When performing industrial hygiene exposure assessments, one of the key things we evaluate is if adequate engineering controls are in place to lower worker exposure. Although a facility may have chemical fume hoods installed, employee exposure can still be elevated. A consultant then has the difficult task of telling a client that the local exhaust ventilation system that they invested in is not effective in reducing their employee exposures.
In some cases, the facility may have designed and installed the ventilation system in-house. They may have a maintenance department, who is tasked with fabricating a hood and running ductwork to an exhaust fan. In other cases, they may have contracted a company to install a ventilation system. In both examples, time and money were invested to reduce worker exposure to chemical vapors, particulates, or fumes. This blog will address a few common pitfalls when trying to design an industrial ventilation system.
Incorrect Hood Usage
Often, when employee exposures are elevated and ventilation is provided, the type of hood installed is incorrect for reducing employee exposure to a contaminant. For example, a facility may install a canopy hood over a dip tank and although this design may be effective for maintaining vapor concentrations of a flammable liquid below 25% of the lower flammability limit (LFL), the hood may not prevent exposure to employees who operate the tank. This is because the employee may be standing in the pathway of the contaminant when it is being pulled away from the surface of the dip tank and up to the canopy hood. In situations like this, a slot hood — positioned behind the tank instead of above it — is the better option for controlling exposures. There are recommended guidelines and standard industry practices regarding the types of exhaust ventilation that will work best for various types of exposure sources.
Inadequate Capture Velocity
Another common pitfall is inadequate capture velocity at the contaminant source. Moveable extraction arms, slot hoods, and other hood types must be positioned close enough to the point of generation to have enough capture velocity to effectively remove the contaminant from the breathing zone of the employee. Often, the location of the hood is the issue and the hood can be repositioned closer to the point of generation. In some cases, ventilation must be increased to achieve the desired capture velocity.
Improper Ductwork Sizing
The last pitfall we will discuss is ductwork. Ductwork with a diameter that is too narrow and/or elongated will not provide the air flow needed to capture the contaminant.
At Braun Intertec we can assist with assessing ventilation performance, including the following:
- Performing ventilation performance verification testing of newly installed equipment.
- Performing routine ventilation measurements of existing equipment.
- Determining exhaust flow rates and capture velocities required.
- Determining the correct type of local exhaust capture system needed and flow rates required.
If you would like to speak with someone in more detail about industrial exhaust ventilation design and/or assessment, please give us a call or click the link below to fill out our Contact Us form.