Cleanroom design in 10 easy steps

Engineered Systems, April, 2008 by Vincent A. Sakraida

OK, so "easy" may not be a word that comes to mind for designing such sensitive environments. However, that doesn't mean you can't produce a solid cleanroom design by tackling issues in a logical sequence. This article covers each key step, down to handy application-specific tips for adjusting load calculations, planning exfiltration paths, and angling for adequate mechanical room space relative to the cleanroom's class.

Many manufacturing processes need the very stringent environmental conditions provided by a cleanroom. Because cleanrooms have complex mechanical systems and high construction, operating, and energy costs, it is important to perform the cleanroom design in a methodical way. This article will present a step-by-step method for evaluating and designing cleanrooms, factoring in people/material flow, space cleanliness classification, space pressurization, space supply airflow, space air exfiltration, space air balance, variables to be evaluated, mechanical system selection, heating/cooling load calculations, and support space requirements.

STEP ONE: EVALUATE LAYOUT FOR PEOPLE/MATERIAL FLOW

It is important to evaluate the people and material flow within the cleanroom suite. Cleanroom workers are a cleanroom's largest contamination source and all critical processes should be isolated from personnel access doors and pathways.

The most critical spaces should have a single access to prevent the space from being a pathway to other, less critical spaces. Some pharmaceutical and biopharmaceutical processes are susceptible to cross-contamination from other pharmaceutical and biopharmaceutical processes. Process cross-contamination needs to be carefully evaluated for raw material inflow routes and containment, material process isolation, and finished product outflow routes and containment. Figure 1 is an example of a bone cement facility that has both critical process ("Solvent Packaging," "Bone Cement Packaging") spaces with a single access and air locks as buffers to high personnel traffic areas ("Gown/Ungown").

STEP TWO: DETERMINE SPACE CLEANLINESS CLASSIFICATION

To be able to select a cleanroom classification, it is important to know the primary cleanroom classification standard and what the particulate performance requirements are for each cleanliness classification. The Institute of Environmental Science and Technology (IEST) Standard 14644-1 provides the different cleanliness classifications (1, 10, 100, 1,000, 10,000, and 100,000) and the allowable number of particles at different particle sizes.

For example, a Class 100 cleanroom is allowed a maximum of 3,500 particles/cu ft at 0.1 microns and larger, 750 particles/cu ft at 0.2 microns and larger, 300 particles/cu ft at 0.3 microns and larger, 100 particles/cu ft at 0.5 microns and larger, and 24 particles/cu ft at 1.0 microns and larger. Table 1 provides the allowable airborne particle density per cleanliness classification table.

Space cleanliness classification has a substantial impact on a cleanroom's construction, maintenance, and energy cost. It is 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.