By accurately sizing fans, you prevent over-pressurizing the system, which wastes energy.
The speed of the air (FPM or m/s), which directly affects friction.
I can help walk you through the calculation or direct you to a specific tool. Duct Pressure Loss Calculator | Plastec Ventilation
They calculate the pressure loss in inches of water gauge ("wg) or Pascals (Pa) for various round and rectangular fittings.
The diameter (round) or height and width (rectangular) of the duct.
you are calculating (e.g., radius elbow, mitered bend, reducer)? What is your airflow rate (CFM) and duct size ?
Helps ensure that the total pressure does not exceed the capacity of the air handling unit (AHU).
Common tools for this include digital apps from industry leaders like Price Industries or online calculators. If you can tell me:
