PEMP RMD 2501

Fans and Blowers Session delivered by: Prof Q.H. Prof. Q H Nagpurwala

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© M.S. Ramaiah School of Advanced Studies

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Session Objectives

PEMP RMD 2501

This session is intended to discuss the following:

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

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What is a Fan?

PEMP RMD 2501

 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” 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. 06a

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

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Fans, Blowers and Compressors

PEMP RMD 2501

Fans, blowers F bl andd compressors are differentiated diff i d by b the h method h d usedd to 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

Up to 1 11 Up to 1.11

1136

Blowers

1.11 to 1.20

1136 to 2066

Compressors

More than 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 for defining the fans and blowers 06a


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Components of Fan/Blower System

PEMP RMD 2501

Turning Vanes (typically used on short radius elbows) Outlet O tl t Diffs Diff

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

Heat Exchanger Baffles

Inlet Vanes Motor Controller

Centrifugal Fan/Blower Belt Drive

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Variable Frequency Drive Motor


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PEMP RMD 2501

Parts of a Fan / Blower 1 1. 2. 3. 4 4. 5. 6. 7 7. 8.

•

•

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I ll Impeller Blade Shroud H b Hub Housing Inlet Outlet Guide Vanes

Centrifugal housing include side plate and scroll ll sheets. h t Axial housing includes the outer and inner cylinder, belt tube ©© M.M.S. S. Ramaiah School of Advanced Studies, Studies Bangalore Ramaiah School of Advanced

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Fan Types

PEMP RMD 2501

Fans are classified F l ifi d according di to the h direction di i off flow fl through h h the h impeller: 

Axial Flow: Air flows through the impeller parallel to, 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 g which may y range g from 300 to 900. The ppressure rise is ppartially 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. periphery 06a


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Centrifugal Fans 

R t ti impeller i ll increases i i velocity l it Rotating air



Air speed is converted to pressure



High pressures for harsh conditions • High temperatures • Moist/dirty air streams • Material M t i l handling h dli



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

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PEMP RMD 2501

8

Centrifugal Fan Impeller Types

PEMP RMD 2501

Open Type Backward inclined Radial Tip Blades

Backward inclined Radial tip Blades

Airfoil Blades with Higher Efficiency

Backward inclined radial di l blade 06a

Open Type Backward inclined Radial Tip Blades


Forward Curved Blades Type

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Centrifugal Fans

PEMP RMD 2501

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

Paddle blade or radial fan Backward curved Radial R di l fans f h have hi high h static t ti pressures (up to 1400 mm WC) and can handle heavily contaminated airstreams. Well suited for high temperatures and medium blade tip speeds

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Forward curved or multi-vane radial fan

Backward-inclined fans are more efficient than forwardcurved fans. Also known as "non-overloading" because changes in static pressure do not overload the motor


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Applications of Centrifugal Fans • • • • • • • • • • • • • • • • • • • 06a

PEMP RMD 2501

Augmenting g g Air Fan Scanner Air Fan Booster Air Fan Burner Air Fan Degasser Blower Combustion Air Fan Oil Vapour Exhaust Fan Purge Gas Blowers Inline Fans Supply Air Fan Exhaust Air Fan Ventilation Fan Radial Blowers Turbo Blowers (Centrifugal) FD Fan ID Fan In-series In series Blowers Igniter Air Fan Seal Air Fan © M.S. Ramaiah School of Advanced Studies

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Centrifugal Fans T Type Radial

Ch Characteristics t i ti

PEMP RMD 2501

T i l A li ti Typical Applications

High pressure, medium flow, Various industrial applications, efficiency y close to tube axial fans, suitable for dust laden, moist power increases continuously air/gases

Medium pressure, high flow, dip Low p pressure HVAC,, packaged p g F Forward d curved d i pressure curve, efficiency in ffi i units, suitable for clean and blades higher than radial fans, power rises dust laden air/gases continuously

Backward curved blades

Airfoil type

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High pressure, high flow, high HVAC, vaious industrial efficiency, power reduces as flow applications forced draft fans increases beyond the point of etc highest efficiency Same as backward curve type, highest efficiency

Same as backward curved, but for clean air application


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Fan/Blower Blade Types  





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, 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 freelyy used pparticularlyy when blade stresses are high and extra stiffening is required

constant thickness blades 06a

PEMP RMD 2501

Aerofoil blades


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PEMP RMD 2501

Axial Flow Fans

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

Vane axial fan Tube axial fan Propeller fan

Higher speeds than propeller fans, highpressures 250 – 400 mm WC and efficiency up to 65%. 06a

Pressures upto 500 mmWC and are highly energy-efficient


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Axial Flow Fans – Applications • • • • • • • • • • • • • • •

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PEMP RMD 2501

Ventilation Fan Airscrew Fan Wall mounted Supply pp y Fan Wall mounted Exhaust Fan Bifurcated Fan Roof Exhaust Fan Inline Fan Spark proof Fans Inline Fans Freshh Air Ai Unit i Ventilation Unit Air washer Unit Smoke Exhaust Unit Toilet Exhaust Fans U Fans


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Propeller Fan

PEMP RMD 2501

Propeller fan also known as 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

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Axial Fans

PEMP RMD 2501

Type

Characteristics Typical Applications low pressure, high flow, low Air circulation, efficiency peak efficiency efficiency, ventilation ventilation, Propeller close to point of free air exhaust delivery (zero static pressure) M di Medium pressure, high hi h flow, fl HVAC, higher efficiency than drying ovens, Tube Axial ppropeller p type, yp dipp in pressure p exhaust systems flow curve before peak pressure point Vane Axial

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Highh pressure, medium Hi di flow, fl High pressure dip in pressure-flow curve, applications including use of guide vanes improves HVAC systems efficiency exhausts © M.S. Ramaiah School of Advanced Studies

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Mixed Flow Fan

PEMP RMD 2501

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 p at the small outlet diameter. air spin 06a


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Axial-Centrifugal Fan Types

PEMP RMD 2501

Single inlet single width impeller

Double inlet double width impeller

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Single inlet single width fan wheel with six radial blades welded to a back plate l © M.S. Ramaiah School of Advanced Studies

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Belt Drive versus Direct Drive Belt drive • Flexibility in operating speeds • The air stream ing over the motor cools it • Large size fans can be operated at low speeds while motor is operated at higher speeds resulting in economical operation A 30 increase in blade angle will result in 10-15% increase in fflow

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PEMP RMD 2501

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


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Blower Types        



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PEMP RMD 2501

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 multi-staging 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, Positive-displacement rotors which "trap" trap air and push it through housing. Positive-displacement blowers provide a constant volume of air even if the system pressure varies. 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. changes © M.S. Ramaiah School of Advanced Studies

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PEMP RMD 2501

Fan Laws

Fan data for geometrically similar fans can be collapsed onto a single curve using dimensionless numbers Q = volumetric flow rate D = ffan di diameter t N = fan rotational speed W = fan power  = fluid density P = fan pressure rise

N  10% Q  10% or N  10% Q  10% 06a

N  10% Ps  19% or N  10% Ps 21% © M.S. Ramaiah School of Advanced Studies

N  10% HP  27% or N  10% HP 33% 22

Fan Laws

PEMP RMD 2501

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 For all F ll fan f laws l ( ( t)1 = ( ( t)2 and d ((point i t off rating) ti )1 = (point of rating)2 Law 3 – shows effect of changing size, volume flow, or density on speed, pressure, and power 06a


P equals either pt or ps q

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Fan Laws

PEMP RMD 2501

O ti Point: P i t Fan F curve andd system t curve intersect i t t Operating

Move to flow Q2 by closing damper (increase system resistance) Flow Q1 at pressure P1 and fan speed N1 Move to flow Q2 by reducing fan speed

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PEMP RMD 2501

Efficiency or BEP Type of Fan

Peak Efficiency Range

Centrifugal fans:

Deviation from BEP results in inefficiency and energy loss y resistance reduces fan Increased system efficiency 06a

Airfoil, Backward curved/inclined

79-83

Modified radial

72-79

R di l Radial

69 75 69-75

Pressure blower

58-68

Forward curved

60-65

Axial fans: Vane axial

78-85

T b axial i l Tube

67 72 67-72

Propeller

45-50


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Fan Efficiency Calculation

PEMP RMD 2501

f l l i ffan efficiency ffi i i parameters • Before calculating measure operating  Air velocity, pressure head, air stream temp, electrical motor input etc., input, etc • Ensure that Is efficiency  Fan is operating at rated speed the onlyy  Operations are at stable condition criteria for fan • Methodology selection? 1 Calculate air/gas density 1. 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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Performance Characteristics

PEMP RMD 2501

The theoretical pressure-quantity curve of an ideal fan (no g line between zero volume and zero p pressure losses) is a straight 06a


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PEMP RMD 2501

System Resistance Sum of static pressure losses in system

•

Configuration of ducts, pickups, elbows

•

Pressure drop across equipment

Increases with square of air volume

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•

Long narrow ducts, many bends: more resistance

•

Large ducts, few bends: less resistance


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System Resistance Curve

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PEMP RMD 2501

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PEMP RMD 2501

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

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Impeller Types and Performance

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

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

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PEMP RMD 2501


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Centrifugal Fans

PEMP RMD 2501

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

PEMP RMD 2501

Schematic S h i sketch k h off typical i l scroll ll housing h i assembly bl for f a 36.5 inch centrifugal fan with airfoil, backward curved blades for general ventilation

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Airflow versus Blade Width

PEMP RMD 2501

Airflow versus blade width for a centrifugal fan with airfoil blades 06a


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Blade Angles and Diameter Ratios

PEMP RMD 2501

Tipp angles g 2, as a function of the inlet blade angle g 1 and of the diameter ratio d1/d2 for straight blades 06a


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Control of Fan/Blower Airflow         

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

Flow control dampers

I l t vane dampers Inlet d 06a


PEMP RMD 2501

Pulley Driven

I l t guide Inlet id vanes 36

Control of Fan/Blower Airflow • Pulley change: reduce motor/drive pulley size

PEMP RMD 2501

Speed Change

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

• Dampers: reduce flow and increase upstream pressure Inexpensive E Easy to install i ll Limited adjustment Reduce flow but not energy consumption Higher operating and maintenance costs 06a


Dampers 37

Control of Fan/Blower Airflow

PEMP RMD 2501

I l t guide id vanes • Inlet  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 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 p problems No stall problems at different flows Applicable to axial fans only Risk of fouling fo ling problems Reduced efficiency at low loads 06a


Lets look at this in detail in subsequent slides

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PEMP RMD 2501

• Variable speed drives (VSDs): reduce fan speed and air flow – Two types; Mechanical VSDs and Electrical VSDs ((includingg VFDs)) 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) Effective and easy flow control Improved efficiency over wide operating range Can be retrofitted to existing motors Compactness No fouling g pproblems Reduced energy losses and costs 06a


39 39


PEMP RMD 2501

• Multiple speed drive (Changes fan speed from one to other) Efficient control of flow Suitable if onlyy 2 speeds p required q Need to jump from speed to speed High investment costs

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

•

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Simple design Feasible in some applications only Operate fans in series Lower average duct pressure Less noise Lower structural / electrical required Not suited for low resistance systems © M.S. Ramaiah School of Advanced Studies

Fans in series 40


PEMP RMD 2501

• Operate more fans in parallel (instead of one large fan) High efficiencies at varying demand Less expensive p and better performance p than one large g fan Risk of downtime avoided Can be equipped with other flow controls O l suited Only i d for f low l resistance i system 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

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Blade Pitch (angle) Setting

Adjustment using pitch plates

PEMP RMD 2501

Adjustment using pitch markings

• Performance control is achieved by altering speed, adjusting • • 06a

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 diffs On-load or off load blade pitch adjustment is possible © M.S. Ramaiah School of Advanced Studies

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Blade Pitch (angle) Setting Adjustment j usingg a protractor p

PEMP RMD 2501

Variable 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 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. blade Depending upon the twist of the blade this could be as much as 30 resulting with the fan performance less than expectations 06a


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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 equilibrium is reached instantaneously



Will follow path 11’, 2 2’, 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

PEMP RMD 2501

Single fan - variable speed

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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 become unstable during this stage



It is i found f d by b experience i that th t fans with long lengths of lay-flat duct reduces excessive power d draw andd prevents the h fan f from f shaking violently

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PEMP RMD 2501

Single fan – Damper Control


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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 non-operating operating fan



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

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PEMP RMD 2501

Two fans in series


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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 and settle on the system



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

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PEMP RMD 2501

Two fans in parallel


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Merits of Axial and Centrifugal Fans

PEMP RMD 2501

 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.

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

 Axial fans are ggenerallyy considered to be more easilyy accessible for maintenance.

 Axial fans generally run faster than centrifugal as a consequence are much h noisier. ii

 Axial fan impellers are generally manufactured from aluminum in an effort to keep weight to a minimum. As a consequence the potential for erosion is greater, particularly if there is water in the shaft. 06a


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Merits of Axial and Centrifugal Fans

PEMP RMD 2501

 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 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 p to become unbalanced, and if allowed to continue it will result in high vibration and eventual failure of the impeller shaft.

 Centrifugal C if l fans f traditionally di i ll require i the h construction i off large l concrete foundations for the motor and ductwork. The cost of these foundations significantly increases the capital cost of the fan. 06a


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Merits of Single and Multiple Fans

PEMP RMD 2501

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

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Session Summary

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PEMP RMD 2501

•

Various types of axial and centrifugal fans and blowers have been described described.

•

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

• • •

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


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PEMP RMD 2501

Thank you

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