Made the DC w/ the thien baffle - questions on the PVC -

[jousley]

hey all, i've been through this forum for a few weeks on and off - and finally made my own thien baffle and of course it works great.  Now i'm looking at how to run some PVC down teh length of my garage, installing some blast gates, and allowing the access to be easier.  I just have a question regarding the diameter of the PVC that I run. 
I am using a Craftsman Shop Vac - 5.0 HP.  model # 113.177690.  I believe the hose size of the shop vac is 1 7/8". I want to run some PVC down the length of my garage about about 10-15' with 3 blast gates to give me 3 access points to the dust collector.  I will be using one tool at a time as it's only me.  If my shop vac hose is 1 7/8", what diameter should my PVC be that i'm running down the garage.  At first, I was thinking of just using 2" PVC straight pipe - but looking on the forums people are talking about 3-6" diameter, which seems way too big for my application - but I'm not sure.

Wouldn't a larger diameter reduce the suction down the length of the run? 
Is 2" good enough for my app - or should I go bigger? 

appreciate all the knowledge and i'll let you know how it turns out.  Just need some informed advice.  Thanks again all.

J.

[jdon]

First, some caveats: I'm new to the site myself, and I'm not a physicist or engineer, so buyer beware. With that in mind, from my readings, regardless of the size of your vacuum hose, ideally you want as large a diameter as possible, particularly if you have a long run.

It's not the suction (vacuum?) you need to be concerned about. Rather, it's the airflow (cfm) you can wring out of your setup. The enemy of airflow is turbulence and "friction" (for lack of a better word), so all the advice about smooth linings, minimal and large diameter bends, etc., and large diameter runs.

I picture the diameter effect as similar to an electrical run from your breaker box to an outlet. You get a lower voltage drop at the outlet with larger gauge wire, even though the appliance/tool plugged into the outlet has smaller gauge wire. Likewise, you'd get better airflow with, say, 4 inch pipe with 1-7/8 inch connections at either end, than with 1-7/8 (or 2 inch) all the way.

The only things affecting suction are the power of your vacuum, and the opening area at the open (far) end of the system- the diameter (volume) of the pipe in between have no bearing on the suction (vacuum).

Just my .02

[retired2]

Using a shop vac as the heart of a stationary DC system has the potential for failure.  I'm not saying it won't work if the runs are not too long and the lines are sized correctly.

You need to understand that shop vacs have high SP (suction), but low CFM, which is just the opposite of a true DC.  In most cases with a shop vac, and sometimes with DC's as well, you don't know what either of those specs are because the manufacturers like to paste big HP ratings on their units, and this tells you next to nothing.

Next thing you need to know is that for any given CFM, if you increase the line size, you decrease the velocity.  And in order to keep wood shavings or dust entrained, you need to maintain 3500 - 4000 FPM.  If you don't the waste will settle out in the line.

Now if you have no idea how many CFM your shop vac is moving, it is a little hard to say how large you can safely make your header.  It is true that larger is better because you have less frictional losses, but bigger is better only until you have dropped the line velocity to the point where waste is no longer being conveyed.

I would try a main that is one size larger than your shop vac port.  There isn't much to be gained by going a lot larger.

[phil (admin)]

You've gotten some great responses so far, I'll just add my two cents.

I use a shop vac (connected to one of my separators) as a centralized DC system in my shop.  I have a couple of clear pipe 2-1/2" networks I purchased at Rockler that I use to connect all my tools to the separator.

The caveat (and it is a big one) is that all my tools have 2-1/2" dust ports.  I have a Ryobi BT3K table saw, a Ryobi portable jointer, a Dewalt planer, a couple of Skil bandsaws, and a belt sander, and every single one of these has a 2-1/2" dust port.  Because everything else was already 2-1/2", I installed a 2-1/2" port on my shop-made router table extension.

For my tools, a conventional DC wouldn't work so well, as necking-down a 4" or 5" line to 2-1/2" reduces the CFM to about what I can achieve with my shop vac-based system.  And the shop vac has oodles of static lift compared to the DC.

I would not, however, try what I'm doing with a conventional cabinet saw or large planer (or large bandsaw).  If a tool comes with 4" ports, it probably should be connected to a collection system powered by a more conventional DC.  A shop vac may be able to do a marginal job on a temporary basis, but it wouldn't be a good long-term solution.

So let your tools dictate your path.  And if you plan on upgrading tools, don't invest too heavily in 2-1/2" piping.