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There are two phases to designing your dust collection system:

The first phase is sizing your duct work for adequate volume and velocity of flow for the type of dust you will be creating

The second phase is computing the static pressure (SP) of your system to determine the size and power of your dust collection unit.

Prior to Making Your Calculations:

Diagram the floor plan of your shop to scale on graph paper. Include the size and location of each machine, and the location of its dust port or outlet; the floor to joist dimension; the location of the dust collecting unit; and the most efficient (fewest number of turns or bends) path for routing your duct lines. This is also a good time to start your take-off list of duct components for your system.

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Dust Collection System

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You will also need to familiarize yourself with the following concepts:

  • CFM (Cubic Feet per Minute) is the volume of air moved per minute.
  • FPM (Feet per Minute) is the velocity of the airstream.
  • SP (Static Pressure) is defined as the pressure in the duct that tends to burst or collapse the duct and is expressed in inches of water gage (˝wg).
  • VP (Velocity Pressure), expressed in inches of water gage (˝wg), is the pressure in the direction of flow required to move air at rest to a given velocity.

CFM is related to FPM by the formula CFM = FPM x cross-sectional area (ft2 ). FPM is important because a minimum FPM is required to keep particles entrained in the air stream. Below this minimum FPM, particles will begin to settle out of the air stream, forming clogs—especially in vertical runs. Table 41-1 (see below) shows the minimum FPM that Spiral Manufacturing recommends for several types of dust in branch and main runs.

Step 1

From the Table 41-1 determine the velocity (FPM) of your system for the type of dust that will be produced. For the purpose of the following examples assume woodworking dust. Wood dust requires 4500 FPM in branches and 4000 FPM in mains.

Table 41-1:  Velocity for Types of Dust
Type of Dust Velocity in Branches (FPM) Velocity in Main (FPM)
Metalworking Dust 5000 4500
Woodworking Dust 4500 4000
Plastic/Other Light Dust 4500 4000

 

Step 2

Determine the diameter of each branch line. You can use the diameter of a factory installed collar or port, or consult the manufacturer. Convert metric ports to the nearest inch. Convert rectangular ports to the equivalent round diameter. Ports less than 3” will require a reducer to 4”. Record any reducers or rectangular to round transitions on your take off list.

Step 3

Using Table 41-2, determine the CFM requirement of each branch. Remember the FPM for wood dust in branch lines is 4500.  Example:

  • Table saw 4” dia. 390 CFM (rounded)
  • Planer 5” dia. 610 CFM (rounded)
  • Lathe 6” dia. 880 CFM (rounded)

Continue for all branches.

Table 41-2:  CFM for Pipe Diameter at Specified Velocity
Diameter 3500 FPM 4000 FPM 4500 FPM
3″ 277 316 356
4″ 305 348 392
5″ 477 546 614
6″ 686 784 882
7″ 935 1068 1202
8″ 1222 1396 1570
9″ 1546 1767 1988
10″ 1909 2182 2455
12″ 2749 3142 3534
14″ 3742 4276 4810


Step 4

Identify your primary or high-use machines. These are the machines that operate simultaneously on a frequent basis. The objective here is to define your heaviest use scenario so you can size your system to meet it. Including infrequently used machines and floor pickups in your calculations will only result in an over-designed system that will cost more to purchase and to operate. At this point, all of your branch lines are sized, and you have a list of all components required for your branch lines.

Step 5

Now you are ready to size the main trunk line. Begin with the primary machine that is furthest from where you will place the dust Table Saw (Primary) Radial Arm Saw Dust collecting unit. In our example, this is the table saw, which has a branch diameter of 4”. Run this 4˝ Spiral pipe to the point where the second primary machine (the planer on a 5˝ branch) will enter the main. (Note: If a non-primary machine or pick-up is added to the system between primary machines, the size of the run is not increased.)

You now have a 390 CFM line (table saw) and a 610 CFM line (planer) combining for a total of 1000 CFM. Using Table 41-2 again, you will see that for 4000 FPM (the velocity requirement for main line that you determined in Step 1) the required pipe diameter falls between 6” and 7”. (Note: Spiral Manufacturing recommends that you round up to 7”. This not only assures adequate air flow but also anticipates a future upgrade in machine size.)

Now calculate for the addition of the third primary machine (the lathe on a 6˝ branch). You have an 1000 CFM main + an 880 CFM branch line (for the lathe) for a total of 1880 CFM. Using Table 41-2 once again, 1880 CFM at 4000 FPM requires between a 9” and 10” pipe. We recommend rounding up to a 10” main after the addition of the lathe. The main going to your dust collecting unit will be 10”, and your dust collection unit must be capable of pulling 1880 CFM through a 10” duct at 4000 FPM.

Figure 41-1

Dust Collection System

Step 6

In this step, you calculate the Static Pressure (SP) or the resistance of your system that your dust collection unit must overcome. Static Pressure is measured in inches of water gage (˝wg). To do this you total the Static Pressures of the following system component groups:

1) The branch line with the greatest SP or resistance (see Figure 42-1). Calculate the SP of all branches to determine which has the greatest SP. Only the branch with the greatest SP or resistance is added to the total.

2) The SP of the main run (see Figure 42-2).

3) The SP for the collection unit’s filter, if any, and for the pre-separator, if any (see Figure 42-3). (You can use the charts on pages 51-60 to assist in your calculations).

Summing the SP loss for the system, we have:

  1. Highest loss branch: 5.17
  2. Loss for main: .90
  3. Filter loss: 1.50

Total SP loss (˝wg) loss in the system): 7.57

You now have the information you need to specify your dust collector. Your dust collection unit must provide a minimum of 1880 CFM through a 10” duct at 4000 FPM, and have a static pressure capability of no less than 7.57 (˝wg).

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The above example is for a small system with few variables. It is recommended that for larger systems a professional engineer be consulted to assure that the system is properly designed and sized.

If the dust collector is located in a separate enclosure, it is essential to provide a source of make-up air to the shop to prevent a down draft through the flue of the heating system. If this is not done, carbon monoxide poisoning could result. If a return duct is necessary from the dust collector, it should be sized two inches larger than the main duct entrance and its SP loss added into your calculations.

Some dust collection units may not include fan curve information that shows CFM or Static Pressure variables. We do not recommend procuring collector equipment without this information.

Blast gates should be installed on all branch lines to maintain system balance.

Dust suspended in air has a potential for explosion, so it is recommended that you ground all of your duct runs, including flex-hose.

If your system has areas where long slivers of material could possibly hang-up and cause a clog, install a clean-out near that area. Many types of dust, including many woods are toxic, so take special care to choose a filtering system that will provide optimal safety.