The design of the air supply duct will depend on the air velocity and

• A. pressure drop
• B. cfm
• C. length of run
• D. all of the above

Answer: (D)

Air duct design centers on getting the required amount of air to move through a space while keeping the air moving at a sensible speed. The velocity you aim for is driven by the desired CFM and the duct size you choose—more flow in the same duct raises velocity, while a larger duct lowers it. Pressure drop is the loss of pressure as air travels through the ductwork, and it grows with higher velocity, longer runs, and many fittings; it matters because the fan must overcome this drop to deliver the needed CFM. The amount of air you must move (CFM) sets the overall flow requirement, which in turn constrains the duct diameter and the velocity range you’ll tolerate. The length of the run increases friction losses; longer routes mean more pressure loss and potentially a need for a larger duct or a more powerful fan to maintain the target velocity and flow. Since velocity, pressure drop, CFM, and length of run all influence duct sizing and system performance, all of these factors come into play.