Dust Extractor Efficiency

I’ve done some preliminary proof-of-concept testing to see if the anemometer I’ve bought is going to provide the sorts of readings that will be useful, and valid.

The anemometer I have can provide airflow rates from 1m/s to 30m/s (200ft/m to 6000ft/m)  When used to measure flow rates in 4″ trunking, this equates to 14cfm to 425cfm (given that dust extractors all seem to be sold with their cfm listed).

My initial trial was using a 1HP / 750W GMC portable dust extractor, with a rated flow rate of 470cfm. My first trial gave a flowrate of 26.7 m/s, or 377cfm. So that seems to validate the initial test (assuming my maths stacks up), but it does seem to reveal a couple of things.  Firstly, the rated airflow of this dust extractor seems to be 20% higher than as tested, and secondly, this isn’t even with the resistance back pressure of the dust bag.  I’m sure when they supplied the flowrate, they assumed you wouldn’t ever actually use the unit with a pointless dust collection bag.

Dust Extractor and Concertina Hose

Dust Extractor and Concertina Hose

Next, I tested the airspeed with a concertina hose attached – 650mm compacted.  The airflow dropped to 346cfm, or a loss of 8%.

Then I expanded the concertina hose to its full 2650mm length, and tested again. The airflow dropped again to 274cfm- a loss of 21% over having the concertina hose closed up.  This demonstrates that there appears to be a reasonable correlation between performance and resistance.

Concertina Hose Expanded

Concertina Hose Expanded

Finally, I coiled the expanded hose through 1x 360 degree loop, and got 195cfm (29% loss over straight hose). So the initial tests seem to hold true.

Next, I tried testing the output airflow from the air cleaner.  The first test was with the filters in place, and not cleaned, and got 8.3m/s  After cleaning the filters (simply banging the dust out), the airflow increased 14% to 9.45m/s  This may be a useful figure in the future – assuming the filter was reasonably clogged at the point I ran this trial, I could in future quickly test the output airflow, and use this figure to determine if the filter needs cleaning.

My main dust extractor is rated for 1200cfm, so well above the max rating of the anemometer.  By the time it gets through all the tubes to the machines, that will probably have dropped well into the range of the anemometer.

My next set of tests will be about real-life dust extraction runs.  I want to know just how expensive (on performance) a 90 degree turn costs, a metre of tubing, the difference between a metre of smooth pipe vs flexible dust pipe, the flow rate loss when reducing pipe size from 4″ to 1″ etc. Finally, it will mean that I will be able to test the condition of the pipes – whether they are getting clogged with uncleared sawdust (woodwork cholesterol).

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