Gas separation: apparatus – Electric field separation apparatus – Including gas flow distribution means
Reexamination Certificate
1999-12-17
2001-11-27
Chiesa, Richard L. (Department: 1724)
Gas separation: apparatus
Electric field separation apparatus
Including gas flow distribution means
C055S486000, C055S521000, C055SDIG005, C095S057000, C096S069000, C264S258000, C264SDIG004
Reexamination Certificate
active
06322615
ABSTRACT:
TECHNICAL BACKGROUND
The present invention relates to filters cleanable by washing or vacuuming or the like for inlet air heating and air conditioning systems used in residential or commercial buildings and, more particularly, this invention relates to an electrostatic filter for such systems having an extended service life.
BACKGROUND ART
Inlet air filters for heating and air conditioning systems have been in use for decades. The inlet air or primary filter was originally intended to protect the heating coils and mechanical devices such as fans from damage by airborne particles. As the harmful effect of inhaled particles on human health became known, inlet air filters were designed remove this particulate matter from the air.
The most popular prior art configuration is a thin, rectangular, disposable filter. The filter contains fiberglass, animal hair, fibrous foam or polymeric media or aluminum mesh encased in a cardboard or plastic frame. Prior art filters can comprised polyester panels, urethane foam or latex coated animal hair. Recently, prior art filters containing static or passive electrostatic media have become available.
Most residential resistance specifications require air filters to have an initial pressure drop of no more than 0.22 inches of water for an air flow rate of 300 feet per minute. The dust spot efficiency for typical prior art air filters tested using ASHRAE 52.1-1992 is 20% or less. The ASHRAE efficiencies of four types of commercially available filters follows:
TABLE 1
Area of Media
Filter Type
Square Feet
ASHRAE EFFICIENCY
Fiberglass Throwaway
4.0<
20%
Electrostatic (Passive)
4.0
20%
Electrostatic (Fibers)
Up to 8.9
30%
Pleated Polyester Blend
Up to 6.9
25%
When these filters are first placed in use across an air stream, they have a very low filtering efficiency. Typically, the exhaust or dust spot efficiency is about 8%. However, as dust particles are collected, the dust collection efficiency can increase to approximately 20%. At this point, the filter is ready to be replaced. Prior art filters are usually difficult to clean since the dust particles become embedded in the media.
Another current concern is the recognition that particles below 10 microns are not filtered by the cilia hairs in the nose and are therefore inhaled into a human lung. Fibrous particles such as asbestos and fiberglass are known to cause respiratory diseases. Most current inlet air filters for heating and air conditioning systems are not very efficient in capturing these small particles. Filters containing layers of electrostatic media perform better within this range of particles but these filters also become clogged. Since they cannot be efficiently cleaned they must be replaced.
The Ashrae 52.0 standard test is concerned with a resistance and dust sport efficiency. This test is concerned with quantity not the quality of the dust captured. This testing method was satisfactory when the emphasis was on protecting the system components. Ashrae standard testing method 52.2 entitled “Method of Testing General Ventilation Air Cleaning Devices for Removal by Particle Size” was developed to certify the ability of ventilation filters to remove particles below 10 microns from inlet air. The filtration efficiency of ventilation air filters used in residential and commercial buildup is measured in MERV's (Minimum Efficiency Rating Value). The U.S. Environmental Protection Agency (EPA) operates six laboratories throughout the U.S. that evaluates and certifies performance of ventilation filters according to Ashrae 52.2 and other testing methods.
The ventilation filters disclosed in pending application Ser. No. 09/155,059 modified the angle of approach of the air stream to effect the particle capture efficiency. This increased the life of the filter between cleaning and eased the difficulty of cleaning. The filter had a non-perpendicular path of air flow resulting from the corrugated or pleated configuration of the novel air filter. Because of the corrugated configuration, the filter contained more surface area than a standard, flat, prior art air filter. The increased surface area of the novel filter also results in a lower pressure drop than experienced with prior art electrostatic air filters. The filter has significantly longer life between cleanings and it is very easy to clean. The efficiency for removing particulate matter is significantly improved over prior ventilation filters.
The improved filter includes an upstream passive electrostatic layer disposed adjacent a core or central filter media layer forming an assembly. The assembly is corrugated or pleated to increase filter surface area as compared to a flat surfaced air filter.
The core or central layer is a bendable mechanical filter media material, from 0.01 to 0.45 inches in thickness selected from polymeric foam or a fibrous polymeric material such as air laid polyester fibers having a density of from 0.5 to 5 ounces per square yard (OPSY). The fibers are resin bonded by a resin such as an acrylic resin or point bonded or needle bonded with electrostically charged fibers. In a preferred embodiment the central or core layer is comprised of a thermoset material. This material removes most of the particles from the air stream, typically up to 50 to 85% of the incoming particles at 3 to 10 microns.
From a morphological standpoint, pollens differ in size, form, moisture content, weight and their content of specific antigens. The size of the pollen grains range from 2-3 microns up to 250 microns and in the majority of allergenic plants is between 20 and 45 microns.
The particle capture efficiency of the core material is enhanced by placing a layer of electrostatic polymer material on each side of the core material. The material can be woven or non woven. More preferably, passive electrostatic materials are used which become charged as air flows past the polymers that tend to have natural static charges. These are preferred since the available active electrostatic fibrous materials produce an undesirably high pressure drop as a filtration media and are not cleanable. The passive material is usually formed by casting, extrusion or weaving from a polymer of a monosaturated alkene monomer having 2-8 carbon atoms. One type of material is woven polypropylene netting having a thickness from 0.01 to 0.20 inches, usually from 0.08 to 0.18 inches. The netting can be woven in a manner to resemble a honeycomb structure.
The outside grill layer does not contribute to filtering but is present to hold the angled media after it has been formed. Again, the layer must be capable of being easily bent or deformed during the assembly process. If the sheet of material is too thick it is difficult to bend and if it is too thin, it is not capable of holding the multi-layer media in its bent shape. The grill can be formed of a high strength, rigid polymer or of metal. Expanded metal such as steel or aluminum having a thickness from 0.010 to 0.25 inches, performs satisfactorily.
Corrugated filters that were first prepared had a MERV efficiency of about 6. Though this was a substantial improvement over the commercially available electrostatic filters, the increasing awareness of the harmful effects of inhaled particles in the less than 10 micron range demanded that ventilation filters have MERV efficiencies over 6.
STATEMENT OF THE INVENTION
It has been discovered in accordance with this invention that the efficiency of corrugated, electrostatic ventilation filters for small particles can be increased by increasing the mechanical particle capturing capability of the central media. This in combination with increased surface area provided by higher density of corrugations and smaller angles between corrugations provides the desired particle capture efficiency. The porosity of the passive electrostatic netting can be increased to reduce the overall initial resistance of the corrugated media to air flow. Another aspect of the invention relates to the ability to achieve the higher density and lower angle corrugations. In the method of the invention,
Chiesa Richard L.
Jacobs Marvin E.
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