Ducting lenght before reduction

[Joe Koren]

Hi, I am in the process of setting up my dust collectior for my shop. I just have a quick question regarding ducting for my dust collection system.

First, the facts: I purchased an Onieda Dust Gorilla 3 HP Cyclone (1713 CFM). It has a 8 inch intake (see web link below for more info). I have 80 feet of main lines running across my shop with the longest single run being about 40 feet. I was thinking of using 8 inch diameter Standard 40 PVC pipes as my mains with 6 inch diameter drops to the individual tools (a Minimax 300 (saw, shaper, planer/jointer), a 16 inch bandsaw, a sanding table, a shop vaccuum for the floor, and a miter saw). My question is do I need to reduce these mains to less than 8 inches to offset the air resistance of the PVC pipes? If so what should I reduce them to?

I have read Bill Pentz's web site. In there he states: "you need to use at least 5" duct for any hobbyist blower rated up to 1100 CFM and 6" duct for blowers rated 1100 CFM to 1800 CFM."

Any recommendations?

Thanks,
Joe

http://www.oneida-air.com/products/systems/super_gorilla/main.htm

[Jim Becker]

Joe, Onieda favors a stepped system and generally has you reduce the main after the first branch or two. I step my 2hp O system down from 7" to 6" at about 8' just because it was convenient to do so.

Schedule 40 is way heavier than you need and will also likely be as expensive as metal. (The fittings are "ching-ching", too...) Find a local supplier for metal spiral pipe (often very reasonable in price that way) and just buy your laterals, etc., from Oneida or KenCraft. It sounds like you have a shop full of great gear. 'No harm in putting in an equally nice duct work setup.

[Bob Dodge]

Hi Joe,

Congratulations on a fine dc.

With a 40 foot run of 8" PVC, you'd have to be flowing 1225 cfm through your 6" drops, in order to have a velocity of 3500 fpm in the 8" main. That will give you a "length resistance" of roughly 0.78" sp. An 8 foot drop-pipe, 6" in diameter, with two 90 degree elbows, will give you a resistance of roughly 4.78" sp at 1225 cfm, depending on how your hood is tapered, and the radius of the elbows. Total static pressure resistance for that run will be 5.56" sp. The system pressure-curve shows roughly 1300 cfm with a clean filter, at that resistance. Very close match. You should be fine with that 8" main, as long as you do not reduce your drops to less than 6" diameter. If you do, you may have to consider a 7" main.

If you're a one-man shop, a 7" main will give you excellent performance on your drops, as long as the drop is flowing 940 cfm or better.(3500 fpm) Resistance will still be quite low in the 7" pipe, even at 1225 cfm, with a total resistance of only 1.5"sp over a 40 foot length. Velocity= 4586 fpm. That's an increase of only 0.7" sp over the 40 foot length.

Bob

[Stephen Dixon]

except to say that if you have bought an Onieda DC, that if you send them a layout of your shop they will send you a duct layout back that will be correct for your system.

Steve

[Joe Koren]

Stephen,

It is true that Onieda gives you free duct design, however, I have had such a poor experience with them I can honestly say I would never purchase anything from them again.

However, they did fulfill their commitment and give me duct design plans. I believe their plans were poorly designed. Among other things, their plans had the ducting do two 90-degree bends in the first four feet out of the intake. This would have caused a massive amount of turbulence in the air entering the DC and greatly reduced the efficiency of the collector. They also had the ducting reduce many times going across the shop from 8 inches to 4 inches in some cases. It really did not seem they know what they are doing and between the horrible ... miserable ... just poor customer service and the poor duct design I decided to do it myself.

To make the system most effective I have shortened the runs, got rid of some of the tight bends, and in the end adjusted my shop a bit for a better DC system.

Joe

[lou sansone]

Hi Joe,

Congratulations on a fine dc.

With a 40 foot run of 8" PVC, you'd have to be flowing 1225 cfm through your 6" drops, in order to have a velocity of 3500 fpm in the 8" main. That will give you a "length resistance" of roughly 0.78" sp. An 8 foot drop-pipe, 6" in diameter, with two 90 degree elbows, will give you a resistance of roughly 4.78" sp at 1225 cfm, depending on how your hood is tapered, and the radius of the elbows. Total static pressure resistance for that run will be 5.56" sp. The system pressure-curve shows roughly 1300 cfm with a clean filter, at that resistance. Very close match. You should be fine with that 8" main, as long as you do not reduce your drops to less than 6" diameter. If you do, you may have to consider a 7" main.

If you're a one-man shop, a 7" main will give you excellent performance on your drops, as long as the drop is flowing 940 cfm or better.(3500 fpm) Resistance will still be quite low in the 7" pipe, even at 1225 cfm, with a total resistance of only 1.5"sp over a 40 foot length. Velocity= 4586 fpm. That's an increase of only 0.7" sp over the 40 foot length.

Bob

Bob
is the 3500 fpm a figure of merit ( or put another way, is 3500 fpm ideal for wood dust and chips ) and one that industry uses for material transfer in air systems ? Should one always try to maintain this velocity ?
thanks
lou

[Bill Webber]

Stephen,

...I have had such a poor experience with them...

Joe
I'm not intending to jump on anyone's bandwagon for bashing companies, but I'll offer a 'me too' for these folks and my other recent and supposedly high end purchases. :(

We are all dis-inclined to public bashing, but I'd suggest folks considering some of these major purchases request private feedback for negative comments. Most public comments are usually laudatory.

Bill W.

[Chris McKimson]

Joe, it sounds like Oneida gave you a copy of the duct design they did for me. :D

Although I made some changes, I did stick with the intent of thier design, namely to reduce pipe diameter at branches. Most of my horitzontal runs are at least 6", I reduced to 5" drops (instead of the 4" Oneida suggested) reducing to 4" right at the machine. I did run 3 feet of straight 8" pipe prior to a 8 inch 45, wye, then a 7" 45 to get back to horizontal. Probably alot like your design. Anyhow, in the end I'm happy with the performance of the cyclone and ducting.

Me three on the CS. The salesperson dropped the ball many times CS wise, but once I got ahold of Jeff, he did everything he could to make me happy and actually provided the type of CS I had heard such positive comments about.

Chris

[Allan Johanson]

Hi Joe,

3500fpm is a typical industry guideline for the minimum desired velocity in a horizontal run to ensure sawdust doesn't fall out of the airstream and start collecting in the bottom of the ducts. 4000fpm is preferred in vertical runs to help compensate for gravity. That's why you'll usually read that a main duct likes 3500fpm and you want 4000fpm in the drops. This is for the typical ceiling-mounted horizontal main duct with vertical drops down to the tools.

I've built my own Bill Pentz cyclone and have tested the airflow and it's similar "cfm-wise" to the Oneida 3HP cyclone. After seeing the performance in my shop, in your case I'd probably go with the following for a nicely balanced system where high airflow is desired and you won't have to worry about sawdust building up:

- 8" main from the cyclone until the first wye
- 7" main afterwards
- 6" drops to the tools for convenience. Use 7" drops for tools where you want max airflow but you will need custom/multiple hoods to make sure you have enough airflow to feed the 7" drop.

BTW, don't worry too much about calculating the SP loss in the pipes to 2 decimal places. Once you fasten the pipe to the tool all that estimated precision will go out the window. Often tools have plenty of internal resistance that fretting about 0.36" SP just won't matter because the tool itself may add another 2-4" SP loss.

Cheers,

Allan

[Bob Dodge]

Bob
is the 3500 fpm a figure of merit ( or put another way, is 3500 fpm ideal for wood dust and chips ) and one that industry uses for material transfer in air systems ? Should one always try to maintain this velocity ?
thanks
lou
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Hi Lou,

Yes, 3500-4000 fpm is an industry standard for general woodworking applications. 3500 fpm in the horizontals, and 4000 fpm in the verticals. This will assure you that your ducts run clean, with no "settling", regardless of waste-density.

You can also configure certain runs to deal with specific waste types, for example, wood-flour from milling MDF etc. 2500 fpm in a horizontal should be fine. This would be categorized as "low-density light waste". This set-up can offer higher CFM at your source machine, with lower resistance in your pipes. Ideally, you'd want to devote special attention to the filtration stage with this set-up.

Bob

[Joe Jensen]

I just installed an Oneida system. I have a 2HP Dust Gorilla and they had me start with 7", and then reduce down after a few feet. But, as I think about this now, the rule of reducing at each "Y" really seems like it would only apply to systems where you are running more than one machine. I have five 5" dust ports on tools, and one 6 inch port on my planer. As I understand it, the 5 inch tool outlet will limit CFM. If I'm only using that machine, and as I would my way back towards the collector, I will eventually hit 7" or 8" duct. Won't my CFM be too low in the 8" pipe if it's only serving a single 5" port? If you had two machines with 5" ports, and they are fed by a 6" or larger line, then I see how that rule of thumb would make sense.

Does this make sense?...joe

[Bob Dodge]

Hi Joe,

Basically, the large diameter inlets you're seeing on a lot of cyclones these days, allows you to start your run in that diameter. You always want to be careful about "balancing" your pipes to your dc's capabilities. You always have to consider your "worst-case scenario" run, with the least cfm.

For example, in a 7" main, you'd have to flow 940 cfm through your most restrictive drop, to get a velocity of 3500 FPM in that main. I see a lot of guys trying to use the largest diameter pipe they can use, in order to get "Max CFM" at the tool. Basically, in dust-collection, we don't have the same considerations that an HVAC technician would have. He'd be looking at CFM only, not necessarily waste-support velocity. You have to balance the two.

In order to have a 3500 FPM flow in an 8" pipe, your "worst-case scenario" drop would have to allow 1225 CFM.

In a small one-man shop, staggered diameters are ok within reason. You wouldn't want to be doing much of that in a large one-man shop with long duct-runs. The 8" pipe is ok as a "starter" over a reasonable distance, however, the static pressure losses in a 7" main are insignificant anyway when using 6" drops. Only 0.92"sp over a 40 foot length (PVC) at 3500 fpm. (0.023"sp per foot)

If your flowing 940 CFM at the tool's 6" drop, that 6" pipe is operating at 4790 FPM. If that 6" drop has a 5" hood-port, velocity there is 6897 fpm. Hood entry-loss will be roughly 3.5"sp- 4"sp, depending on configuration, or 5.73"sp with a "plain-end" pipe.

Bob

[Bob Dodge]

Joe,

I've taken the liberty to calculate the resistance for your 6" drop with 5" reduction at the tool. Using 12 feet of 6" galvanized metal pipe, with two "four-piece" 90 degree HVAC elbows (not including the "take-off" at the main) Resistance for that 6" pipe at 940 CFM, is 0.72"sp.

Resistance at the 5" hood, will be roughly 3.3"sp with a reasonably well-tapered hood. Total so far = 4"sp.

Add 40 feet of 7" main, with a 7" take-off for your 6" drop(90 degrees), and that will add another 1.29"sp.

Bottom-line, if your dc can pull 940 CFM @ 5.3"sp, you're off and running. Low resistance, yet great velocity in all pipes. It's always prudent to allow at least 1" pressure drop for filter loading, so with 940 CFM @ 6.3"sp you're pretty-well covered.

If you were to use PVC as opposed to galvanized pipe, your length losses would drop by roughly 10%. (deduct .3"sp-.2"sp) Hood entry-loss would remain unchanged.

Summary; 40 foot long 7" main (galvanized) with one 90 degree 7" "take-off for the drop; 12 feet of 6" pipe with two 90 degree elbows (4 piece HVAC elbow with radius of 1.5x diameter); and a 5" tapered hood. 5.3"SP at 940 CFM.

If for some reason you feel your last drop is flowing less than 940 CFM, it's not a big deal as long as it's within reason. 3200 FPM in the main should still offer good performance (855 CFM). The 6" drop would still be operating well, at 4357 FPM. As a precaution, you could always extend your 7" main by a foot or so beyond your last "take-off" elbow. Cap the pipe, and every now and then, briefly un-cap that extension to allow max airflow. This will clear any settled waste in the main.

Bob