Dust Collection System Design

      Duct choice and duct sizing are key elements of a working dust collection system for a woodshop. June 15, 2014

Question
We used to have a really nice shop with a really nice dust collector that met our needs. It had two hoppers, one for heavy stuff and one with bags that had a shaker that caught fine stuff. We lost it in a tornado with the rest of the shop. We are in a temporary shop right now using three portable dust collectors that don't meet our needs. They are working right now on an 80x80 addition that will eventually be incorporated into the shop when it is finished.

I would like to abandon our current dust collection and upgrade to a unit similar to what we had. I have no idea to go about this. I am sure there is some vendor that could look at what we have and what we do and tell me what we need. There are probably some used units somewhere that we could get a good deal on that I wouldn't mind looking at. Anyway, I figured rather that shoot blindly at this I would ask for a little direction to get from where we are to where we want to be.

Forum Responses
(Dust Collection and Safety Equipment Forum)
From Contributor C:
Where are you located? How many and what machines are you pulling dust from? I'm sure there are some used collectors on the market today. There have been a bunch of shops go out of business or downsized in the last few years. Sounds like you probably had a cyclone collector with rotary air lock on the first stage and a bag house on the second stage. Cyclones separate the large chips from fine dust which can fall out of the bottom into a hopper with the addition of a rotary air lock. The fine dust then exits the cyclone and moves into a bag house which filters the fine dust into bags. This is probably what you are looking for.



From contributor F:
There is a glut of used bigger dust collectors out there these days. I think the problem you'll have is most companies that can help you design and set up a system are going to want to sell you something they carry. If you know what size and type of collector you'll need you can make out pretty good with used stuff. I used a local company called Dust Technology Inc. to do my setup, but they work all over the country. I already had my collector so they designed and provided the ductwork for me.


From the original questioner:
We are in Greensburg, KS. I put our shop in the Shop Gallery the way it is now. We have two table saws, a jointer, bandsaw, edgesander, spindle sander, wide belt sander, two planers, radial arm saw and mitersaw. We also have a Ritter pocket screw machine but it doesn't have a dust port so we had a floor sweep near it. Our old shop had a couple pipes running down to a couple work benches for sanding. Oh, and a router table, but may move up to a shaper in the next couple years. I don't have a problem installing it myself, but wouldn't mind getting a bid on the whole deal to see if it would be worth my time.

From Contributor J

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Now, if I may suggest, all of the previous posts are good advice, but do not purchase a collector before designing the system. While this is common practice, it can sometimes cause you to end up with a terrible system. It sounds like Contributor F did well with his (I would guess by design). The people at Dust Technology know their craft well and I would not hesitate to recommend them also.

A few things I would warn you about. Don't hire a heating and air company, unless they have extensive experience with dust collection. Positive systems and negative systems are two different fields, completely. Also, don't use HVAC ductwork for dust collection, unless it has a heavy wall. The typical thin wall you have in your home will collapse if put under too much vacuum, and the branches, elbows and Y's are less efficient. Next, do a layout of your facility. Note the placement of the machines and what machines you expect to have in use at any given time. Why? Because proper dust collection is all about CFM (this is why you don't buy a collector before designing the system).

I will try to explain this to the best of my ability, so, please no one take offense if I sound condescending. Let's take the following. Machine 'A' has a 4" port. Only a given quantity of air will travel through a 4"opening at a specific CFM. Ok? This is crucial. If you have a 4" port and you connect it to a 2 1/2 " shop vac you will only pull 2 1/2" of air - obviously. If you connect the same port to a dust collector that has a 6" port, you will only pull 4" of air. Great! Dust collectors with a 6" port pulls more CFM, and therefore, the pickup should increase at machine 'A'. Wrong! Big Wrong! What happens is, only the maximum CFM for a 4" opening will feed the dust collector. If the difference is too great, you will starve the collector of air, and in effect vacuum lock the collector.

How to you determine this? Using basic math we need to find the square area of the given port in question.

The formula is (radius squared x PI)
4" machine port: 2 X 2 X 3.14 = 12.56 sq inches
6" dust collector: 3 X 3 X 3.14 = 28.26 sq inches

In the above example, we supply the dust collector with less than half the air necessary for it to work at it stated CFM. Why is this important? Try running a marathon breathing through a straw. Sure you can do it, but it doesn't work well.
This is where it gets tricky.

Next, letís assume we have machine 'B' it also has a 4" port. Adding the CFM from machine 'A' and 'B' gives us a requirement of 25.12 square inches. Anytime you add CFM load to a system, you must either increase the diameter of the combined piping or restrict that air somewhere in the previous system.

I try to make my increases roughly +/- 10% of the next pipe size. So, following our example 4" + 4" = 6"
12.56 + 12.56 = 25.12
6 square inches = 28.26
Two 4" ports added together will require a 6" port to continue with the correct CFM.

Or, you can restrict the system by the same amount as the expansion. A blast gate closing either of the 4" ports will allow you to continue with a 4" pipe. This is where planning for the use of the machines is critical. This process is followed through your facility all the way back to your dust collector. Add a machine, expand the pipe, add a machine, and expand the pipe again. Once you arrive at the collector, this pipe diameter will determine the collector size.

If you intend to expand the collection to new machines in the future, you need to account for them now and estimate their requirements and the placement within the system, then leave openings within you designs for this. This is literal, you leave an open hole within your wall of your pipe (usually protected with some sort of screen and/or partial cover). Ok so far?

The next issue to conquer is pipe size. When we expand, we only want to expand the requirements at that location within our system.

For example: Three machines. Machine A and B are side by side. Machine C is 12' away and the collector is 12' beyond that.
Mach A = 4" (12.56")
Mach B = 4" (12.56")
Mach C = 8" (50.24")
Taking the total of the 3 we have 75.36 square inches. Doing the reverse math, it requires a pipe of 10" to deliver the correct CFM from the machines to the collector.
75.36 / 3.14 = 24

Square Root of 24 is close to 5". So, we have a 5" radius or 10" diameter pipe. If we use a 10" pipe to run completely from the collector to machine A and B, We will create a lack of CFM immediately after the branch to machine C. This is the exact situations we observed in our first example of using a 2 1/2" shop vac. This lack of CFM will cause the velocity of the air to slow and any dust particles suspended will fall and settle within this pipe. This will continue until the pipe fills and restricts the pipe to a point in which the CFM rises enough to draw the dust away.

The downside to this is that dust is heavy. This creates an overhead safety issue and I have seen piping fall from a ceiling. It is not pretty. It's also a fire hazard. I hope this helps you and I didn't go off in some direction you didn't care about. My hope is that it at least gives you some idea of the complexity that goes into what you want to do.



From contributor J:
There is a much easier way to size the ducting. You just need inlet size and required velocity. Code states minimum of 3,500 FPM, but I normally size it based on 4,500 FPM to ensure there is no settling in the ducting. Also, sizing the ducting using the Constant Velocity Sizing method prevents settling & ensures balanced airflow at all drops.



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