High-efficiency electrostatic air filter device

An improved high-efficiency electrostatic air filter device which has a high efficiency of dust collection and a long service life. The device is composed of a charging section having a plurality pairs of electrodes to charge the dust particles in the gas to be treated; and a dust collecting section which is provided with a corrugated filter medium and spacer electrodes that are disposed in the troughs of the corrugations of the filter medium, and one of each pair of the electrodes in the charging section and the specer electrodes on one side of the filter medium in the dust collecting section are applied with high electric voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved high-efficiency electrostatic air 
filter device. More particularly, the invention relates to an 
electrostatic air filter device for use in the cleaning of air in a room, 
which device has a very high efficiency and a long service life. 
2. Description of the Prior Art 
The so-called HEPA (high-efficiency particulate air filter) is widely used 
in the prior art. It has a high dust-collecting efficiency, however, the 
head loss is quite high when dust-laden gas is passed through the filter. 
If the pores of the filter are made coarse in order to reduce the head 
loss, the efficiency of dust collection is lowered. If the head loss is 
decreased by reducing the velocity of the gas to be treated, the size of 
the filter must be increased. In addition, there have been other 
disadvantages in that the head loss is increased with the filling of 
pores, and that the life of the filter is short. Therefore, a suitable 
pre-filter is often employed in order to extend the life of such a filter. 
BRIEF SUMMARY OF THE INVENTION 
It is, therefore, the primary object of the present invention to provide an 
improved high-efficiency air filter device which is free of the 
above-described disadvantages. 
Another object of the present invention is to provide an electrostatic air 
filter which is able to treat dust-laden gas with high efficiency without 
the need of a pre-filter. 
A further object of the present invention is to provide an air filter 
device which has quite a long life and can be used for a long period of 
time without requiring any troublesome operations or maintenance work. 
Still a further object of the present invention is to provide an air filter 
device which is compact but not complicated in structure and which does 
not require a large floor space. 
Pursuant to the above object, in the embodiment of the electrostatic air 
filter device of the present invention, the suspended particles in the gas 
to be treated are electrically charged before the gas is passed through a 
filter medium; the charged particles are then collected on the surface of 
dust collecting electrodes that are disposed in the space formed by the 
filter medium to which a high electric voltage has been applied. The 
remaining particles are filtered off by the filter medium, thereby 
attaining a quite high efficiency of dust collection and a long service 
life.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the accompanying drawings, the present invention will be 
described in more detail. 
The air filter device of the present invention is composed of a charging 
section 1 and a dust collecting section 5. The frame 2 of the charging 
section 1 is provided with a plurality of plate electrodes 3 that are 
disposed parallel to each other. Each of the plate electrodes 3 is 
grounded, and the plane of the electrode 3 is in parallel relationship 
with the direction of the air to be treated. In the spaces between the 
plate electrodes 3 are disposed charging electrodes 4 which are connected 
to a high d.c. voltage source 10. 
A frame 6 of the dust collecting section 5 is provided with a filter medium 
7 which is folded in a corrugated form. In the troughs of the corrugations 
at both the upstream side and the downstream side are provided spacer 
electrodes 8A and 8B which are made of, for example, corrugated metal 
sheets. The spacer electrodes 8A on the upstream side are grounded, and 
the other spacer electrodes 8B on the downstream side are connected to an 
electric power source 9 for applying a high voltage to them. 
By connecting the outlet of the above-described charging section 1 to the 
intake of the dust collecting section 5, the improved high-efficiency 
electrostatic air filter device of the present invention can be formed. 
The charging section 1 and the dust collecting section 5 may be installed 
together in the same framework, if desired. 
The operation of the above-described air filter device will now be 
explained. 
A high d.c. voltage of 1 kv to 3 kv is applied to the charging electrodes 4 
and the spacer electrodes 8B on the downstream side. The dust-laden gas to 
be treated is supplied from the inlet of charging section 1, in which dust 
particles are electrically charged by corona discharge. The gas carrying 
the charged dust particles then passes through the dust collecting section 
5, as indicated by the dash line arrows in FIGS. 3 and 4. In this process, 
most of the charged particles 13 are attracted to the spacer electrodes 8A 
on the upstream side, release their electric charge, and are deposited on 
the surfaces of the electrodes 8A. Thus, most of the dust particles in the 
treated gas are removed. The larger the particle size, the greater the 
effect of this dust removal action. 
Gas currents 11 containing the remaining charged particles 13 advance as 
shown by the arrow lines in the drawing, that is, the gas currents 11 pass 
across the filter medium 7 along the shortest path owing to the resistance 
of filtration. Therefore, as shown in FIG. 4, the gas currents 11 move 
parallel and opposite to lines of electric force 12 that are directed from 
the spacer electrode 8B to the spacer electrode 8A. At the same time, the 
velocity per unit cross-sectional area of the gas currents 11 through the 
filter medium 7 becomes very low as compared with the velocity on the 
upstream side of this dust collecting section 5. Since the direction of 
the electric field and the direction of the gas currents are opposite to 
each other, the charged particles 13 move oppositely to the direction of 
the gas currents. If the velocity of this opposite movement of the 
particles exceeds the velocity of the gas currents through the filter 
medium 7, the charged particles 13 cannot enter into the pores of filter 
medium 7. However, since the gas current velocity outside the filter 
medium 7 is large, the charged particles 13 are ultimately deposited in 
porous state on the surface of the filter medium 7. Furthermore, even when 
the charged particles 13 are received into the pores of the filter medium 
7, they are deposited in porous state along the lines of the electric 
field applied by the spacer electrodes 8A and 8B, and are distributed 
through the fibers of the filter medium 7. Since the state of deposition 
of dust particles is porous, the amount of dust that is caught by the 
filter medium 7 is quite large as compared with the case in which dust 
particles are deposited irregularly on and in the filter medium 7. 
With the above-described dust collecting mechanism, the efficiency of dust 
collection can be much improved, and clean gas can be obtained from the 
outlet of the dust collecting section 5. Further, since larger particles 
are more effectively removed, the filling of the pores of filter medium 7 
hardly occurs, providing a much longer service life. 
In connection with the efficiency of dust collection, the air filter device 
of the present invention has been tested in order to confirm the 
effectiveness of the device. 
Test Method 
Air supplied from a blower was cleaned by using a high efficiency 
particulate air filter to remove suspended fine particles of foreign 
substances. The cleaned air was then mixed with a suspension of fine DOP 
(dioctyl phthalate) particles obtained by using an aerosol suspension 
generator. The gas mixture thus obtained was passed through a current 
regulating lattice, an upstream density measuring section, a filtering 
test section and a downstream density measuring section, and the tested 
gas was discharged. Test samples were taken through a diluting device at 
the rate of 100 ml per 20 seconds from sampling tubes that were attached 
to the upstream density measuring section and the downstream density 
measuring section. The number of DOP particles was counted by a 
light-scattering particle counter. The efficiency of dust collection was 
calculated from the upstream particle density and the downstream particle 
density in accordance with the following formula. The upstream particle 
density of particles having diameters of 0.3 microns or more was about 
5000/ml. 
##EQU1## 
where Cin is the particle number at the upstream side before filtration 
and Cout is the particle number at the downstream side after filtration. 
Test Results (Efficiency of Dust Collection) 
______________________________________ 
Items Test 1 Test 2 
______________________________________ 
Filter medium only 
99.997% 97.7% 
Air filter device of 
the present invention 
99.999997% 99.998% 
Head loss (in both cases) 
25.4 mmAq 8.5 mmAq 
______________________________________ 
From the above test results, it will be understood that when the air filter 
device of the present invention is used, the efficiency of dust collection 
can be perfected as much as 3 decimal places beyond the efficiency of the 
conventional case of a filter medium alone. 
In the above-described embodiment, the corrugated electroconductive plates 
are used for the spacer electrodes 8A and 8B so as to define spaces 
between crests of the folded filter medium 7. However, if the filter 
medium 7 is made of a hard material, the spacer electrodes 8A and 8B may 
be made of flat plates because the trough spaces of the wave form of the 
filter medium 7 can be maintained by the rigidity of the material. 
Further, in the above-described embodiment, the direction of the electric 
field between the spacer electrodes is opposite to the direction of the 
gas currents. It should be noted, however, that the direction of the 
electric field can be made the same as the direction of the gas currents 
or the former can be inclined relative to the latter. When the direction 
of electric field between the spacer electrodes is made the same as the 
direction of the gas currents, most of the charged particles are deposited 
in the pores of the filter medium. 
In the above embodiment, when the electric voltage applied to the spacer 
electrodes is too high, sparks occur between the spacer electrodes to 
damage the filter medium. On the other hand, if the electric voltage is 
too low, the remakable improvement in dust collecting efficiency cannot be 
expected. Therefore, the electric voltage applied to the spacer electrodes 
may be on such a level that the velocity of movement of the charged 
particles in the direction of the lines of the electric field must be the 
same or larger than the velocity of the gas currents that pass through the 
filter medium. In order to comply with this requirement, it is necessary 
to reduce the velocity of the charged particles through the filter medium 
and, therefore, the filter medium is given a corrugated shape so as to 
enlarge its effective area. 
In addition, if the electroconductive spacer electrodes 8B on the 
downstream side are wrapped with an insulating material, sparks can be 
avoided. Therefore, an intense electric field can be produced between the 
spacer electrodes and the efficiency of dust collection can be much 
improved. Furthermore, since the dust collection is quite effective, the 
density of dust on the downstream side is quite low. Thus, the lowering of 
the dust collection efficiency owing to collected dust on the surface of 
the above insulating material over the spacer electrodes can well be 
avoided. 
In the air filter device of the present invention, the dust particles are 
electrically charged and then collected by three measures, on the spacer 
electrodes, on the surfaces of the filter medium and in the pores of the 
filter medium. In other words, the dust particles are preliminarily 
collected by the spacer electrodes, the remaining dust particles are 
collected on the surfaces of the filter medium, and the still remaining 
dust particles are finally deposited in the spaces among the fibers of the 
filter medium. Therefore, the air filter device of the present invention 
is capable of attaining a very high efficiency of dust collection as well 
as providing a long service life, which two effects are in confrict in the 
conventional art. 
This comes from the three functions of the spacer electrodes: they maintain 
the spaces between the folded portions of the filter medium and 
mechanically reinforce the filter medium; they serve as dust collecting 
plates for the charged particles; and they serve as electrode plates to 
provide an electric field in the spaces between them and the filter medium 
and in the pores of the filter medium. The structure is different from 
conventional dust collecting devices, for example, the device in which the 
filter medium is wrapped in wire netting so as to generate an electric 
field within the filter medium, and electrodes are separately installed; 
or the device in which a double-step electric dust collector is separately 
attached at the upstream side. 
According to the above-described three-stage dust collecting mechanism of 
the present invention, a quite high efficiency of dust collection can be 
attained as compared with the case in which an electric voltage is not 
applied. In addition, owing to the preliminary dust collecting effect by 
the spacer electrodes and the manner of dust deposition in the porous 
structure on the surface of filter medium, a quite effective air filter 
device can be obtained, in which the filling up of filter pores does not 
occur, and in which a very low head loss is provided. 
Furthermore, as will be understood from the foregoing results of Test 2, it 
is possible to produce an air filter having a quite high efficiency of 
dust collection with very low head loss. In other words, when the 
efficiency of dust collection and the size of the device are made the same 
as those of conventional devices, it is possible to produce an air filter 
device having a large treating capacity. 
Although the present invention has been described in connection with a 
preferred embodiment thereof, many variations and modifications will be 
apparent to those skilled in the art. It is preferred, therefore, that the 
present invention be limited not by the specific disclosure herein, but 
only by the appended claims.