Dust collector questions

[Marshall Harrison]

Can someone explain how a dust collector works?

As I see it the hose connects to the tool and then to the blower mower that is providing the suction. Most collectors I've seen have two bags. I assume that the exhaust exits thorough the bags which trap the dust.

I've seen some DCs that have a filter up top and a bag below. If I understand correctly the theory is that the filter traps the dust and the bag will get the larger pieces. I would think the the exhaust would take the path that has the least resistance and would go out through the bag leaving the filter pretty much useless.

Also on say a scale of 1 to 100 with 100 being the best dust collection how would you rate the systems below bags and HP being equal):

1. Stock DS
2. Stock DC with trashcan lid seperator
3. DC with paper filter
4. DC with paper filter and trashcan lid seperator
5. DC with cyclone

I'm going to use some bonus money to buy a DC and I'm trying to decide what is the best route. For economy I'm looking at a DC with bags and a trashcan lid sperator. Could possibly add filter later. My shop area is very small and I hate giving up room for the DC but if I have to I won't t do it right. THe main dust sources for me are TS, BS and mortiser.


Thanks.

[Jeff Sudmeier]

The DCs with a upper filter and a lower bag have a lower plastic bag. They also have a "neutral vane" in them. This causes all but the finest dust to fall into the lower plastic bag. The fine dust is trapped in the filter. The filter may then be cleaned by using a lever that knocks the dust out of the pleats of the filter and into the lower bag.

[Jim Becker]

Dust collectors do not work via "suction", although that's what one would think from putting one's hand over the hose! Dust collectors work by moving a volume of air at a given speed over a period of time...that's where "Cubic Feet Per Minute" or CFM comes from. They also do this at low pressure, unlike a shop vac or other vacuum cleaner.

The more air you can move over a given period of time, the more material (dust and chips) you can move from the tool to the collection point. A larger size path (duct diameter) combined with a blower that can move more air at a given velocity (typically in the 3500-4500 feet per minute range) will provide better performance than small diameter duct or hose and a small blower.

Each tool has a minimum "requirement" relative to CFM that will properly clear the dust and chips while it's in operation. Some of this is due to the nature of the tool (planers and jointers can make a lot of chips really fast, especially on wide material or heavy cuts) and the design of the hood that is doing the collecting from the cutting device. Good articles, books and sites on dust collection will tell you what these demands are and should always be a guide when sizing both the duct work and the necessary collector to drive it. In fact, the LAST step is picking the dust collector in the process! (Or it should be...)

There is great appeal in cyclones because it is much easier to deal with the collected material. The filters only have to catch the finest material on a cyclone system. The downside is that they are typically more expensive to buy or build and require "extra" power to overcome the performance drop they cause by disrupting the air stream...that's necessary because that disruption is actually how they work to separate the big stuff from the very small stuff.

One stage "bag" systems are probably most common in hobbyist shops and are usually effective as "chip collectors" out of the box but often need aftermarket filters to perform safer when it comes to the fines. Further, these single stage systems dump both the big stuff and the small stuff into the same effective place and are generally messier to empty and keep clean from a filter standpoint. A recent trend toward cartridge filters helps with the sub-2 micron filtration and since these flow more air, many of those systems can use a disposable plastic bag on the bottom for easier, although still messy, disposal.

About the best compromise for a very small shop is a single stage system with a cartridge filter and a simple hose to connect to the tool-in-use. a "2hp" system is the best choice if you can accommodate the required 240v power, and if not, a "1.5hp" system that runs on 120v will do. While both will work with duct work, the "2hp" system is better suited to it simply due to performance.

Finally, beware the "specifications" that the vendors put on their systems. With the exception of Oneida and the Pentz design "shop-built system", most CFM ratings are bogus...your actual "real" CFM performance will be no more than half the rating...and that assumes your duct work is appropriate for that volume of air. (4" duct or hose can't move more than about 350 CFM...it just will not fit!)

[Marshall Harrison]

Thanks Jeff and Jim.

Things work pretty much as I though though I wasn't aware that the bottom bag is plastic when there is a filter installed.

How well does the trashcan seperator/cyclone work? I'm leaning towrds the HF 2 HP dust collector as it seems to get good reviews and is only $159 right now. I'm considering hooking this to the trashcan seperator.

My wife and I share shop area, me with WW and her with stained glass. She really wants to get more involved withthe SG so I can't go around coating all of here stuff with sawdust. Or at least that is how I am pitching it to her. Wish me luck.

[David LaRue]

The basics on dust collection system design:

As air moves through a pipe, it slows down do to friction. The longer the pipe the more friction, the greater pressure loss.

The size and length of the pipe has major effect on the overall efficiency of your system

• Your system is only as effecient as your weakest link (smallest restriction which can be at the D/C, in the pipe, or at the machine)
• 6” smooth walled pipe is the best choice, and should be used from the DC to the machine
• 6” has better flow than 5” which is better than 4” pipe ... (Although you can technically go with too big a pipe, it is not typically an issue)
• Do not use 2 ½” pipe for a central D/C it is like trying to suck through a cocktail straw
• A shorter pipe is better than a longer one
• Avoid 90 degree turns at all cost, use two 45 degree connections to make the 90, or use commerial long arc 90’s.
• Do not use flex, except for the last couple of feet to connect to your machines for mobility, and accessibility
• If you need to go a long distance use a bigger diameter pipe (Larger than 6” for main trunks in a large system with a cyclone for example).

In summary, keep the overall length to a minimum, use the largest pipe you can, and avoid 90's wherever possible, and minimize the use of flex-hose, which can have up to twice the resistance, of smooth-walled pipe.

Cyclone separators work on the principle of pressure drop, and centripetal force. A negative pressure gradient is created as the volume is rapidly increased, combined with the circular motion of the air inside the cylinder. When the pressure drops, the ability of the air to hold the particles in suspension decreases, and they start to fall. This combined with a design that makes the air travel as far as possible inside the cylinder, in a circle, the particles further slow down (centripetal force lowers), and the particles run into each other, friction losses increase. Then the particles are then forced into a cone, air pressure increases, force on the particles decreases, and the particles here further squished, until the fall out of suspension into the waste can below. The waste air is then vented into the center vortex area of the cyclone (the eye of the ‘Hurricane’) where there are few particles.

Serarators
There are number of trash can lid dust separators on the market. The best designs are ones that mimic the larger cyclonic dust separators. The principle is the same. If you can get the saw dust to travel in a circular motion around the inside of the collection container, the saw dust particles will come out of suspension and collect on the bottom of the container. Although not as effective as a true cyclonic separator, it is a lot easier to clean a can of saw dust than to change the lower bag on the dust collector. It also keeps the system working a lot more efficiently since some of the larger dust is removed by the 1st stage separator. The efficiency drops off rapidly when the drum, or trash can becomes more than 1/2 full.

Critical Design Features:

• It is important that the saw dust is entered tangentially relative to the center of the top of container so the cyclonic, circular motion can start.
• The outlet to the dust collector is in the center of the collection device

[Charlie Plesums]

The trash can separators cause a significant loss in the airflow (well, the efficiency, at least, since what goes in also goes out). Therefore I use my trash can separator only on the jointer and planer line, where I have a high percentage of chips that will drop. The sander, saw, etc. go directly to the dust collector.

I started with a two bag collector, then have spent a fortune on different bags to get the efficiency up while filtering the fine dust. I wish I had started with a cyclone, like everyone recommended - it probably would not have cost any more than I ultimately spent on my several false starts with bags.

[Peter Gavin]

I have seen simple blowers (like what are attached to DCs) without any collection system at all. If the dirty air could be piped outside (with perhaps some kind of disposable but porous bag to collect the large chips) can you see any major problems? Assuming that diameter, length of run and solid versus flexible pipe arrangements are followed.

Thanks
Peter

[Jim Becker]

I have seen simple blowers (like what are attached to DCs) without any collection system at all. If the dirty air could be piped outside (with perhaps some kind of disposable but porous bag to collect the large chips) can you see any major problems? Assuming that diameter, length of run and solid versus flexible pipe arrangements are followed.

There is no inherent technical problem with exhausting outside...other than heating/air conditioning issues and perceived "environmental" issues if you live in an urban and close suburban area. The former is more serious...if you are heating with a flame...you really need to exhaust back into the shop for safety reasons. If you are not going to just eject the material "into the world", dumping it into a bin, either directly or via a cyclone separator is a good idea as it makes disposal easier. I would not use any kind of bag for this...messy, messy to deal with.

[Peter Gavin]

What I am actually thinking about would be a screened in lid on top of a garbage bag with a plastic bag inside for easy removal. Only the fine particles would escape and they would mostly drop into and disappear in my gravel driveway (I can't believe they would be much more environmentally unfriendly than the leaves and sticks that fall and decompose naturally. Since the basement I'm working in is only indirectly heated by airflow flowing down from the upstairs, I can't see major heating issues.

Peter

[Jim Becker]

Since the basement I'm working in is only indirectly heated by airflow flowing down from the upstairs, I can't see major heating issues.

But where is the heating system and what kind is it? If it has a flame...exhausting outside is a no-no as it could draw dangerious gasses back into your living space from the flue.

[Peter Gavin]

I have a gas furnace in the basement (where my workshop is). Is the danger that the exhaust gases would be sucked out of the furnace into the basement instead of exhausting up the chimney because of negative air pressure caused by venting outside? I never thought of that.

[Jim Becker]

I have a gas furnace in the basement (where my workshop is). Is the danger that the exhaust gases would be sucked out of the furnace into the basement instead of exhausting up the chimney because of negative air pressure caused by venting outside? I never thought of that.

Exactly.....

[Peter Gavin]

Any easy solutions?

[Steven Wilson]

Exhaust the air back into your shop and filter it properly.

[Marshall Harrison]

Thanks to everyone for all of the help. I've made my decisions and purchases - see my Gloat with no pictures (pre- gloat?) post.