# Main trunk line drop downs



## Temp626 (Jan 2, 2013)

I am in the design phase of ductwork planning and have a question or two for the forum. 

General info:
- The collector is the PennState Tempest 1535 w/ 7" inlet. The machine is rated at 1700 CFM through the collector. I will be using the 7" - 6" reducer that is supplied with the machine. That was recommended by the support staff at PennState. My main trunk line will be 6" with some 6" drop downs and some 4" drop downs. 


My main question is, should the "y" portion of the fitting that creates the drop down come off the main trunk horizontally and then transition to vertical downward, OR can the "y" portion of the fitting come off the main trunk vertically downward right away.

I read somewhere that you should come off horizontally so that the bottom of the main trunk doesn't have spots for chips/dust to settle down into like the vertical "y" drop down.

I know this is a bit confusing to picture, but any help will be appreciated.

Thank you.


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## retired2 (Mar 14, 2013)

Temp626 said:


> I am in the design phase of ductwork planning and have a question or two for the forum.
> 
> General info:
> - The collector is the PennState Tempest 1535 w/ 7" inlet. The machine is rated at 1700 CFM through the collector. I will be using the 7" - 6" reducer that is supplied with the machine. That was recommended by the support staff at PennState. My main trunk line will be 6" with some 6" drop downs and some 4" drop downs.
> ...


I have a 1200 CFM DC with 5" mains and 5" drops from laterals (wye's) and all are from the bottom of the main. If you have good velocity in your system, as you should with a 1700 CFM system, these drops will not be a problem. 

Once in a while, when I open a blast gate on one of my drops along the middle of a main, I may notice a little waste that has dropped out from another open gate further downstream, but it is an insignificant amount and it clears instantly. 

In most cases bringing drops off horizontally creates a configuration and space headache, and even if you have a large enough shop where it doesn't, there is a good chance you are adding additional pipe and ells which in turn degrade the system performance.

Just remember, all things being equal, if you increase line size, velocity will slow, so you don't want to go nuts with the size of your mains. I would not go above the 6" recommended by PSI for your system.


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## Temp626 (Jan 2, 2013)

Retired2.

Thank you for your reply. I don't plan to go more than 6" for the trunk lines. You confirm pretty much what I was thinking, that coming off the bottom would be insignificant as long as the velocity is sufficient, and that additional pipe/fittings only degrade the system..... Thanks again.


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## WarnerConstInc. (Nov 25, 2008)

Horizontal then drop down.


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