*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, 1000 Ljubljana, Slovenia, eneja.osterman@fs.uni-lj.si 233 Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 Received for review: 2022-02-15 © 2022 The Authors. CC BY 4.0 Int. Licensee: SV-JME Received revised form: 2022-04-05 DOI:10.5545/sv-jme.2022.68 Original Scientific Paper, Special Issue: SARS-CoV-2 Accepted for publication: 2022-04-07 Analysis of Educational Building’s Ventilation Suitability to Prevent the Spread of Coronavirus (SARS-CoV-2) Osterman, E. – Dovjak, M. – Vaupotič, J. – Verbajs, T. – Mlakar, U. – Zavrl, E. – Stritih, U. Eneja Osterman 1,* – Mateja Dovjak 2 – Janja Vaupotič 3 – Tomaž Verbajs 1 – Urška Mlakar 1 – Eva Zavrl 1 – Uroš Stritih 1 1 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia 2 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Slovenia 3 Jožef Stefan Institute, Department of Environmental Sciences, Slovenia In a larger educational building in Slovenia, we examined the efficiency of ventilation systems by analysing the operation of the heating, ventilation, and air conditioning (HVAC) system in several classrooms. Using the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) COVID-19 ventilation calculator, the probability of infection due to the spread of coronavirus through aerosol particles and the reproduction number were calculated based on the classroom occupancy, ventilation rates, and other parameters (i.e., classroom characteristics, preventive measures). Firstly, different levels of ventilation capacity (50 % and 80 %) were applied. Considering the distance between occupants 1.5 m and wearing the masks of all participants, the probability of infection during lectures was always lower than 1 %. Secondly, the maximum number of students that can attend lectures is about 30 %, as calculated according to the legal requirements, recommendations, and given conditions. Keywords: classroom ventilation, REHVA calculator, probability of infection, reproduction number, HVAC system Highlights • A mixed-mode ventilation in the examined large classrooms is not appropriate for effective control and prevention of transmission risks in the educational environment. • Considering the ventilation efficiency increase from 50 % to 80 %, in a larger size classroom, the probability of infection is reduced from 0.40 % to 0.27 %. • Considering the ventilation efficiency increase from 50 % to 80 %, in a larger size classroom, the reproduction number decreases from 0.11 to an acceptable level of 0.07. • During the restrictions against the spread of coronavirus, maximum occupancy of the classrooms is not recommended. 0 INTRODUCTION It is well known how important the design of heating, ventilation, air conditioning (HV AC) systems is to achieve adequate air quality, while not deteriorating thermal comfort [1]. Since the Coronavirus disease COVID-19 outbreak, preventive measures have been taken to mitigate transmission risks (i.e., airborne, contacts) in buildings. Ventilation solutions present the main engineering controls described in the traditional infection control hierarchy [2] to reduce environmental risks of airborne transmission [3] to [5]. Expelled respiratory droplets that are airborne range from less than 1 µm to more than 100 µm in diameter. Airborne transmission depends on the droplet size and includes i) short-range region for close contact (i.e., large droplets up to 2 mm that fall within 1.5 m) and ii) long-range region (i.e., small droplets less than 50 µm fall beyond 1.5 m distance) [6] and [7]. In indoor air, coronavirus SARS-CoV-2 can remain active for up to 3 hours and up to 2 to 3 days on room surfaces in common indoor conditions [8]. Therefore, the main role of efficient ventilation is to ensure a sufficient amount of fresh air per occupant while simultaneously removing the harmful airborne microbes. A study by Nishiura et al. [9] highlighted that the odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment. Poorly designed and/or not properly maintained HV AC systems enable the airborne droplets to be easily transported around the spaces in buildings, and therefore, such a method of transmission is becoming increasingly important [10] and [11]. Quite a few studies have been done analysing HV AC systems and the impact of natural ventilation (opening of windows) [4], [12] and [13]. It was found that with appropriate measures regarding ventilation, the probability of infection is relatively low (less than 1 %) [14]. Similarly, our study aimed to verify the ventilation efficiency in the selected educational building in Slovenia and to calculate the transmission risks for COVID-19. The main question was whether the existing ventilation system meets the requirements of standards to prevent the spread of SARS-CoV-2 Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 234 Os t erman, E. – Do vjak , M. – V aupo tič, J. – V erbajs, T . – Mlak ar , U . – Za vrl, E. – S tritih, U . during normal occupancy of classrooms and how the probability of infection could be quantified. 1 METHODS To be able to assess the current state of the probability of infection in the selected building and to be able to propose appropriate measures, the REHV A COVID-19 ventilation calculator was used [15]. The calculation is based on the Wells-Riley model [16], which determines the probability of infection for the selected space and human activity. The probability of infection (p) is defined by Eq (1): pe n   1, (1) where n is the number of quanta inhaled. Quantum represents the number of airborne droplet nuclei that cause infection in 63 % of susceptible individuals. This depends on the origin of the viruses, which is defined with quanta emission rate, E, [quanta/h]. The quanta inhaled (n, quanta) depends on the time-average quanta concentration, C avg , [quanta/m 3 ], the volumetric breathing rate of an occupant, Q b , [m 3 /h] and the duration of the occupancy, t, [h] as shown in Eq. (2): nCQt avgb = . (2) C avg is defined in Eq. (3), where V represents the volume of the room [m³], λ is a first-order loss rate coefficient for quanta/h due to the summed effects of ventilation, deposition onto surfaces and virus decay. Values for λ are taken from studies [17] to [19]. Estimated values for E and Q b are based on the studies of the Skagit Valley Chorale event [5] and quanta generation rates for SARS-CoV-2 [6] and are given in Table 1. C E Vt e avg t             1 1 1. (3) Table 1. 66 th percentile SARS-CoV-2 quanta emission rates for different activities [20] Human activity Quanta emission rate, E, [quanta/h/occupant] Resting, oral breathing 0.72 Heavy activity, oral breathing 4.9 Light activity, speaking 9.7 Light activity, singing (or loudly speaking) 62 In addition to the calculation of the probability of infection, it was also necessary to define its acceptable value. For this, several studies propose to define the event reproduction number R. It is defined as the number of new disease cases divided by the number of infectors and its value should be below 0.1 [15]. Table 2. Volumetric breathing rates [21] and [22] Human activity Breathing rate, Q b , [m 3 /h] Standing (office, classroom) 0.54 Talking (meeting room, restaurant) 1.10 Light exercise (shopping) 1.38 Heavy exercise (sports) 3.30 Mentioned should also be the assumptions made in this model. It is assumed that quanta are emitted at a constant rate throughout the event; the infected occupant is present in the room throughout all occupancy time; an infectious respiratory aerosol is evenly distributed throughout the well-mixed room air; infectious quanta are removed by ventilation, filtration, deposition, and airborne virus decay. 2 EXPERIMENTAL AND CALCULATIONS 2.1 Experimental An inspection of ventilation systems with a description of mechanical installations of the selected educational building was made as part of the energy audit in 2012. Mechanical installation systems have not changed much since then, as only service and maintenance works have been carried out in the meantime. We also reviewed some parameters (type of recuperation, surface area, height and volume of the classrooms, air flow rate of air-conditioning (AC) unit, type of air inlet, the maximum number of occupants, number of seats, etc.) and measured them based on the obtained data. The results are given in Table 3. Validation of their Supervisory control and data acquisition (SCADA) system was performed using the reference measuring equipment Testo 400 (Universal IAQ instrument), according to the standard EN ISO 12599 requirements [23]. The cross-checking of temperature, CO 2 and air inlet velocity showed that their system deviates by less than 6 % from the reference. It should be noted that the CO 2 sensors from their SCADA system detect a higher value than the reference one, which in turn means that the ventilation turns on at lower CO 2 concentrations and consequently the ventilation is better. At the time of our measurement, the SCADA was set to increase the power of the ventilation system at elevated CO 2 concentrations (>1000 ppm) in the air of classrooms. Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 235 Analysis of Educational Building’s Ventilation Suitability to Prevent the Spread of Coronavirus (SARS-CoV-2) The inspection followed the Methodology for Regular Inspections of Air-conditioning Systems [24]. 6 AC units supplied air for 6 large classrooms (LCR) on the ground floor (LCR 1_G – LCR 6_G) and 1 AC unit for small classrooms (SCR) in the basement (SCR 1_B – SCR 6_B). 2.2 Calculations: the Probability of Infection and Reproduction Number The following assumptions had to be made when calculating the probability of infection and the reproduction number using the REHV A COVID-19 ventilation calculator [15]: • Proper wearing of the masks of all occupants was envisaged; the value for mask efficiency for susceptible occupant is 0.3, and the value for mask efficiency for the infectious occupant is 0.5. • The virus decay was the default from the study by van Doremalen et al. [8], and its value is 0.63 h –1 . • Deposition to surfaces was defaulted from the studies by Buonnano et al. [20] and Miller et al. [25], where the value could vary between 0.24 and 1.5 h –1 , depending on the aerosol particle size range. For the study, the value taken was 0.24 h –1 . • Additional control measures (such as a removal rate of UV disinfection) were 0 h –1 . • Quanta emission rate was 5 quanta/h. • Breathing rate was 0.54 m 3 /h. • Classroom occupancy was 12 h/day. • The distance between the occupants is at least 1.5 m. • There is only one infected occupant in the classroom. 3 RESULTS AND DISCUSSION As presented in the previous chapter, we analysed the ventilation systems in the educational building and came to the following conclusions: • All larger ventilation devices have rotary heat exchangers, which means that there is a possibility of the virus being transferred back into the classroom in the event of a leak. • There is mixed-mode ventilation in large classrooms, which is not suitable for keeping the sufficient quality of air in the classroom. Small classrooms have a displacement mode of ventilation, which is more suitable from the air exchange point of view. • The windows were opened after each lecture so that a large number of windows were completely opened for several minutes. Windows were also opened if the CO 2 sensor showed values above 1000 ppm. • Ventilation ducts are not being cleaned. • Large classrooms have ventilation efficiency controlled by CO 2 sensors, while small classrooms do not. The results obtained from the computations are shown in Figs. 1 to 4 and Table 4. For LCR 1_G – LCR 6_G, the ventilation capacity was set at 50 % and 80 % (Figs. 1 and 2), and for SCR 1_B – SCR 6_B was assumed the ventilation with the same share of airflow (Figs. 3 and 4). Note: in some figures, the lines overlap. As seen from Fig. 1, the probability of infection after 12 h is the highest in LCR 2_G, when it reaches 0.4 % with 50 % ventilation capacity. If ventilation capacity is increased to 80 %, the probability of infection is reduced to 0.27 %. This means a 28 % lower probability of infection. The lowest probability of infection is in LCR 6_G and LCR 5_G. The same is true when comparing the reproduction number (Fig. 2). At 50 % ventilation capacity, the maximum value is 0.11 in LCR 2_G, and at 80 %, it is reduced to 0.07. The recommended value of the reproduction number is 0.5, and to control the epidemic, it should be kept below 1 [1]. Table 3. Characteristics of AC units, classrooms and analysed ventilation scenarios Classroom Air-conditioning unit The airflow rate [m 3 /h] Surface area of classroom [m 2 ] Classroom height [m] Max number of occupants (with 1.5 m distance) LCR 1_G NP1: IMP KNMD 9/6 D25 4500 197 2.9 28 LCR 2_G NP2: IMP KNMD 9/6 D25 4500 198 2.9 25 LCR 3_G NP3: IMP KNMD 12/6 D25 5800 245 4.5 32 LCR 4_G NP4: IMP KNMD 12/6 D25 5800 267 3.95 39 LCR 5_G NP5: IMP KNMD 12/6 D25 9400 307 4.5 35 LCR 6_G NP6: IMP KNMD 12/6 D25 9400 624 4.5 42 SCR 1_B – SCR 6_B N1: IMP KNMD 9/9 D25 7505 509 2.9 73 Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 236 Os t erman, E. – Do vjak , M. – V aupo tič, J. – V erbajs, T . – Mlak ar , U . – Za vrl, E. – S tritih, U . It was envisaged that the fresh air is distributed equally among SCR 1_B – SCR 6_B. We can see that the probability of infection is still below 1.5 % (Fig. 3). The reproduction number is the highest in SCR 5_B (0.21 – Fig. 4), which is also the most problematic classroom because it has no windows. In Figs. 1 to 4, the values only consider the transmission of the virus by air in aerosols, i.e., assuming a distance of 1.5 m between occupants. Transmission with contact or droplets is not taken into account. The evaluation of the adequacy of the value of ventilation was carried out with the legally required [24] and recommended values [26], where large classrooms (LCR 1_G – LCR 6_G) require 30 m 3 /h air per occupant. 4 loads of classrooms were inspected (scenarios S1 – S4) according to the number of occupants present, which were determined for each classroom separately: a) b) Fig. 1. Probability of infection for the ground floor and the ventilation of a maximum value of; a) 50 %, and b) 80 % a) b) Fig. 2. Event reproduction number for the ground floor and the ventilation of a maximum value of; a) 50 %, and b) 80 % a) b) Fig. 3. Probability of infection for the basement and the ventilation of a maximum value of; a) 50 %, and b) 80 % Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 237 Analysis of Educational Building’s Ventilation Suitability to Prevent the Spread of Coronavirus (SARS-CoV-2) S1: Subject to all regulations and COVID-19 recommendations, safety distance between occupants 1.5 m. S2: Half occupancy of classrooms. S3: Maximum load after the epidemic (full occupancy of classrooms with occupants). S4: Sufficient air volume (30 m 3 /h/occupant), 1.5 m distance between occupants not considered. Table 4 lists the number of seats in each classroom, the maximum airflow that AC units can supply (100 % capability), required airflow rate according to S1 and S2. The amounts of air determined by the REHV A calculator for one-third load of classrooms with users (S1) represent a minimal risk of infection and at the same time meet the requirements of the regulations and recommendations of the standard. The quantities for the anticipated half-load in scenario S2 are sufficient and meet the requirements of the rules and recommendations of the standard, except for LCR 4_G and SCR 5_B. It should be noted that the quantities in S2 are valid only for the time after COVID-19 as minimum distance of 1.5 m is not achieved. The quantities set for the estimated maximum load in scenario S3 do not meet the requirements. The S4 scenario includes the maximum number of students in each classroom to meet the requirements. This scenario is taken into account only in the period after the end of the COVID-19 pandemic, as it does not take into account the distance of 1.5 m between space users. 5 CONCLUSIONS Using the REHV A calculator, the probability of infection due to the spread of coronavirus through aerosol particles and the reproduction number for a) b) Fig. 4. Event reproduction number for the basement and the ventilation of a maximum value of; a) 50 %, and b) 80 % Table 4. Set of scenarios on occupational load, airflow and AC capability Classroom Number of seats [-] AC capability at 100 % [m 3 /h] S1: Covid - needed airflow [m 3 /h] S2: 50 % occupancy - needed airflow [m 3 /h] S3: max occupancy – needed airflow [m 3 /h] S4: max number of occupants at 100 % capability [-] LCR 1_G 210 4500 840 3150 6300 150 LCR 2_G 196 4500 750 2940 5880 150 LCR 3_G 270 5800 960 4050 8100 193 LCR 4_G 330 5800 1170 4950 9900 193 LCR 5_G 304 9400 1050 4560 9120 313 LCR 6_G 200 9400 1260 6000 6000 313 SCR 1_B 63 1250 300 945 1890 41 SCR 2_B 56 1250 240 840 1680 41 SCR 3_B 56 1250 450 840 1680 41 SCR 4_B 42 1250 360 630 1260 41 SCR 5_B 70 1250 600 1050 2100 41 SCR 6_B 12 4500 180 180 360 41 S1: Subject to minimal required airflow and COVID-19 recommendations, safety distance between persons 1.5 m. S2: Half occupancy of classrooms. S3: Maximum load after the epidemic (full occupancy of classrooms with occupants). S4: Sufficient air volume (30 m 3 /h/person), 1.5 m distance between persons not taken into account. Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 233-239 238 Os t erman, E. – Do vjak , M. – V aupo tič, J. – V erbajs, T . – Mlak ar , U . – Za vrl, E. – S tritih, U . each classroom at the selected educational building were calculated. Considering the distance between occupants 1.5 m and wearing the masks of all participants, the probability of infection was always lower than 1 %. The acceptable reproduction number is less than 0.1 which was achieved in most of the cases. The most critical are cases with 50 % capability and when the occupancy time approaches 12 h. In reality, such a case is highly unlikely therefore spread of the virus should not be an issue. In the calculations for the maximum allowed number of people in each classroom assuming all corona measures, i.e. 1.5 m distance and wearing the mask of all participants, about a third of the number of seats could be occupied. The AC system was analysed also according to the required amount of fresh air to define how many people can be in individual classrooms with four specific scenarios (S1 to S4). It should be noted that scenarios S2, S3 and S4 are not appropriate during the COVID-19 situation and do not take into account the distance of 1.5 m, but the prescribed value of the fresh air is guaranteed according to the rules (ventilation rate of 30 m 3 /h/occupant). Due to the construction of the ventilation system at the educational building before 2002, when the Rules on ventilation and air conditioning of buildings [27] were amended, we concluded that the occupancy of large classrooms could be 70 % if the ventilation system is operating at full power. 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