November 23, 2011 STATIC PRESSURE CALCULATION SHEET D. Beale/B.Pentz Version 6.0 Disclaimer!! USE THIS AT YOUR OWN RISK! HIRE A PROFESSIONAL ENGINEER to design, specify, test, and certify performance of any industrial dust collection system if you have a commercial or an industrial application, allergies, other medical problems, people working for you, a large shop, work with hazardous materials, or are subject to regulatory oversight. Don Beale, Bill Pentz, American Air Filter, and all other references and links cannot be held liable for this calculation's applicability to your specific situation. Warning this is only an approximation! This information is to give amateur woodworkers a better sense of what they need to do to collect the wood dust in their hobby shop applications. These calculations are approximations based upon values from industrial fittings, pipes and dust hoods which may be significantly different than those used by hobbyist. Moreover, these calculations do not include other known large losses such as those that occur with the use of flex hose, collectors, separators, cyclones and filters. Introduction: We built and shared this calculator because most woodworkers don't realize how much their duct size and layout impacts dust collection. You probably understand airflow far better than you may realize! Water and air share much in common. A garden hose would empty a city water tank far slower than a 6" diameter pipe because the hose is too small. In fact that is exactly how we control water flow as faucets simply close the pipe opening to get the desired flow. Getting a larger storage tank and faster pump has little effect on the home faucet unless there was too little water to begin with. Instead of a pipe to a water faucet, for air we have ducting leading to a blast gate and dust hood. Just like with water, getting a bigger blower and adding more horsepower does little good as the flow is restricted by the size of the duct. With air, a 4" line puts a restriction on flow that pretty much holds all hobbyist blowers to an air output of 350 to 400 cubic feet per minute (CFM). You will see by using the spread sheet that as the air velocity climbs the pressure drop will skyrocket. With any direct drive fan, higher pressure drop will mean lower CFM. The longer the hose and the more bends and fittings we use, the higher the pressure drop. 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. This simple calculator helps you work through the needs for your own shop. Assumptions, comments, and notes: These loss factors are based on the following assumptions: The system is made of 4", 5" or 6" metal dust collection ducting. Loss factors are based on metal with a roughness factor .0005 feet Pipe loss factor based on 4000 FPM. One VP acceleration factor is included in the hood entry. Hood entry loss is for a flanged pipe entry. OTHERS VARY - SEE FAQ 90 loss factor is for smooth stamped steel, R/D = 2.0 Wye loss factor is for a 30-degtree branch entry, on a 15-degree taper (where it is reduced). Flex pipe losses vary - SEE FAQ 8", 10", and 12" loss factors are for furnace pipe, to be used on RETURN ONLY. Transition losses are not included. Instructions: READ THE EXAMPLE PROBLEM! THIS IS REQUIRED!!! Input values in the yellow cells for CFM, hood, number of 90's, wyes, and total feet of straight pipe, FOR EACH SIZE. Run the calculation for only one branch, from the hood to the collector. For another branch, input those quantities and lengths. Input 1 for the hood duct size - under the hood size only. INCLUDE in the total footage the centerline through the fittings. For a 45 elbow, use half of a 90. Example - two 90's and a 45 is 2.5 90's. For a wye branch, put it in the BRANCH size, do not input it for airflow straight through the body. Return can be added in to each run and it will be included in the total drop. See the sample problem. Target velocity for dirty air is 3500 - 4000 FPM. For clean air, 2000 FPM or less. CFM: 800 Ducting: 4" DIA: 5" DIA: 6" DIA: 8" DIA: 10" DIA: 12" DIA: VELOCITY, fpm: 9167 5867 4074 2292 1467 1019 HOOD: # of 90's: # of Wye: Ft. of PIPE: Ft. of FLEX Other: Trashcan Separator S.P. 1 ( Use 2.5" ) Cyclone S.P. ( Use 4.5" with Wood & Similar, 3.5" with neutral vane, 3.0" with air ramp ) Filters S.P. ( Std. Bag =2.5", <5 Micron Bags=1", 400+ sq ft cartridge filters=0.25") Muffler S.P. ( muffler=0.15" ) Other S.P. Note: if over 12" you need to use bigger TOTAL SP LOSS: 1.00 inches WC diameter pipe! SUBTOTAL LOSSES: 4" DIA: 5" DIA: 6" DIA: 8" DIA: 10" DIA: 12" DIA: Hood: 0.00 0.00 0.00 0.00 0.00 0.00 90's: 0.00 0.00 0.00 0.00 0.00 0.00 Wye: 0.00 0.00 0.00 0.00 0.00 0.00 Pipe: 0.00 0.00 0.00 0.00 0.00 0.00 Flex: 0.00 0.00 0.00 Example problem: For starters, you need a sketch of your shop, with the tools in place, and the centerlines of all the ducts run. The object in layout is to create the shortest runs, with the least number of bends and with the right sized ducting. In this example, we have a 20 x 20 shop, with the main line running up the center from West to East. At the East wall, the main turns south, runs 10' to the S wall. Then W and down, to a 45 elbow into the cyclone intake. The cyclone is located 5' from the SW corner, and the S wall. Next, for each machine, we need to see what CFM is required for each machine and the sizes for each machine's exhaust duct. We get this information from the EXHAUST REQUIREMENTS FOR WOODWORKING page: Click here to see the Exhaust Requirements for Woodworking Document Click here to see the Exhaust Hoods samples If there will be return duct, it can be input in the 8", 10", or 12" size as you wish. Simply select the size to run at 2000 FPM or less with your max CFM machine. Leave it in each branch calc. The same for a cyclone or muffler. We'll size the main run first, so we start by selecting the machine with the largest CFM requirement. At the end of the main is the largest CFM requirement - for our case, a 12" single bladed planer. The planer requires 785 CFM. Input 785 CFM in the spreadsheet, and LOOK AT THE DUCT VELOCITIES. Target velocity is 3500 - 4000 FPM. We will need a 6" hood and main line. The planer sits at the end of the main run, 15' from the E wall. So the duct runs up 5' to the main run level, E 15' , S 10', turns down and W with a 90, into the final 45 to the cyclone inlet. That’s a total of one hood entry, three 90's, one 45, and 35' of duct. Enter one hood, 3.5 90's, and 35'. Total static pressure, 3.4". Note that at the machine on your sketch. Now we'll size a branch. On the middle of the N wall we have a Radial Arm Saw. 500 CFM per the exhaust requirements, so input that in the CFM spot. LOOK AT THE DUCT VELOCITIES! Note that the 6" velocity is too low, and the 4" is too high. We'll need 5" duct to the RAS. Now we'll size a branch. On the middle of the N wall we have a Radial Arm Saw. Per the exhaust requirements, it will need 500 CFM so input that in the CFM spot. LOOK AT THE DUCT VELOCITIES! Note that the 6" velocity is too low and the 4" is too high. We will need 5" duct for the branch. Per the detail shown in the EXHAUST HOODS SAMPLES PAGE, the RAS hood turns up on the wall with a 90 degree, goes 5' up the wall, turns S 8' into a 60 degree elbow (READ THE ASSUMPTIONS AGAIN - that's a 30-degree branch entry! ) and another 2' into a wye branch at the main, for a total of 15' through the fittings. In the 5" size column input 1 hood, 2.66 90's, a wye, and 15' of pipe. In the main, from the wye it runs 8' to the E wall, 10' S as before, 5' down and W as before, into the cyclone. That's no hood, 2.5 elbows, no wye ( it's in the branch ), and 23' of pipe. Input those numbers. This completes the run, and the total static pressure is 2.79". Note that on our sketch, next to the CFM for the RAS. Repeat this process for each machine. Try to maintain 3500 - 4000 FPM velocities, and eliminate as many fittings as possible. After repeating the process for each machine, choose the highest SP loss machine. ADD TO THAT LOSS the loss of the cyclone or other collector, any filters installed, and any other losses in the spots provided. In this example, we have one run at 785 CFM @ 3.4", and one run at 500 CFM @ 4.2". SELECT YOUR FAN FOR THE HIGHEST CFM REQUIREMENT AT THE HIGHEST LOSS REQUIREMENT. So we add the other 2.5" losses to 4.2" SP for a total of 6.7" and select a fan for 785 CFM @ 6.7" SP. No matter that the higher loss might be in a lower CFM branch - the fan will need to be able to meet that pressure drop. One more thing. Given that the assumptions at the top of the page are applicable to you, THIS SHEET WILL TELL YOU WHAT THE LOSSES ARE; IT WILL NOT TELL YOU WHAT IS REASONABLE!!! For example if you input 10000 CFM, the 4" velocity goes to 114592 FPM, which is 1302 MPH. And the hood entry alone in a 4" branch goes to 1220" water, which is 44 PSI.
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