Ventilation systems on trial

In BTR 2/2021, we reported on a research project launched by the German stage technicians’ union, DTHG, and the North Rhine Westphalian Ministry of Culture and Science, to test the effectiveness of indoor ventilation systems and award certificates of approval where appropriate. Moving on from the project’s cornerstones, in this issue, we look at it from a technical perspective, present its findings, and suggest some improvements.

Bühnentechnische Rundschau

Now the project has made the transition from paper to practice, all theatres, even smaller ones, can apply to have their ventilation system certified ( Special offers and conditions are available to institutional members and the membership of Bühnenverein and Inthega. Depending on the scale of the inspection, the whole process including certification costs on average between 2000 and 4000 Euros per theatre. 
Survey and assessment 
The project is based on a survey and assessment concept.

First, participating theatres complete an online questionnaire, entering data on their ventilation technology and its uses, based on the operators’ practical knowledge, and consulting their documents. 
After checking each theatre’s information is complete and plausible, project staff then calculate their air quality values. The key value is the ‘effective air flow rate’ per person in the space. To assess it, the ventilation system air flows, any mobile air cleaning devices, and the room air volume are all factored in. As there are varyingly effective partial airflows in any space, each one is multiplied according to the usual degree of ventilation efficiency. The formula for calculation and correction variables can be found in a newly compiled set of regulations, which is available for download at 
Next, the effective air flow is compared with the minimum air flow necessary to ensure good air quality and a minimal risk of infection, based on the latest findings of recently published research (see 
The effective air flow rate needs to be the same or greater than the requisite minimum to guarantee air quality and pass the test. If this applies, the indoor ventilation classifies as fulfilling pandemic-specific requirements. Prior to conducting this check, aspects of air quality in the formation of the effective air flow are noted, based on the input data, e.g., the proportion of recirculating air, the filter quality of the supply air, the air flow in the stage area, and the system’s service life. 
To sound out these values, any implausibilities, or missing pieces of information, every assessment needs to be dealt with by an expert, though site visits are not necessary. 
Using the data, the project was able to suggest some quick, low-cost possibilities for improvement, above all for spaces without adequate technical or window ventilation. Consider the following example: The auditorium of a theatre is supplied with 10,000 m³/h air input via a central ventilation system. Under pandemic conditions, the building automation would show a proportion of approx. 20 percent recirculating air during performances. The space, with a volume of approx. 2000m³, is equipped with displacement ventilation. Usually, it seats 300, reduced to 150 in pandemic conditions. In both cases, the maximum performance time is fixed at two hours. The used air is extracted from above and mostly around the stage area. Asked about the proportion of recirculating air, the in-house technician said the room air is still adjusted to the pre-pandemic CO2 value of 1200 ppm. By lowering the target CO2 value to 800 ppm, as is currently recommended due to the pandemic, the theatre could switch to a purely outside-air operation and prevent air from recirculating. And since most of the hall’s used air is extracted from above the stage area, the input of aerosols from the stage area is minimized, so the artists and musicians do not need to be factored into the auditorium’s air supply. 
Using the equation given in the regulations, project staff calculated an effective air flow of 36 m³/h per person for a full house and 73 m³/h under pandemic conditions. 
A recommendation for minimum air flow can be found in, e.g., the specialist VBG guidebook, where it is given as 50m³/h for audiences. Meanwhile, Technische Universität Berlin conducted a detailed simulation, factoring in the spread of aerosols from one infected person during a two-hour performance, with social distancing and masks being worn, that came up with a recommendation of 40 m³/h. In these conditions, the risk of infection during a visit to the theatre would be lower than in a supermarket (relative R number <1). 
The regulations are currently based on a minimum value of 40 m³/h. The comparison between effective air flow and the minimum value shows that the air supply in a half-filled auditorium is very generous. 

In our example theatre, the foyer – an area where members of the public linger for 15 minutes or more – is only aired via windows but directly connected to the auditorium, as is frequently the case. Under pandemic conditions, 100 guests are allowed to be in the foyer at once; the remaining audience members are presumed to stay in the auditorium, go to the toilet, smoke outside, or walk up and down the corridors during intervals.
From the point of view of air quality, the regulations are based on the premise that an air flow of 45 m ³/h per person, at a bar where people stay and drink for approx. 30 minutes, is sufficient airing in pandemic conditions. Taking equation 1 in the set of regulations, it can now be reformulated according to the specific air flow required to supply the foyer with fresh air throughout the interval. With a room volume of approx. 600 m³, and 100 guests in the foyer, that would mean a required air flow of 4500 – 600/0.5 = 3300 m³/h. 
This result shows, for one, that large theatre halls have enough air capacity and supply to significantly reduce the exhaled air volume of an infected person. And if the foyer is directly connected to the auditorium, it is possible to ensure sufficient airing of the foyer throughout the interval via an overflow of supply air from the auditorium through the foyer to the opened windows. In this case, the waste air extraction in the auditorium should be reduced during the interval by at least 3300/10,000 m³/h = 33%. As this can be regulated on many ventilation systems, it is a virtually no-cost, immediate solution. 
If the toilets are mechanically ventilated (as is common), it is possible to ventilate all the relevant public spaces in a venue and fulfil the requirements to contain a pandemic. The DTHG’s inspection and certification project aims to consider all these aspects and point the way for operators and the health authorities to concrete, practical, and adaptable solutions.

The findings of some 115 assessments of 26 venues in North Rhine Westphalia have shown that around 80 percent of their indoor public spaces fulfil the requirements for ventilation in pandemic conditions. The number is buoyed by the drastic reductions in audience numbers to ensure social distancing that the venues have been forced to make. On average, theatres can only fill around 30 percent of their available seating. The project found that 20 percent of the venues needed to carry out technical improvements or organizational adjustments to fulfil the requirements. Only three venues did not meet the criteria. They are now installing, or planning the installation, of new indoor ventilation systems.

BTR Ausgabe 3 2021
Rubrik: English texts, Seite 154
von Rüdiger Külpmann and Wesko Rohde

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