ASHRAE Journal - December 2022 - 44

TECHNICAL FEATURE
FIGURE 5 Comparison of temperature and velocity profiles of mixing ventilation and displacement ventilation systems as measured at Tree Location 7.
MV
DV
9
8
7
6
5
4
3
2
1
65
70
75
Temperature (°F)
80
9
8
7
6
5
4
3
2
1
65
70
75
Temperature (°F)
80
9
8
7
6
5
4
3
2
1
MV
9
8
7
6
5
4
3
2
1
10
2 ACH 4 ACH 6 ACH
two thermal mannequins (B and D) and the associated
desk equipment were powered off to maintain a heat
balance in the space.
Further from the occupants (Trees 5, 6, 7 and 8), the
difference in air quality between the MV and DV systems
was inconclusive. The DV system maintained low particle
counts at seated head height (43 in. [1092 mm]) but
showed higher particle counts, and a similar Ez as the
MV system, at standing height (67 in. [1702 mm]). It is
important to note that there were no mannequins near
these measurement trees nor any heat sources at standing
height; thus, there was no thermal plume to pull the
fresh supply air to these tree locations or to a higher level
in DV. In MV, the particle counts remained mixed along
the vertical axis. In both MV and DV, there was a wide
variance in particle counts between the four trees, so it
FIGURE 6 Particle density at the breathing zone of Occupants B, C and D at each
ventilation rate. (Occupants B and D were removed for the 2 ACH condition to
maintain heat balance in the room.)
0.700
0.600
0.500
0.400
0.300
0.200
0.100
0.000
DV
Occupant B
44
2 ACH
4 ACH
6 ACH
was challenging to make a direct comparison of the Ez
values.
Did the Air Change Effectiveness (CO2 Tracer Gas)
Match the Air Distribution Effectiveness (Saline
Particle Count)?
The measured air change effectiveness values closely
matched the findings of previous tracer gas studies
(Figure 7). The MV system resulted in air change effectiveness
values just below 1.0 for all measured heights.
The DV system resulted in air change effectiveness values
ranging from 1.6 to 1.8 depending on height. This
FIGURE 7 Comparison of measured air distribution effectiveness (Ez) and air
change effectiveness; both values were taken as an average of measurements at
the breathing zones of mannequins B, C and D.
Air Distribution Effectiveness vs. Air Change Effectiveness
at Occupant Breathing Zone (B, C, D, 43 in. Above Floor)
12.00
11.00
10.00
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
EZ
Away From Occupants,
Standing Height
(Trees 5, 6, 7, 8:
67 in. Above Floor)
MV
Occupant C
DV
MV
Occupant D
ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 2
DV
MV
2 ACH (DV)
2 ACH (MV)
4 ACH (DV)
4 ACH (MV)
6 ACH (DV)
6 ACH (MV)
EZ
Occupants' Breathing Zone
(Trees B, C, D:
43 in. Above Floor)
Air Change Effectiveness
Occupants' Breathing Zone
(Trees B, C, D:
43 in. Above Floor)
20
30
40
Air Velocity (fpm)
50
DV
10
20
30
40
Air Velocity (fpm)
50
Particle Count (#/cm3)
Height (ft)
Height (ft)
Height (ft)
Height (ft)
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ASHRAE Journal - December 2022

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