4 - Fan Systems and Audit

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FAN SYSTEMS&

FAN AUDIT

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Fan Systems & Fan Audit

LEARNING OBJECTIVES

� At the end of the day the trainee shall be able to :

- Measure and qualify fan performance

• Revise fan curves

• Determine system curves

• Estimate power savings potential

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FAN BASICS &

FAN SYSTEMS

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FAN SYSTEMS - CONCEPTS OF PRESSURE

• Static pressure (ps):• pressure exerted in all directions by a fluid at rest

• Dynamic pressure (pd):• pressure exerted by the velocity of a fluid : pd = ½ρv2

• Total pressure (pt):• the sum of static and velocity pressures

TP SP VP

Air flow

pt = ps + pd

pdpspt

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k = Constant characteristicρ = Gas densityQ = Volume flow

p1 p2∆p

Q Q

∆p = k ρ Q2

FAN SYSTEMS – SYSTEM RESISTANCE

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•Each component in a system offers resistance to the gas flow.

•System resistance :The sum of all resistances to the flow

The system resistance varies with air flow rate

•System curve :The relation between system resistance and flow rate

FAN SYSTEMS – SYSTEM RESISTANCE

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Airflow rate Q

Sys

tem

res

ista

nce

∆∆ ∆∆P kp =∆

• Constant static head• Airflow through a liquid pool• Fluidized bed

Airflow rate Q

Sys

tem

res

ista

nce

∆∆ ∆∆P 2kQp =∆

FAN SYSTEMS – SYSTEM CURVES

• Complete turbulent flow• Standard fan systems

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2Qkp ρ=∆

0

2

4

6

8

10

12

0 20 40 60 80 100

Q (m³/s)

∆∆ ∆∆p

•Limit discussion to the completely turbulent system curve

FAN SYSTEMS – SYSTEM CURVES

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Pressure vs Volumetric flow rate given by the fan

•Defined for fixed specific operating conditions :

• gas density (T, p, composition)

• fan speed•Usually, power curve is given•Sometimes, efficiency curve

Flow rate

Pre

ssur

e

Pow

er/E

ffic

ienc

y

blue = pressurered = powergreen = efficiency

FAN SYSTEMS – PERFORMANCE CURVES

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1

2

•Fan Total Pressure (FTP)The European way

FTP = TP2 - TP1

= (SP2 + VP2)–(SP1 - VP1)

•Fan Static Pressure (FSP)The American way

FSP = TP2 - TP1 - VP2

= (SP2 - SP1)- VP1

•Static Pressure Rise(Good way!)= SP2 - SP1

FAN SYSTEMS – FTP / FSP CONCEPT

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• Total efficiency (ηηηηt) :

• Static efficiency (ηηηηs) :

where Q : fan flow (m³/h)FTP : fan total pressure (Pa)FSP : fan static pressure (Pa)P : fan power (W)

Fan Total Efficiency 83%Fan Static Efficiency 80%Fan “Static Rise” Efficiency 85%

PFTPQ

t ××=

3600η

PFSPQ

FTPFSP

ts ××==

3600ηη

FAN SYSTEMS – FTP / FSP EFFICIENCY

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Performance Curve for IE 250 Fan

FSP

FTP

BHP

Static Efficiency

TotalEfficiency

0

5

10

15

20

25

0 5000 10000 15000 20000 25000 30000 35000

Flow Ra te (cfm)

0

20

40

60

80

100

120

140

FAN SYSTEMS – FTP vs FSP

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BHP

0

12

25

37

50

62

2,5 5 7,5 10 12,5 15 17,5

Flow Rate [m³/s]

0

15

30

45

60

74

88

103

Power

Fan Total Pressure

Efficiency

Operating Point

Pow

er [kW] / %

efficiency

Fan

Tota

l Pre

ssur

e [m

bar]

0

FAN SYSTEMS – FAN PERFORMANCE

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FTPPower

Efficiency

00

Flow Rate [m³/s]

0

Fan

Pre

ssur

e [m

bar]

Pow

er [kW] / %

efficiency

2,5 5 7,5 10 12,5 15 17,5

15

30

45

60

74

88

103

5

10

15

20

25

30

35

40

45

50

FAN SYSTEMS – FAN + SYSTEM CURVES

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•Measured operating point may not fall on the fan curve

•Due to measurement errors and fan system effects

• In Fan Curves spreadsheet, the flow rate is assumed correct

Actual fan curve (XYZ-200)

Operatingpoint

00

Flow Rate [m³/s]

Fan

Tota

l Pre

ssur

e [m

bar]

0

Pow

er [kW]

155 7,5 10 12,52,5

5

10

15

20

25

30

35

74

147

221

295

368

442

Operating point :6,12 m³/s28,62 mbar249,9 kW

FAN SYSTEMS – REALITY

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Peak

Flow Rate

Unstable Stable

FAN SYSTEMS – OPERATING RANGES

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Two large classes •Centrifugal Fans

•Extensively used in cement plants• Clinker cooler fans• ID fan• Mill ventilation fans• Dust collector fans

•Axial Fans• Kiln shell cooling fans• Airplane’s propeller

FAN SYSTEMS – FAN TYPES

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Stationary Inlet

Inlet Bell

Scroll

CutoffBackplate

Blades

Inlet Guide Vanes

Impeller

Inlet Outlet

Side Sheet

HubFlange

Exploded View of a Centrifugal Fan


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Diffuser

Tailpiece (sometimes omitted)

Outlet

Outer Cylinder

Inner Cylinder

Belt Fairing

Discharge VanesInlet

Blades

Hub

Impeller

Inlet Bell

Cutaway of a Vane-Axial Fan


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vr : radial velocity

vt : tangential velocity

v : fluid velocity

vt

vr vCentrifugal Fan Principles

FAN SYSTEMS – FAN PRINCIPLES

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•Different types of fans characterized by their blade type:•Straight radial•Forward curve•Radial tip•Backward inclined - flat blade•Airfoil

•Different applications require different blade type•Each type has different fan performance curves

FAN SYSTEMS – FAN BLADES

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Airfoil (AF) : 85 - 90 %

Backward-curved (BC) : 85 %

Backward-inclined (BI) : 75 - 80 %

Radial-tip (RT) : < 71 %

Forward-curved (FC) : 65 %

Radial blade (RB) : 60 - 63 %

FAN SYSTEMS – FAN BLADES

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•Fan performance curve made for specific conditions:•fan speed•air density (temperature, pressure)•Always AT INLET CONDITIONSAT INLET CONDITIONS

•What happens in other conditions?•What if I change the speed of the fan? (N)•What if the gas density changes? (ρρρρ)•What if I change the size of the fan? (D)

FAN SYSTEMS – FAN LAWS

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• Geometrically similar (impeller) fans• Volume is directly proportional to fan speed

• Pressure is proportional to the square of the speed

• Power is proportional to the speed cubed

QQ

NN

2

1

2

1=

2

1

2

1

2��

��

�=

NN

TPTP

PP

NN

2

1

2

1

3= �

��

�

��

FAN SYSTEMS – FAN LAWS

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•Fan Details :n = 1305 [min-1] v (1) = 7,1 [m³/s]dpT = 7,6 [kPa]P = 66,5 [kW]

Increased flow (2)v (2) = 8,5 [m³/s]n, dp , P =?

Exercise 5.1 – FAN LAWS

Question :

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Exercise 5.1 – FAN LAWS

Solution :

1

1

122 min1562

7.113058.5

QNQ

N −=×==

kPa10.8913051562

7,6NN

TPTP22

1

212 =�

�

��

�×=��

��

�=

kW114.013051562

66.5NN

PP33

1

212 =�

�

��

�×=��

��

�=

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1822 min -1

00

Flow Rate [m³/s]

0

Po w

e r [kW]

Fan

Pre

ssur

e [m

bar]

1518 min -1

1670 min -1

2,5 5 7,5 10 12,5 15 17,5 20

40

75

110

150

185

12,5

25

37,5

50

62,5

75

FAN SYSTEMS – FAN LAWS - SPEED

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•Fans are constant volume machines

•Affects the pressure generated and power consumed

•A change in density affects the system curve

FAN SYSTEMS – FAN LAWS - DENSITY

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1.13 kg/m³

0.56 kg/m³

0

5

10

15

20

25

30

35

40

45

50

0 2,5 5 7,5 10 12,5 15 17,5

Flow Rate [m³/s]

0

15

30

45

60

75

90

105

120

Po w

e r [kW]

Fan

Pre

ssur

e [m

bar]


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Dust collector on inlet sideof fan• total pressure drop: 20

mbar• inlet of fan - 20 mbar

Clinker cooler fan• total pressure drop: 20

mbar• inlet of fan: atmosphere

For same air flow, fan selection would be differentdue to density change at fan inlet


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•Used mostly to compare two similar fans of same type

•Some examples of “tipping out” fans

•Casing often has to be redesigned

FAN SYSTEMS – FAN LAWS (SIZE)

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D

+10% D

0

5

10

15

20

25

30

35

40

0 2,5 5 7,5 10 12,5 15 17,5

Flow Rate [m³/s]

0

7,5

15

22,5

30

37,5

45

52,5

60

67,5

75

Po w

e r [kW]

Fan

Pre

ssur

e [m

bar]

D = Diameter

FAN SYSTEMS – FAN LAWS (SIZE)

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• Requirement to control the air flow from a fan• The system resistance curve governs the fan

output• Air flow can be changed by changing :

• The fan curve• The system resistance curve

• Available methods:• Discharge dampers• Variable inlet vanes• Box-vane control• Variable speed drives

FAN SYSTEMS – FAN CONTROL

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Parallel Blades Opposed Blades

� Yields more uniform profile� More linear response

� The discharge dampers change the system resistance curve

� Power wasting devices

•More flow on one side of duct•Not very linear response

FAN SYSTEMS – FAN CONTROLDischarge Dampers

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40% open

60% open

80% open

Wide open

20% open

0

15

30

45

60

75

90

0 5 10 15 20 25 30 35 40

Flow rate [m³/s]

Fan

Pre

ssur

e [m

bar]

FAN SYSTEMS – FAN CONTROLDischarge Dampers

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Closed Position

Open Position

• Installed at the inlet of the fan•Controls volume and direction

of air flow•Designed to give a spin to the

air in the direction of the impeller

•Pre-spin unload the impeller reducing the pressure

•Less pressure implies less power

FAN SYSTEMS – FAN CONTROLVariable Inlet Vanes

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100%75%50%25% open0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 400

7,5

15

22,5

30

37,5

45

52,5

60

67,5

75

Flow rate [m³/s]

Pow

er [kW]

Pre

ssur

e FAN SYSTEMS – FAN CONTROL

Variable Inlet Vanes

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•Similar to inlet vanes: pre-spin effect

•Blades always parallel •Must be used with an inlet box•Power efficiency: a little less

than inlet vanes.•Easier to maintain than inlet

vanes (hot and dusty applications)

FAN SYSTEMS – FAN CONTROLBox Vane Control

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Box vane controls


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•Direct application of the fan laws:

•Maintain same efficiency at different speeds

•The most efficient method of controlling fans

•Higher initial cost

1

2

1

2

NN

QQ =

2

1

2

1

2��

��

�=

NN

TPTP

3

1

2

1

2 ��

��

�=NN

PP

FAN SYSTEMS – FAN CONTROLVariable Speed Fans

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η22 2

2

12

11

2

1

2

12

1

31 1

1= × =

�

��

�

�� × �

��

�

��

�

��

�

��

= ×Q TPP

QNN

TPNN

PNN

Q TPP

Q QNN2 1

2

1= �

��

�

�� TP TP

NN2 1

2

1

2= �

��

�

�� P P

NN2 1

2

1

3= �

��

�

��

Fan Laws :

η11 1

1= ×Q TP

PEfficiency :

FAN SYSTEMS – FAN CONTROLVariable Speed Fans

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Dischargedamper Box v

ane control

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110

% Flow

% P

ower

Variablespeed

Inlet vanes

FAN SYSTEMS – FAN CONTROLPower Savings

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• (a) Plot the operating point on the fan curve• (b) What is the speed of the fan?• (c) What is the shaft power of the fan?• (d) Is the fan selection good? Why?

Exercise 5.2 – VARIABLE SPEED ID FAN

Question :

Fan InletP = -70mbar

T = 350C

Fan OutletP = -2mbarT = 350C

Flow = 144.4m3/s

100% open

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Solution :•REMEMBER – ALWAYS CONVERT TO INLET

CONDITIONS !• 144.4m3/s x 3600 = 519840m3/hr at the outlet• Remember ‘Ideal Gas Law’• PV = nRT or P1V1/T1 = P2V2/T2• So, Flow at inlet Conditions,

• = 519840 x (1013-2)/(1013-70) = 557327m3/hr• From Performance curves,• Delta P = 68mbar => Speed is 990rpm• Shaft Power is 2025kW

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Belt drive•Standard speed motors can

be used • no need for slow speed motors (expensive )

•Exact fan speed for required air and volume can be obtained

•Speed can be adjusted by simply changing pulley ratio

Direct drive•Reduces initial cost if

standard speed motor could be used

• no extra supports, pulleys, bearings, shafts

•Elimination of power loss by belt drive (5 to 10%)

•No maintenance required from stretching belts

FAN SYSTEMS – FAN CONTROLDrive Arrangements

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• Important points are :• Safety margin : 10% on volume & 10% on pressure• Recommended maximum fan speed

• Far enough to be from critical speed• Variable inlet vane dampers(clean gas) or variable speed

• V-belt drive• Often required :

• Piezometer• Silencer

FAN SYSTEMSLAFARGE PREFERRED SPECIFICATIONS

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FAN AUDIT

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Objective :

Reduce energy consumption

Indicators:• kWh absorbed by the motor• Fan efficiency• Fan reliability factor

FAN AUDIT

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FAN AUDIT

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FAN AUDIT

Hardfacing was removed Deformation Crack

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• Many fans don’t operate at optimal point• Operating conditions has changed since installation• Bad initial selection at installation

�� Energy is wasted• Audits every five years or more frequently

• To check operating conditions of the fan• Identify improvements to bring it to optimal

performance (efficiency)• Identify solutions if fan is a process bottleneck

• Coordination between Maintenance, Production and Process departments

FAN AUDIT PROGRAM

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•List fans that make up for 80% of power consumption

•Operating conditions, new and future, adapted to Process requirements

•Maintenance history of the fans

•Fan curves

•Ducting arrangement / flowsheet

•Adequate position of measuring points

•Creation of a common (Maintenance, Process) file

FAN AUDIT - PREPARATION

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•Flow rate• For one operating point• More could be required for fans with wide ranges of operating point

•Static pressure at fan inlet• after damper if any• before variable inlet vane if any

•Static pressure at fan outlet • before damper if any

•Static pressure on other side of damper• to determine pressure drop through damper

•Damper opening

FAN AUDIT - MEASUREMENT

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•Gas temperature and composition• For density

• Moisture content if significant

• Correction for dust load if significant

•Fan speed•Absorbed power •Fan elevation •Atmospheric pressure•Ambient air temperature

FAN AUDIT - MEASUREMENT

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MEASUREMENT FOR FAN STATIC PRESSURE

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• External inspection (fan running):

• Bearings temperature

• Vibrations of bearings and housing

• Noise level

• Leakage (holes in housing, ducting,…)

• Even air flow distribution at fan inlet

• Pressure drop through dust collector (if any)

FAN AUDIT – MECHANICAL EVALUATION

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• Internal inspection (fan stopped) :

• Fan impeller alignment

• Impeller and housing: wear and material accumulation

• Dampers: proper opening / closing, damages and material accumulation

• Louvers: configuration of blades, functioning of individual blade

• Turning vanes condition


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• Internal inspection (fan stopped) :

• Belt drive : tension and wear

• Coupling alignment

• Internal cone adjustment (too large a gap?)

• Accumulation of material in the duct

• Type of fan wheel –

To confirm drawings / fan curve

• Validation of existing drawings


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• Fan system diagram showing :

• fan

• damper (position and type)

• position in relation with its environment

• elbows

• duct expansions, contractions

• turning vanes

FAN AUDIT – ADDITIONAL INFORMATION

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• Statistics on flow, pressures and damper opening

• Aspen / IP21

• Fan curve

• Correct to actual conditions (elevation, temperature, density, RPM)

• Fan and motor name plate information

FAN AUDIT – ADDITIONAL INFORMATION

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•Position operating point on fan curve

• Does it match? Why?

•Calculate efficiency

• Is it good?

•How can we improve the efficiency?

•How can we increase flow or pressure, if required?

FAN AUDIT – EVALUATION

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Duty Cycle :

•For fans with highly variable flow demand such as:

• Cooler exhaust

• Kiln I.D.

• Some cooler fans

�� Variable Frequency Drive (VFD) ?


20/01/2009 - 67


• For constant duty fans such as:

• Mill draught

• Primary air

�� Find most cost effective solution to

gain efficiency

• Replace belt (and motor?)

• Modify impeller?


Duty Cycle :

20/01/2009 - 68


What Is The Damper Saying?

• If fan damper never open > 75%

then fan is oversized and wasteful

�� Downsizing?

�� Replace belts?

�� VFD?


20/01/2009 - 69


• Poor efficiency fan design costs all the time

• Many pre-1985 fans are straight radial :

• 60-75% efficiency

• Current technology :

• Curve radial (dirty air) : to 82% efficiency

• Airfoil (clean air) : to 85% efficiency

Fan Design –


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Low Cost Solutions –• Is Hermit Crab solution possible?

• Can shaft & bearing be retained?

• Can you take advantage of the need to replace an impeller for maintenance reason?

• Is there a retired fan that could do the job?

• Inlet turning vanes can improve efficiency by 2%

• Is there a way to modify ducting configuration to reduce system effect?


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•Summary• Measurements• Mechanical observations• Other information• Findings

•Fan curve• Original• Corrected to actual conditions with operating point and duty cycle

•Recommendations•Follow up of actions• Include action plan in PIP

FAN AUDIT – REPORT

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Documents

4 - Fan Systems and Audit