Why coil specification matters
The single most common commissioning-day disaster on a new HVAC duct fabrication line is wrong coil specification — a buyer who orders the wrong width, wrong thickness, or wrong galvanising weight discovers on the first production run that the machine cannot accept the steel. Wasted commissioning time, missed project deadline, and angry buyer. The correct coil specification is set by the machine manufacturer and must be confirmed before steel is ordered. This guide is the framework for getting the spec right the first time.
Slit coil width by machine type
SBKJ spiral tubeformer (SBTF series)
- SBTF-1500: 137 mm slit coil for standard diameter range (Φ80-1500 mm). Saw-blade cutting variant.
- SBTF-1602: 137 mm slit coil for Φ80-1600 mm. Flying-shear variant for high-volume production.
- SBTF-2020: 137 mm slit coil for Φ100-2000 mm. Large-diameter variant.
- Specialty wider coils: 165 mm or 220 mm available for specific large-diameter or thick-wall applications. Confirm with SBKJ during quotation.
SBKJ auto duct production line (SBAL series)
- SBAL-V: 1,250 mm or 1,550 mm wide coil. The 1,250 mm width is most common; 1,550 mm for very large duct dimensions.
- SBAL-III: 1,250 mm wide coil standard.
- SBAL-II: 1,250 mm wide coil standard.
Other SBKJ equipment
- TDF flange machine: cleat-cut blanks from sheet stock or coil, typically 50-200 mm wide
- Lockformer (Pittsburgh): blanks cut from 1,250 mm wide sheet
- CNC plasma cutter: full sheet input, typically 1,500 × 3,000 mm or 1,500 × 6,000 mm bed
- Hydraulic shear: 1,250 mm or 1,550 mm width capacity
Thickness tolerances
Per ASTM A924 (US standard) and EN 10143 (European standard) and AS/NZS 4791 (Australia/NZ):
- 0.4-0.6 mm: ±0.04 mm tolerance (standard)
- 0.6-0.8 mm: ±0.05 mm
- 0.8-1.0 mm: ±0.06 mm
- 1.0-1.2 mm: ±0.07 mm
- 1.2-1.6 mm: ±0.08 mm
- 1.6-2.0 mm: ±0.10 mm
Restricted thickness tolerance: half the standard tolerance, available from premium mills at 5-15% cost premium. Required for:
- Spiral duct where seam consistency depends on uniform thickness
- TDF flange forming on thin gauges (0.5-0.7 mm) where the rolled flange profile is sensitive to thickness variation
- Cleanroom and pharma fabrication where gauge consistency affects appearance
Gauge correlation (US / metric)
- 26 ga ≈ 0.5 mm
- 24 ga ≈ 0.6 mm
- 22 ga ≈ 0.85 mm
- 20 ga ≈ 1.0 mm
- 18 ga ≈ 1.2 mm
- 16 ga ≈ 1.5 mm
- 14 ga ≈ 2.0 mm
Galvanising weight (Z designation)
Galvanising weight is total coating mass in g/m² (both sides combined):
- Z80 (80 g/m²): light commercial, indoor only, dry climate
- Z140 (140 g/m²): standard commercial HVAC — workhorse specification
- Z180 (180 g/m²): light outdoor or humid indoor
- Z200 (200 g/m²): moderate outdoor / coastal indoor
- Z275 (275 g/m²): severe coastal, exposed outdoor
- Z350 (350 g/m²): marine atmosphere, heavy industrial
- Z450 (450 g/m²): extreme marine
For typical commercial HVAC: Z140 indoor, Z200 coastal indoor or light outdoor, Z275+ outdoor or marine atmosphere. Per ASTM A653 (HDG steel), EN 10346, JIS G 3302, and AS/NZS 4791.
Stainless steel grades
- 304L: low-carbon austenitic. Workhorse for ISO Class 7-9 cleanroom HVAC. Good corrosion resistance, weldability and cost. Density 7.93 g/cm³.
- 316L: low-carbon austenitic with molybdenum. ISO Class 5-6 cleanroom, pharma sterile, chloride-exposed locations. 15-25% premium over 304L.
- Duplex 2205: ferritic-austenitic. Semiconductor wet-process exhaust where standard austenitic is attacked by acid/solvent fumes. Higher strength permits lighter gauge.
Stainless surface finish for HVAC duct:
- 2B: cold-rolled, annealed, pickled, skin-passed. Ra 0.5-1.5 μm. Standard for commercial cleanroom (ISO Class 7-9).
- 2D: matte finish. Lower cost but less common in HVAC.
- BA (bright annealed): high-reflectivity finish. Ra 0.1-0.5 μm. Premium cleanroom (ISO Class 5-6).
- Electropolished (EP): post-fabrication smoothing. Ra < 0.4 μm. Highest cleanroom grade.
Aluminium grades
For aluminium HVAC duct (rare but used in marine, mining and special architectural applications):
- 5052-H32: most common for HVAC. Good corrosion resistance, weldable, formable. Density 2.68 g/cm³.
- 3003-H14: lighter gauge applications, slightly cheaper than 5052.
- 6061-T6: structural applications where higher strength is needed.
Coil packaging and handling
HVAC coil typically supplied:
- Slit coil for spiral tubeformer: small spool 200-500 kg, OD typically 600-900 mm, ID 508 mm (20 in) or 460 mm (18 in) — confirm decoiler ID compatibility
- Wide coil for SBAL-V: master coil 5-15 tonnes, OD 1,800-2,400 mm, ID 508-610 mm — requires 5+ tonne overhead crane and powered decoiler
- Sheet stock for plasma cutter: 1,250 × 2,500 mm or 1,500 × 3,000 mm sheets, palletised, accessible by forklift
Coil ID (inside diameter) compatibility with the decoiler is critical. Standard mill IDs are 508 mm (20 in) and 610 mm (24 in). SBKJ decoilers are configured for 508 mm by default; specify 610 mm at quotation if local mills only supply that ID.
Material standards
- ASTM A653: hot-dip galvanised steel sheet — North American standard
- ASTM A924: general requirements for steel sheet (tolerances, dimensions)
- EN 10346: continuously hot-dip coated steel flat products — European harmonised
- EN 10143: tolerances on dimensions and shape
- JIS G 3302: hot-dip zinc-coated steel sheets — Japanese standard
- AS/NZS 4791: hot-dip galvanised steel sheet — Australia/NZ
- EN 10346: continuously hot-dip galvanised steel — global standard
- IS 277: galvanised steel sheets — Indian standard
Major coil mills (global)
- Europe: ArcelorMittal (multiple plants), Tata Steel Europe (UK, Netherlands), Thyssenkrupp (Germany), Salzgitter (Germany), SSAB (Scandinavia), Voestalpine (Austria)
- Asia: Nippon Steel (Japan), JFE Steel (Japan), Kobe Steel (Japan), POSCO (Korea), Hyundai Steel (Korea), CSC (Taiwan), Tata Steel India (TSL), JSW Steel (India)
- North America: Cleveland-Cliffs, US Steel, Nucor (USA), ArcelorMittal Dofasco (Canada), Stelco (Canada)
- Latin America: ArcelorMittal Brasil, CSN (Brasil), Ternium (Mexico, Argentina)
- Australia / NZ: BlueScope Steel (Zincalume, Galvabond), New Zealand Steel
- Middle East: Hadeed (Saudi Arabia), Emirates Steel (UAE), Esmark (Egypt)
- Africa: ArcelorMittal South Africa, Mittal Steel
Local distributor model
HVAC duct fabricators rarely buy direct from mills (mill MOQ is typically 100-1,000 tonnes per order). Instead, fabricators buy from local steel service centres / slitting houses that:
- Buy master coil from mills
- Slit to machine-specific widths (137 mm slit coil for SBTF, etc.)
- Stock common gauges and galvanising weights
- Deliver in lot sizes from a few tonnes upward
- Provide mill certificates traceable to original heat
For SBKJ machine deployments, the local steel service centre / slitting house relationship is critical to ongoing operations. Identify the supplier before commissioning, confirm slit coil capability (137 mm for SBTF in particular), and establish credit terms.
Common coil specification mistakes
- Wrong slit width for spiral tubeformer: ordering 1,250 mm sheet when SBTF needs 137 mm slit coil. Day-one disaster.
- Wrong ID for decoiler: decoiler configured for 508 mm ID, mill supplies 610 mm. Coil cannot be loaded.
- Wrong thickness tolerance: ordering standard tolerance for SBTF spiral seam consistency, getting variable thickness within the standard ±0.05 mm — seam quality varies. Specify restricted tolerance.
- Wrong galvanising weight: Z80 ordered for coastal application; corrosion within 6-12 months. Specify Z200 minimum for coastal.
- Wrong stainless grade: 304 specified (with high carbon) instead of 304L for welded HVAC. Heat-affected zone sensitisation causes intergranular corrosion at welds.
- Mismatched coil weight to decoiler capacity: 5-tonne master coil delivered to a 2-tonne decoiler. Cannot be loaded without splitting.
- Mill heat traceability missing: cleanroom or Aramco project requires mill cert per heat — coil delivered without heat number documented. Project rejects the material.
SBKJ coil compatibility documentation
Every SBKJ machine quotation includes a coil specification page listing:
- Required slit width (or sheet width for SBAL-V)
- Acceptable thickness range
- Acceptable coil ID for decoiler
- Acceptable maximum coil weight
- Standards reference (ASTM/EN/AS-NZS as appropriate)
- Recommended galvanising weight for the deployment region
Send this page to your local steel service centre when sourcing material. Following the spec eliminates commissioning-day surprises.
Get an SBKJ machine quote with coil specification →
FAQ
What is slit coil and why does it matter?
Slit coil is master coil cut into machine-specific widths. SBTF tubeformers need 137 mm slit coil; SBAL-V auto duct lines accept 1,250-1,550 mm wide coil. Wrong width = day-one commissioning disaster.
What thickness tolerance applies?
Per ASTM A924 / EN 10143 / AS-NZS 4791: ±0.04 to ±0.10 mm depending on thickness. Restricted tolerance (half) available at 5-15% premium for spiral seam consistency.
What galvanising weight should I specify?
Z140 for standard commercial. Z200 coastal/light outdoor. Z275+ severe outdoor/marine. Per ASTM A653, EN 10346, AS/NZS 4791.
What surface finish for HVAC duct?
Regular spangle for concealed duct. Minimised spangle for exposed architectural. 2B stainless for ISO 7-9 cleanroom, BA or electropolished for ISO 5-6.
Where do I source coil?
Local steel service centres / slitting houses buy from mills (ArcelorMittal, Nippon Steel, POSCO, BlueScope etc.) and slit to machine-specific widths. Establish local supplier before commissioning.
