ASHRAE Journal - August 2020 - 28

TECHNICAL FEATURE

Normalized Particle Concentration

MS2 aerosol was introduced upstream of the mixing
FIGURE 2 Particle size distribution of MS2 bacteriophage aerosol.
baffles. An 8 ft (2.4 m) duct section was inserted between
0.12
the mixing baffles and the sampling probe of the
upstream impactor to improve the mixing. The distance
0.1
between the aerosol injection point and the upstream
0.08
sampling probe was about 10 ft (3 m), allowing uniform
0.06
mixing of the clean air and aerosol and complying with
0.04
the ASHRAE Standard 52.2-2017 requirement for achiev0.02
ing adequate mixing.
0
Figure 2 shows the particle size distributions of the MS2
1
10
100
1,000
bacteriophage aerosol in the test duct measured using
Particle Size (nm)
a scanning mobility particle sizer (SMPS) at three difC 1,024 cfm
C 1,200 cfm
C 2,000 cfm
ferent duct airflow rates. The particle size distributions
were similar, indicating the stability of the MS2 aerosol
TABLE 1 PFU measurements in empty duct.
in the test duct at different residence times, i.e., 1.3 s at
3
3
SAMPLE
UPSTREAM
DOWNSTREAM UPSTREAM/DOWNSTREAM
3400 m /h (2,000 cfm) and 2.5 s at 1740 m /h (1,024 cfm).
COUNTS (#)
COUNTS (#)
The peaks of the distributions are between 30 nm and
1
167
159
1.05
60 nm, larger than the MS2 virion size of 27 nm, indicat2
306
321
0.95
ing that some droplets contained multiple virions.
3
98
89
1.10
The sampling point of the SMPS was at the same plane
Average
1.03
as the upstream sampling probe of the impactor. For
each filter test, three upstream air samples
TABLE 2 Filter information.
and three downstream air samples were
FILTER MEDIA ELECTROSTATIC
PLEAT
MERV
FILTER QUANTITY
taken using impactors for 5 min at calibrated FILTER ID BASIS WEIGHT,
G/M 2
CHARGE
SPACING, MM RATING
SIZE (IN.)
sampling rates of 28 L/min (7.4 gal/min). The
A
50
Yes
16.7
5 20 × 25 × 1
6
impactor was the sixth stage of a six-stage
B
65
Yes
7.1
12 20 × 25 × 1
6
sampler that had a cutoff size of 0.65 µm.
C
65
Yes
7.1
13 20 × 25 × 1
6
Although the cutoff size of the impactor was
D
75
Yes
5.6
14 20 × 25 × 1
6
larger than the peak size of the MS2 aerosol,
a portion of the MS2 aerosol had particles
larger than the cutoff size of the impactor and were colThe efficiency (E) was then calculated using the
lected by the impactor.
formula.
The collection plates, having a double layer of agar
consisting of a hard Lysogeny broth (LB) bottom layer
Upstream PFU − Downstream PFU
Efficiency =
and a soft top layer incorporating E. coli, were then incuUpstream PFU
bated at 35°C (95°F) and 96% relative humidity for 24
hours. After incubation, the recovered plaque-forming
Filter Information
units (PFU) were enumerated. Only PFUs 1.0 mm or
Four types of residential HVAC filters were investigated
larger were counted.
Because viability of microorganisms in the test duct may in this study. Because our intention was to focus this
affect the test results, the upstream PFUs and downstream study on high-efficiency HVAC filters, i.e., MERV 12 and
higher, we selected MERV 12, MERV 13 and MERV 14 filPFUs of the empty test duct were measured and comters because these high-efficiency filters are commonly
pared. The results are shown in Table 1. Although significant fluctuation existed in the upstream and downstream used in homes. A MERV 5 filter was selected as a comparative filter. All filters were made with electrostatically
PFUs, the ratio of the upstream PFUs to the downstream
charged filter media. Filter information is shown in
PFUs remained relatively close together, indicating good
Table 2.
viability of MS2 bacteriophage in the test duct.
28

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ASHRAE Journal - August 2020

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