ASHRAE Journal - August 2023 - 40

TECHNICAL FEATURE
offer obstructions to the airflow in the cleanroom.
Supply air is distributed through eight ceiling diffusers
placed around the isolator. The impact of diffuser
locations on the contamination control performance
is evaluated by varying the layout of the ceiling diffusers.
Figure 1a shows the ceiling laminar diffusers placed
Spread Index (SI)
The contamination control performance of the
cleanroom is analyzed by using a newly developed
CFD-based metric called the Spread Index (SI). It
is the ratio of the volume of the cleanroom space
occupied by the particle concentration levels above
the desired ISO class concentration level to the
total volume of the cleanroom. Ideally, the cleanroom
ventilation systems should effectively remove
the particles from a cleanroom and minimize the
spread of particles by containing them close to
the sources. If the entire cleanroom space meets
the desired ISO class, then the SI value would be
close to zero. Lower values of SI indicate effective
removal of contaminants at the desired rate. Note
that the desired ISO class cleanliness level depends
on the type of cleanroom. In this study, the desired
ISO class corresponds to the target concentration of
10,000 particles/ft3 (300 particles/m3), and hence,
the Spread Index is computed for the ISO 7 class.
The Spread Index provides a normalized value
of the contamination control performance for
comparing the impact of various design and operational
parameters of a cleanroom. It also provides
visual verification of the cleanliness level of the
critical zone of a cleanroom. Other indices for the
ventilation effectiveness usually assign a single
value of effectiveness for the entire space without
considering a certain cleanliness requirement of
the space. The Spread Index, on the other hand,
provides both a quantitative as well as a qualitative
evaluation of the ventilation effectiveness.8 Since
this is a CFD-based approach, the contamination
control performance and the subsequent design
optimization of cleanrooms can be performed
during the early stages of the design and do not
require a physical facility to conduct field tests and
measurements.
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closer to the cleanroom walls, whereas in Figure 1b, the
diffusers are placed closer to the isolator over the operators.
In both cases the air exits through three low wall
returns placed on the three walls. Air leakage passages
are created surrounding the entry door on the fourth
wall of the cleanroom.
The impact of air change rate (ach) is evaluated by
varying the three levels of supply airflow rate for the
Figure 1a configuration. The three levels of supply airflow
rates are 1,000 cfm (472 L/s, 10 ach); 2,500 cfm
(1180 L/s, 25 ach); and 4,000 cfm (1888 L/s, 40 ach).
The impact of diffuser location is evaluated by keeping
the supply airflow rate constant at 25 ach for the Figure
1b configuration. The cleanroom is assumed to be positively
pressurized with a leakage rate of 100 cfm (47 L/s)
through the door leakage passages. The air leakage
rate through the isolator mouse hole is assumed to be
negligible.
The operators are assumed to be the primary sources
of particles in the cleanroom with a release rate of
1 million particles per min per operator.2 The actual
rate of particle release depends on the type of clothing
and the air velocities surrounding the operator.
The particles are assumed to be airborne with particle
diameter less than 5 micron, and thus, the rate of
particle settling and surface deposition is assumed to
be negligible. The cleanroom design is intended for a
cleanliness level of the ISO 7 class, which corresponds
to 10,000 particles/ft3 (300 particles/m3) at 0.5 μm.
The theoretical average particle concentrations,
which are calculated by dividing the total particle generation
rate by the total supply airflow rate, yield the
space average values of 5,000, 2,000, and 1,250 particles/ft3
(176 500, 70 600, and 44 125 particles/m3) for
the supply airflow rates of 1,000 cfm (472 L/s, 10 ach);
2,500 cfm (1180 L/s, 25 ach); and 4,000 cfm (1888 L/s,
40 ach), respectively. It indicates that even at the lowest
supply airflow rate of 10 ach, the cleanroom would
meet the ISO 7 class requirement. Note that such dilution
calculations assume perfectly mixed conditions,
which rarely occur in the cleanrooms. The airflow patterns
resulting from the locations of supply and return
combined with the locations of the particle generation
determine the contamination concentration at a critical
location in the cleanroom.
The realizable k-epsilon turbulence model, which is
suitable for recirculating flows was used to compute
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ASHRAE Journal - August 2023

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