Airflow, hoses and dust collectors

[Szymon Balasz]

Hi everyone,

My 12 year old vacuum cleaner (normal household but w/ dust deputy) finally died yesterday - halfway through sanding a big project. While I try and figure out what to replace it with I tried to improvise by taking a 4" hose from the dust collector and rigging two or three adapters and smaller hoses, and eventually connecting it to my orbital sander. It worked, but I only got tiny amounts of suction.

I get the basic principles. i.e. shopvacs = static pressure, dust collectors = airflow. So I'm not surprised it wasn't effective. But it got me thinking about how different piping and ports affects airflow and if I can optimise my setup. I have a 3HP dust collector with a 6" with it's stock 3-way splitter, each taking a 4" hose to a different tool. One of those is my contractor style tablesaw with an enclosure I built for the motor and expanding foam plugging everything else. Pretty much the only airflow is through the insert plate (mostly zero clearance) and through the gap by the tilting arbor blade lift thingy in front (most of which is covered by a magnetic sheet). I'm wondering if I should make any openings for air to enter the table saw enclosure to let the DC pull more air through. I don't think I get a lot through those two openings I do have and now I'm concerned I'm negatively affecting it's performance.

Also, I heard somewhere in a youtube video that you should have as thick as possible piping/ducting right up until your tool. So should I have 6" piping running till a metre or so before the tool, and then 4" flex hose the rest of the way?

Cheers

[Charles Lent]

I think you should make some additional air inlet openings in your contractor saw enclosure, but you need to also place them where the air flowing through them to the dust the collector hose goes across or near and past the blade area, so that the sawdust from the blade is carried directly to the collector hose. Other tools need to be connected and vented in a similar manner. The long run of pipe across your shop from the dust collector should be 6" if possible, but the drops to the tools can be reduced to 4". All of this piping needs to be smooth walled piping, metal or plastic, and only a short run of flexible piping run the last couple of feet between the machine and the rigid pipe, if the tool might need to move around some. Flexible piping will handle much less air flow than the smooth piping and should be minimized as much as reasonable possible. It will also help to keep one 4" port at the opposite end of your long run of 6" open most of the time so the air flow from it to the dust collector can keep the saw dust from settling in the long and relatively level 6" pipe. Having a 3 hp fan on the dust collector will do you little good if you close off everything and prevent air from flowing through the ducts. With 3 hp you will likely need a minimum of a couple of 4" ports open, the one at the end, plus one or two more on the machines that you are using. The dust won't be collected without the air flow no matter how big the dust collector fan motor is.

Charley

[John K Jordan]

should I have 6" piping running till a metre or so before the tool, and then 4" flex hose the rest of the way?

Szymon, I do that with several tools. I have a 5HP cyclone with drops at various tools. For my drum sander, disk sander, lathe, bandsaw, and long shopvac hose for cleanup. I bring down smooth wall 6" ducts to close to the tools then split with blast gates as needed into PVC or flex. (I need flex on this one that goes to the drum sander since I wheel it out for use.)

electrical_shop_s.jpg

This provides more suction that I ever imagined. The open space around the bottom of the drum sander is so small the 4" is perfect; I can't imagine needing 6" to that machine. The bandsaw gets zero sawdust inside the cabinet with two 4" ducts (and a moveable one on top the table to catch dust from skimming cuts). I do have 6" directly into the table saw and will convert the jointer when I have time. At the lathe the suction is so hard I have to wear hearing protection and be careful to hold onto things or they will disappear.

lathe_PM_Jan17_IMG_5751.jpg

I have no idea how well this would work with a smaller DC. If possible, more capacity would not hurt (up to a point!).

I did think about a way to pick up more dust at the table saw but it would make it hard or impossible to tilt the blade more than a little. As well as the main port in the cabinet, I'm imagining splitting off a smaller hose and bringing it horizontally through the side of cabinet so the pickup is inside right next to the blade. Another tube from the outside could feed air close to the opposite side of the blade. I am imagining in my head that this "wind" will tend to blow some of the dust from the kerf as the blade descends before it can be carried around and thrown back up into the air. Blast gates would have to be added to both lines to balance the flow. It's on my list of things to try...

JKJ

[James Gunning]

Syzmon,

It is always better to run the 6" duct/hose all the way to a matching size port. If you step down to 4", you will only get the flow possible through that size port. (4"=12.56 sq.in.) (6"=28.27 sq.in.) In Johns case he has a 5HP Clearvue cyclone that moves enough air to allow working at a higher static pressure. He is not seeing as much drop in airflow with 4" hose because that big blower can partially compensate. However, I notice he has upgraded the ports on many of his machines to 6" or equivalent.

You may have sealed up your saw a bit too tight. You need about the same open area on the saw as your pickup ducting to not impede the airflow. Remember, DC's are low pressure, high flow machines. Restrictions to airflow reduce their efficiency. I'll bet that ZC plate really whistles.

[Szymon Balasz]

Heh, I thought it might be the case about limiting my airflow @ the tablesaw, but I had no idea about the flex piping reducing airflow. My shop is going to get an overhaul soon and putting in some 6" PVC is definitely going to the top of the list.

Charles, by saying I need a port at the end open do you literally mean at the end? So if my ducting will have a few splits to machines, then instead of finishing off with an elbow at the last machine I should have another 2 way split and leave the one end open?

If two piped join at the DC, does having air coming in from Pipe B increase suction/flow in Pipe A? Always? Or only if Pipe A is too small? (like 4")

[Doug Garson]

Without getting too technical, I think a little basic fluid dynamics would be helpful here. Pressure drop is made up of two components, friction loss and shock loss. In any fluid flow there are a set of basic laws of physics that apply. One of them is that pressure drop friction loss varies as the square of the velocity and velocity is basically volumetric flow (CFM) divided by flow area. Thus if you reduce the pipe size (flow area) by say 1/2 you would increase the pressure drop per foot of pipe length by a factor of 4 so 10 feet of the smaller pipe would have the same pressure drop as 40 feet of the larger. Friction loss also varies depending on the smoothness of the inside of the pipe so smooth pipe has less pressure drop per foot than flexible hose which has a corrugated inside surface. Shock loss occurs any time the direction of flow is changed or the flow area is changed (generally referred to as expansion/contraction losses).

[John K Jordan]

If two piped join at the DC, does having air coming in from Pipe B increase suction/flow in Pipe A? Always? Or only if Pipe A is too small? (like 4")

From my experience adding a little extra air near the pickup far from the DC can help. In my case, I use a long small diameter hose connected to a 4" port at a split by my lathe. The long shop-vac (2-1/2" I think) hose has no problem picking up shavings off the floor. However, due to the decreased airflow the entire system DOES have a problem pulling those shaving vertically and down the almost 40' run to the cyclone. What I do is crack the blast gate at the lathe. The extra air injected there is enough to "energize" the whole run.

I'm having trouble imagining how extra air from Pipe B right at the DC would do anything but further reduce the airflow in Pipe A. But I don't speak fluent fluid dynamics.

From my research one thing that impacts performance is sharp changes of direction, as Doug so described. The experts recommend against using short 90s. Unfortunately, I don't know of a source for long-radius 90s in PVC. What I did instead to make 90-deg bends was connect several 45s and/or 22.5-deg elbows to attempt to make a more gradual bend. I have no easy way to evaluate the effectiveness other than to say it really sucks, (That's technical terminology) The unknown and the long runs contributed to my decision to go with a 5hp rather than smaller DC. I only bought it once, and the difference in cost was not significant compared to the building, equipment, electrical, taxes, insurance, security and climate control.

JKJ

[Charles Lent]

Heh, I thought it might be the case about limiting my airflow @ the tablesaw, but I had no idea about the flex piping reducing airflow. My shop is going to get an overhaul soon and putting in some 6" PVC is definitely going to the top of the list.

Charles, by saying I need a port at the end open do you literally mean at the end? So if my ducting will have a few splits to machines, then instead of finishing off with an elbow at the last machine I should have another 2 way split and leave the one end open?

If two piped join at the DC, does having air coming in from Pipe B increase suction/flow in Pipe A? Always? Or only if Pipe A is too small? (like 4")

No, any open port at the end of your main duct will work. You just want some wind blowing in the full length of the main duct to pick up any saw dust that may settle in the main duct. It can be the last machine or a port specifically made for this. It also does not have to be straight off the end of the main duct. A dust collector needs an inlet for air to flow and move the dust along. A 3 hp dust collector can move a lot of air if you give the air a place to get in, but you want the air flow to move fast enough for it to carry the saw dust along. Cut off all the air flow and the saw dust falls to the bottom of the duct instead of moving to the collector. I'm purposely avoiding velocity and pressure drop and all the math that goes with it to make this as simple to understand as possible. Your dust collector can likely handle 3-4 4" ports wide open at the same time, if the duct work and flex hoses do not restrict it. Open too many ports at once and all the air rushing in will slow the speed down and allow dust to fall out of the stream. Its kind of a balance that needs to be determined based on how efficient and large the dust collector is as well as the duct size and design. There are many variables. Just experiment with your system after you get it together to see how many open ports achieves the best flow. If too many are open it will leave saw dust behind, but not enough open will result in dust being left behind too.

Charley