If you’ve shopped industrial ductwork, you’ve seen spiral duct quoted at roughly half the price of clamp-together systems like Nordfab Quick-Fit. That price gap is real, and on a clean-air HVAC project, spiral is often the right call.
For a combustible dust system under NFPA 660, it’s the wrong one — and not for the reasons most people assume. The problem isn’t strength. The problem isn’t gauge. The problem is that spiral duct was engineered for HVAC, and the four things that make it cheap and fast for HVAC are the same four things that turn it into a compliance liability when you’re moving fugitive dust through it.
You get authorized dealers for Nordfab Quick-Fit, MicroAir, Plymovent, and six other manufacturers when you work with us — and we don’t carry spiral duct at all. Most of what we know about spiral, we learned walking customer facilities where it had already been installed and was about to become someone’s compliance problem. Here’s why.
What spiral duct actually is
Spiral duct is made by feeding a continuous coil of sheet metal through a forming machine that bends, folds, and locks the edges together into a helical seam — picture a candy cane stripe wrapped around a tube. The “lockseam” is mechanical: two folded edges crimped together, no weld. That manufacturing method is fast, cheap, and uses very little energy, which is why spiral is the workhorse of commercial HVAC.
Quick-Fit pipe is built differently. Each pipe section has one longitudinal seam running end-to-end — and that seam is fully laser welded. Nordfab’s spec sheet describes the QC step explicitly: “All laser welded seams will undergo a light test to ensure there are no voids or imperfections in the system.” They shine a light inside the pipe in a dark room and look for any pinpoint where light escapes. A failed seam doesn’t ship.
That’s the core difference. One seam is folded. The other is welded and inspected.
What Nordfab says about it
You don’t have to take our word for the leakage difference. Nordfab — who manufactures both clamp-together and flanged ductwork and has zero reason to disparage the broader category — puts it directly in their published engineering specifications:
“In comparison to other ducting typically used in fit-together / clamp-together systems, Nordfab’s QF duct pipe has fully welded, leak-tight laser or plasma welded seams. Spiral and other ducting with lockform seams are not fully welded at the pipe seams and can be expected to have higher leakage rates than QF.”
Nordfab Ducting General Engineering Specification
That’s the foundation everything else rests on. The four problems below are downstream of that one engineering fact.
Problem 1: The lockseam leaks — and the math gets ugly fast
You can quantify the difference because both products have been tested to the same SMACNA standard. Here’s how spiral compares to Quick-Fit on the same 100-foot run of 12″ duct at 10″ of static pressure:
- Spiral (SMACNA Class 3): approximately 4 CFM of leakage per 100 feet of pipe
- Quick-Fit: approximately 0.40 CFM per joint × 20 joints per 100 feet = 8 CFM total
Wait — Quick-Fit looks worse on a 100-foot bench test. So what’s the catch?
The catch is where the leakage happens. Spiral leaks along the entire helical seam of every single foot of pipe, because the lockseam is the leak path. Quick-Fit leaks only at clamp joints, and those joints are gasketed and rated. On a complex industrial system with branches, elbows, and equipment connections, the spiral leakage rate compounds across every linear foot, while Quick-Fit leakage stays bounded to known joint counts.
But the bigger issue isn’t the airflow loss. It’s what comes out of those leaks. In a dust collection system running at -10″ WG, every leak point becomes an inward air infiltration point — which is fine for the fan but invisible to operators. In a positive-pressure return air or transfer scenario, those same leak paths become fugitive dust emission points along the entire helix of every spiral pipe in the building.
That’s the compliance problem. Fugitive dust accumulation on roof beams, light fixtures, and overhead conduit is one of the most common dust hazard analysis (DHA) findings. A duct that leaks along its entire length contributes directly to the fugitive dust layer that creates secondary explosion risk under NFPA 660.
Problem 2: The lockseam is on the inside of the pipe
This one rarely makes it into engineering conversations but it matters more than the leakage data.
The helical lockseam doesn’t just live on the outside of the pipe. It protrudes into the interior — typically 1/8″ to 3/16″ of a folded sheet metal edge sticking into the airstream, wrapping in a spiral pattern from end to end of every pipe section.
For HVAC air, that’s a minor pressure drop and nothing else. For dust collection, it’s a snag point. Fines, fibers, hygroscopic powders, and combustible dust accumulate on that internal ridge because the lockseam disrupts the laminar airflow that keeps particulate suspended. Over time, that buildup becomes:
- A reservoir of accumulated dust inside the duct itself — which is exactly what NFPA 660 wants you to eliminate
- A flame propagation path during a deflagration event (the accumulated layer is a fuel source)
- A hidden inspection failure — you can’t see internal lockseam buildup without disconnecting the duct, and most facilities never do
Nordfab’s marketing says it more politely: “Smooth, fully welded seams prevent leaks, snags, and possible ‘bug harbors.’” The “bug harbor” language is about insect harborage in food processing, but the engineering principle is identical. A protruding internal seam catches whatever’s flowing past it. In combustible dust service, what catches inside the duct can hurt you.
Quick-Fit pipe has no internal seam. The longitudinal weld is flush on both surfaces because it’s laser welded edge-to-edge with no overlap. The interior is smooth.
Problem 3: Static bonding becomes a per-joint chore
NFPA 77 paragraph 8.4.1.1 requires that all parts of a continuous metal piping system carrying flammable or combustible material shall have a resistance level not exceeding 10 ohms — measured end-to-end across the entire duct system. That’s the static bonding rule, and it’s what prevents an accumulated charge on the duct from arcing and igniting the dust inside.
Quick-Fit handles this in the design. Nordfab’s spec sheet describes it: “Die-formed rolled edges are uniform in shape which provides the most consistent contact. The ears of the clamp contact with the rolled edges and provide maximum conductivity.” The clamp itself is the bond. Continuous metal-to-metal contact happens by default when you close the clamp.
Spiral duct doesn’t bond reliably at joints. The lockseam has paint, oil, or zinc coating folded into it from manufacturing, which insulates the metal-to-metal contact. Spiral installations in combustible dust service typically require bonding jumpers — copper braid or wire — bolted across every single joint, every elbow, every branch, every machine connection. Then someone has to test continuity end-to-end with a megohmmeter and document it.
That’s not impossible. It’s just expensive and chronically maintained. Every time the duct moves, vibrates, or gets disturbed during cleaning, a jumper can loosen. Every loosened jumper is a potential bond failure that won’t show up until someone tests for it — or until something else does.
Problem 4: You can’t reconfigure it
Spiral duct joints are typically sealed with mastic and either riveted, screwed, or welded at every connection. To move a branch, you cut the pipe, refabricate the connection, re-seal it, and re-bond it. Plan to lose the pipe section you cut.
Quick-Fit comes apart in seconds with no tools. Nordfab’s literature says it: “Easy to remove, clean, and reinstall – without tools… Easy to modify or relocate as your needs change.” Pop the clamp, disconnect the joint, move the section, re-clamp.
For a stable facility running the same product on the same equipment for the next twenty years, this difference doesn’t matter much. For any facility that’s:
- Adding equipment
- Relocating machine cells
- Going through a capacity expansion
- Modifying the system after a DHA finding
- Cleaning internally on a regulated schedule
- Replacing a worn pipe section without rebuilding the run
…the total cost of ownership story for spiral changes considerably. You paid less up front. You’ll pay more every time anything changes.
When spiral duct IS the right answer
You don’t get oversold here. Spiral has legitimate applications, and pretending otherwise would be dishonest:
- HVAC supply and return air. This is what spiral was built for. Clean conditioned air at low pressure, no combustible dust, no static bonding requirement. Spiral wins on cost and is perfectly suited to the job.
- Light-duty ventilation. Welding booth exhaust at low static pressure with non-combustible welding fume, simple commercial kitchen exhaust, basic room ventilation. The economics favor spiral and the compliance stakes are low.
- Temporary installations. Construction site dust extraction, short-term remediation work, anything you’re going to tear out in 90 days. Spiral does fine.
- Non-combustible material handling at low loadings. Clean inert dust at light concentrations in a facility with no NFPA 660 jurisdiction (very rare — most industrial dust is combustible by default, but it does happen).
The line we draw is combustible dust under NFPA 660. If the material is combustible and the system falls under NFPA 660 jurisdiction — which covers virtually all industrial dust collection installed after January 1, 2026 — we won’t quote spiral.
What we quote instead
For combustible dust systems, we build with Nordfab Quick-Fit clamp-together duct (3″ through 24″) and Nordfab flanged ductwork (above 24″) as the standard. Both share the same laser-welded longitudinal seam, the same conductivity-by-design clamp connection, and the same modular reconfigurability.
For higher abrasion service — abrasive grit, hardwood at high velocity, recycled material with metal contamination — we move to heavier-gauge Nordfab (14ga or 12ga) or US Duct heavy-gauge product. The connection method stays the same; the wall thickness goes up.
Pricing depends on diameter, gauge, material (galvanized vs. stainless), and the fitting count for your specific layout. Our ductwork cost breakdown walks through the variables in detail.
Every engineered system we quote includes a complete duct layout drawing — floor plan, cut list, hanger schedule, and bill of materials — at no charge as part of the proposal. You see exactly what you’re paying for before you commit.
Frequently asked questions
Isn’t Nordfab spec data biased? They manufacture both clamp-together and flanged duct.
Nordfab also manufactures spiral-style products in their European market and has nothing to gain by understating spiral performance broadly. The leakage data on their spec sheet is tested per SMACNA “HVAC Air Duct Leakage Test Manual” — an industry-standard procedure that any manufacturer’s lab can replicate. The Class 3 spiral leakage rates Nordfab references are SMACNA’s published numbers, not Nordfab’s.
Will spiral duct pass an NFPA 660 inspection?
Possibly. NFPA 660 doesn’t ban any specific duct construction — it sets performance requirements for fugitive dust control, static bonding, isolation, and explosion protection. Spiral can meet those requirements with bonding jumpers, sealed joints, and rigorous housekeeping. The question isn’t whether it can pass; it’s whether the long-term cost of making it pass is lower than building it right the first time. In our experience, it’s not.
What about flanged duct? Is it better than clamp-together?
For pipe diameters above 24″ or for very high vacuum applications (above -28″ WG sustained), flanged is the right call and Nordfab makes it. Below 24″ and inside normal operating pressures, clamp-together gives you the same leak-tight pipe seam with faster installation and easier modification. We spec flanged where the application calls for it.
Can spiral duct be retrofitted to comply with NFPA 660?
Sometimes. If the existing spiral is in good condition, properly sized, and the only deficiencies are bonding and joint sealing, we’ve added bonding jumpers and re-sealed joints on existing systems as part of a compliance retrofit. If the spiral is undersized, the wrong material for the dust, or has accumulated internal buildup we can’t see, replacement usually beats retrofit. A dust hazard analysis (DHA) is the right starting point for that decision.
Why is laser welding better than plasma welding or stitch welding?
Laser welding produces a fully fused seam with no overlap and minimal heat-affected zone, which is why Nordfab can light-test it. Plasma welding is used on Nordfab pipe above 24″ diameter and produces a similar fully-welded result. Stitch welding (welds spaced at intervals along the seam) is what some lower-cost manufacturers use, and it leaves un-welded gaps between welds — small leak paths that compound across the run.
Get a duct layout built around your actual application
If you’re sizing a new dust collection system or evaluating whether your existing duct meets NFPA 660, you get a complete engineered duct layout — sized for your dust, your machinery, and your facility — at no charge as part of any system quote.
Every engineered system we install comes with our pass-or-free compliance guarantee: if the system doesn’t pass its post-installation compliance verification, we fix it for free. The duct material we spec is part of that guarantee.
Or download our NFPA 660 readiness checklist and walk your own facility first.