On 02/24/2016 1:38 AM, OFWW wrote:
....

In a vacuum strange things happen, given a spark, and a whoosh of dust
already suspended in the pipe (your cloud) and I can see things happen
that wouldn't happen in a positive pressurized pipe.

If it were a vacuum, there'd be no oxygen, hence no combustion.

Even in the highest CFM DC systems the air pressure is only a minimal
amount below atmospheric pressure; yes it's a big "vacuum" cleaner, but
there's really not a lot of vacuum, it's just slightly lower pressure
air moving at a pretty high velocity. Hence, there's not going to be
any strange things happening owing to anything other than that there is
a concentration of dust created and given a large enough ignition
source, one could potentially cause a boom. But, static electricity
from PVC for at least home-shop-sized duct work simply doesn't have
sufficient energy to do so. Metal hitting an iron impeller, _maybe_,
but still unlikely. More likely would be an overheated bearing or
another open ignition source like a steaming tube or the like that gets
away but getting it into the necessary location is the trick there...

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