A is the incoming fresh air from outside, TO the filter. B is a valve that allowed incoming air to come from either the intake pipes outside (through the tee at C) or from the intake 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 would allow filtered air to be 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 draawn from the airlock back through the filters to clean it.