Cleanrooms are critical spaces within many industries that are dedicated to uniquely sensitive processes or high-value products such as computer chips and lifesaving vaccines. They typically have very high air change rates related to the ISO rating class required for the processes they house. The various ISO classes represent the airborne particle concentrations allowed in the space and range from ISO 9 (the “dirtiest”) to ISO 1 (the “cleanest”).
The main source of contaminants in a cleanroom is people, generated from the particles that are shed from exposed skin and clothing fibers. The particle-shedding rate depends on the clothing being worn (such as specialized cleanroom garments) and how much the person is moving around.
| Figure 1. A CFD model for a cell-processing lab |
The airborne particle concentration in a cleanroom is a function of the air change rate and air distribution in the space. Therefore, simply specifying an air change rate assuming full dilution is not enough to ensure that cleanrooms operate properly and achieve the particle counts required for their ISO class rating.
The effect of garments is an interesting one. Clothing can drastically reduce the particles shed from human skin by covering more parts of the body, and having personnel wear nonwoven fabrics can greatly reduce the amount of clothing fibers that are shed. Recommendations of which cleanroom garments to wear can vary – one source mentions using a face mask, lab coat and hair cover for ISO 7 spaces while recommending an ensemble consisting of coveralls, booties, gloves, face mask and hood for ISO 5 spaces.
A computational fluid dynamics (CFD) model of a cell-processing lab (shown in figure 1) was created to assess the impact of two different particle-shedding rates on the particle concentrations relative to the ISO 7 criteria. The space is conditioned with nine fan filter units in the ceiling and with low-level return grilles. The lab is positively pressurized with the offset airflow assumed to exit under the doors. In addition to heat-generating equipment such as incubators, computers and centrifuges, the lab also contains two class II biosafety cabinets (BSCs).
| Table 1. Simulation Inputs |
The air volumes and heat gains for the lab are summarized in table 1. Some of the internals of the BSCs have been included in the CFD model (figure 2) for added accuracy, since it contains both a downflow and an exhaust component. The low-shedding scenario corresponds to clothing typical of ISO 5 spaces, while the high-shedding scenario corresponds to clothing recommended for ISO 7 spaces. The particles are assumed to be generated from the occupants’ faces in the CFD model, and the particle-shedding rates are shown in table 2 for both scenarios.
| Table 2. Particle-Shedding Rates |
| Figure 2. Detailed models of class II biosafety cabinets | |||
Airflow patterns for the fan filter units and biosafety cabinets are shown in figure 3. The flow patterns from the fan filter units look reasonable, although the placement of the units could be fine-tuned to better distribute the air in the work areas of the lab. Some acceleration of the supply air is evident due to the 6°F temperature difference between the supply and room air.
The flow from the BSC exhaust impinges on the ceiling and gets mixed into the supply airstreams from the fan filter units. There is no sign of containment failure at the sash opening, as evidenced by the flow streamlines all moving into the biosafety cabinet (and not out of it). The flow inside the BSC cabinet is quite complex, with intake air coming in across the sash opening and being mixed with the cabinet’s downflow before being drawn in through the various exhaust points inside the cabinet. This underscores the need for including the BSC interior in the CFD model.
| Figure 3. Airflow patterns in the lab: BSC exhaust (top left), BSC intake (bottom left) and fan filter units (right) | |
The particle concentration plots at the seated occupant breathing height (43 in. above the floor) are shown in figure 4 for the two particle size categories (above 0.5 µm and above 5 µm). As expected, there is a drastic difference in airborne particle counts between the high- and low-shedding cases. Although the plots appear to show more large particles (larger than 5 µm) in the space, their concentration is lower and is related to the particle size distribution of the sources (occupants), which tend to shed more particles in the smaller (smaller than 5 µm) size range.
| Figure 4. Particle concentrations at a seated breathing level (43 in. above the floor): high shedding for sizes ≥ 0.5 µm (top left), low shedding for sizes ≥ 0.5 µm (top right), high shedding for sizes ≥ 5 µm (bottom left) and low shedding for sizes ≥ 5 µm (bottom right) | |
A rigorous pass/fail analysis relative to the ISO 7 requirements is presented in table 3, which examines concentrations at predetermined measurement points placed at important locations throughout the lab (figure 5). Although not all locations fail in the high-shedding case, most of them do due to high particle concentrations. In the low-shedding case, all the locations pass, and many of the measurement locations show concentrations near zero.
| Table 3. ISO 7 Pass/Fail Evaluation |
| Figure 5. Particle sampling locations for ISO compliance verification |
Key takeaways:
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