So just how is dust collector CFM calculated?

[Jim Laumann]

This has puzzled me for a while......

There must be a forumla which is used - but I'm not math +/ physics fluent.....

There is fan size, motor rpm, inlet size (diameter), and ?????????????

It would be handy to know the formula and be able to compare to published spec's.....

Any thoughts?

Thanks

Jim

[Jim Laumann]

dis-regard - much more complex than I thought.... sorry for the bother.

Jim

[ian maybury]

It starts with what's called a fan curve Jim. This is a graph of CFM output for the versus resistance to flow. http://www.greenheck.com/library/articles/10 Resistance is measured in inches of water gauge - the height of the column of water a given total pressure drop can support in a tube. (google 'manometer' to see what this means)

You need a given CFM to get decent dust collection at the point of work - Bill Pentz figures it's as much as 800 - 1000cfm on the major floor machines given the fairly ineffective hoods on most machines. That said most DIY systems run at more like 400 cfm or even less which is enough for chip collection, but doesn't do a great job on fine dust.

You need an airspeed of something approaching 4,000 feet/min to get good chip transportation, especially in verticals - but much more makes noise and creates extra pressure drop.

Take say a six inch duct. The cross section of that is 0.5X0.5X 3.142/4 = 0.196 square feet. (the formula for the area of a circle:area = radius x radius X Pi/4)

That means that a six inch duct flows 0.196 x 4,000 = 784cfm at 4,000 ft/min. Luckily at our low pressures (actually up to about 25in water gauge) you can treat air as not being compressible.

To design your ducting you need figures for the pressure drop/100ft run of ducting, and for the various types of bends, cyclone and other stuff in the size that will make up your system. This comes from tables, Bill Pentz has quite a lot of information on his pages too. That means laying out your system on paper, and counting up the feet of ducting, and the numbers of each type of fitting - in each run from each machine back to the filters. The makers should be able to provide pressure drop figures for the cyclone and filters.

Most low pressure blowers we use start to fairly rapidly lose CFM once you get above about a total of 12in of water gauge total pressure drop. So you basically add up the total pressure drop for the run of ducting to each machine, and make sure that none gets above 12in - doing what you can by way of layout to keep the runs as short as you can and the number of bends minimum to minimise the total pressure drop...

In practice fan HP and the total pressure drop (roughly determined the size of your ducting, and the length of your longest duct run) determine the CFM likely to be delivered by a given system - just about all of the blowers used in dust systems are the low pressure centrifugal type.

Bill Pentz more or less summarises the issues surrounding this topic here: http://billpentz.com/woodworking/cyclone/DCBasics.cfm The table in section 5. lists roughly the cfm likely to be delivered by a given fan HP for a given size/length of system using 6in ducting

ian

[Bruce Wrenn]

For most dust collectors, 1-you take the size of the inlet, 2-multiply it by the diameter of blower wheel, and 3-divide by HP of blower motor, and then 4-multiply by square root of blower speed. 5-Then choose any number that you want to list as CFM. Or you can skip steps 1-4 and go directly to step 5

[Mike Cruz]

Bruce, that was PRICELESS!

Jim, if you want to know why Bruce said that, and why I thought it was "priceless", reseach dust collectors. Scratch you head at why one manufacturer with a 2 hp motor, 6" inlet, and 12" impeller will claim 1200 cfm, and anther with the same specs will claim 1550 cfm. From what I've heard/read/understand, some manufacturers test with 10 feet of pipe (likely a minimum that anyone would actually have on their DC), and others go so far as to remove the bags (which restrict flow).

I know you said "never mind", but you opened a can of worms... Bottom line is what to know about how well a dust collector will push air, look at the impeller size...not the motor (many will put a bigger motor on and claim higher cfm...baffling). Manufacturers may be happy to charge you more to put a larger motor on, which in essence doesn't hurt the DC, but gets them more money. But I have a real hard time believing that any manufacturer would put an impeller that is too big for the motor.

[Bob Wingard]

AN anemometer on a piece of pipe sized to match the blower's inlet is hart to refute.

[Mike Cruz]

Yeah, what he said...

[ian maybury]

I guess as in all these things there's the data/information (a fan curve is the result of a properly conducted test in a standard format), and then the engineering. Then there's the misleading stuff published by many of those selling systems. There's no substitute for honestly derived data, but understanding the engineering and having access to good data (the essential fact of Bill Pentz's stuff is that while nobody is all knowing it's honestly put together) does help in figuring out when the published data is bum....