Dual-dipole electrostatic air filter

A filter assembly (10) for removing particulate matter from air forced therethrough comprises two pairs of oppositely charged woven electrostatic filtering material. Each pair of electrostatic filtering material comprises a positively charged and a negatively charged layer (22, 24, 28 and 30). An open cell foam layer (26) is disposed between the two pairs. Metal grids (20,32) enclose the filtering elements.

TECHNICAL FIELD OF THE INVENTION 
This invention pertains in general to air filtering devices, and more 
particularly to an electrostatic air filter. 
BACKGROUND OF THE INVENTION 
The principle of electrostatic filtering has been known for many years. 
Under the influence of an air stream, certain materials which are normally 
dielectric will obtain an electrostatic charge. The electrostatic charge 
attracts dust particles from the air stream, thereby filtering air. Tests 
have shown that self-charging electrostatic air filters provide superior 
filtration as compared with conventional impingement type filters which 
collects dust through contact with the dust particles. 
U.S. Pat. No. 4,115,082 to Newell discloses an air cleaner assembly having 
outer sheets of polypropylene overlying two series of rigid thermoplastic 
rods. A central element of open-cell foam rubber is sandwiched between the 
two series of rods. It is claimed that the rods and central element 
develop a positive electrostatic charge while the outer polypropylene 
material develops a negative charge, increasing the efficiency of the 
filtration. 
Because of the thermoplastic rods, the construction of the filter disclosed 
in U.S. Pat. No. 4,115,082 has proven to be quite expensive. Furthermore, 
while it is doubtful that the plastic rods can develop enough charge to 
significantly effect the filtration; they may increase the resistance to 
airflow. Independent tests have shown that the filtration efficiency of 
the unit of this type is only about 85% using ASHRAE 52-76 test 
procedures. 
Therefore, a need has arisen in the industry to provide a low-cost 
electrostatic air filter having enhanced filtration capabilities. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an air filter is provided which 
substantially eliminates or prevents the disadvantages and problems 
associated with prior electrostatic air filters. 
In the first embodiment of the present invention, the air filter comprises 
two layers of woven electrostatic material, typically polypropylene, which 
are disposed adjacent each other. The first layer of electrostatic 
material is such that a positive charge will develop in response to air 
flow therethrough, while the second layer of electrostatic material is 
chosen such that a negative charge will develop in response of air flow 
therethrough. This filter has been found to have superior air filtering 
characteristics. 
In a second aspect of the present invention, two pairs of electrostatic 
woven material are provided, each pair having a positively charged 
electrostatic material and a negatively charged electrostatic material. 
The pairs of electrostatic filter layers are separated by an open-cell 
foam material, such as polyurethane, which also has electrostatic 
properties. Typically, the polyurethane has a thickness of approximately 
one-quarter of an inch. This embodiment of air filter of the present 
invention has been found to offer improved filtering characteristics. 
In a third embodiment of the present invention, metal grids are placed 
before and after the electrostatic filtering material. It is believed the 
metal grids provide a grounding plane which increases the electrostatic 
effect, thereby increasing the filtering efficiency.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment of the present invention is best understood by 
referring to FIGS. 1-2 of the drawings, like numerals being used for like 
and corresponding parts of the various drawings. 
FIG. 1 illustrates a perspective view of an air filter 10, with a cut-away 
view of the various layers thereof. The air filter 10 comprises an outer 
frame 12 having a top portion 14 attached thereto by screws 16 secured in 
an integral screw boss 18. The frame 12 contains a plurality of filtering 
elements. A first wire grid 20 is placed in front of a first layer of 
woven electrostatic material 22. A second layer of woven electrostatic 
material 24 is placed between the first layer of woven electrostatic 
material 22 and a layer of open-cell foam 26. Third and fourth layers of 
woven electrostatic material 28 and 30 are placed behind the open-cell 
foam layer 26, and are held within the frame by a second wire grid 32. 
In the preferred embodiment, the first layer of electrostatic material 22 
comprises a material which will develop a positive electrostatic charge in 
response to air flow therethrough (hereinafter a "positive electrostatic 
material"). The second layer 24 comprises a negative electrostatic 
material, the third layer 28 comprises a positive electrostatic material 
and the fourth layer 30 comprises a negative electrostatic material. 
Typically, the layers of woven electrostatic material 22, 24, 28 and 30 
are formed from interwoven fibers of polypropylene. Sheets of the 
polypropylene material, both positive and negative, can be obtained under 
the tradename LUMITE. Other materials exhibiting electrostatic properties 
are listed in W. Orman and H. Endress, "Self-Charging Electrostatic Air 
Filters," Heating, Piping and Air Conditioning, Jan. 1952. 
The open-cell foam layer 26 has a preferable thickness of approximately 
one-quarter to three-eighths of an inch. The open-cell material will 
acquire an electrostatic charge in response to the air flow therethrough, 
and it is believed that the charge developed across the open-cell foam 
layer 26 enhances the polarity difference between the two pairs of woven 
electrostatic material layers 22 and 24 and 28 and 30. By providing a 
relatively thick separation, the foam layer 26 also prevents accumulated 
dust from discharging the electric field between the two pairs of woven 
material. Retaining the electrostatic charge throughout the filter 
maintenance cycle increases the dust holding capacity of the filter. In 
addition to its electrostatic filtering properties, the open-cell foam 
layer 26 acts as a conventional impingement type filter and also diffuses 
the dust particles in the air stream to create vertical as well as 
horizontal movement of the particles, thereby increasing the efficiency of 
woven layers 28 and 30. 
The wire grids 20 and 32 serve two purposes. First, the wire grids serve to 
retain the filtering layers 22, 24, 28 and 30 within the outer frame 12. 
Second, test have shown that the wire grids improve the filtering 
capabilities of the air filter 10. It is believed that the wire grids 20 
and 32 act as grounding planes which enhance the electrostatic effects of 
the filtering layers 22, 24, 28 and 30. In the preferred embodiment, the 
wire grids 20 and 32 comprises painted metal wires of approximately 1/20- 
1/16 inches in diameter, arranged in a grid. The spacing between parallel 
wires is approximately one-half of an inch. It is believed that other 
configurations of wire will also serve to be an effective grounding plane. 
The filter of the present invention has been proven in accordance with 
ASHRAE 52-76 test procedures to exceed the performance of a commercial 
embodiment of the filter of U.S. Pat. No. 4,115,082. The prior art filter 
has been tested as having a resistivity of 0.17 wg and filtering 
efficiency of 85% (percentage of particles removed) in Air Filter Testing 
Report #4371. By contrast, the filter of the present invention has been 
tested to have a resistance of 0.15 wg and a filter efficiency of 93% as 
evidenced by Air Filter Testing Report #4792, dated May 25, 1988. Hence, 
the filter of the present invention provides significant filtering 
capabilities of that of the prior art. 
Furthermore, the air filter 10 can be easily and inexpensively assembled 
and disassembled. Cleaning is accomplished merely by forcing water through 
the filter elements using an ordinary water hose. All filter elements are 
extremely durable and will therefore require no maintenance by the user 
other than routine cleaning. 
Although a preferred embodiment of the present invention has been described 
in detail, it should be understood that various changes, substitutions and 
alterations can be made therein without departing from the spirit and 
scope of the invention as defined by the appended claims.