5 inch elbow inlet in 31 gallon can?

[mj2736]

I'm planning to use a 31 gallon metal trashcan and 5 inch plumbing (metal HVAC). From what I've read here it seems like for 5 inch the preferred or at least usual approach is a side inlet, with or without a "top hat". But in my particular setup this isn't really practical so I've decided to go with the "original" Thien separator using a top-mounted 90 degree elbow as the inlet.

With the dimensions involved (5 inch inlet, 5 inch outlet, and approx. 20 inch diameter can) do I need to be concerned about the inlet and outlet being too close to each other, and the incoming chips and dust getting caught up in the outlet's suction before they have a chance to slow down over the baffle's cutout and fall below?

If so, would a second vertical baffle, or even just extending down one side of the outlet pipe, help to prevent the premature mixing of the two air streams, or would it somehow just get in the way of the overall separation process? Or am I just over-thinking this?  ???

I attached a few Sketchup drawings that will hopefully make this more clear.

[bill70j]

mj2736:
These look like very interesting and potentially valuable upgrades.  I have a 31 gal set-up, but am using 4" plumbing.  However, I like your ideas, and since my unit is already finished, I can easily run some tests with both of your suggested upgrades.  Directionally, should your upgrades work on 4", then they should work on 5".  I already have the ability to determine the additional pressure loss, if any, that would come from the enhancements, and can also figure out how to determine the efficiency of each of your suggested alternatives.

If you are interested, please post as so, and I will get it done.

Thanks again for your great ideas!!
Bill

[phil (admin)]

The phenomenon you're referring to is "short circuiting."  Basically, incoming dust exits the outlet tube before it has a chance to enter a spin.

The biggest causes of short circuiting are:  (1) Poor alignment of the inlet (you need to align this so it is 90-degrees to the edge of the can).  (2) Inadequate airspeed.

The extra baffle will work against you, because it will reduce the rotation speed inside the can.  Some of the incoming air coming into the can will want to go backwards around the vertical baffle to the outlet tube.  This will reduce the speed of the air going in the correct direction.

[mj2736]

Hi Phil. Thanks for the reply. And thanks also for sharing the results of your research, and for making this forum available for discussion and Q&A.

So just to make sure I understand are you saying that as long as the airspeed entering the can isn't too slow and the elbow is properly aligned in relation to the can then it should be possible to make this combination of can size and elbow size work without a "short circuit"? I don't think (?) airspeed should be an issue in my case since I will have just a single run of about 15 total feet of mostly rigid metal duct and I have an upgraded canister filter with a lot of surface area. The biggest impediment to airspeed in the whole system will probably be the elbow inlet itself. I'll try to align it as close to 90 degrees as I can get it but I think it will necessarily end up being a little less than 90.

I kind of suspected that like you explained the vertical baffle on one side might throw the cyclone effect out of balance and have an overall negative impact of one kind or another. So in any case I decided to go ahead with the standard build and experiment with add-ons only if I did end up experiencing short circuiting problems, and then only after trying to mitigate other factors like leaks and alignment issues. My build is still in progress but after it's finished and I've had a chance to use it a little I'll update with how it went. Thanks again!

[mj2736]

Hi Bill. I would absolutely be interested in the results of any testing you cared to perform and share. I think another baffle (whether one of the sketches above or some other design) could be considered to add value if-and-only-if (1) short circuiting was indeed occurring and in the absence of the two conditions Phil described above that usually cause it and (2) if the additional baffle had its intended effect without also introducing some form of "fatal flaw" in terms of airspeed, SP, CFM, separation efficiency, etc. And Phil's reply all but confirmed for me my suspicion that #2 is a pretty big "if".

Having said that, suspicions and theories are one thing but real-world testing is the only way to know for sure. So by all means if you're interested enough to perform the testing then I'm certainly interested in hearing the results! Thanks!

[bill70j]

mj2736:
Very good.  I will do the tests.  Please PM me your email address and I will keep you informed of progress, including testing protocols and results.
Regards, Bill