1. Fans and Blowers

2. 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
Velocity triangles
Performance characteristics
Fan laws

3. 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 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.
“ ”

4. Fans, Blowers and Compressors.
Fans, Blowers and Compressors
Fans, blowers and compressors are differentiated by the method used to move 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
1136
Blowers
1.11 to 1.20
1136 to 2066
Compressors
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 .

5. Components of Fan/Blower System.
Components of Fan/Blower System
Turning Vanes (typically used on short radius elbows)
Outlet Diffusers
Provide air for ventilation and industrial
Heat
Exchanger
processes that need air flow
Baffles
Filter
Inlet Vanes
Motor Controller
Centrifugal Fan/Blower
Variable Frequency Drive
Belt Drive
Motor .

6. Parts of a Fan / Blower Impeller Blade Shroud Hub Housing Inlet Outlet.
Parts of a Fan / Blower
Impeller
Blade
Shroud
Hub
Housing
Inlet
Outlet
Guide Vanes
Centrifugal housing include side plate and scroll sheets.
Axial housing includes the outer and inner cylinder, belt tube . .

7. .
Fan Types
Fans are classified according to the direction of flow through the impeller:
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 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. . .

8. Centrifugal Fans Rotating impeller increases air velocity.
Centrifugal Fans
Rotating impeller increases air velocity
Air speed is converted to pressure
High pressures for harsh conditions
High temperatures
Moist/dirty air streams
Material handling
Categorized by blade shapes
Radial
Forward curved
Backward inclined . .

9. Centrifugal Fan Impeller Types.
Centrifugal Fan Impeller Types
Open Type Backward inclined Radial Tip Blades
Backward inclined Radial tip Blades
Airfoil Blades with
Higher Efficiency
Open Type Backward inclined Radial Tip Blades
Forward Curved Blades Type
Backward inclined radial blade . .

10. Well suited for high temperatures and medium blade tip speeds.
Centrifugal Fans
Forward-curved fans are used in clean environments and operate at lower temperatures. Well suited for low tip speed and high- airflow at lower pressures
Paddle blade or radial fan
Backward 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
"non-overloading" because changes in static pressure do not overload the motor
Forward curved or multi-vane radial fan .
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11. Applications of Centrifugal Fans.
Applications of Centrifugal Fans
Augmenting 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 Blowers
Igniter Air Fan
Seal Air Fan . .

12. HVAC, vaious industrial applications forced draft fans etc.
Centrifugal Fans
Type
Characteristics
Typical Applications
High pressure, medium flow, efficiency close to tube axial fans, power increases continuously
Various industrial applications, suitable for dust laden, moist air/gases
Radial
Medium pressure, high flow, dip Low pressure HVAC, packaged
Forward curved
in pressure curve, efficiency
units, suitable for clean and dust laden air/gases
blades
higher than radial fans, power rises continuously
High pressure, high flow, high efficiency, power reduces as flow increases beyond the point of highest efficiency
HVAC, vaious industrial applications forced draft fans etc
Backward curved blades
Same as backward curve type, highest efficiency
Same as backward curved, but for clean air application
Airfoil type . .

13. Fan/Blower Blade Types.
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, 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 . .

14. Pressures upto 500 mmWC and are highly energy-efficient.
Axial Flow Fans
Less efficient, large airflow and low speeds
Vane axial fan
Tube axial fan
Propeller fan
Higher speeds than propeller fans, high- pressures 250 – 400 mm WC and efficiency up to 65%.
Pressures upto 500 mmWC and are highly energy-efficient . .

15. Axial Flow Fans – Applications.
Axial Flow Fans – Applications
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
Ventilation Unit
Air washer Unit
Smoke Exhaust Unit
Toilet Exhaust Fans
CPU Fans . .

16. Propeller Fan 24”propeller fan with belt drive.
Propeller Fan
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 . .

17. Axial Fans Type Characteristics Typical Applications.
Axial Fans
Type
Characteristics
Typical Applications
low pressure, high flow, low efficiency, peak efficiency
Air circulation, ventilation,
Propeller
close to point of free air delivery (zero static pressure)
exhaust
Tube Axial
Medium pressure, high flow, higher efficiency than propeller type, dip in pressure flow curve before peak pressure point
HVAC,
drying ovens, exhaust systems
Vane Axial
High pressure, medium flow, dip in pressure-flow curve, use of guide vanes improves efficiency exhausts
High pressure applications including HVAC systems . .

18. .
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. . .

19. Axial-Centrifugal Fan Types.
Axial-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 plate . .

20. Belt Drive versus Direct Drive.
Belt Drive versus Direct Drive
Belt drive
Flexibility in operating 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 speeds resulting in economical operation
A 30 increase in blade angle will result in 10-15% increase in flow
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
Results in more flow since motor does not obstruct flow
Performance flexibility of belt drive can be obtained by adjustable pitch blades and increasing number of blades . .

21. .
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
One characteristic is that airflow tends to drop drastically as system pressure increases
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 to clogging,
They turn much slower than centrifugal blowers (e.g. 3,600 rpm), and are often belt driven to facilitate speed changes.
" " . .

22. .
Fan Laws
Fan data for geometrically similar fans can be collapsed onto a single curve using dimensionless numbers
Q = volumetric flow rate D = fan diameter
N = fan rotational speed W = fan power
 = fluid density
P = fan pressure rise
N  10% Q  10% or
N  10% Ps  19%
N  10% HP  27% or
N  10% Q  10%
or N  10% Ps 21%
N  10% HP 33% . .

23. Fan Laws P equals either pt or ps.
Fan Laws
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 fan laws (t)1 = (t)2 and (point of rating)1 = (point of rating)2
P equals either pt or ps
Law 3 – shows effect of changing size, volume flow, or density on speed, pressure, and power . .

24. .
Fan Laws
Operating Point: Fan curve and system curve intersect
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 . .

25. Increased system resistance reduces fan.
Efficiency or BEP
Peak Efficiency Range
Type of Fan
Centrifugal fans:
Airfoil, Backward
79-83
curved/inclined
Modified radial
72-79
Radial
69-75
Pressure blower
58-68
Forward curved
60-65
Axial fans:
Deviation from BEP results in inefficiency and energy loss
Increased system resistance reduces fan
Vane axial
78-85
Tube axial
67-72
efficiency
Propeller
45-50 . .

26. Fan Efficiency Calculation.
Fan Efficiency Calculation
Before calculating fan efficiency measure operating parameters
Air velocity, pressure head, air stream temp, electrical motor input, etc.,
Ensure that
Fan is operating at rated speed
Operations are at stable condition
Methodology
1. Calculate air/gas density
Is efficiency the only criteria for fan selection?
Measure air velocity and calculate average
Calculate the volumetric flow in the duct
Measure the power drive of the motor
Calculate fan efficiency (Mechanical and Static efficiency) . .

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

28. System Resistance Configuration of ducts, pickups, elbows.
System Resistance
Configuration of ducts, pickups, elbows
Pressure drop across equipment
Sum of static pressure losses in system
Increases with square of air volume
Long narrow ducts, many bends: more resistance
Large ducts, few bends: less resistance . .

29. System Resistance Curve.
System Resistance Curve . .

30. Fan Characteristic Curve.
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 usually including: fan volume, system static pressure, fan speed, efficiency and BHP required to drive the fan under the stated conditions . .

31. Impeller Types and Performance.
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 . .

32. .
Centrifugal Fans
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
 = blade angle
U = blade velocity
W = relative air velocity V = Absolute air velocity
l = blade length
1 is usually 10o to 30o .

33. .
Scroll Casing
Schematic sketch of typical scroll housing assembly for a 36.5 inch centrifugal fan with airfoil, backward curved blades for general ventilation . .

34. Airflow versus Blade Width.
Airflow versus Blade Width
Airflow versus blade width for a centrifugal fan with airfoil blades . .

35. Blade Angles and Diameter Ratios.
Blade Angles and Diameter Ratios
Tip angles 2, as a function of the inlet blade angle 1 and of the diameter ratio d1/d2 for straight blades . .

36. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
Speed change by pulley change
Dampers
Inlet guide vanes
Variable pitch fans
Variable speed drives (VSD)
Multiple speed drive
Flow control dampers
Pulley Driven
Disc throttle
Operating fans in parallel
Operating fans in series
Inlet vane dampers
Inlet guide vanes . .

37. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
Pulley change: reduce motor/drive pulley
size
Speed Change
Permanent speed 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 Easy to install
Limited adjustment
Reduce flow but not energy consumption
Higher operating and maintenance costs
Dampers . .

38. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
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 airflow Cost effective at % of full air flow Less efficient at <80% of full air flow
Variable pitch fans: changes angle incoming
airflow and blades – Axial fan only
Lets look at this in detail in subsequent
High efficiency at range of operating conditions No resonance problems
slides
No stall problems at different flows Applicable to axial fans only
Risk of fouling problems
Reduced efficiency at low loads . .

39. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
Variable speed drives (VSDs): reduce fan speed and air flow
– Two types; Mechanical VSDs and Electrical VSDs (including 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 problems
Reduced energy losses and costs .
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3399

40. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
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
Feasible in some applications only
Operate fans in series Lower average duct pressure Less noise
Lower structural / electrical support requi
red
Not suited for low resistance systems .
Fans in series .

41. Control of Fan/Blower Airflow.
Control of Fan/Blower Airflow
Operate more fans in parallel (instead of one large fan)
High efficiencies at varying demand
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
Comparing Fans in Parallel and Series
Comparing the impact of different types of flow control on power use . .

42. Solidity – Axial Flow Fans.
Solidity – Axial Flow Fans . .

43. Blade Pitch (angle) Setting.
Blade Pitch (angle) Setting
Adjustment using pitch plates Adjustment using pitch markings
Performance control is achieved by altering speed, adjusting 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 . .

44. Blade Pitch (angle) Setting.
Blade Pitch (angle) Setting
Adjustment using a protractor
Variable pitch blades
Each setting has a different performance characteristic
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 much as 30 resulting with the fan performance less than expectations . .

45. Start Up  Single Fan – Variable Speed RMD 2501
PEMP
Start Up  Single Fan – Variable Speed RMD 2501
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 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 . .

46. Start Up  Single Fan – Damper Control.
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 found by experience that
fans with long lengths of lay-flat duct reduces excessive power draw and prevents the fan from shaking violently
Single fan – Damper Control . .

47. Start Up  Two Fan in Series.
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
At the start-up of the second fan the system resistance is lowered and the first fan comes down the curve whilst the second fan moves from a free flow (air from the first fan) situation until both fans are at the same speed and
contributing to the combined fan curve
Two fans in series . .

48. Start Up  Two Fan in Parallel.
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 both fans are at the 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
Two fans in parallel
dip as well as a hump . .

49. Merits of Axial and Centrifugal Fans.
Merits of Axial and Centrifugal Fans
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 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 maintenance.
Axial fans generally run faster than centrifugal as a consequence are much noisier.
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. . .

50. Merits of Axial and Centrifugal Fans.
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 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.
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. . .

51. Merits of Single and Multiple Fans.
Merits of Single and Multiple Fans
Single fan installations are generally less expensive than multiple fan 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, blades etc) is good management and should be included as upfront capital expenditure. . .

52. Session Summary .
Various types of axial and centrifugal fans and blowers have been 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.
Fans and blowers follow affinity laws which help in scaling of the machines. . .