I have lost count of how many duct shop owners told me they would never make flat oval, then called me two years later because a retrofit job in a parking garage had 400 mm of headroom and a round duct simply would not fit. Flat oval keeps winning the awkward spaces: shallow ceiling plenums, risers in slabs, service corridors. It keeps the aerodynamics of round duct, mostly, while squeezing into places rectangular would dominate.
The manufacturing side is where people get surprised. Flat oval is not harder than round spiral, exactly, but it has its own rules, and breaking them shows up as out-of-round ends, wrinkles at the flat, and fittings that do not line up. Here is what I have learned setting up flat oval production on both common routes.
You make a normal round spiral tube on your spiral tubeformer, then pass it through an ovalizer that squeezes the round tube into the flat oval profile. This is the route most existing spiral shops take, because the ovalizer is a bolt-on addition. The roll set controls the final width and height ratio, typically something like 3:1 or 4:1 aspect.
The catch: ovalizing works the seam. The lock seam goes through a squeeze cycle, and on thin gauges or low lock tension you get seam opening or a slight spiral twist. I have seen 0.5 mm tube come out of the ovalizer with a visible corkscrew. The fix is usually more lock tension on the tubeformer and slower ovalizer feed, roughly 2 to 3 meters per minute instead of 6.
A dedicated flat oval duct forming machine forms the oval profile directly from coil, locking the seam along one flat. Sizes change faster, aspect ratios are more consistent, and there is no seam-working problem. The trade is that a dedicated machine only makes oval, so it ties up floor space and capital for a product line that may be 15 percent of your volume.
| Factor | Ovalizer on existing spiral line | Dedicated flat oval machine |
|---|---|---|
| Capital outlay | Low, bolt-on unit | Medium to high |
| Size changeover | Change tubeformer dies plus ovalizer roll set, 20-40 min | Quick-change roll set, 10-20 min |
| Aspect ratio range | Limited by ovalizer roll set, usually 1-2 profiles | Wider, multiple roll sets |
| Seam condition | Seam worked during ovalizing | Seam formed in-line, low stress |
| Best for | Shops with spiral volume, occasional oval orders | Regular oval production, 20%+ of mix |
My rule of thumb: if flat oval is under 15 percent of your output, buy the ovalizer. If a market segment, say underground parking or shipbuilding, keeps sending you oval work, the dedicated machine pays back fast in changeover time alone.
Designers size flat oval by equivalent diameter, and there are two of them: friction-loss equivalent and capacity equivalent. They are not the same number, and drawing offices mix them up more than you would believe. As a manufacturer, your job is simpler but strict: hold the stated width and height within tolerance, because the contractor's fittings were ordered against those numbers.
Practical tolerances that keep everyone happy: width plus or minus 3 mm, height plus or minus 2 mm, and end squareness within 1.5 mm over the major dimension. Ends matter more than people expect, because flat oval joints, whether beaded slip couplings or flanges, have less adjustment room than round ones. A bell-mouthed end that would slide fine on round duct will fight you all day on oval.
Flat oval behaves like round duct for pressure, with one wrinkle: the flats between the curves see bending, so the long dimension of a heavily flattened profile wants more gauge than the same diameter in round. Common practice runs 0.6 mm for small sizes up to 1.0 mm for large or higher-pressure work. For 2 kPa and above, most specs call for either thicker gauge or spot reinforcement beads along the flats.
On dedicated machines the lock seam runs along one flat, which is the best place for it: invisible from most viewing angles, easy to seal, and away from the curved corners where the lock would fight the bend. On ovalized spiral the seam follows the helix, so it crosses the curves. It works, but sealant adhesion on a helical seam in a curved zone is less reliable. Plan on testing a sample before committing to a leakage-critical job.
Standard joints for flat oval are beaded slip couplings, draw bands, and angle-ring flanges on the two flat ends for flanged systems. Beaded couplings are the workhorse: the bead forms on the tube end with a beading roll and the mating piece slides over it. What breaks in practice is the corner transition, where the bead must stay continuous through the curve. Cheap beading rolls produce a bead that thins or disappears at the corners, and that corner is exactly where the leak shows up on test.
Flanged flat oval is less common but appears on plant rooms. Flange corners are formed, not welded, on good tooling; welded corners work too but distortion makes gasket seating uneven.
Long flats on large profiles flex under pressure pulsing. Beading along the flat, spaced roughly every 200 to 300 mm, restores most of the stiffness at almost no material cost. This is the same beading logic as rectangular duct, and if you already run a beading machine the tooling adapts with an oval-specific roll set.
Straight duct is the easy 60 percent. The fittings decide your margin. Elbows are typically built as flat oval gores or as round-throat conversions with one flat kept. Takeoffs branch off the curved side wherever possible, because tapping the flat with a standard round saddle leaves an oval-to-round transition you have to fabricate anyway.
My advice to shops adding oval: quote fittings at 2.5 to 3 times your round-fitting labor hours until your team has done ten jobs. Then recalibrate. The first flat oval tee always takes a day. The tenth takes ninety minutes.
Lubricant choice matters more than on round. The ovalizing squeeze drags the outer skin over the rolls, and a dry tube scorches the coating. A light vanishing oil on the outer surface before the ovalizer, wiped or evaporated afterward, keeps pre-painted skins clean.
Support the tube through the ovalizer. Round spiral tube is rigid in bending; a half-flattened tube mid-process is a wet noodle. Infeed and outfeed roller conveyors adjusted to the exact output height, within 2 mm, prevent the droop that shows up as a wavy flat.
Slit coil width tolerance drives everything. If your slitter wanders plus or minus 1 mm, that error doubles into oval height tolerance on some ratios. When a customer complains about height tolerance, audit the coil width first. Nine times out of ten the duct machine is innocent and the slitting line is the culprit.
Three checks on every size run: width and height with a caliper at both ends of one tube per twenty; a bead-continuity visual at the corner transitions; and a lay-flat check on a surface plate for the first tube after each size change. Total time under five minutes. Skipping it once cost a shop I know a full container of 6-meter sticks that arrived on site with one size 4 mm narrow and no fitting would mate.
Buyers and shop owners keep sending each other vague RFQs for oval duct, so let me write down the short list that prevents most of the emails. State the profile by width and height, not just equivalent diameter. State the gauge per size, and which standard governs it, SMACNA or DW/144 or a local code. State the joint type per size: beaded slip, draw band, or flanged. Ask for the seam location in writing if appearance matters, because a helical seam on an ovalized tube looks very different from a straight seam on a formed one.
Ask how the fittings are made. Not who makes them, but from what: a flat oval elbow built from gores behaves differently at the corners than a round elbow with two flats kept, and the pressure drop between honest versions of the two is small but the leakage difference is not. Finally, ask for one sample tube per profile before the production run. Every serious shop does this without being asked twice, and any shop that resists a sample request has told you something useful about the next twenty containers.
Most trade catalogs standardize on profiles with equivalent diameters from 150 mm to 1500 mm and aspect ratios from about 2:1 to 4:1. Extreme flattening beyond 4:1 is possible with dedicated tooling but stiffness on the flats drops sharply and reinforcement needs increase. Stay in the standard range unless the project truly forces otherwise.
Yes, with an ovalizer, provided your tubeformer produces consistent lock tension and your diameters fall within the ovalizer's roll set range. Budget for at least two roll sets to cover the common profiles, and expect feed speeds around 2 to 3 m/min during ovalizing. Thin gauges below 0.6 mm need extra care with seam tension.
Well-made flat oval with beaded couplings and sealed corners tests comparable to round spiral, comfortably meeting typical class A or B targets. The risk points are seam corners and bead gaps at curves, so sample-test each new profile before a leakage-critical contract rather than assuming round performance carries over.
Material-wise flat oval usually wins: less metal than rectangular for the same capacity and lower fitting count in many layouts. Labor is the swing factor. If your shop is set up for rectangular and rarely makes oval, per-piece costs run high until volume justifies the tooling. For pure supply contracts from a specialist, flat oval pricing often lands 10 to 25 percent over round spiral.