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Objectives

Objectives. Airflow in Ducts Fluid dynamic review Duct Design. Static pressure distribution. Fan. P+. P atm. P atm. P-. Static pressure distribution in a residential HVAC system. Supply 2. Supply 1. For non-uniform duct diameter - apply Bernoulli equation. Dynamic pressure. 0. 0.

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Objectives

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  1. Objectives • Airflow in Ducts • Fluid dynamic review • Duct Design

  2. Static pressure distribution Fan P+ Patm Patm P-

  3. Static pressure distribution in a residential HVAC system Supply 2 Supply 1

  4. For non-uniform duct diameter - apply Bernoulli equation Dynamic pressure 0 0 Fan We need fan for this Dynamic pressure ∆P total ∆P static Patm

  5. Relationship Between Static and Total Pressure

  6. Use of Static to Dynamic Pressure Conversion (and reverse)

  7. Duct design • Two objectives: • Define the size of duct sections • Define the pressure drop • Process: • Position and size diffusers • Layout desired pattern for supply • and return ducts • Calculate the flow rates for each duct • section • Select the duct diameter for each section • Measure the length for each section • Calculate the pressure loss for each section • Count the number, and define the type, of fittings and elbows • Calculate the pressure drop in fittings and • elbows • - Calculate the total pressure drop

  8. Local losses • Losses associated with • Changes in velocity • Obstructions • Bends • Fittings and transitions

  9. Fittings and elbows

  10. Pressure drop in duct v D L MOODY'S DIAGRAM Relative roughness e/D For unit length L=1

  11. Frictional Losses Shaded area

  12. Ductulator Same as the table in previous slide

  13. Non-circular Ducts • Also you can use hydraulic diameter

  14. Local Loss Coefficients ΔP local = CoPdynmaic

  15. Example Size the ducts and calculate pressure drop from - A to C - B to C A a c C b B Tabulated calculation: ……..

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

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