Why cleanroom duct is different
Cleanroom HVAC ductwork serves a controlled airborne particulate environment, where the duct interior is part of the cleanliness chain — not just a conduit. Particles shed from interior surfaces, harboured at joint discontinuities or generated by mastic sealant outgassing all directly affect the cleanroom class. A cleanroom duct fabrication shop is therefore operating to a different specification than commercial HVAC: the steel grade, the welding regime, the surface finish, the sealing material, the QA documentation and the field test all change.
SBKJ has supplied cleanroom-grade duct fabrication equipment to pharmaceutical clients in Saudi Arabia, India and Australia, semiconductor fabs in Vietnam and Egypt, and hospital projects globally. This guide is the configuration framework we walk through with cleanroom buyers.
ISO 14644 cleanroom classes — what each one means for duct
ISO 14644-1 defines cleanroom cleanliness by maximum allowable airborne particle counts at specified particle sizes. Classes are numbered 1 (cleanest, semiconductor lithography) to 9 (least clean, packaging hall). Most cleanroom HVAC duct work operates in Classes 5 through 9; Classes 1 through 4 are very specialised and use HEPA-filtered terminal supply with limited duct exposure.
- ISO Class 5: pharmaceutical aseptic processing, semiconductor lithography, sterile compounding. Stainless steel duct mandatory, continuous TIG welding, electropolished interior surface, zero-leak sealed joints.
- ISO Class 6: pharmaceutical sterile filling, advanced semiconductor assembly, gene therapy production. Stainless steel duct strongly recommended, continuous welding standard, polished interior, SMACNA Class A or tighter.
- ISO Class 7: pharmaceutical solid dosage, hospital operating theatres, life sciences laboratories. Stainless steel preferred, welded longitudinal seams, SMACNA Class A sealed.
- ISO Class 8: pharmaceutical packaging, hospital recovery, medical device assembly. Galvanised steel acceptable with sealed joints; stainless preferred for pharma. SMACNA Class A.
- ISO Class 9: pharmaceutical warehouse, hospital ward, light cleanroom adjuncts. Galvanised steel standard, SMACNA Class A sealed.
Stainless steel grade selection
Three stainless grades cover the great majority of cleanroom duct work:
- 304L: low-carbon austenitic stainless, the workhorse for ISO Class 7–9 cleanroom HVAC. Good corrosion resistance, weldability and cost. 304L is preferred over 304 because the lower carbon content reduces sensitisation during welding and minimises chromium carbide precipitation at the weld heat-affected zone.
- 316L: low-carbon austenitic with molybdenum addition, specified for ISO Class 5–6 cleanrooms, pharmaceutical sterile process facilities, and any duct exposed to chloride compounds (coastal locations, certain sterilisation gases). 316L costs approximately 15–25% more than 304L.
- Duplex 2205: ferritic-austenitic stainless used for semiconductor wet-process exhausts where standard austenitic stainless is attacked by acid or solvent fumes. Higher strength allows lighter gauge construction.
Some specialised semiconductor processes require polypropylene-lined steel, fibreglass-reinforced plastic (FRP) or PVC duct. SBKJ supplies the spiral tubeformer tooling and lockformer tooling for FRP and PVC; PP lining is typically post-fabrication.
Surface finish requirements
Surface roughness Ra (arithmetic average roughness) is the binding specification for cleanroom duct interior finish. Particles adhere more readily to rough surfaces and are harder to wipe down at periodic cleaning. Typical surface finish bands by ISO class:
- ISO Class 5: Ra ≤ 0.4 µm interior, achieved by 2B mill finish stainless plus electropolishing
- ISO Class 6: Ra ≤ 0.8 µm interior, achieved by 2B mill finish plus mechanical polishing or electropolishing
- ISO Class 7: Ra ≤ 1.6 µm interior, achieved by 2B mill finish without additional polishing
- ISO Class 8–9: Ra ≤ 3.2 µm interior, achieved by 2B mill finish or 1D mill finish
Mill finish designation: 2B is cold-rolled, annealed, pickled and skin-passed — a smooth, slightly reflective finish standard for cleanroom work. 1D is hot-rolled, annealed and pickled — duller and rougher, suitable for less stringent applications.
Joint and seam construction
Cleanroom duct construction differs from commercial duct in joint and seam method. The general principle is to eliminate or minimise discontinuities where particles can shed or be harboured.
- Longitudinal seams: Pittsburgh lock and snap-lock seams are unacceptable for ISO Class 5–7. Use continuous butt welding (TIG), continuous lap welding, or for spiral round duct, a continuous closed lockseam with the inside seam sealed.
- Transverse joints: continuously TIG-welded inside and out for ISO Class 5–6. For ISO Class 7, a sealed flanged joint with cleanroom-grade silicone sealant is acceptable. ISO Class 8–9 accepts standard SMACNA Class A flanged joints.
- Penetrations: all duct penetrations through cleanroom walls are welded or sealed with cleanroom-grade silicone. Mastic sealant is generally prohibited because of volatile outgassing.
- Hangers: external supports only — no interior bracing or reinforcement that could shed particles. Use stainless threaded rod and stainless angle bracket.
Field testing and acceptance
Cleanroom duct is acceptance-tested at both the leakage level and the cleanliness level. Leakage testing follows the SMACNA Duct Air Leakage Test, EN 1507 or AS/NZS 4254 method depending on project specification. Cleanliness testing is done after installation and before commissioning, by particle count surveys at design airflow conditions.
For ISO Class 5–6 facilities, the duct interior is also typically swabbed for surface contamination testing (TOC, total organic carbon) and rinse-water analysis to confirm no residual fabrication oils or contaminants. The fabrication shop's degreasing protocol becomes part of the project QA pack.
Machine configuration for cleanroom duct fabrication
SBKJ supplies a complete cleanroom-grade duct production line:
- Stainless coil decoiler: rated for stainless coil weight (typically heavier than galvanised), with stainless rollers to avoid contamination cross-transfer
- Spiral tubeformer (SBTF-1500 with stainless package): hardened polished tooling, lockseam with sealed inside surface, available for Φ80–Φ1500 mm round duct
- Rectangular duct line (SB-FS1535L): dedicated stainless duct forming machine with polished tooling
- TDF flange machine (stainless tooling): rolls TDF flange directly onto stainless duct; hardware (nuts, bolts, washers) supplied as stainless
- Continuous TIG welding station: handheld and benchtop TIG units with argon shielding, water-cooled torch
- Electropolishing tank: optional, sized for the largest duct section produced
- Degreasing line: ultrasonic or alkaline cleaning before final pack
See SBKJ stainless duct forming machine, SBTF-1500 spiral tubeformer and handheld laser welding product pages for technical specifications.
Documentation pack for cleanroom projects
Cleanroom projects expect a structured documentation pack delivered with the duct. SBKJ supplies the following as standard:
- Mill certificates for all stainless steel coils used
- Welder qualification records (WQR) for all TIG welders involved
- Weld procedure specifications (WPS) for each weld type
- Surface finish test reports (Ra measurements at multiple sample points)
- Pickling and passivation certificates (for fabrications requiring chemical surface treatment)
- FAT report including leakage test at project specification class
- Cleaning and degreasing procedure used and the certificates of cleaning solutions
- Packaging and transport protocol (cleanroom-grade poly bagging, sealed end caps)
Common cleanroom duct mistakes
- Specifying 304 instead of 304L. Standard 304 sensitises during TIG welding, causing intergranular corrosion at the weld heat-affected zone. Always specify L grades for welded cleanroom duct.
- Using mastic sealant on transverse joints. Mastic outgases volatile organic compounds (VOCs) into the air stream. Use cleanroom-grade silicone or eliminate the sealant by continuous welding.
- Pittsburgh lock seams in ISO Class 6. The seam discontinuity harbours particles and is impossible to wipe-clean. Use butt welding or closed-seam spiral duct.
- Galvanised duct in pharmaceutical sterile rooms. Zinc galvanising is not acceptable for ISO Class 5–7. Stainless steel is mandatory.
- Skipping the post-fabrication degreasing step. Forming oil residue contaminates the cleanroom and shows up in surface contamination tests at handover. Always include alkaline cleaning or ultrasonic degreasing.
Get an SBKJ cleanroom duct fabrication quote →
FAQ
What stainless steel grade is used for cleanroom HVAC duct?
304L for ISO Class 7–9, 316L for ISO Class 5–6 and chloride-exposed locations. Some semiconductor wet-process exhausts require duplex stainless or polypropylene-lined steel.
What surface finish is required for cleanroom duct?
ISO Class 5–6: Ra ≤ 0.4–0.8 µm with electropolishing or mechanical polishing. ISO Class 7–9: Ra ≤ 1.6–3.2 µm 2B mill finish.
How is cleanroom duct sealed differently?
Continuously TIG-welded transverse joints and longitudinal seams for ISO Class 5–6. SMACNA Class A flanged with cleanroom-grade silicone for ISO Class 7. No mastic sealant.
Can SBKJ machines produce cleanroom-grade stainless duct?
Yes — full stainless production line including coil decoiler, spiral tubeformer, rectangular duct machine, TDF flange machine with stainless tooling, TIG welding stations, electropolishing tank and degreasing line.
What leakage class applies?
SMACNA Class A or EN 1507 Class C for most cleanroom duct. Pharma sterile and semiconductor wafer fabs often specify a tighter local class with continuous-weld construction.
