Description
Above Injection Molding: The Fabricated Range
Beyond DN1000, injection molding is no longer feasible — the mold size, clamping force, and polymer cooling time make it impractical. This is the domain of fabricated duct: PP, PVC, or PPS sheet rolled to diameter and welded with a longitudinal seam using hot-gas extrusion welding. The manufacturing method changes. The quality requirements do not.
Fabricated large-diameter duct is not inherently inferior to injection-molded — it’s different, and the difference must be managed. A properly fabricated DN1200 duct with full-penetration welds, proper circularity control, and adequate reinforcement will perform reliably for 10-15 years. A poorly fabricated one — with incomplete weld penetration, excessive ovality, or inadequate stiffening — will leak at the seam, collapse under vacuum, or both. The quality is in the welding, the reinforcement engineering, and the inspection. See our PP Round Duct main page for smaller diameter injection-molded options.
DN1000-DN1500 Capacity Table
| DN | Airflow at 10 m/s | Airflow at 14 m/s | Fabrication Method |
|---|---|---|---|
| DN1000 | 28,260 m3/h | 39,570 m3/h | Injection molded (standard) or fabricated (budget) |
| DN1100 | 34,200 m3/h | 47,880 m3/h | Fabricated — sheet rolled, longitudinal weld |
| DN1200 | 40,700 m3/h | 56,980 m3/h | Fabricated — sheet rolled, longitudinal weld |
| DN1300 | 47,760 m3/h | 66,860 m3/h | Fabricated with external rib reinforcement |
| DN1400 | 55,390 m3/h | 77,540 m3/h | Fabricated with external rib reinforcement |
| DN1500 | 63,580 m3/h | 89,010 m3/h | Fabricated with external rib reinforcement; multi-piece circumference for larger |
These capacities assume a single duct. For air volumes above 90,000 m3/h, multiple parallel ducts are typically more cost-effective than a single duct above DN1500 due to fabrication complexity and shipping constraints.
What Matters in Fabricated Large-Diameter Duct
When you cannot injection-mold, you must fabricate. When you fabricate, three factors determine whether the duct performs like a molded product or fails prematurely:
- Weld penetration — full thickness, full length. The longitudinal weld is the defining feature of fabricated duct. A proper hot-gas extrusion weld uses PP welding rod of the same material grade as the parent sheet, applied at 220-280degC with precise speed and pressure control. The result should be a weld bead with full penetration through the sheet thickness, with a smooth internal profile — no gaps, no voids, no undercut. We ultrasonic-test longitudinal welds on DN1200 and above as standard quality verification.
- Circularity control. Sheet rolling produces a cylinder that is approximately circular. The final circularity depends on the rolling technique, the number of roll passes, and the sheet thickness. For duct sections that will be flange-connected, the ends must be truly circular — within 0.5% of diameter — to ensure uniform gasket compression. We use internal mandrels during flange welding to maintain end circularity.
- External rib reinforcement. Large-diameter duct under negative pressure needs stiffening. The flat or slightly curved surface between supports acts like a panel under external pressure — the larger the unsupported area, the lower the buckling pressure. External PP ribs, welded to the duct exterior at calculated spacing, increase the buckling pressure by dividing the unsupported span. Rib spacing is engineered for your specific vacuum level and safety factor — not copied from a standard that may not match your conditions. Proper reinforcement is essential for meeting EU Industrial Emissions Directive structural integrity requirements for industrial exhaust ductwork.
Applications for DN1000-DN1500 Duct
These diameters handle the largest air volumes in industrial ventilation — the main headers that collect from entire buildings or process areas:
- Central waste gas treatment plants. Multiple process exhaust streams collected into a DN1200-DN1500 main header feeding a central scrubber or thermal oxidizer. These are typically the largest-diameter ducts on a site — and the most expensive to repair or replace. Getting the fabrication quality right is essential because access for future repair is often limited (the duct runs through occupied areas, above operating equipment, or in confined mechanical spaces).
- Steel mill and foundry exhaust. Sinter plant off-gas, electric arc furnace exhaust, and casting area ventilation — high air volumes at elevated temperature. PPS is specified when exhaust temperature exceeds 80degC. Fabricated PPS duct with external reinforcement handles these conditions where PP would soften and PVC would degrade.
- Power plant FGD. Flue gas desulfurization system ductwork connecting the boiler outlet to the FGD absorber and from the absorber to the stack. These ducts carry saturated flue gas at 50-80degC with SO2, SO3, and chloride content — conditions that destroy carbon steel within 2-3 years of unprotected service. PP duct with external reinforcement provides corrosion immunity without the cost of high-alloy steel or rubber lining.
- Tunnel and underground ventilation. Road and rail tunnel ventilation shafts — large diameters (DN1200-DN1500+) for emergency smoke extraction and normal ventilation. PP with fire-retardant additives or PPS for sections requiring fire rating per NFPA 90A.
Shipping and Site Assembly
DN1000-DN1500 duct sections present logistical challenges that smaller diameters do not. A DN1500 duct section is 1.5 meters in diameter — it occupies most of a standard shipping container’s width. Sections are typically 2-3 meters long for container compatibility. Site assembly requires:
- Field welding. Butt-weld joints between sections performed on site by qualified PP welders. We can provide welding procedure specifications and welder qualification criteria for your local installation contractor, or supply our own installation team.
- Flanged connections at equipment interfaces. Welded flange pairs at scrubber inlets, fan connections, and damper locations allow future disconnection without cutting. Flange faces are machined flat after welding to the duct section.
- Lifting and handling. Large-diameter PP duct is lightweight relative to steel but still requires mechanical lifting for DN1200+. At 3 meters length, a DN1500 PP duct section weighs approximately 100-150 kg — manageable with a small crane or forklift with a spreader bar.
Send your large-diameter duct requirements to xicheng023@outlook.com. We’ll provide an engineered proposal with fabrication details, reinforcement design, and installation support. WhatsApp: +86 18927456906.
