313D Airflow and Duct Sizing 2

Question 1

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

Accepted Answer

900 FPM

Answer

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

Question 2

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

Accepted Answer

90-degree square elbow without turning vanes

Answer

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

Question 3

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

Accepted Answer

Static pressure drop per 100 feet

Answer

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

Question 4

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?

Accepted Answer

0.18 in. w.g.

Answer

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

Question 5

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

Accepted Answer

To recover pressure after a velocity reduction

Answer

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

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

313D Airflow and Duct Sizing 2