building my first Thien baffle - a few questions

[baldguy]

First, thanks in advance for your help - this forum and the info on this site has helped me quite a bit!

I recently got a great deal on a Shop Fox W1685 DC (1.5HP, 16A 120V) at a garage sale.  it came from a fellow woodworker who had upgraded his system, and is in great working order.  I added a Wynn filter to it with a plastic bag and it performs admirably with my new plumbing configuration.  I'm in the later stages of remodeling my garage to allow half of that space to serve as my wood shop.  I built cabinets along two walls and plumbed 4" PVC through them with 45* wyes and risers to blast gates on the work surface, as well as a floor sweep at the end of the run and one wye that connects to my table saw.  My band saw and jointer/planer are not convenient to that run, so there is a blast gate to shut off the main run and another to open a secondary (and much shorter) run that serves the two aforementioned tools.  It's hard to tell exactly how well it's performing, but the floor sweep will suck up whatever dust and chips I throw into it, and all of the ports seem to work great... that part I'm pretty happy with (at least for now). 

I'm wanting to add a trash can separator, so of course I want to build a Thien style baffle for it.  I bought a standard metal 30gal trash can at the local home store, and I have some MDF, 4" fittings, and threaded rod set aside for the project.  I'm a bit concerned with losing suction, though.  Everything I've read says that I should expect a decrease in performance... by my calculations I still have over 450 CFM at the end of my 15-ish foot 4" PVC run.  it seems to work at least as well as my 6 gallon shop vac.  so, here are my questions:

1) should I really expect to lose a noticeable amount of performance by incorporating this separator?  Could it drop me below acceptable numbers?
2) is a gasket between the lid and the can a good idea?  I don't see a lot of people mention it, but there has to be some loss there, right?
3) what is the main reason people choose to build a top hat style instead of the kind that drops into the can a few inches?

I'm very much looking forward to your responses!

[baldguy]

so maybe I posted this in the wrong forum... I'm going to start this build tomorrow though so if anyone has any tips I'd sure appreciate them :)

[BernardNaish]

1.There will be a substantial loss of suck! The advantage is that the filter will stay clear for much longer so maintaining performance. Only by building a separator will you know how much is being lost. I would say it is probably better to build one.

2. As the system must be air tight a gasket between the can and the separator is essential. There is quite bit about this on the forum. The usual way is to trap a strip of closed cell foam rubber in a groove. The kind use to draft seal doors works well.

3. It is much easier to remove a top hat separator from the top of the can to empty it. Top hats allow very smooth air flows to be established close to the chamber wall so are more efficient at removing debris. The best build is retired2's design here:

  http://www.jpthien.com/smf/index.php?topic=563.msg3034;topicseen#msg3034

This is quite long but well worth reading and includes all you need to know about seals.

Hope the build goes well. Please let us know about it.

[baldguy]

awesome, thanks for the reply!  I have been reading a ton about these things but honestly have never seen mention of a gasket.  Thanks for setting me straight.  I think I might go for a top hat - I need some acrylic for another projects, so buying a small (half?) sheet should cover both efforts.

as for the loss of suction, perhaps I can offset some of those losses by tightening up in other places.  I'm thinking about making wooden caps for the portable tool blast gates I installed... they close off most of the airflow but I think there is still some loss at those points.  and I think I could do a better job mating the PVC to the various DC fittings in my main run.  hopefully when all is said and done I will be able to maintain enough airflow for the planer and all other tools.

thanks again!

[BernardNaish]

Acrylic (Plexiglass, Perspex etc) is not suitable for the chamber wall as it is too rigid and will crack when bent. What you need is 1/8" thick polycarbonate. Search for polycarbonate in the box at top right to get lots more about this.

[baldguy]

yes, polycarbonate is what I bought.  I didn't get to start the build, though, since other issues came up and my weekend plans went down the tubes.  Hopefully I will find time, this week.