Transcript Lecture 14 Fans 1 - Biosystems and Agricultural Engineering

Fans Part 1
• Reading:
– Chapter 5 in Henderson/Perry
– Greenheck Product Application Guide
– Dwyer Manometer Guide
– AMCA Fan Curve Guide
BAE2023
Dr. C. L. Jones
Biosystems and Ag. Engineering
1
Pumps, Fans, Compressors-Turbomachines
• Turbomachines: change energy level of
flowing fluid by means of momentum
exchange
• Wide spread in ALL industries
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Power units: cars/trucks, tractors
Computers
Grain elevators
Oilfield
Medical facilities
Food processing
And…….
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Dr. C. L. Jones
Biosystems and Ag. Engineering
2
Pumps, Fans, Compressors—
Turbomachines (Review)
• Differences between pumps, fans,
compressors
– Pumps: move liquids
– Fans: move gases with little increase in
pressure
– Compressors: move gases with greater
increase in pressure
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Dr. C. L. Jones
Biosystems and Ag. Engineering
3
Fan Standards
• ASHRAE: American Society of Heating,
Refrigeration, and Air conditioning
Engineering…research and standards
• AMCA: Air Movement and Control
Association…standards
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Biosystems and Ag. Engineering
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Fan Operation
• Fans cause pressure increase by:
– Centrifugal force created by rotation of the
column of air trapped between two blades
– Kinetic energy is supplied to the air through
the impeller
– Total pressure = velocity head + static
pressure
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Dr. C. L. Jones
Biosystems and Ag. Engineering
5
Selection Considerations
• Quantity of air to be moved per unit time
• Estimated system resistance and
expected variations
• Amount of noise permitted
• Space available for fan
• Economic implications
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Biosystems and Ag. Engineering
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Fan Classification
• Designated as dynamic pumps
– Centrifugal
• Further classification by entry and exit of fluid/gas
through impeller
• 4 aerodynamic classifications:
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Axial flow
Radial flow : referred to as “centrifugal fan”
Cross flow
Mixed flow
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Biosystems and Ag. Engineering
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Axial-flow Fans
• Propeller
• Tube-Axial
• Vane-Axial
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Propeller Fans
• Used in many agricultural ventilation
applications with free delivery or low
pressure conditions
• Propeller has a pitch (twist). Distance of air
travel depends on pitch and # of blades
• Warping the blades prevents back flow of
air (page 121)
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Biosystems and Ag. Engineering
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Tube-Axial Fans
• Wheel or impeller within a tube
• Blades are warped for efficiency
• Operates at higher pressures and higher
ME than propeller fans
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Biosystems and Ag. Engineering
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Vane-Axial Fans
• Like a tube axial with guide vanes before or
after the impeller
• Higher pressures and efficiencies ( up to
2200 psi and 85% +) because…
• Vanes direct air flow through a gradual turn
until tangential velocity component is
eliminated
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Biosystems and Ag. Engineering
11
Centrifugal Fans
• Consists of wheel or rotor within a spiral
housing
• Air makes a 90 degree turn
• Forward-curved-tip
• Backward-curved-tip
• Straight or radial tip
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Straight or Radial-Tip Fans
• 6 – 20 blades
• Blades are 2 to 3 x as long as they are
wide
• Larger housing, more expensive
• Can handle dirty air and higher pressures
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Biosystems and Ag. Engineering
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Backward-Curved-Tip Fans
• Approx. 12 blades
• Flat and tilted backwards
• High speed fan with self-limiting power
– If sized correctly, motor won’t overheat if
conditions change (curve pg. 132)
• Most efficient
• Cannot handle dirty air
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Biosystems and Ag. Engineering
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Forward-Curved-Tip Fans
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•
•
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Up to 60 blades
Narrow radially but wide parallel to shaft
Face forward in direction of rotation, scoop
Low-speed, moderate pressures, can be
unstable
• Clean air only
• Lowest noise!!!!!
• Used in many furnace blowers
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Biosystems and Ag. Engineering
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Cross-Flow
(or Tangential) Fans
• Uniform discharge, high air rate
• Use in ovens, baseboard heaters, unit
heaters, drying equipment, kitchen hoods
• Larger housing, more expensive
• Can handle dirty air and higher pressures
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16
Mixed-Flow Fans
• Aka In-line or Tubular Centrifugal fans
• Combines features of centrifugal and vaneaxial fans
• Straight-through air flow but performance
characteristics of centrifugal
• Quiet like centrifugal
• Used as exhaust fans in homes,
commercial and agricultural
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Performance
• Tested at shutoff and free delivery
conditions
• Shutoff: discharge is blanked off
• Free delivery: outlet resistance is 0
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Fan Curves
• Manufacturer provides fan curve
• Predicts the pressure-flow rate performance
of each fan
• Choose fan according to volumetric flow
rate for your system pressure drop
• Choose fan with peak efficiency at or near
your operating point
• Sometimes manufacturers use tables
instead (Greenheck Handout)
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Fan Curves
• Fans follow the affinity laws (ASHRAE)
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Fan Law Example
• Fan speed N changes
from 600 to 650 rpm for
a fan of a given size
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Fan Curves
• If we know static pressure and outlet area, we
can find total pressure:
v 2
pt  ps 
2
v
Q
A
• Total pressure includes static and velocity
pressure
• If we know power, static efficiency, or total
efficiency, the others can be calculated:
et 
ptQ
P
es 
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psQ
P
Dr. C. L. Jones
Biosystems and Ag. Engineering
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Sound Power Levels
• Energy transfer
Sound
• In dB:
W 
L  10log 

 W0 
• W0 = 1 x 10-12 Watts
• W = power (W)
• For additional units, see eqtn. 5.6 (H/P)
• Sones: One sone represents the
loudness of a 1 kHz tone at 40 dB SPL.
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More
Fans…
• Fans in parallel: moves curve to the right…
pressure stays the same, flow rate is the
sum
• Fans in series: moves curve up…
flow rate stays the same, pressure is the
sum
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Biosystems and Ag. Engineering
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