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Objectives

Objectives. Air Distribution Systems Diffuser selection Duct design . Air Distribution System Design. Describe room distribution basics Select diffusers Supply and return duct sizing. Forced driven air flow Diffusers. Grill (side wall) diffusers. Linear diffusers. Vertical.

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Objectives

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  1. Objectives Air Distribution Systems • Diffuser selection • Duct design

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

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

  4. Diffusers types Valve diffuser swirl diffusers ceiling diffuser wall or ceiling floor

  5. Coanda effect on ceilingand walls Coanda effect https://www.youtube.com/watch?v=AvLwqRCbGKY

  6. 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

  7. Low mixing Diffusers Displacement ventilation

  8. 18.7

  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. Select Register • Pick throw, volumetric flow from register catalog • Check noise, pressure drop http://www.titus-hvac.com/utility/getfile2.aspx?fileid=8967

  14. 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

  15. Example 18.3 • Qtot = 38.4 kBTU/hr • Δh = 9.5 BTU/lbma omission in text

  16. Pressures • Static pressure • Velocity pressure • Total pressure – sum of the two above

  17. Relationship Between Static and Total Pressure

  18. Total and static pressure drops are proportional to square of velocity Plot of pressure drop vs. volumetric flow rate (or velocity) is called system characteristic Duct Design

  19. System Characteristic

  20. Electrical Resistance Analogy

  21. Frictional Losses

  22. Non-circular Ducts • Parallel concept to wetted perimeter

  23. Dynamic losses • Losses associated with • Changes in velocity • Obstructions • Bends • Fittings and transitions • Two methods • Equivalent length and loss coefficients

  24. Loss Coefficients ΔPt = CoPv,0

  25. Example 18.7 • Determine total pressure drop from 0 to 4

  26. Conversion Between Methods

  27. Reading asignement • Chapter 18 • 18.1-18.4 (including 18.4)

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