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Fans & Fan LawsFans & Fan Laws

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Fans and Fan Laws

WHAT WE WILL SEE THIS SESSION

• Recognize the various types of fans• Understand fan laws• Significance of static Pressure• The right fan for an application

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FansFeatures

An air pump that creates a pressure difference and causes airflow.

An impeller does work on the air, imparting both static and kinetic energy, which vary in proportion depending on the fan type

Ptot 1

Pdyn

1

Psta

t 1

Pto t 1

Pdyn

2

Ptot

2Pt

ot 1

Po

Pto

t

Pressure Profile in a Ventilation System

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Cappedduct

Static Pressure

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Static Pressure

If we consider a very thin disc placed in the fluid, and moving with it, the static pressure as it is called will act on the two faces of the disc.

If the disc is stationary the pressure acting on the two surfaces will be the same providing the presence of the disc does not disturb the flow.

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Static Pressure

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Static Pressure & Velocity Pressure

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Static Pressure & Velocity PressureIf the stationary disc is turned through 90° its front face willbe subjected to not only static pressure, but also an additional dynamic pressure equal to the velocity pressure in incompressible flow, due to the impact.

The velocity pressure is due entirely to the motion of the flow and depends upon the velocity and the density of the fluid.

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Static Pressure & Velocity Pressure

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Static Pressure & Total Pressure

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Fan Curves

Sta

tic P

ress

ure

in P

asc

al

250

500

750

1000

1250

1500

Airflow x 1000 m3/hr

1.7 3.4 5.1 6.8 8.5 10.2 11.9 13.6 15.3

A

‘Block off’ / ‘shutoff’

Stall RegionB C

Recomm

ended Selection Range

D

Free Delivery

The “RIGHT SIDE” of the fan curve

For a given SP, each fan has a

corresponding airflow quantity

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Point A is known as “block off” “shut off”, “no flow” and “static no delivery”

Operation in the ‘stall’ region (B) is discouraged because of erratic airflow that generates excessive noise and vibration. An “UNSTABLE” area of operation. Occurs usually when the flow is too low for the selected fan (unduly oversized, lack of intake air, poor performance)

Point C represents the “PEAK” of the static pressure curve.

Point D is the point of max airflow. Point D is also known as ‘Free Delivery’ / ‘free air’ / ‘wide open performance’/ ‘wide open volume’

CD is that portion of the curve where the fan must be chosen to operate.

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Fan Curves

Sta

tic P

ress

ure

in P

asc

al

250

500

750

1000

1250

1500

Airflow x 1000 m3/hr

1.7 3.4 5.1 6.8 8.5 10.2 11.9 13.6 15.3

A

C

D

Sha

ft kw

/Bra

ke H

orse

pow

er

2

4

6

8

10

12BHP

SYSTEM CURVE

OPERATING POINT

OPERATING POINT MUST BE BELOW THE SURGE LINE

1400 RPM

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NOTE : The values shown here for the various axes are not true for any particular manufacturer. The curves and the scales are shown merely as a teaching tool and does not have mathematical formulae linking the numbers.

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Fan LawsAffinity Laws

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Fan Laws

Changing Operating Conditions

A fan produces 18,700 CMH against 65 mm. H20 at 900 rpm, and requires 9kw. The airflow needs to be increased to 23,800 CMH.

Using the fan laws: what fan speed, static pressure, and fan power do you predict will be required?

CMH1

CMH2

=RPM2

RPM11 RPM2 =RPM11 CMH2

CMH1

X

RPM2 =900 23800

18700

X

RPM2 =1145

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Affinity Laws

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Fan Laws

Changing Operating Conditions

A fan produces 18,700 CMH against 65 mm. H20 at 900 rpm, and requires 9kw. The airflow needs to be increased to 23,800 CMH.

Using the fan laws: what fan speed, static pressure, and fan power do you predict will be required?

SP1

SP2

=RPM2

RPM11

2RPM2= X

RPM11

SP2 SP1

2

= 651145

900X

= 105.2 MM

SP2

2

SP2

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Affinity Laws

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Fan Laws

Changing Operating Conditions

A fan produces 18,700 CMH against 65 mm. H20 at 900 rpm, and requires 9kw. The airflow needs to be increased to 23,800 CMH.

Using the fan laws: what fan speed, static pressure, and fan power do you predict will be required?

KW1

KW2

=RPM2

RPM11

3RPM2= X

RPM11

KW2 KW1

3

= 91145

900X

= 18.4 KW

3

KW2

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Affinity Laws

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Fan PrinciplesStall

When a Fan operates at an air quantity significantly lower than what it is supposed to deliver, the fan operates in the STALL region. (highly oversized fan)

Noise increases

Mechanical damage – structural metal fatigue (increased vibration)

Operates at lesser efficiency (running costs are high)

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Fan PrinciplesSurge

A system in surge is like an oscillator. Sometimes air blows back through the inlet

Periods when air tends to blow back FANS - by R Sekar

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Measurement of Static Pressure & Total Pressure

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Measurement of Static Pressure & Total Pressure

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Measurement of Differential Pressure

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U TUBE MANOMETER

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INCLINED MANOMETERFANS - by R Sekar

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Pitot Tube with Mag Gauge

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Read here

Inclined Manometer

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Fan Classification

Centrifugal Airfoil

Backward Inclined

Radial

Forward Curved

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

Centrifugal Airfoil

Highest efficiency of all centrifugal fan designs

Airfoil blades counter curved away from direction of rotation

Air leaves impeller at velocity less than tip speed

For given duty, has highest speed of centrifugal fan designs

Scroll Design for efficient conversion of velocity pressure to static pressure

Maximum efficiency requires close clearance and alignment between wheel and inlet

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

Centrifugal Airfoil

Highest efficiencies occur at 50-60% of wide open volume. This Volume also has good pressure characteristics

Power reaches maximum near peak efficiency and becomes lower towards free-delivery

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

Centrifugal Backward Inclined / Backward Curved

Efficiency only slightly less than airfoil fans

Ten to 16 single thickness blades curved or inclined away from direction of rotation

Same housing configuration as airfoil design

Performance similar to airfoil fan, except peak efficiency slightly lower

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

Centrifugal Radial

Higher pressure characteristics than airfoil, BC/BI fans

Curve may have a break to left of peak pressure and fan should not operated in this area

Scroll is usually narrowest of all centrifugal designs. Since wheel design is less efficient, housing dimensions are not critical as for airfoil and BI / BC fans

Not common for HVAC. Used in Industrial / material handling applications

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

Centrifugal Forward Curved

Flatter pressure characteristics and lower efficiency than airfoil, BC/BI fans

Do not rate fan in the pressure curve dip to the left of peak pressure.

Scroll similar to all centrifugal designs.

Highest efficiency to right of peak pressure at 40 to 50% of wide open volume

Power curve rises continually towards free delivery

Low pressure HVAC applications, Packaged air-conditioners

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Impeller produces pressure by

1.centrifugal force rotating air column between the blades and

2. Kinetic energy imparted to the air by its velocity leaving the impeller (combination of rotational velocity and airspeed relative to the impeller

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Fan Classification

AxialPropeller

Tube Axial

Vane Axial

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Axial-flow fan impellers produce pressure principally by the change in air velocity as it passes through the impeller blades, with none being produced by the centrifugal force.

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Fan Classification

Axial PropellerLow Efficiency

Limited to low pressure applications.

Usually low cost impellers have two or more blades of single thickness attached to relatively small hub.

Simple circular ring, orifice plate, or venturi.

Optimum design is close to blade tips and forms smooth airfoil into wheel.

Hi flow rate, low pressure capability

Max efficiency near free delivery.

Air-moving applications such as air circulation in a space or ventilation through a wall with ductwork.

Makeup air applications FANS - by R Sekar

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Fan Classification

Axial Tube AxialMore efficient and capable of producing more useful static pressure than propeller

4 to 8 blades with airfoil or single thickness

Cylindrical tube with close clearance to blade tips

Hi flow rate, medium pressure capability

Performance dips to left of peak pressure. Do not operate fan in this region

Low and medium pressure ducted HVAC applications where air distribution downstream is not critical.

Industrial applications – spray booths, tunnel ventilation, fume exhausts

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Fan Classification

Axial Vane AxialGood Blade design gives medium to high pressure capability at good efficiency.

Blades have fixed, adjustable of controllable pitch

Cylindrical tube with close clearance to blade tips

Guide vanes upstream or downstream from impeller increase pressure capability and efficiency

Hi pressure capability with medium flow rate

Performance dips to left of peak pressure due to stall. Do not operate fan in this regionLow medium and high pressure HVAC applications where straight through flow and compact installation are required.

Good down stream air distributionFANS - by R Sekar

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Fan Classification

Special Design FansTubular CentrifugalSimilar to BC, except capacity and pressure are lower.

Efficiency lower than BC fan

Cylindrical tube with not sonot so close clearance to blade tips

Low return air systems in HVAC applications.

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Fan Classification

Special Design FansCentrifugal Power Roof VentilatorPositive Exhaust ventilation .

Housing designed specially to discharge air from impeller in full circle

Operated without ductwork, operates at very low pressure and high volume.

Low pressure exhaust systems, such as general factory, kitchen, warehouse.

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Fan Classification

Special Design FansAxial Power Roof VentilatorLow pressure, Positive Exhaust ventilation .

Propeller fan mounted in a supporting structure

Hood protects fan and acts as safety guard

Operated without ductwork, operates at very low pressure and high volume.

Low pressure exhaust systems, such as general factory, kitchen, warehouse.

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Summary – Fans and Fan Laws

•Static Pressure

•Velocity Pressure

•Total Pressure

•Measurement of static Pressure and Total Pressure

•Pressure measurement devices

•Affinity Laws

•Fan Curves

•Surge and stall

•Type of Fans

•Performance of various types of fans.

•Applications of various types of fans