PEMP
RMD 2501
Fans and Blowers
Session delivered by:
Prof Q.H.
Prof.
Q H Nagpurwala
06a
© M.S. Ramaiah School of Advanced Studies
1
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
06a
© M.S. Ramaiah School of Advanced Studies
2
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
3
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
©©
M.M.S.
S. Ramaiah
School
of Advanced
Studies, Studies
Bangalore
Ramaiah
School
of Advanced
4
Components of Fan/Blower System
PEMP
RMD 2501
Turning Vanes
(typically used on
short radius elbows)
Outlet
O tl t Diffusers
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
06a
Variable Frequency Drive
Motor
©©
M.M.S.
S. Ramaiah
School
of Advanced
Studies, Studies
Bangalore
Ramaiah
School
of Advanced
5
PEMP
RMD 2501
Parts of a Fan / Blower
1
1.
2.
3.
4
4.
5.
6.
7
7.
8.
•
•
06a
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
6
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
© M.S. Ramaiah School of Advanced Studies
7
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
06a
© M.S. Ramaiah School of Advanced Studies
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
© M.S. Ramaiah School of Advanced Studies
Forward
Curved
Blades Type
9
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
06a
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
©©
M.M.S.
S. Ramaiah
School
of Advanced
Studies, Studies
Bangalore
Ramaiah
School
of Advanced
10
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
11
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
06a
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
© M.S. Ramaiah School of Advanced Studies
12
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
© M.S. Ramaiah School of Advanced Studies
13
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
© M.S. Ramaiah School of Advanced Studies
14
Axial Flow Fans – Applications
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
06a
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
CPU Fans
© M.S. Ramaiah School of Advanced Studies
15
Propeller Fan
PEMP
RMD 2501
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
06a
© M.S. Ramaiah School of Advanced Studies
16
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
06a
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
17
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
© M.S. Ramaiah School of Advanced Studies
18
Axial-Centrifugal Fan Types
PEMP
RMD 2501
Single inlet single
width impeller
Double inlet
double width
impeller
06a
Single inlet single width
fan wheel with six radial
blades welded to a back
plate
l
© M.S. Ramaiah School of Advanced Studies
19
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 fflow
06a
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
© M.S. Ramaiah School of Advanced Studies
20
Blower Types









06a
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
21
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
© M.S. Ramaiah School of Advanced Studies
P equals
either pt or ps
q
23
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
06a
© M.S. Ramaiah School of Advanced Studies
24
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
© M.S. Ramaiah School of Advanced Studies
25
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)
06a
© M.S. Ramaiah School of Advanced Studies
26
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
© M.S. Ramaiah School of Advanced Studies
27
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
06a
•
Long narrow ducts,
many bends: more
resistance
•
Large ducts, few bends:
less resistance
© M.S. Ramaiah School of Advanced Studies
28
System Resistance Curve
06a
© M.S. Ramaiah School of Advanced Studies
PEMP
RMD 2501
29
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
06a
© M.S. Ramaiah School of Advanced Studies
30
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
06a
PEMP
RMD 2501
© M.S. Ramaiah School of Advanced Studies
31
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
06a
© M.S. Ramaiah School of Advanced Studies
32
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
06a
© M.S. Ramaiah School of Advanced Studies
33
Airflow versus Blade Width
PEMP
RMD 2501
Airflow versus blade width for a centrifugal fan with airfoil blades
06a
© M.S. Ramaiah School of Advanced Studies
34
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
© M.S. Ramaiah School of Advanced Studies
35
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
© M.S. Ramaiah School of Advanced Studies
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
© M.S. Ramaiah School of Advanced Studies
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
© M.S. Ramaiah School of Advanced Studies
Lets look at this
in detail in
subsequent
slides
38
Control of Fan/Blower Airflow
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
©©
M.M.S.
S. Ramaiah
School
of Advanced
Studies, Studies
Bangalore
Ramaiah
School
of Advanced
39
39
Control of Fan/Blower Airflow
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)
•
06a
Simple design
Feasible in some applications only
Operate fans in series
Lower average duct pressure
Less noise
Lower structural / electrical support required
Not suited for low resistance systems
© M.S. Ramaiah School of Advanced Studies
Fans in series
40
Control of Fan/Blower Airflow
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
06a
© M.S. Ramaiah School of Advanced Studies
41
Solidity – Axial Flow Fans
06a
© M.S. Ramaiah School of Advanced Studies
PEMP
RMD 2501
42
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 diffusers
On-load or off load blade pitch adjustment is possible
© M.S. Ramaiah School of Advanced Studies
43
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 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.
blade Depending upon the twist of the blade this could be as
much as 30 resulting with the fan performance less than expectations
06a
© M.S. Ramaiah School of Advanced Studies
44
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
06a
© M.S. Ramaiah School of Advanced Studies
45
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
06a
PEMP
RMD 2501
Single fan – Damper Control
© M.S. Ramaiah School of Advanced Studies
46
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
06a
PEMP
RMD 2501
Two fans in series
© M.S. Ramaiah School of Advanced Studies
47
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
06a
PEMP
RMD 2501
Two fans in parallel
© M.S. Ramaiah School of Advanced Studies
48
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
© M.S. Ramaiah School of Advanced Studies
49
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
© M.S. Ramaiah School of Advanced Studies
50
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.
06a
© M.S. Ramaiah School of Advanced Studies
51
Session Summary
06a
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.
© M.S. Ramaiah School of Advanced Studies
52
PEMP
RMD 2501
Thank you
06a
© M.S. Ramaiah School of Advanced Studies
53