Early Concepts
The development of filters was driven primarily by the need to protect workers dealing with dust and smoke, the demand for medical respirators, and protection in extreme conditions. Thus, the history of filter development can be traced to ancient times. One of the earliest documented mentions of filters dates back to the records of Pliny the Elder around 50 c.e. (Davies 1948). The works of Julius Pollux (c.e. 150) refer to the problem of industrial dust in the description of working conditions of Egyptian miners. According to these accounts, the protection was ensured via filters made of loose bladders. Works of Leonardo da Vinci contain descriptions of wet fabric used for protection against smoke in warfare. According to Agricola, the issue of dust in mines was addressed using basic respirators and ventilation (Davies 1948). The earliest reference to adverse health effects of particulate matter was the description of dust disease in the work of Paracelsus. Later records contain detailed descriptions of protective measures from rock dust as well as lists of professions dealing with the risk of exposure to particles. Nevertheless, the earliest description of a respiration device can be found in the early nineteenth century’s document. The device in question is a compartment with cotton filament and a breathing tube.
The need for medical respirators became apparent in the second half of the nineteenth century after the link between putrefaction and airborne organisms was established by Pasteur. As a result, the concept of a cotton respirator was significantly improved by the early twentieth century. At the same time, the use of protective masks for firefighters has become commonplace in several European countries. The filter in the mask was filled with cotton. At this time, the first qualitative performance tests were performed by John Tyndall (Feldhaus 1929).
Filter Types
In 1930, an important design update was introduced by N. Hansen in the form of powdered colophony resin added to the filter padding, resulting in a significant performance boost. Importantly, the modification made possible the electric insulation characteristic for electret filters. For a certain time, Hansen’s filters were considered the most advanced means of protection from smoke. Several countries have adopted the solution for use in warfare.
Several types of fibrous materials were used as filters during World War II, including felt, paper, and soft pads. Empirical tests have demonstrated that asbestos fibers were particularly effective filtration materials (SpurnĂ˝ 1965). As a result, asbestos-containing filters were used in a wide range of filtering devices, including gas masks, respirators, ventilation system filters, and filters for cleaning various liquids, among others. Once it became apparent that asbestos was highly carcinogenic, it was substituted by other sources of fibers, such as ceramics, glass, organic materials, and carbon. Currently, glass and polymer fibers are utilized in the majority of commercially available filters.
Various fibrous structures were included in filters to separate fine particles in gaseous and liquid substances. This type of filters is widely used for analytical purposes, such as sampling of dust for chemical and physical analysis. Currently, the most common porous filters are based on colloidal and silver membranes, which provide highly accurate results both for single-particle and bulk chemical analyses. An early example of porous filters is a cellulose nitrate membrane introduced in the late nineteenth century and successfully used in colloid chemistry studies in the 1950s (Gelman 1965; Kruse 1948).
Metal membranes with uniform porosity were developed in 1964 by Selas Corporation (Mateson 1987). The technology made filters usable in a wide range of temperatures and ensured electric conductivity. Simultaneously, polycarbonate filters were introduced by an unrelated research group. The cylindrical structure of pores allowed for high selectivity of particles, offering new opportunities for analysis via electron microscopic methods (Kaufmann 1936).
Early Theories
The basic approach to fine particle movement was laid out by Robert Brown, who observed the interaction of particles in liquid currently known as Brownian motion. The concept was applied to gaseous substances by Bodaszewsky as collision and adhesion of particles (Bodaszewsky 1881). Fibers of a typical filter occupy a relatively large area, leading to a high number of collisions. It is important to note that diffusion effect-based filtration is relevant only for relatively small particles (e.g. those smaller than 0.2 µm) which ensure frequent collisions with surfaces.
The understanding of mechanisms responsible for filtration was further improved in the early twentieth century. Specifically, it was demonstrated that particles with a radius between 0.1 and 0.2 µm were more likely to penetrate filtering membranes whereas larger and smaller ones could be filtered relatively effectively (Engelhard 1925; Freundlich 1921). The reasons behind the removal of larger particles were determined after the formulation of Stokes’ law. Nevertheless, the basic principles were laid out by Albrecht. According to the researcher, greater size of larger particles became captured in cylindrical surfaces due to their inability to follow airflow curves (Sell 1931). The theory was further refined by the introduction of concepts of direct interception and mathematical model accounting for viscous flow of fluids (Langmuir 1962). Eventually, the concepts were combined into a unified theory that incorporated the mechanisms of direct interception and particle diffusion (Davies & Peetz 1956; Friedlander 1958; Yoshioka et al. 1969).
At this point, it is important to consider the phenomenon of a change in pressure throughout a filter at a given airflow rate known as filter resistance. Early theoretical approaches estimated filter resistance using the Darcy law, which did not provide sufficiently accurate results. In response, a more suitable measurement was proposed based on the Knudsen number. As a result, the majority of subsequent theories accounted for the filter’s bulk structure by incorporating various geometrical models. Another important improvement was made in the second half of the twentieth century with the solution of an equation that could be applied to flow of particles in parallel cylinders.
Recent History
One of the key changes in the development of particle filtration theory was the recognition of significance of particles’ internal disposition in fibrous filters. The result of experiments suggested that existing filters were largely unsuitable for the study of filtration mechanisms in fluid mechanical theories. In response, model filters were utilized by the researchers. This approach was eventually used to experimentally determine the maximum size of a particle capable of penetrating a fiber filter (Dyment 1970; Whitby 1965). At the same time, partial separation mechanisms and their combinations were identified. The obtained knowledge was used to complete the theoretical understanding of the mechanisms responsible for a direct interception (Fuchs & Stechkina 1963; Kirsch & Fuchs 1967; Stechkina & Fuchs 1966). Based on these findings, a fan filter model was developed, in which successive equidistant layers are rotated at random angles (Kirsch & Fuchs 1968). Unfortunately, despite significant progress made in this direction, the structure of a model filter contained non-negligible differences from the real-world model due to the existence of irregularities in the dispersion of fibers.
Another important property of aerosol particles is their ability to be electrically charged. A particle with an electric charge is attracted to an uncharged element of a filter, increasing its effectiveness. The effect was described and demonstrated experimentally in the late twentieth century. The application of an electric field to the filter results in the polarization of both the fibers and the particles attached to them, enhancing the filtering potential. These findings were used to develop commercially available electrically charged filters.
The accumulation of particles in filters leads to significant changes in their characteristics. This process, known as filter clogging, introduces a time-dependent variable into the calculations of statistics of a clean filter. This phenomenon was described and confirmed in a series of experiments by a number of researchers.
Testing Methods
The earliest method for testing filters involved impinger sampling and microscopic particle quantification (Michaelis 1890). Light-scattering methods were used to evaluate the penetrated concentration of particles (Fieldner et al. 1919). The approach was enhanced by the addition of a carbon smoke penetrometer until substituted by a more accurate methylene blue test in the first half of the twentieth century (Gucker et al. 1947; Walton 1940). Subsequent improvements included the use of an optical particle counter and the utilization of automation and computer-driven analysis. As can be seen, the core principles and underlying philosophy remained largely unaltered.
As can be seen, the main principles of particle filter construction and operation were established in the late nineteenth and the early twentieth century. The technology was improved mainly through the use of automation and the utilization of more accurate testing procedures. At the same time, a wide range of enhancements was made in the field of fibrous and porous materials used in filters. Finally, the use of mathematical models was allowed to obtain a sufficiently accurate estimation of outcomes pertinent to different filter models.
Reference List
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