# Comparison of Portable and Large Mobile Air Cleaners for Use in Classrooms and the Effect of Increasing Filter Loading on Particle Number Concentration Reduction Efficiency

^{1}

^{2}

^{*}

## Abstract

**:**

^{3}). The decay rates of the particle number concentration (PNC) were measured simultaneously at up to four positions in the room. It was found that the different air outlet configurations of the units have an effect on the actual PNC removal in the room when operated at the same volume flow rates. This effect of the airflow efficiency of the air cleaners (AP) in a classroom is quantified with an introduced Air Cleaning Efficiency Factor in this study to identify beneficial airflows. In this context, the effect of filter loading in long-term operation on the cleaning effect is also investigated. The emitted sound pressure levels of the APs are given special attention as this is a critical factor for use in schools, as well as power consumption. A total of six different devices were tested—two portable APs and four large APs. In order to achieve the necessary volume flow rates, three or four of the portable units were used simultaneously in one room, while only one of the large units was used per room. When used at the same air circulation rates in the room, the portable APs exhibit higher sound pressure levels compared to the large APs. At air circulation rates of 4–5 h

^{−1}, the portable APs exceeded a value of 45 dB(A). Two of the four large units reach sound pressure levels below 40 dB(A) at air circulation rates of 4–5 h

^{−1}, whereby both large units, which are positioned on the rear wall, realize a homogeneous dilution of the room air. This is achieved by an air outlet directed horizontally at a height above 2 m or diagonally towards the ceiling, which points into the room and partly to the sides. On the other hand, an air outlet directed exclusively to the sides or horizontally into the room at floor level to all sides achieves lower particle decay rates. To investigate the influence of the filter loading, three large APs were operated in a school for a period of one year (190 days with 8 h each). For the three APs, long-term operation leads to different changes in PNC reduction efficiency, ranging from −3% to −34%. It is found that not only the size of the prefilter and main filter has a significant influence, but also whether there is a prefilter bypass that negatively affects the loading level of the main filter. At the same time, it was shown that one type of AP, measuring the pressure drop across the filters and readjusting the fan, kept the circulation rate almost constant (up to −3%) over a year.

## 1. Introduction

^{−1}) [21,22]. If viruses are attached to small airborne particles, which in turn have half-lives of more than one hour [23], an atmosphere can be created in closed rooms that favors infections [20]. A critical measure of the probability of infection indoors is the ratio of people to room volume [6], as well as their length of stay [24]. If both are high, such as in classrooms, it must be ensured that the respiratory aerosols, but also humidity, and emitted CO

_{2}are removed from the room. Although at the beginning of the pandemic the role of schoolchildren in the spread of SARS-CoV-2 was sometimes considered to be minor [25], there were documented reports that schools contributed to the spread of infection [26,27,28]. Exhaled particles, humidity, and CO

_{2}can only be removed in a controlled manner indoors by pre-installed ventilation systems that supply the room with fresh air and heat it if necessary. Such pre-installed systems are not widespread in German schools, which is mainly due to the high construction costs and the sometimes complex reconstruction measures. Therefore, the most widespread means of improving indoor air quality is window ventilation. This natural ventilation depends on factors such as ventilation area, temperature gradient between indoor and outdoor air, and wind direction and speed. Whether the wind speed (wind pressure) or the temperature difference is the dominant factor for the air exchange rate depends on the angle of incidence, according to [29]. In addition, studies show that the human factor also limits effective window ventilation. For example, on cold days, when window ventilation is more effective than on warm days [30], significantly less ventilation takes place as people are less likely to open windows for thermal comfort reasons [31]. Guidelines published by the German Federal Environmental Agency on ventilation state that window ventilation should be performed every 20 min [32]. However, a study involving more than 7000 measurement days in classrooms, ref. [33] showed that this guideline was followed in practice in less than 8% of cases. Thus, it is not possible to ensure a controlled and defined air exchange with solely natural ventilation.

_{2}concentration nor on the air humidity, which is why they can only be considered as a supporting measure to natural ventilation. Similarly, masks and physical distance indoors can result in a reduction in the probability of infection [36], but this does not replace ventilation either.

^{3}. To evaluate the usefulness of APs, it is not sufficient to determine the theoretical air exchange rate from the AP volume flow rate and the room size. Rather, the actual cleaning effect in real room conditions should be evaluated or compared. This aspect is taken up by the so-called ‘expert recommendation’ of German scientists that specifically deals with air cleaners. This recommendation states that in order to achieve a significantly high cleaning effect, an AP should remove at least 90% of the particles from the room air within 30 min [43], which corresponds to a decay rate of λ

_{min}= 4.6 h

^{−1}. The decay rate of the particle number concentration (PNC) in a room can be described according to Equation (1), where C

_{0}is the PNC at the start and C

_{t}is the concentration at a time t.

^{3}/h/person are recommended [44], although these refer to supplied outdoor air. The value can therefore only be considered under the assumption that at an acceptable CO

_{2}concentration according to [44], the exhaled PNC can also be kept at a low level. With a volume flow of 36 m

^{3}/h/person, decay rates of 4.5–4.8 h

^{−1}result in a room volume of 186 m

^{3}and 23–25 persons.

_{t}does not allow a direct statement about the particle decay rate that will occur in a room when an AP is in operation. A more conclusive local CADR in the room can be determined using Equation (3). Here, the CADR

_{m}is determined considering the room volume (V

_{room}) and the practically measured decay rates (λ) resulting from Equation (1). Additionally, the decay rates are divided into those caused by the air cleaner (AP) and those caused by natural processes (nat). How the CADRs from Equations (2) and (3) can relate to each other is discussed in Section 2.4 (Scenario 2).

_{t}is supposed to be kept constant; the other units do not readjust. The filter areas of the units without automatic adjustment of the fan power differ significantly, so conclusions can be drawn about the influence of the filter area or whether an increasing pressure drop of the filters have a relevant influence on the volume flow rates and therefore on the CADR.

## 2. Air Cleaners and Setup

#### 2.1. Portable/Mobile/Large Air Cleaners Used

#### 2.1.1. Portable Air Cleaners

#### 2.1.2. Large and Mobile Air Cleaners

^{3}/h. The devices considered are the RLC2000-X from AFS Airfilter Systeme GmbH (Uebrigshausen, Germany), hereafter referred to as AP A, the AirCO2NRTOL from KEMPER GmbH (Vreden, Germany), hereafter AP K, the TAP-L from TROX GmbH (Neukirchen-Vluyn, Germany) (AP T), and the AirPurifier from WOLF GmbH (Mainburg, Germany) (AP W).

^{2}and the areas of the HEPA filters from 13.0 to 30.8 m

^{2}. The prefilters are intended to separate coarser particles in the first step before the aerosol passes through the HEPA filter. This results in different replacement intervals for the filter types, which according to the standard [46], are one year for prefilters and two for the main filters. However, reference is also made to the manufacturer’s specifications, which may differ in some cases. Only AP A has a different specification for the HEPA filter, which stipulates a replacement interval of one year. All units have an EC fan, which is installed after the filters (AP A, AP K, AP T) or between the prefilter and the main filter (AP W).

^{3}/h (AP T, AP W) or 1060 m

^{3}/h (AP A).

^{3}/h (AP T, AP W) and 1060 m

^{3}/h (AP A), respectively.

^{2}). However, the power consumption of this AP is comparatively high (Figure A2).

#### 2.2. Measurement Equipment

#### 2.2.1. Aerosol Spectrometer and Generator

^{3}, with a mass-related maximum value of 20,000 µg/m

^{3}.

_{1}, PM

_{2.5}, PM

_{4}, PM

_{10}), as well as temperature, pressure, relative humidity, CO

_{2}concentration, and volatile organic hydrocarbons (TVOC). The specifications given by the manufacturer can be found in the appendix of [41].

^{9}particles/s (1.6 × 10

^{7}particles/cm

^{3}) with a size of ≥0.2 µm are generated.

#### 2.2.2. Sound Analyzer and Power Meter

#### 2.3. Setup of the Test Series

^{3}with a ceiling height of 3.05 m. For more information on the arrangement of the furniture and lamps, we refer to [41].

#### 2.4. Experimental Procedure and Scenarios Investigated

^{3}on average can be detected. This is followed by the measurement with air cleaners turned on and with the aerosol generator and the fan switched off. The following measurement scenarios are covered:

**Scenario 1:**In the first step, the local homogeneity of the time-resolved PNC is measured by means of four measuring positions distributed in the room (Figure 3). Measurements are also carried out without APs to map the natural decay behavior (λ

_{nat}) of the particles in the room. Especially with large air cleaners where only one device is used, it must be proven by measurement that the decay rates of the PNC are similar everywhere in the room. Furthermore, the set volume flow rates are examined by separate measurements in order to be able to classify any differences in the measurement results obtained. The volume flow rates are calculated from the flow velocities in an attached pipe by means of a small vane anemometer. Here, eight measuring points per radius are used, while the measurement radii in the circular pipe are determined according to the trapezoidal rule [58].

**Scenario 2:**In the second step, the measured PNC decay behavior in the room, caused by the applied air cleaners at different set volume flows, is addressed. Recommendations already introduced in Section 1 provide for air circulation rates of 4–5 h

^{−1}, which should be realized in the room. Based on the measured decay rates at different measuring points, statements about beneficial air flow patterns and separation efficiencies of the different devices can be derived. This comes up against its limits when different units cannot be set to the same volume flow rate. Therefore, an efficiency factor is introduced below to compare the particle decay rate caused by different APs at different volume flows in each case.

_{s}can also be determined by the CADR

_{m}measured at a measuring point in the room in relation to the CADR

_{t}theoretically provided by the unit, which is then multiplied by the filter efficiency. Here, the CADR

_{t}is related to the volumetric flow rate ${\dot{V}}_{s}$ set on the unit, and thus to the manufacturer’s specification. It is convenient to use the set volume flow rate, since in practical applications the true volume flow rate would have to be determined with relatively high effort. The ACEF is proportional to filter efficiency according to Equation (4). It should be mentioned here that lower filter efficiencies in a certain range (e.g., >99%) are not necessarily disadvantageous, as the lower flow resistance leads to lower power consumption and lower noise emissions, which is further explained in Section 3.3. Therefore, the ACEF must always be assessed in relation to noise emissions as a critical measure.

_{t}≈ CADR

_{m}from Equations (2) and (3) applies. An ACEF of less than 1 indicates a non-optimal turbulent mixed flow, for example, due to “short-circuiting” or filter efficiencies that are significantly lower than 1. If the ACEF is >1, the condition of partial displacement flow is reached, which, in this case, means that the air sucked in by the AP would have a higher PNC than the average room air. A real displacement flow with a low degree of turbulence in the conventional sense cannot be generated by APs, however.

_{t}(Equation (2)) therefore refers to the measured (m) volume flow.

**Scenario 3:**The third phase of the study focuses on the long-term performance of those APs that were able to prove their suitability for classrooms in the previous phase. In that case, the APs were tested again after one year in operation. The units were in use every school day, which results in an operating duration of approximately 190 days of 8 h. According to the guideline [46], the prefilters must be changed annually, and the HEPA filters every second year unless otherwise specified by the manufacturer. The effect of the filter loading of the different APs varies due to different filter areas or the use of a volume flow control technology.

## 3. Results and Discussion

#### 3.1. Evaluation of the Spatial Homogeneity of the PNC in a Classroom (Scenario 1)

_{nat}was measured separately. Without the use of APs, the average natural decay rate (λ

_{nat}) is 0.092 h

^{−1}, whereby the deviations at MPs 1–3 are small (0.092–0.136 h

^{−1}); MP 4, however, with 0.028 h

^{−1}, is significantly lower. One possible reason is the position of MP 4, which is in a corner. Here, a lower convective flow velocity is to be expected in the corner, opposite the windows and heaters, so that deposition processes on surfaces might be lower.

^{3}/h (AP K, AP T, AP W) or 1060 m

^{3}/h (AP A), the decay rates at measurement points (MP) 1–4 (Figure 3, Table A1) vary less for the large APs with the air outlet at the top of the unit than for AP K, where it is at floor level. The maximum deviation between the decay rates at the different MPs 1–4 is 2% for AP A, 2.5% for AP T, and 4% for AP W. It is found that a forward discharge direction of the filtered air into the room is advantageous (AP A, AP T) compared to an exclusively lateral discharge direction (AP W). If the filtered air is supplied to the room at floor level, as in the case of AP K, there is a systematic deviation of the decay rates at the measuring points. For example, the decay rate at MP 4 (Figure 3), which is positioned closest to AP K on the right, is up to 11% higher than that of MPs 1–3. MP 3 is almost the same distance from AP K as MP 4, but there are several chairs and a table between MP 3 and the AP.

^{3}) with filtered air from the APs can thus be demonstrated for the large air cleaners at a set volume flow of around 1000 m

^{3}/h, with the mentioned limitation of AP K.

#### 3.2. Verification of the Manufacturer’s Specifications Regarding the Volume Flow Rates

^{3}/h, AP K showed 1001 m

^{3}/h, AP T 1026 m

^{3}/h, and AP W 968 m

^{3}/h. In our measurements, AP A has a volume flow of 967 m

^{3}/h with a set flow rate of 1060 m

^{3}/h.

^{3}/h with a set of 333 m

^{3}/h. AP X shows a measured volume flow rate of 379 m

^{3}/h with a set volume flow according to the manufacturer of 500 m

^{3}/h. Due to the strong deviation between the nominal value and the measured value of AP X, the measurements were carried out on a second device, with the same outcome. At this point, it must be mentioned that the measurement method used has a measurement uncertainty of up to 10%. However, the significant deviation for AP X cannot be explained by this. The measured volume flows of the other adjustment levels are summarized in Table 4.

#### 3.3. Decay Rates of Investigated APs at Different Set Volume Flows (Scenario 2)

**I**.**Large APs**

^{3}/h, with deviations of a maximum of 1.5%. AP W shows deviations between the measuring points of 8.5% at set 690 m

^{3}/h. At higher set volume flows, however, the deviation between the measuring points is significantly lower (<3%) when using AP W.

_{s}of 0.62, 0.67, and 0.76 could be measured at set volume flows of 690, 1000, and 1200 m

^{3}/h, respectively. The filtering efficiency with the prefilter and the HEPA (H14) filter is >99.995% and, therefore, is not the factor that leads to the low ACEF

_{s}. Due to the ACEF

_{s}of well below one and the predefined fixed setting levels of AP W (levels ‘4’, ‘5’, ‘6’; see Table 2), a volume flow of 1200 m

^{3}/h must be set here for a room volume of 186 m

^{3}in order to achieve a decay rate of ≥4 h

^{−1}. With a set volume flow of 1000 m

^{3}/h, this is not achieved with 3.66 h

^{−1}.

^{−1}at a set flow rate of 800 m

^{3}/h, while AP A achieves 4.3 h

^{−1}at a set flow rate of 790 m

^{3}/h (Table 3). The high values are achieved due to the good flow distribution in the room, which can be explained by the large discharge distance of the filtered air into the room, which, unlike AP W, can use the entire length of the room without reaching a wall (Figure 2). This is also reflected in the ACEF

_{s}(Equation (4)) with an ACEF

_{s}

_{,A}= 0.99 for AP A and an ACEF

_{s}

_{,T}= 1.07 for AP T. The value quantifying the efficiency remains the same for both air cleaners in the measured volume flow range (790–1200 m

^{3}/h). Thus, for higher volume flows of 1200 m

^{3}/h, decay rates of up to 7 h

^{−1}result for AP T (Table 3).

_{m}at the measuring points is 25% lower than the set and measured volume flow of the unit (ACEF

_{s}

_{,K}= 0.75). The CADR

_{m}results from the decay rate of the particles in the room of 4.1 h

^{−1}at a set volume flow of 1000 m

^{3}/h. It should be noted that the design of the air outlet, which is also partly directed towards the wall (Figure 2), can also be expected to reduce the ACEF

_{s}

_{,K}. The flow of filtered air directed towards the room wall cannot spread unhindered in the room and is partly drawn in again by the unit with a particle load that is significantly below the average PNC in the room (“short circuit”). It cannot be conclusively clarified here what share of the air outlet at floor level or the partial airflow towards the wall is responsible for the low ACEF

_{s}

_{,K}, but the results (e.g., Section 3.1) indicate that both play a significant role.

^{−1}. At decay rates of up to 4.7 h

^{−1}, a value of 35 dB(A) is not exceeded. The air cleaners AP K and AP W have higher continuous sound levels so that no decay rates ≥ 4 h

^{−1}at 40 dB(A) or less can be realized for a room volume greater than or equal to 186 m

^{3}. The measured sound levels are summarized in Table 3.

^{3}/h. The use of an H13 filter using AP K results in a reduction in the A-weighted equivalent continuous sound level of 1.5 dB(A), compared to an H14 filter, measured at a distance of 1 m at a height of 1.1 m. This is reasonable because a lower pressure drop across the filter reduces the required fan power.

^{−1}at low continuous sound levels of ≤40 dB(A) (see Section 2.4), which is a key factor for use in classrooms.

**II**.**Portable APs**

^{3}) were used. Since the manufacturers do not specify any volume flow rate information for the individual operating levels, except for the maximum level in each case, several levels were tested (Table 4). The volume flow rates were additionally measured (Section 3.2) and, due to the incomplete volume flow rate data provided by the manufacturers, the efficiency of particle removal in the room by the devices is thus related to the measured volumetric flow rates (Equation (5)), not to manufacturer specifications as is the case with the large APs.

^{−1}. In ‘turbo’ mode, the decay rate averages 6.59 h

^{−1}. The Air Cleaning Efficiency Factor (ACEF

_{m}), here related to the measured volume flow rate, remains at a comparable level for stages ‘2’–‘t’ with 0.94–1.01. These results must be seen in the context of sound emissions, again. For four units used, the continuous sound levels are 47.9 dB(A) (level ‘3’) and 53.9 dB(A) (level ‘turbo’), measured at a distance of 1 m at a height of 1.1 m from one air cleaner, while the other air cleaners were positioned according to Figure 3 and switched on. The values are summarized in Table 4 and considerably exceed the maximum values from the guidelines introduced in Section 2.4. If AP P is operated at levels ‘1’ or ‘2’, the sound emissions decrease significantly, but decay rates of less than 3.5 h

^{−1}are not sufficient (meaning < 4 h

^{−1}) for classrooms.

^{3}), like AP P, can efficiently remove particles from the room. The ACEF

_{m}is in the range of 1.02–1.09 for levels ‘8’, ‘12’, and ‘17’. A decay rate ≥ 4 h

^{−1}could be reproducibly demonstrated from level ‘12’ with 5.04 h

^{−1}, whereby levels ‘9’–‘11’ were not examined due to a large number of adjustable levels. Level ‘8’ realizes a particle decay rate of 3.63 h

^{−1}at a measured continuous sound level of 45 dB(A). According to the guidelines introduced in the introduction, the decay rate is not sufficient for classrooms; moreover, the sound level is already too high. At the continuous sound level of 35 dB(A) (level ‘4’) to be aimed for in classrooms, the measured decay rate is 2.43 h

^{−1}. The results are summarized in Table 4.

^{−1}, which here refers to a room volume of 186 m

^{3}. Based on the measured volume flow rates provided by the units, however, the resulting decay rates are high, which results in a high ACEF

_{m}in the range of 0.94–1.09.

^{−1}can only be realized at continuous sound levels significantly above 40 dB(A), the portable devices were not investigated in a long-term test in schools and are thus missing in the investigations of the following scenario 3.

#### 3.4. Influence of the Filter Loading Level after an Operating Period of One Year (190 Days of 8 h) (Scenario 3)

^{2}; AP T 16.8 m

^{2}; AP W: 3.0 m

^{2}) and in the classification according to the international standard [63] (see Figure A2). The same standards apply to the replacement interval, which is specified as one year. The main filters (AP A: 16.0 m

^{2}(H13); AP T 30.6 m

^{2}(H13); AP W: 13.0 m

^{2}(H14)) of the HEPA class according to [34] do not have to be changed in the first year either (every second year unless the manufacturer’s specifications differ). Due to the increasing degree of loading of the filters over the year, the pressure drop across the filters increases with constant fan power, resulting in lower cleaning performance. This can be seen in the PNC decrease at MP 1 and 2 when using AP A (set volume flow 1060 m

^{3}/h) in Figure 6a. The diagrams show the decay curve (according to Equation (1)) of the PNC when using new prefilters and new main filters, as well as new prefilters and main filters used for one year, and prefilters and main filters used for one year.

_{s}can be calculated according to Equation (4), where the volume flow rate of the respective AP is related to the set volume flow rate which refers to the operation with new filters. With a new prefilter and main filter, AP A achieves an Air Cleaning Efficiency Factor (ACEF

_{s}) of one, (Equation (4)). After one year of use (190 days of 8 h), this efficiency is reduced by 34% (Table 5). If the prefilter is replaced after one year, with the HEPA filter remaining in place, the ACEF

_{s}drops by 5% compared to use with a new HEPA filter. Thus, the prefilter with an area of 0.3 m

^{2}has a significant influence on the decay rate of the PNC in the room and, thus, the efficiency (−30%). At an initial decay rate of 5.8 h

^{−1}with a set volume flow of 1060 m

^{3}/h, a decay rate of less than 4 h

^{−1}is achieved after one year of use (3.9 h

^{−1}). Due to the unplanned disposal of the prefilter, no measurements of the changed continuous sound level and power consumption through the usage time of one year could be carried out.

^{3}) due to the prefilter area being larger by a factor of 10 (3 m

^{3}). Thus, the prefilter results in a loss of efficiency of 11% after one year. The HEPA filter has a minor influence with a decrease in efficiency of 1.5%. The operating life also influences the continuous sound level, which increases by 1.6 dB(A) on average, while the power consumption decreased by 6.5% (Table 5).

_{s}, which is −30% for small filter areas (AP A with 0.3 m

^{2}). Larger filter areas reduce the efficiency losses significantly so that with a filter area of 3 m

^{2}(AP W), the measured CADRs in the room is 11% lower in relation to the set volume flow. The filter area is not always the decisive factor, as the comparison of the main filters of AP A (16 m

^{2}) and AP W (13 m

^{2}) shows. Despite the larger main filter area, the particle decay rate of AP A decreases stronger (−5%) than that of AP W (−1.5%). A visual inspection of the filters provides a possible explanation for this. A prefilter bypass on AP A resulted in the deposition of coarse dust particles on the main filter. This was the case with all five units of AP A investigated in this study. The HEPA filters of AP T (6 units) and AP W (5 units) did not show any visible contamination during a visual inspection.

#### 3.5. Limitations of This Study

## 4. Conclusions

^{3}. All devices examined were able to realize particle decay rates > 4 h

^{−1}in the room. Due to fundamental differences in the discharge direction and height of the air outlet of the units, there are differences in the Air Cleaning Efficiency Factor (ACEF), which result from the calculated decay rate and the room volume related to the volume flow rate of the APs.

^{3}/h resulted in increasing ACEF

_{s}from 0.62 to 0.76. At filter efficiencies of one, an ACEF of one represents an optimum turbulent mixed flow in the room. If this value is considerably lower, this indicates a so-called ‘short-circuiting’ of the filtered air. The other type of large unit (AP T) has, in addition to the air outlet to the sides, also one to the front, in the direction of the center of the room, which leads to significantly higher ACEF

_{s}of 1.07, which are also constant in the investigated volume flow range of 800–1200 m

^{3}/h. Similarly, high Air Cleaning Efficiency Factors can be demonstrated for an air outlet directed exclusively into the room (AP A), initiated diagonally towards the ceiling, although these are slightly lower on average at 0.99, but are also robust over the volume flow range investigated (790–1190 m

^{3}/h). An air outlet horizontally at floor level (AP K), with an air outlet on all sides, showed a low ACEF of 0.75. The respective ACEF must always be seen in the context of the continuous sound level. Only for the two unit types with ACEF

_{s}≈ 1 are continuous sound levels of 35 dB(A) not exceeded at particle decay rates of >4 h

^{−1}(up to 4.7 h

^{−1}); for increased volume flow rates, at 40 dB(A), even decay rates of up to 5.8 h

^{−1}can be achieved for a room volume of 186 m

^{3}.

^{3}) are used here simultaneously. The portable APs with a vertical discharge direction archive a homogeneous reduction in the particle number concentration, which reaches efficiencies (ACEF

_{m}) of 0.94–1.09, where these efficiencies were calculated on the basis of measured (m) volume flow rates. The portable units could only achieve desired particle decay rates of ≥4 h

^{−1}at continuous sound levels significantly above 40 dB(A), making them unsuitable for continuous operation in classrooms.

_{s}) decreases by 30% after the operating time of one year (190 days of 8 h) with a prefilter area of 0.3 m

^{2}(AP A), and by 11% with a filter area of 3.0 m

^{2}(AP W). The degree of loading of the main filters after one year of use in school operation has a smaller influence, with a drop in efficiency of 5% and 1.5%, for filter areas of 16 m

^{2}(AP A) and 13 m

^{2}(AP W), respectively. Here, the filter size of the main filter is not necessarily the decisive measure for decreasing efficiency, but also possible leakage around the prefilter, which revealed contamination by coarse dust particles through a visual inspection of the main filter in all units of a unit type (AP A). If a control system automatically adjusts the fan’s capacity to compensate for increasing pressure losses across the filters, the Air Cleaning Efficiency Factor (ACEF

_{s}) of the AP (AP T) in the room can be kept almost constant (fluctuations around 3%).

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Appendix A

**Figure A1.**Manufacturer’s data of the two portable air cleaners used in this study [40]. Data in the area of ‘air intake’ and ‘air outlet’ are partly from the manufacturers and partly from fog tests and measurements carried out by the authors.

**Figure A2.**Manufacturer’s data of the four large air cleaners used in this study. Data in the area of ‘air intake’ and ‘air outlet’ are partly from the manufacturers and partly from fog tests and measurements carried out by the authors.

**Table A1.**Coordinates (in meters) of measurement points (MP) 1–4 according to Figure 3, where z represents the height.

x in m | y in m | z in m | |
---|---|---|---|

MP 1 | 0.70 | 3.25 | 1.10 |

MP 2 | 6.00 | 3.25 | 1.10 |

MP 3 | 4.00 | 1.20 | 1.10 |

MP 4 | 7.70 | 5.30 | 1.10 |

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**Figure 2.**Illustration of the position of the investigated large APs within the classroom ((

**a**). AP A, (

**b**). AP K, (

**c**). AP T, (

**d**). AP W) including the qualitative flow direction at the outlet. Note: the length of the arrows and the marked area are intended to qualitatively represent the outlet direction and therefore they do not allow any conclusions to be drawn about the throw distance of the filtered air.

**Figure 3.**Sketch of the classroom with tables and chairs and the locations of the large (L) APs as well as the small (S) devices according to Table 2, including the positions of 4 different measuring points (MP 1–4). The grey rectangles represent tables and the squares with grey contours represent chairs.

**Figure 4.**Normalized aerosol number concentration profile measured at measurement point MP1 and MP2 in a 186.4 m

^{3}classroom during operation of air cleaner (

**a**). AP A; (

**b**). AP T; (

**c**). AP W with different AP volume flow settings. The markers (·, ×, Δ) indicate the measurement results, and the lines the regression function according to Equation (1).

**Figure 5.**Time decay behavior of the particle number concentration when using portable air cleaners ((

**a**). 4×AP P; (

**b**). 3× AP X) at different operating levels as well as without air cleaners in a classroom (186 m

^{3}). The markers indicate the measurement results and the lines the regression function according to Equation (1).

**Figure 6.**Normalized aerosol number concentration profile measured at measurement point MP1 and MP2 in a 186.4 m

^{3}classroom during operation of air cleaner, (

**a**). AP A; (

**b**). AP T; (

**c**). AP W at set volume flow 1000 m

^{3}/h with new prefilters and heap filters (0 a) and filters used for one year (1 a corresponds to 190 days of 8 h). The markers (·, ×, Δ) indicate the measurement results and the lines the regression function according to Equation (1).

**Table 1.**Overview of studies on air cleaners (AP) in schools from different years and countries with different types of APs, all of which have HEPA filters.

Source/Year | Type of AP | Country | Measurement Results |
---|---|---|---|

[37], 2017 | portable AP (HEPA) | US | PM_{2.5}, black carbon (BC), noise level |

[38], 2020 | not specified | Korea | PM_{2.5}, PM_{10}, CO_{2} |

[39], 2021 | large AP (HEPA) | Germany | particle number concentration, particle size distribution |

[40], 2021 | portable AP (HEPA) | Germany | particle number concentration, particle size distribution, PM_{10}, CO_{2}, noise level |

[41], 2021 | 3 large APs (HEPA) | Germany | particle number concentration, particle size distribution, noise level |

[42], 2023 | mobile AP (HEPA) | Germany | particle number concentration, air velocity, noise level |

**Table 2.**Assignment of the different air cleaners to the set-up positions in the classroom according to Figure 3.

Type of AP | Position of Air Cleaners | ||||
---|---|---|---|---|---|

(L) | (S1) | (S2) | (S3) | (S4) | |

AP A | x | ||||

AP K | x | ||||

AP T | x | ||||

AP W | x | ||||

AP P | x | x | x | x | |

AP X | x | x | x |

**Table 3.**Decay rates, resulting CADR

_{m}(according to Equation (3)), efficiency ACEF

_{s}(Equation (4)), and time after 10% of initial particle number concentration is reached as well as A-weighted equivalent continuous sound level (L

_{Aeq}) and power consumption (P) at different set volume flows of AP A, AP K, AP T, AP W.

Manufacturer’s Information | Measured Values | ||||||||
---|---|---|---|---|---|---|---|---|---|

AP | Level | Volume Flow (m^{3}/h) | Decay Rate (λ_{AP}) in 1/h | t(C_{t}/C_{0} = 0.1) in min | CADR_{m}in m ^{3}/h | L_{Aeq}in dB(A) | P in W | ACEF_{s} | |

MP1 | MP2 | ||||||||

AP A | 5 | 790 | 4.28 | 4.25 | 32.4 | 776.4 | 33.5 | 85.80 | 0.98 |

6 | 1060 | 5.83 | 5.83 | 23.7 | 1068.1 | 39.7 | 158.70 | 1.01 | |

6.5 | 1190 | 6.28 | 6.23 | 22.1 | 1147.3 | - | - | 0.97 | |

AP K | 1000 | 1000 | 4.12 | 4.13 | 33.5 | 752.7 | 48.7 | 248.30 | 0.75 |

AP T | 800 | 800 | 4.72 | 4.66 | 29.5 | 855.6 | 35.1 | 67.30 | 1.07 |

1000 | 1000 | 5.89 | 5.89 | 23.5 | 1079.3 | 40.7 | 106.15 | 1.08 | |

1200 | 1200 | 6.98 | 7.00 | 19.8 | 1284.3 | - | - | 1.07 | |

AP W | 4 | 690 | 2.30 | 2.51 | 57.4 | 429.7 | 38.7 | 81.20 | 0.62 |

5 | 1000 | 3.61 | 3.71 | 37.7 | 663.6 | 43.2 | 162.15 | 0.67 | |

6 | 1200 | 4.94 | 4.97 | 27.9 | 905.0 | - | - | 0.76 |

**Table 4.**Decay rates determined by the measurement results (room volume 186.4 m

^{3}) at measuring points (MP) 1 and 2, the resulting CADR

_{m}, the measured (m) volume flow rate $\dot{V}$, the efficiency ACEF

_{m}(Equation (5)), and time after 10% of initial particle number concentration is reached (Equation (3)), the power consumption of all (n) units used at the same time, A-weighted equivalent continuous sound levels (L

_{Aeq}) measured at one unit and n units measured at a distance of 1 m at a height of 1.1 m for the portable air cleaners AP P and AP X.

Level | Decay Rate in 1/h (Range in 1/h) | t(C_{t}/C_{0} = 0.1)in min | CADR_{m}in m ^{3}/h | ${\dot{\mathbf{V}}}_{\mathbf{m},\mathbf{n}}$ in m^{3}/h
| L_{Aeq,n = 1}in dB(A) | L_{Aeq,n}in dB(A) | Power Consumption (P _{n}) in W | ACEF_{m} | |
---|---|---|---|---|---|---|---|---|---|

MP 1 | MP2 | ||||||||

AP P (n = 4) | |||||||||

1 | 2.35 (0.01) | 2.38 (0.13) | 58.4 | 423.7 | - | <30.0 | 32.1 | 22.4 | - |

2 | 3.36 (0.08) | 3.45 (0.05) | 40.6 | 617.5 | 614 | 38.7 | 41.1 | 39.8 | 1.01 |

3 | 4.48 (0.18) | 4.53 (0.06) | 30.7 | 822.6 | 857 | 45.2 | 47.9 | 70.0 | 0.96 |

t | 6.58 (0.01) | 6.59 (0.08) | 21.0 | 1210.3 | 1289 | 53.9 | 56.4 | 174.9 | 0.94 |

AP X (n = 3) | |||||||||

4 | 2.36 (0.23) | 2.49 (0.09) | 57.0 | 434.9 | - | 32.4 | 35.2 | 22.2 | - |

6 | 3.12 (0.02) | 3.31 (0.03) | 43.0 | 582.1 | - | 40.2 | 43.2 | 36.1 | - |

8 | 3.55 (0.10) | 3.71 (0.02) | 38.1 | 659.5 | 606 | 42.1 | 45.1 | 48.2 | 1.09 |

12 | 4.94 (0.04) | 5.03 (0.03) | 27.7 | 912.1 | 885 | 49.6 | 52.5 | 96.5 | 1.03 |

17 | 6.31 (0.02) | 6.38 (0.01) | 21.8 | 1165.6 | 1138 | 55.3 | 58.0 | 168.4 | 1.02 |

**Table 5.**Decay rates determined by the measurement results (room volume 186.4 m

^{3}) at measuring points (MP) 1 and 2, the resulting CADR

_{s}, the ACEF

_{s}(Equation (4)), and time after 10% of initial particle number concentration is reached (Equation (3)), the power consumption and the A-weighted equivalent continuous sound levels (L

_{Aeq}) measured at a distance of 1 m at a height of 1.1 m depending on the usage time of the prefilters and HEPA filters of the air cleaners AP A, AP T, and AP W when new (0 a) and after one year (1 a) (190 days of 8 h).

Usage Period in a | Measured Values | ||||||||
---|---|---|---|---|---|---|---|---|---|

AP | Prefilter | HEPA Filter | Decay Rate (λ_{AP}) in 1/h | t(C_{t}/C_{0} = 0.1) in min | CADR_{m} in m^{3}/h | L_{Aeq}in dB(A) | P in W | ACEF_{s} | |

MP1 | MP2 | ||||||||

AP A | 0 a | 0 a | 5.80 | 5.75 | 23.9 | 1058.6 | 38.7 | 158.7 | 1.00 |

0 a | 1 a | 5.50 | 5.50 | 25.1 | 1007.6 | - | - | 0.95 | |

1 a | 1 a | 3.86 | 3.88 | 35.7 | 703.9 | 38.6 ^{(a)} | 156.2 ^{(a)} | 0.66 | |

AP T | 0 a | 0 a | 5.90 | 5.87 | 23.5 | 1079.5 | 40.7 | 106.2 | 1.08 |

0 a | 1 a | 5.94 | 5.84 | 23.5 | 1080.9 | 40.6 | 109.0 | 1.08 | |

1 a | 1 a | 5.77 | 5.73 | 24.0 | 1053.9 | 40.5 | 122.0 | 1.05 | |

AP W | 0 a | 0 a | 3.64 | 3.70 | 37.6 | 667.4 | 43.2 | 162.2 | 0.67 |

0 a | 1 a | 3.42 | 3.85 | 38.0 | 660.2 | - | - | 0.66 | |

1 a | 1 a | 3.14 | 3.35 | 42.6 | 587.4 | 44.6 | 151.7 | 0.59 |

**Note:**

^{(a)}: the values refer to a usage period of 3 months.

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**MDPI and ACS Style**

Duill, F.F.; Schulz, F.; Jain, A.; van Wachem, B.; Beyrau, F.
Comparison of Portable and Large Mobile Air Cleaners for Use in Classrooms and the Effect of Increasing Filter Loading on Particle Number Concentration Reduction Efficiency. *Atmosphere* **2023**, *14*, 1437.
https://doi.org/10.3390/atmos14091437

**AMA Style**

Duill FF, Schulz F, Jain A, van Wachem B, Beyrau F.
Comparison of Portable and Large Mobile Air Cleaners for Use in Classrooms and the Effect of Increasing Filter Loading on Particle Number Concentration Reduction Efficiency. *Atmosphere*. 2023; 14(9):1437.
https://doi.org/10.3390/atmos14091437

**Chicago/Turabian Style**

Duill, Finn Felix, Florian Schulz, Aman Jain, Berend van Wachem, and Frank Beyrau.
2023. "Comparison of Portable and Large Mobile Air Cleaners for Use in Classrooms and the Effect of Increasing Filter Loading on Particle Number Concentration Reduction Efficiency" *Atmosphere* 14, no. 9: 1437.
https://doi.org/10.3390/atmos14091437