Contamination Control in an Upgraded Laboratory
A small area of a manufacturing plant had been upgraded to a laboratory environment and was equipped with a clean bench within which was to be performed assembly of sensitive prototype electronic components. The clean bench was a downflow design with a HEPA filter installed above the work surface. Acceptance testing showed the filter to be leak-free and the technicians involved in product assembly were observed by this consultant to be working in an appropriate manner. However the client indicated that the quality of products assembled on this bench was erratic and particulate contamination was suspected. At the consultants' suggestion, a particle counter was installed within the bench on the work surface and particle counts were observed over time. Within a short time it was noted that periods of low particle counts were interspersed with periods of exceptional high counts.
This consultants' inspection of the laboratory space showed that it had been meticulously cleaned and all surfaces were either of new, non-shedding materials, or had been coated with a non-shedding epoxy finish. A pattern of high vs low counts was observed and ultimately related to the cycling of the air conditioning system serving the laboratory. Further inspection showed that while new, clean filters, had been installed in the air conditioning system during the upgrade process, the filters were direct replacements for the low efficiency furnace filters commonly found in air conditioning equipment of that type. Whenever the air conditioner cycled "ON" to maintain room temperature, particle laden air delivered from ceiling mounted air diffusers would blow into the clean bench work area and mix with the highly filtered air flowing from the HEPA filtration system of the clean work bench with resulting high particle counts. The recommended solution, of replacing the diffusers with ceiling mounted HEPA filter units, cleaned up the conditioned air being delivered to the lab space. While there was an increased pressure drop caused by the new HEPA filters, it was not enough to require a larger, replacement, fan motor. In this case the high pressure drop of the HEPA filters was overcome by adjusting the air conditioning fan speed. Particle counts were noted to be consistently within specification from that point forward and product quality met expectations.
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