Note that 90 degree bends greatly restrict airflow and should be avoided as much as possible. Long sweeps, less severe bends, and diagonal runs of dust collection pipe will cause a smaller drop in static pressure.
Edited and condensed from my answer on using a shop-vac vs. dust collector...
CFM vs. Static Pressure, defined
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).
Is that enough CFM and static pressure?
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.
- Table saw, radial arm saw, miter saw, bandsaw: 350 CFM, 165 l/s
- Belt, disc, or drum sander: 350-550 CFM, 165-260 l/s (depending on size)
- Table-mounted router: 195 CFM, 92 l/s
- Thickness planer, up to 13": 400 CFM, 190 l/s
- Thickness planer, 14"-20": 785 CFM, 370 l/s
Now let's look at some contenders for an entry-level dust collection system. 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.
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.