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113 dust collectors




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This article is from the Woodworking FAQ Collection 2, by multiple authors.

113 dust collectors

From: jeffj@pedsga.UUCP
Date: 8 Dec 88 00:12:29 GMT

I have an interest in pneumatics and I think I can help illuminate
the differences between shop vacs and dust collectors.

One of the informative graphs about a blower is the ratio
of air moved in Cubic Feet Per Minute (CFM),
and the pressure or vacuum created in inches of water or mercury.

They tend to look like this:
air   |  .
moved |    .
      |     .
      |      .
      |       .
      |        .
      |          .
      |            .
      |_________________
          pressure/vacuum

The amount of air moving decreases as you obstruct the flow and
call on the blower to suck it in or blow it out.

A dust collector is on the left of this graph. It is hardly
obstructed.

A shop vac covers the entire graph. It is often obstructed.

Dust collectors (and leaf blowers) have modest vacuum ability.
This is because the impeller (fancy word for fan blade)
is a flat disk with flat broad vanes.
It has a lot of surface area, so a lot of air is moved.
The amount of power per square inch that is put to the blades
depends on the motor, number of vanes, size of vanes.
If you block the intake, not much suction is created.
The motor keeps spinning as the efficiency of the impeller
drops to almost zero.
(Ventilation fans take this to an extreme using squrrel
cages for great vane surface area with little power
behind it).

The filtering system for both leaf blowers and dust collectors
seems to be the same. The stuff goes through the impeller
and is blown into a container with an air filter (usually
just a cloth bag). This implies that everything that is
sucked up also gets chewed up. You risk damaging the
impeller by picking up hard debris.
Perhaps a leaf blower (like the 1HP electric handheld ones
Toro, Sears, etc sell) would make a dust collector as well?
Dual purpose tools sound economical, no?

Shop vacs use a disk with small curved vanes
(the disk is about 6" is diameter with vanes less than
1/2 inch high). This moves less air (less surface area
of vanes pushing the air), but there is great power
to each square inch (depending on the power of the motor).
If you block this, it creates a lot of suction.

The filtering system blocks everything before it reaches
the impeller. This means that the impeller is protected
from damage, and you can suck up stuff that can't be chewed
up (like nails, scraps) or would damage the works (like water).

Shop vacs are good at collecting/concentrating.
All the air is going through a 2" hole with a filter
covering it. Dust will quickly cover the filter and
decrease efficiency. But mud and debris will collect
in the bucket rather then splattering all around.

Dust collector: The filter is large since its on the blowing side.
Not as easily blocked by dust.
A nice benefit of dust collectors is that you can blow the stuff
directly into a cloth bag or even a disposable plastic bag for easy cleaning.
The bag simply inflates and doesn't need a drum.

One I saw in a store was like this:

                        ___
                blower (   )
                     [](   )   <- cloth bag
               OOOOOO[](===)+  <- hoop holds bags together
                       (   )|
                hose   (   )|  <- plastic bag
                       (___)|
		       -----+-  <- base

I don't see why a leaf blower can't be used for the blower.


while mentioning power per square inch:
Increasing the power of the motor will increase the
lifting ability (in water)as well as the air moved (cfm).
Vac manufacturers like to tell you the power of the motor (in horsepower).
This is now a game, where 'peak' horsepower is given.
'Peak' is the moment you jam the impeller, using the impulse
of the motor, a far cry from normal running power.

Sears is "inconsistent"(to be nice) in the nameplate data.
Two vacs that claimed to have different horsepower had nameplates
that claimed that both drew the same current at 115VAC.




 

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