Fans & Blower.pdf

PEMP RMD 2501

Fans and BlowersFans and Blowers

Session delivered by:Session delivered by:

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

© M.S. Ramaiah School of Advanced Studies06a 1

PEMP RMD 2501Session 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


PEMP RMD 2501What 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.

© M. S. Ramaiah School of Advanced Studies, Bangalore

PEMP RMD 2501Fans, Blowers and Compressors

F bl d diff i d b h h d d

Difference Between Fans Blower and Compressors

Fans, blowers and compressors are differentiated by the method used tomove the air, and by the system pressure they must operate against.

Difference Between Fans, Blower and Compressors

Equipment Specific Ratio Pressure Rise (mmWC)

Fans Up to 1 11 1136Fans Up to 1.11 1136

Blowers 1.11 to 1.20 1136 to 2066Compressors More thanCompressors More than

1.20

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


discharge pressure to the suction pressure is used for defining the fans and blowers

© M. S. Ramaiah School of Advanced Studies, Bangalore

PEMP RMD 2501Components of Fan/Blower System

O tl t Diff

Turning Vanes (typically used on

short radius elbows)

Outlet Diffusers

Heat

Provide air for ventilation and

industrial

Baffles

Exchangerindustrial processes that need air flow

Inlet Vanes

Filter

Motor

Variable Frequency Drive

Motor Controller

Centrifugal Fan/Blower

© M.S. Ramaiah School of Advanced Studies06a 5© M. S. Ramaiah School of Advanced Studies, Bangalore

Belt Drive Motor

PEMP RMD 2501Parts of a Fan / Blower

1 I ll1. Impeller2. Blade3. Shroud4 H b4. Hub5. Housing6. Inlet 7 Outlet7. Outlet8. Guide Vanes

• Centrifugal housing include side plate and

ll h tscroll sheets.• Axial housing includes

the outer and inner


cylinder, belt tube

PEMP RMD 2501Fan Types

F l ifi d di h di i f fl h h hFans 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 g y g p p yby 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.

PEMP RMD 2501Centrifugal Fans

R t ti i ll i i l it 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

M t i l h dli• Material handling

Categorized by blade shapes• Radial• Forward curved• Backward inclined


PEMP RMD 2501Centrifugal Fan Impeller Types

Open TypeBackward inclined Radial tip Blades

Open Type Backward inclined Radial Tip Blades

Airfoil Blades withwith

Higher Efficiency

Backward inclined

di l

Open Type Backward

inclined Radial Tip Blades

Forward Curved

Blades Type


radial blade

Tip Blades


Forward-curved fans are used in clean environments and

operate at lower temperatures Well suited fortemperatures. Well suited for

low tip speed and high-airflow at lower pressures

Paddle blade or radial fan

R di l f h hi h t tiBackward curved

Radial fans have high static pressures (up to 1400 mm

WC) and can handle heavily contaminated airstreams.

Backward-inclined fans are more efficient than forward-curved fans. Also known as

Well suited for high temperatures and medium

blade tip speeds

Forward curved or multi-vane radial fan

curved fans. Also known as "non-overloading" because changes in static pressure do

not overload the motor


PEMP RMD 2501Applications of Centrifugal Fans

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

Fan• Purge Gas Blowers• Inline Fans• Supply Air Fan• Exhaust Air FanExhaust 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


T Ch t i ti T i l A li tiType Characteristics Typical Applications

RadialHigh pressure, medium flow,

efficiency close to tube axial fans, Various industrial applications, suitable for dust laden, moist y

power increases continuously air/gases

F d dMedium pressure, high flow, dip

i ffi i Low pressure HVAC, packaged Forward curved blades

in pressure curve, efficiency higher than radial fans, power rises

continuously

p , p gunits, suitable for clean and

dust laden air/gases

Backward curved blades

High pressure, high flow, high efficiency, power reduces as flow

increases beyond the point of

HVAC, vaious industrial applications forced draft fans

etchighest efficiency etc

Airfoil type Same as backward curve type, highest efficiency

Same as backward curved, but for clean air application


highest efficiency for clean air application

PEMP RMD 2501Fan/Blower Blade Types

Impeller blades are manufactured either laminar (flat, constant Impeller blades are manufactured either laminar (flat, constantthickness) or aerofoil shape and generally hollow

Aerofoil blades have greater efficiencies (up to 90%) comparedto constant thickness blades with the advantages of efficiencyto constant thickness blades, with the advantages of efficiencyspread over the characteristic and lower noise generation

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

Aerofoil blades are freely used particularly when blade stressesy p yare high and extra stiffening is required

© M.S. Ramaiah School of Advanced Studies06a 13constant thickness blades Aerofoil blades

PEMP RMD 2501Axial Flow Fans

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

Vane axial fan

Propeller fan

Tube axial fan

Higher speeds than propeller fans, high-

pressures 250 – 400 mm WC and efficiency up to

Pressures upto 500 mmWC and are highly

energy-efficient


65%.

PEMP RMD 2501Axial Flow Fans – Applications

• Ventilation Fan • Airscrew Fan• Wall mounted Supply Fanpp y• Wall mounted Exhaust Fan• Bifurcated Fan • Roof Exhaust Fan • Inline Fan • Spark proof Fans• Inline Fans

h Ai i• Fresh Air Unit• Ventilation Unit• Air washer Unit• Smoke Exhaust Unit• Smoke Exhaust Unit• Toilet Exhaust Fans• CPU Fans


PEMP RMD 2501Propeller Fan

Propeller fan also known as panel fan is commonly used to exhaust hot orPropeller 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


PEMP RMD 2501Axial Fans

Type Characteristics Typical Applicationslow pressure, high flow, low efficiency peak efficiency

Air circulation, ventilationPropeller efficiency, peak efficiency

close to point of free air delivery (zero static pressure)

ventilation, exhaust

M di hi h fl

Tube Axial

Medium pressure, high flow, higher efficiency than

propeller type, dip in pressure

HVAC, drying ovens,

exhaust systemsp p yp p pflow curve before peak

pressure point

exhaust systems

Hi h di fl

Vane Axial

High pressure, medium flow, dip in pressure-flow curve,

use of guide vanes improves

High pressure applications including

HVAC systems


efficiency exhausts HVAC systems

PEMP RMD 2501Mixed 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.


p

PEMP RMD 2501Axial-Centrifugal Fan Types

Single inlet single width impeller

Double inlet double width

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

l


plate

PEMP RMD 2501Belt Drive versus Direct Drive

Belt drive• Flexibility in operating speeds

Direct drive• Lower number of componentsresulting in lower costsspeeds

• The air stream passing over the motor cools it

resulting in lower costs• Requires no regular checkups foradjustment of belt

• Large size fans can be operated at low speeds while motor is operated at higher

• Higher fan efficiency since noslippage due to belt drive

R l i fl imotor is operated at higher speeds resulting in economical operation

• Results in more flow since motordoes not obstruct flow• Performance flexibility of beltPerformance flexibility of beltdrive can be obtained by adjustablepitch blades and increasing numberof blades

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


of bladesf

PEMP RMD 2501Blower Types

Centrifugal blowers typically operate against pressures of 0.35 to 0.70kg/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 Positive displacement blowers have rotors which "trap" air and push it Positive-displacement blowers have rotors, which trap air and push it

through housing. Positive-displacement blowers provide a constant volume of air even if the

system pressure varies They are especially suitable for applications prone tosystem pressure varies. They are especially suitable for applications prone toclogging,

They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and areoften belt driven to facilitate speed changes


often belt driven to facilitate speed changes.

PEMP RMD 2501Fan Laws

Fan data for geometrically similar fans can be collapsed onto aFan data for geometrically similar fans can be collapsed onto asingle curve using dimensionless numbers

Q = volumetric flow rateD f di tD = fan diameterN = fan rotational speedW = fan power = fluid density = fluid densityP = fan pressure rise

N 10% Q 10% or N 10% P 19% N 10% HP 27% or


N 10% Q 10% or N 10% Q 10%

N 10% Ps 19% or N 10% Ps 21%

N 10% HP 27% or N 10% HP 33%


Law 1 – relates to effect of changing size, speed, orLaw 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

F ll f l ( ) ( ) d ( i t f ti )For all fan laws (t)1 = (t)2 and (point of rating)1 = (point of rating)2

P equals either pt or psLaw 3 – shows effect of changing size, volume


q pt psLaw 3 shows effect of changing size, volumeflow, or density on speed, pressure, and power


O ti P i t F d t i t tOperating Point: Fan curve and system curve intersect

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

Flow Q1 at pressure P1 and fan speed N1N1

Move to flow Q2 by reducing fan speed


PEMP RMD 2501Efficiency or BEP

Type of FanPeak

Efficiency Range

Centrifugal fans:Airfoil, Backward

79-83Backward curved/inclinedModified radial 72-79R di l 69 75Radial 69-75Pressure blower 58-68Forward curved 60-65Axial fans:Vane axial 78-85T b i l 67 72

Deviation from BEP results in inefficiency and energy loss

Increased system resistance reduces fan


Tube axial 67-72Propeller 45-50

yefficiency

PEMP RMD 2501Fan Efficiency Calculation

f l l i f ffi i i• Before calculating fan efficiency measure operating parametersAir velocity, pressure head, air stream temp, electrical motor

input etcinput, etc.,• Ensure thatFan is operating at rated speed

Is efficiency the only

Operations are at stable condition• Methodology

1 Calculate air/gas density

ycriteria for fan

selection?1. Calculate air/gas density2. Measure air velocity and calculate average3. Calculate the volumetric flow in the duct4. Measure the power drive of the motor 5. Calculate fan efficiency (Mechanical and Static efficiency)


PEMP RMD 2501Performance Characteristics

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


g p

PEMP RMD 2501System Resistance

Sum of static pressure losses in system

• Configuration of ducts, pickups, elbows

• Pressure drop across equipment

Increases with square of air volume

• Long narrow ducts, many bends: more resistanceresistance

• Large ducts, few bends: less resistance


PEMP RMD 2501System Resistance Curve


PEMP RMD 2501Fan Characteristic Curve

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

Typically a curve will be developed for a given set of

di i llconditions usually including: fan volume, system static pressure fanstatic pressure, fan speed, efficiency and BHP required to drive the fanto drive the fan under the stated conditions


PEMP RMD 2501Impeller Types and Performance

Efficiency limited to 60%

Non overloading power characteristic.

to 70% at most.Steeply rising

power characteristic

characteristic. (i.e. power input does not peak at eitherfree flow orfree flow or no flow)



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

d1 = blade inner diameterd2 = blade outer diameter

i

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

= blade angle U = blade velocity

b = blade widthl = blade length

W = relative air velocity V = Absolute air velocity

1 is usually 10o to 30o


PEMP RMD 2501Scroll Casing

S h i k h f i l ll h i bl fSchematic sketch of typical scroll housing assembly for a 36.5 inch centrifugal fan with airfoil, backward curved

blades for general ventilation


PEMP RMD 2501Airflow 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

PEMP RMD 2501Blade Angles and Diameter Ratios

Tip angles 2, as a function of the inlet blade angle 1 and of the


p g 2, g 1diameter ratio d1/d2 for straight blades

PEMP RMD 2501Control of Fan/Blower Airflow

Speed change by pulley change Dampers Inlet guide vanes V i bl it h f Variable pitch fans Variable speed drives (VSD) Multiple speed drive

Pulley DrivenFlow control dampers

Multiple speed drive Disc throttle Operating fans in parallelp g p Operating fans in series

I l t idI l t d


Inlet guide vanesInlet vane dampers


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

Permanent speed decreaseSpeed Change

pReal energy reductionFan must handle capacity changeOnly applicable if V-belt system or motor

• Dampers: reduce flow and increase upstream pressure

InexpensiveE i llEasy to installLimited adjustmentReduce flow but not energy consumption


Reduce flow but not energy consumptionHigher operating and maintenance costs Dampers


I l t id• Inlet guide vanes Create swirls in fan direction Reduce angle air and fan blades Lowering fan load, pressure, air flow

Improve efficiency: reduced load and airflowCost effective at 80-100% of full air flowCost effective at 80-100% of full air flowLess 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 conditionsNo resonance problems

Lets look at this in detail in subsequent p

No stall problems at different flowsApplicable to axial fans onlyRisk of fo ling problems

slides


Risk of fouling problemsReduced efficiency at low loads


• 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 controlSpeed adjustments over continuous rangehigh 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 controlImproved efficiency over wide operating rangeCan be retrofitted to existing motorsCompactnessNo fouling problems


g pReduced energy losses and costs

© M. S. Ramaiah School of Advanced Studies, Bangalore 39


• Multiple speed drive (Changes fan speed from one to other)• Multiple speed drive (Changes fan speed from one to other)Efficient control of flowSuitable if only 2 speeds requiredy p qNeed to jump from speed to speedHigh investment costs

• Disc throttle (Sliding throttle that changes width of impeller exposed to air stream)

Simple designSimple designFeasible in some applications only

• Operate fans in seriesLower average duct pressureLess noiseLower structural / electrical support required


Lower structural / electrical support requiredNot suited for low resistance systems Fans in series


• Operate more fans in parallel (instead of one large fan)• Operate more fans in parallel (instead of one large fan)High efficiencies at varying demandLess expensive and better performance than one large fanp p gRisk of downtime avoidedCan be equipped with other flow controlsO l i d f l iOnly suited for low resistance system

Comparing Fans inParallel and Series

Comparing the impact of

different types


of flow controlon power use

PEMP RMD 2501Solidity – Axial Flow Fans


PEMP RMD 2501Blade Pitch (angle) Setting

• Performance control is achieved by altering speed, adjustingimpeller blade pitch angle or adjusting variable inlet guide vanes

Adjustment using pitch plates Adjustment using pitch markings

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

PEMP RMD 2501Blade Pitch (angle) Setting

Adjustment using a protractor Variable pitch bladesVariable pitch bladesj g p Variable pitch bladesVariable pitch blades

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

Each setting has a different performance characteristic

Some manufacturers specify the blade pitch angle in terms of the “tip chord”. Thosewho refurbish fans often incorrectly set these angles as the at the blade root ratherthan 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 asmuch as 30 resulting with the fan performance less than expectations

PEMP RMD 2501Start 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

PEMP RMD 2501Start 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 mayover the hump and the fan may become unstable during this stage

It i f d b i th t It is found by experience that fans with long lengths of lay-flat duct reduces excessive power d d h f f

Single fan – Damper Control

draw and prevents the fan from shaking violently


PEMP RMD 2501Start 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 fancaused by the non operating fan

At the start-up of the second fan the system resistance is lowered

d th fi t f d thand the first fan comes down the curve whilst the second fan moves from a free flow (air from h fi f ) i i il b h

Two fans in series

the first fan) situation until both fans are at the same speed and contributing to the combined fan


Two fans in seriescurve

PEMP RMD 2501Start Up Two Fan in Parallel

If started simultaneously they will actin the same manner as a single fan

When one fan is started it will run up When one fan is started it will run upand settle on the system

The second fan (no flow) will startand when acceleration is sufficient itand when acceleration is sufficient itwill move to the right at the sametime that the first fan is moves up its

til b th f t thcurve until both fans are at the samespeed and contributing to thecombined fan curve

Two fans in parallel

Note that the second fan must moveover the hump and could causeserious instability if the curve has a


Two fans in paralleldip as well as a hump

PEMP RMD 2501Merits 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.

Th f f i l d t if l f i th The performance of a single speed centrifugal fan requires the installation of variable inlet vanes.

Axial fans are generally considered to be more easily accessible for g y ymaintenance.

Axial fans generally run faster than centrifugal as a consequence are h i imuch noisier.

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.

PEMP RMD 2501Merits 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 haveconditions 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 punbalanced, and if allowed to continue it will result in high vibration and eventual failure of the impeller shaft.C if l f di i ll i h i f l Centrifugal fans traditionally require the construction of large concrete foundations for the motor and ductwork. The cost of these foundations significantly increases the capital cost of the fan.


PEMP RMD 2501Merits of Single and Multiple Fans

Single fan installations are generally less expensive than multiple fan installationsinstallations.

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


PEMP RMD 2501Session Summary

• Various types of axial and centrifugal fans and blowers have been describedbeen 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 explainedFan starting characteristics have been explained.

• Fans and blowers follow affinity laws which help in scaling of the machines.


PEMP RMD 2501

Thank you


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