is completely
converted into water at constant pressure. A
hot well is used to collect the condensate.
 A pump which raises the pressure of the
liquid water to the boiler pressure and
pumps it to boiler for conversion into steam.

Net work done & thermal efficiency of
Rankine cycle
( )
( )23
14
1
hh
hh
Rankine
−
−
−=η
( ) ( )1243 hhmhhmWWW pumpturbinenet −−−=−=

Comparison of Carnot cycle and Rankine
cycle for the same temperature limits
RankineCarnot ηη >

Why only Rankine cycle?
 Carnot cycle needs a compressor to handle wet
steam mixture whereas in Rankine cycle, a
small pump is used.
 The steam can be easily superheated at
constant pressure in a Rankine cycle.
 Superheating of steam in a Carnot cycle at
constant temperature is accompanied by a fall
of pressure which is difficult to achieve in
practice because heat transfer and expansion
process should go side by side.
 Therefore Rankine cycle is used as ideal cycle
for steam power plants.

1) Increasing the boiler pressure
2) Decreasing the condenser pressure
3) Superheating the steam
4) Reheating the steam between stages
5) Regeneration process
Methods of improving performance of Rankine cycle

 Increasing the boiler pressure

 Decreasing the condenser pressure

Schematic layout of
Rankine cycle with superheating

 Superheating the steam

Schematic layout of reheat Rankine cycle

 Reheating the steam between stages

Schematic layout of
regenerative Rankine cycle

 Regeneration process

Practical regenerative Rankine cycle

Features of regenerative Rankine cycle
 There is improvement in cycle economy with relatively
much smaller capital expenditure.
 With the infinite number of heaters, the heating process
becomes reversible and the efficiency approaches to
Carnot efficiency.
 The supply of feed water to the boiler is at increased
temperature. That reduces the temperature range in the
boiler and keeps thermal stresses low.
 However, the work done per kg of steam decreases and as
such large capacity boiler is needed for a given output.
 The system also becomes complicated, less flexible and
involves greater maintenance and capital cost due to
installation of feed water heaters.

Binary vapour power cycle
A cycle works with two working substances producing dual work
but at only one heat supply is called binary vapour power
cycle.
The heat that is rejected in the condenser can be used to
generate vapors of some other working substances at high
pressure to run another Rankine cycle that produces work.
To run a binary vapour power cycle, the two working fluids
should have high temperature difference.
The substance having high boiling point acts as working medium
in topping cycle and the other in bottoming cycle.

 Fluids used:
 Water (@ 12 bar, sat. temperature is 187°C)
 Mercury (@ 12 bar, sat. temperature is 560°C)
 Di-phenyl ether
 Aluminium bromide (@ 12 bar, sat. temperature is 482.5°C)
 Liquid metals like sodium and potassium
 Since critical pressure and temperature of Hg are 1080 bar &
1460°C respectively, it is suitable working fluid in topping cycle

Heat supplied,
( )121 hhmQ Hg −=
,
Total work done in the cycle,
( ) ( )6532 hhmhhmW stHgnet −+−=
( ) ( )
( )12
6532
1 hhm
hhmhhm
Q
W
Hg
stHgnet
binary
−
−+−
==η
Thermal efficiency of the binary cycle is
( ) ( )9543 hhmhhm stHg −=−
Energy balance equation gives,
( )( )stHgcombined ηηη −−−= 111
Combined efficiency,

Work ratio
It is defined as the ratio of net work
output to the gross turbine work.
turbine pump pumpnet
turbine turbine turbine
W W WW
WR 1
W W W
−
= = = −

Effect of irreversibilities on cycle efficiency
( )
( )
( )
( )
( )'
pump 2 1ideal
pump isentropic
pump 12actual
W h hIdeal work required
Actual work required W h h
η
−
= = =
−

Effect of irreversibilities on cycle efficiency
( )
( )
( )
( )
( )21
21 '
hh
hh
W
W
developedIdeal work
dk developeActual wor
idealturbine
actualturbine
isentropicturbine
−
−
===η

Requirements for a good working fluid
 Large latent heat of vaporization
 Critical temperature should be well above the metallurgical limit so that
latent heat can be supplied at maximum temperature of the cycle
 The condensation pressure should not be too low so that leakage
problems are minimized
 The freezing point should be below the room temperature to facilitate the
filling and draining of the equipment.
 Highest saturation temperature for a moderate pressure
 Low specific heat
 High density
 Steep saturated vapour line
 Higher saturation pressure than atmospheric pressure at the minimum
cycle temperature
 Non-toxic, non-corrosive and not excessively viscous

Problem - 1
A basic steam power plant works on ideal Rankine
cycle operating between 30bar and 0.04bar. The initial
condition of steam being dry saturated, calculate
pumping work required, work developed from turbine,
cycle efficiency, work ratio and specific steam
consumption. Assume the flow rate of steam as
10kg/s.
(30.07 kW, 9402.2 kW, 35%, 0.9968, 3.84 kg/kWh)

Problem - 2
In a Rankine cycle thermal power plant, superheated steam
is supplied at 1.5MPa and 300°C to a turbine and expands
to a condenser pressure of 80kPa. The saturated liquid
coming out from condenser is pumped back to the boiler by
a feed pump. Assuming ideal processes, determine the
condition of steam after expansion, cycle efficiency, mean
effective pressure, ideal steam consumption per unit kWh
and actual steam consumption per unit kWh. Take relative
efficiency as 0.6 and neglect pump work.
(0.916, 21.27%, 293.63 kPa, 6.39 kg/kWh, 10.66 kg/kWh)

Problem - 3
In a reheating cycle, steam at a pressure of 90bar
& 480°C is expanded in a steam turbine in first
stage up to 12bar and reheated to its original
temperature before expanding to the condenser
pressure of 0.07bar. If the mass flow rate of
steam is 0.5kg/s, find the power developed and
efficiency neglecting pump work.
(791 kW, 41.76%)

Problem - 4
In a single heater regenerative cycle, the steam enters
the turbine at 30bar, 400°C and exhaust pressure is
0.1bar. The feed water heater is a direct contact type
which operates at 5bar. Find (i)efficiency and steam
rate of cycle (ii)the increase in mean temperature of
heat addition, efficiency and steam rate as compared to
Rankine cycle without regeneration. Neglect pump
work.
((i) 35.36%, 3.93 kg/kWh (ii) 27.4°C, 1.18%, 0.476 kg/kWh)

Problem - 5
Tsat
(°C)
hf
(kCal/kg)
hg
(kCal/kg)
sf
(kCal/kgK)
sg
(kCal/kgK)
540 1.80 87.3 0.0360 0.1217
205 6.92 78.7 0.0188 0.1675
A binary vapour plant uses mercury between temperatures
205°C and 540°C. The mercury is dry and saturated at high
temperature. The steam cycle works between 17.35bar and
73.66mm of Hg. Steam is supplied to boiler at 370°C and feed
water is used at 200°C in economizer & is evaporated to dry
steam in the condenser and is superheated by gases. Assume
ideal cycle, find mass of mercury circulated per kg of steam
generated, the work done by mercury and steam per kg
separately and plant efficiency. Use following properties for
mercury.
(9.42 kg/s per kg of steam generated, 1210.47 kW/kg of steam, 1152.7
kW/kg of steam, 66%)

Definition of a boiler
A boiler is a device used to generate steam at
a desired pressure and temperature by
transferring heat energy produced by burning
fuel to water to change it to steam.
It is a combination of apparatus used for
producing, furnishing or recovering heat
together with the apparatus for transferring
the heat so made available to the fluid being
heated and vaporized.

The fluid is contained in the boiler drum
called shell and the thermal energy released
during combustion of fuel is transferred to
water and this converts water into steam at
the desired temperature and pressure.
Function of a boiler

Applications of boilers
 Power generation: Mechanical or electrical power may be
generated by expanding steam in the steam engine or steam
turbine.
 Heating: The steam can be used for heating residential and
industrial buildings in cold weather and for producing hot
waters for hot water supply.
 Industrial processes: Steam can also be used for industrial
processes such as for sizing and bleaching etc. in textile
industries and other applications like sugar mills, cement,
agricultural and chemical industries.

Factors to be considered for selection of good boiler
1) The working pressure and quality of steam required
2) Steam generation rate
3) Floor area available
4) Accessibility for repair and inspection
5) Comparative initial cost
6) Erection facilities
7) The portable load factor
8) The fuel and water available
9) Operating and maintenance costs

Requirements of an efficient boiler
1. The boiler should generate maximum amount of steam at a required
pressure and temperature and quality with minimum fuel consumption and
expenses
2. Steam production rate should be as per requirements
3. It should be absolutely reliable
4. It should be light in weight
5. It should not occupy large space.
6. It should be capable of quick starting
7. It should conform to safety regulations.
8. The boiler components should be transportable without difficulty
9. The installation of the boiler should be simple
10. It should have low initial cost, installation cost and maintenance cost.
11. It should be able to cope with fluctuating demands of steam supply.
12. All parts and components should be easily accessible for inspection, repair
and replacement.
13. The tubes of the boiler should not accumulate soot or water deposits and
should be sufficiently strong to allow for wear and corrosion
14. The water and gas circuits should be such as to allow minimum fluid
velocity (for low frictional losses)

Classification of boilers
Relative position of hot gases and water
•Fire tube boilers (Cochran, Lancashire, Cornish, Locomotive)
•Water tube boilers (Babcock and Wilcox boiler, Stirling boiler)
Method of firing
•Internally fired boilers ( Lancashire, Locomotive)
•Externally fired boilers (Babcock and Wilcox boiler)
Pressure of steam
•High pressure boilers(>80 bars-Babcock and Wilcox boiler, Lamont boiler)
•Low pressure boilers (<=80 bars-Cochran,Lancashire,Cornish, Locomotive)
Method of circulation of water
•Natural circulation boilers (Lancashire, Locomotive, Babcock & Wilcox
boilers)
•Forced circulation boilers (Two large fire tubes Lancashire boiler, Single
large fire tube Cornish boiler, Cochran boiler, Many small tubes Locomotive
boiler, Babcock Wilcox water tube boiler)
Nature of service to be performed
•Land boilers
•Mobile boilers (or) Portable boilers

 Once through boilers
 Position and number of drums
• Single drum boilers
• Multi-drum boilers(Longitudinal or crosswise)
 Design of gas passages
• Single pass boilers
• Return pass boilers
• Multi-pass boilers
 Nature of draught
• Natural draught boilers
• Artificial draught boilers
 Heat source
• Combustion of solid, liquid or gaseous fuels
• Electrical and nuclear energy
• Hot waste gases of other chemical reactions
 Fluid used
• Steam boilers
• Mercury boilers
• Special boilers for heating special chemicals
 Material of construction of boiler shell
• Cast iron boilers
• Steel boilers

Particulars Fire tube boiler Water tube boiler
Position of
water and hot
gases
Hot gases inside the tubes
and water outside the tube
Water inside the tube and
hot gases outside the tubes
Mode of firing Generally internally fired Externally fired
Operating
pressure
Operating pressure limited
to 16 bar
Can work under as high
pressures as 100 bar
Rate of steam
consumption
Lower Higher
Suitability for
large power
plants
Not suitable Suitable
Risk on
bursting/explos
ion
Involves lesser risk on
explosion due to lower
pressure
Involves more risk on
bursting due to high
pressure
Floor area
For a given power,
occupies more
For a given power,
occupies less
Differences between Water-tube and Fire-tube boilers

Particulars Fire tube boiler Water tube boiler
Construction Difficult Simple
Transportation Difficult Simple
Shell diameter Large for same power Small for same power
Chances of
explosion
Less More
Treatment of water Not so necessary More necessary
Accessibility of
various parts
Various parts are not so
easily accessible for
cleaning, repair and
inspection
Various parts are more
accessible
Requirement of skill
Require less skill for
efficient and economic
working
Require more skill and
careful attention
Differences between Water-tube and Fire-tube boilers
Contd…..

COCHRAN BOILER
Features of Cochran boiler:
1)Vertical
2)Multi-tubular
3)Internally fired
4)Natural circulation
5)Fire tube boiler
6)Up to maximum steam pressure of 6.5 bar
7)Maximum evaporative capacity of 3500 to 4000 kg of
steam per hour.

Construction of COCHRAN BOILER
Cochran boiler consists of a vertical cylindrical shell, fitted with a
hemispherical crown at its top which form the steam space, and a
hemispherical dome which forms the furnace of fire box.
A platform over which the fuel burns called fire gate is provided in the
furnace. Beneath the grate there is a space, called ash pit to facilitate the
collection of ashes. The fuel is charged through the fire door provided at
the front end of the furnace.
The combustion chamber at the rear end in the middle portion of the
boiler is lined with the fire bricks which prevents the overheating of the
combustion chamber plate.
The furnace and the combustion chamber are interconnected by the
elliptical flue tube. The unburnt volatile matter leaving the furnace along
with the hot gases are burnt in the combustion chamber.
Number of flue tubes connects the combustion chamber and the smoke
box fitted at the front end. The chimney provided above the smoke box
serves for the escape of gases.
The man hole provided at the crown of the boiler facilitates the
inspection and repair of the interior of the boiler.

Working of COCHRAN BOILER
The Cochran boiler is filled with water to the specified level and
maintained at that level by charging with makeup water using a feed water
pump and when the water level drops below its specified level. The entire
surface of the furnace except the openings for the fire door and the
combustion chamber will be surrounded by water. The flue tubes will also be
completely submerged in the water.
The hot gases from the furnace along with the unburnt volatile matter pass
to the combustion chamber through the elliptical flue tube where the unburnt
volatile matter burns completely. From the combustion chamber they pass
through the horizontal flue tubes to the smoke box. The gases from the smoke
box escape to the atmosphere through the chimney.
The hot gases while passing through the flue tubes transfer their heat to the
water which is also heated by the furnace directly, gets converted into steam
and accumulates in the steam space. The steam stop valve allows the steam
from the boiler to the steam supply pipe.
The Cochran boiler is mounted with the essential mountings and
accessories like steam stop valve, safety valve, pressure gauge, water level
indicator, fusible plug, blow off valve, feed check valve. The working
pressure and steam capacity of Cochran boiler are 6.5 bar and 3500 kg /hr
respectively.

Advantages
1)Cochran Boiler occupies less floor space.
2)Construction cost of Cochran Boiler is Low.
3)Cochran boiler is semi-portable and hence easy to install and
transport.
4)Because of self contained furnace no brick work setting is
necessary.
Disadvantages
1)The capacity of the Cochran boiler is less because of the
vertical design.
2)Cochran Boiler requires high head room space.
3)Because of the vertical design, it often presents difficulty in
cleaning and inspection.
Advantages & Disadvantages of COCHRAN BOILER

BABCOCK & WILCOX BOILER
Features of Babcock & Wilcox boiler:
1)Horizontal, Straight & Stationary
2)Externally fired
3)Natural circulation
4)Water tube boiler
5)Minimum steam pressure of 10 bar to 20 bar
6)Minimum evaporative capacity of 4000 to 7000
kg of steam per hour.

Construction of BABCOCK & WILCOX BOILER
Babcock and Wilcox boiler is a horizontal, externally fired, natural circulation, stationary, and
water tube boiler. The Babcock and Wilcox water tube boiler consists mainly four parts such as water
and steam drum, water tubes, chain grate stoker, superheater tubes.
The water and steam drum is suspended from iron girders resting on the iron columns, and is
independent of the brick work setting. This arrangement prevents unequal expansion troubles and
facilitates repair of the brick work. A number of inclined water tubes at a very low inclination are
connected at right angles to the end boxes called headers. The water tubes will be arranged in a
number of vertical rows, each row consisting of 40 to 5 tubes. In each vertical row the tubes will be
arranged one below the other in a serpentine form. There will be a number of such vertical rows one
behind the other. Each one such vertical row of inclined water tubes are connected to one set of two
headers. The header at the right end of the water tubes is called down take header and the other at the
left end of the water tubes is called uptake header. Each of the vertical rows of water tubes which are
arranged one behind the other are connected to one set of headers which are also arranged one behind
the other. Each set of the headers are inturn connected to the boiler drum by one set of two tubes, on
eat the uptake end and the other at the downtake end. A mud box is provided just below the downtake
header. Any sediment in the water, due to its heavier specific gravity will settle down in the mud box
and is blown off from time to time through the blow off pipe.
The grate is provided at the front end below the uptake header. The boilers of higher capacity are
usually provided with a chain grate stoker, which consists of a slowly moving endless chain of grate
bars. The coal fed on at the front end of the grate is burnt on the moving grate in the furnace and the
residual ash falls at the outer end of the grate into the ash pit. The boiler is fitted with a superheater.
The superheater consists of number of U-tubes secured at each end to the horizontal connecting boxes
and placed in the combustion chamber below the boiler drum. The upper box of the superheater tube
is connected to a T-tube, the upper branches of the T-tube being situated in the steam space in the
drum. The lower box of the superheater tubes is connected to the steam stop valve mounted over the
drum through a vertical tube passing outside the drum.

Working of BABCOCK & WILCOX BOILER
The water is introduced into the boiler drum through a feed valve. A constant water level is
maintained in the boiler drum. The water descends at the rear end into the downtake headers and
passes up in the inclined water tubes, uptake headers and in the tubes connecting the uptake
header and the drum. Thus a circuit is established between the drum and the water tubes for the
flow of water.
The hot gases from the furnace grate are compelled by the baffle plate to pass upwards around
the water tubes lying in between the combustion chamber under the water drum, then downwards
around the water tubes in between the baffle plates, then once again upwards between the baffle
plate and the downtake header, and finally passes out of the boiler through the exit door and the
chimney.
During this path of the hot gases, the hottest gases emerging directly from the grate come in
contact with the hottest portions of the water tubes. The water in these portions of the water tubes
gets evaporated. The water and the steam mixture from this portion of the water tubes ascend
through the uptake headers and reach the boiler drum.
The steam from the steam space in the boiler drum is led into the branches of T-tube, and then
it passes into the upper connecting box of the superheater, then through its U-tubes. Since the
superheater tubes are fitted in the combustion chamber and exposed to the hot gases, the steam
passing in it will be superheated. The superheated steam from the superheater tubes are passed to
the steam stop valve through the lower connecting box and the vertical tube fitted outside the
drum. From the steam stop valve the superheated steam is passed to the prime-mover. When the
superheated steam is not required the steam from the steam space directly passes out to the
prime-mover through the steam stop valve.

BOILER MOUNTINGS
1) Pressure gauge
2) Fusible plug
3) Steam stop valve
4) Feed check valve
5) Blow off cock
6) Man and mud(sight)holes
7) Two safety valves
8) Two water level Indicators

Man and mud(sight)holes
These are used to allow men to enter inside the
boiler for inspection and repair.

Two safety valves
The commonly used safety valves are:
1)Dead weight safety valve
2)Lever safety valve
3)Spring loaded safety valve
4)High steam and low water safety valve

Loading arrangement for Lever Safety Valve
VFapVFWGFWAFW vl ××=×+×+×
Taking moments about the fulcrum F, we get
Where
2
4
da ×=
π

BOILER ACCESSORIES
1) Economiser
2) Air Preheater
3) Superheater
4) Feed Pump
5) Steam Separator
6) Steam Trap

Advantages of economizer:
1. The temperature range between various parts
of the boiler is reduced which results in
reduction of stresses due to unequal expansion
2. If the boiler is fed with cold water it may
result in chilling the boiler metal. Hot fed
water checks it.
3. Evaporative capacity of the boiler is
increased.
4. Overall efficiency of the plant is increased.

definition
A steam turbine is a prime mover in
which the potential energy of the steam is
transformed into kinetic energy and later in
its turn is transformed into the mechanical
energy of rotation of the turbine shaft.

 Power Generation
 Transport

According to the action of steam:
 Impulse turbine: In impulse turbine, steam coming out
through a fixed nozzle at a very high velocity strikes the
blades fixed on the periphery of a rotor. The blades change
the direction of steam flow without changing its pressure. The
force due to change of momentum causes the rotation of the
turbine shaft. Ex: De-Laval, Curtis and Rateau Turbines
 Reaction turbine: In reaction turbine, steam expands both in
fixed and moving blades continuously as the steam passes
over them. The pressure drop occurs continuously over both
moving and fixed blades.
 Combination of impulse and reaction turbine

According to the number of pressure stages:
 Single stage turbines: These turbines are mostly used for
driving centrifugal compressors, blowers and other similar
machinery.
 Multistage Impulse and Reaction turbines: They are made in
a wide range of power capacities varying from small to large.
According to the type of steam flow:
 Axial turbines: In these turbines, steam flows in a direction
parallel to the axis of the turbine rotor.
 Radial turbines: In these turbines, steam flows in a direction
perpendicular to the axis of the turbine, one or more low
pressure stages are made axial.

According to the number of shafts:
 Single shaft turbines
 Multi-shaft turbines
According to the method of governing:
 Turbines with throttle governing: In these turbines, fresh
steam enter through one or more (depending on the power
developed) simultaneously operated throttle valves.
 Turbines with nozzle governing: In these turbines, fresh steam
enters through one or more consecutively opening regulators.
 Turbines with by-pass governing: In these turbines, the steam
besides being fed to the first stage is also directly fed to one,
two or even three intermediate stages of the turbine.

According to the heat drop process:
 Condensing turbines with generators: In these turbines, steam at a
pressure less than the atmospheric is directed to the condenser. The
steam is also extracted from intermediate stages for feed water heating).
The latent heat of exhaust steam during the process of condensation is
completely lost in these turbines.
 Condensing turbines with one or more intermediate stage extractions: In
these turbines, the steam is extracted from intermediate stages for
industrial heating purposes.
 Back pressure turbines: In these turbines, the exhaust steam is utilized
for industrial or heating purposes. Turbines with deteriorated vacuum
can also be used in which exhaust steam may be used for heating and
process purposes.
 Topping turbines: In these turbines, the exhaust steam is utilized in
medium and low pressure condensing turbines. These turbines operate at
high initial conditions of steam pressure and temperature, and are mostly
used during extension of power station capacities, with a view to obtain

According to the steam conditions at inlet to turbine:
 Low pressure turbines: These turbines use steam at a pressure
of 1.2 ata to 2 ata.
 Medium pressure turbines: These turbines use steam up to a
pressure of 40 ata.
 High pressure turbines: These turbines use steam at a
pressure above 40 ata.
 Very high pressure turbines: These turbines use steam at a
pressure of 170 ata and higher and temperatures of 550°C and
higher.
 Supercritical pressure turbines: These turbines use steam at a
pressure of 225 ata and higher.

According to their usage in industry:
 Stationary turbines with constant speed of rotation: These
turbines are primarily used for driving alternators.
 Stationary turbines with variable speed of rotation: These
turbines are meant for driving turbo-blowers, air circulators,
pumps, etc.
 Non-stationary turbines with variable speed: These turbines
are usually employed in steamers, ships and railway
locomotives.

advantages of steam turbines
over steam engines
1. The thermal efficiency is much higher.
2. As there is no reciprocating parts, perfect balancing is possible and
therefore heavy foundation is not required.
3. Higher and greater range of speed is possible.
4. The lubrication is very simple as there are no rubbing parts.
5. The power generation is at uniform rate & hence no flywheel is
required.
6. The steam consumption rate is lesser.
7. More compact and require less attention during operation.
8. More suitable for large power plants.
9. Less maintenance cost as construction and operation is highly
simplified due to absence of parts like piston, piston rod, cross head,
connecting rod.
10. Considerable overloads can be carried at the expense of slight
reduction in overall efficiency.

impulse turbine Vs reaction turbine
 steam completely expands in the
nozzle & its pressure remains same
during its flow through the blade
passages
 steam expands partially in the nozzle
and further expansion takes place in
the rotor blades
 The relative velocity of steam
passing over the blade remains
constant in the absence of friction
 The relative velocity of steam passing
over the blade increases as the steam
expands while passing over the blade
 Blades are symmetrical  Blades are asymmetrical
 The pressure on both ends of the
moving blade is same
 The pressure on both ends of the
moving blade is different
 For the same power developed, as
pressure drop is more, the number of
stages required are less
 For the same power developed, as
pressure drop is small, the number of
stages required are more
 blade efficiency curve is less flat  blade efficiency curve is more flat
 steam velocity is very high & hence
speed of turbine is high.
 steam velocity is not very high and
hence speed of turbine is low.

 It primarily consists of a nozzle or a set of nozzles, a
rotor mounted on a shaft, one set of moving blades
attached to the rotor and a casing.
 A simple impulse turbine is also called De-Laval
turbine, after the name of its inventor
 This turbine is called simple impulse turbine since the
expansion of the steam takes place in one set of
nozzles.

 The impulse turbine consists basically of a rotor
mounted on a shaft that is free to rotate in a set of
bearings.
 The outer rim of the rotor carries a set of curved
blades, and the whole assembly is enclosed in an airtight
case.
 Nozzles direct steam against the blades and turn the
rotor. The energy to rotate an impulse turbine is
derived from the kinetic energy of the steam flowing
through the nozzles.
 The term impulse means that the force that turns the
turbine comes from the impact of the steam on the
blades.

 The toy pinwheel can be used to study some of the basic principles of
turbines. When we blow on the rim of the wheel, it spins rapidly. The
harder we blow, the faster it turns.
 The steam turbine operates on the same principle, except it uses the
kinetic energy from the steam as it leaves a steam nozzle rather
than air.
 Steam nozzles are located at the turbine inlet. As the steam passes
through a steam nozzle, potential energy is converted to kinetic energy.
 This steam is directed towards the turbine blades and turns the rotor.
The velocity of the steam is reduced in passing over the blades.
 Some of its kinetic energy has been transferred to the blades
to turn the rotor.
 Impulse turbines may be used to drive forced draft blowers, pumps,
and main propulsion turbines.

 In impulse turbine, steam coming out through a fixed nozzle at a
very high velocity strikes the blades fixed on the periphery of a
rotor.
 The blades change the direction of steam flow without changing
its pressure.
 The force due to change of momentum causes the rotation of the
turbine shaft.
 Examples: De-Laval, Curtis and Rateau turbines.

The uppermost portion of the diagram
shows a longitudinal section through the
upper half of the turbine.
The middle portion shows the actual shape
of the nozzle and blading.
The bottom portion shows the variation of
absolute velocity and absolute pressure
during the flow of steam through passage of
nozzles and blades.
The expansion of steam from its initial
pressure (steam chest pressure) to final
pressure (condenser pressure) takes place in

 The steam leaves the nozzle with a very high velocity and strikes the blades
of the turbine mounted on a wheel with this high velocity.
 The loss of energy due to this higher exit velocity is commonly known as
carry over loss (or) leaving loss.
 The pressure of the steam when it moves over the blades remains constant
but the velocity decreases.
 The exit/leaving/lost velocity may amount to 3.3 percent of the nozzle
outlet velocity.
 Also since all the KE is to be absorbed by one ring of the moving blades
only, the velocity of wheel is too high (varying from 25000 to 30000
RPM).
 However, this wheel or rotor speed can be reduced by adopting the method
of compounding of turbines.

1. Since all the KE of the high velocity
steam has to be absorbed in only one
ring of moving blades, the velocity of
the turbine is too high i.e. up to 30000
RPM for practical purposes.
2. The velocity of the steam at exit is
sufficiently high which means that
there is a considerable loss of KE.

V - Absolute Velocity of steam in m/s
u - Blade velocity in m/s
Vr - Relative velocity of steam w.r.t blade in m/s
Vw - Tangential velocity (Whirl) of steam in m/s
Va - Axial velocity of steam in m/s
α - Nozzle angle in degrees
β - Blade angle in degrees
Suffix-1 - Inlet condition
Suffix–2 - Outlet condition
K - Blade velocity coefficient =
r2
r1
V
V

( ) Newtons
Newtonsyin velocitchangesecondperMass
direction.ltangentiain themomentumofchangeofRatedirectionltangentiain theForce
2ww1 VVm ±=
×=
=
( ) Newtonsthrust)(axial
direction.axialin themomentumofchangeofRatedirectionaxialin theForce
2aa1 VVm −=
=
( ) m/s-NbladesonsteambydoneWork 2 uVVm ww1 ±=
( ) kW
uVVm ww1
1000
turbineby thedevelopedPower 2±
=
( ) ( )
2
1
ww1
2
1
ww1
V
uVV2u
mV
uVVm 22
2
1blade(s)thetosuppliedEnergy
blade(s)on thedoneWork
efficiencyBlade
±
=
±
==
( ) m/s-N
2
1
frictionbladetoduelostEnergy
2
r2
2
r1 VVm −=
( )
( )
( )
d
ww1
1
ww1
H
uVV
HHm
uVVm 2
2
2
stagepersuppliedenergyTotal
blade(s)on thedoneWork
efficiencyStage
±
=
−
±
==
ringnozzlein thedropHeat=−= 21d HHHwhere

From the combined velocity triangle (diagram), we have
uVVV rw +== 11111 coscos βα uVVV rw −== 22222 coscos βαand
( )KCV
V
V
VVVVV r
r
r
rrrww +=





+=+=+∴ 1cos
cos
cos
1coscoscos 11
11
22
11221121 β
β
β
βββ
1
2
r
r
V
V
K =where and
1
2
cos
cos
β
β
=C
( )( )KCuVVV ww +−=+ 1cos 1121 α(or)
( )( )uKCuV +− 1cos 11 α
ratiospeedBlade
1
==
V
u
ρ
Rate of doing work per kg of steam per second =
( )( )
2
1
11 1cos
V
KCuV
b
+−
=
α
η∴Diagram efficiency,
Let,
( )( )KCb +−= 1cos2 2
1 ραρηThen, Diagram efficiency,

 If the values of α1, K and C are assumed to be constant, then diagram
efficiency depends only on the value of blade speed ratio, ρ
 In order to determine the optimum value of for maximum diagram
efficiency,
 Then, ρ becomes, ρ =
0=
∂
∂
ρ
ηb
2
cos 1α

( ) ( ) ( )
2
cos
1
4
cos
cos.
2
cos
12 1
2
1
2
1
1
max
αα
α
α
η KCKCb +=





−+=
Maximum diagram efficiency =
( )( )uKCuV +− 1cos 11 αWork done/kg of steam/second
=
2
2u=Then maximum rate of doing work/kg of steam/second
Note: If the blade is symmetrical & friction is absent, then, we have
β1=β2 and K = C = 1
Then, maximum diagram efficiency, (ηb)max = cos2
α1

 If high velocity of steam is allowed to flow through one row of moving
blades, it produces a rotor speed of about 30000 rpm which is too high
for practical use.
 It is therefore essential to incorporate some improvements for practical
use and also to achieve high performance.
 This is possible by making use of more than one set of nozzles, and
rotors, in a series, keyed to the shaft so that either the steam pressure or
the jet velocity is absorbed by the turbine in stages. This is called
compounding of turbines.
 The high rotational speed of the turbine can be reduced by the
following methods of compounding:
1) Velocity compounding
2) Pressure compounding, and
3) Pressure-Velocity compounding

 It consists of a set of nozzles and a few rows of moving blades which are fixed to the
shaft and rows of fixed blades which are attached to the casing.
 As shown in figure, the two rows of moving blades are separated by a row of fixed
blades.
 The high velocity steam first enters the first row of moving blades, where some
portion of the velocity is absorbed.
 Then it enters the ring of fixed blades where the direction of steam is changed to suit
the second ring of moving blades. There is no change in the velocity as the steam
passes over the fixed blades.
 The steam then passes on to the second row of moving blades where the velocity is
further reduced. Thus a fall in velocity occurs every time when the steam passes over
the row of moving blades. Steam thus leaves the turbine with a low velocity.
 The variation of pressure and velocity of steam as it passes over the moving and fixed
blades is shown in the figure. It is clear from the figure that the pressure drop takes
place only in the nozzle and there is no further drop of pressure as it passes over the
moving blades.
 This method of velocity compounding is used in Curtis turbine after it was first
proposed by C.G. Curtis

 It consists of a number of fixed nozzles which are incorporated between
the rings of moving blades. The moving blades are keyed to the shaft.
 Here the pressure drop is done in a number of stages. Each stage consists
of a set of nozzles and a ring of moving blades.
 Steam from the boiler passes through the first set of nozzles where it
expands partially. Nearly all its velocity is absorbed when it passes over
the first set of moving blades.
 It is further passed to the second set of fixed nozzles where it is partially
expanded again and through the second set of moving blades where the
velocity of steam is almost absorbed. This process is repeated till steam
leaves at condenser pressure.
 By reducing the pressure in stages, the velocity of steam entering the
moving blades is considerably reduced. Hence the speed of the rotor is
reduced. Rateau & Zoelly turbines use this method of compounding.

1) In this method of compounding, both
pressure and velocity compounding
methods are utilized.
2) The total drop in steam pressure is carried
out in two stages and the velocity
obtained in each stage is also
compounded.
3) The ring of nozzles are fixed at the
beginning of each stage and pressure
remains constant during each stage.
4) This method of compounding is used in
Curtis and More turbines.

 A turbine in which steam pressure decreases gradually
while expanding through the moving blades as well as the
fixed blades is known as reaction turbine.
 It consists of a large number of stages, each stage
consisting of set of fixed and moving blades. The heat drop
takes place throughout in both fixed and moving blades.
 No nozzles are provided in a reaction turbine. The fixed
blades act both as nozzles in which velocity of steam
increased and direct the steam to enter the ring of moving
blades. As pressure drop takes place both in the fixed and
moving blades, all the blades are nozzle shaped.
 The steam expands while flowing over the moving blades
and thus gives reaction to the moving blades. Hence the
turbine is called reaction turbine.
 The fixed blades are attached to the casing whereas moving
blades are fixed with the rotor.
 It is also called Parson’s reaction turbine.

( ) smNVVmu ww /21 −+
( )
H
VVu
,Efficiency
pairperstagetheindropEnthalpy
pairperstagetheinsteamofkgperdonework
,Efficiency
ww
1000
21 +
=∴
=
η
η
( ) mNVVu ww −+ 21
The work done per kg of steam in the stage (per pair) =
The work done per kg of steam per second in the stage (per pair) =
( ) kW
VVmu ww
1000
21 +
Power developed (per pair) =
where, H = Enthalpy drop in the
stage
per pair in kJ/kg
where, m = mass of steam flowing over blades in kg/s

Governing is the method of maintaining the speed of
the turbine constant irrespective of variation of the
load on the turbine.
A governor is used for achieving this purpose which
regulates the supply of steam to the turbine in such a
way that the speed of the turbine is maintained as far
as possible a constant under varying load conditions.
The various methods of governing of steam turbines
are:
1) Throttle governing
2) Nozzle governing
3) By-pass governing
4) Combination of (1) & (2) or (2) & (3)

 Residual velocity loss
 Losses in regulating valves
 Loss due to steam friction in nozzle
 Loss due to leakage
 Loss due to mechanical friction
 Loss due to wetness of steam
 Radiation loss

Reheat factor:
It is defined as the ratio of cumulative heat drop to the
adiabatic heat drop in all the stages of the turbine. The
value of reheat factor depends on the type and efficiency
of the turbine, the average value being 1.05.
DA
BABABA
dropheatAdiabatic
dropheatCumulative
factorReheat
1
332211 ++
==
Overall efficiency:
It is defined as the ratio of total useful heat drop to the total
heat supplied.
DA1
332211
h-H
CACACA
suppliedheatTotal
dropheatusefulTotal
efficiencyOverall
++
==

Definition
 A device (steam to water heat exchanger) in
which heat from exhaust steam is transferred
(by removing heat from steam) to circulating
cooling water at a pressure less than
atmosphere.

Function
1) To reduce the turbine exhaust pressure so as to
increase the specific output and hence increase the
plant efficiency and decrease the specific steam
consumption.
2) To condense the exhaust steam from the turbine and
reuse it as pure feed water in the boiler. Thus only
make up water is required to compensate loss of
water

Advantages of condensers
1) High pressure ratio provides larger enthalpy drop
2) Work output per kg of steam increases and hence
specific steam consumption decreases
3) Condensate can be reused as hot feed water to the boiler
thus reduces the time of evaporation and hence fuel
economy
4) No feed water treatment is required and hence reduces
the cost of the plant
5) The formation of deposits in the boiler surface can be
prevented with the use of condensate instead of feed
water from outer sources

Elements of steam condensing plant
1) Condenser
2) Air extraction pump
3) Condensate extraction pump
4) Circulating cooling water pump
5) Hot well
6) Cooling tower
7) Make up water pump
8) Boiler feed pump

Elements of steam condensing plant

Classification of condensers
1) Jet condensers (mixing / contact / direct)
a) Parallel flow type (Low level)
b) Counter flow type (High & Low levels)
c) Ejector type
2) Surface condensers (non-mixing / non-contact / indirect)
a) Down flow type
b) Central flow type
c) Inverted type
d) Regenerative type
e) Evaporation type

Jet Condenser Vs Surface Condenser
Jet Condensers
(Direct Contact type/
Mixed type)
Surface Condensers
(Indirect Contact type/
Non-Mixed type)

Jet Condensers Surface Condensers
1) Cooling water and steam are mixed up
2) Low manufacturing cost
3) Requires small floor space
4) The condensate cannot be used as feed
water to boiler unless it is free from
impurities
5) More power is required for air pump
6) Less power is required for water pump
7) Requires less quantity of cooling water
8) The condensing plant is simple
9) Less suitable for high capacity plants
due to low vacuum efficiency
10) Lower upkeep
1) Cooling water & steam aren’t mixed up
2) High manufacturing cost
3) Requires large floor space
4) The condensate can be used as feed
water to boiler as it is not mixed with
cooling water
5) Less power is required for air pump
6) More power is required for water pump
7) Requires large quantity of cooling water
8) The condensing plant is complicated
9) More suitable for high capacity plants
as vacuum efficiency is high
10) Higher upkeep
Jet Condenser Vs Surface Condenser

Jet condensers
 Jet condensers are used in small capacity units
where clean fresh water is available in plenty.
 In jet condensers, water is in direct contact with
exhaust steam. Hence these are also called direct
contact type (or) mixed type

Jet condensers
1) As a result of effective mixing, it requires less
circulating cooling water
2) Equipment is simple and occupy less space
3) Maintenance is cheap
Advantages
Disadvantages
1) Not suitable for higher capacities
2) Condensate cannot be used as feed water to boiler
3) Air leakages are more
4) Requires larger air pump
5) Less vacuum is maintained

Surface condensers
 Surface condensers are used in large capacity plants
 In surface condensers, exhaust steam and water do
not mix together. Hence they are also called
indirect contact type (or) non-mixed type

Surface condensers
1) Can be used for large capacity plants
2) High vacuum can be created
3) Condensate is free from impurities and can be reused as
feed water to boiler
4) Impure water can also be used as cooling medium
5) Air leakage is comparatively less, hence less power is
required to operate air pump
Advantages
Disadvantages
1) Design is complicated and costly
2) High maintenance cost
3) Occupies more space
4) Requires more circulating water

Parallel flow low level Jet condenser

Counter flow low level Jet condenser

Central flow Surface condenser

Inverted type Surface condenser
 In this type of jet condensers, steam enters at the
bottom of the shell and flows upwards.
 Air extraction pump is placed at the top.
 The condensate flows down and removed at the
bottom where condensate pump is located.

Regenerative type Surface condenser
 The condensers used in a regenerative method of
heating the condensate are called regenerative type
condensers.
 In this type of condensers, the condensate after leaving
the condenser is passed through the exhaust steam
where the temperature is increased.
 The condensate at high temperature can be reused as
feed water to the boiler.
 This increases the efficiency of the plant and minimise
the fuel consumption.

Sources of air in Condenser
1) Air leakage from atmosphere at the joints
of the parts which are internally under a
pressure less than atmosphere
2) Air accompanied with steam from the
boiler into which it enters dissolved with
feed water
3) In jet condensers, a little quantity of air
accompanies the injection of water in
which it is dissolved

Effects of air leakage in a condenser
1) Lowered thermal efficiency
2) Increased requirement of cooling water
3) Reduced heat transfer
4) Corrosion

Methods of obtaining maximum vacuum
1) Air pump
2) Steam air ejector
3) De-aerated feed water
4) Air tight joints

Vacuum Efficiency
It is defined as the ratio of actual vacuum to the maximum
obtainable vacuum.
Actual vacuum
Vacuum efficiency
Maximum obtainable vacuum
Vacuum efficiency
Barometer pressure - absolute pressure of steam
Actual vacuum
=
=

Condenser Efficiency
It is defined as the ratio of difference between the outlet
and inlet temperatures of cooling water to the difference
between the temperature corresponding to the vacuum in
the condenser and inlet temperature of cooling water
































watercoolingof
etemperaturInlet
-
vacuumto
ingcorrespond
eTemperatur
watercoolingof
etemperaturinRise
=efficiencyCondenser

Air Pumps
 An air pump maintains vacuum in the condenser as
nearly as possible equal to that corresponding to the
temperature of condensate by removing the air from
the condenser.
 Air pumps may also remove condensate together with
air from the condenser.
 A dry air pump removes the moist air alone
 A wet air pump removes both air and condensate.

Types of air Pumps
1) Reciprocating piston pumps (or) bucket pumps
2) Rotary pumps
3) Steam jet air pumps (or) ejectors
4) Wet jet pumps

Features of Edward’s Air Pump
1) Reciprocating piston pump
2) Wet air pump
3) Absence of foot and bucket valves
4) Limited speed of operation
5) Very bulky for higher vacuum or larger powers

Fundamentals
Refrigeration
 Refrigeration is the science of producing and maintaining
temperatures below that of the surrounding atmosphere.
 In simple, refrigeration means the cooling of or removal of
heat from a system by employing an equipment to maintain
the system at a low temperature known as refrigerating
system.
 The system whose temperature is kept at low temperature is
called refrigerated system.
 Refrigeration is generally produced in one of the following
ways:
 By melting of a solid
 By sublimation of a solid
 By evaporation of a liquid

Need for refrigeration
1) Ice making
2) Transportation of foods above and below freezing
3) Industrial air-conditioning
4) Comfort air-conditioning
5) Chemical and related industries
6) Medical and surgical aids
7) Processing food products and beverages
8) Oil refining and synthetic rubber manufacturing
9) Manufacturing and treatment of metals
10) Freezing food products
11) Miscellaneous applications:
a) Extremely low temperatures
b) Plumbing
c) Building construction etc.

Definitions
Refrigerating effect
The rating of a refrigeration machine is obtained by
refrigerating effect or amount of heat extracted in a given time
from a body. The rating of a refrigeration machine is given by
a unit of refrigeration called standard tonne of refrigeration.
A standard tonne of refrigeration is defined as the
refrigeration effect produced by melting of 1 tonne of ice from
and at 0°C in 24 hours. Since the latent heat of fusion of ice is
equal to 336kJ/kg, the refrigerating effect of 336×1000 kJ in
24 hours is rated as one tonne, i.e.
(or)
(or)
3.888kW=tonne1
3.5kW=ton1 ( )tonne0.9=ton1

Definitions contd…
COP
 The performance of a refrigeration system is
expressed by “Coefficient of Performance”. It
is defined as the ratio of heat absorbed by the
refrigerant while passing through the
evaporator to the work input required to
compress the refrigerant in the compressor.
InputWork
EffectionRefrigerat
COP =

Refrigerator vs Heat Pump
( )
LH
L
frigeratorRe
QQ
Q
COP
−
= ( )
LH
H
PumpHeat
QQ
Q
COP
−
=

Elements of a refrigeration system
1. A low temperature thermal sink to which heat
will flow from the space to be cooled.
2. Means of extracting energy from the sink,
raising the temperature level of this energy
and delivering it to a heat receiver.
3. A receiver to which heat will be transferred
from the high temperature high-pressure
refrigerant.
4. Means of reducing the pressure and
temperature of the refrigerant as it returns
from the receiver to the sink.

Methods of refrigeration
1) Ice refrigeration
2) Air refrigeration system
3) Vapour compression refrigeration system
4) Vapour absorption refrigeration system
5) Special refrigeration systems
1) Adsorption refrigeration system
2) Cascade refrigeration system
3) Mixed refrigeration system
4) Vortex tube refrigeration system
5) Thermoelectric refrigeration system
6) Steam jet refrigeration system

Reverse Carnot cycle for refrigeration

1) The air is expanded isentropically from points 1 to 2.
This causes the temperature to fall from T1
to T2
.
2) The air is now expanded isothermally to point 3 at
temperature T2
. During this process, heat is absorbed
from the cold body.
3) The air is now compressed isentropically to point 4
by the help of external power which causes the
temperature to rise to T1
. During this process no heat
is absorbed or rejected by the air.
4) The air is now compressed isothermally from 4 to 1.
During this process, heat is rejected by the air to the
hot body.





 − 1s4s1T=bodyhottorejectedHeat











−=
−
1s4s2T
2s3s2T=bodycoldfromabsorbedHeat
1s2sand4s3s 




 ==




















−−=
−−−=
=
1s4sTT
1s4s2T1s4s1T
absorbedHeat–rejectedHeatcycleperrequiredWork
21
From T-s diagram,

Ordinary Household Refrigerator

Air Refrigeration System
• In air refrigeration system, air is used as the
refrigerant to remove the heat from a refrigerated
place and discharge the same to the atmosphere.
• Air refrigeration is one of the earliest methods of
refrigeration and was obsolete for several years
because of its low COP and high operating costs.
• However it has been applied to aircraft refrigeration
system, where with low equipment weight, it can
utilize a portion of the cabin according to the
supercharger capacity.
• The main characteristic feature of air refrigeration
system is that throughout the cycle the refrigerant
remains in gaseous state.

Principle of operation:
• In air refrigeration system, the compressor draws air
from the cold chamber, compresses it and then
delivers it to the air cooler.
• The high pressure cooled air is then expanded in the
expansion cylinder (air motor).
• The low temperature air leaving the expansion
cylinder then enters the cold chamber and abstracts
heat from the refrigerated place.
• The air coming out from the cold chamber again
enters into the compressor and the cycle is repeated.

(Bell-Coleman cycle)

Bell-Coleman cycle
 Process a - 1 represents the suction of air into the
compressor
 Process 1 - 2 represents isentropic compression of air by
the compressor
 Process 2 - b represents the discharge of high pressure air
from the compressor into the air cooler
 Due to the cooling of air in the air cooler, there is reduction
in volume from 2 to 3 as represented by the process 2 - 3
 Process 3 - 4 represents the isentropic expansion of air in
the expansion cylinder. The air enters the cold chamber at
condition 4
 Process 4 - 1 represents the absorption of heat at constant
pressure.

Bell-Coleman cycle
Heat absorbed or abstracted from cold chamber per kg of air is equal to 





−= 411 TTpCN
Heat rejected to the cooler per kg of air is equal to 





−= 322 TTpCN
Work required for the compressor = Hat rejected – heat absorbed












−−−= 4132 TTpCTTpCinW
( )
( ) ( )
( )
( ) ( )4132
41
4132
411
TTTT
TT
TTpCTTpC
TTpC
inW
N
COP
−−−
−
=
−−−
−
==∴
Applying isentropic law for the processes 1-2 and 3-4, COP becomes,
43
4
TT
T
COP
−
=

Advantages & Disadvantages of A-R System
Advantages :
1) The refrigerant used air is non-poisonous, cheap and easily available
2) There is no danger of any kind of air leakage
3) The system is highly reliable
4) The system is highly useful for aircraft refrigeration system due to its
light weight and less space requirements in comparison to other
systems
Disadvantages :
1) Very low COP in comparison to other systems
2) Running cost is very high compared to other refrigeration systems
3) Large volume of air is required to be handled per ton of refrigeration as
compared to other systems results in larger size of compressor and
expander.
4) There is a danger of frosting at the expansion valve as air may contain
some water vapour in the case of open air system. This problem can be
partly reduced by passing air through silica gel that can highly absorb
water vapour.

Types of Air Refrigeration Systems
1) Closed air refrigeration system
2) Open air refrigeration system

Closed Air Refrigeration System
In this system (or dense air system), the air refrigerant is
contained within the piping or component parts of the
system at all times and refrigerator with usually pressures
above atmospheric pressure.

Open Air Refrigeration System
In the open system, the refrigerator is replaced by the actual
space to be cooled with the air expanded to atmospheric
pressure, circulated through the cold room and then
compressed to the cooler pressure. The pressure of
operation in this system is inherently limited to operation at
atmospheric pressure in the refrigerator.

Advantages of Closed A-R System
1) The suction to compressor may be at high pressure
and hence the sizes of expander and compressor can
be kept within reasonable limits by using dense air.
2) In open air system, the air picks up moisture from the
products kept in the refrigerated chamber; the
moisture may freeze during expansion and is likely to
choke the valves whereas it does not happen in
closed system.
3) In open air system, the expansion of the refrigerant
can be carried only up to atmospheric pressure
prevailing in the cold chamber but for a closed
system there is no such restriction.

Vapour Compression Refrigeration System
1) It is the most practical form of refrigeration in
which the working fluid used as refrigerant in the
vapour form alternately undergoes a change of
phase from vapour to liquid and liquid to vapour
during the working in the cycle.
2) In evaporating, it absorbs heat from the cold body
which is used as source of latent heat and gets
converted from liquid into vapour.
3) While condensing, it rejects its latent heat to the
circulating water of the cooler.
4) The refrigerants generally used in this system are
ammonia, carbon dioxide and sulphur dioxide.

Different components of VCR System
1) Compressor: The compressor used in VCR system may be either reciprocating
type, centrifugal type or rotary type. The function of compressor is to draw the
vapour through the suction valve from the evaporator at low pressure and low
temperature at point 1. The vapour is compressed isentropically to point 2. During
compression, the pressure and temperature increases and the vapour is discharged
through the delivery valve and enters the condenser at point 2.
2) Condenser: In the condenser, heat is transferred to the cooling fluid which is
generally water or air. The compressed vapour is cooled and condenses at
saturation temperature which corresponds to the pressure in the condenser. The
high pressure saturated liquid leaves the condenser and enters the throttle valve at
point 3.
3) Expansion valve (or) Throttle valve: The function of the throttle valve is to allow
the liquid refrigerant under high pressure to pass at a controlled rate into the low
pressure part of the system known as evaporator. The expansion in the throttle
valve takes place from point 3 to 4.
4) Evaporator: An evaporator consists of pipes in which the liquid evaporates at the
lower temperature and takes up heat from cold brine which produces the
refrigerating effect. The liquid (vapour) will thus leave the brine tank (evaporator)
as a fairly dry vapour and enters the compressor at point 1 thus completing the
cycle.

Effect of various parameters on VCR System
1) Effect of decreasing suction pressure: When the suction pressure is decreased,
the refrigerating effect is decreased and the work required is increased. The net
effect is to reduce the refrigeration capacity of the system with the same amount
of refrigerant flow and the COP.
2) Effect of increasing delivery pressure: The effect of increasing the delivery /
discharge pressure is just similar to the effect of decreasing the suction pressure.
The only difference is that the effect of decreasing the suction pressure is more
predominant than the effect of increasing delivery / discharge pressure.
3) Effect of sub-cooling of liquid: Sub-cooling is the process of cooling the liquid
refrigerant below the condenser temperature for a given pressure. The effect of
sub-cooling is to increase the refrigerating effect results in increase of COP
provided that no further energy has to be spent to obtain the extra cold coolant
required.
4) Effect of increasing vaporizing temperature and decreasing condenser
temperature: The capacity and performance of the refrigerating system improve
as the vaporizing temperature increases and condensing temperature decreases.
Thus the refrigerating system should always be designed to operate at the
highest vaporizing temperature and lowest condensing temperature keeping in
view of the requirements of the application.

Advantages of VCR System
1) The COP is better because the cycle using a vapour as
refrigerant absorbs and rejects heat at constant temperature
like the reversed Carnot cycle.
2) The temperature at which heat is to be absorbed can be
changed conveniently by altering the boiling pressure.
3) The pressure to which the refrigerant is to be compressed is
determined by the cooling water temperature and not by the
level of refrigeration as in the case of air refrigeration
system.
4) The same refrigerant can be used over and over again.
5) The heat transfer coefficient is high because of the presence
of liquid refrigerant in the condenser as well as in the
evaporator.
6) The expander is eliminated.

Vapour Absorption Refrigeration System
1) In this system, the refrigerant is absorbed on
leaving the evaporator, the absorbing medium
being a solid or a liquid.
2) The compressor is replaced by an absorber, a
pump and a generator.

Vapour Absorption Refrigeration System

Operation of VAR System
1) The vapour at low pressure leaving the evaporator passes to the absorber where it is
dissolved in the weak ammonia solution contained in the absorber.
2) The absorber is cooled by circulating cold water.
3) The strong ammonia solution formed in the absorber is then pumped to the generator and
circulated through the system by the pump. Which increases the pressure of the solution to
that desired in the condenser (about 10 bar).
4) The strong ammonia solution is heated in the generator by the steam or heating coil and the
ammonia vapour is driven out of the solution and a satisfactory condenser pressure is
produced.
5) The ammonia vapour then passes to the condenser to be condensed.
6) The high pressure liquid ammonia then passes through the expansion valve or throttling
valve.
7) The high pressure liquid is converted into a very wet vapour at low pressure (about 3 bar) &
temperature -10°C during this process.
8) The cold and wet ammonia vapour then passes through the evaporating coils in the
evaporator, where it extracts the latent heat of evaporation from the brine or substance to be
cooled.
9) The ammonia vapour coming out from the evaporator is fairly dry and enters the absorber
where it mixes with the cold water contained in the absorber thus completing the cycle.
10) The hot weak ammonia solution left at the bottom of the generator is first throttled to low
pressure by passing it through a pressure reducing valve and then passed into the absorber.

To improve efficiency of VAR System
In actual practice, the vapour absorption refrigeration system is fitted
with a heat exchanger, an analyser and a rectifier to improve the efficiency of
the plant.
1)Heat exchanger: The capacity of water at high temperature for
absorbing ammonia vapour is low. So the hot weak solution coming out from
the generator to the absorber must be cooled. The heat removed from the weak
solution may be used to raise the temperature of the strong solution coming
from the absorber and going to the generator. The heat transfer is accomplished
by placing a counter flow heat exchanger between the pump and the generator.
It increases the economy of the plant.
2)Analyser: The ammonia vapour contains water vapour while leaving
the generator. The water vapour is to be removed before the ammonia vapour
enters the condenser otherwise it will freeze at the throttle valve. The water
vapour is partly removed by passing the ammonia vapour through an analyser
containing the series of trays.
3)Rectifier: The rectifier removes the remaining water vapour from the
ammonia vapour coming out from the analyser by providing water cooling.
The condensed liquid is returned to the upper part of the analyser by a drip
return pipe. Rectifier is fitted before the condenser.

Advantages of VAR over VCR
1) As there is no moving part in the system, the operation is
quiet and there is very little wearing.
2) The maintenance cost is very low.
3) The system does not depend on the electrical power.
Exhaust steam from the other equipments may be
economically used.
4) It can be built in capacities well above 1000 tons each.
5) At reduced loads, the absorption system is almost as
efficient as at full load. The COP of the compression
system decreases as the load decreases.
6) VAR system can operate at reduced evaporator temperature
by increasing the steam while is supplied to the generator
with little decrease in capacity. The capacity of the
compression system drops rapidly with lower evaporator
temperature.

VCR vs VARS.NO. PARTICULARS VCR SYSTEM VAR SYSTEM
1
Type of energy
supplied
Mechanical-a high grade energy Mainly heat – a low grade energy
2 Energy supply Low High
3 Wear and tear More Less
4
Performance at part
loads
Poor
System not effected by variations of
load
5 Suitability
Used where high grade
mechanical energy is available
Can also be used at remote places as it
can work even with a simple kerosene
lamp
6 Charging of refrigerant Simple Difficult
7 Leakage of refrigerant More chances
No chances as there is no compressor
or any reciprocating components to
cause leakage
8 Damage
Liquid traces in suction line may
damage the compressor
Liquid traces of refrigerant present in
piping at the exit of the evaporator
constitute no danger

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