Changing inlet/outlet dimensions within tophat

[pricem0595]

Hey all, this is my first post! I've scoured the forums for an answer but can't seem to find one so I'm hoping someone knows..

Is it possible to have a 4" inlet with a 6" outlet on a tophat?

Is it better to just transition the 6" piping to 4" (or 2" or whatever) at the tool rather than at the tophat or does it make a difference?

I'm trying to understand what exactly happens to the airflow when one transitions sizes..

Any help would be appreciated!  :D

[retired2]

Hey all, this is my first post! I've scoured the forums for an answer but can't seem to find one so I'm hoping someone knows..

Is it possible to have a 4" inlet with a 6" outlet on a tophat?

Is it better to just transition the 6" piping to 4" (or 2" or whatever) at the tool rather than at the tophat or does it make a difference?

I'm trying to understand what exactly happens to the airflow when one transitions sizes..

Any help would be appreciated!  :D

An oversimplification, but the 4" line will have higher velocities and greater frictional losses than a 6" line.

[alan m]

any restriction at any point on either the feed or out feed side will effect the air movement.
large lines allow more air to move and to move more easily

personally I wouldn't run a line any less than 6". if the tool can only take 4" then look at ways to put larger ports on it or more ports in other areas on the tool.

[pricem0595]

any restriction at any point on either the feed or out feed side will effect the air movement.
large lines allow more air to move and to move more easily

personally I wouldn't run a line any less than 6". if the tool can only take 4" then look at ways to put larger ports on it or more ports in other areas on the tool.

Hey all, this is my first post! I've scoured the forums for an answer but can't seem to find one so I'm hoping someone knows..

Is it possible to have a 4" inlet with a 6" outlet on a tophat?

Is it better to just transition the 6" piping to 4" (or 2" or whatever) at the tool rather than at the tophat or does it make a difference?

I'm trying to understand what exactly happens to the airflow when one transitions sizes..

Any help would be appreciated!  :D

An oversimplification, but the 4" line will have higher velocities and greater frictional losses than a 6" line.

Hey thanks for getting back to me! On Christmas of all days lol

So after a bit more digging I found that the "optimal" pipe size for my DC (Harbor Freight 2HP) is 5". The 5" pipe is going to be too expensive to source for me so I'm left with either 4 or 6..

Given my machine and I'm sure you know its capabilities, do you think it will be able to handle 6" pipe or should I stick with 4"? I'll be mainly using it for fine dust (CNC, table saw)

[alan m]

i think you will get away with 6 " with fine dust. planer shavings could build up in the pipe

[nucww]

Unless you have to pay for shipping HVAC 5" metal pipe is a reasonable cost.  You may have to look at a local hvac commercial distributor rather than the box stores.  Plastic 5" wyes are now available from woodcraft for ~$15.

[retired2]

Let me echo some of nucww's comments.  I purchased all my 5" 26 ga. snaplock pipe from an HVAC supply house for only a few bucks more than the big boxes were charging for 30 ga.  "Y"s were also available at a slightly higher cost, but since airflow in HVAC systems is almost always the opposite direction, the internal fabrication tabs can lead to snags in a dust collection system.  That problem is easily fixed with body filler.  I may have posted on that subject with photos, can't remember for sure.

Ell's are the most critical part of the dust collection system and they will eat your lunch no matter how you do it.  If you buy cheap, 3 segment, 1 or 1.5xD bends they will eat your SP with friction losses that are significantly higher than long radius bends.  If you buy the 5 segment 2.25xD bends, you will have a much more efficient system, but 5 segment bends are expensive and I was not able to get them from my HVAC supplier, strictly mail order, so shipping costs are a factor. I think I got mine from Oneida, and they might have lower shipping costs for multiple units.

I would recommend a 5" system because it does seem to be the sweet spot for 1-1/2hp and 2hp blowers.  It substantially reduces the friction losses inherent to a 4" system, but still maintains the necessary conveying speeds to avoid dropout of heavier waste.   6" pipe offers only marginally better frictional losses, but with the risk of lowering velocities to the point where plugs can occur.  Just because a blower has a 6" connection port does not guarantee its performance can support a stationary 6" piped system. 

Whatever you end up doing, go out of your way to reduce wye's and ell's.  And where you must use ell's use long radius.  And while you have not mentioned flex hose, keep it to an absolute minimum.  It has about 3x the frictional losses of an equivalent length of smooth pipe.

Much of the above advice becomes less critical if you are only piping to one or two tools and the distance is short, but if your system is more complex the above advice will avoid a lot of disappointment, and money spent for a system that doesn't work very well.