Why fittings deserve their own production strategy
On a typical commercial HVAC project, fittings are 25-40% of total ductwork by linear metre but absorb 50-70% of fabrication labour because each fitting is unique to its location and orientation. Straight duct sections are produced in standard lengths on SBAL-V or spiral tubeformer; fittings are made one at a time from cut blanks and folded panels. Optimising fitting fabrication is therefore the single biggest leverage point for HVAC shop productivity.
Round duct fittings
Round elbow construction
Round HVAC elbows are constructed from mitred segments welded or lockseamed together. Common configurations:
- 3-piece (90° quick elbow): 2 mitred segments plus 2 end pieces. R = 0.5×D. Used in tight spaces; high pressure drop (Co ≈ 0.6-0.9).
- 5-piece: 4 mitred segments plus 2 end pieces. R = 1.0×D. Most common in commercial HVAC. Co ≈ 0.31-0.43.
- 7-piece: 5 mitred segments plus 2 end pieces. R = 1.5×D. Lower pressure drop. Co ≈ 0.21-0.27.
- Smooth (pleated): continuously curved bend. R = 1.5×D or larger. Lowest pressure drop (Co ≈ 0.11-0.15) but requires specialised equipment.
Mitred segments are fabricated by cutting circular pipe sections at the appropriate angle on a saw or plasma table, then joining the cuts on a gore-locker (e.g. SBKJ SBEM-1250) which forms a Pittsburgh-style lockseam between adjacent segments. The resulting elbow has a continuous lockseam following the helix of the bend.
Round reducers
Concentric (centreline-aligned) and eccentric (one-side-aligned) reducers are fabricated by rolling a flat pattern blank into a cone shape and lockseaming the longitudinal seam. Reducer angle: 30° total included angle is standard for low pressure drop; 45° acceptable for tight installations; over 60° creates significant turbulent loss. SBKJ supplies oval roller and ovalizer machines that handle reducer forming up to Φ1,500 mm.
Round branch takeoffs
- 45° lateral tap: branch enters main at 45°. Lowest pressure drop. Standard for supply-side branches.
- 90° straight tap: branch enters perpendicular. Higher pressure drop but simpler to fabricate. Acceptable for return and short branches.
- Conical tap (bellmouth): branch starts at smaller diameter, expands into the main. Very low pressure drop; specialty fitting.
- Bullhead tee: two equal branches off a main, both perpendicular. Used in symmetric supply distribution.
Round end caps and special fittings
End caps are flat or domed. Domed end caps are formed on a press brake or hydraulic shaping press. Other special fittings include offsets (parallel displacement), saddle taps (branch onto an existing main), and transition pieces (round to rectangular or vice versa).
Rectangular duct fittings
Rectangular elbow construction
- Square-throat without vanes: simplest construction, just four flat panels welded or seamed at corners. High pressure drop (Co ≈ 1.2-1.4). Used only for low-velocity return air.
- Square-throat with turning vanes (TDC): aerofoil-shaped vanes inserted at the bend. Pressure drop reduced to Co ≈ 0.18-0.35. Most common rectangular elbow in commercial HVAC.
- Radius elbow (smooth curve): curved inner and outer panels. Lower pressure drop than square-throat (Co ≈ 0.15-0.25 at R/W=1.5). Higher fabrication labour because curved panels require press-brake forming or roll-bending.
- Mitred elbow with vanes: hybrid construction, 45° mitre at corner with internal vanes. Compromise between square-throat and radius.
Rectangular reducers (transitions)
Concentric and offset transitions reduce or change cross-section. The flat pattern is cut on the CNC plasma cutter from sheet stock, folded on a press brake or pneumatic folding machine into the four-sided trapezoidal shape, and seamed at the corners. SBKJ supplies hydraulic folding machines (sblf series) that handle large-format reducer panels.
Rectangular branch takeoffs
Most rectangular branch takeoffs are 45° or 90° rectangular branches with a connecting box (saddle) for installation onto the main duct. Conical-throat takeoffs (with airfoil splitter) reduce pressure drop on the branch side but require specialised fabrication.
Pressure drop dominates fitting selection
Approximate dynamic loss coefficients (Co) at standard conditions, from ASHRAE Duct Fitting Database (DFDB):
- Square-throat elbow no vanes: Co ≈ 1.2-1.4
- Square-throat elbow with vanes: Co ≈ 0.18-0.35
- Radius rectangular elbow R/W=1.5: Co ≈ 0.15-0.25
- Radius rectangular elbow R/W=1.0: Co ≈ 0.30-0.45
- Round 5-piece elbow: Co ≈ 0.31-0.43
- Round 7-piece elbow: Co ≈ 0.21-0.27
- Round smooth elbow R/D=1.5: Co ≈ 0.11-0.15
- Round 45° lateral tap (branch): Co ≈ 0.40-0.60 on branch side
- Round 90° tap: Co ≈ 0.80-1.20 on branch side
- Concentric reducer 30° total angle: Co ≈ 0.05-0.10
- Concentric reducer 45° total angle: Co ≈ 0.10-0.20
Selecting low-pressure-drop fittings on the supply side typically saves 15-25% of fan energy over the system life. For a 50 MW hyperscale data centre cooling system, a 5% fan energy reduction can save USD 200-400K/year — easily justifying premium fitting selection.
Gauge selection for fittings
Fittings are typically fabricated one gauge heavier than the connected straight duct because they experience higher local stress and turbulence. Per SMACNA tables:
- Straight duct 22-ga at +2 in.wg ⇒ fittings at 22-ga or 20-ga
- Straight duct 20-ga at +2 in.wg ⇒ fittings at 20-ga or 18-ga
- Reinforcement spacing on fittings is typically 50% of straight duct spacing on the same pressure class
Equipment for efficient fitting fabrication
Round duct fittings
- SBEM gore-locker (SBEM-1250): Pittsburgh-style lockseam between mitred segments. Diameter range Φ100-1,250 mm. SBEM-1250 product page.
- SBTF spiral tubeformer: short pipe sections for elbow segments and transition pieces.
- Ovalizer (SBHT-3100): oval-to-round transitions and oval reducers.
- Hoop machine (SBKT-12): ring forming for elbow ends and large-diameter reinforcement.
- Plasma cutter: blank cutting for elbow segments and reducer patterns.
Rectangular fittings
- SBAL-V auto duct line: handles straight rectangular duct at 1,500-2,500 m²/day. Many fittings can be produced via panel cutting on the SBAL-V's integrated plasma.
- CNC plasma cutter: high-throughput blank cutting from sheet stock.
- Hydraulic folding machine (SBLF series): large-format folding for transition panels and elbow components.
- Pneumatic folding machine: smaller batch folding for fittings.
- TDF flange machine: terminal flanges on fittings.
- Drive cleat former: transverse seams between fitting sections.
- Auto corner mounting: corner installation on rectangular fittings.
- Welding station: continuous welding for cleanroom and high-pressure fittings.
BIM-to-CAM workflow
Modern HVAC fabrication shops typically use CAM software that generates fabrication-ready files directly from BIM models. The workflow:
- HVAC engineer creates Revit / AutoCAD MEP model with all duct routing
- Model exports to CAM software (East Coast SheetMetal, Quickpen, Pipefitter, FastTrak)
- CAM software generates flat-pattern blank cutting files (DXF) for CNC plasma cutter
- CAM generates bend programs (G-code) for folding machine
- CAM generates a fabrication ticket (BOM, drawings, label) for each fitting
- Fabricator scans ticket, cuts blanks, folds panels, assembles fitting
- Each fitting carries a label tying it back to the BIM model and project location
BIM-to-CAM workflow reduces fabrication labour by 30-50% on complex projects (data centre, semiconductor cleanroom, hospital), eliminates layout errors and shortens project lead time. SBKJ machines accept standard DXF and the major CAM proprietary formats.
Common fitting fabrication mistakes
- Wrong elbow R/D ratio: specifying 5-piece on supply side increases fan energy by 5-10% over 7-piece.
- Square-throat without vanes on supply: fan energy penalty of 15-30% — always specify turning vanes on supply.
- Concentric reducer at wrong angle: 60°+ angle creates separation and high pressure drop. Use 30° standard or 45° in tight spaces.
- Branch takeoff too close to main elbow: turbulent flow into the takeoff. Maintain at least 3-4 duct diameters between elbow and takeoff.
- Forgetting reinforcement on fittings: fittings need reinforcement at half the spacing of straight duct.
- Skipping BIM-to-CAM workflow: manual layout and cutting on complex projects is slower and error-prone.
- Wrong gauge on fittings: using same gauge as straight duct rather than one gauge heavier.
Get an SBKJ fittings fabrication equipment quote →
FAQ
What is the difference between a 5-piece and 7-piece elbow?
5-piece (4 mitred segments + 2 ends) gives R=1.0×D and Co ≈ 0.31-0.43. 7-piece (5 mitred segments + 2 ends) gives R=1.5×D and Co ≈ 0.21-0.27. 7-piece has 30-40% lower pressure drop.
How is a rectangular duct elbow fabricated?
Square-throat with turning vanes (most common, Co ≈ 0.18-0.35), radius elbow (curved panels, Co ≈ 0.15-0.25), or mitred elbow with internal vanes. Square-throat is easier to fabricate; radius has lower pressure drop.
What pressure drop should I expect?
See ASHRAE Duct Fitting Database. Range from Co ≈ 0.05 (concentric reducer 30°) to Co ≈ 1.4 (square-throat elbow no vanes). Selecting low-Co fittings on supply saves 15-25% fan energy.
What equipment do I need?
Round: gore-locker (SBEM-1250), spiral tubeformer, ovalizer, hoop machine, plasma cutter. Rectangular: SBAL-V, plasma cutter, folding machine, TDF flange, drive cleat former, welding station.
Can fittings be CAM-driven from BIM?
Yes. Modern shops use East Coast SheetMetal, Quickpen, Pipefitter, FastTrak to export from Revit and generate DXF blank files plus bend programs. Reduces fabrication labour by 30-50% on complex projects.
