Performance evaluation of personalized ventilation personalized exhaust (PV PE) system in air conditioned healthcare settings 6

Chapter 6: Performance evaluation of PV-PE system in
conjunction with background MV and DV systems —
airborne infection control

6.1 Background
As introduced in Chapter two, in the context of airborne infection control, it is critical
that the ventilation system is able to extract the contaminated exhaled air immediately
and within the shortest possible time. This will minimize the spread of the
contaminated air into the room air. Studies have found that normal breathing can
produce respirable contaminated droplets (Edwards et al., 2004). Gao et al. (2010)
found that PV has the possibility to increase the intake fractions of exhaled air and
droplets from other occupants. In order to prevent or minimise the spread of
contaminated air exhaled by occupants efficiently, the ideal strategy is to exhaust the
exhaled air as much as possible right around the infected person, which further
supports the novel PV-PE system. The feasibility and potential of PV-PE system
were studied experimentally for a particular healthcare setting consulting room
geometries and the normal medical consultation positions of an Infected Person and a
Healthy Person. This chapter, as well as chapter 7 examine the issue mentioned above.
In particular, this chapter deals with the originally stated Objective 2a, which is
reproduced as follows:
• Determine the effectiveness of airborne infection control of the combined PV-PE
system in conjunction with background MV or DV systems in terms of the
localized extraction of the contaminated exhaled air from an Infected Person in
healthcare settings - Infected Person seated facing the seated Healthy Person.
During a normal consultation process, the Healthy Person (doctor, nurse, staffs, etc.)
are seated behind the desk, thus, the PV is used for providing fresh air. It is
hypothesized that combined PV and PE for healthy person can achieve the highest
PEE; PV for healthy person helps to reduce exposure from Infected Person; PE for
Infected Person with PV for Healthy Person provides the best exposure reduction; PE
for Infected Person is more effective than PV for Healthy Person. PE for Infected
Person helps reduce exposure for Healthy Person.

As described in Section 3.5.1, two types of PE are examined: Top-PE and Shoulder-
PE; two different background ventilation systems are used. It is hypothesized that
Top-PE is better than shoulder-PE; and in the presence of PE for Infected Person, DV
system lead to a better exposure reduction than MV system
The experimental design and evaluation index are described in Chapter 3 (Section
3.5.3). Experimental chamber is described in Section 3.4.1.1.
6.2 Performance of combined PV and PE for Healthy Person
The Personalized Exposure Effectiveness (PEE) of the Healthy Person calculated
based on the concentration of tracer gas measured in the experimental chamber from
Equation 3.1 are shown in Figures 6.1 and 6.2. The index is used to evaluate the
ability of the novel PV-PE system in bringing conditioned outdoor air and protect
occupants from indoor contaminants at the breathing zone.


Figure 6.1 Comparison of PEE with MV

0.29%
0.36%
0.35%
0.4%
0.39%
0.41%
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Figure 6.2 Comparison of PEE with DV

Compared with totally mixing ventilation which has 0 value for PEE, PV alone can

increase the amount of the clean personalized air to 26% and 31% when PV flow rates
are 5 l/s and 10 l/s. This means PV alone is able to protect the healthy person by
supplying conditioned outdoor air in the breathing zone. The percentage of PV air or
the percentage of outdoor air in the inhaled air changes when different PV air flow
rates (10 l/s and 5 l/s) are used. However, when comparing the PEE in pairs of 10 l/s
and 5 l/s without PE, the PEE does not change a lot. For example, with DV, PEE is
0.26 when PV flow rate is 5 l/s while it is 0.31 when PV flow rate is 10 l/s. The
difference of PEE between these two PV air flow rates is only 0.05; while the
difference with MV is 0.07. Compared with the cases when PE is working together
with PV, PV air flow rate has a stronger effect on the PEE with DV. It was observed
that with DV, PV-PE system is always able to deliver a much higher percentage of PV
air at 10 l/s flow rate. With a top-PE added, an increase of 66.7% is observed when
PV flow rate increases from 5 l/s to 10 l/s. The percentage is even higher, 79.4%,
when shoulder-PE is applied. However, this obvious increase pattern is not found with
MV background system. The difference of PEE is only 0.05 and 0.02 for top-PE and
shoulder-PE respectively. This is because the mixing effect of MV disturbs the
pulling effect of the PV-PE system.

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0.3%
0.32%
0.52%
0.34%
0.58%
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The results in Figure 6.1 and Figure 6.2 also show that the shoulder-PE performs
better than top-PE in terms of pulling more PV air towards the breathing zone. For the
case with 5 l/s for PV with DV, the PEE increases at 26.9% after activating the top-PE

and 30.8% if shoulder-PE is utilized instead. A similar pattern is observed with MV
when better performance is achieved with combined shoulder PE-PV. This is when
compared with PV alone and top PE-PV, which has a PEE of 0.01 (PV 5 l/s) and 0.06
(PV 10 l/s) higher when shoulder PE is activated compared to top-PE
6.3 Performance of the PV- PE for preventing transmission of exhaled
contaminated air during normal medical consultation process
6.3.1 Exposure Reduction
The first evaluation index, Exposure Reduction, was described in Section 3.5.3.
It is useful to first explain the manner in which Figure 6.3 to Figure 6.6 is plotted to
display the experimental results; for each type of PE used for Infected Manikin
(shoulder-PE or top-PE), there are four clusters of data. Each cluster consists of two
bars representing two different PV flow rates, 5 l/s and 10 l/s. The first cluster shows
the reduction in exposure to the exhaled tracer gas from Infected Manikin when only
PV is implemented. Note that the base case is when neither PV nor PE is utilized;
there is only background ventilation system (MV or DV) in operation; that is, the data
of exposure reduction with MV and the data with DV have different base data. Thus,
the bars for the same configuration for the two manikins with MV and DV (data in
Figure 6.3 and Figure 6.4; data in Figure 6.5 and Figure 6.6) are not comparable.
Comparing the Exposure Reduction with MV in Figure 6.3 and Figure 6.5, it is found
that the Healthy Manikin experiences roughly 25% reduction in exposure to the
Infected Manikin exhaled contaminated air when PV supply air is at 5 l/s. The
reduction percentage is increased to 55% if the PV flow rate is increased to 10 l/s. The
Exposure Reduction is observed with DV in Figure 6.4 and Figure 6.6 as 12.5% and
29.17% for the PV flow rate at 5 l/s and 10 l/s respectively. This implies that the use
of PV alone has the ability to protect the Healthy Manikin from inhaling contaminated
air from the Infected Manikin sitting opposite.
After switching on the top-PE for infected manikin while switching off the PV for
healthy manikin, a more than 70% reduction of exposure is found under MV, and
more than 75% for DV base case. This is encouraging not only because of the
significant reduction in Healthy Manikin’s exposure, but also because of the much

higher reduction than using PV alone. In this case, the exposure reductions under two
different flow rates of PE do not have much difference. They are almost the same
with MV and a slight increase of reduction with the increase of PE flow rate with DV.
For the cases of combined PV for Healthy Manikin and top-PE for Infected Manikin,
Figure 6.3 and Figure 6.4 show that the best performance is found after combining PV
and top-PE for cases with both MV and DV, yielding an 81% and 88% exposure
reduction for case AIp
!!"
Hp
5
with MV and case AIp
!!"
Hp
5
with DV respectively.
However, the increase in exposure reduction between the top-PE alone and combined
top-PE-PV is not much, for example, the increase of Exposure Reduction with DV
between case AIp
!!"
Hp and case AIp
!!"
Hp
10
is only 3.4%. This shows that, not only
is the top-PE system for Infected Manikin efficient, but also that it does not help a lot
to use PV for Healthy Manikin while top-PE is utilized for Infected Manikin.
For the case of utilizing shoulder-PE, Figure 6.5 indicates exposure reductions of 25%
and 75% for the PE only cases AIp
!!!"
Hp and AIp

!!!"
Hp with MV. These results of
AIp
!!!"
Hp are close to the case of PV alone AIpHp
5
. The use of shoulder-PE alone at
10 l/s in terms of reducing the exposure does not surpass the use of a small flow rate
(5 l/s) of PV. PV alone for Healthy Manikin at 10 l/s (case AIpHp
10
) can achieve a
much higher reduction of exposure than shoulder-PE alone for Infected Manikin at 10
l/s (case AIp
!!!"
Hp) with MV. With the increase of PE flow rate from 10 l/s to 20 l/s
with MV, the advantages of shoulder-PE is seen, which shows a much higher local
removal ability of 75% exposure reduction. Different performance pattern of
shoulder-PE alone is found when the room is ventilated by DV. The shoulder-PE
suction at 10 l/s still does not prove to be as efficient as 20l/s case (exposure reduction
is 63% while shoulder-PE is 10 l/s and 78% for the case with 20 l/s), but it is able to
achieve a higher exposure reduction than PV alone (case AIpHp
5
and AIpHp
10
) .
Clearly, the flow pattern of overall interaction of exhaled air, thermal plume and the
PE flow are still largely influenced by the background air type, making the shoulder-
PE suction less effective with MV.
The combined PV for Healthy Manikin and shoulder-PE for Infected Manikin also
performs differently with different background ventilation systems. Figure 6.5 shows

that when the shoulder-PE flow rate is 10 l/s, the exposure reduction increases with
the increase of PV flow rate. However, for the case of shoulder-PE 20 l/s, Figure 6.5
indicates a 3.1% less exposure reductions when switching on the PV compared with
the cases using PE alone. Considering the sitting positions of Infected Manikin which
is below the supply diffusers with a distance of 0.5 m closer to the floor centre, the
flow from MV four-way diffuser and the flow of PV for healthy manikin tend to pull
the exhaled contaminated air further towards the healthy person. If the combined PV
and shoulder-PE is applied with DV, the trend of the results is close to that of top-PE
with DV, which shows a best reduction of exposure by combing PV for Healthy
Manikin and PE for Infected Manikin. An additional interesting point to note is that
the exposure of reduction at the higher flow rate (20 l/s) of shoulder-PE, is 13% (PV 5
l/s) and 21.2% (PV 10 l/s) more than that at the 10 l/s flow rate of shoulder-PE (As
different to the top-PE which does not have much increase between the two flow rates
of PE).
Indeed, Figures 6.3 through 6.6 show that the top-PE can greatly reduce the exposure
of exhaled air at a lower flow rate compared with shoulder-PE. This could be
attributed to the top-PE enhancing the ability of thermal plume to bring up the exhaled
air towards the suction opening to mitigate the release of exhaled contaminated air to
the room air.
!
Figure 6.3 Comparison of Exposure Reduction when utilizing top-PE with MV
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Figure 6.4 Comparison of Exposure Reduction when utilizing top-PE with DV


Figure 6.5 Comparison of Exposure Reduction when utilizing Shoulder-PE with
MV
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Figure 6.6 Comparison of Exposure Reduction when utilizing Shoulder-PE with
DV

6.3.2 Intake Fraction
The second evaluation index, Intake Fraction (iF), was described in Section 3.5.3.
Figure 6.7 to Figure 6.9 yield the iF results computed for Healthy Manikin based on
the exhaled contaminant source from Infected Manikin. The data are divided into
three groups: PE alone for Infected Manikin; PE for Infected Manikin while 5 l/s of
PV for Healthy Manikin; and PE for Infected Manikin while 10 l/s of PV for Healthy
Manikin. Here, the iF results of each group have been compared with their respective
PE-off case to allow the visualization of the effect of increase of PE flow rate.

The first point to address is the drop of iF in all cases listed in the three figures, which
means that the inhaled amount of exhaled contaminated air is reduced with the
increase of PE flow rate. When considering 10 l/s of PE flow rates, the performance
of shoulder-PE cases is not as good as that of top-PE, which means a higher iF is
observed with shoulder-PE in the three figures. As discussed previously in Exposure
Reduction section, this indicates that the top-PE performs better than shoulder-PE in
terms of exhausting contaminated exhaled air before it mixes with the room air. And
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top-PE has an energy saving potential compared with shoulder-PE since it can achieve
a lower iF with lower flow rate.
An interesting feature to note in the three Figures is the comparison between iF with
DV and MV when PE is not switched on. It shows that the iF values are obviously
higher with DV than MV, which means more amount of contaminated air traveling to
the breathing zone of Healthy Manikin. This is because the exhaled air from normal
breathing process could concentrate maximally just at the breathing height level with
DV and lead to a higher infection risk. This finding complies with the previous
research conducted by Qian et al. (2006), Gao et al. (2008), and Pantelic (2011).
Although DV may lead to a higher risk of exposure to cross-contaminated air, the
performance of PE with DV does not show any weakness of infection control. In fact,
after switching on the PE for Infected Person, the type of PE (top-PE or shoulder-PE)
has a larger effect on the iF than the type of background ventilation system. Thus, it
may be encouraging to utilize the PE system in rooms ventilated by DV to avoid
the longer penetration of exhaled air.

Figure 6.7 iF with PE for Infected Manikin
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Figure 6.8 iF with PE for Infected Manikin and 5 l/s PV for Healthy Manikin


Figure 6.9 iF with PE for Infected Manikin and 10 l/s PV for Healthy Manikin

As depicted in Figures 6.10 through 6.13, the iF varies with different configuration for
Healthy Manikin. When PE for Healthy Manikin is not activated (flow rate is 0 l/s),
the Intake Fractions are, for all the four figures, decreasing with the increase of PV
flow rate for Healthy Manikin. By adding a PE working together with PV for the
Healthy Manikin, it is found that the iF is further reduced. This is due to the pulling
effect of combined PV-PE system, which can increase PV air in the inhaled air, thus
protecting the Healthy Manikin. However, as the PE flow rate for Infected Manikin
increases, the advantages of combined PV-PE for Healthy Manikin is not the same.
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For example, when top-PE is used for Infected Manikin with MV, the iF of the case
AIp
!!"
Hp
!!!"

!
is more than the case AIp
!"#
Hp
!
; and when shoulder-PE is used for IP
with DV, the iF of the case AIp
!!!"
Hp
!!"
!
is higher than the case AIp
!"#$
Hp
!
. This
indicates that when PE is applied for both Healthy Manikin and Infected Manikin, the
combined PV-PE for Healthy Manikin may bring more amount of contaminated air to
the Healthy manikin due to the pulling and suction effect of the PV-PE system for
Healthy Manikin.
Considering now the cases in Figure 6.10 and Figure 6.11 of 10 l/s top-PE suction, the
impact of PE for Infected Manikin on the Intake Fraction is obvious. With the further
increase of PE flow rate from 10 l/s to 20 l/s, the reduction of iF begins to decrease
slower as the slopes look more gentle. Different trend of the data is observed in Figure
6.12 and 6.13, a much gentler slope is found from the point where shoulder-PE is
exhausting at 0 l/s to the point where 10 l/s are set for shoulder-PE. And a sharper
decrease in iF is seen from shoulder-PE 10 l/s to 20 l/s. In these cases, the shoulder-
PE works less effectively in terms of exhausting the exhaled contaminated air from
the Infected Manikin, thereby changing the trend of the data as compared the top-PE
cases.

From Figures 6.10 through 6.13, it is clear that, the Intake Fraction of a Healthy
Manikin is largely dependent not only on the PE flow rate, but also on the types of PE
used for the Infected Manikin. Before activating the PE for Infected Manikin, the PV
or the combined PV-PE for Healthy Manikin plays an important role in protecting the
Healthy Manikin from exhaled contaminated air. However, after activating the PE for
Infected Manikin, not much differences of Intake Fraction is found between the case
with or without PV-PE for Healthy Manikin. This again indicates that in terms of
airborne transmission control in health care settings, using a PE for Infected Person is
more critical than using PV for a Healthy Person.

Figure 6.10 iF with different configurations for Healthy Manikin with top-PE
for Infected Manikin with MV

Figure 6.11 iF with different configurations for Healthy Manikin with top-PE
for Infected Manikin with DV

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PE for Infected Manikin with MV


Figure 6.13 iF with different configurations for Healthy Manikin with shoulder-
PE for Infected Manikin with DV
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6.4 Air temperature and velocity
The room air temperature and mean velocity are recorded at heights of 0.1 m, 0.3 m,
0.7 m, 1.3 m, 1.65 m and 1.95 m using the DANTEC measurement system. The
vertical room air temperature distribution is presented in Figure 6.14.


Figure 6.14 Room air temperature with MV and DV
From the data collected at the two points of the room, it can be concluded that the
room air temperature is stratified along the vertical height of the room space with DV.
In the region close to the floor surface, the room air temperature is cooler with the
cool DV supply air. Due to the heat source (thermal manikin), the vertical temperature
gradients are larger in the height from 0.7 m to 1.35 m than those in the bottom and
upper part of the room. The convection flow generated in this region results in higher

level of temperature gradient than in the space away from the manikin. With MV, the
temperature keeps quite stable through the room height, which is due to the well-
mixed flow effect.
Figure 6.15 presents the profiles of the mean room air velocity during the experiments
with both MV and DV. The higher air movement is observed with MV than DV.
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The local air velocity at the neck/ear region of the Infected Manikin is shown in
Figure 6.16 and Figure 6.17. Generally, the air velocity increases with the increase of
PE flow rate. In cases without activating the PE, the air velocity at the neck/ear region
is almost the same as the room air, so that the environment around the human head is
mostly calm. In Cases with the flow rate 10 l/s of shoulder-PE, air is being sucked by
the PE, and flow field of about 0.31m/s in velocity extends to the area around the
human face with MV. A higher velocity of 0.38 m/s is observed when the PE flow
rate is increased to 20 l/s. Compared with shoulder-PE, the increase of local air
velocity when using top-PE instead is smaller, which is 0.24 m/s when flow rate is 10

l/s and 0.27 m/s when the flow rate is 20 l/s. Similar trend is observed with DV, the
shoulder-PE tends to generate more velocity increase at neck/ear region than MV. In
Cases with 20 l/s of PE flow rate, in which air is blowing at a higher velocity of 0.28
m/s and 0.17 m/s with shoulder-PE and top-PE respectively. This is because the
location of shoulder-PE is closer to the neck/ear region and the air movement of the
surrounding air is enhanced.

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Figure 6.16 Local air velocities at neck/ear region with MV


Figure 6.17 Local air velocities at neck/ear region with DV
In the space close to the manikin’s neck/ear region (1.2 m), draft rating (DR) at the
face is presented in Figure 6.18 and Figure 6.19. Draft is an unwanted local cooling
of the body caused by air movement. The figures show that the activation of PE

always brings a higher value of DR at the neck/ear than the cases without PE.
Although the velocities are higher with MV, there is not much difference between the
DR with MV and DV. This means that the effect of the background ventilation type
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on DR is not strong. After activating either top-PE or shoulder-PE, the DRs increase
to more than 15%, even with the lower flow rate of 10 l/s; and a flow rate of 20 l/s of
PE would result in a slightly higher DR. This means that the use of either shoulder-PE
or top-PE may lead to some draft discomfort.

Figure 6.18 Draft rating at neck/ear region of Infected Manikin with MV

Figure 6.19 Draft rating at neck/ear region of Infected Manikin with DV

6.5 Key Findings
The Infected Manikin keeps exhaling the tracer gas into the room air, which is cooled
by 90% re-circulated air. During the half hour of experiments (the maximum duration
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of a typical consultation process), there is no steady-state achieved. In this chapter,
only data at the end of the 30 minutes of each experiment were used for analysis; The
Intake Fraction and Exposure Reduction at 10 mins and 20 mins after the start of each
experiment are listed in Appendices 1 and 2.
As a summary, the key findings in this chapter and Appendices 1 and 2 are listed as
follows:
During a normal consultation process in healthcare centers, where the Infected Person
and the Healthy Person are sitting face to face, the results indicate that the combined
PV and PE for HP can achieve the highest PEE. And the shoulder-PE performs a little
better than top-PE in terms of increasing PV air in inhaled air.
In the context of airborne transmission control, the use of PV alone has a potential to
protect the HP; the use of combined PV-PE alone for HP can achieve the lowest
Intake Fraction; and the use of PE for IP alone shows much better performance than
using PV for HP alone. However, after activating the PE for IP, the use of PV for HP
may lead to higher or lower exposure. The increase or decrease depends on the
effects of background ventilation type, PE type and PV flow rates.
For better airborne transmission control, top-PE is preferred than shoulder-PE because
it can achieve better exposure reduction and lower Intake Fraction with a lower flow

rate.
%