From what I know, dust collectors are generally quieter, hold more chips, and can support a larger hose than a shop vac.
Besides this, are there any advantages related to suction? Can dust collectors pick up or filter things that a shop vac can't?
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Short answer: A shop-vac is better than nothing, but barely adequate for dust collection on almost any stationary power tool. On the other hand, a dust collector would be inappropriate and wouldn't work very well for most handheld tools.
Long answer: read on...
The others mentioned that a Shop-Vac produces low-volume air flow with high static pressure, and a dust collector produces high-volume air flow with low static pressure. But what does any of this really mean in practical terms?
Air volume is measured in cubic feet per minute (CFM) or liter per second (l/s)--the volume of air that the device can move from its intake to its exhaust in a given amount of time.
Static pressure is measured in inches of water (often referred to as inches water column, or inches WC), as in, how many inches the suction device can lift water up a pipe, or in millibar (mbar).
In simple terms, static pressure determines how much resistance (“static pressure loss”) your dust collector can overcome in order to move a given volume of air through the hoses and pipes. When it comes to designing a dust collection system, the raw CFM figure is initially more important than the raw static pressure figure, though static pressure does come into play once you start designing your ductwork (more on this later).
WOOD Magazine has a great article on calculating CFM and static pressure requirements for dust collection. Although the worksheets in the article may look intimidating at first glance, they aren't too difficult to fill out. The article includes some typical CFM requirements for various tools, some of which I've included or summarized below.
Now let's look at some contenders for an entry-level dust collection system. As of this writing, a ”6.5 peak horsepower” Shop-Vac® vacuum rated at 9.3A@120V produces 185 CFM (87 l/s) of airflow and 64 inches (160 mbar) of pressure. That's not even adequate for the least-demanding machine in the list, falling 10 CFM (5 l/s) short for a router table. In practical terms, this means the vacuum won't be able to produce the recommended 4000 FPM (feet per minute)/2 m/s (meters per second) linear air velocity necessary to capture most of the dust particles produced by the tool. (Note that the 4000 FPM/2 m/s figure somewhat oversimplifies things but it is a good, convenient figure for making realistic estimates.)
By comparison, a 1hp Grizzly dust collector rated at 8A@110V produces 500 CFM/235 l/s and 2.76"/7 mbar of static pressure. As you can see from the CFM figures above, under ideal conditions this dust collector should be able to capture most of the dust from most tools except larger sanders and 14" or larger planers.
Remember, I said that was under ideal conditions. This is where static pressure comes into play. Every inch of pipe or hose, every turn, and every change in diameter produces resistance which your dust collector (or vacuum) must overcome. You can use one of the various static pressure calculators such as Bill Pentz's static pressure Excel spreadsheet to calculate the static pressure drop produced by your ductwork and hoses. For the Grizzly 1hp dust collector we looked at earlier, the total static pressure drop can be no more than 2.76"/7 mbar. If you plug only 500 CFM/235 l/s and 7 feet (2 m) of 4" (10 cm) flex hose into the calculator, you'll see the static pressure drop is almost 3"/7.5 mbar, so we already need to look at a more powerful dust collector.
Although this is the simplest way to size a dust collector, you don't suddenly go from 500 CFM to 0 CFM. The excessive static pressure loss simply reduces the airflow through the system. Some manufacturers or magazines provide the CFM curves at various static pressures, which is helpful in determining whether you can sacrifice a few CFM for a longer run of pipe or hose while maintaining the recommended 4000 FPM air velocity.
In addition to the fact that a Shop-Vac just isn't suitable for stationary power tools, one of the dust collector's greatest advantages over a Shop-Vac is that it has a larger hose and is less likely to clog from chips and shavings. Whereas a Shop-Vac typically has a maximum hose size of around 2-1/2 inches/6 cm, a dust collector hose (or pipe) is typically 4 inches/10 cm or larger and is less likely to clog.
Getting back to your questions, you asked whether a dust collector can (1) pick up or (2) filter things that a shop-vac can't. We've already answered the first question (yes, a dust collector can “pick up” more). HEPA-rated filters can remove 99.97% of particles down to 0.3 microns. You can buy these types of filters for both Shop-Vacs and dust collectors, so technically the answer to the second question is yes but it just depends on what type of filter you buy (and, in some cases, how determined you are to adapt such a filter to your machine if your manufacturer doesn't offer one).
Of course, even the best dust collector will not capture all the dust, and even if you pony up the cash for a HEPA filter your dust collector will disperse some amount of fine dust back into the air. That is why you should try to exhaust your dust collector outside if it is practical (while also venting outside air back into the shop), and you should still wear a well-fitted respirator with replaceable P100 (HEPA-equivalent) filter, regardless of any other precautions.
Shop vacs pull a smaller volume of air (cubic feet/minute) than dust collectors, but with a higher pressure differential. That might not mean much to you, but what it amounts to is this: shop vacs are better at sucking dust from a close source (hooked into the exhaust of your handheld sander) and dust collectors are better at collecting from a diffuse source (under your tablesaw, near a radial arm saw).
Dust collectors also have a larger volume for holding sawdust/chips than a vacuum, so if you're producing a lot of dust, you probably want a dust collector, but at that point you're probably in a shop big enough to have both.
If you're using a shop vac, you may want to invest in filter bags to help protect both the vacuum and your lungs.
And just so we're clear: dust collection is not an excuse not to wear a mask.
Dust collectors have higher air flow but lower static pressure than a shop vac. In other words, dust collectors are better at sucking in all the dust chips and fine dust. They will have a harder time with heavier objects (rocks, metal, etc...). With that being said, you shouldn't collect those heavier objects with your dust collector; you could damage the propeller and it could be a fire hazard.
With a shop vac, you can pretty much collect what ever you want, but it won't be as good at collecting the fine dust produced by your tools.
As for filtering, it all depends on the filters you buy. The best filters have the lower micron size. A canister is also better than a bag, it has more surface area so it won't clutter as fast as a bag and this will keep your suction power for a longer time.
Overall, I think it depends on your intention and on your tools. If you want to use it to clean your garage of small rocks as well as wood dust, shop vac it is. If you only plan on collecting wood dust, dust collector it is. If your tools have 4" ports and are well build for dust collection, a dust collector is a step up from a shop vac. If your tools have 2½" ports, I would stay with a shop vac.
A DC uses higher velocity and volume to capture dust/chips vs a shop vac, and typically has a larger storage bin for dust/chips. DC's are not necessarily quieter. DC's are able to handle the higher chip loads of planers and jointers, which will overload a shop vac almost immediately. The higher volume of air is better able to capture dust from larger tools, such as a table saw, due to the velocity of air pulled in through the openings of the tool and past the cutting edge.
I don't have much experience with a dust collector, but I recently switched to one after extensive use of a Shop-vac and here's what I discovered:
I had purchased the Shop-vac 14-Gallon 5.5-Peak-HP Shop Vacuum, Model # 9441411 from Lowes. I quickly discovered the necessity for a dust separator since using it directly for woodworking will quickly clog the filter and using it without a filter is a bad idea since it can damage the motor.
I tried a few DIY separator options, such as the Thien-baffle design, but I didn't have good results. In hindsight, I think the issue was not getting an air-tight seal on the separator and without one, none of them will work correctly.
I finally gave in and bought the Oneida Dust Deputy. I tried a Rubbermaid trash can, but the vacuum caused it to collapse in on itself. I eventually found a 55 gallon drum with a lid for sale on Craigslist and that became my container with the Dust Deputy attached. I use a thickness plainer, so the container had to be large, otherwise I would have to stop every few minutes to empty a small one. With the Dust Deputy, the Shop-Vac stayed clean just as long as the dust separator container didn't get too full.
The Shop-vac worked great for a while, however it recently died on me. The motor isn't serviceable, so you have to purchase a whole new Shop-vac when it dies. I don't feel like I got enough use out of it before it failed, so I didn't want to go that route again. I purchased the portable dust collector from Harbor Freight and was very impressed how much quieter it was. The motor looks like it's a lot more durable. The problem is it has 4-inch inlets vs the 2-1/2 inches the Shop-vac uses.
So when I tried retrofitting the dust collector to 2-1/2 inches for the Dust Deputy, I lost too much CFM to where it was useless. Oneida makes a "Super" Dust Deputy, however this is substantially more expensive. The new 4 inch diameter hoses are an additional expense and cost more than the 2-1/2 inch one for the Shop-vac.
So the biggest differences I see so far are:
The question I faced was "Do I want to have to keep buying Shop-vacs when one dies or pay a little more and get a proper machine that won't need replacing near as often?". I opted for the latter, however I will forewarn you that the cost of doing so will be quite a bit higher, especially if you're wanting a very efficient separator stage. And I highly suggest factoring in a dust separator stage, which I've found the Dust Deputy to be the most efficient one so far.
I hope that helps.
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