Minimizing Noise, Vibration and Lighting Problems in Cleanrooms
Designing cleanrooms must include noise, vibration and lighting consideration. Overlooking these areas may, in the long run, be more costly than addressing them in the design phase and planning them into the budget.
By John F. O`Connor and William Flaherty, Jr.
Noise, vibration, and light all play an important part in the design of a cleanroom. Whether in a new room or a retrofit, each environmental element can influence both the productivity of the worker and the quality of the process. Overlooking these areas may, in the long run, be more costly than addressing them in the design phase and planning them into the budget.
The focus of a typical cleanroom design is on size, class requirements, temperature, and humidity. Decisions are usually made by management with product quality and productivity in mind. In determining final designs and costs, worker considerations, while important, may not weigh as heavily.
Worker comfort, however, can have as much to do with product quality and productivity as the size and class of the cleanroom. A worker who is not comfortable in his or her surroundings is not likely to be functioning at full capability. Lighting that causes eye fatigue or impairs product visibility will not be effective in locating and correcting defects. Thus, quality and productivity may be below standard. Certainly, reducing noise and controlling light are ways to improve worker productivity.
What are the options?
What then are the options available to minimize the problems associated with noise, vibration, and lighting? Can these areas be addressed economically, and how difficult is it to retrofit an existing location to improve these elements?
In many instances, noise in a cleanroom is the result of the air circulation required to maintain low levels of contamination within the room environment. These rooms typically employ modular wall construction. Many relatively low-cost modular cleanrooms use motorized ceiling modules to provide air flow recirculation. Large rooms, the size of a hotel ballroom or larger, use remote air systems that move air into the room. The focus of this article is primarily on modular rooms, but many of the suggestions also apply to larger, stick-built rooms.
Controlling Noise
Ceiling modules are prime candidates for exploring ways to reduce noise. The basic concept of a ceiling module is to provide a self-contained blower-powered HEPA filter that produces filtered air. The filter media can be either standard HEPA grade (99.997 percent efficiency at 0.3 microns) or ULPA grade (99.999 percent efficiency at 0.12 microns). Clean air is typically recycled through the cleanroom a certain number of times per hour to reduce contamination to a pre-specified level that determines the cleanroom class. The class and room size determine how many modules will be needed. Federal Standard 209 defines cleanroom classifications and techniques for measurement of residual contamination levels. As previously stated, the more modules required, the greater the increase in noise.
Ceiling modules are available from a variety of manufacturers at varying levels of noise. It is important to specify low-noise modules. Sound levels printed in manufacturers` literature can be a good means of comparison as long as the cleanroom designer is satisfied that the data has been taken under controlled conditions and is accurate and repeatable. It is important to remember that a module which exhibits a low noise level when tested in an anechoic sound chamber, will not perform the same in a real cleanroom environment. The reason for this difference is the additive effect of sound reflection from acoustically “hard” cleanroom surfaces.
The performance specification for the module must also be considered in relation to noise. A very quiet module that only moves 650 cfm may require more modules to achieve the class levels demanded. A module that moves 800 cfm may do the job with fewer modules, resulting in lower overall room noise levels. The module manufacturer should be able to provide a curve of static pressure versus air flow, so the user can be assured that the module will meet pressure and flow requirements. Data for the fan alone is not adequate, and the designer should make sure that the measurements being relied on are for the total module and not just the internal fan. The addition of ductwork and room pressurization can add considerable static pressure resistance to the flow circuit, reducing the flow each module provides. A typical module operating performance characteristic is shown in Figure 1.
Ceiling modules can produce noise levels as high as 75 to 80 dBA. In a hard surface room, the noise level can be estimated by adding 10 x log base 10 of the number of modules to the overall sound level for one module,
Ladd = Lbase + 10 * log (N)
Where, Ladd is the number to be added, Lbase is the base sound level of one module, and N is the number of modules.
For example, if the overall level of a module is 70 dBA, then a hard enclosure with six modules would have an overall level of almost 78 dBA. Fortunately, most rooms are not perfectly reflective, so that some of the noise scattered by transmission and absorption causes the actual sound level to be somewhat less than the above estimate. Figure 2 shows the relationship for added sound level with an increasing number of modules. This rule of thumb gives a maximum level that would be expected.
Noise generation in large cleanrooms with remote air systems is usually easier to control, because blowers are usually very large and can be operated at low speeds. Also, there is usually enough space for installing noise-attenuating acoustic material between the blower and the point of delivery to the room. Filters located at the point of entry to the cleanroom space (usually in the ceiling) can help to absorb remaining duct noise before it enters the room. The best alternative is still to make sure that blower design and application are appropriate for low noise. Usually, an efficient blower installation will have a low-noise level as well. Remember, a lot of noise can be generated by only a few watts of power. The most common design mistake is to use a blower that is too large for the flow requirement. The large blower may operate in an unstable region, where buffeting and turbulence may produce unwanted noise. Failure to allow enough room for air flow at blower inlets or driving the discharge flow immediately into an elbow at the blower discharge are common mistakes. Conditions like these tend to increase flow velocity and produce “organ pipe” noise or cavity resonance.
The location of the cleanroom is another factor that must be carefully considered. In a manufacturing area, equipment vibration and noise could be transmitted into the cleanroom. If possible, the room should be located away from areas that might generate noise and vibration. A high ambient background level may negate increased productivity in the cleanroom altogether.
Noise generated by cleanroom processes can be controlled by outfitting equipment with special custom-made enclosures made to absorb and scatter sound energy. Cost is usually based on the size of the equipment and the degree of noise control required. It is always better to eliminate noise at the source rather than try to use attenuation methods later. Machinery and other cleanroom equipment should be carefully evaluated for noise at the time of purchase. Process noise should be evaluated after the room is completed and in operation to best determine attenuation needs.
Wall construction is important when designing a room for low noise. In a modular design, walls are available in thicknesses from 1-3/4 to 3 in. and are usually Styrofoam filled to reduce outside noise transmission into the cleanroom. Further noise reduction can be obtained by adding Gypsum panels to the cleanroom walls. Various surfaces such as painted steel, anodized aluminum, or vinyl can be applied to offer the designer durability and a choice of colors. Sealing any openings or cracks in the enclosure is important to the maintenance of a clean environment and very important in excluding ambient noise.
Designing to Reduce Vibration
Vibration can interfere with the manufacturing process in the cleanroom and can also cause structural components to emanate noise. The problems caused by vibration in a cleanroom vary, depending on the processes and the industry. While not always thought of as related to personnel comfort, vibration can cause worker fatigue and affect product quality. Examining a semiconductor wafer under a 1,000¥ magnification, for example, can be impacted if even slight vibrations are present.
Vibrations emanating from poorly designed ceiling modules can cause additional noise by setting up sympathetic vibrations in the ceiling or other structures. Ceiling module vibration can be minimized by using good impeller balance and resilient supports that isolate the rotating system. A typical module will have a vibration level of about 0.03 in./sec. measured with the module on resilient supports. At this level, the subjective severity of vibration is between “good” and “very good.”
Installing the module in the ceiling is also critical to reduced sound and vibration. The ceiling is constructed from cleanroom-rated tiles, which should be gasketed and clipped down. This construction minimizes noise and eliminates contamination. Also, the face of the module filter is gasketed into the ceiling support, and the weight of the module is supported by the overhead structure.
The use of an air shower can also add to noise and vibration. The usual cure is proper vibration isolation of the air shower`s blower system. In extreme cases, the air shower blower system can be remotely located and the air supply ducted back to the shower.
Vibration emanating from the machinery used for processing in the cleanroom can also interfere with productivity. The best defense against process vibration is the use of special tables designed to dampen vibration and isolate a sensitive instrument or process.
Choosing Lighting
Lighting for most cleanrooms is fluorescent and is designed for 100 ft. candles measured 3 ft. from the floor, in accordance with minimum lighting levels specified in Air Force Technical Order 00-25-203. The fluorescent fixtures used are usually mounted flush with the ceiling and can be relamped from within the cleanroom. Ceiling height can change the amount of lighting required to meet the standard. Light levels at the work surface will be reduced as distance from the light fixtures is increased. However, when considering the comfort level of cleanroom workers, it may be wise to deviate from standard general lighting requirements for cleanroom working surfaces. Levels of 60 to 80 ft. candles are more consistent with general office lighting standards. If higher intensity lighting is required, it can be designed into the room as auxiliary lighting and incorporated into work benches or process equipment. Auxiliary lighting to increase light intensity only where it is needed is usually the least expensive alternative and allows lower, more comfortable light levels in other areas of the cleanroom. Lower general lighting levels will result in cost savings that can be applied to offset the cost of workstation spot-lighting. The spectral frequency of the light used is also important. In a solid-state photoetching process, the frequency of the light used can have a direct bearing on process results. Sensitive processes may require a special lighting spectrum. An economical method of adjusting the frequency of light is the use of tinted plastic tube shields, which can be placed over the standard fluorescent tubes.n
John F. O`Connor is executive vice president of Torrington Research Co. (Torrington, CT). He has a bachelor`s degree in Mechanical Engineering from the University of Hartford and a master`s degree in Mechanical Engineering from Rensselaer Polytechnic Institute. A member of the American Society of Heating, Refrigerating and Air Conditioning Engineers` Committee on Fans, he has been working in the research and development of fluid flow systems for about 40 years.
William Flaherty, Jr. is manager of Engineering with Liberty Industries (East Berlin, CT). He has a bachelor`s degree in Mechanical Engineering and has specialized in HVAC and cleanroom design for the past 15 years. He presented a paper at Cleanrooms `91 and has participated in other cleanroom seminars. A member of the American Society of Heating, Refrigerating and Air Conditioning Engineers, The Institute of Environmental Sciences, and the International Society of Pharmaceutical Engineering, he has done extensive research and development work in the area of cleanroom design.
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Figure 1 (above) shows that during typical ceiling module air performance, a module that moves at 800 cfm may be able to do the job with fewer ceiling modules, which results in lower overall room noise levels. Figure 2 (below) shows that as more ceiling modules are added, there is an increase in the dB level of noise.
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A ceiling module is installed in a cleanroom ceiling. Ceiling modules are one of the best ways to reduce noise because they provide a self-contained blower-powered HEPA filter.
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Within a typical cleanroom, sensitive instruments such as microscopes are often completely isolated from vibration so that the product being examined is not impacted by even the slightest vibration.