Lecture10-StreamTurbines-ReactionTurbine

8/9/2019 Lecture10-StreamTurbines-ReactionTurbine

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Page 1of 7Lecture 10

Lecture 10: Steam Turbines

Reaction TurbineReaction Turbine

A reaction turbineis one that is constructed of rows of xed and rows ofmoving blades. The xed blades act as nozzles. The moving blades move as aresult of the impulse of steam received (caused b change in momentum!and also as a result of expansion and acceleration of the steam relative tothem. "n other words# the also act as nozzles. The enthalp drop per stage ofone row xed and one row moving blades is divided among them# oftene$uall. Thus a blade with a %0 percent degree of reaction# or a %0 percent

reaction stage# is one in which half the enthalp drop of the stage occurs inthe xed blades and half in the moving blades. The pressure drops will not bee$ual# however. The are greater for the xed blades and greater for thehigh&pressure than the low&pressure stages.

The moving blades of a reaction turbine are easil distinguishable from thoseof an impulse turbine in that the are not smmetrical and# because the actpartl as nozzles# have a shape similar to that of the xed blades# althoughcurved in the opposite direction. The schematic pressure line ('ig. 10.1!shows that pressure continuousl drops through all rows of blades# xed andmoving. The absolute steam velocit changes within each stage as shownand repeats from stage to stage. 'igure 10. shows a tpical velocit diagram

for the reaction stage.

Pressure and enthalp drop both in thexed blade or stator and in the movingblade or Rotor

Degree of Reaction=


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R= 1 2

) ver widel used design has half degree of reaction or %0* reaction and

this is +nown as Parson,s Turbine. This consists of smmetrical stator androtor blades.

The velocit triangles are smmetrical and we have

'rom the inlet velocit triangle we have#

= -or+ done (for unit mass ow per second!

Therefore# the /lade ecienc

b= 1 1

2 2

Figure 10.1: Three stages of reaction turbine indicatingpressure and velocity distribution


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Put= then

b= 1 2

'or the maximum ecienc b =0 and we get

opt= =cos 1

can be found out b putting the value of in the

expression for blade ecienc

(b )max= cos 1

2

=is greater in reaction turbine. nerg input per stage is less# so there is

more number of stages.

Stage Efciency and Reheat actor

The Thermodnamic e2ect on the turbine ecienc can be best understoodb considering a number of stages between two stages 1 and as shown in'igure 10.3.

The total expansion is divided into four stages of the same ecienc

and pressure ratio.


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Page !of 7Lecture 10

1 = x = y = z

The overall ecienc of expansion is . The

actual wor+ during the expansion from 1 to is

4r#

o= a =

Reheat !actor (R " # " )=

Reheat !actor (R " # " )= 1( x) y$ z*

5.' is 1.06 to 1.03

"f remains same for all the stages or is the mean stage ecienc.

s=1x = xy = yz = z2

s=1x xy yz z 2

s=

-e can see7

"igure 10#!: $i%erent stage o asteam turbine


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Page &of 7Lecture 10

This ma+es the overall ecienc of the turbine greater than the individualstage ecienc.

The e2ect depicted b e$uation is due to the thermodnamic e2ect called8reheat8. This does not impl an heat transfer to the stages from outside. "tis merel the reappearance of stage losses an increased enthalp during the

constant pressure heating (or reheating! processes )9# /:# ;< and =.

Losses in steam turbines

>team turbines extract energ from high&pressure steam and convert it intoshaft wor+. The total energ content available in steam supplied to the steamturbine is not completel recovered in the form of mechanical energ. Thereare certain losses in energ of steam which occur inside a turbine.

1. )dmission losses

. Lea+age losses

6. 'riction losses

3. xhaust loss

%. 5adiation and convection losses

?. Losses due to moisture

@. ;arr over losses

Admission 'osses

"n practice the ow of steam through nozzle is not isentropic# butaccompanied with losses which decrease the +inetic energ of steam comingout of the nozzle.

The decrease in +inetic energ is due to the following reasons

Aiscous forces between steam particles

Beat loss from steam before entering the nozzle

=eection of ow in the nozzle

/oundar laer development in the nozzle

Turbulence in the nozzle


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Page (of 7Lecture 10

The friction in the nozzle which reduces available enthalp drop andhence actual velocit leaving the nozzle is less than that obtained withisentropic expansion

Lea)age 'osses

>team leaves the boiler and reaches the condenser after passing through themain valve# regulating valves# nozzles# clearance spaces between nozzlesand moving blades# diaphragm and rotating shaft etc. 'urther there is largepressure di2erence between inside of steam turbine and the ambient andalso from one location to another location across these devices.

Therefore steam lea+age ta+es place through

Cain valve and regulating valve

>eals and glands

>paces between nozzles and moving blades

>paces between diaphragm and shaft of turbine

>pace between moving blade rings and turbine casing

Lea+age of steam through these is a direct loss of energ.

"riction 'osses

'rictional resistance is o2ered during ow of steam through nozzles onmoving and stationar blades. "n most of the turbines the blade wheels rotatein a space full of steam. The viscous friction at the wheel surface causesadmission losses as steam passes from nozzle to wheel. The e2ect of partialadmission creates eddies in the blade channels.

The surface of curved moving blades and stationar blades o2ers resistance#which increases with increase in roughness of blade surface and relative

velocit between steam and rotating blade.

The energ loss also ta+es place when the steam Dets turns along thecurvature of the blade surface. The turning losses depend on the angle ofturning.

E*haust 'oss


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The energ content of steam is not full utilized in the turbine. =espite ofbeing at ver low pressure the exhaust coming out of the turbine andentering the condenser carries some of +inetic energ and useful enthalp#which is direct energ loss.

Radiation and con+ection 'osses

The steam turbine operates at a relativel high temperatureE therefore someof the heat energ of steam is radiated and convected from the bod of theturbine to its surrounding. These direct losses and are minimized b properinsulation,s.

Losses due to moisture

The steam passing through the last stage of turbine has high velocit andlarge moisture content. The li$uid particles have lesser velocit than that ofvapor particles and hence the li$uid particles obstruct the ow of vapor

particles in the last stage of turbine and therefore# a part of +inetic energ ofsteam is lost. "f the drness fraction of steam falls below 0.FF# the erosionand corrosion of blades can also ta+e place.

,arry o+er 'osses

-hen steam passes from one stage to another through the diaphragm# someenerg losses ta+es place# which are referred to as carr over losses andtherefore# the +inetic energ of steam available at succeeding stage ofmoving blades for utilization is less than that of the exit at preceding stage.

This is due to formation of eddies in annular space between the nozzle andmoving blades.

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Lecture10-StreamTurbines-ReactionTurbine