Engineered Systems - April 2008 - (Page 45)

Cleanroom Design Class name Class 1 10 100 1,000 10,000 100,000 ISO 14644 3 4 5 6 7 8 Allowable number of particles per particle size 0.1 microns (m3) 1,000 10,000 100,000 1,000,000 *1 *1 *1 *1 (ft3) 35 350 3,500 35,000 *1 *1 0.2 microns (m3) 237 2,370 23,700 237,000 *1 *1 (ft3) 7.5 75 750 7,500 *1 *1 0.3 microns (m3) 102 1,020 10,200 102,000 *1 *1 (ft3) 3 30 300 3,000 0.5 microns (m3) 35 352 3,520 35,200 352,000 3,350,000 (ft3) 1 10 100 1,000 10,000 100,000 1.0 microns (m3) 8 83 832 8,320 83,200 832,000 (ft3) 0 2 24 236 2,360 23,600 5.0 microns (m3) 0 0 29 293 2,930 29,300 (ft3) 0 0 0 7 70 700 *1 - There are no particle limits for Class 10,000 and 100,000 in 0.1, 0.2, and 0.3 micron ranges. Table per IEST standard ISO 14644-1. TABLE 1. Allowable airborne particle density per cleanliness classification. important to carefully evaluate reject/contamination rates at different cleanliness classifications and regulatory agency requirements, such as the Food and Drug Administration (FDA). Typically, the more sensitive the process, the more stringent cleanliness classification should be used. Table 2 provides cleanliness classifications for a variety of manufacturing processes. Your manufacturing process may need a more stringent cleanliness class depending upon its unique requirements. Be careful when assigning cleanliness classifications to each space; there should be no more than two orders of magnitude difference in cleanliness classification between connecting spaces. For example, it is not acceptable for a Class 100,000 cleanroom to open into a Class 100 cleanroom, but it is acceptable for a Class 100,000 cleanroom to open into a Class 1,000 cleanroom. Looking at our bone cement packaging facility (Figure 1), “Gown/Ungown” and “Final Packaging” are less critical spaces and have a Class 100,000 (ISO 8) cleanliness classification, “Bone Cement Air Lock” and “Sterile Air Lock” open to critical spaces and have Class 10,000 (ISO 7) cleanliness classification; “Bone Cement Packaging” is a dusty critical process and has Cass 10,000 (ISO 7) cleanliness classification, and “Solvent Packaging” is a very critical process and is performed in Class 100 (ISO 5) laminar flowhoods in a Class 1,000 (ISO 6) cleanroom. STEP THREE: DETERMINE SPACE PRESSURIZATION Maintaining a positive space air pressure, in relation to adjoining dirtier cleanliness classification spaces, is essential in preventing contaminants from infiltrating into a cleanroom. It is very difficult to consistently maintain a space’s cleanliness classification when it has neutral or negative space pressurization. What should the space pressure differential be between spaces? Various studies evaluated contaminant infiltration into a cleanroom vs. space pressure differential between the cleanroom and adjoining uncontrolled environment. These studies found a pressure differential of 0.03 to 0.05 in w.g. to be effective in reducing contaminant infiltration. Space pressure differentials above 0.05 in. w.g. do not provide substantially better contaminant infiltration control than 0.05 in. w.g. Keep in mind, a higher space pressure differential has a higher energy cost and is more difficult to control. Also, a higher pressure differential requires more force in opening and closing doors. The recommended maximum pressure differential across a door is 0.1 in. w.g. At 0.1 in. w.g., a 3- ft by 7-ft door requires 11 lb of force to open and close. A cleanroom suite may need to be reconfigured to keep the static pressure differential across doors within acceptable limits. Our bone cement packaging facility is being built within an existing warehouse, which has a neutral space pressure (0.0 in. w.g.). The air lock between the warehouse and “Gown/Ungown” does not have a space cleanliness classification and will not have a designated space pressurization. “Gown/Ungown” will have a space pressurization of 0.03 in. w.g. “Bone Cement Air Lock” and “Sterile Air Lock” will have a space pressurization of 0.06 in. w.g. “Final Packaging” will have a space pressurization of 0.06 in. w.g. “Bone Cement Packaging” will have a space pressurization of 0.03 in. w.g., and a lower space pressure than “Bone Cement Air Lock” and “Final Packaging” in order to contain the dust generated during packaging. A Class 100,000 (ISO8) cleanroom having a low occupancy rate, low particle generating process, and positive space pressurization ep s in relation to adjacent dirtier cleanliness spaces might use i s 15 ach, while the same cleanroom having high occupancy, frequent in/out traffic, high particle generating process, or tg neutral space pressurization will probably need 30 ach. The air infiltrating into “Bone Cement Packaging” is coming from a space with the same cleanliness classification. Air infiltration should not go from a dirtier cleanliness classification space to a cleaner cleanliness classification space. “Solvent Packaging” will have w w w. esmag a zin e. c o m 45 http://www.esmagazine.com

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Engineered Systems - April 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips This Time, Water Cooled Was All Wet Cleanroom Design In 10 Easy Steps It's Not The Heat, It's The Humidity Issues & Events Computers and Software Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - April 2008

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