Thread: Help validate my understanding of DC and critique my intended setup

Originally Posted by lemerv

Interesting thought. Does that mean you have a mobile dusty to move near each machine? Or it's stationary but you have long flexy to reach everywhere you need? If it's the latter, does the length of flexy start to impact on overall performance vs smooth ducting from a stationary machine? In my case the dusty would be immediately outside the garage, so it would only add say 50cm to the overall length of ducting I require.

I have a ducted Clearvue that could suck start a whole fleet of Jumbo Jets. I did warn you I was anal about capture at source. Capture at source needs lots of airflow and a big negative pressure bubble, so I decided to bite the bullet and buy a ClearVue. It hurt, as I paid the bill, but it's a decision I have never regretted ... except to complain that the agent at the time, Chris, did not convince me to buy a CV MAX.

Originally Posted by lemerv

Good to know. doug3030 is suggesting the same. I probably need to revisit this. However, I'm not sure how this works considering I have the ventilation fans doing 20 room air changes per hour? Would the gains from doing the 6" mods on everything be worth it if I am ensuring I get the 20 room air changes an hour? My thinking was stock standard DC setup plus the ventilation would produce a similar outcome in terms of visible clean and healthy clean?

Every bit of dust you capture at the source improves your system and your health. I would go so far as to say that regardless of whether you installed a ClearVue or a 2 HP system, you are throwing a lot of your capacity and your money away if you don't modify the machines and ports. In fact, the smaller is your system, the bigger the impact of not opening up the ports becomes because a smaller system does not have the power to overcome resistance. It makes a very big difference to dust capture. A huge difference! My dust collection improved remarkably once I modified machines and ports, and my system has oodles of power.

As the dust enters the 6 inch duct, it drops in velocity dramatically if you haven't opened up the machine and it's ports, because you are not getting enough air INTO the duct. I see this with my thicknesser/planer, the only machine I have not modified because I can't find a way to get the extra air through the machine. If you stick to a 4 inch port at the machines, the airflow is choked because at the pressures we are using air is barely compressible. This means your negative pressure bubble is much smaller and that over half of your potential airflow is lost at the machine. As the dusty air flows through ductwork, it will deposit dust at the wyes etc along the way.

I know it is a lot of work. I've done it, but it is essential work if you want good dust capture. I get between 800 and 1,000 CFM through the machines (depending on the machine) and into the ducts. If I left the 4 inch ports in place, those figures would drop to something close to one third of those figures. A 6 inch pipe has a cross section of about 28 sq in. A 4 inch pipe has a cross section of about 12 sq in. Then we need to make allowance for the fact that the disturbed air near the pipe walls is a bigger proportion of a 4 inch pipe than it is in a 6 inch pipe. A good rule of thumb is that to replace a single six inch pipe's flow you need three 4 inch pipes. I am a plumber by training. I have known this to be true since I was 15. The same rule that applies to fluids applies to air that is (largely) incompressible.

If we are to get good dust capture we start with the max airflow of our dusty. Then we need to figure out how to get this amount of air through the machine, through it's port/hood/shroud, into the duct and finally, through the dusty. I hate to be the bearer of bad news, but if you don't open up the machines and their ports, you are likely to pull no more than the equivalent of one half to one third of the potential of the dusty. Sad, but true.

We could hook up four ClearVues to your duct, to develop huge suction, but if they are trying to pull air through a 4 inch port we have wasted three and a half ClearVues. BobL has all the figures on airflow, including the maximum airflow through 4 and 6 inch pipes. These figures can also be found at Bill Pentz's site. BobL is a superior source, I think, because he has years of experience and can quote systems that he has measured himself. So BobL is a source of real data, not the inflated figures the machine manufacturers serve us. That is why I was so happy when Ronboult brought his clever instruments to my shop to actually measure the airflow, machine by machine. The data are conclusive. The biggest airflow losses occur not in the straight duct--not even in the duct fittings--but at the machines themselves, including ports/hoods/shrouds.

Here endeth the epistle.

EDIT: Oops, just noticed that BobL beat me to the punch. Sorry to waste your time, and mine.

Originally Posted by John Samuel

I have a ducted Clearvue that could suck start a whole fleet of Jumbo Jets. I did warn you I was anal about capture at source. Capture at source needs lots of airflow and a big negative pressure bubble, so I decided to bite the bullet and buy a ClearVue. It hurt, as I paid the bill, but it's a decision I have never regretted ... except to complain that the agent at the time, Chris, did not convince me to buy a CV MAX.




Every bit of dust you capture at the source improves your system and your health. I would go so far as to say that regardless of whether you installed a ClearVue or a 2 HP system, you are throwing a lot of your capacity and your money away if you don't modify the machines and ports. In fact, the smaller is your system, the bigger the impact of not opening up the ports becomes because a smaller system does not have the power to overcome resistance. It makes a very big difference to dust capture. A huge difference! My dust collection improved remarkably once I modified machines and ports, and my system has oodles of power.

As the dust enters the 6 inch duct, it drops in velocity dramatically if you haven't opened up the machine and it's ports, because you are not getting enough air INTO the duct. I see this with my thicknesser/planer, the only machine I have not modified because I can't find a way to get the extra air through the machine. If you stick to a 4 inch port at the machines, the airflow is choked because at the pressures we are using air is barely compressible. This means your negative pressure bubble is much smaller and that over half of your potential airflow is lost at the machine. As the dusty air flows through ductwork, it will deposit dust at the wyes etc along the way.

I know it is a lot of work. I've done it, but it is essential work if you want good dust capture. I get between 800 and 1,000 CFM through the machines (depending on the machine) and into the ducts. If I left the 4 inch ports in place, those figures would drop to something close to one third of those figures. A 6 inch pipe has a cross section of about 28 sq in. A 4 inch pipe has a cross section of about 12 sq in. Then we need to make allowance for the fact that the disturbed air near the pipe walls is a bigger proportion of a 4 inch pipe than it is in a 6 inch pipe. A good rule of thumb is that to replace a single six inch pipe's flow you need three 4 inch pipes. I am a plumber by training. I have known this to be true since I was 15. The same rule that applies to fluids applies to air that is (largely) incompressible.

If we are to get good dust capture we start with the max airflow of our dusty. Then we need to figure out how to get this amount of air through the machine, through it's port/hood/shroud, into the duct and finally, through the dusty. I hate to be the bearer of bad news, but if you don't open up the machines and their ports, you are likely to pull no more than the equivalent of one half to one third of the potential of the dusty. Sad, but true.

We could hook up four ClearVues to your duct, to develop huge suction, but if they are trying to pull air through a 4 inch port we have wasted three and a half ClearVues. BobL has all the figures on airflow, including the maximum airflow through 4 and 6 inch pipes. These figures can also be found at Bill Pentz's site. BobL is a superior source, I think, because he has years of experience and can quote systems that he has measured himself. So BobL is a source of real data, not the inflated figures the machine manufacturers serve us. That is why I was so happy when Ronboult brought his clever instruments to my shop to actually measure the airflow, machine by machine. The data are conclusive. The biggest airflow losses occur not in the straight duct--not even in the duct fittings--but at the machines themselves, including ports/hoods/shrouds.

Here endeth the epistle.

EDIT: Oops, just noticed that BobL beat me to the punch. Sorry to waste your time, and mine.

No apologies necessary. A very thorough post! The weight of evidence at this point tells me (as do all of you!) to do the mods to make everything 6”, so I will endeavour to do that.

Appreciate everyone’s input on this.

Is pvc pipe the best option for the ducting? Or is there a cheaper alternative that will stand up to the task? I will certainly go dumpster diving but my efforts so far have only turned up 100mm (4”) pipe, which I guess is standard for houses?

Originally Posted by BobL

Most of the world is still operating under 1960's OHS standards which are outdated. The AUS OHS standard is based on 1960's UK OHS standards involving beech and oak and make no allowance for the greater potential toxicity of our timbers. The newer EU OHS standards can be met on most machines using 5" ducting which is why this is starting to be be more available on EU machines.

You don't need 6" for everything.
EG drilling produces relatively little fine dust and 4: is enough for this process.

To determine what to use you should look at this Bill Pentz chart.
If a 4" duct can nominally perform at 400 CFM then you can see that it basically does chip collection and not much else.
Just a heads up that even the Medical Euro standards may still not meet the needs of seniors and people with health complications.


Machinerydustflowrequirements.jpg

Thanks BobL, that’s a good table. I will have a further read of Bill’s site.

Originally Posted by lemerv

Is pvc pipe the best option for the ducting? Or is there a cheaper alternative that will stand up to the task? I will certainly go dumpster diving but my efforts so far have only turned up 100mm (4”) pipe, which I guess is standard for houses?

All the data I have seen suggests PVC is the best option, both in terms of cost and in terms of losses to airflow. That's easy to believe because it has such a smooth surface with only minor disturbance at the joints. It is also dead easy to modify later because the joints are taped rather than glued or riveted.

Get the stormwater variety, because it is cheaper than the variety made for sewerage. When I was setting my system up the cheapest supplier, by far, was Total Eden, now known as Nutrien Water. They have changed their name, but I hope the pricing policy remains unchanged. Look around. At the time I was doing my job the cheapest were half the cost of the most expensive. The most expensive were the plumbing supply outfits. Try agricultural irrigation and similar places. Also look for outfits that sell pumps and similar for agricultural and rainwater etc purposes.

I note you are already all over the issue of smoothness of the internal walls of flexy. Great! My 6 inch flexy has quite smooth internal walls, and this does make a significant difference.

Good luck.

That's looking like a very satisfactory result will be achieved. Sherwood don't actually say the size of the inlet on that DC, or if they do I couldn't find it, so it might even have a 6" inlet. Yes, I'm an optimist. Regarding the forced ventilation fans, two 6" will not move the same amount of air as a single 12", you would need 4 x 6" or 2 x 8" to replace a single 12". Could be worthwhile to scour the local builders who do commercial work to see if any of them have an offcut of 12", or failing that at least some 8".

One of the things that bothered me about my installation was my inability to measure real airflow, machine by machine. Ronboult solved that problem when he came to my shop with his instruments.

I had measured Amps before Ron arrived, so today I correlated Amps and airflow in CFM to see how good the correlation was. With an R-squared of 80% it's not perfect, but its not too shabby, either. If you are conducting improvements, Amps would likely tell you if the airflow had actually improved, and by about how much. It would also allow you to compare the performance of your system, machine by machine. Nothing replaces direct measurement, but very few have access to a Pitot tube or other airflow instruments.

Originally Posted by John Samuel

One of the things that bothered me about my installation was my inability to measure real airflow, machine by machine.

Ideally, accurate measurement is the way to go but not everyone has access to them or knowledge to operate them effectively.

The important thing to be able to do is to tell whether a change you have made has made a positive difference or not. IN the absence of the proper measuring equipment I have often resorted to primitive but effective methods.

One method I used to determine whether it was worth while boring out the 2hp dusty to 6" was to use paper streamers. I held up the streamers in front of the intake before modification and then measured the distance from the intake I could hold the streamers and have them deflect to about 45 degrees. Then after I did the mod, I repeated the test with the streamers and found that the distance was quite a bit longer. This proved to me that a significant improvement had been made. I could not quantify it in terms of air speed or CFM or anything else that the instruments would measure; all I knew was that an improvement was made so the mod was worthwhile.

I guess what I am saying is don't blow the tool budget on testing equipment if you don't need to measure precise units. Simple and cheap methods can be used to reassure yourself that there has been a gain from your efforts.

Before you buy any measuring gear read this
DRAFT: FAQ - Dust Extraction (Practical Aspects)

Measuring air flow accurately is PITA and takes a long time and in many cases you will just end up chasing your tail.

Best thing would be to follow the leads in the dust forum and if you are going to spend money on measuring something get your self a PM2.5 detector.
That will tell you if you have anything to worry about.

Originally Posted by aldav

That's looking like a very satisfactory result will be achieved. Sherwood don't actually say the size of the inlet on that DC, or if they do I couldn't find it, so it might even have a 6" inlet. Yes, I'm an optimist. Regarding the forced ventilation fans, two 6" will not move the same amount of air as a single 12", you would need 4 x 6" or 2 x 8" to replace a single 12". Could be worthwhile to scour the local builders who do commercial work to see if any of them have an offcut of 12", or failing that at least some 8".

Yep good point. I’ll probably work backwards - see what size ducting I can get and then get the same size fan and make sure I get the right number of them. Lots of construction going on in the area so I might get lucky.

Any advice (or good resources) on the best way to vent the duct outside and keep it weather proof seeing as the advice is to remove the baffles so as not to obstruct air flow?

Extractor motor Current is a guide for larger changes to extraction but it has some limitations for smaller changes.
Unless you have access to a good quality clamp meter the max current resolution you can get is 0.1A.

For the average 2HP the max current drawn, with nothing attached to the impeller (ie max flow) is about 7.0A (When I measured mine it was about 6.2A).
The current with zero air flow was about 4.0A.
The difference between the 2 positions is thus 3.0A

With a 0.1A resolution, for a "before and after" test of "changing something" and seeing what it does to the air flow, the uncertainty in any difference measurement is the sum of the two +/-0.1A or +/- 0.2A

The minimum % uncertainty is thus (0.2/3.0) x 100 = 6.7%
OR
To see a significant difference in air flow whatever you do to the DC system must change the current by >6.7%
In practice it's more like 10%

Most gains in improvement to air flow are achieved by lots of small (<10%) changes that all add up, but trying to identify between doing something "this way" versus "that way" becomes tricky and may mean you may even end up changing things the wrong way.
The smaller the DC the lower the flow the harder it gets to use current for these purposes.