UNIT I
STEAM GENERATORS
Types and classification Fire tube – Water tube
Low Pressure – High pressure Stationary – Mobile
Power generation – Processing Coal fired – Oil and
gas fired Vertical – Inclined – Horizontal
 Fire





tube boilers
Cochran
Cornish
Lancashire
Marine
Locomotive
 Water



Tube Boilers
Simple vertical boiler
Babcock Wilcox
Stirling
1.
2.
3.
4.
5.
6.
7.
8.
Safety valve
Pressure gauge
Water level indicator
Steam stop valve
Fusible plug
Manholes, handholes
Blow off cock
Feed pump
 Superheater
 Economiser
 Steam
water separator
 Air preheater
 Performance
 To




testing
find
Equivalent evaporation
Boiler efficiency
Losses
Heat balance sheet
 Factor
of evaporation
 h-hf /2257
 E = total heat required to evaporate feed
water from and at 100oC
 E= me(h-hf)/2257, where me is mass of steam
actualy produced in kg/kg of fuel or like
units
 Efficiency of boiler = ms (h-hf)/mf.C
 Capacity
required, pressure and temperature
of steam
 Base load or peak load
 Place of erection of boiler
 Fuel and water available (Quality and
quantity)
 Probable permanency of the station
 Losses
due to unburnt coal
 Losses due to moisture present in coal
 Losses due to sulphur like elements
 Heat lost in flue gases
 Radiation heat loss
Fire Tube boiler
Water Tube Boiler
Low pressure boiler p<80 bar
High pressure boiler p>80 bar
Shell must be present
Shell need not be there
Forced circulation very difficult
Forced circulation makes the heat transfer
more effective
Explosion risk less
Explosion risk more
Transportation and Erection difficult
Transportation and Erection easy
Fixed capacity
Capacity can be increased by increasing
the pressure
Scale formation and thus less heat Forced circulation and less or no scale
transfer
formation
Lancashire bolier, Cochran boiler
Babcock and Wilcox boiler
STEAM NOZZLES
A convergent nozzle
Steam out
A convergent – divergent nozzle
A divergent nozzle
 In
steam turbines to increase velocity of
steam
 In steam injectors to pump water into the
boiler
 In processing plants for drying the chemicals
etc
 Isentropic
expansion
1/2
 C2 = [2(h1-h2)]
m/s


where C2 is the exit velocity,
h1 and h2 are the enthalpy of steam at inlet of
the nozzle and at the exit of the nozzle
respectively (in J)
Effect of friction
•To increase dryness fraction of the steam
•To reduce the total heat drop and thus reduce the exit velocity of the
steam coming out of the nozzle
STEAM TURBINES
 Rotary
machine to convert heat energy of
steam in to shaft work
 Impulse turbine and reaction turbine
 Used in power plants
 First reaction turbine is hero engine
 Single stage – multistage
 Governing is needed to control the speed visà-vis load
ROTARY
 Balancing and
lubrication easy
 Less vibration
 Less linkages
 Does not Need
flywheel
 Used in power plant
 Less losses
 Costly

Steam TURBINE
RECIPROCATORY
 Balancing and
lubrication difficult
 More vibration
 More linkages
 Need flywheel
 Used in only small
engines
 More losses
 cheap

STEAM ENGINE










Works on impulse principle
Small in size
More losses
More power per stage
Nozzle present
Symmetric blades
Does not need pressure
tight casing
Flow only through nozzle
Cheap
DeLaval turbine
Impulse TURBINE









Works on reaction
principle
Big in size
Less power per stage
No nozzles only guide
blades
Aerofoil blades
Air tight casing needed
Flow through the entire
annular space
Costly
Parson turbine
Reaction turbine
I C ENGINES
A
reciprocating device that converts heat
energy into shaft work
 As per thermodynamic cycle



Otto cycle
Diesel cycle
Dual Cycle
 As


per Stroke
Two stroke
Four stroke
 Vertical
engines
 Horizontal ingines
 Incline engines
 Inline engines
 Radial engines
 V-engines
 Opposed cylinder engines
 Single cylinder
 Multi cylinder engines
 Automobiles
 Agricultural
equipments
 Power generation
 Earth movers
 Marine applications
 Rail locomotives
To Cool the IC engine
To lubricate the moving parts of an IC Engine
To inject diesel into the
combustion chamber at very
high pressure for atomisation
 Pushing
out the burnt gases out of the
cylinder before taking the fresh charge is
called as scavenging.
 In 4-stroke engine scavenging takes place in
exhaust stroke.
 If scavenging is poor, then power produced
will be reduced
 Supplying
more air during the inlet or suction
stroke by pressure is called supercharging.
 This is done to improve volumetric efficiency
 This increases the net power produced by
the engine.
 Supercharging is carried out by turbocharger,
which is driven by the exhaust gas from the
engine
 In
SI engine ignition takes place before the
TDC of the piston due to certain
circumstances (like preignition). This is
called as detonation.
 Isooctane has zero detonation characteristics
and any fuel is measured in octane rating.
 Due
to the combustion, different wave fronts
are formed inside the cylinder and the
wavefronts compress the already compressed
fuel. This increases the temperature and the
compressed but yet to be ignited fuel burns
and opposes the wave front thus producing
knocking
 Knocking is measured in Cetane rating
To find the power and performance
characteristics, the performance tests such as
brake power test, Morse test are conducted
 Indicated power (IP) is the power produced
inside the cylinder – measured by indicator
 IP = pLANk/60 (Watt)
 Brake power (BP) is the power obtained in a
dynamometer outside the flywheel shaft
 BP = 2πNT/60 (Watt)
 Friction power = indicated power – Brake power

 Air
standard efficiency
 Indicated thermal efficiency
 Brake thermal efficiency
 Mechanical efficiency
 Volumetric efficiency
 Heat
carried out by exhaust gases
 Heat carried out by cooling fluid
 Heat lost due to friction power
 Unaccountable losses

SI ENGINE

CI ENGINE

Compression ratio 1:8

Compression ratio 1:22

Petrol fuel

Diesel fuel

Spark ignition

Compression ignition

Carburetor

Fuel injector

Need current for ignition

Does not need current

More air std efficiency

Less air std efficiency

Lighter cylinder

Heavier cylinder

Less heat and vibration

Vibration and heat more

Lighter flywheel

Heavier flywheel

Cooling, balancing and
lubrication easy

Cooling, balancing and
lubrication difficult












One power stroke in one
revolution
Lighter flywheel
Suitable for small engines
Lubrication difficult
High specific power
High speed
More pollution, scavenging
difficult
Starting easy
Special design for piston
No valves only ports
High specific fuel consumption
Low volumetric efficiency












One power stroke in TWO
revolutions
Heavier flywheel
Suitable for heavy engines
Lubrication easy
Low specific power
Low speed
Less pollution, separate
exhaust stroke
Starting difficult
Simple design for piston
valves present
Low specific fuel consumption
High volumetric efficiency
GAS TURBINES
A
rotary device, (a prime mover) transforms
heat energy of gases into mechanical work or
shaft work
 An external combustion engine
 Works on Brayton thermodynamic cycle (or
reverese Joule’s cycle)
 Used in airplanes, turbochargers and power
generation
 Two types of gas turbines are
 Open cycle
 Closed cycle
Processes
1-2 Isentropic compression
2-3 Constant pressure heat addition
3-4 Isentropic expansion (power process)
4-1 constant pressure heat rejection
Fue
l
Gas
Turbine
Starti
ng
motor
Generato
r
Air
Compressor
Exhaust gases
Atmospheric air
Open
cycle
Mixing type
combustion chamber
 Air and gas as
medium
 Aviation fuel as fuel
 Relatively cheap
 High specific power
 Used in airplanes
 Power cannot be
increased

Closed
cycle
Non-mixing type
 Helium or liquid
sodium medium
 Any low quality fuel
 Costly
 Low specific power
 Power plants
 Power can be
increased by
increasing the
pressure ratio

Gas turbine
 Rotary device
 High speed prime
mover
 Aviation fuel as fuel
 Less balancing
 Difficult to start
 Used in airplanes
 Lubrication easy
 No flywheel
 Governing difficult
IC Engine
 Reciprocating device
 Low speed
 Petrol, diesel as fuel
 Complicated
balancing
 Easy to start
 Automobiles, Power
plants
 Lubrication difficult
 Flywheel must
 Governing easy
Net Power Produced =
Work done by Turbine – Work
done on compressor
W = Wt – Wc
Work ratio = W /Wt
Efficiency of the Turbine
system
= (Qs – Qr) /Qs
= [(T3-T2) – (T4-T1)] / (T3 –
T2)
=
1 – (1 / rp (γ-1)/ γ)
Intercooling
Reheating
Regeneration
Combination
of the above