Top Hat Opinions

[drxlcarfreak]

Man, this site is awesome. There is so much info here, and after reading through I think I am almost ready to start my own build for my HF DC. I am aiming for a 4" rectangular inlet and a 5" outlet 1-1/2 height inlet top hat on top of a 30 gallon trash can.

Before I start building mine, I had a few ideas that seem like easy items to implement to increase separation efficiency, but didn't want to recreate the wheel on ideas that actually made the design worse. I looked around, but didn't see any threads on some ideas I had to hopefully improve separation. I was wondering if anyone has tested any of these ideas and had promising results, or epic failures.

1. It seems like most rectangular inlets don't have a very high aspect ratio W/H, it seems like most are closer to a square. If the transition was closer to a 2H:1W or even more approaching the width of the slot, wouldn't more fines be caught instead of sucked into the DC. I know that making it extremely narrow would require a larger area due to skin friction. In my head with a 4" inlet a 5.5" high by 2.75" wide inlet would be a good middle ground. Anyone have any experience with this?

2. This one is assuming that I understand the function of the bellmouth on the outlet is to create more laminar flow which reduces velocity to increase efficiency and better separation. Going on that idea, instead of a bellmouth what if a larger pipe overall was used for the outlet then transitioned down closer to the impeller. I imagine that the flow efficiency wouldn't be nearly as high as a bellmouth, but if the velocity inside the tophat is much slower, would that encourage the entapped fines to stay inside the separator until they fall into the slot. For example, it looks like most bellmouths being discussed are around 9" in diameter, and seem to have some issues picking up more fines. What if an 8" pipe is used for the outlet and 4" for the inlet. The velocity being sucked up the outlet should be about 1/4 of the inlet speed. Does this impose other issues I am not thinking of?

3. My last idea for now, to help increase the amount of separation is a second baffle of sorts within the tophat. For this idea, a picture is probably easier than me explaning it, but I will try to lay it out. For the example lets assume a tophat 6" tall with 4" inlet and outlets. So reading from before the depth of the outlet should be at 1/2-1d from the baffle, so lets say it is 4" from the baffle. What if we take an 8" diameter pipe and mount it to the baffle 4" high. I imagine that this would increase turbulence quite a bit, but I imagine that it should greatly reduce the amount of fines making it past the impeller since they have to change directions multiple times and hopefully drop out before getting sucked up the tube. I am not even sure what to search for, for this one to see if it has been tried before, but it seems pretty obvious to me and pretty easy to implement so I am guessing that it has major drawbacks that I am not aware of?

Thanks!

[retired2]

1) If I were building another tophat I would stretch the aspect ratio slightly from my previous build because I believe it would be beneficial.  However, a "tidy" transition piece gets to be a fabrication problem for most woodworkers.  That is why I bit the bullet and paid a sheet metal fabricator to do it for me. 

2) The primary purpose of the bellmouth is to keep exit port entry losses minimized. 

3) A separator is a wonderful thing, but it introduces significant SP losses.  If you have plenty of hp, it is not an issue, but most 1-1/2 or 2 hp blowers don't have SP to spare.  So, what you are proposing sounds like it could cause a lot more harm than good, but give it a try that is the only way you will know for sure.

[BrianHood]

From R2-
"If I were building another tophat I would stretch the aspect ratio slightly from my previous build"
First off I'm impressed you have been able to retire twice- I'm only up to retired 1/2.

As soon as I can find time in the project flow I'll build a top hat, probably this one. So, any thoughts you share are being studied carefully. One question I need to search through the forum for will be finding a bell mouth fitting. I have the exact same DC as you do. I'll do the straightener of course, I guess the cross blade or the three blade.

BradD
thanks for posting the file. I imported the .dfx file into Vectorworks, made a few mods, then exported to Fusion360, so now I have a 3D model. Super easy and I'm not even very good at software. I'll import that file into a Shaper Origin handheld CNC 'robot router' and zip it out of 1/2" ply.  Really appreciate you efforts and the file, I will post the SVG file here when I get it ready in case that's of use to anyone.

[drxlcarfreak]

Sorry guys, I didn't receive a notification that there was a response to this!

1. How much do you think is too much? For my 4" inlet, my current thought is 2-1/4" wide, just for the sheer fact that its double the size of the slot in the baffle. Adding 20% of area (10% for the round to square inefficiency, and ~8% for the loss of surface friction of this aspect ratio that I found looking at duct charts), this gives me a height of 6-3/4". Anymore aggressive in the width and it starts to get silly with the added friction of the increased perimeter. For example, using a round to square duct calculator, to keep the same pressure drop in the transition, the area almost has to double and the height creeps up to just over 18", pretty much just a linear slot diffuser at that point. Which thinking about it, could actually be a decent test for someone who has the room and time to test it out, not me though, I want to get this garage cleaned up as quickly as possible! Almost serves a double duty. It makes the height of the unit higher, and theoretically, ignoring internal vortexes the dust never makes it out of the area above the slot.

I also think the dust could be persuaded a bit more with an eccentric transition to try and throw it at the outside walls and down, so I may give that a try as well. Does ratio this sound like too much?

2. Fair enough, I wasn't thinking SP losses, I was thinking another attempt to slow the air temporarily to persuade the dust to drop.

3. I am running the HF blower, so I don't have room to spare power wise. Yeah, thinking about it, forcing the air to change directions two more times would significantly increase the static pressure of the separator.

Brian - Not sure what size exhaust you are aiming for, but check out a subwoofer port flare. They sell them at parts-express pretty cheaply.

I had one more question if people are up for it. I have seen notes that separation goes up with increased overall diameter, and not really a loss in SP, but I haven't seen anyone try to make a separator wider than a trash can on here. Granted most people are tight for space, but I am envisioning a separator larger than a trashcan with a cavity below and a trashcan sealed to that. Im guessing that since no one has done it, there are very good reasons for it. Just an idea I had reading through the forums.

[DustySanders]

drxlcarfreak, I think most people make the top hats the size of the trash can or other container so that the dust falls directly into the can below. If you made the separator bigger than the can, you'd need to build some sort of funnel or other means to get the dust into the can. There has been some discussion about airflow patterns in the can, and I think the conclusion is that it doesn't matter a whole lot. There should be a thread titled "Square Collection Box" or something along those lines. 

I briefly considered basing my soon to be built system on some cans I've seen at Home Depot, that are about 36" diameter, and about as tall as a Roughneck. I decided not to use them because the top looked like it would be hard to seal to.