Filter media are characterised by many different chemical and mechanical properties, and the right combination can usually be found for most applications. Purchas (1980) identified some 20 significant properties divided into three major categories: (a) machine-orientated properties (Table 8.2), (b) application-orientated properties (Table 8.3) and (c) filtration-specific properties (Table 8.4). Although all are important one way or another, the machine-orientated properties are often considered in discussion with the media supplier and the user may not have access to meaningful information on these properties.
In order to improve the separation characteristics of finer and colloidal suspensions, primary particle size can be increased by some form of aggregation process where the addition of a chemical agent alters either surface properties the surface properties of the particles, the properties of the suspending liquid or the manner in which the solids and liquid interact.
A layer of fluid lines the individual alveoli and is in contact with their contained gases. One of the physical properties exhibited by an air-liquid mixture is surface tension. This is a force which acts to reduce the area of gas-liquid contact to a minimum. Thus, air bubbles collapse in water because of the surface force developed by the fluid. However, bubbles can be maintained in water to which soap has been added, since the soap diminishes this active surface force.
Filter media are characterised by many different chemical and mechanical properties, and the right combination can usually be found for most applications. Purchas (1980) identified some 20 significant properties divided into three major categories: (a) machine-orientated properties (Table 8.2), (b) application-orientated properties (Table 8.3) and (c) filtration-specific properties (Table 8.4). Although all are important one way or another, the machine-orientated properties are often considered in discussion with the media supplier and the user may not have access to meaningful information on these properties.