I'm not the guru you're looking for, and I've forgotton all the math I learned in High School (didn't have to have any for my degree in college) BUT: If I read what you wrote correctly, you are asking what happens to the air flow when the 4" pipe dumps into the 6" pipe. Does the CFM actually drop particles out of the stream because of the flow speed drops when it hits the larger pipe. I would say, yes this would/could happen. To keep this from happening, there needs to be some air flowing in the 6" pipe from a source further away from the DC than where the 4" dumps into the pipe. That would keep the air flow up in the 6" pipe so that the heavier particles don't drop out. Say, a blast gate partially open, so that when the 4" pipe dumps in, the flow in the 6" pipe is close to it's max capability. Does this make sense? Again, no proof to back it up, just my common sense kicking in. Jim.
edit. Stu's post makes sense too. I never understood the step down in the duct work of a DC, except in big commercial installations, and it is stepping down as more pipes go off to machines so that when 2 6" pipes that are collecting at the same time come together, you need more pipe to handle the flow and keep it up. Otherwise it would slow down in the feeding pipes and create problems there. Just like a HVAC handler steps down as it goes along it's path because there are outlets letting part of the flow out of the pipe to cool/heat the room. DCs just work backwards because the flow is backwards. Jim.
Last edited by Jim O'Dell; 05-14-2009 at 03:21 PM.
Reason: add edit.
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