Spiral duct selection starts with duty classification, not diameter. HVAC stock galvanised spiral typically spans 4"–24" (102–610 mm) for commercial supply runs, while industrial extraction systems can run from 52 mm up to 559 mm in flexible acoustic formats [S4][S6]. The decision tree narrows fast once operating pressure, conveyed material, and code jurisdiction are locked in.
Spiral duct is roll-formed from a continuous metal strip into a helically locked tube, producing a rigid round section with a single helical seam. That seam geometry is the reason it is the dominant choice for dust collection, paper trim, and high-velocity HVAC: the round cross-section has lower friction loss per unit air than rectangular duct of equal cross-sectional area, and the spiral lock resists positive pressure without flange reinforcement at every joint. A spec-first buy starts by defining the duty, then matching material, gauge, diameter, and certification to it.
Step 1: Classify the Duty and Match the Wall Type
Spiral duct is specified across three duty bands: HVAC supply/return, dust and trim collection, and industrial exhaust of corrosive or high-temperature airstreams. The duty drives wall construction first: single-wall galvanised is the default for HVAC stock, double-wall insulated is used where condensation control or thermal loss matters, and stainless or coated steel is mandated for corrosive or high-temperature service [S3][S4].
Spiralite, the patented non-metallic insulated ductwork held by Khansaheb Industries (UAE, established 1935), claims up to 48% combined energy, space, and time savings versus equivalent sheet-metal installations in commercial and residential HVAC projects [S3]. The figure is vendor-reported and should be treated as one data point, not a universal benchmark. For trim and dust collection, G.F. Puhl (Gallatin, TN) fabricates heavy-duty fittings engineered to industrial ventilation guidelines and explicitly markets them as the connection point between components and spiral duct in paper-waste and trim systems [S2].
For readers cross-referencing ductwork against other process piping decisions, the wiring duct encyclopedia page covers the cable-management cousin, while industrial valve selection shares the same duty-first logic.
Step 2: Lock Diameter, Gauge, and Material
Stock diameter ranges are the first hard number. U.S. HVAC fabricators such as Spiral Products (NC) typically stock thousands of feet of galvanised single-wall spiral in 4"–24" diameters plus hundreds of fittings, with rectangular duct produced for smaller commercial and residential projects on the same line [S4]. Flexible acoustic spiral duct from Foshan Chuangxing covers 52–559 mm diameters, which lines up with the residential and light-commercial branch-duct end of the market [S6].
Material grade is the second hard number. Galvanised steel to ASTM A653 with G60 or G90 coating is the HVAC default; 304 stainless is common for food and pharma exhaust; 316 stainless is required for chloride exposure. Gauge follows velocity and pressure class. For low-pressure HVAC (≤2" w.g. class), 26–24 gauge galvanised is typical in 4"–12" diameters, stepping to 22 gauge in 14"–24". For dust collection running 6"–12" w.g. negative pressure, 22–20 gauge is more common. These ranges are industry practice; specific jobs must be checked against SMACNA 2005 or the equivalent local standard.
The spiral duct encyclopedia entry covers the geometry, lock-seam mechanics, and typical gauge tables in more detail. Buyers comparing duct choices against adjacent process equipment often also weigh pressure transmitter and flow meter sizing on the same duct run, since the instrument tap location is part of the duct spec, not an afterthought.
Step 3: Verify Standards, Certification, and Sealing Class
SMACNA 2005 HVAC Duct Construction Standards is the explicit fabrication baseline used by U.S. shop fabricators for galvanised spiral and rectangular stock [S4]. Leakage class per SMACNA governs seal requirements: Class 4 (4 CFM per 100 sq.ft. at +1" w.g.) is the commercial default; Class 3 and tighter are used in healthcare and pharmaceutical supply; Class 6 or 8 is common on dust collection where negative pressure would otherwise pull ambient air through seams [S4].
For process plant buyers, the relevant harmonised standards also include DW-143 (UK ductwork) and EN 12237 for hygiene-class duct in healthcare. The forming equipment side, relevant for OEMs rather than buyers, is dominated by suppliers such as SBKJ Spiral Tubeformer (Jiangyin, China, ISO 9001:2000 and CE certified, exporting to 60+ countries) [S5].
Sealant and gasket material is itself a sub-spec: butyl rubber is the residential default, while food-grade silicone or PTFE gaskets are mandated in sanitary service. SMACNA 2005 also locks in transverse joint reinforcement intervals, hanger spacing, and pressure-class gauges; deviating from it should be a deliberate, documented call. For buyers sourcing from outside North America, the analogous European baseline is EN 1507 for rectangular and EN 12236 for circular ductwork, both of which share the same leakage-class philosophy as SMACNA.
Step 4: Compare the Realistic Options on Cost, Lead Time, and Fit
The mainstream options in a typical RFQ sit on a 2-by-2: galvanised single-wall (cheapest, fastest lead time, HVAC default) versus galvanised double-wall insulated (mid-cost, mid-lead-time, condensation control), versus stainless single-wall (highest cost, longest lead time, corrosive or sanitary duty), versus non-metallic insulated pre-fabricated (Spiralite-class systems, lowest on-site labour, vendor-reported 48% combined savings claim) [S3][S4].
On lead time, a U.S. stock program running 4"–24" galvanised can ship within days from a regional fabricator [S4]. Custom diameters, stainless construction, or double-wall insulated runs typically run 3–6 weeks depending on gauge and slot availability. On fit, the helical lock seam means spiral duct tolerates diameter changes through standard swedge reducers, but does not easily accept branch take-offs in the field; tees must be factory-formed or shop-fabricated as fittings [S2].
For adjacent selection work on a process line, a flow meter spec'd to the same duct run needs the same inner diameter tolerance, and a pressure sensor tap location must be planned before the duct is roll-formed, not cut in afterwards.
Who Should NOT Pick the Default Galvanised Single-Wall Option
The galvanised single-wall default is wrong for at least four duty cases. First, any airstream above roughly 200°C or carrying corrosive condensate (chlorides, acids) will eat the zinc coating: specify 304 or 316 stainless, or a coated alternative. Second, any exterior or cold-humidity run where condensation will drip: specify double-wall insulated to keep the inner liner above dew point. Third, food, pharma, or healthcare supply where leakage class tighter than SMACNA Class 3 and a cleanable internal surface are mandated: stainless or non-metallic with welded seams. Fourth, any application requiring a published energy or thermal performance warranty, where insulated pre-fabricated systems (e.g. Spiralite-class) carry a vendor-backed thermal figure rather than a calculated one [S3].
Buyers who only need a few branch runs under 12" diameter in conditioned space can also save by going flexible acoustic spiral in the 52–559 mm range rather than rigid galvanised [S6]. The trade-off is lower mechanical durability and a lower pressure class.
Sourcing, Fabrication Equipment, and the Shortlist Logic
U.S. buyers typically source from regional sheet-metal fabricators (e.g. Spiral Products in central NC, Hopewell Sheet Metal, G.F. Puhl in TN) that hold SMACNA 2005 as their baseline and stock 4"–24" galvanised single wall for fast-turn orders [S4][S7]. International buyers and OEMs often procure forming equipment directly, with SBKJ's spiral tubeformer line (SBTF-1602 and related models) and CE/ISO 9001:2000 certification being one of the more visible supply chains out of the Yangtze River Delta [S5].
The shortlist logic for a spec-first RFQ: (1) define duty and pressure class; (2) pick material and gauge against the conveyed stream; (3) lock diameter range and confirm it is in the fabricator's stock or lead-time envelope; (4) name the standard (SMACNA 2005, EN 1507, EN 12237, or project-specific) on the RFQ; (5) require leakage-class certification and seam-locking method in the submittal. Process engineers should treat the spiral duct spec as one node in a wider instrument-and-piping RFQ, alongside flow, pressure, and valve decisions, rather than a standalone commodity line item. For related spec-first selection work on a process line, see this sight glass sizing and selection reference and the flame arrester deflagration vs detonation duty breakdown.