Use clean benches to control particulate contamination
09/01/2007
Various types of clean benches are available that can serve as a more economical means to achieving specific cleanroom class areas within a critical environment
By Kevin Weist, Clean Air Products
Clean benches are one type of mini clean environment and offer a convenient method of controlling particulate contamination within a controlled work area. They do not require a user to be gowned, but individuals will often wear gloves and arm covering when using the bench as an extra measure of protection against particulate contamination.
Clean benches are used in a variety of industries, including semiconductor, pharmaceutical, medical device, health care, food, aerospace, nanotechnology, disk drive, injection molding, and automotive, among others. They can be used as a stand-alone solution, or inside a cleanroom-it is often more economical to supplement a cleanroom with clean benches for a localized Class 100/Class 10 area, rather than making the entire cleanroom Class 100 or Class 10 (ISO 5 or 4).
Benches are available in a variety of configurations and modes of operation, with additional environmental controls available as necessary depending on their type and configuration. They range in size from compact tabletop versions to larger cabinets. Sizes are available from 2 to 10 ft in width, work area heights from 22 to 46 inches, and table depths up to 48 inches. Clean benches can have a vertical or horizontal airflow, and with smaller micro systems the air is often purged with a hose and removed through a HEPA filter system.
Clean benches with either vertical or horizontal laminar airflow will provide the cleanest working environments, because the HEPA filtered air within these benches is unidirectional and turbulence free. Unidirectional is defined by the Federal Standard 209E as airflow having generally parallel streamlines, operating in a single direction, and with uniform velocity over its cross-section-also referred to as laminar.
Laminar airflow is the most efficient option for removing particulate contamination from the controlled area because the airflow uniformity and lack of turbulence keeps “clean air streams” from mixing and being contaminated by a contaminated air stream. An object being bathed with clean, turbulence-free, laminar airflow will remain clean and protected from nearby contamination as long as the two air streams remain unidirectional and devoid of turbulence.
Vertical and horizontal airflow each have strengths and weaknesses when it comes to keeping objects free from contamination. The challenge is that anytime an object is placed into an air stream it is not perfectly aerodynamic and will block some of the airflow, creating turbulence around the object. Where there is turbulence there is a reduced air exchange rate and a mixing with adjacent air streams. That turbulence can extend down the air stream from the object, and the energy from the turbulent area can draw air back upstream. If the turbulence is caused by an object in the air stream, the contamination can be drawn upstream back toward the object. When using a clean bench where the distance between the object within the clean environment and dirty ambient air is relatively short-typically less than 12 inches-one must be careful to locate these objects within the air stream to minimize the turbulence.
To minimize turbulence within the clean bench, objects should be placed within the airflow such that 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. Careful positioning of items on the work surface can prevent turbulence and inflow contamination. When possible, setting objects on a raised platform will allow air to flow under the object and reduce downstream turbulence.
Horizontal airflow is generally best when the object has a large horizontal surface and/or a narrow cross-section perpendicular to the airflow. Horizontal airflow is also well suited when working directly over the object such that there is constant clean airflow flowing between the work object and the operator’s hands or work instruments. When using a clean bench with a standard 90 LFPM airflow velocity and a 30-inch work area, it takes less than two seconds for a complete air exchange to occur.
A typical horizontal flow clean bench has a table top and a three-sided hood; the air flows out the end of the hood like a “wall of air.” The wall of air extends beyond the end of table top, creating a semi-clean zone directly in front of the clean bench. In the event that contamination is created, if laminar airflow can be maintained everything upstream of the contamination will remain clean; the only area affected by the contamination would be that directly downstream of the contamination.
Vertical flow is used for any number of reasons, with the size and type of object being the primary considerations. Large objects can block the airflow from a horizontal flow bench, creating large turbulent areas on the downstream side of the object. These large turbulent areas often become contaminated as ambient contamination is drawn toward them. 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 HEPA filtered air. Vertical flow air then continues down through a perforated or rod-type table top, or turns and flows out the front of the bench if it’s a solid-top table.
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 HEPA filtered air toward the critical work area. The shield will reduce the open frontal area to increase the exit air velocity and help prevent infiltration of contaminated air into the clean space.
The front shield also protects the vertical airflow air 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 and will deteriorate the laminar flow.
When using solid table tops or minimum perforated tops, most air flows out the front of the bench. There will be some airflow compromise in that it is not completely unidirectional, as the air turns toward the front and there will be an area of turbulence in the rear of the bench between the table top and back panel. Although this is not ideal, it is typically not problematic in most applications. Care should be taken to avoid placing critical objects within this turbulent area and doing work in this area that would generate contamination. This air will eventually clean itself, but does not do so as efficiently as in the laminar areas.
Adding some perforation to the table top or the rear of the back panel, or a small space between the back panel and table top, can minimize the turbulent area. Note that if holes are added to the rear of the hood, one must ensure that drafts from other air currents do not cross-flow through the bench. Adding a perforated or rod top allows the air to flow down through the table top in a more laminar manner. However, this does reduce the outflow of air from the front of the hood, making a front face shield or window even more important.
Not all clean benches are designed for a Class 100 (ISO 5) interior space. Even with HEPA filtered air feeding the work area, they will require greater perimeter shielding to contain the critical workspace. With vertical and horizontal flow hoods that need to be Class 100 (ISO 5 or better), the air discharging from the HEPA filter must be inline with the hood or side walls of the support frame to eliminate turbulence. The air will become turbulent if non-moving air comes in contact with the moving laminar airflow. This is accomplished 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 hood’s surfaces. If turbulence exists, the resulting energy can flow along the vertical surface and draw in contaminated air from outside the bench.
On a horizontal flow bench this would exist on all four sides: hood, top, sides, and table top. On a vertical flow cabinet the air in shear should be on the sides and front. As previously mentioned, face shields help the laminar flow direct the clean air toward the work surface, prevent frontal turbulence, maintain higher exit velocities, and help the laminar flow fully develop.
Vertical flow hoods are available in many different configurations, including non-recirculation airflow, partial recirculating airflow, or recirculating with partial exhaust (odors can be controlled through charcoal filters or 100 percent exhaust). Exhausting units can be made of stainless steel or polypropylene, with specialty vertical flow cabinets almost exclusively made of polypropylene for trace metal analysis or work in which concentrated acids are used within the enclosure.
Biosafety cabinets are a type of clean bench designed for both product and personal protection, typically used with biological agents. They supply HEPA filtered air into the clean space, with a slight inflow at the front of the cabinet. The inflow is done in a manner that contamination does not get past the front grill, where a second HEPA filter cleans that air. Some units require this air also be 100 percent exhausted. Exhausting the HEPA filtered air from biosafety cabinets is a secondary measure of protection to ensure no biological agents within the bench enter the ambient space.
Location, location, location
The clean bench should be located away from obvious drafts, as well as areas where air velocity will exceed 100 ft per minute (the same airflow velocity coming out of the HEPA filter). If outside air is blowing directly toward the clean bench, it will blow into the unit and contaminate the space.
Clean benches are commonly used inside a cleanroom or other clean space. Persons with Class 100,000 (ISO 8) or Class 10,000 (ISO 7) cleanrooms will often utilize clean benches in critical areas to make them Class 100 (ISO 5), or even Class 10 (ISO 4). If the surrounding space is clean, the clean bench performs better, with significantly less particulate contamination for the HEPA filter to handle and less ambient contamination threatening to infiltrate into the work zone.
Additional considerations
The following lists other important factors to consider when specifying a clean bench.
- Look for a UL listing on the assembled cabinet. Local code typically requires UL listing of products.
- Request a speed control switch for the blower to adjust airflow velocity over time.
- Make certain there is negative pressure around the HEPA filter.
- Pay attention to where the clean bench will be installed to ensure that the location of the HEPA filter is compatible with future servicing needs.
- Know whether the operator will be sitting or standing because this will impact the table height.
- Confirm whether your process only requires particulate contamination control and at what level.
- Confirm whether or not the operator needs protection from the process within the bench itself.
- Confirm any physical restrictions you have when bringing the equipment into the space such as doors, hallways, and other equipment. If need be, order casters as part of the equipment.
- Consider the workflow into, within, and out of the clean bench and how the bench is going to interface with the other operations.
Your cleanroom equipment supplier can help you determine which type of clean bench will best serve your application needs. Additional information is also available from the Institute of Environmental Sciences IEST-RP-CC002.2, ISO Standard 14644, and Federal Standard 209E.
Kevin Weist is president of Clean Air Products (Brooklyn Park, MN; www.cleanairproducts.com).
Potential challenges to consider when using a horizontal flow clean bench
- 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.
- A horizontal clean flow 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.
What class is your clean bench?
How clean your bench needs to be is dependent upon the particular operation. It is not just the quality of the filter that makes a clean bench perform; it is also related to the airflow within the enclosure.
Using a HEPA filter does not guarantee that the clean bench interior will be Class 100 (ISO 5); you need the proper airflow within the bench.
Many clean bench users need to get to Class 10 (ISO 4); upgrading to an ULPA filter will typically help them reach this goal. Although an ULPA filter will be more expensive than a HEPA filter, the cost is relatively nominal considering the overall costs of a clean bench system.
How the HEPA filter seals to the cabinet
Many bench manufacturers utilize a negative pressure area around the HEPA filter to create a high pressure seal against the filter some distance away from the second gasket that seals against the cabinet. The space between these two gaskets is under negative pressure, such that if either of the gaskets should leak the air would flow away from the clean space and back toward the blower. This system eliminates troublesome gasket leaks and also serves to simplify future service to the unit.