Ductwork Construction
Rectangular and round duct fabrication, aspect ratios, transitions, elbows, tees, reducers, and beading.
- Fabricate rectangular duct using standard seam and connection methods
- Fabricate round duct and common round fittings
- Lay out transitions, elbows, and tee fittings
- Apply beading, cross breaking, and reinforcement for duct rigidity
Lesson 1
Rectangular Duct Fabrication
Rectangular Duct Standards
Rectangular duct is the most common duct type in commercial HVAC systems. It is fabricated from galvanized steel sheet in standard gauges based on the duct size and pressure class. SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) standards define gauge, reinforcement, and connection requirements.
The current edition is the SMACNA HVAC Duct Construction Standards 4th Edition. This is the authoritative reference for all rectangular and round duct fabrication. The 4th Edition incorporates Technical Resolution Bulletin TRB #01-25, which updates corner fastening requirements for T-25a and T-25b flanged transverse joints used in TDC (Transverse Duct Connection) systems.
TRB #01-25 - T-25a and T-25b Corner Fastening Update
SMACNA TRB #01-25 revises the corner fastening requirements for T-25a and T-25b flanged transverse joints. The updated requirements address corner clip spacing and fastener placement to improve joint integrity under positive and negative pressure cycling. When fabricating TDC flanged joints, always reference the 4th Edition tables rather than older editions.
Aspect Ratio
The aspect ratio of rectangular duct is the ratio of the longer side to the shorter side. An aspect ratio of 1:1 (square) is ideal for airflow and structural strength. SMACNA recommends keeping the aspect ratio at 4:1 or less. Higher ratios waste material, increase pressure loss, and require more reinforcement.
Aspect Ratio Rule
A 48" x 12" duct has a 4:1 aspect ratio - the maximum recommended. A 48" x 48" duct (1:1) carries the same airflow with less material, lower friction, and less reinforcement needed. Always try to keep the ratio as close to 1:1 as possible.
Fabrication Steps
The basic rectangular duct fabrication process:
- Calculate stretchout (2W + 2H + seam allowances)
- Shear the blank to size on the squaring shear
- Mark bend lines on the blank
- Notch corners for seam clearance
- Form the Pittsburgh seam on the Pittsburgh machine
- Bend on the brake to form the duct section
- Close and lock the Pittsburgh seam
- Apply TDC or drive slip flanges to each end
Rectangular duct gauge is determined by duct size and pressure class per SMACNA HVAC Duct Construction Standards 4th Edition. Keep the aspect ratio at 4:1 or less - lower is better. A square duct (1:1) is the most efficient shape for airflow and material use. Always apply TRB #01-25 corner fastening requirements for T-25a and T-25b flanged transverse joints.