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Fans Part 1

Fans Part 1

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Fans Part 1

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  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 BAE2023

  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……. Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  3. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  4. Fan Standards • ASHRAE: American Society of Heating, Refrigeration, and Air conditioning Engineering…research and standards • AMCA: Air Movement and Control Association…standards Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  5. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  6. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  7. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  8. Axial-flow Fans • Propeller • Tube-Axial • Vane-Axial Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  9. 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) Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  10. Tube-Axial Fans • Wheel or impeller within a tube • Blades are warped for efficiency • Operates at higher pressures and higher ME than propeller fans Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  11. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  12. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  14. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  15. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  16. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  17. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  18. Performance • Tested at shutoff and free delivery conditions • Shutoff: discharge is blanked off • Free delivery: outlet resistance is 0 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  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) Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  20. Fan Curves • Fans follow the affinity laws (ASHRAE) Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  21. Fan Law Example • Fan speed N changesfrom 600 to 650 rpm fora fan of a given size Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  22. 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: Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  23. 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. Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023

  24. 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 Dr. C. L. Jones Biosystems and Ag. Engineering BAE2023