Another Question Regarding Dust Collection Ducting Size

[Mark R Webster]

I have read several threads regarding ducting for dust collection. Can someone help me understand why it is recommended to run 6" ducting as far as possible to a tool that has a 4 inch port. Doesn't the 4" hose or opening at the end of the run reduce the airflow to what can get through a 4" opening. Why not just use 4" ducting all the way. Please be gentle.... I am not a math major.
Thanks

[Jebediah Eckert]

Somebody will be able to answer much better then me for sure. Anyhow, I'm just finishing up a system in my home shop now. It was designed by Oneida and you are correct, you want to have the larger pipe as far as possible. They explained it that DC work of volume. Unlike a shop vacuum that is high suction. On a DC you want to move as much volume of air as possible.

My main run is 8". My first tool is my jointer that has a 4" port. They said I will get much better results if I make the port 6". So the main is 8" then down to a 6" about 5' from tool on the drop. Then 6" into the tool.

There is is so much thought and pondering that goes into your first DC system I was just "OK" with basically saying "it works better that way, and that's the way it is." I didn't need anything else to ponder.

i will ill check back here to see if there is a simple explanation, as I am curious as well. My logic says if take say a 6" line and neck it down to 2" you'd get more suction like a shop vac, but I know that's now correct.

[Cody Jensen]

Simple explanation is that smaller diameter ducting has higher friction losses, so you have a decrease in your suction pressure. You lose energy and see decreased performance. Going as big as possibly isn't the correct way to look at it, as you can go too big for your blower and the velocity in the pipe would get low enough so solids (dust/chips) will fall out of the moving air and rest of the surfaces of the ducting. I think for for typical home use (1 - 3 HP), you can probably get by with 6" - 8" ducting.

[Jim Becker]

There is certainly more than one "school of thought" with this. On one hand, it's extremely common for duct system designs by dust collection specialists to be "stepped down" systems where the larger trunks move smaller as they get farther out to the tools. For large shops with multiple workers using multiple tools simultaneously, this is a good way to balance "available air" with "available air flow capacity" at the collector end. Most hobbyists work by themselves on single tools so "the other hand" comes into play, especially if they are using PVC duct work which as you likely know, doesn't offer the sizing flexibility that is available with metal duct. So running larger lines all the way out makes sense in maximizing air flow to any and every specific tool that's in use "in the moment" and makes installation simpler. (one blast gate open at a time) Yes, the 4" port on a tool is a bottleneck for airflow even though there might be a slight venturi effect, depending on how the port is designed. Some folks replace the smaller port with a larger one. Some don't. The "ideal" would obviously be 6" all the way to the tool in this example because it can handle substantially more air flow (which is what dust collection is singularly dependent on...not "suction") but that's not always practical or easy to accomplish with some tools. Too many tools come "standard" with 4" ports despite the increased sensitivity to dust collection and the fact that a 4" port tops out at about 400 CFM...which is inadequate for excellent dust mitigation.

For the record, my system is a "stepped design" originally suggested by Oneida, but modified by me over time to best accomodate the tools I have in my shop after buying my MiniMax equipment. From the 7" inlet to the cyclone that goes out about 6', it does run 6" all the way past the major tools with 5" drops to the slider (120mm port) and a 6" drop to the J/P that transitions to 5" at the tool (120mm port). Beyond that it's mostly 5" with 4" drops to other tools.

[Andy Giddings]

Mark, I would recommend reading some of the material on Bill Pentz' web site for a more detailed rundown of why this is important. Lots of good answers above. You'll also see a calculator on the site which allows you to calculate your optimum duct size for the DC you have. It also helps to understand which ducting units are the least efficient.

[John K Jordan]

I have 6" ducts from the cyclone down to the machines. On some machines, like the bandsaw and lathe, I go down to 4" flex to the ports. This is not a problem for the bandsaw since I split the 6" drop into three 4" ports and get plenty of airflow. For the tablesaw and 8" jointer my plan is to run 6" right to the machine. Eventually.

However, I also connect a hose to a splitter for vacuuming up chips and dust. With just that one 4" blast gate open I don't get enough air flow to suit me. I partially open a second blast gate to get more air moving. I judge this by watching and listening as the vacuumed chips move through the ducts but I haven't measured the airflow or pressure in the ducts. After a lot of use I opened an inspection port and could see no buildup in the long main duct.

If building a new system, I recommend putting in a couple of inspection ports to keep an eye on things, at least during the initial checkout. I just added threaded PVC plugs at the ends of the longest straight runs.

JKJ

[Chris Padilla]

If you cannot or don't want to modify the dust collection port on your machine, then taper the pipe down from, for example, 6", to, for example, 4". Taper that down over approximately a foot if possible. Try to avoid an abrupt step down. You want to change ports sized gently so the air-flow is interrupted as minimally as possible. Frankly, I'd work the port on the machine to accept the larger pipe.

John also suggested another way and that is going from 6" pipe to a 4" wye. The cross-sectional area of a 6" pipe is just a tad larger than the area of 2 4" pipes so it is a good compromise assuming both 4" pipes are used at the same time although sometimes you can just open both 4" ports even if you're only using one. That will be better than just having the single 4" port open.

[Mark R Webster]

Thanks guys for the responses. Maybe I should ask my question behind the question. I have a 1.5 HP collector and have all my major tools on a trunk line of 6". I break off for each tool with 4 inch gates and various sized flex depending on the tool. My delima is that I have a lone chopsaw on the other side of the shop. A horizontal distance of about 25 feet. The most direct route is up one wall, 7' over the ceiling 25' and stub out half way down 3' on the opposite wall. I can put a 6' "Y" in near the collector and run 6" 30ish feet reduce on the opposite wall to the 4" gate and then 4" flex. Or go the cheaper route with 4" the whole way. That is the only tool that will be on that line and all other blast gates will be closed when the chopsaw is running. 6? or 4? for this application?

[James Gunning]

I'll add to the chorus recommending larger than 4" pipe. As others have said, it's all about the airflow. Assuming it has a 12" impeller, your 1.5 HP blower can probably pull about 800 CFM at the end of a properly designed 6" duct. If you choke it down to 4", it won't even get close to that. Also, if at all possible, increase the airflow at your machines. I know the manufacturers generally use 4" fittings and that has become the defacto standard. The problem is that it provides an airflow that is inadequate. If you can enlarge the ports to 6" or provide more than one port Y'ed into a 6" duct, you will se a dramatic increase in the airflow. Consider this: the area of the cross section in a 4" duct is 12.57 Sq. In. A 6" duct is 28.27 Sq. In.

[Scott DelPorte]

Hi Mark, would you be connecting to something smaller than 4" on your chop saw?

[Mark R Webster]

Yes, it is a Dewalt with a 1.5 port.

[Mark R Webster]

Thanks everyone for your suggestions and comments.

[Joe Jensen]

I have read several threads regarding ducting for dust collection. Can someone help me understand why it is recommended to run 6" ducting as far as possible to a tool that has a 4 inch port. Doesn't the 4" hose or opening at the end of the run reduce the airflow to what can get through a 4" opening. Why not just use 4" ducting all the way. Please be gentle.... I am not a math major.
Thanks

Yes, there is so much misinformation on the web. You are mostly correct in your observation. Commercial shops have a giant main branch that steps down to smaller ducts to each machine because that system is designed to run multiple machines at the same time. If you have time on the Bill Pentz website suggested there is a calculator that will help you. Basically the smaller the duct the more resistance to flow so you'd think you want as big of a duct as possible. But you also need to keep the air speed in the duct fast enough so the dust doesn't settle out. 3500 feet per minute is considered the minimum air speed. A 6" duct will probably be fine with a 4" port but if you do the math an 8" duct on 4" port will result in too slow air flow. I have a paper I wrote with some examples and I will email it if you PM me you address...joe

[Jim Dwight]

If all you connect is the dust port of your CMS then I don't see why you should run a 6 inch line to then go to a 1.5 inch port. But most CMSs, perhaps all, leak sawdust fairly badly. So many build hoods. A 6 inch with a Y to hook to the saw dust port and the big pipe to the hood would be good. My last DC was a 1hp and I used 4 inch. I haven't decided to replace but if I do, I am not sure if I'll use 4 inch or 6 inch.

DCs are sensitive to the pipe size because they are designed to move LOTS of air but not pull against much resistance. Shop vacs move much lower amounts of air unless there is a lot of resistance then they may be similar. In other words, a shop vac is limited more by the motor and fan than the resistance. The DC airflow falls of rapidly with resistance.

[Mark R Webster]

Thanks guys for all the good info. A lot to ponder