Created: May 12, 2002
Updated: October 4, 2011

Table of Contents

1. Introduction
2. Background
3. Air Flow Requirements
4. Basic System Design
5. Frequently Asked Questions (FAQs)

Introduction

Don Beale was an air engineer who worked with me to build and share this calculator because most small shop woodworkers don't realize how much their duct size and layout impacts dust collection. Most wrongly think of their dust collector as a huge shop vacuum. That causes all kinds of problems. A powerful shop vacuum has enough suction to lift a column of water up to about 100”, but the blowers used in dust collection work with only 7” to 10” of suction. At dust collector blower pressures air will barely compress at all, so almost any small pipe, bend, wye fitting, small port, restriction, or other obstruction will act just like a partially opened water valve and kill our airflow. This leaves us with two choices. We can add horsepower and larger blower until we overcome all that resistance or design a system with minimal resistance to permit us to use the smallest, most cost efficient blower. This StaticCalc can help you design and size your ducting system to give the best flow.

Background

To meet government air quality mandates that went into effect in the late eighties, the major suppliers of dust collection equipment had to take a fresh look at dust collection. Until then dust collection meant keeping shop floors clear of the dust and chips that would otherwise be swept up with a broom. We now call this “chip collection”. These firms found that to also ensure collecting the fine airborne dust they had to collect the dust right at the source, meaning each tool as the dust was made. If they let the fine airborne dust escape into the air, it took hours for a good exhaust fan or air cleaner to bring the dust levels down to low enough to meet government standards. They found the only way to capture this fine dust was to increase the airflow to our tools and keep the air streams from our tool blades, bits, cutters, belts, motor fans, etc. from spraying this fine dust all over. They found they had to start by fixing almost every stationary tool. Almost all required a new hood, larger ports, better internal ducting, and sometimes new better panels. Small shop owners must make similar changes to our tools if we want good fine dust collection. Before these web pages it was very difficult to find this information. This spreadsheet also helps by providing the calculations to help you make sure you properly size your ducting plus pick the right sized blower and blower motor needed to get good fine dust collection in your shop.

Air Flow Requirements

The major dust collection suppliers also did the testing to determine what was needed for good fine dust collection. They knew that traditional dust collection requires enough air volume to collect the dust and then enough airspeed in the ducts to keep vertical runs from plugging and horizontal runs from building dust piles that can lead to dangerous ducting fires. Air engineers long ago learned that it takes an air volume of 350 cubic feet per minute (CFM) to collect sawdust and chips at most small shop stationary tools and an air speed of 3800 feet per minute (FPM) to keep the collected dust moving vertical ducting and roughly 2800 FPM to keep it moving down horizontal duct runs. Most air engineers add a little safety factor so design traditional dust collection systems to have at least 4000 FPM airspeed in vertical runs and 3000 FPM in horizontals.

Just a little testing showed just 50 FPM airspeed will pull in the fine dust before normal room air currents dispersed that dust. Most suspected the air volume would be small as well, so existing systems would work just fine. The testing showed a totally different story. If a tool was built from the ground up with good fine dust collection engineered in to protect and control the fine dust until it can be collected, a good shop vacuum that only moves 50 cubic feet per minute (CFM) provides good fine dust collection. Unfortunately, in our real world most small shop owners buy older and hobbyist tools that have little to no fine dust collection built in and come with at best poor dust collection. Our larger tools are identical to smaller commercial tools. Air engineers worked with these same tools and figured out what we need to modify to provide good fine dust collection. Click here to see samples of good tool exhaust hoods.

After working through the physics, doing lots of testing, then years of experience the air engineers who design systems that will meet fine dust collection air quality standards, have built and refined their air volume tables that show what we need for good fine dust collection at each of these tools. These tables show that to meet the minimum OSHA air quality standards requires at least 800 CFM airflow at the same traditional stationary tools that get good “chip collection” with just 350 CFM. To meet the ACGIH tougher air quality standard requires about 900 CFM and the medical recommended standard adopted by the European Union and the EPA requires moving a full 1000 CFM at these same stationary tools. Click here to see the Exhaust Requirements for Woodworking document. It shares what airflow these air engineers find we need at each tool to meet the different fine dust collection standards. If we then look at a good blower table we see we really need at least a 3 hp dust collector or 5 hp cyclone to move the 1000 CFM needed to get good fine dust collection at most small shop stationary tools.

At first this does not make sense. It takes almost triple the volume of air to collect fine dust which we know we can blow around with the lightest breath. We already know this is true if we think about using our shop vacuums. They only pick up right next to the inlet. This is because sucked air comes from all directions at once causing airspeed to decrease at roughly four time pi times the distance from the inlet squared (4*Pi*r^2). In fact, moving less air will not produce the 50 FPM we need to overcome the room air currents and collect the fine dust just a few inches away!

In summary, to keep commercial shops from being closed due to poor indoor air quality air engineers had ensure they collected the fine dust at the source. To do so they nearly tripled the total air volume (CFM) plus had to redo most machine hoods, ports and internal ducting. Small shop owners must do the same if we want good fine dust collection. Most respiratory doctors recommend we build our systems to provide 1000 CFM.

Basic System Design

We have two ways to design your dc system in order to get optimum results.

1. Commercial: Size your ducting/dc for the total requirements of the entire shop with NO BLAST GATES all. Air engineers design commercial dust collection systems with all ducting runs are open at once to collect the dust from all machines running at the same time. All ductwork drops are sized to provide the minimum CFM requirements for each machine. The main becomes a collection of ever bigger sized pipes sized to meet the CFM of all downstream ductwork. The Air Laws show us that to double the airflow we need eight times the power. Just a two person shop running a couple of runs at the same time requires at least a 7.5 hp motor turning at least an 18" diameter blower impeller. Motors this big normally only work with three-phase power, require commercial power, and require much heavier wiring than found in most residences.

2. Small Shop: Most small shop owners cannot build dust collection systems similar to commercial systems because we lack the power service, wiring, or don't want to pay the cost to buy or run a huge motor. As a result, our systems are designed to use smaller, much more efficient blowers that can only collect from one machine at a time. All other ducting runs are closed off with blast gates. This lets us use a blower just big enough to meet the airflow needed for fine dust collection at our largest machine and highest resistance ducting run. We should size our ducting and dust collector for our max CFM requirements at our largest machine and size the ductwork for the longest possible run. This will allow only one machine at a time but there will be no build up and all dust and lets you move machines around without needing to buy a bigger blower and motor.

   1. Main Duct: If you want the most possible CFM you need the biggest duct you can get that keeps the air velocity ample to keep the dust moving instead of clogging your ducting. Most hobbyist vendors sell 4" dust collection duct and flex hose as their standard. This works well to collect chips, but strangles the airflow needed for good fine dust collection. A typical 2 hp hobbyist dust collector with a 12" diameter impeller moves a maximum of about 1200 CFM, but a 4" duct airflow drops that airflow to only about 450 CFM. It takes a 5 hp motor turning a 16.5" diameter impeller to force a 4" duct to carry 800 CFM. Most prefer a less expensive solution. In addition to maintaining that airflow, our blowers must keep the duct airspeed high enough so we don't get plugging or piles of wood dust. Dust piles in ducts pose a fire hazard and ruin both blowers and filters when they break loose and go slamming around. Air engineers found designing for a duct speed of 4000 FPM keeps the ducting clear. Because FPM=CFM/duct size, a little math shows we need 6" diameter ducting to move 800 CFM at 4000 FPM. Most hobbyists need all 6" hoods, 6" main ducting lines, and 6" down drops.

   2. Down Drops: Commercial dust collection systems size each down drop to carry just the air needed for good collection for each specific tool. They also use many different sized ducting runs for the mains to keep the airspeed ample to avoid plugging and piles. This ends up being so complex that most ducting engineers use commercial programs to compute all the ducting sizes, parts, etc. Sadly, a number of firms now offer similar services to hobbyists using these same commercial programs. The results are a nightmare because any time we use a down drop smaller than our main in a one ducting run open at a time hobbyist system, the small down drop strangles our needed airflow. A 4" down drop connected to a 6" main looks pretty and appears to work well, but drops the main airspeed to under 2500 FPM immediately building up dangerous dust piles that can grow huge. These piles pose a serious fire hazard and when the airflow gets restored, these piles break loose and slam into things potentially ruining motor bearings, impellers, and your filters.

   3. Blast Gates: Normally we put a blast gate as close as we can to the main for each machine to leave the least possible pipe to build up piles when their runs are not open. Putting the blast gate down low lets lots of debris build up in the pipe. When the gate gets opened this material can slam around potentially ruining blowers and especially filters.

   4. Return Duct: If there will be return duct moving the cleaned air back inside your shop from an outside cyclone, it only needs to provide 2000 FPM speed. To determine that return duct size simply select the size to run at 2000 FPM or less with your max CFM machine. Leave it in each branch calc. The same applies when sizing the return for a cyclone or muffler.

   5. Tool Ports: It takes one 6" port to support the minimum 800 CFM required for larger stationary small shop tools and a 7” port to support the 1000 CFM required for good fine dust collection. What I do with my cyclone design is use a larger blower with increased pressure to pull a full 1000 CFM through a 6” duct. This allows using 6” ducting mains and 6” tool ports. For machines with two pickups, hobbyists should generally use a wye that splits their 6" ducting into two runs. The smaller should use 3.5" diameter ducting and port. The larger should use 5" ducting connected to a 5" port. This combination maintains the same area as the 6" main to prevent airflow restrictions, plugging, dust piles, and poor dust collection. Generally, we have to modify our hobbyist tools to add the 3.5", 5", and 6" ports. Dust collectors do not have enough pressure to provide good fine dust collection for tools that cannot be modified to have at least 3.5" diameter ports. Tools with smaller ports require use of a shop vacuum that generates at least 60" of pressure to force the air collection needed plus often a movable hood and downdraft table.

   6. Resistance Calculation: Small shop owners only use one ducting run open at a time, so we size our systems based upon what it takes to power our largest need. Knowing that most will move their machines around over time, it is best to figure what it takes to power our largest machine overhead and our longest ducting run. The longer the hose and the more bends and fittings we use in our ducting runs and machine collection, the higher the pressure drop and bigger blower it will take. We need to calculate the resistance of our tool with the most hoods and ducting overhead, then add to that the resistance of all the pipe and fittings in their largest ducting run, and then add the other overhead resistance including cyclone, muffler, and filters. That total resistance corrected for air flow loss gives the worst pressure drop in our system. We use that total worst case resistance with a standard engineering fan table to size our blower motor and impeller diameter. This simple resistance calculator helps you work through what you need for your own shop.

Frequently Asked Questions (FAQs)

1. I need to run two branches at one time. How do I use the Static Calculation sheet to size a system for that?
   I need 4000 CFM, and when I try to use the larger sizes the losses skyrocket. How do I design larger systems?
   These two questions and many similar questions all have the same answer. This calculator is not appropriate to use to answer your questions. This is static calculator designed to help small shop woodworkers build a simple dust collection system that blocks the airflow in all but one branch at a time. You can estimate by adding the individual resistance of the flows then dividing by how many flows, but that still does not address the need to balance your system so you get the flows at each machine as needed. To balance the flows you need a different type of calculator. The only other calculator I’ve seen for small shops is provided by a small shop cyclone vendor. That calculator makes lots of serious errors and does a terrible job of balancing. I say that based upon buying one of those ducting designs that looked pretty and most professional then measuring the individual airflows. Just about all who made similar purchases and tested their airflows found similarly bad news where the larger pipes left the smaller tools without enough airflow to even do good "chip collection".

2. I’m getting a loss number of 186” (or some other absurdly high number). Can this be correct?
   In theory it can. In reality, no centrifugal blower can run at losses that high so it has not been tested. Look at the velocities, and the CFM input. Read the example at the bottom of the example problem, about 10,000 CFM in a 4” duct. The sheet will not differentiate between an absurd input and a reasonable one.

3. You keep talking about velocity, velocity, velocity. I’m thinking in terms of CFM. How are they related?
   Velocity is measured in feet per minute (FPM). To get good pickup of sawdust and chips plus keep our vertical runs from plugging we need a target airspeed of at least 3800 FPM. Air engineers try to provide 4000 FPM airspeed to provide a little extra without forcing the use of too large of a blower that will cost more to buy and run. Pressure drop is calculated based on velocity in FPM, not CFM. CFM and FPM are related by the cross sectional area of the duct. FPM = CFM / AREA, where CFM is in cubic feet per minute and area is in square feet. For example, a 4” duct has a cross sectional area of 0.087266 square feet (144 sq. in. / pi*r*r) Dividing 350 CFM by 0.087266 gives 4010 FPM velocity. With our needing about 349 CFM moving at 4000 FPM to ideal "chip collection" this is why most small shop ducting and tool ports are sized for 4" diameter pipe.

4. The target velocities are 3800 - 4000 FPM. When I size a 4” branch on a 6” main, the main velocity drops too far below that and I get plugging in my vertical runs and dust piles in my horizontal. How do I correctly size the system, and stay within the target velocities?
   This will usually be the case when running a main more than one size up from the branch, meaning more than 1” difference in diameter. This big loss in airspeed in the mains is why for hobbyist and small shop systems that only collect from one machine at a time, we recommend using all the same sized duct with no more than one inch difference between mains and down drops. Maintaining the transport velocity is more critical in the vertical rise than it is in the horizontal. Try increasing the CFM a little and look for a happy medium. If you let the main vertical runs drop below 3800 FPM you risk plugging. This is why most commercial systems put their mains up high where there are no vertical runs. With no vertical runs we only need to maintain enough airflow to keep the horizontal runs clear, roughly 2800 FPM. Be aware of the potential for plugging and building up dust piles. Plugging can force you to have to take your ducting apart. Ducting dust piles and plugging also create a double dangerous hazard. There is the potential of a ducting fire, plus when a large amount of dust breaks loose at once we end up with one of the few instances in small shop woodworking where the dust to air ratio can become explosive. You should check and clean your ductwork periodically. Installing a cleanout at the end of the main run is good idea.

5. Will the sawdust tend to fall out of the vertical runs? Do I need to add anything different for a vertical rise than a horizontal run?
   As long as the transport velocity is maintained the sawdust will remain entrained in the air stream. You do not need to do more than maintain the minimum transport velocity. Normally, in a system with multiple duct sizes, the highest velocity will be in the smallest duct - which is the vertical rise anyway. This is why the velocity in the vertical is more critical. In the horizontal, as dust falls out of the air stream, it will tend to roll along the bottom of the duct and either stop at an obstruction or get where it is supposed to go. In the vertical, it will float around in the duct until you shut off the blower and then drop back down to the hood.

6. I see that as velocity drops, static pressure drops significantly. Why not size for a maximum of 3800 FPM?
   Think of a cloud in the air as like sawdust. And the air temperature is like velocity. As the temperature drops, the cloud begins to rain. Drop the temperature a little more, and it rains harder. Drop it a little more yet, and it rains REALLY HARD. This is similar with the velocity. Dust contains all different sizes and weights of particles. As the velocity comes down below 3800 FPM, the biggest particles will not make it up vertical runs. As the velocity drops below 3500 FPM, particles in horizontal runs will roll along and hang up on obstructions, beginning to form a plug. Drop the velocity a little more, and it “rains” a little harder. Somewhere between 3000 FPM and 3500 FPM, it will all of a sudden rain REALLY HARD. So long as you understand the risk, and are prepared to deal with the plugging, you can get away with lowering the velocity. But again, in the vertical, that’s different. The dust MUST be entrained in the air stream to be picked up vertically, as there is no surface for it to roll along.

7. I have sized my system using this spreadsheet, and now it is running. Putting a manometer on the cyclone inlet, I read a number significantly lower than what the calculation showed. Why?
   If your reading is low, the most likely explanation is that you don’t have the airflow you think you do. Remember most small shop vendors provide a maximum blower airflow in their advertising, but after adding the resistance of the cyclone, ducting, and filters you end up with about half that airflow. Now add the overhead of your ducting and tool and you can drop to a third of that maximum advertised airflow. Take a look at my Measurement web pages for more information on how to measure the air velocity, which by now you realize is directly related to CFM by the cross sectional area of the duct.

8. I have sized my system using this spreadsheet, installed it, accurately measured the CFM, and the SP reading is significantly higher than the sheet showed. Why?
   There are many possible reasons for this. The most likely reason is that the fitting loss factors in the spreadsheet are different than the loss factors on the fittings you installed. Fittings are the single biggest component of losses in most cases. The factors used here represent the LOWEST LOSS FITTINGS AVAILABLE! The best industrial fittings are welded, smooth interior walled pipe. These fittings are generally best ordered from a local firm that provides dust collection parts as shipping can often be as expensive as the cost of the parts. These parts are called “PREFERRED”, meaning ‘expensive’. Using furnace (HVAC) fittings will increase the pressure drop by several inches (or decrease the volume until the pressure drop is the same). The next most likely reason is that the losses NOT included in the spreadsheet are different than the assumptions you used. Another possible reason is due to a bad blower installation. The blower should be installed with straight runs into and out of it, for a minimum of 3 duct diameters. Longer is better. Putting an elbow on the inlet or outlet of a blower will significantly degrade its performance.

9. I see that the assumption for the hood entry is a flanged pipe entry, and that others will vary. How much can that assumption affect the result?
   This can be significant. The best hood entry available is a bell mouth, and the worst is a sharp-edged orifice. If you eliminate the orifice from consideration then the flanged pipe entry used represents a reasonable middle ground guess. Eliminating the orifice from consideration gives a margin of error at 4000 FPM of plus or minus ½”.

10. How does the spreadsheet calculate the static pressure?
    The spreadsheet uses the straight friction loss method. It does NOT use static regain. This will induce a small error, but it is a much simpler method and is commonly used in the industry. Specifically, the sheet calculates velocity pressure using the formula (V/4005) squared, which assumes standard temperature air. All loss factors are published as a function of the velocity pressure. For example, the loss factor for a 90 elbow, in smooth stamped steel R/D >2.0, is 0.13. So the losses for the 90 elbows are a formula Q*VP*LF, where Q is quantity, VP is velocity pressure, and LF is loss factor.
    The entire formula looks like this: (Q1*VP1*LF1)+(Q2*VP2*LF2)+(Q3*VP3*LF3)+… where 1, 2, and 3 represent different loss factors. In this case there are four loss factors in each size pipe; one for a hood entry, one for an elbow, one for a wye, and one for straight pipe.

11. The introduction notes say that flex hose losses can vary greatly. What does that mean in terms of the margin of error in the spreadsheet?
    There are many ways to manufacture flex hose. If the hose is made with a smooth interior it will have significantly less resistance than a pipe made with an interior covered in ridges and valleys. Also, if the hose allows tight bends it will have far more resistance than a flex hose that severely limits bending. Understandably, flex hose makers are reluctant to share static pressure losses and generally only share loss factors as roughly three times the losses for straight pipe. The only published losses I could find are based on a hose reel manufacturer. They worked out to 2.25x the pipe loss in 4”, 3.2x the pipe loss in 5”, and 3.45x the pipe loss in 6”. Those are the factors used in the spreadsheet. Without knowing precisely what kind of flex those are based on, I can only assume it is the smoothest inside wall available. Generally you want to use as little flex pipe as possible, and then use only the smoothest inner wall flex pipe you can find. Also, try to install the flex pipe as straight as possible. Using as little flex as possible will reduce the margin of error, but I do not have any idea how much the differences are between flex pipe manufacturers. One should assume those differences are significant.

12. I was cruising the woodworking forums and was referred to a static calculator that looked far more detailed than yours. I foolishly did not write down the URL for that calculator. Do you know the address and what are your thoughts on this calculator?
    Although there are a number of on-line static pressure calculators, there are only a few that provide fairly good accuracy. Most are well intended, but omit a number of fairly high overhead concerns. The static calculator I have most seen referred to that appears pretty well made is the FreeCalc.com spreadsheet. It seems to work well, but presumes you know quite a bit about the overhead, resistance and details of your system that many woodworkers just do not know. For instance, you need to know you should input a 4000 FPM transport velocity, but do you put that in as ACFM or one of the other three options? The Calculator that Don Beale and I setup shares most of the values you need to dig pretty hard to find in order to use this calculator and uses default units typical for dust collection instead of trying to cover flow rates for a wide range of other alternatives. Many air engineers have praised our calculator as being both easier to use and a little more accurate for setting up dust collection systems. All should use one of these better calculators when configuring their shops to size their blower and ducting appropriately then actually measure and test to verify they were successful.

13. Bill there is a fellow who answers almost every dust collection question on a couple of the Internet Woodworking forums I like to read. He keeps saying you and your site are full of it and then sites the FreeCalc.com spreadsheet you referred to in your Static Cal pages FAQ #12. He sounds very knowledgeable and puts out some very convincing information. Is there anything to what he is saying?
    I know this guy fairly well as he at one time volunteered his time to help me with the overwhelming volumes of forum questions I received. He agreed to use my most frequently asked questions and my responses for his posts plus defer questions he could not handle to me. He had excellent success for about six months and grew ever more brazen shifting to also respond to questions he lacked the information to answer. He lacks an engineering or science background, so his logical sounding responses were often wrong. Whenever he got in trouble, which was often, he terminated his discussions attributing his responses as coming from me. His help generated such a mess and so much email I asked him to stop responding on my behalf in 2004. He continued to do so anyway, so I began responding to questions he got wrong. This left him embarrassed and upset to the point he began bashing me and my web pages at every opportunity. The president of one of the better known firms admitted he hired this guy as a paid shill to help sell cyclones by giving focused dust collection advice, but fired him soon after because the guy had no interest in giving out correct information. Meanwhile, without my pages and me as an authority source this fellow shifted to use the FreeCalc program along with some fairly complex engineering sources that he does not seem to understand as I find most of his airflow and filter information just plain wrong. Meanwhile, many air engineers have used both FreeCalc and the spreadsheet on my pages that Don Beale and I built. They say ours is easier to use and it provides the same results as FreeCalc if given the same input. Unfortunately, feeding FreeCalc incorrect input can easily prove just about anything you want in terms of airflow.