ASHRAE Journal - August 2023 - 39

TECHNICAL FEATURE
maximum permitted values of particle concentration,
which are measured at several critical locations in a
cleanroom.5,6,7
The distribution of particles is seldom uniform, and
the cleanroom environment is not often well mixed, as
is usually assumed in the dilution theories. In reality,
the highest cleanliness level is required in the vicinity
of the products being manufactured , which is the most
critical zone for ventilation
in a pharmaceutical
cleanroom.
Therefore, simply
meeting the requirement
of average particle
concentration
in a cleanroom as
predicted by dilution
theories would not
serve the real purpose
of the contamination
control. Moreover,
high ach does not necessarily ensure the highest level
of cleanliness everywhere in a cleanroom.5
Since air is the primary carrier of particles, heat and
other contaminants in indoor spaces, the airfl ow patterns
play a crucial role in determining the indoor
conditions in general and the distribution of particle
concentration in particular. The contamination control
performance of a cleanroom depends on the clean
supply air properly sweeping the contaminant sources
without short-circuiting (that is, the supply air leaving
the cleanroom without passing through the sources
of contaminants) and ideally with it following a single
pass-through pattern without recirculating the contaminated
air.
Airfl ow patterns and the resulting fl ow path of airProduct
Exit
Carts
Returns (3)
Air Leakage Passages
Around the Door Frame
Racks (2)
(a)
Persons (5)
(b)
the airfl ow patterns and the resulting distribution of
particles in a typical pharmaceutical cleanroom. The
contamination control performance of various cases
is evaluated by using a newly developed metric called
Spread Index and by comparing the particle distribution
at a critical location in a cleanroom.
borne contaminants depend on several factors including
location of supply diffusers, supply airfl ow rates
and associated diffuser throws, supply air temperature,
size and locations of room return, leakage areas,
and associated leakage airfl ow rates, locations and
strengths of various heat sources in a room, location
and size of obstructions to airfl ow, and relative location
and strength of particle generating entities.
Systematic evaluation of the impact of all these
parameters on the contamination control performance
of cleanrooms is time-consuming, labor-intensive and
Cleanroom CFD Models
Figure 1 shows two sets of CFD models developed for
a typical pharmaceutical cleanroom. These threedimensional
models represent steady-state, isothermal
operating conditions of a cleanroom. The cleanroom
design has a fl oor area of about 600 ft2 (55.7 m2) with a
10 ft (3 m) ceiling height. The cleanroom has a centrally
located isolator, which is primarily used for vials fi lling,
stoppering and capping operations and provides a
complete barrier to ensure aseptic conditions and containment.
The fi nished product exits through a " mouse
hole " from the end wall of the isolator. The exit opening
and product tray are surrounded by fl exible plastic
curtain strips. The isolator has an independent ventilation
system, which is isolated from the cleanroom
ventilation system. Figure 1 shows that the isolator is
surrounded by fi ve operators who are the main sources
of particles in the cleanroom. A few carts and racks are
positioned at various locations in the cleanroom, which
A U G U S T 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
39
even impossible during the design stage. In such situations,
computational fl uid dynamics (CFD) simulations
can help optimize the contamination control performance
of a pharmaceutical cleanroom under a variety
of design and operating conditions.
As mentioned earlier, this study with the help of CFD
simulations systematically evaluates the impact of
air change rates and locations of laminar diffusers on
FIGURE 1 Schematic of the cleanroom CFD models. a) Supply diffusers located near the cleanroom walls. b) Supply diffusers located
close to the isolator over the operators.
Laminar Diffusers (8)
Laminar Diffusers (8)

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ASHRAE Journal - August 2023

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