Dust-Collection System Sizing

More examples of dust-collector math. August 29, 2006

Question
I'm looking into a new dust collector for my shop. We usually run two to three machines at a time. I noticed that the cyclone collectors tend to have less cfm's than the bag style collectors. Do the cyclones have a comparable draw to them? I don't understand that much about cfm's or fpm's. Should I be looking for a collector with higher cfm's or static pressure or what?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor R:
If you plan to return the air to your building, you will definitely need a filter (perhaps a cyclone filter) to meet OSHA rules. You may need one for outside to meet EPA rules.

CFM, SP, and FPM are all related and need to be considered when sizing a filter. When you are quoted the values for a specific filter, ask about the values after a year or two of use. Some filters do not maintain their values.

Another area needing investigation is Interstitial Velocity. If this value is too high, problems will occur. Look at the FAQ section of our website, wolfeaire.com.

Static Pressure is confusing for many people but is important. A machinery list would be helpful to size what you are talking about.

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From contributor B:
Single-stage dc's have lower built-in resistance than cyclones, so with the same blower, a single-stage will outdraw the cyclone. A cyclone is merely a pre-separator with a blower mounted to it. Think of it as two components built into one unit. One beneficial characteristic of a cyclone is that you will (should) pre-separate the bulk of your waste before it comes into contact with the filters.

Now, with regard to your specific requirement of three machines at a time, here's what to do. Determine how many cfm you want at each of those machines. Just as a guide, let's say you want 800 cfm at each machine. Since they are operating simultaneously, your main duct will have to be sized to allow 2400 cfm total. A decent 5hp dc should easily get you there.

The diameter of the ducting will determine velocity through the duct. You want 4000fpm velocity for ideal particle suspension. Look for a 9"-10" intake on the dc (depending on suction capability), to attach your main. From there, at the first 6" drop to a machine, you need a 10"x9"x 6" fitting. The 6" is the drop. The 9" is the continuation of your main to the next two machines.

At the next drop, use a 9"x6"x6" fitting (wye). One 6" is the drop, and the other 6" is for the continuation of your duct to the final machine. If the dc has a 9" intake, use a 9"x8"x6" wye at the first drop, followed by a 8"x6"x6" wye for the next drop, then 6" all the way to the last machine.

Remember, the sizes I've suggested are only for the example of 800 cfm per machine.