Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Background of the Invention
~arious types o~ systems for separating solid
particles of material, such as dust from gas from a boiler,
industrial process, or similar sources, are in use. If
5the temperature of the gas carrying the dust is low enough
the gas may be passed through porous filter bags or through
a spray chamber in which a liquid spray separates the dust
from the gas. If the gas is hot these systems are not suitable 5
because of the danger of fire or rapid evaporation of the
lQspray materials.
Gravel bed filters have been used in high temperature
systems. In a gravel bed system the raw gas (gas carrying
the dust to be collected) is drawn, usually downwardly,
through a bed of particulate material. The dust lodges
15on the particles of filter material. As the process continues
the dust collected in the ~ilter bed impedes-the flow and
the dust must then be removed. This is done by replacing
the bed or by back flushing it by flowing clean air through
the filter bed in a direction reverse to the previous gas
20flow while gently agitating the bed. Much of the collected
dust agglomerates into a filter cake that is carried off ;
in pieces during the back ~lush operation. In the usual
systems, the back flush gas at a pressure higher than the
pressure of the raw gas is forced through the bed, then
through a raw gas precleaner such as a cyclone for the -
individual filter bed and into the raw gas duct. It is
intended thak the pieces of filter cake be collected in
the inlet precleaner or cyclone. However~ a separator, such
as a cyclone separator, is e~ficient only for flow in one
3 direction and as a result much of the filter cake is broken ;
and dust is carried back into the raw gas duct to be collected
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~,i another fllter bed. As a reault the ~ystem tends to load up
wlth dust and the actual dust collectlng capacity of the system
i~ reduced.
SUMMARY OF THE INVENTION
.
According to ~he invention a number of filter modules,
each with a filter bed, are arranged such that a separate
collector and fan, in cooperation with simple valves in the ducts
of ~he system, may draw back flush gas through a selected filter
bed and the collector and discharge the gas from the collector
into a raw gas duct of the system. The invention further
includes arranging the back flush gas supply duct of the system
in heat exchange relation wi~h at least one of the raw gas duct
- and cleaned gas duct to heat the back flush gas and thus avoid
cooling and condensing any vapours from the raw gas that might
be in the filter systems. In a preferred form of the invention
the system is arranged so that at least the raw gas duct and the
back flush receiver duct from the modules are substantially
vertical and extend upwardly from collected dust discharge means.
Broadly speaking, therefore, the present invention ;~
provides a gravel bed dust collector system comprising in combina-
tion, at least two collector modules each comprising an upper
and a lower chamber, a perforated partition separating the
chamber~ and a bed of particulate fileer material on the partition,
a raw gas duct that receives the gas to be cleaned and that has
sollds material discharge means, a back flush receiver duct tha~
extendo generally parallel to the raw ga~ duct, first branch
ducts includi~g valves arran8ed to selectively connect the upper
chamber of each module to the raw gas duct and the back flush
receiver duct, a cleaned ga~ duct that extends generally parallsl
to the raw gas duct, a back flush 8as supply duct, second branch -
~ducts lncluding vslves arranged to s~lectively connect the lower
chamber of each module to the cleaned gas duct and back flush gas
~upply duct, and a dust collector and fsn arranged a~d connected
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ln series betwee~ the back flush receiver duct and the raw gas
duct, the fan servin~ to malntain the pre~sure in the back
flush receiver duct less than the Pressure in the raw ~as duct,
whereby, when the valves are in first positions ~as may be drawn
f rom the raw gas duct, downwardly throu~h the bed of particulate -
filter material and into the cleaned gas duct, and when the
valves are in second positions back flush gas from the back flush
gas supply duct is drawn upwardly through the filter beds and the
gas together with collected dust from the bed is carried through
the back flush receiver dust into the dust collector.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of the improved dust
collector system.
~ig. 2 is a perspective view, wîth parts broken away, `-
of a filter module of the improved system.
Fig. 3 is a front elevation, with part broken away,
of a preferred arrangement of filter modules and connectin~ ducts.
Fig. 4 is a side elevation, with parts broken away,
of the preferred arrangement.
Fig. 5 is a top view, with parts broken away, of the
preferred arrangement.
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Description of a Preferred Embodiment
A gravel bed dust collecter system operates on
the principle that the fine dust in the gas to be cleaned
can be caught in the gravel bed of a module as the gas
flows from the raw gas duct through the module to the cleaned
gas duct. The collected dust serves to catch more of the
dust so that a layer of dust forms on the surface of the
gravel bed. The dust in this layer tends to agglomerate.
When the pressure drop for a given gas flow exceeds a given
value, i.e. a substantial quantity of dust has been collected
on the bed, the collected dust is removed from the bed.
This is done by back flushing the bed, i.e. drawing or blowing
clean air or gas through the bed in a reverse direction. -
The reverse air flow, perferably aided by gentle mechanical
agitation of the bed, lifts the collected dust out of and
off the bed. In accordance with the invention~ the back
flush air or gas after leaving the module is drawn through
a collector, preferably of the cyclone type, and discharged
into the raw gas duct. A cyclone type collector can easily
collect the agglomerated dust although it is quite inefficient
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in collecting the dust before agglomeration.
The gas flow paths in the improved dust collector
system are shown in Fig. 1. As shown, the raw gas to be
cleaned enters the system through a raw gas duct 1. Just
before it enters an upper chamber 2 of a filter module
3 it passes a diverter valve 4 which normally is in its
first position with the duct 1 open.
~ rom the upper chamber 2 the raw gas flows down
through a gravel bed 5 that is supported on a perforated
divider plate or partition 6 and into a lower chamber
7 of the module 3. As the raw gas is passing through the
gravel bed 5 dust in the gas is trapped by the particulate
material of the bed 5 while the now cleaned gas continues
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through the bed and partition into the lower chamber 7 and
out through a normally open back flush valve 8 into a cleaned
gas duct 9. An induction fan 10 maintains a subatmosphere
pressure in the cleaned gas duct 9 to maintain the just
described gas flow. `
In a commercial system a plurality of modules
3 are provided, the raw gas duct 1 and the cleaned gas
duct 9 are in fact manifolds with short branch ducts leading
to the individual filter modules 3. A diverter valve 4 is ~
lQ provided in each branch duct from the raw gas duct while ~;
each branch duct from the cleaned gas duct 9 includes a
back flush valve 8. :
When the bed 5 is to be back flushed the associated
diverter valve 4 and back flush valve 8 are moved to their '`
second positions. Air or cleaned gas from the output of
the fan 10 is then drawn from a back flush gas supply
duct 11, through the back flush valve 8, the lower chamber ~;-
7, the perforated partition 6 and filter bed 5 into the ~'r .:
upper chamber 2. While passing through the bed 5 the gas
2Q picks up the collected dust and carries it from the chamber
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2 through a short branch duct and the diverter valve 4 into
a back flush receiver duct 12 that leads into a dust collector
13, preferably a cyclone, and thence through a back flush
fan 14 that discharges into the raw gas duct 1.
While a bed is being back flushed it is gently
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agitated by a motor driven rake 15 to loosen the dust adhering
to the particles without breaking up or reducing the agglomerated
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material to dust.
It may be noted that any leakage in the diverter
valve 4 merely passes raw gas from the raw gas duct 1 into
the back flush receiver duct 12 from which it is returned
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by way of the collector 13 and fan 14 to the raw gas duct.
Thus the only effect of the leakage is to slightly increase
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the load on the fan 14. Likewise any leakage through the
back flush valve 8 passes clean air or gas from the back
flush supply duct 11 to the cleaned gas duct 9 with no -
ill effects other than a slight increase in load on the
induction ran 10.
Each rilter module 3, as shown in greater detail ~
in Fig. 2, comprises a generally cylindrical tank 16 that `
i5 divided into the upper chamber 2 and lower chamber 7
by the perforated partition 6. The gravel bed 5 resting ;
on the partition 6 may be any suitable granular material,
such as, for example, silica sand. During back rlush~
to loosen the collected dust, the bed 5 is gently agitated
by the rake 15 having tines 17 that clear the partition
6.
The rake 15 is driven by any suitable drive such
as a gear motor 18 mounted on the top of the tank or container
16.
The upper chamber 2 is connected by branch duct
19 and diverter valve 4 to either the raw gas duct 1 or
the back ~lush receiver duct 12. Pre~erably the branch
duct 19 is rectangular in cross section so that a simple
valve plate 20 hinged at a side of the duct can alternatively
close the portions 21, 22 of the duct leading to the raw
gas duct 1 and receiver duct 12 respectively. Any suitable
operator 23 may be provided to move the valve plate 20.
It is desirable but not necessary that the path
of the back flush gas be as straight as possible to avoid
fragmentation o~ the agglomerated dust loosened ~rom the
filter bed.
A similar branch duct 24 connects the lower chamber
7 to the back ~lush supply duct 11 and the cleaned gas duct
9. A suitable valve plate 25 moved by an operator 26 controls
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the flow through branches 27 and 28 connected to the cleaned
gas and back flush supply ducts respectively.
In a typical installation a plurality of modules
3 are operated in parallel during the dust collecting time
and are back flushed individually. That is~ while one
module is being back flushed or regenerated the remaining
modules are collecting dust.
A preferred arrangement for a commercial installation
is shown in Fig. 3, 4 and 5. The filter modules 3 are
arranged in groups, each group occupying one level or one
floor of a tower arrangement. The tower comprises an open
frame 30 including a horizontal support frame 31 at each
level to carry the filter modules of that graoup. The modules ^;
can also be arranged in a spiral like the steps of a spiral
staircase. A raw gas duct 32, corresponding to the duct -
1 of Fig. 1 and shown as rectangular in cross section,
extends vertically up the center of the tower. As shown,
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it has an inlet 33 (Fig. 3) leading into its lower end. .
The inlet may be at any elevation depending upon the particular
installation. The duct 32 has a solids discharge gate
34 at its bottom. The gate 34, which is provided with
tipping valves or other suitable means forming an air lock,
discharges into a solids collector conveyor 35 to dispose
o~ any solids matter that may drop out of the raw gas
in the raw gas duct.
At each module branch ducts, similar to the duct `~
19, 22, 23 lead from the raw gas duct 32 into the upper :
chamber of the module and from the module to a back flush
receiver duct 37. Diverter valves corresponding to the
valve 20 (Fig. 2) are arranged to direct the flow into the
chamber as indicated by an arrow 38 ( Fig. 5) or from the
chamber to the back flush receiver ducts as shown by arrow
39 ( Fig. 5).
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The back flush receiver duct 37, which may be
either one or several individual ducts, leads into a cyclone
collector 40. The output of the cyclone passes through
a fan 41 and into the raw gas duct 32. When several individual
5 back flush ducts are used it may be more convenient to supply
more than one cyclone collector and fan than to supply
the duck work required to lead all of the ducts into one
cyclone. Also the provision of two or more cyclones and
fans allows an equal number of modules to be back flushed
10 simultaneously.
The cleaned gas from the lower chamber 7 of each
module flows through a branch duct 42 and a back flush
~ ~ va ~e
4~ 1 vl~vc 43 (shown as a disk valve) into a cleaned gas duct
44. As shown, there are two of the cleaned gas ducts to
15 simpli~y the duct work from the modules. Also, the modules
shown in Fig. 3, 4, 5 are double units, each having two
complete filter beds and upper and lower chambers.
The cyclone 40 or each cyclone if more than one
are used has a solids discharge gate 45, equipped with
20 an air lock, connected to the conveyor 35 to dispose of
the dust collected during back flush of the several filter
beds.
One feature of this particular arrangement is
the means for preheating the back flush gas to avoid cooling
25 the filter beds during back flush. Cooling the beds may
cause condensation of vapors and lead to failure of the
beds. In this arrangement a first run 46 of the back
; flush duct extends upwardly between and in heat exchange
relation to the raw gas duct 32 and the cleaned gas duct
30 44. Thus heat is transferred from the hot raw gas and
the hot cleaned gas to the back flush gas.
At the top of the tower, or at some other convenient
place, a cross over duct 46 leads to a second run of the
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back flush supply duct leading downwardly past the valves ~.
43. This duct is connected through the valves 43 to the .
adjacent modules for back flush gas flow as indicated by
the arrow 47. :
The preferred arrangement with the modules stacked
at several levels above each other and supplied through
vertical ducts takes advantage of gravity to prevent solids ~ . ,
build up in the ducts as well as requiring relatively small '-
area.
The duct work may also be arranged horizontally
to provide the same flow paths except that means would :
then be required along each duct to dispose of solids that
may settle out of the gas particularly in the raw gas duct
and the back flush receiver duct.
This system thus provides a simple compact arrangement
for a dust collector having a minimum number of fans or
blowers, and high efficiency means for cleaning the collected ..
dust from the filter beds.
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