Fans Part 1

1
Fans Part 1

• Reading
• Chapter 5 in Henderson/Perry
• Greenheck Product Application Guide
• Dwyer Manometer Guide
• AMCA Fan Curve Guide

Dr. C. L. Jones Biosystems and Ag. Engineering
2
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.

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3
Pumps, Fans, CompressorsTurbomachines (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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Fan Standards

• ASHRAE American Society of Heating,
  Refrigeration, and Air conditioning
  Engineeringresearch and standards
• AMCA Air Movement and Control
  Associationstandards

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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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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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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

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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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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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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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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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13
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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Backward-Curved-Tip Fans

• Approx. 12 blades
• Flat and tilted backwards
• High speed fan with self-limiting power
• If sized correctly, motor wont overheat if
  conditions change (curve pg. 132)
• Most efficient
• Cannot handle dirty air

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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

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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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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

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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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19
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 changesfrom 600 to 650 rpm fora fan
  of a given size

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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

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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.

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More Fans

• Fans in parallel moves curve to the
  rightpressure stays the same, flow rate is the
  sum
• Fans in series moves curve upflow rate stays
  the same, pressure is the sum

Dr. C. L. Jones Biosystems and Ag. Engineering