Aerosol Science: Technology and Applications

Chapter 5

Filtration Mechanisms

Sarah Dunnett

Department of Aeronautical and Automotive Engineering, Loughborough University, UK

5.1 Introduction

Filtration may be defined as a process of separating particles from a gas (usually air) flow by means of a porous medium. The needs for such a process are many; for example, in many working environments people are exposed to harmful particles, and so protection must be provided. This might be in the form of a large-scale industrial filter or of personal protective equipment such as respirator filters. Some industries, including parts of the pharmaceutical and electronic sectors, rely on clean air of exceptional quality, which is provided through filtration. This is also true of the medical sector, where the control of infection is crucial. Filtration can also improve the quality of air in homes, workplaces, schools and transport.

There are two principal types of filtration: surface filtration and depth filtration. In surface filtration, particles are collected on a permeable surface; as the deposit builds, it forms a cake on the filter, which increases the resistance to flow. Depth filtration removes particles both from the surface and throughout the depth of the medium. One of the most important types of depth filter is one made up of fibres. Such filters consist of pads of fibres in an open three-dimensional network. Porosity is high, generally from 70 to 99%. An example of such a filter is shown in Figure 5.1. Because of their high ...