Spiral duct is a round tube formed by helically winding a continuous metal strip into a 4-ply lockseam, with common diameters from 80 mm to 1600 mm and machine capacity reaching 2500 mm, per Alnor's spiral ductwork guide [S1].
The product is a subcategory of round duct, distinct from rectangular duct in shape, friction loss, and joinery method, and is the default rigid metal duct for commercial HVAC, dust collection, and industrial exhaust [S5][S9].
Material Classifications and Steel Grades
Galvanized steel is the default spiral duct material, with European mills typically supplying hot-dip coated grades DX51D (standard formability), DX53D, and DX54D (deep-draw formability) at coil widths of 137 mm or 140 mm slit from 1250-1500 mm master coils [S1].
Where corrosion or hygiene rules out galvanizing, spiral duct is also produced from stainless steel (commonly 304/304L and 316/316L), aluminum (alloys 3003, 3105, 5052), and copper; fiberglass board is a niche non-metallic option for low-pressure clean exhaust [S1][S5]. The same coil stock feeds both spiral lockseam and longitudinal-seam round duct, but the spiral process uses a narrower strip wound under tension, which is why strip-width tolerance directly controls lockseam quality [S1].
Seam and Wall Construction Types
The defining structural feature of spiral duct is its 4-ply spiral lockseam, with the groove entirely on the outside and the interior left smooth for low friction loss, per Sheet Metal Connectors' product specification [S2].
Single-wall spiral pipe has a crushing resistance roughly 2.5 times that of equivalent longitudinal lockseam or welded pipe, and adding optional corrugations increases rigidity by approximately 300% [S2]. Three outer-seam variants are in production: spiral lockseam (baseline), spiral lockseam with a standing rib (heavier gauge reinforcement), and a true longitudinal seam used only on special short sections [S6]. On the fitting side, SMC lists five seam types used on elbows, reducers, and tees: stitch weld, lap seam (riveted, tack weld, or solid weld), elbow lock seam, standing seam, and butt weld seam, with or without duct sealant [S2].
Connection Systems and Diameter Bands
Spiral pipe joints are not one-size-fits-all: connector selection is driven by diameter and by the airtightness target, and most suppliers publish three bands [S2].
Complete Seal fittings with a double-legged EPDM gasket cover 3" to 24" diameter (galvanized only) and are rated to deliver a virtually airtight joint; E-Z Flange Jr. with Barrel Clamp covers 6" to 24", standard E-Z Flange with Barrel Clamp covers 26" to 96", and companion angle rings are used for field-flanged connections on the largest bores [S2]. Plain spiral pipe ends are typically female, so a separate male coupler is required to mate two pipe sections in the field, and long-run installations on diameters above 24" usually transition to angle-ring flanges with gaskets [S1][S2].
Spiral vs Rectangular vs Flanged vs Clamp-Together
Selection between spiral, rectangular, welded flanged, vanstoned flanged, and clamp-together duct is driven by run length, retrofit access, and total installed cost rather than by raw material price [S3][S5].
Spiral duct is cheapest to fabricate and ships in continuous lengths, but it is best suited to long straight runs between 16 ft and 36 ft with few fittings, because long sections complicate handling, demand exact field cuts, and require one of five difficult joint methods (barrel clamp, spiral connector, sheet-metal screws, angle ring, or crimp-and-force) [S3]. Rectangular duct wins where ceiling plenum depth is constrained, since a 100 mm-deep rectangular cross-section fits spaces a round duct cannot, but it pays a friction-loss penalty at the same airflow [S5][S8]. Welded or vanstoned flanged duct is the right answer for tight, skilled-fitter retrofits because flanges can be sealed very tightly with a gasket and allow small modular sections; clamp-together rolled-lip duct is the fastest to install and easiest to reconfigure on dust-collection and welding-fume jobs [S3].
Airtightness, Leakage, and SMACNA-Class Implications
Leakage is the single biggest lifecycle cost driver in low-pressure duct, and spiral duct's airtightness depends entirely on the connector system rather than on the pipe itself [S1][S2].
A bare spiral lockseam pipe jointed only with a sheet-metal screw or a crimped end is the worst-case leakage path; the same pipe jointed with a Complete Seal fitting and EPDM gasket is rated by the manufacturer as "virtually airtight" [S2]. For European specifiers, airtightness is generally demonstrated against EN 12237 (the ductwork airtightness class standard referenced across the HVAC industry), while North American projects typically call out SMACNA HVAC Air Duct Leakage Test manual pressure classes, with the seal class dictating whether single-wall spiral with sealed flanges is acceptable or whether welded stainless is required [S1]. Galvanized stock supplied under ASTM A-653 lockforming quality is the standard substrate for these sealed fitting systems [S2].
Typical Applications, Limits, and Failure Modes
Spiral duct is the workhorse for commercial HVAC supply and return, restaurant kitchen exhaust (when grease-rated), dust and shavings collection from woodworking lines, welding fume extraction, and chemical-fume exhaust in non-corrosive service [S3][S4][S8].
It is a poor fit where: (a) the run is short and has many directional changes, since each joint is a labor cost spiral duct cannot absorb; (b) shipping length restrictions force field assembly of long sections, negating the fabrication-cost advantage; (c) the conveyed stream is abrasive or condensable, because the external lockseam groove is a particle snag point; or (d) the process gas is hot enough to exceed the galvanized zinc coating's service ceiling, typically cited around 200 °C continuous, after which stainless or aluminum becomes mandatory [S1][S3]. The standing-rib and corrugated variants exist specifically to push the diameter ceiling and the negative-pressure collapse rating upward, but they do not change the material or temperature limits of the base coil [S2][S6].
Specifying a Spiral Duct: A Decision Checklist
A clean spiral duct specification reads in this order: material and grade (e.g. DX51D galvanized to ASTM A-653, or 304 stainless), diameter range, wall gauge, seam type (4-ply lockseam baseline), connector system keyed to diameter (Complete Seal 3"-24", E-Z Flange 26"-96"), seal class target, and length-of-run expectation [S1][S2].
For most commercial HVAC jobs the default is single-wall galvanized, 4-ply spiral lockseam, with sealed flanges on the 26"-96" range; for industrial dust and fume, default to clamped or flanged joints and reserve spiral for the long trunk runs between collection points [S2][S3]. When sizing a project like a battery plant, a wastewater treatment hall, or a chemical-packaging line, it is worth scanning the parallel spec maps for adjacent equipment, for example construction machinery and equipment layouts that constrain ceiling clearance, before locking the duct geometry. For an at-a-glance refresher on how the round and spiral families relate, the spiral duct encyclopedia entry and the wiring duct entry together clarify the difference between air-handling spiral tube and cable-management channel, which are often confused in vendor catalogs. Two trackable signals to watch on the next sourcing cycle: the September 2026 DuctQuote tool update adding corrected flanged-pipe BOM counts and 3D model fixes, indicating active vendor-side configurator work in this category [S4], and the steady push of Complete Seal and E-Z Flange systems into the 26"-96" band that previously had to be welded in the field [S2].
For related coverage, see Wind Turbine Shaft Collar Selection: Material, Clamp Style, and Holding Power.