313D Airflow and Duct Sizing 2 — Questions and Answers
Question 1: What is the recommended maximum air velocity in a main supply trunk duct for a low-noise residential HVAC system?
- 600 FPM
- 900 FPM (Correct answer)
- 1200 FPM
- 1500 FPM
Correct answer: 900 FPM
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?
- 45-degree elbow
- 90-degree square elbow without turning vanes (Correct answer)
- Straight tee
- Reducing collar
Correct 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.
Question 3: When using the equal friction method for duct design, what parameter is held constant throughout the system?
- Air velocity
- Static pressure drop per 100 feet (Correct answer)
- Total airflow CFM
- Duct cross-sectional area
Correct 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.
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?
- 0.13 in. w.g.
- 0.18 in. w.g. (Correct answer)
- 0.22 in. w.g.
- 0.30 in. w.g.
Correct 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.
Question 5: What is the purpose of a static pressure regain section in a duct system?
- To increase air velocity before a branch
- To recover pressure after a velocity reduction (Correct answer)
- To filter particles from the airstream
- To add humidity to supply air
Correct answer: To recover pressure after a velocity reduction
Pressure regain occurs when velocity decreases and kinetic energy converts back to static pressure.
Question 6: For a residential system serving multiple zones, which device helps balance airflow between branches without mechanical dampers?
- Plenum box
- Splitter damper
- Balancing collar (Correct answer)
- Duct board liner
Correct answer: Balancing collar
A balancing collar allows adjustable restriction at branch takeoffs to equalize airflow across multiple runs.
Question 7: When calculating duct heat gain for a cooling system, which factor most significantly increases heat gain into supply ducts?
- Duct color
- Duct location in unconditioned attic space (Correct answer)
- Duct material thickness
- Supply air temperature
Correct answer: Duct location in unconditioned attic space
Ducts routed through hot unconditioned attics are exposed to extreme temperature differentials, dramatically increasing heat gain.
313D Airflow and Duct Sizing 2
What is the recommended maximum air velocity in a main supply trunk duct for a low-noise residential HVAC system?