HVAC Duct Systems and Airflow
HVAC Duct Systems and Airflow
This domain tests basic airside design: airflow, velocity, pressure, duct sizing, and leakage. You need to use Q = V x A with consistent units.
Core rules
- Airflow equation: Q (cfm) = V (fpm) x A (ft2). Convert inches to feet before calculating area.
- Pressure: static pressure pushes outward on duct walls; velocity pressure is due to motion; total pressure is the sum. Fans produce total pressure; fittings and friction consume it.
- Sizing methods: equal friction, velocity reduction, and static regain. Equal friction is common for simple systems.
- Leakage: SMACNA pressure classes and seal classes define construction and sealing. Higher pressure needs stronger joints and better seals.
- Fan laws: cfm varies with rpm, pressure with rpm squared, and horsepower with rpm cubed.
Worked example
A 24 x 12 in. duct carries 1,200 cfm. Area = (24/12) x (12/12) = 2.0 ft2. Velocity = 1,200 / 2.0 = 600 fpm. If the design limit is 700 fpm, the duct is acceptable. The reasoning: convert dimensions to feet first; using 24 x 12 = 288 as area would give a meaningless velocity.
Traps
Using inches instead of feet; confusing radius and diameter; adding static and velocity pressure incorrectly; ignoring fitting losses; and assuming a bigger duct always lowers fan energy without checking the system curve.