Woodshop Dust Collection System Specs

This discussion supplies a wealth of detail on duct sizing, vacuum capacity, cyclone separator design, and more. November 25, 2005

I have a small shop and am purchasing a dust collector to handle a 15" planer and a Unisaw. The dust collector will be about 20' away from the machines. Most of the small dust collector's I see have 4" ports. Is it wise to use 6" pipe even with the 4" ports? Also, what cfm would you recommend for my small shop needs?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor B:
A dust collector capable of delivering an honest 800 cfm to the machine would be just the ticket for your requirement. When operating this class of dust collector, 6" pipe is recommended. 6" pipe will flow 800 cfm at just over 4000 fpm, which is desirable and recommended. Look for a dust collector that has both 6" intake and outlet, and at least a 12" impeller. A large filter surface will flow more air at a lower resistance (pressure drop) and improve small particle filtration.

When doing your ducting, you could have a 6" drop to the saw, terminated by a 6" x 5" x 3" wye. The 5" would serve the saw's base, and 3" for an over-arm blade cover. Alternatively, a 6" x 4" x 4" would be fine if you have 4" porting on your over-arm blade cover (shop-built). The commercially available over-arm blade covers, such as the Excalibur, use 3" internal porting despite having a 4" connector at the back end of the boom.

As for the planer, you could run 6" right to the planer with the appropriate hood. Avoid using reducers to neck-down to an undersized (less than 6") hood. If you must use the 4" connector, use a 6 x 4 x 4 wye on the end of that drop. Hook one 4" port to the planer and leave the second 4" port open. This will allow enough air to the 6" duct for proper transport velocity. If you were to close that second 4" port velocity through the open port it would be a bit higher, but the added resistance (static pressure) will reduce the total amount of air flowing through the 6" duct to possibly well below the required 4000 fpm velocity required, even though the entry suction would appear quite strong.

I'd strongly recommend not using less than a 5" hood/and drop duct on the planer. If your coming off a 6" main to the planer, and are using a 5" hood/drop on the planer, you could use a 6" x 5" x 3" wye at the main and have a 5" drop to the planer. You could also have a 3" to a drill-press, or other low requirement machine. Both of those should be left open to feed the main.

Avoid using 4" connectors if possible. They will possibly "air-starve" the dc's blower, unless you have two 4" hoods simultaneously feeding the 6" main, or 6" drops. The total area of the hoods should not be less than the area of the duct (drop or main).

Even putting a 5" hood on the end of a 6" drop is not a good idea, unless your dust collector is capable of handling the extra resistance (static pressure resistance). Very few consumer model dust collectors are capable of pulling the 800 cfm that the 6" drop-duct requires through a 5" reducer. They all will appear well at the hood however.

Two 4" hoods are almost the same size in area as one 6" duct. They are about 10% less, actually. Velocity through those 4" hoods will be 10% greater than the 6" pipe. Alternatively, a 5" and a 3" combined are also the same size as a single 6" pipe. Wyes with that balanced proportion are readily available off the shelf.

From the original questioner:
I have ordered an import 2 hp single stage dust collector from Harbor Freight that is rated at 1600 CFM. It has a dual 4" inlet that I believe can be removed to use a 6" inlet. I read several recommendations for a garbage can type "cyclone" separator to be installed between the dust collector and the tools in order to drop out the larger material before it hits the impeller. What do you think of these type units? If I build one myself, would 4" fittings in the top be ok if the rest of the ducting is 6"? Where is a good source for 5" and 6" ducting and connectors? I live in Santa Rosa in Northern California.

From contributor B:
I'm not familiar with the specific dust collector you mentioned, but typically these should move about half of the claimed rating. As you're probably aware, those ratings are "free air" ratings. That's just the blower, with no filters or ducting attached.

You should be fine using 6" pipe, but be careful to keep your network as efficient as possible. Make sure they are short, with few bends. Use large radius elbows, and 45's, instead of 90 degree elbows.

Making an effective respirator is easy. You can use a well sealed box, or a drum. Make a lid from MDF, and attach an inlet duct and outlet duct. Inexpensive HVAC fittings will do. In order for the waste to stay in the waste-bin, the air velocity has to be reduced. This will allow particles to fall out of the airstream, settling into the waste-bin. To ensure that is the case, just use a larger diameter exit duct than the entry duct.

If you have a 6" pipe from the tool to the respirator, use an 8" exit duct. That duct could be an inexpensive HVAC 8" elbow. On that, you have to neck back down to 6" pipe, so use a reducer (8"x7"). Add a second reducer to the first one (7"x6"), and you'll be able to attach the 6" pipe from your dust collector to that second reducer. Assuming your air enters the separator at 4000f pm (800 cfm/6" pipe), air velocity will slow as soon as it enters the larger cavity, shedding its load.

If the exit duct were the same size as the inlet, escaping air would also travel at 4000 fpm, and the waste might just blow through to the dust collector. By using an enlarged outlet (8"), that same volume of air will travel at a much lower speed; 2293 fpm, as it goes through that portion of the pipe. That's well below particle suspension level, ensuring the waste stays where intended.

As that air continues through the reductions, it picks up speed again. It's always the same volume (800 cfm), it’s just traveling at different speed through the various diameters. I live a few thousand miles away from you, Leon, so I couldn't really recommend where to get 5" and 6" pipe. Oneida, or Woodcraft, or Air Handling, would be good mail-order choices. I'd contact a local supplier first. The HVAC fittings are at stores like Home Depot, and etc.

From contributor K:
I have a small shop also and use a 2 hp dust collector. It is hooked up to a 13" planer, table saw, edge sander, jointer, shaper and router table. I have limited space so the system is between these machines. I only use 4" flex pipe and have zero problems. I do use a cyclone/separator and would highly recommend you get one for your size operation. It makes it so easy to dump waste and not have to fool with detaching the bag. Grizzly, for one, sells them (and you can buy the 30 gallon can separately). I don't see why you would need to change from easily accessible 4" pipe to 6". You're only running two machines.

From contributor B:
4" pipe is readily available, inexpensive, and quite effective, when used properly. 4" pipe is recommended for a flow of roughly 350-400 cfm. 6" pipe, on the other hand, is designed to deliver a flow of 800 + cfm, at woodworking velocities of 3500-4500 fpm.

From contributor K:
The longest run I have is 10' to my edge sander. My planer is on a mobile stand and I connect to the floor sweep off-shoot when I use it. This run is only 5'. The dust collector system is set up in the middle of an island of machines. Therefore, the runs are all very short. I've actually wondered if 2 hp was a little overkill for my usage.