When I'm lucky (wealthy?) enough to have a two-bag DC, I'll let you
know. OK, really, there are a lot of caveats that are important in
practice, and not quite knowing what you're describing, I'll just chalk
it up as "it's quite possible."

There is one thing I'd like to point out and that is that the reason a
shopp-vac has much higher static pressure is that the impeller speed is
much higher. Pressure rise at zero flow goes something like [rotation
speed * radius]^2 (I think--again, don't quote me). Despite the larger
diameter of DC's, the high speed of the shop-vac is more than enough to
compensate. Obviously, when there is airflow, things change, but you get
the idea.

Greg


GerryG wrote:
Greg, while I agree with your statements, per se, I'd like to toss in one more
item. Specifically, the intake bypass in a 2-bag DC. We have a single fan
(impeller), and if the air line to that was fully (or even mostly) blocked for
some reason, the upper bag would collapse. To avoid this, there appears to be
a partial intake bypass. The air movement would then split between the main
duct and the bypass by the relative resistance of the two paths.

Now, I imagine a pressure limit valve could be used in the bypass, but I doubt
they do this.

Haven't seen this mentioned before in discussions. But it explains why a 2-hp
DC cannot match the static vacuum of even a medium shop vacuum, no matter how
much you restrict the opening. It would also impact some of your conclusions
(by degree, not type), in that moving from a 5- to 4-inch hose would be worse
than expected since more air would flow though the intake bypass.

Does this make sense, or am I missing something?
GerryG

On Mon, 19 Jul 2004 08:28:54 -0400, "G. Lewin" wrote:


No, the system is not limited by the narrowest pipe. It's not a "weakest
link" analogy. The reason is that the speed of the air will vary
inversely to the area of the duct. So the 4 inch restriction will just
accelerate the air. There is some (small) loss with restrictions, mostly
because it is easy to accelerate flow efficiently but hard to slow it
down.

Others have properly noted the fact that while large ducts can pass a
lot of air, the speed of the air drops so that dust can settle out in
the pipe.

Greg


Fly-by-Night CC wrote:

In article , "George" george@least
wrote:



Not an engineer, but imagine the optimum transport pipe is probably ~5".
Force/unit area calcs show 6" less than half the four.


Perhaps someone can point out the error of my thinking on this subject...

The system can only flow as much as the smallest port in the factory
design. Take my Jet 1.5hp for example, what I'm getting at is that the
port and hose from the blower housing to the bag hanging ring is, I
believe, 5" diameter. To my thinking whatever size of the system outside
of the factory setup is limited by this 5" - in other words, one can't
fully draw 6" of main trunk air before the blower through a 5" hose
after the blower - therefore the appropriate size of the main trunk
should be no larger than 5" - or whatever the size of the smallest port
in the manufactured assembly.

Wadya think?