Dust Collector Pressure and Suction

A discussion of basic pressure-balancing and air-sealing issues in a small shop dust collection system. June 28, 2007

Question
I have an 1100 cfm Jet dust collector (with cartridge filter and cleaning handle on top). I also have a 20 gallon garbage can type separator with one of those inexpensive plastic things on top that is supposed to make it act somewhat like a cyclone. The inlet at the blower is 6 inches but has a plastic sleeve that fits over it with two 4 inch inlets. I want to remove the plastic sleeve with the 4 inch inlets and just use the 6 inch inlet. I was told that this would increase suction but would also suck out all the contents of the separator. Can I place a cyclone separator (like the one Oneida sells) in line with the jet without having all the larger chips sucked out of the drum? I also built a shed outside my shop to house the collector but did not place any vents or openings between the shop wall and shed that houses the collector. I read something about negative pressure and don't quite understand the purpose of a vent between the two walls. Do I really need to have an opening/vent between the walls? Could someone in dummy terms please explain why and what the results would be if I didn't put in the vent?

Forum Responses
(Cabinetmaking Forum)
From contributor J:
I imagine that a cyclone separator would work, but you would have to check the specs of the cyclone to see if it will handle the cfm you want to run. As for the vents in your wall, they are used to return the air that you have just sucked out of the shop. If you are sucking out 1500 cfm, where is it going? How is it getting back? If your shop is small and airtight, then you could create a negative air pressure in your shop. I really don't think that the negative air pressure will be a big deal. I do think that you will suck out all of your hot air in the winter.


From contributor D:
The biggest problem with negative air pressure is if you have any type of combustion heating (i.e. gas or propane) in your shop. Negative air pressure would suck fumes back down the chimney into your space, and that would be very bad. If no heating is involved, the only other problem I could imagine is if there is not enough air being returned to the shop. Then the collector may not work to its full potential, as it is having to work increasingly harder to get air out. Imagine sucking on a straw where one end is in an airtight container. That's kind of what would be happening on a much bigger scale.

As for adding a cyclone inline, I think that's your best bet for chip removal.

The other reason to do some research is, you'll want to calculate how much cfm you'll lose with your ductwork. You may find after all this, your collector is not quite up to the challenge of getting all the dust out.


From contributor L:

If you don't have either return air or an opening to the outside for air makeup, you will find the shop door slams really hard! I can't see that going to one 6" over two 4" is going to change much. The cross sectional area of the ducts adds up to about the same, so the velocity would be about the same. For a cyclone to work well it needs to be sized to match the air velocity. Too small and you will have too much loss, too big and you will get poor separation.

From contributor B:
Are you having a problem with insufficient suction? I have a similar setup to you and was having a sufficient suction problem, most noticeable on my planer. At the time I was using plastic blast gates. I went through the entire system checking for possible locations for air leaks. This included where the 4" goes into the cyclone top above the garbage can. I duct taped and sealed all locations for possible vacuum loss and switched from plastic to metal blast gates. System works great now with no issues. You may want to take these easy steps before doing something more drastic. Also make sure your blast gates are closing completely and not hanging a little open due to clogs.
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