Wednesday, 12 September 2012

Really sucks

The dust shroud shown in the previous post works really well but only if you have suction and can keep it. Last summer I pimped my shop vac with a cyclone because it would clog in seconds especially when doing wood work. Thought I should actually show it:

Cyclone vacuum systems seem a bit mysterious but are really not. First how do vacuums work? Well they move air with an impeller, speeding it up and creating a region of low pressure. Atmospheric pressure (the pressure caused by having a sky full of air above us) then pushes air into that region. So the impeller creates an air flow. This picks up dust simply because the dust becomes entrained in the flow of air. So in my shop vac the bag is in a chamber from which the air is sucked, this creates a flow of air through the nozzle and through the bag and out of this chamber. The bag acts as a filter. The problem is that it fills quickly and soon becomes clogged as fine particles get stuck in the pores.
A cyclone system uses the fact that the dust only moves within moving air. If you watch the video and understand the construction then air must flow from the tangential intake on the side of the cone to the vertical tube which ends about halfway down the cone. The high velocity of the air, the tangential inlet and the shape of the cone means the flow has to turn. Particles that are neutrally buoyant will follow the flow perfectly but anything with a bit of mass to it will tend not to turn and carry on straight, leaving the flow before hitting the cone's side and flowing down into the collection bin.  So most of the dirt ends up in the bucket rather than going up the tube into the vacuum. Actually as a side note, cyclones are used to extract neutrally buoyant bubbles from bubble generators for use in flow visualization, they use tiny 3mm or so bubbles full of helium, they follow the flow so well that even when streamed over a wing at high speed they do not hit the wing and pop, they will even go through fans etc. In those devices the bucket is just collecting soap solution from popped bubbles which are flung outwards.

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