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Air Pressure Drop (APD) - airflow pressure drop in HVAC ductwork. Air Pressure Drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream, this also applies to Indoor Air Quality (IAQ) when we talk about air filtration. When air is drawn through an HVAC system with a filter installed, that filter will create some resistance to the airflow. Air resistance in ventilation system is usually determined by its air speed, as a result air resistance grows proportional to air flow. This phenomenon is known as pressure loss. Pressure drop indicates too much resistance to the airflow and that an HVAC system is struggling to process the air, this can lead to lower energy efficiency of a system.


How does pressure drop affect flow rate?

  • Under laminar flow conditions, pressure drop is proportional to volumetric flow rate
  • Under turbulent flow conditions, pressure drop increases as the square of the volumetric flow rate
  • Pressure drop decreases as common mode pressure increases
  • Pressure drop increases as gas viscosity increases
  • Pressure drop increases as gas temperature increases

What causes a pressure drop?

Dirty filters are still the most common cause of excessive pressure drop. This is the main reason why it is so important to change air filters on a regular basis.

Any type of obstruction, restriction, or roughness in ventilation system will cause resistance to air flow and cause pressure drop.
The maximum pressure drop from the supply side to the points-of-use will occur when the compressed air flow rate and temperature are highest.