Why are they then often ineffective in practice in controlling the fumes/dust?
Because the operator often works outside the Control Zone (the Captor Distance)!
How can we determine the extent of the Control Zone?
In TExT work we usually use smoke release progressively from the hood entrance until control is lost (then usually moving back 50mm towards the hood entrance) and measure that distance.
Some advocate using a hot wire anemometer on a centreline curve at the captor distance required to measure Captor Velocity as a direct measurement
We could also measure the Face Velocity at the entrance to the Hood and use Fletcher’s Equations to calculate the Captor Control Distance (Control Zone).
This video considers these options and shows some worrying considerations but also some suggested options!
How to Measure Captor Hood Control Distance
Download the Excel Spreadsheet:- Fletchers Nomogram Calculations – to go with this Video