Note: Descriptions are shown in the official language in which they were submitted.
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; This invention relates generally to canopy fume
hoods for use in industrial applications, and has to do
particularly with the design of a canopy fume hood particu-
larly suited to fumes with irregular flow rates.
The problem of efficient capture of fumes in canopy
fume hoods has recently become acute due to environmental
protection legislation which has forced primary metals
industries to install fu~ne collection equipment on smelting
furnaces. Previously fume collection systems have been
relatively small, with suction capacities ranging from a few
thousand to less than 100,000 CFM.
Large financial burdens are placed on the smelting
industry when fume collection systems of up to one million
CFM and over are contemplated.
, In the primary metals industry, the charging opera-
tion in a medium capacity electric arc furnace generates a
sudden surge of fume, which of course the collection system
should be capable of containing. However, the previous
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- fume collection systems have generally been sized to accommo-
date the regular fume flow during the operation of an elec-
tric arc furnace, and this has meant that the considerable
extra surge produced during the charging operation has not
been taken care of. In some installations, only about 50
of the fume generated during the charging operation of a
medium capacity electric arc furnace can be collected.
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In view of the foregoing difficulties encountered
in the collection of fumes in canopy hoods, it is an aspect
of this invention to provide a canopy fume hood design
which is adapted to minimize the "spill-out" of fume ~hen a
fume surge is encountered, and which can reduce the cost of
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installation of a collection system which is intended to
collect substantially all fume generated, including the
extra surge upon the charging of an arc furnace.
Accordingly, this invention provides a fume hood
which comprises:
an upper wall,
a suction openi.ng in said upper wall,
fan means for withdrawing gas upwardly through
. . said suction opening,
side walls extending generally downwardly from
edges of said upper wall., thereby to define with said upper
wall a space enclosed on all sides but the under side,
and baffle plates substantially within said space,
said baffle plates sloping upwardly and outwardly with
respect to the part of said space under said suction
~, opening, the plates surrounding said part and at leas~
some plates having ~heir upper edges spaced from the walls
defining said space, the plates defining between them a cen- :
tral passageway through which gas can pass to said suction
opening.
Three embodiments of this invention are illustrated
in the accompanying drawings, in which like numerals denote
like parts throughout the several views, and in which: -
Figure 1 is a vertical sectional view through a
conventional fume hood, representing the prior art, showing
the velocity profile for gas flow into and through the hood;
Figure 2 is a velocity profile diagram for a
typical fume rising from a fume-generating source;
Figure 3 is a vertical sectional view through a
conventional fume hood showing the circulation pattern of
gas when a surge of fume enters the hood;
Figure 4 is a somewhat diagrammatic representation
showing velocity profiles around and between the baffle
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arrangement of this invention;
Figur~ 5 is a vertical sectional view through the
first embodiment of this inventioni
Figures 6, 7 and 8 are vertical sectional views
through the first, second and third embodiments of this
invention, respectively; and
Figure 9 is a perspective view of an installation
using the first embodiment of this invention.
In a general way, there is a "doughnut-shaped"
circulation pattern existing around every upward hot air or
gas flow emanating from a heated source. This circulation
pattern is caused by the non-uniform velocity distribution
within the plume, as can be seen in Figure 2. The greatest
vertical upward velocity exists in the centre, which is
the hottest part of the plume, while the lowest velocity
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exists in the outer fringes of the plume where the plume
merges with the surrounding still air.
Figure 1 illustrates the velocity profile across a
typical canopy fume hood 10 which represents the prior art.
The hood 10 includes an upper wall 12, a suction opening 14
centrally of the upper wall 12, and side walls 16 extend-
ing generally downwardly from the edges of the upper wall 12,
thereby to define with the upper wall 12 a space enclosed
on all sides but the under side.
The velocity profile shown in Figure 1 can be taken
as that occurring when a suction means, such as a fan, is
connected to the suction opening 14 to draw a particular
flow rate of air through the opening 14, represented as QH.
The condition of Figure 1 would occur when no specific hot-
gas plume were being captured by the hood 10. In other words,
the flow pattern shown in broken lines in Figure 1 is merely
that of the surrounding still air in the unheated condition,
which is drawn through the hood 10 by virtue of the fan or
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other suction means.
If the hood 10 in Figure 1 were now to be exposed
to a particular plume of rising air or gas for which it had
. been specifically designed, the flow lines represented in
broken lines in Figure 1 would alter somewhat, but generally
all of the rising plume would be taken into the hood 10 and
expelled through the opening 14.
However, where the overall flow rate in the rising
plume, designated as Qp in the Figures, is greater than the
maximum evacuation capac.ity QH of which the fan or other
suction device is capable, the doughnut-shaped circulation
pattern existing around the fringes of the plume would give
rise to the pattern shown in broken lines in Figure 3 of
the drawings, in which a part of the total flow spills out-
wardly under the bottom of the side walls 16. In the situa-
tion shown in Figure 3, only a fraction of the total plume
flow Qp finds its way out through the suction opening 14.
In accordance with this invention, baffle means
consisting of baffle plates are provided substantially within
the space defined by the upper wall 12 and the depending
side walls 16 of the hood 10. The baffle plates slope upward-
ly and outwardly and have their upper edges spaced from the
. : walls defining the space.
Figure 4 is a diagrammatic illustration of the
velocity profile imposed upon an upwardly flowing plume of
hot air or other gas when the same passes between side walls
20 and aroundtwo obliquely sloping baffle plates 22. As is
illustrated by the broken lines representing the flow
contours, the outwardly diverging configuration of the
. 30 baffle plates 22 causes the portion of the plume flowing
upwardly between the plates 22 to expand and thus move
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more slowly, while the upwardl~ constricting space defined
outwardly of the baffle plates 22 between the plates and
the side walls 20 causes an increase in the velocity of the
gases flowing in these locations. In the centre between the
baffle plates 22 there remains a differential flow with the
maximum flow rate still in the centre of the plume, but the
flow profile has been evened out as compared to the flow
depicted in Figure 2, which represents the upwardly moving
plume by itself.
The increase in the upward speed of the peripheral
gases which pass outside of the baffle plates 22 serves to
counteract the "doughnut~shaped" recirculation pattern which
would normally be found at that location, and thus outward
spillage of part of the flow underneath the lower edges of
the slde walls 20 is eliminated.
. Referring now to Figure 5, it has been found that
when the baffles 22 are located substantially as shown with-
in a hood 10 similar to that depicted in Figure 1, and when
a standard or "design" plume flow rate Qp is allowed to
. 20 move upwardly and centrally into the hood 10, most of the
- plume passes up the centre between the baffle plates 22
with only a minor, marginal portion passing outside of the
baffle plates 22. This assumes that the suction flow rate
QH matches or is greater than the standard plume flow rate
QPS. Under these circumstances, by far the greater propor-
tion of the canopy space defined between the side walls 16
and the baffle plates 22 is not occupied by any of the hot
gas from the plume, but rather is filled with ambient air
as represented by the inflowing arrows 25 in Figure 5.
Now, when the standard plume flow rate QPS suddenly
changes to a surge flow rate Qsur~ the additional flow over
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and above the noLmal capacity of the suction apparatus
(operating at flow rate QH) will overflow outwardly into
the spaces between the baffle plates 22 and the side walls
16 (hatched in Figure 5), and will begin to displace out-
wardly the a~bient air located in these spaces at the begin-
ning of the surge. The excess flow will gradually fill up
these spaces, and if the hood is adequately designed all of
the excess gas in the surge will be accommodated in the
spaces which are hatched in Figure 5, and the flow will
return to the normal flow QPS before any of the excess gas
begins to spill or leak o~twardly under the bottom edges of
the side walls 16.
When the surge has passed and been taken up through
the suction opening 14, the flow would return to normal with
surrounding ambient air again entering the hatched spaces
; between the baffle plates 22 and the side walls 16.
It has been found generally that an angular dispo-
sition of the baffle plates 22 at about 45 with respect to
the side walls 16 provides satisfactory performance of the
- 20 modified collecting hood of this invention. Some departure
from this angle is contemplated within the scope of this
- invention.
In Figure 6, the first embodiment of this invention
is again shown, with a fan 28 being illustrated above the
suction opening 14.
In Figure 7, the second embodiment of this invention
is illustrated, in which the depending side walls of the
hood extends downwardly and somewhat outwardly, at an angle
to the vertical. Again, the baffle plates 22' would be
disposed at an angle of roughly 45 with respect to these
sloping side walls.
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Figure 8 ,hows a third embodiment of this inven-
tion, in which the hood space is defined by curved surfaces,
but in which the baffle plates 22" are nonetheless disposed
at about 45 to the main direction of what can be considered
the equivalent of the side walls of the hoods in Figure 6
and Figure 7.
An advantage of the construction just described
relates to the tendency for particles in the plume to settle
out on any surface associated with the hood which may be
horizontally disposed or substantially so. There are
some prior art installations in which flat, hori70ntal bands
or strips are placed across the mouth of a canopy hood in
order to restrict flow. Such surfaces, however, would
naturally constitute resting places for particulate material
in the plume. In the present construction, the baffle plates
- are disposed at a substantial angle to the horizontal, and
under normal circumstances would not be capable of collecting
- significant amounts of particulate material from the plume.
In certain installations, the suction opening
~ 2~ would not be located centrally of the canopy hood. For
`~ example, the suction opening could be adjacent a wall, with
the fume-generating source being also directly under the
wall. The canopy could in such circumstances be built out
from and supported by the wall, with the suction opening
displaced from the geometric centre of the canopy area, as
seen in plan view. In such a circumstance, it could be
appropriate to provide angulated baffle plates along three
sides of the rectangle, i.e., only along the three down-
wardly depending side walls of the canopy hood, it being
understood that the wall of the building constitutes the
fourth "side". In other words, there would be no baffle
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plate provided adjacent the wall of the building, but
only on the other three sides of tlle rectangle.
An example of this kind of application is illu-
strated in Figure 9, in which a fume-emitting hot process
is illustrated diagramatically at 31 on the floor 32 of
a building 34. Some of the building walls have been
removed for clarity of illustration. Two roof panels 36
and 37 are located above the fume-emitting process 31.
As can be seen, the fume 38 gradually increases in cross
section as it rises, due to the entrainment of surround-
ing ambient air. This entrainment dilutes the fume and
lowers its average temperature, but at the same time
it increases the volume which must be taken off by the
canopy hood. Suspended under the roof panel 36 is a
canopy hood shown generally at 40, seen to consist of
four vertically downwardly depending side walls 42, 43,
44 and 45. The roof panel 36 itself constitutes the
, upper wall of the canopy, and it contains a suction open-
ing 48 which is connected to a conduit 50 leading to a
suction device such as a fan (not shown).
Mounted within the space defined by the side walls
42-45 and the roof panel 36 are four downwardly and
- inwardly converging baffle plates 53, which are dis-
posed at substantially 45 to the side walls 42-45.
~he baffle plates have their upper outer edges spaced
both from the corresponding side wall and from the
roof panel 36, so that fume entrained to the outside of
the baffle plates 53 can pass around the upper edge of
the baffle plate and enter the suction opening 48.
The velocity profile of the plume is shown at
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60 in Figure 9.
It is to be understood that the number of
baffle plates to be utilized in an installation in
accordance with this invention is extremely variable.
In certain cases, due to cross currents and so forth
in the space in which the fume-generating process is
located may give rise to a tendency for fume to spill out
of a canopy hood only on one or two sides, but not on
the other sides. In this kind of circumstance, it may
be suffieient to provide baffle plates only adjacent
the sides where the spillage tends to occur.
A further advantage of the construetion aceord-
ing to this invention becomes clear when considering
what is ealled the "entrainment height" of a given fume
hoodO Generally speaking, the entrainment height for a
given plume xising to be accommodated in a given canopy
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fume hood is the height at which the marginal edges of
; the plume aetually contact a portion of the fume hood.
In a substantially rectangular eanopy hood, if
the same has been eonstrueted large enough to extend
beyond the normal position of the marginal edges of the
rising plume, these marginal edges do not eome into
eontaet with the hood until they reaeh the upper or
top wall of the hood. This eonsideration effeetively
inereases the minimum eapaeity of the fume hood whieh
is required to handle the plume, beeause the total
volume of the rising plume is eonstantly inereasing, as
ean be seen in Figure 9. Theoretieal eonsiderations
(eonfirmed by empirieal studies) indieates that the
suetion volume required to handle a given plume inereases
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as the 5/3 power of the height between the source cf the
plume and the location of first contact between the plume
and the collection device. In effect, the insertion of
baffles in accordance with this invention lowers the
effective height between the fume-generating device and
the "first contact" with the canopy hood to the location
of the lower edges of the canopy hood, rather than the
upper wall as is usually the case. Where the height
; of the canopy hood is ir the region of 20 to 30~ of the
total distance between the fume-generating process and the
hood, the minimum capacity for the hood could be almost
doubled due to the effect of the 5/3 power. The reason
why the baffles bring the effective height down to the
lower limit of the canopy rather than the top has to do
with the fact that the baffles constitute a wall preventing
entrainment of ambient air with the plume beyond their
lower edges. In effect, they constitute a partition
which prevents further mixing above their lower edges.
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