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THERMAL ENGINEERING
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EFFICIENCY OF IMPULSE TURBINE
In general
Efficiency=(Output)/(Input)
Output ispower developed by turbine
Input= Energy supplied to the rotor blade in the form of
kinetic energy
OR
Total Energy supplied per stage in the form ofEnthalpy
Drop
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According to various outputs and inputs efficiency are of
3 types
1. Diagram Efficiency
2. Nozzle Efficiency
3. Stage Efficiency
http://localhost/var/www/apps/conversion/tmp/scratch_10/Diagram%20Efficiency.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_10/Nozzle_Efficiency.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_10/Stage_Efficiency.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_10/Stage_Efficiency.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_10/Nozzle_Efficiency.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_10/Diagram%20Efficiency.pptx7/27/2019 Thermal Engineering.pptx
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REACTION TURBINE
1. Casing
2. Fixed Blades Performs the function of
Nozzle in Impulse turbine.
It directs steam to
adjacent moving blade.3. Moving Blades-produces
impulse action on blades
4. Shaft
5. Rotor
6. Draft tube
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REACTION TURBINE
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WORKING
High pressure steam directly supplied to boilers with out
nozzles.
Steam expands( drum diameter increases) as it flows through
fixed and moving blades Continuous drop of pressure.
Produces reaction forces on blades
Reaction causes rotor to rotate.
Propulsive force causing rotation of turbine is the reaction
force. Hence called reaction turbine.
Eg: Parsons Turbine
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IMPULSE-REACTION TURBINES
Continuous pressure drop as steam passes through blades
Fixed blades action similar to nozzles Steam suffers
change in direction as it passes through moving blades
Change in momentum Impulse to blades.
Pressure drop of steam in moving blades causes reaction
force also.
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DEGREE OF REACTION
D.R=(Enthalpy Drop in rotor Blades)/Enthalpy drop
in stages
D.R(impulse turbine) = 0 as enthalpy drop in rotor
blades is 0(as pressure remains constant)
D.R>0 as enthalpy drop and pressure variation
occurs in rotor blades too
If D.R= 0.5 implies half the enthalpy drop occurs in
nozzles and remaining half in rotor
Mathur page 866
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LOSSES IN STEAM TURBINE
Factors reducing the output of a turbine is known aslosses
Internal losses:- losses which will affect the steam
condition
External Losses:- Losses that doesnt affect the steamcondition
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CLASSIFICATION OF LOSSES
LOSSES
Internal
Nozzleloss
Residualvelocity
loss
Bladeloss
DiscFriction
Loss
CarryoverLoss
MoistureLoss
External
Leakagelosses
Mechanicalfriction losses
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INTERNAL LOSSES
Nozzle loss:- Actual velocity is less than the enthalpy
drop occurred
Reasons are
Frictional resistance of walls
Viscous friction between fluid particles
Turbulence effect
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Contd
Residual loss
Kinetic energy loss in the in the exhaust steam
Reduced by having multistage turbine
Carry over loss
Loss occurred while carrying the steam between
stages
Blade loss
Friction over the blades
Leakage over the annular space
Turning of jet
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Contd
Disc friction
o Due to the resistance offered by steam for turbine wheel
rotation
o Natural frictional resistance against rotation
Moisture Loss
o Moisture content wont do any work and have different
velocity with steam
o Forms a mixture with steam so that steam should drag the
water content too
o
Reduce the life of turbine blades
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EXTERNAL LOSSES
Mechanical losses
Frictional losses in the bearings
Defined by mechanical efficiency
Leakage loss
Leakage through turbine gland where the turbine shaftpass through the casing
Labyrinth packing- for prevention
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GOVERNING
To maintain the speed of turbine fairly constant at
designed speed irrespective of the load
By varying the amount of steam passed through turbine
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TYPES OF GOVERNING
Throttle(servomotor) Governing- reducing the mass
flow rate by adjusting the position of spear
Main parts are servo-motor, control valve and
centrifugal governor
Nozzle Governing- set of nozzles and valves controlled
by relay cylinder or relay governor
Cannot be used for reaction turbine as it is not a partial
admission turbine
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Contd
By-pass Governing:- by-pass line with by pass valve and
speed governors
Mostly used for multistage turbine
Here the additional mass of steam is provided to the
corresponding stage of turbine, not at initial stage
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Axial Thrust
Force due to the velocity of flow produces an
axial thrust on the rotor
=mass*change in flow velocity
Since the rotor is not allowed to move tin axial
direction this axial thrust should be taken by the
thrust bearings
Mathur page 864