Safe office ventilation during a pandemic

31 of August '20

FläktGroup rotary heat exchangers

FläktGroup's heat recovery systems meet all the recommendations of the Federation of European Heating, Ventilation and Air-Conditioning Associations (REHVA) for reducing the spread of COVID-19 through supply and exhaust systems.

To date, there has been no adequately documented outbreak of SARS-CoV-2, the source of which would be aerosol transmission through a central supply and exhaust system. The spread of infection caused by air leakage from heat recovery systems is highly unlikely. A great deal of scientific research is currently underway to understand the transmission of the virus by airborne particles, so it is reasonable to take precautions to reduce internal leaks and recirculate air in ventilation systems. The most comprehensive, independent advice can currently be obtained from REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations (https://www.rehva.eu/activities/covid-19-guidance). FläktGroup agrees with REHVA's main recommendations, which can be summarized as follows:

Make sure that exhaust ventilation in restroom areas is working properly. These are high-traffic areas where, in addition, human contact with surfaces is inevitable. In addition, COVID-19 may enter the atmosphere when flushing the toilet.
The amount of supply and exhaust air should be increased. Diluting with fresh air and exhausting any particles to the outside are key factors in reducing indoor transmission.
All recirculation sections in air handling units should be turned off. Recirculation of contaminated exhaust air should be avoided.
The indoor environment should be maintained at 50% relative humidity (ideally), with relative humidity optimally between 40% and 60%. There is ample evidence that many viruses (mainly influenza A) lose their ability to become infectious most rapidly when the relative humidity at normal indoor temperatures is 50%. There is growing evidence that COVID-19 exhibits the same response.
General ventilation filters are not designed to capture virus particles, but it is good practice to make sure that dirty filters are replaced and disposed of properly. The addition of HEPA filters may be beneficial for critical areas, but it is unlikely to be a practical solution for general ventilation systems.

FläktGroup's comprehensive systems

FläktGroup can offer solutions for all of the above requirements.

Photo: © FläktGroup

Of the three most commonly used heat recovery systems - plate heat exchangers and glycol heat recovery systems do not recover moisture from the exhaust air stream and therefore dehumidify the building in winter. Rotary heat exchangers can be supplied as hygroscopic heat exchangers, which can feature a moisture recovery of 50%, or sorption heat exchangers, which can recover up to 80% of available moisture and are the best solution for maintaining indoor humidity if there are no direct humidifiers.

Leaks are always a problem in rotary heat exchangers. However, with the use of a cleaning sector and maintaining the correct pressure values inside the air handling unit, the exhaust air exchange rate (EATR) can be 0% according to EN 16798-3:2018. And that means no exhaust air recirculation.

All rotors manufactured by FläktGroup are always equipped with a cleaning sector. In addition, we can provide a pressure balancing system on the room side of the rotor, according to the recently published REHVA guide (https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID-19_specific_guidance_document_-_Limiting_internal_air_leakages_across_the_rotary_heat_exchanger_.pdf). This system also prevents leakage in the wrong direction, even when the rotary heat exchanger is turned off.

Regular inspection is important

With a properly designed, installed and maintained rotary heat exchanger, leakage of potentially pathogen-contaminated exhaust air into the supply air stream is usually very small and virtually negligible. However, if the fans are not properly arranged in the air handling unit or the pressure balance is not properly set, leakage can be much higher. Therefore, it is worth checking the configuration of fans in any existing air handling unit.

If the fans are placed on both sides of the rotor so that the rotor is on the suction side of both fans, then pressure balancing can lead to an exhaust air exchange rate (EATR) of 0%, and the cleaning sector itself will work very effectively.

If both fans are on the building side, there is no pressure balancing across the rotor, and the exhaust air exchange ratio (EATR) can be up to 35%.

If both fans are on the outside side, there is no need for pressure balancing, as air always leaks from supply to exhaust, but in this case the overall capacity of the air handling unit is lower.