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Irreversible Flow from Turbine Exit to Condenser

P M V Subbarao

Professor

Mechanical Engineering Department

I I T Delhi

Irreversibilities due to Closed Cycle Policy ..

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The Last Stage of LP Turbine

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First Stage of A Turbine : Governing Stage

A governing stage is the first stage in a turbine with nozzle

steam distribution. The principal design feature of a governing stage is that its

degree of partiality changes with variations of flow rate throughthe turbine.

The nozzles of a governing stages are combined into groups,

each of them being supplied with steam from a separategoverning valve.

A governing stage is separated by a spacious chamber from thesubsequent non-controlled stages.

Governing stages may be of a single-row or two-row type.

Single row impulse governing stage is employed for an enthalpydrop of 80-120 kJ/kg.

Two row governing stages are used when enthalpy drop is high,100 250 kJ/kg.

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Governing Stage

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Selection of Enthalpy Drop & Type of Governing

stage

The enthalpy drop & type of governing stages are selected by

considering the probable effect of the governing stage on the

design and efficiency of the turbine.

Higher the number of governing stages, lower will be the

number of other stages.

A high enthalpy drop in governing stage ensures a lower

temperature of steam in its chamber and permits application of

less expensive materials.

In high capacity steam turbines, a single-row governing stages

are preferred, since the advantages of elevated enthalpy drop

are justified economically.

The efficiency of governing stages,

in

inVustageg

T

p

m

k0002.0

83.0/,

in

inVustagesg

T

p

m

k0002.0

8.0/2,

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Steam Path in Non-Controlled Stages

Estimate approximate mass flow rate of steam by assuming an

overall turbine internal efficiency of 0.85.

Calculate flow through the condenser, using optimum of number

of FWHs. (Using Cycle Calculations).

Calculate Modified Efficiency of Low volume and intermediate

volume stages.

For a group of stages between two successive FWHs.

Z

h

m

lgroup

iso

steam

avgroup

listages

,1

2sin1

2000

6001

5.0925.0

groupegroupiav ..

Average density is calculated as

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The efficiency of groups of very high volume stages:

While designing the steam path, it is essential to consider the

pressure losses in the following:

Pressure loss in reheater: 0.1 prh.

Pressure loss in connecting pipes between turbine

cylinders:0.2ppipe.

group

iso

ev

group

isogegigroup

hvh

hhxx

10000

4001

28.01870.0

,,

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Internal Reheating due to Irreversibilities

3

4s

4IIs

4IIIs

4Is

4Vs

4IVs

4Ia

4IIa

4IIIa

4IVa

4Va

4VIs4VIa

T

s

Governing group

Group 1

Group 2

Group 3

Group 4

Group 5

Macro available enthalpy:

Micro available enthalpy:

shh 43

...444443

sasas IIIIIIIII

hhhhhh

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Macro available enthalpy:

Micro available enthalpy:

s

hh43

N

Ijsas jjI

hhhh 14443

Reheat Factor:

N

Ijsas

sh

jjI hhhh

hhR

14443

43

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Internal Reheating due to Irreversibilities : HP

3

4s

4IIs

4IIIs

4Is

4Vs

4IVs

4Ia

4IIa

4IIIa

4IVa

4Va

4VIs4VIa

T

s

Governing stage

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

22.33 MPa,3379.0

15.74 MPa,3303.0 k J/kg

13.77 MPa, 3269.0 k J/kg

12.12 MPa, 3236.5.0 k J/kg

10.56 MPa, 3203.8 k J/kg

9.2 MPa, 3171.0 k J/kg

7.94 MPa, 3140.4 k J/kg

4VIIa6.9 MPa, 3104.9 k J/kg

4VIIIa

5.17 MPa, 3036.7 k J/kg4IX

a

5.95 MPa, 3070.9 k J/

Pho=5 %

Pho=19.5%

Pho=21%

Pho=22%

Pho=23.5%

Pho=25%

Pho=30%

Pho=32%

Stage 6

Stage 7

Stage 8

Pho=35%

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0

2

4

6

8

10

12

14

HP1(1s

tSt.)

HP4

HP7

HP10

HP13

HP16 IP

1IP

4IP

7IP

10

IP13

LP2

LP5

Stages

Loss

(kJ/kg) Cumulative loss

Cumulative Losses for All Stages : 500 MW

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Definition of Efficiency

Relative blade efficiency is calculated as:

Internal Relative Efficiency is calculated as:

dropEnthalpyEffective

lossBladeMoving&Nozzle-dropEntalpyEffectiverel

dropEnthalpyEffectivelossprofile-lossleakage-lossesBladeMoving&Nozzle-dropEntalpyEffectiveint, rel

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Blade Efficiency & Internal Relative Efficiency: 800 MW

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Efficiency

Stage No

Relative Blade efficiencyRelative internal efficiency

LP Cylinder

efficiency=78.0

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LP Turbine Exhaust System

In a condensing steam turbine, the low-pressure exhaust hood,consisting of a diffuser and a collector or volute!, connects the last

stage turbine and the condenser. The function of the hood is to transfer the turbine leaving kinetic

energy to potential energy while guiding the flow from the turbineexit plane to the condenser.

Most of exhaust hoods discharge towards the downward condenser.

Flow inside the hood therefore must turn about 90 deg from theaxial direction to the radial direction before exhausting into thecondenser.

The 90-deg turning results in vortical flow in the upper half part ofthe collector and also high losses.

The exhaust hood is one of the few steam turbine components thathas the considerable aerodynamic losses.

It is a challenge for engineers to operate a hood with high pressurerecovery and low total pressure loss in a compact axial length.

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Exhaust Hood

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Exhaust Diffuser For L P Turbine

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Steam Turbine Exhaust Size Selection

The steam leaving the last stage of a

condensing steam turbine can carry

considerably useful power to the

condenser as kinetic energy.

The turbine performance analysis needs to

identify an exhaust area for a particular

load that provides a balance between

exhaust loss and capital investment inturbine equipment.

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Path Lines in Exhaust Hood

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Exhaust Losses

Exhaust losses are losses which occur between last

stage of turbine and condenser.

Exhaust losses made up of four components:

Actual leaving losses Gross hood loss

Annulus restriction loss

Turn up loss

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Residual velocity loss

Steam leaving the last stage of the turbine has certain velocity, which

represent the amount of kinetic energy that cannot be imparted to theturbine shaft and thus it is wasted

Exhaust end loss

1. Exhaust end loss occur between the last stage of low pressure turbineand condenser inlet.

2. Exhaust loss depends on the absolute steam velocity.

Turbine Exhaust end loss

= Expansion-line -end point - Used energy at end point.

T i l h t l h i di t ib ti f t l

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Turn-up loss

Total Exhaust

Loss

Gross hoodloss

Actual leaving

loss

Annulus

restriction loss

Annulus Velocity (m/s)

ExhaustLoss,

kJ/k

gofdryflow

0 120 150 180 240 300 360

10

20

30

40

50Annulus velocity (m/s)

Condenser flow

rateAnnulus area

Percentage of Moisture atthe Expansion line endpoint

Typical exhaust loss curve showing distribution of component loss

SP.Volume

an

steamexan

A

xvmV

3600

01.01.

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Optimal Design of Exhaust Hood

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Performance Analysis of Power Plant

Condensers

P M V Subbarao

Professor

Mechanical Engineering DepartmentI I T Delhi

A Device Which makes Power Plant A True Cycle..

A Device Which set the limit on minimum cycle

pressure..

T S Di R ki C l ith FWH

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T-S Diagram : Rankine Cycle with FWHs.

?,, exitcondincond pp

inCWT ,outCWT ,

?TTD

exhaustturbinep ,

hoodp

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A Device to Convert Dead Steam into Live Water

Water ready to take

Rebirth

Dead Steam

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Steam Condenser

Steam condenser is a closed space into which steam exits the turbine and is forcedto give up its latent heat of vaporization.

It is a necessary component of a steam power plant because of two reasons. It converts dead steam into live feed water.

It lowers the cost of supply of cleaning and treating of working fluid.

It is far easier to pump a liquid than a steam.

It increases the efficiency of the cycle by allowing the plant to operate on largest

possible temperature difference between source and sink.

The steams latent heat of condensation is passed to the water flowing through thetubes of condenser.

After steam condenses, the saturated water continues to transfer heat to coolingwater as it falls to the bottom of the condenser called, hotwell.

The difference between saturation temperature corresponding to condenservaccum and temperature of condensate in hotwellis called condensate depression.

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Two-Pass Surface Condenser

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Layouts of A Condenser

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Layouts of A Condenser

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An Integral Steam Turbine and Condenser System