Note: Descriptions are shown in the official language in which they were submitted.
~0575Z3
METHOD AND APPARATUS FOR CLEANING FABRIC
FILTERS OF BAG TYPE OR THE LIKE
This invention relates to a method of cleaning fabric filters Or
bag type or the like by exposing the filter bags to a pressure
pulse Or cleaning medium, which is supplied to the filter bags
through a cleaning apparatus comprising a pressure tank for con-
taining the cleaning medium in the rorm of gaseous medium under
pressure, preferably compressed air, a distribution passagaway
communicating with said tank and provided w;th nozzles or orifices
directed to the apertures Or the bags, a valve means and control
means for producing the pressure pulse.
A plurality Or difrerent cleaning principles are applied in con-
junction with fabric filters, for example cleaning by vibration,
shaking, return air injection, compressed air pulses, sound pulses,
and combinations Or said principles. The principle substantially
being dealt with in the following is cleaning by compressed air
pulses, hereinafter also called pressure pulses.
In ~rinciple, cleaning by compressed air pulses is carried out
in such a manner, that the compressed air is distributed rrom a
tank via a syst.em Or passageways to the fabric filter configur-
ation in quefition, which e.g. may consist Or bags, and is injected
into the bags through some kind of nozzle. The cleaning air flow
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having opposed direction relative to the operation gas flow
cleans the bag of collected dust particles. The jet energy
of the pressure pulse in the nozzle outlet is utilized for
the co-ejection of surrounding air in order to rapidly fill
the bag volume and obtain a large reversible through-flow
(so-called ejected pulse). In most cases ejectors are utilized
at the bag inlet for producing a good co-ejection effect. The
pressure in the pressure tank usually is chosen to lie in the
high-pressure range, i.e. that the excess pressure is between
0,4 MPa and 0,8 MPa. There exist also systems operating with
lower pressure, for example between 0,1 MPa and 0,2 MPa, and
with a smaller or none ejecting flow (so-called direct-pulse).
The object in such cases is to utilize the greater part of the
jet flow directly for bag cleaning. One disadvantage of the
conventional systems, however, is that the compressed air
consumption is higher than at systems operating according to
the ejected-pulse principle. Moreover, the cleaning effect
obtained at the known systems often has been unsatisfactory
and thereby has jeopardized the serviceableness of the filter
installation.
By a detail study of the dynamic procedures in a
conventional direct-pulse system, the way in which the
cleaning effects are obtained, has been elucidated in detail.
It was a.o. found, by registration and evaluation of the
pressure developments in the tank, piping and bags and by
direct comparisons with results obtained from pilot-scale and
full-scale tests in real installations, that the most essential
cleaning effect was obtained by the pressure chock in the bag
which preceded the air through-flow proper, i.e. the acceler-
ation-retardation procedure, which is forced onto the filter
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medium with the dust particles collected thereon, is more
essential from the cleaning aspect than the subsequent through-
flow. It was, thus, discovered that, for rendering the filter
cleaning more efficient, it is essential to produce an improved
acceleration effect on the bag, and that this increased
acceleration is to be brought about during the build-up of
the pressure pulse in the bag.
The invention is based on the understanding of that
the time for the pressure pulse to reach its maximum value is
as much as possible to be shortened, while the maximum value
of the pressure pulse is to be set as high as possible. For
realizing this object, a certain geometric relation between
nozzle and filter medium configuration, for example bag, is
required. The pressure transfer from nozzle to bag which can
be described by the impulse law and has been attested by
practical tests, is most effective when the bag inlet and
nozzle location are so chosen that as little as possible
surrounding air is co-ejected. The velocity in pressure
increase and the size of the maximum pressure pulse in the
bag, further, are influenced by the flow losses of the air
system, i.e. the energy available must be concentrated to the
greatest possible extent to the air jet proper ejected from
the nozzle. Of course, this is technically selfevident, and
it is conventionally carried out also with a view on the
manufacturing cost aspects. In a conventional system, the
flow losses can be said being concentrated
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to the valve, distribution pipe (friction and air distribution
losses) and nozzles (inlet nozzles). The losses in the
distribution pipe and nozzles can be affected in a conventional
manner by changing the dimensions. This is, of course, also
the case for the valver but in order to, besides, bring about
the higher velocity in pressure increase in the bag and
possibly be able to interrupt the procedure immediately after
the maximum and increased pressure pulse in the bag has been
obtained, a more rapid and fully controlled opening and
closing function than obtained with a conventional system
build-up is required.
The present invention, therefore, relates to a method
of cleaning fabric filters according to the direct-pulse
principle and has as its object to provide a method, by which
the efficiency of the cleaning is substantially improved and
the air consumption is reduced.
One broad aspect of the invention relates to apparatus
for cleaning bag-type fabric filters comprising a pressure tank
for containing pressurized gaseous medium, a gas distribution
conduit provided with at least one flow opening for applying
pressurized gas to the surface of a filter opposite the surface
having dust particles collected thereon, said distribution
conduit having an end opening into the pressure tank and
forming a valve seat within the tank, a movable valve member
disposed within the tank for opening and closing the valve seat,
and fluid pressure control means for exerting a fluid pressure
on the movable valve member to move the same into engagement
with the valve seat.
The invention and its relation to known art is
described in greater detail in the following, with reference to
the accompanying drawings, in which
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Fig. 1 shows in a schematic way the construction of a con-
ventional blow cleaning system,
Fig. 2 is a diagram showing the pressure development in the
tank and bag as a function of the time at a conventional
system,
Fig. 3 shows a nozzle and bag at a direct-pulse system,
Fig. 4 shows in a schematic way constructions of the blow
cleaning system according to the invention,
Eig. 5 shows in detail the valve with the diaphragm in open
position,
Fig. 6 shows the valve diaphragm,
Fig. 7 is a diagram showing the pressure relation in the tank
and bag as a function of the time at a system accord-
ing to the invention,
Fig. 7a is a diagram showing the control impulse to the valve,
Fig. 8 is a diagram showing the control impulse to the valve
at a so-called pulse train,
Fig. 9 shows in a schematic way a pressure tank divided into
sections .
In Fig. 1, which refers to a conventional blow clean-
ing system according to the direct-pulse principle, the numeral
1 designates a pressure tank for cleaning medium in the form
of compressed air. To said pressure tank is connected a
pipe 2, which is coupled to a valve 3. Upon opening the valve,
a pressure pulse is produced which is led via the distribution
passageway 4 to nozzle pipes 5, which are directed to the
openings of the bags 6. The diagram in Fig. 2 shows more
clearly the pressure conditions in the tank and bag when
the valve is being opened. The curve A represents the pressure
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drop in the tank after the valve has been opened, and the
curve B represents the pressure development in the bag. The
time Tl represents the time for the bag pressure to rise from
operation pressure to maximum pressure, which is designated
by Pb. After the pressure has reached its maximum, a contin-
uous decrease in pressure takes place owing to the air flow-
ing o~t through the filter medium. It was proved by a plur-
ality of pilot - as well as full-scale tests, that the clean-
ing effect was not influenced when the time for which the
valve had been held open was shortened from 0,7 second to
about 0,2 second. These time intervals are indicated in Fig. 2
by T3 and T2, respectively. It was found that it were the
velocity in pressure increase represented by the time Tl, and
the maximum value Pb of the bag pressure chocks which render
the essential cleaning effect. The subsequent flow of air
through the filter medium is of minor importance in this
respect. This was also confirmed by means of theoretical
calculations.
In Fig. 3 is shown the location of the nozzle 5
in relation to the bag 6. It was found that, in order to
obtain a minimum co-ejection of surrounding air, the distance
h between the outlet of a nozzle and the bag inlet must be
chosen being between 25mm and 175mm for relations between
nozzle and bag diameter dl/d2 of 0,012-0,030. Fig. 4 shows
the construction of a blow cleaning system according to the
invention. The pressure tank 1 contains the cleaning medium
in the form of compressed air. The distribution passageway
4, which communicates with the pressure tank, is provided with
nozzle pipes 5 or alternatively apertures 7 directed to the
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bag opening. Said distribution passageway also comprises a
portion 9, which projects into the tank and the end of which,
thus, opens into the tank. The two passageway portions 4 and
9 can be manufactured integral or be connected to each other
by a coupling means 10, which may be designed, for example,
with bayonet socket or as a flexible coupling with rubber
sleeve and hose clips. At the end of the distribution pass-
ageway a valve means 8 is provided, which comprises a valve
diaphragm 11, which in the position shown sealingly abuts a
valve seat 12 disposed at the end of the distribution passage-
way. An O-ring may serve as a sealing between the distribution
passageway and the valve seat. For fixing the end of the
distribution passageway (and valve seat) with the shell surface
of the tank, a connection 13 is provided. The valve diaphragm
is actuated by a pilot valve 14, which in its turn is control-
led by a control system (not shown). The main re~uirement ~o
be met by the control system is to emit control signals of
sufficient speed. This can be realized in different ways by
known art. It is presupposed in the following that the
signals are emitted in the form of electric pulses. The
valve means may also, within the scope of the invention idea,
be given a location other than that at the embodiment shown.
The extended portion 9 of the distribution passageway, for
example, can be made very short so that the valve seat in
practice will be located close to the tank shell surface
where the passageway penetrates the tank wall. In such a
case, the main part of the valve means will be located within
the tank.
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~0575Z3
In Fig. 5 the valve means is shown in detail when the diaphragm
11 is in open position. An annular gap t is then formed be-
tween the valve seat 12 and diaphragm 11. In order to render
the function satisfactory, the annular area Ao = ~ do t
for the air inlet is about the same as the cross-sectional
area in the distribution passageway which is equal to ~do2/4.
As a result of assembling the valve with the pressure tank,
as shown in Figs. 4 and 5, very low flow losses are obtained.
This, together with a rapid opening function of the valve,
provides the prerequisites for both the high velocity in
pressure increase and an increased, maximum pressure pulse
in the bag. As an example can be mentioned that at measure-
ments made for a 3-inch valve a pressure drop coefficient
(defined according to the relation ~P = ~ P dyn) for the
integrated valve function was obtained which was 20 per cent
lower than the valve for the conventional valve function
according to Fig. 1. Due to the fact that the valve is
completed with a rapid control system, also a very rapid
closing of the valve is obtained. This, adding up, renders
it possible to obtain a time interval between opening and
closing of the valve which is very short, compared with
conventional systems. Hereby the procedure can be interrupted
immediately after the maximum pressure pulse has been ob-
tained in the bag and thereby renders possible a substantial
reduction of the air consumption.
Fig. 6 shows in detail the valve diaphragm 11 provided with
a so-called blow-off cock.15. The diaphragm can be modified
so as to match the opening and closing times with each other
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to an optimum combination. At measurements made on a
commercially available valve make, for example, a diaphragm
opening time of 0,005 secand and closing times of 0,03-0,05
second at a tank excess pressure of 0,11 MPa were obtained.
By providing the diaphragm with three to four blow-off
holes of 3 mm diameter, certainly a twice as long opening time
was obtained, but the closing time was reduced to about its
half, which resulted in a shortened total of opening and
closing times. The figures mentioned, thus, refer to a
certain diaphragm mass, diaphragm rigidity and tank pressure.
For higher pressures, for example, a thicker (=stronger)
diaphragm is required which, consequently, has a greater
mass and requires other combinations of blow-off holes or
corresponding measures.
In the following, the development of a pressure
pulse is described in greater detail, reference being made
to the Figures 7 and 7a. Fig. 7 is a diagram showing pres-
sure ~ as a function of the time T, and Fig. 7a, superimposed
in Fig. 7, shows the control impulse S as a function of the
time. In Fig. 7, the curve C represents the pressure
relation in the pressure tank, the curve D represents the
pressure relation in the filter-media configuration, which
e.g. may be a bag, and curve E in Fig. 7a indicates the
electric impulses controlling the opening and closing of the
valve. The impulse level SO corresponds to impulse for
closed valve, and the impulse level Sl refers to impulse for
open valve. After the electric impulse for valve opening
has been released, a certain time TO, the
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~0575Z3
so-called dead t;me, lapses before the physical valve opening
commences. The opening time for the valve is T4 whereafter the
dynamic flow is fully developed and causes the pressure rise
in the bag to the maximum value Pb. ~en after the release Or the
electric control impulse a certain time has lapsed, the closing
procedure commences thereby that the electric control impulse
is broken. The length Or the electric pulse is designated by
Te. When again a dead time T0 has lapsed, the physical valve
closing is commenced which takes the time T5. The time during
which the valve is open, thus, is corresponded by the time T7.
The time T6 is re~!uired for emptying the system. As pointed out
earlier, it is the procedure portion being essential from the
cleaning point Or view, which is to be utilized, viz. the rapid
pressure pulse increase in the bag, i.e. the pressure rise which
takes place during the time T8. Therefore, the procedure is to be
interrupted as soon as the pressure pulse in the bag has reached
its maximum value. This may imply, due to the shirting in time
between the procedures in the valve and in the bag, that the
electric impulse for valve closing must be given even before the
pressure pulse in the bag has reached its maximum value. The
electric pulse time Te between opening and closing, therefore, is
made very short, 0,02 to 0,10 second , compared with conventional
systems whcre the time is about 0,15 to 1,0 second. As an example
can be mentioned that at tests with a system described above the
electric impulse time for opening/closing was chosen at one
occasion 0,040 second, at which occasion the time during which
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~0575Z3
the valve was open, inclusive of the opening and closing
time, was about 0,075 second. Times as short as about 0,020
second (electric impulse time) could be applied before a
decrease in size of the compressed air pulse of the bag
occurred. A corresponding pressure drop ~p in the tank (tank
volume 0,5 m3) was 5000-40 000 Pa at an excess pressure in
the tank which in the starting position was about 110 000 Pa
corresponding to a compressed air consumption of 0,020-0,20 m3
free air per blowing. Corresponding measurements in a con-
ventional system according to Fig. 1 rendered air consumption
figures of 0,40-0,60 m3 free air per blowing and a maximum
pressure pulse in the bag which was lowe~ by as much as 60%.
The velocity in pressure increase, defined as
~p bag/~t, which is achieved in the bag, has also been
measured. As an example of the average velocity in pressure
increase, i.e. Pb/T8, can be mentioned that by application
of the invention a value exceeding 400 000 Pa/s (Pascal per
second) has been obtained at 0,11 MPa (megapascal) excess
tank pressure and more than 1 200 000 Pa/s at 0,25 MPa excess
tank pressure. It can, further, be mentioned that this
velocity in pressure increase is four to six times higher
than that obtained with known art.
Compared with known systems the invention, thus,
offers both a substantial improvement of the cleaning effect
and a reduction of energy consumption.
It should, further, be pointed out that the maximum
pressure pulse Pb in the bag, of course, also is affected by
the pressure prevailing in the tank. The object with the
invention is to utilize primarily the low-pressure range with
a tank excess pressure of 0,05-0,3 MPa, but it may be necessary
1057523
for certain applications also to utilize the high-pressure
range (0,3-1,0 MPa). Such utilization, thus, lies within
the scope of the invention idea. The decision which tank
pressure is to be chosen, is in practice a problem of
optimizing, for which the entire filter function and the
process application in question must be taken into con-
sideration.
The diagram in Fig. 8 shows a variant of the control
principle at which two or more pulses tightly following each
other, so-called pulse trains, are produced. The time Te
designates the length of a control pulse, and the time Ts
refers to the time interval between the beginning of two
subsequent pulses. The pulse train can be obtained in a
simple manner by electric forced control, so that a sub-
sequent pulse already begins before the pressure in the tank
has reassumed its original value, or first after said value
has been reassumed. In order to achieve the greatest effect
in relation to the air consumption, short time intervals are
to be chosen. Suitable values are 20-50 ms electric impulse
time Te and a time difference Ts about twice as great between
the pulse train chocks. For a specific case, the values
Te = 35 ms and Ts = 70 ms have been tested. The effect of
such a pulse train system, of course, depends to some extent
on the capacity of the pressure producing system available,
but irrespective thereof has been noted at tests in instal-
lations, that an additional improvement of the cleaning
effect, compared with only one pulse, is obtained. In order
to limit the compressed air consumption, it is possible to
limit the volume of the tank, instead of substantially short-
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ening the time during which the valve is held open. For
special applications and sizes, the air consumption figures
thereby obtainable are almost as low as if the valve is
given a short holding-open time. The smallest tank volume,
which can be used without reducing the amount of the maximum
pressure pulse in the bag, is five to ten times greater than
the volume of the air distribtion passageways. Fig. 9
shows how the limited tank volume can be brought about at
the construction of a full-scale installation. The pressure
tank 1 is provided with distribution passageways 4 (shown
partially). The tank is divided by a partition wall 16 into
sections, so that the volume of each tank section is so
adjusted to the volume of the associated distribution
passageways that the aforesaid requirements are met.
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