Optimal capture zones in dust extraction – downdraught benches or flexible arms?

Selecting a dust extraction system is a complex process. An expert needs to consider many different options before making their final choice.  These options range from walk-in dust control booths, through mid-sized solutions such as downdraught benches and extraction arms, all the way down to compact arms and on-tool extraction. Each of these systems comes with its own combination of hood size, capture velocity, and system pressure.

Factors to consider when choosing your dust extraction system:

There are many factors that help narrow down this selection including:

• The type of dust or fume which must be captured
  
• How the cloud of contaminant is generated
  
• How it is moving
• Where the point of generation is located in relation to breathing zones
  
• How these factors may vary over time. 

Consideration should also be given to the wider working environment:

• Available space
• Available services
• Location within a building
• Background noise levels

All are factors to help narrow down the selection of a dust extraction system suited to your specific workspace.

When combined these factors help us start to determine the solution. A useful starting point is to consider the area over which the dust must be extracted. If we consider welding as an example, and compare the traditional flexible arm solution to a high volume, low velocity system such as a downdraught bench, we can understand one of the known problems experienced when specifying extraction systems – a failure to understand operator behaviour.

Welding Fume Issues – Dust Extraction Solutions 

An ‘easy’ solution to a welding fume issue is to install a flexible extraction arm. These welding fume extraction systems are relatively affordable, can be installed quickly, and – when working with multiple workstations – can be connected to a single central fan. This is appealing to factory managers. However, we need to look at how an operator works. In many welding operations, the welder will work at various locations on the part. This might involve rapidly welding in positions a meter or more apart. The small inlet area of a flexible arm gives an effective capture zone the size of a football. So, we are relying heavily on the welder moving the arm every time he or she works in a new position. This ideal behaviour tends to be unreliable – welders will not always move the arm to the new weld position. We very quickly find that the newly installed arm system is ineffective and a cloud of weld fume is hovering in the workshop.

In this situation, there are great advantages to the use of a system which does not rely on operator intervention to be effective. High volume, low velocity systems tend to solve this issue. A walk-in dust control booth is a good example to begin with – while the velocity required in an extraction booth is very low, around 0.5m/s, the size of the booth means that total airflow is very high. Even high velocity particles will slow down while still within the booth, meaning they can be captured by much lower velocity airflow. As a comparison, the very small extraction tips present on high vacuum systems need airflows of perhaps 40m/s to be effective.

A downdraft bench provides a middle ground. The capture zone remains large, airflow can remain relatively low at perhaps 1.5m/s depending on the process, but costs (and space requirements) are significantly lower than for a full walk-in dust extraction booth. With the proper specification, correct filtration, and correct airflow – and when applied to the right processes – a downdraught bench can provide most of the advantages of a walk-in booth, in a more compact format, and at a lower cost. This can be achieved without the disadvantages of flexible arms.

This short example demonstrates the value of using expert advice. Every application is different; even with our example focussing on welding, the appropriate solution may vary from on-torch extraction through to a full walk-in booth or even masks and large hall air cleaners. A good supplier will be honest about their own extraction solutions, be able to talk from experience, and will offer you options – even when the best option when it comes to dust extraction equipment is simply to refer you elsewhere. At AirBench we’re pleased to be able to demonstrate most of our mist, dust and fume extraction products on site and allow you to trial them in operation on your own processes so you can buy with confidence, and your whole team can determine if our solutions will work in practice; contact us to discuss your options or to book a demonstration.