EXHAUST - dust collector vs inline fan

[Robert Walters]

Could someone please fix the typo in the subject line please, should be inline, not inlie. thanks.


Since I've been having issues with my laser for a few weeks, I've had things torn apart. As such,
I've set off the smoke alarm a couple of times on some 10 second tests too. I got this thing about fire, so I REALLY don't like the smoke alarms going off. Even if I know/am the cause and it's under control.

I've been thinking about this for a while and I hit craigslist and found a slightly used 10" activated carbon filter for $20. It's the same kind you can get at any hydroponics store for $250. It came with a polyester pre-filter.

This one is rated at 1800 CFM, has a 10" duct flange, 44" tall, and 13.5" wide, weighs about 30 lbs I'd guess. While I can't hear pellets rattling around (it might use rolled carbon fabric for all I know), I have a local source for 4mm activated carbon pellets that I could drill out the rivets holding the flange on, and just replace the pellets as needed in the future, and no more setting off the smoke alarm. Plus the added bonus of just recirculating the warm air back into the shop.

Well, they don't make a 10" to 4" adapter that's available off the shelf, so I'll have to come up with something. This got me to thinking about inline fans as I know some of you are using them.

I use the little red harbor freight dust collector. It's rated at 900CFM, 1hp (Ha!), 107dB @ 3ft.
I'm guessing it's about 8-9" static pressure if/when I dial in down (on a speed controller).


I'm going to make a Manometer (yes, that's a real thing =), and test things out later on:
http://www.dwyer-inst.com/Products/M...troduction.cfm

If you engrave a ruler on a piece of 18"+ long acrylic, you can read up to 9" static pressure.
Just add some holes on both side of the "U" for zipties to feed thru. Then you can adjust the tubing up/down after you have filled it with water (you can add a drop of red food dye to make it easier to read) so the level of water on both sides are at the ZERO mark, instead of having to pour out a little bit of water till you get it right.

If you know a simple way to read CFM, let me know.
OTHER THAN filling my shop with smoke and timing how long it takes to exhaust that is


From the manual for my machine:
"This system MUST be capable of supplying 250 CFM (cubic feet per minute) @ 6 inches of static pressure (water)."

It then goes on to say:

"In order to meet the laser system’s CFM requirements, a high-pressure, high static pressure rated, exhaust blower must be installed. This type of blower has self-cleaning blades and can maintain airflow even though restrictions are introduced. Length of exhaust pipe, exhaust pipe diameter, number of 90-degree angles, and other restrictions must be calculated when determining the correct exhaust blower unit. Installing an incorrect or undersized blower is not only unsafe, but it can also lead to premature and excessive wear and tear to the laser system.

DO NOT install forward incline, backward incline, in-line, or ventilator fans because these types of air handlers are inadequate and inappropriate for this type of installation."




That last part is a bit of a gotcha ya.... "DO NOT use inline fans"


Also if you look at the specs for inline fans:
http://atmosphere.com/english/html/p...s/vtx/vtx.html

For a 12" inline fan at 388CFM you only have 1.5" static pressure.
(blue line in the graph shown)


That's not even close to the 6" SP requirement.
I'm sure other system have something close to that static pressure as well.


For me, it be easier to mount an inline fan on top of the filter, and find a 10" to 6" reducer (a 10" to 4" doesn't exist for all practical purposes), then run 6" duct to the laser, and then use a 6" to 4" reducer" (which is the recommended way in the manual due to airflow restrictions). I prefer to "blow" (positive pressure) on the longest run, rather than "suck" (negative pressure), but I could totally have that backwards for all I know in regards to performance/efficiency.


I do realize inline fans are quieter and probably why many are using them, but has anyone tested the SP (static pressure) or CFM on their system including any/all the duct work (end to end)?

For those using inline fans, have you had any issues?

[Scott Shepherd]

I've found all those static pressure requirements to be useless. I've seen all the arguments for high static pressure and I've been standing at a machine that had issues with clearing the smoke while an airflow engineer uses his $2,500 instrument to measure it all. In the end, higher static pressure didn't change a single thing about the airflow. If it's clearing the cabinet, then I wouldn't worry about anything else.

[Don Corbeil]

I am using an inline 6" (450cfm) on my speedy 300 and have had no problems clearing the cabinet with any medium. It works very well. Specifically, my exhaust run is only about 3.5 feet, and I'm using 4" hard ducting, with two slight elbows. Longer runs would probably not have worked as well. I had doubts about using it, but during my research on sawmill creek, I found that there are members that have been successfully using an inline fan for some time, so I thought I'd try it. I'm glad I did, as it is efficient and quiet. I don't know what the static pressure is, but I can say it clears the cabinet quickly without any noticeable smoke.

http://www.sawmillcreek.org/showthre...ion&highlight=

[Scott Shepherd]

I will also add that many of the spec's given aren't for the blower, they are for the cabinet. 300cfm is very different INSIDE the cabinet vs. the actual blower rating. Of course no one tells you that, they just say "300CFM".

[Niklas Bjornestal]

You cant increase the pressure while keeping the flow the same (if you dont block the air inlets). If you want to double the flow you have to have 4 times higher pressure, and the fan will need 8 times more power ( Flow = k * Pressure^(1/2))

[Dan Hintz]

I use the little red harbor freight dust collector. It's rated at 900CFM, 1hp (Ha!), 107dB @ 3ft.
I'm guessing it's about 8-9" static pressure if/when I dial in down (on a speed controller).

Divide by 10 and you might have realistic SP numbers for that... you're not getting that kind of SP out of it at full bore, and at low speeds it's hopeless.

That said, I use the same unit with my fume filter and it works just fine.

[Robert Walters]

I went and read the thread Dan linked to (March 2014).

I see where Steve was having an issue and brought in the Trane guy.
I see where you were saying in the cabinet is FPM (which your orange gauge measured), not CFM.
*I* don't know the technical difference other than speed (FPM) and volume (CFM).

I truly understand your reservations Steve, after all the BS you've been "authoritatively" told, if I had heard someone say 29", I would have told them their are full of bull right to their faces and said ok, find one.


Dan, my only issue is even if I derate the HF Red DC at 50%, when I add this monster filter, will I be restricting it to the point that one day I may not be paying attention and not realize something is amiss.


After I get my laser tube issues sorted out, For giggles I'll go ahead and make a manometer (as I already have the tubing anyway) and see what SP I have at various points.

I read that if I have a known diameter (let's say) 4", and insert a known reduction 3.5" and put the manometer on both sides that I can get the CFM as well. I dont know if that's true, but would be interesting to find out.

[Scott Shepherd]

Yeah, Robert, I'm convinced that airflow is a black art to laser manufacturers. I haven't talked to a single person at ANY company that appeared to know and understand airflow. All I ever get from any of them are talking points from the manual or maybe a tiny bit of additional information like "We have a guy who's using this and it works great". Well, not everyone is running the same thing. A 300CFM blower might be great if you were engraving anodized aluminum all day, but if you're cutting and engraving wood all day, then it might fail miserably. It's all relative to what you're doing.

What I determined was that the static pressure number was worthless, but the important number was FPM and it took something like 400 or 500 FPM to keep smoke moving horizontal. Less than that and smoke could move vertically, which put the lens and mirror at risk from contamination. I've seen a machine with less than 3" of static pressure, measured at the machine, not at the motor, running, and I've seen the same machine with an exhaust system that had over 20" of static pressure running and it made zero difference as far as an improvement. The higher static pressure was a lower CFM so it actually made the air speed in the cabinet go down, not up.

The only thing that's important to me is air speed. Without it, the smoke won't clear. I don't care how you get there, you have to have that air speed and I believe that's a far better measurement than anything else.

I also heard all the "you have ribbed tubing" and that's a major problem. They said if you put smooth walled tubing in, it would change that number by 10-20%. It changed it like 3%. I heard "you have too many elbows", "your run it too long", and all those things. When they were all tested, not a single thing moved the needle more than 3%, despite the claims from the manufactures and field service people.

It's all about airspeed.

[Phil Thien]

Divide by 10 and you might have realistic SP numbers for that... you're not getting that kind of SP out of it at full bore, and at low speeds it's hopeless.

That said, I use the same unit with my fume filter and it works just fine.

The little red one is more like a shopvac blower than a DC blower. It uses a geared universal motor driving a small (maybe 6"?) radial tipped blower wheel to high RPM's. 9" of SP is believable.

It is incredibly loud, though. It requires hearing protection. And nothing with a universal motor is going to stand up to long-term use clearing an engraving cabinet, I imagine.

[Bill George]

Scott you have it right, its about FPM feet per minute or air speed and CFM cubic feet per Minute or volume of air. This was discussed in depth a few months ago, no need to rehash.

[Robert Walters]

A ways back, I built a cabinet on casters that fits perfectly under my laser.

It's purpose was to:
Collect solids that came from the laser.
Hold the dust collector on rubber shock mounts for sound/vibration dampening.
Act as a carbon pellet air filtration and HEPA filter as the last stage.

I spent a lot of time in SolidWorks (I rarely use) painstakenly measuring everything to make sure it fit under the laser and everything fit inside. It had bins in it to hold and easily replace the pellets as needed.

Intake was from the rear left, exhaust was at the right bottom. I made sure that there was enough bottom clearance not to be an obstruction. The hepa filter was huge enough so even if it got full/clogged it wouldn't be an excessive burden on the system.

I figured that even if the HF DC was only 7" instead of 9" SP, that it still had enough pressure to push through it.

What I didn't know/realize is that 4mm carbon pellets are dense, I had never worked with them before.
They were so dense that even when I reduce the amount of pellets by half, I still didn't have any airflow.
And this was with nothing connected to the inlet side of the DC. I was seriously disappointed.


So yes, to me, static pressure DOES matter as a whole.


I now have this monster 1800CFM rated carbon filter and I want to make sure I can push my way thru it.
I see 12" inline fans having less thatn 2" SP at MAXIMUM, and that's with low CFM as well.
They are also not designed to deal with solid particles as far as I can tell, like a DC is.

I really have no clue on the science/voodoo behind airflow,
just picking up the art/math here and there as I can find it.

Ribbed duct vs straight pipe does make sense to me from a resistive context.
Using 6" duct on a 4" exhaust (from laser) makes sense too, it's less resistive to flow.

Two 45 elbows vs one 90 elbow sorta kinda makes sense in a "I hit a brick wall and need to change directions" sorta thing.

Now, I have to go from 10" to 6" to 4" which I may or may not do, just depends on what I come up with.

10" inline fan makes sense from an audio and practical POV, I just don't want to relive what I experienced before.

4" round duct is 12.56 square inches,
1"x24" exhaust plenum is 24 square inches,
0.5" x 24" intake plenum is 12 square inches.

So, does 12 to 24 to 12 increase the flow (FPM) across the bed?
Is it just the path of least resistance?

If my laser had a 6" exhaust, I'd love it, but I'm not taking a torch to it either (not yet anyway

I'd just like some hard numbers for *my* system as a whole, then at least I can tell down the road if/when something changes and what/where to look. http://www.dwyer-inst.com/Products/A...troduction.cfm


Scott,

Your engineer's hot wire anemometer sounds an awful lot like an automotive MAP sensor.

I realize you went thru a bunch of engineering malarky to figure out it's much simpler than everyone makes it out to be. I get it, but on the other hand I do recognize that CFM and SP play a part in that as well ON A WHOLE. If one exhaust to the outside of a room, you have to have more than that coming into the room; is 4" vs 8" linear, etc.

[Joe Hillmann]

If you want to measure your airspeed and cfm you can look up pitot tubes. They are nearly the same thing as a manometer, the difference is where you put both ends of the tube. With a bit of math you can figure out the air speed from the numbers the pitot tube gives you. From air speed you can figure out air speed in a tube (there are specific formulas that take into account friction between the air and pipe causing the air to move slower the closer it is to the pipe wall.)


Since you can't do any testing for air speed or pressure until you have it installed I don't think it is all that valuable to know your airspeed and pressure. In my opinion if the blower sucks the smoke out of cabinet and keeps your lens clean-ish then it is good and the numbers don't matter. If the blower can't clear the cabinet of smoke then you need a bigger blower or less restriction in the system.

[Scott Shepherd]

Robert, all I'm saying is that all the numbers that people throw around are strictly math numbers and formulas. They aren't the real world. If you knew how many things we tried that we were told would fix the issue, without question, only to see things get worse or not move much at all, you'd be shocked.

A HUGE part of it all has to do with the design of the intake and exhaust ports on the laser cabinet itself. You can have the best motor with the best rating in the world on it, hook it up to a cabinet and everything changes radically. It changes to match the math in the cabinet. The only way to get the perfect flow inside the cabinet is to design the cabinet in software that shows and simulates the airflow, which costs a lot of money from what I gather. I don't believe anyone is using that software because I've seen things that make be believe it's all just an afterthought.

The motor is only one piece of it, I'd argue that the design of the cabinet, the intake and exhaust ports inside the cabinet play a much larger role than anything else. You can stick a 300CFM blower on a machine and stick a 2100 CFM blower on the same machine and it may not change anything. The cabinet becomes the factor, and that's something no one is discussing.

[Robert Walters]

Robert, all I'm saying is that all the numbers that people throw around are strictly math numbers and formulas. They aren't the real world. If you knew how many things we tried that we were told would fix the issue, without question, only to see things get worse or not move much at all, you'd be shocked.

A HUGE part of it all has to do with the design of the intake and exhaust ports on the laser cabinet itself. You can have the best motor with the best rating in the world on it, hook it up to a cabinet and everything changes radically. It changes to match the math in the cabinet. The only way to get the perfect flow inside the cabinet is to design the cabinet in software that shows and simulates the airflow, which costs a lot of money from what I gather. I don't believe anyone is using that software because I've seen things that make be believe it's all just an afterthought.

The motor is only one piece of it, I'd argue that the design of the cabinet, the intake and exhaust ports inside the cabinet play a much larger role than anything else. You can stick a 300CFM blower on a machine and stick a 2100 CFM blower on the same machine and it may not change anything. The cabinet becomes the factor, and that's something no one is discussing.

Steve,

I really do understand what you are saying.

I'm not look at the maths to come up with a system-as-a-whole,
I'm looking at the maths to COMPARE against, down the road, to what I have already "that works".

All this stuff is relative. We can read specs all day long. I'm looking this from a "common sense" pov.

I KNOW that a DC can tolerate solid particles in the airstream, I don't know if an inline fan can.

An inline fan "works for YOU", I don't know if it'll work for this monster of a filter I plan on using.
I got bit on the assets the last time I played with activated carbon, I don't want to be bit again =)
I'm just down right curious what this lil HF Red DC really does, and not just we derate it for.



I just found this (scroll down to filters)...

http://canfilters.com/faq.html

While laser engravers may be a million dollar industry, growing pot is a billion dollar one!
They don't care about engineering, they just don't want to get busted! lol

And I learned a little something about refilling the filter I have, which is what I intended to do.
At least now I'm aware of it

[Bill George]

SP or Static pressure is the resistance to air flow in a duct system. Manufactures will publish the SP in IWC for the equipment they make. Getting those numbers is another thing. Ductwork has resistance, fittings have resistance and so on. Fans are rated at so much fpm output against a given SP. There are also fan curves published that will allow you to do some estimation of performance from that model of fan.

A lot of Blue sky and fluff from people who think they understand ductwork design and air flow. Only those who work with it in the field really know. Books don't tell you everything.
I worked with it in the field and then taught it for another 12 years.... still a long ways to being an expert.