Controlling airflow

Woodworkers use air to run tools, collect dust and even create vacuum clamps. But there are two very different physical processes at work here and understanding their basics can make woodworking more enjoyable, safer and a lot more efficient. The two biggest differences between air power that is being used for pneumatics as opposed to that being used for dust collection are volume and pressure.

The numbers that manufacturers use to describe dust collectors are supposed to let woodworkers compare machines and systems to each other, but they can be misleading and confusing. Some collectors are rated in horsepower (hp) while others are graded according to cubic feet per minute (cfm). Neither measure really describes what the machine can do.

Horsepower refers to the size of the motor that is running the fan. It’s a fuzzy number for many reasons, but quite convenient for manufacturers because it’s simple and makes potential buyers believe they’re comparing apples to apples. The problem is that a 3-hp motor can run at almost any speed and torque, and its blades can be designed in a hundred different ways. They can also vary widely in diameter, so not all 3-hp motors move the same amount of air at the same rate and under the same degree of compression.

The volume of air is still not a perfect measure, but it’s at least a little more accurate. Cubic feet per minute describes the number of cubic feet being moved by the collector through the dust collection hoses or pipes. Here, the diameter and length of the pipes is crucial. The air in a 12” diameter pipe drawing 1,000 cfm has a much, much slower speed than one of 4” diameter processing the same volume. Understanding this is critical because speed is what keeps dust in suspension. If the air is moving too slowly, the dust falls out of the stream and never makes it all the way from the machine to the dust collector. (That’s why we have to get down on the floor and empty the dang table saw so often.) The area of a circle is π r² (that is, pi or 3.142 times the radius squared), so the 12” diameter pipe has a cross-section area of 113 square inches. A 4” pipe encloses only about 12.6 square inches. So, while the diameter is only three times as large, the area is about nine times as large. That means that our 1,000 cubic feet will take nine times as long to pass through a 4” pipe as it will a 12” pipe, if the force (fan) pushing them is the same. When a woodworker is installing or extending ductwork, it’s essential that he/she asks a professional designer to help size the pipe. Most ductwork suppliers will do so at no charge.

On those smaller one-machine collectors that have a 6” outlet that wyes into two 4” outlets, you’ll have to do some math to figure out exactly what you’re getting. A good rule of thumb is that a 4” pipe will carry about 350 cfm and a 6” one will be closer to 800 cfm if the fan can move 4,000 feet per minute.

Another rule of thumb is that a collector connected to a single machine (belt sander, router table, etc., and here we’re still talking about small equipment in a one-man shop, but the principle can be scaled up) must draw around 350 cfm just to collect the visible residue. However, that’s just convenience and not safety. The collector needs to draw at least 1,000 cfm to collect the fine dust that escapes to the surrounding air. That’s because the dust that we see is basically too coarse to breathe and doesn’t cause problems, but those essentially invisible fines are what will harm you over time.

That brings up another concern. Many manufacturers say that their 3-hp machine will draw somewhere in the vicinity of 2,000 to 2,500 cfm. But when you read the fine print, there’s always a codicil. That huge volume assumed there is no resistance, or static pressure, in the system. They’re basically saying the machine performs well before you hook it up to a hose or a pipe. Static pressure in the dust collection industry describes the resistance that the airflow meets, and it is usually measured as inches of water in a water gauge. It includes the total of all the obstacles that a volume of air will meet while traveling from the machine to the collector. Then there are bends, reducers, ribs in flexible hose and so on, plus the length of the system (add up all those feet of pipe). Dirty filters and hoppers can dramatically affect the system’s performance. So, can sizing the bags properly and using the best fabric and micron rating (yup, another number to research). Too much moisture in the shop air can clog up filters in a hurry, too.

Large shop owners may want to swing by the SLY Inc. website and check out a blog post on troubleshooting central dust collection systems (blog.slyinc.com/dust-collector-problems).

Thunk! It’s such a satisfying sound when a pinner or brad nailer does in a split second what we once had to do with hammers and finish nails. From holding parts together while the glue cures to fixturing on the CNC with plastic nails, pneumatic guns save an incredible amount of time and effort. Compressed air also run veneer clamps, spray guns and even sanders, but no matter what the task, two numbers are critical – volume and pressure.

Just as it is with dust collection, volume in compressed air is measured in standard cubic feet per minute (scfm). The ‘standard’ here measures the volume at sea level, at 68 degrees F, and at 36 percent relative humidity. Each of those numbers is critical because they effect the ‘size’ of the air. Moist and warm air is, well, bigger. And the higher one goes from sea level, the less condensed the air becomes. So, if your woodshop is in Denver (5,280 feet), the air’s volume is going to be significantly different than it would be in, say, Miami. The atmosphere at sea level is roughly 14.7 psi. In Denver it’s roughly 18 percent smaller, at around 12.2 psi. That’s a huge difference, so compressor manufacturers have settled on standard scfm as a baseline. It allows us to compare compressor ratings without having to account for their location, and the ambient atmospheric conditions.

Volume is the amount of air needed to make a tool work. Some tools such as small nail guns can be extremely efficient, while others (such as traditional rotary sanders) can require a huge volume of air. The concept here is that a compressor squeezes air into a smaller volume than it naturally wants to be, and then releases it back into the atmosphere through a tool. The speed at which it escapes and regains its volume creates energy that is harvested by the tool. It takes a lot of compression to squeeze air into that compact form, and that pressure is measured in pounds per square inch (psi).

So, remember that 14.7 psi at sea level? Well, even a small portable compressor can squeeze air into a holding tank that is only 10 percent the size of the air’s natural volume. Doing so would raise the pressure tenfold, to 147 psi. Of course, compressors are regulated so that the pressure doesn’t continue to rise past the capacity of the tank to contain it, and most smaller units deliver the contents to a tool at a nice, even pace in the 60 to 125 psi range.

Because the pressure is regulated and controlled, the variable here is the volume. Most small compressors deliver the same pressure range, and most of the tools work within those ranges, but for how long? If you’re just blowing a little dust out of joinery, or using a pin nailer, you won’t need much more than a gallon or two of compressed air in the storage tank. The reservoir in the tank equalizes demand, so as you allow small volumes of compressed air to escape back to the atmosphere through the tool, the compressor keeps topping up the reserve and maintains the pressure and volume. If you’re using a high-pressure low-volume (HVLP) spray gun for more than touch-ups, you’ll need to upgrade to something larger than a portable, oil-free compressor and keep at least ten gallons of compressed air in reserve. Pneumatic sanding (which is more of an autobody shop requirement than a woodshop one) might have you shopping for something with at least a 50-gallon tank.

One way for small shops to add compressed air on a budget is to use flexible jobsite hoses to distribute compressed air through the shop. Simply run a pneumatic hose across the ceiling from the compressor to the workbench and secure it with pipe clips. If demand increases, the shop will need to upgrade to a safer system that can handle higher pressure and volume, and the material of choice nowadays seems to be L grade copper piping (not the thinner M grade). Traditional black gas pipe works well, too, but it can rust and is a bit harder to install or modify (unless you don’t know how to solder the copper). Either way, make sure the pipe drains to a cleanout or moisture will accumulate in the pipe. 

This article was originally published in the September 2020 issue.