Among the many types of solid particle filtration systems available to industry in general, there are two main categories that comprise the vast majority of equipment supplied for this purpose: 'dry' and 'wet' filtration units.
We will examine the distinguishing features, limitations and application of each type of technology to provide an overview of the main considerations to take into account when making a choice.
Fundamentals of separation with dry filtration units
These types of units typically utilise elements made of permeable filter media with varying degrees of efficiency and finish to separate solid particles from a gas stream using the four main components of membrane filtration mechanics: impact, interception, diffusion and electrostatic attraction. Usually, dry filtration units used in industrial applications use an automatic cleaning method to remove the filtered particles from the medium (dynamic filters), but this is not always the case: some units are designed to be simple barriers without the need for medium regeneration (static filters).
Inertial impact occurs when the gas flow passes around the filter fibre, but inertia causes a dust particle to come into contact with the fibre
Interception occurs when a dust particle follows the gas path and comes into contact with a filter fibre
Diffusion is characterised by the random movement of a dust particle coming into contact with a filter fibre
Electrostatic attraction causes a particle to be drawn into contact with a filter fibre
High humidity levels in the gaseous state, i.e. the relative humidity of the air passing through the filter, are not necessarily a problem, as long as the temperature differences between the exhaust air and the filter unit are taken into account so that the air does not cool down to the 'dew point', where condensation into liquid occurs. This problem is usually solved by adding a heating system and/or thermal insulation to minimise dew point problems.
Other considerations regarding the application of dry filters concern cases where potential ignition sources are introduced into the system by the process itself, e.g. metal grinding producing sparks, and the limitations of protection when handling highly explosive and volatile dust.
Most dry filter media are flammable and the introduction of ignition sources may pose a fire risk unless some means of protection is implemented, such as spark extinguishing or similar. If the extracted powder is explosive, the risks are even greater if ignition sources are known to be present, since there is a potential mixture of fuel and a coexisting ignition source. Even with adequate explosion protection in line with ATEX regulations, this type of application may pose a sufficiently high risk that the use of a dry filter is not recommended.
A limiting factor of dry filters is also the maximum level of explosion protection available. In terms of explosiveness, the vast majority of particles handled in industry are well within the capabilities of a dry filter system, however care must be taken
Fundamentals of separation with wet filtration units
A wet filter unit typically separates solid particles from a gas stream by passing it through an atomised liquid, so that the particles are encapsulated by droplets and subsequently immersed in a tank where sedimentation takes place. Since the separation method is provided by a liquid that is recovered and reused within the unit, there is no need for replaceable filter elements like those found in a dry unit.
