A 12-inch round duct doesn't fit in an 8-inch ceiling plenum. The engineer switches to 20×6 rectangular — same cross-sectional area. Same CFM. Should work, right?
The friction penalty
Round ducts have the lowest perimeter-to-area ratio. Less perimeter = less surface contact = less friction.
12" round duct:
Area: 113 in²
Perimeter: 37.7 in
20×6" rectangular:
Area: 120 in² (slightly larger)
Perimeter: 52 in (38% more surface)
That extra perimeter means 40% more friction at the same airflow.
Aspect ratio penalties
Aspect ratio = long side / short side. As it increases, friction climbs:
Aspect Ratio Friction vs Round
────────────────────────────────────
1:1 (square) +12%
2:1 +20%
3:1 +35%
4:1 +50% ← ASHRAE max recommended
6:1 +80%
8:1 +120%
Above 4:1, friction and sealing problems get severe.
The equivalent diameter trap
When converting round to rectangular, use the hydraulic equivalent — not equal area:
De = 1.30 × (a × b)^0.625 / (a + b)^0.250
20×6 rectangular:
De = 1.30 × 120^0.625 / 26^0.250 = 15.9"
This is NOT equivalent to a 12" round.
It's equivalent to a ~16" round.
To match a 12" round at the same friction, you need something like 14×8 (aspect ratio 1.75:1) — not 20×6 (3.3:1).
When to use each
Round:
✓ Lowest friction
✓ Strongest structure
✓ Easiest to seal
✗ Needs clearance height
Rectangular:
✓ Fits tight plenums
✓ Custom around obstructions
✗ Higher friction
✗ Keep aspect ratio ≤ 4:1
Flat oval:
✓ Lower profile than round
✓ Less friction than rectangle
✗ More expensive to fabricate
The rule
If space forces a flat duct, increase dimensions to compensate for friction — or accept higher fan power and noise. A 20×6 is not "the same as" a 12" round. The perimeter says otherwise.
Top comments (0)