8" port tophat design

[Lance]

Hello all, 

My plans to build a tophat seperator got stalled a few years ago.  Finally, I'm starting this project back up.  I have a homemade impeller blower based on a Pentz design with 8" inlet and outlet.  I have most likely overlooked it, but I could not find any other posts or examples of others making any tophats this big.  Anyway, at this stage in my planning I wanted to make sure my dimensions for the top hat are big enough to handle the volume of air moving thru that 8" pipe. I am going to use Retired2's build as a reference to compare sizes. Here are my numbers, Please let me know if my logic looks sound:

(for those pedantic math nerds I will be rounding number on occasion.)

Retired2's tophat:
5" ports with 22.5 diameter which is 6" tall
  5"diamiter= (2.5*2.5)*pi=20sq"
  22.5" diameter = (11.25*11.25)*pi=397.6sq"
  397.6*6=2386ci
  The ratio between the area of the port and the volume of the tophat is 119:1  (397.6/20=119.3).  (I know I am comparing area to volume, but I think it works for what I am trying to accomplish)

My tophat:
8" ports with 22.5" diameter with a height of ???
   8" diameter= (4*4)*pi=50sq"
   using the Ratio above 397.6*50 gives me a target volume of 5950ci
   I then take the Volume and divide it by the area of the tophat:  5950/397.6=15"
   Now I have my height at 15"
   the rectangular port size should be 50"/15"=3.3"

There for I think I am going to make my tophat 22.5" in diameter by 15" tall with a port transitioning from a round 8" pipe to a square that is 15" tall x 3.5" wide.  Does that sound good or do some of you see anywhere I really went astray? 

I know everyone here like the eye candy so here are some pics of the outlet pipe I created a few years ago when I started this whole thing.  It is an 8" thick-walled PVC pipe with a small bell mouth stretched into it.







The thick wall only let me stretch it a small amount, but I think it is enough to help.  Some of the whitepapers only showed the bell mouth curving about that far.

Thanks, Lance

[nucww]

Your scaling calculations appear correct for the assumptions you made, however other considerations may be needed.  Does the diameter need to change of the top hat? I haven't seen any info on 8" piping so you may need to experiment. The distance from the 8" pipe to the tangential inlet is narrower and could affect dust collection. Also, an 8" trunk line for a vacuum system requires a large amount of HP to keep the air moving at 45 mph to keep saw dust airborne.  Too slow and dust will collect in the 8" line. The advertised CFM on dust collectors is with no load and you can't trust them for your system design.  Scaling my HF 5" existing piping system would mean that I would need a 4-5 HP motor with an 8" line to keep dust from accumulating in the trunk line. The velocity of the air at the 4" intakes would be much higher than needed. The pressure losses of the diameter, length, number of elbows, and number of lines affect the HP requirements as well.  The top hat design is made for small DIYers with small systems.  The size of this system appears to be more of a commercial system.  Some DIYers with big shops will use a few portable smaller systems with a top hat rather than this large system with a lot of piping.  If you haven't already, look at all your needs and what designs can meet them before you go much farther. Also, small affordable cyclones are now available which were not available when Phil proposed this design.  If you need more information, there is a huge amount of info on the internet on any of these subjects with simple searches. 

[Lance]

I am using the 22.5" diameter for the tophat as that fits my 50 gal plastic drum. In the long run using a top hat may not work and I may have to go back to a cyclone.  I am hopeful that the tophat works as it is much shorter than the cyclone for this size of system.  The original cyclone I built had a body that was 18" in diameter with an 8" outlet port so I hope the 22.5 tophat will still work with the 8" port.  The original Blower, Cyclone and bin stood just under the available 9 foot ceiling in my old shop and I only had about 2 feet of height for the bin at the bottom. I was never happy with that.  Using a tophat design will allow the full-size bin to fit while keeping with an acceptable over all height. 

I know for some "Pentz" can be a dirty word around here, but I do believe there is some validity to his claims about the need to remove the fine dust particles from our shop air (or keep them out of the air to begin with).  I you haven't been there before it is worth going to billpentz.com and check out the basics tab.

I'm not Just attempting to clean up the bigger dust and chips, I'm trying to remove as much fine dust from the air around the tool as possible.  This means I am shooting for something closer to 1000 CFM at the tools.  Most of my tools will have custom larger ports added, but also in most cases multiple ports for each tool (for instance one above and one below the blade).  My blower was specifically made to move the volume of air you are talking about. It uses a 5hp motor with a 16" impeller.  I took readings for air speed at the inlet of the blower years ago when I made it, but I don't remember the numbers now.  I will have to re-test that when I start putting all of this together.  needless to say, it was well over the 1000 CFM at the blower, the trick is to keep the losses to a minimum as I add filter, seperator, ducting ports, etc.

[Lance]

I have been thinking about what to do if the 22.5" diameter does not work out with the 8" outlet... 

The cone portion of the cyclone design is there to aid in the separation of the chips and the height of that cone is crucial. if it is too short the separator does not stop the swirling air from disturbing the bin and lifting dust back into the separator.  That is where Phil's baffle comes in.  It eliminates the need for the cone.  It looks like everyone sizes their tophats to the bin it goes on top of.  That is what I am looking at doing, even with my 8" port sizes.  What popped into my mind is, just because I am using a baffle it doesn't mean I couldn't still use a cone also, Not the 30' tall cone a cyclone uses but a shorter cone just to go from a 30" tophat to a 22.5 in bin.  Maybe 8 to 10" tall?
Sorry for the crude drawing...

[nucww]

Absolute collection of fines for my system seems to be the function of the filter.  I crudely tested my system with flour and collected ~2/3 in the baffle can.  I have to use pressurized air to blow into the filter from the outside ~1 every two years to remove fines.  My recollection of the benefits of the thien baffle was that it collected the bigger stuff with a smaller pressure drop and smaller than the cyclone but wasn't as good as collecting fines as a cyclone. The only time big stuff gets past is when my can fills up and I don't notice it. Your idea of tapering the outside is interesting.  Since your top hat is taller, the inlet to the internal pipe to the blower could be set at a higher height so that the distance from the pipe to the tapered wall is similar to the smaller design typically shown here.  However, this is just a guess.