I’ve been out on site this week with Deb, getting our hands dirty doing some ventilation testing.
A number of the systems were quite large, with several hoods and branches. Most of the hoods couldn’t be reached when the process is operating due to safety considerations. This makes testing difficult. It’s only possible to measure face and capture velocities when it isn’t running and as the process itself will affect the airflow, measurements taken when it’s down are not representative. In such cases it’s often best to use static pressure behind he hoods (in a location that can be accessed safely) but these are only meaningful where there is data from an initial evaluation when the system is performing as intended and when it is known that control is being achieved. You also need to know what variance in pressure is acceptable (i.e. at what pressure the system stops being effective). Of course, most companies do not have such data!!