In a previous post I discussed why captor hoods are ineffective at controlling contaminants generated by most common industrial processes. Partial enclosures, or booths, are another common type of hood which, in principle, should be more effective for many situations. This is because they don’t have to actively capture the contaminant as it’s generated inside the hood. The airflow provided by the system doesn’t have to pull the contaminant into the hood, but is intended to
- prevent the contaminant escaping into the workplace and
- remove it from the hood.
One of the main problems that is commonly experienced with partial enclosures is that the presence of the user in front of the hood disturbs the airflow and can lead to contaminants being drawn into the user’s “breathing zone”. This is illustrated in the following diagram. The blue lines represent the airflow while the red arrows show how contaminants generated in front of the operator just inside the booth are likely to behave.
There are a number of ways that this problem can be overcome, which are discussed in the Health and Safety Executive publication HSG258 – Controlling airborne contaminants at work: A guide to local exhaust ventilation
1. Move the source of the contaminants further into the booth so that they are not entrained by the turbulent airflow near to the user. The designer needs to make sure that the booth is deep enough for this arrangement
2. Insert a transparent barrier at the front of the booth – i.e. increase the degree of containment.
3. Use a side flow booth where the operator stands at right angles to the horizontal airflow
4. Use a downdraught booth. Here he worker stands inside the booth where the airflow is vertical (from top to bottom) so the contaminants are pulled down away from the breathing zone.
Care needs to be taken with the design and application of these downdraught booths to ensure that the presence of the operator, work-piece and work equipment does not disturb the flow too much so that eddies are created that draw the contaminants into the breathing zone, thereby negating the reason for using this type of booth. It can also be difficult to control highly energetic contaminants (e.g. the spray from a paint spraying operation) with this type of set up.