Flow Rate Dropoff

I’m starting to work on a collection of different air flowrates in the DC system, to see the effects of different fittings, fixtures, and layout on the resulting flowrates.  With this set of data, you can start planning a dust collection run which takes into account the power of the dust collector itself, and just how much you can add to it / distances you can run it and still get an adequate collection rate.

Some initial figures that came out already have me rethinking the location of the dusty itself.  I have it in a separate shed, partly space, partly so any dust escaping doesn’t end up in my breathing air (although with a 1 micron filter on it, that is not really a concern), and the main reason is the noise!  The bigger the unit, the noisier it is.  Air flowing through pipes is noisy enough, let alone hearing the suction unit.  However, the dropoff in performance is remarkable over the distance of run that I have, so I’m wondering now whether I should just bite the bullet, accept the additional noise and have it relocated back to the main shop.  Having it there has the advantage that I can easily turn it on and off as I need it, and clearing blockages does not require a expedition to the other shed.

Hmm, decisions decisions.

The figures that I got on the first couple of DC runs were:

Running the dust collector through the 1st stage collector, and measuring near the tablesaw collection point: 16.1 m/s

Running the dust collector through all the same pipes, but with the 1st stage bypassed: 19.3 m/s

So there is a bit of a loss (expected) of about 15% caused by the 1st stage collector.  That is probably not too bad, so long as you have that extra capacity spare of course.

However, when I measured the flow directly at the DC, I got a reading of 34.3 m/s – double the flowrate that I am currently achieving at the tablesaw.  That is significant, and although I don’t expect to achieve a perfect flowrate, I’m not sure if I should accept such a dropoff.  So my trial is going to be to return the DC to the main shed, and connect it into my fixed collection system with straight PVC piping, rather than having about 2m of the flexible tube at the end that I have at the moment.  Future tests will show just how much of a difference that makes, but I’m just interested in seeing if it makes a significant increase in the TS collection flowrate.

Hmm, much more testing required!

7 Responses

  1. What size pipe are you using for the main run?

    Id consider trying 6″ if your using 4″

  2. It’s all 100mm (4″) – majority is downpipe PVC, with the standard DC flexible hose over short runs to each machine from that. Each machine has a blast gate obviously.

    Do you have some experience with 6″ trunking being more effective on a 4″ collector?

  3. I agree with going with larger pipe, and I’m betting the PVC approach will definitely cut down on loss. Those flexible pipes have a major disadvantage with all of those striations inside (e.g. the wire-wrap) that restrict airflow. Having an interior with a smooth surface, PVC should move the air without the suction loss of flexible ducting. Actually, I’m sure much of this has been worked out for HVAC installations. In my house, all of the ducts are flexible, and this defintely takes a toll on the output pressure at the end of a long duct. I have read that metal ductwork doesn’t have this disadvantage.

    Another thing is to try to minimize bends. Why not run a 6″ PVC pipe along the roof of the shed (e.g. over the collar ties), and then run your flexible ducting down to each piece of gear from above. It won’t look as nice as having it strung up along the wall, but I’m sure it would be more efficient.

    • If you see my earlier video on the current dust system, the majority of the dust run is already laid out and functional (and is based around a straight run of 100mm PVC pipe).

      However there have been some interesting conversations about larger pipe, and the inlet to the DC has 2 x 4″ connectors. Instead of running all the dust to a single one, I might at least use the 2nd connector for the thicknesser or something.

      The only flex components in place are short runs between the machine and the PVC ducting. (And the current final run to the dusty, which I considering relocating to remove its necessity).

  4. Stu,
    My wifes family has been in timber for a few generations now. Her Grandfather claimed it was easier to lift dust and chips vertically than through a horizontal run. The clear implication is that the friction of the horizontal run has an impact and, therefore, I’d assume that a PVC pipe will have a measureable effect to throughput.

  5. Stu,
    I’ve been toying with an idea of how to deploy my DC. DCs have long leads to the machines and a short pipe to the bags. Maybe this should be reversed. I am thinking of placing the DC very close to the machines and pushing the output outside of the shed. Maybe have a cyclone next to the DC and then just carry the fine dust to bags outside. Not sure if it would work but maybe it worth an experiment. Have you ever checked the airflow on the output of the DC to compare with the inputs>

    • Different fans are designed in different ways – some designed to create suction and airflow, others to be more of an inline unit (blowing and sucking).

      I would be very hesitant in thinking that the fan on a DC would be good for downstream airflow.

      However, the idea is not bad – have you considered an amalgamation of your ideas though? Keeping the dust extractor unit near the wall where you plan to exhaust the fine dust entrained air, and use the preseparator near the tool. Heavy chips are collected early, so they don’t need to be carried along the pipes, and the exit from the preseparator then heads off to the DC with only the lightest of dust to carry.

      Might work very well.

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