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
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 D v 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

16. 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:
……..

17. Reading asignement
Chapter 18
(including 18.4)