1. Why heavy steel fabrication HVAC is its own engineering discipline
An Australian heavy steel fabrication shop is not a commercial office HVAC job in disguise. It is not a foundry either, although it shares a great deal with one. It is a discrete category in its own right, sitting at the intersection of the welding-fume problem (every welding bay in the country has it), the particulate problem (every cutting table, every grinder, every blast booth has it), the chemistry problem (pickling, passivation, plating, paint), and the heat problem (post-weld heat treatment, stress relief, induction normalising). The shop that builds bridge girders for a state main-roads contract, the boilermaker that pressure-tests a vessel for an alumina refinery, the tank fabricator stamping out atmospheric storage shells for a fuel terminal, and the structural steel workshop pre-assembling a high-rise core all share the same five HVAC demands at once.
Heavy fabrication ductwork lives or dies on five things in parallel. Welding-fume capture at every arc, sized per process and per consumable rather than per square metre of floor. Particulate capture at every cutting, grinding and blast station, sized for the abrasive load. Acid mist and chromium VI control on any in-house pickling, passivation or plating line, with the right materials downstream of the scrubber. Solvent and isocyanate control in the spray paint booth and bake oven, classed to AS/NZS 60079 inside the booth volume. And, where the workshop carries post-weld heat treatment in-house, NFPA 86 oven exhaust on a dedicated stack. Each demand alone is manageable; together they explain why a generic commercial-HVAC contractor who treats a fabrication shop as just another industrial fit-out leaves the job over-promised and the operator non-compliant on day one.
This guide is written for fabricators, consulting mechanical engineers, plant managers, work-health-and-safety advisors and the procurement officers who put projects out to tender. It walks through the regulatory backbone, zones the workshop bay by bay, addresses every fume and dust source with capture geometry and material selection, and closes with the SBKJ machine configuration that gives a duct fabricator the production envelope to serve this market profitably.
2. The Australian regulatory stack — everything that drives the duct design
Australian fabrication HVAC sits at the intersection of about a dozen standards. Ignoring any one is an infringement notice, a SafeWork prosecution, an ISO 3834-2 non-conformance, or a fire-services failure on inspection waiting to happen.
2.1 AS 1668.2 — mechanical ventilation for buildings
AS 1668.2 is the umbrella mechanical-ventilation standard for all building classifications, and Table 4 sets the minimum extract for industrial occupancies including welding, sandblasting, grinding, spray painting, plating and pickling. In a heavy fabrication shop, AS 1668.2 building-volume figures are almost always exceeded by the sum of localised LEV requirements at each individual source. Where AS 1668.2 matters most is the make-up air rule: every cubic metre extracted must be replaced by tempered, filtered air, with the workshop floor at neutral or slightly positive pressure relative to office, NDT and laboratory zones. Where the shop runs in a heavy build-up rule against winter heat loss, supply make-up is direct-fired or indirect-fired gas heating; in tropical Brisbane and Townsville workshops, supply make-up is filtered ambient or chilled-water cooled to keep operator core temperatures inside acceptable bounds. Our AS 1668.2 reference covers the calculation and dispersion criteria in detail.
2.2 AS 1668.1 — fire and smoke control
AS 1668.1 governs fire-mode operation: smoke spill, stair pressurisation, fire dampers, smoke dampers and the controls that drive them. A heavy fabrication shop with a sprinklered roof needs smoke-spill ventilation rated to the AS 1668.1 capacity, and the supply make-up handlers need fire/smoke dampers at every penetration. Fire dampers carry AS 1851 maintenance obligations. AS 1530.4 sets the fire-rated duct construction test method that proves the assembly. Our fire and smoke damper integration reference covers detection, actuation and inspection on these systems.
2.3 AS 4254 — ductwork construction
AS 4254 sets the construction class for ductwork in two parts: Part 1 for flexible duct and Part 2 for rigid duct. Pressure class drives gauge selection, seam selection, reinforcement spacing and leakage class. Most heavy fabrication shop extract sits at AS 4254 medium-pressure (1000 Pa) on welding mains and high-pressure (2500 Pa) on dust-collector inlet trunks; supply make-up is typically low-pressure (500 Pa). The SBAL-V automatic duct line fabricates AS 4254 low-pressure and medium-pressure galvanised rectangular duct without operator intervention beyond coil loading and panel pickup. For high-pressure stainless duct on the chrome and solvent-bearing branches, the SBSF-1525 stitchwelder fabricates AS 4254 high-pressure class leak-tight in 304L or 316L. Our AS 4254 reference walks the class table.
2.4 AS 1554 series — structural welding qualification
AS 1554 is the structural welding qualification series. AS 1554.1 covers general carbon-steel structural welding (the bread and butter of most fabricators). AS 1554.6 covers stainless steel structural welding. AS 1554.7 covers hardfacing — tungsten carbide and chromium carbide overlay for wear-resistant components. The relevance to the HVAC envelope is that the duct system must support the welder-qualification audit trail: each WPS (weld procedure specification) has a fume-generation profile that the LEV has to handle, and each procedure qualification record (PQR) needs the welder to be working under representative ventilation. A WPS run on a duct that pulls fume away from the arc gives a different inspection result than the same WPS run in still air; the practical implication is that the welding bay LEV must run during qualification testing and the test setup must be the same as production.
2.5 AS 1796 and AS 2980 — welder qualification and certification
AS 1796 covers welder qualification certification for pressure equipment, and AS 2980 covers welder qualification for structural steel. Welder certification is renewed periodically and is tied to specific WPS, position and process. The HVAC envelope is incidental to the certification audit, but the welder’s working environment is a SafeWork compliance question. A fabricator that certifies welders under AS 1796 to a high level needs to demonstrate that those welders are working under ventilation that meets the Safe Work Australia welding-fume WES and any tighter consumable-specific limits.
2.6 AS 4458, AS 4037 and AS 1210 — pressure equipment codes
AS 4458 is the pressure equipment design and construction code. AS 4037 covers boiler and pressure equipment classification (Class A, B and C) and operator competency. AS 1210 is the older unfired pressure vessels code, still referenced. For a workshop that fabricates boilers and pressure vessels, these three codes layer on top of AS 1554, AS 1796 and AS 2980 with state plant registration requirements (SafeWork NSW, WorkSafe Victoria, WorkSafe Queensland) and nominated inspector sign-off at design, fabrication, post-weld heat treatment and hydrostatic test stages. The HVAC system has to serve these workshops without contaminating the weld zone, without disturbing helium leak testing on Class A vessels, and without affecting NDT radiograph or ultrasonic inspection. The duct routes are typically segregated from the pressure-equipment fab cells by physical and air-pressure barriers.
2.7 AS/NZS 60079 — hazardous areas
AS/NZS 60079 classifies any zone where flammable vapour or combustible dust can reach explosible concentration. In a fabrication shop, the canonical hazardous areas are the spray paint booth interior (Class I Zone 1), the solvent room and paint mix room (Zone 1 or 2), the area within 1 m of any paint booth opening (Zone 2), and any zone in a workshop that grinds or blasts aluminium, magnesium or titanium fines (Zone 22 dust). The classification drives Ex-rated electrical equipment requirements for fans, motors, instrumentation and lighting on the duct system, and it drives bonding and grounding of every duct segment to prevent static-discharge ignition.
2.8 AS 1940 — flammable and combustible liquids
AS 1940 governs storage and handling of flammable and combustible liquids, including the paint storage room, the solvent decant area and any tank used for thinners, primers and topcoats. Storage volume drives the segregation distance to ignition sources and the fire-rated separation from the rest of the workshop. AS 1940 ventilation requirements are typically continuous extract at 12 air changes per hour minimum, with explosion-protected fans on the discharge side.
2.9 AS 4114 — spray painting booth construction
AS 4114 (in two parts — AS 4114.1 design construction and testing, AS 4114.2 installation and maintenance) sets the booth construction standard for spray painting in Australia. Booth face velocity 0.5 m/s minimum downdraft, plenum airflow uniformity within 20%, exhaust filter at the booth outlet, paint overspray containment to dry filter or wet wash. Our automotive paint booth HVAC guide covers booth construction in detail with adaptations for heavy fabrication context.
2.10 AS 3957 — dust hazard classification
AS 3957 classes industrial dust by ignition risk, explosibility and toxicity. Sandblast dust, fettling dust, grinding dust, plasma cutting fume and welding fume each have a class with corresponding LEV design requirements. AS 3957 Class C dust covers the bulk of fabrication-shop particulate. Class B and Class A apply where the dust is more toxic or more explosible — lead from sandblasting old painted steel, aluminium fines from finishing, magnesium turnings from machining.
2.11 NFPA 660 — combustible dust
NFPA 660 (the consolidated standard published in 2025 to replace the former NFPA 484 combustible metals and NFPA 654 combustible particulate solids) is the de-facto international reference for combustible dust handling and is widely referenced by Australian insurance underwriters. NFPA 660 mandates wet-bath collection for fine aluminium, magnesium and titanium dust, prohibits dry baghouses without engineered deflagration venting, sets the bonding and grounding requirements for ductwork, and drives the isolation-damper layout that prevents a baghouse explosion from propagating back into the workshop. For a heavy fab shop that machines aluminium structural sections or magnesium die-casting tooling, NFPA 660 is the governing document for the grinding and machining LEV.
2.12 NFPA 33 — spray application
NFPA 33 covers spray application of flammable and combustible materials — effectively the spray paint booth. NFPA 33 layers on top of AS 4114 with specific requirements for explosion-protected dampers, LEL monitoring at the exhaust stack, isolation valves between the booth and the bake oven, and an audit trail of paint-line solvent inventory. Together with AS/NZS 60079 it sets the duct-construction envelope on every paint booth exhaust branch.
2.13 NFPA 86 — industrial ovens and furnaces
NFPA 86 governs every gas-fired or electric oven and furnace above 100 degC in industrial service. In a fabrication shop, NFPA 86 applies to post-weld heat treatment (PWHT) and stress-relief ovens, normalising and annealing furnaces, paint bake ovens (where the bake temperature is above the threshold), and aluminium T4 solution heat treatment ovens. The standard mandates LEL monitoring, purge cycles before light-off, explosion venting on the oven shell, and dedicated exhaust risers that do not share trunk capacity with general workshop extract.
2.14 AS 4801 — occupational health and safety management
AS 4801 is the Australian OHS management system standard (broadly aligned with ISO 45001). It sits behind every duct-design decision in the workshop because the HVAC system is the engineering control that delivers compliance with workplace exposure standards. The audit trail required under AS 4801 includes the LEV design calculation, the commissioning report and the routine recommissioning data; the duct designer’s deliverables feed directly into the operator’s AS 4801 documentation.
2.15 AS 1657 — platforms, walkways, stairs and ladders
AS 1657 governs the workshop access platforms and is incidental to HVAC except where the ductwork crosses access routes, where platform access is needed for filter maintenance and for inspection of overhead branches, and where the supply make-up handler sits on a mezzanine. Filter banks, fire dampers, smoke dampers, balancing dampers and inline cleaning ports all need AS 1657-compliant access. SBKJ duct routing on every Australian fabrication-shop fit-out is coordinated with the access platform design from the start.
2.16 AS 1851 — fire damper maintenance
AS 1851 sets routine inspection and maintenance intervals for fire protection equipment including fire and smoke dampers in HVAC ductwork. Annual drop-testing of fire dampers, biannual inspection of smoke dampers, recorded outcome filed against building approval. The duct designer’s deliverable is access panels at every damper to AS 1851 inspection standard.
2.17 Safe Work Australia workplace exposure standards (WES)
Safe Work Australia’s WES is the table that the HVAC designer is actually designing against. The values that matter in a heavy fabrication shop are:
- Welding fume general — 1 mg/m3 TWA
- Hexavalent chromium Cr(VI) — 0.005 mg/m3 TWA (stainless welding)
- Manganese (and inorganic compounds, as Mn) — 1 mg/m3 TWA
- Nickel (metal and insoluble compounds) — 1 mg/m3 TWA
- Iron oxide fume (as Fe) — 5 mg/m3 TWA
- Zinc oxide fume — 5 mg/m3 TWA
- Lead inorganic — 0.05 mg/m3 TWA (sandblasting old painted steel)
- Respirable crystalline silica — 0.05 mg/m3 TWA (sandblast, abrasive blasting with silica media)
- Isocyanate — 0.005 ppm STEL (PU paint hardener)
- Styrene — 50 ppm TWA (GRP composite repairs and lining work)
- MEK — 200 ppm TWA
- Toluene — 50 ppm TWA
- Xylene — 50 ppm TWA
- Ozone — 0.1 ppm TWA (welding by-product, especially TIG on stainless)
- Nitrogen oxides — 3 ppm STEL (welding by-product, plasma cutting)
- Carbon monoxide — 30 ppm TWA (gas-fired ovens, oxy-fuel cutting, holding furnaces)
The design target on every duct in the workshop is 60% of the WES at the breathing zone under worst-case process load — a margin that absorbs filter loading, cartridge change-out drift and unintended infiltration without pushing the operator over the standard.
2.18 ISO 9001 and ISO 3834-2
Most Australian heavy fabricators that work for the major resource and infrastructure tier carry ISO 9001 quality management and ISO 3834-2 welding QMS certification. ISO 3834-2 is the welding-specific QMS standard and includes ventilation and workplace environment as a third-party audit item. A fabricator’s renewal audit will request the LEV design calculation and recent commissioning balance data — the duct designer’s file becomes part of the QMS evidence.
3. Zoning the workshop — sixteen functional groups
Before any duct can be sized, the workshop has to be zoned. A general fabrication shop is not a single ventilation zone, and treating it as one is the most common mistake we see in early-stage tender drawings. The zoning logic that SBKJ uses on every Australian heavy-fabrication-shop fit-out has sixteen functional groups, and every duct run in the building belongs to exactly one of them.
3.1 Steel receiving and stock storage
Raw plate, structural section, pipe stock, plate stock and bar stock arrive at the receiving end and are stored on racks or stillages. The HVAC envelope is general workshop comfort — supply air at 22 to 26 degC depending on season, exhaust to dilute any infiltration. The duct is conventional galvanised on the SBAL-V at low pressure.
3.2 Plate cutting bay — plasma, laser, oxy, waterjet
The plate cutting bay is the highest-precision particulate source in the shop and the heaviest single LEV duty after the sandblast booth. CNC plasma cutting on a 6 to 50 mm plate bed, laser cutting on a 1 to 25 mm plate bed (fibre laser), oxy cutting on 25 to 200 mm plate, and waterjet cutting on a 1 to 200 mm plate bed (no fume but water aerosol). Each technology has its own capture geometry. Section 4 covers it in detail.
3.3 Bandsaw, ironworker and shear
Cold cutting stations — horizontal bandsaw for round and beam stock, ironworker for punching and notching, hydraulic shear for plate — produce minimal fume and limited dust. General workshop ventilation is sufficient. The exception is bandsaw cutting on stainless or duplex where cutting fluid mist drives a localised mist-collector requirement.
3.4 CNC press brake and plate roll
Press brakes and plate rolls are cold forming operations with minimal HVAC demand beyond general workshop ventilation. The exception is heated bending of heavy section, which is a hot work activity that generates fume and is captured under section 7.
3.5 General welding bay — manual stations
The manual welding bay is where the bulk of the fab shop’s welding hours happen. MIG, MAG, FCAW, SMAW (stick) and TIG on individual bench stations or floor-positioned fabrication. Each station is an LEV source. Section 5 covers the welding fume capture in detail.
3.6 Robotic welding cell
Higher-volume operators (Civmec Henderson, BAE Williamstown, ASC Osborne) run robot welding cells for high-repetition work. Enclosed cell with positive door interlocks, integrated fume hood on the robot frame, dedicated extract on a HEPA-bag downstream filter for any chrome-bearing stainless work. Section 5.6 covers it.
3.7 Submerged Arc Welding (SAW) line
SAW is used on heavy-section longitudinal and circumferential welds — bridge girders, pressure vessel shells, tank walls, naval frigate keel plate. The fume mass per metre is the highest of any welding process, and the capture is a dedicated slot hood at 1.0 m/s capture velocity within 200 to 400 mm of the leading edge of the flux blanket. Section 5.4 covers it.
3.8 Pressure vessel and boiler fabrication bay
The pressure vessel bay is segregated from general fab by the AS 4458 audit trail. Heavy-plate stitch welding, pre-heat and PWHT, dimensional inspection, hydrostatic test pad. The HVAC envelope serves the bay without contaminating the weld zone, and the heat-treatment exhaust runs on its own NFPA 86 stack. Section 7 covers it.
3.9 Pipe spool fabrication shop
The pipe spool shop produces process piping for oil and gas, alumina refining, water and wastewater, and brewery and food-processing clients. Stick, TIG and orbital TIG welding on the root pass and hot pass, with internal argon purge gas captured and recovered on the exit side. Section 7.4 covers it.
3.10 Tank fabrication bay — atmospheric storage
Atmospheric storage tanks (water, fuel, chemical, beer wort) are fabricated to AS 1692 (LPG), AS 1657 access, and the API 650 standard (referenced for cross-certified petroleum work). Confined-space welding inside the tank shell drives the LEV envelope. Section 7.7 covers it.
3.11 Sandblast booth and shot blast cabinet
Sandblast booths range from walk-in rooms with the operator inside in PAPR (powered air-purifying respirator) hood, to fully automated cabinet blasting on structural section. Face velocity 5 m/s (1000 ft/min) at the open face per AS 3957 class C; dust mains at 18 to 22 m/s; cartridge filter MERV 16 minimum. Section 8 covers it.
3.12 Acid pickling and passivation line
In-house pickling and passivation is used by stainless and duplex fabricators to remove heat-affected-zone discolouration after welding. HCl or H2SO4 pickle, HNO3 passivation, water rinse. Acid mist drives FRP duct upstream of the scrubber and 304L stainless downstream. Section 9 covers it.
3.13 Hot-dip galvanising plant (where integrated)
Some heavy fab shops carry an in-house galvanising line for structural steel and tank components. Acid pickle, flux bath, zinc spelter at 450 degC, quench. Each step has its own LEV. Section 9.5 covers it.
3.14 Plating shop — zinc, cadmium, nickel, chromium
Plating shops are less common in general fab but appear in workshops that build precision mechanical equipment (Komatsu Wacol, Caterpillar). Chromium VI mist drives the design target to 0.005 mg/m3 hex-chrome WES; stainless 304L duct minimum on the cleaned discharge side. Section 9.7 covers it.
3.15 Spray paint booth and bake oven
The paint booth is the single largest HVAC capital item in the shop. Downdraft at 0.5 m/s, hazardous area Class I Zone 1 inside, AS/NZS 60079, NFPA 33, isocyanate hardener on PU systems. Section 10 covers it.
3.16 NDT laboratory and engineering office
The NDT lab and engineering office are clean zones requiring positive pressure relative to the workshop floor. HEPA-prefiltered supply air, controlled temperature for dimensional inspection. Section 11 covers it.
4. Plate cutting bay — plasma, laser, oxy and waterjet
The plate cutting bay is the heaviest sustained particulate LEV in the shop after the sandblast booth. Each cutting process produces a different fume profile and a different capture geometry.
4.1 CNC plasma cutting
Plasma cuts through plate by ionising a gas stream to 20,000 to 30,000 K and using the resulting plasma jet to melt and blow out the kerf. Air plasma, nitrogen plasma and oxygen plasma each have characteristic fume profiles, with oxygen plasma the cleanest on carbon steel and air plasma the cheapest. Fume is dominated by iron and steel oxide aerosol in the sub-micron range, with a high thermal plume driving natural convection at the cutting head. Local extract is via a downdraft cutting table where the plate sits on a grid above a plenum, and the fume is drawn through the plate. Plenum airflow is sized to 1.5 to 2.5 m3/s per square metre of cutting bed. The duct from the plenum is at 22 to 25 m/s transport velocity to handle the fine particulate. Filtration is MERV 16 cartridge or higher. SBKJ duct for the plenum-to-filter run is on the SBSF-1525 stitchwelder in 304L stainless where the plasma cuts galvanised or zinc-coated stock (zinc oxide fume is corrosive on galvanised duct), and on the SBAL-V in 1.0 mm galvanised on bare carbon-steel plasma work where the fume chemistry is benign to zinc coating.
For workshops that carry in-house duct fabrication alongside their primary structural or pressure vessel business, the SBPC1500 plasma cutter is the SBKJ-supplied cutter that pairs with the rest of the duct fab line. The SBPC1500 cuts the same plate stock that the workshop uses for HVAC sheet metal — flange blanks, gussets, reinforcement plates — and feeds the SBAL-V coil line for finished duct.
4.2 Fibre laser cutting
Fibre laser has displaced CO2 laser across most Australian heavy fabrication over the last decade. Cut quality is better on stainless, throughput is higher on thin plate, and the fume mass is lower than plasma per metre of cut. Capture is by the same downdraft table topology, with airflow 1.0 to 1.5 m3/s per square metre of cutting bed. Filtration is MERV 17 cartridge or HEPA H13 on galvanised stock (zinc fume) and on stainless (chromium and nickel fume). Duct fabrication is the same as plasma. We cover laser cutting extract in our welding and cutting methods reference.
4.3 Oxy-acetylene and oxy-LPG cutting
Oxy cutting persists on plate above 25 mm where plasma struggles and laser cannot reach. The process is dominated by heavy iron oxide fume with a high thermal plume, and the cut is slower than plasma. Local extract is by overhead canopy hood at 0.5 to 1.0 m/s capture velocity at the operator’s working face, ducted at 18 to 22 m/s transport velocity. Filtration is MERV 14 minimum — the fume is coarser than plasma so the filter does not need to go to MERV 16. The duct is on the SBAL-V in galvanised because oxy fume is non-corrosive to galvanising over typical workshop lives. Carbon monoxide is the watch-out: oxy cutting generates measurable CO and the workplace CO concentration must stay below the 30 ppm WES.
4.4 Waterjet cutting
Abrasive waterjet (garnet abrasive in a high-pressure water stream at 300 to 600 MPa) produces no fume and no thermal plume. The HVAC demand is limited to local extract for any aerosol generated above the cutting head, which is modest. The catch tank below the cutting bed catches the garnet and the cut waste, and waterjet is the cleanest of the four cutting processes from a workshop air-quality perspective. The downside is cut speed — slower than plasma or laser — and the additional waste-water handling that goes with the catch tank.
5. Welding fume capture — the dominant LEV duty in any heavy fab shop
Welding is the dominant fume load in every general fabrication, boilermaking and pressure-vessel workshop in the country. Safe Work Australia’s welding-fume WES is 1 mg/m3 TWA, and the design target is 0.6 mg/m3 at the breathing zone for the 60% margin that absorbs cartridge loading and infiltration. Welding on stainless and duplex pulls the design target to 0.003 mg/m3 hex-chrome at the breathing zone — three orders of magnitude tighter than general carbon-steel welding.
5.1 MIG and MAG welding
MIG (metal inert gas, argon shielded) and MAG (metal active gas, CO2 or argon-CO2 shielded) are the highest-volume welding processes in Australian heavy fabrication. The fume rate ranges from 0.5 to 2 g/min per gun depending on wire feed rate, shielding gas and consumable chemistry. On-torch fume extraction is the standard solution on every new build: a vacuum-extraction nozzle integrated into the torch head captures the fume at 50 to 100 L/s per gun within 50 mm of the arc. Where on-torch extraction is impractical (heavy-section work, restricted access, multi-pass weld, cellular geometry inside a pressure vessel), the backup is a slot hood within 200 to 400 mm of the arc at 0.5 to 1.0 m/s capture velocity, or an overhead canopy hood at 0.4 m/s capture velocity for static welding bays where the operator is stationary. Articulated extraction arms (the Plymovent, Nederman and Kemper systems are the common AU brand choices) at 1.5 to 2.5 m reach are deployed at every manual welding bay where the operator moves around the workpiece. Branch material is galvanised on the SBAL-V where the fume is iron-oxide dominated; stainless 304L on the SBSF-1525 where the consumable carries chrome or nickel.
5.2 TIG welding
TIG (tungsten inert gas) is used for fine work on stainless and aluminium, for root passes on pipe and pressure vessels, and for any weld where appearance and cleanliness matter. TIG produces less fume than MIG/MAG but the fume is finer (sub-micron) and contains more chromium and nickel on stainless work. On-torch extraction at 100 to 200 L/s per gun or a slot hood at 0.5 m/s capture velocity within 300 mm of the arc. Stainless welding requires the duct to be in 304L stainless construction because chromium and nickel fume condense on duct walls and produce a corrosion risk over time. Ozone is a TIG by-product, especially on aluminium TIG; the 0.1 ppm WES is the design target. Our welding methods reference covers TIG capture and post-weld pickling.
5.3 Stick welding (SMAW) and FCAW
Shielded metal arc welding (stick, SMAW) and flux-cored arc welding (FCAW) produce 2 to 3 times the fume mass per metre of solid-wire MIG. Stick is the workhorse of the boilermaker on heavy plate and is still common on structural steel site work. FCAW is used on positional welding inside pressure vessels and tanks where the self-shielding flux core allows welding in tight access. Both processes need stronger capture than MIG: 1,000 to 1,500 L/s per slot hood at 0.5 to 1.0 m/s capture velocity within 200 to 400 mm of the arc. Articulated arm extraction at 2 to 3 m reach. Branch transport velocity 18 to 22 m/s. The bulky particulate from stick welding (slag flake from the electrode coating) is heavier than MIG fume and the duct must be sized for the heavier load. Manganese content in stick electrodes (E7018, E11018) is high and the manganese WES is the controlling exposure limit.
5.4 Submerged Arc Welding (SAW)
SAW is used on the long longitudinal and circumferential welds where mass deposition matters — pressure vessel shells (Saunders International in Sydney, Civmec in Henderson), atmospheric storage tank walls, bridge girder webs and flanges, ship and submarine hull plate. The arc is buried under a granular flux blanket that masks the visible arc and absorbs most of the radiant heat, but the fume mass per metre of weld is the highest of any welding process. Capture is by a slot hood within 200 to 400 mm of the leading edge of the flux blanket at 1.0 m/s capture velocity, with a parallel vacuum recovery system collecting the trailing un-melted flux for recycling. SAW extract is 300 to 500 L/s per slot hood with a duct transport velocity of 22 m/s to handle the iron oxide and silicate particulate. The riser is on SBSF-1525 stainless in 600 to 800 mm diameter on chrome-bearing consumables, or SBAL-V galvanised on carbon-steel SAW. Twin-wire and triple-wire SAW (used for the fastest deposition on tank and vessel shells) produces 2 to 3 times the fume mass and pulls the capture sizing accordingly.
5.5 Stainless and duplex welding — the hex-chrome problem
Welding on 304, 316, 2205 duplex and 2507 super-duplex stainless generates hexavalent chromium Cr(VI) fume. Safe Work Australia’s WES for Cr(VI) is 0.005 mg/m3 TWA, three orders of magnitude tighter than general welding fume. The design target at the breathing zone is 0.003 mg/m3. The implications are significant: every stainless welding station needs dedicated on-torch extraction or close-coupled slot-hood capture, the duct is stainless 304L minimum, the cartridge filter is HEPA H13 or H14, and the discharge stack is monitored against the relevant EPA Cr(VI) limit for the state. Nickel content in stainless adds the 1 mg/m3 nickel WES on top. The stainless welding bay is the highest-spec LEV in the workshop and the duct fabrication is on the SBSF-1525 stitchwelder in 304L with handheld laser finishing on the visible side using the SBLR-600 handheld laser welder.
5.6 Robotic welding cells
Higher-volume operators (Civmec Henderson on naval frigate hulls and submarine modules, BAE Williamstown, ASC Osborne) run robot welding cells for high-repetition longitudinal and stitch welds. The cell is an enclosed booth with positive door interlocks, an integrated fume hood on the robot frame, and a dedicated extract that runs continuously during the cell duty cycle. Cell extract sizing is 800 to 2,500 L/s per cell depending on weld density and dwell. Duct construction is the same as manual MIG — SBAL-V galvanised on carbon steel, SBSF-1525 stainless on chrome-bearing work. Downstream HEPA filtration is mandatory on stainless work; cartridge filter on carbon steel is sufficient. The cell controller ramps the extract from idle (200 L/s) to peak when the robot starts a weld and ramps back down on completion, with a 30-second tail to clear retained fume. VFD-controlled demand reduces fan energy by 40 to 60% across the cell duty cycle.
5.7 Filtration and discharge for welding extract
The captured fume cannot simply be discharged. AS 1668.2 dispersion criteria require filtration to limit ground-level concentration at the building boundary. The default for welding fume is a cartridge filter bank rated MERV 14 minimum, MERV 16 on carbon-steel work and HEPA H13 on stainless. Cartridges are pulse-jet cleaned on a programmed cycle, with the dislodged dust dropping into a sealed drum for disposal. Discharge is at roof level via the spiral riser, with stack height set 3 m above the eaves, discharge velocity 12 to 15 m/s, and stack-to-intake separation at least 8 m. Where the workshop is in a residential-adjacent zone (Brisbane inner-suburban fab shops, inner-Melbourne legacy industrial), the discharge needs to be modelled against EPA SEPP air-quality criteria to confirm acceptable downwind concentration.
6. Grinding, fettling and surface preparation
Grinding follows almost every welding and cutting operation and produces a continuous dust load that has to be captured at source. The grinding wheel speed (typically 80 m/s for resin-bonded wheels) atomises the metal into a fine dust at the wheel periphery, with the heaviest dust falling within 1 m of the wheel and the finest fraction carrying 5 to 10 m on natural convection. Local extract is via a slot hood at 0.5 m/s capture velocity within 300 mm of the wheel, ducted at 18 to 22 m/s transport velocity. Filtration is MERV 14 cartridge. Iron oxide fume WES is 5 mg/m3 TWA, which is more permissive than welding fume, but the dust load is continuous rather than intermittent and the time-weighted exposure is what matters. Stainless grinding generates Cr(VI) and the design pulls to the hex-chrome WES.
Hand-held angle grinder work is the hardest to capture because the operator and the work move together. The practical solution is a downdraft bench (workpiece on a perforated table with the dust drawn down into a plenum) or a backdraft bench (workpiece on a flat table with the dust drawn back into a slot hood at the rear of the table). Face velocity 0.5 to 0.7 m/s across the working aperture; transport velocity 18 to 22 m/s in the branch. The capture works only if the operator is upstream of the air movement, and an effective fettling station includes operator-position marking on the floor.
For workshops that machine aluminium, magnesium or titanium components alongside their primary steel work, NFPA 660 combustible metal dust risk applies. Aluminium fines from grinding 5xxx and 7xxx series alloys are explosible at concentrations above 30 g/m3. The dedicated capture for these materials is a wet-bath dust collector with bonded and grounded duct on every segment. Spark-resistant duct construction (no internal protrusions, smooth wall transitions, non-ferrous fasteners on critical zones) is mandatory under NFPA 660 for the combustible-metal-dust branch.
7. Pressure vessel and tank fabrication bays
Workshops that fabricate boilers, pressure vessels and atmospheric storage tanks operate under a separate regulatory stack from general steel fab, and the HVAC envelope reflects that. AS 4458 design and construction, AS 4037 boiler classification, AS 1210 unfired pressure vessels, AS 1554 welding qualification, AS 1796 and AS 2980 welder qualification, state plant registration (SafeWork NSW, WorkSafe Victoria, WorkSafe Queensland) and nominated-inspector sign-off at design, fabrication and pressure-test stages. American Petroleum Institute API standards (API 650 atmospheric storage tanks, API 510 pressure vessel inspection, API 570 piping inspection, API 653 tank inspection) are cross-referenced on workshops that export to oil and gas markets or work on cross-certified imports.
7.1 Pressure vessel fabrication bay layout
The PV bay sits on a dedicated cell of the shop with a clear span overhead crane (typically 20 to 100 tonne for shop-built work, with site work up to 1,000 tonne) and a vessel-rotation positioner to roll the shell during longitudinal and circumferential welding. The HVAC envelope is heavier than general fab because the welding hours per square metre of bay are higher: a Class A vessel under AS 4458 carries full radiographic inspection, full PWHT and a hydrostatic test pad, and the welding programme is dense. SAW on the longitudinal seams (section 5.4), FCAW or stick on the heavy-section root passes (section 5.3), TIG root pass on stainless cladding (section 5.2), and overhead canopy fume extraction to capture the rising plume from the heated work.
7.2 Pre-heat and post-weld heat treatment (PWHT)
Heavy-wall pressure vessels need pre-heat to 100 to 200 degC before welding to control hydrogen cracking, and PWHT at 600 to 700 degC after welding to relieve residual stress. Pre-heat is typically with gas-fired torches or induction blankets and generates moderate radiant heat with combustion-product fume (carbon dioxide, water vapour, modest CO if the burner is poorly tuned). The HVAC response is overhead canopy at 0.4 m/s capture velocity over the pre-heat zone. PWHT is a different problem: the vessel is fully insulated and the oven (or local box furnace) is at full temperature for 4 to 16 hours. NFPA 86 governs the oven design, with dedicated stack discharge, LEL monitoring on the stack, and explosion venting on the oven shell. The duct on the PWHT stack is refractory-lined mild steel for the first 5 m above the oven, transitioning to insulated carbon steel further downstream. Section 12 covers heat-treatment exhaust in detail.
7.3 Radiography and ultrasonic inspection
Class A and B pressure vessels and many boilers require 100% radiographic inspection of butt welds. Industrial radiography uses X-ray sources (up to 450 kV) and gamma sources (Iridium-192, Selenium-75) in shielded rooms or with mobile shielding curtains. AS/NZS 2243.4 governs the radiation protection envelope. The HVAC response is segregated exhaust: the radiography room has its own air-tight envelope at slight negative pressure to ensure that no radioactive contamination (in the case of gamma source handling) can migrate into the rest of the workshop. Ozone produced by X-ray equipment is a watch-out, with the 0.1 ppm WES driving the extract rate inside the radiography room. Ultrasonic testing is non-hazardous and uses ambient comfort ventilation.
7.4 Pipe spool fabrication
The pipe spool shop produces process piping for oil and gas (Worley, Monadelphous Worley clients), alumina refining and mining infrastructure, water and wastewater treatment plants, and brewery and food-processing clients. Stick, TIG and orbital TIG welding on the root pass with internal argon or argon-helium purge gas, and stick or FCAW on the fill and cap passes. The HVAC envelope is dense: each fab station carries its own articulated arm extraction at 1.5 to 2.5 m reach, the internal purge gas is recovered on the exit side of the pipe (a separate small-bore extract on the open end of each spool), and the welding bay extract is sized for the sum of all stations running concurrently. Material is SBAL-V galvanised on carbon-steel pipe spool work and SBSF-1525 stainless on stainless and duplex.
7.5 Boilermaking shop
Boilermaking is the heaviest single welding duty in the workshop on a fume-mass basis. Stick welding on heavy-section boiler plate with 4 to 5 mm E11018-G electrodes, FCAW on positional welds inside the firebox, and SAW on longitudinal seams. The HVAC response combines heavy local extract at every station (1,000 to 1,500 L/s per slot hood on stick, 300 to 500 L/s per SAW station), articulated arm extraction at every bench, and high-volume overhead canopy on the bay as a whole to dilute fugitive emissions. Filtration is MERV 16 cartridge on the carbon-steel work, with HEPA H13 on the dual-shielded electrode work where the consumable carries higher manganese.
7.6 Hydrostatic test pad
Every pressure vessel and boiler is hydrostatic tested at 1.25 to 1.5 times the design pressure before despatch. The test pad is a contained area with floor drainage to handle the test water release, and any pinhole release path during the test must be captured to keep mist out of the breathing zone. The HVAC envelope is comfort-level ventilation with localised mist capture at any test point. The test pad is typically segregated from the main fab bay by a roll-up door and an air-pressure differential.
7.7 Atmospheric storage tank fabrication
Atmospheric storage tank fabrication (water tanks, fuel storage, chemical storage, beer wort and process vessels) sits at the intersection of pressure vessel and structural steel work. Shells are typically built on-site for large tanks (above 5 m diameter), with the workshop pre-fabricating courses and shop-welding the bottom plate to the courses. AS 1692, API 650 (cross-referenced for petroleum work), AS 1657 access, and the AS 1554 welding series govern. The unique HVAC challenge is confined-space welding inside the tank shell: as the tank grows course-by-course, the welder is increasingly working in an enclosed volume where natural ventilation is inadequate, and dedicated portable extract or low-volume forced ventilation through the manway is required. SafeWork confined-space entry permits drive the extract design.
8. Sandblast booth and shot blast cabinet
Sandblasting is used to prepare the steel surface before painting and to clean weld scale from heat-treated work. The booth captures the blast media and the dust through downdraft extract, separates the blast media for recycling, and discharges the cleaned air through a cartridge filter.
8.1 Booth construction under AS 3957
AS 3957 sets the dust hazard classification and the booth design parameters. Booth face velocity at the open face is 1000 ft/min (approximately 5 m/s) minimum per the class C requirement that covers most blast media. The booth is enclosed with rigid panel walls, a perforated floor that catches the spent media for recycling, and downdraft extract drawing the airflow downward across the workpiece. Where the workshop blasts inside the booth with operator entry, the operator wears a PAPR hood with continuous fresh-air supply at 6 to 10 L/s; the booth itself runs at slight negative pressure.
8.2 Blast media chemistry
Five common blast media in Australian heavy fab. Silica sand is the legacy choice and the cheapest, but the respirable crystalline silica WES of 0.05 mg/m3 has effectively pushed silica sand out of modern booths. Garnet is the most common replacement; iron oxide and chromite content drives the dust chemistry. Steel grit and steel shot are used in shot-blast cabinets for the heaviest substrate cleaning and recycle 80 to 95% of the spent media. Glass bead is used for finish work on stainless and aluminium where surface texture matters. Aluminium oxide is used on stainless where iron contamination must be avoided. Each media has its own dust chemistry and the LEV filtration must match.
8.3 Lead from painted steel substrate
Workshops that strip old painted steel (mining bucket repairs, locomotive overhaul, rail bogie refurbishment, naval refit) face a lead-based-primer risk that is often missed in scoping. Lead inorganic WES is 0.05 mg/m3, half the welding-fume general WES. Where the substrate is suspected of carrying lead-based primer (anything pre-1990 in heavy industrial service is suspect), the booth must be sealed wet-collection and the spent media goes to hazardous-waste disposal rather than recycling.
8.4 Duct material and filtration
Blast booth dust mains carry abrasive particulate at high velocity and they wear quickly. Wear-resistant material on the primary mains is critical: 6 to 10 mm mild steel with replaceable wear liners (chromium carbide overlay or boron carbide ceramic tile on the heaviest-duty installations). Branches and ducting downstream of the cyclone pre-separator can be lighter (3 to 5 mm carbon steel) and may be galvanised. The plenum below the booth is fabricated in 304L stainless on the SBSF-1525 to handle moisture infiltration and any condensation from compressed-air carryover. Filtration is MERV 16 cartridge minimum with high-frequency pulse-jet cleaning. Where combustible metal dust applies (aluminium-substrate cleaning, magnesium fines), the entire booth is NFPA 660 wet-collection with spark-resistant duct on every segment.
9. Pickling, passivation, galvanising and plating
Surface treatment processes that follow fabrication generate aggressive chemical fume that destroys ordinary carbon steel and galvanised duct rapidly. Each process has its own HVAC envelope.
9.1 Pickling — HCl, H2SO4
Pickling removes mill scale and heat-affected zone discolouration from steel and stainless. Hydrochloric acid (HCl, typically 15 to 18% w/w) is the most common pickle on stainless; sulphuric acid (H2SO4) is used on carbon steel. Tank temperatures 20 to 80 degC drive vapour pressure and the LEV demand. Acid mist is corrosive to almost every metal except titanium, 904L super-austenitic stainless, and acid-resistant plastics. FRP (fibreglass reinforced plastic) duct is the standard upstream of the scrubber; 304L stainless on the SBSF-1525 stitchwelder is acceptable downstream. SBKJ does not fabricate FRP duct — that section is fabricated by a specialist FRP supplier and integrated into the stainless system at the scrubber outlet.
9.2 Passivation — HNO3
Passivation removes free iron from a stainless surface and restores the chromium oxide passive film. Nitric acid (HNO3, typically 20 to 50% w/w) is the standard passivator. The fume is dominated by NO and NO2 oxide vapour, which is corrosive and toxic (3 ppm STEL). Stainless 316L on the SBSF-1525 handles the cleaned discharge side; FRP upstream of the scrubber.
9.3 Wet scrubber design
Acid mist control is by wet scrubber — packed-tower or venturi — with caustic neutralisation. Residence time in the scrubber is 1 to 2 seconds, packing depth 1.5 to 2.5 m, scrubber blowdown to a chemical effluent system at pH 6 to 9. The scrubber discharge is monitored against the state EPA acid-mist licence condition. Roof discharge with stack-to-intake separation per AS 1668.2 dispersion criteria.
9.4 Roof discharge under AS 1668.2
The scrubber discharge stack height is sized for ground-level concentration at the building boundary below the relevant trigger level. For HCl, the standard is 7 mg/m3 ground-level peak; for HNO3, 5 mg/m3. Building-wake plume dispersion modelling drives the stack height. Larger workshops in metropolitan zones often run two scrubber stages in series on the pickling line to keep the stack height manageable.
9.5 Hot-dip galvanising plant
An in-house galvanising plant carries a pickle bath (HCl), a flux bath (zinc ammonium chloride at 60 to 80 degC), a zinc spelter bath at 450 degC, and a quench tank. Each station has its own LEV. The pickle and flux baths share the acid-mist envelope above. The zinc spelter generates zinc oxide fume (5 mg/m3 WES) and ammonium chloride fume from the flux residue burning off as the workpiece enters the molten zinc. Capture is by side-draft slot hoods at the bath rim drawing fume across the bath surface, with 304L stainless duct on the SBSF-1525 because acidic chloride condensate is the worst-case material attacker on this branch. Discharge through a baghouse with caustic scrubbing where the chloride load is high.
9.6 Phosphating and chromate conversion
Lighter-duty surface treatment lines for component-scale work (Komatsu Wacol, Caterpillar Australia) include phosphating (zinc or iron phosphate) and chromate conversion (yellow, clear or olive drab chromate, with the legacy hex-chrome systems still in use under SafeWork variance). Each generates fume that drives a wet scrubber on the discharge. The hex-chrome WES of 0.005 mg/m3 governs the chromate conversion line; 304L stainless duct on the SBSF-1525 is mandatory.
9.7 Plating shop — zinc, cadmium, nickel, chromium
The electroplating line is the most demanding chemical HVAC in the workshop. Hex-chrome plating (decorative chrome and hard chrome) generates Cr(VI) mist with the 0.005 mg/m3 WES driving the design target. Nickel plating generates nickel salt mist with the 1 mg/m3 nickel WES. Cadmium plating (largely phased out in Australia under environmental restrictions but still used on some defence and aerospace work) generates cadmium fume with a 0.01 mg/m3 WES. Zinc plating is the most benign of the four. Each line has dedicated push-pull slot hood capture at the tank rim with 0.4 to 0.6 m/s capture velocity across the tank surface, ducted to a wet scrubber with caustic or sulphite neutralisation. Material is 304L stainless on the SBSF-1525 minimum; some operators specify 316L on the highest-spec chromium discharge.
10. Spray paint booth and bake oven
The spray paint booth is the single largest HVAC capital item in most heavy fabrication shops. AS 4114 governs booth construction, NFPA 33 governs spray application, AS/NZS 60079 governs the hazardous-area classification.
10.1 Booth configuration
The standard configuration is downdraft — supply air from a perforated ceiling plenum, downward airflow across the workpiece, exhaust through a floor or low-side grille, paint overspray captured in dry filters or a wet wash, exhaust discharged through a stack. Booth face velocity 0.5 m/s, plenum airflow uniformity within 20% across the working area, paint overspray containment to AS 4114 dry filter or wet wash.
10.2 Hazardous area classification
Inside the booth volume is Class I Zone 1 under AS/NZS 60079.10.1 whenever paint is being applied. Within 1 m of any booth opening (door, exhaust grille) is Zone 2. The classification drives Ex-rated electrical equipment, bonded and grounded ductwork, and explosion-protected dampers at the booth inlet and outlet. NFPA 33 layers on top with LEL monitoring at the exhaust stack and isolation valves between the booth and any downstream bake oven.
10.3 PU paint and isocyanate hardener
Two-component polyurethane paint is the dominant topcoat on Australian heavy fabrication — high-build epoxy primer, high-build polyurethane topcoat. The hardener is methylene diphenyl diisocyanate (MDI) or isophorone diisocyanate (IPDI), with the Safe Work Australia STEL of 0.005 ppm. The design target at the operator breathing zone is 0.003 ppm. Air-supplied respirator hoods are the standard PPE inside the booth, with the booth ventilation alone treated as a secondary engineering control. The duct material is stainless 304L on the SBSF-1525 for the paint exhaust because solvent partitioning at the duct wall causes galvanised to corrode within months. The supply make-up duct can be galvanised on the SBAL-V because the supply side is clean.
10.4 Solvent management
MEK, toluene, xylene and naphthalene are the workhorse solvents. WES is 200 ppm for MEK, 50 ppm for toluene and xylene. The booth LEV is sized for solvent vapour dilution as well as overspray capture, with the dominant factor typically being the safety-margin against the isocyanate STEL rather than the solvent WES. Solvent paint is decanted in a dedicated paint mix room rated to AS 1940 with continuous extract at 12 ACH minimum.
10.5 Paint bake oven
The bake oven downstream of the booth cures the paint at 60 to 180 degC depending on the system. Forced air convection oven with gas-fired or electric heating, dedicated exhaust to atmosphere via a stack, LEL monitoring at the stack per NFPA 86. The oven exhaust carries residual solvent vapour and any thermal decomposition products of the paint chemistry, and is treated separately from the booth exhaust because the chemistry is different (oven exhaust is dominated by VOC and combustion products; booth exhaust is dominated by paint aerosol overspray). The oven duct is on the SBSF-1525 stainless because the elevated temperature and condensation on cool restart cycles attack galvanised. Our automotive paint booth HVAC guide covers booth-and-oven integration in detail.
11. NDT laboratory, engineering office and clean zones
The NDT lab, the engineering office, the metrology room and the welder certification test bay are the clean zones in the shop — positive pressure relative to the workshop floor, controlled temperature, HEPA-prefiltered supply air.
11.1 NDT laboratory ventilation
The NDT lab covers radiography (X-ray and gamma source), ultrasonic, magnetic particle and dye penetrant inspection. X-ray and gamma rooms need concrete-lined exhaust, personnel-access interlocks under AS/NZS 2243.4, and segregated air handling that prevents any radioactive contamination from migrating into the rest of the workshop. Magnetic particle inspection benches use a dry magnetic-powder or wet-suspension medium and need slot-hood capture at 0.4 m/s within 300 mm of the part to control airborne medium during inspection. Dye penetrant benches use solvent-based penetrant and developer; the LEV is similar to magnetic particle with the addition of solvent dilution. Solvent inventory in the NDT lab pulls the room ventilation rate to 6 to 8 air changes per hour at minimum.
11.2 Engineering office
The engineering office and drawing office sit at the front of the shop with positive pressure relative to the production floor (5 to 10 Pa typical) to keep airborne particulate out. HEPA-prefiltered supply at 2 to 4 ACH, exhaust through return-air grilles tied back to the main supply handler. Comfort design temperature 22 to 24 degC year-round. The duct envelope is the lightest in the building — conventional galvanised low-pressure on the SBAL-V.
11.3 Welder certification test bay
Workshops that certify welders to AS 1796 and AS 2980 in-house need a dedicated test bay with HVAC representative of production conditions. The bay carries the same articulated arm extraction as the production welding bays, and the certification test pieces are welded under the same air movement that the welder will encounter in routine work. The test bay is captured on the workshop’s ISO 3834-2 QMS as a controlled environment.
12. Heat treatment and post-weld stress relief
Workshops that carry post-weld heat treatment (PWHT), normalising, annealing or stress-relief furnaces in-house operate under NFPA 86. The exhaust topology is distinct from general workshop extract.
12.1 PWHT oven
PWHT for pressure vessel and boiler work runs at 600 to 700 degC for 4 to 16 hours depending on wall thickness and material. Gas-fired or electric ovens with full insulation; the vessel is loaded into the oven, the temperature ramped up, held, and ramped down on a controlled schedule that depends on the AS 4458 procedure. The exhaust carries combustion products (CO2, water vapour, modest CO) and any residual solvent or oil from the vessel surface. Refractory-lined mild steel duct for the first 5 m above the oven, dedicated stack discharge separate from general workshop extract, LEL monitoring at the stack, purge cycles before light-off, explosion venting on the oven shell.
12.2 Normalising and annealing
Normalising (air-cool from above the upper critical temperature) and annealing (slow-cool to refine grain structure) run at 800 to 950 degC and follow similar NFPA 86 exhaust topology. Quench oil tanks (for oil-quench heat treatment) generate oil mist that requires localised mist capture at the tank rim, with the mist discharged to a coalescing filter before stack discharge.
12.3 Aluminium age hardening
T6 age hardening of aluminium structural sections runs 150 to 200 degC and falls below NFPA 86 thresholds; the exhaust is comfort-level extract. T4 solution heat treatment at 470 to 540 degC is in scope of NFPA 86 and carries the same audit trail as ferrous heat treatment.
12.4 Induction heat treatment
Induction heating is used for localised PWHT on field-welded site work and on shop-welded heavy-section work where a full oven is impractical. Mobile induction blankets at 5 to 50 kW power deliver localised heat to the weld zone. The HVAC response is overhead canopy at 0.4 m/s capture velocity over the heated area.
13. Compressed gas storage and dive cylinder fill
Workshops that carry an in-house gas storage and cylinder-fill operation (welding gases, cutting gases, inert purge gases for pipe spool work) operate under AS 2030 and AS 2299. Storage of compressed gas cylinders in segregated racks with adequate ventilation, separation distances from ignition sources, and explosion-protected electrical fittings in the gas room. The HVAC response is continuous extract at 6 ACH minimum, with the room at slight negative pressure to keep any leak contained.
Workshops that fill diving cylinders or breathing-air cylinders (defence, marine, commercial dive) operate under AS 2299 dive standard and AS 2030 cylinder filling. The fill room is a high-spec clean zone with HEPA-prefiltered supply at 8 to 12 ACH, dedicated CO monitoring, and the compressor on a dedicated air handling cell separate from the fill room.
14. Plant registration and the SafeWork audit trail
Heavy fabrication shops carrying pressure equipment work in Australia operate under state plant registration regimes. SafeWork NSW, WorkSafe Victoria, WorkSafe Queensland, RTW SA and equivalent regulators in WA, Tasmania, NT and ACT each maintain a register of pressure equipment manufacturers with nominated inspectors at design, fabrication and pressure-test stages. The HVAC system is incidental to the pressure equipment audit but is a SafeWork compliance question in its own right under AS 4801.
The audit trail that a duct designer must provide includes the LEV design calculation (every branch sized against the relevant WES), the commissioning balance report (every branch measured with calibrated hot-wire anemometer at the capture face), the personal air monitoring campaign (24-hour sampling across a representative production day at each fume and dust source), and the routine recommissioning data on a 3-year cycle (or annual where the operator’s SafeWork conditions require it). Industry bodies including the Welding Technology Institute of Australia (WTIA), the Australian Steel Institute (ASI), the Australian Welding Institute (AWI) and the Independent Pressure Equipment Inspectors (IPEI) publish guidance that informs the inspector’s expectation.
15. Operator type and process intensity — what changes
The Australian heavy fabrication market covers a wide spectrum from the single-boilermaker jobbing shop to the multi-acre prime defence contractor. The HVAC envelope shifts substantially across that spectrum.
15.1 Heavy structural and naval prime contractors
Civmec (ASX:CVL, Henderson WA) is the largest dedicated heavy fab operation in Australia and carries the prime contract on the Arafura-class OPV programme and a major work share on the Hunter-class frigate hulls at Williamstown alongside BAE Systems Australia. Forgacs Engineering (NSW, acquired into the Civmec group) carries a major shipbuilding and rolling-stock fabrication portfolio. ASC Pty Ltd (Osborne SA) is the submarine prime under the AUKUS programme. BAE Systems Australia operates at Williamstown VIC on naval programmes alongside Henderson WA. Each of these operators runs welding bays measured in hundreds of arcs concurrently, robotic SAW lines on hull-plate longitudinal welds, blast booths on full bogie and module scale, and downdraft paint booths sized for full module entry. The HVAC duct fabrication for an operator at this scale runs to 5,000 to 20,000 m of duct on a single fit-out and the duct fabricator on the project carries the full SBAL-V plus SBSF-1525 plus SBPC1500 plus SBLR-600 configuration to produce galvanised, stainless and laser-finished work concurrently.
15.2 Mid-size structural steel and metal fabrication
Liberty Primary Steel (formerly OneSteel, Whyalla SA and Newcastle NSW), Bisalloy Steels (ASX:BIS, Unanderra NSW) on quenched-and-tempered armour and mining plate, BlueScope Steel (ASX:BSL, Port Kembla NSW) on primary steel and adjacent fab businesses, and InfraBuild (under Liberty Primary) on steel processing and fab all sit at the mid-to-large end of the market. These operators run mixed-product workshops with structural, rolled-section and plate-fab in parallel, often integrating their own galvanising or surface-treatment lines. The HVAC duct envelope is similar to the prime contractors at unit operation level but with fewer concurrent stations.
15.3 Boilermaking, pressure vessel and tank fabricators
Saunders International (ASX:SND, Sydney HQ) is the largest dedicated tank and pressure vessel fabricator in the country. Worley (ASX:WOR, Brisbane and Perth) provides engineering and EPCM services for oil and gas including significant fab subcontract pathways. Monadelphous (ASX:MND, Perth) operates the largest single fab base in WA for mining and oil-and-gas construction. McConnell Dowell, John Holland (CIMIC Group), Acciona Australia, CPB Contractors (CIMIC), Laing O’Rourke Australia and Lendlease Engineering run major project fab pathways with either in-house workshops or sustained subcontractor relationships. Watpac (BESIX-owned) carries a structural fab portfolio. Each of these operators runs pressure vessel and boiler fab cells alongside general fab and the HVAC envelope reflects the section-7 requirements above.
15.4 Mining and heavy machinery OEMs
Komatsu Australia (Wacol QLD) and Komatsu Mining (Caboolture QLD) build and rebuild dragline buckets, dump truck trays, shovel components and longwall mining equipment. Caterpillar Australia builds and overhauls mining and construction equipment. Liebherr Australia operates a major rebuild facility. WesTrac (Seven Group Holdings) and Hastings Deering carry the Caterpillar dealer rebuild pathways. Sandvik Mining Australia (Heatherbrae NSW and Toowoomba QLD), Komatsu Mining (Caboolture), Atlas Copco / Epiroc Australia (Tomago NSW) and Outotec Metso (Perth and Newcastle) run OEM fab and refurb. Each of these operators carries a mixed-process workshop with welding, machining, blast and paint in parallel, and the HVAC envelope is closer to the prime-contractor scale.
15.5 Engineering services and SME fabricators
WACOM Australia, Engenco (ASX:EGN), Vix Technology, Allight Sykes, Ampcontrol (Newcastle), TEAM Industrial Services and Welltech Industries plus several thousand smaller engineering services firms across Australia carry a mix of fab, machining and assembly work. The HVAC envelope for a 5 to 50 person fab shop is a scaled-down version of the prime-contractor template: a single welding bay with 4 to 12 articulated arm extraction posts, a single sandblast booth (or none, with grit-blast outsourced), a single paint booth, and modest NDT capability. The SBAL-V handles the workshop’s in-house galvanised duct fabrication and the SBSF-1525 handles the stainless work, with a single SBLR-600 for laser finishing.
16. SBKJ machine configuration for heavy fabrication shop HVAC duct supply
A fabricator who wants to serve the Australian heavy fabrication, boilermaking and pressure vessel workshop market with HVAC ductwork supply needs a machine configuration that handles the full range of materials, gauges and seam types that the application requires. The SBKJ envelope below is the configuration we install on Australian projects in this market.
16.1 SBAL-V automatic duct production line — the galvanised workhorse
The SBAL-V automatic duct production line is the cornerstone machine for galvanised rectangular ductwork supply to heavy fab shops. It handles 0.5 to 1.2 mm GI coil with integrated decoiler, leveller, beading, Pittsburgh lock seam former, TDF flange former, notch shear and corner cutter, with throughput typically 12 to 20 metres of duct per minute depending on cross-section and seam configuration. AS 4254 low-pressure and medium-pressure classes are produced without operator intervention beyond coil loading and finished panel pickup. The SBAL-V is the right machine for general workshop supply air, welding bay primary extract on the clean side of the cartridge filter, NDT and office supply, and any zone where the duct sees clean air or low-aggressor fume. We cover the SBAL-V configuration in detail in our SBAL-V vs SBAL-III comparison.
16.2 SBSF-1525 stitchwelder — stainless and high-spec
The SBSF-1525 stitchwelder fabricates AS 4254 high-pressure class leak-tight rectangular ductwork in 304L and 316L stainless and in heavier galvanised gauges where the SBAL-V capacity is exceeded. It handles 0.7 to 2.0 mm sheet thickness with continuous stitch welding on the longitudinal seam, producing a leak-tight result without the post-weld cleanup that conventional rolled-and-welded fabrication requires. The SBSF-1525 is the right machine for stainless on every chrome-bearing welding bay extract, the paint booth exhaust, the chrome conversion and plating shop extract, the pickling discharge downstream of the FRP-to-stainless transition, and the SAW capture branches on stainless pressure vessel work.
16.3 SBPC1500 plasma cutter — in-house plate cutting
The SBPC1500 CNC plasma cutter handles in-house cutting of duct components, flange blanks, gussets and reinforcement plates from the same coil stock that the workshop fabricates duct from. It also cuts the access panels, inspection ports and fire-damper frames that the workshop needs for AS 1657 platform access and AS 1851 damper inspection. The SBPC1500 ties to the SBAL-V coil line for finished duct and removes the dependency on subcontracted plate cutting for HVAC duct ancillaries.
16.4 SBLR-600 handheld laser welder — clean stainless finish
The SBLR-600 handheld laser welder is the finishing tool on stainless duct work where the visible side of the duct must be presentable and the post-weld pickling step is unwanted. The SBLR-600 produces a smooth, narrow bead at 3 to 5 times the speed of TIG, with low heat input that does not distort the panel. It is the right tool for the visible face of paint booth supply ducts, NDT lab supply ducts, and any duct in a customer-facing zone of the workshop.
16.5 Spark-resistant duct construction
For sandblast booth primary mains and any zone with combustible metal dust risk (aluminium or magnesium fines from grinding), spark-resistant duct construction is mandatory under NFPA 660 and AS/NZS 60079. SBKJ duct in this application is fabricated to spark-resistant tolerances: no internal protrusions, smooth wall transitions, non-ferrous fasteners on critical zones, and bonded-and-grounded joints on every segment. The fabrication is on the SBSF-1525 stitchwelder for the leak-tight high-pressure class that the NFPA 660 envelope demands.
16.6 What sits outside the SBKJ range
FRP duct upstream of the pickling and passivation scrubbers is fabricated by a specialist FRP supplier (typically Australian-based; the FRP market is well-served domestically by half a dozen specialist fabricators). Refractory-lined mild steel duct on heat-treatment furnace flues is fabricated by a specialist refractory contractor — SBKJ supplies the steel shell on the SBPC1500-cut blanks and the SBSF-1525 stitchwelded long seam, and the refractory lining is poured or sprayed in-situ by the refractory specialist. Wear-resistant chromium-carbide overlay liners on blast booth primary mains are fabricated by a specialist hardfacing contractor under AS 1554.7.
17. Worked sizing example — a 1,200 m2 boilermaking and pressure vessel workshop
To make the section-by-section sizing concrete, here is a worked example for a 1,200 m2 heavy fabrication workshop with mixed boilermaking, pressure vessel and tank fabrication work, similar in scale to a mid-tier Saunders International or Monadelphous shop cell.
Welding stations: 12 manual welding bays (mix of MIG, MAG, FCAW, stick) at 1,000 to 1,500 L/s each = 15,000 L/s peak. 3 dedicated stainless TIG bays at 200 L/s on-torch each = 600 L/s peak. 2 SAW stations at 400 L/s each = 800 L/s peak. Total welding extract design: 16,400 L/s peak.
Cutting: 1 CNC plasma cutting table (3 m x 12 m bed) at 30 m2 x 2.0 m3/s/m2 = 60 m3/s = 60,000 L/s peak. 1 fibre laser cutting table (1.5 m x 3 m bed) at 4.5 m2 x 1.25 m3/s/m2 = 5,600 L/s peak. Total cutting extract design: 65,600 L/s peak.
Grinding and fettling: 8 downdraft fettling benches at 1,500 L/s each = 12,000 L/s peak.
Sandblast booth: single walk-in booth (6 m x 4 m x 4 m) with downdraft extract at 1,000 ft/min face velocity over 24 m2 working face = 122 m3/s, but the practical sizing on a walk-in booth runs lower at 30 to 50 m3/s peak with operator inside in PAPR. Design value 40,000 L/s.
Spray paint booth: downdraft booth (8 m x 4 m x 4 m) at 0.5 m/s downward velocity over 32 m2 floor = 16 m3/s = 16,000 L/s plus 25% peak overspray = 20,000 L/s.
Heat treatment: 2 stress-relief ovens at 4,000 L/s each = 8,000 L/s.
NDT and office: 2,000 L/s combined supply with HEPA prefiltration.
Total extract: approximately 164,000 L/s peak, with VFD-controlled demand bringing average operating load to 60 to 80% of peak. Total make-up air to balance, with tempered direct-fired gas heating on the supply make-up handler in Melbourne winter and indirect-fired or chilled-water cooling on the supply make-up handler in Brisbane and Townsville summer. Total HVAC duct fabrication on the SBAL-V plus SBSF-1525 plus SBPC1500 plus SBLR-600 configuration: approximately 3,500 to 5,000 m of finished duct on the project.
18. Commissioning, balancing and the ongoing audit cycle
The duct designer’s deliverable does not stop at handover. The commissioning balance is the design-validation step that the SafeWork notice of operation depends on. At commissioning, every LEV branch is measured against its capture-velocity target with a calibrated hot-wire anemometer; every hood is smoke-tested to confirm that the capture envelope reaches the breathing zone; and a 24-hour personal-air-monitoring campaign at each fume and dust source generates the documentary baseline for AS 4801 OHS compliance. Welding bay sampling for total welding fume, manganese, hex chrome (stainless work) and nickel. Blast booth sampling for respirable crystalline silica and respirable iron oxide. Paint booth sampling for isocyanate, toluene, xylene, MEK. Plating shop sampling for hex chrome and nickel mist. Each sample is referenced against the relevant WES with a 60% margin target.
The audit cycle continues on a routine basis. SafeWork state regulators typically require recommissioning every 3 years for high-risk LEV (blast booth, paint booth, plating shop, chemical pickle); every 5 years for general welding extract. ISO 9001 and ISO 3834-2 surveillance audits run on a 12-month cycle and request the commissioning data and any recommissioning updates. The fabricator’s industry-association memberships (WTIA, ASI, AWI, IPEI) provide guidance documents that inform the inspector’s expectations and the audit-trail templates. The duct designer’s deliverables — design calculation, commissioning report, recommissioning schedule — form the foundation of the operator’s long-term compliance position. Our HVAC commissioning and air balancing guide covers the process in detail.
19. The compliance position SBKJ delivers
An SBKJ Group duct package on a heavy fabrication, boilermaking or pressure vessel workshop project hands the operator a complete compliance dossier on day one. The dossier includes the LEV design calculation against every Safe Work Australia WES that applies in the workshop; the AS 4254 pressure class evidence for every branch with weld-procedure records; the AS 1554 weld procedure interaction notes for the welding bays; the AS 4458 plant-registration-compliant routing through the pressure vessel cell; the AS/NZS 60079 hazardous-area documentation for the paint booth and solvent room; the NFPA 660 combustible-metal-dust risk assessment for any grinding zone that machines aluminium or magnesium; the NFPA 33 spray booth conformance; the NFPA 86 heat-treatment furnace exhaust documentation; the AS 4114 paint booth construction conformance; the AS 3957 dust hazard classification; the AS 1668.2 building extract and make-up calculation; the AS 1851 fire damper schedule; the AS 1657 platform access plan; and the AS 4801 OHS audit trail.
From Box Hill North in Victoria, our engineering team works directly with consulting mechanical engineers, fabricator owners, project managers and SafeWork inspectors on every Australian heavy fab fit-out we engage on. ARBS 2026 in Sydney is our scheduled meeting point for the year — the team will be available for face-to-face design reviews during the show and across the surrounding weeks at workshops in Melbourne, Sydney, Brisbane, Adelaide, Perth and the regional centres where most of Australia’s heavy fab capacity sits.
FAQ
What Australian standards apply to HVAC ductwork in a steel fabrication or boilermaking workshop?
AS 1668.2 sets mechanical ventilation requirements for industrial occupancies including welding bays, blast rooms, paint booths and pickling lines. AS 1668.1 covers fire and smoke control. AS 4254 governs duct construction and pressure classification. AS 1554 series covers structural welding qualification (1554.1 general, 1554.6 stainless, 1554.7 hardfacing). AS 1796 and AS 2980 cover welder qualification and certification. AS 4458 and AS 4037 cover pressure equipment design, construction and Class A/B/C boiler classification. AS 1210 covers unfired pressure vessels. AS/NZS 60079 classifies hazardous areas for paint booths and solvent rooms. NFPA 660 (consolidating the former NFPA 484 and NFPA 654) governs combustible metal dust in workshops that grind aluminium, magnesium or titanium. NFPA 33 governs spray booths. AS 4114 sets booth construction requirements. AS 3957 sets the dust hazard classification. Safe Work Australia workplace exposure standards drive the local-exhaust ventilation sizing at every fume and dust source.
Why does generic galvanised ductwork fail in heavy fabrication shops?
Three reasons. First, abrasion from blast media and grinding dust strips zinc coating within months in any duct carrying particulate from sandblast booths or fettling stations. Second, acid mist from in-house pickling and passivation lines (HCl pickle, HNO3 passivation) attacks zinc immediately and eats through carbon steel rapidly. Third, ozone and nitrogen oxide by-products of welding combine with humidity to produce mild acidic deposits inside extract duct over time. Galvanised SBAL-V duct works perfectly for general workshop supply air, welding bay primary extract on the clean side of the cartridge filter, and process air not exposed to particulate or chemical aggressors. Stainless 304L or 316L fabricated on the SBSF-1525 stitchwelder belongs on paint booth exhaust, plating shop chromium VI streams, plenums for sandblast, and primary cooling-water mains. FRP belongs upstream of acid pickling scrubbers. Refractory-lined mild steel belongs on heat treatment furnace flues.
How much air does a welding bay actually need?
It depends on welding process, wire feed rate and consumable chemistry, not on bay area. A single MIG/MAG gun running 0.9 to 1.2 mm wire at 6 to 10 kg/hr produces 0.5 to 2 g/min of fume, which needs 50 to 100 L/s of on-torch capture or 800 to 1,500 L/s of slot-hood capture at 0.5 to 1.0 m/s capture velocity within 300 mm of the arc. Submerged arc welding (SAW) running 4 to 6 mm wire produces the highest fume mass per metre of any welding process and needs 300 to 500 L/s per slot hood at 1.0 m/s capture velocity. Stick welding (SMAW) with 3.2 to 5.0 mm electrodes on heavy plate produces moderate fume but with much higher manganese content per kilogram of consumable. Flux-cored arc welding (FCAW) produces 2 to 3 times the fume mass of solid-wire MIG. Stainless welding (TIG or MIG on 304/316/duplex) produces hexavalent chromium fume that pulls the design target to 0.005 mg/m3, which means 0.003 mg/m3 at the breathing zone for a 60% margin against the Safe Work Australia workplace exposure standard. SBKJ designs every welding bay around the heaviest process actually run, not the building volume.
What does an in-house pressure vessel and boiler workshop need that a general fab shop does not?
Six things. First, dedicated heavy-plate SAW capture sized for continuous 4 to 6 mm wire welding on circumferential and longitudinal seams. Second, shielded radiography (X-ray and gamma) rooms with concrete-lined exhaust and personnel-access interlocks under AS/NZS 2243.4. Third, pipe spool fabrication bays with internal-purge gas recovery (argon, helium) on root passes per AS 1554 and AS 4458. Fourth, post-weld heat treatment (PWHT) ovens and stress-relief furnaces under NFPA 86 with LEL monitoring and dedicated exhaust risers. Fifth, hydrostatic test pads with floor drainage and any release path captured to keep mist out of breathing zones. Sixth, an audit trail of welder qualification (AS 1796, AS 2980), weld procedure specifications (WPS) and procedure qualification records (PQR) tied to the boiler inspector’s Class A/B/C certification under AS 4458 and AS 4037. Each of these flows into the HVAC envelope as a discrete branch with its own material, capture velocity and discharge requirements.
What SBKJ machines does a heavy fabrication shop need to fabricate its own HVAC ductwork?
For general workshop extract and supply air in galvanised, the SBAL-V automatic duct production line handles 0.5 to 1.2 mm GI coil with integrated Pittsburgh seam, TDF flange and notching, with throughput typically 12 to 20 metres of duct per minute. For paint booth exhaust, plating-shop chromium streams and the plenum on the sandblast booth in stainless 304L or 316L, the SBSF-1525 stitchwelder fabricates leak-tight seams to AS 4254 high-pressure class. The SBPC1500 plasma cutter handles in-house cutting of duct components, flange blanks, gussets and reinforcement plates. The SBLR-600 handheld laser welder finishes the stainless work to a smooth bead that does not need post-weld pickling on the visible side. For sandblast booth primary mains and any zone with combustible metal dust risk (aluminium or magnesium fines from grinding), spark-resistant duct construction is mandatory. Where primary acid pickling generates HCl mist upstream of the scrubber, FRP is the material of choice and that section is fabricated by a specialist FRP supplier outside the SBKJ range.
