Quạt và Máy thổi công nghiệp: Tổng quan về Fans and Blowers

PEMP RMD 2501

Fans and Blowers Fans and Blowers

Session delivered by: Session delivered by:

Prof. Q.H. Nagpurwala Prof Q H Nagpurwala Prof. Q.H. Nagpurwala Prof Q H Nagpurwala

1 06a

PEMP RMD 2501

Session Objectives

This session is intended to discuss the following:

• Types and working principle of fans and blowers • Applications of various fans and blowers • Pressure rise, flow coefficient and efficiency • Pressure rise flow coefficient and efficiency • Velocity triangles • Performance characteristics • Fan laws

2 06a

PEMP RMD 2501

What is a Fan?

 Any device that produces a current of air by the movement of broad surfaces can be called a fan.  Fans fall under the general classification of “turbomachinery”  Fans fall under the general classification of turbomachinery and have a rotating impeller at least partially encased in a stationary housing.  Fans are similar in many respects to pumps. Both are turbomachines that transfer energy to a flowing fluid. It is easy to distinguish between fans and pumps: pumps handle liquids; fans handle gasses.  Broadly speaking, the function of a fan is to propel, displace, or move air or gas. move air or gas.

3 06a

PEMP RMD 2501

Fans, Blowers and Compressors

diff

d b

F Fans, blowers and compressors are differentiated by the method used to h d move the air, and by the system pressure they must operate against.

Difference Between Fans, Blower and Compressors Difference Between Fans Blower and Compressors

Equipment

Specific Ratio

Pressure Rise (mmWC)

Fans Fans

Up to 1.11 Up to 1 11

1136 1136

Blowers

1.11 to 1.20

1136 to 2066

Compressors Compressors

More than More than 1.20

As per American Society of Mechanical Engineers (ASME) the specific ratio – the ratio of the is used discharge pressure to the suction pressure – is used discharge pressure to the suction pressure for defining the fans and blowers

4 06a

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Components of Fan/Blower System

Turning Vanes (typically used on short radius elbows)

Outlet Diffusers O tl t Diff

Heat Exchanger

Baffles

Provide air for ventilation and industrial industrial processes that need air flow

Filter

Inlet Vanes

Motor Motor Controller

Centrifugal Fan/Blower

Variable Frequency Drive

Belt Drive

Motor

5 06a

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Parts of a Fan / Blower

1 I ll Impeller 1. 2. Blade 3. Shroud 4. Hub 4 H b 5. Housing Inlet 6. 7 Outlet 7. Outlet 8. Guide Vanes

• Centrifugal housing

include side plate and

scroll sheets. ll h t

• Axial housing includes the outer and inner cylinder, belt tube

6 06a

PEMP RMD 2501

Fan Types

h di

ifi d

h h

f fl

F Fans are classified according to the direction of flow through the impeller:

 Axial Flow: Air flows through the impeller parallel to, and at a  Axial Flow: Air flows through the impeller parallel to and at a constant distance from the axis. The pressure rise is provided by the direct action of the blades  Centrifugal or radial flow: Air enters parallel to the axis of the fan and turns through 900 and is discharged radially through the blades. The blade force is tangential causing the air to spin with the blades and the main pressure is attributed to this centrifugal force  Mixed flow: Air enters parallel to the axis of the fan and turns through an angle which may range from 300 to 900. The pressure rise is partially y by direct blade action and partially by centrifugal action  Cross Flow: air enters the impeller at one part of the outer periphery flows inward and exits at another part of the outer periphery. flows inward and exits at another part of the outer periphery

7 06a

PEMP RMD 2501

Centrifugal Fans

R t ti

it i  Rotating impeller increases air velocity

 Air speed is converted to pressure

 High pressures for harsh conditions  High pressures for harsh conditions

• High temperatures • Moist/dirty air streams • Material handling i l h dli

M t

 Categorized by blade shapes

• Radial • Forward curved • Backward inclined

8 06a

PEMP RMD 2501

Centrifugal Fan Impeller Types

Backward inclined Radial tip Blades

Open Type Open Type Backward inclined Radial Tip Blades

Airfoil Blades withwith Higher Efficiency

Forward Curved Blades Type Open Type Backward inclined Radial Tip Blades Tip Blades

Backward inclined radial di l blade

9 06a

PEMP RMD 2501

Centrifugal Fans

Paddle blade or radial fan

Backward curved

Forward-curved fans are used in clean environments and operate at lower temperatures. Well suited for temperatures Well suited for low tip speed and high- airflow at lower pressures

Forward curved or multi-vane radial fan

Radial fans have high static hi h t ti R di l f pressures (up to 1400 mm WC) and can handle heavily contaminated airstreams. Well suited for high temperatures and medium blade tip speeds Backward-inclined fans are more efficient than forward- curved fans. Also known as curved fans. Also known as "non-overloading" because changes in static pressure do not overload the motor

10 06a

PEMP RMD 2501

Applications of Centrifugal Fans

• Augmenting Air Fan • Scanner Air Fan • Booster Air Fan • Burner Air Fan • Degasser Blower • Degasser Blower • Combustion Air Fan • Oil Vapour Exhaust

g g

Fan

• Purge Gas Blowers • Inline Fans • Supply Air Fan • Exhaust Air Fan Exhaust Air Fan • Ventilation Fan • Radial Blowers • Turbo Blowers (Centrifugal)

• FD Fan • ID Fan • In-series Blowers In series Blowers • Igniter Air Fan • Seal Air Fan 06a

PEMP RMD 2501

Centrifugal Fans

Type T

i ti

ti

Radial

Characteristics Ch t High pressure, medium flow, efficiency close to tube axial fans, power increases continuously

Typical Applications T i l A li Various industrial applications, suitable for dust laden, moist air/gases

, p

ffi i

Forward curved F d d blades

Low pressure HVAC, packaged g units, suitable for clean and dust laden air/gases

Medium pressure, high flow, dip in pressure curve, efficiency i higher than radial fans, power rises continuously

Backward curved blades

HVAC, vaious industrial applications forced draft fans etc etc

High pressure, high flow, high efficiency, power reduces as flow increases beyond the point of highest efficiency

Airfoil type

Same as backward curve type, highest efficiency highest efficiency

Same as backward curved, but for clean air application for clean air application

12 06a

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Fan/Blower Blade Types  Impeller blades are manufactured either laminar (flat, constant  Impeller blades are manufactured either laminar (flat, constant

thickness) or aerofoil shape and generally hollow

 Aerofoil blades have greater efficiencies (up to 90%) compared to constant thickness blades, with the advantages of efficiency to constant thickness blades with the advantages of efficiency spread over the characteristic and lower noise generation

 However with careful attention to design of blade curvature, inlet eye detail and impeller shrouding, comparable efficiencies can be achieved with constant thickness blades

 Aerofoil blades are freely used particularly when blade stresses

are high and extra stiffening is required

constant thickness blades Aerofoil blades

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Axial Flow Fans

i fl

ffi i L t l Less efficient, large airflow and low speeds

Vane axial fan

Tube axial fan

Propeller fan

Pressures upto 500 mmWC and are highly energy-efficient

Higher speeds than propeller fans, high- pressures 250 – 400 mm WC and efficiency up to 65%.

14 06a

PEMP RMD 2501

Axial Flow Fans – Applications

pp y

• Ventilation Fan • Airscrew Fan • Wall mounted Supply Fan • Wall mounted Exhaust Fan • Bifurcated Fan • Roof Exhaust Fan • Inline Fan • Spark proof Fans • Inline Fans • Fresh Air Unit i h Ai • Ventilation Unit • Air washer Unit • Smoke Exhaust Unit • Smoke Exhaust Unit • Toilet Exhaust Fans • CPU Fans

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PEMP RMD 2501

Propeller Fan Propeller fan also known as panel fan is commonly used to exhaust hot or Propeller fan also known as panel fan is commonly used to exhaust hot or contaminated air or corrosive gases from factories, welding shops, foundries, furnace rooms, laboratories, laundries, stores or residential attics or windows

24”propeller fan with belt drive

16 06a

PEMP RMD 2501

Axial Fans

Type

Propeller

Typical Applications Air circulation, ventilation, ventilation exhaust

Tube Axial

HVAC, drying ovens, exhaust systems exhaust systems

Characteristics low pressure, high flow, low efficiency, peak efficiency efficiency peak efficiency close to point of free air delivery (zero static pressure) Medium pressure, high flow, M di hi h fl higher efficiency than p p propeller type, dip in pressure flow curve before peak pressure point

Vane Axial

High pressure applications including HVAC systems HVAC systems

High pressure, medium flow, Hi h dip in pressure-flow curve, use of guide vanes improves efficiency exhausts

17 06a

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Mixed Flow Fan

Mixed flow fan with barrel shaped spun housing for small diameters of inlet and outlet ducts. Direct drive, the fan wheel has a conical back plate. Outlet guide vanes prevent excessive air spin at the small outlet diameter.

18 06a

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Axial-Centrifugal Fan Types

Double inlet double width impeller

Single inlet single width fan wheel with six radial blades welded to a back plate l

Single inlet single width impeller

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Belt Drive versus Direct Drive

R l

Belt drive • Flexibility in operating speeds speeds • The air stream passing over the motor cools it • Large size fans can be operated at low speeds while motor is operated at higher motor is operated at higher speeds resulting in economical operation

f

A 30 increase in blade angle will result in 10-15% increase in flow

Direct drive • Lower number of components resulting in lower costs resulting in lower costs • Requires no regular checkups for adjustment of belt • Higher fan efficiency since no slippage due to belt drive • Results in more flow since motor i i does not obstruct flow • Performance flexibility of belt Performance flexibility of belt drive can be obtained by adjustable pitch blades and increasing number of blades of blades

20 06a

PEMP RMD 2501

Blower Types

 Centrifugal blowers typically operate against pressures of 0.35 to 0.70

kg/cm2, but can achieve much higher pressures

 Also used to produce negative pressures for industrial vacuum systems  Major types are; centrifugal blower and positive-displacement blower  The impeller is typically gear-driven and rotates as fast as 15,000 rpm  Efficiency drops with multi-staging due to the path taken from stage to stage  Efficiency drops with multi staging due to the path taken from stage to stage  One characteristic is that airflow tends to drop drastically as system pressure

increases

air and push it  Positive displacement blowers have rotors which "trap" air and push it  Positive-displacement blowers have rotors, which

trap

through housing.

 Positive-displacement blowers provide a constant volume of air even if the system pressure varies They are especially suitable for applications prone to system pressure varies. They are especially suitable for applications prone to clogging,

 They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and are

often belt driven to facilitate speed changes. often belt driven to facilitate speed changes

21 06a

PEMP RMD 2501

Fan Laws Fan data for geometrically similar fans can be collapsed onto a Fan data for geometrically similar fans can be collapsed onto a single curve using dimensionless numbers

Q = volumetric flow rate D = fan diameter di D f N = fan rotational speed W = fan power  = fluid density  = fluid density P = fan pressure rise

N  10% Q  10% or N  10% Q  10% or N  10% Q  10% N  10% HP  27% or N  10% HP  27% or N  10% HP 33% N  10% Ps  19% N  10% P  19% or N  10% Ps 21%

22 06a

PEMP RMD 2501

Fan Laws

Law 1 – relates to effect of changing size, speed, or Law 1 relates to effect of changing size, speed, or density on volume flow, pressure, and power level

Law 2 – relates to effect of changing size, pressure, or density on volume flow rate, speed, and power

Subscript 1 and 2 denotes the variable for the fan under consideration and for the tested fan respectively

ll f i t d ( ti ) ( f ) l

P equals either pt or ps ps

F For all fan laws (t)1 = (t)2 and (point of rating)1 = ) ( (point of rating)2

Law 3 shows effect of changing size, volume Law 3 – shows effect of changing size, volume flow, or density on speed, pressure, and power

23 06a

PEMP RMD 2501

Fan Laws

P i t F

i t

O t Operating Point: Fan curve and system curve intersect

Move to flow Q2 by closing damper closing damper (increase system resistance)

Flow Q1 at pressure P1 and fan speed N1N1

Move to flow Q2 by reducing fan speed

24 06a

PEMP RMD 2501

Efficiency or BEP

Type of Fan

Peak Efficiency Range

Centrifugal fans:

79-83

Airfoil, Backward Backward curved/inclined

Modified radial

72-79

Radial R di l

69-75 69 75

Pressure blower

58-68

Forward curved

60-65

Axial fans:

Vane axial

78-85

T b i l Tube axial

67 72 67-72

Deviation from BEP results in inefficiency and energy loss y Increased system resistance reduces fan efficiency

Propeller

45-50

25 06a

PEMP RMD 2501

Fan Efficiency Calculation

ffi i

• Before calculating fan efficiency measure operating parameters  Air velocity, pressure head, air stream temp, electrical motor

input, etc., input etc

• Ensure that

 Fan is operating at rated speed  Operations are at stable condition

• Methodology

Is efficiency y the only criteria for fan selection?

1. Calculate air/gas density 1 Calculate air/gas density 2. Measure air velocity and calculate average 3. Calculate the volumetric flow in the duct 4. Measure the power drive of the motor 5. Calculate fan efficiency (Mechanical and Static efficiency)

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PEMP RMD 2501

Performance Characteristics

p

g

The theoretical pressure-quantity curve of an ideal fan (no losses) is a straight line between zero volume and zero pressure

27 06a

PEMP RMD 2501

System Resistance

• Configuration of ducts,

Sum of static pressure losses in system

pickups, elbows • Pressure drop across

equipment

Increases with square of air volume

• Long narrow ducts, many bends: more resistance resistance

• Large ducts, few bends:

less resistance

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System Resistance Curve

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Fan Characteristic Curve

The fan curve is a graphical representation of a number of inter-related parameters under a specific set of conditions

di i

Typically a curve will be developed for a given set of conditions usually ll including: fan volume, system static pressure, fan static pressure fan speed, efficiency and BHP required to drive the fan to drive the fan under the stated conditions

30 06a

PEMP RMD 2501

Impeller Types and Performance

Efficiency limited to 60% to 70% at most. Steeply rising power characteristic

Non overloading power characteristic. characteristic. (i.e. power input does not peak at either free flow or free flow or no flow)

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PEMP RMD 2501

Centrifugal Fans Schematic sketch of a typical centrifugal fan wheel with ten backward-curved airfoil blades Schematic sketch of a typical centrifugal fan wheel with ten backward-curved airfoil blades

d1 = blade inner diameter d2 = blade outer diameter b = blade width i l = blade length  = blade angle U = blade velocity W = relative air velocity V = Absolute air velocity

1 is usually 10o to 30o

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Scroll Casing

h f

ll h

Schematic sketch of typical scroll housing assembly for S h l a 36.5 inch centrifugal fan with airfoil, backward curved blades for general ventilation

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Airflow versus Blade Width

Airflow versus blade width for a centrifugal fan with airfoil blades Airflow versus blade width for a centrifugal fan with airfoil blades

34 06a

PEMP RMD 2501

Blade Angles and Diameter Ratios

g 1

2,

Tip angles 2, as a function of the inlet blade angle 1 and of the diameter ratio d1/d2 for straight blades

35 06a

PEMP RMD 2501

Control of Fan/Blower Airflow

Pulley Driven

 Speed change by pulley change  Dampers  Inlet guide vanes  Variable pitch fans  V i bl it h f  Variable speed drives (VSD)  Multiple speed drive  Multiple speed drive  Disc throttle  Operating fans in parallel g  Operating fans in series

Flow control dampers

Inlet vane dampers I l t d Inlet guide vanes I l t id

36 06a

PEMP RMD 2501

Control of Fan/Blower Airflow

• Pulley change: reduce motor/drive pulley • Pulley change: reduce motor/drive pulley

size

p Permanent speed decrease Real energy reduction Fan must handle capacity change Only applicable if V-belt system or motor

• Dampers: reduce flow and increase

Speed Change

Dampers

upstream pressure Inexpensive Easy to install E ll Limited adjustment Reduce flow but not energy consumption Reduce flow but not energy consumption Higher operating and maintenance costs

37 06a

PEMP RMD 2501

Control of Fan/Blower Airflow

id

• Inlet guide vanes

I l t  Create swirls in fan direction  Reduce angle air and fan blades  Lowering fan load, pressure, air flow

Improve efficiency: reduced load and airflow Cost effective at 80-100% of full air flow Cost effective at 80-100% of full air flow Less efficient at <80% of full air flow

• Variable pitch fans: changes angle incoming

airflow and blades – Axial fan only

High efficiency at range of operating conditions No resonance problems No stall problems at different flows Applicable to axial fans only Risk of fouling problems Risk of fo ling problems Reduced efficiency at low loads

Lets look at this in detail in subsequent slides

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PEMP RMD 2501

Control of Fan/Blower Airflow • Variable speed drives (VSDs): reduce fan speed and air flow • Variable speed drives (VSDs): reduce fan speed and air flow

– Two types; Mechanical VSDs and Electrical VSDs

) (including VFDs) g ( Most improved and efficient speed control Speed adjustments over continuous range high costs

• Variable frequency drives (Change motor’s rotational speed

by adjusting electrical frequency of power) by adjusting electrical frequency of power)

Effective and easy flow control Improved efficiency over wide operating range Can be retrofitted to existing motors Compactness g p No fouling problems Reduced energy losses and costs

39 39 06a

PEMP RMD 2501

Control of Fan/Blower Airflow • Multiple speed drive (Changes fan speed from one to other) • Multiple speed drive (Changes fan speed from one to other)

Efficient control of flow Suitable if only 2 speeds required Need to jump from speed to speed High investment costs

• Disc throttle (Sliding throttle that changes width of impeller

exposed to air stream) Simple design Simple design Feasible in some applications only

• Operate fans in series

Lower average duct pressure Less noise Lower structural / electrical support required Lower structural / electrical support required Not suited for low resistance systems

Fans in series

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Control of Fan/Blower Airflow

• Operate more fans in parallel (instead of one large fan) • Operate more fans in parallel (instead of one large fan)

High efficiencies at varying demand p Less expensive and better performance than one large fan Risk of downtime avoided Can be equipped with other flow controls Only suited for low resistance system O l

i d f

i Comparing Fans in Parallel and Series

Comparing the impact of different types of flow control on power use

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Solidity – Axial Flow Fans

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PEMP RMD 2501

Blade Pitch (angle) Setting

is achieved by altering speed, adjusting

• Performance control

impeller blade pitch angle or adjusting variable inlet guide vanes impeller blade pitch angle or adjusting variable inlet guide vanes • Performance is enhanced by installation of inlet cone, inlet or outlet

guide vanes, tail fairings, and diffusers

• On-load or off load blade pitch adjustment is possible

Adjustment using pitch plates Adjustment using pitch markings

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PEMP RMD 2501

Blade Pitch (angle) Setting

Each setting has a different performance characteristic

Adjustment using a protractor g p j Variable pitch blades Variable pitch blades Variable pitch blades Variable pitch blades

Impellers without markings require the use of a protractor to set the appropriate angle

Some manufacturers specify the blade pitch angle in terms of the “tip chord”. Those who refurbish fans often incorrectly set these angles as the at the blade root rather than the tip of the blade. Depending upon the twist of the blade this could be as than the tip of the blade Depending upon the twist of the blade this could be as much as 30 resulting with the fan performance less than expectations

44 06a

PEMP RMD 2501

Start Up  Single Fan – Variable Speed

 On start up the fan goes

from standstill to full speed

 Will follow path 1,2,3,4 if  Will follow path 1,2,3,4 if equilibrium is reached instantaneously

 Will follow path 1’ 2’ 2’  Will follow path 1 , 2 , 2 , 4 if equilibrium is not established instantaneously

 In any case all points are

on the negative part of the curve and therefore stable

Single fan variable speed Single fan - variable speed

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PEMP RMD 2501

Start Up  Single Fan – Damper Control

 Dampers set to open when a predetermined pressure is reached (i.e no flow until operating pressure is reached)

 Fan must follow the parabola over the hump and the fan may over the hump and the fan may become unstable during this stage

 It is found by experience that  It i f th t

d b

h f

fans with long lengths of lay-flat duct reduces excessive power draw and prevents the fan from d d shaking violently

Single fan – Damper Control

46 06a

PEMP RMD 2501

Start Up  Two Fan in Series

 If started simultaneously they will act in the same manner as a single fan

 If one fan is started the operating fan sees a higher resistance caused by the non operating fan caused by the non-operating fan

t f

d th fi

) i

 At the start-up of the second fan the system resistance is lowered and the first fan comes down the th curve whilst the second fan moves from a free flow (air from the first fan) situation until both h fi il b h fans are at the same speed and contributing to the combined fan curve

Two fans in series Two fans in series

47 06a

PEMP RMD 2501

Start Up  Two Fan in Parallel

 If started simultaneously they will act in the same manner as a single fan

 When one fan is started it will run up  When one fan is started it will run up

and settle on the system

contributing

t th to

 The second fan (no flow) will start and when acceleration is sufficient it and when acceleration is sufficient it will move to the right at the same time that the first fan is moves up its til b th f curve until both fans are at the same speed the and combined fan curve

 Note that the second fan must move over the hump and could cause serious instability if the curve has a dip as well as a hump

Two fans in parallel Two fans in parallel

48 06a

PEMP RMD 2501

Merits of Axial and Centrifugal Fans  Axial fans offer better efficiency over a wider range of duties whereas  Axial fans offer better efficiency over a wider range of duties whereas the centrifugal fans can have a higher efficiency, albeit over a smaller range, on a single performance curve.

 The performance of a single speed axial fan can be altered simply by

adjustment to the impeller blade pitch angle.

t if

l f

 The performance of a single speed centrifugal fan requires the  Th

d installation of variable inlet vanes.

 Axial fans are generally considered to be more easily accessible for

maintenance.

 Axial fans generally run faster than centrifugal as a consequence are

much noisier.

i i

 Axial fan impellers are generally manufactured from aluminum in an

effort to keep weight to a minimum. As a consequence the potential for effort to keep weight to a minimum. As a consequence the potential for erosion is greater, particularly if there is water in the shaft.

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PEMP RMD 2501

Merits of Axial and Centrifugal Fans

 The light material used in the blades along with the high rotational speed

of axial fans make them prone to erosion, and even in good (dry) conditions it is reasonably expected that this erosion will have conditions it is reasonably expected that this erosion will have significantly reduced the fan performance within five years.

 Centrifugal fan impellers are fabricated from plate and are generally hollow. As a consequence when there is water in the shaft the nose of the blade is prone to pitting allowing water to enter the hollow section. Sufficient water in this section will cause the impeller to become unbalanced, and if allowed to continue it will result in high vibration and eventual failure of the impeller shaft.

l f

di i

f l

 Centrifugal fans traditionally require the construction of large concrete i C foundations for the motor and ductwork. The cost of these foundations significantly increases the capital cost of the fan.

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Merits of Single and Multiple Fans

 Single fan installations are generally less expensive than multiple fan

installations. installations

 Multiple fan installations have the advantage of airflow redundancy, i.e. a percentage of airflow will always be available whilst a fan is off line for maintenance or component change out.

 Single fan options do not provide any capacity for redundancy

airflow. The purchase of spares (motor, impeller, shafts, bearings, airflow The purchase of spares (motor impeller shafts bearings blades etc) is good management and should be included as upfront capital expenditure.

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Session Summary

• Various types of axial and centrifugal fans and blowers have

been described. been described

• Fans and blowers are low speed machines with low pressure rise, and the flow through them is treated as incompressible. rise, and the flow through them is treated as incompressible.

• Fan / blower characteristics have been discussed. • Fan starting characteristics have been explained Fan starting characteristics have been explained. • Fans and blowers follow affinity laws which help in scaling of

the machines.

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