CFM airflow Guru's please help

[Hoakie]

I just got done with my router table and was thinking about the dust collection. In Norm's plan, he brings his DC in reduced from 4" to 2" then uses a wye to hookup the lower and upper DC ports. To me it would make more sense to use a 4" wye THEN reduce the top and bottom ports to 2". Am I correct in thinking that both will see more CFM in this configuration?

TIA

[LCHIEN]

I just got done with my router table and was thinking about the dust collection. In Norm's plan, he brings his DC in reduced from 4" to 2" then uses a wye to hookup the lower and upper DC ports. To me it would make more sense to use a 4" wye THEN reduce the top and bottom ports to 2". Am I correct in thinking that both will see more CFM in this configuration?

TIA

In the order of least impact to airflow to worst:
4 inch wyed to two 4" ports
4" wyed to two 4" ports then reduced on each leg to 2"
4" Wyed reduced to 2" the wyed to two 2" ports.


Any time you go thru a reducer you take a big hit on airflow. in the second, half the air flow is throttled in each reducer so its not as bad as the third.

The first is hugely better, can't you put 4" ports on your router? or at least 4" on the main box? allowing you to keep one path all the way at 4"? reducing is so very bad for air flow.

[Hoakie]

I thought about that and can still do it, however it seemed to me like I would really be sacrificing the airflow at the bit. Or is this false logic? Would I still have good airflow from 2" port?

[gjat]

Wouldn't the best configuration be to wye a 2" off the 4" and use the 4" on the bottom and 2" on the top like many have done with the BT?

[LCHIEN]

Wouldn't the best configuration be to wye a 2" off the 4" and use the 4" on the bottom and 2" on the top like many have done with the BT?

that was sort of what i had in mind when i made my suggestion about going 4" to the bottom of the box.

[poolhound]

John,

While airflow math is always interesting its ultimatley the result that matters. I have a DC setup on my router table thats pretty much identical to Nahms. 4" to a 2"T and then one half goes to the fence with the other evacuating the cabinet.

So far I have not had any problems with this arangement. I guess it primarily depends on how much suck you have coming from your DC in the first place. I have a 50-760 BTW.

[LCHIEN]

there's always the "good enough" approach. With dust collection, the primary force to overcome restriction due to length and constriction (like size reduction or orifices) is the static head of the DC impellor, which though it varies some, is not all that different for machines of similar size.

The principle advantage of air flow is the ability to prevent air bearing fine particles from leaving your tool's cutting area. You do this by sucking as much air as possible so that none leaves except by the one-way action of the DC air flow, and two, by having sufficient velocity at the tool cutter to prevent it from flinging particles against the prevailing air flow into the machine, and escaping into the room. A router turns as much as 25,000 RPMs on a 1" diameter bit so flinging velocities can be high.

[Hoakie]

I think for now I'm gonna see how it goes. I used a piece of paper towel and was getting good pull through all the door orifices. Enough so that I imagine the fines could not escape. I do like the idea of having the 4" below, but I fear I'd miss a lot of fines above the table, or maybe like Loring indicates, the bit is spinning so fast and spitting dust at such a velocity it would not make that much of a difference above the table.

[LCHIEN]

I think for now I'm gonna see how it goes. I used a piece of paper towel and was getting good pull through all the door orifices. Enough so that I imagine the fines could not escape. I do like the idea of having the 4" below, but I fear I'd miss a lot of fines above the table, or maybe like Loring indicates, the bit is spinning so fast and spitting dust at such a velocity it would not make that much of a difference above the table.

ah, but the point of more airflow is so that the air slipping in the bit opening will be a greater flow rate and hence faster velocity than air slipping in the same opening with a lower flow rate. The particles would have less chance of escaping against higher velocity air.

[drillman88]

I talked to my plant engineer once about this, the way he explained it to me was basically volume=velocity. The more air you can pull between the intake and the collector the more velocity at the intake. I think this is what Loren is trying to explain the 2" wye will reduce the volume of flow considerably therefore reducing the velocity of the intake. The way he explained reducing the volume of the flow with very small changes pipe diameter would affect the velocity exponentially. I am not an engineer so my explanation is lagging I am sure, but I hope this helps.

[Hoakie]

ah, but the point of more airflow is so that the air slipping in the bit opening will be a greater flow rate and hence faster velocity than air slipping in the same opening with a lower flow rate. The particles would have less chance of escaping against higher velocity air.

I believe we are thinking the same thing but to be sure I'll run through my options and thinking in more detail. The way I see it is that I have a 4" pipe from the DC that needs to be in split to capture dust from two places. So I have the 3 options:

Option #1
4" reduced 2" then branch with 2" wye (Nahm's setup). One branch above table fence port and one branch in cabinet. To me this is not an option because really makes no sense compared to the other two.

Option #2
4" branched with a 4" wye. Each branch is subsequently reduced to 2" hoses one running to the above table fence port and the other to the cabinet. The assumption here is that I would get roughly equal draw to capture fines from above table and below table.

Option #3
4" branched with 4" wye. The branch to the above table fence port is reduced to 2" and the below cabinet branch remains at 4". This would give me much more airflow through the cabinet to draw fines. However, with a 4" opening below the table would cause a significant decrease in the volume of air being drawn through the 2" fence port since it would be the path of least resistance.

If this is the case, then the velocity of airflow at the bit, above the table, would suffer to the point where a lot of fines would be missed. So the question is; Does the increased draw from below the table offset the reduced airflow above the table for effectively collecting fines? Or is it a wash?

I can rationalize and justify both as viable options in my non-engineer head.

[Bill in Buena Park]

Disclaimer: I am not an engineer.

Hose length and curve restrictions aside, I've been thinking about the intake area of the ducts/ports being discussed...

4in opening = 12.57 sq in

2in opening = 3.14 sq in

So when reducing to two 2in openings, you only get 6.28 sq inches total area, which is only half of what the 4in line wants. But where pressure being pushed through smaller openings = higher velocity, I think here you end up without noticeable airflow velocity increase, because I think you're just stalling the impeller some, and you've lost the higher volume (sq in) of airflow intake - correct?

Also - 3in opening = 7.09 sq in

So one 2in opening (above table) + one 3in opening (below table) would give you 10.23 sq in, which is closer to what the 4in line wants, so would maintain better airflow velocity at the heads of both?

Engineers, is my logic close on this?