What describes the average pressure drop per hundred feet of duct?

The concept of pressure drop per hundred feet of duct is primarily related to the friction rate in the duct system. Friction rate refers to the loss of pressure that occurs as air moves through the ductwork due to the internal surface roughness and the shape of the duct. This loss of pressure is important for HVAC systems, as it affects the overall efficiency and performance of air distribution throughout a building.

Understanding the friction rate is crucial for designing duct systems because it allows HVAC professionals to estimate how much pressure will be lost over a certain distance, ultimately impacting airflow and system effectiveness. A higher friction rate indicates more resistance within the duct, which can lead to inadequate airflow if not properly accounted for.

Other options like flow rate, velocity factor, and static pressure relate to different aspects of HVAC systems—flow rate measures the volume of air moving through the system, velocity factor relates airflow speed, and static pressure measures the pressure within the system at a given point, but none specifically describes the average pressure drop due to friction in the ductwork as directly as friction rate does. Thus, identifying friction rate as the average pressure drop per hundred feet aligns correctly with industry practices and standards.