Blow Suction Dust Removal for Large Halls
Blow suction dust removal is what is specified when the workpiece is too large for a hood and the welding position changes every hour — a controlled air path across the hall instead of an extraction point chasing fume that has already spread.
40–80%
capture efficiency measured with a push jet across a 10 m wide source area
40% → 60%
average capture improvement when the push airflow was switched on
20,000–30,000 m³/h
air volume across the blow-suction units in this range
When the workpiece is bigger than any hood
There is a point in fabrication where source capture stops being practical. The weldment is fifteen metres long, it turns on a rotator, and the welder works his way along it all day. A hood can be positioned for one part of that job and for no other.
Blow-suction — also called push-pull ventilation — solves it by giving the air a direction. A duct on one side of the hall discharges a plane of clean air across the working zone; a duct on the other side pulls it back, contaminated. Fume generated anywhere in between is carried along the path and captured, whatever the welder is doing and wherever he happens to be standing.
What makes it work is the balance between the two. Too little push and the plume rises through the air path into the roof space; too much and the resulting draught disturbs the shielding gas and makes the welder uncomfortable. The measured effect is large: on a foundry casting line where the source area was ten metres wide, capture efficiency with the jet running varied between 40% and 80% depending on how far the source was from the exhaust, and switching the push airflow on raised the average capture from 40% to 60% with no change to the exhaust.
What the push-pull measurements show
40–80%
Capture efficiency measured by tracer gas on a large-area installation with the push jet running, varying with the distance between the source and the exhaust hood — the design range for a large-area system, not a single figure.[1]
40% → 60%
Average capture on the same installation before and after the push airflow was switched on, at the same exhaust volume. The push side of the system is where the performance comes from.[1]
> 90%
Capture efficiency measured for a push-pull arrangement in bench trials, against 38–58% for the same exhaust airflow without the push — and the arrangement proved relatively insensitive to obstructions in the path and to cross-draughts.[2]
4 regimes
Flow regimes identified in aerodynamic studies of push-pull systems — dispersion, transition, encapsulation and strong suction. Operating in the strong-suction regime is the strategy for capture efficiency, and the design guidelines provide the critical push and pull velocities.[3]
Full sources are listed under References at the end of this page.
Push the air, pull the air, stay out of the crane way
A blow-suction installation is designed in section, not in plan. The height of the ducts, the distance between them, the velocity of the supply air and the pull at the return all come from the cross-section of the hall and the height of the working zone.
A controlled air path across the working zone
FDCX-20P at 20,000 m³/h and FDCX-30P at 30,000 m³/h are the units for hall-level duty, with 259 m² and 345 m² of filter media. The supply duct discharges above the working zone and the extraction duct returns on the opposite side, so the air path crosses the area where fume is generated rather than the space above it. Ducts typically sit below the crane rail and above head height, which is the constraint that usually sets the design: a blow-suction system in a hall with an overhead crane lives or dies on the envelope it is allowed to occupy.
Design it as the second layer, not the only layer
Blow-suction is a general ventilation method that captures fume after it has left the arc, so it protects the hall but not the welder’s breathing zone on its own. The best installations combine both: local capture at the arc for the welder, blow-suction for the fume that escapes and for the people working elsewhere in the hall. That combination is also how the system stays economic — you only ventilate the volume the local extraction could not cover.
Blow-suction units for high bays
Two units cover hall-level duty up to 30,000 m³/h each, with 259 m² and 345 m² of media.
What a blow-suction installation includes
- Supply and extraction ducting with discharge nozzles arranged to give a plane of air across the working zone, sized from the hall section
- Duct heights set below the crane rail and above head height, with the working-zone height confirmed against your crane envelope
- Automatic pulse-jet cleaning and a dust discharge arranged for the volumes a hall system collects
- Commissioning with airflow and air-path velocity measured in the working zone, and capture efficiency checked by smoke or tracer at several positions in the hall
Send the hall cross-section, the crane envelope, the working-zone height and where fume is generated. Blow-suction designs are driven by the section, and by what the crane will let you install.
Halls and work this suits
High-bay fabrication halls, shipyards and ship repair, wind tower and monopile fabrication, heavy structural steel and bridge fabrication, and large machine and vessel assembly halls. Operations include MIG/MAG and flux-cored welding on large weldments, cutting and gouging, and grinding and dressing at height or on large parts. The method suits halls where the welding position moves, the parts cannot be brought to a hood, and the crane rules out ducting at low level. It is generally a complement to source capture, not a substitute for it.
In the hall
Questions from fabrication hall managers
Send us the hall section and the crane envelope
Send the hall cross-section, the crane envelope, the working-zone height, the processes used and where the fume is generated. We will come back with the duct positions, the supply and extraction velocities and the unit size — and say where local capture still needs to be added.
References
- Hynynen P, et al. Local ventilation solution for large, warm emission sources. Ann Occup Hyg 2007. — push-pull ventilation across a 10 m wide casting area: capture efficiency 40–80% with the push jet, average improvement 40% to 60%. ovid.com
- Watson SI, Cain JR, Cowie H, Cherrie JW. Development of a push-pull ventilation system to control solder fume. Ann Occup Hyg 2001;45(8):669–676. doi:10.1093/annhyg/45.8.669 — capture efficiency above 90% with push airflow, 38–58% without it at 420 mm from the hood.
- Huang RF, Lin SY, Jan SY, et al. Aerodynamic characteristics and design guidelines of push-pull ventilation systems. Ann Occup Hyg 2005;49(1):1–15. doi:10.1093/annhyg/meh065 — flow regimes and critical push and pull velocities.
- Health and Safety Executive (UK). Controlling airborne contaminants at work: a guide to local exhaust ventilation. HSG258, 3rd ed. — Table 9 capture velocities for welding fume, and the commissioning checks for a moveable extraction arm. hse.gov.uk
- IARC Working Group. Welding, molybdenum trioxide, and indium tin oxide. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 118, 2018. Summary: Guha N, et al. The Lancet Oncology 2017;18(5):581–582. doi:10.1016/S1470-2045(17)30255-3
- Health and Safety Executive (UK). Change in enforcement expectations for mild steel welding fume. Safety alert STSU1-2019. hse.gov.uk