Fundamentals of air filtration

Particle separation

The most common method of separating particles from gas streams in HVAC systems and in industrial applications is through fibre filters. Other processes for particle separation such as cyclones, scrubbers or electrostatic precipitators are generally more complex and are therefore only used in specific industrial areas. The following section examines fibre filtration in detail.

The efficiency of a filter for separating particles is usually described using separation efficiency, also referred to as fractional efficiency. This is defined as the ratio of the number of particles of a particular size that have been deposited in the filter to the total number of particles of this size upstream of the filter. Filter efficiency can be defined both in terms of quantity and mass (the mass of arrested dust in relation to the total dust mass fed to the filter is named gravimetric arrestance). When specifying filter performance, it is therefore important to always be sure about the particle size on which the data is based and whether it is defined in terms of quantity or mass. Values can only be compared that have been measured according to the same standard. This is because different standards are usually based on different measurement methods and are therefore not directly comparable.

Typically, in building ventilation and HVAC applications, fibre filter media is used, where the average pore size is significantly larger than the size of particle to be arrested. Particles can enter the filter medium and could pass through it, if they could follow the air streamlines perfectly. Since this is not the case, there is a certain probability that on their way through the filter media, particles will hit a fibre, where they will be deposited and remain.

These are the mechanisms that lead a particle to hit a fiber. The child mechanisms:

Interception: This principle means that the path on which a particle’s center of gravity moves passes the fiber at a distance of less than half the particle diameter. The particle therefore hits the fibre and is deposited there. The probability of a particle hitting a fibre due to interception increases with the particle size. Interception dominates arrestance for particles with diameters between 0.5 and 1 μm.

Inertia: Due to inertia forces, particles cannot fully follow the air streamlines that flows around a fibre. Instead, they hit the fibres at a certain proximity on a less curved path. The importance of inertia for particle collection increases with increasing particle mass (i.e. particle diameter) and increasing particle velocity. In the case of the typical air velocity in air filtration, the inertia effect becomes dominant from a particle diameter of › 1 μm.

Diffusion: Due to the irregular thermal movement known as Brownian motion, the particles oscillate. This means that some very small particles that would otherwise pass a fibre hit them and are deposited there. Diffusion-based particle collection increases with decreasing particle size and decreasing air velocity. Assuming there is no predominant electrostatic interaction, nanoparticles (i.e. particles with a diameter of ‹ 100 nm) are deposited almost exclusively by diffusion.

Electrostatics: Electrostatic interaction causes particles to be attracted to the fibres. If the particles and fibres have opposite electrostatic charges, they will attract each other. However, if only the fibre or particle is electrostatically charged, it is also sufficient to polarize the respective counterpart to generate a force of attraction.

Electrostatic interaction-based particle collection decreases with increasing air velocity. In industrial air filtration, this effect is used in electret media, in which the fibres are selectively electrostatically charged in the production process. Because electrostatic fields increase the filtration efficiency of filter media without increasing flow resistance, and hence pressure drop, such filter media are particularly energy-efficient. However, there is a possibility that under certain conditions (e.g. very high humidity or a very high proportion of submicron particles in the air to be filtered) the electrostatic charge could be reduced during the filtration operation. This could ultimately lead to a decrease in the filtration efficiency of the filter.

It is therefore necessary to ensure a certain minimum filtration efficiency, based purely on mechanical particle collection mechanisms and which remains effective even after complete removal of all electrostatic charges. It is important to find an optimum balance between energy-efficient electrostatic particle collection and purely mechanical collection. In practice, however, any loss of filtration efficiency caused by the reduction of the charge can be at least partly compensated by the increase in filtration efficiency associated with the higher dust loading of the filter.

The phenomena of increased filter efficiency due to the electrostatic charge is illustrated in Figure 3.

Curve 1 shows the efficiency of the filter not depending on the electrostatic charge, whereas the Curve 2 corresponds to the filter with a charge decreasing after short period of time.

Figure 1: Increase of efficiency due to electrostatic charge.

In practice, the particle collection mechanisms described above all occur simultaneously and superpose accordingly. This results in total in a dependency for the fractional efficiency as a function of particle size as shown in Figure 4. The curve with a distinct minimum in the particle size range between 0.1 and 0.5 μm is typical of depth-loading filtration with fibre filter media. Smaller particles can be arrested very efficiently due to diffusion. For larger particles, high levels of particle collection are achieved due to inertia and interception. The particle size with the lowest arrestance and the greatest penetration is usually referred to by the abbreviation MPPS (Most Penetrating Particle Size). With increasing air velocity, minimum fractional efficiency is decreased and shifts toward smaller particles.

Figure 2: Transport mechanisms in particle separation on fibres.


Removal of gaseous contaminants

In addition to particles, a wide variety of types and concentrations of contaminant gases can be found in the air.

  1. Adsorption and absorption

Technically speaking, these are largely removed from the air by sorption, i.e. adsorption or absorption. Adsorption refers to the accumulation of substances from the gas phase (adsorbate) on the surface of a solid (adsorbent). This is different from absorption, in which substances penetrate the interior of a solid or a liquid and dissolve therein.

Adsorption is generally a physical process, in which atoms or molecules attach themselves to the surface of a solid via van der Waals forces. In this case, we talk of physisorption. The strength of this adhesive force depends on the material combination.

The rate of sorption (i.e. the quantity of contaminant gas deposited or released per time unit) depends on temperature, contaminant gas concentration, rate of diffusion from the gas phase to or away from the surface of the adsorbent.

  1. Common absorbents

The most commonly used adsorbent in technical applications is activated carbon, which usually adsorbs a large, very nonspecific number of different gases, such as aliphatic or cyclic hydrocarbons (VOC) and alcohols. For this reason, it is often used to arrest odors in industrial air handling and building HVAC systems. In practical use, the high affinity of water vapour (humidity) to activated carbon is problematic as, in the case of pure physisorption, water can displace (desorb) many other substances and decrease the adsorption efficiency of the activated carbon.

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