ASHRAE Journal - August 2023 - 46

TECHNICAL FEATURE
FIGURE 6 Airflow patterns in a cleanroom for the two layouts of the supply diffusers.
(a)
(b)
Particles (#/ft3)
>10,000.0
9,000.0
8,000.0
7,000.0
6,000.0
5,000.0
4,000.0
3,000.0
2,000.0
1,000.0
0.0
FIGURE 7 Airflow streamlines in a cleanroom passing through a central plane for the two layouts of the supply diffusers.
(a)
(b)
low wall returns. Though increasing the ach levels
enhances dilution and reduces the overall particle
concentration, it does not ensure the effective removal
of contaminants and the required level of cleanliness
in the critical zone of a cleanroom.
Analysis of Supply Diffuser Layout
Airflow Patterns. Figure 6 and Figure 7 shows the
impact of supply diffuser locations on the resulting
airflow patterns in the cleanroom. The airflow patterns
are colored by the concentration of the particles
in the room. In the previous analyses, the supply
diffusers were located close to the wall and away from
the operators and isolator. In the modified layout as
shown in Figure 1b, the supply diffusers are moved over
the operator and close to the isolator. In both cases
the supply airflow rate is 25 ach. Relocation of the
supply diffusers over the operators helps reduce the
short-circuiting and the flow of supply air toward the
low wall returns. The supply of air over the operators
(sources of particles) helps the effective sweeping of
the particles away from the isolator.
46
ASHRAE JOURNAL ashrae.o rg
A U G UST 2023
Particles (#/ft3)
>10,000.0
9,000.0
8,000.0
7,000.0
6,000.0
5,000.0
4,000.0
3,000.0
2,000.0
1,000.0
0.0
Similar to the previous
analyses, the airflow
patterns in the modified
case still show nonunidirectional
airflow
patterns. Modified locations
of the supply diffusers still
form recirculating airflow
patterns; however, unlike
in the previous cases, the
zones of air recirculation
are located closer to the
walls away from the isolator.
Discrete locations of the low
wall return combined with
the high momentum of the
incoming air form local air
recirculation zones. Return
air cannot find a ready path
to exit the cleanroom. The
location, number and size
of returns can also play
a significant role in the
contamination control.
Particle Distribution
and Spread Index. Figure 8 shows the distribution of
particle concentration at a critical plane level, which
is at 3 ft (0.9 m) elevation. Keeping the same supply
airflow rate (25 ach), relocation of the diffusers shows
a reduction of about 19% in the average concentration
of particles. Moreover, as shown in this figure, most
of the region at this elevation satisfies the ISO 7 level
of cleanliness (particle concentration of 10,000 #/ft3).
As shown in Figure 6, sweeping of the contaminant
sources (operators) promotes the downward motion
of particles, which, in turn, shifts the zones of high
particle concentration from the higher elevation to the
lower elevation closer to the floor.
Figure 9 shows the impact of diffuser location on the
Spread Index of particle concentration at the ISO 7
level. Simple relocation of the diffusers over the operators
without increasing the supply airflow rate reduced
the Spread Index from 11% to 1%, about 91% reduction.
It means the overall cleanliness level of the entire space
increased significantly in meeting the desired ISO 7
class level. In the modified layout of the supply diffusers,
the zones of high particle concentration are limited

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ASHRAE Journal - August 2023

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