Aerosol transmission in rotary wheel heat exchangers

Abstract

Transmission by aerosols is considered the main route of COVID-19 infections indoors. Therefore, limiting air transfer between supply and extract air in ventilation systems is critical. Heat recovery components (HRCs) are used as standard in new ventilation systems. Rotary wheels are very efficient, but have a higher exhaust air transfer ratio (EATR) compared to other types. The fact that the surface of rotary wheels is touched by both supply and exhaust air allows humidity recovery, but also carries a risk of transferring undesirable substances. Aerosols can
deposit on the surface if they come into contact with it. The characteristics of rotary wheels raise the question whether a relevant transfer of aerosols can take place and whether this is different from the EATR. Experimental investigations were carried out with two rotary wheels. The aerosol used was a water-glycol mixture whose properties compare well with human lung aerosols. Particle sensors were installed at all four duct connections of the HRC. In parallel, the EATR was determined with tracer gas. In 16 measurement series, the air velocity, rotor speed and air conditions were varied. The determined aerosol transfer ratio was typically 1 to 2 percentage points below the EATR. The results allow the conclusion that rotary wheels designed and operated according to current standards transfer only a non-relevant small amount of aerosols and thus do not pose an infection risk for COVID-19 in applications such as offices where the frequency of highly infectious individuals is low to moderate. However, aerosol transmission in hygienically relevant quantities is conceivable in poorly designed systems with incorrect pressure ratios and inadequate filters at the same time, but this does not only affect rotary wheels. Although first positive results are available, further investigations are planned. The understanding of aerosol absorption and HRCs surface's properties is to be deepened.