Insights · Reference
Round duct can be made three ways: continuous spiral lockseam, longitudinally welded, or gored from rolled segments. They are not interchangeable — each suits a different pressure class, gauge and material, and each is made on different equipment. This reference sets out when to use each method and which SBKJ machine makes it. Machine figures are taken verbatim from the SBKJ Product Catalog 2026.
Before this gets confused with the round-versus-rectangular question, note that this page is about how to make round duct — the seam and forming method — not whether to choose round over rectangular cross-section. For the cross-section trade-off (friction loss, aspect ratio, fan energy) see Round vs Rectangular Duct Compared. Here we assume you have decided on round, and the question is: spiral lockseam, welded, or gored?
Almost all round HVAC duct produced today is spiral lockseam. The other two methods exist for the jobs spiral cannot do: longitudinal welding for heavy, high-pressure and stainless work, and gored/segmented construction for fittings and one-off large sections. The table below is the short version; the sections after it explain the why.
| Attribute | Spiral lockseam | Longitudinally welded | Gored / segmented |
|---|---|---|---|
| Seam | Helical mechanical lockseam (cold-formed) | Single straight welded seam along the length | Lockseam, rivet or weld between rolled segments |
| Blank | Continuous coil strip (137 mm) | Flat sheet rolled to a cylinder | Individual rolled gores |
| Typical gauge | 0.4–2.0 mm GI; to 3.0 mm aluminium | Heavy gauge / plate, to 3.0 mm stainless | Matches the parent duct gauge |
| Pressure class | Low to medium pressure, most commercial HVAC | High pressure, gas-tight (Seal Class A) | Per parent duct & joint method |
| Speed | Continuous, fastest — no-stop flying-shear cut | Slower — roll, then a welding pass | Slowest — piece-by-piece |
| Relative cost / m | Lowest | Higher (rolling + welding labour) | Highest (one-off fabrication) |
| Typical use | Commercial supply/return, exposed architectural, dust extraction | Stainless, pharma/lab fume, high-temp industrial, mine | Elbows, reducers, tees, large-bore fittings |
| SBKJ machine | SBTF-1602 / SBTF-2020 tubeformer | Roll bending + seam welder / MF welder | Roll bending + lock/weld closure |
Machine figures: SBKJ Product Catalog 2026, manufacturer nameplate specifications. Round duct conforms to EN 1506 / SMACNA round-duct construction. Pressure-class and seal-class guidance follows SMACNA and EN 1506 practice. Achievable gauge depends on material temper.
A spiral tubeformer takes a coil strip and runs it through a forming head that progressively rolls it into a helical tube while continuously locking the spiral seam — a four-fold mechanical lockseam, formed cold, with no heat and no filler. Because the strip feeds continuously from a decoiler, the machine never has to start and stop a sheet: it produces an unbroken tube and the patented flying slitter cuts it to length on the move. That single fact — continuous forming with a no-stop cut — is why spiral is the fastest and lowest-cost way to make round duct, and why it dominates commercial HVAC.
The lockseam itself does useful structural work. The helical fold stiffens the wall against buckling, so spiral duct needs less reinforcement than a plain cylinder of the same diameter and gauge, and the seam is inherently low-leakage for the low- and medium-pressure supply and return air that makes up the bulk of any building's ductwork. Spiral is also increasingly specified for architecturally exposed runs because the continuous helical line is clean to look at. Where it stops is at the top of the gauge and pressure range: a mechanical lockseam is not a gas-tight weld, and once you need Seal Class A integrity or material heavier than a former economically rolls, you move to welding.
The machines. The SBKJ SBTF-1602 covers Φ80–Φ1600 mm and the SBTF-2020 reaches Φ80–Φ2500 mm for large-bore industrial, mine-ventilation and tunnel duct. Both run galvanised 0.4–2.0 mm, stainless 0.4–1.2 mm and aluminium 0.4–3.0 mm from a 137 mm strip (142 mm for heavy gauge), with plasma cutting and a flying slitter for continuous, no-stop length control. The SBTF-1602 draws 15 kW (plus a 0.25 kW cooling-water pump) and weighs 5000 kg; the larger-diameter SBTF-2020 draws 22 kW. For the full four-machine spiral range and how diameter, cutting method and power scale, see the spiral duct machine spec comparison.
Welded round duct starts as a flat sheet or plate, rolled into a cylinder and closed with a single straight seam running the length of the tube. The seam is a continuous weld rather than a mechanical fold, which buys two things spiral cannot: a genuinely gas-tight joint to SMACNA Seal Class A, and the ability to work material far heavier than a spiral former is built to roll. That is exactly what high-pressure, high-temperature and corrosive-service duct needs.
This is the round duct of industry rather than commerce. Stainless 304/316 fume and exhaust duct for laboratories, hospitals and pharmaceutical plants is welded because the joint has to be cleanable and leak-free. High-temperature and high-pressure industrial extract, dust collection on heavy plant, and large-bore mine and process ducting are welded when the wall thickness or the duty rules out a lockseam. The trade-off is throughput and cost: rolling a cylinder and then running a welding pass over its full length is slower and more labour-intensive than continuous spiral forming, so welded round duct is reserved for the work that genuinely requires it rather than used as a default.
The machines. Two stations in sequence. A roll bending machine (SBKJ SBW11G series) rolls the sheet into the cylinder — the range handles 0.4–2.0 mm at up to 1500 mm wide, down to a minimum Φ100 mm, on a compact 1.5 kW drive. The longitudinal seam is then closed on a seam welder: the SBKJ SBFN range runs from the SBFN-35 (0.4–1.0 mm, 45 KVA) to the SBFN-100 (0.4–2.0 mm, 110 KVA), all at a continuous weld speed of 0.5–3 m/min. For the heaviest sections — stainless and carbon steel up to 3.0 mm, aluminium to 2.5 mm — the Medium Frequency Welding Machine delivers the higher, more stable current that thick stainless needs for a sound seam. Welding process selection itself (resistance, stitch, laser) is covered in the duct welder comparison.
The third method is not a tube at all. Gored construction builds a round section from individual rolled segments — gores — joined edge to edge. It is how round fittings are made: elbows are mitred gores, reducers are rolled cones, and large tees and laterals are cut-and-joined segments. It is also the fallback for one-off large-diameter or non-standard sections that fall outside a spiral former's diameter range or that are needed in quantities too small to justify setting up the line.
Each gore is rolled on the same roll bending machine used for welded duct — it forms arcs, cylinders and cones and is the essential tool for round fittings and cylindrical work. The segments are then joined by lockseam, rivet or weld depending on the gauge and the leak class required. Gored work is the slowest and most labour-intensive of the three methods per finished part, which is why it is confined to fittings and specials rather than straight-run duct: you make the straight runs on the spiral former and the fittings by goring.
Work through three questions in order:
1. Pressure and seal class. If the duct must be gas-tight to SMACNA Seal Class A — pharma, laboratory fume, high-pressure or high-temperature industrial extract — the seam is welded. If it is ordinary low- or medium-pressure commercial supply and return, a spiral lockseam is sufficient and far cheaper.
2. Material and gauge. Galvanised up to 2.0 mm, stainless to 1.2 mm and aluminium to 3.0 mm are squarely in spiral-former territory. Stainless and carbon steel beyond that — into the 3.0 mm range — is welded, because the former is not built to roll it and the duty usually wants a weld anyway.
3. Straight run or fitting. Straight duct is spiral (or welded, by case 1). Elbows, reducers, tees and large-bore specials are gored and rolled. Every round-duct shop therefore carries at least a spiral former and a roll bender; a shop that also serves industrial and stainless work adds a seam welder.
In practice most fabricators run the spiral line as their volume product and keep the welder and roll bender for the specialist and fitting work — spiral pays the bills, welding wins the jobs spiral can't touch. SBKJ supplies all three as matched modules — the spiral tubeformer range, roll bending and the welding stations — with a 2D workshop layout drawing so the line, decoiler, run-out table and welding cell are sized to your floor before you commit.
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Spiral lockseam round duct is formed continuously from a coil strip that is helically wound and mechanically lock-seamed into a tube on a spiral tubeformer — no heat, very fast, the dominant method for commercial HVAC. Longitudinally-welded round duct is a flat sheet rolled into a cylinder and joined with a single straight welded seam along its length, used for heavy gauge, high pressure and stainless industrial duct where a continuous gas-tight joint is required.
Weld the seam when the duct is heavier than a spiral former economically rolls, when the pressure or temperature class demands a continuous gas-tight joint (SMACNA Seal Class A), or when the material is stainless 304/316 for pharmaceutical, laboratory, fume or high-temperature industrial exhaust. For ordinary low- and medium-pressure commercial supply and return air, spiral lockseam is faster and cheaper and is the default.
A spiral tubeformer. The SBKJ SBTF-1602 forms Φ80–Φ1600 mm and the SBTF-2020 forms Φ80–Φ2500 mm, both from a 137 mm strip (142 mm heavy gauge), in galvanised 0.4–2.0 mm, stainless 0.4–1.2 mm and aluminium 0.4–3.0 mm, with a patented flying slitter that cuts the tube while the machine keeps forming.
Two in sequence: a roll bending machine (SBKJ SBW11G series, 0.4–2.0 mm thick, to 1500 mm wide) rolls the sheet into a cylinder, then a seam welder (SBKJ SBFN-35 to SBFN-100, 45–110 KVA, 0.5–3 m/min) closes the longitudinal seam. The Medium Frequency Welding Machine handles the heaviest stainless and carbon steel to 3.0 mm.
Gored duct is built from rolled segments rather than a continuous tube — most commonly elbows, reducers and large-diameter fittings. Each gore is rolled on a roll bending machine and the segments are joined by lockseam, rivet or weld. It is the method for fittings and for one-off large or non-standard sections a spiral former cannot produce.
Spiral lockseam is substantially cheaper per metre for the gauges it covers, because one operator produces continuous tube with no secondary welding, deburring or filler. Welded round duct carries the cost of rolling plus a slower welding pass and is reserved for heavy-gauge, high-pressure and stainless work. A shop that makes both runs spiral as the volume product and the welder for the specialist jobs.
Not sure whether your round-duct mix needs a spiral former, a welder, or both? An SBKJ mechanical engineer replies within 12 hours — not a salesperson.
Ask an engineerThe round-duct methods above run on standard SBKJ equipment: spiral tubeformers for continuous lockseam tube, duct welding machines for longitudinally-welded and stainless duct, and roll bending for gores and fittings — or browse the full machine catalog.
