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Upgrading Dust Extractor Motor. Good/Bad Idea??

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PearlyKing

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I have an old Record Power DX750 dust extractor ( 550W, 2800rpm, 650m3 per hr). It was fine when connected directly to a machine. However, I have now installed some 100mm ductwork, about 10mtrs, and it doesn't seem to be as efficient. I expected some drop but it seems a lot. I have already checked all the ductwork seals, blast gates etc and straightened the runs as much as possible.

My question is, would I see any improvement by upgrading the motor to 1 or 1.5kw, same rpm? I cannot get a bigger impeller due to cabinet size. I am sort of thinking there is no benefit as the rpms would stay the same, or have I got that all wrong?
 

Rorschach

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Assuming your RPM's are remaining constant, changing the power of the motor will have no effect. If you are getting an drop in RPM's though then you would benefit from a power increase.
 

PearlyKing

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Assuming your RPM's are remaining constant, changing the power of the motor will have no effect. If you are getting an drop in RPM's though then you would benefit from a power increase.
Thanks. That's what I thought. Back to the drawing board.
 

clogs

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just get a bigger unit.....
I have a low air speed high Volume for the planer (big tube100-150mm) and High air speed low volume for the router bench 50-75mm.....
 

Inspector

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You have 2 problems. First being the small DC and the second being the 100mm duct. The small DC hasn't got the grunt to pull the maximum flow over that distance and the 100mm duct limits the amount of airflow to 680 cubic meters per minute no matter how powerful a DC you have.

Pete

Error on my notation so the above should read 680 m3/hr.
 
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PearlyKing

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You have 2 problems. First being the small DC and the second being the 100mm duct. The small DC hasn't got the grunt to pull the maximum flow over that distance and the 100mm duct limits the amount of airflow to 680 cubic meters per minute no matter how powerful a DC you have.
Thanks, I hadn't thought of those sort of physical constraints. Someone pointed me to a website run by Bill Pentz that I am now slowly working my way through.
 

Inspector

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@Inspector 680m3 a minute seems a lot, no?
I converted 400 CFM and I might have written it down wrong, Should it be 680 m3/hr? 400 CFM is optimistic and it usually flows 350 CFM (594m3) in most smaller DCs and less with longer ducts, elbows, hose and small machine ports, etc.

Pete
 

DBT85

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I converted 400 CFM and I might have written it down wrong, Should it be 680 m3/hr? 400 CFM is optimistic and it usually flows 350 CFM (594m3) in most smaller DCs and less with longer ducts, elbows, hose and small machine ports, etc.

Pete
Oh I have no idea at all, but 680m3 a minute is 60 times what pearlykings extractor claims to be able to do. I think most machines only seem to spec a minimum of around 1000m3/h while also only having 100mm ports.

On a quick Google hunt it seems 400cfm is 680m3h
 

Inspector

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Doesn't matter what they spec they can't deliver those volumes. The method they use to get those claimed numbers is to use a short test duct to the impeller that has no hose adaptor, hose or bags on it. Then they take a single reading in the centre of the duct where the flow is the fastest. If you take the readings with the bags etc on and at a number of points across the diameter of the duct and average them you will have about half the claimed flow rate. Now a 4"/100mm duct can only flow 400CFM/680m3h maybe slightly more with any DC unless it can pull the high vacuum pressure you get with vacuum cleaners and shop vacs and they don't come in powerful enough 4"/100mm sizes. So even a 5hp DC with a 15" or 16" impeller still can't coax more airflow. Move up to 6"/150mm and you can move 3 times as much air if the DC is up to it.

Pete
 

DBT85

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Sorry, Pete, I meant machines like table saws or P/Ts asking for 1000m3h while only providing 100mm ports, not DCs that claim they can do it.

Seems daft to spec a minimum airflow that can't possibly be achieved. I mention it as I just watched a new video from Keith Brown talking about his new HVLP extractor which he bought after buying a new Axminster table saw and them quoting the 1000m3/hr rate required for the saw. His existing LVHP extractors naturally couldn't get near it but from what you're saying nothing ever would given the 100mm port on the back of the saw.

My very old DX4000 dual motor LVHP claims around 380 m3h and is better than nothing but once the workshop is done one thing to do is buy a HVLP as required.
 

Inspector

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The port sizes are from specs decades old that were only concerned with the dust you could see, not the invisible dust we now know to be unhealthy. Tool makers are probably keeping them small because the average woodworker is buying the cheap small DCs. How do you tell a customer they need a big DC that costs more than the saw or thicknesser they want to buy? The other thing they don't do is allow enough air to flow into and through the machine to carry the dust out. Big straw or little straw if the cherry blocks it you can't suck up your milkshake.

Pete
 

Derek Cohen (Perth Oz)

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It is a path, with limits at each step.

Dust/particle size produced by machine > dust port size > hose size/wall smoothness >run length > impeller size > motor h.p. > exhaust filtering.

At each stage compromises are made. However, efficiency is determined by the lowest level of each step: all the grunt in the world will be throttled by small diameter hoses (anything under 5" ..... 5" hose carries double the air from 4" hose; 6" hose doubles 5", etc). Smaller impellers fail to utilise the potential of higher h.p.

One can use lower power, etc. However it is all about finding the best balance from the combination at your disposal: short runs, and at least 5" hoses with smoother walls.

Regards from Perth

Derek
 

OldWood

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Derek
I'm interested in your statement about 'double of air' - if you do pi x r^2 and percentage the increase in diameter, that is significantly below 'double'. I remember doing something about laminar flow at university 60 years ago, but is that why you can say 'double'?
Rob
 

J-G

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Derek
I'm interested in your statement about 'double of air' - if you do pi x r^2 and percentage the increase in diameter, that is significantly below 'double'. I remember doing something about laminar flow at university 60 years ago, but is that why you can say 'double'?
Rob
Similarly, the first thing I did on seeing Derek's post was to calculate the area and relative ratio's of 3, 4 and 5" rings and found that a 4" is 56.25% larger than a 3" and a 5" 44% larger than a 4".

It may well be that the relative areas are not the salient factor but I can't see why not.
 

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The flow is in part limited by the surface of the duct. The duct walls will have more drag than the middle of the duct. For illustration purposes and not actual numbers (because it is way above my level of education :rolleyes:) lets say the air is turbulent for half an inch deep against the wall. A 4" duct would then have a clear zone of happy air 3" in diameter with the 1/2" all around the duct being miserable and digging its feet. The 5" duct will have 4" of clean air an a 6" duct will have a 5" zone of clean air. The area of miserable air becomes a lower percentage of the total as the duct diameter increases. So a 4" duct will flow a max of 400CFM, a 5" duct is about 600CFM and the 6" duct flowing just about 900CFM. All of them based on a airflow speed of 4,500 FPM. You guys can do the conversions if you like. So Derek's numbers are out a touch but the principle remains. If you decrease the airflow by having a long duct with lots of turns and a lot of flexible hose your flows will drop. Just like you will do better with a big DC and shorter clean runs.

Pete
 
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Derek Cohen (Perth Oz)

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My percentages were not to be taken literally, but to get the message across. The point being that there is a significant gain to to be had with larger diameters and, conversely, significant reductions with smaller diameters. Keep in mind that this is part of the equation only.

Regards from Perth

Derek
 
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