walks like a duct, quacks like a duct, but doesn't look like a duct

[David Kumm]

We need someone to do a flow test and stop guessing. If flow drops over a length of duct then any impedance should be magnified? If a real world test says that two 45's make no difference then we have been led up the garden path so to speak.

Chris, while guessing is something I do best or worst,in this case I was reading charts from Air Handling Systems. They put out info on dust collection at www.airhand.com that I use as a reference. It has some handy charts about pipe and fitting sizes, velocity, cfm, and static pressure. I'm not nearly as scientific as Alan or Ian but do measure velocity and cfm at each machine port to insure I give myself enough room for error that error won't be fatal. Dave

[Alan Schaffter]

Chris, while guessing is something I do best or worst,in this case I was reading charts from Air Handling Systems. They put out info on dust collection at www.airhand.com that I use as a reference. It has some handy charts about pipe and fitting sizes, velocity, cfm, and static pressure. I'm not nearly as scientific as Alan or Ian but do measure velocity and cfm at each machine port to insure I give myself enough room for error that error won't be fatal. Dave

On the 90 vs dual 45's, it depends on the radius of both. Since we are talking PVC plumbing pipe, I suspect the radii are close if not the same for both and that the difference between 2 45's and a 90 is likewise minimal- since there are less turbulence causing joints a 90 might actually have less impact on SP if measured with precision instruments. The goal should be to do the best you can with what you have and avoid stupid or bad configurations based on faulty reasoning or incorrect information.

I'm not nearly as scientific as Alan or Ian but do measure velocity and cfm at each machine port to insure I give myself enough room for error that error won't be fatal. Dave

I've read and understand most of the science, and tried to design my ducting using best practices - on the other hand I have a set of Dwyer Magnehelic gauges and pitot tube that have been sitting in a box somewhere in my shop for years- I never installed them- I've got what I've got. The only place they might have come in handy but could have easily been replaced by a slack tube manometer, is to keep tabs on my filters.

[Chris Parks]

On the 90 vs dual 45's, it depends on the radius of both. Since we are talking PVC plumbing pipe, I suspect the radii are close if not the same for both and that the difference between 2 45's and a 90 is likewise minimal- since there are less turbulence causing joints a 90 might actually have less impact on SP if measured with precision instruments.

Not in Oz, the normal single 90 is far shorter than two 45's. All sorts of issues raise their head when a fluid changes direction, boundary layers, differential speed between inner and outer parts of the fluid due to distance through the bend etc. BTW my comment of being led up the garden path was tongue in cheek and I meant no dis respect to anyone who has trodden this path before us including BP.

The length of a duct will eventually define how much air will travel down that duct but whether the lengths we talk of generally use have an impact I don't know. The biggest problem I see is that everyone is using something different and that something has differing capabilities. person A comes on here and complains that his TS has not got effective dust extraction while person B can't understand why not as his identical saw works ok. Person A has 2HP plus bags and 100mm ducting and B has 5HP/150mm.

To say that the length of duct or radius of a bend has no effect is a bit misleading in my view, it might not have an effect to anyone using top equipment but to someone who is using something a bit marginal it will have an effect. This point is glossed over to a large extent by some of us and I have done it in the past myself. let's face it not everyone has the resources to afford the best but we should be educating them to do the best with what they have and maybe looking at novel cheaper ways to do that.

Given the disparity of equipment and the lack of education which exists I am surprised that the companies who are selling this stuff have not tried to be more pro active in education. They rely on people asking questions not getting out there and putting the information in the public arena for all to see. We are all in the pioneering stage of this stuff as serious dust awareness has only become an issue in the last few years as far as hobby woodworking goes so learning is part of the fun and we mostly enjoy helping each other, that's the bit I like, helping other people and making sure that they continue to enjoy this great hobby.

I will get off my soapbox now and hope I haven't bored you to death.

[David Kumm]

Chris, I'm not sure that if companies educated buyers they would sell many stationary systems less than 3 hp 14" impellers. I know started with a 2hp 12" bagger, went to a 3 hp cyclone, and finally a 5 hp 15+" impeller on a vfd as the shop and my education expanded. Second in brilliance only to my 6 to 8 to 12 to 16" jointer purchases. Dave

[ian maybury]

Much like David I'm not sure that the situation is all that confused Chris - but it sure as hell is confusing.

There's such a wide variety of types of DIY/hobby system out there - many (most?) of which are highly marginal in terms of the amounts of air they move and the quality of filtration they deliver. It's damn hard given the cacophony of competing and contradictory claims for a non engineer that isn't in possession of a solid technical framework to overcome the conditioning and accept this reality - never mind move on to assess the specific claims of the various makers.

You see lots of variations of experience reported, but this is again not too surprising. In that marginally specified systems are going to be disproportionately disturbed by what may actually be relatively minor changes in design or operating conditions, and that most ducting installations are DIY and follow no very clearly defined code of practice. (one of the most common issues is the tendency to reduce the size of the drops to machines) Machine hooding is another major source of variation.

Strip away the dust cloud though and it's clear (a) that this is a mature branch of engineering - the practices applied by those contracted to deliver systems delivering specified air quality levels in regulated industrial environments are very well established, and pretty much standardised. An indication of this is the way that there's often multiple manufacturers supplying generically very similar pieces of equipment (standardised industrial solutions) like cartridge filters, blowers, cyclones, mobile dust collection units etc.

It's also clear that (b) safety critical systems like these need a decent factor of safety built in if they are to perform reliably - stuff like the blinding of filters, unexpectedly changed operating circumstances, less than perfect maintenance and the like should not so far as is reasonably possible leave people at risk. ('robust design' is the term) This cushion means that minor variations like changing a bend radius here or there probably isn't going to be a show stopper.

Even the design methods have become standardised and streamlined. Experience and expertise must still play their part, but as before it's fairly typical to design systems like these using stock tables to total the expected pressure drop, and to then size the blower from the fan curves. (rather than from first principles) The chances are for example that the data is relatively easily accessible out there for the variations in bend types we've been discussing.

The problem that still remains is as before that these industrial solutions are relatively high cost, high power and hardware heavy by the standards of many in the DIY market. One major question is how effectively this gap can be closed - maybe we're just going to have to bite the bullet and accept that it's worth investing in our health.

My personal view is that we're going as a group to have to step up to the mark - but that it's going to take much more carefully designed machine hooding (which will then become much more situation specific) to reduce air volumes to enable this. Maybe increased sales volumes can bring down costs too.

The reality right now though is that the technology is where it is - it's a pays your money takes your pick sort of deal.

I hope we can do it the intelligent way - the prospect of type approval and the like applied to machine hoods and the like doesn't bear thinking about....

ian

[Chris Parks]

Much like David I'm not sure that the situation is all that confused Chris - but it sure as hell is confusing.

From my position as a retailer of DE equipment in the colonies I get three types of inquiries, those who have tried to educate themselves with varying degrees of success and most still not getting it and those who have heard it all, not done any homework but realise that DE might be a good idea. The third group are those who on inquiry find the financial side either too much of a hurdle or just can't justify spending the required amount of coin on a dust system due to all sorts of reasons. When a bloke tells me that money is not the issue but still does not buy a system I liken him to someone who smokes and won't give it up. Believe me when I say that there are lots of people who have the money but because the result is intangible in lots of ways they don't see the end result except maybe a clean workshop and I reckon for that reason alone it is worth it. I had a woman ring me who ordered a system for her husband because she was sick and tired of hearing him coughing from wood dust and he reckoned that it was not a problem. It must have been interesting when the system arrived and they had the argument about who was going to install it!