By Robert P. Scaringe, Ph.D., P.E. and John Meyer, M.S.

Mainstream Engineering Corporation
Rockledge, Florida 32955

The presence of indoor air particulates is a concern, especially for those people afflicted with breathing or lung disorders such as asthma. These diseases are characterized by an excessive respiratory sensitivity to various stimuli of which a major trigger is particulate matter in household air. Sources of these particulates are varied, but a major fraction consists of cigarette smoke, pollen, mold, human/animal dander, feather particles and dust.

As air is circulated through the house via heating, ventilating, and/or air conditioning (HVAC) systems, particulate matter will accumulate inside the system, especially in cooling systems, where they serve as medium for bacteria and fungal growth. The dispersion of microbes such as bacteria, virus, mold, and fungus can be the source of sickness to exposed occupants in the climate-controlled area. For example, Legionella pneumophilia has been found to exist in such an environment and has been linked to Legionnaire's disease. Other microbes can contribute to "sick home" or "sick building" syndrome. Many people are also allergic to the molds and fungus entrained in the dwelling's ventilation as the air passes over contaminated condensate drain water and wet evaporator cooling coils.

The process of reducing indoor air particulates is accomplished by a filter which permits air to pass through the porous, typically fiber-like, material that essentially blocks the path and captures the particles before they enter the heating, ventilating, and air conditioning (HVAC) system. Since the pores between fibers are typically much larger than the airborne particles, the filter relies on the random chance that the particle will impact a fiber. If the thickness of the filter is increased, or the pores made smaller (through the use of a tighter fiber weave), filter performance does increase. However, because the resistance to the airflow has increased, the pressure drop across the filter has increased resulting in reduced airflow. This reduced airflow translates into reduced system efficiency, increased strain on the system, and increased energy consumption. Clearly, this is not the best method of improving air quality.

One method of increasing particulate removal efficiency without decreasing pore size fiber density is electrostatic attraction. Active electrostatic filters impart a high voltage charge between plates, and any charged particles passing through are electrostatically withdrawn from the passing gases, and captured on the charged surface (plates). This type of electrostatic system works very well. However, the cost to install and maintain such systems, as well as the safety issues, make them impractical for typical residential and small commercial applications.

To alleviate the need for an applied voltage but still obtain the advantages of electrostatic dust removal, passive electrostatic systems have been developed. A passive electrostatic system relies on electrostatic charges produced from air friction as the air is drawn through the filter. As taught by Savell, in United States Patent 5,336,299, air passing through dielectric fibers will generate friction that induces a static charge that builds up to become substantial enough to draw out any passing charged particles, such as household dust. Yanagawa also uses this passive electrostatic charge approach in U. S. Patent 4,702,752, Hodge in U.S. Patent 5,690,719 and Yanagawa in U.S. Patent 4,944,778 where dielectric fibers are used to capture dust and enhance filter performance.

The passive electrostatic filter behaves like active electrostatic filters, in that dust and other contaminates are drawn from the air and adsorbed to the fibers by electrostatic forces. These filters perform much better than normal filters, which as we have said, simply rely on the random chance that a contaminate will hit a filter fiber. With the electrostatic filter, contaminates are now drawn from the air and to the filter surface by these electrostatic forces. Just like static cling, which makes clothes stick together when you take them from the dryer, electrostatic forces drive the airborne contaminates to the electrostatic filter.

Unfortunately, electrostatic filters typically cost much more that ordinary filters, and so are made re-usable to minimize this cost impact. These filters which range in cost from $30 to $100.00 must be washed and reused. This is not only a dirty and inconvenient process, it is also a difficult process. Because of the same electrostatic forces, it is very difficult to completely clean these filters. That means filter performance degrades after the initial use.

Realizing this, a few manufacturers have begun to mix electrostatic fibers with traditional fibers to produce a disposable filter with some electrostatic benefit. While these filters are disposable, the full electrostatic potential is not achievable, since not all of the filter surface is electrostatic.

Rather than fabricate a costly passive electrostatic filter from electrostatic filter material, the unique patent-pending PuraCleanÔ formulation creates a passive low-cost electrostatic filter surface on any ordinary low-cost non-electrostatic disposable filter. This provides all the performance benefits of a 100% passive electrostatic filter, without the problems of filter cleaning and reuse. An ordinary low cost filter is transformed into a highly efficient electrostatic filter. Because the cost is low, this filter can simply be discarded when dirty.

There is no other product on the market like Puraclean, and we did not start out to develop a product to improve indoor air quality. Rather, PuraClean was discovered somewhat by accident, during similar but related indirect research to support both the U.S. Army Medical Core and NASA.

Mainstream engineers and scientists were looking for a means to improve the air quality in future NASA spacecraft, and later we expanded this investigation to include potential methods to remove chemical warfare agents from the soldier's environment.

What we ended up with was a patent-pending liquid formulation which after application to an ordinary non-electrostatic filter (such as a metallic filter, disposable spun-glass filter, or foam filter) will produce a dielectric filter surface and hence, turn an ordinary filter into a passive electrostatic filter.

It is well known that ordinary (and widely used), fiber-filters installed in heating or air conditioning systems only remove about 20% of the large dust particles, meaning 80% of these large contaminates pass directly through the filter. While a denser filter would improve filtration, the resulting increased pressure drop across the filter would restrict the airflow, which in turn causes a decrease in system efficiency and a decrease in air circulation.

It is also well know that these ordinary, coarse, fiber- filters are ineffective at removing smaller, respirable particles such as smoke particles and pollen, which are a major source of discomfort for people with respiratory conditions, such as asthma.

Most scientists would further agree that passive and active electrostatic filters improve the performance. However a more measurable means of determining the improvement in performance is needed. As a consumer you should be asking, how much better is one filter compared to the next, and how do these filters affect system air circulation and thus efficiency. The American Society of Heating Refrigerating and Air-conditioning Engineers, Inc., which is abbreviated as ASHRAE, is an engineering society that develops and continually updates tests standards for most aspects of heating and cooling equipment. When you read about the efficiency or measured performance of a certain air conditioner, furnace, or refrigerator, the test method to determine this performance was most likely developed by ASHRAE.

ASHRAE has a standard test method for determining the performance of air filters, referred to as ASHRAE 52.1 and titled "Gravimetric and Dust Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter". Among other things, this test method looks at the ability of the filter to capture contaminates, and the pressure drop, that is the resistance to airflow of the filter.

There are approved laboratories, throughout the United States that can perform this ASHRAE 52.1 test method on air filters. One such laboratory, Research Triangle Institute (RTI), located in Research Triangle Park, North Carolina, was contracted by Mainstream to perform the ASHARE 52.1 filter tests on ordinary course filters, treated with the PuraClean spray, and untreated. Their results were reported in RTI Report No. AX10080101, and summarized briefly below.

Figure 1 contains a bar graph of the measured filtration efficiency of the treated and untreated filter when exposed to 3 through 8-micrometer particles in the ASHRAE test performed by RTI. This bar graph has been created from data obtained from Figure 1 in RTI report No. AX10080101. It is significant to point out that the untreated filter has a zero filter efficiency at removing 8-micrometer particles and for all sizes. The PuraClean treated filter provided a substantial improvement in filtration efficiency, ranging from a 200% improvement in filtration efficiency when exposed to 3 micrometers particles, to more than a 1,200% improvement for 7 micrometer particle sized contaminates. Test results also indicated no significantly measurable difference in resistance to airflow.

The author's acknowledge the support of the United States Army Medical Command and the National Aeronautic and Space Administration for directly and indirectly supporting Mainstream's R&D research in technology areas which have indirectly fostered this unique development.