Inline Exhaust Vent?

I'm setting up my new (to me) Trotec Speedy 300 CO2 80w (with air assist) in my home office and trying to determine the best way to vent the exhaust outside.

I have searched this forum and read a bunch of previous posts. Based on my reading, I think an inline fan may be the right solution.

It is my plan to go up vertically through the ceiling and out the roof using 4 to 6" rigid metal pipe except for a short "whip" of flexible metal at the machine.

It would be a total run of 25 to 30 feet with the fan in the middle (unless it's better to place it elsewhere). I've used Fantech's FG series for remote kitchen & bath vents in the past but am open to other brands.

Trotec is recommending 500 cfm at 6 inWG at the machine port. This is for a 3" port and mine actually has a 4" fitting if that matters. It's been difficult to get any info from them beyond this.

Because the laser is located in air conditioned space (hot, humid Houston, Texas), I do not want to oversize the exhaust. Obviously, I don't want to undersize it either as that creates all kinds of problems. I need to get this right so I get the smoke out without drawing all the air conditioning out as well.

I was concerned to read that axial fans (aka inline) can gum up with suspended particles. I'm not sure if this is also an issue for centrifugal fans but they seem to move too much air.

Has anyone else done it this way? Am I on the right track or the wrong track?

Any suggestions would be greatly appreciated.

Thanks!

Joe

Any fan you use will need cleaning from time to time. Try and keep the run as short as possible and the intake or suction side of the fan should be inside the building and the pressure side outside. Caulking the joints will help and only run the fan when needed.

Joe

My inline blower is 23 years old and going strong. My run is less than half of yours but I take mine from 4" from the laser to 6" into and out of the blower back to 4" through the wall.

I opted for a dust collector instead of an in-line fan. It has a higher static pressure. I can put a small filter box that I made that uses cheap 12 in ac filters to remove most of the crap before the blower. Also it is designed somewhat to have things pass through. Maybe overkill maybe not.

I’m also in Houston. Good luck with balancing the smells and the ac bill!

You might consider a valve of some sort where you can restrict the flow or open it up depending on what you are processing.

I'm considering some kind of inline filter to try to protect the fan from flying debris and soot buildup on the blades.

Fantech makes a MERV 5 unit that looks interesting. https://shop.fantech.net/en-US/fgr--...ssette/p115883

Has anyone tried this or something like it?

My inline Fantech inline is now 16 years old. I bought it at Grainger, and for about 5 years had a 25’ run using 4” PVC pipe after the flexible metal coming out of my laser. The fan was mounted on the wall next to the laser. Currently it’s mounted on the wall in my home shop, but only a total of about 6’. I have cleaned the vent pipe 3 times over the last 11 years, but the fan works fine so I haven’t opened it up.

Thank you for all the feedback.

Ultimately, I have come to realize that an inline fan is not the right solution for my situation.

One of the best answers I got was from my Trotec rep who summarized it as follows: "The downside of an underpowered exhaust is a possible lens or mirror breaking due to debris building up. Also cutting thick material is going to be limited because of more likely flare ups. The downside of overpowered exhaust is a potentially higher a/c bill. In just about every situation, I’d recommend overpowering your exhaust because it gives you greater cutting capacity and protects your optics much better."

Trotec is recommending 500 cfm at 6 inWG at the machine port.

The Fantech technical support rep literally laughed at me and said their fans are rated at "tenths" of inWG (ex. 0.1 through 0.9) rather than whole number inWG. They may move a lot of CFM's but they do it at low pressure ratings.

Soooooo... I have decided to use a centrifugal fan with a radial bladed wheel that is rated to move the specified CFM's at a higher pressure rating. It's very similar to the popular Penn State and Harbor Freight models that are often recommended.

I am also installing a blast gate that I am hoping will allow me to modulate the airflow as needed and maybe save a little air conditioning.

For the record - I'm using a 5-foot run of 4" high temp flexible hose from the machine to the blast gate. It connects to a 6.5-foot run of 4" Nordfab pipe (chosen for appearance) going up the wall to the attic where it will connect with a 1hp exhaust fan. The output from the fan will switch to 6" snap-lock pipe (chosen for budget) and run another 14-feet to a backdraft damper and then out a roof vent.

I decided against using any kind of filter although I can always add one later.

Thanks again for your help.

I truly appreciate the advice.

Joe

FYI from over 30 years in the HVAC/R field. Static pressure is a measure of resistance to air flow in a system If a fan is rated at 500 cfm against 6 IWC pressure, when you lower the static pressure by making the duct work have Less resistance you will move more Cfm. More Cfm means the fan motor will need more amps or watts because its doing more work, as moving more air. Yes I have books and static pressure meters, air flow meters and the such, but you got the Readers Digest version.

Residential HVAC systems are usually designed to operate at 1 IWC pressure or less. Commercial 2 - 3, some much high pressure as those 2 inch PVC or metal high pressure systems that are sometimes installed in existing old historic buildings where they want to hide the pipes and supply registers.

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Originally Posted by Bill George

FYI from over 30 years in the HVAC/R field. Static pressure is a measure of resistance to air flow in a system If a fan is rated at 500 cfm against 6 IWC pressure, when you lower the static pressure by making the duct work have Less resistance you will move more Cfm. More Cfm means the fan motor will need more amps or watts because its doing more work, as moving more air. Yes I have books and static pressure meters, air flow meters and the such, but you got the Readers Digest version.

Residential HVAC systems are usually designed to operate at 1 IWC pressure or less. Commercial 2 - 3, some much high pressure as those 2 inch PVC or metal high pressure systems that are sometimes installed in existing old historic buildings where they want to hide the pipes and supply registers.

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Bill,

I really appreciate the explanation. Honestly, trying to understand airflow dynamics has been frustrating to me.

Am I correct in thinking that my Trotec Speedy 300 requires high pressure to move the smoke out of the laser chamber? Their rating requirement is "at the exhaust port" of the machine. They specify a 3" port but mine actually has a 4" port.

I was originally pursuing a low pressure inline fan like some folks on here use but became concerned it would not be sufficient.

Thanks!

Joe

No you need CFM Cubic Feet per Minute. Static pressure is the resistance to air flow in the duct work. You design duct work to have low resistance not high static pressure. These folks here have been using inline fans for years, mainly because it works. Commercial fans have "fan curves" published ratings at a given static pressure. Remember whatever air your removing from a space has to be replaced by air from someplace or you will have a vacuum. A lot of the air being exhausted is being made up from infiltration. Some folks may have a make up air duct either directly into the machine from the outside or close by.

The fan will work a lot better with a short inlet and longer exhaust. Pressure loss on inlet (vacuum) is harder to overcome that the positive pressure loss on the discharge. That is why you need larger duct on inlet than outlet for most systems. The fan closer to the unit is probably going to be easier to maintain also.

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Originally Posted by Thomas Canfield

The fan will work a lot better with a short inlet and longer exhaust. Pressure loss on inlet (vacuum) is harder to overcome that the positive pressure loss on the discharge. That is why you need larger duct on inlet than outlet for most systems. The fan closer to the unit is probably going to be easier to maintain also.

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Generally the larger inlet has to do with the volume of air, the suction side is slow moving air, the exhaust side is faster moving. Additionally, I think it is better to have the minimum length of exhaust, both to minimize friction losses (faster moving air) and to reduce the impact of any leaks that might develop. Leaks on the suction side do not contaminate the workspace, leaks on the exhaust side do - if they are indoors. Of course, you can change the velocity of the air on either side of the fan by changing the size of the ducting but for best efficiency a larger suction side is better. An exception would be for chip collection, where the air needs to be moving quickly enough to keep chips from settling. Not an issue for fumes.