Why bakery HVAC is its own engineering discipline
Bakery air handling sits at the intersection of three engineering domains that rarely meet anywhere else in a food factory. The first is food safety — a bakery is a food premises under the FSANZ Food Standards Code and every cubic metre of air that contacts open product is regulated. The second is combustion engineering — every continuous tunnel oven, deck oven, rack oven or convection oven on the line is a fired appliance with hot exhaust that has to be captured, transported and discharged under NFPA 86 and local environmental rules. The third is combustible dust — flour, starch, sugar, milk powder, cocoa and most other dry ingredients in a bakery are classified St1 explosible dusts under NFPA 484 and AS/NZS 60079, which means any extraction or transport system in the dry-side of the plant is a dust explosion control problem before it is a ventilation problem.
Get any one of those three wrong and the consequence is not a comfort complaint. It is a closed-down factory after a fire marshal walk-through, a recall after a humidity spike grows mould on a finished loaf, or in the worst case a deflagration through a sifter and into a mixer room. Australian regulators have all three on their radar after a decade of grain industry dust incidents and the broader food-safety modernisation push following the 2019 cantaloupe recall and the 2021 listeria notices.
This guide consolidates how SBKJ specifies and fabricates duct for each room of a working bakery. It applies whether you operate a single deck oven and a stone bench in a regional Victorian arcade or whether you run a 24-hour continuous bread line spanning three football fields. The physics is the same. The duct gauge, fabrication tolerance and material changes with the scale.
The Australian bakery industry — what we actually build for
To specify the right ducting you first have to know what kind of bakery you are building for. The Australian sector splits into four clear tiers, each with a different HVAC profile.
Tier 1 — Industrial bread plants
The top tier of the Australian baking sector is the big-loaf bread plant — facilities running continuous tunnel ovens at 50,000 to 200,000 loaves per day. Goodman Fielder operates more than fourteen bread and baking plants across Australia, supplying the Wonder White, Helga's, Vogel's, Mighty Soft and Country Life brands plus a long list of supermarket private labels. Tip Top Bakeries, owned by George Weston Foods with an ARYZTA stake, runs the second-largest network producing sliced loaf bread, English muffins, breakfast crumpets and a growing line of baked snacks for the on-the-go retail channel. Allied Pinnacle, formed from the merger of Allied Mills and Pinnacle Bakery, supplies both the flour milling side and a large industrial bakery product set including par-baked breads and pizza bases for the food service channel. Lawson's Bakery and adjacent contract bakers fabricate private label loaves for the Coles and Woolworths supermarket channels.
These plants are characterised by continuous high-throughput tunnel ovens, in-line dough handling, pneumatic flour conveying from external silos, and frozen distribution wings that store par-baked product and breakfast crumpet inventory at minus 18 to minus 25 degrees Celsius. A single industrial bread line typically draws between 15 and 35 megawatts of total ventilation, heating and refrigeration load, and the ductwork run can exceed five kilometres of supply, return and exhaust trunk.
Tier 2 — Franchise bakery chains
The second tier is the franchise retail bakery — Bakers Delight, with more than 550 franchise stores across Australia, New Zealand, the United Kingdom and Canada, sets the template here. Brumby's Bakeries, owned by Quadrant Private Equity through Retail Food Group, runs the second-largest franchise network. Banjo's Bakery operates a Tasmanian-anchored chain also under Quadrant ownership. Beechworth Bakery runs a smaller regional Victorian network anchored on its original Beechworth premises. The Country Cob Bakery group runs multi-store regional bakeries through Victoria and New South Wales.
These stores typically operate a small flour silo or bag dump station, a single horizontal mixer, a bench dough table, one or two retarder-proofer cabinets and two or three deck or rack ovens. The store front sits directly behind the production room — meaning the ductwork has to handle a much higher cross-contamination risk between hot oven exhaust, cooling racks, customer-facing display cabinets and the front counter. Production throughput typically runs 300 to 1,000 loaves per day per store with additional pastry, slice and roll volumes that double the air-handling demand.
Tier 3 — Artisan and patisserie
The third tier is the artisan bakery and patisserie — premises where the loaves are hand-shaped, the laminated doughs are folded on a marble bench and the croissants are proofed by skilled bakers who watch the dough rather than the timer. Bourke Street Bakery in Sydney sits in this tier, as does Brasserie Bread, Iggy's Bread of the World, Sourdough Cafe and the Wild Sourdough operation. The Melbourne patisserie scene includes LuxBite among others — premises that handle chocolate, sugar, fondant and creme patissiere alongside laminated pastry.
These bakeries have a different air problem entirely. The laminated doughs are temperature-sensitive — butter starts to soften above 22 degrees Celsius and the layered structure of a croissant collapses if the bench temperature drifts. The chocolate work demands stable humidity below 50 percent. The customer-facing patisserie counter sits in a different air zone again — front-of-house cool and dry, back-of-house warm and damp. The duct geometry has to thread fresh air into the back, capture oven exhaust without dragging cool front-of-house air over the proofer and keep the chocolate room separated from the bread side.
Tier 4 — Specialty and ingredient supply
Underneath the bakeries themselves sits the Australian flour milling and ingredient supply chain. Allied Pinnacle runs the largest milling network across New South Wales, Victoria, Queensland and Western Australia. Manildra Group, through Manildra Milling and the Shoalhaven Starches operation, runs the second-largest flour and starch operation. GrainCorp Foods sits in the broader grain processing space. The mills generate their own HVAC challenges — flour dust at higher concentration than any bakery, and a hazardous area classification that runs through nearly the entire production floor. The HVAC and dust handling rules that apply to a flour mill are stricter than the bakery rules but flow from the same NFPA 484 and AS/NZS 60079 framework.
The regulatory stack — what you have to comply with
Australian bakery HVAC sits under a layered regulatory stack. Each layer applies independently. Compliance with one does not waive the others.
AS 4674 — Design, construction and fitout of food premises
AS 4674 is the controlling Australian standard for the physical construction of any premises where food is prepared, processed or sold. It specifies the wall, floor, ceiling, drainage, lighting, equipment and ventilation requirements at a level above the headline FSANZ requirements. For ductwork, AS 4674 drives three core decisions. First, all ductwork above an exposed food zone must be smooth, cleanable, non-corroding and free of horizontal ledges that can accumulate dust. Second, all ductwork penetrations of food zone ceilings must be sealed against insect and pest ingress. Third, the ductwork surface must be a food-grade material — in practice 304 stainless steel for the ducting that runs directly above open product.
AS 1668.2 — Mechanical ventilation in buildings
AS 1668.2 is the Australian mechanical ventilation code. It specifies the minimum outdoor air rates for each space type, the calculation method for exhaust hoods, the safe discharge of exhaust to atmosphere, the spacing between exhaust and intake terminals, and the smoke control overlay. For a bakery, AS 1668.2 drives the supply air volume to each room, the makeup air volume that balances the oven exhaust and dust extraction systems, and the discharge geometry for oven exhausts and odour control fans.
FSANZ Food Standards Code Chapter 3.2.3
The FSANZ Food Standards Code is the Australian and New Zealand binding regulation for all food production. Chapter 3.2.3 specifically deals with food premises and equipment, and it interlocks with AS 4674. It is the document a food safety auditor will quote when inspecting your bakery. From an HVAC perspective the relevant clauses concern positive pressure in clean zones, the prevention of condensation drip from cold ductwork onto product, the separation of high-risk and low-risk areas, and the cleanability and inspectability of all overhead ductwork.
HACCP and Codex Alimentarius
The Hazard Analysis and Critical Control Points framework drawn from Codex Alimentarius applies to every Australian bakery that supplies a major retail customer. From the HVAC standpoint, HACCP requires the operator to identify air-borne hazards (mould spores, dust, condensation, environmental allergens, foreign-body contamination from filter degradation), apply controls, monitor those controls and record the monitoring. The ducting layout, the filter classes, the differential pressure regime between rooms and the inspection access on ductwork all feed into a documented HACCP plan.
NFPA 86 — Standard for ovens and furnaces
NFPA 86 is the international standard for fired industrial ovens. It applies to every continuous tunnel oven, large rack oven and most deck ovens used in commercial baking. From an HVAC perspective it specifies the safety ventilation rate to prevent flammable gas accumulation in the oven enclosure, the post-purge cycle on shutdown, the explosion relief on direct-fired ovens, the materials and construction of the oven exhaust duct, the leak rate of the exhaust system, and the fail-safe behaviour on fan failure. The exhaust duct geometry — including dampers, fans, joints and cleanouts — must be detailed to NFPA 86 in any bakery operating an industrial-grade oven.
NFPA 484 — Standard for combustible metals (and the dust hazard analysis methodology)
NFPA 484 is most often cited for metal dusts but the dust hazard analysis methodology runs across the entire NFPA combustible dust framework including NFPA 61 (agricultural and food processing facilities) and NFPA 652. The reference here applies to the bakery flour dust exposure — a documented dust hazard analysis, isolation between dust collectors and processing rooms, conductive grounded ducting, explosion venting or chemical suppression on the dust collector and removal of any ignition sources from the dust cloud zone.
AS/NZS 60079 — Hazardous area classification
Where flour dust is generated, captured or transported in concentrations above the minimum explosible level, the surrounding electrical and mechanical zone is classified as a hazardous area under the AS/NZS 60079 series. The classification determines the type of motor, light fitting, sensor, switchgear and ductwork bonding that is permitted in that zone. Typical bakery flour silo loading bays, sifter rooms and dust collector enclosures fall into Zone 21 or Zone 22 dust classifications.
AS 5812 — Where applicable to baked snack pet products
AS 5812 (Manufacture and supply of pet meat) applies to a narrow subset of bakery operations — specifically the baked pet biscuit, treat and snack production lines that some Australian bakers operate as a side stream. Where a bakery produces baked products intended for animal consumption (dog biscuits, baked treats, pet meal toppers), AS 5812 imposes additional controls on cross-contamination between human-food and animal-food zones, including the HVAC separation of the two streams.
AS/NZS 4254 — Ductwork
The duct itself, regardless of what flows through it, is fabricated to AS/NZS 4254. Part 1 covers flexible duct and Part 2 covers rigid duct. The standard specifies gauge, joint construction, support spacing, leakage class, pressure class and insulation. SBKJ fabricates all bakery ducting to AS/NZS 4254 with the appropriate pressure and leakage class for each duty.
The bakery as a process — and the HVAC zones that result
A working bakery is best understood as a process line rather than a building. Flour enters at one end and packaged bread leaves at the other, with each intermediate stage demanding a different air condition. Below we walk the process and specify the ductwork for each zone.
Zone A — Flour intake, silo, sifter and dust collection
Flour arrives at an industrial bread plant by bulk tanker, blown into one or more vertical silos by the delivery driver. Each silo loading point is a primary dust generation source — the loading event entrains hundreds of cubic metres per minute of dusty air at flour concentrations well above the minimum explosible level. The vent on the silo must therefore discharge through a dust filter sized for the loading rate, with explosion venting back to atmosphere through a deflagration vent panel sized to NFPA 68. The vent panel discharge zone has to be located so that an explosion does not vent toward an occupied building or vehicle access.
Below the silo, flour is conveyed to sifters and pre-mix hoppers. Each transfer point — silo discharge, screw conveyor inlet, sifter inlet, sifter outlet, pre-mix hopper inlet — is a secondary dust generation source that must be captured. Capture velocity at the source needs to be 0.5 to 1.0 metres per second to entrain the dust without aerosolising it more aggressively than necessary. The captured air is conveyed through conductive ductwork (bonded carbon steel or stainless, no plastic flex) to a centralised dust collector sized for the total airflow.
The dust collector is the highest-risk component in the system. It concentrates the dust at the bag house or cartridge filter face, and a single ignition source inside the collector can propagate back through the ducting to the source. The mitigation strategy under NFPA 484 includes explosion venting or chemical suppression on the collector itself, isolation valves (rotary valve, fast-acting damper or chemical isolator) between the collector and each branch line, removal of ignition sources, and a dust monitoring programme that ensures the dust loading inside the ducting is kept below the minimum explosible concentration.
SBKJ supplies the dust extraction ducting in two configurations. For straight runs and trunks, SBTF-1602 round spiral duct in conductive steel is the standard product — round geometry minimises dust accumulation on flat ledges, the spiral lockseam gives an inherent leakage class better than rectangular construction, and the conductive carbon steel can be bonded and earthed continuously without a return path through the building structure. For branch tap-offs and fitting work, fabricated rectangular sections are used selectively where space requires it, with internal radiusing on the bends to maintain transport velocity.
Zone B — Mixer room
The mixer room is where formulated flour, water, yeast, salt and minor ingredients are blended into dough. Industrial bakeries use spiral mixers or horizontal high-speed mixers — Tweedy-style continuous mixers in the largest plants. During the charging phase the mixer generates a visible flour cloud as the dry ingredients land on top of the water in the bowl. Mixer rooms therefore need both local exhaust capture above the mixer and general ventilation to manage residual airborne flour.
Local exhaust hoods over each mixer are sized for the bowl diameter and the charging method, with a capture velocity at the bowl rim of around 0.75 metres per second. The hood is fabricated from 304 stainless steel under AS 4674 because it sits directly above an open food product. The hood ducting connects either back to the central dust collection system (if dust concentration permits and the system is rated for it) or to a dedicated mixer extract trunk with its own bag filter discharge.
General ventilation runs at 10 to 15 air changes per hour. Supply air is filtered to MERV 13 or higher to prevent outdoor dust ingress and to manage allergen control where the bakery handles mixed product (gluten and non-gluten lines, for example). The supply diffusers are positioned to avoid disturbing the dust cloud during charging — high level supply, low velocity, with diffuser face velocity below 1.5 metres per second.
The room temperature is held at 22 to 24 degrees Celsius. Mixer rooms run warm from the kneading energy of the mixer drive and the warm water dosing — without cooling supply air the room can drift to 28 degrees Celsius which then warms the dough, which then accelerates fermentation and disrupts downstream process timing. The supply duct above the mixer is 304 stainless. The return duct in non-product areas can be galvanized.
Zone C — Dough handling, dividers and rounders
From the mixer, dough is transferred to dough dividers, rounders, intermediate provers and final shapers. This zone is less dusty than the mixer room but still classified as a food contact zone. Ductwork above the conveyors is 304 stainless. The room is held at 22 to 24 degrees Celsius with 50 to 60 percent relative humidity to prevent the dough skinning on the surface as it travels between machines.
An important detail on this zone is the supply air diffuser velocity. Dough surfaces are sensitive to direct air movement — a high-velocity air jet will skin a freshly divided dough piece in seconds, creating a rough surface that does not bake to the desired crust. Diffusers are positioned to wash the room air without impinging directly on the conveyor product. SBKJ's typical specification uses linear slot diffusers on supply duct above the conveyor at face velocity under 1 metre per second.
Zone D — Final proofing
Final proofing is where the shaped dough rises to its full volume before baking. It is the single most demanding HVAC zone in the bakery. The conditions have to be held tightly: 27 to 32 degrees Celsius dry bulb, 70 to 85 percent relative humidity, with low air movement to avoid wind-drying the surface of the dough. Croissant and laminated dough proofers run cooler around 26 to 28 degrees Celsius to protect butter layers.
The proofer is fed by a dedicated air handler with steam or atomised humidification. The supply duct downstream of the humidifier sees saturated air at high temperature — meaning the duct itself must be insulated externally to avoid surface condensation in adjacent spaces, and fabricated from 304 stainless to handle the corrosive moisture loading. SBKJ's standard proofer supply trunk is round spiral 304 stainless from the SBTF-1602 mandrel set, externally insulated with a polyurethane or rockwool jacket and a vapour barrier wrap.
A small constant exhaust bleed of 5 to 10 percent of the supply volume removes the CO2 generated by yeast fermentation. Without the bleed the CO2 level inside the proofer can rise to several thousand parts per million, which depresses fermentation rate and produces an inconsistent dough rise. The bleed exhaust duct is also 304 stainless with external insulation and is routed to a discharge point that does not let saturated moist air back-flow into the supply intake.
Proofer doors are a major HVAC loss point. Every door opening for the loading and unloading sequence dumps cubic metres of warm humid air into the surrounding production hall. This is why industrial proofers operate as continuous tunnels or spirals with air locks at the inlet and outlet rather than as batch cabinets — the air loss per loading cycle is dramatically lower. The HVAC ducting around the proofer accounts for the regular bleed to the surrounding hall during loading and includes a buffer capacity in the make-up air supply.
Zone E — The oven
The oven is the heart of every bakery and its exhaust is the single largest HVAC duty in the plant. Industrial tunnel ovens for sliced loaf bread run 30 to 70 metres long with three or four temperature zones, baking at deck temperatures between 220 and 250 degrees Celsius. The exhaust stream from such an oven discharges at 200 to 280 degrees Celsius, carrying combustion products from the gas burners, water vapour from the dough surface, ethanol vapour from yeast fermentation residue, and the volatile organic compounds that give bread its aroma.
The exhaust duct must handle the temperature continuously. Below 200 degrees Celsius continuous, heavy-gauge 316 stainless (2 mm wall) is sufficient. Between 200 and 350 degrees Celsius the trunk is either heavy-gauge 316 stainless or refractory-lined carbon steel with internal ceramic fibre. Above 350 degrees Celsius (pizza ovens, specialty deck ovens) refractory-lined is the only practical option.
NFPA 86 governs the entire exhaust path. The damper between oven and exhaust fan must be fail-safe-open. The fan must be rated for maximum operating temperature plus a safety margin. Duct supports must be spaced for dead weight plus flow loads plus thermal expansion — a 30 metre run expands approximately 100 mm on cold start, taken up by purpose-designed bellows. Cleanouts go at every change of direction and every 6 to 8 metres of straight run. The exhaust must terminate above the roof line at a setback meeting the local environment protection authority emissions standard. For plants in mixed-use precincts a thermal oxidiser is fitted to combust VOCs and reduce both odour and visible plume.
SBKJ supplies oven exhaust in three configurations. For deck and rack ovens in retail bakeries, a 316 stainless trunk fabricated on the SBAL-V coil line is standard. For industrial tunnel ovens up to 250 degrees Celsius, the same stainless trunk is supplied with external rockwool insulation and a stainless skin. For tunnels above 250 degrees Celsius, refractory-lined carbon steel is supplied as fabricated heavy-plate sections with internal ceramic fibre.
Zone F — Cooling tunnel and spiral cooler
Hot bread leaves the oven at 95 to 98 degrees Celsius internal temperature. Before slicing or packaging the loaf must cool to between 25 and 35 degrees Celsius — slicing a warm loaf gummies up the blade and packaging warm bread traps steam inside the bag which fogs the bag, drives surface moisture into the crumb and creates a mould growth environment.
The cooler is held at 8 to 12 degrees Celsius ambient with a high air change rate, typically 30 to 60 air changes per hour. Modern industrial coolers use a spiral conveyor that moves the loaf through a vertically stacked path, with cooled and dehumidified air blowing through the matrix. The HVAC system feeding the cooler is a dedicated air handler with a chilled water coil sized for the latent load from the bread and a reheat coil to prevent over-cooling at the loaf core. The supply air dew point is held at 5 to 8 degrees Celsius so that surface condensation on the loaf is avoided.
The cooler supply ducting must be insulated externally to prevent surface condensation in the surrounding factory space. Above the product belt, the ducting is 304 stainless to satisfy AS 4674 cleanability rules. The main supply trunks are SBTF-1602 round spiral, which gives the lowest pressure drop per metre and the best leakage class — critical for a cold air system where leaked supply air carries the cooling capacity into the wrong zone.
Return ducting from the cooler picks up warm moist air laden with bread aroma. The return system filters out crumb debris and recirculates a fraction of the air through the chilled coil, with a fresh air bleed to manage CO2 and to dilute aroma loading. The recirculation fraction is balanced against energy cost (recirculating cold air is cheaper than cooling fresh air) and product safety (high recirculation rates concentrate any airborne contamination from upstream zones).
Zone G — Slicing and packaging
Slicing and packaging is the most sensitive food-safety zone in the bakery. The bread is fully baked, fully cooled, and now being cut and bagged — meaning any contamination introduced here goes directly into the consumer pack. The room is held at 18 to 22 degrees Celsius and 60 to 65 percent relative humidity. Higher humidity drives surface moisture which grows mould on the bag-stored loaf. Lower humidity dries the loaf crust and reduces shelf life. The HVAC therefore has to thread a narrow band on both axes.
Positive pressure is maintained relative to all adjacent zones, with supply air filtered through a final HEPA-grade filter (typically H13 or H14 under EN 1822) to remove airborne mould spores, yeast cells and bacteria. The supply ducting downstream of the final filter is 304 stainless with smooth internal welds — any ledge or rough surface inside the duct downstream of the HEPA becomes a colonisation site for the very organisms the filter is meant to exclude.
Air movement in the room is engineered to avoid disturbing the slicer blade airflow (which is a critical control point in the bagging operation — a poorly aimed supply diffuser can blow crumb dust into the open bag at exactly the wrong moment) and to maintain a sweep from clean side (bagging machine) to less clean side (return air grille on the room wall). The room is therefore a unidirectional flow zone in the strictest packaging sites, and a controlled mixed flow zone in less critical applications.
Differential pressure between the packaging room and adjacent zones is held at 10 to 15 pascals positive — high enough that any door opening drives air outward rather than inward, but not so high that doors are difficult to open. The pressure is monitored continuously and alarmed if it drifts outside the band. SBKJ supplies the supply and return ducting in 304 stainless with internal smooth welds and externally accessible cleanouts at every change of direction.
Zone H — Frozen distribution
Where the bakery operates a frozen distribution wing for par-baked product, breakfast crumpets, frozen pastry or finished frozen loaves, the freezer zone holds minus 18 to minus 25 degrees Celsius depending on product. The HVAC ducting in this zone is fundamentally different — a vapour-tight insulated duct with a continuous vapour barrier on the warm side, designed to handle the heavy thermal load of cold conditioned air and the very large temperature differential to the surrounding factory space.
The duct construction is normally double-skin insulated panel rather than fabricated sheet metal — the joints between sections must be vapour sealed to prevent moisture migration into the insulation, where it would freeze, expand, and eventually rupture the duct wall. SBKJ cross-references the cold-chain duct guide for the detailed construction of this zone — see the dedicated cold storage and cold-chain HVAC duct guide for the materials, joint detail and commissioning protocol.
Coordination between the freezer HVAC and the surrounding bakery HVAC is critical. The loading dock between the freezer and the despatch truck bay needs a controlled air-lock with dock shelters, fast-acting roller doors and a make-up air strategy that prevents cold air dumping into the bakery proper every time a truck loads. The make-up air supply to the freezer is typically pre-cooled in a buffer zone to reduce the latent load on the freezer evaporators.
Materials — when galvanized is fine, and when only stainless will do
Material selection in a bakery is more nuanced than a simple "stainless everywhere" rule. Stainless is more expensive, harder to fabricate, harder to insulate, and not always necessary. The right material depends on zone classification, temperature, moisture and food contact.
Galvanized steel (Z275 or higher coating)
General supply and return ductwork in non-product areas, return ducting in product areas where the duct does not run above exposed product, exhaust ducting in non-product areas, dust extraction ducting in the dry side of the plant (where the dust itself protects the surface), and outside air intake ducting all use galvanized steel built to AS/NZS 4254. SBKJ fabricates this on the SBAL-V coil line in galvanized stock — the same machine that fabricates the bulk of our commercial HVAC ducting worldwide.
The advantage of galvanized for non-product zones is cost (roughly 30 percent of stainless), fabrication speed (the lockseam closes cleanly and consistently), and weight (lighter than stainless of equivalent gauge, easier to handle and install). The Z275 coating is suitable for the moderate moisture loading of a bakery — it is not suitable for the higher chloride environments of meat and seafood processing where stainless is mandatory throughout.
304 stainless steel (food-grade ducting above product zones)
All ducting that runs above exposed food product, all ducting within designated food contact zones under AS 4674, all ducting downstream of HEPA filters in clean packaging rooms, all ducting carrying high humidity supply air to proofers, and all ducting carrying chilled high humidity supply air to cooling tunnels uses 304 stainless. SBKJ fabricates the stainless ducting on the same SBAL-V coil line with a stainless tooling set — the changeover from galvanized to stainless takes around 90 minutes for a coil-line crew and the lockseam quality on 304 is comparable to galvanized when the tooling is properly set up.
The grade is 304 rather than 316 for general food zones because chloride exposure in a bakery is low. 316 is used for oven exhaust trunks and ducting in zones using chlorine-based sanitisers where 304 would pit.
Refractory-lined carbon steel (high temperature oven exhaust)
Above approximately 250 degrees Celsius continuous operating temperature — encountered on industrial tunnel ovens, large rack ovens and pizza ovens — neither galvanized nor stainless is the most economic choice. Galvanized starts to oxidise and loses its zinc protection. Plain stainless is workable but heavy gauges become expensive. The standard solution is refractory-lined carbon steel — a fabricated outer carbon steel skin sized to AS/NZS 4254 with an internal ceramic fibre or refractory cast lining sized for the operating temperature. SBKJ supplies these as fabricated heavy-plate trunk sections with internal linings installed at the workshop and field-jointed on site with flanged connections.
Aluminium and other materials
Aluminium ducting is occasionally used for very low temperature applications below freezing — primarily in retrofit work where weight matters. It is not standard for bakery applications because it does not resist abrasion from flour dust as well as steel and it does not handle the temperature swings of an oven exhaust environment. SBKJ does not supply aluminium duct as a standard product for bakery work.
Polyvinyl chloride (PVC) and similar plastic duct is not acceptable in any bakery zone that handles flour dust or runs above exposed product. PVC is not conductive (it cannot be earthed in a hazardous area), it is not food-grade cleanable to AS 4674, and it ages poorly in industrial environments. SBKJ never specifies PVC duct for bakery work.
Acoustics — keeping a bread plant under NC-50
Industrial bakeries are mechanically intensive — mixers, conveyors, dividers, ovens, fans, refrigeration compressors and packaging machines all contribute to the noise floor. The HVAC ductwork itself can be a significant contributor if it is not designed for low velocity and well-attenuated transmission. SBKJ specifies bakery ducting for an in-room noise criterion of NC-50 (around 55 dBA) for the production hall and NC-40 (around 45 dBA) for the front-of-house retail counter and customer-facing areas.
The main acoustic strategies are velocity control (supply duct velocity below 6 metres per second in the main trunks, below 4 metres per second within 5 metres of a diffuser), in-duct attenuators on each supply branch into a critical noise zone, vibration-isolated fan mountings on every air handler, and flexible canvas connections between the fan and the rigid duct. The retail zones additionally receive lined duct (perforated stainless or galvanized with absorbing fill behind) in the final 3 to 6 metres of supply ducting before each diffuser.
The franchise retail bakery — different scale, same physics
A typical Bakers Delight or Brumby's franchise store occupies 80 to 150 square metres of total floor area, of which around 30 to 50 square metres is the production back-of-house. The HVAC equipment is sized for one or two rack ovens, one or two retarder-proofers, a single mixer, a small flour silo or bag dump station and a customer-facing display zone with the bagging counter, the bread display shelving and the EFTPOS point.
The HVAC ducting in a franchise retail bakery has to do everything the industrial plant does but in a fraction of the space and at a fraction of the equipment count. The oven exhaust from a single rack oven is still 200+ degrees Celsius and still needs an NFPA 86 compliant trunk to roof. The proofer still needs 27 to 32 degree humidified supply at 70 to 85 percent RH. The customer-facing display still needs to be air conditioned and pressurised positively. The difference is that in a small site all these zones share walls and ducts — meaning the ducting layout has to be tighter, the transitions cleaner, the cross-talk between zones more carefully controlled.
SBKJ supplies the franchise retail bakery package as a kit of standard duct sections fabricated on the SBAL-V coil line — galvanized for the general work, stainless for the proofer and packaging zones, and a refractory section for the single rack oven exhaust trunk. The kit is sized for a typical 600 to 1,000 loaf per day store and is installed by a local mechanical contractor with the SBKJ engineering team available for remote support during commissioning.
The artisan patisserie — handling the laminated dough problem
Artisan patisseries operate at a smaller scale again but with a much higher value per gram of product and a much narrower tolerance on temperature and humidity. A laminated dough — croissant, pain au chocolat, kouign-amann — depends on a sandwich structure of dough and butter that has to be kept cool throughout the fold-and-roll process. If the dough temperature drifts above 22 degrees Celsius the butter softens and merges with the dough rather than staying as discrete layers, which destroys the final pastry structure.
The HVAC for a laminated dough bench therefore has to hold the bench area at 18 to 20 degrees Celsius. The supply diffuser cannot blow directly on the bench because the air movement will skin the dough surface — instead the supply is delivered through a downward-flow ceiling slot at low velocity, with the room-air return at low level on the opposite side of the room. The supply duct is 304 stainless above the bench. The capacity is typically 8 to 12 air changes per hour for a 30 to 60 square metre laminated dough room.
Adjacent to the laminated dough room sits the chocolate work area for the patisserie's pralines and ganache items. Chocolate work demands stable humidity below 50 percent relative humidity — higher humidity dulls the chocolate temper, causes sugar bloom and frosts the finished surface. The chocolate room therefore has a dedicated air handler with active dehumidification and supply duct routed through 304 stainless to avoid any moisture re-introduction.
The front-of-house display counter in a patisserie has its own air problem — the display cabinets are refrigerated, the customer flow is constant, the front door opens to street air, and the air condition above the display has to keep the meringues crisp and the eclairs unfogged. SBKJ supplies a separate, smaller-scale duct package for the front-of-house with stainless ducting in the cabinet vicinity and galvanized in the customer-side ceiling cavity.
Cross-contamination control — keeping the gluten line separate
Many modern Australian bakeries operate both gluten-containing and gluten-free production. The two streams must be kept physically and pneumatically separate to satisfy FSANZ allergen control and to avoid contaminating the gluten-free product with airborne wheat dust. The HVAC plays the central role.
The two production stages operate at different pressures. The gluten-free hall is held at +15 pascals positive relative to the gluten hall, with a dedicated supply air system filtered through HEPA, dedicated return ducting that does not connect to the gluten side, and a physical wall (not just a curtain) between the two zones. The single point of crossover is the personnel access — typically a small ante-room with two interlocked doors, sufficient air change rate to clear any wheat dust that enters with personnel, and a controlled negative pressure relative to the gluten-free hall but positive relative to the gluten hall.
The duct material strategy reinforces the segregation. The gluten-free hall uses 304 stainless throughout including in non-product zones, partly for HACCP audit confidence and partly for cleanability. The gluten hall uses galvanized in non-product zones and stainless above product, per the standard bakery specification.
SBKJ machine configuration for bakery work
SBKJ supplies the fabricating machinery for bakery ductwork from three main product platforms. The full machine catalogue is at the SBKJ machine catalogue and the bakery-specific configuration is summarised below.
SBAL-V auto coil line — primary supply, return and exhaust trunk fabrication
The SBAL-V is SBKJ's flagship auto duct production line. It feeds galvanized or stainless coil, levels, decoils, slits, notches, beads, cleats, folds and produces complete rectangular duct sections in a single operation. For bakery work, the machine is typically configured with the standard galvanized tooling for the bulk of the ducting, and a quick-change stainless tooling set for the food-contact zones above product. The galvanized stock is run on Z275 coil, the stainless on 304 grade in 0.8 to 1.2 mm thickness for general ductwork or 1.5 to 2.0 mm for high-pressure exhaust trunks. Total throughput on a single shift exceeds 800 metres of finished duct, sufficient to supply a complete franchise bakery in two days or a complete industrial plant section in two to three weeks.
SBTF-1602 spiral tubeformer — round trunks for low-leakage applications
The SBTF-1602 fabricates round spiral lockseam ducting in 250 to 1,600 mm diameter from galvanized, stainless or carbon steel strip stock. For bakery applications, round spiral is the preferred geometry for the cooling tunnel supply trunks, the proofer supply trunks, the dust extraction main runs, and any trunk where leakage class is critical. The spiral lockseam achieves Class A leakage rates (under 0.5 percent at design pressure) inherently — no field caulking required. SBKJ ships the SBTF-1602 with stainless and carbon steel tooling included as standard, with field calibration during installation.
Refractory-lined trunk fabrication — fabricated heavy plate sections
For the high-temperature oven exhaust above 250 degrees Celsius, SBKJ supplies refractory-lined trunk sections as fabricated heavy-plate components rather than coil-line product. The carbon steel outer skin is laser-cut and welded to AS/NZS 4254 dimensions, the internal ceramic fibre or refractory cast lining is installed at the workshop, and flanged connections are machined for field jointing. These trunks are project-specific rather than catalogue items because the size, lining thickness and connection geometry vary with the oven type and exhaust gas conditions.
Commissioning and food safety validation
A bakery HVAC system is not commissioned in the traditional building-services sense of "balance the dampers and walk away." The validation is a food safety exercise that has to satisfy the bakery's HACCP plan and the food safety auditor.
The commissioning sequence runs in four stages. Stage one is the AS/NZS 4254 ductwork leakage test, performed on each main duct system at design pressure with a calibrated leakage tester. Stage two is the air volume balancing — each diffuser and grille is measured against the design schedule, with adjustment at the dampers until every zone is within tolerance. Stage three is the temperature, humidity and pressure differential commissioning — each zone is held at design conditions for at least 24 hours under stable load, with calibrated dataloggers recording the result for the food safety record. Stage four is the food safety walk-through — a joint inspection by the bakery's quality team, the SBKJ commissioning engineer and the external food safety auditor against the HACCP plan and the AS 4674 cleanability checklist.
The deliverable from commissioning is an as-built documentation pack that includes the duct layout drawings, the AS/NZS 4254 leakage test certificates, the air balance report, the 24-hour environmental log, the food safety auditor sign-off, the HACCP plan reference and the maintenance schedule with cleaning frequency for each duct section. This pack lives with the food safety records and is reviewed at each annual audit.
Maintenance — what changes the operating cost over 10 years
HVAC maintenance in a bakery is heavier than in a comfort-cooling office. The flour dust loading and humidity exposure mean filters fill faster, ductwork accumulates film, and ovens generate carbon deposits in the exhaust path. Supply air filters in the production area are replaced every 1 to 3 months. HEPA final filters in the slicing and packaging room are replaced every 12 to 24 months with integrity testing to AS 1807. Dust collector cartridges or bags are replaced every 6 to 12 months with pressure drop monitoring driving exact timing.
Oven exhaust ducting requires annual inspection and cleaning under NFPA 86. SBKJ supplies cleanouts at every change of direction and every 6 to 8 metres of straight run so the duct interior is accessible without disassembly. Proofer ducting accumulates yeast cell films on the internal surface; the 304 stainless smooth-welded interior is cleaned with a food-safe sanitiser every 3 to 6 months. Cooling tunnel supply ducting accumulates crumb dust at the diffuser face — weekly or monthly cleaning depending on product, with the duct interior on the same sanitiser regime as the proofer.
A worked example — duct package for a mid-size franchise bakery
Below is a worked example for a typical Bakers Delight or Brumby's-scale franchise store producing 800 loaves per day plus 1,200 pastry items. Production floor 45 square metres, customer area 70 square metres, total store 115 square metres.
Production hall supply uses a dedicated 4 kW air handler at 1,200 cubic metres per hour with MERV 13 filter, supply trunk 350 by 250 mm galvanized SBAL-V fabricated, dropping into linear slot diffusers on 304 stainless extension above the mixer and bench. Return trunk 400 by 300 mm galvanized. Proofer supply is a dedicated humidified air handler at 600 cubic metres per hour, supply trunk 250 mm diameter SBTF-1602 round spiral 304 stainless externally insulated 50 mm polyurethane, with a 100 mm diameter bleed exhaust to roof.
Oven exhaust is a 250 mm diameter 316 stainless heavy-gauge trunk externally insulated 100 mm rockwool with stainless skin, NFPA 86 fail-safe-open damper, to roof terminal. Customer area supply uses a 6 kW reverse-cycle air handler at 1,600 cubic metres per hour, supply trunk 400 by 250 mm galvanized into ceiling diffusers. Display cabinet zone has a separate 400 cubic metres per hour air handler with 200 mm diameter SBTF-1602 stainless trunk to the cabinet face.
Total duct fabrication for the store is approximately 62 metres of mixed galvanized and stainless rectangular and round, plus the refractory-lined exhaust trunk. Fabrication time on the SBAL-V coil line and SBTF-1602 spiral former is approximately 1.5 days. Field installation by local mechanical contractor approximately 3 to 4 days. Commissioning and food safety sign-off approximately 1 day.
What to specify when you put a bakery duct package out to tender
The single best lever for keeping a bakery duct package on budget and on schedule is a tight specification at tender stage. The specification below is the SBKJ engineering template used as a starting point for new bakery projects.
- State the bakery type, daily throughput, product mix (loaf, pastry, laminated, frozen, gluten-free) and operating hours.
- Reference AS 4674, AS 1668.2, FSANZ Food Standards Code 3.2.3, NFPA 86, NFPA 484 and AS/NZS 60079 in the compliance clause.
- Specify duct fabrication to AS/NZS 4254 with pressure class and leakage class for each duct system.
- Specify material by zone — galvanized Z275 for non-product zones, 304 stainless above product and in proofer/cooler/packaging zones, 316 stainless for oven exhaust below 350 degrees Celsius, refractory-lined carbon steel for oven exhaust above 350 degrees Celsius.
- Specify the dust hazard analysis under NFPA 484, including conductive ducting, isolation valves, vented or suppressed dust collector and removal of ignition sources from the dust cloud zone.
- Specify the hazardous area classification under AS/NZS 60079 for the flour intake, sifter, mixer charging and dust collector enclosure zones.
- Specify the proofer conditions (27 to 32 degrees Celsius, 70 to 85 percent RH) with dedicated humidified air handler and externally insulated 304 stainless supply duct.
- Specify the oven exhaust to NFPA 86 with refractory-lined trunk or 316 stainless trunk, NFPA 86 dampers, fail-safe-open behaviour and an emissions discharge meeting the local environment protection authority.
- Specify the cooling tunnel ambient at 8 to 12 degrees Celsius with externally insulated 304 stainless supply duct above the product belt and SBTF-1602 round spiral main trunks.
- Specify the packaging room at 18 to 22 degrees Celsius and 60 to 65 percent RH, with HEPA H13 or H14 final filter, 304 stainless smooth-welded supply duct downstream of the filter and 10 to 15 pascals positive pressure.
- Specify the frozen distribution interface per the cold-chain duct guide, with vapour-tight insulated panel construction and a controlled air-lock loading dock.
- Specify the commissioning sequence — leakage test, air balance, environmental conditioning, food safety walk-through — and the as-built documentation pack.
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FAQ
What ductwork standards apply to Australian bakery HVAC design?
AS 4674 (food premises), AS 1668.2 (ventilation), FSANZ Food Standards Code 3.2.3, HACCP from Codex Alimentarius, NFPA 86 for industrial ovens, NFPA 484 for combustible dust, AS/NZS 60079 for hazardous area classification, and AS/NZS 4254 for the ductwork itself. Each layer applies independently — compliance with one does not waive the others.
Is flour dust really a combustible dust hazard?
Yes. Flour dust is a Class St1 combustible dust under NFPA 484 with a documented minimum ignition energy that overlaps common ignition sources in mixer rooms, sifter rooms and pneumatic conveying. Where the system entrains flour above the minimum explosible concentration, the duct must be conductive, bonded and earthed, isolated from the dust collector, and the collector itself must be explosion vented or suppressed. AS/NZS 60079 governs the hazardous area classification of the surrounding zone.
Should bakery ductwork be stainless or galvanized?
It depends on the zone. Galvanized Z275 is fine for general supply and return in non-product areas. 304 stainless is required above exposed product, in food-contact zones under AS 4674, downstream of HEPA filters in clean packaging rooms, and in saturated humid air zones (proofer, cooling tunnel). 316 stainless or refractory-lined carbon steel is used for oven exhaust trunks above 250 degrees Celsius.
What temperature and humidity should a dough proofing room hold?
27 to 32 degrees Celsius at 70 to 85 percent relative humidity for yeasted bread doughs, with control bands of plus or minus 1 degree Celsius and plus or minus 5 percent RH. Laminated dough (croissant, kouign-amann) proofs cooler at 26 to 28 degrees Celsius to protect the butter layers. The supply duct downstream of the humidifier must be 304 stainless and externally insulated, and a small constant exhaust bleed of 5 to 10 percent removes the CO2 from yeast fermentation.
How hot is bread oven exhaust and what duct material is required?
200 to 280 degrees Celsius continuous on most continuous tunnel ovens, with peaks above 300 degrees Celsius on deck reload. NFPA 86 governs the exhaust geometry, dampers, fans and supports. Below 250 degrees Celsius continuous, heavy-gauge 316 stainless is the standard. Above 250 degrees Celsius continuous, refractory-lined carbon steel is normally specified. A thermal oxidiser may be added where local environment protection authority emissions conditions require it.
