The front shield protects the vertical airstreams
from the contaminated stationary air just in front of
the bench. If these two air masses come in contact —
one moving and one stationary — a turbulent boundary will develop that will result in the two air masses
mixing. This mixing will deteriorate the laminar flow.
Horizontal laminar flow workstations: No front
shield is needed in a horizontal clean bench because
air flows out the end of the hood creating a wall of
air. The wall of air extends beyond the end of tabletop, making a semi-clean zone directly in front of the
clean bench. If contamination is created, everything
upstream of the contamination will remain clean as
long as laminar airflow is maintained. The only area
affected by the contamination would be that directly
downstream of the contamination.
Size/dimensions of clean bench
The size of the clean bench will be determined by
the size and configuration of the work piece and the
nature of the work to be done. Also, whether the
operator will be sitting or standing will impact the
table height selected.
To minimize turbulence within the clean bench,
objects should be placed so the smallest cross section
is perpendicular to the airflow with the air able to
flow around as much of the object as possible. If the
air path is long enough, the clean air will regain its
uniform airflow subject to the size of the object. As
a general rule, when clean air is
traveling on two sides of an object
the air will regain unidirectional
flow at approximately three times
the distance of the object’s width.
If clean air can only get at one
side of the object, this distance
increases to approximately six
times the object’s thickness.
Clean benches range in size
from compact tabletop versions
to larger cabinets. Sizes are available from 2 ft. to 10 ft. in width,
work area heights from 22 in. to
46 in., and table depths that go up
to 48 in.
A horizontal flow clean bench takes up more
floor space because the filter is located behind the
unit, rather than on the top as with a vertical flow
Cleanroom class/filtration requirements
How clean your bench needs to be depends on the
particular operation. Government regulations and
customer specifications may be factors in the decision.
Cleanroom benches are usually classified
according to the number and size of particles per-
mitted per volume of air in a specific amount of
time. This is related to not just the quality of the
filter, but also the airflow within the enclosure.
Class 100 (ISO 5) — Using a HEPA filter does
not guarantee that the cleanroom bench interior
will be Class 100 (ISO 5). You need the proper airflow within the bench.
With clean benches that need to be Class 100
(ISO 5) or better, the air discharging from the
HEPA filter must be in line with the hood or side
walls of the support frame to eliminate turbulence.
If turbulence exists, the resulting energy can flow
along the vertical surface and draw in contaminated
air from outside the bench. (The air will become
turbulent if non-moving air comes in contact with
the moving laminar airflow.)
Turbulence is eliminated by having the media
edge of the HEPA filter in line with the edge of the
hood or by putting in a secondary grill that diffuses
the air so it is in shear with the bench’s surfaces.
On a horizontal flow bench this would exist on
all sides — hood, top, sides, and table top. On a
vertical flow cabinet, the in shear should be on the
sides and front. Face shields help the laminar flow
direct the clean air towards the work surface, prevent frontal turbulence, maintain higher exit velocities, and help the laminar flow fully develop.
Class 10 (ISO 4) — Many clean bench users
need to get to Class 10 (ISO 4), and upgrading to a
UPLA filter will typically get them there. While an
ULPA filter will be more expensive than a HEPA
filter, the cost is relatively small considering the
overall costs of a clean bench system.
Configuration and construction materials options
Clean benches are available in a variety of configurations and modes of operation with additional
environmental controls available as necessary
depending on the type.
Vertical-flow bench options: Vertical flow clean
benches are available in many different configurations, including non-recirculating airflow, partial
recirculating airflow, or recirculating airflow with
partial exhaust (odors can be controlled through
charcoal filters or 100 percent exhaust). Exhausting
Clean Air Products CAP201
Series horizontal laminar
flow clean bench.