Basics of fans

Basics of Fans
Tahseen Qamhieh
Sr. Mechanical Engineer

 What is Fan?
 Fans Basic Terms
 Fan Performance Evaluation
 Fan Types
 Fan Basic Components
 Fan System Components
 Fan Design and Selection
 Fan Flow Control Strategies
 AMCA Association
 Fans Maintenance
Page 2
Presentation Outline

• A fan is a rotating device that creates
pressure differential that results in air
movement.
• Fans generate a pressure to move air
against a resistance caused by ducts,
dampers, or other components in a fan
system.
• The fan rotor receives energy from a
rotating shaft and transmits it to the air.
BASICS OF FANS Page 3
What is Fan?

• Air Flow (Q): Amount of air moved per rate of time, typically measured in cubic
feet of air per minute (CFM).
• Static Pressure (Ps): Resistance against airflow by the system (ductwork, fittings,
dampers, filters, etc.). Typically measured in inches of water gauge (in. wg.)
• Total Pressure (Pt): The amount of pressure exerted by airflow on anything
directly in the airstream.
• Velocity Pressure (Pv): Directly related to the velocity of the airflow at any given
point in a system. Used to calculate the airflow at any point in a system. Cannot be
measured directly and is calculated as the difference between Total Pressure and
Static Pressure.
Pv = Pt - Ps
Basic Terms

• Velocity (V): Speed of air in the direction of flow. Measured in feet per minute
(FPM).
• Power (HP): Rate of doing work, typically measured in Horsepower. For rotating
machinery power is the amount of torque applied to a shaft to maintain a given
rotating speed (RPM).
HP = RPM × torque (ft-lb) / 5252.
1 HP = 33,000 foot-lbs per minute.
• Brake Horsepower (BHP): (as listed in a fan performance table) The amount of
HP required at the fan shaft to move the specified volume of air against a given
system resistance. It does not include drive losses.
Basic Terms

Fan Performance
Evaluation and
Efficient System
Operation

– The system effect is the change
in system performance that
results from the interaction of
system components.
– Typically, during the design
process, the system curve is
calculated by adding the losses
of each system component
(dampers, ducts, baffles, filters,
tees, wyes, elbows, grills,
louvers, etc.)
System Effect

– The governing equation for pressure loss across any particular component is:
System Effect

– System resistance varies with
the square of the volume of air
flowing through the system.
Thus, the system resistance
increases substantially as the
volume of air flowing increases.
Conversely, resistance
decreases as flow decreases.
– In existing systems the system
resistance can be measured,
while in new designed systems
the system resistance must be
calculated.
System Resistance characteristics

– Performance curve for a particular
fan under specific conditions.
– Typically a curve will be developed
for a given set of conditions
including: fan volume, system
static pressure, fan speed, brake
horsepower, and efficiency.
– The intersection of the system
curve and the fan curve defines
the operating point.
Fan Performance Curve(s)

– The fan actual operating point “A”
showing a flow (Q1) at pressure (P1)
and fan speed (N1).
– Two methods can be used to reduce
air flow from Q1 to Q2:
1) Closing a damper in the system.
This increase system resistance and
causes a new system performance
curve (SC₂). The fan will operate at
“B” to provide the reduced air flow
(Q2) against higher pressure (P2)
System Performance Curve

2) Reducing fan speed to (N2) while
keeping the damper fully open.
The fan will operate at “C” to
provide the same (Q2) air flow,
but at lower pressure (P3).
– Thus, reducing the fan speed is a
much more efficient method to
decrease airflow since less power
is required and less energy is
consumed.
System Performance Curve

– Fan efficiency is the ratio the power
imported to the airstream to the power
delivered by the motor.
– The power of the airflow is the product of
the pressure and the flow, corrected for
units consistency
Best Efficiency Point (BEP)
– On the BEP a fan operates most cost effectively in terms of both energy efficiency
and maintenance considerations.
– Operating a fan near its BEP improves its performance and reduces wear, allowing
longer intervals between repairs. Moving a fan’s operating point away from its BEP
increases bearing loads and noise.

– Fan rotational speed
is typically measured
in revolutions per
minute (RPM).
– A change in speed
(RPM) of any fan will
predictably change
the flow rate, the
pressure rise and
power necessary to
operate it at the new
RPM.
Fan Laws

 As per American Society of Mechanical Engineers (ASME), the Specific Ratio
is used for defining the fans, blowers and compressors.
 Specific Ratio is the ratio of the discharge pressure over the suction pressure.
Difference Between Fans, Blowers and Compressors

Centrifugal Axial
Fans
Forward
Curved
Radial
Backward
Curved
TubeaxialPropeller Vaneaxial

 Fan Characteristics:
– Blades are in a radial direction from the hub
– Low/Medium airflow rates
– High static pressure, High temperature
– High sound levels
– Low maintenance cost
– Efficiency 65-75%
 Typical Applications:
Various industrial applications suitable for dust
laden, wood chips, metal scrap and moist air/gases.
Radial Fan

– The large clearances between the
blades allow this fan to operate at
low airflows without the vibration
problems that usually accompany
operating in stall.
– In many cases, the blades can be
inexpensively coated with protective
compounds to improve erosion and
corrosion resistance.
– The characteristic durability of this
fan type is a key reason why it is
considered an industry workhorse.
Radial Blade Fan Performance Curve

– Blades curve towards the direction of rotation
– Large airflow rates against relatively low
static pressures
– Medium sound levels
– Medium maintenance cost
Low pressure HVAC, Packaged units, suitable
for clean and dust laden air/gases.
Forward Curved Fan

– The dip in the performance curve
represents a stall region that can create
operating problems at low airflow rates.
– Fan output is difficult to adjust accurately
(note how the fan curve is somewhat
horizontal), and these fans are not used
where airflow must be closely controlled.
– Forward curved fans have a power curve
that increases steadily with airflow
toward free delivery; consequently,
careful driver selection is required to
avoid overloading the fan motor.
Forward Curved Fan Performance Curve

– Blades are inclined opposite to the
direction of fan
– High pressure
– Low sound levels
– High maintenance cost
HVAC various industrial high pressure
applications, forced draft fans.
Backward Inclined Fan

– Within backward inclined fans are three
different blade shapes: flat, curved, and
airfoil.
– Flat blade types are more robust. Curved
blade fans tend to be more efficient. Airfoil
blades are the most efficient of all, capable
of achieving efficiencies exceeding 85%.
– The motor brake horsepower increases
with airflow but drops off at high airflow
rates. Because of this non-overloading
motor characteristic, this fan type is often
selected when system behavior at high
airflow rates is uncertain.
Backward Inclined Fan Performance Curve

– High flowrates
– Low pressure
– High sound levels
– Light weight and inexpensive
Air circulation, ventilation exhaust, air-
cooled condensers, cooling towers.
Propeller Fan

– The power requirements of
propeller fans decrease with
increases in airflow.
– They achieve maximum efficiency,
near-free delivery, and are often
used in rooftop ventilation
applications
Propeller Fan Performance Curve

– Close clearance between blade and
housing to improve airflow efficiency
– High flowrates
– Medium pressure
– Moderate sound levels
– Moderate cost
HVAC, drying ovens, exhaust systems.
Tubeaxial Fan

– Much like propeller fans, tubeaxial
fans have a pronounced instability
region that should be avoided.
– Tubeaxial fans are frequently used
in exhaust applications because
they create sufficient pressure to
overcome duct losses and are
relatively space efficient.
– Because of their low rotating mass,
they can quickly accelerate to rated
speed, which is useful in many
ventilation applications.
Tubeaxial Fan Performance Curve

– Equipped with inlet or outlet guide vanes to
improve efficiency by directing & straightening
the flow.
– Medium flow rate
– High pressure
– Low sound levels
– High cost
High pressure applications including HVAC systems
Vaneaxial Fan

– Vaneaxial fans have performance
curves that have unstable regions to
the left of the peak pressure.
– Vaneaxial fans are often equipped
with variable pitch blades, which can
be adjusted to change the angle of
attack to the incoming airstream.
– When equipped with airfoil blades
and built with small clearances, they
can achieve efficiencies up to 85%.
Vaneaxial Fan Performance Curve

 In Centrifugal flow; air flow changes direction twice, once when entering and
second when leaving
Centrifugal Fan Basic Components

BASICS OF Fans Page 36
Single Inlet and Double Inlet Centrifugal Fans

BASICS OF Fans Page 37
Centrifugal fan Common Configurations

 In Axial flow; air
enters and leaves
the fan with no
change in direction.
Axial Fan Basic Components

– A typical fan system consists of a fan,
an electric motor, a drive system,
ducts or piping, flow control devices,
and air conditioning equipment (filters,
cooling coils, heat exchangers, etc.)
– There are two principal types of drive
systems: direct drive and belt drive.
– In direct drive systems, the fan is
attached to the motor shaft. This is a
simple, efficient system but has less
flexibility with respect to speed
adjustments
Fan System Components

– Applications with low temperatures and
clean system air are well-suited for direct
drives because the motor mounts directly
behind the fan and can be cooled by the
airstream
– Belt drives offer a key advantage to fan
systems by providing flexibility in fan speed
selection.
– There are four principal types of belts: Flat
belts, V-belts, cogged V-belts, and
synchronous belts. There are different cost
and operating advantages to each type.
Fan System Components

– Flat belts have a uniform cross-
section and transmit power through
friction contact with flat pulley
surfaces.
– V-belts are an improvement over
the flat belt, using a wedging action
to supplement friction-based power
transfer.
Types of Belt Drives

– Cogged V-belts offer the same advantages as V-
belts; however, their notched design provides
additional flexibility that allows the use of smaller
pulleys. Cogged V-belts are slightly more efficient
than conventional V-belts, because of their added
flexibility and the fact that the notched surface
transfers force more effectively.
– Synchronous belts offer many advantages over
standard flat belts and V-belts. By using a mesh
engagement, synchronous belts are the most
efficient type of belt drive because they do not suffer
efficiency losses through slip. Synchronous belts
have teeth that engage with grooves in the sheave.
Types of Belt Drives

– In most applications, ducts are used on
one or both sides of a fan and have a
critical impact on fan performance.
Friction between the airstream and the
duct surface is usually a significant
portion of the overall load on a fan.
– As a rule, larger ducts create lower
airflow resistance than smaller ducts.
– Round ducts have less surface area per
unit cross sectional area than rectangular
ducts and, as a result, have less leakage.
Ductwork and Piping

– Flow control devices include inlet dampers on
the box, inlet vanes at the inlet to the fan, and
outlet dampers at the outlet of the fan.
– Inlet vanes adjust fan output, while dampers
can be used to throttle the air entering or
leaving a fan and to control airflow in branches
of a system or at points of delivery.
– The inlet vanes and dampers must be
designed for proper fan rotation and are to be
installed in such a way that these inlet vanes
and dampers open in the same direction as the
fan rotation.
Airflow Control Devices

– Conditioning equipment influences fan
performance by providing flow resistance and,
in some cases, by changing air density.
– In many systems, poor performance is a direct
result of inadequate attention to filter
cleanliness.
– The effects of heating and cooling coils on fan
system performance depend largely on where
in the system the heat exchangers are located,
the extent of the temperature change, and how
the heat exchangers are constructed.
Air Conditioning and Process Equipment (Filters,
Heat Exchangers, etc.)

1) Flowrate or volume required (CFM)
2) Fan static pressure (in.wg)
3) Airstream characteristics; moisture, particulate content,
flammable environment…etc.
4) Space constraints and limitations.
5) Drive arrangements, direct drive or belt drive.
6) Noise levels.
7) Operating temperature range.
8) Operation life and costs.
9) Safety and accessories.
10)Efficiency; operate close to Best Efficiency Point (BEP).
Factors affecting Fan design and Selection

– Choose the right fan considering all factors
affecting the fan design.
– Avoid oversized fans. Indications of oversized
fans; high capital costs, high energy costs,
poor performance, frequent maintenance,
high noise/vibration levels.
– Reduce the system resistance.
– Operate close to Best Efficiency Point (BEP).
– Maintain fans regularly.
– Control the fan airflow.
Energy Saving Measurements and Opportunities

1) Pulley Change:
– The fan must be driven by a motor through a
V-belt system.
– The fan speed can be increased or decreased
with a change in the drive pulley or the driven
pulley or in some cases both pulleys.
– As shown in the figure, a higher sized fan
operating with damper control was downsized
by reducing the motor (drive) – Pulley size
from 8” to 6”. The power reduction was 12 kW.
Flow Control Strategies

2) Damper Controls:
– Can be used to throttle the air entering or leaving a
fan and to control airflow in branches of a system or
at points of delivery without changing fan speed.
– Dampers control airflow by changing the amount of
restriction in an airstream. Increasing the restriction
creates a larger pressure drop across the damper
and dissipates some flow energy, while decreasing
the restriction reduces the pressure differential and
allows more airflow.
– Damper controls are not particularly energy efficient.

3) Inlet Guide Vanes:
– Inlet vanes adjust fan output in two
principal ways: by creating a swirl in the
airflow that affects the way in which the
air hits the fan blades, or by throttling the
air altogether, which restricts the amount
of air entering the fan.
– The pre-rotation or swirl of the air helps
reduce the brake horsepower of the fan.
– Guide vanes are energy efficient for
modest flow reductions, reduced load
and airflow.

4) Variable Frequency Drives (VFDs):
– VFD operation involves reducing the speed of the fan to meet reduced flow
requirements.
– When fan speed decreases, the curves for fan performance and brake horsepower
move toward the origin. Fan efficiency shifts to the left, providing an essential cost
advantage during periods of low system demand.
– Another system benefit of VFDs is their soft-start capability. It allow the motor to be
started with a lower start-up current (usually about 1.5 times the normal operating
current), thus reducing wear on the motor windings and the controller.
– Although VFDs offer a number of benefits in terms of lower operating and
maintenance costs, they are not appropriate for all applications. VFD may not be
economical for systems which have infrequent flow variations, and systems with
high static pressure.

5) Variable Pitch Blades:
– Applicable to axial fans only.
– Control fan output by adjusting the fan blade angle of attack with respect to
the incoming airstream. This allows the fan to increase or decrease its load
in response to system demand
– Can be hydraulically or pneumatically controlled to change blade pitch while
the fan is at stationary.
– Variable pitch fans provide a highly efficient means, and modify the fan
characteristics substantially.

6) Series and Parallel Operation:
– Parallel operation is defined as having two or more fans blowing together side by
side. The performance of two fans will result in doubling the volume flow, but only
at free delivery.
– Fans in parallel are only suited for low resistance system.
– Series operation is defined as using multiple fans in a push-pull arrangement. By
staging two fans in series the static pressure capability at a given airflow can be
increased but not to double at every flow point.
– In series operation the best results are achieved in systems with high resistance.

 Lower Average Duct Pressure – the series-configurations fans along different
points in a system minimize the average static pressure in a duct.
 Lower Noise Generation – Lower pressure requirements can decrease the
noise generated by fan operation.
 Redundancy – Failure of one unit does not force a system shutdown. With a
multiple-fan arrangement, one can be repaired while the others serve the system.
 Efficiency – Allowing each fan to operate close to its BEP can provide
substantial energy savings. In addition, a potential advantage of multiple fans is a
higher overall efficiency level
 Structural and Electrical Constraints – Two smaller fans in series may be
more suitable in terms of structural and electrical requirements than a single one.
Advantages of Multiple-Fan Arrangements

Roof Exhaust Fan Installation (Good vs. Poor)
Good Poor Poor

Centrifugal Fan Installation (Good vs. Poor)
Good Poor PoorGood Poor

Axial Fan Installation (Good vs. Poor)
Good Poor Poor

Air Movement and
Control Association
International, Inc.
(AMCA International)

– AMCA International is a non-profit international
association of the world’s manufacturers of related
air system equipment.
– Such equipment primarily includes fans, louvers,
dampers, air curtains, airflow measurement
stations, acoustic attenuators, and other air system
components for the industrial, commercial, and
residential markets.
– AMCA International provides a variety of services
to its members and the air movement and control
industry, including its Certified Ratings Program,
Standards, and Testing Laboratories.
Introduction to AMCA International

– The CRP assures that a product line has been tested and rated in conformance
with AMCA International or Industrial Standards Organization (ISO) test standards
and rating requirements.
– Only after a product has been tested and the manufacturer’s catalogued ratings
have been submitted to and approved by AMCA International staff can
performance seals be displayed in literature and on equipment.
– Currently, AMCA International has the world’s only international CRP for air
system components.
– AMCA International maintains the Directory of Products Licensed to Use the
AMCA International Certified Ratings Seal on its Web site at www.amca.org
AMCA International’s Certified Ratings Program (CRP)

 The AMCA International test laboratory is located in Arlington Heights, Illinois,
and accredited AMCA International laboratories are located around the world.
Independent accredited AMCA International laboratories, located in the United
Kingdom and Taiwan, function much like AMCA International’s primary laboratory.
 The AMCA International test laboratory is equipped to test fans in accordance with
the following standards:
• ANSI/AMCA 210, Laboratory Method of Testing Fans for Aerodynamic
Performance Rating
• AMCA 220, Test Methods for Air Curtain Units
• ANSI/AMCA 230, Laboratory Method of Testing Air Circulator Fans for Rating
• ANSI/AMCA 240, Laboratory Method of Testing Positive Pressure Ventilators
The AMCA International Test Laboratory

 Common maintenance tasks on fan systems include:
 Periodic inspection of all system components
 Bearing lubrication and replacement
 Belt tightening and replacement
 Motor repair or replacement
 Fan cleaning.
Maintenance Items

Belts – Check belt condition, tightness, and alignment.
Also check sheave condition.
Bearings – Determine bearing condition by listening for
noises that indicate excessive wear, measuring bearing
operating temperature, or by using a predictive
maintenance technique, such as vibration analysis or oil
analysis. Lubricate bearings in accordance with fan
manufacturer instructions. Replace bearings, if necessary.
System Cleaning – Fans and system components that
are susceptible to contaminant build-up should be cleaned
regularly.
Basic Maintenance Checklist

Leaks – Check for ductwork leakage that can lead
to energy losses and poor system performance.
Motor Condition – Check the integrity of motor
winding insulation. Generally, these tests measure
insulation resistance at a certain voltage or measure
the rate at which an applied voltage decays across
the insulation. Also, vibration analysis can indicate
certain conditions within the motor windings, which
can lead to early detection of developing problems.
Basic Maintenance Checklist

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