Even the ambient air has to be monitored
and maintained at proper levels. HEPA-filtered
ventilation systems, assisted by preventive
measures, help manufacturers limit
airborne contamination. However, in order
to assure environmental purity, regular
housecleaning procedures are also necessary.
Cleaning with both a HEPA-filtered vacuum
and traditional wipe-down methods are
standard housekeeping procedures in most cleanrooms. Yet in critical cleaning situations,
vacuuming may be the more efficient
method because particles are retained inside
the machine with little chance of being exhausted
into the atmosphere (provided the vacuum has a HEPA-filtered exhaust
stream).
So, what should a cleanroom vacuum include?
According to the people at Nilfisk
CFM, which manufactures vacuums for Class
1 and Class 10 cleanrooms, any vacuum
cleaner used in a biotech, parenteral cleanroom
must be HEPA-filtered to ensure that
99.97 percent of all particles down to and including
0.3 microns are collected and retained.
In addition, it is absolutely critical that the
HEPA filter be installed after the motor to
filter the exhaust stream. The motor’s commutator
and carbon brushes generate dust,
and if the exhaust stream is not filtered that
dust will be released into the environment.
A word of caution: not all HEPA-filtration
systems are created equal. Nilfisk recommends
that the vacuum contain a
multi-stage, graduated filtration system for
peak operating efficiency. A graduated filtration
system uses a series of progressively finer filters to trap and retain particles as they
move through the vacuum.
The largest particles are captured first by
coarser filters; smaller particles are then
caught and retained by the finer HEPA filters.
This multi-stage system protects the
HEPA filters from blockage and excessive
wear-and-tear, maintaining peak performance.
(When equipped with an ultra low
penetration air (ULPA) filter, the system
should retain up to 99.999 percent of all ultra-
fine particles, down to and including 0.12
microns in size).
Additionally, the filtration system in your
vacuum should use oversized filters, which slow airflow across the larger surface area
and optimize the air-to-cloth ratio. This allows
the vacuum to easily collect large volumes
of debris over extended periods of time
with minimal maintenance.
Besides having an exceptional filtration
system, any vacuum used in a cleanroom
should be constructed of non-particle-generating
materials. For example, non-porous,
stainless steel vacuums – equipped with
smooth hoses and attachments – enable personnel
to quickly wipe down and decontaminate
equipment for faster, simpler
sanitization and validation. And, it must be
specially packaged to prevent contaminants
from entering the cleanroom environment
when delivered.
Filtration
Filtration is an enormous issue when it
comes to industrial vacuum cleaners. For
particle filtration, the airborne particles that
have been vacuumed must come in contact
with the filter media. According to Nilfisk CFM, there are five basic mechanisms
by which this can happen: Straining
or screening, impaction, interception, diffusion,
and electrostatic enhancement.
Straining, also know as screening, occurs
when the spaces between the fibers of the
filter media are smaller than the particles,
therefore they are captured.
Impaction takes place when larger particles
with adequate momentum are unable to
follow the airstream around the fibers of the
filter media and therefore collide into it and
are captured.
Interception occurs when particles are
small enough to follow the airstream, yet
come within a half-particle diameter of the
fiber. The particle is captured by the fiber by
means of molecular surface attraction.
Diffusion, or the Brownian movement,
takes place when small particles that don’t
have sufficient momentum because of their
low mass are bombarded by air molecules,
interrupting the particles’ pathway, therefore causing them to move about randomly. The
irregular path of the particle increases the
likelihood of being captured by the fibers of
the filter. The smaller the particle, the
stronger this effect.
Electrostatic Enhancement occurs when
fibers have a permanent electrostatic
charge. Since particles are attracted to the
opposite charge, they gravitate toward the
filter fibers, allowing the fibers to capture the particles.
The key word for cleanrooms is contaminant.
While the contaminant may be a particle
of perfectly legitimate, high-quality
material, in the cleanroom it earns a status
no greater than the lowly dirt speck. To prevent
such contaminants, consider cleaning
floors and surfaces with a HEPA-filtered
vacuum.
