Environmental Engineering Reference
In-Depth Information
ppmv. With intermittent operation, in-duct electronic air cleaners are
unlikely to cause any significant elevation of O
. However, elevated levels
may be expected when electronic air cleaning devices are operated contin-
uously, particularly in a closed room.
3
6. Clean rooms
The need for nearly particle-free environments has led to the development
and utilization of clean rooms. They are widely used in the semiconductor
industry, pharmaceutical manufacture, aerospace and military applications,
and for some medical purposes. Their early history reflects the parallel needs
for very high efficiency filters in the nuclear and aerospace industries.
In first-generation clean rooms of the 1950s, very high levels of airborne
particle control were achieved by the installation of HEPA filters in AHUs
of clean room units. Such filters, even at that time, had DOP efficiency ratings
of 99.97% at 0.3
m. Clean rooms utilizing laminar flow principles were
developed in 1962. In laminar flow, air moves uniformly in what is, in
essence, parallel streamlines across a space. Laminar flow is used to minimize
cross-streamline turbulence. Particles present or generated in the space tend
to stay in a streamline until they are removed. Laminar flow patterns are
induced by the uniform introduction of low-velocity air through a wall or
ceiling area. Laminar flow patterns in a simple clean room design can be
seen in
Figure 12.16
.
In an ideal situation, the entire ceiling would serve as
a perforated supply air plenum and the floor as a perforated exhaust plenum.
In conventional rooms, particles deposited on room surfaces can be
resuspended by foot traffic and other movements that produce turbulent
eddies. Under laminar flow conditions, air moves in predictable paths, and
turbulent eddy formation is minimized. Laminar flow patterns are disturbed
by the presence of people and objects. When a streamline is broken by an
object, it may reform some distance downstream. If broken streamlines are
not reformed, other streamlines transport airborne particles across the room.
The purpose of laminar flow is to remove (purge) particles generated in
a clean room by workers and their activities and to prevent the resuspension
of particles from horizontal surfaces, particularly the floor. Laminar flow
systems facilitate the use of high purging air flows without the particle-
disturbing turbulence typically associated with such air flows. However, as
volumetric air flows increase to a critical level, they begin to resuspend more
particles than they purge. An optimal flow rate widely used in clean rooms
is 20 air changes per hour (ACH).
Clean rooms are limited in size and volume to optimize environmental
control. As laminar air flow travel distances increase, particles are purged
less effectively.
High recirculation rates are used to move air through HEPA filters.
Sufficient makeup air is also provided to adequately dilute human bioefflu-
ents. Recirculation rates on the order of 75% are typically employed.
HEPA filters are located in AHUs on the discharge side of blower fans
(rather than the suction side) to prevent inward leakage of particles through
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