Airflow and Duct Sizing Flashcards

What is the recommended maximum air velocity in a main supply trunk duct for a low-noise residential HVAC system?

Answer: 900 FPM

Main trunk ducts in residential systems are typically designed for 900 FPM to balance noise and efficiency.

Which duct fitting generates the highest equivalent length of straight duct pressure loss?

Answer: 90-degree square elbow without turning vanes

A 90-degree square elbow without turning vanes creates significant turbulence and has the highest equivalent length among common fittings.

When using the equal friction method for duct design, what parameter is held constant throughout the system?

Answer: Static pressure drop per 100 feet

The equal friction method maintains a constant pressure drop per 100 feet of duct length across all branches.

A supply register is rated at 0.10 inches w.g. pressure drop at 150 CFM. If airflow increases to 200 CFM, approximately what is the new pressure drop?

Answer: 0.18 in. w.g.

Pressure drop varies with the square of airflow change; (200/150)² × 0.10 ≈ 0.18 in. w.g.

What is the purpose of a static pressure regain section in a duct system?

Answer: To recover pressure after a velocity reduction

Pressure regain occurs when velocity decreases and kinetic energy converts back to static pressure.

For a residential system serving multiple zones, which device helps balance airflow between branches without mechanical dampers?

Answer: Balancing collar

A balancing collar allows adjustable restriction at branch takeoffs to equalize airflow across multiple runs.

When calculating duct heat gain for a cooling system, which factor most significantly increases heat gain into supply ducts?

Answer: Duct location in unconditioned attic space

Ducts routed through hot unconditioned attics are exposed to extreme temperature differentials, dramatically increasing heat gain.