ASHRAE Journal - November 2019 - 62

TECHNICAL FEATURE

FIGURE 9 Perspective view for the predicted (a) velocity and (b) temperature
contours for the convection HVAC system, at x = 6.33 ft.

fpm
110.7396
103.8197
96.9000
89.9801
83.0604
76.1405
69.2209
62.3010
55.3813
48.4614
41.5417
34.6218
27.7022
20.7823
13.8624
6.9427
0.0228

PMV

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PPD

92.18377
86.73479
81.28580
75.83681
70.38783
64.93884
59.48986
54.04087
48.59188
43.14290
37.69391
32.24493
26.79594
21.34696
15.89797
10.44899
5.000000

is similar to that of the temperature variation. This
should not be surprising because the flow field is buoyancy induced. In the buoyancy-induced flow field, the
flow field is driven by the temperature variation such
that warm and lighter air moves upward while cold and
heavier air moves downward. The steeper the temperature difference, the stronger the flow field. In this case
study, this steep temperature difference is satisfied close
to the table (45% to 53% of the room height.) Counteracting this upward flow is a downward flow induced by
the cooling effect near the ceiling or next to the walls. As
a result of these two competing flow fields, the average
velocity stays nearly constant in the region spanning 53%
to 80% of the room height.
Figures 12a and 12b show the resulting comfort indices,
which are reflections of the velocity and temperature
values indicated in Figures 11a and 11b. It can be seen from
Figure 12a that for all the proposed arrangements, the PMV
values along the vertical axis are within the acceptable
range of comfort which is from -0.5 to +0.5. It can also
be seen from Figure 12b that for all the proposed arrangements, the PPD values along the vertical axis are within
the acceptable range of comfort, which is below 10%.
62

the convection HVAC system at x = 6.33 ft.
2.209297
1.918469
1.627641
1.336814
1.045986
0.755159
0.464331
0.173503
-0.117324
-0.408152
-0.698979
-0.989807
-1.280635
-1.571462
-1.862290
-2.153117
-2.443945

°F

87.21986
85.34361
83.46738
81.59113
79.71489
77.83866
75.96241
74.08616
72.20993
70.33368
68.45745
66.58120
64.70496
62.82873
60.95248
59.07625
57.20000

FIGURE 10 Perspective view for the predicted (a) PMV and (b) PPD contours for

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N O V E M B E R 2 0 19

However, the radiant ceiling arrangement is expected to
produce the most comfortable environment as its PMV
values are around 0 and its PPD values are around 5%. In
the radiant walls arrangement, the values of PMV change
dramatically from -0.5 to +0.5 along the vertical axis and
the values of PPD oscillate from 5% to 10% along the vertical axis. This means a less stable environment in terms
of comfort caused by relatively high temperature difference between the head and the feet (about 7°C [12.6°F]),
in comparison to the first case (about 3°C [5.4°F]), as
shown in the graphs of the temperature contours in Figure
11. In the convection arrangement, the values of PMV are
around -0.25 along the vertical axis and the values of
PPD oscillate from 5% to 8% along the vertical axis, which
results a relatively cold environment.

Conclusion
This paper compares the cooling of a cleanroom using
three different methods. In the first and second method,
radiant cooling panels are used, while in the third
method conventional convection HVAC systems are
used. The parameters compared are the air velocity profiles and temperature distribution inside the room. The

ASHRAE Journal - November 2019

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