Ventilation effectiveness in classroom infection risk control

Abstract

The benefits of a good ventilation in classrooms are a well-studied topic regarding health and learning outcomes. However, many studies still show poor results regarding air quality, air change rate and air velocities. In this paper, typical Estonian classroom air distribution solutions were studied in an air distribution laboratory at Tallinn University of Technology. The air change efficiency was measured with CO2 tracer gas concentration decay method. For determining the contaminant removal effectiveness, continuous dose method was used to create a constant contaminant source. In addition, by using air velocity probes, we conducted draught measurements in the mock-up classroom. Tests were conducted using dedicated room-based air handling unit and thermal mannequins for imitating heat sources from students. We found that all solutions studied ensured the air change efficiency roughly corresponding to fully mixing air distribution, but local ventilation effectiveness values of contaminant removal showed large variation from 0.6 to 1.7 stressing the impact of source location. Grouped ceiling supply circular diffusers and single vertical supply grille air distribution commonly used in renovated educational buildings resulted in higher draught risk on the border of the occupied zone. High air velocities recorded in some areas of the classroom perimeter, well explain why draught is considered as one of the main reasons why the airflow rates are reduced, or supply air temperatures are lifted compared to designed values. Perforated duct diffusers resulted in acceptable air velocities. In conclusion, local ventilation effectiveness of contaminant removal showed that fully mixing is not a case with a point source, although air change efficiency determined with equally distributed source showed fully mixing conditions. Therefore, in those cases, the air change rate should be increased to achieve the same ventilation effectiveness. Based on the experiments conducted, a point source ventilation effectiveness measurement method for the breathing zone is proposed. This value determined at least with two source locations can be used in infection risk-based ventilation design.