Engineered Systems - April 2008 - (Page 50)

Cleanroom Design 7- ft door is 190 cfm with a differential static pressure of 0.03 in. w.g. and 270 cfm with a differential static pressure of 0.05 in. w.g. STEP SIX: DETERMINE SPACE AIR BALANCE Space air balance consists of adding all the airflow into the space (supply, infiltration) and all the airflow leaving the space (exhaust, exfiltration, return) being equal. Looking at the bone cement facility space air balance (Figure 2), “Solvent Packaging” has 2,250 cfm supply airflow and 270 cfm of air exfiltration to the “Sterile Air Lock,” resulting in a return airflow of 1,980 cfm. “Sterile Air Lock” has 290 cfm of supply air, 270 cfm of infiltration from “Solvent Packaging,” and 190 cfm exfiltration to “Gown/Ungown,” resulting in a return airflow of 370 cfm. “Bone Cement Packaging” has 600 cfm supply airflow, 190 cfm of air infiltration from “Bone Cement Air Lock,” 300 cfm dust collection exhaust, and 490 cfm of return air. “Bone Cement Air Lock” has 380 cfm supply air, 190 cfm exfiltration to “Bone Cement Packaging”, and 190 cfm exfiltration to “Gown/Ungown.” “Final Packaging” has 670 cfm supply air, 190 cfm exfiltration to “Gown/ Ungown,” and 480 cfm of return air. “Gown/Ungown” has 480 cfm of supply air, 570 cfm of infiltration, 190 cfm of exfiltration, and 860 cfm of return air. We have now determined the cleanroom supply, infiltration, exfiltration, exhaust, and return airflows. The final space return airflow will be adjusted during start-up for unplanned air exfiltration. STEP SEVEN: ASSESS REMAINING VARIABLES Other variables needing to be evaluated include: • Temperature – Cleanroom workers wear smocks or full bunny FIGURE 3. Class 100,000 (ISO 8) HVAC process flow diagram. Packaging” is a very critical process which is performed in Class 100 (ISO 5) laminar flow hoods within a Class 1,000 (ISO 6) cleanroom. “Solvent Packaging” has very limited in/out travel and low process particulate generation, resulting in a 150 ach rate. STEP FIVE: DETERMINE SPACE AIR EXFILTRATION FLOW The majority of cleanrooms are under positive pressure, resulting in planned air exfiltrating into adjoining spaces having lower static pressure and unplanned air exfiltration through electrical outlets, light fixtures, window frames, door frames, wall/floor interface, wall/ ceiling interface, and access doors. It is important to understand rooms are not hermetically sealed and do have leakage. A well-sealed cleanroom will have a 1% to 2% volume leakage rate. Is this leakage bad? Not necessarily. First, it is nearly impossible to have zero leakage. Second, if using active supply, return, and exhaust air control devices, there needs to be a minimum of 10% difference between supply and return airflow to statically decouple the supply, return, and exhaust air valves from each other. If the valves are not statically separated, their controls can end up fighting each other. The amount of air exfiltrating through doors is dependent upon the door size, the pressure differential across the door, and how well the door is sealed (gaskets, door drops, closure). We know the planned infiltration/exfiltration air goes from one space to the other space. Where does the unplanned air exfiltration go? The air relieves within the stud space and out the top. Looking at our example project (Figure 1), the air exfiltration through the 3- by FIGURE 4. Class 100 (ISO 5) HVAC process flow diagram. 50 En gi neer ed S y stem s April 2008

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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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