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If you are installing an indoor recirculating dust collector you need not calculate any more duct diameters. If you are attaching ductwork to the exhaust side of your dust collector it is accepted practice to use a duct diameter two inches larger on the exhaust side than on the inlet side, thus minimizing exhaust and duct resistance.
TOTAL CFM = Static pressure of two 4" branches (350 CFM each) and one 5" branch (550 CFM) pulling vacuum simultaneously = Total 1,250CFM.
- 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 main duct, and static pressure of the return duct.
- There are more complicated ways to figure the entry loss of your system, but we find it usually equals a loss of 1" watergauge. (Use 1" as a constant).
- If your system has filters, add in a 2" loss. (If you do not have filters add zero).
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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 and elbows. Static pressure of worst branch and main duct can be calculated by using Chart 2. Chart 2 is based on 100 feet of pipe; therefore, you have to convert all elbows to an equivalent of pipe.
| Chart 2 |
| Static Pressure based on 100' of Pipe. |
Elbow to Straight Pipe Conversion |
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To convert 90o and 45o elbows to equivalent feet of pipe use Chart 2. When figuring the feet of pipe count lateral type branches as 45o elbows. Flexhose has a lot of resistance depending on the corrugation. 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.
- 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. $21.95, (see page 15).
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