Is it really necessary to tape the pvc joints of your DC plumbing?

[Brian Henderson]

While I'm sure you can get away without it, all of my joints are screwed and taped, just to make sure they never come loose or leak. If you're sure about your setup, you can always just glue it together and ignore the rest. PVC glue is cheap.

[Alan Schaffter]

While I'm sure you can get away without it, all of my joints are screwed and taped, just to make sure they never come loose or leak. If you're sure about your setup, you can always just glue it together and ignore the rest. PVC glue is cheap.

PVC glue is kinda permanent!

My horizontal runs didn't need little screws, but I used them on a few of the short 45° runs connecting to the machine ports in my shop above. Most ports are in the knee walls except for the ones servicing the tablesaw and jointer, which are in the floor. The 4th photo is a very old one showing manual blast gates on the tablesaw and jointer. All gates were converted to autogates and relocated behind the knee walls or below the floor.

[Brian Henderson]

PVC glue is kinda permanent!

That's why I said if you're really sure of your setup, that will solve the problem and it's cheap.

[Alan Schaffter]

Yes, but what if you suck up a stick (done that), tape measure (done that too! but it made all the way to the blower, Yikes!), or something else that can't be retrieved unless you disassemble the duct? Saw time?

[lee cox]

I thought with 6 inch pipe you were suppose to leave 2 blast gates open? I don't have my DC finished yet.

[Ole Anderson]

I thought with 6 inch pipe you were suppose to leave 2 blast gates open? I don't have my DC finished yet.

Where did you hear that? All it will do is reduce the suction at the machine in use. Suction is the name of the game.

[John K Jordan]

I thought with 6 inch pipe you were suppose to leave 2 blast gates open? I don't have my DC finished yet.

I've never heard that either. The amount of air and suction will depend on the type and power of the DC. The only time I leave a second blast gate open on purpose is when I want more airflow in the main line when I use a shop vac hose to clean up dust around the bandsaw and lathe.

JKJ

[Jim Becker]

Where did you hear that? All it will do is reduce the suction at the machine in use. Suction is the name of the game.

Actually, "suction" isn't where it's at for dust collection...exactly the opposite. Dust collection is about air movement at low static pressure. The more air you move from a CFM (cubic feet per minute) perspective, the more dust and chips get moved with it. That said, I agree that opening up multiple blast gates generally will reduce performance in the smaller systems we use in a typical one-person shop. Ideally, the air movement needs to come from the tool being serviced. Having a 6" port for a 6" drop is ideal, but even a smaller port where the reduction is right at the machine still benefits from the larger duct size because of venturi effect.

[lee cox]

I guess I should qualify if you are using a 6 inch pipe with a 4 inch end then it is best to leave 2 4 inch open ports. One 4 inch open port does not feed a 6 inch pipe properly. I only have 4 inch ports on my tools.

[Andy Giddings]

I guess I should qualify if you are using a 6 inch pipe with a 4 inch end then it is best to leave 2 4 inch open ports. One 4 inch open port does not feed a 6 inch pipe properly. I only have 4 inch ports on my tools.

Depends on what you are trying to achieve - I would agree that 2 x 4 is about the same as the 6 in area, but if you only have one open, its not going to cause an issue - just means the DC motor is not going to work as hard (draw as many amps). As others have pointed out, your duct should collect from the machine in use and only that machine

[Ole Anderson]

Actually, "suction" isn't where it's at for dust collection...exactly the opposite. Dust collection is about air movement at low static pressure. The more air you move from a CFM (cubic feet per minute) perspective, the more dust and chips get moved with it. That said, I agree that opening up multiple blast gates generally will reduce performance in the smaller systems we use in a typical one-person shop. Ideally, the air movement needs to come from the tool being serviced. Having a 6" port for a 6" drop is ideal, but even a smaller port where the reduction is right at the machine still benefits from the larger duct size because of venturi effect.

Yes, it is all about air flow volume, measured in cfm for us in the states. For a given size pipe, the more suction (static pressure) you maintain, the more air you are going to flow as you try to offset the pipe and entrance losses with horsepower and static pressure. The very high static pressure you get from a shop vac is needed to get a decent airflow from a small hose. The same principal applies to the larger pipes in a dust collection system, except instead of a 2" hose and 100" of static pressure, we deal with a 6" pipe and hopefully 8" or 10" of static pressure. By opening a second gate, you are splitting the available airflow between two ports, reducing the airflow at the tool being used. As far as opening a second port to maintain adequate flow in a main duct, which is more important: collecting dust as the source or keeping the inside of your pipes squeaky clean? All you need to clean the pipes is a quick opening of the second gate when you are done using the dust producing machine.

[Bill Jobe]

From what I've read here there are a lot of members that use 2 45s to make a 90 degree turn as opposed to a long sweep 90. I'm curious to know if anyone has actually ran tests on the 2 and came away with real numbers to back the choice up.
My guess is that a long sweep 90 would come out on top, but I don't know that.
For one thing, using 2 45s would double the cavities that create turbulence.

Also,and this may sound a bit silly, but have any of you tested some type of vortex generator, such as the Tornado they used to advertise on tv to increase gas mileage in an automobile.
I've read some reviews that stated they actually lost milage using one.
But I can't help but wonder if such a device was placed shortly before a turn in a pvc run that it might reduce the loss of flow created by the cavities at the union.

[Peter Christensen]

BobL on the woodwork forum in Australia discussed a PDF engineering book from a US company with a lot of data on duct, elbow, wyes, etc. It showed that 2 45s did have more drag than a tight 90 elbow. A 1.5 x diameter radius 90 was best. Using the cell or I would link the PDF.


PDF is Loren Cook Engineering Cookbook. Page 54.

[Bill Jobe]

Thank you, Peter.

[Chris Padilla]

BobL on the woodwork forum in Australia discussed a PDF engineering book from a US company with a lot of data on duct, elbow, wyes, etc. It showed that 2 45s did have more drag than a tight 90 elbow. A 1.5 x diameter radius 90 was best. Using the cell or I would link the PDF.


PDF is Loren Cook Engineering Cookbook. Page 54.

Peter,

Looking at the PDF you attached on Page 54, a 3-piece 45 is 15' and a 2-piece 45 is 20'. An R/D of 0.75 gives a best case of 37' (tightest on the chart) for a smooth, 90.
I assume it is fine to simply double the 45 to get a 90 and that gives 30' and 40', respectively. However, I wonder about the R/D ratio for the 45s as they claim the same numbers regardless. Two 45s look like fine to me. Long-radiused 90s are usually more expensive than two 45s--at least in 6" DWV PVC fittings last I built my system over 10 years ago.

That is a great PDF...lots of information in there!