What flat-oval duct is
Flat-oval duct — sometimes written flat oval, or called obround duct — is round spiral duct that has been flattened into an oval section: two parallel flat sides joined at each end by a constant-radius semicircle. It is not a separate kind of duct made from scratch. It starts life as ordinary round spiral duct, with the same continuous helical lock-seam running along its wall, and is then reshaped. That origin is the whole point: the finished oval keeps the structural behaviour of round duct.
Round duct is the strongest, stiffest and lowest-leakage shape in HVAC because pressure acts evenly on a curved wall and the spiral lock-seam stiffens the tube along its length. Its drawback is height — a round duct needs as much vertical space as it does horizontal. Rectangular duct solves the height problem but pays for it: long flat panels flex under pressure, need cross-breaking or reinforcement, and the four corner seams are the classic leakage path. Flat-oval is the deliberate middle ground. By flattening a round tube you trade a little of round duct's efficiency for a much lower profile, while keeping most of its rigidity and air-tightness and avoiding the flat-panel and corner-seam penalties of rectangular.
Where flat-oval duct is specified
Flat-oval earns its place wherever vertical space is the binding constraint but the project still wants round-duct performance:
- Low plenum and tight ceiling voids. The most common driver. When the slab-to-ceiling space cannot accept the equivalent round duct, a flat-oval section of the same airflow capacity fits in a shallower zone, leaving room for sprinkler mains, cable tray and lighting.
- Retail and commercial fit-outs. Shopping-centre tenancies, supermarkets and open-plan offices frequently combine high airflow with a low finished-ceiling height. Flat-oval lets the designer move the air without dropping the ceiling.
- Refurbishment and heritage work. When new HVAC has to thread through an existing structure with fixed floor-to-floor heights, the shallower oval profile is often the only metal duct that fits the available void.
- Exposed architectural HVAC. Flat-oval reads as a cleaner, more deliberate line than rectangular duct when ductwork is left visible, while still behaving like round duct acoustically and structurally.
Where there is no height constraint, plain round spiral duct is usually the better and cheaper choice — it skips the second forming step. Flat-oval is specified when the low profile genuinely buys something. For the underlying trade-off between round and rectangular, see our round vs rectangular duct comparison.
How flat-oval is made from spiral round
Flat-oval fabrication is a two-stage sequence, and understanding it is the key to specifying the machinery.
Stage 1 — form the round spiral tube. A spiral tubeformer takes a continuous galvanised, stainless or aluminium strip from a decoiler and runs it through a series of forming heads that roll the strip into a helical tube while continuously closing the spiral lock-seam. The output is round spiral duct in whatever diameter the forming head is set for. This is the same process used for all round spiral duct; the spiral duct forming guide covers it in detail, and the spiral duct machine spec comparison sets out the diameter, gauge and cutting options across the SBKJ SBTF series.
Stage 2 — ovalize the round tube. The finished round tube is fed into an ovalizer. The machine presses the tube between a matched forming die and a forming tray and stretches the circular section outward into a flat-oval, holding the constant-radius ends while flattening the sides. Because the spiral lock-seam is not cut or opened during this step, the wall stays continuous and the duct keeps the strength and low leakage of the round tube it came from. The die controls the radius of the oval ends and the tray sets the flattened dimension, so the two together define the oval section produced.
A practical consequence: the oval's geometry is bounded by the tooling. You cannot ovalize a tube smaller than the machine's minimum diameter, you cannot exceed its working length in one piece, and the achievable oval dimensions are limited to the die and tray sets the machine carries. That is why sizing has to be checked against the forming machine, not assumed — covered below.
Aspect ratios and sizing
A flat-oval section is described by three numbers: the major dimension (overall width), the minor dimension (overall height, equal to the diameter of the round ends), and the flat length between the two semicircular ends. The ratio of major to minor — the aspect ratio — is the single figure that captures how flat the duct is.
The design logic is a balance. The flatter the section (higher aspect ratio), the more ceiling height it saves — but the more it gives back the advantages that made flat-oval attractive in the first place. As the flat sides get longer relative to the curved ends, the section behaves more like rectangular duct: the flat walls become more prone to flexing under pressure and the forming and reinforcement demands rise. So designers keep the aspect ratio modest, sizing the oval for the airflow first and then confirming that the chosen major and minor dimensions actually fall within the forming machine's die and tray capability before the section is committed to drawings.
Two sizing rules follow directly from the fabrication method. First, the minor dimension can never be smaller than the smallest round tube the machine will accept — the oval starts as a round tube, so the round-tube minimum is the floor. Second, the major dimension and the run length are both capped by the machine: the tray set fixes the range of flattened widths, and the working length fixes the longest single piece. Anything beyond those limits is made as joined sections rather than one piece. Because flat-oval keeps the continuous spiral wall, it does not need the cross-breaking and panel reinforcement that rectangular duct of the same capacity would, which is a large part of why it leaks less and installs cleaner.
The SBKJ machinery that makes flat-oval duct
Flat-oval is a two-machine job, and SBKJ supplies both stages as a matched pair: an SBTF-series spiral tubeformer to make the round tube, feeding the Ovalizer SBHF-3100 to flatten it. The figures below are taken verbatim from the SBKJ Product Catalog 2026.
| Ovalizer SBHF-3100 specification | Value |
|---|
| Material thickness | 0.4–1.2 mm |
| Maximum working length | 3100 mm |
| Minimum round-tube diameter | Φ330 mm |
| Forming die set | Φ150, Φ200, Φ250, Φ300, Φ350, Φ400, Φ450, Φ500 |
| Forming tray set | 50, 100, 200, 300, 400, 500, 600 mm |
| Drive power | 18.5 kW |
| Machine weight | 2720 kg |
| Footprint (L×W×H) | 5630×1250×1130 mm |
| Supply voltage | 380 V / 50 Hz / 3-phase (60 Hz on request) |
Source: SBKJ Product Catalog 2026, manufacturer nameplate specification for the Ovalizer SBHF-3100. The die and tray sets together define the oval section produced; achievable dimensions depend on material and temper. Confirm your section against the machine configuration when you enquire.
How the two stages pair up. The spiral tubeformer produces the round tube, and its diameter has to land within the ovalizer's range — the SBHF-3100 ovalizes round tube from a Φ330 mm minimum, so the tubeformer is set accordingly. A hydraulic press supplies the forming force, an aluminium die avoids jamming, and an aluminium cast tray carries the work, with a single trained operator running the machine. For projects that need to reshape smaller round duct, SBKJ also offers the Oval Roller SBOR-1.2×1500, a lighter round-to-oval machine that works from a Φ100 mm minimum diameter up to a 1500 mm maximum width on a 4 kW drive — useful where the oval sections are smaller than the SBHF-3100's range. Match the machine to your section: confirm minimum diameter, the die and tray dimensions for the oval you need, and material gauge before you commit.
Heavier, higher-power machinery needs a stronger floor, a larger electrical supply and more floor space, so the ovalizer and its feeding tubeformer should be planned together. SBKJ provides a 2D workshop layout drawing with every quotation, so the decoiler, spiral former, run-out and ovalizer are sized to the available floor before you commit — the same way the spiral duct line is laid out for round duct.
Get a flat-oval machine recommendation in 12 hours →
FAQ
What is flat-oval duct?
Flat-oval duct is round spiral duct flattened into an oval (obround) cross-section — two flat sides joined by semicircular ends. It keeps the continuous lock-seamed spiral wall of round duct, so it retains round duct's stiffness and low air leakage, while the flattened profile fits the shallow ceiling space that would otherwise force the designer onto rectangular duct.
Why use flat-oval instead of round or rectangular?
Flat-oval is chosen when ceiling or plenum depth is too tight for an equivalent round duct but the project still wants round duct's low leakage, rigidity and lower pressure loss. For a given airflow the oval section is shallower than the round duct of the same area, yet it leaks less and needs less reinforcement than rectangular duct of the same capacity. It is common in retail, commercial fit-outs, refurbishments and exposed architectural HVAC.
How is flat-oval made from round spiral duct?
In two stages. A spiral tubeformer first rolls and lock-seams strip into continuous round spiral tube. That round tube is then fed into an ovalizer, which presses it between matched dies and a forming tray to stretch the circular section into a flat-oval. The spiral seam is undisturbed, so the finished duct keeps the strength and air-tightness of the original round tube.
What machine forms flat-oval duct at SBKJ?
SBKJ forms flat-oval on the Ovalizer SBHF-3100, fed by an SBTF-series spiral tubeformer. Per the SBKJ Product Catalog 2026 the SBHF-3100 handles 0.4–1.2 mm material, a working length up to 3100 mm, a minimum round-tube diameter of Φ330 mm, a die set of Φ150–Φ500 mm and a tray set of 50–600 mm, on an 18.5 kW drive at 380 V / 50 Hz / 3-phase.
What aspect ratios are typical for flat-oval?
Flat-oval is described by its major dimension (width), minor dimension (height) and the constant-radius ends. The flatter the section, the higher the major-to-minor aspect ratio and the more height it saves — but the more it loses round duct's advantage and the more forming and reinforcement it demands. Designers keep the aspect ratio modest and confirm the achievable range against the machine's die and tray sets rather than assuming any width fits any height.
Does flat-oval leak less than rectangular duct?
Generally yes. Flat-oval keeps the continuous spiral lock-seam of round duct and has no long flat panels and no four-corner seams, so it is inherently stiffer and tighter than rectangular duct of the same capacity, which usually needs cross-breaking, reinforcement and sealed transverse joints to reach a comparable leakage class.
