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Objectives

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  1. Objectives • Duct Design AND Diffuser Selection

  2. Relationship Between Static and Total Pressure

  3. 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

  4. System Characteristic

  5. Electrical Resistance Analogy

  6. Frictional Losses

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

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

  9. Loss Coefficients • ΔPt = CoPv,0

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

  11. Conversion Between Methods

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

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

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

  15. 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

  16. Low mixing Diffusers Displacement ventilation

  17. 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

  18. Find Characteristic Length (L)

  19. 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)

  20. Ideal and Reasonable Throws

  21. Select Register • Pick throw, volumetric flow from register catalog • Check noise, pressure drop

  22. 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

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