Laser Venting and Noise Concerns

[Tim Bateson]

For what it's worth - My in-box air flow increased when I switched from metal piping to PVC. I might add, when I switched to PVC, I also increased the run 2x.

My unscientific test - Start smoky job, then turn on the blower:
Short run with metal piping - smoke would be pulled out, but at a lingering pace.
Long PVC run - smoke is yanked out.

[Scott Shepherd]

How does this help Chris...... Chris... how far do you need to move the exhaust to vent to the out side?. how many turns will the pipes be making? what is the height difference from machine to vent?

None of that matters Mark, that's my point. The number of turns and the height different to machine made no difference.

We had a 2,300 CFM blower hooked up, running over 2 stories of metal pipe, measured it, unhooked the metal pipe so there was no run of exhaust pipe, other than 5 feet from the machine to the blower, and it didn't change any numbers at all inside the cabinet.

My point in raising any of this was merely based on the information that was provided telling him that 300CFM was all that was required. Yes, that's all that's required INSIDE the cabinet, not on the spec of the blower motor hooked to it. Two VERY different things.You won't get 300CFM inside the cabinet with a 300CFM rated blower.

[Mike Lassiter]

something else to consider is if you are using a cutting table and the air is being pulled through it then you lay a full size sheet of material on top of it. You have the proper sized blower and ductwork, but you just choked the airflow by covering the cutting table. It is like having a dirty plugged filter on your central heat & air unit. How can it breath?
Having the potential to move 1500 cfm but restricting the inlet (cutting table covered) will not let 1500 cfm flow.

[Bill George]

something else to consider is if you are using a cutting table and the air is being pulled through it then you lay a full size sheet of material on top of it. You have the proper sized blower and ductwork, but you just choked the airflow by covering the cutting table. It is like having a dirty plugged filter on your central heat & air unit. How can it breath?
Having the potential to move 1500 cfm but restricting the inlet (cutting table covered) will not let 1500 cfm flow.

Amen.
The design of the machine air intake is very important. You are in effect designing.... in Reverse a supply air diffuser or register. They are designed for face velocity and throw to cover the wall or window or whatever you want to cool or heat with air. In the case of a air intake only under the cutting table, you need a way to bring in air from other areas of the machine, and a way to open and close those air intakes is needed. The more you restrict the air intake the higher negative static pressure and there is a limit as to how as to much air (cfm) a forward curve fan can move. Most if not all published fan ratings are in "free air", you need the fan curve to see what happens in a real world situation.

There is "friction loss" in pipe, fittings and anything else in the air flow path. That was proved by Tim when he replaced the metal pipe and fittings in his system with PVC.

[Scott Shepherd]

There is "friction loss" in pipe, fittings and anything else in the air flow path. That was proved by Tim when he replaced the metal pipe and fittings in his system with PVC.

I'm not sure I agree with that totally Bill. I agree there is friction loss in pipe, but going from metal to PVC, I can't imagine it's a noticeable difference. In fact, I saw less than 3% difference in any style pipe. That was going from a ribbed pipe to a smooth pipe. We took elbows out, smoothed out the entire intake side of things. Didn't make a difference. I suspect there's a point in which no matter what you do, not much changes, and that's because you're pulling such a high CFM through a system. I suspect at lower CFM's, you'd probably see more noticeable results, but that's just my gut feeling.

I have a number of youtube videos that are private from when we did some of the testing. I'll watch through them when I get some time.

I don't disagree with the theory, but I think you can pull so much on a system that changes start to become very minor. A 2300 CFM system on a laser with a 4" duct, that's then necked down to 19 sq. inches of intake inside the cabinet and 18 sq. inches from the front of the cabinet (letting fresh air in) is going to struggle to show any improvements, I think, because you're maxing out the system, so to speak (although not technically maxing it out, but rather pushing past the top of the bell curve).

[Scott Shepherd]

I think the number is 580 fpm inside the cabinet. That's where we saw smoke clear on various machines. Forget any numbers I stated earlier, they were from memory (or lack there of). I just watched a video again.

[Bill George]

You can't push or pull 2300 cfm (cubic feet per Minute) normally thru a single 4 inch pipe. It would need to be a special (expensive more Hp) blower designed for high static pressure. Why not just go to a larger pipe to more air with a standard blower?

Scott 600 fpm is not a un-realistic number for a standard blower system.

High static, high velocity HVAC systems do use flexible and hard PVC piping. OK it might not be PVC because of the fire hazard, but it is plastic.

[Scott Shepherd]

Bill, their system is lower CFM, but it pulls 5,800 PA, or about 24 inches of water (if I said that right). They said that high static pressure would solve the problem. It didn't. It got worse. The high CFM, low static pressure out performed the low CFM high static pressure system by a significant amount.

That's when I came up with measuring the air velocity and determining what actually worked in real life, rather than on a computer sitting in an engineering lab. What I did learn is that 580 fpm, smoke clears properly from all the systems I tested. Below that, not so much. Didn't matter what was pulling the air through it, high or low static pressure, it just mattered that it was hitting that magic sweet spot of 580 fpm.

I'm sure someone way smarter than me can tell me why it works and map it all out, but for me, I just needed to try and get it working and that's where we ended up.

[Bill George]

Scott it does not surprise me that a person in the field doing what you did, solved the problem. I will take someone who tries it out and makes changes until it works over someone who just has the book learning like some engineers I had to work with. Maybe next time they need to come and ask you what to do!

Static pressure is the air pressure pushing outward on the duct walls and out any openings. On the intake side of the fan, its the opposite.

SP is measured as Inches of Water Column. Most commercial HVAC systems have 5 IWC or less, residential usually less than 2, exhaust systems.. well I have never seen them at 24 but...

[Kev Williams]

I'm sure I'm in the minority here, but it seems like way too much rocket science for sucking some
smoke out of a metal box...

I have a sure fire method that works great for me-
If the blower I'm using sucks out all the smoke, I'm golden

[Scott Shepherd]

I'm sure I'm in the minority here, but it seems like way too much rocket science for sucking some
smoke out of a metal box...

It is until it's your box it won't suck smoke out of

[Bill George]

You know Kev, that's kind of the way I look at it. I hooked the blower up... the one I had on hand to the new to me ULS and the smoke went outside. True its a little noisy but working is the key word. I need to get back to work, had a large order come in and the promise of another Monday.

[Mark Sipes]

With all said....... I hooked up a 2 hp blower to my machine and ran it the 4 feet to the vent and the material on the table keep getting sucked into the exhaust. Solution was to run 87 feet of pipe around the room from the table and then out the vent. That worked......then I woke up from this nightmare and went to my laser that runs a 300 cfm inline fan and went to makin' money.



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