Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~his invention is concerned with improvements
in or relating to fume extraction.
- In industry, fume emitted from various sources
, . .
is collected and extracted via a canopy. The ideal
shape of canopy is believed to be in the form of a
sharp pitched conical or pyramidal hood with a vertical
fume outlet at the apex. Alternatively, a horizontally
elongated canopy may be provided with a pitched roof
portion; preferably the angle of pitch of the roof
portion is less than 90.
However, height considerations sometimes
dictate a flatter canopy profile, which it has been
found can lead to problems of non-uniformly distributed
fume extraction rates, requiring excessive power
consumption to achieve adequate ~ume extraction from
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the areas of lower extraction rate. Apart from the
increased power consumption which this involves, it
also leads to excessive final gas volumes and the
increased entrainment of atmospheric air resulting
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compensate for the areas of lower extraction rate.
In sorne instances in areas remote from the off-ta~e
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fume can escape from the canopy altogether.
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The economic effects of these disadvantages
are specially important where generally high gas volumes
are involved as, for example, in the case of secondary
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ventilation systems in the steel industry. ~or example,
in buildings where basic oxygen steel-making is carried
out or electric arc furnace shops, the extracted gas
volumes may be in excess of one million cubic feet
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~ er minute, and the high power consumption involved in handling
such volumes will be readily appreciated.
The ~roblems of the prior art are overcome by the presen-t
invention which broadly provides a fume extraction assembly which comprises
(a) an elevated canopy adapted to collect fume, and (b) elongated
extraction duct means offset from an apex of the canopy, the extraction
duct means having at least one aperture provided therein whereby
to allow for extraction of fumes from the canopy, and wherein the
extraction area of the at least one aperture per unit length of
the extraction duct means decreases in the horizontal d~wnstream
direction of the assembly whereby to compensate for a tendency to
uncontrolled extraction rates along the extraction duct means.
The invention also provides a method of extracting fu~e
using an assembly according to the invention; the fume emanating, for
example, from a metallurgical process, e.g. iron or steel making.
There now follows a description, to be read with reference
to the accompanying diagrammatic drawings of fume extraction assemblies
embcdying the invention. This description, which is illustrative
of apparatus and method aspects of the invention, is given by way
of example only and not by way of limitation of the invention.
- In the accompanying drawings;-
- Figure 1 shows an end view of a first assembly embodying
the invention;
Figure 2 shcws a plan view corresponding to Figure l;
; Figure 3 is a view on the line III-III of Figure 2;
Figure 4 shows an end view of a second assembly embodying
the invention;
Figure 5 shows a plan view corresponding to Figure 4; and
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Figure 6 shows a side view of a side wall of a
third assembly embodying the invention.
The first fume extraction assembly embodying the
,~ invention (Figures 1 and 2) is located in a building
(not shown) and comprises an elongated canopy 10 elevated
' above floor level and adapted to collect fumes emanating
e.g. from a steel making process carried out in the build-
ing, such as basic oxygen steel-making or an electric arc
furnace. The canopy 10 is rectangular in plan view and
comprises vertical side walls 14, 16, and vertical end
walls 18, 20. The canopy 10 comprises a pitched roof 12
extending between the side walls 18, 20 with an angle of
pitch (~) greater than 90.
An extraction duct 22 offset (,Figure 1) from the
apex 11 of the canopy extends along the side wall 16 from
the end wall 20 to the end wall 18 below the level of the
roof 12, and then leads vIa an extraction fan (not shown)
to gas cleaning plant (,not shown); alternatively the ex-
traction fan may be downstream of the gas cleaning plant.
Fume extracted via the canopy 10 and duct 22 is discharged
to the atmosphere following treatment in the gas cleaning
- plant. The duct 22 comprises (,in cross-section) a horizon-
''` tal base wall 24, an outwardly downwardly sloping top wall
26, and a vertical outer side wall 28; the side wall 16
' defines a boundary between the duct 22 and the canopy 10,
and the side wall 28 diverges from the side wall 16 in the
downstream direction from the region of the end wall 20
to the region of the end wall 18, where the cross-section
of the duct 22 becomes uniform; it will be realised that
as the side wall 28 diverges so the cross-section of the
duct 22
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` increases.
The side wall 16 is pxovided with a plurality
of parallel uniformly spaced vertical slots or apertures
30, each of which extends longitudinally for the full
height of the side wall 16. Each slot 30 after the one
closest to the end wall 20 is narrower (as viewed in
Figures 2 and 3) than the preceding upstream slot 30;
or instead of each slot 30 being narrower than the
preceding one the slots may be provided in banks each
` 10 comprising a plurality of adjacent slots of uniform
width, but the slot width narrowing from bank to bank;
it will be realised that in either case the slot width
narrows progressively in the downstream direction. In
a modification, the slot width is uniform throughout,
but they are spaced progressively wider apart in the
downstream direction.
With uniformly spaced slots 30 the uniform
spacing between adjacent slots is, for example, 1 to
3 feet with a minimum slot width of 2 inches. In a more
specific example the spacing is 2 feet with a slot width
; decreasing from ~6 inches to 3 inches over a 200 foot
long canopy.
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The progressively narrowing slots 30 provide
gas flow characteristics which compensate for a tendency
to uncontrolled non-uniform extraction rates along the
side wall 16. It will be realised this tendency is
towards increased extraction rates at the downstream end
of the wall 16, and progressively reduced extraction rates
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towards the upstream end; and the wider the vertical
slot 30 the less resistance to gas flow it presents;
hence the compensation.
The progressive downstream widening of the duct
22 provided by the diverging side wall 28 also provides
gas flow characteristics which assist in compensation
for the tendency to non-uniform extraction rates, since
suction from the fan will be increased by reduct~on in
the cross-sectional area of the d~lct 22.
In a modification, however, the outer side wall
of the duct 22 is parallel to the side wall 16 as shown
in chain line at 28a, for cases where sufficient compen-
sation is provided by the slots 30.
The assembly also comprises, facing the slots 30,
a concave curved baffle plate 32 which extends inside the
:
canopy 10 along the full length of the side wall 14, and
downwardly from the roof 12 adjacent the top of the side
wall 14 to merge with a lower portion of the side wall 14.
The baffle plate 32 serves to direct gas transversely from
the region of the baffle plate towards the side wall 16
and the slots 30 therein. In a modification, the baffle
plate is inclined planar rather than concave.
In some cases it may be desirable to provide a skirt
attached to lower portions of the assembly to
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aid in fume entrainment; this is illustrated in chain
dot at 29.
The second fume extraction assembly embodyina
the invention (Figures 4 and 5) resembles the assembly
shown in Figures 1 and 2 in many respects, and is
described in so far as it differs therefrom.
The assembly shown in Figures 4 and 5 comprises
a canopy 40 comprising a twin symmetrical pitched roof
42, having an angle ~ greater than 90 . The canopy 40
comprises opposed side walls 46 each corresponding to the
side wall 16 and having slots 50 corresponding to the slots
30. Ducts 43 each corresponding to the duct 22 extend
along the side walls 46, and each duct 43 may have an outer
side wall 48 parallel to the side walls 46 as shown in
,~ Figure 5, or may have an inclined outer side wall correspo-
nding to the side wall 28. Twin symmetrical baffle plates
52 corresponding generally to the baffle plate 32 are
''~! provided in a central region as viewed in Figure 4, and
the lower ends of the baffle plates 52 are connected to-
gether in merging relationship. It will be noted that each
baffle plate 52 faces one of the side walls 46.
The third fume extraction assembly embodying the
invention (Figure 6) resembles the first or second assembly
in many respects, and is described in so far as it differs
therefrom.
In the third assembly the slots 30 or 50 are
replaced by a single V-shaped opening or apertures 60 in a
side wall 62 corresponding to the side walls 16,46; the
opening is defined by upper and lower boundaries 64,66
respectively, the vertical spacing of which varies along
the side wall 62.
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It will be realised that the ~-shaped opening converges
in the downstream direction.
In other assemblies embodying the invention, smaller
low height profile canopies, e.g. square or rectangular
in plan view, have an extraction duct fully encircling the
canopy with slots corresponding to the slots 30 suitably
sized to optimize uniformity of extraction rate according
- to the particular conditions obtaining. In such cases,
there may be more than one off-take from the encircling
extraction duct, the off-takes then leading to a single
further duct which itself leads towards the fan.
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-- In appropriate cases remotely operable rotatable
louvres may be provided to define and vary slot width when
required; this may be useful for example where because of
process conditions it is desired to extract preferentially
in certain areas; this may apply, for example, when a long
canopy extends over a plurality of furnaces and it is
desired to extract preferentially from one furnace. In
this case, for example louvres of uniform width and axial
spacing may be used with a louvre width from 1 to 3 feet
to give a corresponding maximum slot width also from 1 to
3 feet. It will be realised that variations in width
between the several slots according to requirements is
achieved by different angular settings of the louvres.
In some cases the canopy may be provided by the
roof of a building itself.
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