Filtration of the exhaust is not needed if there is an existing filter built into the exhaust port of the Hollis.  Many of them had this feature.  Slide gate dampers work best for consistent performance and repeatability.

Changes to exhaust settings can have significant effects on a well tuned process.  I have had an experience where the change in barometric pressure as a storm front moved through, with a positive pressure facility, caused a repeatable 50-75 ppm increase in defect rate due to the additional air flow cooling my preheaters. ---------------------------------------------------
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