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Comprehensive Guide to Duct and Diffuser Design in Air Distribution Systems

This detailed guide covers the essential principles of duct design, diffuser selection, and air distribution system efficiency. It reviews frictional and dynamic losses associated with various fittings, bends, and transitions, emphasizing the importance of effective air distribution in contributing to thermal comfort. You'll learn about selecting the right diffusers (including linear, swirl, and ceiling diffusers) based on design loads and airflow characteristics. The guide also discusses fan performance, including centrifugal and axial fans, along with critical parameters for optimizing system performance.

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Comprehensive Guide to Duct and Diffuser Design in Air Distribution Systems

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  1. Objectives • Finish with Duct Design • Review the design procedure and explain the theoretical background • Diffuser Selection

  2. Frictional Losses

  3. Dynamic losses • Losses associated with • Changes in velocity • Obstructions • Bends • Fittings and transitions

  4. Air Distribution System Design • Describe room distribution basics • Select diffusers • Supply and return duct sizing

  5. Forced driven air flowDiffusers • Grill (side wall) • diffusers • Linear diffusers • Vertical • Horizontal one side

  6. Diffusers types • Valve diffuser • swirl diffusers • ceiling diffuser • wall or ceiling • floor

  7. Diffusers • Perforated ceiling diffuser • Jet nozzle diffuser • Round conical ceiling diffuser • Square conical ceiling diffuser • Wall diffuser unit • Swirl diffuser • Floor diffuser • Auditorium diffuser • Linear slot diffuser • DV diffuser • External louvre • Smoke damper • http://www.titus-hvac.com/techzone/ • http://www.halton.com/halton/cms.nsf/www/diffusers

  8. Low mixing Diffusers Displacement ventilation

  9. V = maximum volumetric flow rate (m3/s, ft3/min) Qtot = total design load (W, BTU/hr) Qsen = sensible design load (W,BTU/hr) ρ = air density (kg/m3, lbm/ft3) Δt = temperature difference between supply and return air (°C, °F) Δh = enthalpy difference between supply and return air (J/kg, BTU/lbm) Diffuser Selection Procedure • Select and locate diffusers, divide airflow amongst diffusers

  10. Find Characteristic Length (L)

  11. Indicator of Air DistributionQuality • ADPI = air distribution performance index • Fraction of locations that meet criteria: • -3 °F < EDT < 2 °F or -1.5 °C < EDT < 1 °C • Where, EDT = effective draft temperature • Function of V and Δt (Eqn 18.1) • EDT=(tlocal-taverage)-M(Vlocal-Vaverage) , M=7 °C/(m/s) ADPI considers ONLY thermal comfort (not IAQ)

  12. Ideal and Reasonable Throws

  13. Diffuser testing

  14. ADPI

  15. Select Register • Pick throw, volumetric flow from register catalog • Check noise, pressure drop http://www.titus-hvac.com/ecatalog/subcategory.aspx?refid=186 http://www.nailor.com/ http://www.price-hvac.com/

  16. Summary of Diffuser Design Procedure • Find Q sensible total for the space • Select type and number of diffusers • Find V for each diffuser • Find characteristic length • Select the diffuser from the manufacturer data

  17. Fans • Driving force to move air in buildings • Raise pressure and produce flow • Two main types: • Centrifugal • Axial

  18. Relevant Fan Parameters • Total pressure rise • Static pressure rise • Power requirement • Efficiency • Note ηtot does not account for motor efficiency

  19. Balance Point

  20. Fan Performance

  21. Centrifugal fans

  22. Axial Fans

  23. Fan Laws • “Approximate relationships” • Given two aerodynamically similar fans, can extrapolate performance data • Be very careful

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