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Don Corbeil
03-07-2014, 11:09 AM
I am preparing my workspace for the arrival of my Speedy 300, and have been busy preparing for both the electrical & exhaust reqts.
I did the electrical yesterday, and put in a dedicated 20A circuit/plug for the laser, and another for the blower.

I'm now looking at the exhaust system, which I will port outside through the wall. Right now I have an existing 4" slot that was previously used for a dryer exhaust. If possible, I'd like to utilize that existing slot. The rep told me I have a choice on the 300 to go with either a 3" or 4" port outlet from the laser. I'm thinking of running that 4" line through the blower to a reduced 3" line into the machine. The rep told me this reduction would increase the suction, but then when I talked to a penn state guy, he said it would restrict it more... :confused:

Just wondering what your thoughts are on either keeping it 4" throughout, or doing that reduction to a 3" into the machine.

*edit: the total ducting length will be less than 5' from laser to wall exit

Louie Alvarez
03-07-2014, 6:06 PM
The 3" will indeed restrict air flow, so I would recommend you stay with the 4" connecting point from the laser to your wall opening. This not only will eliminate the need to obtain a 3" - 4" adapter but will keep air flow at optimal levels.

My laser machines are 6" connector at back of machine to 6" input connector of exhaust blower for optimal flow. The output of the exhaust blower is 4" direct to the 4" wall outlet. This provides a huge boost in air flow which is essential to remove as much debris your laser projects will create.

Don Corbeil
03-07-2014, 7:30 PM
My laser machines are 6" connector at back of machine to 6" input connector of exhaust blower for optimal flow. The output of the exhaust blower is 4" direct to the 4" wall outlet. This provides a huge boost in air flow which is essential to remove as much debris your laser projects will create.

Louie,

Thanks.

So you have a reduction in size from 6" to 4" coming out the blower, which actually helps the airflow? I'm not a fluids expert, but I would have thought that reduction might restrict airflow...?

Louie Alvarez
03-07-2014, 7:49 PM
The suction from the exhaust blower connected to my laser machines are both at 6" in diameter, so lots of volume of air is being "sucked" into the exhaust blower, The exit/output side of the exhaust blower is the standard 4" to go outdoors.

I've been using laser systems since 1993 or so and all the laser machines I've every used, owned or sold have had 4" input and output on the exhaust blower. BUT those laser machines have never been larger than 2'x3' with max depth of maybe 12-14", not so with my laser systems, my most popular laser machines is 3'x4' work area with a depth of up to 21.5" that creates a huge volume of air that needs to be moved so I increased the laser connection in back to 6" and use an exhaust blower with the matching 6" input connection, then I include the huge 6" hose to connect these to connectors together to allow the most volume of air to be moved.

How it expels outdoors is not what I'm concerned about so the 4" is more than enough to get outdoors. BUT to make life easier for my customer base I wanted them to be able to easily obtain 4" components if/when needed, such as the longer 4" aluminum dryer hose that is typically 25' to 50' in length as well as a standard dryer vent can be used, such as the type that has flaps that open when air flow is moving and these flaps close when there is no air flow. A longer dryer hose and vent can easily be obtained for nominal price at your local Lowes or Home Depot.

After all that, I would recommend you keep your connector and hose from laser to exhaust as large as available (4" correct) to maximize air flow into the exhaust blower, the output isn't nearly as critical. Hope that helps.

Dave Sheldrake
03-08-2014, 9:26 AM
Don,

The fan efficiency will be hit hard, very hard.

What CFM is the blower rated at? for example a non compensating 400 CFM basic fan will lose 25% of it's efficiency by dropping from say a 6 inch duct to a 4 inch duct (a 4 inch is half the area of a 6 inch not 2/3rds) or it will use 25% more power if it's a pressure compensating version.

Moving up to bigger fans like a 1200 CFM (that are available) that would require a straight run of 14" diameter and if reduced to a 6" even the most powerful industrial extraction systems (10's of HP) would be running at 350% ideal static pressure.

In the case of the Trotech machine, the rep is plain wrong, a 4" hole direct to 4" outlet with the correct fan will pull a lot more air than a 4 inch reduced to a 3 inch. Try to use straight wall ducting, coiled duct drops around 15% efficiency as well (fluid dynamics treats exhaust movement as line of sight or unobstructed diameter)

hope that makes some kind of sense :)

cheers

Dave

Don Corbeil
03-08-2014, 9:50 AM
OK, makes sense, thanks Louie.
One last thing I was debating. As I mentioned, I have only about a 5' run from the 4" laser exhaust port to the wall. The speedy exhaust recommendations are a minimum of 330 cfm with a static pressure of 5.8. Instead of the typical blower, I was considering using a high volume inline fan to move air out. They are typically quieter and draw less amperage and they do have models that pull considerable CFM, although the static pressure is rarely noted. I've seen some on the forum here using them successfully. With the short run I have, it seemed reasonable to consider. I was thinking of using either a 6" (450 CFM) or an 8" (700 CFM) inline fan mounted at the wall outlet, reducing it to 4" at both inflow and outflow. Maybe this 'double' reduction is self defeating, but I wanted to ask if it might work. If that's too questionable, the 1 HP blower I was looking at pulls 660 CFM with a static pressure of 8.5. Over a 5' run, this should provide good venting. I'd rather err on the side of overkill than under, so what do you think of those options?

Mike Null
03-08-2014, 11:23 AM
Don

I use 4" from the machine to a 6" inline blower then about 3 feet of 4" to outside. My entire distance is not more than 8 ft. I do not use dryer duct, only hard duct with adjustable bends. Flexible works but you should keep it as straight as possible. I also use a 4" blast gate at the machine.

Do not use the 3" outlet. As Dave says, your rep is wrong.

I would advise staying close to the 330 cfm. Too big a unit in your climate could present other problems.

Don Corbeil
03-08-2014, 12:38 PM
Dave, Mike, Louie
Thanks.
OK, I will keep the outlet from laser at 4". I will also be looking at hard duct options, since it is a short run. I know the foil type dryer pipe tends to create a lot more friction along the airflow.

Mike, your setup is very similar to what I'm looking at installing. That is encouraging. Is your inline fan rated around 400-435 CFM? That's the typical capacity I am seeing available in a 6" inline fan. I am assuming you are getting good draw, ie a curtain of air across your entire work area?

Dave Sheldrake
03-08-2014, 12:45 PM
A 6" 450 is going to pull about 330 with a single reduction IF it has enough power to do so. The flow cross section of inline fans is designed to work as straight through not reduced so the lower power inlines will lose a LOT of efficiency if you go changing out sizes of the inlet/outlet or running it anything less than direct. A 550cfm inline will work if it's reduced to 4" as in Mikes set up quite well. As Mike said though, keep the minimum of bends, a single 90 degree drops 30% of your available flow.

A big 660/1hp is going to bring it's own problems, it's unlikey you will be getting enough infeed to balance the flow so it will exert force on things like the machine window while not really giving much in the way of benefits :(

cheers

Dave

Don Corbeil
03-08-2014, 1:09 PM
If I could find a 6" inline that did 550 CFM, I'd buy it. I only seem to be seeing 400-450CFM in that size. Mike, what do you have?
I can't get my exhaust to go exactly direct, but I only need to drop 18" vertical and maybe 24" horizontal distance.

Mike Null
03-08-2014, 4:00 PM
Mine is 300 cfm and it's now about 17 years old. I first used it on a smaller ULS machine before changing to the Trotec. If I had a longer run or more el's I would have needed a larger unit but this one evacuates smoke and fumes very efficiently and quietly. Not so quiet now as I'm beginning to get some vibration in the duct which I'll be remounting in a week or so.

I've had occasions when I forgot to turn it on and the cabinet filled with smoke. It was clear within just a few seconds of turning the blower on.

Don Corbeil
03-08-2014, 7:03 PM
OK thanks, that gives me a lot of good information since I'll also be exhausting the very same machine...

Kev Williams
03-08-2014, 7:58 PM
The rep may be wrong, or maybe not so wrong. A smaller diameter in the right place can increase air velocity-- This may- or may not- increase airflow, and/or suction at the machine. Maybe this is what the rep meant?

The blower that I got with my Triumph looks a bit weird, to me anyway- it's a squirrel-cage blower, the cage being about 15" diameter and maybe all of 2" wide. The air inlet and outlet are both 6" diameter. But the 6" outlet is an add-on to accommodate the air tubing ,the actual outlet is just over 2-1/2" in diameter. I forget the exact measurement I came up with, but the cubic meters per minute rating translated to around 1060 cfm. And I can tell you, it does blow a TON of air! I have to assume the restricted outlet size to increase air velocity is a big reason why...

Of course, I'm sure everyone is correct that, if I were to plumb this blower with 4" pipe instead of 6", I'd lose actual air flow. It would actually be fun to test. If only I had the time.. ;)

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Dave Sheldrake
03-09-2014, 7:41 AM
if I were to plumb this blower with 4" pipe instead of 6", I'd lose actual air flow.

Yup, you would be losing 50% of the cross section of the outlet pipe, a given cross section has an upper limit on how much air can flow through it at a given pressure. Look at it like the hand dryers in public men's rooms, lots of speed, high pressure , low volume.
Extraction is about mainly volume or number of full swaps per minute.If you take a hypothetical cabinet volume of 1 cubic foot, compress it down to 1 cubic inch, on release you have a LOT of pressure through a small hole and a lot of velocity but still only 1 cubic foot of air at atmospheric pressure being transferred.

cheers

Dave

Martin James
03-10-2014, 4:21 PM
go to grainger .com and look at specs on any blower, they are all listed with multiple ratings for different levels of static pressure.
I use one like this grainger # 7AT87T. it is hooked to a big sheetmetal box with attachment collars to go to different areas of the shop. I run aluminized flex duct which can be moved where needed, although for a laser I would run metal out from the laser to reduce fire risk. The plenum box has 5 or 6 duct hoses hooked to it. The ends of the ducts can be capped with a piece of cardboard when not in use.


I run a 2 speed motor 1/4hp and 1/2 (I think) and the top is 1400 cfm.
It won't suck up stray cats, but it will slam the doors and pull smoke back in from chimneys.


The # 7AT87T I pasted is just an example. That one is 208-220 volrts and has a SP rating of 250 cfm at 1" static and 150 at 3 inches.


A bigger wheel will put the top cfm rating way up and the pressure rating way down.


This might cost us$600-700 which while not over kill is cheaper than excessive engineering.


The grainger tech support can be a good place to get the basics. And they can tell you if you can use a 2 speed motor, which will save on the electric cost.


Noise is a very important concideration. My blower make a low tone. It does not sound like a jet airplane, or worse a helicopter. Granger lists the decibel ratings.

Don Corbeil
03-10-2014, 5:38 PM
It won't suck up stray cats, but it will slam the doors and pull smoke back in from chimneys.


LOL. I'll make sure the pets are secured before operating...
and I'll take a look at the grainger web site. thanks!

Bill George
03-11-2014, 9:11 AM
First post but I just had to add the amount of good and accurate information posted in this Thread. Yes, flex does cut down on the amount of air flow, but needed for connections. The metal hard ductwork is much preferred for max flow and less resistance.

Gee I wonder why my Signature did not show up? Yes retired commercial HVAC and more....

Richard Rumancik
03-11-2014, 11:10 AM
. . . The speedy exhaust recommendations are a minimum of 330 cfm with a static pressure of 5.8. Instead of the typical blower, I was considering using a high volume inline fan to move air out. They are typically quieter and draw less amperage and they do have models that pull considerable CFM, although the static pressure is rarely noted. . .

Don, when they give a flowrate without specifying a static pressure, they are implying that the flowrate given is at 0" static pressure. This means an unloaded condition (basically nothing attached to the inlet or outlet that would lower the performance.)

Inline blowers are typically used when the static pressure (loading, or "system impedence") is very low. The system impedance is a result of all tubing, bends, dryer flaps, gates, obstructions in the laser cabinet, losses through grilles etc.

For some reason many laser manfacturers target a system impedence of around 6" H20. I assume that they have found that a typical installation will have a 6" system impedance so set a recommended flowrate at that amount of pressure.

If you want to get anywhere close to the manufacturer's recommendation, you won't get it with a 4" or 6" inline fan. It is just not possible.

Here is a typical inline fan series:

http://www.atmosphere.com/english/html/product/fans/vtx/vtx.html

They show a fan curve as well as a table. For sake of argument look at the VTX 600. You can get 452 cfm at 0" H20 pressure. At 1.5" pressure you get only 169 cfm. If you follow the VTX600 curve, it maxes out at about 2.4" pressure at which point you get the maximum pressure and zero flow. This blower was not indended to be used in an application requiring 6" SP. Typically you should be operating more towards the middle section of a flow curve.

That said, a few people use inline blowers and swear that they work well for them. I don't know the reason for the discrepancy; I can speculate that either the manufacturers are asking for more airflow than necessary, or else these users are satisfied that the much lower flow is still clearing the smoke and gases adequately for their particular needs.

So you need to decide if you want to experiment with inline blowers or try to meet the manufacturer's recommendations. If you want to achieve 330cfm @ 5.8" you will need a more conventional exhaust system. You may still have some difficulty achieving these specs, depending on location, permitted noise level, and budget.

Some people have reported good success with dust extractor equipment which is relatively inexpensive.

I am using a belt-driven Grainger (Dayton) 4C129. You can use a 1/2hp motor and get 235 cfm@ 6" SP or a 1hp motor and get 625 cfm@6" SP. The nice thing about belt driven is that you can select the motor to suit your needs.

Mike Null
03-11-2014, 11:54 AM
Richard

I respect what you say and believe that you might well be right. I do not believe the laser manufacturers have a clue what is required. I have been using my 300 cfm inline for many years with first my ULS and now my Trotec. I have a very short flow with minimal obstruction and my result is quite good.

Someone with a longer line or more obstructions could not use the unit I'm using.

I tried to do my homework before I bought my unit and was stunned at how many mfrs (fans) could not provide technical advice. After a number a calls I finally found an engineer who was knowledgeable and I bought his blower.

Scott Shepherd
03-11-2014, 12:46 PM
I went through a pretty exhaustive exercise with a friend that had a laser that had issues clearing the cabinet from smoke. He had a 900 CFM blower. He upped it to 1200 CFM. Still didn't clear it. He upped to to 2100 CFM. Didn't solve a thing. We brought in an air flow engineer with his $3,000 instrument that measured anything to do with air flow and pressures. We used 1 line coming into the machine (4"), 2 lines, shortened things up, switched to all smooth pipe. Didn't change virtually anything. Static pressure was given as the reason for it not working properly by the manufacturer. So he installed a system that had low CFM and very high static pressure (above 20"). That actually made things far worse.

At best I'd say any changes that people are recommending here made about 3-5% of a difference. What I noticed that I believe was far more critical was air speed. When you can get the airspeed to about 500 CFM (there's a handy little, reasonable tool you can buy for that), then the smoke will clear. If you get into the 600,700, or 800 CFM air speed INSIDE THE CABINET, then you won't have any issues at all. Below 450 or so and you'll start to see issues, you'll see lenses getting dirty more often because the smoke is bellowing up and getting on the lens or mirrors.

A lot of what's mentioned ini this thread is meaningless because no one's yet mentioned the actual opening size in the plenum of the machine. On the Speedy 300, it's about 13" sq. in if I recall correctly. So pulling it all kinds of larger ports isn't going to overcome the bottleneck of the 13" sq. in opening in the machine cabinet.

I haven't found anyone at any manufacturer that actually knows what they are talking about in regards to any of it. They tend to find something that works through trial and error and then tell everyone to get that same model. The thing that varies is not everyone is doing the same work. 600 CFM might be great when you are engraving 1/16" thick stock. But put a 6" tall box in there to engrave on and you've wrecked the aerodynamics of the cabinet and you'll get issues with the smoke clearing.

Get that airspeed to 500 or better CFM INSIDE THE CABINET and you'll be just fine.

A VERY important point you need to understand. When the manufacturer tells you they need 300 CFM, they mean 300 CFM INSIDE the cabinet, not 300 CFM blower. Two VERY VERY VERY different things.

Just my opinion.

Bill George
03-11-2014, 1:42 PM
I am assuming that the folks with the high CFM exhaust fans are using in an outside shop. The make up air to replace the air being exhausted has to come from somewhere, in the old days it was infiltration from around the windows and doors. Today with tight construction being the norm it may not be enough. Exhausting 200 - 300 or more CFM in a structure without make up air or outside air directly piped into the cabinet can create an unsafe condition where you draw products of combustion and carbon monoxide back into the building from gas water heaters, gas dryers and the older single vent pipe gas furnaces.

Dave Sheldrake
03-11-2014, 2:59 PM
From my post #9


A big 660/1hp is going to bring it's own problems, it's unlikey you will be getting enough infeed to balance the flow so it will exert force on things like the machine window while not really giving much in the way of benefits :(

Extraction is based on two things, flow volume / air swaps, get either one wrong and it simply won't work.

cheers

Dave

Bill George
03-11-2014, 3:17 PM
Dave I must of missed your post, you've got it right. I dealt with air flow issues and more in my service related field work. Toughest part was working with the old timers in the trades who learned the "rule of thumb" way.

Dan Hintz
03-11-2014, 3:53 PM
I do not believe the laser manufacturers have a clue what is required.
I won't be as harsh, but overall I agree with the overall sentiment. To tie into Steve's comments about the required airspeed being quoted as inside the cabinet, there's some issue with that line of thought. If they're quoting cabinet CFM rates, they should say that... and I have never once heard them do that. On top of that, you can't have 500 CFM at every part of the cabinet as the flowrate at the 4" outflow nozzle would be tremendous to make up for the change in area. Therefore, specified flow rates must be measured at a known and consistent areal quantity, the output port. The 1HP red beast from Harbor Freight was no king of collectors, but it has no problem collecting thick smoke from my ULS, and that was with my fume filter. Smoke never rose more than about 3/4" from the substrate before being pulled towards the back.

As I mentioned to Steve when making my own suggestions for his friend's smoke collection issues, airflow is not a simple problem. Rules of thumb will get you by for a generic circumstance, but when you start dealing with the details, it becomes cloudy really fast. It's a fluid dynamics problem, a problem usually solved with high-powered desktop computers and CFD software costing many many thousands of dollars. This takes a lot of time to do right, and time is money. What works on one cabinet design will often not work at all for a cabinet that has had its dimensions increased, but is, for all intents and purposes, the exact same design. Without that design time properly scheduled, most companies use trial and error, just as Steve said. Sometimes they get lucky, others not so much.

For a cabinet that encompasses, say, eight square feet, moving a few hundred CFM is plenty adequate... the trick is to move all of the air within the cabinet evenly, and that's where the real design work comes into play. Corners stagnate. Flat plates (like the bed) quickly get laminar airflow across them (like, within several inches of the front), and air stagnates close to the bed. To get air flowing properly in these areas, the inputs and outputs need to be designed and located in specific ways. Most machines have a large, thin hole at the front and a centrally located round hole at the back... not ideal by any stretch of the word.


I haven't found anyone at any manufacturer that actually knows what they are talking about in regards to any of it. They tend to find something that works through trial and error and then tell everyone to get that same model. The thing that varies is not everyone is doing the same work. 600 CFM might be great when you are engraving 1/16" thick stock. But put a 6" tall box in there to engrave on and you've wrecked the aerodynamics of the cabinet and you'll get issues with the smoke clearing.

Get that airspeed to 500 or better CFM INSIDE THE CABINET and you'll be just fine.

A VERY important point you need to understand. When the manufacturer tells you they need 300 CFM, they mean 300 CFM INSIDE the cabinet, not 300 CFM blower. Two VERY VERY VERY different things.

By the way, Steve, how did that turn out? Did you guys ever get an acceptable flow pattern?


I am assuming that the folks with the high CFM exhaust fans are using in an outside shop. The make up air to replace the air being exhausted has to come from somewhere

Some of us exhaust back into the environment after first filtering the air, negating that issue. For those who exhaust outside, yes, they need makeup air.

Scott Shepherd
03-11-2014, 4:03 PM
By the way, Steve, how did that turn out? Did you guys ever get an acceptable flow pattern?

Nope. Air speed went to zero on the front left corner and down to 200 on the right side. We semi-solved the issue in theory by opening the front door about 1". When we did, the air speed shot up and got close to the levels it needed to be to clear the cabinet. It also balanced out the airspeed across the table fairly well. We thought with 1 or 2 more minor modifications that we'd have it solved. He wasn't able to get it solved through them, so they ended up buying his machine back. It was a real shame. We were at the cusp of fixing it all and couldn't get to the right people to get it worked out.

We also asked the air flow engineer about the sucking too much air out of the room, since it was about a 900 sq. ft. room and he was running a 2100 CFM exhaust. He looked around and said "You're fine". We also tried opening doors and windows to see if it changed the air speed inside the cabinet and it didn't move it at all. Once again, in theory, that all sounds right, but in reality, it didn't change a thing.

Mike Null
03-11-2014, 6:21 PM
The biggest variable of all in determining the type and size of blower is from the machine to the exit point. A long run or several obstacles change the equation.

Scott Shepherd
03-11-2014, 7:10 PM
The biggest variable of all in determining the type and size of blower is from the machine to the exit point. A long run or several obstacles change the equation.

Mike, I thought so to until the engineer showed up on the job. My buddy was traveling about 40 ft after the blower to a roof top of a 2nd story building. I asked him about it and he said it meant nothing. He proved it by unhooking it and having 0' feet of tube after the blower. Didn't change a single reading.

Mike Null
03-12-2014, 6:18 AM
Steve

You know I'm too old to learn new things.:confused:

Bill George
03-12-2014, 9:28 AM
Mike, I thought so to until the engineer showed up on the job. My buddy was traveling about 40 ft after the blower to a roof top of a 2nd story building. I asked him about it and he said it meant nothing. He proved it by unhooking it and having 0' feet of tube after the blower. Didn't change a single reading.

Perhaps your engineer should use the same resources I did when working in the "real": world and then teaching it for 12 years before retirement. #1 The ACCA Manual "D" for ductwork and air flow design available here> http://www.acca.org/

Or #2 the ASRAE books on the subject available here> https://www.ashrae.org/

I still have a ACCA Manual "D" around someplace and the Ductulator, my ASHRAE books I donated to my department when I retired. Its been a few years since I've cracked a book open but I know there is no way your going to get 2100 CFM (Cubic Feet per Minute) through a 6 inch duct, spiral high pressure or otherwise. Resistance to air flow in a duct system or Friction Loss
is the sum of the loss in the fittings and other restrictions, as filters and registers or grills. Straight pipe has little restriction to air flow as your engineer proved.

Scott Shepherd
03-12-2014, 12:44 PM
Bill, the problem isn't that it doesn't matter in theory. The problem he said, and it was proven, was that this isn't a sealed duct system like HVAC. He was a high level air flow guy at Trane working on very large projects, so no doubt he understands it all. The problem isn't that what you are saying isn't true, the problem, as stated to me, is that it simply doesn't apply to the types of exhaust systems we're using for lasers. If you had 100 CFM flowing through things, then it might matter, but the airspeed was so high on dust collector motors that those principles didn't move anything to any measurable degree.

My example is that everyone says "You shouldn't use corrugated tubing" and he was told specifically by the manufacturer that his main issue was that piece of 8' long corrugated duct. They told him he would see a 20-30% increase by going to smooth hard pipe. We did it and it moved things 3%. That's exactly my point. In theory, that's all true, right angles, 45's, rough tubing, it all matters, but when you put the gage in the machine, it's just not moving the needle in any appreciative way.

They kept saying "You don't have enough static pressure". We measured it and it was around 4 at the machine. Well, they said that was unacceptable and it needed to be 29 inches. 29 inches? The engineer looked at that data and said "that's impossible with this type of setup and you'd have to have a turbine style setup that would easily be 30-50 HP to get those sorts of numbers". So he installed the low CFM, high static pressure system they recommended and the air speed actually dropped and things got worse.

I'm just saying that in reality, I haven't run across many (if anyone) that seem to understand how to spec out a system properly at any of the manufacturers.

Dave Sheldrake
03-12-2014, 1:02 PM
The lack of appreciable benefit will be the inflow Scotty, it's back to the swaps function, to take 100 cfm out you need to allow 100 cfm in or you end up with a negative pressure system that will restrict flow but increase velocity. It's the reason fitting a 2hp 1200 m^3Hr blower won't give much better results on a 6" pipe than a 550 m^3Hr system will given the size of the intake area.

cheers

Dave

Scott Shepherd
03-12-2014, 1:10 PM
I understand that Dave, but my point is that from the manufacturer, they said a 900 CFM system wasn't enough. They recommended 1200 or whatever it was. That changed nothing. Then they said 2100. That changed nothing. Then they said it was too high, and then he changed that to their specs again and it got worse. My point being that I don't think any of these manufacturers have a handle on any of it. Just to have people swapping systems out that costs 1000's of dollars only to say "Oh, that didn't work, try this" is exactly my point.

I understand to a small degree, air flow and some of the mechanics behind it. I also understand all the tests we did under so many circumstances that changed almost nothing at all, however, they were touted as being the fix for it.

Dave Sheldrake
03-12-2014, 1:20 PM
I tend to agree, few of the laser companies have fluid dynamics people I would think (until you get to the real industrial manufacturers)

A true 900 CFM is going to suck in passing children in an open system, I doubt there are any closed case lasers that need that kind of flow ;) My open bed flatbeds use about 750 M^3Hr flow rates and catch just about everything in the way of fumes and smoke.

cheers

Dave

Ray Scott
03-12-2014, 1:38 PM
It is good that you added a picture to your post... You should really look at getting rid of the spiral slinky hose. It hurts the efficiency of the air flow. I know it is convenient for moving around the fan, but you could also find some plastic hose somewhere. The inside wall of plastic hose is much smoother and reduces the noise.

My advice... Use the larger hose when pushing/pulling air. Keep the suction side of the airflow as short as possible. Make sure the seal the air leaks on the hoses. Purchase the 6" ductwork from the Heating/AC section of HomeDepot or Lowes. They even have some nice wall mounts for outside the garage (prevents squirrels from visiting your workshop).

Ray Scott
Rabbit Laser USA

Bill George
03-12-2014, 2:17 PM
The lack of appreciable benefit will be the inflow Scotty, it's back to the swaps function, to take 100 cfm out you need to allow 100 cfm in or you end up with a negative pressure system that will restrict flow but increase velocity. It's the reason fitting a 2hp 1200 m^3Hr blower won't give much better results on a 6" pipe than a 550 m^3Hr system will given the size of the intake area.
cheers
Dave
Dave you are right on track.
Scott airflow is airflow and the theory and math holds up on exhaust fans or HVAC, BTW The V stands for ventilation. He proved exactly what Dave and I are staying when he quote " opened the door an inch or so and we got the air flow". Putting a larger HP motor on a squirrel cage forward curve blower and adjusting the speed to run it faster is counter productive as there is a certain RPM where you stop moving air, and a fan curve would prove that point.
Static pressure on a standard heating and cooling system would be around 1" wc, on commercial systems and exhaust systems it can go as high as 5" wc. If he told you 29 inches he is full of something, and its not knowledge!

I suggest doing some online research on duct and air flow design or get the books I suggested above.

Scott Shepherd
03-12-2014, 3:18 PM
Static pressure on a standard heating and cooling system would be around 1" wc, on commercial systems and exhaust systems it can go as high as 5" wc. If he told you 29 inches he is full of something, and its not knowledge!

I suggest doing some online research on duct and air flow design or get the books I suggested above.

Thanks Bill, the machine is gone so no need to learn anything else about it. You're proving my point. When the factory tells you that it needs to be 29" WC, then obviously they don't know what they are talking about, which is what I keep saying is the problem at hand, not being able to get good, solid, concise, accurate, repeatable information from the manufactures selling you lasers that require exhaust systems.

Bill George
03-12-2014, 3:40 PM
Scott Just got back in from a painting project on the bucket build for my John Deere 318. What I am really thinking now, IF they opened up the intake opening like Ray says (your expert already proved that would work ) and then concentrate on the airflow INSIDE the cabinet I bet it would work gang busters with whatever you had. I can not for the life of me find my Manual N book which lists airflow exhaust requirements for welding processes and printing and so on. I think it also listed the face velocity of the air intakes.

They make smoke tapers that you can use to check airflow direction, or smoke candles if your really brave. But I think a little chunk of dry ice in small cup with some water and you can use as many as you need, the smoke would show the air flow inside the case and cost nearly nothing.

Anyway get back to making laser smoke and hopefully some money!!

Dan Hintz
03-12-2014, 3:54 PM
He proved exactly what Dave and I are staying when he quote " opened the door an inch or so and we got the air flow".

For the sake of clarity, Bill, I believe Steve was talking about opening the door of the laser, not the door to the shop. I mention this due to your earlier comment about needing makeup air, open doors/windows, etc. Not sure if you were thinking in those terms when quoting him...

Bill George
03-12-2014, 4:25 PM
Dan, Yes I understood that part and Ray said the same thing, they needed more intake openings. I suggested smoke tracers to actually map the air flow inside the cabinet. Its too bad the problem could not have been solved.

BTW I see you have a Jet 9x20 lathe, I have an old Emco Maximat Super 11, Made in Austria that I had to rebuild after purchasing off Ebay.

Scott Shepherd
03-12-2014, 5:45 PM
Bill, opening the door solved some of the problem, but not all of it. What is did was cause the airspeed to jump up inside the cabinet.

I only mention any of this in order to help others understand more about what's required. I do know what we tested and it was really extensive. You name it, we tried it. I even called Dan in the middle of it all to get his input and the things he suggested, we were already doing. I do know that going from a 900 CFM blower to a 2100 CFM blower made the airspeed go up substantially. I also know that all the other things people say make a huge difference made little to no difference at all, like the smooth tubing versus the flex. In theory, yes, it matters. Put the gage in the cabinet and you just don't see any benefit from it. I'm all for theory, but I'm telling you what I measured and saw, and what people say matters a lot, didn't matter much at all. A 3% increase in air speed isn't worth spending the money or time to do. Show me 10% or better and I'd say it's worth doing.

I also saw the exhaust run over 40 ft, straight up, then saw it with 0' as well. Didn't change the air speed 1%. Nothing, zero. Opening the office door to the outside. Nothing. Opening the window. Nothing. 3" ducts, nothing. 4" nothing. It just didn't change any of the readings. We worked on this for over a month, so I have a pretty good handle on what moved the needle and what didn't.

Dan Hintz
03-12-2014, 7:29 PM
I also know that all the other things people say make a huge difference made little to no difference at all, like the smooth tubing versus the flex. In theory, yes, it matters. Put the gage in the cabinet and you just don't see any benefit from it. I'm all for theory, but I'm telling you what I measured and saw, and what people say matters a lot, didn't matter much at all. A 3% increase in air speed isn't worth spending the money or time to do. Show me 10% or better and I'd say it's worth doing.

I also saw the exhaust run over 40 ft, straight up, then saw it with 0' as well. Didn't change the air speed 1%. Nothing, zero. Opening the office door to the outside. Nothing. Opening the window. Nothing. 3" ducts, nothing. 4" nothing. It just didn't change any of the readings. We worked on this for over a month, so I have a pretty good handle on what moved the needle and what didn't.

I meant to address this earlier but got pulled sideways and forgot. Without seeing the numbers for every location, my guess would be "those things didn't matter because you weren't yet in a position for them to matter". A 250hp car will get up to around 150mph, but to get a 5% increase in speed, you need to add 20% more hp (and it goes up exponentially from there). In the same vein, heavy corrugation in the tubes isn't very noticeable when the air flowing through them isn't moving very fast to begin with. If you could get the air really flowing though that tube, you'd see a more marked difference in the flow rates between corrugated and smooth.

Since the main limiting factor in this entire exercise was likely the exit port from the cabinet, changing the external tubing really wasn't having a major effect on the overall numbers. I'd bet that once that port was widened, however, the corrugated tube would have an increasingly larger effect on the overall flow rate, at which point swapping it out for smooth would have shown a larger increase.

But it's difficult to visualize everything as an armchair quarterback having never seen the setup.

Scott Shepherd
03-12-2014, 7:34 PM
Yes Dan, and I don't want to get sidetracked any more than we already are. My repeated point is that none of the manufacturers are giving the best advice for dealing with exhaust issues so I'd take what they are saying with a grain of salt. When a factory engineer tells you that you need 29 WC, there's something wrong.

I personally would follow my own method. Take the little velocity checker, put it in the cabinet. If you see numbers above 500 feet per minute on air speed, you're going to be fine. I couldn't care less what the sticker on the motor says, if that air speed says 500 CFM (The airspeed, NOT the rating on the dust collector), then you'll be just fine in most cases.

I said earlier, CFM, it's not 500 CFM that I'm reading, it's 500 feet per minute air speed. I'll try to go back and edit that. Tried to, but time has passed.

I went back and watched a video clip I had of it and saw the "ft minute" reading on the gage. My mistake. I should have looked at that before saying CFM, but it's not CFM, it's ft. per minute I'm talking about.

Bill George
03-12-2014, 8:13 PM
Scott, you are reading fpm just take that times the area of the opening you are measuring and then convert to CFM. You are using the right meter, just need to finish the math. I think you will be an expert before long! Good Luck.

Scott Shepherd
03-12-2014, 8:59 PM
Scott, you are reading fpm just take that times the area of the opening you are measuring and then convert to CFM. You are using the right meter, just need to finish the math. I think you will be an expert before long! Good Luck.

Bill, it was a measurement taken in 9 places across the table to measure the consistency of the air speed across the entire table. Air speed at the plenum at the back of the machine was 1400 ft. per minute. It would actually suck the gage against the grate and hold it there. Then we'd move to the back of the table, left edge, middle, right, then same thing in the middle of the table, and then the front of the table.

We found that a reading of 500 ft. per minute is enough to clear the cabinet. It various across the table on my Speedy 300 by about 100 ft. per minute across the table. In the case we were troubleshooting, the air speed dropped to 0 ft. per minute in multiple places on the table, which was a bad thing. Masking intake vents off in key places trying to force the air speed to increase in those areas is something we worked hard on, but in the end, we couldn't get the airspeed up to 500 ft. per minute.

I think specifying CFM and all that stuff is worthless for the most part. You can hook up 5000 CFM dust collector but if the inside air speed is less than 500 ft. per minute, it's not going to clear the cabinet, which causes premature lens fouling. I think the requirements to get that number vary greatly from machine to machine due to the cabinet geometry and aerodynamics of the placement of the inlet ports and the plenum intake at the back of the machine. So saying a 300 CFM dust collector works on an Epilog 18" x 24" machine doesn't mean a 300 CFM dust collector will work on a 18" x 24" Universal.

I just don't believe anyone is measuring or talking about the right things from the factories. In my opinion, the factories should take their machine, measure the inside air speeds using a handful of options and they ought to post those results and let the customer pick one. Instead, they all mumble things like "penn state, pick this one".

Bill George
03-13-2014, 8:36 AM
Scott when you get time will you post where you purchased your air flow meter and the make and model. It would really help others who might have the same problem..

When figuring total CFM you need to take several fpm readings across the duct or opening and average them out. Then figure the area in square feet or inches times your average, converting to cubic feet per minute. Air flow measurements can vary a lot either in a duct, across the face of a grill or inside the cabinet as you pointed out.

Scott Shepherd
03-13-2014, 9:03 AM
Scott when you get time will you post where you purchased your air flow meter and the make and model. It would really help others who might have the same problem..

When figuring total CFM you need to take several fpm readings across the duct or opening and average them out. Then figure the area in square feet or inches times your average, converting to cubic feet per minute. Air flow measurements can vary a lot either in a duct, across the face of a grill or inside the cabinet as you pointed out.

Bill, it wasn't mine, but it's one like this : http://www.celestron.com/sports_outdoors/windguide-yellow.html

That was the cheap version we used when the engineer wasn't there with his incredibly expensive device.

The problem with taking the average is that it would have masked the issue we were seeing. When you have an airspeed go to zero, that's devastating to the inside of a laser cabinet. If we averaged out the measurements across the table, then it would have appeared to be at an acceptable level. That's exactly my point, understanding what needs to happen inside a laser cabinet is not the same as an HVAC system. While I understand the principles are the same, they are very different. Having airspeed drop at the edges of the inside of ductwork in an HVAC system isn't going to stop your home from heating or cooling. Having the airspeed drop inside the edges of a laser cabinet can cost you a $350 lens and wipe out the mirror at the same time.

Bill George
03-13-2014, 10:03 AM
I just ordered one from Amazon, it was $28 and change. Not as good as the $1200 plus ones I had to use when working but good enough. I agree 100% with your posting about air flow or lack of inside the case. I was just pointing out the way to figure CFM for those interested parties.

David Somers
03-13-2014, 10:36 AM
Steve (Scott)

Thanks for laying your tests out so well!

One fault I have is I will try to reason something out and then eventually find out I was way off the mark. Often there is some other factor there I am not seeing, or my assumptions, though natural, are not correct. So you laying out your tests like this where it really matters, the inside of the cabinet, was terrific!
I have to admit, as I was reading most of this thread I kept running to manuals about fluid dynamics and thinking "what?" But as I said, once you explained you were metering inside the cabinet it all made sense, especially since that is where it counts.

Anyway....Thanks!!

Dave

Richard Rumancik
03-13-2014, 2:05 PM
If anyone wants to take velocity readings then I would suggest using a hotwire anemometer rather than a vane (propeller) type anemometer. I don't know if you can buy a cheap version but I would not really try to use a vane type anemometer to measure velocities in a small enclosure, as it will disturb the flow too much. It might be useable for qualitative measurements (finding dead zones.)


Scott, you are reading fpm just take that times the area of the opening you are measuring and then convert to CFM. . . . .

Well, sort of. Within a laser enclosure, doing this arithmetic is not very meaningful as it is a local velocity that is being measured and there is no defined area it is flowing through.

Now if you take a round exhaust pipe and measure the velocity at the center (using a hotwire anemometer or pitot tube) and multiply the velocity by the area you will get a flow in CFM. But it will be wrong. That is because the flow is not linear all across the pipe. It will be zero velocity at the walls and be a maximum at the center. So the actual CFM will be less than predicted by the simple arithmetic.

If the results are just being used for comparisons it might be fine to measure center (peak) velocity but if one is trying to get accurate flowrates then the math is more involved.

Scott Shepherd
03-13-2014, 2:23 PM
If anyone wants to take velocity readings then I would suggest using a hotwire anemometer rather than a vane (propeller) type anemometer. I



That's what our engineer was using. But he didn't leave it for us to tinker with, so we just used that version to give us some readings.

Bill George
03-13-2014, 3:44 PM
If anyone wants to take velocity readings then I would suggest using a hotwire anemometer rather than a vane (propeller) type anemometer. I don't know if you can buy a cheap version but I would not really try to use a vane type anemometer to measure velocities in a small enclosure, as it will disturb the flow too much. It might be useable for qualitative measurements (finding dead zones.)



Well, sort of. Within a laser enclosure, doing this arithmetic is not very meaningful as it is a local velocity that is being measured and there is no defined area it is flowing through.

Now if you take a round exhaust pipe and measure the velocity at the center (using a hotwire anemometer or pitot tube) and multiply the velocity by the area you will get a flow in CFM. But it will be wrong. That is because the flow is not linear all across the pipe. It will be zero velocity at the walls and be a maximum at the center. So the actual CFM will be less than predicted by the simple arithmetic.

If the results are just being used for comparisons it might be fine to measure center (peak) velocity but if one is trying to get accurate flowrates then the math is more involved.

BG Said: When figuring total CFM you need to take several fpm readings across the duct or opening and average them out. Then figure the area in square feet or inches times your average, converting to cubic feet per minute. Air flow measurements can vary a lot either in a duct, across the face of a grill or inside the cabinet as you pointed out.



Thought I covered that pretty well when I said you need to take more than one reading and then do an average. I had my students taking up to twelve readings on a smaller ductwork section. Very aware of how to do air flow measurements as that was part of our test, start and balance work, in the real world. . :) .
Scott was dealing with a company that did not have a clue of what air flow or exchange rate they even needed. Especially when they told the gentleman working for Trane he needed 29"wc static.