Another DC ducting question....

[Jim O'Dell]

Hi all! I hope everyone had a good weekend. After I got to a quiting spot today in the shop, I just sat there with my floor plans and started looking at some of the next steps I would be doing, that is if I can ever get the doors built for the shop! I spent a little time looking at where the DC lines will run, and this question came to mind. What is better, myltiple 22 1/2 degree elbows with some straight pipe between , 45 degree elbows, or bending the S&D pipe to a smooth but gentle curve? I have access to a heat gun, and I've read that filling the pipe with sand will help keep it from collapsing, and help hold the heat for the bending process. But which option is better for the cyclone? TIA for your thoughts. Jim

[Steve Cox]

I'm not an engineer but.....:rolleyes: . Probably the bent pipe would be best from an air flow point as long as they are long radii. That way, you eliminate the turbulence from multiple joints as you put in 45 and 22.5 degree elbows. After that 22.5 is probably best but good luck trying to fit them in everywhere. I went with 45s and straight pieces in between and it has worked fine for me. BTW, be careful if you go the heat gun route. The fumes are extremely poisonous from what I've read. Good ventilation and a vapor respirator.

[Allan Johanson]

If you have a Dust Gorilla or stronger and only have a couple bends on a run, and if your goal is around 800cfm at the tools, I wouldn't even bother with the 45s if the run is decent. Slap on a short radius elbow and don't worry.

Going to 45s with a piece of pipe between them will help increase airflow, but probably not by enough for the vast majority of folks to care. And I doubt if you could reliably measure the difference between 45s with straight pipe and a homemade long radius elbow.

But if you are concerned about airflow, by all means do everything you can to reduce the resistance.

Check out these two graphs:

Oneida Dust Gorilla:
http://www.oneida-air.com/products/systems/gorilla/performance.htm

Belfab JJ 2HP "Industrial" unit:
http://www.belfab.net/perf-jj.html

These two graphs show the difference between two machines and what the impact of more resistance is. The Oneida can handle more resistance while still maintaining high airflow compared to the Belfab. Just look at that broad, wide performance curve. At 6" of SP resistance, the Oneida is still flowing close to 900cfm. On the other hand, the Belfab is flowing....well....according to the graph, zero? In reality it'll flow something, but it'll be way less than the Oneida.

So ideally you want to know your DC and what kind of happy operating range it can handle. With all you folks buying Dust Gorillas, you should be pleased to know you can almost abuse the duct run and still get the target of 800cfm many folks want. With a smaller machine like this Belfab JJ, you need to be careful to keep the duct runs simple so it can handle it. It has a very narrow operating range and if you go beyond that, the performance will be quite poor.

BTW, to give you an example of abusing the ductwork, I fired up Bill Pentz' SP calculator to get an estimate:

Considering all 6" ducting and a 6" port on a free-breathing tool, you could have in theory:

1 - 6" hood
8 - 90 deg elbows
2 - wye branches
35 feet of solid pipe
10 feet of flex

...and you will probably still have over 800cfm with the Oneida.

Like I said, for most folks I wouldn't worry about it. :D

Cheers,

Allan

[Jim O'Dell]

Thanks Allan. I've not been able to down load Bill's calculator. Seems you need to have a certain program, and I don't. The cyclone I have is the Clear Vue that is Bill's design, with the new impeller that he and Ed Morgano of Clear Vue designed. I'll probably go with the 45's with some straight pipe between them, given the info you related. It will be easier, and safer. And yes, I knew about the toxic fumes. Thanks for the info from both you and Steve. Jim.

[Allan Johanson]

Hi Jim,

You need MS Excel to use Bill's static calculator program.

With your cyclone, it's even more powerful than the Dust Gorilla. Don't worry about the elbows, just use what you want for the turns your ducting has to make. You'll be more than happy with the cyclone as I am with the Bill Pentz cyclone I built.

Tonight I've just completed my main run to the SCMS and I'm getting around 1250cfm at that station (I have my own testing gear). And I'm using a major "no-no" by some folks - HVAC short radius 90's. When you've got the power to spare it doesn't matter. :cool:

Enjoy your cyclone. Soon you'll have this goofy smile on your face every time you turn it on. :D

Cheers,

Allan

[mike malone]

Thanks for all that info Alan
mike

[Ken Garlock]

Hi Jim.

With a searching, I found a document (http://www.oneida-air.com/design/ductguide.pdf) at Oneida that states that a 90 degree elbows is equivalent to 6 to 8 feet of ducting. Likewise, a 45 is equivalent to 3 to 4 feet of ducting. One could generalize that a 22.5 elbow is equivalent to 1.5 to 2 feet of ducting.

Looking at the numbers, no matter how you cut it, you will still have a 6 to 8 feet duct loss when turning a 90 degree corner. Given the turbulence introduced by the joints of the fittings, it would/might be more.

In my duct system, using 6" PVC, I used two 45's and a 2 ft straight section to turn the one corner I had to negotiate. It didn't gain me anything, but I felt better about it. :)

[JayStPeter]

I don't really understand the argument against using 45s. I understand that an overpowered cyclone can make up for poorly designed ductwork. But, what are we really talking here. For each 90* bend, it's around $5-6 to use two 45s as opposed to a single 90. Even with HVAC pipe, you can use 2 adjustable 90s to make a single large radius 90. They cost what, $3.
I don't know about you, but my ductwork only has a couple of 90* bends in it. All the drops are done with wyes which need a 45 to complete the bend anyway. If I had used 90s instead of 45s and Ts instead of wyes, I think I'd have saved $30 in my entire system. Foolish way to save a small amount of money IMO. I more understand if you just don't have the space and need the 90s to fit. Then, you do what you've got to do.
Heck, if you really want to make your ductwork inefficient, there's lots more money to be saved using 4" flex instead of 6".

Jay

[JayStPeter]

Hi Jim.

With a searching, I found a document (http://www.oneida-air.com/design/ductguide.pdf) at Oneida that states that a 90 degree elbows is equivalent to 6 to 8 feet of ducting. Likewise, a 45 is equivalent to 3 to 4 feet of ducting. One could generalize that a 22.5 elbow is equivalent to 1.5 to 2 feet of ducting.

Looking at the numbers, no matter how you cut it, you will still have a 6 to 8 feet duct loss when turning a 90 degree corner. Given the turbulence introduced by the joints of the fittings, it would/might be more.

In my duct system, using 6" PVC, I used two 45's and a 2 ft straight section to turn the one corner I had to negotiate. It didn't gain me anything, but I felt better about it. :)

Ken,

I think you're (via Oneida document) comparing a large radius 90 (radius = twice the diameter of the pipe) to two 45s. I don't think anyone will argue that they will be similar.
But, in PVC, the inner radius of the 90* fittings is next to nothing. In this case, the two 45s with a small straight section would act more like the numbers you presented. I don't have a number to say what a short-radius 90 would be equivalent to, but I think it is safe to assume worse :).

Jay

[Jim O'Dell]

Thanks guys! Some good info here. Thanks for relieving my pain Allan. I just come from the point that if I'm spending this much on the system, I want to get all I can get out of it!!! Plus the fact that I usually go overkill on most things I decide to do. Thanks again everyone! Jim.

[Allan Johanson]

Jim: I hear ya. I've done the same with trying to get what I can from some runs. For my system, I'm using 45* bends in several places and have gone to larger ducting where I can to flow more air. Making custom hoods, etc. Here's a pic of a drop for my jointer (and other machines will be added here later) where I've made the overhead pipe come in at an angle. I chose to do this to gain a little extra airflow, but it was also easy to do. And I'm pretty lazy. ;)

Jay: I didn't mean to come across as "against" 45* compound bends, but more from the angle that I've seen many folks start to stress out over this topic. They'll have a strong DC and they've only heard bad things about 90* elbows and how they'll drastically cut your performance in half, etc.

When you have a strong DC, this won't have as much of an impact. I've tested this myself.

I was just trying to reduce the stress level for the folks with strong DCs, I wasn't talking about saving money. If they can afford to spend over $1000 on a DC and ducting then $30 isn't going to matter. :D

Cheers,

Allan