Why tunnel ventilation is unique
Commercial HVAC engineers walking onto their first tunnel project tend to underestimate the scale and severity of the loads. A fifty-storey commercial tower might run 300,000 litres per second of total supply air at 750 Pa external static pressure. A single Australian motorway tunnel running at peak emergency-mode extraction can move 800 cubic metres per second — nearly three times the entire airflow of that fifty-storey tower — at typical static pressures of 250 to 600 Pa, through ducts up to 2.5 metres in diameter, with the entire system required to survive a 100 MW heavy-goods-vehicle fire for a minimum of two hours.
Tunnel ventilation answers four engineering questions that almost no other HVAC system has to answer simultaneously. First, it has to remove the heat dumped into the tunnel envelope by traffic — a stationary heavy goods vehicle in congestion idles at 30–60 kW of waste heat, and a rail tunnel station can absorb 2–8 MW of regenerative braking heat per peak hour. Second, it has to manage smoke during fire emergencies, maintaining tenable evacuation conditions for the time required to clear passengers from rolling stock or self-rescue from vehicles. Third, it has to control dust loading from tyre wear, brake pad debris and aerosolised mineral particulate. Fourth, it has to dilute combustion pollutants — carbon monoxide, oxides of nitrogen, particulate matter from diesel exhaust — to within prescribed exposure limits along the tunnel length and at portal exits where ambient air-quality regulators monitor compliance.
The key consequence for HVAC duct fabrication is that tunnel duct is heavy. Wall thicknesses run 1.5 to 3.0 mm in galvanised steel for general supply and exhaust applications — three to five times the gauge of commercial spiral duct — with stainless 316L specified at the same heavy gauges for corrosive environments. Joints have to survive both the steady-state pressure differential and the cyclic aerodynamic loading from train piston effects in rail tunnels. Fire-rated sections move into welded construction with full-penetration seam welds and intumescent flange gaskets. Procurement timelines move out to 18 to 24 months because the volume of material and fabrication is too large to compress, and Factory Acceptance Tests are mandatory on first-of-type and on every batch of fire-rated duct.
This guide walks an HVAC contractor or fire engineer from tunnel category through to hot smoke test, with reference to the standards that apply in Australia (AS 4825, AS 1668.4, AS 1530.4, AS 4391), the United States (NFPA 502, NFPA 130), Europe (EN 14067, EN 1366-8) and international guidance (PIARC). It covers project examples from Snowy 2.0, the West Gate Tunnel, North East Link, Sydney Metro, Melbourne Metro Tunnel, Crossrail / Elizabeth Line, the Brenner Base Tunnel and Mumbai Coastal Road. The objective is to give the reader a single reference they can use to brief a tunnel ventilation duct package, write a specification, evaluate a fabricator, and walk into the hot smoke test with confidence.
Tunnel categories and their ventilation profiles
The first decision in any tunnel HVAC scope is to fix which type of tunnel you are working in, because the ventilation strategy, duct material and fire engineering basis change radically across categories. The four major categories are road, rail (with several sub-categories), hydropower, and mining, with utility tunnels (cable, water, sewer) as a fifth, smaller category.
Road tunnels — urban and motorway
Road tunnels split into urban and motorway sub-categories. Urban tunnels — Sydney's WestConnex M4-M5 link, the Melbourne CityLink Domain Tunnel, the Burnley Tunnel — are typically two to four kilometres long with congested low-speed traffic at peak hours and free-flowing traffic at off-peak. The dominant load is pollutant dilution under congestion (CO and NOx peak at low speeds because catalytic converter efficiency drops at idle) plus heat removal during long stationary queueing. Motorway tunnels — the Sydney Eastern Distributor, the Melbourne West Gate Tunnel, the planned Western Harbour Tunnel — are designed for higher continuous traffic speeds and have lower idle-pollution loading per metre but higher continuous heat from sustained vehicle movement.
Both classifications fall under NFPA 502 in the United States and AS 4825 in Australia, with PIARC reports providing the international engineering reference. The design fire size — the most consequential single number in the entire HVAC scope — is typically set at 30 MW for car and light vehicle fires, 50 MW for medium goods vehicles and 100 MW or higher for heavy goods vehicles carrying flammable cargo. AS 4825 references these sizes and gives Australian tunnel engineers the basis for ventilation airflow sizing under fire mode.
Rail tunnels — metro, intercity, freight, high-speed
Rail tunnels split into four sub-categories with different ventilation profiles. Metro and light rail tunnels — Sydney Metro, Melbourne Metro Tunnel, Cross River Rail in Brisbane — are short-headway high-frequency systems where the train piston effect dominates normal ventilation and mechanical ventilation comes into play primarily during emergencies. Intercity passenger rail tunnels — sections of the Sydney Trains network, the V/Line Geelong tunnel — have lower frequency but longer rolling stock and different fire load profiles. Freight rail tunnels — including upcoming Inland Rail tunnel sections through northern New South Wales and southern Queensland — carry diesel locomotives and require continuous mechanical ventilation under normal operation because of locomotive exhaust loading. High-speed rail tunnels — currently planned for the East Coast HSR project linking Sydney, Canberra and Melbourne — have unique micro-pressure-wave issues at portal exits and require specific aerodynamic detailing per EN 14067-5.
Rail tunnels are governed by NFPA 130 in the United States, by sections of AS 4825 plus operator-specific standards in Australia (Transport for NSW T HR series, Victorian Department of Transport standards), and by EN 14067 for aerodynamic design and various TSI documents in Europe. Design fire sizes are typically lower than road tunnels — 15 MW for older rolling stock, 20–35 MW for modern rolling stock with composite materials — because rolling stock fire loads are bounded by the maximum fuel load on a single train.
Hydropower tunnels
Hydropower tunnels are a different beast altogether. The waterway tunnels themselves — penstocks carrying water from headrace to turbine, draft tubes carrying spent water back to the tailrace — are full of water during operation and do not require HVAC. The HVAC scope is concentrated in the dry zones around the waterway: valve halls housing main inlet valves, transformer chambers, control rooms, machine halls in the powerhouse cavern, access tunnels, and emergency egress shafts. Snowy 2.0 — the largest pumped-hydro project in the world by reservoir volume — is the major Australian example, with two reservoirs (Tantangara and Talbingo) connected through 27 kilometres of headrace and tailrace tunnels and a powerhouse cavern roughly 800 metres underground.
Hydropower HVAC scope splits into two phases. Construction-phase ventilation supplies large volumes of fresh air to working faces during tunnel boring and excavation — typically 20 to 40 cubic metres per second per advancing heading, with flexible Layflat ducting for the headings themselves and rigid main duct on the access tunnel. Permanent-operation ventilation handles transformer heat rejection, hydrogen leakage from generator cooling, electrical room cooling, personnel access ventilation and the powerhouse cavern itself.
Mining declines and underground mine ventilation
Mining ventilation deserves its own treatment — see the companion guide on mining ventilation HVAC duct — but for the tunnel-engineering audience the key point is that a mine decline is a tunnel that connects the surface to the underground workings, and the ventilation system has to handle diesel exhaust from haul trucks, dust from drilling and blasting, methane and carbon dioxide from the rock mass, plus heat from auto-compression at depth. The ventilation strategy is typically push-pull with main intake and return drifts and forcing or exhausting auxiliary fans into individual headings. The duct scope at the surface and along the decline portal is large-diameter rigid duct in heavy-gauge galvanised — almost identical fabrication scope to a road or rail tunnel main supply duct.
Utility tunnels — cable, water, sewer
Utility tunnels carrying high-voltage power cable, telecommunications fibre, water, sewer or gas have small-scale HVAC scope by comparison but the demands are exacting. Power cable tunnels — the Sydney North Shore cable tunnel, the planned Western Sydney Energy Link cable tunnel — have continuous heat-rejection loads from the cables themselves (typically 1–3 watts per metre per cable circuit at maximum continuous rating) and the ventilation system has to keep cable surface temperatures below the manufacturer's continuous rating temperature, usually 90 degrees C for XLPE-insulated cable. Telecommunications tunnels are lower-load but have very tight environmental control requirements for splice chambers. Water and sewer tunnels typically have minimal HVAC except at access shafts.
Ventilation strategies — longitudinal, transverse, semi-transverse, hybrid
Once the tunnel category is established, the next decision is which ventilation strategy to deploy. The four primary strategies — longitudinal, transverse, semi-transverse and Saccardo nozzle — and their hybrid combinations cover the great majority of modern tunnel ventilation designs.
Longitudinal ventilation
Longitudinal ventilation pushes air along the tunnel axis from one portal to the other, using either jet fans suspended from the tunnel ceiling at intervals or shaft fans drawing air at strategic intermediate points. The strategy is the cheapest to install, most efficient under normal traffic flow, and best suited to unidirectional traffic where the natural draught created by moving vehicles supplements the jet fan thrust. Most modern road tunnels under three kilometres in Australia use longitudinal ventilation as the primary strategy. The Sydney Cross-City Tunnel, the Sydney Eastern Distributor, the Brisbane Clem Jones Tunnel and the Brisbane Airport Link all use predominantly longitudinal systems.
Under fire mode, longitudinal ventilation pushes smoke downstream of the fire location toward the upstream portal, allowing evacuees to walk against the smoke flow toward the upwind portal. The strategy works for traffic moving in one direction but breaks down for bidirectional traffic because evacuees can be trapped on either side of the fire. Longitudinal ventilation also requires high-power jet fans able to overcome the fire-induced thermal gradient — typical jet fan thrust on a major Australian motorway tunnel runs 1,200 to 2,500 newtons per fan with banks of six to twelve fans per traffic direction.
HVAC duct scope on a purely longitudinal tunnel is concentrated at portals, ventilation shafts and jet fan power supplies — there is no continuous duct along the tunnel itself. Where shaft ventilation is provided, the shaft duct is typically large-diameter rigid duct (1,800 to 2,500 mm diameter) in heavy-gauge galvanised, with transition to rectangular duct at the surface fan plant.
Transverse ventilation
Transverse ventilation runs parallel supply and exhaust ducts along the tunnel ceiling or under the carriageway, with supply air entering through registers along one duct and exhaust air leaving through registers along the other. The strategy is best suited to long tunnels (five kilometres and above), bidirectional traffic, and tunnels where ambient air quality regulations require high-volume continuous fresh air injection. The transverse strategy decouples evacuation direction from prevailing flow direction — under fire mode the local exhaust register over the fire location can extract smoke vertically, leaving the longitudinal flow undisturbed for evacuation in either direction.
The HVAC duct scope on a transverse system is enormous — full-length supply and exhaust ducts running the entire tunnel length, typically rectangular section in concrete-cast plenum or structural steel duct. The Eurotunnel between Folkestone and Calais, the Mont Blanc Tunnel between France and Italy, and the older Sydney Harbour Tunnel use elements of transverse ventilation. Modern Australian transverse-ventilation projects are uncommon because longitudinal-with-shaft-extraction has become the default for cost reasons.
Semi-transverse ventilation
Semi-transverse ventilation runs a single duct — usually supply only or exhaust only — along the tunnel length, with the other direction handled by longitudinal flow at the portals. Supply-only semi-transverse pushes fresh air uniformly along the tunnel and lets it exit at the portals; exhaust-only semi-transverse draws smoke and pollutants from points along the tunnel and exhausts via the duct to a surface plant. The strategy gives some of the benefits of full transverse at lower cost and is used in tunnels where one direction (usually fresh-air supply for pollutant dilution) is the binding constraint.
Saccardo nozzle injection
The Saccardo nozzle is a high-velocity slot injection system that introduces fresh air at the tunnel portal at a steep angle to the tunnel axis, using the entrained jet to drive bulk longitudinal flow without the need for distributed jet fans along the tunnel. The strategy is mechanically elegant, uses less duct volume than transverse, and is particularly efficient on shorter tunnels with constrained ceiling height. Several European tunnels and a few Australian projects use Saccardo nozzles as the primary or supplementary ventilation strategy.
Hybrid systems
Most modern major-project tunnels use hybrid configurations — longitudinal jet fans along the running tunnel with transverse-style smoke extraction at intervals through ceiling exhaust dampers connected to a single overhead exhaust plenum. This hybrid approach gives the cost efficiency of longitudinal ventilation under normal mode with the smoke-extraction discipline of transverse ventilation under fire mode. The Melbourne West Gate Tunnel, North East Link and Sydney WestConnex M4-M5 use hybrid configurations of various subtypes, with HVAC scope reflecting the combination — jet fans at intervals, ceiling exhaust dampers feeding intermittent extraction shafts, plus shaft ducts at major intermediate ventilation buildings.
Smoke control and emergency ventilation modes
Tunnel ventilation systems run in two distinct modes — normal mode for pollutant dilution and heat removal under steady-state traffic, and emergency mode for smoke management during a fire incident. The transition between modes is governed by fire detection and operator override, with mode-switch response times typically required to be under 90 seconds from fire confirmation.
Fire-emergency longitudinal flow
Under longitudinal smoke management, jet fans drive smoke from the fire downstream toward the downwind portal, leaving the upwind portal as the egress route. The critical design parameter is the critical velocity — the minimum longitudinal velocity required to prevent smoke backlayering against the prevailing flow. PIARC and NFPA 502 give critical velocity correlations as a function of fire heat release rate, typically 2.5 to 3.5 metres per second for design fires up to 50 MW and 3.0 to 4.0 metres per second for fires up to 100 MW. Jet fan banks have to be sized for the worst-case combination of design fire size, opposing natural draught and traffic-stoppage condition.
Smoke extraction via ceiling exhaust
In transverse and hybrid systems, smoke is extracted vertically from the tunnel through ceiling exhaust dampers into a longitudinal exhaust plenum and then to a surface fan plant. The dampers immediately downstream of the fire location open to draw smoke into the plenum; dampers further from the fire stay closed to maintain extraction velocity at the active dampers. The control logic is a critical part of the SCADA / BMS configuration and is one of the items hot smoke testing is specifically designed to verify.
Refuge area positive pressurisation
Refuge areas — emergency egress passages running parallel to the main tunnel and separated by fire-rated walls — are positively pressurised under fire mode to prevent smoke ingress when refuge doors open during evacuation. Pressurisation airflow is typically supplied through dedicated refuge-supply duct routed to door openings, with the supply rate calculated to maintain a minimum velocity through any open door (commonly 1.0 metre per second per AS 1668.1 / NFPA 92 principles) and a minimum overpressure of 30 to 50 Pa relative to the tunnel.
The standards landscape
Tunnel HVAC duct sits at the intersection of fire engineering, mechanical engineering and civil tunnel design, and the standards that apply reflect that. The list below covers the most important documents and what they govern. A specifying engineer will hold all of these in scope on any major tunnel project.
NFPA 502 — Standard for Road Tunnels, Bridges, and Other Limited Access Highways
NFPA 502 is the dominant North American standard for road tunnel fire safety and ventilation. It covers fire scenarios, design fire heat release rates, ventilation strategy options, emergency egress provisions and operational requirements. Chapter 7 covers fire protection and life safety; chapter 11 covers emergency ventilation. NFPA 502 design fires are 5 MW for cars, 30 MW for buses and small goods vehicles, 50 MW for medium goods vehicles, and 100 MW for heavy goods vehicles carrying flammable liquid or compressed gas — the heavy-goods scenario being the binding constraint for most modern motorway tunnel ventilation sizing.
NFPA 130 — Standard for Fixed Guideway Transit and Passenger Rail Systems
NFPA 130 is the dominant standard for fixed-guideway rail systems including metro, light rail, heavy rail and rapid transit. It covers tunnel ventilation, station HVAC, emergency egress, and the rolling stock fire performance criteria that bound the design fire size. Chapter 7 covers emergency ventilation. NFPA 130 design fire sizes vary with rolling stock material — typically 5–15 MW for older steel-bodied stock and 15–35 MW for modern composite-bodied stock with non-compliant interior materials. NFPA 130 also addresses platform-screen-door integration, station overpressurisation under emergency mode, and over-track exhaust.
PIARC tunnel ventilation guidelines
The World Road Association (PIARC) publishes a series of technical reports on road tunnel ventilation, fire protection and operations that are the international engineering reference for tunnel ventilation design. PIARC Report 2017R02EN on road tunnel ventilation, Report 2008R02EN on tunnel safety and human behaviour, and the periodic updates from Technical Committee 4.4 on tunnel operations are core references. Most major Australian, European and Asian road tunnel projects use PIARC as the engineering basis with NFPA 502 or AS 4825 as the local compliance overlay.
AS 4825 — Tunnel Fire Safety
AS 4825 is the Australian Standard for tunnel fire safety, covering road tunnels and mass-transit rail tunnels. It is the binding compliance document for major Australian tunnel projects and references PIARC and NFPA documents for technical detail. Section 5 covers ventilation and smoke management. AS 4825 sets out the fire engineering process, design fire scenarios, performance criteria and verification methodology — including the requirement for hot smoke testing as the final pre-handover acceptance milestone.
AS 1668.4 — The use of mechanical ventilation in buildings
AS 1668.4 is the Australian Standard for mechanical ventilation in buildings and applies to the building-integrated portions of tunnel HVAC scope — ventilation buildings, fan plant rooms, electrical equipment rooms and station HVAC where applicable. Its companion AS 1668.1 covers fire and smoke control in buildings and is referenced for refuge pressurisation calculations.
EN 14067 — Railway applications, aerodynamics
EN 14067 is the European Standard series covering railway aerodynamics. EN 14067-5 is specifically about tunnel aerodynamics and addresses the train-induced piston effect, blockage ratio, micro-pressure waves at portal exits, and pressure transients on rolling stock and tunnel infrastructure. The standard is the binding reference for high-speed rail tunnel design and a common reference for metro and intercity tunnels where pressure transient loading on duct supports and seals is a design concern.
Other key references
Other documents commonly in scope: AS 1530.4 for fire-resistance testing of building elements (used for fire-rated duct); AS 4391 for hot smoke testing methodology; ISO 9705 for room corner fire test (referenced for tenability calibration); EN 1366-8 for fire-resistance tests of smoke extraction ducts; BS 476 Part 24 for fire test of ventilation ducts (UK heritage but still cited in Australian specifications); EN 12237 and EN 1507 for ductwork leakage classification; AS 1668.2 for ventilation design of buildings; and the various ISO TR documents on tunnel fire safety.
Hot smoke testing per AS 4391 and ISO 9705
Hot smoke testing is the final pre-handover commissioning event for most major Australian tunnel projects, and the single biggest reason that tunnel HVAC duct must be specified for full design temperature performance. The test introduces a controlled fire load into the tunnel — typically methanol pool fires of known dimension and burn rate, calibrated to deliver between 1 and 10 MW of convective heat release — and validates the entire smoke management system end-to-end against the fire engineering report design intent.
AS 4391 governs hot smoke testing in Australia and gives the test methodology, instrumentation requirements, calibration procedures and acceptance criteria. ISO 9705 provides the international room-corner-test reference used to calibrate smoke generation rates. The test sequence typically runs over multiple nights during low-traffic windows and includes baseline measurements (no fire), small-scale fires (1–2 MW for sensor and damper response validation), and full-scale fires (5–10 MW for end-to-end smoke management verification). Instrumentation includes thermocouple trees through the smoke layer, optical density meters along the egress path, pitot-static probes at ventilation duct cross-sections, and high-definition video for smoke-layer behaviour.
The implication for HVAC duct procurement is twofold. First, the duct must be specified, fabricated and installed to survive the design fire — there is no margin for under-specifying material thickness, joint design or support spacing because the hot smoke test will expose any weakness. Second, the documentation trail back to the fabricator must be complete — weld procedures, weld inspector reports, mill certificates for fire-rated material, FAT sign-offs on first-of-type and on every batch — because if the hot smoke test fails on duct integrity, the contractor needs to be able to trace the failure mode to a specific weld, a specific batch, a specific procedure. SBKJ supplies the full document trail with every fire-rated duct package as standard.
Material specifications for tunnel ventilation duct
Material selection for tunnel HVAC duct is governed by four factors: pressure and aerodynamic loading, corrosion environment, fire integrity requirement and fabrication economics. The four common materials and their typical applications are described below.
Heavy-gauge galvanised steel G275
The workhorse material for general tunnel HVAC duct is heavy-gauge hot-dip galvanised steel to G275 coating mass (275 g/m² total both sides), in wall thicknesses from 1.5 mm to 3.0 mm depending on duct diameter and pressure class. Galvanised steel gives an economic balance of mechanical strength, corrosion resistance for typical tunnel environments, weldability and fabrication speed. Spiral-wound round duct on a heavy-gauge spiral tubeformer can be produced at 1.0 to 2.5 metres per minute even at 2.0 mm wall thickness, which is the only practical fabrication speed for the kilometres of duct a major tunnel project requires.
316L stainless steel
Stainless 316L is specified for highly corrosive environments — rail tunnels with acid leachate from brake dust and track ballast, road tunnels exposed to chloride attack from de-icing salts (less common in Australia but standard in northern hemisphere alpine tunnels), tunnels passing through saline or sulphate-bearing rock formations, and the wet electrical environments around hydropower valve halls. 316L pricing is typically four to six times galvanised, so its use is targeted at the specific high-corrosion zones rather than general duct.
Aluminised steel
Aluminised steel — carbon steel sheet hot-dip aluminised on both surfaces — is specified for exhaust ducts that need to survive prolonged high-temperature service during a fire event. The aluminium-iron intermetallic surface layer protects the underlying steel up to roughly 700 degrees C for short durations, well above the temperature at which galvanised steel zinc coating volatilises (around 420 degrees C). Aluminised steel is common in road tunnel ceiling exhaust ducts and rail tunnel emergency exhaust paths.
Concrete and concrete-lined steel
For very large transverse exhaust plenums, the most economic construction is often concrete cast as part of the tunnel civil works, with steel duct only at the connections to surface fan plant and at fire damper bays. Snowy 2.0's powerhouse cavern HVAC uses a combination of concrete-cast plenum sections and rigid steel duct at the connection points. The HVAC contractor's fabrication scope on a concrete-plenum tunnel is concentrated at the steel-to-concrete transitions, the damper bays and the surface fan plant duct.
Fire-rated ductwork — A60, A120 and the test standards
Fire-rated ductwork is duct that has been tested and certified to maintain integrity (E rating) and insulation (I rating) for a defined period under standard fire test conditions. The two integrity ratings most commonly cited in Australian tunnel specifications are A60 (60 minutes of fire integrity) and A120 (120 minutes of fire integrity), with test methodologies from EN 1366-8, BS 476 Part 24 and AS 1530.4 forming the compliance basis.
EN 1366-8 is the European Standard specifically for fire-resistance testing of smoke extraction ducts — that is, ducts that are required to extract smoke from a burning compartment — and is the closest match to the operational requirement on a tunnel smoke exhaust duct. BS 476 Part 24 is the older UK reference for fire-resistance testing of ventilation ducts, still cited in Australian fire engineering reports because it gives a longer-established evidence base. AS 1530.4 is the Australian Standard for fire-resistance testing of building elements and is used as the local compliance test for fire-rated duct.
The fabrication implications of A60 / A120 fire rating are substantial. Joint design typically moves to fully welded continuous seam construction or to flanged joints with documented intumescent gasket and fire sealant; mechanical lock-seam joints are not normally fire-rated to A120. Material thickness is typically the heavy end of the gauge range — 2.0 to 3.0 mm in galvanised or stainless. Hangers and supports must be fire-rated separately and the full assembly (duct, supports, fasteners, penetration seals) must be tested as a system, not as components in isolation. The test certificate becomes a project-controlled document and the fabricator's installation has to match the tested configuration in every detail.
Tunnel exhaust duct configurations
Tunnel exhaust duct configurations vary with the ventilation strategy and the smoke management mode. Three configurations dominate Australian tunnel projects.
Ceiling-mounted continuous exhaust (transverse strategy)
In a full transverse system, a continuous exhaust duct runs the length of the tunnel along the ceiling, with extraction registers at intervals (typically 50 to 100 metres) feeding the duct from the carriageway. The duct cross-section is typically rectangular, sized for the bulk extraction flow plus margin for damper actuation transients. Construction is heavy-gauge welded steel or cast concrete plenum integrated with the tunnel structure. This configuration is uncommon in modern Australian projects because of cost.
Interval extraction with fire-rated dampers (longitudinal with smoke extraction)
The hybrid configuration used on most modern Australian motorway tunnels — Melbourne West Gate Tunnel, North East Link, Sydney WestConnex M4-M5, the planned Western Harbour Tunnel — is interval extraction. A longitudinal exhaust plenum runs along the tunnel ceiling but only opens to the carriageway through fire-rated dampers at specific intervals (typically 100 to 200 metres). Under normal mode all dampers are closed and the plenum carries no flow. Under fire mode, the dampers immediately downstream of the fire location open while all others remain closed, drawing smoke vertically into the plenum and extracting it via shaft fans at intermediate ventilation buildings.
The HVAC duct fabrication scope on an interval-extraction system is the longitudinal exhaust plenum (typically rectangular, heavy-gauge galvanised or aluminised, fire-rated to A120), plus the local damper bays and the shaft duct at each ventilation building. The damper bays are particularly demanding because they incorporate fire-rated dampers, fire-rated transitions and fire-rated penetration seals through the tunnel ceiling slab into the plenum.
Ventilation shaft duct
At each major ventilation building or surface fan plant, the tunnel duct transitions through a vertical or inclined shaft to the surface. Shaft duct is typically very large diameter (1.8 to 2.5 metres) round duct or rectangular section, in heavy-gauge galvanised or aluminised. The shaft duct sees the highest pressures and the largest cyclic loading from train piston effects (in rail tunnels) or pressure-mode-switch transients (in road tunnels), and is specified to the highest pressure class of the project.
Major Australian tunnel projects and their HVAC scope
Australian tunnel infrastructure is in a peak construction cycle through the late 2020s, with major projects on both coasts at various stages of design, tendering, construction and commissioning. The summary below covers the projects most relevant to HVAC contractors and fabricators bidding tunnel ventilation duct packages.
Sydney Metro
Sydney Metro is delivered as a series of staged extensions to a unified driverless metro network. The City and Southwest extension has linked Chatswood through North Sydney and the central business district to the existing Bankstown line, with services already operational on the inner segments. The Western Sydney Airport line is under construction connecting St Marys to the new Western Sydney International Airport at Badgerys Creek. The Northwest line connects Tallawong through Macquarie University to Chatswood. The HVAC scope across Sydney Metro is concentrated at stations, ventilation shafts and emergency egress — the running tunnels themselves use train-piston-effect natural ventilation supplemented by shaft fans for emergency mode. Station HVAC integrates with platform screen doors and over-track exhaust per NFPA 130 principles, with emergency ventilation per the operator's T HR series standards. Duct material is predominantly heavy-gauge galvanised with stainless 316L at corrosive transition zones.
Melbourne Metro Tunnel
The Melbourne Metro Tunnel adds a new nine-kilometre rail tunnel under the city of Melbourne with five new stations — Tom Town (Footscray-side portal), Anzac (St Kilda Road), State Library (city centre), Parkville (Royal Parade) and Arden (North Melbourne) — connecting the Sunbury line in the west to the Cranbourne and Pakenham lines in the east. The project is in late commissioning at the time of writing with phased service introduction underway. HVAC scope includes station ventilation, over-track exhaust, ventilation shafts at intermediate locations, and emergency ventilation per AS 4825 with fire engineering basis aligned to Victorian Department of Transport standards.
West Gate Tunnel
The West Gate Tunnel in Melbourne is a 4-kilometre twin-tube road tunnel under the Yarra River and inner west, providing a second crossing alternative to the West Gate Bridge. The HVAC scope uses a hybrid longitudinal-with-interval-extraction configuration with significant fire-rated duct at ceiling exhaust plenums and major surface fan plants at the western and eastern portals plus an intermediate ventilation building. Fire engineering is to AS 4825 with PIARC reference for ventilation strategy.
North East Link
The North East Link in Melbourne is a major motorway tunnel project linking the Eastern Freeway in Bulleen to the M80 Ring Road at Greensborough, with twin-tube tunnels of approximately 6.5 kilometres under residential suburbs and the Yarra River. The HVAC scope is among the largest of any current Australian motorway project, with multiple ventilation buildings, kilometres of fire-rated ceiling exhaust plenum, and duct material specifications running to heavy-gauge galvanised G275 at 2.0–3.0 mm wall as the bulk material with stainless and aluminised at fire-rated and high-temperature zones.
Snowy 2.0
Snowy 2.0 is the largest infrastructure project currently under construction in Australia by some measures and is the world's largest pumped-hydro project by reservoir energy storage. The scope includes 27 kilometres of headrace, tailrace and access tunnels connecting the existing Tantangara reservoir (at higher elevation) and the existing Talbingo reservoir, plus a powerhouse cavern roughly 800 metres underground housing six 350 MW reversible pump-turbines for a total of 2,200 MW of generating capacity and 350 GWh of energy storage. The HVAC scope splits between construction-phase tunnel ventilation (large-volume Layflat duct at the working faces, rigid main duct on the access tunnel) and permanent-operation ventilation for the powerhouse cavern, transformer chambers, valve halls, control rooms and access tunnels. Material specifications are heavy-gauge galvanised G275 for general scope with stainless 316L at the wet electrical environments.
Inland Rail
Inland Rail is the major national freight rail project linking Melbourne to Brisbane via inland routes through Victoria, New South Wales and Queensland. While the bulk of Inland Rail is at-grade or in cutting, several tunnel sections through difficult terrain in northern New South Wales (the Liverpool Range) and southern Queensland (the Toowoomba Range) require ventilation engineering. Diesel-locomotive freight operations require continuous mechanical ventilation under normal mode, distinct from the train-piston-effect natural ventilation used on metro lines.
WestConnex and Western Harbour Tunnel (Sydney)
WestConnex M4-M5 link, the planned Western Harbour Tunnel and Beaches Link, and the Sydney Gateway road project together form the largest road tunnel programme in Australian history. WestConnex uses hybrid longitudinal-with-interval-extraction across multiple tunnel sections with fire-rated duct scope in the thousands of metres. The Western Harbour Tunnel will be one of the largest single-project HVAC duct packages in Australia at the time of construction.
Bruce Highway upgrade tunnel sections (Queensland)
Various sections of the Bruce Highway upgrade in Queensland include short tunnels and underpasses with HVAC scope at the smaller end of the spectrum. The Brisbane Cross River Rail tunnel project — a 5.9-kilometre rail tunnel under the Brisbane River and CBD — is the larger Queensland project of relevance and is at advanced commissioning at the time of writing.
Major global tunnel projects
Beyond Australia, several major international tunnel projects are reference examples for tunnel HVAC engineering and are worth understanding for context.
Crossrail / Elizabeth Line (London)
Crossrail, marketed as the Elizabeth Line, is a 118-kilometre east-west rail crossing of London with 42 kilometres of new tunnel under central London. The project is fully operational and the HVAC scope is one of the largest single rail tunnel ventilation deliveries in modern history. Configuration is predominantly longitudinal with shaft-based emergency extraction at intermediate ventilation buildings. Materials and configuration are largely consistent with Sydney Metro at larger scale.
Grand Paris Express
The Grand Paris Express is a 200-kilometre metro extension around Paris with four new metro lines and 68 new stations, primarily in tunnel. The project is in staged delivery through the late 2020s and represents the largest urban metro construction project in Europe. The HVAC scope reflects the size — long, deep tunnels with multiple intermediate ventilation buildings and emergency egress shafts.
Brenner Base Tunnel (Italy/Austria)
The Brenner Base Tunnel is a 64-kilometre rail tunnel under the Alps between Innsbruck in Austria and Fortezza in Italy, currently under construction and one of the longest rail tunnels in the world. The project requires substantial mid-tunnel ventilation and rescue station ventilation, with the longest underwater rail rescue platform configuration of any current tunnel project. HVAC duct is specified to European standards including EN 14067 for aerodynamics and EN 1366-8 for fire integrity.
Fehmarnbelt Link (Denmark/Germany)
The Fehmarnbelt Link is an 18-kilometre immersed-tube tunnel under the Fehmarn Belt connecting Denmark and Germany, currently under construction. The immersed-tube tunnel form factor — pre-cast concrete tunnel elements floated into position and lowered onto a prepared seabed — gives a different HVAC scope from bored tunnels, with simpler geometry but substantial dehumidification and pressure equalisation requirements at the joints.
Mumbai Coastal Road Tunnel
The Mumbai Coastal Road tunnel in India is a 2.07-kilometre twin-tube road tunnel under the Arabian Sea, recently opened. The project used standard NFPA 502 / PIARC engineering basis with hybrid longitudinal-with-interval-extraction configuration and is a recent example of a major Asian road tunnel HVAC delivery.
Aerodynamic forces in tunnels
Tunnel HVAC duct sees aerodynamic loading that has no commercial-HVAC equivalent. Three load sources — train piston effect, blockage ratio effects and micro-pressure waves — drive design decisions on duct gauge, joint integrity and support spacing.
Train piston effect
A train moving through a tunnel pushes air ahead of it and pulls air behind it, creating a strong axial flow proportional to train speed and inversely related to the blockage ratio (train cross-section divided by tunnel cross-section). On a typical Sydney Metro tunnel, train piston effect generates flow velocities of 8 to 15 metres per second along the running tunnel, with associated static pressure transients of 2 to 5 kPa at the running tunnel cross-section. HVAC duct in the running tunnel and at adjacent ventilation buildings sees this cyclic loading at every train pass — typically every 90 seconds at peak headway — and duct supports, flange gaskets and damper actuators have to survive several million load cycles over the project lifetime.
Blockage ratio
Blockage ratio is the ratio of the train cross-section to the tunnel cross-section, and it controls the magnitude of the piston-effect transients. Typical metro tunnels run blockage ratios of 0.20 to 0.30; high-speed rail tunnels run lower blockage ratios (0.10 to 0.15) to reduce micro-pressure-wave issues at portal exits. Design of duct geometry around the running tunnel — particularly at ventilation shaft connections and station throats — has to take blockage ratio into account in the local pressure transient calculation.
Micro-pressure waves at portal exits
When a high-speed train enters a tunnel, a compression pressure wave travels ahead of the train along the tunnel at the speed of sound. When this wave reaches the far portal, it can radiate as a sonic boom — the so-called micro-pressure wave or tunnel sonic boom — which is a significant noise nuisance for neighbouring communities. EN 14067-5 gives the design methodology for managing micro-pressure waves through portal hood geometry, distributed perforation along the tunnel entry and bulk-flow modification. While micro-pressure waves are not a duct integrity issue per se, the portal geometry design to control them often interfaces with the ventilation duct termination at the portal, and the HVAC contractor needs to be aware of the constraint.
Hydropower tunnel HVAC in detail
Hydropower tunnel HVAC is sufficiently distinct from transport tunnel HVAC that it warrants its own treatment. The four major sub-systems are penstock and waterway tunnels (no HVAC scope, listed for completeness), valve hall ventilation, transformer chamber ventilation, and powerhouse cavern HVAC.
Penstocks and waterway tunnels
Penstocks (carrying pressurised water from headrace to turbine) and tailrace tunnels (carrying spent water from turbine back to the lower reservoir) are full of water during operation and carry no air-side load. The only HVAC scope on the waterway tunnels themselves is for inspection and maintenance access during dewatering — typically temporary ventilation deployed during outage windows.
Valve hall ventilation
The valve hall houses main inlet valves, draft tube isolation valves and the upper portions of the surge shafts. The hall is dry under normal operation but can experience humidity transients from valve operation and from leakage past valve seats. Ventilation requirements are dehumidification, electrical equipment cooling and personnel access. Material specifications often move to stainless 316L because of the wet electrical environment.
Transformer chamber ventilation
The transformer chamber houses generator step-up transformers and reactor banks, with continuous heat-rejection loads of 0.3 to 0.8 percent of the transformer rated capacity (typically 1 to 5 MW per transformer for hydropower applications). The chamber is also a fire-risk zone because of the substantial volume of dielectric oil in transformer tanks. Ventilation scope includes both normal heat-removal and fire-mode smoke extraction with fire-rated duct configuration.
Powerhouse cavern HVAC
The powerhouse cavern houses the main pump-turbines, generator-motors, cranes, control rooms and personnel access infrastructure. HVAC scope is the largest single sub-system on a hydropower project and includes machine hall ventilation, control room HVAC, electrical room cooling and personnel-access ventilation. On Snowy 2.0, the powerhouse cavern is roughly 250 metres long, 30 metres wide and 50 metres tall, requiring substantial bulk ventilation airflow.
Mining decline ventilation — overlap with tunnel ventilation
Mining decline ventilation overlaps significantly with tunnel ventilation at the surface portal and along the upper sections of the decline, where the duct configuration and material specification are essentially identical to a road or rail tunnel main supply duct. The companion guide on mining ventilation HVAC duct covers the full mining ventilation scope including auxiliary fans, flexible Layflat duct at the headings and the strata-ventilation regulatory framework. For tunnel-engineering audiences the takeaway is that the surface and decline-portal HVAC scope is fabricated on the same heavy-gauge spiral tubeformer used for road and rail tunnel main duct, giving a natural cross-project material and fabrication standardisation.
Cable tunnel and utility tunnel HVAC
Cable tunnels carrying high-voltage power transmission cables have continuous heat-rejection loads from cable I²R losses, with the ventilation system designed to maintain cable surface temperatures below the conductor manufacturer's continuous rating (typically 90 degrees C for XLPE-insulated cable). Cable tunnel ventilation is typically simpler than transport tunnel ventilation — uniform supply along the tunnel length, exhaust at one end, fire-rated cable barriers at compartment boundaries — but the demand for thermal accuracy is high because cable life is sensitive to operating temperature. Telecommunications conduit and utility tunnels carrying water or sewer mains have lower-load HVAC scope, primarily at access shafts and equipment rooms.
SBKJ machinery for tunnel ventilation duct fabrication
Tunnel HVAC duct fabrication uses a small number of high-throughput machines because the volume of material is too large for any other approach to be economic. SBKJ supplies three primary machine families to tunnel duct fabricators worldwide.
SBTF spiral tubeformer for round tunnel main duct
The SBTF spiral tubeformer is the workhorse machine for round tunnel ventilation duct, producing spirally-wound round duct from 80 mm to 2,500 mm diameter in heavy-gauge galvanised, stainless and aluminised steel up to 3.0 mm wall thickness. The SBTF runs at 1.0 to 2.5 metres per minute on heavy-gauge stock and can be configured with run-out conveyors, automated cut-off and integrated coil handling for high-volume tunnel projects. Spiral construction gives a continuous helical lock-seam joint with documented leakage class performance, suitable for both supply and exhaust applications.
SBAL-V auto duct line for rectangular tunnel exhaust
The SBAL-V auto duct production line handles rectangular duct in heavy-gauge for transverse exhaust sections, ventilation shaft duct and station HVAC. The line covers material from 0.5 mm through to 1.6 mm in standard configuration with options to 2.0 mm for heavy-gauge tunnel applications. Output is up to several hundred square metres of finished rectangular duct per shift, with automated TDF flange forming, length cut-off and material handling.
Heavy-duty welded duct lines for fire-rated tunnel applications
For fire-rated duct, fully-welded construction is often required to achieve A60 or A120 fire integrity. SBKJ supplies duct welding machines including longitudinal seam welders, spiral seam welders and rotary girth welders for fire-rated duct fabrication in heavy-gauge galvanised, 316L stainless and aluminised steel. The welded duct scope for a major tunnel project can run into thousands of metres of fire-rated assembly.
Snowy 2.0 specifics — construction and operation HVAC
Snowy 2.0 is a unique project in scale and configuration. Several specific points are worth flagging for HVAC contractors and fabricators bidding into Snowy 2.0 packages or future similar pumped-hydro projects.
First, the construction-phase HVAC scope is enormous and runs for several years before permanent-operation HVAC is commissioned. Tunnel boring machine support ventilation, machine hall excavation ventilation, and access tunnel ventilation during construction generate significant duct demand independently of the permanent operation scope. Second, the permanent-operation HVAC scope is dominated by the powerhouse cavern and transformer chambers, which require continuous high-volume ventilation for transformer heat rejection and cavern thermal management. Third, the access tunnel HVAC connects the surface to the underground workings via several kilometres of access drift, with the ventilation strategy a hybrid longitudinal system with intermediate fan stations.
The HVAC duct material specification is heavy-gauge galvanised G275 for the bulk scope with stainless 316L at the wet electrical environments around the valve halls and the lower transformer levels. Snowy 2.0 has been a significant reference project in the Australian heavy-gauge spiral duct market and demonstrates the production-volume requirement that drives fabricators toward high-throughput spiral tubeformer machines like the SBTF.
Rail tunnel specifics — train piston effect, station HVAC, platform screen doors
Rail tunnels have several specific design considerations that road tunnels do not share, and they drive HVAC duct decisions that the project specification has to anticipate.
Train piston effect on duct loading
The cyclic pressure transients from train piston effect — typically 2 to 5 kPa peak at every train pass — load duct supports, flange gaskets and damper actuators with several million cycles over the project lifetime. Duct gauge, support spacing and gasket material selection have to allow for fatigue, particularly at the connection between running tunnel duct and ventilation shaft duct where the pressure transient is concentrated.
Station HVAC and over-track exhaust
Station HVAC is a substantial scope on metro projects, with three main sub-systems: platform ventilation (handling passenger thermal comfort and contaminant dilution), over-track exhaust (extracting heat from rolling stock at standstill and during braking), and concourse / mezzanine HVAC. Over-track exhaust is fire-rated because the scenario it has to handle is a rolling stock fire with the train at the platform.
Platform screen door integration
Platform screen doors — full-height glass barriers between platform and trackway, opening only when a train is correctly stopped at the platform — change the station HVAC engineering substantially. With platform screen doors, the platform becomes an isolated zone from the trackway, and platform HVAC becomes a building-style zone with controlled airflow rather than a tunnel-style high-volume system. Sydney Metro and Melbourne Metro Tunnel both use platform screen doors at their underground stations.
Ventilation shaft locations
Rail tunnel ventilation shafts are typically located at intervals of 1 to 2 kilometres along the running tunnel, between stations, and serve dual purposes — emergency ventilation and emergency egress. The shaft duct and the ventilation building above-ground are part of the HVAC scope and the geometry interfaces tightly with the local civil and urban planning constraints.
Procurement timeline for tunnel HVAC duct packages
Tunnel HVAC duct procurement typically runs 18 to 24 months from contract award to final delivery and commissioning, broken into the following phases.
- Months 0–3 — Contract award and design coordination. Detailed shop drawing development, coordination with civil and structural designers, sign-off on duct routing, fire engineering basis lock-down, material specification confirmation.
- Months 3–6 — First-of-type and FAT. Manufacture of first-of-type duct sections, Factory Acceptance Test witnessed by the principal contractor's engineering representative, sign-off on fire-rated assembly procedure, baseline weld inspector qualification.
- Months 6–14 — Bulk fabrication. High-throughput fabrication of the bulk supply, exhaust and fire-rated duct on heavy-gauge spiral tubeformers and auto duct lines. Continuous QA witnessed at the fabricator, mill certificates collected, weld maps documented.
- Months 12–20 — Staged delivery. Phased delivery aligned to tunnel boring machine progress and civil completion. First installation typically at portals and ventilation shafts, with main tunnel duct following civil concrete cure milestones.
- Months 18–22 — Installation and commissioning. Duct installation, pressure and leakage testing, fan and damper commissioning, integrated system testing, snag close-out.
- Months 22–24 — Hot smoke test and handover. Hot smoke test per AS 4391, final acceptance, as-built drawing handover, warranty start.
Commissioning — hot smoke test, fan curve verification, integrated system test
Tunnel HVAC commissioning is a multi-stage process running over several months and culminating in the hot smoke test as the final pre-handover acceptance milestone.
The first stage is component commissioning — fan startup, damper actuation testing, sensor calibration, leakage testing of installed duct sections, and verification of fail-safe positions on power loss and fire alarm signal. The second stage is sub-system commissioning — fan curve verification on installed shafts, damper sequence testing on installed plenums, control system tuning at the local PLC level. The third stage is integrated system testing — coordinated response to simulated fire alarm signals from the tunnel SCADA / BMS, verification of mode-switch response time, validation of pressure profile and air-flow direction across the full system. The fourth and final stage is the hot smoke test, validating end-to-end performance under realistic fire conditions.
The Factory Acceptance Test on first-of-type duct is the project-controlled document that ties the on-site commissioning back to the fabrication baseline. SBKJ supports witnessed FAT on first-of-type duct as standard, with full documentation including weld procedure qualification records, weld inspector reports, mill certificates for raw material and dimensional inspection records.
How SBKJ supports tunnel ventilation duct projects
SBKJ Group supplies the heavy-gauge duct fabrication machinery used by tunnel HVAC contractors and fabricators worldwide. The relationship typically runs through one of two routes — direct supply of fabrication machinery to a contractor with in-house duct manufacturing, or supply through a fabricator partner who is bidding into the tunnel project's HVAC package.
Our engineering team in Box Hill North VIC supports tunnel duct projects in several ways: machine sizing for the project's specific duct material, gauge and pressure class; fabrication consultation including weld procedure development for fire-rated duct; FAT witnessing on machinery destined for tunnel projects; and ongoing field service support during the project's fabrication and installation phases. Our heavy-gauge machine portfolio covers round duct fabrication via the SBTF spiral tubeformer family up to 2,500 mm diameter and 3.0 mm wall, rectangular duct via the SBAL-V auto duct line at heavy gauge configuration, and welded duct fabrication via the SBKJ duct welding machine family.
For HVAC contractors bidding into Australian tunnel projects, the natural starting point is a conversation about scope — duct quantities, material breakdown, fire-rated proportion, schedule and FAT requirements — followed by a sizing exercise to confirm the right machine portfolio for the project. Visit our Australia page for the regional contact details, or go directly to contact to start the conversation. We typically reply within 12 hours from a senior engineer rather than a salesperson.
Related guides on SBKJ
This guide on tunnel ventilation HVAC duct sits alongside several related references on the SBKJ insights library:
FAQ
What is AS 4825 and how does it apply to tunnel ventilation HVAC duct design?
AS 4825 is the Australian Standard for tunnel fire safety, covering road tunnels and mass-transit rail tunnels. It governs fire scenarios, design fire heat release rates (typically 30 MW for cars, 100 MW for HGVs in road tunnels), smoke management strategy, evacuation provisions and ventilation system performance under fire. For HVAC duct, AS 4825 sets the requirement that smoke extraction ductwork must maintain integrity through the design fire duration — typically 2 hours at 250–400 degrees C — which drives material selection (heavy-gauge galvanised or 316L stainless), fire-rated jointing and damper specifications.
What are the differences between NFPA 502 and NFPA 130 for tunnel HVAC duct?
NFPA 502 covers road tunnels and limited-access highways. NFPA 130 covers fixed-guideway transit and passenger rail systems including metro, light rail and heavy rail. The two differ on design fire size, tenability criteria for evacuating passengers, and ventilation strategy preference. NFPA 130 also addresses station HVAC, platform-screen-door interfaces and over-track exhaust which have no equivalent in NFPA 502.
What is a hot smoke test and when is it required for tunnel commissioning?
A hot smoke test introduces a controlled fire load (methanol pool fires sized for 1–10 MW of convective heat release) into the tunnel during commissioning to validate the smoke management system end-to-end — fan response, damper actuation, smoke layer behaviour and tenability at evacuation routes. AS 4391 governs hot smoke testing in Australia. Hot smoke tests are required as the final pre-handover acceptance milestone for most major Australian tunnels.
What materials are used for tunnel ventilation ductwork?
Tunnel HVAC duct uses heavy-gauge galvanised G275 (1.5–3.0 mm) for normal supply and exhaust, 316L stainless for highly corrosive environments, aluminised steel for high-temperature exhaust during fire, and concrete-cast or steel-lined concrete for ceiling exhaust plenums in transverse-ventilated tunnels.
How long is the lead time for a tunnel ventilation HVAC duct package?
Tunnel HVAC duct packages typically run 18–24 months from contract award to final delivery — months 0–6 detailed design and FAT, months 4–14 bulk fabrication, months 12–20 staged delivery, months 18–22 installation and commissioning, months 22–24 hot smoke test and handover.
How does Snowy 2.0 ventilation differ from a road or rail tunnel?
Snowy 2.0 is a pumped-hydro project with deep tunnels and a powerhouse cavern roughly 800 metres underground. The HVAC scope splits between construction-phase ventilation (large-volume supply to working faces) and permanent-operation ventilation for the powerhouse cavern, transformer hall, valve chambers and access tunnels. Permanent ventilation handles transformer heat rejection, hydrogen leakage, electrical room cooling and personnel access — a fundamentally different load profile from a transport tunnel.
Does Sydney Metro use longitudinal or transverse ventilation?
Sydney Metro primarily uses longitudinal ventilation supplemented by ventilation shafts at intermediate locations and station-based emergency ventilation. The HVAC duct scope is concentrated at stations, ventilation shafts and emergency egress — not continuous along the running tunnel — which is the typical pattern for modern metro systems following NFPA 130 emergency ventilation principles.
What duct fabrication machines does SBKJ supply for tunnel ventilation projects?
SBKJ supplies three machine families. The SBTF spiral tubeformer handles round duct from 80 to 2,500 mm diameter in heavy-gauge galvanised up to 3.0 mm. The SBAL-V auto duct line handles rectangular duct in heavy-gauge for transverse exhaust sections, ventilation shaft duct and station HVAC. Heavy-duty welded duct lines and seam welders handle fire-rated duct, stainless 316L and aluminised steel where fully welded construction is required.
