Why paper mill HVAC is its own engineering problem
A paper mill is the only manufacturing environment we work in where the building atmosphere is, in routine operation, simultaneously above 90 percent relative humidity, above 35 degrees Celsius dry bulb at the machine hall floor, contains hydrogen sulphide and methyl mercaptan in the parts-per-million range on the kraft side, contains chlorine dioxide carryover on the bleaching side, and is loaded with airborne paper fibre dust and Yankee creping dust depending on the product. There is no other industrial sector where a single HVAC contractor has to specify duct material against four incompatible chemistries within walking distance of each other.
This guide is written for HVAC fabrication contractors, mill mechanical engineers and procurement teams quoting Australian pulp and paper projects. It covers the process zoning, material selection, dryer hood mass balance, kraft TRS and ClO2 segregation, control room positive pressure, recycled board mill specifics and the machinery scope you will need to source — including where the SBKJ standard machinery range fits and where it deliberately does not. We assume the reader knows what an air handler is, has used SMACNA or AS/NZS 4254, and is now trying to translate generic HVAC training into a kraft mill or a recycled board operation.
The reference frame throughout is the Australian operating environment in 2026: Visy at Tumut and Coolaroo, Orora at Botany, Opal Australian Paper at Maryvale (the former Australian Paper, now Nippon Paper-owned and rebranded), Norske Skog at Boyer in Tasmania (in transition from newsprint to packaging following the AUD 300M / USD 200M conversion announced in 2024), Solaris Paper at Penrith, and the smaller tissue and specialty mills around the country. Where we cite standards, AS 1668.2 governs occupied-zone ventilation, AS/NZS 4254 governs rectangular duct fabrication, SMACNA HVAC Industrial Duct Construction Standards govern higher-pressure and corrosive exhaust, TAPPI test methods govern the laboratory HVAC, and the US-based NCASI (National Council for Air and Stream Improvement) is the most thorough open-literature reference for kraft mill emissions and ventilation engineering.
The five HVAC challenges that define a paper mill
Strip the marketing brochures away and a paper mill HVAC engineer is solving five overlapping problems. None of them are individually unique to paper, but the combination is.
1. Extreme humidity in the machine hall and dryer hood
A paper machine dryer hood is the wettest air an HVAC system in any industry has to handle. Inside the hood, air sits at 95 percent relative humidity at 90 degrees Celsius dry bulb, and that air has to be captured, conditioned and exhausted continuously while the machine runs at 1,200–2,000 metres per minute web speed. The hood itself is typically supplied at 90–110 degrees Celsius and 110–140 grams of water per kilogram of dry air, and exhausts at 80–95 degrees Celsius and 160–180 grams of water per kilogram. Outside the hood, the machine hall floor sits 30–35 degrees Celsius and 70–85 percent relative humidity unless the make-up air system is correctly sized.
A 1,000 tonne per day fine-paper machine — comparable to the larger machines at Maryvale or Tumut — evaporates 700 to 800 tonnes of water per day across the dryer section. That is roughly 8 kilograms per second of water vapour entering the hood air. Every duct that touches that exhaust stream is, in cold-climate terms, condensing surface waiting to happen. Sloped construction, drain points at every low spot, and insulation thick enough to keep duct skin temperature above the dew point at all expected ambient conditions are non-negotiable.
2. Corrosive chemistry — kraft TRS and ClO2 bleach
Kraft pulping uses sodium hydroxide and sodium sulphide to dissolve lignin from wood chips. The byproducts are total reduced sulphur (TRS) compounds — hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide. These are present in the digester relief, brown stock washing area, evaporator vents, and any non-condensable gas (NCG) handling system. They corrode galvanised steel within 18–36 months and pit 304 stainless within 5–10 years. The default duct material for kraft TRS exposure is 316L stainless on the cold runs and welded carbon steel with internal lining or solid 316L on the hot mains.
Bleaching is now almost universally ECF (Elemental Chlorine Free), meaning chlorine dioxide (ClO2) replaces elemental chlorine. ClO2 is more aggressive than chlorine for HVAC duct purposes — it attacks 304 stainless rapidly, attacks 316L within 1–3 years on hot vapour exposure, and requires PVDF-lined carbon steel, solid PVDF, FRP with vinyl-ester resin and synthetic veil, or titanium for high-criticality runs. The Australian context: Maryvale, Tumut and any other kraft mill operating ECF bleaching has these zones; recycled board mills generally do not.
3. Heavy steam systems and high heat load
The paper machine dryer section runs on 0.7–1.0 MPa(g) saturated steam supplied to internally-rotating steam-heated dryer cylinders. A 1,000 t/day machine consumes roughly 2.5–3.5 tonnes of steam per tonne of paper produced. The recovery boiler supplies the steam by burning concentrated black liquor. Excess steam from the recovery boiler typically heats the entire HVAC system — outdoor air pre-heat coils, dryer hood supply air heaters, building space heating where applicable — through dedicated steam coils with steam trap and condensate return.
The thermal consequence for HVAC: the building never needs heating in operation. The challenge is rejecting the heat from the dryer section, the press section radiation, the recovery boiler hot zone and the lime kiln area without driving the machine hall floor temperature past the AS 1668.2 worker comfort threshold. Dryer hood heat recovery is therefore not optional — it is structural.
4. Fibre dust and OCC dust hazards
Airborne paper fibre dust is generated at the dry end of every paper machine — at the calenders, the reel-up, and the rewinder. Tissue Yankee dryer creping generates fine creped fibre dust that is more abrasive than fine-paper dust. Recycled board mills handle old corrugated container (OCC) bales at the front end, and the OCC baling and pulping area generates coarse fibre dust mixed with grit, staples, plastic flakes and contaminants from the recovered paper stream.
The HVAC consequences: bag filters and cyclones on the dust extraction systems, abrasion-resistant lined elbows on extraction ducts, careful intake location for make-up air to avoid recirculating fibre dust, and explosion protection (NFPA 654) on dust collectors handling dried paper fibre. Paper fibre is combustible at sufficient concentration and the dust extraction system is the highest-risk fire path in any paper mill.
5. 24/7 operation with no shutdown windows
Australian paper mills operate continuously. Visy Tumut, Maryvale and the major recycled board mills run 24/7/365 with single annual shutdowns of 7–14 days for major overhaul. HVAC systems must be designed for continuous operation, redundant air handlers (N+1 minimum on critical zones), and maintainability without process shutdown. Replacing a corroded duct on the live side of a kraft mill while the digester runs is not a credible plan. Material selection that lasts 10–15 years between major refurbishments is the only economically viable approach.
Paper mill process types and HVAC implications
Not every paper mill is a kraft mill. The four major process types operating in Australia each have distinct HVAC requirements.
Integrated kraft pulp and paper mill
Wood chips enter the digester. Chips are cooked under sodium hydroxide and sodium sulphide (white liquor) at 160–170 degrees Celsius and 0.7–0.9 MPa(g) for 60–120 minutes. The dissolved organic content (black liquor) is concentrated in evaporators and burned in the recovery boiler to recover sodium and produce process steam. The brown pulp is washed, oxygen delignified, and bleached to ECF brightness through ClO2 stages. Bleached pulp goes to the paper machine wet end. The Maryvale (Opal Australian Paper) site is a classic integrated kraft mill of this type. Visy Tumut is also kraft-based at the front end and feeds linerboard production.
HVAC scope is the most demanding of any paper mill type: kraft TRS exposure on the digester and brown stock side, ClO2 exposure on the bleach plant side, dryer hood heavy humidity on the paper machine, recovery boiler high-temperature exhaust, lime kiln high-temperature exhaust, and a large balance-of-plant. Material selection ranges from galvanised G90 in the warehouse to 316L stainless in TRS zones to PVDF-lined and FRP in the ClO2 zones.
Recycled board mill (containerboard, packaging)
Old corrugated container (OCC) bales are pulped in a high-consistency hydropulper, screened to remove contaminants, and fed to the paper machine wet end. No digester, no kraft chemistry, no bleach plant. Visy Tumut linerboard machine, Visy Coolaroo, Orora Botany and the smaller recycled converters all fit this profile. The chemistry is far less aggressive than a kraft mill — galvanised G90 is acceptable for general HVAC and the worker zone, with 304L or 316L only on the water-treatment plant odour exhaust and any anaerobic pulper zone.
The dominant HVAC challenges are OCC dust handling, dryer hood humidity (lower than fine-paper because the basis weight is higher and the moisture removal per kilogram of paper is lower, but still substantial), and the large make-up air requirement for the machine hall. The Australian recycled board sector has been growing for two decades and is structurally the most active investment area in paper.
Tissue paper mill
Tissue machines run a Yankee dryer — a single very large rotating cylinder, typically 4.5–6.0 metres in diameter, with a hood that can run hood air at 150–200 degrees Celsius. Solaris Paper Penrith and the smaller tissue producers fit this profile. The Yankee hood HVAC is hotter than any fine-paper or board machine hood, and the exhaust carries Yankee creping dust which is fine, fibrous and capable of building up on heat exchanger surfaces.
HVAC implications: high-temperature hood supply duct (typically Inconel or 316L for the hottest ductwork), filtration ahead of any heat recovery exchanger, and care with hood balancing because the Yankee creping process is sensitive to hood air conditions. Tissue mill atmospheres are otherwise less aggressive than kraft — galvanised general HVAC is normal.
Specialty paper, printing and writing
Maryvale runs printing and writing paper grades. Speciality grades — security paper, fine art, label stock — exist at smaller mills around Australia. The HVAC engineering is closest to the integrated kraft case, but with additional consideration for the size press and surface treatment ductwork (water-based coating dryer hoods, occasional solvent-based coating exhaust on speciality grades).
The pulp mill process — HVAC zoning
A kraft pulp mill is a chemical plant that happens to make pulp. Each stage has distinct HVAC requirements.
Chip preparation
Logs are debarked, chipped, and screened. The chip pile is outdoors. The chip handling building is a dust and fibre environment but chemically neutral. Galvanised G90 ductwork is standard, with abrasion-resistant elbows on the dust extraction system.
Digester
Continuous digesters (Kamyr-type) and batch digesters cook chips under white liquor at 160–170 degrees Celsius and elevated pressure. Digester relief gases are condensed to recover heat and turpentine, and the non-condensable gas (NCG) stream is incinerated in the recovery boiler or a dedicated NCG burner. The HVAC system around the digester building handles process radiation heat, occasional vent releases of TRS, and personnel protection during gas releases. 316L exhaust ductwork from the digester area, positive pressure on the operator pulpit.
Brown stock washing
Brown stock washers (vacuum drum washers, diffusion washers, or pressure washers) wash spent cooking chemicals from the pulp. The washer hood is a TRS exposure zone — H2S and methyl mercaptan are present continuously at low concentration and at higher concentration during process upsets. Hood exhaust is dedicated 316L stainless to the NCG handling system. Worker zone supply air is conditioned with carbon filtration.
Oxygen delignification
Oxygen delignification (O2 stage) is a pre-bleaching stage that removes additional lignin under oxygen pressure and alkali. The chemistry is less aggressive than the bleach plant — 316L stainless is standard for any duct exposed to vent gases.
Bleach plant — ECF bleaching
ECF bleaching uses ClO2 in two or more stages (typically D0-EOP-D1-EP-D2 or similar configuration) to brighten the pulp from ISO 30 brown to ISO 88+ white. ClO2 generation is on-site from sodium chlorate and either sulphuric acid (R2 process) or methanol (R8 process). The ClO2 generator and the bleach tower vents are the most aggressive HVAC zones in any paper mill. PVDF-lined or FRP duct, dedicated ClO2 scrubber, never combined with TRS exhaust.
Pulp drying or transfer to paper machine
Market pulp mills dry the bleached pulp on a flash dryer or air dryer for export. Integrated mills transfer the bleached pulp directly to the paper machine wet end. The flash dryer is a hot-air dryer at 200–400 degrees Celsius dryer air; the HVAC scope is the supply air heating, the exhaust to bag filter, and the worker zone.
The paper machine — section by section
The paper machine is the heart of the mill. HVAC requirements vary dramatically across the machine length.
Headbox and wet end
The headbox delivers dilute pulp slurry (0.5–1.5 percent consistency) onto the wire forming section. The atmosphere is wet but cold — typical 30–40 degrees Celsius dry bulb at high humidity. Galvanised general HVAC is acceptable here in most mills. Acid-mist atmospheres can develop where alum is used as a retention aid; 304L or G90 with epoxy coating are appropriate.
Press section
Press rolls remove water from the wet web through mechanical pressing. The atmosphere is moist and warm. Air-knife dewatering systems and Uhle box vacuum systems generate exhaust requirements but the chemistry is neutral.
Dryer section
The dryer section is the largest single HVAC consumer in the mill. Steam-heated dryer cylinders at 100–150 degrees Celsius surface temperature evaporate water from the web. The dryer hood encloses the dryer cans and captures the evaporated water in the hood air stream. Hood supply: 90–110 degrees Celsius, 110–140 g/kg humidity ratio, 12–18 m/s duct velocity. Hood exhaust: 80–95 degrees Celsius, 160–180 g/kg humidity ratio. Heat recovery to incoming air via plate or run-around glycol heat exchanger, 50–65 percent efficiency.
The hood enclosure is segmented along the machine — pocket ventilation between dryer cans, single-tier or two-tier hood depending on machine vintage, and air-knife sealing at the hood ends. The HVAC contractor's scope on the hood is normally the worker-zone supply duct outside the hood and the exhaust mains downstream of the hood; the hood enclosure itself is paper-machine OEM scope.
Size press and coating section
Modern fine-paper machines have a size press for surface starch application and may have an on-machine coater for coated grades. The size press is a low-temperature wet zone. The on-machine coater includes a coating dryer hood, typically infrared and air-impingement combined, with a dryer hood HVAC system similar in principle to the main dryer hood but smaller. Water-based coating exhaust is moist; legacy solvent-based coating (rare in 2026) requires explosion-rated exhaust to a thermal oxidiser.
Calender and reel-up
The calender stack applies surface finish through high-pressure rolls. The reel-up winds finished paper onto the parent reel. Both are dry-end zones with paper fibre dust. Local exhaust to bag filters, abrasion-resistant elbows, and worker zone make-up air supply.
Off-machine winder
Parent reels are slit and rewound to customer roll widths. The winder room is dusty and noisy. HVAC scope is dust extraction and worker zone supply.
Australian paper mills — operating context for HVAC contractors
The Australian pulp and paper sector has consolidated significantly over the past decade. As of 2026, the operating landscape is defined by a small number of large sites and a longer tail of specialty and tissue mills.
Visy Tumut, NSW
Visy Tumut is the largest single-site investment in Australian paper. Integrated kraft pulp and linerboard production, with multiple paper machines and a significant share of Australian containerboard capacity. HVAC scope at Tumut covers the full kraft process — digester, brown stock, oxygen delignification, bleach plant for the kraft fraction, recovery boiler, lime kiln — plus the linerboard paper machine dryer hood and the recycled fibre line. Material selection spans the full range from galvanised general HVAC to 316L kraft TRS to PVDF-lined ClO2.
Visy Coolaroo, Victoria
Visy Coolaroo is a recycled containerboard mill in northern Melbourne. No kraft chemistry — OCC pulping, screening, paper machine. HVAC scope is dryer hood, OCC dust handling, water treatment plant odour, MCC and control room. Galvanised general HVAC dominates with 304L on the water treatment exhaust. Coolaroo is a useful reference site for a recycled-only fabrication scope.
Orora Botany, NSW
Orora operates a recycled containerboard mill at Botany in Sydney's south. Comparable HVAC profile to Coolaroo: recycled fibre, no kraft, dryer hood, OCC dust, water treatment odour. Galvanised general HVAC standard.
Opal Australian Paper, Maryvale, Victoria
Maryvale is the largest integrated kraft mill in Australia, producing kraft pulp and printing/writing paper grades. Now Opal Australian Paper, owned by Nippon Paper Industries since the rebrand from Australian Paper. HVAC scope spans the full integrated kraft mill — digester, brown stock washing, oxygen delignification, ECF bleach plant with ClO2, recovery boiler, lime kiln, multiple paper machines with steam-heated dryer hoods, size press, calenders. Material selection is the most demanding in the Australian sector, with 316L for kraft TRS, PVDF-lined or FRP for ClO2, and galvanised for general HVAC. Maryvale is a long-cycle HVAC refurbishment opportunity for fabrication contractors with the chemistry knowledge to specify correctly.
Norske Skog Boyer, Tasmania
Norske Skog Boyer near Hobart was historically a newsprint mill. In 2024 Norske Skog announced a USD 200M (approximately AUD 300M) conversion project to repurpose the Boyer site from newsprint to packaging grades, reflecting the global decline in newsprint demand. The conversion project is significant HVAC scope — paper machine dryer hood reconfiguration, size press addition, possibly recycled fibre handling additions — and is structurally the largest paper-mill HVAC opportunity in Australia in the current decade. Project execution is in progress through 2026 and beyond.
Solaris Paper, Penrith, NSW
Solaris operates a tissue mill at Penrith with Yankee dryer technology. HVAC scope is dominated by the Yankee hood — high-temperature hood supply, fine creped fibre dust on the exhaust, hood air balancing. Galvanised general HVAC for the balance of plant.
Carter Holt Harvey
Carter Holt Harvey operates pulp and paper assets across Tasmania and NSW. The CHH portfolio includes specialty grades and feeds related building product operations. HVAC scope varies by site — kraft on some, mechanical pulping on others.
Smaller tissue and specialty mills
Pratt Recycling operates recycled fibre handling at multiple sites including Tumut. A long tail of specialty paper mills produces filter paper, fine art paper, security paper and label stock. HVAC scope at these mills is typically in the AUD 1–5M range per project rather than the AUD 20M+ scope at the major sites.
Key standards and references
Paper mill HVAC sits at the intersection of several standards bodies. The HVAC engineer should have ready reference to all of the following.
AS 1668.2 — The Use of Ventilation and Airconditioning in Buildings, Part 2
The Australian standard for mechanical ventilation in occupied zones. Governs control room, MCC room, locker room, lunchroom, laboratory and office HVAC. Sets minimum outdoor air rates per occupant and per floor area, defines kitchen and laboratory exhaust requirements, and specifies general indoor air quality criteria. Read this standard cover to cover for the worker-zone HVAC scope. Australian regional context for HVAC contractors.
AS/NZS 4254 — Ductwork for Air Handling Systems in Buildings
The Australian and New Zealand standard for rectangular and round duct fabrication. Governs gauge selection, joint construction, sealant specification, hanger spacing and pressure class. Maps closely to SMACNA HVAC Duct Construction Standards. SBKJ SBAL-V auto duct lines are configured to AS/NZS 4254 tolerance presets for Australian fabrication contractors.
SMACNA HVAC Industrial Duct Construction Standards
The Sheet Metal and Air Conditioning Contractors' National Association industrial duct standard is the international reference for higher-pressure and corrosive exhaust duct construction. Where AS/NZS 4254 stops short for an industrial application — bleach plant exhaust at higher static pressure, kraft TRS mains at elevated temperature — SMACNA Industrial fills the gap. SBKJ machinery supports SMACNA, EN 1505 and AS/NZS 4254 tolerance presets configurable on the SBAL-V.
TAPPI Test Methods
Technical Association of the Pulp and Paper Industry test methods. Most relevant to HVAC: TAPPI T 402 (Standard Conditioning and Testing Atmospheres) which sets the laboratory HVAC target of 23 degrees Celsius and 50 percent relative humidity, and TAPPI T 411 (Thickness of Paper, Paperboard, and Combined Board) which references conditioned-atmosphere testing. The mill laboratory HVAC must achieve TAPPI conditioning continuously for paper testing to be valid.
NCASI — National Council for Air and Stream Improvement
US-based industry research body for pulp and paper environmental performance. Issues the most thorough open-literature references on kraft mill TRS emissions, recovery boiler stack chemistry, NCG handling and bleach plant emissions. Australian engineers without direct kraft mill experience use NCASI publications as the benchmark reference for kraft chemistry and ventilation design.
NFPA 70 — National Electrical Code (Hazardous Area Classification)
Where kraft TRS may be released through digester relief, NCG handling failures or bleach plant ClO2 carryover, NFPA 70 hazardous area classification governs MCC room, lighting and electrical equipment selection. Class I Division 2 is typical for the digester area; Class I Division 1 for the immediate vicinity of NCG burners.
NFPA 654 — Combustible Dust
Paper fibre dust at sufficient concentration is combustible. NFPA 654 governs design of dust collection systems, including explosion venting, isolation and the spacing of collection units relative to the building. Mandatory reference for any dust extraction scope on a paper machine dry end or recycled fibre handling system.
ASTM D685 — Standard Practice for Conditioning Paper
The ASTM equivalent of TAPPI T 402, used internationally for paper conditioning before testing. Same conditions: 23 degrees Celsius, 50 percent RH, with conditioning periods specified by paper grade.
AS 1530 series — Fire performance of building materials
Governs fire performance of duct insulation, lining and the duct wall itself where fire-rated penetrations are involved. Relevant for the warehouse and office portions of the mill.
Paper machine dryer hood HVAC — heat and mass balance
The dryer hood is the single most engineered HVAC system in the mill. The mass balance starts from the production rate.
Evaporation load calculation
For a paper machine producing P tonnes of paper per day at moisture inlet to the dryer section M_in (kg water per kg dry fibre, typically 1.5–2.0 for fine paper after the press section) and final moisture M_out (typically 0.05 for fine paper), the evaporation load E is approximately:
E (tonnes water per day) = P × (M_in − M_out) / (1 − final paper moisture fraction)
For a 1,000 t/day fine-paper machine at M_in = 1.7 and M_out = 0.05, evaporation is approximately 700–800 tonnes of water per day, equivalent to 8 kg/s. This is the load that the hood air must absorb and transport out of the machine to the heat recovery exchanger.
Hood air flow rate
Hood air flow rate is conventionally expressed in kg dry air per kg paper produced. Typical values range 0.8–1.4 kg dry air per kg paper for fine-paper machines, 1.2–1.8 for tissue Yankee machines, and 0.6–1.0 for board machines. The lower end is achieved on modern high-humidity hoods; the upper end on legacy lower-humidity hoods.
Hood supply conditions
Hood supply air is typically heated to 90–110 degrees Celsius and 110–140 g/kg humidity ratio. The high humidity ratio is a deliberate design choice — it reduces the temperature differential across the dryer hood and improves the energy efficiency of the dryer section. Heating is by mill steam (0.7–1.0 MPa(g) saturated) on a steam coil, or by direct gas firing in some installations. Duct velocity 12–18 m/s on insulated round or rectangular spiral construction.
Hood exhaust conditions
Hood exhaust runs 80–95 degrees Celsius at 160–180 g/kg humidity ratio. Sloped construction back to the hood, drain points at every low spot, and insulation rated to keep skin temperature below 60 degrees Celsius for worker zones. The exhaust passes through plate or run-around glycol heat exchanger to pre-heat incoming air, then exhausts to atmosphere through an electrostatic precipitator (ESP) on tissue and high-fibre-content machines, or directly to atmosphere on cleaner fine-paper applications.
Heat recovery
Plate heat exchangers are standard for hood air-to-incoming-air heat recovery. Plate construction in 316L stainless or aluminium is typical. Recovery efficiency 50–65 percent is realistic on hood air; the limitation is the high water content of the hood exhaust which condenses on the cold-side plates and requires drainage. Run-around glycol coils are an alternative with lower recovery efficiency (35–50 percent) but better separation between exhaust and supply streams. Fouling resistance and washdown access are non-negotiable on the hood-side surfaces.
Make-up air
Machine hall make-up air replaces the air exhausted through the hood and the localised exhaust systems (calender, reel-up, winder). Make-up air is typically supplied through ceiling-level diffusers along the machine length, conditioned to 25–35 degrees Celsius depending on outside ambient. The make-up air system is the dominant HVAC duct scope for the machine hall — large rectangular trunks running the machine length at moderate velocity (8–12 m/s).
Pulp mill HVAC challenges by zone
Beyond the paper machine, the pulp mill side has a series of high-load HVAC zones each with distinct requirements.
Lime kiln area
The lime kiln burns calcium carbonate to produce calcium oxide (lime), which reacts with green liquor in the causticising plant to regenerate white liquor. Internal kiln temperature is 1,200–1,400 degrees Celsius. The HVAC scope is not the kiln itself — that is a process system — but the surrounding control room, MCC, lubrication area and personnel zones. High ambient heat load from the kiln shell radiation, conditioned air supply with high cooling capacity, and dust and TRS filtration on the intake air.
Recovery boiler area
The recovery boiler burns concentrated black liquor to recover sodium chemicals and produce process steam. Boiler internal temperatures exceed 1,000 degrees Celsius and the stack handles substantial flue gas. The HVAC scope is the boiler control room (separate from the main mill control room in larger installations), the precipitator room, and the personnel access platforms. High redundancy on cooling because recovery boiler safety-critical equipment must remain operable during process upsets.
Brown stock washer area
The brown stock washing area is the largest contiguous TRS exposure zone in the mill. Multiple stages of vacuum drum washers or pressure washers in series. Hood exhaust on each washer to the NCG handling system in 316L stainless. Worker zone supply air to the operator pulpit with carbon filtration.
Evaporator area
Multi-effect evaporators concentrate weak black liquor (15–18 percent solids) to strong black liquor (65–75 percent solids) for combustion in the recovery boiler. Evaporator vents are TRS exposure. 316L exhaust ductwork to NCG handling.
Causticising and white liquor preparation
Green liquor (sodium carbonate solution) reacts with quicklime to produce white liquor (sodium hydroxide). The chemistry is alkaline and corrosive but less aggressive than the kraft TRS or ClO2 zones. 304L or 316L is standard for any duct exposed to vent gases from this area.
The bleach plant — the most aggressive HVAC zone in any mill
Chlorine dioxide is the most aggressive routine chemistry in any HVAC duct system in the pulp and paper sector. Material selection in the bleach plant is unforgiving.
ClO2 generator
Sodium chlorate reacts with sulphuric acid (R2 process) or methanol (R8 process) to produce ClO2 gas, which is absorbed in chilled water to produce ClO2 solution. The generator vents are concentrated ClO2 — the most aggressive duct atmosphere in the mill. Solid PVDF, PVDF-lined carbon steel, or titanium are the only viable materials. 316L will fail in service.
Bleach tower vents
The bleach tower D0, D1, D2 stages and any ClO2 reactor have tower vents with ClO2 carryover. PVDF-lined or FRP duct to a dedicated ClO2 scrubber. The scrubber typically uses sodium hydroxide solution to neutralise ClO2 to chloride and chlorate. Never combine ClO2 vent ductwork with TRS exhaust because the chemistries are incompatible at the scrubber.
Washer hood exhaust on bleach stages
Each bleach stage has a washer hood. The hood exhaust contains ClO2 carryover at lower concentration than the tower vent. PVDF-lined or FRP duct, dedicated routing to the ClO2 scrubber.
Worker zone supply
The bleach plant operator pulpit and the supporting offices run positive pressure with conditioned supply air. Intake location must avoid the ClO2 scrubber stack at all expected wind directions. Carbon filtration on the intake air, ClO2 monitoring in the operator pulpit, and emergency shutdown of the supply fan triggered by ClO2 sensor activation in the pulpit.
Material selection summary for the bleach plant
Solid PVDF or PVDF-lined carbon steel for ClO2 vent duct on the generator and tower. FRP with vinyl-ester resin and synthetic veil for larger-diameter ClO2 exhaust runs where PVDF is not commercially viable. Titanium for the highest-criticality runs where weight or fire performance excludes PVDF. 316L stainless for the supply air and worker zone exhaust portions of the system. SBKJ does not fabricate FRP, PVDF or titanium duct — those are sourced from specialty composite fabricators — but the SBAL-V supplies 316L rectangular duct for the supply-air and worker-zone portions.
Materials by zone — the paper mill duct material matrix
The single most important engineering decision in paper mill HVAC is the material selection per zone. Get this wrong and the duct fails in service within two to five years.
Galvanised G90 carbon steel
Standard for general HVAC: control room supply, MCC ventilation, machine hall make-up air, warehouse, office, locker room. Acceptable in recycled board mill atmospheres including the OCC baling area (with abrasion-lined elbows on dust extraction). Not acceptable in any kraft TRS zone or any bleach plant zone. SBKJ SBAL-V auto duct line in galvanised configuration is the standard fabrication machinery for this scope.
304L stainless
Acceptable for some moderately corrosive zones — water treatment plant odour exhaust on a recycled board mill, evaporator vent on a kraft mill at lower temperature, and any zone where galvanised would corrode but full kraft TRS exposure is not present. 304L is an upgrade path from galvanised at moderate cost premium.
316L stainless
The default material for kraft pulp mill TRS exposure zones — digester area, brown stock washing, evaporator vents, causticising, NCG handling. Also the default for the supply-air and worker-zone portions of the bleach plant, and for high-temperature paper machine hood ductwork. SBKJ SBAL-V auto duct line in 316L stainless coil configuration is available with stainless tooling and is the standard fabrication machinery for this scope. Tolerance per AS/NZS 4254 or SMACNA Industrial.
PVDF-lined carbon steel
The default for ClO2 exposure — bleach tower vents, ClO2 generator vents, washer hood exhaust on bleach stages. Carbon steel duct lined with PVDF (polyvinylidene fluoride) sheet bonded to the duct interior. Not fabricated on the SBAL-V — sourced from specialty fabricators with PVDF lamination capability.
FRP — fibreglass reinforced plastic
Fibreglass reinforced plastic with vinyl-ester resin and synthetic veil. Used for larger-diameter ClO2 exhaust runs where PVDF is not commercially viable, and for some highly corrosive applications such as recycled board white-water exhaust. Not fabricated on SBKJ machinery — sourced from specialty composite fabricators.
Titanium
Used for the highest-criticality ClO2 runs and for some specialty bleaching applications. Cost premium 5–10x over 316L. Reserved for runs where PVDF is not viable and FRP fire performance is inadequate.
Coal tar epoxy lined carbon steel
An older lining specification still seen on some recycled board mill zones — particularly on duct exposed to white-water condensate from the pulper or savealls where digester acid leachate analogues are present. Modern specifications generally favour solid 304L or 316L over coal tar epoxy lined carbon steel for new builds, but coal tar epoxy remains a refurbishment option.
Inconel and high-nickel alloys
Reserved for the hottest paper machine hood ductwork (Yankee dryer hood supply on tissue machines at 200+ degrees Celsius) and for any ductwork exposed to combined high temperature and TRS chemistry. Cost premium and lead time both significant.
For deeper material selection guidance on galvanised versus stainless across general HVAC, see our galvanised versus stainless steel duct guide.
Worker comfort — control room and machine pulpit HVAC
The mill control room is where the operators live for 12-hour shifts in 24/7/365 operation. Get the HVAC right or risk losing trained operators to easier sites.
Positive pressure
Main control room and machine pulpit run positive pressure relative to the mill atmosphere — typically 25–50 Pa above ambient. Hermetic door seals, single point of entry through an air-lock vestibule on critical zones (bleach plant operator pulpit, kraft mill central control room).
Particulate intake filtration
MERV 13 minimum on intake air, MERV 14 typical for kraft mills. Pre-filter at MERV 8 to extend final filter life. Fibre dust accumulation on intake filters is heavy in any paper mill — filter replacement intervals 3–6 months for the pre-filter, 12–18 months for the final.
Gas-phase carbon filtration
Activated carbon filters on the intake air for any control room downwind of a kraft mill recovery boiler stack, lime kiln stack or NCG flare. Carbon filter sizing per ASHRAE Handbook (HVAC Applications) for TRS and ClO2 capture. Carbon replacement on TRS sensor activation or on a fixed time interval (typically 6–12 months).
Redundant cooling
N+1 redundancy on cooling units serving critical control rooms. Loss of cooling on a kraft mill central control room is a process safety event — the operators cannot abandon the room during a shift change. Two air handlers each sized for 100 percent of the load, with automatic failover.
Fire suppression integration
HVAC dampers integrated with the building fire alarm system. On fire detection, dampers close, the supply fan stops, and the control room isolates from the building atmosphere. The control room own pressurisation continues from a dedicated fresh-air make-up unit.
Substation and electrical room HVAC
The mill electrical infrastructure includes one or more high-voltage substations, multiple MCCs (motor control centres), and dedicated VFD (variable frequency drive) rooms for the paper machine drives. All require dedicated HVAC.
Heat load
Substation and MCC room heat load is dominated by transformer losses, busbar I²R losses and VFD switching losses. A large VFD room serving paper machine drives can carry 100–300 kW of waste heat that must be rejected continuously.
Redundancy
N+1 redundancy on cooling units. Loss of cooling on a major MCC room within minutes drives ambient temperature past the 40-degree-Celsius rating of most VFDs and can trip the paper machine. The cost of an unplanned paper machine trip easily exceeds the cost of redundant cooling over the life of the system.
Filtration
Dust filtration to MERV 13 minimum. In kraft mills near the brown stock area, gas-phase carbon filtration to remove TRS that would otherwise corrode busbar contact surfaces and electronic boards.
Fire suppression integration
Clean agent fire suppression (FM-200, Novec 1230 or inert gas systems) is standard for major substations and VFD rooms. HVAC dampers integrated with the suppression release.
Tissue paper machine HVAC — Yankee dryer specifics
Tissue machines run a single very large rotating cylinder — the Yankee dryer — instead of the steam-heated cylinder bank of a fine-paper machine. The HVAC implications are significant.
Yankee hood
The Yankee hood encloses the dryer cylinder and supplies high-temperature impingement air at typically 150–200 degrees Celsius and high velocity (50–80 m/s on the impingement nozzles). The hood air provides drying through forced convection in addition to the conduction through the cylinder shell.
Hood supply duct
Hood supply duct from the burner to the hood plenum is high-temperature 316L stainless or Inconel for the hottest runs. Insulation rated to keep duct skin temperature below the burn threshold. Burners are typically gas-fired direct combustion, with combustion air supply ducting in stainless.
Yankee hood exhaust
Yankee hood exhaust carries fine creped fibre dust generated during the creping process. The dust is fine, fibrous, and capable of building up rapidly on heat exchanger surfaces. Bag filtration ahead of any heat recovery exchanger is mandatory. Heat recovery to incoming combustion air via plate heat exchanger or run-around glycol coil.
Building exhaust
Tissue machine hall has substantial fine-fibre dust loading at the dry end. Local exhaust to bag filters at the calenders, the reel-up and the off-machine winder. NFPA 654 explosion protection on dust collectors handling dried fibre.
Recycled board mill HVAC — OCC and pulper specifics
The recycled board mill is a different beast from a kraft mill — far less aggressive chemically, but with its own HVAC challenges.
OCC baling and bale storage
Old corrugated container bales arrive at the mill from waste paper recyclers. The bale storage area is dusty, contains insect and rodent contamination, and has high fire risk from compacted recovered paper. HVAC scope is dust extraction, fresh air supply, and explosion protection on dust collectors.
Pulper
The high-consistency hydropulper breaks the OCC bales into individual fibres in a water slurry. The pulper area is wet, warm and contains plastic flakes, staples, paperclips and other contaminants released from the bales. HVAC scope is local exhaust on the pulper hood and worker zone supply.
Screening and cleaning
Coarse screens, fine screens and centrifugal cleaners remove contaminants from the pulp slurry. The atmosphere is wet but chemically neutral. Galvanised general HVAC is standard.
Fibre recovery
Saveall systems recover fibre from the white-water loop. The white-water can become anaerobic if not turned over fast enough, producing hydrogen sulphide and other reduced sulphur compounds. Where this is observed in service, upgrade exhaust ductwork from galvanised to 304L on the affected runs.
Water treatment plant
Effluent water treatment includes anaerobic and aerobic biological stages. The anaerobic stage produces hydrogen sulphide. Odour exhaust to a dedicated odour scrubber in 316L or PVDF-lined duct. This is the most aggressive HVAC zone in any recycled board mill.
Steam plant integration — the heating energy of the mill
A pulp mill produces excess steam from the recovery boiler — far more than the paper machine and the process consume. The HVAC system is a beneficiary of this excess.
Steam supply to HVAC coils
Heating coils on outdoor air units, dryer hood supply heaters, and building space heating where applicable use mill steam directly. Typical supply pressure 0.7–1.0 MPa(g) saturated, condensate return to the deaerator. Steam coil sizing and steam trap selection are part of the HVAC scope and need to coordinate with the mill steam balance.
Cogeneration and CHP
Modern recovery boilers and bark/biomass CHP boilers generate electrical power as well as steam. Maryvale, Tumut and the major mills are net electricity exporters or close to balanced on most operating days. The HVAC engineer should be aware of the steam balance to negotiate steam supply pressure and flow with the mill energy team.
Australian sector transitions — Boyer conversion and ownership changes
The Australian paper sector has been in active transition for several years. Two changes matter for HVAC contractors planning project pipeline.
Norske Skog Boyer newsprint-to-packaging conversion
In 2024 Norske Skog announced a USD 200M / approximately AUD 300M conversion of the Boyer mill in Tasmania from newsprint production to packaging grades. The conversion reflects the global decline in newsprint demand and the growth in containerboard and other packaging substrates. Project execution is in progress through 2026 and beyond, and the HVAC scope on this conversion is substantial — paper machine dryer hood reconfiguration for different basis weight, possible recycled fibre line additions, and balance-of-plant modifications. The Boyer conversion is structurally the largest paper-mill HVAC opportunity in Australia in the current decade.
Australian Paper to Opal Australian Paper
Australian Paper at Maryvale was rebranded to Opal Australian Paper following ownership stabilisation under Nippon Paper Industries. The site continues to operate kraft pulp and printing/writing paper grades. Periodic refurbishment cycles on the Maryvale paper machines and bleach plant are long-term opportunities for HVAC fabrication contractors with the chemistry knowledge to specify correctly.
Tissue and specialty growth
Tissue capacity in Australia has been growing slowly over the past decade. Solaris and the smaller producers have been incrementally expanding. Specialty grades — security paper, fine art, label stock — show modest growth. None individually rival the Boyer conversion in scope but cumulatively they support a steady project pipeline.
Sustainability and the paper mill HVAC system
The Australian paper sector has substantial sustainability commitments and the HVAC system is part of the response.
Black liquor recovery and biomass CHP
The recovery boiler burns black liquor (a renewable fuel by definition — it is the lignin and hemicellulose content of the wood chips). Bark and wood waste are burned in dedicated biomass CHP boilers. Combined, kraft mills are typically net electricity exporters and have minimal fossil fuel consumption beyond startup and emergency backup. The HVAC system benefits from low-carbon process steam.
Water conservation
Paper mills consume substantial water — typically 20–80 cubic metres per tonne of paper depending on grade and white-water closure. Effluent treatment is a significant infrastructure investment. The HVAC scope on the water treatment plant is the odour exhaust, which is the most aggressive HVAC zone outside the kraft and bleach plant areas.
Heat recovery and energy efficiency
Dryer hood heat recovery, recovery boiler economiser, and lime kiln waste heat recovery are all standard energy-efficiency features of modern Australian mills. The HVAC engineer participates in the heat recovery scope where the recovered heat displaces other building heating loads.
SBKJ machinery for paper mill HVAC fabrication
The SBKJ machinery range covers the rectangular and round duct fabrication scope on a paper mill HVAC project. Heavy-gauge welded fabrication, FRP duct and PVDF-lined duct are deliberately outside the SBKJ standard machinery range — those are sourced from welded fabrication shops and specialty composite fabricators.
SBAL-V auto duct production line
The SBAL-V is the SBKJ flagship rectangular duct fabrication line. Standard configuration handles galvanised G90 coil for general HVAC scope on any paper mill. Stainless coil configuration with 304L or 316L coil and stainless-rated tooling handles the kraft TRS exposure zones and the supply-air portions of the bleach plant. Tolerance presets for SMACNA, EN 1505 or AS/NZS 4254. Single-shift output measured against your nominated coil at FAT, not brochure ideal conditions. Full SBAL-V specifications and coil compatibility.
SBTF spiral tubeformer
The SBTF is the SBKJ spiral tubeformer for round return-air mains and round duct in the 80–1,500 mm diameter range. Galvanised standard, 304L or 316L stainless on request. Used on paper mill projects for the round return-air trunks in the machine hall, the round MCC ventilation mains, and the laboratory fresh-air supply. Full SBTF specifications.
Welded fabrication scope (out of SBKJ standard scope)
Welded carbon steel and welded stainless duct for kraft TRS exhaust mains, recovery boiler hood, lime kiln exhaust, and any heavy-gauge plate work is sourced from welded fabrication shops. SBKJ does supply duct welding stations as a separate machine line, but the standard machinery range is rectangular and round forming, not heavy welded plate fabrication. SBKJ duct welding machine range.
FRP and PVDF duct (out of SBKJ scope)
FRP duct, PVDF-lined duct and titanium duct for the bleach plant ClO2 zones are sourced from specialty composite fabricators and titanium fabricators. SBKJ machinery does not fabricate composite or PVDF-lined duct. Specifying the contractor with FRP and PVDF capability separately from the contractor with rectangular and round metal duct capability is the standard procurement approach on a paper mill project.
Material handling and ancillary scope
Coil decoilers, slitters, automatic stackers and run-out tables to integrate with the SBAL-V are part of the SBKJ ancillary scope. Spec the workshop floor footprint with run-out tables and coil storage at quotation stage — retrofitting a workshop is more expensive than specifying correctly.
How the SBKJ scope fits in a typical paper mill HVAC fabrication contract
A fabrication contractor on a paper mill HVAC project typically holds three to five separate machinery lines to cover the full duct material range. The SBKJ scope normally covers the largest two of those lines.
The galvanised general HVAC line
One SBAL-V auto duct line in galvanised configuration, plus one SBTF spiral tubeformer, covers 60–80 percent of the duct fabrication tonnage on most paper mill projects. This is the workhorse machinery for control room supply, MCC ventilation, machine hall make-up air, warehouse, office, locker room, and recycled board mill general scope.
The 316L stainless line
One SBAL-V auto duct line in 316L stainless configuration with stainless-rated tooling, plus one SBTF spiral tubeformer in stainless configuration, covers the kraft TRS exposure zones and the supply-air portions of the bleach plant. This represents 15–30 percent of the duct fabrication tonnage on a kraft mill project, less on a recycled board mill project.
The welded fabrication scope (separate contractor)
Welded carbon steel exhaust mains, welded stainless on the kraft TRS hot mains, and any heavy-gauge plate fabrication is sourced from a welded fabrication shop. This is typically a separate contractor on a paper mill project, working from issued-for-construction drawings.
The FRP and PVDF scope (separate contractor)
FRP duct and PVDF-lined duct for the bleach plant ClO2 zones is sourced from a specialty composite fabricator. Typically a separate contractor on a paper mill project, working from issued-for-construction drawings.
The fabrication contractor holding the SBAL-V and SBTF scope is normally the lead HVAC contractor on the project, coordinating the welded and composite fabricators as subcontractors. This is the procurement structure used on Visy Tumut, Maryvale and the Boyer conversion as observed from public information.
Lead times and project planning for an Australian paper mill HVAC project
Total project timeline from kick-off to first duct off the line is normally 18–28 weeks for the SBKJ scope.
SBAL-V galvanised configuration
10–14 weeks ex works from order confirmation. 6–8 weeks ocean freight and customs into Melbourne, Sydney or Brisbane. 2 weeks installation and commissioning at the fabrication contractor's workshop.
SBAL-V 316L stainless configuration
14–18 weeks ex works from order confirmation (longer than galvanised because stainless tooling is built to order). 6–8 weeks ocean freight and customs. 2 weeks installation and commissioning.
SBTF spiral tubeformer
8–12 weeks ex works. 6–8 weeks ocean freight. 1 week installation.
Project milestones
Typical project sequence: contractor wins the HVAC fabrication scope at month 0; contractor specifies machinery requirements and issues RFQ to SBKJ at month 1; contract signed and 30 percent T/T deposit at month 2; SBKJ fabricates and FATs the machinery from month 2 to month 6; ocean freight and customs from month 6 to month 8; installation and commissioning at the workshop at month 8; first duct off the line at month 9 to month 10. Site fabrication and installation continues from month 9 onwards through the project execution. Get an itemised SBKJ quote against this timeline.
Cross-industry references
Paper mill HVAC engineering shares techniques with other heavy industrial sectors. Fabrication contractors with experience across these sectors find paper mill projects more approachable.
Mining and mineral processing
The dust handling, abrasion-resistant elbows and bag filter scope on a recycled board mill OCC area is similar to a mineral processing plant. Mining HVAC industry context. For deeper detail on mining ventilation duct material selection see our mining ventilation HVAC duct guide.
Food processing
The hygienic stainless duct fabrication on the laboratory and the white-water-clean recycled fibre handling shares techniques with food processing. Food processing HVAC industry context.
Steel mill and smelter
The high-temperature exhaust on the recovery boiler and the lime kiln shares heat-resistant duct material selection techniques with steel mill and smelter HVAC. Steel mill and smelter HVAC duct guide.
FAQ
What duct material should I use for a kraft pulp mill brown stock area?
316L stainless steel is the default for any duct routed through or exhausting from kraft pulp brown stock washers, digester relief lines, evaporator vents and any zone where total reduced sulphur (TRS) compounds are present. Galvanised will pit and perforate within 18–36 months in a kraft TRS atmosphere. For ducts that see ClO2 carryover, upgrade to PVDF-lined carbon steel or solid PVDF.
How do I size duct for a paper machine dryer hood?
Start with the evaporation rate — a 1,000 t/day fine-paper machine evaporates roughly 700–800 tonnes of water per day, or about 8 kg/s. Hood supply: 90–110 degrees Celsius and 110–140 g/kg humidity ratio. Hood exhaust: 80–95 degrees Celsius and 160–180 g/kg before heat recovery. Duct velocity 12–18 m/s on insulated round or rectangular construction. Hood enclosure flow rate 0.8–1.4 kg dry air per kg paper produced.
What HVAC standards apply to an Australian paper mill?
AS 1668.2 governs occupied-zone ventilation (control room, MCC, locker room, lab, office). AS/NZS 4254 governs rectangular duct fabrication. AS 4485 covers chemical bulk storage area ventilation. SMACNA HVAC Industrial Duct Construction Standards apply to higher-pressure exhaust. TAPPI T 402 governs lab conditioning at 23 degrees Celsius and 50 percent RH. NCASI is the open-literature reference for kraft mill emissions. NFPA 70 governs hazardous-area classification.
What is the lead time for HVAC duct machinery on a paper mill project?
SBKJ SBAL-V auto duct line in 316L stainless: 14–18 weeks ex works, 6–8 weeks freight to Australia, 2 weeks installation. Galvanised configuration: 10–14 weeks ex works. SBTF spiral tubeformer: 8–12 weeks ex works. Welded duct fabrication for kraft TRS hot mains and FRP for ClO2 are out of SBKJ scope. Total project timeline 18–28 weeks from kick-off to first duct off the line.
Can a galvanised duct line handle a recycled board mill atmosphere?
Yes for general HVAC, conditional for some zones. Galvanised G90 is acceptable for control room, MCC, machine hall make-up air and most recycled board mill atmospheres. Where the white-water loop becomes anaerobic and produces hydrogen sulphide, upgrade to 304L or 316L on the affected exhaust runs. OCC baling area is abrasive but not chemically aggressive — galvanised with abrasion-lined elbows is standard. Water treatment plant odour exhaust is the most aggressive zone in a recycled board mill and should be 304L minimum.
How is a chlorine dioxide bleach plant duct specified?
ClO2 vent ductwork (generator vent, tower vent, washer hood exhaust on bleach stages) is PVDF-lined carbon steel, solid PVDF, FRP with vinyl-ester resin and synthetic veil, or titanium for highest-criticality runs. 316L will fail in service in a ClO2 carryover line. Dedicated exhaust to a dedicated ClO2 scrubber — never tied into a TRS exhaust because the chemistries are incompatible at the scrubber. SBKJ does not fabricate FRP, PVDF or titanium duct; SBAL-V supplies 316L for the supply-air and worker-zone portions.
Which Australian paper mills are operating in 2026?
Visy Tumut NSW (integrated kraft and linerboard, the largest single-site investment in Australian paper). Visy Coolaroo Victoria (recycled board). Orora Botany NSW (recycled containerboard). Opal Australian Paper Maryvale Victoria (kraft pulp and printing/writing, Nippon Paper-owned). Norske Skog Boyer Tasmania (newsprint, in transition to packaging following the USD 200M conversion announced 2024). Solaris Paper Penrith NSW (tissue). Carter Holt Harvey across Tasmania and NSW. Plus a long tail of specialty and tissue mills.
Does SBKJ have experience with paper mill HVAC ductwork machinery?
Yes. SBKJ Group has supplied SBAL-V auto duct lines and SBTF spiral tubeformers to fabrication contractors serving pulp and paper plants globally since 1995. Standard scope is the rectangular duct fabrication line for control room, MCC, machine hall and dryer hood worker-zone supply, plus the SBTF for round return-air mains, and 316L stainless coil-handling configurations for kraft TRS and bleach plant zones. Heavy-gauge welded duct, FRP duct, PVDF-lined duct and titanium duct are outside SBKJ standard machinery scope.
Get an SBKJ quote for your paper mill HVAC fabrication scope →
