10 system. Clean benches also can be
used in lieu of a cleanroom; it is sometimes more cost-effective to build a
lower-class cleanroom and supplement
it with clean benches than to create a
higher class cleanroom.
Type and positioning of work surface
Careful selection of the clean bench
work surface and positioning of items
on the work surface can prevent turbulence and inflow contamination.
Horizontal bench options: With
a horizontal flow clean bench the
work surface is solid; however, the work piece may
be mounted on a platform or supports to allow the
laminar airstreams to bathe the piece on all sides.
Vertical bench options: In a vertical flow clean
bench, laminar flow airstreams may continue down
through a perforated or rod-type tabletop. If a solid
tabletop is used, the airstreams must turn.
When using a solid tabletop or a tabletop with
minimum perforations, most air flows out the front
of the bench. There will be some airflow compromise at this point since the flow is not completely
unidirectional as it turns toward the front. There also
will be an area of turbulence in the rear of the bench
between the table top and back panel. While this
is not ideal, it usually is not problematic if critical
objects are not placed in these turbulent areas. The
air will eventually clean itself but not as efficiently as
in the laminar areas.
Adding perforations to the tabletop, the rear of
the back panel, or to a small space between the back
panel and tabletop can minimize the turbulent area.
A perforated or rod tabletop allows the airstreams to
flow through the tabletop in a more laminar manner.
However, this reduces the outflow of air at the front
of the hood making the need for a front face shield
or window even more important (see below). If holes
are added to the rear of the hood, care needs to be
taken to ensure that drafts from other air currents do
not cross flow through the bench.
Front shield usage
Vertical laminar flow workstations: The front of a
vertical flow clean bench is open and relies on the
outflow of air to maintain the clean environment.
Adding a rigid or flexible front face shield or front
sliding window will contain the work area and help
direct the filtered air towards the critical work area.
The shield will reduce the open frontal area, increase
the exit air velocity and help prevent infiltration of
contaminated, ambient air into the clean space.
Clean Air Products CAP412
Series vertical-flow open
base clean bench.
horizontal airflow benches each have strengths and
weaknesses when it comes to keeping objects free
from contamination. The configuration of the work
piece and the nature of the work to be done will
impact whether vertical or horizontal is best.
The challenge is that when an object is placed
into an air stream it is not perfectly aerodynamic and
will block some of the airflow creating turbulence
around itself. Where there is turbulence there is a
reduced air exchange rate and possible mixing with
adjacent air streams. Turbulence can extend down
the air steam from the object. Then energy from the
turbulent area can draw air back upstream towards
Horizontal flow: Horizontal airflow is generally
best when the work piece has a large horizontal
surface and/or a narrow cross section perpendicular
to the airflow. Horizontal airflow is also well suited
for applications in which an individual must work
directly over the work piece. In a horizontal flow
clean bench there is constant clean airflow between
the work object and the worker’s hands or instruments. When using a clean bench with a standard 90
LFPM (Linear Feet per Minute) airflow velocity and
a 30 in. work area, it takes less than two seconds for
a complete air exchange to occur.
The general-purpose horizontal laminar flow
clean bench is designed to provide Class 10 or Class
100 (ISO Class 4 or 5) environments suitable for
laboratories, testing, manufacturing, inspection, and
pharmaceutical processing. The clean HEPA-filtered
air flows outward from the cabinet, washing out particulates and preventing contamination from entering the work zone.
A typical horizontal laminar flow workstation has
a tabletop and a three-sided hood. The HEPA filter is
located on the vertical rear side of the work surface
and is susceptible to damage. A secondary grill is
sometimes needed for filter protection.
Vertical flow: Vertical flow is used for a number
of reasons but the size and type of object are the primary considerations. Large objects can block the airflow in a horizontal laminar flow workstation, creating large areas of turbulence on the downstream side
of the object. These turbulent areas often become
contaminated as ambient air is drawn in. With vertical flow, air enters the work zone from the top and
flows downward, surrounding the object within the
work area. Air flows on the front, sides, and rear,
bathing the object on all sides with clean, filtered air.
A vertical laminar flow clean bench creates a
mini-environment within a cleanroom. The open
base allows the unit to be placed over existing work
tables or equipment. Models range from open interior to exhausting clean bench with wet process to