HVAC Sheet Metal and Fabrication 4 — Questions and Answers
Question 1: What does the term 'duct aspect ratio' refer to, and why does it matter?
- The ratio of duct width to height; high ratios increase friction loss (Correct answer)
- The ratio of duct length to diameter; affects insulation thickness
- The ratio of supply to return air volume; affects system balance
- The ratio of velocity pressure to static pressure
Correct answer: The ratio of duct width to height; high ratios increase friction loss
Aspect ratio is the width-to-height ratio of rectangular duct; ratios above 4:1 significantly increase friction losses and material use.
Question 2: When fabricating a conical reducer in round ductwork, what maximum taper angle minimizes turbulence?
- 15 degrees or less per side (Correct answer)
- 45 degrees per side
- 30 degrees per side
- 60 degrees per side
Correct answer: 15 degrees or less per side
Keeping the taper angle to 15 degrees or less per side (30 degrees total included angle) minimizes flow separation and turbulence in a conical reducer.
Question 3: What is a 'snap lock' seam used for in HVAC sheet metal fabrication?
- Joining round duct spiral seam longitudinally (Correct answer)
- Connecting two rectangular ducts end-to-end
- Attaching a duct liner to the duct interior
- Sealing a duct at a penetration point
Correct answer: Joining round duct spiral seam longitudinally
The snap lock seam is the longitudinal seam used on spiral wound round duct, formed continuously by interlocking the strip edges as it is wound.
Question 4: Which material is NOT typically acceptable for use as HVAC duct in a commercial building per IMC/SMACNA?
- PVC plastic (Correct answer)
- Galvanized steel
- Aluminum
- Stainless steel
Correct answer: PVC plastic
PVC plastic duct is generally not acceptable for supply and return HVAC systems in commercial buildings due to flammability concerns, though it may be allowed for specific exhaust applications.
Question 5: A duct system tested at 1.0 inch WC positive pressure is classified under SMACNA as what pressure class?
- 1-inch WC (low pressure) (Correct answer)
- 2-inch WC (medium pressure)
- 3-inch WC (high pressure)
- 0.5-inch WC (very low pressure)
Correct answer: 1-inch WC (low pressure)
SMACNA classifies duct systems tested at 1.0 inch WC as the 1-inch pressure class, requiring specific gauge and reinforcement standards.
Question 6: When cutting galvanized sheet metal with power shears, what safety practice is most critical?
- Wearing cut-resistant gloves and eye protection to guard against sharp edges and metal fragments (Correct answer)
- Applying oil to the blades before every cut
- Using only right-handed shears for safety
- Clamping the sheet to a wooden workbench before cutting
Correct answer: Wearing cut-resistant gloves and eye protection to guard against sharp edges and metal fragments
Cut-resistant gloves and eye protection are essential when using power shears because edges are razor-sharp and metal chips can become airborne projectiles.
Question 7: What is the purpose of a turning vane inside a rectangular duct elbow?
- Reduce pressure drop by guiding airflow smoothly around the bend (Correct answer)
- Increase airflow velocity through the elbow
- Filter dust and debris from the airstream
- Dampen sound transmission through the duct
Correct answer: Reduce pressure drop by guiding airflow smoothly around the bend
Turning vanes guide air smoothly around sharp radius elbows, significantly reducing the static pressure loss associated with abrupt direction changes.
HVAC Sheet Metal and Fabrication 4
What does the term 'duct aspect ratio' refer to, and why does it matter?