i get a fair amount of questions about dust collection ductwork some from folks who think that the size of the ducts don't matter all and others who think that the bigger the duct work the better both points of view though can lead to a massive waste of money and loads of regret i've learned that the hard way so today i just want to offer a simple easy to understand explanation of this one part of the larger dust collection subject specifically why the size of your dust collection hoses and ducts do matter and what's right for your shop may not be what you think i think this is best broken down into four factors volume velocity resistance and pressure you must balance these four factors between your ductwork your tool dust ports and the capabilities of your vacuum or your dust collector it is very important to understand this before you spend any money on dust collection equipment or ductwork so let's consider these factors quickly one at a time beginning with volume air may be invisible but it occupies space a balloon for example has to stretch to create more space as the volume of air inside increases a dust collection hose or pipe can't expand like a balloon it can only hold so much air within its walls while it's true that air can be compressed it takes a lot more pressure to substantially compress air than a wood dust collector generally can generate so the only way to really increase the volume limitations of your duct sizes is to use larger ducts or larger hoses air volume or air flow is measured in cubic feet per minute cfm generally a four inch duct can accommodate four times the air volume as a two inch duct and a six inch duct can accommodate about twice the volume as a four inch duct don't forget that the air isn't the only thing inside your ducts and hoses sawdust and wood chips also compete for that space with the air but before you go out and you buy the biggest pvc pipe and hoses you can find consider velocity while it takes a certain volume of airflow to gather the dust and the chips up from your tools it also takes a certain amount of air velocity to keep those chips suspended and moving through the air stream on their way to the collection bin if the air velocity is insufficient dust will drop out of the airstream and it will pile up inside your ducts you might test this principle by blowing sawdust off your bench through a drinking straw then trying the same thing except this time blowing through a larger cardboard tube like you'd find in a roll of paper towels the smaller diameter of the straw obviously restricts the volume of airflow that can go through it compared to the larger tube but the velocity of the air coming out of the smaller straw is much greater and it moves more dust to compensate for the larger tube you would have to increase the volume of air from your lungs basically blow a lot harder to move the same amount of dust the key points are if the volume of air is the same you will get more velocity from a smaller duct likewise if you increase the size of the duct you will need more volume to maintain the same velocity now let's talk about resistance if you look at the specs on your vacuum or your dust collector you're likely to see a cubic feet per minute or cfm rating that indicates the volume it can move when the blower is completely unresisted but in the real world we had all sorts of resistance to our dust collection systems so focusing again just on the ducts and the hoses every bend and turn every rib in the hose even the walls of the ducts themselves along their length add resistance resistance slows velocity and as i explained velocity is what keeps the chips moving so in our first section we learned that volume is closely related to velocity now we learned that velocity is closely related to resistance and in our final factor we learned that resistance is closely related to pressure it takes more pressure to push an equal amount of air through a smaller space than a larger one you might test this by drinking water through a regular straw as compared to those little cocktail straws you have to suck a lot harder on the ladder and even then you won't drink as much volume because your lungs are not capable of generating a lot of negative pressure blower style dust collectors such as the big cyclones or the single stage harbor freight style ones they produce very little static pressure so like your lungs they struggle more when you connect them to smaller ductwork and hoses vacuum cleaners on the other hand generate vastly more static pressure and they work better when you connect them with smaller hoses and ducts so we've covered our four points and clearly you see you have to find the right balance between those four factors based upon the system you have take a shop vacuum for example shop vacuums typically come with two and a half inch or smaller hoses for a reason they're not designed to move enough air volume to maintain sufficient velocity through the larger ducts to keep that dust moving but they do generate enough static pressure to pull a decent amount of volume through a small hose or a small dust board on a tool a blower style dust collector on the other hand like your harbor freight dust collectors or your big cyclones don't generate enough static pressure to pull enough volume through a small hose or a dust port to effectively collect sawdust and chips from your tool but if you increase the size of the ducts or the hoses or the tool ports you remove the volume restrictions and you dramatically increase that type of system's effectiveness until you reach the system's tipping point where the size of the blower can no longer move enough volume of air to maintain the velocity required to keep the dust moving in the ducts i know this is sounding a little bit complicated so let me simplify it as much as possible big six inch ducts go best with big dust collectors medium four inch ducts go best with medium dust collectors shop vacuums go best with little hoses and all of them work better with as little added resistance as possible from excessive duct length bends or corrugated hoses anytime you mix up those duct sizes as you sometimes have to do when you build a shop dust collection system you have to deal with the changes in performance that i just outlined in those principles so let's apply that to some real world examples here in my shop in the main area i have a powerful cyclone system with a big blower that can move a lot of volume so i used large six inch ducts to accommodate that volume some of my tools though have four inch ports on them i'm obviously not going to get my maximum air flow through those four inch ports but i have to reduce down to them still i use the six inch on the main ducts because i do have other larger connections in the shop and i also have some tools that have dual four-inch connections so i need the larger main duct to handle the combined air volume from those dual connections in fact with a
