Differentiating Filter Media by Capillary Flow Porometry
Mechanical filters are devices that use porous media, such as paper, foams, synthetic fibers, cotton, or spun fiberglass, to remove solid particles and other contaminants from fluid streams. The Porometer 3G series of instruments, utilizing the technique of capillary flow porometry, can measure and analyze the through-pore characteristics of materials used in different applications. This application report compares and contrasts the results for two similar looking filters used to remove contaminants from an in-house air source.
Filters and membranes are devices which remove particulates from a continuous stream of either a gas or liquid. Important applications requiring air purification are ventilation systems, engine air intakes, air compressors, gas turbines and vacuum cleaners. Importantly, cleanrooms that manufacture the latest electronics also require nearly particle-free air for material fabrication. The four main materials used in mechanical air filter media include paper, foam, synthetic polymers, and cotton. Some buildings, as well as aircraft and other man-made environments (e.g., satellites and space shuttles) use foam, pleated paper, or spun fiberglass filter elements to keep people or individual components protected from unwanted contaminants. In addition, some filters employ a static electric charge to attract dust particles to the filter.
Polyester and glass fibers are also commonly used to make air filters. Both materials have high temperature ratings near 120 °C, and are widely used in commercial, industrial and residential applications. In some cases polypropylene is used to enhance chemical resistance, but has a lower temperature tolerance. These materials can also be blended with cotton or other synthetic fibers to produce a wider range of performance characteristics. Tiny synthetic fibers known as micro fibers are also used in many types of HEPA (High Efficiency Particulate Air) filters.
The efficiency of air filters is normally reported as Minimum Efficiency Reporting Value (MERV) - a measure of the efficiency with which particulate filters remove particles of a specified size from an air stream (Fig 1 and Appendix). The higher the MERV number, the better the removal efficiency, particularly of smaller particles. MERV levels 1 through 16 are determined using the American National Standards Institute / American Society of Heating, Refrigerating and Air Conditioning Engineers (ANSI / ASHRAE) Standard 52.2-2017 test method. However, this does not address HEPA filters or Ultra Low Penetration Air (ULPA) filters (MERV 17 – 20). Instead, HEPA / ULPA filters are assigned MERVs based on their performance in accordance with standards published by the Institute of Environmental Sciences and Technology (IEST) [1, 2].
Both types of test are known as “challenge” tests which require standard dust particles of a known size in which the filters removal efficiency can be tested. However, these tests do not truly measure pore size but their abilities to remove certain particulates. To truly know a material’s ability to filter particles over a given size range, the through-pore size distribution should be determined. This can be done quickly and automatically by capillary flow porometry experiments.
1. ASHRAE 52.2-2017 Standard 52.2-2017 - Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size
2. ISO 14644:2015 Cleanrooms and associated controlled environments - Part 1: Classification of air cleanliness by particle concentration
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