Bill Pentz is pretty firm in pointing out that lots of air is needed to capture all the dust (not just chippings) from machining operations. He appears to say that a hood placed to surround the stream of waste thrown off by the cutter connected to a 100mm extracion pipe wont do the whole job. Much better if the hood is connected to a 150mm pipe. If the extractor motor/fan is pulling hard enough to keep the airflow at the same speed in teh bigger pipe, the air speed in the hood will now be faster. This should improve capture within/close to the hood. The airspeed within the hood itself (slower than the pipe due to its larger X sectional area) might be measured with an hand anemometer, cost around 60 quid.
Typical practical in-hood velocities are in the range 3 to 10 m/sec. If you take a RAS or MS as an example, with a large hood 300x300mm at the rear, this figure of 3-10m/sec is reduced by an order of magnitude to 0.3 to 1 m/sec just 300 mm away from the hood*. This is estimatable by the 'fag test'. Bear in mind fine dust 'just floating about' needs an air flow of 0.3 to 0.5 m/sec to get itself captured.
* Local Exhaust Ventilation, HSE 1999
air speed reduces by 10x for each diameter distant from the pipe/hood end, (hence at 300mm for a 300x300mm hood)
Capture velocities, abstract from publication above:
difficult to do table, data given as pairs, in following format:
particle velocity/airspeed to capture in m/sec
practically none, into quiet air/0.25-0.5m/s (evaporation - i.e. brush painting)
low velocity into slow moving air/0.5-1.0m/s (i.e. paint spraying in booth)
high velocity into rapid air/2.5-10m/s (typical of wood machining)
