Induced Draft Fan

INDUCED DRAFT FAN

RITESH KUMAR CONSTRUCTION , M6K221 MECHANICAL ENGGINEER

Reference : “shaft alignment hand book", ‘john piotrowski’.

DRAFT SYSTEM

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Supply air for combustion

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Remove air from furnace

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Transporting pulverized coal to furnace

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Maintain speed for heat transfer

DRAFT SYSTEM

TYPES OF FANS Axial fan

Centrifugal fan

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Fluid comes axially .

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Fluid comes axially.

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Get compressed while moving axial.

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Get compressed because of  centrifugal action.

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Move out axially.

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Comes out of fan radially.

CENRIFUGAL FANS

ERECTION OF INDUCED DRAFT FANS 

Blue matching

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Erection of packer plates

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Primary alignment

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Secondary alignments

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Inlet cone erection

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Erection of casing

Blue matching of machine foot , wedge plates and packer plates is done

Leveling device

Packer plate

Erection of packer plate 

Elevation

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Flatness

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Co planarity

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positioning

Master level

Installation of induced draft fan

Result of improper installation 

Lose of production

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Premature failure of machine components

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Loss of energy

To insure ,before alignment some primary checks are done

Preliminary checks 

Coupling half runout

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Soft footing

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Bearing

Runout check  Runout standards

 Maximum times these standards are provided by manufacturer 

Shaft speed

Maximum runout

0-1800 rpm

0.13 mm

1800-3600 rpm

0.08 mm

3600+ rpm

0.05 mm

Ways of detecting soft foot Dial indicator

shims

Checking procedure Dial indicator

shims

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First tight the bolt

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Place dial indicator

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Loose bolts

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See dial show less then 0.06 mm.

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Check gap between foot and pedestal by shims at four sides of foot.

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They should come same other wise soft footing is there.

Repeat this for all foots Eliminating soft footing is most time consuming and skilled job. Machines on maximum occasion have vibration problem due soft footing This is very much a trial and error method because every time you need to put shims as per calculation and check for soft footing till you eliminating. •

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Example of soft footing 

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This condition is outcome of  bad blue matching, leveling and complaining of packers

Reasons of soft footing Insufficient contact between machine foot and pedestal.  Noncoplanarity foot or pedestal .

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 Alignment basics

Types of misalignments

Measured in “mm”

Measured in “degree of angle”

Calculation of angular misalignment 

 Alignment methods

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Face to Rim method

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Reverse indicator method

There other method to align shafts but these are predominantly used. Reverse indicator method is more accurate then Face to Rim method.

Face to Rim method  Advantage of Face to Rim method above reverse indicator method , it can be used for alignment  when rotating both shaft is not possible

Both shafts are moving so facial reading is going to be effected. To overcome ,we are using two extra dial indicator .

Set of readings

 Making compensation for shaft movement is important .

Other arrangement used in Face to Rim method 

 Axial reading = A1- A2/2

Why compensation is this? 

Dimension required

 Accuracy of Face to Rim method depends on diameter at which facial reading is taken.

How to align •

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Electrical machines have magnetic center , so motor shaft first needs to be positioned to it magnetic center , only then alignment is done. With above mentioned formula calculate position of Driver and Driven machine inboard and outboard foots. Find out movement of inboard and outboard foots to make shafts collinear and coplanar. This can easily be done by using graph paper Individuals do it on hit and trial basis but by using above process can be done in less time.  Magnetic center of electric machines is specified by manufacturer 

Interpretation of dial reading in 3-D view

Example

Page 361

Reverse indicator method

Same process of finding movement of Drive and Driven machine inboard and outboard foot to align shafts we follow here.

Why to rotate both shaft 

 Alignment specification 

Most of time provided by manufacturer

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Depends on coupling ,speed and power. NTPC allowable tolerance Parallel misalignment

Angular misalignment

Output of rotor in KW  Coupling

Flexible coupling

Rigid coupling

Speed rpm

Below 400 (mm)

400-1000 (mm)

Over 1000 (mm)

2500-4000

0.01

0.02

0.03

1300-2500

0.025

0.04

0.06

Below 1300

0.04

0.06

0.10

2500-4000

0-01

0.02

0.03

Below 2500

0.015

0.03

0.04

Output of rotor in KW  Coupling

Flexible coupling

Rigid coupling

Speed rpm

Below 400 (mm)

400-1000 (mm)

Over 1000 (mm)

2500-4000

0.02

0.03

0.04

1300-2500

0.06

0.07

0.10

Below 1300

0.08

0.10

0.15

2500-4000

0.02

0.03

0.04

Below 2500

0.03

0.04

0.05

OPERATION PROBLEM 

Part load

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Fluctuating weather

FLOW CONTROLING 

Dampers

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Inlet vane

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Hydraulic variable speed coupling

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Variable frequency drive

Damper

Inlet vane

Inlet control valve is also controlled by motor actuators  Actuator

Mechanical comparison Speed (flow)

Hours

Load KW 

Fluid coupling KWh

 Variable speed coupling

100 %

400

933

439,000

439,000

90%

800

680

725,000

648,000

80%

4000

477

2,935,000

2,229,000

70%

2000

320

1,143,000

780,000

60%

800

201

350,000

201,000

 Annual KWh

5,559,000

4,367,000

 Annual energy cost

$279,600

$218,350

Maintenance costs

$20,000

$10,000

Total annual costs

$299,600

$228,350

Capital costs

$235,000

$295,000

Total five year cost

$1,733,000

$1,436,000

Total ten year cost

$3,231,000

$2,578,000

Cost summary 

The application considered above is an induced draft fan •

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Horse power 1250 hp. Cost of energy is $0.05.

Reference : “Fan drive system efficiencies", Universal Dynamics Limited, ‘ADAM CREERY ‘.