Brownian motion, also known as diffusion is the random movement of the aerosol in the gas stream caused by the collision of aerosols with atoms of the gas on a molecular scale. In this, a particle of 1 micron diameter has 15 times the motion energy compared with a particle with a diameter of 5 micron. This means in practice that, the smaller the particle, the higher the probability that this particle will collide with a filter element fibre and thus be filtered out. Filter elements making use of Brownian motion achieve an improved filtration performance in practice as far as the smallest particle sizes to be separated are concerned. For separating particles below 0.3 micron, filter elements are manufactured which filter out more than 99.99% of all particles. Oil is stored in the fibre depth, the liquid particles add up to form droplets which flow as a cohesive film of liquid along the fibres to the lowest point of the filter. Suitable gas velocities prevent the separated liquid particles from following the airflow and being re-entrained.
Figure 4.5.1.1
Because of the Brownian motion, the aerosol particles do not follow the flow lines of the gases around the fibre. They deviate from the zone of these flow lines in the direction of the fibre surface, where they are deposited. The effect of Brownian motion grows with diminishing particle size and, with this, diffusion separation of smaller particles increase. The diffusion effect of particle deposition applies particularly to particles smaller than 1 micron. It is caused by their motion. As a result, some particles adhere upon contact with the fibre. The concentration gradient created causes diffusion flow in the direction of the fibre surface. This effect depends mainly on the particle size Dt and the gas flow velocity wo. The influence of temperature also affects the dynamic viscosity of the air. With small and very small aerosol particles, the diffusion coefficient is a function of the mean free path of the air molecules, which is inversely proportional to the air pressure.
