C) Determine Diameter of Main Duct

1. Determine which machines are your primary machines. A primary machine is the machine(s) that will operate at the same time under the worst conditions. (If you normally operate two machines, but once a week need to operate a third machine at the same time, then you must size your system for all three machines.) We generally highlight the primary machines on the drawing.

2. Sizing the Main Trunk Line. When sizing the main truck line start with the primary machine farthest from the duct collector. Run that size duct until the next primary branch enters the main. Increase the main size at that junction to accommodate the CFM total of the two primaries. You will follow this practice all the way to the collector, sizing all primary junctions, to accommodate total CFM of all primaries at that point. Do not increase main duct size when a branch other than a primary enters. Your total CFM requirement is the total of all primary branches. When not using a primary machine you will close blastgate and divert suction to a secondary machine.

A 4” branch will be run from the Table Saw until it joins with the 4” branch from the Lathe. At this point your main starts and you need to increase the pipe to handle the combined CFM (350+350 = 700). Using the CFM Chart 1 look up 700 CFM under the appropriate velocity (3500 FPM in the main for wood dust), then look at the corresponding diameter (6"). Run 6" pipe in the main from the Lathe until the branch of the Radial Saw joins the main.

Here again you need to increase your main to handle the total CFM (700+550=1250 CFM). Using the chart again you will see that 1250 CFM is slightly more than volume for 8” diameter. Drop back to 8” diameter so as not to go below transport velocity. Run the 8" duct in your main from the Radial Saw to your Dust Collector.

If you are installing an indoor re-circulating dust collector you need not calculate any more duct diameters. If you are attaching ductwork to the exhaust side of your duct collector it is accepted practice to use a duct diameter two diameters larger on the exhaust side than on the inlet side, thus minimizing exhaust and duct resistance.

D) Figure System Resistance (SP)

The total static pressure is several factors added together. They are entry loss, dirty filter loss, static pressure of the worst branch duct, static pressure of the main duct, and static pressure of the return duct.

1. There are more complicated ways to figure the entry loss of your system, but we find it usually equals a loss of 1" water gauge. (Use 1" as a constant.)

2. If your system has filters, add in 2" loss. (If you do not have filters add zero).

3. The Worst Branch, is the branch with the greatest resistance. The branch with the greatest resistance is usually a smaller diameter with the most lineal footage of pipe elbows. Static pressure of worst branch and main duct can be calculated by using the following Chart 2. Chart 2 is based on 100 feet of pipe; therefore, you have to convert all elbows to an equivalent of pipe.  To convert 90^o and 45^o elbows to equivalent feet of pipe use Elbow to Straight Pipe Conversion chart (see chart, click here). When figuring the feet of pipe count lateral type branches as 45^o elbows. Flexhose has a lot of resistance depending on the amount of bends included in the installation. For this reason we suggest you keep hose to a minimum.  Multiply your length of flexhose on your worst branch by 3 for equivalent length of straight pipe.

3. If clean air return duct is required, duct resistance should also be calculated. Now you have all the information you need to make an educated decision in purchasing your dust collector. You have determined the Velocity, CFM, Static Pressure and the size of the ductwork. To develop your list of materials required, go through the system; this time starting at the dust collector and list each part you will need. Don't forget the assembly equipment such as: pop rivets, hangers, strapping, caulking, and couplings. To order please call: (800) 367-3828; fax: (800) 438-7135; or order on-line at www.airhand.com. If you have any questions while you are designing your system give us a call.

For additional information on designing your Air Handling System, we recommend reading Woodshop Dust Control, by professional woodworker Sandor Nagyszalanczy. Sandor provides practical, shop-tested solutions to total dust control so you can build the right system without complex calculations.

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