Thread: Fletty's new shed WIP ....or is it a retrospective??

Originally Posted by Chris Parks

Its flow limits are too low for this purpose despite what the salesman says.

It is a proven fact that all generic dust extractors sold with no exception never flow the amount of air that salesman tell you they will.

Originally Posted by GraemeCook

Their airflow figures is a theoretical (read fictional) figure calculated without any ducting or filters in place.

So is the power rating:
"How much power you want us print this week? We do 1, 2, 2½, even 3 hp. Even better - cost no extra - no change to motor! 1hp, 3hp all same same!"

I think that all the figures they try to sell us are theoretical BS and wishful thinking at 60hz not 50hz. For those who are now truly befuddled and heading off to get a pill for the migraine that is developing I best explain. An electric motor has its speed controlled by the electricity supply and the frequency per minute that supports that supply. In Oz our supply is done at 50 hz a minute whereas the Americans where most of this equipment is sold use 60hz a minute. The 50hz supply means a motor (as a rule) will turn at 2850 revs per minute and at 60hz that same motor will turn at 3600 revs per minute so the different speed at which the dust extractor works in both situations is hugely different and very important.

Originally Posted by FenceFurniture

What Chris is trying to say is that using 90mm sucks......no, no....it doesn't suck! To be clear - its suck sucks.

Cross sectional areas:
90mm is 63.64 cm²
150mm is 176.78 cm² which is 2.77 times bigger than the 90mm.
That means it will carry 2.77 times the volume of air at any given speed. Even going to 100mm represents a 23% increase in air flow. .

Cross sections areas alone will significantly underestimate the differences in flow so its better to forget about area and look a flow charts like this
DRAFT: FAQ - Dust Extraction (Theory).

What happens is the the wall circumference to cross sectional area ratio is much greater for narrow pipes than for wide pipes which generates far greater wall friction in smaller pipes.

At 8" of water column pressure the nominal air flow rates for
90 mm ducting is about 300 CFM
100 mm ducting is about 400 CFM
150 mm ducting is around 1200 CFM

So 100 mm duct duct carries ~1/3 (33%) more air than a 90 mm duct
150 mm carries 3 times (300%) more than a 100 mm duct and 4 times more than a 90 mm duct.

Thanks Bob but I think we need to keep it simple, going into that stuff is going to cause confusion at this stage. The pipe is bigger as can be seen by comparing the two pieces and it flows more, 90mm is simply too small to support the sort of flow needed to do a good job of extracting the small micron particles. The complicated why is it so stuff can come later.

Here is a thought, let's male it dust extraction without mathematics. It will help if the OP chimes in and asks questions also.

I understand what you are trying to do but I didn't introduce any maths until FF did and my response was more for his benefit.

The main issue I see time and time again is that most folks think that as there's only 10% difference between the 90 and 100 mm (it's 11% but lets not go there) so there's only a 10% difference in flow. Telling them that there's a larger area and hence a larger flow difference without the maths doesn't seem to convince them either - they still thinK its only a 10% improvement.

In my experience and I have done this many times over the phone, every customer needed to have it explained and maths was never used. I simply told them what was needed and that is what happened. I have most probably complicated it more than necessary already, simply put, use 150mm pipe and if the dust extractor won't support flow through that size buy a bigger dust extractor. My advise to the disbelievers was get the pipe and compare them and they always came back and said they could see what I meant.

All good from my perspective - the maths needs to be correct (thanks Bob), but I also agree that explaining it without maths is very worthwhile for those who are not bent that way. Both approaches are valid.

Originally Posted by BobL

So 100 mm duct duct carries ~1/3 (33%) more air than a 100 mm duct

Just a typo in there that needs to be edited for clarity.

Ian needs to step in and tell us if we have made it any clearer, some people find it so confusing they simply can't get into the subject and I understand that. What we take for granted and forget to explain is sometimes the missing piece(s).

It's a pity pipe diameters has come up in this thread. This is such a good thread about Fletty's shed and all the posts about duct diameter should be moved to a thread in the Dust forum.

Originally Posted by BobL

It's a pity pipe diameters has come up in this thread. This is such a good thread about Fletty's shed and all the posts about duct diameter should be moved to a thread in the Dust forum.

Yes I agree that all of this current discussion should really be moved to another thread - I was actually going to say the same thing a day or two ago.

Good call.

I ran a 3m 100mm offshoot off my 150mm main and did some flow testing. Should I post this in the genetic 2hp thread or the new thread (if there is one)?

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Joy is when the early morning Sun shines upon your bench....

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FRUSTRATION is when you can't do a bl**dy thing with it!

Joy is when the photos are not loaded sideways.