ASHRAE Journal - April 2022 - 24

TECHNICAL FEATURE
FIGURE 1 Example particle injection and decay test conducted to measure loss rate constants and CADR of a portable HEPA air cleaner operating in a 1,296 ft3 (36.7 m3)
chamber.
A. Particle Injection and Decay
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
60
Air
Cleaner
On
Air
Cleaner
Off
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
120
Time (Minutes)
with and without an air cleaner operating. The CADR
(in units of m3/h or cfm) is calculated as the difference
between the two loss rate constants times the volume of
the chamber (Equation 3). The CADR can also be calculated
by multiplying the flow rate through an air cleaner
by its efficiency, but it is not necessary to measure flow
rates in Equation 3.
CADR ()
=−
on
VL Loff
(3)
where Lon and Loff are the first-order contaminant loss
rates measured in a space with an air cleaner on and off,
respectively (h-1 or min-1), and V is the volume of the
space in which an air cleaner is tested (m3 or ft3). To estimate
loss rate constants (Lon and Loff), we take the natural
logarithm of the contaminant concentration in the
chamber at each time step (Ct ) minus the steady-state
background concentration in the chamber (Cbg ) divided
by the concentration at the initial time step (C0) minus
Cbg (Equation 4).


−ln 
CC
CC
t
−
−
bg
bg


 = Lt
(4)
This is a simple first-order exponential decay model,
assuming the chamber is well-mixed. We then fit a
straight-line regression through these calculated data
points versus time to estimate first-order loss rate
constants.
24
ASHRAE JOURNAL ashrae.o rg
A P R I L 2022
180
240
15
Air Cleaner Off
Air Cleaner On
y = -0.229x
R2 = 0.9989
y = -0.0303x
R2 = 0.9991
CADR = 258 ft3/min
(0.229/min - 0.030/min) × 1,296 ft3
30
45
Time (Minutes)
The above performance metrics are interrelated. For
example, a subtractive air cleaner with a high-efficiency
filter and high airflow rate will have a high CADR,
resulting in a high realized effectiveness in the space.
However, it is the CADR specifically that allows for a
quantitative comparison of pollutant loss rates with and
without an air cleaner operating in a space, which also
allows for extrapolating performance from lab tests to
field conditions and incorporating into mass balances
for equipment sizing (e.g., CADR per room size).21,22
Figure 1 shows an example of a " smoke " -sized
(0.09 µm-1 µm) particle injection and decay test conducted
with and without a portable HEPA air cleaner
operating in a 1,296 ft3 (36.7 m3) chamber. The resulting
first-order loss rate constants during air cleaner
on and off conditions were 0.229 per minute and
0.030 per minute, respectively, resulting in a CADR of
258 cfm (438 m3/h).
Next, we explore how electronic air cleaner test reports
often show their efficacy data and will attempt to translate
them to these more conventional performance metrics
of loss rate constants and equivalent CADR.
Issues in Electronic Air Cleaner Performance
Testing and Reporting
Common generic examples of efficacy statements in
test reports include statements such as:
* Air Cleaner Technology A uses reactive molecules to
destroy pathogens. Independent testing demonstrates
60
75
90
B. Loss Rate Constants and CADR
Particle Concentration (#/cm3)
ln[(Ct - Cbg)/(C0 - Cbg)]
http://ashrae.org
ASHRAE Journal - April 2022

Table of Contents for the Digital Edition of ASHRAE Journal - April 2022
