Airflow Control of Containment Systems

Until alternative methods for total containment became prevalent, airflow control was, and is still, the primary method for attempting to control fugitive releases in the small-scale environment, e.g. laboratories and Kilogram scale pilot plants. Airflow remains the primary choice for control of dispensing. Even the recent development of Remote Access Barriers (RAB’s) relies on airflow, using a two-stage process to attempt to isolate the worker from the emission source.

In our previous Technical Bulletin we presented a collection of findings that have yet to be considered by practitioners at large, yet they have relevance to current practices. Of real significance is our inability to measure the mass of all particle sizes quantitatively.

Visual evidence comes from a 1997 study on a variety of airflow containment approaches. These studies led to questioning conventional beliefs. The test challenge was <3μm particles of lactose monohydrate which was generated in air at a concentration of 4mg/m3 entering the test environment through a particle effusor modeled after the conventional gas diffuser described in the ASHRAE hood test. A static mannequin was placed at the workers location. Three tests were conducted for each device and test condition

Particle effusor emitting lactose test material into a test enclosure

The visual evidence shown was not significant enough to identify after a single 30 minute test cycle, but after a full set of cycles (3 for each enclosure), deposition patterns were observed. Of note is the fact that all breathing zone samples taken across the front of all enclosures showed non-detectable values at <4ng/cu.m. All enclosures were airflow performance tested by independent experts before the studies began.

Most surprising were the visual results from the Biological Safety Cabinet (BSC). The airflow obviously did an excellent job of collecting emissions from the workspace in front of the mannequin. The disturbing fact is that while the airflow constrained the emissions from spreading, it immediately drew them across the work area immediately in front of the static mannequin at a visibly high concentration. An operators hands spend most time adjacent to the emission source crossing the airflow path frequently, thereby becoming contaminated. The visible air path then exited the enclosure through the vent slots but left a clearly discernible deposit on the metal grid where an operators sleeve covering can brush. Deposits were noted on the side of Petri dish located on the worker side of the grid. With the impact force the particles bounce, contaminating the sleeves and midriff of an operator working in the enclosure. Even ignoring potential transport of particles along the thermal boundary, the operator has high probability of becoming contaminated in a manner not normally anticipated. Such contamination did not show up as a breathing zone measure since the mannequin remained static throughout. Add the potential for under-sampling as discussed in the previous Technical Bulletin and it becomes clear that our trust in BSC’s is misplaced.

Biological Safety Cabinet deposition immediately in front of work zone

Both balance enclosures tested revealed powder return within the enclosure along a direct path to where an operators hands and sleeves are positioned during use. As well as creating air vortices, the air and powder flow were perturbed by the utility cutout for power and data cables in the rear of the enclosures, which focused the powder flow to the front of the enclosure. A supplementary study using airflow vanes along the edges of the enclosure did not prevent this occurrence, merely resulting in a more disperse coating over the entire surface in front of the operator – both sideways and vertically. Normal balance hood use has the worker seated with the arms bent, which creates a disturbance in the convection path along the workers sleeve. Hand and body movement within the enclosure will add to the dispersion, creating potential for the workers midriff to become contaminated. As in the BSC study, the breathing zone samples would not reveal this contamination profile.

Deposition inside the Balance Enclosure work zone
Dispersed deposition in front of worker using Balance Enclosure

Particles do not behave in the same manner as test gases. They show classical behavior as colloidal particles displaying Brownian Motion with movement counter to the airflow. This can be observed using a laser pointer to observe Tyndall light scattering created by airborne particles.

These findings helped in understanding a contaminant migration profile found during a laboratory study of a researcher working in a laboratory hood, discussed in an earlier Bulletin.

Although Downflow Booths were not included in the study, the same particulate characteristics will impact all open airflow control technologies.

RAB’s were still in a trial stage when the study was conducted. Nevertheless the same dynamic behavior can be anticipated. The interstitial barrier airflow is penetrated by the operators arms during use, creating vortices and disturbing the overall airflow local to this disturbance, giving rise to similar release pathways. The impact of particulate release outside the barrier will be less than observed for conventional airflow hoods but still makes them unpredictable for containment purposes., Once a release path is created, any particles following the thermal boundary along the arms of the worker will continue to follow the breach path. With the worker standing, the breach will allow an unobstructed convection path along the sleeve into the workers headspace, in addition to the possibility of contamination deposits on the workers hands and sleeve.

Any containment device having a physical opening to the outside environment will have unpredictable performance capability. Persons who have observed contamination outside airflow controlled containment zones without successfully identifying the source should consider the phenomena in this and the preceding Technical Bulletin.

FabOhio Inc. can guide you to devices that will help to reveal and better quantify potential performance. Open face devices frequently require replacement to achieve desired containment performance. We have a successful history of identifying, quantifying, and solving containment issues – from small scale laboratory enclosures to complete containment suites. We are also noted for providing the most economical solutions that minimize validation and maintenance efforts.

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