1. Fans, Blowers and Compressors
P M V Subbarao
Professor
Mechanical Engineering Department
I I T Delhi
Machines to Generate Lively Gases….
2. First Law Analysis & ASME Classification
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SSSF :
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Hardware
5. Centrifugal Fan Operation
• Fans cause a pressure increase through two methods
• Centrifugal force is created by the rotation of the column of air
trapped between two blades.
• Kinetic energy is supplied to the air through the impeller
• Total pressure = static pressure + velocity head
• Blades are backward-curved, forward curved, or radial (straight)
Airfoil-types are
complex and expensive
but very efficient;
they’re used for large
systems where the cost
is justified.
6. Axial Flow Fans
• Common types: propeller, tubeaxial, vaneaxial
• Tubeaxial: impeller is inside a tube to guide airflow and
improve performance.
• Vane axial: like a tube axial except vanes downstream of the
impeller are used to reduce swirl and improve performance
• Used to deliver large flow rates but small increase in pressure
• Examples include fans used for ventilation without ductwork,
mobile room fans, and fans used to cool computers.
• Tube-axial fan for computer cooling
• Tube-axial fan for ventilation
• Vane-axial fan for high air resistance electronics cooling
• Straightening vanes are located inside tube.
9. Fan Laws
Operational and performance data of different but geometrically
similar fans can be cast onto a single universal curve using
dimensionless numbers
N
D
Q
rate
flow 3
2
2
N
D
p
pressure
3
5
N
D
P
power
Pressure
rise
Power
11. Fan Flow Control
• Fans rarely are operated continuously at the same pressure and
volume discharge rates.
• There exists a need for a system which can vary the vane
output according to requirements.
• Common methods of controlling fan output are:
• Damper control
• Inlet vane control
• Variable speed control.
• Axial fans can control flow using variable pitch blades.
19. Duty of Compressors
• A two step life infusion into gas.
• Rotor : Mostly Energy Transfer
• The centrifugal compressor
consists essentially of a
stationary casing containing a
rotating impeller which imparts
a high velocity to the air.
• Only Energy Conversion:
• A number of diverging passages
(Diffuser) in which the air is
decelerated with a consequent
rise in static pressure.
20. Centrifugal Compressors
• Air is sucked into the impeller eye and
whirled round at high speed by the
vanes on the impeller disc.
• Centripetal acceleration leads to
increase in rise of some amount of
static pressure.
• The remaining increase in rise of
pressure occurs in diffuser.
24. Performance of centrifugal compressors
• Total Pressure Ratio
1
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V
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Design clue for Blades:
29. Remarks on Centrifugal Compressors
• High pressure ratio per stage (r0p= 3 to 4).
• Less flow area per unit mass flow rate.
• A multistage centrifugal compressors have shorter lengths.
• They are best suited for low mass flow rates.
• As the flow is turned by 900, the efficiency of centrifugal
compressor is relatively low.
• The impeller is an integral unit of blades and a disk.
• Even if one blade is damaged the entire unit is to be replaced.
• Look for other fluid dynamic phenomenon for ingestion of life
into fluids.
• Aerofoil Theory.
30. Evolution of Compression Phenomenon
Centrifugal Aerofoil
Compact & Less Efficient Bulky & More Efficient
35. Axial Flow Fluid Machines
The power, P of a fluid Machine
in
w
out
w V
V
U
m
T
P ,
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36. Similarity Analysis
• Experimental performance curves or maps provide a versatile
data for compressor engineers.
• The data is translated into most general form.
• Data in general form is minimally effected by air speed,
altitude etc.
• The total pressure ratio of a compressor is:
R
p
A
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Using dimensional analysis & Buckingham Pi Theorem:
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44. Axial Flow Compressors: Current
Design Practice
Parameter Range
Pressure ratio for single
stage
1.5 – 2.0
Pressure ratio for two
stages
2.0 – 3.5
Pressure ratio for three
stages
3.5 – 4.5
Inlet mass flow rates 195 – 205 kg/m2.s
Tip speed 427 – 457 m/s
Fan or low pressure Compressor
45. Axial Flow Compressors: Current
Design Practice
Parameter Range
Stage loading
coefficient
0.3 – 0.35
Flow coefficient 0.45 – 0.55
Hub/tip ratio 0.6 – 0.75
Inlet mass flow rates 175 – 185 kg/m2.s
Tip speed 386 – 457 m/s
High pressure Compressor
