Mobile air purifiers in the fight against corona

Why are air purifiers needed?

As soon as there is more than one person in a room, there is a potential risk of infection. Face masks and maintaining a safe distance help to avoid droplet transmission. However, these measures do not prevent viruses from being carried through the air via airborne aerosols. The longer an infected person stays in a room, the more viruses spread there. This is displayed by numerous simulations on the Internet. Air purifiers and cross ventilation can remove these airborne viruses. With an air purifier, a powerful fan circulates the air in the room and the viruses collect in integrated HEPA filters, while purified air is released. The abbreviation HEPA stands for High Efficiency Particulate Air. These HEPA filters are available in different filter classes. Air purifiers usually use an H13 or H14 HEPA filter, which have a separation efficiency of 99.95% to 99.995%. The air purification can be supported by additional measures, such as UVC radiation or thermal disinfection, which kill the viruses.

Advantages and disadvantages of the air purifier versus window ventilation

In addition to removing viruses, window ventilation has the advantage that fresh air is also supplied to the room again. This can be measured by the carbon dioxide concentration (see e.g. CO2 traffic lights in classrooms). Especially during the cold season, the heat energy is virtually blown out the window if there is no heat recovery. With the air purifier, the room air is permanently cleaned of viruses, pollen, bacteria, without heat loss.

The more people there are in a room, the faster the CO2 concentration increases. In the graph shown, the CO2 progression in a classroom was recorded (source: ILK Dresden, Prof. Franzke). The black line shows the average value. At the beginning, the CO2 concentration was about 500ppm, i.e. clean ambient air. The CO2 concentration then rose to 1,700ppm. After 39 minutes the windows were opened (yellow line) and one can see a slow decrease in the CO2 concentration, which is representative of virus degradation. The greater the temperature difference between outside and inside, the faster the exchange works. In this case the outside temperature was 0°C, inside about 20°C (at the beginning).

Window ventilation: faster reduction of viral load is possible, but heat loss in the classroom is not optimal.

Where are air purifiers used?

Typical places of use for air purifiers are:

Day care centers and classrooms
nursing homes, hospitals, doctors' offices
Restaurants, bars, pubs, dance schools
Halls for events, company celebrations, conferences
Wellness and fitness studios
retail stores, pharmacies, bakeries, hotels
Open-plan offices and workplaces

Air purifier requirements: small versus large devices

At the pro.vention trade fair in Erfurt, a supplier of air purifiers once told us: "Everyone wants a small Alexa, which then completely cleans the entire exhibition hall!" This is probably also the reason why there are so many small air purifiers. According to the motto "I now also did something for it", you can purchase these small devices inexpensively on the Internet. But the following should be noted:

The air purifier should be able to circulate 6 times the volume of the room within one hour. A typical room with 30 m² and 2.5m room height has the following volume:
L x W x H = 6 m x 5 m x 2.5 m = 75 m³ volume.
So, if the room air is circulated 6 times, the air purifier should be able to provide an airflow of 6 x 75 m³ = 450 m³ per hour. At this performance, the noise level should be within the acceptable range.

The fan in small devices usually runs at higher speeds, which then also leads to a higher noise level. If possible, the noise level should not exceed 45dB(A), measured at a distance of one meter, depending on the room. A library is to be evaluated differently than an open-plan office. How loud 45dB(A) is can be seen from the list below. An increase of 10dB(A) corresponds approximately to a doubling of the noise perception.
- 30 dB: Whispering, own breathing sound
- 35 dB: Rustling leaves
- 40 dB: In the living room with the window closed
- 45 dB: Residential area without road traffic
- 60 dB: Entertainment (individual conversation)
- 70 dB: Open-plan office

Small air purifiers up to approx. 1m in height tend, with a few exceptions, to generate a short-circuit flow. This means that the cleaned air is immediately sucked in again on the other side before it is distributed into the room. Air purification happens here only in the immediate vicinity of the air purifier and not everywhere in the room. See the sketch on the right.

So for air purification in larger rooms, large air purifiers are more suitable. They also offer more power reserves so that the device does not constantly run at maximum power, and is thus quieter than its smaller counterparts.

Who supplies large air purifiers?

Manufacturer: SIDOPA
Product: AIR GUARD 5000
Functionality: HEPA H14 filter
Air volume: up to 5.000m³/h
Sound pressure at 1 m distance: 42 - 45dB(A)
Dimensions: 80 x 80 x 230 cm

Manufacturer: SIDOPA
Product: AIR GUARD 3000
Functionality: HEPA H14 filter
Air volume: up to 3.000m³/h
Sound pressure at 1 m distance: 38 - 45dB(A)
Dimensions: 57 x 69 x 230 cm

Manufacturer: AFS
Product: AFS 2000 RLC-X
Mode of operation: HEPA H13 or H14 filter
Air volume: 600 - 2.100m³/h
Sound pressure at 1 m distance: 35 - 51dB(A)
Dimensions: 114 x 51 x 189 cm

Manufacturer: Greentec
Product: CAS1000
Functionality: HEPA H14 and thermal disinfection
Air volume: 200 - 1.350m³/h
Sound pressure at 1 m distance: 36 - 48dB(A)
Dimensions: 71 x 71 x 192 cm

Manufacturer: IVAT
Product: Hygiene-Tower
Functionality: HEPA H14 and UVC cleaning
Air volume: up to 5.500m³/h
Sound pressure at 1 m distance: 37dB(A) @ 1.000m³/h
Dimensions: 95 x 110 x 205 cm

Is the purchase of such equipment subsidized by the German Government?

Funding is available for the rental or purchase costs of air purifiers in the following German states:

Bavaria: in daycare centers, large daycare centers, therapeutic daycare centers and schools
Hesse: in schools
Rhineland-Palatinate: in schools
North Rhine-Westphalia: in schools and sports halls

What role do sensors play in air purifiers and ventilation systems?

1. pressure sensors for filter monitoring:
As with a vacuum cleaner, the filter becomes more and more contaminated with increasing hours of operation. The increase in particles in the filter causes greater pneumatic resistance. As a result, either the volume flow becomes smaller, or the current consumption increases. The pressure sensor measures the pressure difference before and after the filter. The greater this differential pressure, the greater the contamination of the filter. This means that a replacement filter can be provided in

2. multisensors for a comfortable climate:
For the fully automatic operation of air purification and air conditioning devices, several individual sensors are combined to form multisensors. This results in modules that record several measured variables simultaneously. In this way, cooling, heating, air humidification and cleaning can be controlled accordingly, so that a pleasant and clean climate always prevails:

Pressure (differential pressure at the filter)
Temperature (room temperature)
Humidity (room humidity)
VOC (Volatile Organic Compounds)
CO2 (carbon dioxide)

3. CO2 sensors for controlling decentralized ventilation systems with heat recovery:
According to VDI6040, air quality in classrooms should not exceed 1,000ppm on average. Higher concentrations above 2,000ppm are considered unacceptable. For this reason, the so-called CO2 traffic lights have come into being. The CO2 concentration is usually divided into 3 areas, like a traffic light. CO2 sensors are ideal to control decentralized ventilation systems with heat recovery.