Pressure Drop and Collection Efficiency of an Irrigated Bag Filter Tetsuo Yoshida, Yasuo Kousaka,* Shigeo Inake, and Shigeyuki Nakai Faculty of Engineering, University of Osaka Prefecture, Osaka, Japan
An irrigated bag filter has been developed to improve performances of existing dry bag filters. Irrigation to filter surface by spraying or overflowing water prevents filter media from firing in handling hot gas and makes it possible to wash away the precipitated dusts from filter surfaces. Some characteristics regarding pressure drop and dust collection of an irrigated filter which were quite different from dry ones were studied, and then basic mechanisms of them were discussed. A series of studies suggested that this kind of collector will be useful in certain industrial fields.
Introduction
Discussion
An irrigated bag filter described here is quite different from existing dry bag filters, because the surface of filter media is covered by water. I t has been reported that an irrigated bag filter can treat high-temperature and highly humid gas, and that sweepage procedures of dust cake necessary for a dry filter are not needed because of water falling along the filter surface (Minami, et al., 1969). It has been also reported that the relation between pressure drop and gas flow rate is peculiar compared with that of dry filters (Muhlrad, 1970) and that collection efficiency is fairly high (Minami, et al., 1969). In this paper, pressure drop and collection efficiency of irrigated bag filters were tested and their basic mechanisms were studied by using nets of standard wire meshes instead of bag cloths.
In this section, some basic mechanisms of pressure drop and dust collection of an irrigated filter are studied by using nets of standard wire meshes instead of bag cloths. Pressure Drop. Figure 5a indicates a model of the mesh over which a water film covers. The equilibrium of force is given as follows when pressure difference exists between the two sides of the film.
Experimental Section One of the experimental apparatuses used in this study is shown in Figure 1. Water is supplied along the inside of a ring dam to the top of the bag cloth. Gas flows out from the inside of the bag cloth just contacting with falling water in the manner of crossflow. Superficial filtering gas velocities were varied within 20 cm/sec and water rates were from 2 t o 20 l./min. The dust particle used was CaC03 having a median diameter of 3.6 p (in weight base) and concentrations a t the inlet were from 2 to 8 g/m3. Some physical properties of bag cloths are shown in Table I. Pressure Drop. Figure 2 shows the comparison of the pressure drop of irrigated bag filters and that of dry ones when they are clean. As is shown in Figure 2 the characteristics of pressure drop considerably differ from each other. Figure 3 indicates the same comparison but with dust loads. Because of washing action against deposited dusts by the down stream of water, almost 'no pressure rise occurred in irrigated bag filters. For certain dusts, however, which contain some tar substances, the pressure drop increased with operation period. Collection Efficiency. Figure 4 indicates the collection efficiency of irrigated bag filters. The collection efficiencies seem to be correlated to the pressure drops as shown in Figure 3. Although other experimental conditions of various water rates ranged from 2 to 10 l./min and those of superficial gas velocities from 1.5 to 8 cm/sec were also examined, almost no differences among them were found.
APDADBg, = 2(DA
+
D , ) ( T ~C O S
+
( ~ 1
1 1 2 7 2 COS ~ p 2 )
(1)
or
The pressure difference AP gives the critical one a t which the film is just broken. In existing wire meshes, because of a three-dimensional structure shown in Figure 5b, the direction of the force of surface tension varies with positions of a mesh. Then this factor was included in in eq 2. The coefficient, t , however, must be constant when the material of the meshes, the manner of weaving, and liquid, respectively, are the same. Equation 2 indicates that the pressure drop to break a film is inversely proportional t o the opening size of a mesh. An irrigated net of wire meshes whose openings have a size distribution is next discussed. When the pressure difference between both sides of the net is gradually raised, the film covering over a mesh with the maximum opening size, in this case, will be broken first because of the minimum pressure to break it as shown in eq 2. Subsequently, with a slight pressure rise, the film over a mesh with the next larger opening size is then broken. Thus films are broken in order of their opening sizes as pressure rises. When the film over the mesh having a hydraulic diameter of D H of~ the ith size is just broken a t the pressure of
Apt
(3)
gas must flow out through the opening with the velocity of u L to keep pressure drop in AP,. The pressure required to break the ith film against the force of surface tension is caused by the resistance of gas flow through the openings over which films are already broken. Then
This equation indicates the flow resistance on the ith opening, and on other openings there must be the following relations Ind. Eng. Chern., Process Des. Dev., Vol. 14, No. 2, 1975
101
Table I. Properties of Filters Used Filter cloth A B C D E
Resistance coefficient, l / m
Fabric
Teviron Tetoron Tetoron Kanekalon Saran
Filament, plain Spun, plain Spun, plain Spun, plain Filament, satin
Hydraulic mean radius, cm
0.432 X 10' 2.10 x 107 1.48 x 10' 2.66
76.9 48.5 65.8 64.1
x 10'
Porosity
x 10-4 x 10-4 x lo-' x
6.07 4.46 7.67 4.47
- b#
240
-
x >:
x x 10-2
0
FILTER A C
e
D
9
E
---DI RR YR I G A T E D
,,
I,I
kglm'
DUST L O A D
Figure 3. Comparison of pressure drop of irrigated and dry bag
filters with dust load.
SLURRY OUT
Figure 1 . Experimental apparatus. ,'OO",yy--~
20
*
k
LJ
,I
%
---
~
l5 E
-
-00
"0 00 -- -- - - - 64.
>
10
E a g D In
40-j
d
5 2
,;
b
20-
-
0
SUPERFICIAL GAS
0
-
VELOCITY
6crnlsec
a
FILTER A
C 0
D E
--- DRY -IRRIGATED
0 0 SUPERFICIAL
G A S VELOCITY
U cmlsec
Figure 2. Comparison of pressure drop of irrigated and dry bag filters with no dust load. APi =
d
(4f - + K, + K, d (4f - + K, + K , DHl
AP, =
DH2
When meshes with a narrow size distribution, such as standard wire meshes, are concerned, each gas velocity through openings is expressed as
Then superficial gas velocity will be written by
102
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2, 1975
When all of openings may be regarded as almost square,
A I is represented as Ai
+
i
D H i 2 = A' jDHif(DH.o')DHzdDH(8) i=l
DH i
The above equations give the relation between superficial gas velocity and pressure drop, when the size distribution of meshes, tno-dimensional porosity of a mesh, surface tension of liquid, and its correction factor 5 are known. When A , is first assumed, then D H ~AP,, , vl, and Ut are given respectively by using eq 8, 3, 4, and 7 . Repeating the same procedure for various A , , the correlation of A P and u i s obtainable. The experimental apparatus to test the above analysis was essentially similar to that of Figure 1, but a net of standard wire meshes was installed instead of a bag cloth and the size of the apparatus was about half of that of Figure 1. Table I1 shows some physical properties of wire meshes used in the experiment. The dust particle used in
Table 11. Properties of Standard Wire Meshes Hydraulic mean diameter, DH,mm Porosity, E,
Wire mesh
Fabric
JIS 500
Plain
0.499
M 60
Plain
0.172
JIS 149
Plain
0.149
JIS 74
Plain
0.073
Size distribution
A
I
Figure 5. Model of wire mesh.
-
:250 E
O U
150
20
30
SUPERFICIAL GA5 VELOCITY E crnisec
-
,200 0
IO
0
Figure 7. Comparison of experimental and calculated results of pressure drop.
-
U
m 3
El00 cc a 50
SUPERFICIAL
-
GAS VELOCITY 0
-
15 cmlsec
lo 0
u
OO
4
a
12
>
16
RECIPROCAL OF MEAN HYDRAULIC DIAMETER llmm
Figure 6. Relation between pressure drop and mean hydraulic diameter of wire mesh.
I
“L
- 2
0
the experiment was clay having a median diameter of 7.5 1.1 in weight basis. Figure 6 shows the relation between the mean hydraulic diameter of meshes and the pressure drop under a constant superficial gas velocity. The figure suggests that eq 2 is valid. Figure 7 shows the comparison of the experimental result with the calculated one on A P us. U for standard wire meshes. In the calculation, the value of [ was put as 0.72 for each mesh in Table 11. The broken lines in which Figure 7 indicate the relation between A P , and is calculated by eq 4 and 7 . The values of N , / N in the figure indicate the ratio of the number of openings having broken film to that of the total openings of meshes. It is one of the interesting characteristics that the number of openings having broken film is extremely small. This phenomenon was also observed on photographs. Dust Collection. The observation of the surface of an irrigated net of wire meshes by high-speed camera suggested that two marked mechanisms of dust collection existed. As discussed in the former section, the number of openings having broken film is extremely small, so one of the two mechanisms may be modeled after that of the collection by a n orifice in a n infinite plane. It was also found with precise observation that each opening having broken film was not stable but after an instant, after 1/1000 sec or less, it was recovered by a water film. The period of films being kept broken, which was observed by high-
ut
rm
2
.
.
0.5 0.75
0
1
At
1.25 1.5
Csed
Figure 8. Interval of water film being kept broken (JIS 500).
speed camera, is shown in Figure 8. This sudden opening and closing action of an orifice, that is shutter action of an orifice, may be considered as another mechanism of collection. Collection by an Orifice in an Infinite Plane. The collection efficiency by an orifice in an infinite plane can be obtained when the stream line around the orifice and then the trajectory of a particle in the stream are calculated. In order to estimate the collection efficiency, potential flow for stream line and Stoke’s law for drag of a particle are here assumed. The stream function of ideal gas around an orifice in an infinite plane is given by (Lamb, 1932)
The gas velocities are also given as
.
n=
(10‘)
When the stream is thus given, trajectories of a particle in Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2,1975
103
the stream are computed as‘follows by assuming Stokes’ law and no disturbance in the stream by the particle. The dimensionless equations of motion of a particle are expressed as follows when any external forces including gravity may be ignored d2R d R 2 * E 2 + -dT d2X
2a-
dT2
ax +dT
-
v, = v, =
0
I-
R--
(11)
0
(12)
where
Figure 9. Illustration of collection efficiency of an orifice in an infinite plane.
>
1.0 INTERCEPTION
0
\E = PDdD2VI2
361a
At a point far away from the orifice, the velocity of a particle is assumed to be equal to that of fluid, so the initial conditions are
(13)
(13 ’1
*
The trajectories of a particle for various values of are obtained by performing numerical calculation of above equations. Figure 9 illustrates the relation of a stream line and a trajectory of a particle. The collection efficiency of a particle of d, in diameter is consequently given as the ratio of the volumetric gas flow rate QP to the total flow rate Q as shown in Figure 9. Figure 10 indicates the result of calculation of the efficiency defined above. The figure suggests that the interceptional collection is important in this case, and so collection efficiency cannot be expected to be too great unless the interceptional parameter is sufficiently large. Collection by Shutter Action of an Orifice. One may suppose a case where a shutter which covers an orifice is suddenly opened and then gas flows out through the orifice. In this case, if dust particles are contained in gas, dust-free gas only may flow out a t first while it takes some instants to accelerate the dust particles. When the orifice is recovered by a shutter in the next instant, the particles go straight ahead to be caught to the shutter. If the interval of the shutter being kept open is short enough, a fair contribution to dust collection is expected. The analysis of this mechanism of collection may be accomplished by calculating the unsteady particle motion in an unsteady velocity field of fluid around the orifice. It will be difficult, however, to estimate the unsteady velocity profile of gas around the orifice which is confronted with the sudden opening and closing action. It is assumed here that a steady velocity profile of gas may be instantaneously built up a t opening and that the flow may also be instantaneously stopped a t closing. This assumption may be valid only for rough estimation of the extent of the collection efficiency described above. Under the assumption, the collection efficiency of sudden closing of the orifice may be defined as follows (see Appendix)
(14) In the equation qstop represents the volume surrounded by a stopping distance shown in Figure 11. The stopping distance of a particle with dp in diameter and pp in density is given by
104
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2, 1975
lo-2k
16’
,
,
,
.
,
,
.
,
,
,
,I
1 10 I N E R T I A L PARAMETER
Figure 10. Collection efficiency by an orifice
Figure 11. Illustration of collection mechanism by sudden closing of an orifice.
uo is the initial velocity of the particle which is regarded as same as the surrounding gas velocity just before the flow stops. Figure 12 shows the result of numerical calculation of qs. From the figure the shutter action seems to be very effective for collection, especially when the opening period is very short. If the collection by a shutter being just opened is considered in addition, the efficiency will be about twice as much as that in Figure 12. Figure 13 indicates the experimental result of collection efficiency for an irrigated net of standard wire meshes, the conditions of which are almost same as those of Figure 12. I t was pointed out previously that dust collection by an orifice in an infinite plane could not be expected unless the interceptional parameter was large. In the case of Figure 13, the interceptional parameter is small and thus the efficiency in the figure may be roughly compared with that by shutter action of an orifice. Although good agreement is not expected because of the simplified analysis of the phenomena, some resemblances may be found between Figures 12 and 13.
$1 0
'I
PARTICLE DIA 8 micrm D E N S I T Y 2 6 g/cm'
V c
U
Fo8
OPENING INTERVAL
3
Ci, Q
In
$0 6
-
U >
w
Qstop
Co,Q --c
Cm
Collection chamber
:04 LL
z 002
ORIFICE D I A ' 0 5 mm
u,
10 20 30 40 ACTUAL VELOCITY THROUGH ORIFIC
I
Figure 14. Illustration of collection efficiency by sudden closing of orifice.
U 0
C
I
mlSeC
Figure 12. Collection efficiency when shutter being just closed
where n is the number of orifices whose shutters are open. Q is also expressed as Q = aa2nDo. When the collection by a n orifice in an infinite plane without shutter action is negligible, the dust concentration before and behind the orifice is equal, then co = c m . By using these correlations, collection efficiency is given as
li 3 5
RANGE OF ACTUAL VELOCITY THROUGH O R I F I C E
z
I?
: I-
205-21
30
5 misec
-.
s
Nomenclature
F I L T E R J I S 500
I
1 0
100 150 SUPERFICIAL GAS VELOCITY 50
200 cmisec
Figure 13. Collection efficiency obtained by standard wire mesh experiment.
Conclusion The manner of change in pressure drop with gas velocities of irrigated bag filters was first tested and was found to be very peculiar compared with the dry ones. The mechanism of pressure drop was then analyzed from the equilibrium of forces such as surface tension and static pressure of gas, and the result of the analysis was found to agree well with that of experiments using a net of standard wire meshes. The slight change in pressure drop over the wide range of gas velocities seemed to be one of the interesting characteristics of this type of filter. It was also found that almost no pressure rise occurred after a long operation for dusts not containing tar substances. The collection efficiency of an irrigated bag filter was found to be fairly high. It was impossible to give a full explanation of the experimental results because of the complexity of collection mechanisms. However, two mechanisms of collection, one by an orifice in a n infinite plane and another by shutter action of an orifice, were pointed out by a simplified analysis in the case of standard wire meshes being irrigated.
Greek Letters cc = two-dimensional porosity 1 = viscosity of fluid, kg/sec m ( = coefficient defined by eq 2 p = density of gas, kg/m3 pp = density of particle, kg/m3 us
Acknowledgment
= surface tpnsion, kg/sec2 = force by surface tension shown in Figure 5, kg/
TI, 7 2
S. Magono, K . Yamadaki, T. Yasumune, and Y. Adachi were very helpful in the experimental work. Appendix The material balance before and behind the collector with orifices having shutter action is expressed as CiQ
a = orifice radius, m A = total area of a net of wire meshes, m2 A , = area defined by eq 8, m2 d, = particle diameter, m d, D.A?D B = values illustrated in Figure 5, m D H ,D H = hydraulic and hydraulic mean diameters, m f = friction factor I = interception parameter = d p / 2 a K,, Ke = friction loss factor of contraction and expansion rn = correction factor of wetted length N , N , = number of total openings and of openings with broken film AP = pressure drop, mm Aq q s t o p = volume surrounded by stopping distance shown in Figure 11, m3 Q = volumetric gas flow rate, m3/sec QP = gas flow rate shown in Figure 9, m3/sec r = radial distance, m At = interval of shutter being open, sec U = superficial gas velocity, m/sec u, ur, u x = gas velocity, m/sec u o = 1/aa2, m/sec 00 = mean actual velocity through orifice, m/sec x = axial distance, m
-
~ m Q s t o p = COQ
where cI, CO, and Cm represent respectively the dust concentration a t inlet, outlet, and a t collection chamber shown in Figure 14. Q represents the volumetric gas flow rate, and Qstop the total volume of qstop formed in collection chamber in unit time and is given by
sec2 PI,p2 = angle shown in Figure 5 9 = inertial parameter
Subscripts 1,2,3, . . . , i = order of opening size Literature Cited Lamb, H . , "Hydrodynamics," 6th ed, p 138, McGraw-Hill, New York, N. Y . , 1932.
Minami, T., Suzuki, T . . Funsai. No. 14.74. Hosokawa Micromeritics Laboratory. 1969. Muhlrad, W., Staub, 3 0 , 74 (1970)
Received for review September 23, 1973 Accepted November 21, 1974 Ind. Eng. Chem., Process Des. Dev., Vol. 1 4 , No. 2, 1 9 7 5
105
