Objectives Airflow in Ducts Fluid dynamic review Duct Design

Static pressure distribution Fan P+ Patm Patm P-

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

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

Relationship Between Static and Total Pressure

Use of Static to Dynamic Pressure Conversion (and reverse)

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

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

Fittings and elbows

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

Frictional Losses Shaded area

Ductulator Same as the table in previous slide

Non-circular Ducts Also you can use hydraulic diameter

Local Loss Coefficients ΔP local = CoPdynmaic

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

Reading asignement Chapter 18 18.1-18.4 (including 18.4)