Why are CDF analyses of air-cleaning systems so complex?
Article from graduate engineer Sven Rentschler
Fig.1 – CFD analysis of the X-CYCLONE® technology
Figure 1 shows a CFD analysis of an X-CYCLONE® aerosol separator. What does the picture reveal? The flow tracks of the particles are represented in different colours. The colours show the flow behaviour of different particle sizes. It is a very important finding that airborne aerosols (particles) do not always behave in the same way as the air flow. If the flow behaviour of the air and the airborne particles were always the same, the separating rate of each filter and each separator would be equal to zero!
In many cases, the air-flow behaviour is not identical to the flow behaviour of airborne aerosols!
It sounds logical, but many misinterpretations regarding commercial kitchen ventilation and industrial air cleaning are based on this fact! Far too often, only the flow behaviour of the air is examined and it is automatically assumed that the airborne pollutants such as aerosols and soot particles behave exactly in the same way. This assumption is dangerous and, in most cases, simply wrong. Such unspecific CFD analyses might be appropriate for manufacturers of air curtains or supply air systems because only the air-flow behaviour is important for these kinds of systems. In an air-cleaning system, however, more complex conditions have to be considered!
A central requirement of the new standard EN 16282 is evidence of functionality! If evidence of functionality is provided, the new standard EN 16282 even allows the air-flow rate to be reduced, which is done indirectly when an air-induction system is used. This at least reduces the supply air flow through the room!
But how can we provide evidence of the functionality of an air cleaner? When can we guarantee that an extractor hood functions appropriately? When pollutants are kept away from the operating staff and the machine operator or chef is not exposed to smoke, aerosols and steam, we can say that there is evidence of functionality. A functional guarantee can only be given, if the staff is protected from all pollutants such as aerosols, steam, soot particles, etc.!
To achieve this goal we must examine and analyse the behaviour of the airborne particles. It is not enough to examine the behaviour of the air flow!
A complex examination requires sophisticated measuring technology and renders CFD analysis quite difficult because examining just the air flow through the whole kitchen is not enough. The conclusion that the staff is safe if the air-flow conditions in the kitchen comply with regulations would only be correct if all airborne pollutants always behaved in the same way as the air molecules. But we have seen at the beginning of the present report that this is not true!
Therefore, we undertake complex CFD analyses of air-cleaning systems such as collecting hoods equipped with an air-induction system! Consider the following illustration taken from the current REVEN® catalogue (www.reven-epaper.de):
Fig. 2 – CFD analysis of the REVEN® technology
In this figure, the different colours do not represent different particle sizes but different residence times. It may happen that some airborne particles have a residence time equal to zero! What does that mean? A particle carried along with the thermal air flow with a residence time equal to zero is floating on the spot. A negative residence time means that the particle is falling, etc. These are very important aspects especially in the CFD analyses of collecting systems because spatial distances of up to three meters often have to be examined. Therefore, you should be aware that airborne particles do not automatically rise with the hot air up to the collecting hood and are extracted there! A particle can “grow” due to condensation effects and become bigger and heavier. If so, it changes its flow behaviour and no longer follows the flow route of the air molecules.
Therefore, we have to use CFD to examine which particles follow the flow of the air molecules and which do not. We have to find out the size at which a particle’s flow behaviour changes. These findings will help us to analyse in practice which particles sizes are generated in the different processes and released to the air flow and which parameters have an influence on this. And we will be able to find out what happens to these particles when flowing through the respective air cleaners and collecting systems
For these reasons, we do not share the opinion of some manufacturers and the German Association of Kitchen Ventilation according to which CFD analyses should always examine the entire room and not the individual collecting system. Of course it is interesting to know how the air flow behaves throughout the whole room and there is no question that good air-flow conditions guarantee the good collection of pollutants. However, this kind of simplified examination can never provide evidence of the functionality and can never prove that airborne pollutants are perfectly under control.
The final evidence of functionality can only be provided when focusing on the examination of airborne particles and aerosols in the analysis and measurement work.
Sersheim, 29 September 2015
Rentschler Reven GmbH
Copyright © 2015, All Rights Reserved – Dipl.-Ing. Sven Rentschler