Air Flow Measurement and Balancing Procedures
Core concepts
Know the balancing sequence: verify design and installation, calibrate instruments, establish total system airflow at the fan, then proportion branch and terminal flows, then set the outside-air/return/exhaust balance, and finally document. Traverse ducts using the equal-area (log-Tchebycheff or equal-area) method; take readings at multiple points and average velocity pressures before converting to velocity-never average velocities from individual points when the profile is non-uniform.
Understand fan laws: flow ∝ RPM, pressure ∝ RPM², power ∝ RPM³ (for fixed system). Use them to predict the effect of speed changes and to diagnose system-effect problems. Know the difference between proportional and proportional-balance methods and the use of balancing dampers, volume dampers, and pressure-independent terminal boxes.
Traps
- Averaging velocities instead of averaging velocity pressures before taking the square root.
- Traversing too close to elbows, transitions, or dampers (need adequate straight duct, typically several diameters upstream and downstream).
- Ignoring density corrections: readings taken at non-standard temperature/altitude must be corrected to standard or to design.
- Confusing total, static, and velocity pressure; velocity pressure is always positive in the direction of flow.
- Forgetting that a 10% RPM increase raises power ~33% (1.1³).
- Not allowing the system to stabilize before reading.
Worked example
A rectangular duct traverse yields average VP = 0.16 in. w.c. Velocity V = 4005 × √VP = 4005 × √0.16 = 4005 × 0.4 = 1,602 fpm. For a 24 in. × 12 in. duct (2 ft²), flow = 1,602 × 2 = 3,204 CFM. If density is below standard, correct: actual CFM = standard CFM × (standard density ÷ actual density).
Rule of thumb
Always average velocity pressures, then convert; confirm straight-duct length before trusting a traverse.