Created: May 12, 2002
Updated: February 6, 2008

Static Calculator Frequently Asked Questions

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 you 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 3700 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 3700 - 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 3700 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 3700 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 3700 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.