Ductwork and port size questions

[Anthony Whitesell]

What is the advantage of ductwork diameter that is larger than port diameter in a one-man one-tool-at-a-time shop?

[John Lanciani]

Many of my tools have 2 - 4" ports feeding into 6" duct

[Steve Peterson]

The larger duct size will reduce friction and allow more air movement. There is a risk of going too large and reducing airspeed so the dust can settle out. The solution is to do like John mentioned and open 2 4" ports into a 6" duct. They are practically the same size. Also 2 2.5" ports is about the same size as a 4" duct.

I don't think it is an accident that the common flex hose sizes are 2.5", 4", and 6".

Steve

[Marty Tippin]

The larger duct size will reduce friction and allow more air movement.

You'll get more air movement in a larger duct *only* if you open multiple smaller ports -- once you've restricted to a single 4" port, you've limited the amount of air that can move through the system, regardless of how big your main ductwork is. And, as you've mentioned, larger ductwork with the same amount of airflow will have lower velocity and potentially allow dust to settle out and collect within the duct.

[Ole Anderson]

A 6" duct with a 4" port will move a whole lot more air than a 4" port and 4" duct. How much more depends on the length of the duct. Using Bill's static calculator, assuming 600 cfm and a square edged 4" port (orifice) you get 2.97" of loss with 20' of 6" duct, if you go with 20' of 4" duct instead of 6" duct you get 6.39" of loss. Big difference. To work backwards to determine how much flow you will actually get in each case you need more information. Sorry Marty, a common misconception I am sure was picked up from someone else.

[Mike Chalmers]

You'll get more air movement in a larger duct *only* if you open multiple smaller ports


I would just like to point out that the airflow increase will be in the main duct line only, not in the 4" part of the line.

[Anthony Whitesell]

A 6" duct with a 4" port will move a whole lot more air than a 4" port and 4" duct. How much more depends on the length of the duct. Using Bill's static calculator, assuming 600 cfm and a square edged 4" port (orifice) you get 2.97" of loss with 20' of 6" duct, if you go with 20' of 4" duct instead of 6" duct you get 6.39" of loss. Big difference. To work backwards to determine how much flow you will actually get in each case you need more information. Sorry Marty, a common misconception I am sure was picked up from someone else.

What more information would you need?

What is the differential in port versus duct size that can be tolerated, it there is not a "sacrificial" gate open?

[Dan Friedrichs]

A 6" duct with a 4" port will move a whole lot more air than a 4" port and 4" duct. How much more depends on the length of the duct. Using Bill's static calculator, assuming 600 cfm and a square edged 4" port (orifice) you get 2.97" of loss with 20' of 6" duct, if you go with 20' of 4" duct instead of 6" duct you get 6.39" of loss. Big difference. To work backwards to determine how much flow you will actually get in each case you need more information. Sorry Marty, a common misconception I am sure was picked up from someone else.

Ole, this is interesting. I may be one of the people promulgating that incorrect information, as my back-of-the-napkin math and wild assumptions about how smooth ductwork is led me to believe that frictional losses were pretty negligible. I spent some time with the Pentz worksheet and it's equations, though, and I was wrong - as you said, it makes a big difference in 20' of pipe.

That said, a single 6" port with 20' of pipe has, like, 2" of SP loss, but a 4" port with 20' of (6") pipe has, like, 8" of SP loss. So, it's a really bad idea to reduce down to 4", at all, but a worse idea to do it right at the blower inlet :)

[Chris Padilla]

Any math showing that tapering down over some distance from 6" to 4" port is better than 6" to the port, and step "instantly" down to 4"? My understanding is that a fair amount of the SP (friction) comes from unwielding, wildly violent air flow at these large 'discontinuities' whereas gently tapering helps to tame the flow a bit and keep it somewhat smooth.

[Anthony Whitesell]

So the question becomes, when does the difference between the port diameter and the duct diameter reach "critical mass"? When does the difference between them mean the port restricts the flow too much to keep the linear velocity up and the dust from settling?

[Ole Anderson]

So the question becomes, when does the difference between the port diameter and the duct diameter reach "critical mass"? When does the difference between them mean the port restricts the flow too much to keep the linear velocity up and the dust from settling? There is really no pat answer. You need to model each system with pipe sizes, lengths, types (flex or smooth), types of fittings and ports, and so on, then see how the flow vs suction (pressure) curve of your fan/cyclone/filter combination interacts with the duct/port system. It is complicated and you need to have a feel for how the model works. And be warned that the Pentz model has some quirks that I had to tweek to have it make sense to me. I am no expert in pneumatic systems, but in my previous life I spent years modelling the hydraulics of open channel flows (rivers) and water distribution systems. But frankly I have forgotten much of what I learned over the years.

[David Kumm]

As has been said, the answer depends on the size and design of the impeller. Download the Cincinnati Fan PB series tables at 3450 rpm and look at the cfm per SP tables for various size backward curved and straight blade impellers. They will give you an idea of how diameter, inlet size, and blade design affect how an impeller delivers cfm under pressure. You will find that diameter increases cfm under pressure but that the curved impeller delivers more cfm/hp at low pressure but a radial outperforms at high pressure. You will also find that cfm reduces significantly as altitude increases. A collector in Denver won't deliver the same flow as one at sea level. Dave

[Anthony Whitesell]

For a given layout of duct work and impeller, I don't think it should be that complicated.

What is the maximum CFM that can flow through and open:
2.5" gate
3" gate
4" gate
6" gate

?

[John Lanciani]

Hi Anthony,

If you're interested in doing some real world testing on your system I have all the equipment you need to come up with actual performance numbers. It really puts all of the theoretical information into perspective.

(I've made this offer before, but if anyone else is interested I'm more than happy to lend my anemometer, manometer, and ammeter to anyone that wants to put the effort in and report back to the group)

[Ole Anderson]

For a given layout of duct work and impeller, I don't think it should be that complicated.

What is the maximum CFM that can flow through and open:
2.5" gate
3" gate
4" gate
6" gate

? But it is. Depends on the performance curve of your dust collector and the piping size, length and fittings between the port and the DC. A 5 hp system will pull a lot more air through the port than a 2 hp unit

[Anthony Whitesell]

But it is. Depends on the performance curve of your dust collector and the piping size, length and fittings between the port and the DC. A 5 hp system will pull a lot more air through the port than a 2 hp unit

I'm not trying to compare dust collectors or piping lengths. Choose one of each, then those variables are now constant. The piece of information I'm looking for now is what is the maximum CFM that can flow through an open:2.5" gate
3" gate
4" gate
6" gate

It has been said there is a maximum for each as duct collects work at such low SP values. I thought it was on Bill's website, but I haven't found it yet.

Anthony

[Malcolm McLeod]

I'm not trying to compare dust collectors or piping lengths. Choose one of each, then those variables are now constant. The piece of information I'm looking for now is what is the maximum CFM that can flow through an open:2.5" gate
3" gate
4" gate
6" gate

It has been said there is a maximum for each as duct collects work at such low SP values. I thought it was on Bill's website, but I haven't found it yet.

Anthony

After you choose the variables (now they ARE constants), you will be able to reliably ESTIMATE the SP generated by your system. Based on this you can reliably calculate the CFM thru the end-device. But you have to choose first. Or, build and then measure flow.

Otherwise, your question has no definitive answer. ...I could get a 2.5" gate to pass 5000 SCFM - - if the SP is high enough. No one could afford the air handler for such, but it can be done.

[Ole Anderson]

OK, assuming a 2 HP Oneida SDG (what I have) with the orifice connected directly to the 7" inlet of the cyclone no duct or fittings. Using the Pentz calculator and the Oneida fan curve, new filter: 3.5" (as small as his tables go) 780 cfm, 4" =930 cfm, 6" = 1330 cfm.. Sorry I don't have the time now to figure out how to unlock the spreadsheet and modify it for a smaller gates.