When sizing an exhaust fan, what describes the relationship between CFM, duct size, and static pressure?

When sizing an exhaust fan, what describes the relationship between CFM, duct size, and static pressure?

• A. The CFM should be fixed regardless of duct size.
• B. Static pressure has no effect on CFM.
• C. The fan must deliver the required CFM against the static pressure given the duct size; larger ducts reduce velocity and pressure, while smaller ducts increase them.
• D. Smaller ducts reduce both velocity and pressure.

Answer: (C)

The main idea is that the fan’s job is to move a required amount of air (CFM) while overcoming the resistance the duct system creates, which is the static pressure. The duct size sets how hard the air has to work to move through it. With larger ducts, air moves more slowly for the same CFM, and friction losses drop, so the static pressure the system presents is lower. With smaller ducts, air speeds up, friction losses rise, and the static pressure increases. So the fan must be able to deliver the required CFM against whatever static pressure the duct size causes. That’s why delivering the needed CFM against the duct’s static pressure matters, and why larger ducts lessen the pressure burden while smaller ducts raise it.

The main idea is that the fan’s job is to move a required amount of air (CFM) while overcoming the resistance the duct system creates, which is the static pressure. The duct size sets how hard the air has to work to move through it. With larger ducts, air moves more slowly for the same CFM, and friction losses drop, so the static pressure the system presents is lower. With smaller ducts, air speeds up, friction losses rise, and the static pressure increases. So the fan must be able to deliver the required CFM against whatever static pressure the duct size causes. That’s why delivering the needed CFM against the duct’s static pressure matters, and why larger ducts lessen the pressure burden while smaller ducts raise it.