What’s wrong with captor hoods?

Captor hoods are one of the most common types of inlet that you’ll find on extraction systems. With these hoods, the contaminant is generated and disperses outside the hood. It has to be drawn into the system by the airflow.

Unfortunately, in most cases, they are largely ineffective at controlling contaminants. The following diagram illustrates how airflow drops off very rapidly with distance from the face of the hood. This happens because air is drawn in from every direction – not just from the region where the contaminant is generated.

As a rough rule of thumb, the velocity a distance equivalent to the diameter of the hood opening will be around 10% of that at the hood face (although the diagram suggests that a captor hood’s performance is even worse than that) .

Here is a common process – a stonemason carving a block of stone with a chisel. This process generates fine dust which will include some respirable crystalline silica – a highly hazardous dust. We can see a captor hood being used to control the dust. The nature of this process means that the dust is created at different locations as the mason works along the block of stone. A dust lamp has been used to visualise the dust created.

The capture zone for the hood (the area where the air velocity is high enough to capture the contaminant) is marked with the dashed yellow circle.

As you can see when the mason is working close to the hood the dust is created in the capture zone and there is a good chance that it is being controlled. However as the working position changes, dust is created further from the hood and outside the capture zone.

Remember that the dust is invisible to the naked eye, so the mason doesn’t realise that this is happening. As far as he is concerned the dust is being controlled by the LEV. We can see that this is only true when he is working close to the hood.

The mason in the picture is wearing a respirator, so, hopefully, he is protected. However, this will not always be the case and many workers can be exposed to significant concentrations of hazardous contaminants when using captor hoods.

Dusts are particularly difficult to control as they are relatively heavy and often have momentum. A relatively high velocity is needed to draw the particles into the hood. As the velocity drops off very quickly, any dust more than a few centimetres away from the hood is unlikely to be captured effectively.

With gases and vapours lower capture velocities are needed. But as the contaminant tends to disperse over a wide area, they can still be difficult to achieve control with a captor hood.

Captor hoods are often installed as an “off the shelf” solution, and because the purchasers have a mistaken belief that they will control the contaminants. Unfortunately, this is rarely the case. The first step in effective control is to understand the problem –

• how the contaminant is created,
• where it is generated (making sure that ALL sources are identified), and
• how it behaves once released

If LEV is the best, or most practicable solution to the problem (it isn’t always) then the hood should be designed around the process using this information. In many cases it will be possible to design a full or partial enclosure which will almost always be more effective than a captor hood.

Where captor hoods have to be used:

• they need to be positioned as close as possible to the source of the contaminant
• the capture velocity needs to be adequate to pull the contaminant into the hood from the furthest point where it is likely to be present
• the capture zone needs to be clearly defined, bearing in mind that it is easily disrupted, particularly by draughts. It can shrink and expand depending on conditions.
• 

The capture zone for a flexible arm captor hood

For further details on LEV design, see the HSE LEV website and HSG258 – Controlling airborne contaminants at work: A guide to local exhaust ventilation – (which can be downloaded free of charge)