How To Specify A Clean Bench
KEY PRINCIPALS OF CLEAN BENCH PERFORMANCEClean benches employ two key principals that are critical to maintaining a contaminant-free environment — laminar airflow and filtration.
Laminar airflowLaminar flow refers to a state in which non-turbulent streams of air flow in one direction parallel to one another. Laminar airflow is the most efficient option for removing particulate contamination from a controlled area because the airflow uniformity and lack of turbulence keep clean airstreams from mixing with contaminated airstreams. An object being bathed with clean, turbulence-free, laminar airflow will remain clean and protected from nearby contamination.Clean benches with either vertical or horizontal laminar airflow provide the cleanest working environments because HEPA-filtered air within these benches is unidirectional and turbulence-free. (Unidirectional is airflow having generally parallel stream lines, operating in a single direction with uniform velocity over its cross section.)
FiltrationThe air that bathes a product in a clean bench needs to be free of contaminants. This requires filtration. Clean benches are usually classified according to the number and size of particles permitted per volume of air in a specific amount of time. A HEPA (High Efficiency Particulate Air) filter is often used to filter the airstreams of a clean bench. To qualify as HEPA by U.S. government standards, an air filter must remove 99.97% of 0.3 micrometer particles. With the proper airflow and HEPA-filtered air, a clean bench can be rated Class 100 or ISO 5 (99.99% are typically used). An ULPA (Ultra-Low Particulate Air) filter can be used when a Class 10 or ISO 4 rating is required. An ULPA filter removes at least 99.999% of any airborne particles 120 nanometers (0.12 micron) or larger.
There are many possible configurations for clean benches. Specifying the ideal environment for a particular application will require careful decisions regarding airflow, bench design and filtration options.
CONSIDERATIONS FOR SPECIFYING CLEAN BENCHES
Direction of laminar flowThe first decision a specifier of clean benches faces is horizontal or vertical laminar flow. Vertical and 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 airstream 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 airstreams. Turbulence can extend down the air steam from the object. Then energy from the turbulent area can draw air back upstream towards the object.
Horizontal flowHorizontal 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. 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 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.
Type and positioning of work surfaceCareful 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 table top 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.
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. 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 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 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 cabinet. 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. Clean room benches are usually classified according to the number and size of particles permitted 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 lamina