Ventilation is the most common control for confined space atmospheres. OSHA requires the air supply for forced-air ventilation to come from a clean source and not increase hazards in the space (1910.146(c)(5)(ii)(E)). Equipment choice and setup decide whether that actually happens.
Types of confined space blowers
| Type | How it works | Strengths | Watch for |
|---|---|---|---|
| Axial fan | Propeller-style blade moves large volumes of air | High CFM with short duct runs | Airflow drops quickly with long ducts and bends |
| Centrifugal blower | Squirrel-cage wheel builds pressure | Holds airflow better through long duct runs and bends | Lower free-air CFM than an axial fan of similar size |
| Venturi (air horn) | Compressed air pulls ambient air through a horn | No motor in the airstream; common in flammable atmospheres | Needs a large compressor supply; compressor intake must be clean |
| Explosion-proof electric | Motors and wiring rated for hazardous locations | Use where flammable vapors may be present | Must match the hazardous location classification |
CFM and duct losses
Blower ratings are usually free-air CFM, measured with no duct. Every foot of duct and every bend reduces delivered airflow. Use the manufacturer's performance chart to find actual airflow for your duct length and number of bends, then estimate purge time with the ventilation purge calculator.
Supply or exhaust?
- Supply (push): fresh air delivered to the work area or bottom of the space. The standard choice for general ventilation, since entrants breathe air that just came from outside.
- Exhaust (pull): draws contaminated air out, often near the source, such as welding fumes or solvent work. Make sure the exhaust discharges away from people and intakes.
- Push-pull: both, for large or complex spaces.
Setup rules
- Intake upwind, away from vehicle and generator exhaust, process vents and the space's own exhaust.
- Duct to the work area or bottom of the space so air sweeps the whole volume, not just the opening.
- Never use pure oxygen to ventilate.
- Ground and bond equipment and use rated equipment where flammable atmospheres are possible.
- Test after purging, at all levels, before entry, and keep ventilating during entry.
- Evacuate if the blower stops, unless the permit's procedures say otherwise.
Maintenance
Inspect ducts for tears and crushed sections, check blower guards, cords and plugs, and clean intakes. Damaged duct leaks air and reduces delivered CFM.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.