BAE2023 1
Dr. C. L. JonesBiosystems and Ag. Engineering
Fans Part 1
• Reading: – Chapter 5 in Henderson/Perry– Greenheck Product Application Guide– Dwyer Manometer Guide– AMCA Fan Curve Guide
BAE2023 2
Dr. C. L. JonesBiosystems and Ag. Engineering
Pumps, Fans, Compressors--Turbomachines
• Turbomachines: change energy level of flowing fluid by means of momentum exchange
• Wide spread in ALL industries– Power units: cars/trucks, tractors– Computers– Grain elevators– Oilfield– Medical facilities– Food processing– And…….
BAE2023 3
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 4
Dr. C. L. JonesBiosystems and Ag. Engineering
Fan Standards
• ASHRAE: American Society of Heating, Refrigeration, and Air conditioning Engineering…research and standards
• AMCA: Air Movement and Control Association…standards
BAE2023 5
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 6
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 7
Dr. C. L. JonesBiosystems and Ag. Engineering
Fan Classification
• Designated as dynamic pumps– Centrifugal
• Further classification by entry and exit of fluid/gas through impeller
• 4 aerodynamic classifications:– Axial flow – Radial flow : referred to as “centrifugal fan”– Cross flow– Mixed flow
BAE2023 8
Dr. C. L. JonesBiosystems and Ag. Engineering
Axial-flow Fans
• Propeller
• Tube-Axial
• Vane-Axial
BAE2023 9
Dr. C. L. JonesBiosystems and Ag. Engineering
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)
BAE2023 10
Dr. C. L. JonesBiosystems and Ag. Engineering
Tube-Axial Fans
• Wheel or impeller within a tube• Blades are warped for efficiency• Operates at higher pressures and higher
ME than propeller fans
BAE2023 11
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 12
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 13
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 14
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 15
Dr. C. L. JonesBiosystems and Ag. Engineering
Forward-Curved-Tip Fans• 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
BAE2023 16
Dr. C. L. JonesBiosystems and Ag. Engineering
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
BAE2023 17
Dr. C. L. JonesBiosystems and Ag. Engineering
Mixed-Flow Fans
• Aka In-line or Tubular Centrifugal fans• Combines features of centrifugal and vane-
axial fans• Straight-through air flow but performance
characteristics of centrifugal• Quiet like centrifugal• Used as exhaust fans in homes,
commercial and agricultural
BAE2023 18
Dr. C. L. JonesBiosystems and Ag. Engineering
Performance
• Tested at shutoff and free delivery conditions
• Shutoff: discharge is blanked off• Free delivery: outlet resistance is 0
BAE2023 19
Dr. C. L. JonesBiosystems and Ag. Engineering
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)
BAE2023 20
Dr. C. L. JonesBiosystems and Ag. Engineering
Fan Curves
• Fans follow the affinity laws (ASHRAE)
BAE2023 21
Dr. C. L. JonesBiosystems and Ag. Engineering
Fan Law Example
• Fan speed N changesfrom 600 to 650 rpm fora fan of a given size
BAE2023 22
Dr. C. L. JonesBiosystems and Ag. Engineering
Fan Curves
• If we know static pressure and outlet area, we can find total pressure:
• Total pressure includes static and velocity pressure
• If we know power, static efficiency, or total efficiency, the others can be calculated:
2
2t s
vp p
Qv
A
tt
pQe
P s
s
p Qe
P
BAE2023 23
Dr. C. L. JonesBiosystems and Ag. Engineering
Sound Power Levels• Energy transfer Sound• In dB:
• 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.
0
10logW
LW