I have had the same conversation with contractors in maybe a dozen countries. They quote a job built the way they have always built it, rectangular duct with TDF flanges, and then a competitor undercuts them by fifteen percent. Half the time the difference is not labor or overhead. It is the shape of the duct. When a shop switches a suitable project from rectangular to spiral, the material bill drops, the joint count drops, and the crew on site finishes faster. But it does not work for every job, and pretending it does is how people end up with a tubeformer sitting idle. Let me walk through where each shape wins, with numbers I have seen on real orders rather than brochure figures.
For a given cross-section area, a circle has the shortest possible perimeter. That single geometric fact drives most of the cost story. Take 0.5 square meters of free area, which is a common size for a main branch. A round duct needs a diameter of about 800 millimeters, and the circumference works out to 2.51 meters of sheet per meter of duct. A rectangular duct at the same area, say 1000 by 500, has a perimeter of 3.0 meters. That is roughly sixteen percent more galvanized steel for every meter you ship, before you even talk about flanges.
Smaller ducts show the same pattern. For 0.125 square meters, a 400 diameter round duct uses 1.26 meters of sheet per meter of length, while a 500 by 250 rectangle uses 1.5 meters. About seventeen percent more. The ratio stays in that fifteen to twenty percent band across the normal commercial range, and it is the single most reliable saving in the whole comparison.
Put money on it. Galvanized coil at 0.6 millimeter weighs about 4.7 kilograms per square meter. If your coil costs around 750 dollars per ton landed, then a meter of 400 spiral consumes about 5.9 kilograms of steel, roughly 4.40 dollars. The equivalent 500 by 250 rectangular blank consumes about 7.1 kilograms, roughly 5.30 dollars, and then you add a TDF flange set, corner inserts, and gasket on every 1.2 meter section. On a 3000 meter project that gap alone is around 2700 dollars in raw material, and flange hardware pushes it wider.
There is a second material effect people miss. Spiral is made from coil directly, so there is no offcut skeleton from cutting rectangular blanks out of sheet. A rectangular line always leaves some scrap between parts unless your nesting is very disciplined. Spiral converts nearly one hundred percent of the coil into duct, minus the starter and cutoff waste at each coil change. On thin gauges that pushes the real saving closer to twenty percent.
Rectangular duct comes in sections, usually 1.2 meters long, and every section carries four flange corners, a gasket, and fasteners. Over ten meters of duct run, that is eight joints. Spiral comes in three meter standard lengths, so the same run has three joints. Fewer joints means fewer leak paths, less site labor, and fewer parts in the truck.
The seam itself matters too. A spiral lock seam runs continuously along the duct and, on a decent tubeformer with properly adjusted forming heads, it is close to airtight as manufactured. Rectangular duct depends on every corner of every flange being seated right, and the corners are exactly where leakage concentrates. In our leak tests, an unsealed rectangular system with TDF flanges typically runs two to three times the leakage of an unsealed spiral system at the same test pressure. Once you seal both properly to a class A target the gap narrows, but spiral starts from a much better place, which means less sealant and less rework when the test crew shows up.
Lower leakage is not just an engineering nicety. Leaked air is air your fan paid for and the space never received. On a system running eight hours a day, cutting leakage in half shows up on the building's electricity bill for the next twenty years, and more consultants are writing tight leakage classes into the spec. If your market is moving that way, spiral has the easier path.
Here is where honest comparisons get uncomfortable for spiral. Rectangular fittings are fabricated from flat blanks on the same line as the duct, so a shop with an auto duct line makes elbows, reducers, and transitions in-house with no new equipment. Spiral fittings require either a gorelocker for segmented elbows, a plasma cutter for patterns, or a supplier. The fittings are usually cheaper to buy for round than the equivalent fabricated rectangular piece, but if you fabricate rectangular yourself the cash cost difference is smaller than it looks on a price list.
My rule of thumb after watching shops make this transition: count the fittings. If a project is more than about thirty percent fittings by length, or if it has a lot of transitions and offsets, rectangular gets more competitive. If it is long straight runs with simple tees and ninety degree elbows, spiral wins comfortably. Warehouses, parking garages, open-plan offices, and corridors are spiral country. Plant rooms and cramped ceiling voids are not.
A meter of 400 spiral at 0.6 millimeter weighs about 5.9 kilograms. The 500 by 250 rectangular equivalent runs about 7.1 kilograms plus flanges, corners, and sealant, call it 8 kilograms assembled. Lighter duct means bigger sections per lift, fewer hangers in some designs because of better rigidity, and less fatigue for the crew. Spiral also joins with simple band connectors or companion flanges that go together in a fraction of the time a TDF flange takes, because there are no corners to close and no gasket to seat along two axes.
Crews I have timed on mixed projects hang round duct thirty to fifty percent faster per meter than rectangular. That is a real number you can put in your tender. It is also the reason mechanical contractors who self-perform keep pushing designers toward round: their own labor is the biggest line they control.
Do not throw out your auto duct line. Rectangular wins in specific situations and it is worth naming them. First, headroom. A 500 by 250 duct fits under a beam where a round duct of equal area needs 800 millimeters of clear height. In tight ceiling voids above corridors, rectangular or flat oval is often the only shape that fits. Second, big aspect ratios. Air handling plant connections and large low-velocity trunks are naturally rectangular, and nobody builds a two-meter round duct for a plenum connection. Third, flexibility of modification on site. A rectangular duct can be notched, tapped, and reworked with hand tools; round duct modifications need proper takeoffs and sometimes new fittings. Fourth, some markets simply expect rectangular. If your customers are agencies with old standards on the books, you fight the shape, not the spec.
These are the numbers I use when a customer asks for a straight answer. Assume 0.6 millimeter galvanized, coil landed at 750 dollars per ton, equivalent free area of 0.125 square meters, and a 3000 meter project.
| Item | Spiral 400 | Rectangular 500x250 TDF |
|---|---|---|
| Sheet used per meter | 1.26 m², about 5.9 kg | 1.50 m², about 7.1 kg |
| Material cost per meter | about 4.40 USD | about 5.30 USD plus flange set |
| Joint hardware | band connector, 1 per 3 m | flanges, 4 corners, gasket per 1.2 m |
| Joints per 10 m run | 3 to 4 | 8 |
| Leakage, unsealed | low, tight seam as formed | 2 to 3 times higher, corners dominate |
| Erection speed | 30 to 50 percent faster per meter | baseline |
| Material saving on 3000 m | baseline | about 2700 USD more in sheet alone |
| Fitting supply | needs gorelocker or supplier | made on same auto line |
No single row decides a tender. Read the table against your project mix and your own shop's equipment, and the answer usually becomes obvious within a minute.
If the comparison convinces you, the entry cost is a spiral tubeformer, and the honest way to size it is by diameter range and gauge. A machine forming 80 to 1250 millimeter diameters from 0.5 to 1.2 millimeter coil covers most commercial HVAC. Check the forming head count and how long a diameter changeover takes, because a job with many sizes will eat your afternoon if changes are slow. Add a plain and beaded elbow machine or a hydraulic gorelocker for fittings, and a band or flange crimper for joints.
Tubeformers run at 15 to 30 meters per minute depending on gauge, so one operator can produce more duct meters in a shift than a rectangular line crew, and the coil feeding is continuous. Power draw for the tubeformer itself is modest, typically under 10 kilowatts. The real cost drivers are forming head wear and lock seam quality control, which is why I tell buyers to put seam sample checks at the start of every shift and every coil change. A seam that opens under bending pressure will cost you a warranty call, not a warranty letter.
No. For long straight runs at equal area it is usually fifteen to twenty percent cheaper in material and faster to install, but jobs with heavy fitting content, tight ceiling voids, or large low-velocity trunks often stay rectangular. Count fittings and check available height before you switch.
Most commercial work sits between 100 and 800 millimeters. A machine forming 80 to 1250 millimeters gives you headroom without paying for range you will rarely use. Match the gauge range to your thickest regular sheet, usually up to 1.2 millimeter for galvanized.
Yes, and that is the most common setup we see. The auto line keeps serving rectangular work and fittings, and the tubeformer picks up straight round runs. Shops that do both quote more of each tender instead of walking away from half of them.
Unsealed spiral typically leaks two to three times less than unsealed rectangular with TDF flanges, mainly because there are fewer joints and the lock seam is continuous. Sealed to the same leakage class, spiral needs less sealant and less rework to pass the test.