Anyone who has dealt with a shop dust collection system has likely encountered a lot of letters, numbers and equations not seen since high school physics class. If figuring out the SP to coordinate ducting to a machine with a certain CFM to evacuate a dust with a certain velocity gives you a bit of a headache, here is an explanation of what it all means in English.

Three key terms

According to Curt Corum at Air Handling Systems, three terms are most important in putting together a dust collection system. They are velocity, CFM and SP.

Velocity is only important in relation to what material, or type of dust, is being collected. "The velocity is the speed of the air, how fast the air has to go to keep the material entrained in the air stream to get it from point A to point B," explains Corum. In the case of wood dust, that speed is 4,000 feet per minute. For metal dust, it's 4,500 feet per minute (since metal dust is heavier than wood dust, the speed is higher), and so forth. The numbers are all predetermined for a given material and won't change.

Machine requirements

What a given material's velocity is, affects what CFM is needed to move the dust out of your shop. CFM stands for cubic feet per minute, and Corum says, "It's critical." CFM tells you the volume of air a machine needs for proper evacuation.

Most of the time, machines come with a CFM pre-calculated by the manufacturer for a certain diameter of ducting. But be careful: "In many of the machines produced today, the round diameter (duct) is already prevalent. But if the machinery has a rectangular or square opening, which some of them do, the first objective is to take that and convert it to an equal area, or round duct diameter," warns Corum.

That diameter, paired with the velocity required for a certain material, can be used to calculate the CFM of a machine, if it's not known. The easiest way is to use the industry-standard charts issued in the Industrial Ventilation Manual (see sidebar on industry resources), or use a slide rule designed for this purpose. (These slide rules are manufactured by Datalizer,

The total CFM of your shop is the total CFM of all the machines that run simultaneously. Adding another machine increases your CFM and, unless the increase is accommodated, it will make your dust collection system insufficient.

Static pressure

The last term that's important to know is SP or static pressure. Static pressure acts in much the same way as static friction does. The force acts perpendicular to the ducting and creates resistance in a system.

Corum likes to use the analogy of curly drinking straws: "The ones that kids drink soda out of that go in a circle and a circle and a circle. You see them trying to drink the soda, and their cheeks are ready to implode. Well, what they did is overcome the resistance, or the static pressure, of that straw. If it was just a straight straw, it has a low level of resistance, and you're going to be able to drink your soda quite easily."

Keeping that analogy in mind while performing SP calculations helps make sense of the numbers.

Every diameter of ducting has a corresponding static pressure. For straight ducting, multiply that corresponding SP by the feet of ducting being used. Similarly, every elbow in a certain diameter of ducting has a corresponding length of straight ducting. Figure out all the straight ducting numbers, add in the straight numbers corresponding with elbows, factor an inch of SP for the initial hood entry loss and two inches SP for filters, and you have total feet of static pressure for a system.

Common mistake

It's important to remember that unlike CFM, SP isn't cumulative for individual branches. "The theory is that as long as you calculate the resistance for that worst branch, all of your branches with a lesser level of resistance will pull vacuum as long as you compensate for the worst one," explains Corum. Adding all the numbers together instead of merely calculating the worst branch is a common mistake people make, Corum says.

After the SP of the worst branch is calculated, it's necessary to calculate the SP in the mainline. Starting furthest away from the dust collector, main duct size increases as each primary branch enters the main. You isolate each diameter of the main and calculate the SP for each duct size according to the length of run.

If your dust collector is located inside the shop, these are the only SPs to calculate add them together for your total SP. If the dust collector is located outside the shop, it is necessary to calculate the SP of the return duct as well.

"Typically in a woodshop dust system it's going to range between 12- and 14-inches of static pressure," says Corum. Keeping those averages in mind could help you avoid overcalculation.

Armed with all of this information, and you're still hesitant to design or upgrade your dust collection system on your own? There are alternatives. You can hire an engineer to make a blueprint and tell you exactly what kind of dust collector is needed. Or you can go to a major dust collection company that will calculate your dust collection needs to figure out which collector to sell you. But even if you get help for your dust collection system, at least now you'll understand what they are talking about.

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