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Air Distribution Systems and Diffuser Selection in Duct Design

Learn about diffuser selection, duct design, and air distribution systems. Explore videos, computational fluid dynamics, and examples of airflow. Understand the factors affecting air contamination and buoyancy-driven flow. Discover various types of diffusers and their characteristics. Study the calculation of volumetric flow rate, design load, air density, and temperature difference. Follow a step-by-step procedure for selecting and locating diffusers. Evaluate air distribution quality with the air distribution performance index. Select the appropriate diffuser based on the manufacturer's data.

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Air Distribution Systems and Diffuser Selection in Duct Design

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  1. Objectives - Air Distribution Systems • Diffuser selection • Duct design Reading Assignment: Chapter 18

  2. Near Human Body flow • Diffuser flow • https://www.youtube.com/watch?v=LFgJn75LhGc • https://www.youtube.com/watch?v=5e3gixnmNzM • https://www.youtube.com/watch?v=550fA39PpbQ

  3. Computational Fluid Dynamics

  4. CFD in Air Distribution Design Contaminant concentration in a kitchen

  5. Buoyancy driven flow:Example of airflow in a stairway Heater (radiator)

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

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

  8. Low mixing Diffusers Displacement ventilation

  9. 18.7

  10. 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

  11. Find Characteristic Length (L)

  12. 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)

  13. Ideal and Reasonable Throws

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

  15. 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

  16. Example 18.3 • Qtot = 38.4 kBTU/hr • Δh = 9.5 BTU/lbma

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

  18. Relationship Between Static and Total Pressure

  19. 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

  20. Frictional Losses

  21. System Characteristic

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