# 4" versus 6" duct



## cburdick1 (Jan 22, 2010)

Hello all,

I've read on Pentz's website that 6" duct offers better fine dust collection than 4"duct. As much as I'd like to use 6" duct, my basement shop/laundry room just wont allow for it. So, if you have a DC system that uses 4" duct, can you offer your opinions as to it's performance?

Thanks!


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## rrbrown (Feb 15, 2009)

To many variables to know for sure but in general if you set up everything right you should have a working system.

*Type collector* *(Brand/HP)* - Not all units are created equal.

*Type and length of hose -* smooth pipe and shorter runs works best.

*No sharp turns *- Use long sweeping elbows or two 45's no sharp 90's. Use WYE fittings instead of TEE's

*Filters* - Canister filters work much better then bag filters.

*Chip/dust separators* -Keeps the filter cleaner and working more efficient. Cyclone, Thien separator or other mods work best but a plain lid type separator works ok as well. 

6" works better especially for multi-port tools but many of us used 4" for a long time or still use it. That site while good info, would have you spending some serious dollars on a system that may be more then you need or can afford.


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## Itchytoe (Dec 17, 2011)

Assuming a 4 inch duct can handle the maximum flow from your blower, a 6 inch will only provide the same flow. If 4 inch isn't big enough, then you're not getting enough flow, and 6 is definitely better. A 6 inch will have a lower velocity of air for the same amount of flow than a 4 inch will. That lower air velocity makes it easier to remove particles from the air, but harder to move those particles around corners and through rough areas in the pipe. Over long distances (like 50 feet or more) a 4 inch will create significantly more drag than a 6 inch, so I'd suggest using 6 inch pipe for your main header, and 4 inch for all of your tie-ins. If you can only use 4", try and keep the overall length down.


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## rrbrown (Feb 15, 2009)

Itchytoe said:


> Assuming a 4 inch duct can handle the maximum flow from your blower, a 6 inch will only provide the same flow. If 4 inch isn't big enough, then you're not getting enough flow, and 6 is definitely better. A 6 inch will have a lower velocity of air for the same amount of flow than a 4 inch will. That lower air velocity makes it easier to remove particles from the air, but harder to move those particles around corners and through rough areas in the pipe. Over long distances (like 50 feet or more) a 4 inch will create significantly more drag than a 6 inch, so I'd suggest using 6 inch pipe for your main header, and 4 inch for all of your tie-ins. If you can only use 4", try and keep the overall length down.


Thanks I forgot the main trunk part. However if your blower will handle a 6" line it's best to use 6 " all the way to the tool then reduce to 4" at the tool. By doing that it increased the suction and velocity on my system by 50%. Before that I had 4" running everywhere and what happens is the air slows down as the length gets longer, starving the blower.

The canister over a bag filter also gave another 50% increase.


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## dbhost (Jan 28, 2008)

I have taken a different approach. My DC, as is the case with a large percentage of single stage DCs, is fitted with a 5" inlet, and a wye splitter to 2 4" connections. I run 2 4" runs to my equipment. 1 upper, 1 lower. On the equipment I have hooked up it works fantastically, on the stuff I have yet to get to, dust is a disaster... Keep your runs as short and straight as possible, and use the shortest lengths of flex hose that you can get away with...


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## WillemJM (Aug 18, 2011)

cburdick1 said:


> Hello all,
> 
> I've read on Pentz's website that 6" duct offers better fine dust collection than 4"duct. As much as I'd like to use 6" duct, my basement shop/laundry room just wont allow for it. So, if you have a DC system that uses 4" duct, can you offer your opinions as to it's performance?
> 
> Thanks!


a 4" duct will kill your system.
On the other hand, if you are using a vacuum cleaner, the volume will be so low, that a 6" duct may cause collection of dust remaining in the 6" line.
For most dust collection systems though, a 4" line of any substantial length will seriously impede the performance of your system.

Any Engineers over here?

Did the math in 6th grade and the basic principles of fluid dynamics in first year of college, no pun intended.

So here goes:

6" duct versus a 4" duct

Cross sectional area of a 6" duct is 28.3 square inches.
Cross sectional area of a 4" duct is 12.6 square inches.

Air velocity in inches per minute = Volume (CIM)/ Area

Without bothering to convert to CFM and actually doing the calculation, we can see that the cross sectional area of the 6" duct is more than twice that of the 4" duct, therefore the velocity in the 4" duct will be more than twice that in the 6" duct.

For friction loss (pressure loss), we work with the square of flow velocity, and the inverse of diameter so it follows that 2 times squared gives a number of 4 and for inverse of diameter, we multiply by 3/2.

In short your pressure losses in your 4" duct will be more than six times that in your 6" duct.

If this confusing, I can post actual collection numbers with actual CFM and friction loss numbers to clarify further.


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## cburdick1 (Jan 22, 2010)

Thanks for the replies everyone. I understand that 6" duct is the preferred way to go. 

I'm not arguing that 4" duct is more effective, that 6" duct is not effective, or anything in between. 

All I would like to know is: For those who have 4" duct systems, how do they feel the system works?

Thank you.


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## bob sacamano (Jan 24, 2012)

lets assume the shop is 20 x 20 . roughly the size of a 2 car garage. 2 legs of 4inch hose with the collector in the corner. does it really pay to re-do the system with 6 inch and get rid if the 4 inch only to work it back down to 4 inch hose near the tool ? i can see if its a 50 x 50 shop with 5 legs there might be a big difference. but in a small shop ??


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## bob sacamano (Jan 24, 2012)

cburdick1 said:


> Thanks for the replies everyone. I understand that 6" duct is the preferred way to go.
> 
> I'm not arguing that 4" duct is more effective, that 6" duct is not effective, or anything in between.
> 
> ...


i thought my 4 inch worked well till i started reading more on the subject 

now my wife wants me to get 2 more inches !


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## rrbrown (Feb 15, 2009)

bob sacamano said:


> lets assume the shop is 20 x 20 . roughly the size of a 2 car garage. 2 legs of 4inch hose with the collector in the corner. does it really pay to re-do the system with 6 inch and get rid if the 4 inch only to work it back down to 4 inch hose near the tool ? i can see if its a 50 x 50 shop with 5 legs there might be a big difference. but in a small shop ??


You would be amazed at how much better the suction is in my system since I switched to the 6" duct and switched to a canister filter.

The biggest increase in efficiency is for the TS and BS that have a 4" and a 2" or 2 1/2" port Both of those running on a 4" duct just kills the suction and velocity. Where as when they are on a 6" duct everything works correctly.


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