When Standard Profiles Don’t Fit
Off-the-shelf duct sizes work for new construction — the architect designs the building around standard dimensions. Retrofit is different. You’re working inside an existing structure with fixed obstructions: columns where the duct should run, beams at the wrong elevation, existing services occupying the obvious route. The duct must adapt to the building; the building will not adapt to the duct. Custom square duct — fabricated to your specific cross-section dimensions, not selected from a catalog — solves the problem that standard sizes cannot.
Our fabrication process starts with your space dimensions and airflow requirement, not a catalog of pre-existing profiles. Any width, any height, any aspect ratio — within the limits of PP/PVC/PPS sheet fabrication (up to 1,500 mm side dimension). Standard profiles up to 1,000 mm are injection-molded where the mold exists; all other sizes and aspect ratios are fabricated from sheet with full-penetration hot-gas extrusion welding. See our PP Square Duct main page for standard product specifications.
The Custom Fabrication Process
Custom duct is engineered per order. The process is different from selecting a standard product — it requires dimensional input, material selection, reinforcement design, and quality verification:
- You provide the constraint. Available space dimensions (width × height × length), airflow requirement (m3/h), negative pressure specification (Pa), and chemical exposure conditions. We work backward from your constraint to the duct cross-section that maximizes airflow within the available space.
- We calculate the cross-section. Aspect ratio, hydraulic diameter, pressure drop per meter, and required wall thickness. The output is a duct schedule: cross-section dimensions, wall thickness, reinforcement spacing, and flange locations.
- We fabricate. PP/PVC/PPS sheet cut to developed width, rolled or bent to form the rectangular cross-section, longitudinal corner welds, transverse butt welds for section joining. External rib stiffeners welded at calculated spacing. Flanges welded at ends and equipment connection points.
- We inspect. Dimensional check against the approved drawing. Weld visual inspection (full penetration, no voids, no undercut). Flange face flatness check. Vacuum box test on longitudinal seams for critical exhaust applications.
When Custom Duct Is the Only Option
| Situation | Why Custom | Example |
|---|---|---|
| Retrofit into existing building | Standard sizes won’t clear obstructions | 700 × 180 mm duct to fit between existing cable tray and sprinkler main in a 1950s factory |
| Equipment-matching connections | Equipment nozzle dimensions don’t match standard duct sizes | Scrubber inlet flange is 920 × 620 mm — no standard duct matches. Custom duct with matching flange connects directly. |
| Irregular cross-section | The available path is not rectangular — it’s L-shaped, stepped, or tapered | Duct rising through a triangular corner behind an elevator shaft requires a custom-formed cross-section |
| Multi-function plenum | The duct also serves as a distribution plenum with multiple take-offs at non-standard locations | Custom plenum box with 6 branch connections at specified positions along the length |
| Aspect ratio beyond standard | Required ratio exceeds what standard profiles can provide | 1,200 × 120 mm duct (10:1 ratio) for under-floor exhaust extraction in a semiconductor fab |
Square-to-Round and Other Transitions
Custom duct often requires transition pieces that connect rectangular sections to round equipment or duct. These are fabricated as part of the same order:
- Square-to-round transitions. Rectangular duct connecting to a round fan inlet or scrubber nozzle. The transition length is designed to prevent flow separation — typically 2-3× the difference between rectangular width and round diameter, with a taper angle under 30deg.
- Concentric and eccentric transitions. Concentric (centerline maintained) for vertical runs. Eccentric (one face flat) for horizontal runs where the duct bottom must remain level or where liquid pooling must be avoided.
- Multiple branch connections. Custom-fabricated plenum boxes with welded branch stubs at specified positions and angles. Eliminates field-fabricated tap connections that are difficult to make leak-tight.
For standard connection components, see our Air Duct Connections & Flanges and PP Duct Elbow pages.
Send your space constraints, airflow requirements, and a sketch of the installation area to xicheng023@outlook.com. We’ll provide a custom duct design, pressure drop calculation, and quotation within 24 hours. Custom-fabricated exhaust duct design considers OSHA 29 CFR 1910.94 ventilation requirements for industrial exhaust system integrity. WhatsApp: +86 18927456906.
