bae20231 dr. c. l. jones biosystems and ag. engineering fans part 1 reading: –chapter 5 in...

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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

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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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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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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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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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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 won’t 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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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:

2

2t s

vp p

Qv

A

tt

pQe

P s

s

p Qe

P

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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.

0

10logW

LW

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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

bae20231 dr. c. l. jones biosystems and ag. engineering fans part 1 reading: –chapter 5 in...

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