Fabrication and Layout Conflicts on a 3,000 cfm Trunk
A shop is fabricating a 60 ft supply trunk for a 3,000 cfm low-velocity commercial system. The drawing shows 24 x 12 in. rectangular 22 ga galvanized duct in 4 ft joints, plus a 14 in. round branch feeding a 1,400 cfm zone. The trunk must rise 9 in. over a 24 in. horizontal run to clear a structural beam, using two 45° elbows. The specification requires hangers at 8 ft maximum spacing and within 2 ft of every transverse joint. The installer proposes hanging the trunk on 8 ft centers starting 1 ft from the plenum, and wants to roll the 24 x 12 in. trunk down to 12 x 24 in. flat to gain ceiling clearance.
1. Check the velocities in the 24 x 12 in. trunk and the 14 in. round branch against low-velocity design practice, and state what you would change.
Read the worked answer
Trunk
Area = 24 × 12 = 288 in² ÷ 144 = 2.00 ft² Velocity = 3,000 / 2.00 = 1,500 fpm
For a low-velocity commercial supply main, 1,500 fpm is above good practice (mains are typically held to roughly 1,000–1,200 fpm to control regenerated noise and pressure drop; higher velocities are reserved for high-velocity systems with the matching fittings and terminal units).
Options that keep the same 12 in. depth:
- 30 x 12 = 2.50 ft² → 1,200 fpm
- 36 x 12 = 3.00 ft² → 1,000 fpm (best acoustically, needs the ceiling space)
Branch
14 in. round: area = π × (7/12)² = 1.069 ft² Velocity = 1,400 / 1.069 = 1,310 fpm - too high for a branch serving a zone, and too high for the flex or takeoff that will inevitably be added in the field.
- 16 in. round = 1.396 ft² → 1,003 fpm, a reasonable branch velocity
Judgment: upsize the trunk to at least 30 x 12 in. (or 24 x 16 in. if depth allows) and the branch to 16 in. Note that the proposed "roll" to 12 x 24 in. flat does not change area or velocity - 12 x 24 = 24 x 12 - so it is a legitimate clearance move only if the reinforcement and hanger design are updated for the new aspect ratio; it buys nothing hydraulically.
2. For the 9 in. rise using two 45° elbows, compute the length of the diagonal duct section and how much of the 24 in. horizontal run the offset consumes. Does the offset fit, and what do you order from the shop?
Read the worked answer
Geometry of a two-elbow 45° offset
The offset (rise) is 9 in. A 45° elbow turns the duct 45° from horizontal, so the diagonal section runs at 45°:
- Diagonal length = offset / sin 45° = 9 / 0.7071 = 12.73 in. → order a 13 in. section (round up; you cannot buy 12.73 in. of duct)
- Horizontal advance of that diagonal = offset / tan 45° = 9 / 1.0 = 9.0 in.
This is the part installers get wrong: the diagonal is 12.73 in. long but it only eats 9 in. of horizontal run, because at 45° the horizontal and vertical components are equal.
Does it fit in 24 in.?
Horizontal consumed = 9.0 in. for the diagonal, plus the takeout (heel/cheek) length of each 45° elbow - typically 2–4 in. each on a 24 x 12 in. duct depending on the fitting style:
24 − 9.0 − (2 × 3) ≈ 9 in. left for straight sections
So yes, the offset fits with two short straights. If the takeouts turn out larger than assumed, the fallback is a single 45° elbow plus a field-fabricated transition, or shifting the beam penetration.
Order: two 45° elbows, 24 x 12 in., same gauge and joint type as the trunk; one 13 in. long diagonal section; and note on the cut sheet that the offset assembly occupies 9 in. of horizontal run so the field crew does not over-cut the straight duct ahead of it.
3. Evaluate the proposed hanger layout (8 ft centers starting 1 ft from the plenum) against the specification. How many hangers does the 60 ft trunk actually need?
Read the worked answer
Two criteria have to be satisfied at the same time:
- Maximum 8 ft spacing.
- Within 2 ft of every transverse joint.
The duct is built in 4 ft joints, so there are transverse joints at 4, 8, 12, … 56 ft, plus the connections at each end (0 and 60 ft).
Test the installer's proposed layout: hangers at 1, 9, 17, 25, … ft.
- Spacing is 8 ft - criterion 1 passes.
- The joint at 4 ft is 3 ft from the hanger at 1 ft and 5 ft from the hanger at 9 ft - criterion 2 fails. Same for joints at 12, 20, 28 ft, etc.
Working layout: with 4 ft joints, a hanger on every joint means hangers at 0, 4, 8, … 60 ft. Each joint then sits directly on a hanger, and spacing is 4 ft - both criteria satisfied.
- Intervals: 60 / 4 = 15 → 16 hangers (including one at each end).
Judgment note: a joint does not have to be on a hanger to comply - a hanger at 2 ft from a joint satisfies the 2 ft rule - so a 4 ft pattern starting at a joint is the clean, buildable answer. The 8 ft pattern would only work if the duct were fabricated in 8 ft joints. Also confirm that the hanger type matches the 22 ga duct and that hangers are not placed where they interfere with the volume dampers or the beam penetration; the offset section needs its own support within 2 ft of each elbow.