Cleanroom design in 10 easy steps

Engineered Systems, April, 2008 by Vincent A. Sakraida

Looking at Figure 1, "Gown/Ungown" has the most in/out travel but is not a process critical space, resulting in 20 ach. "Sterile Air Lock" and "Bone Cement Packaging Air Lock" are adjacent to critical production spaces and in the case of the "Bone Cement Packaging Air Lock," the air flows from the air lock into the packaging space. Though these air locks have limited in/out travel and no particulate generating processes, their critical importance as a buffer between "Gown/Ungown" and manufacturing processes results in their having 40 ach.

"Final Packaging" places the bone cement/solvent bags into a secondary package which is not critical and results in a 20 ach rate. "Bone Cement Packaging" is a critical process and has a 40 ach rate. "Solvent 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.

[FIGURE 3 OMITTED]

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 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.