On Wed, 24 Feb 2016 10:04:59 -0600, dpb wrote:

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.


At sea level the air pressure is 15PSI. Every 2" of vacuum is
equivalent to a 1 PSI drop in pressure. So at what point do you
consider a vacuum a vacuum? And how does a gasoline engine operate
when its intake manifold is in a vacuum if there was no oxygen?
IOW's how many inches of vacuum required before all oxygen is boiled
off, and that includes the oxygen from moisture.

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...

An overheated bearing would have to be above 425 degF as I recall to
burn wood. A spark is a different animal. A spark hitting a muffler
packed with steel wool sets it on fire which sets the sawdust on fire
and minor implosion in a vacuum system, and hey! It was my imagination
so don't be a party pooper, K?