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How to Select the Correct Ductwork Diameter

How to Select the Correct Ductwork Diameter
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How to Select the Correct Ductwork Diameter

Many dust collection systems suffer due to poor ductwork designs. There is a large amount of misinformation about what the proper size pipe is to run. In reality, proper pipe sizing can get complicated and depends on many variables; tool size, air requirements, length of pipe run required, number of machines running at one time etc.

A very common and very disastrous problem in duct design is to simply run all 4" diameter pipe. People run 4" pipe for several reasons... usually not the right ones! Most "import" tools and chip collectors are outfitted with 4" diameter ports, 4" pipe is common, 4" pipe is inexpensive, 4" pipe is the most common size of PVC pipe. None of these reasons means that 4" pipe is the "right" size to run for optimal performance.

Tools are sold with 4" ports because the chip collectors have 4" ports. That doesn't mean that a tool wouldn't be much better off with a 5" or 6" port.

Let's look at an example of a small shop that has a large tool in it. In this case a 20" planer. Now I realize that not everyone has a 20" planer but more and more small shops are able to afford "large" tools. Planers, drum sanders, more powerful table saws all require a lot more airflow than they did a few years ago. This simplified layout only shows the duct run from the system to the planer. We will assume this is the run in question or the worst case for this shop.

Dust collection ductwork diagram for 20 in. Planer
A 20" planer needs to have around 800CFM to clean it properly.

The question stands, what size pipe should I run? Let's assume this is a single operator shop so the planer will not run with any other tool. All we want to do is deliver 800 CFM to the planer, that's the worst case our system will have to support. Let's also assume that the system will be run with a 2HP fan.

Here are two examples: a right and a wrong way to design this ductwork. The first (right) way uses 6" diameter pipe run all the way to the planer from the cyclone. This design will work and be able to deliver the desired 800 CFM. The second (wrong) way uses all 4" pipe run from the cyclone to the planer. This design will not get the desired 800CFM to the planer. In fact, it will deliver about half that amount.

The RIGHT way to do it…

6 inch ducting to 20 inch Planer
This layout shows the 6" pipe run. We've also letter-coded each piece or section of the system (see table below).

ID Description Part Number
A Hood Entry From Planer #DOR050000
B 5' of 6" Hose #DHF060500
C 6" 90 Deg. Large Radius Elbow #DEA900600
D 20' of 6" Straight Pipe #DPT260660
E 2 x 6" 90 Deg. Large Radius Elbows #DEA900600
F 2' x 6" Pipe #DPT260624
G Cyclone Dust Collector #XGK020105H
H Plenum and Filters #FCS133695HF

The graph below shows how pressure is lost through each section of the system assuming we are moving at 800CFM. Each component of the system adds resistance whether it's the corrugations of the flexible hose or the fabric of the filter media... it all creates friction losses for the moving air.

Pressure loss in 6 inch ducting layout

Note that the fan adds pressure. Points on the suction side of the fan experience negative pressure (vacuum). Points past the fan in the system experience positive pressure since the air is being pushed from there forward.

The total loss from all the components in the system is calculated to be 8.4". This means that if we are to actually move 800CFM through this system we will have to have a fan that can move 800CFM @ 8.4" Static pressure.

Let's check our 2HP's Fan Curve and see if it will work. We determined we need 800CFM @ 8.4" pressure. Look at the fan curve below. This is a valid point on the curve so we know the setup will work as expected.

2HP Fan Performance Curve

The WRONG way to do it…

4 inch ducting to 20 inch Planer

This setup is an impossibility as you will see in a moment. In order for the system to move 800 CFM through the 4" pipe, elbows etc. the air would have to move twice as fast. The simple formula that shows this is: CFM = Speed × Cross Sectional Area

We want the same CFM but we only have half as much cross sectional area in a 4" pipe as a 6" pipe, so the speed has to be twice as high. So just make the air go twice as fast right? Well, you can't - Not with any typical 2HP fan blower. The reason is that as airspeed in a duct increases, friction losses increases exponentially. The following chart shows pressure losses per 100' of 4" diameter pipe at various CFM ratings. Notice how the pressure loss increases sharply as CFM goes up.

CFM (airflow) Pressure Loss
100 CFM 0.65"
200 CFM 2.4"
300 CFM 5.1"
400 CFM 9.0"
800 CFM 26"

We'll look at fan curves again but realize that most dust collection fans only operate at a maximum of 8-12" total pressure. You can see that trying to move 800CFM very far through a 4" pipe will eat up all of your available pressure... and that's before you hook up any hose, elbows, filters, etc!

What really will happen with the 4" pipe setup is that the airspeed will try to increase in the pipe but very quickly friction losses will eat up all of the pressure the fan can supply and the air will only move a little faster, if any, than compared to the 6" pipe. Since the air is moving at roughly the same speed through a pipe that has half the cross section... you get half the total CFM.

Pressure loss in 4 inch ducting layout
This is the pressure loss chart for the 4" system at 400CFM.

Notice how each section of the system uses up more pressure than before and that our total required pressure to move the 400CFM is more than when we were working with 6" pipe. This system will need the fan to be able to move 400CFM @ 10.4" Static Pressure.


Let's look at the fan curve again…

2HP Fan Performance Curve
This situation will also work since 400CFM @ 10.4" is a valid point on the curve, but look what happened! It delivers 50% of the CFM needed with 25% greater air resistance placed on the system.

By using the same exact dust collector and simply running the correct diameter piping we were able to get twice the amount of CFM to our tool. Had one actually set up the system with 4" pipe they would have very poor pickup at the planer and they would be wasting most of the system's energy to friction loss.

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Edward Zappen
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How to Select the Correct Duct Diameter? Your article does a good job explaining why it is important to select the correct size duct. However it offers absolutely no advice on "How to select the correct duct diameter" . Could you send me something that will help me design my system correctly, please. I would like to buy my products from you and am aware that you have such a service. but until I am in my small basement shop (450 sq ft) except for the table saw, I am not sure where the rest of the tools will be placed.
Hollywood Engineer
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Plan for 3,000 to 4,000 feet per minute of velocity in the duct to help prevent buildup inside the duct. In this instance, the 800 cfm divided by 4,000 ft/min calls for a duct area of 0.2 sq feet. 0.2 sq ft x 144 sq in/sq ft = 28.8 sq inches (cross sectional area of duct). The 6 inch diameter duct area is 3*3*3.14 [rad x rad x pi] = 28.3 sq inches, which meets the need.
Oneida Air Systems
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We can start the early stages of our shop design service, even if you don't know where the tools will be located exactly. If you do know the tools and the approximate overall dimensions of your shop, our team can begin to make estimations and revise as your shop develops. Please note that we can only offer our shop design service for customers with Oneida Air dust collectors. Visit oneida-air.com/shop-plan-design for more details.
Pat Salvo
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I am purchasing an Oneida cyclone. Can you help me design my woodworking dust collections system. I plan to upgrade to your upgrade system eventually but need to start with my own shop vacuum which will currently feed into the cyclone and from the oneida cyclone out to the pvc piping? (What size?). I have my tool details, and my space measurements as well ready to go. Are you a company that is willing to help the small guy get started Ao that they can build up & hopefully eventually upgrade to your dust collection systems?
Stuart De Jong
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I have used one of your systems for 20 years without any problems. Now I am moving it into a retirement community woodshop and would like to use the info in your "How to select the correct Duct Diameter but I cannot print it. It is not a printer error as I printed another document of 10 pages right after trying to print yours. Could you check your file and see if there is a problem with it? Otherwise please send or e-mail me a copy.
Oneida Air Systems
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Sure thing Stuart. We'll email you a copy today. I'm sorry for the difficulty you've been having printing this document.
Denis U.
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Hi, I've been looking in to one of your 5HP cyclone collectors. I'd also be interested in getting the duct system schematic designed by you. What steps do you suggest me to take first?
Oneida Air Systems
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Hi Denis, if you go to oneida-air.com/shop-plan-design you'll find a form-fillable PDF that can be downloaded. Fill that out and return it back to us and one of our reps will touch base with you to proceed with your design.
Christoph
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So if my system has a 4" intake can I run 6" pipe all the way up to the intake port, then reduce it to 4"? Would that increase my CFM rather than running 4" pipe to my tools?
Jake
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I would like to know this as well. None of my tools: table saw, planer, jointer, NONE of them have anything higher than a 4” port. Modification is impossible with the planer. I constantly see people finger wagging about this crap but never see the actual tool connections talked about
Matthew Rutherford
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I added a 6" flange to my 6x89 Jet belt sander, based on the info I learned in this article. I used the 6" flange cut from the 6 to 8" reducer that came with my 2 hp dust collector. I did not use the reducer and ran 8" duct off the DC instead. I have a total of 5 ports on my system, three 4" and two 6", and can use any two simultaneously.
Oneida Air Systems
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No, this would not increase your overall CFM as your airflow is restricted by the size of your dust collector's intake. Ideally you should only use 6" dia. piping if your dust collector has an intake of that size or larger.
Nevil Knupp
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I have purchased a planer, jointer, and table saw and all have 4” dust connector ports. The 2HP collector I bought has two 4” intakes and one is capped off. Although I read the whole 6” vs 4” advice above I see no alternative other than to use 4”. My run will be 20’ horizontal with 6’ drops to each tool. Is this going to work well??
Oneida Air Systems
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Nevill, if your collector is splitting into two 4" ports at the inlet, is this coming from a larger port at the start? If it is, you could consider modifying your collector to cut off the splitter and just use the larger port. If your collector is splitting a 4" diameter port into two additional 4" diameter ports, then your best option is to run the 4" pipe and keep your ducting/hose lengths, turns, and reductions to an absolute minimum to eliminate resistance and preserve your system's air pressure. Alternatively, you could invest in the larger pipe and plan to upgrade your dust collector to a larger inlet down the road.
Adam Dexter
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So building off Christoph’s Questions. I’d i have a blower with a 5” port but a cyclone chip collector with 4” in/out ports sitting in front of it, running 5” ducts in the shop is pointless because I’m already capped by my 4” cyclone ports?
Oneida Air Systems
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Adam, You are mostly correct. You won't gain any noticeable improvement by running 5" duct with your cyclone's 4" ports. It won't negatively impact your performance, however, and could be considered if you foresee upgrading your cyclone in the future.
Wayne Bunker
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You would increase the CFM by running the larger diameter pipe because you would reduce the friction loss in that section of pipe. You would also reduce the velocity FPM in the larger diameter pipe. Trane Corporation makes a circular cardboard “slide rule” called a Ductulator that makes it easy to determine pressure drops for different diameter ductwork. $10.95 on eBay.
Sonal Kumar
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Good helpful information given such a nice blog thanks for sharing keep updating more
John Sanford
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I am interested in the 7.5HP Direct Drive Cyclone Dust Collector and it comes with a 10" round intake. I would be running my duct work approximately 30'. Should I use 10' duct work for the entire length of my run and break off into 6" duct to a 4" slide gate. I would be running 4" hose to my tools.
Oneida Air Systems
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John, generally you will want to run the largest ductwork diameter possible and then reduce down to your tools. With the 30 foot run you describe, however, it may be better to reduce to 8" at some point after your first 5-10 feet; The smooth, gradual transition of your duct is what is most important to ensure efficient airflow. Other conditions may vary based on your number of tools, shop layout, and budget. One of our technical reps will reach out to you to better assist you.
Aaron Gatzke
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One more time around the mulberry bush... According to one of your replies, if all my tool ports are 4 inch, it is no use running 6 inch ducts. Yet, you have another post where you ell someone to reduce their duct size down to match the tool. What is the real answer? Why the discrepancy?
Greg W.
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So if my dust collectors main port is 8" then splits into three 4" ports, I should just remove the splitter and either run an 8" pipe directly off of that and then split it to 4" into each tool? Or reduce it to 6" at the collector then branch it off that way?
Marvin McConoughey
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I have had only good experiences with my Oneida-designed existing duct system but I may have to expand it to fit new tools. Like another reader, I cannot download your guide with my printer, A Lexmark. Two thoughts: 1. Can you email me a copy and, 2, can you put a PRINT button on the website. Possibly that will help. I've never before had a "won't print" issue with the Lexmark.
Greg Lundin
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As a fairly new woodworker, I am pretty new to dust collection systems...well done, sir. My engineering background appreciated both the thoroughness and detailed explanation you provided. But I do have a couple of questions I am hoping you have time to respond to. First, one factor for static pressure is the 90 degree elbow and how many CLRs are involved. Rather than a 90 degree elbow with a 1.0 CLR, can I use two 45 degree elbows with straight pipe to simulate a 90 degree elbow with a 2.5 CLR and its associated pressure loss? The second question has to do with the pressure loss due to the type of ducting used. For a straight 4 inch pipe, what is the loss for both PVC and metal pipe? Thanks so much for letting me be a pest...Greg
Jay Schubert
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Download McQuay DuctSizer. If you plug in your CFM and the FPM you can size out the entire system, starting from the furthest point and working your way back to the vacuum. This program performs the same function as the Ductulator someone mentioned in an above comment, but it's free and easier to use (imho). Two 45s equal one 90. Friction loss does change based on type of pipe, but it won't be a significant factor unless you're talking about hundreds of feet of ductwork. What should matter most for a vacuum system is being able to break it apart to clear any clogs that develop. Jay Schubert, P.E.
Don reynolds
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This article is great for people with 4" connections on their tools but most of us I would bet have table saws, router tables with 2 1/2" connections so a big wet/ dry vac is about the best we can do..I have a one horse dust collector that's hooked up to my table saw and I've ported it under and behind the saw..currently I have the router and miter saw hooked up to my shop vac and it does a fair job. Your Dust Deputy is on the shop vac. Without buying anything new I think this may be about the best I can do..Any thought?
Stan Marshall
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I have a cyclone collector that has an 8" port with a splitter with 4" ports. If i run 8" duct and make a 4' drop to my machines will that work? The total runs is about 50' with a couple of 90's turns. Machine CFM is 2100CFM?
John dunn
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Designing two banks of downdraft tables for sanding fine particle materials. (Bondo) 24' long per side. Roughly 30" x 96" per section, total of 6 sections. What size unit will supply enough suction to satisfy the demands if all tables were in use at once? I had planned on running 6" pipe, but considering larger after doing some research into suction demands and after reading your friction loss comment. Thanks very much!
James Baker
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My application is dust collection for a table saw only. I will pick up a Dust Deputy tomorrow. The DD and shop vac will be colocated within 3' of the table saw. I am trying to pair a Ridgid HEPA shop vac to the DD for maximum effectiveness. Ridgid offers a 161 CFM with 2.5" hose and a 89 CFM with a 1.875" hose. I have not been able to find a reference to help me know which is the best fit for the DD. Suggestions? Is there such a thing as pulling too much CFM through the DD, or too little?
Oneida Air Systems
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Our standard 2" Dust Deputy cyclones are suitable for either vacuum, so we would recommend using the vacuum with a higher CFM rating for your table saw application. While there is an upper and lower limit for the Dust Deputy's CFM, in practice any wet/dry vacuum on the market is going to fall safely within that range.
Ed
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I'm confused about one thing: Air moves from the room into the tool and then into the collection system. For example, air moves from the room, across places like the throat plate of a saw, through the cabinet, and into the collection hose/pipe. So, there is a pressure drop across the tool itself and I never see this discussed or added in. I'll see statements like 500 cfm are needed at the inlet to a given tool, but no specification of the pressure drop of the tool itself even though the tool is part of the circuit. In your calculation, you show 2"-water SP for "entry loss." Is this the drop across the tool itself? If so, where do I find this information for any given tool, e.g., planar vs. bandsaw vs. table saw?
Oneida Air Systems
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The 2" static pressure loss for "entry loss" is a general number for any type of tools. While this value certainly varies for any given type of tool (e.g. planer, bandsaw, table saw), 2 inches is a good general number to plan your duct design by. It would unfortunately be very difficult to give a specific number for each and every type of woodworking tool as it could vary wildly even among those of the same type (different port sizes, different shapes and routes from the tool head to the dust port, integrated fan blowers on the tool, etc.).
Joseph
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What type of round duct is recommended for high pressure/high velocity (>3Kfpm) Dust Collection Systems? The client has expressed concern on using Spiral Round Duct. What other options do we have?
Oneida Air Systems
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We would not be able to give a firm recommendation without know the pressure value for your clients system, as that cannot be determined from the air velocity alone. In general, Spiral ducting (such as the 24 gauge pipe sold on our website) is sufficient for general dust collection applications, including our high-vacuum and SMART boost systems. If your client is concerned however, the last option we would be able to supply would be our Quick Clamp ductwork, which is usually 20 gauge or thicker.
Brad Ungerer
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I have a one man shop looking to get a sp of 2.76 at 500 cfm system to use on one tool at a time what size pipe diameter and length should I use and will it change from flex to smooth wall?
Oneida Air Systems
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Brad, our Mini-Gorilla dust collector could easily meet that requirement and you could use 5" flex hose or ducting with your setup - whatever works best for your shop's layout. I highly recommend you reach out to one of our technical sales reps if you're looking to purchase ductwork, a ductwork design, and/or a dust collector from Oneida Air Systems.
Ryan Hutchins
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I’m confused as there are conflicting answers. If my tools all have 4” ports, can I only run 4” ducting from my dust collector? Or, can I run 5” or 6” ducting up to the tool and then reduce down to the 4” tool port? Which will flow more from? I need to get this correct so I buy the correct size Dust Deputy.
Oneida Air Systems
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To clarify: You should run the largest possible ductwork/hose to your tool and then reduce down at the tool port. However, you shouldn't run ducting/hoses that are larger in diameter than your dust collector's inlet as you won't see any performance improvement by going larger than the collector and it will be more costly to purchase those components. Match your cyclone to the inlet on your dust collector.
Rick
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What's the word on height? I've got 15' ceilings, and to do work up there I have a rolling scaffold. I'd like to mount the main ductwork above the scaffold railings (so, about 13' high). I've got 6" pipe for it, and currently a 2 HP Harbor Freight dust collector on a Super Dust Deputy (5" intake fitting). This system will get upgraded later this year to 5 HP and a larger cyclone.
Oneida Air Systems
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You shouldn't see any issue running ductwork vertically. Compared to water - where it might make a difference - air has such a low density that running it up or down won't make a significant difference on the air pressure. The overall length of the duct is the most important factor, so do keep in mind that you'll be adding a bit more length by running it up through the rafters and then back down to your tools.
David de Jager
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Can I buy a program to calculate the duct size required Thanks David
Oneida Air Systems
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We unfortunately don't have anything like that that we can offer. There are professional engineering programs out there that could do it, but it would be much more cost effective for most customers to utilize our shop plan design service. Visit oneida-air.com/shop-design for more details.
Roger
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Does the friction actually increase exponentially with decreasing pipe diameter? I came up with head loss scaling between ~1/D^3 to 1/D^4. Not trying to quibble, but rather to test my understanding of the physics involved as I design my system. The Darcy-Weisbach formula for frictional loss goes as ~f_s*v^2/D. The v^2 carries a factor of 1/D^2. If laminar, then the friction coefficient f_s~1/Re~1/D, where Re is the Reynolds number. So there's a total of 1/D^4. But the fluid is likely in the turbulent regime, where f_s for a smooth pipe varies very slowly with Re (by a Lambert w function), so it's closer to a 1/D^3 scaling. Still, the 4" pipe has over 3x the loss of a 6" pipe, so the point of the blog post still stands.
Roger
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I made an error in my comment earlier- the head loss from the velocity^2 should go as D^-4 (not D^-2), so the overall loss for turbulent flow seems to be roughly D^-5, which I think I read somewhere else as well. I'm still naturally curious if friction goes exponentially in some cases.
Larry
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I have a 2 hp. with a 1550 CMF With A 4" hose inlet/outlet would I be better off with a 4" pvc or 6" pvc.
Oneida Air Systems
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You will not see any benefit from using a duct diameter larger than your dust collector's inlet as you'll still have to reduce down at the end. Additionally, 6" duct is going to be more expensive than 4" duct so the loss is compounded. 4" hose/duct would be sufficient in the description of your application.
Tom
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I have a 1hp 240v 700cfm rated dc with a 6" inlet. Should I run a 6" or 5" trunk?
Oneida Air Systems
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For best airflow performance we highly recommend running as large of a duct diameter as possible (up to the size of your collector's inlet) and then reducing once you get to the tool.
Dwight Moldenhauer
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Oneida, At present my existing collector has two 4 inch inlets, I would like to replace it with a oneida collector in the next year, In the meantime would runnng a trunk line at 6 inches be terribly inefficient, until i replace my unit?? I need to get started and i already own the current collector and want to place my order with you and build my ducting...
Oneida Air Systems
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Dwight - You can run a 6" main trunk line if you have the ability to remove the 2 - 4" wye on your dust collector inlet. Usually there is a 6" inlet behind that. If the wye cannot be removed then switching to the 6" main line will not work efficiently for you as you will not want to reduce at the inlet.
chip
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Would it be possible to put the cyclone and catch can outside the shop and run the filter to be in the shop for return air?
Oneida Air Systems
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Hi Chip, The Dust Collector can go outside and yes, you would need to have return air back into the shop. This would normally be done by bringing the filter back inside. The dust collector is not going to be weather proof but weather resistant. It would be recommended to have some sort of roof over the system to protect the motor. If you have further questions, please reach out and let us know.
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