The Role of Air Filters in HVAC Units regarding the Coronavirus

Introduction:

The threat by the virus (SARS-CoV-2) to the human health is currently a hot debated topic. One central question is how infections and spreading could reasonably be restricted. The role of air filters in this context is discussed in the following:

Either way – it is undisputed that the use of effective air filters is indispensable for maintaining a hygienically impeccable condition of HVAC systems. Many pollutants like particulate matter, gases and germs can be separated from the air. With regard to viruses, a successful elimination depends on the virus itself, the air filter and the environmental conditions.

Properties of the virus:

The COVID-19-virus is transmitted via droplets. People release those droplets in different sizes. Additionally the size changes during the lifetime by processes like vaporisation. Big droplets with a diameter bigger than a few dozen micrometres fall down rather quickly and can travel a distance about 1-2 metres. They are the main source for a direct transmission and the smear infection. If the virus settles down on surfaces, it can survive for several days. On top of that, we also release finer droplets. They build up aerosols and can travel over much longer distances.

For air filters, the virus is a particle like any other. The minimum particle size is important for the separation. Unfortunately, it is not known up to which particle size a coronavirus is still infectious. A single virus unit has a diameter of approx. 0.1 µm. As soon as this is surrounded by a shell of water, the diameter increases significantly. Even if the shell evaporates completely, it is likely that several viruses will remain as a larger agglomerate. Besides it has been proven that the viruses themselves can also settle on fine dust. Thus the diameter also increases. It is therefore probable that the virus occurs to a large extent in/on particles or agglomerates with a diameter significantly larger than 0.1 µm.

The Role of Air Filters in HVAC Units regarding the Coronavirus

Basically, a distinction must be made between ventilation systems in fresh air- and recirculating air operation and all intermediate stages. A high rate of air change with fresh air is advisable in order to dilute a possible virus concentration. Operation can be recommended without restrictions for systems with fresh air. It can be helpful to start the operation some time before using the room. An air conditioning system operating in accordance with current specifications such as VDI 6022 or EN 16798, guarantees the delivery of hygienic supply air.

Systems operating in recirculation-air mode have to be considered differently. Especially in case of unfiltered air being transported over long distances or several rooms, the spread of viruses can be encouraged. If such ventilation is indispensable, the highest possible filter class should be used. In most times a changeover to HEPA filters is not possible, as this is usually accompanied by a significant increase of pressure drop. This endangers the proper operation of systems that are not designed for this. But even if this is not possible, there may be reasonable alternatives.

Filter classes for recirculating air:

Filters of class ISO ePM1 80% separate over 80% of particles from the air in the range 0.3 – 1 µm. As described above, this is probably the range which airbone SARS-CoV-2 is also present in. Therefore, such filters are suitable to keep the virus concentration in rooms low. Compared to H13 filters, significantly higher air exchange rates can often be achieved. This has two advantages. On the one hand, the air in the room is filtered more often. If, for example, the air passes two times through a filter with an efficiency of 80%, it is like passing for one time through a filter with an efficiency of 96%. On the other hand, a high air exchange rate prevents possible viruses from remaining in the room for a long time and thus spreading.

Conclusion:

In combating SARS-CoV-2, all aspects of transmission must be considered. Airborne viruses are only one aspect. The aerosol concentration can be minimized by adequate ventilation. Air filters play a major role in this context, especially in circulating air systems. The interaction between a high filter class and a high air exchange rate is decisive. Viruses that are separated by fibre filters are not released again and can be immobilized. When changing the filter, the usual safety measures must be observed. An overview and product recommendations for the respective cases are listed in the following.


Kalthoff Solutions
Fresh Air
Recirculating Air
MULTIFOLD Pocket Filter TCC
Air conditioning design, solid
and efficient
TCC 95
ISO ePM1 65%
TCC 98
ISO ePM1 80%

MULTIFOLD Pocket Filter TC
Industrial design,
high dust-storage capacity with low initial pressure drop

TC 85
ISO ePM1 60%
TC 98
ISO ePM1 85%

Panel Filter RELIM
Space-saving with high filter surface and
moderate service lives
RELIM RV 7
ISO ePM1 55%
RELIM RV 9
ISO ePM1 85%

Compact Filter RELIM VRK
Very solid, long service lives and
low pressure drop
VRK 85
ISO ePM1 55%
VRK 98
ISO ePM1 80%

Compact Filter FLUWID VSK ETA
Highest energy efficiency and
very low pressure drop
VSK 85 - ETA
ISO ePM1 60%
VSK 98 - ETA
ISO ePM1 85%