Difference between volume and pressure -- dust control

[ReneS]

I have seen a few things about this on YouTube where people talk about high volume and low pressure, and low volume and high pressure, I think. But I have a hard time grasping the differences.

Mainly, I am trying to understand how volume and pressure affect the effectiveness of dust collection and how that applies to using a shop vac versus a dust extractor, and similarly, what makes a dust extractor different than a "vacuum cleaner."

If anyone can help me with this, I would appreciate it.

 

[Tom the Remodeler]

Hi Rene,

In very broad terms, a shop vac, dust extractor, and "vacuum cleaner" would all fall into the "Low Volume/High Static Pressure" category, where they don't move a lot of air, but they produce a lot of "suction", which is necessary for moving air through small orifices like portable sanders, routers, saws, etc.... Generally, collectors with 2" and smaller ports fall into the "Low Volume/High Static Pressure" category.

Larger "Dust Collectors" with 4" and larger tend to fall into the "High Volume/Low Static Pressure" category, where the primary goal is moving much larger volumes of air through larger, less constricted spaces typical of larger "shop" machinery such as jointers, thickness planers, shapers, wide belt sanders, etc....

The distinction between the two very broad categories are relative.

 

[luvmytoolz]

I have a HVLP extractor hooked up to my drum sander, so long as it's close it captures most of the dust. If I extend the distance, the pressure drops off and it then misses a percentage of the dust.

Anything below 2hp is going to be a trade-off between HVLP-LVHP and is likely something that benefits from a short ducting run and being physically close to the dust making source.

 

[Mini Me]

All your DE questions can be answered here https://www.woodworkforums.com/forumdisplay.php?f=200

That link takes you to the DE section of the Australian WW forum which has the world's largest knowledge base for dust extraction. YT I have found to be full of advice that 99% of the time is misleading and mostly straight wrong with only a few exceptions.

 

[Steve1]

I have seen a few things about this on YouTube where people talk about high volume and low pressure, and low volume and high pressure, I think. But I have a hard time grasping the differences.

Mainly, I am trying to understand how volume and pressure affect the effectiveness of dust collection and how that applies to using a shop vac versus a dust extractor, and similarly, what makes a dust extractor different than a "vacuum cleaner."

If anyone can help me with this, I would appreciate it.

I would suggest that all the Youtube talk about high pressure vs low pressure is just people trying to sound smart. Ignore them.

What you need for good dust collection is lots of airflow.
The maximum static pressure is an irrelevant number for me.

Yes, a shopvac or Festool extractor will achieve a higher static pressure than my Laguna PFlux 1 ---- so what ?
I am figuring that is simply a result of the type of blower used in a small portable unit compared to the type of blower normally put in a large stationary dust collector.

My PFlux-1 has a 1.5 HP motor in it --- that's half the size of the motor on any shop-vac (if you believe those shop-vac HP ratings), but the PFlux runs circles around my CT-36 for dust collection capacity. Based on airflow ratings, that is what one would expect. On my table saw I have one hose above the table top and one hose below. I had to put a steel screen at the inlet of the above-table collector because PFlux kept sucking up my off-cuts from the table (makes a terrible racket as they rattle around the steel cyclone)

 

[Jim_in_PA]

Easy rule of thumb...small hand-held tools and other things with small ports need the higher SP of a shop vac/extractor because you can't get volume though a small port. Larger tools with 4" or larger ports need a dust collector that moves a lot of air because that's what actually moves the dust and chips when SP is lower. Festool's CT extractors are the equivalent of a shop vac, albeit a better engineered product. You can use them for an overhead blade guard with extraction on a big table saw as that's usually a small port application. but not for the actual saw cabinet, for example

 

[Hipplewm]

Think of it like a milkshake and a straw

normal straw - you need a lot of suction to get the milkshake to flow - but not much volume - high suction pressure - low flow

Now, replace that straw with a 4" PVC pipe and try and suck up the milkshake - You would need a move a TON more air to get the milkshake up the straw and due to it having a much bigger volume to travel, but, you would need less suction pressure

Typically, but not always vacuums, dust extractors etc (high pressure low volume stuff) run on the suction pressure side to make them work
Dust collection systems (4" pipe and up) run on the air flow side of things and use molecular entrainment to keep the pieces moving rather than suction pressure

My everyday job is pumps (water kind) and most of this is crossover - we can get into physics and friction, size of pipes etc - but that is way overthinking this stuff - unless you have a large shop and need an engineer to design for multiple pieces of equipment runnign simultaneously etc

 

[ChuckS]

I would suggest that all the Youtube talk about high pressure vs low pressure is just people trying to sound smart. Ignore them.

What you need for good dust collection is lots of airflow.
The maximum static pressure is an irrelevant number for me.

Yes, a shopvac or Festool extractor will achieve a higher static pressure than my Laguna PFlux 1 ---- so what ?
I am figuring that is simply a result of the type of blower used in a small portable unit compared to the type of blower normally put in a large stationary dust collector.

My PFlux-1 has a 1.5 HP motor in it --- that's half the size of the motor on any shop-vac (if you believe those shop-vac HP ratings), but the PFlux runs circles around my CT-36 for dust collection capacity. Based on airflow ratings, that is what one would expect. On my table saw I have one hose above the table top and one hose below. I had to put a steel screen at the inlet of the above-table collector because PFlux kept sucking up my off-cuts from the table (makes a terrible racket as they rattle around the steel cyclone)

Why do you need a side opening on the dust shroud (inlet)?

 

[ReneS]

Thank you for the replies, everyone. I appreciate it.

 

[Steve1]

Why do you need a side opening on the dust shroud (inlet)?

You mean, as opposed to coming from the top?

Because the vacuum hose is coming from the left, and it's a cleaner set-up coming from the side rather than the top. If coming from the top, it would tend to obstruct my hand/arm pushing parts through, unless the hose were offset to the left, and that would be a rather awkward set-up in my opinion.

 

[ChuckS]

I mean the opening that you put a steel screen on if the goal is to extract dust coming out of the table saw kerf.

 

[Mini Me]

Most vacuum cleaners shred the extracted material into smaller bits and because the filtration is poor spit the small bits out into the work area as micro dust particles. The reason I linked the Australian forum is because this is such a wide ranging discussion with multiple questions it is best to read what has already been found to work with the proof of the answers and discussion that has already been done supplied. Nothing less than 6" ducting is effective on a low pressure system and the proof is supplied in that link. It is a huge subject and most people do not understand all the background issues that go along with it. When I was supplying Clearvue cyclones in Oz I had a simple way to answer what is needed, a 15" impeller, 6" ducting to the machines and the job is done and no one ever came back and said it did not work.

 

[Steve1]

I mean the opening that you put a steel screen on if the goal is to extract dust coming out of the table saw kerf.

Yes, it is effective in capturing sawdust.

And also effective as a blade guard.

 

[bobtskutter]

Transporting solid material in a gas stream is about making the gas stream fast enough so the solid material stays suspended.
Which means, the more dust you're trying to move, the more air you need to move it. (Unless you're talking about dense phase conveying but that's a whole other topic).
Air flows from a high pressure area to a low pressure area. If you have a fixed length of hose and increase the vacuum from the vacuum pump (make the static pressure lower) you will get more air flow.
Fan designs are optimised to move a certain amount of air at a certain static pressure, it's called the fan curve.
If you have a large amount of dust you'll need a large air flow to transport it.
You can get a large air flow through a narrow hose if you have a vacuum cleaner type fan motor - these are small, and often multistage. They are high pressure low volume devices.
You can get a large air flow through a large hose with a paddle wheel type fan - these are quite large fans. They are low pressure high volume devices.

The naming HPLV and LPHV is do with the design of the FAN.
The power consumption of the fan is mostly affected by the mass of air it moves, not the suction pressure.

To get the dust extraction, ducting and filter to work you need to look at the whole system.

Regards
Bob

 

[Crazyraceguy]

I mean the opening that you put a steel screen on if the goal is to extract dust coming out of the table saw kerf.

I would assume that it is intended to keep the air moving. Concentrating all of the air-flow, to come from under the enclosure would probably be too restrictive. I know it "seems like" drawing through a smaller opening would increase the suction (static pressure) but that is not the case. The higher volume units rely on movement of air, which doesn't necessarily "feel" like higher suction.
The higher volume of air holds bigger chips in suspension, which keeps them moving toward the container. It's not that smaller hoses can't pick up bigger chips, but they are far more likely to clog.

 

[Tom the Remodeler]

I would suggest that all the Youtube talk about high pressure vs low pressure is just people trying to sound smart. Ignore them.

What you need for good dust collection is lots of airflow.
The maximum static pressure is an irrelevant number for me.

Yes, a shopvac or Festool extractor will achieve a higher static pressure than my Laguna PFlux 1 ---- so what ?
I am figuring that is simply a result of the type of blower used in a small portable unit compared to the type of blower normally put in a large stationary dust collector.

My PFlux-1 has a 1.5 HP motor in it --- that's half the size of the motor on any shop-vac (if you believe those shop-vac HP ratings), but the PFlux runs circles around my CT-36 for dust collection capacity. Based on airflow ratings, that is what one would expect. On my table saw I have one hose above the table top and one hose below. I had to put a steel screen at the inlet of the above-table collector because PFlux kept sucking up my off-cuts from the table (makes a terrible racket as they rattle around the steel cyclone)

"Lots of airflow" is irrelevant if your system doesn't have the static pressure to pull it through the opening in a tool. Yes, high CFM is important for the machines you reference, but they're tools with 4"+ ports, so of course they'll benefit. Now, taper down your dust collection from your PFlux1 to a 2" port, then connect a 12' 27mm hose and see how well it handles pulling dust out of a sander or router. You'll find that, at the end of a 27mm hose, your CT-36 dramatically outperforms your PFlux.

So, both CFM, and static pressure are absolutely relevant to a discussion of dust collection. As already stated, high CFM/low static pressure are generally well suited to machine with ports 4" and up, whereas lower CFM/high pressure extractors are needed for tools with 2" and smaller ports.

 

[ChuckS]

I would assume that it is intended to keep the air moving. Concentrating all of the air-flow, to come from under the enclosure would probably be too restrictive. I know it "seems like" drawing through a smaller opening would increase the suction (static pressure) but that is not the case. The higher volume units rely on movement of air, which doesn't necessarily "feel" like higher suction.
The higher volume of air holds bigger chips in suspension, which keeps them moving toward the container. It's not that smaller hoses can't pick up bigger chips, but they are far more likely to clog.

Thx. Your explanation makes sense. I was thinking the dust shroud on my SawStop PCS is enclosed without any opening on the sides and why one on the OP's.

 

[Crazyraceguy]

Thx. Your explanation makes sense. I was thinking the dust shroud on my SawStop PCS is enclosed without any opening on the sides and why one on the OP's.

Yeah Chuck, the one in the blade guard is the smaller port.
I don't know about the smaller job-site saws, but the full sized units come with a Y branch in the dust collection hose. 4" for the main cabinet and smaller up top. 1 1/2" ?
I don't think they work as well as having them separated, but that is too much hassle for most people. That small port never sucks as well, because it's choked down from a high-volume line, and the 4" hose is compromised by having the "leak". IMHO, it's a bad compromise.
It works reasonably well, but it can be better. I tested it with a CT on the upper port, it does matter.

 

[Mini Me]

The question before anything else is to define what is meant by dust extraction. Is it to pick up the sawdust and keep the area tidy or is it to capture both the sawdust and the micro dust particles. I have always regarded vacuum cleaners as useless when it comes to connecting them to big machines unlike when they are connected to sanders. I could write pages on this subject but that seems pointless because it has already been done elsewhere with all the proof needed showing what works and what doesn't.

 

[Steve1]

Now, taper down your dust collection from your PFlux1 to a 2" port, then connect a 12' 27mm hose and see how well it handles pulling dust out of a sander or router. You'll find that, at the end of a 27mm hose, your CT-36 dramatically outperforms your PFlux.

Why would anybody want to do that ?
PFlux is the wrong tool for the job when running a random orbit sander. You won't be able to hear yourself think. 6" inlet dust collector for a tool with 20mm port.