Believe it or not, there is friction involved with airflow through ducting. Here is the process for calculating the impact of this friction.
Friction loss in ducts is the pressure drop resulting from the air contacting the inside surface of the duct. For the purpose of duct design, friction loss is typically expressed in units of inches of water column per 100 feet of duct.
The friction rate for a duct system is the friction loss along the total effective length that results in a pressure drop equal to available static pressure. The friction rate is calculated by the following formula:
friction rate =
Problem:
Static pressure = 0.18 in
effictive length = 285 feet
friction rate = (static pressure / effective length) x 100
= (0.18 / 285) x 100
= 0.0632 in water column per 100 feet of duct