Abstrict
A filter cake cleaning method in a process for filtering gases
containing fine particles, as well as a device for practicing the
filter cake cleaning method. A filter cake adhering to a filter
cloth is cleaned off by passing an overall amplified high-speed
gas flow along the primary or outer side of the filter cloth parallel
to the filter cloth surface on which the filter cake forms, the
surface serving as the wall of a gas passage. A device emits a jetted
stream into a collecting vessel at an open space upstream of a gas
passage for entraining gas existing in the collecting vessel and
for forming the overall amplified high pressure gas, and is controlled
by a single automatic open/close valve. A device provided on the
secondary or inner side of the filter cloth for jetting backwashing
air can be used in conjunction with the above cleaning operation
to enhance the cleaning effect.
Claims
What I claim is:
1. A method of filtering fine particles, particularly adherent
particles, from gases, and cleaning a filter cloth of collected
particles, the method comprising the steps of:
(a) passing a gas containing particles to be filtered through a
filter cloth in a collecting vessel from an outer side to an inner
side of the filter cloth, the particles being filtered forming a
filter cake on the outer side of the filter cloth; and
(b) cleaning the filter cake formed on the outer side of the filter
cloth with an overall amplified high-speed cleaning gas flow by
using a gas passage formed and lying substantially parallel along
the outer side of the filter cloth, the gas passage having inlet
and outlet ends and having the outer side of the filter cloth as
one wall thereof, the cleaning step including
(i) momentarily jetting a pressurized high-speed cleaning gas toward
the inlet end of the gas passage from an open space in the collecting
vessel upstream of the inlet end of the gas passage;
(ii) mixing additional gas existing at the open space in the collecting
vessel upstream of the inlet end of the gas passage with the jetted
cleaning gas due to the pneumatic ejector effect of the high-speed
cleaning gas; and
(iii) providing the overall amplified high-speed cleaning gas flow
formed by the jetted mixed gas through the gas passage past the
outer side of the filter cloth having the filter cake.
2. A method of filtering gases containing fine particles according
to claim 1, in which said gas passage is limited and defined by
the outer side of the filter cloth and a guide member positioned
opposite to the outer side of the filter cloth.
3. A method of filtering gases containing fine particles according
to claim 2, further comprising the step of controlling the pressurized
cleaning gas by opening and closing an automatic open/close valve
provided in a pressurized gas supply pipe communicating with a source
of pressurized gas.
4. A method of filtering gases according to claim 1, wherein the
flow rate of the pressurized high-speed cleaning gas to the filter
cloth in the cleaning step (b) (i) is within the range of from approximately
equal to about twice that of the flow rate of the filtering gas
to the filter cloth in the filtering step (a).
5. An apparatus for filtering fine particles, particularly adherent
particles, from gases, and cleaning a filter cloth of collected
particles, the apparatus comprising:
(a) a filter cloth having an outer side and inner side;
(b) a collecting vessel housing having a gas inlet and a gas outlet
and containing the filter cloth, the filter cloth being positioned
in the housing; a guide member positioned in the housing adjacent
the outer side of the filter cloth such that a gas passage is formed
substantially parallel along the outer side of the filter cloth
and said guide member and which has side walls and inlet and outlet
ends thereto with one side wall of the gas passage defined by the
outer side of the filter cloth, and wherein the gas to be filtered
passes through the filter cloth from the outer side to the inner
side thereof; and
(c) pressurized cleaning gas supply means for emitting a jetted
gas stream into the housing in such a manner that an overall amplified
cleaning gas flow passes through the gas passage substantially parallel
along the outer side of the filter cloth, wherein the pressurized
cleaning gas supply means has an inlet opening into the housing
at an open space upstream of and adjoining the inlet end of the
gas passage for pneumatically drawing additional, existing gas inside
the housing and entraining the additional gas with the jetted cleaning
gas stream before the mixed gas flows through the gas passage past
the outer side of the filter cloth as the overall amplified cleaning
gas flow.
6. An apparatus for filtering gases containing fine particles according
to claim 5, in which said pressurized gas supply means comprises
a pressurized gas supply pipe communicating at one end with a source
of pressurized air and at the other end with a pressurizad gas delivery
pipe, and wherein said pressurized gas delivery pipe has an orifice
comprising the inlet opening into the housing.
7. An apparatus for filtering gases containing fine particles according
to claim 6, in which said pressurized gas supply means includes
an automatic open/close valve provided in said pressurized gas supply
pipe.
8. An appatatus for filtering gases containing fine particles according
to claim 7, wherein said filter cloth and guide member are formed
into similarly shaped columnar members, and said pressurized gas
delivery pipe having said orifice is disposed in such a manner that
the jetted stream from said orifice is fed into the open space at
the inlet end of the gas passage limited and defined by said filter
cloth and said guide member.
9. A method of filtering gases containing fine particles, particularly
adherent particles, from gases, and cleaning a filter cloth of collected
particles, the method comprising the steps of:
(a) passing a gas containing particles to be filtered through a
filter cloth in a collecting vessel from an outer side to an inner
side of the filter cloth, the particles being filtered forming a
filter cake on the outer side of the filter cloth; and
(b) cleaning the filter cake formed on the outer side of the filter
cloth with an overall amplified high-speed cleaning gas flow by
using a gas passage formed and lying substantially parallel along
the outer side of the filter cloth, the gas passage having inlet
and outlet ends and having the outer side of the filter cloth as
one wall thereof and with a backwashing gas flow, the cleaning step
including
(i) momentarily jetting a pressurized high-speed cleaning gas toward
the inlet end of the gas passage from an open space in the collecting
vessel upstream of the inlet end of the gas passage;
(ii) mixing additional gas prevailing at the open space in the
collecting vessel upstream of the inlet end of the gas passage with
the jetted cleaning gas due to the pneumatic ejector effect of the
high-speed cleaning gas;
(iii) providing the overall amplified high-speed cleaning gas flow
formed by the jetted mixed gas through the gas passage past the
outer side of the filter cloth having the filter cake; and
(iv) concurrently with cleaning step (iii) hereof, momentarily
jetting a backwash pressurized gas toward the inner side of the
filter cloth.
10. A method of filtering gases containing fine particles according
to claim 9, in which the pressurized gas on the inner side of said
filter cloth is jetted from a backwashing gas orifice.
11. A method of filtering gases according to claim 9, wherein the
flow rate of the pressurized high-speed cleaning gas to the filter
cloth in the cleaning step (b) (i) is within the range of from approximately
equal to about twice that of the flow rate of the filtering gas
to the filter cloth in the filtering step (a).
12. An apparatus for filtering fine particles, particularly adherent
particles, from gases, and cleaning a filter cloth of collected
particles, the apparatus comprising:
(a) a filter cloth having an outer side and inner side;
(b) a collecting vessel housing having a gas inlet and a gas outlet
and containing the filter cloth, the filter cloth being positioned
in the housing, a guide member positioned in the housing adjacent
the outer said of the filter cloth such that a gas passage is formed
substantially parallel along the outer side of the filter cloth
and said guide member and which has side walls and inlet and outlet
ends thereto with one side wall of the gas passage defined by the
outer side of the filter cloth, and wherein the gas to be filtered
passes through the filter cloth from the outer side to the inner
side thereof;
(c) pressurized cleaning gas supply means for emitting a jetted
gas stream into the housing in such a manner that an overall amplified
cleaning gas flow passes through the gas passage substantially parallel
along the outer side of the filter cloth, wherein the pressurized
cleaning gas supply means has an inlet opening into the housing
at an open space upstream of and adjoining the inlet end of the
gas passage for pneumatically drawing additional, existing gas inside
the housing and entraining the additional gas with the jetted cleaning
gas stream before the mixed gas flows through the gas passage past
the outerside of the filter cloth as the overall amplified cleaning
gas; and
(d) means for jetting a pressurized gas toward the inner side of
the filter cloth simultaneously with the emitting of the jetted
gas stream from the pressurized cleaning gas supply means.
13. An apparatus for filtering fine particles, particularly adherent
particles, from gases, and cleaning a filter cloth of collected
particles, the apparatus comprising:
(a) a fluted filter cloth having gaps between the mutually adjacent
portions of the fluted filter cloth to form outer sides and inner
sides;
(b) a collecting vessel housing having a gas inlet and a gas outlet
and containing the fluted filter cloth, the fluted filter cloth
being positioned in the housing, wherein the gaps formed between
the mutually adjacent portions of the fluted filter cloth define
gas passages lying substantially parallel along the outer sides
of the fluted filter cloth and having inlet and outlet ends, and
wherein the gas to be filtered passes through the fluted filter
cloth from the outer sides of the inner sides thereof; and
(c) pressurized cleaning gas supply means for emitting a jetted
gas stream into the housing in such a manner that an overall amplified
cleaning gas flow passes through the gas passages formed by the
gaps of the fluted filter cloth substantially parallel along the
outer sides of the fluted filter cloth, wherein the pressurized
cleaning gas supply means has an inlet opening into the housing
at an open space upstream of and adjoining the inlet ends of the
gas passages for pneumatically drawing additional, existing gas
inside the housing and entraining the additional gas with the jetted
cleaning gas stream before the mixed gas flows through the gas passages
past the outer sides of the fluted filter cloth as the overall amplified
cleaning gas flow.
14. An apparatus for filtering gases containing fine particles
according to claim 13, further comprising a plate-like support member
positioned and arranged with respect to the filter cloth so as to
support the fluted filter cloth and to spread the fluted portions
of said filter cloth at least on the inlet end thereof.
15. An apparatus for filtering gases containing fine particles
according to claim 13, further comprising means for jetting a pressurized
gas toward the inner sides of the fluted filter cloth simultaneously
with the emitting of the jetted gas stream from the pressurized
cleaning gas supply means.
16. An apparatus for filtering fine particles, particularly adherent
particles, from gases, and cleaning a filter cloth of collected
particles, the apparatus comprising:
(a) a plurality of filter units, each filter unit having two substantially
flat surfaces at least one of which is formed of a filter cloth
with an outer side and an inner side, said filter units being arranged
side-by-side with mutually adjacent flat surfaces disposed substantially
in parallel with a gap formed therebetween;
(b) a collecting vessel housing having a gas inlet and a gas outlet
and containing the filter units, the filter units being positioned
in the housing, wherein the gaps formed between the flat surfaces
of the filter units define gas passages lying substantially parallel
along the outer sides of the filter cloth and having inlet and outlet
ends, and wherein the gas to be filtered passes through the filter
cloths from the outer sides to the inner sides thereof; and
(c) pressurized cleaning gas supply means for emitting a jetted
gas stream into the housing in such a manner that an overall amplified
cleaning gas flow passes through the gas passages formed by the
gaps of the filter units substantially parallel along the outer
sides of the filter cloths, wherein the pressurized cleaning gas
supply means has an inlet opening into the housing at an open space
upstream of and adjoining the inlet ends of the gas passages for
pneumatically drawing aditional, existing gas inside the housing
and entraining the additional gas with the jetted cleaning gas stream
before the mixed gas flows through the gas passage past the outer
sides of the filter cloths as the overall amplified cleaning gas
flow.
17. An apparatus for filtering gases containing fine particles
according to claim 16, further comprising means for jetting a pressurized
gas toward the inner sides of the filter cloths of each of the filter
units simultaneously with the emitting of the jetted gas stream
from the pressurized cleaning gas supply means.
Description BACKGROUND OF THE INVENTION
This invention relates to a method of filtering a gas containing
fine particles by means of a filter, and to a filtration apparatus
for practicing the method.
Various methods of filtration are known in the art for completing
removing fine particles from a gas to fully recover the particles
and clean the gas. Such methods rely upon an electrostatic Cottrell
precipitator or an absorption tower which uses a liquid-based system,
but the simplest and most reliable method in wide use is to perform
filtration by means of a filtration apparatus having a filter consisting
of a filter cloth. A typical example of such a filtration apparatus
has a plurality of the filters, each in the form of a cylinder,
so arranged as to depend vertically from the ceiling plate of a
collecting vessel. The contaminated gas is caused to pass through
each cylindrical filter from the outer to the inner side thereof,
whereby the particulate matter contained in the gas is removed.
The cleaned gas then exits from the apparatus through passages penetrating
the ceiling plate above respective ones of the cylindrical filters.
The filter cloth used in the aforementioned filters may be broadly
classified into two types. The first is a thick unwoven fabric having
an extremely coarse texture and a thickness of from 15 to 30 mm,
and the second also is an unwoven fabric having a fine surface and
a thickness of from 1 to 2 mm.
With the first filter cloth, particulate matter such as dust contained
in the gas (which we will hereinafter assume to be air for the purpose
of description) is caused to lodge within the filter texture thereby
to be absorbed within the walls of the filter. The purpose of this
filter is to incompletely collect the dust contained in the air
stream only at a low concentration. Thus the particulate matter
is removed imperfectly and cannot be reutilized.
The second filter cloth, on the other hand, does not ordinarily
have a pore diameter as small as the particle diameter of the particles
that are desired to be collected. Specifically, the mean pore diameter
in many cases ranges from 5 to 20 microns. Filtration of this kind
belongs to the category of so-called cake filtration. Specifically,
using a filter cloth of a pore diameter greater than the particle
diameter of the particulate matter, a very minute proportion of
the particulate matter passes completely through to the secondary
side (inner side) of the filter cloth at the instant filtration
begins, but immediately thereafter the particulate matter forms
a layer, known as a filter cake, on the primary or outer surface
of the filter cloth. The filter cake itself then begins acting as
a filter to thenceforth enable complete collection of the particles.
The cake filtration method using the second filter cloth is beset
by a number of problems when filtration is conducted at a high rate.
With a high rate of filtration, the concommitant wind pressure causes
the particles in the filter cake to penetrate through the pores
in the filter cloth and emerge from the secondary side. This is
referred to as so-called filtering break-through, making it impossible
to achieve full collection of the particles. The high wind pressure
also drives the particles deeply into the filter cloth texture,
clogging the pores so that recovery by means of shaking or backwashing
cannot be achieved with satisfactory results.
Owing to the aforementioned problems, the flow velocity through
the second filter cloth that is adopted for particle collection
is ordinarily from 1 to 2 m/min (1.67 to 3.33 cm/sec). This is much
lower than the flow velocity of 1 to 2 m/sec that can be realized
with the first filter cloth. Accordingly, for a constant air flow
rate, it is required that the second filter cloth have a very large
area to assure proper operation. However, for certain particulate
matter which is easy to deal with, depending upon such factors as
the particle diameter and adherability, a flow velocity of up to
6 m/min (10 cm/sec) can be achieved.
There are other reasons for requiring the very low filtration flow
speed mentioned above. For instance, a high flow velocity and small
filter area causes an abrupt rise in the pressure differential across
the wall of the filter cloth owing to its fine texture. Also, a
high flow velocity makes it difficult to collect the particles fully
and to recover dust following removal.
When filtration is carried out with a filtration apparatus that
relies upon the filter cloth of the second type, the particles extracted
from the air form a filter cake by attaching to and accumulating
on the upstream side (namely the primary or outer side mentioned
above, the opposite or downstream side being the secondary or inner
side) of the filter cloth with the passage of time. The formation
of the filter cake causes the pressure differential, measured across
the primary to the secondary sides, to rise. Since an excessive
rise in the pressure differential would be undesirable in terms
of the inherent limitations upon the facilities that provide the
air supply pressure and in terms of a deterioration in filter performance,
it is required that the filter cake be removed from the filter cloth.
It is also required that the particulate matter be recovered. For
these reasons, it is general practice to provide a so-called dust
removal mechanism for knocking the filter cake off the filter cloth
automatically at such time that the filter cake attains a certain
thickness.
Two conventional techniques are available for effecting such dust
removal. One is a so-called shaking method wherein the filter cloth
is vibrated mechanically to shake off the filter cake. The other
is a so-called backwashing method wherein pressurized air is momentarily
blown down toward the second side of the filter, that is, from the
inner side thereof, to dislodge the accumulated particles from the
primary side of the filter by means of the reverse air flow. This
latter method is gaining wider popularity owing to its simpler construction.
Before continuing, it should be noted that a filtration apparatus
with a somewhat different structure also is available, wherein a
number of longitudinally extending bags consisting of filter cloth
are provided perpendicular to the walls of the collecting vessel.
However, in terms of the overall construction and dust removal technique,
this apparatus is essentially no different from that described above.
Generally speaking, in a case where particles have a particle diameter
of as large as 10 microns, there is little adhesion among the particles
and between the particles and the surface of the filter cloth. For
particles of this size, therefore, the filter cake can be dislodged
with just slight vibration, irrespective of whether the shaking
or backwashing method is employed, and the filtration apparatus
may operate without difficulty. For finely divided powders such
as pigment power most of whose particle diameters are less than
one micron, or carbon black having a particle diameter generally
in the millimicron class, the particles exhibit a high physicochemical
attraction as well as a high degree of cross-linking which occurs
when the particles are irregular in shape, rather than circular.
The end result is that the particles have a greater tendency to
adhere to one another and to the surface of the cloth filter, making
it difficult to dislodge them from the filter. Such particles will
be referred to as adherent particles hereafter.
Let us consider the filtration process with respect to such adherent
particles. At the early stages of filtration, the filter cake is
capable of being dislodged from a comparatively large region of
the filter cloth by backwashing. As the filtration and backwashing
cycles are repeated, however, the area of the filter cloth from
which adherent particles are removed grows rapidly smaller until
the filter cake is no longer capable of being dislodged from virtually
any part of the filter. This makes a continuously running operation
impossible. The cause of this phenomenon is presumed to be that
since the wind pressure which acts upon the overall filter cloth
area during backwashing is so small, the influence of internal pressure
upon the overall filter cloth is almost nil, with the wind employed
in the backwashing operation exiting from the primary or outer side
of the filter cloth solely from those points offering least resistance.
It is from these points alone that the filter cake falls away. As
the next filtration cycle proceeds, the flow of contaminated air
concentrates at these relatively exposed areas of the filter cloth,
from which time onward filtration takes place primarily at these
points at a rate much higher than that designed for originally.
These occurrences allow the filter cake to harden and cause partial
blockage or clogging of the filter cloth pores so that, when the
next backwashing cycle takes place, the affected areas of the filter
experience almost no air flowing backwardly across the cloth. This,
coupled with the hardening of the cake, appears to account for the
failure of the dust removal operation. In short, it seems that if
dust removal is allowed to take place only in part, then the area
of the filter cloth that can be used effectively diminishes until
the flow velocity through the filter is no longer true to the proper
filtration flow velocity selected when the filtration apparatus
was designed. As a result, the filter cake clinging to the filter
cloth hardens and grows, gradually diminishing the circulation of
air through the filter.
In a case where backwashing is employed to dislodge the filter
cake attached to the filter cloth, an outlet pipe through which
clean air exits from the filtration apparatus employs flow velocities
of from 1 to 3 m/sec for liquids and from 20 to 30 m/sec for gases,
which are appropriate for ordinary fluid transport in terms of eccnomy
and space limitations. However, when a comparison is made between
the outlet pipe flow velocity (say 20 m/sec) and the filtration
flow velocity (say 3 cm/sec), it may be understood that the area
ratio of the filter cloth to the cross-sectional area of the outlet
pipe passage is 2000/3, namely that the filter cloth has 667 times
the cross-sectional area of the pipe passage. Moreover, the reversely
directed stream of air jetted during the brief backwashing cycle
emerges from a backwashing orifice of a diameter smaller than that
of the outlet pipe, so that the jetted air stream does not have
a flow velocity much different from the flow velocity of the air
passing through the outlet pipe. This, coupled with the fact that
the filter has a very fine texture, causes the air stream jetted
during backwashing to be sealed off so that the air stream can neither
flow the length of the cylindrical filter nor pass through the filter
cloth from the jetted side. Instead, the air stream acts upon substantially
the entire surface area of the filter cloth and develops a flow
velocity near that of the abovementioned filtration flow velocity.
Consequently, the jetted air stream cannot free the filter cake
from the filter cloth, with the result that the pores of the cloth
become clogged. A continuously running operation cannot take place
unless the filter cloth is cleaned by manually removing the filter
cake. Prior efforts at a solution to this problem have resulted
in a much larger, complicated and expensive apparatus, for whatever
improvement in performance has been achieved.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a
method of filtering a gas containing fine particles or powder, through
which all varieties of fine particles and powders can be collected
completely and a filter cake clinging to a filter cloth can be dislodged
from the cloth automatically.
Another object of the present invention is to provide a method
of filtering a gas containing fine particles and powders, wherein
the gas to be filtered is passed through a filter cloth at a low
speed and a filter cake formed by particles attached to the filter
cloth is cleaned off by an overall amplified cleaning gas flow passing
along the primary side of the filter cloth, which gas flow is formed
by a high-speed pressurized air stream jetted by opening and closing
a valve.
Still another object of the present invention is to provide a method
of filtering a gas containing fine particles and powders, wherein
the flow rate of air introduced externally to single filter cloth
during cleaning is held equal to or about twice that of the normal
filtration flow rate of a single filter cloth, and wherein the entire
filter cake on the filter cloth is placed within the region of the
overall amplified cleaning gas flow at the instant of cleaning,
whereby the filter cake may be freed and lifted away from the filter
cloth in its entirety.
Yet another object of the present invention is to provide a structurally
simple filtration apparatus for practicing the foregoing method
of the invention.
Another object of the present invention is to provide a method
of filtering a gas containing fine particles and powders, wherein
when, an overall amplified cleaning gas flow acts upon the primary
side of a filter cloth, pressurized air is jetted from a backwashing
air orifice disposed in an outlet pipe to enhance the effect of
cleaning a filter cake off the filter cloth.
Another object of the present invention is to provide a structurally
simple filtration apparatus for practicing the foregoing method
of the invention.
A further object of the present invention is to provide an apparatus
for filtering a gas containing fine particles or powders adapted
so that an overall amplified cleaning gas flow, created by a high-speed
gas stream jetted from an orifice passes between the fluted portions
of a fluted filter cloth.
Still another object of the present invention is to provide a filtration
apparatus adapted so that an overall amplified cleaning gas flow
passes between filter cloths disposed in parallel.
Other features and advantages of the present invention will be
apparent from the following description taken in connection with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view illustrating a filtration apparatus
and collecting vessel used in practicing a conventional backwashing
method;
FIG. 2 is a sectional view useful in describing the filtering method
of the present invention;
FIGS. 3 and 4 are vertical and horizontal sections, respectively,
illustrating an embodiment of a filtration apparatus according to
the present invention;
FIGS. 5 and 6 are vertical and horizontal sections, respectively,
illustrating another embodiment of a filtration apparatus according
to the present invention;
FIG. 7 is a longitudinal section illustrating a third embodiment
of the present invention; and
FIG. 8 is a side view as seen viewing along the arrows A--A' of
FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENT
To gain a better understanding of the present invention, a brief
discussion of typical prior art will be presented first for the
purpose of comparison.
Referring to FIG. 1 which illustrates a filtration apparatus and
collecting vessel for backwashing according to the prior art, two
cylindrically-shaped filter cloths 1, 2 are shown for describing
the operating principal, although a number of such filter cloths
are provided in actual practice. The collecting vessel, designated
by numeral 3, has a ceiling plate 4 from which the filter cloths
1, 2 depend vertically, an outlet 5 for the collected fine particles
or powder, and a pipe 6 for supplying the contaminated air which
is to be filtered. Outlet pipes 7, 8 for the filtered clean air
extend from the ceiling plate 4. The outlet pipes 7, 8 are penetrated
by respective conduits 9, 10 each of which communicates with a source
of pressurized air, not shown. Automatic valves 11, 12 are provided
in the flow passages of the respective conduits 9, 10 for opening
and closing these passages to the source of pressurized air. The
arrow indicated at 13 shows the direction of contaminated air flow
as it enters the apparatus, while the arrows at 14, 15 show the
direction of the cleaned air flow exiting from the apparatus. Numeral
16 denotes the fine particles or powder collected at the bottom
of the vessel 3, where a discharge value 17 is provided for discharging
the collected particles.
In terms of air flow, the air contaminated with the particulate
matter is fed in continuously from the supply pipe 6. The major
portion of the particulate matter is separated from the air flow
by the influence of gravity and settles at the bottom of the vessel
3 for collection. The separating effect of gravity may be enhanced
greatly by disposing the supply pipe 6 along a line which is tangent
to the cylindrical wall of the collecting vessel 3. Since the valve
17 is closed, the air accompanied by the remaining free particles
passes through the cylindrical filter cloths 1, 2 from the outer
(primary) to the inner (secondary) sides thereof and then is discharged
from the outlet pipes 7, 8 as clean air completely removed of particulate
matter. During the course of this filtration cycle, the particles
attach themselves to the outer surfaces of the filter cloths 1,
2 to be collected thereby. After a certain period of time, one of
the filter cloths, say cloth 1, is subjected to a backwashing operation.
This entails expelling a momentary jet of pressurized air for backwashing
from the conduit 9. The purpose of this step is to blow off the
particulate matter from the filter cloth 1 by the reverse flow of
backwashing air travelling from the secondary side to the primary
side of the filter cloth. During the backwashing cycle of operation,
jetted air which passes completely through the filter cloth 1 and
exits from the primary (outer) side thereof is excess air that proceeds
to flow through the filter cloth 2 in the normal direction (i.e.,
primary to secondary side) and then out through the outlet pipe
8. As this is taking place, there is a corresponding increase in
the load acting upon the filter cloth 2, which at this time is not
being subjected to backwashing. For this reason, a number of the
filter cloths generally are provided, as mentioned above, and the
method followed is to successively backwash the filter cloths in
small groups, thereby diminishing the rate of increase in load.
Let us now split the above process into the filtration and backwashing
cycles and consider these in quantitative terms. We will take the
filtration cycle first.
The apparatus of FIG. 1 is designed for a flow rate of 3 m.sup.3
/min (50,000 cm.sup.3 /sec) with regard to the contaminated air
being continuously fed into the collecting vessel. Since the flow
rate processed per cloth filter will be 25,000 cm.sup.3 /sec, each
of the cloth filters will require a large surface area of 25,000/3.33=7,500
cm.sup.2 (0.75 m.sup.2) where the filtration flow velocity is 3.33
cm/sec. By comparison, a suitable cross-sectional area for the flow
passage through the outlet pipe 7 or 8 is a small 25,000/2000=12.5
cm.sup.2 for a flow velocity of 20 m/sec (2000 cm/sec). For a hollow
circular pipe, this is an inner diameter of 4.0 cm. In designing
the outer geometry of a filter cloth, the cylindrical shape is often
adopted because of structural simplicity. In such case, the surface
area obtained would be 7,500 cm.sup.2 for a cylinder having a diameter
(D) of 20 cm and a length (L) of 119 cm. The result is a slender
filter in which L/D=6. In order to increase the filtering surface
area relative to the diameter of the collecting vessel 3, it is
necessary to enlarge L/D and insert a number of the filter cloths
into the vessel. In this connection, increasing the diameter (D)
of the filter cloth would make it impossible to accommodate a large
number of them in the vessel. It is inevitable, therefore, that
the filter cloth configuration will be long and slender since the
conventional method demands that L/D be large.
With regard to the backwashing cycle of operation, the pressurized
air is expelled from the conduit 9 for a very short period of time
of, say, 0.2 sec. To this end, an arrangement is required wherein
the automatic valve 11 located upstream of the conduit 9 is rapidly
opened for the time required, and then closed. Introducing the pressurized
air for a long period of time is undesirable since it would send
a large quantity of air into the collecting vessel 3 on the primary
side of the filter cloth, adding to the air entering from the supply
pipe 6 for the purpose of being treated. Furthermore, the short
burst of backwashing air, with as low a flow rate as possible, is
preferred in order to reduce pressurized air consumption. The initial
flow velocity of the pressurized air itself when jetted from the
conduit 9 can attain a high speed approaching that of sound if the
air pressure upstream of the conduit is two atmospheres (absolute)
or more. Owing to this kinetic energy, an ejector effect is produced
within the outlet pipe 7, whereby the excess air is made to join
the expelled air to form an increased flow of air which exits from
the primary side of the cloth filter via the secondary side thereof.
However, because of various limiting conditions set forth hereinbelow,
the total air flow rate in the reverse direction for the purpose
of backwashing is limited to approximately the filtration flow rate
(25,000 cm.sup.3 /sec in this example) or, at best, to a value which
is two to three times this figure. The reasons are as follows:
(a) The outlet pipe 7 proper is designed for the normal filtration
flow rate. The reason forbidding a large increase in the size thereof
is set forth in (d) below.
(b) As mentoned earlier, it is necessary to provide a number of
the filter cloths in common practice. The number of such filter
cloths required will be quite large if the ratio of total air flow
rate during backwashing to the normal flow rate is increased to
a marked degree. For example, if the flow rate of air introduced
backwardly through one filter cloth in a backwashing operation is
ten times normal, then the filtration flow rate for air passing
through the other filter cloths in the normal direction at the instant
of backwashing will increase by 6.5 times for a total of three filter
cloths, and 2.2 times for ten filter cloths. Thus, in order to increase
the ratio of the total air flow rate during backwashing to the normal
rate in an effort to enhance the dust removal effect, the only expedient
is to increase the number of filter cloths which, in turn, means
increasing the number of outlet pipes. This leads to a major drawback,
however, as will be mentioned below.
(c) The automatic valve for controlling the pressurized air is
required to have an instantaneous open-close characteristic. While
a satisfactory characteristic can be obtained with a small-size
electromagnetic or solenoid valve, it is not possible to achieve
such a characteristic with a valve having a large opening diameter
using an inexpensive solenoid valve. Instead, obtaining the desired
valve raises costs, complicates the apparatus and increases its
size. This problem is compounded by the fact that an automatic valve
for controlling the pressurized air must be installed for each filter
cloth and outlet pipe combination, of which the total number is
large.
(d) When a large number of outlet pipes are provided, the piping
for supplying the pressurized air to each one becomes quite complicated
in actual practice. Since the spacing among the filter cloths is
narrowed in order to minimize the overall size of the apparatus,
sufficient space for piping is not available in the area above the
apparatus. As the number of filter cloths is increased, moreover,
even less space is available for the automatic valves that control
the pressurized air and for the piping (namely for supplying the
pressurized air and for withdrawing the cleaned air).
Thus, as set forth above, it may seem possible in theory to markedly
increase the ratio of the total air flow rate for backwashing to
the normal air flow rate, but the higher costs and increased facility
size actually prohibit this from being achieved. With the outlet
pipe 7 shown in the arrangement of FIG. 1, the usual high flow velocity
for the transport of a gas through a tube is achieved, specifically
a flow velocity of 20 m/sec at normal filtration and of 40 to 60
m/sec for flow in the reverse direction during backwashing. Since
the filter area is some 600 times greater than the cross-sectional
area of the flow passage of the outlet pipe 7, however, the major
problem encountered here is that the flow velocity of the air passing
through the filter in the reverse direction during backwashing is
only 3.33.times.2=6.66 cm/sec to 3.33.times.3=10 cm/sec at most.
This is an extremely low or even gentle wind speed on a par with
that which prevails during normal filtration, and does not afford
the strong wind effect needed to uniformly scatter the filter cake
of adherent particles from the entire surface of the filter.
A brief discussion of what is meant by a strong wind will be instructive.
When pneumatically conveying particles having a particle diameter
of less than 3 to 4 mm, the air flow velocity through a pipe usually
adopted to avoid settlement and blockage is 10 to 25 m/sec for a
circular pipe having an opening diameter of from one to six inches.
This flow velocity is known as the so-called critical velocity.
At a flow velocity of 20 m/sec, problem-free pneumatic transport
of particles is perfectly feasible even with respect to the adherent
particles dealt with in the present invention, except for special
cases where the particles may be affected by dampness or the like.
At such an air flow velocity, the particles will not adhere to and
build up on the walls of the pipe. In other words, rather than clinging
to the pipe walls, the particles are blown off the walls. This condition,
unbalanced in favor of the latter state, is maintained continuously.
Thus, a strong wind is that representative of a wind velocity that
will not allow particles to adhere to the walls of a pipe.
Let us now suppose that the abovementioned flow velocity of 20
m/sec is adopted for backwashing the filter cloth shown in FIG.
1. Since the area of the filter cloth is 7,500 cm.sup.2, the quantity
of air will be 7,500.times.2,000=15.times.10.sup.6 cm.sup.3 /sec.
Hence, the flow velocity within the outlet pipe (having a passage
cross-sectional area of 12.5 cm.sup.2) would have to be 12,000 m/sec,
and even the flow velocity of air directly downwardly through the
filter cloth cylinder (diameter 20 cm, cross-sectional area=314
cm.sup.2, perpendicular to the cylinder axis) would have to be 477
m/sec. Since these are values greatly exceeding the speed of sound,
which is the upper limit on gas flow velocity, it may be understood
that such speeds are theoretically impossible. Moreover, the quantity
of air would be 600 (15.times.10.sup.6 /25,000) times that prevailing
during normal filtration. This also is a figure that would be impossible
to achieve in view of the technical problems set forth above.
The filtering method of the present invention, which may be understood
from FIG. 2, is hereinafter referred to as a booster (or amplification)
fluidization cleaning process or BFC, process. The apparatus for
practicing this process is hereinafter referred to as a BFC filtration
apparatus.
Referring to FIG. 2, the apparatus includes a filter cloth 21,
a collecting vessel 22, a ceiling plate 23 provided on the collecting
vessel, a supply pipe 24 for leading contaminated air into the collecting
vessel, an outlet pipe 25 penetrating the ceiling plate for discharging
cleaned air, and pressurized air delivery piping 26 communicating
with a pressurized air supply pipe 28. The top side of the pressurized
air delivery piping 26 is provided with a multiplicity of jet orifices
27 positioned in an open space in the vessel 22 upstream of a gas
passage 35 as will be described below. An automatic valve 29 is
provided in the supply pipe 28 to open and close the pipe to the
pressurized air. A sleeve-shaped guide plate 30 is disposed to surround
the filter cloth 21 so as to delimit the passage 35 defined between
itself and the filter, the guide plate 30 being supported from the
ceiling plate 23 in the illustrated position by a small number of
appropriate support means, not shown. The arrows shown at 31 indicate
the direction in which the pressurized air is jetted from the orifices
27. Arrows shown at 32 indicate the direction in which air circulates
within the collecting vessel 22 owing to the ejector or entrainment
effect accompanying the pressurized air jetted as shown by the arrows
32. Arrows 33 indicate the direction in which the combined air streams
31, 32 (referred to hereinafter as an overall amplified cleaning
gas flow) are discharged, together with the filter cake, from the
passage between the filter cloth 21 and guide plate 30, the combined
air streams exiting back into the collecting vessel 22 from the
top of said passage. It should be noted that only one filter cloth
is shown in order to simplify the drawing.
The filter cloth 21 is affixed along the inner edge of its open
upper end to a flange 23.sub.1 projecting downwardly from the bottom
side of the ceiling plate 23. Although the filter cloth 21 is cylindrically
shaped in FIG. 2, any desired shape may be adopted. Accordingly,
the guide plate 30 surrounding the outside of the filter cloth 21
is similarly shaped in order that the overall amplified cleaning
gas may flow through the passage 35 in a uniform manner. Thus the
portion of the filter cloth 21 that is parallel to the guide plate
30 is represented as a cylinder.
In order that the overall amplified cleaning gas developed by the
air jetted from the orifices 27 may flow uniformly into the passage
35 between the filter cloth 21 and guide plate 30, the pressurized
air supply pipe 28 is supported on the collection vessel 22 and
passed, through the wall thereof in such fashion as to position
the orifices 27 in the open space just upstream from the passage
opening.
Let us now describe a filtering cycle of operation, followed by
a description of a cleaning cycle. Quantitatively, we will assume
that the filter cloth 21 has dimensions the same as those described
in connection with FIG. 1, and that the filtration air flow rate
per filter also is the same.
As for the filtration cycle, the contaminated air enters from pipe
24 and reaches the surface of the filter cloth 21 after flowing
into the passage 35 between the filter cloth and the guide plate
30 from the upper and lower ends thereof. In all other aspects the
filtration cycle is as described earlier with reference to FIG.
1.
The cleaning cycle will now be described quantitatively, with emphasis
being placed upon the removal of the filter cake. In the cleaning
operation, the automatic valve 29 is opened momentarily, for a period
of from 0.2 to 0.3 sec, and then closed. As a result, a predetermined
air pressure, preferably of 2 atm (absolute) or more, is applied
upstream of the orifices 27, causing pressurized air to be jetted
from the orifices 27 in the direction of the arrows 31 at a velocity
equal to or near the speed of sound. We will assume that the flow
rate of the pressurized air is the same as the filtration flow rate,
namely 25,000 cm.sup.3 /sec under atmospheric conditions following
the ejection of the pressurized air. Since the air travelling at
this flow rate possesses great kinetic energy owing to the sonic
speed involved, an amount of existing air in the collecting vessel
22 a number of times greater than that jetted from the orifices
27 drawn pneumatically from the area or open space below the filter
cloth 21 and is forced upwardly through the passage 35 between the
filter 21 and the guide plate 30. Though this results in an overall
cleaning air flow rate amplified tens of times from the flow rate
of the jetted air from orifices 27. This increase or amlification
in the flow rate is not due to air brought in from the outside but
results from the air inside the collecting vessel 22 on the primary
(outer) side of the filter cloth 21. Hence, it is not required that
an amount of air corresponding to the increased flow rate be discharged
or purged from the system. Thus, one characterizing feature of the
present invention resides in the above-described magnifying or amplification
action and in the fact that circulation is produced within the system
on the primary side of the filter cloth.
The second important feature of the present invention will now
be described. In order for the amplified air at the total flow rate
for cleaning to be passed substantially parallel to the surface
of the cloth filter 21, the air is caused to flow longitudinally
of the cloth filter. When this is done, the cross-sectional area
of the cleaning air passage 35 can be made very small. Then, if
the total air flow rate as magnified above is available, the prescribed
flow velocity of 20 m/sec can be obtained with facility. It should
be noted that the passage 35, which has the primary (outer) surface
of the filter cloth 21 as one wall thereof, is provided for the
purpose of causing the air to flow in a direction parallel to the
surface of the primary side of the filter, so that no air passes
through the filter in a direction perpendicular thereto. The passage
therefore is substantially parallel to the surface of the primary
filter side, and participates in the cleaning of the filter cloth.
When the outer diameter of the filter cloth 21 is 20 cm, the dimensions
of the guide plate 30 for obtaining the flow speed of 20 m/sec are
computed as shown in the following table:
______________________________________ Guide Total Cleaning Air
Plate Cleaning Air Filter Cloth Passage Sec- Inner Flow Rate Outer
Diam. tional Area Diam. (cm.sup.3 /sec) (cm) (cm.sup.2) (cm) ______________________________________
Case 1: Tenfold 250,000 20 125 23.6 Magnifi- cation Case 2: Twentyfold
500,000 20 250 26.8 Magnifi- cation ______________________________________
The clearance between the filter cloth 21 and guide plate 30 is
1.8 cm in case 1 and 3.4 cm in case 2, both of which are technically
appropriate values. During filtration, the contaminated air travelling
at the flow rate of 25,000 cm.sup.3 /sec enters the passage 35 from
the top and bottom of the guide plate 30. Accordingly, the flow
velocity through the clearance during filtration is 1 m/sec in case
1 and 0.5 m/sec in case 2, both of which are sufficiently low so
that there are no problems. In comparison with the overall cloth
filter area of 7,500 cm.sup.2, the cross-sectional area of the cleaning
passage 35 is very small. Even in a case where the pressurized air
is jetted from the orifices 27 for a net length of time of 0.2 sec,
each point of the surface of the filter cloth is exposed to air
corresponding to a length of four meters (20.times.0.2=4). With
the conventional backwashing method, a backwashing flow velocity
of only 10 cm/sec at best is obtained at right angles to the filter
cloth, so that each point of the filter cloth surface is exposed
to nothing more than 2 cm of air movement.
Some preliminary calculations will be made with regard to the amplification
factor achieved by the jetting of pressurized air. The amplification
factor depends upon the pressure differential of the pressure rise
from the ejector inlet to the ejector outlet, and upon the ejector
efficiency. As for the value of the pressure differential, the likely
components are 100 mm ag at the inlet and outlet of the passage
35 between the filter cloth and guide plate, and 60 mm ag for the
pneumatic transport along the filter cloth length of 1.2 m, for
a total of 160 mm ag. Since this is probably the largest value attained,
it is almost negligible in the ejector calculations. Omitting energy
attributed to pressure and position, and letting the initial jetted
air velocity be 300 m/sec, the final air velocity be 20 m/sec, and
the associated ratio be X, we need consider only the energy attributed
to velocity, so that the following equation holds:
Here, X represents the weight of air associated with the weight
m of the jetted pressurized air. If we assume that m=1, then X will
represent the associated ratio. From the above equation, we have:
Ordinarily, the ejector efficiency is no less than 20%. However,
since even an ejector efficiency of 10% would result in an associated
ratio 22 times greater, an associated ratio 20 times greater as
used in the foregoing explanation is a reasonable value.
Next, as shown in the following table, Reynolds numbers will be
used to check the earlier mentioned fact that wind speeds of 10
or 20 m/sec are in a so-called "strong wind region" in
which an excellent particle dispersing or scattering effect can
be obtained without difficulty. In compiling the chart, the values
of air viscosity and density are those prevailing at 20.degree.
C.
______________________________________ REYNOLDS NUMBERS Pipe inner
diam. 2.5 cm Pipe inner diam. 1.5 cm ______________________________________
3.33 cm/sec 56 336 10 cm/sec 168 1,008 10 m/sec 16,800 100,800 20
m/sec 33,600 201,700 ______________________________________
The Reynolds numbers at 10 to 20 m/sec greatly exceed the boundary
value of 2300 between turbulent flow and laminar flow. It should
be noted that the backwashing wind speed achieved with the conventional
method is on the order to 10 cm/sec, which is much lower than the
turbulent flow region, even for a pipe having an inner diameter
of 1.5 cm.
An embodiment of the present invention will now be described with
reference to FIGS. 3 and 4, in which FIG. 3 is a vertical section
of the filtration apparatus and FIG. 4 a sectional view thereof
taken along the line A--A'. The apparatus includes a base plate
41, a filter cloth attaching flange 42 provided on the bottom side
of the base plate 41, a cylindrical filter cloth 43, a rod 44 for
supporting the filter cloth 43, a plate 45 for supporting the filter
cloth, a clamping ring 46 for attaching the upper end of the filter
cloth 43 to the flange 42, a guide plate 47 surrounding the filter
cloth, and rods 48 for supporting the guide plate 47. An outlet
49 for cleaned air is provided on the base plate 41, and a passage
50 is provided for the jetting of cleaning air. The base plate 41
is provided with orifices 51. A pipe 52 communicates at one end
with the passage 50, and at the other end with a cleaning air inlet
pipe 53 through an automatic value 54. A conduit 56 for jetting
backwashing air penetrates into the outlet pipe 49, with the flow
of backwashing air being controlled by an automatic valve 57. The
outlet pipe 49 is provided with an automatic cut-off valve 58 for
closing off the outlet passage. It should be noted that a collecting
vessel, though not shown, is provided in the same manner as depicted
in FIG. 2. The collecting vessel also is provided in the other embodiments
of the invention, described later, but will likewise be deleted
from the drawings.
The filtration apparatus is used by being attached at an opening
in the side wall or, preferably, the ceiling plate, of another vessel
such as the collecting vessel, with the cylindrical filter cloth
43 being faced toward the inner side of the collecting vessel, and
with the base plate 41 being affixed air-tightly to the wall of
the collecting vessel by means of bolts passed through bolt holes
55.
The cleaning air inlet pipe 53 is connected to a source of pressurized
air under a pressure of, say, 4 to 7 kg/cm.sup.2 G. The outlet pipe
49 is opened to the atmosphere directly or through an exhaust duct.
First, with regard to the filtration cycle, air containing fine
particles or powder flows from the supply pipe (not shown) into
the clearance or passage between the guide plate 47 and cylindrical
filter cloth 43 from both ends of the clearance and travels longitudinally
along the filter cloth. Then, utilizing the entire surface of the
cylindrical filter cloth 43, the air passes through and flows into
the filter cloth cylinder and exits from the outlet pipe 49 as clean
air. During this filtration cycle, the particles proceed to form
a filter cake on the primary-side surface of the filter cloth 43.
In the cleaning cycle of operation, automatic valve 54, which was
fully closed during filtration, is opened momentarily and then closed.
At such time, compressed air at a pressure of 4 to 7 kg/cm.sup.2
G upstream of the automatic valve 54 is supplied for cleaning purposes.
The air is fed into the cleaning air jetting passage 50 where the
air is jetted downwardly via a number of the orifices 51 into the
open space just upstream of the clearance or passage between the
guide plate 47 and the outer periphery of the cylindrical filter
cloth 43. The overall amplified cleaning air flow is formed and
acts as described in connection with FIG. 2, scattering the filter
cake clinging to the primary side of the filter cloth 43 and expelling
the filter cake into the collecting vessel from the opposite end
of the cylindrical filter cloth.
The cleaning mechanism in this embodiment of the invention is augmented
by a backwashing effect, provided by momentarily opening the backwashing
automatic valve 57 simultaneously with the jetting of the cleaning
air, for a period of time substantially equal to the air jetting
period. By opening and then closing the valve 57 in this manner,
the air from a source of pressurized air connected upstream of the
valve 57 is expelled from the backwashing jetting outlet 56 toward
the secondary (inner) side of the filter cloth. Thus, according
to this embodiment of the invention, the cleaning effect is greatly
enhanced. In this embodiment, the instant the valve 57 is opened
and the length of time it is left open can be suitably adjusted.
As for the automatic cut-off valve 58 for cutting off the discharge
passage of the outlet pipe 49, full closure is effected for a length
of time equivalent to the duration of the backwashing operation
when backwashing is to supplement the air jetting process. By such
full closure of the valve 58, only the air attributed to the pressurized
air flow is correctly and forcibly expelled into the collecting
vessel, thereby serving to prevent the inflow of excess air from
outside the system.
In the embodiments that follow, the backwashing air jetting outlet
56, backwashing automatic valve 57 and the discharge passage automatic
cut-off valve 58 are disposed and operate in the manner described
as set forth in the above embodiment and are not described again.
A second embodiment of the present invention is illustrated in
FIGS. 5 and 6. Here the apparatus includes a base plate 61, an outlet
pipe 62, a flange 63 formed integral with the outlet pipe 62 on
the outer wall thereof, and a fluted or pleated filter unit 64.
The filter unit 64 comprises upper and lower wheel-like support
plate members 65 and 66 on the upper and lower sides thereof, respectively,
a filter cloth 67 folded into a fluted configuration and connected
at its circumference, and a perforated support plate 68. Numeral
69 denotes a support equipped with a nut, 70 a blind flange, and
71 a bolt. The bolt 71 fastens the blind flange 70 to the support
and nut combination 69 and serves to fix the fluted filter unit
64 by pulling it against the flange 63. The base plate 61 is provided
with orifices 72 and a cleaning air jetting passage 73 communicating
with the orifices 72. A pipe 74 connects the passage 73 with a cleaning
air inlet pipe 76 through an automatic valve 75. Numeral 77 designates
a bolt hole provided in the base plate 61. The inclined lines shown
at number 78 indicate the outermost boundary of the overall amplified
cleaning air flow caused by the jetted air from the orifices 72,
which are flared downwardly and inclined inwardly to jet the entrained
air in the indicated region. The fluted filter cloth has an inner
diameter d, an outer diameter D, and a height H.
In the present embodiment, the treating flow rate per filter unit
also is 1.5 m.sup.3 /min (25,000 cm.sup.3 /sec), the same as in
the first embodiment, thereby facilitating a comparison of the two.
While this embodiment has almost the same function and operates
in almost the same manner as the first embodiment, there are certain
distinctions which will now be set forth.
(a) By adopting a fluted filter unit, the same filter cloth surface
area can be obtained with a filter cloth having much smaller outside
dimensions.
(b) The reduction in the size of the filter cloth portion is accompanied
by a shortening in the length of the filter from the inlet portion
thereof for the overall amplified cleaning air flow to the outlet
portion thereof. Accordingly, cleaning may be executed without disposing
a guide plate around the filter cloth and in parallel with the direction
of overall amplified cleaning air flow.
(c) The filter cloth surfaces (on the primary or outer side) of
the mutually adjacent fluted portions of the filter construct narrow
paths which force the overall amplified air for cleaning to flow
substantially parallel to the filter cloth surface along the length
of the filter cloth in a predetermined direction.
In the following description, we will emphasize these factors,
attributed to the filter unit configuration, that distinguish the
present embodiment from the first embodiment.
The required filter cloth area of 7,500 cm.sup.2 may be obtained
using a rectangular filter cloth which is 15 cm wide and 500 cm
long. The opposing lateral (short) edges of the filter cloth are
adhered to each other to form the filter cloth into a ring, which
is then folded to form 50 flutes of a width of 5 cm each at right
angles to the long sides of the cloth. This provides the fluted
filter cloth 67 which is shown installed in the filter unit in FIG.
6. For an ordinary fluted filter unit, the the plate members 65,
66 are disk-shaped members with cut-out portions having a round
hole provided at the center thereof. The long sides (namely the
upper and lower edges) of the filter cloth are affixed to the members
65, 66, whereby the primary and secondary sides of the filter cloth
are completely isolated from each other and passages are formed
solely by means of the filter cloth. As a result, the dimensions
are, with reference to FIG. 5, inner diameter d=9.5 cm, outer diameter
D=19.5 cm, height H=15 cm. It will be appreciated that a major reduction
in size is achieved over the first embodiment, wherein the dimensions
of the filter cloth cylinder are a diameter of 20 cm and a height
of 119 cm. The area of the donut-shaped region of the outer diameter
D and inner diameter d is 228 cm.sup.2, half of which (114 cm.sup.2)
is the cross-sectional area of the passages for the cleaning air,
which passages are defined by the spaces between the flutes of the
filter cloth. This is 1/68 of the filter cloth area of 7,500 cm.sup.2.
The cleaning operation is similar to that practiced in the first
embodiment. Specifically, by momentarily opening and then closing
the automatic valve 75, pressurized air from the inlet pipe 76 passes
through the pipe 74 and jetting passage 73 and is jetted from the
orifices 72 in the spread pattern indicated by the lines 78, whereby
the overall amplified cleaning air flow passes through the spaces
between the flutes on the primary side of the filter cloth 67, thereby
to clean the filter. In accordance with this cleaning operation,
the filter cake is cleaned off the filter cloth fully, without the
filter cake continuing to build up and harden on the flutes of the
filter, as in the conventional backwashing method.
Despite the fact that the fluted filter cloth has the above-mentioned
merits along with its very small size, it has not been put into
practical use with the conventional backwashing method because the
filter cake hardens when such method is adopted, as will now be
described.
During the filtration cycle using the fluted filter cloth, a layer
of the filter cake forms in the usual fashion on each of the closely
opposing surfaces on the primary side of the filter cloth in the
narrow spaces between the neighboring flutes. Since the spaces become
particularly narrow near the core of the filter, namely at the inner
peripheral portion thereof, it is here that the filter cake layers
on the adjacent flute surfaces draw near to each other. From observations
of a cleaning cycle using the backwashing technique, it has been
found that filter cake over only a limited area is peeled off and
caused to fall since the force of the backwashing air is extremely
weak, as mentioned earlier. This is similar to the situation experienced
with the cylindrical type filter cloth. However, one characteristic
of a fluted filter unit is that the impact of the jetted backwashing
air causes each of the fluted portions to inflate on the secondary
side, thereby narrowing the spaces between the fluted portions on
the primary side. Following the instantaneous jetting of the cleaning
air, the associated valve is closed, allowing the spaces between
flutes to more or less return to their original condition. As the
filtering and cleaning cycles are repeated, it has been observed
that the mutually adjacent filter cake surfaces on the fluted portions
are compressed, compacted and unified at the narrow spaces approximate
the inner peripheral portion of the filter where the filter cake
is particularly difficult to peel off. In the next filtering cycle,
particles are added to the portions between the filter cloth and
the compressed, hardened filter cake and are themselves compressed
by the action of the next backwashing cycle, until the inner peripheral
portion of the filter cloth is eventually clogged with a hardened
particulate mass. The filter cloth at this portion is no longer
in its original condition and is urged toward the secondary side.
Thenceforth, this portion of the filter obviously is completely
incapable of performing filtration.
The foregoing phenomenon spreads from the inner to the outer peripheral
portion of the filter cloth as the filtering and cleaning cycles
progress, until even the partial filter cake peeling effect initially
exhibited by the backwashing operation is lost completely. As a
result, there is a very large rise in the filter cloth pressure
differential and a sudden drop in the filtered air flow rate, eventually
making it impossible to keep the equipment running. This may occur
in as short a time as 30 minutes, depending upon the type of particulate
matter involved. It is a surprising fact that the filter cake at
this stage may be completely solidified up to the outer circumference
of the filter cloth (i.e., up to the outer diameter D in FIG. 5),
the state of solification being such that the filter cake can be
made to fall away, with some difficulty, by prodding with a rod.
In other words, even though the backwashing air possesses only a
weak force in so far as dust removal is concerned, it does cause
the development of sufficient pressure for closing the gaps between
the fluted portions of the filter cloth on the primary side, thereby
bringing about the foregoing phenomenon. It is surmised from observations
that even the initial filter cake peeling effect is the result not
of wind force but of deformation sustained by the fluted portions.
Regardless of efforts made to achieve technical improvements, therefore,
it appears to have been quite impossible to utilize a fluted filter
unit for the repeated collection of strongly adherent particles
through use of the prior art arrangements.
A third embodiment of the present invention is shown in FIGS. 7
and 8. The apparatus includes a base plate 81 having bolt holes
82, an inlet pipe 83 connected to a pipe 85 by an automatic valve
84 for opening and closing the passage between the pipes, a pipe
86 for jetting cleaning air communicating with the pipe 85 and having
a number of orifices 87, and a filter mounting member 88 provided
on the base plate 81. Numeral 89 denotes an outlet hole, and 90
an outlet pipe communicating with the outlet hole 89. Also included
are a filter cloth supporting rod 91, a filter cloth 92 and a packing
93. Character L indicates the width necessary for installing one
filter unit.
The filter unit is installed on the wall of a collecting vessel
(not shown) by means of the base plate 81. To this end, the collecting
vessel wall is provided with a rectangular opening one side of which
is somewhat shorter than the distance between the bolt holes 82,
thereby allowing the bolts passed through the bolt holes 82 to engage
the collecting vessel wall. The length of the sides of the opening
adjacent the above-mentioned side is found by multiplying L by the
number of filter units to be installed.
The filter cloth 92 has the form of a bag obtained by overlapping
two rectangular filter cloths which are then stiched together along
three sides. The open side of the bag-like filter cloth 92 is fit
around the outer periphery of the mounting member 88 and is joined
to the mounting member in such fashion as to maintain a hermetic
seal. The remaining three sides of the filter cloth 92 are held
in an extended and tensioned state by the filter cloth supporting
rod 91, which consists of a bent round rod supported on the base
plate 81 and located within the filter cloth to be covered thereby.
The filter cloth 92 supported and tensioned in this fashion has
the configuration as shown in FIG. 7. The outlet hole 89 is provided
in the central portion of the filter cloth mounting member 88, whereby
the outlet pipe 90 is brought into communication with the interior
of the filter cloth bag, whose inner side serves as the secondary
side.
Since the filter units are arranged side-by-side with the filter
cloth bags being spaced apart by L, the flat filter cloth surfaces
of the filter units are held close together and in parallel, whereby
a cleaning air passage of a small crosssectional area is formed
between adjacent ones of the flat filter surfaces. The overall amplified
cleaning air flow, formed by the air jetted from the orifices 87
and the entrained peripheral air existing in the collecting vessel,
passes through these passages at high speed and exits from the opposite
side thereof, thereby cleaning the primary side of each filter cloth.
Since the outermost filter units of the filter unit array will not
have an adjacent filter cloth surface on one side thereof, a guide
plate having the same shape as that of the filter cloths may be
disposed at this location. Otherwise, the outermost filter cloths
may be regarded as being non-active portions of the apparatus.
If the filter cloth is of great length as measured from the edge
facing the orifices 87 to the opposite edge thereof, the effect
of the overall amplified cleaning air flow can be enhanced by providing
a guide plate opposite the rectangular base plates 81 for interconnecting
the outermost edges of the filter cloths.
In the foregoing embodiments, the filter cloth may consist of a
fabric, paper, unwoven fabric, wire mesh or the like. What is essential
is that the filter member be of such type that the surface on the
primary side thereof facing the open space utilize its fine porosity
to collect fine particles or powders, and that the internal structure
of the filter member, from the surface of its primary side to its
secondary side, have passages through to the secondary side, which
passages have an opening diameter substantially equal to or larger
than the pore size of the primary side. Any filter member satisfying
these requirements may be employed.
In terms of the attitude or direction of the surface on the primary
side of the filter cloth, the possible choices are (a) vertical,
(b) horizontal and facing downwardly, (c) horizontal and facing
upwardly, (d) inclined and facing downwardly and (e) inclined and
facing upwardly. As long as the overall cleaning air flow velocity
is appropriate, any of these attitudes may be adopted as desired.
The critical point is to design cleaning air flow paths which direct
the filter cake toward the bottom of the collecting vessel as the
filter cake is removed in conjunction with the departure of the
cleaning air from the terminus of each filter cloth, and to suitably
decide the positions of the orifices and the direction of the jetted
air relative to the filter cloth surface. The filter cloth may be
of a cylindrical, hexagonal or any other multilateral configuration.
Furthermore, the object of filtration is not limited to air, for
the invention may be employed for filtering gases of all kinds.
In accordance with the present invention as described and illustrated
hereinabove, the overall amplified cleaning air flow is made to
flow parallel to the surface of a filter cloth in a filtration apparatus
for collecting fine particles from a gas containing them, which
surface, on the primary or inner side of the filter cloth, develops
a filter cake due to particles adhering thereto. Owing to such an
arrangement, the filter cake is completely removed even if it consists
of highly adherent particles, unlike the conventional method based
on backwashing, where the apparatus is large in size and high in
cost regardless of the fact that the cake removal effect is virtually
nil or minimal at best. Moreover, since the cleaning stream is jetted
at high speed, a so-called wake-flow or amplification effect is
produced by the jetted stream so that, with respect to the overall
quantity of cleaning gas, the amount of cleaning gas required for
introduction toward the primary side from an outside source is greatly
reduced. This facilitates handling remarkably, in comparison with
the prior art, in so far as the discharge of gases from the system
is concerned. Furthermore, since the introduction of a small quantity
of cleaning gas suffices, it is possible to reduce the overall size
of the system concerned with that cleaning gas introduction as well
as the overall size of the filtering system.
As many apparently widely different embodiments of the present
invention may be made without departing from the spirit and scope
thereof, it is to be understood that the invention is not limited
to the specific embodiments thereof except as defined in the appended
claims. |