Calculating Airflow and Velocity in Ducts Joined by a Wye

Detailed advice on how to set up a two-duct dust-collector system for good air-flow volume and air speed. December 6, 2006

I am a small picture frame shop. I am trying to design my layout. Is it okay to locate my dc in a corner and wye two six inch mains off of the dc? Each run will be at floor level, 20' and 25' long with six small machines total. Am I better off putting the dc at the end of the main? It is just easier on my space to place it in the middle. I have a 2hp Shop Fox.

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor S:
The end of the run is best, but if you have to, make sure you place your wye about 3' from the fan inlet and you will want blast gates on each machine.

From contributor R:
Is this set up in an 'L' shape? If so, I think you'd be better off with the collector in the corner for two shorter runs. Keep pipe large, but maintaining fpm to reduce static pressure.

From the original questioner:
Yes, it is an L shape. I planned on running 6 inch main along the floor with short 4 inch branch lines maybe a couple feet long. Should I use a 6+6+6 wye or will a 6+6+4 wye be alright for my branch lines?

From contributor R:
I'm not familiar with the Shop Fox. In my first shop (20x25 off of garage), I had a 2hp and then a 3hp collector. I ran 4", but somewhere along the line, realized that 6" would be better. What's the inlet to the 2 HP? I'd go with a 6x6x6 wye. You're probably not going to run many machines at one time, are you? I think I'd run 6" pretty much the whole length and neck to 5" or 4" right at the machine.

From the original questioner:
Yes, it is a 6 inch inlet, and I do only run 1 machine at a time. Am I going to maintain enough FPM to keep the dust moving with all 6" right up to the machines?

From contributor R:
You should probably dig for some formulas on the web and also try to determine realistic cfm for your machine at a given SP. But when I had my layout professionally designed this summer, the main trunk was sized to run at a slower velocity than the branch lines. You need maximum velocity at the machine to capture dust, then slow it down somewhat so power isn't lost trying to move air at high speed. I think he shot for 3500 fpm in the main trunk. Again, there are tables showing what that should be for different types of dust. For example, metal dust must move at a much higher velocity. When your wood dust is moving, it will stay that way without a lot more energy. Also, since it's picture frames, I'm assuming you don't have a lot of heavy shavings. Perhaps that's a wrong assumption. Keeping planer shavings in suspension in the air stream will take more than light dust from a chop saw.

From contributor B:
A 2 hp ShopFox should pull roughly 800 cfm through the 6" inlet, with a short length of pipe attached to the blower (about 5'). You might get a bit more, but there is no current info available on that dc. Typically, those 2hp dc's with a 5" outlet (blower to bag-tree) net approximately that result, 800 cfm. You might get a wee bit more, but that's the general performance level, based on roughly .22"sp external resistance (5 feet of 6" pipe).

If your ducting is horizontal (no vertical ceiling drops), you can do very well at 3500 fpm in a 6" pipe. That requires a minimum of 685 cfm flowing through the pipe. We all know that adding pipe length adds resistance. That added resistance also reduces airflow. Choking down from 6" to 4" will really increase resistance, and dramatically reduce flow.

Now, regarding your 4" drops off the 6" main… A 4" pipe has only 44% the area of a 6" pipe. That will increase static pressure resistance by a substantial margin. The net result will be reduced airflow. That could vary, but in general, expect 50-60% of the initial flow rate. At 60% of 800 cfm, you'd flow 480 cfm through the 4" pipe.

In the 4" pipe, velocity will be stellar, 5503 fpm, but when that volume enters the 6" main, velocity will be reduced to 2446 fpm. Almost 1100 fpm below minimum. The solution is to avoid 4" drops, and go with a single 6" drop split to two 4" pipes, serving two 4" machines simultaneously. In other words, a 6" pipe split with a 6x4x4 wye, and operating both of those 4" pipes at full open. This will dramatically reduce static pressure resistance, and feed more air to the 6" main, increasing velocity. A drop like that would be fine to serve a router-table and a drill press, for example. It could also serve a small lunch-box planer, and a 6" jointer.

Two 4" pipes will flow almost the same amount of air as one single 6" pipe at that resistance (about 90%). That would give you 720 cfm through the 6" pipe, for a velocity of 3669 fpm. Nice low resistance flow, and totally adequate in the horizontal main. Through each of the 4" pipes, you'd be drawing 360 cfm, with a velocity of 4127 fpm. Again, nice strong velocity, fairly low resistance.

For a table-saw drop, I'd recommend a 6" drop, split with a 6x4x4 wye, with one 4" pipe to the saw's base, and a second 4" to an overarm blade cover. Another nicely balanced split at the saw would be a 6x5x3, with 5" to the base, and 3" to the blade cover. For a larger planer or jointer, try to go all the way to the machine with a 6" hood. Same for a drum sander.

If you use 5" hoods at those machines, you may want to look at using a 5" main. Much depends on your sequence of drops, and your dc's suction capability. If your first drop is the table-saw, you could go with a 6" main to that 6" drop, then continue with a 6" main to the next 6" drop that splits to two 4" pipes, serving two small machines. From that point, to serve machines with 5" hoods, reduce the main to 5". When using that 5" portion of the main, partially open the blast-gate at the 6" drop closest to that 6-5" main reduction, if your velocity through the 6" main is low.

A 6" pipe is 45% larger than a 5" pipe, and unless you're pulling 685 cfm at the planer/jointer, you won't have enough air reaching the 6" main to operate at desired velocity. If you've reduced flow to roughly 600 cfm at those machines, velocity through the 5" drop would be 4403 fpm. Once that same 600 cfm reaches the 6" portion of the main, velocity will be 3057 fpm. That's about 450 fpm less than minimum recommended speed of 3500fpm. That could start to leave a waste buildup in the 6" main. That builds up slowly over time. Most of the time, you'd be fine, but if some clogging occurs, just partially open that blast-gate in the 6" drop closest to the 6-5" mains reduction.

Another option would be to leave that 6" gate closed, plane or joint your wood, then to clear any settled waste in the main, open that 6" drop's blast-gate to full open for a moment.

From the original questioner:
Thank you for your response - it is of great help! I have not started my duct system yet - still planning. I have also modified my dc to a 6 inch outlet. The original outlet was equivalent to 7 inches reduced to 5. Hope it helps - I have no way to tell. I also have Pen State 1 micron felt bags.

From contributor B:
Many so-called 2hp dc's, especially the consumer imports, use a 5" outlet, even though they use a 6" inlet and 12" impeller. The only reason I can see for this design is to protect cheap Asian motors. Those motors have virtually no nameplate info, and often, you have no idea of exactly what you're getting. Essentially, that dc's 5" porting makes it a 1 1/2hp design air-wise, and the 2hp nominal rating is pure marketing.

You will see a substantial gain in performance with that 6" outlet mod. Probably a net gain of roughly 200 cfm, from 800 cfm to about 1000 cfm, if yours has the 12" impeller. Twin fabric filters are a part of making that mod work efficiently. A non-breathable plastic lower bag would increase pressure-drop, lowering cfm, and reducing filter capture efficiency.

One caveat. Monitor your amp draw, especially if you have no manual thermal overload protection. If you don't have an ammeter, check to see that the motor's not overheating.
Moving more air requires more horsepower. As long as you're aware of the possible consequences and do your part in monitoring that, you'll be fine.

For the most part, resellers of these types of dc's have no control over how the end-user utilizes his/her dc. Restrictions like 5" outlets are deliberately built-in, and intended to guarantee that the end user doesn't overload the motor by running it unrestricted. This avoids costly warranty repairs.

I've often recommended that outlet modification on those 2hp dc's, but I always tell the user to not run the dc unless the necessary restriction is replaced, in the form of an attached duct network. A guy who runs it with no filters (venting outdoors), and a very short pipe, may get a nasty surprise.

One way of doing that modification requires no bag-tree surgery, no 6" transfer-hose from the blower to the bagtree, and no fittings. All you do is remove the blower's 5" outlet transition flange. Now build a small shallow plenum (air chamber) to replace the bag-tree. That plenum will sit on a drum or waste-box. Cut a hole out of the bottom to allow waste to fall through into the drum. Cut a small hole in one end of that plenum, and bolt the blower directly to that plenum. Add your filters to the top of the plenum, and you're done. You could even have two drums side-by-side, and mount several filters to the top of that plenum.