thermal engineering.pptx

7/27/2019 Thermal Engineering.pptx

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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.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

thermal engineering.pptx

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