This is one of the essential parts that help ensure optimum air quality inside buildings. In this article, we analyze the different types of air filters in ventilation systems, their differences, the regulations that govern them, and how to choose the one that provides maximum filtration in each circumstance. Because never before, with sustainable construction and its improvements in insulation, has it been so important to ensure excellent ventilation with the highest levels of air health.

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What is the purpose of the filters in a ventilation system?
It is worth remembering that the air we breathe, in addition to oxygen and CO2, contains different types of suspended particles. Our body has its own natural filtering mechanisms to prevent larger particles (10 µm and larger) from entering our respiratory system. But how can we prevent others from harming our health? We are not only affected by the size of the particles, but also by their concentration level.

In ventilation systems, it is the air filters that take care of the air purification process, preventing the concentration levels of particles in the air that are considered healthy from being exceeded. In this sense, both the WHO and the various national authorities have established classifications that serve to measure and control the quality of indoor air, in order to prevent problems such as allergies or respiratory pathologies. Air filters, therefore, must take into account two factors to be effective:
-The size of the suspended particles.
-The degree of concentration of the different PM: dust, smoke, pollen, odors, humidity, etc.

Dust separators and their use
Taking into account one of the two basic factors on which air filters must act, that of the size of the particles in suspension, a first differentiation is established:
-Dust separators, for particles with a diameter greater than 1 µm.
-Air filters, for particles with a diameter of less than 1 µm.

Dust separators use mechanical systems to precipitate the larger particles. They generally purify the outside air, i.e. they also have an environmental function. This is the method that has proven to be most efficient in this specific process, which is called dust separation and which is mainly used in industrial environments or activities that emit large quantities of pollutants into the atmosphere. In turn, two groups of dust separators can be distinguished:
-Those that work dry. They can use different types of mechanical force to separate the particles. The most common are gravity, inertial, centrifugal, and electrostatic dust separators.
-Wet scrubbers are also known as scrubbers. These wet dust separators operate with water, with a specific chemical reagent (acidic or alkaline medium) or with a mixture of both. They are used especially to reduce emissions of pollutant gases caused by shipping.

Classification of ventilation system filters according to their applications
If we focus on indoor air quality, where the presence of smaller particles predominates, it is when we have to refer specifically to air filters in ventilation systems. Here, too, there are different options, among which four types of air filters stand out:
-Activated carbon filters are considered one of the best air purifiers. Therefore, their use is recommended to filter particles of molecular size (viruses, bacteria, and harmful gases, among others). They are usually used in equipment with air recirculation systems. They are especially effective in absorbing bad odors.
-Electrostatic air filters work with active electronic plates that act as magnets, attracting particles smaller than 1 µm, without reducing airflow. They work very effectively to filter soot and tobacco smoke.
-Wet or viscous filters are made with a filtering material soaked in grease or oil that retains contaminating particles. Very suitable for retaining pollen or dust.
-Dry filters, very similar to the previous ones, use a mesh of fine fibers to filter PM. Their most effective application is also the filtering of dust mites and pollen.

Filters, a further step in air treatment
In the last year, and as a consequence of the COVID-19 pandemic, everyone has become familiar with terms such as "aerosols", "ventilation" and "air purification". Moreover, Filters are efficient not only in quantity but also in the variety of pollutants they filter, since they retain particles with a diameter of up to 0.3 microns, the most harmful ones because they have a more penetrating effect. Of course, they prevent us from breathing dust and dust mites, but they also retain mold, bacteria, and viruses. In fact, this is the filtering system that has been used for decades in such sensitive spaces as ICUs and airplanes.

Once again, to determine the most suitable type of filter in each specific case, all the factors affecting air quality must be studied, as we analyze below.

What factors should be considered when selecting a filter in a ventilation system?
As mentioned above, air filters are classified according to the percentage of particles of a certain size that they retain. However, this is not the only parameter to consider when choosing air filters for a ventilation system. The concept of pressure drop is also essential for assessing their efficiency.

In the case of ventilation systems, pressure drop refers to the decrease in airflow rate as it passes through the filter; this is a consequence of the resistance to which any fluid is subjected when it is introduced into a duct or incorporated into an installation. This pressure loss has to be compensated to re-establish a uniform flow, which results in an increase in energy consumption.

In addition, the pressure drop is directly related to the clogging that exposure to various particles eventually generates in the air filters of ventilation systems. Therefore, it is essential to calibrate the type of substances and the levels of contamination to which the filters are going to be exposed in order to choose the most suitable for each situation and optimize their useful life. Finally, let us remember that there are different elements that increase the presence of PM, among which the activity carried out indoors stands out, an issue that must also be included among the factors that influence the choice of air filters.

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