A is the incoming fresh air from outside, TO the filter. B is a valve that determined whether incoming air came from the outside intake pipes (through the tee at C) or from the suction pipe in the airlock via D. E is a cap on the bottom of the main intake line, perhaps a cleanout? F is the line FROM the filters into the facilitly. G is a valve that controlled whether filtered air was directed via H to the airlock for decontanimation of personnel or through I where it would split to the two lines inside the building, J and K. The valves at B and G normally fed air from the outside into the filters, then into the building. The valves were connected and would have operated together. When G directed air to the airlock for decontamination, B would change at the same time so that instead of outside air, intake air would be drawn from the airlock back through the filters to clean it. The valves were operated by cables running from a switching mechanism, they both turned at the same time. If the airlock was in use ALL the filtered air went there, and ALL the incoming air came from there.

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