pdp1@EARTHLINK.NET on wed 12 may 04
Hi Joseph,
Oooooo...nice foray...thorough and informative.
I felt like mentioning, that for those wishing to have a
simple spray Booth as will not cost much of time or trouble
to do...
One may make a four sided shape, as a rectangle or cube
which is open on two ends, of either some Plywood, or, some
Cardboard, or some sticks and cloth or viscolene or
something, as somehow may be made to stay put as that
shape...
And, having that on a little table of some kind, to set
behind it one of those square, 2 ft bt 2 ft sized Fans they
sell at all the homeclubs and so on, and as cost like $17.00
or so...and as usually have three or four speeds.
One may also set a little lamp in there for enhanced
lighting...and...maybe have a little dias or turn-table
stand for the piece to be rotated on while spraying, since,
of course, the booth sides effectually prevent one from
walking around it in a circle.
The effeciency may be tested with some burning insense, or
rope or whatever may smoulder well and make smoke to gauge
the flow with.
Having this set where the Air may egress out of a narrowish
doorway or Window is ideal. Or, some viscolene curtain or
the like may be used to restrict a larger doorway or other.
It would not have the full benifits if the exhaust Air were
merely being blown back around the outsides of the little
Boothlet, so the fit as to the opening it blows out of is
important...too other doors or windows may need to be nearly
closed or paid attention to anyway, when useing it, to
ensure the outside air currents do not interfere.
That, and as Sam of Daly City kindly advises, consider to
wear either a resperator or a mask of some kind while useing
it, for prudence sake.
I made one like this for Brazing, or sometimes Painting, and
I have been happy with it. Even a low speed setting proves
excellent for the positive flow of Air 'out'...
Allthough I did upgrade the Fan recently to one as has more
handsome looks, the old cheapie extempore one worked very
well for years on end of frequent use.
Phil
el ve
----- Original Message -----
From: "Joseph Herbert"
> Paul wrote: "Does anyone know if a squirrel cage would
pull air out of a
> spray booth better than an attic fan? "
>
> There are three general forms of air movers that are
called fans or blowers:
> Propeller, squirrel cage, and vane axial. The actual
difference between a
> fan and a blower seems to depend on the purpose for which
the device is
> intended and the naming practices of the industry applying
the air mover.
> The propeller fan is what it sounds like, an arrangement
of blades, usually
> curved, numbering from 2 to lots. This kind of fan may be
open, with no
> flow directing devices at all, of may have a ring of
material around the
> circumference of the fan to prevent back flow from the
fast side to the slow
> side. Attic fans and other exhaust fans are often
constructed like this.
> In this kind of fan, the flow of air is parallel to the
axis of rotation of
> the fan blades.
>
> A Squirrel cage fan is composed of a cylinder of curved
vanes that rotates
> around a central axis inside an enclosing scroll. Air
enters the fan along
> the axis of rotation inside the cylinder of vanes and
leaves through an
> opening in the scroll tangential to the cylinder. The air
may enter from
> one or both ends of the cylinder but usually only exits in
one direction.
> The version of this fan we are used to seeing, as in home
heating furnace
> blowers, have thin vane blades compared to the diameter of
the cylinder of
> the blower. There are other kinds which can have very
wide (long?) blades
> operating in a relatively narrow scroll case.
>
> The third kind is called vane axial and is composed of one
or more propeller
> types blades mounded on a central shaft in an enclosing
duct. Usually the
> duct will have vanes at least at the entrance to the duct.
The enclosing
> duct may be relatively long compared to the diameter of
the blades. This
> kind of fan may have the motor mounted in the center of
the duct so there is
> no drive belt entering and leaving the duct.
>
> Each of these have advantages and are applied mostly on
the basis of cost.
>
> Propeller fans are relatively cheap to buy but have
problems with low
> efficiency in some applications, Propeller fans are
fairly noisy and that
> noise represents a loss of energy. Propeller fans tend to
recalculate air
> from the fast side to the slow side. When total volume of
air moved from
> one side to the other is important, this cuts the
efficiency of the fan.
>
> Squirrel cage fans are relatively quiet and effective at
moving air. The
> lack of noise is probably the reason for their selection
in home furnaces.
> Long bladed centrifugal blowers are not at all quiet but
may be chosen
> because of dust or ash in the air stream. Both wear to
the vanes and
> clogging are factors in this selection.
>
> Vane axial fans are the most expensive to buy because of
the tolerances in
> manufacture and their relatively infrequent application.
>
> One consideration if wishing to apply a squirrel cage
blower to a spray
> booth is the problem of clogging the vanes. The dust from
the spraying, if
> not effectively captures, will tend to coat the parts of
the blower and
> especially the edges and concave surface of the vanes.
This coating reduces
> the efficiency of the blower very quickly. A propeller
fan is less effected
> by coatings on the blades but they will still have an
effect.
>
> I think the real problem with spray booth design is a lack
of attention to
> the way the air flows through the booth. One approach is
the use a really
> large fan mounted in a duct removed from the booth so the
velocity of air at
> every point entering the booth is pretty high. It will be
higher in some
> areas but high enough everywhere. Where problems develop
is in an
> application where the fan is mounted in the back of the
booth. This can
> easily produce dead spots on the sides of the booth and
could even produce
> some backflow if a propeller fan is mounted without a
shroud to prevent
> recirculation.
>
> In an equivocating answer to the original question, you
may be able to get
> more volume of air moved by a squirrel cage for a given
diameter of blade.
> That is, if the scroll for the squirrel cage is 20 inches
it will probably
> move more air and control it better than a 20 inch
propeller bladed fan.
> However, if you could apply a 30 inch propeller fan with
an appropriate
> shroud, there might not be an appreciable difference. The
squirrel cage can
> stand downstream restrictions a little better than
propeller fans, That is
> they will work with some differential pressure across them
whereas propeller
> fans don't like that at all. So: it depends.
>
> Joseph Herbert
>
>
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