Fans and Blowers

Differentiation between fans and blowers

A typical fan A typical blower

Air moving devices are generally described as being either a type of
fan or blower.
The main difference between fans and blowers is in their flow and
pressure characteristics.
Fans deliver air in an overall direction that is parallel to the fan blade
axis and can be designed to deliver a high flow rate, but tend to work
against low pressure.
Blowers tend to deliver air in a direction that is perpendicular to the
blower axis at a relatively low flow rate, but against high pressure.
 Differentiation between fans and blowers
Fans, blowers and compressors are differentiated by the
method used to move the air and by the system pressure on
which they must operate against.
The American Society of Mechanical Engineers (ASME)
uses the specific ratio, which is the ratio of the discharge
pressure over the suction pressure, to define fans, blowers
and compressors (see Table).
Equipment Specific Ratio Pressure Rise (mm Wg)
Fans Up to 1.11 1136
Blowers 1.11 to 1.20 1136-2066
Compressors More than 1.20 --
 INTRODUCTION
• Fans and blowers provide air for ventilation and
industrial process requirements.
• Fans generate a pressure to move air (or gases) 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.
• Fan and blower selection depends on the volume flow
rate, pressure, type of material handled, space limitations
and efficiency.
• Fans are widely used in industrial and commercial
applications such as ventilation, material handling,
boilers, refrigeration, dust collection, cooling
applications and others.
 INTRODUCTION Cont….
• The performance of fans can have a significant impact
on plant production. The importance of fan reliability
often causes system designers to over-design fan
systems to avoid under-performing systems.
• This practice of oversizing fan systems creates
problems that can increase system operating costs while
decreasing fan reliability.
• Fans that are oversized for their service requirements
do not operate at their best efficiency points. Oversized
fans generate excess flow, resulting in high airflow
noise, increased stress on the fan and the system and
excessive energy consumption.
 SELECTION & APPLICATION
OF
FANS & BLOWERS
Air handling systems are normally designed to deliver a
certain amount of air under specific operating
conditions.
• In some cases, the air requirements are constant.
• In other cases, the air requirements may vary up or
down or may even be zero at times.
• A variety of fan types is available to the system
designer, as well as various types of motors and drive
systems that provide flow control in an efficient manner.
• The system to carry the air through a building’s duct
system or an industrial process can also be designed with
features that reduce the ‘‘friction’’ or the backpressure of
the flowing air and which minimize air leakage.
• Once built, making changes to the ductwork or piping
can become costly. For this reason, when duct systems
are expanded or refurbished, it pays to review the
various features that could be incorporated into the
design to further reduce pressure losses and leaks and
thus reduce the amount of energy required to drive the
process.
• Finally, keen observation of the performance and
behaviour of the system along with proper maintenance
can reduce energy and maintenance costs and can
increase reliability and service life.
HOW TO RECOGNIZE FAN & BLOWER TYPES

• There are two primary types of fans:

• Centrifugal fans, and
• Axial fans.
• These types are characterized by the path of the
airflow through the fan.
 Centrifugal Fans
• Centrifugal fans use a rotating impeller to move air first
radially outwards by centrifugal action, and then tangentially
away from the blade tips.
• Incoming air moves parallel to the impeller hub and it turns
radially outwards towards the perimeter of the impeller and
blade tips.
• As the air moves from the impeller hub to the blade tips, it
gains kinetic energy. This kinetic energy is then converted to
a static pressure increase as the air slows before entering the
tangential discharge path.
• Centrifugal fans are capable of generating relatively high
pressures. They are frequently used in ‘‘dirty’’ airstreams
(high moisture and particulate content), in material handling
applications and in systems operated at higher temperatures.
 Centrifugal Fans
• Centrifugal fans are rugged, are capable of
generating high pressures with high efficiencies
and can be manufactured to accommodate harsh
operating conditions.
 Centrifugal Fans
• These are the most commonly used types of
industrial fans.
• Centrifugal fans have several types of blade
shapes, including:
1) Backward-inclined flat blade;
2) Backward-inclined curved blade;
3) Backward-inclined airfoil blade;
4) Forward curved;
5) Radial-blade; and
6) Radial-tip.
 Backward-inclined flat blade
Characteristics Applications
➢ Flat blades are inclined in the direction ➢ Suitable for forced-draft service.
opposite to the rotation ➢ Unsuitable for airstreams with airborne
➢ Considered more robust than other particulates. “Safe” choice because of
types its non-overloading motor characteristic
➢ The low angle of contact with the
airstream facilitates the accumulation
of deposits on the fan blades
➢ Performance drops off at high airflow
rates
Backward-inclined curved blade
Characteristics Applications
➢ Curved blades inclined away from the ➢ Suitable for forced-draft service
direction of rotation ➢ Because of its non-overloading motor
➢ More efficient than flat blades characteristic, this fan type is often
➢ Low angle of contact with the airstream selected when system behavior at high
promotes the accumulation of deposits airflow rates is uncertain
on the fan blades
➢ Performance drops off at high airflow
rates
 Backward-inclined Airfoil blade
Characteristics Applications
➢ Airfoil blades tilt away from the ➢ Suitable for forced-draft service
direction of rotation ➢ Because of its non-overloading
➢ Most efficient with thin blades motor characteristic, this fan type
(~85%), but most unstable because is often selected when system
of stall behavior at high airflow rates is
➢ Low angle of impingement with uncertain
the airstream promotes the
accumulation of deposits on the
fan blades as well as erosion.
➢ Performance drops off at high
airflow rates
 Forward curved
• Sometimes called "squirrel cage" fans, have blades with leading
edge curved toward the direction of rotation.
• Usually contain 24 to 64 impeller blades.
• Air leaves the impeller at velocities greater than the impeller tip
speed i.e. air tip velocity > wheel peripheral velocity.
• Operate at lower speed than other centrifugals, which make them
quiet.
• Smaller size relative to other fan types.
• Lighter in construction and less expensive.
• Because these fans generate high airflow at relatively low speeds,
these require a relatively accurate estimate of the system airflow and
pressure demand. If, for some reason, system requirements are
uncertain, then an improper guess at fan rotational speed can cause
underperformance or excessive airflow and pressure.
• Characterized by relatively low efficiency between 55 and 65
percent and is somewhat less than airfoil and backward-curved fans.
 Forward curved Cont .....
• Well suited for low pressure heating, ventilating and air
conditioning applications such as domestic furnaces, central
station units and packaged air conditioning equipment.
• Exhibit ‘overloading’ power curve characteristic i.e. the power
increases steadily with airflow toward free delivery; consequently,
careful driver selection is required to avoid overloading the fan
motor.
• Fan output is difficult to adjust accurately and these fans are not
used where airflow must be closely controlled.
• Usable only with clean air applications because blades easily
accumulate dirt.
• Not constructed for high pressures or harsh service.
• Not recommended for fumes or dusts that would stick to the
short curved blades because they would cause unbalance and
would make cleaning difficult.
 Radial-Blade and Radial-Tip
Radial fans have blades, which extend straight from the shaft, and
typically have 6 to 16 blades.
• Simplest of all centrifugal fans and the least efficient. Radial fans
have efficiencies of 50 – 65 percent.
• Operate at medium speed and can move air against higher
pressures than other centrifugal fans.
• Pressures up to 55in-wg are common; custom-designed fans can
achieve pressures over 100in-wg.
• Produce high sound levels with pronounced blade passage tones
at high pressure applications.
• Usually have medium tip speed and noise factor and are used for
buffing exhaust, woodworking exhaust, or for applications where a
heavy dust load passes through the fan.
• Because of their simple blade shape, these can be ruggedly
constructed with expensive alloys that are strong and corrosion
resistant, or they can be less expensively constructed with fiber
glass reinforced plastic or coated with a corrosion resistant
material. These are widely used in corrosive applications and in
high-temperature environments
Summary
 Centrifugal Fan Centrifugal Radial Blade Fan
(FanAir Company) (Canadian Blower)

Forward-Curved Fan Backward Inclined Fan

(Canadian Blower) (Canadian Blower)
 Axial Fans
• Axial fans, as the name implies, move air parallel to the
shaft, or axis, of the fan.
• Axial-flow fans impart energy to the air by giving it a
twisting motion. The air is pressurized by the aerodynamic
lift generated by the fan blades, much like a propeller and an
airplane wing.
• Typically provide large air volumes at relatively low
pressures.
• Key advantages are compactness, low cost, and
lightweight.
• Axial fans are frequently used in exhaust applications
where airborne particulate size is small, such as dust streams,
smoke, and steam.
• Less bulky than a centrifugal fan of comparable capacity
and have the advantage of straight through airflow.
 Axial Fans
• Typically designed to generate flow in one direction
but they can operate in the reverse direction.
• This characteristic is useful when a space may require
contaminated air to be exhausted or fresh air to be
supplied.
• Axial fans must rotate faster than comparable
centrifugal fans to achieve the same airflow capacity.
• This characteristic makes them noisier than
comparable centrifugal fans. However, high noise of
axial fans tends to be dominated by high frequencies,
which are easier to attenuate.
• Axial fans have a severe stall region that makes them
particularly unsuitable for systems with widely varying
operating conditions
 Axial Fans
• Axial fans are commonly used in ‘‘clean air,’’ low-
pressure, high-volume applications.
• Axial fans have less rotating mass and are more
compact than centrifugal fans of comparable capacity.
• Additionally, axial fans tend to require higher
rotational speeds and are somewhat noisier than inline
centrifugal fans of similar capacity.
• Axial airflow fans have a number of advantages over
other types including:
✓Compactness;
✓Light weight;
✓Low cost;
✓Direct-drive units operating near the synchronous speed of
the induction motor; and
✓Belt-drive units offering flexibility in fan speed selection.
 Axial Fans
• Usual applications for axial fans are:
✓Exhausting contaminated air or supplying fresh
air;
✓Unidirectional or reversible air-flow applications;
✓Exhaust applications where airborne particulate
size is small, such as dust streams, smoke and
steam.
• Types of Axial Fans
✓Propeller Fans
✓Tubeaxial Fans
✓Vaneaxial Fans
 Propeller Fans
• Have two or more blades that generate very high airflow volumes.
• Low static pressure up to ¾ in-wg.
• Can provide reversed air flow at reduced volumes and pressure by
reversing the direction of rotation.
• Exhibit ‘overloading power’ characteristic i.e. the power required to
drive the impeller continues to increase as the resistance to airflow
increases.
• Very low efficiencies of approximately 50 percent or less.
• Light weight and inexpensive because of their simple construction.
• Noise levels of the propeller fan are only slightly higher than those of
the tubeaxial and vaneaxial fans, but such of the noise is at low
frequencies and therefore is difficult to attenuate.
• Primary applications include low pressure, high volume air moving
applications such as air circulation within a space or ventilation through a
wall without attached ductwork.
• Used for replacement air applications. Ideal for exhaust applications
and are often used in rooftop ventilation applications. Also used for make
up or replacement air applications
 Tubeaxial Fans
Tubeaxial fans have a wheel inside a cylindrical housing, with
close clearance between blade and housing. Generally, the
numbers of blades range from 4 to 8 with the hub normally less
than 50 percent of fan tip diameter.
• Capable of developing a more useful static pressure range (to 10
– 15in-wg)
• Capable of delivering operating efficiencies up to 75 percent.
• The downstream profile is uneven with a large rotation
component. This airflow characteristic is accompanied by
moderate airflow noise.
• Suited for ducted applications where downstream velocity
profiles is not very critical.
• Can be either connected directly to a motor or driven through a
belt configuration. Because of the high operating speeds motors,
most tubeaxial fans use belt drives to achieve fan speeds below
1,100 revolutions per minute.
 Tubeaxial Fans
• Most good design blades are aerofoil shaped.
• Tubeaxial fan generates a somewhat higher noise level than the
vaneaxial fan.
• Used in medium-pressure, high airflow rate applications and are
frequently used in exhaust applications because they create sufficient
pressure to overcome duct losses and are relatively space efficient.
Also used in some industrial applications such as drying ovens, paint
spray booths and fume exhaust systems.
 Vaneaxial Fans
Vane-axial fans are similar to tube-axial fans with guide vanes
on down streamside to improve flow profile.
• Typically have 5 to 20 aerofoil type blades with a large hub
diameter.
• Very close blade tip to housing clearance.
• Blades are fixed or adjustable pitch types and the hub is
usually greater than 50 percent of the fan tip diameter.
• Very efficient. When equipped with airfoil blades and built
with small clearances, they can achieve efficiencies up to 85
percent.
• High-pressure capability up to 8 in-wg. Custom equipment
is capable of 20 -40 in-wg.
 Vaneaxial Fans
• Typically used in medium- to high-pressure applications,
such as induced draft service for a boiler exhaust.
• Vaneaxial fans generate somewhat higher noise levels than
centrifugal ventilating fans of comparable output.
• Adjustable pitch blades allow capacity control.
• Like tubeaxial fans, vaneaxial fans tend to have a low
rotating mass, which allows them to achieve operating speed
relatively quickly. This characteristic is useful in emergency
ventilation applications where quick air removal or supply is
required.
Summary
• The overall system energy efficiency for an existing
system can be expressed in terms of the specific fan
power (SFP). The science of SFP evolved in Europe
and is gaining acceptance in North America.
• The SFP is defined as the installed motor power of
all the fans in the air distribution system divided by
the design air flow rate.
• SFP is expressed in terms of kW per 1000 CFM or in
kW per (m3/s):
✓An efficient system has a low SFP of usually under 0.7
kW/1000 CFM [1.5 kW/(m3/s)];
✓A medium efficiency system has an SFP in the range of
0.7 to 1.9 kW/1000 CFM [1.5 to 4.0 kW/(m3/s)]; and,
✓A low efficiency system has an SFP above 1.9 kW/1000
CFM [4.0 kW/(m3/s)].
• Using SFP can help identify opportunities to increase
fan system efficiency.
• Principles of Operation
✓Systems that require air flow are normally supplied by one
or more fans of various types driven by a motor.
✓The motor rotates the fan which delivers air to the system
as it develops a pressure in the ductwork (or air pathways)
that causes the air to move through the system.
✓Moving air in a streamline has energy due to the fact that
it is moving and it is under pressure.
✓In terms of air movement, Bernoulli’s theorem states that
static pressure plus velocity pressure as measured at a
point upstream in the direction of airflow is equal to the
static pressure plus velocity pressure as measured at a
point downstream in the direction of airflow plus the
friction and dynamic losses between the two measuring
points.
 ✓The motor imparts energy to the fan, which in turn
transfers energy to the moving air.
✓The duct system contains and transports the air. This
process causes some losses in static pressure due to
friction with the walls and changes in the direction of
flow (due to elbows and other fittings), as well as air
losses through unintentional leaks.
• Fan Laws
✓Rotational Speed: Fan rotational speed is measured in
revolutions per minute (RPM). Fan rotational speed
affects fan performance, as shown by the following fan
laws.
✓Airflow rates vary in direct proportion to the rotational
speed of the fan:
• Fan Laws

✓Pressure built up by the fan varies as the square of the

rotational speed of the fan:

✓Power required by the fan varies with the cube power

of the rotational speed of the fan:
• Care needs to be taken when using the fan laws to
calculate the effects of changes in fan speed, since these
laws apply to a specific density of gaseous medium.
• When fan speed changes are accompanied by significant
changes in other parameters such as gas composition,
moisture content and temperature, the fan laws will need
to be adjusted accordingly to compensate for the
resulting change in medium density.
• Rotational speed must be considered concurrently with
other issues, such as:
✓ Air stream density (pressure, temperature, moisture content,
gas composition);
✓ Ambient noise;
✓ Mechanical strength of the fan; and,
✓ Variations in the fan load.
• Fan Characteristics
1. Fan Performance Curve
✓The fan performance curve expresses the power required
over the range of airflow rates. Individual points on a fan
performance curve are determined by plotting the
developed pressure against the air flow rates. This curve is
essential in the design of the system, in the selection of the
equipment and in the operation of the system.
✓The Fan Characteristics section describes the various
types of fans available, along with their pressure-flow
characteristics.
✓When the actual air flow in a system cannot be predicted
with some accuracy, or the air flow is expected to vary
considerably, it is very important to select the type of fan,
motor and control system that will prevent equipment
overloads.
• Fan Characteristics
2. Fan Efficiency
✓Fan efficiency is defined as the ratio of power
transferred to the airstream to the power delivered to
the shaft of the fan.
✓The power of the airflow stream is the product of the
pressure and the flow, corrected for consistency of
units.
✓The fan efficiency can be expressed in terms of Total
pressure (Total Efficiency) or in terms of static
pressure (Static Efficiency).
• Total Efficiency
✓Total Efficiency is the ratio of power of the airflow
stream (using total pressure) divided by power
delivered to the fan shaft in consistent units, or

Where:
✓Total Pressure is in inches of water (in. WG)
✓Airflow is in cubic feet per minute (cfm)
✓bhp is brake horsepower
✓6,362 is the unit consistency factor
• Static Efficiency
✓Static Efficiency is the ratio of power of the airflow
stream (using static pressure) divided by power
delivered to the fan shaft in consistent units, or

Where:
✓Static Pressure is in inches of water (in. WG)
✓Airflow is in cubic feet per minute (cfm)
✓bhp is brake horsepower
✓6,362 is the unit consistency factor
Since the two defined efficiencies are quite distinct and
different from each other, one must be clear to identify the type
of efficiency referred to when comparing performance values.
• Best Efficiency Point
✓Air stream density (pressure, temperature, moisture
content, gas composition); The best efficiency point
(BEP) is a point on the operating characteristics of the
fan where a fan operates most efficiently and cost-
effectively in terms of both energy use and cost of
maintenance/replacement.
✓Operation of a fan near its BEP results in high
efficiency and reduced wear and tear on the
equipment. Operation far away from the BEP results
in lower fan efficiency, increased bearing loads and
higher noise levels.
➢ BLOWER

• Blowers can achieve much higher pressures than fans, as high

as1.20 Kg/cm2.
• The impeller is typically gear-driven and rotates as fast
as15,000 rpm.
• They are also used to produce negative pressures for industrial
vacuum systems.

1. Centrifugal blowers
• Typically operate against pressures of 0.35 to 0.70 Kg/cm2.
• They are most often used in applications that are not prone
to clogging.
2. Positive-displacement blowers
• They are especially suitable for applications prone to
clogging, since they can produce enough pressure up to 1.25
Kg/cm2 – to blow clogged materials free.