Design criteria

When designing a vacuum dust extractor, there are many factors which can influence its performance in real working situations. Drawing upon our many years of experience with products from a wide variety of manufacturers, including practical testing and independent review of every aspect of their construction, TNO has compiled a set of comprehensive guidelines for the design of good extractors.

The full guidelines can be downloaded below. Some of their key points are summarised on this page.

Airflow

As a rule, the airflow through the extraction tube or hose should be as high as possible. Specifically, the following guidelines should be observed.

 

  • Ideally, the airflow as measured at the end of the extraction tube should be at least 150 m³/h.
  • The greater the diameter of the tube, the better (Ø 50 mm, flow unchanged; Ø 38 mm, flow reduced by 25%; Ø 19 mm, flow reduced by 65%).
  • The shorter the tube, the better (the standard length is 3-5 metres).
  • A smooth inner wall is better than a rough or ribbed one.

Filters

There are a number of different possible filter concepts.

 

  • Three-stage filtration (cyclone, fine filter, type H HEPA filter).
  • Simple filtration (L, M, H).
  • Disposable filtration system.

 

The most important design requirement is to ensure that the dust filters do not clog easily and so seriously impair the performance of the extractor whilst in operation. This can be achieved by applying one or more of the following solutions.

 

  • Mechanical cleaning (vibration).
  • Reverse pulse cleaning (airflow inversion).
  • "Continuous" filter replacement and/or dust collection filters.
  • Use of preseparators (cyclones).

 

It is vital that the airflow in the extractor be maintained during filter cleaning. For example, by cleaning half of the filtration system whilst air continues to flow through the other half. We also recommend that an airflow indicator, acoustic or visual, be provided so that performance can be monitored on a constant basis.

Dust collection

There are a number of possible options.

 

  • "Open" container.
  • "Open" plastic bag.
  • Sealed paper bag.
  • Sealed plastic bag.

 

The drawback with the "open" solutions is that they cause greater fouling of filter systems. On the other hand, sealed paper or plastic bags reduce the extractor’s effective capacity as they fill up. One advantage they offer, of course, is that less dust clogs the L, M and H filters. And once full there is little or no danger of exposure to hazardous substances during disposal. That is not the case with “open” systems.

Dust-source enclosure

Dust-source enclosure and the use of dust extractors are closely related topics. With the source completely enclosed, no dust should be able to enter the immediate working environment. Otherwise, it is very likely that so-called “fugitive” dust will escape. The extent to which the dust is contained in this way affects the performance required of the extractor.

The photographs below show examples of total enclosure, partial enclosure and non-enclosure.

 

Totally enclosed system                   Partially enclosed system                Unenclosed system

 

In the totally enclosed system (left), the dust is fully contained around the source. The dust hood on an angle grinder (middle) provides partial enclosure, guiding the dust in a particular direction. In the case of the chipping hammer (right), there is no enclosure of the source and the extraction hose is some distance away. The best choice of vacuum extractor to some extent depends upon the degree of enclosure available. The blue hammer drill on the left operates dust-free at an extraction rate of approximately 20 m3/h. For the angle grinder with dust hood, that figure is 150 m3/h. And for the chipping hammer it is more than 250 m3/h.

Conclusion

It is the system as a whole (tool, extraction unit and vacuum extractor) which determines overall exposure to dust in the “employee inhalation zone”. The better the dust is contained at its source, the fewer demands are placed upon the extractor.

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