What really happens in filters, separators and air purifiers?

Why are CFD analyses of air filters and aerosol separators so important and at the same time so complex?

Fig. 1: The pathways of aerosols in a separator
Fig. 1: The pathways of aerosols in a separator

Figure 1 shows a CFD analysis of an X-CYCLONE aerosol separator. In this figure, the flow paths of the particles are shown in different colours, i.e. the different colours indicate the areas where the different particles will be separated from the airflow, depending on their size (mass). It is a very important finding that the airborne aerosols (particles) do not always behave like the air flow. If the flow behaviour of the air was always the same as that of the airborne particles, the degree of separation of each filter and each separator would be zero! Why?

Well, because the air always finds a way through the filters and separators and if that were the case with the airborne aerosols and particles, they would always flow through the filters or separators and would not be stopped, i.e. filtered or separated!

The flow behaviour of the air is often not identical to the flow behaviour of the airborne aerosols!

Fig. 2: Highly efficient aerosol separator in use
Fig. 2: Highly efficient aerosol separator in use

That sounds logical, but it is precisely on this fact that many misinterpretations in the area of commercial kitchen ventilation and industrial air purification are based! Too often, the flow behaviour of the air is “only” examined and is then tacitly discussed one-to-one on the flow behaviour of the airborne pollutants, such as aerosols or soot particles.

This is dangerous and in most cases it is simply wrong.

Such undifferentiated CFD analyses, which only examine the flow behaviour of the air, may be sufficient for manufacturers of ventilation or air conditioning systems, because such systems really only concern the air flow. An air purification system is about more!

Commercial and industrial air cleaners, separators, filters, capturing hoods and similar systems are never “just” about the most efficient and comfortable ventilation possible, but also about very efficient cleaning of the exhaust air streams, i.e. the extremely efficient separation of airborne pollutants such as aerosols or particles.

This is the part that is often very complex, very complex in terms of the measuring technology, very complex in research and development and also very complex in verifying the function. That is exactly why too many manufacturers do a very wide arc around these tasks and then they start to guess, then they begin to estimate and spread too many semi-wisdoms without any well-founded scientifically processed analysis and measurements.

„If you cannot measure it, you cannot improve it“  William Thomson 1st Baron Kelvin
Fig. 3: „If you cannot measure it, you cannot improve it“  William Thomson 1st Baron Kelvin

To obtain well-founded knowledge and results, it must be analysed and examined how the airborne particles behave. It is not enough to just examine how the air flow in a separator, filter or air cleaner behaves!

Such a differentiated examination requires complex measuring technology and makes a CFD analysis extremely complex, because in no case it’s sufficient, as explained above, to analyse, for example, only the air flows in an oil mist separator or filter. Simply creating a suitable CFD simulation model is very time-consuming and requires a lot of experience.

Fig. 4: From CAD model to CFD simulation model
Fig. 4: From CAD model to CFD simulation model

That is why we carry out complex CFD analyses of our systems on the one hand and then additionally validate these analyses in practice using complex particle measurement technology. All of this then leads to extremely efficient separators, as can be seen in Figure 2 from above. If you have any comments, questions or further interest, please contact Sven Rentschler at ceo@reven.de.