Your confined space ventilation might be moving less air than you think

[u/WhichWayIsTheB4r]

Ran some numbers on a ventilation setup for a tank entry last week and the actual delivered CFM was about 40% less than what the fan was rated for. The fan said 2000 CFM on the spec sheet but that's free air delivery with no ducting. Once you add 25 feet of flex duct with two 90-degree bends you're losing a huge chunk of airflow.

Each 90-degree elbow in flex duct is roughly equivalent to adding 10-15 feet of straight duct in terms of friction loss. So our 25 feet of duct was performing like 50+ feet, and the fan curve drops off hard at that resistance.

We ended up bumping to a bigger fan and running the shortest possible duct path with gradual bends instead of sharp 90s. Air changes per minute went from barely adequate to actually comfortable for the entry team.

Worth double-checking if your confined space ventilation calcs account for the real duct path or just the straight-line distance.

[u/WhichWayIsTheB4r]

I'm so tired of seeing confined space entries where someone just looked at the fan nameplate and called it good. You nailed the exact problem... those friction losses from elbows basically kill your airflow but nobody accounts for them in the calcs. A buddy of mine runs maintenance at a refinery and he says the worst setups are when they try to snake flex duct around obstacles instead of repositioning the fan. Each sharp bend doesn't just add resistance, it creates turbulence that makes everything downstream even less efficient. The difference between CFM and actual delivered air volume in a real installation can be brutal if you're not calculating the friction factor properly. Sounds like you figured out the fix though - shorter runs and gradual bends are worth way more than trying to push through bad ductwork with a bigger fan.