HELP what to use for exhaust line

[Shawn Cavaretta]

my laser is in my shop which is in a strip mall. i can not put a permentant vent out. what should i use for a exhaust line. it has to be 6" dia the run is about 75 feet. i am using a 1 1/2 hp DC blower pushing about 1400 cfm

[Joe De Medeiros]

my laser is in my shop which is in a strip mall. i can not put a permentant vent out. what should i use for a exhaust line. it has to be 6" dia the run is about 75 feet. i am using a 2 1/2 hp DC blower pushing about 1400 cfm

if you are going to run 75ft, it should be smooth wall sheet metal (snap lock),
the duct is too small for that long a run, the friction loss will be very high, you need at least 8" duct

Round duct
friction loss for 75' @ 6" is 12.9 in/wc and 14.7in/wc for flex duct
friction loss for 75' @ 8" is 2.68 in/wc and 4.4in/wc for flex duct

I don't know what your fan is rated at but it's probably 12" max static pressure.

fan curve for 2.5hp DC
http://i30.tinypic.com/fdr4hz.jpg

What I would recommend is build a filter like Dan's (perhaps larger) and pump the air back into the room.
http://www.sawmillcreek.org/showthread.php?t=143725

[Shawn Cavaretta]

the blower is with in 6 feet of the laser using a 4" metal duct to the laser. then there is 75 feet to the outside door

[Dan Hintz]

The problem with pushing the air (blower so close to the laser) that far rather than pulling it (blower at the end of the piping) is leaks in the piping will be immediately evident... a sucker setup will only drop in pressure with leaks, but a pusher setup lets you know with the smallest of leaks (but it's not kind enough to let you know WHERE the leak is).

[Mike Null]

If you can't use a permanent duct out why are you worrying about 75 feet of duct?

It would seem that a filter is your answer.

[Joe De Medeiros]

if you attempt to push or pull air at higher that the rated fan pressure, it will cause an aerodynamic stall of the fan, in the case of 75 feet @ 6" the fan will not be able to push enough air. Most dust collectors have a maximum length for pulling air (sucking), rated in a 4" inlet, example a 1 hp 750cfm DC will have a max static pressure of 6" wc, what that means is the maximum before stall would be 15 feet before the fan would stall. The way around this is to increase the fan size (expensive), or increase the duct size (cheaper). using a short 6 feet and feeding it to a air scrubber is your best bet, unless you want to increase your fan size to something that will over come the static pressure.

PS. I use to design ventilation and extraction systems for industrial and commercial systems.

[Joe Pelonio]

If you can't use a permanent duct out why are you worrying about 75 feet of duct?

It would seem that a filter is your answer.
Perhaps cost is a factor, maybe $100 for duct vs $thousands for filter?

I had a 25 foot run and made sure the seams were all duct taped, a slight leak will quickly cause alarm for other tenants (and a visit from the local fire department).

[Shawn Cavaretta]

if i went with a filter i would be replacing them weekly all i do is vector cutting.

i have been running this setup for 3 years now. but today the ductline tore and filled the shop with smoke.

[Dan Hintz]

a 1 hp 750cfm DC will have a max static pressure of 6" wc, what that means is the maximum before stall would be 15 feet before the fan would stall.

PS. I use to design ventilation and extraction systems for industrial and commercial systems.
This does not compute.

My POS Harbor Fright 1HP blower is used by quite a few people here and they have no problem with runs well exceeding 15' total length (that also includes several bends)...

[James Terry]

DIY it might cost you $50 to cut a hole in the sheet metal roof and install a good vertical vent with waterproofing. And it might cost you $200 to have it removed when the time comes if anybody even remembers about it. Just do it. I'd rather ask for forgiveness. They ALWAYS make exceptions anyway. What if a Subway was going in and they needed exhaust for their ovens... Or whatever.

You can also go through the back wall and use a dryer vent cap. Owner would probably never even notice either of these.

I'll bet if you stick your head up above the ceiling you will find old vents that are no longer in use somewhere within plugup reach.

Isnt there an existing toilet vent somewhere?

Find a matching door for the rear, mount it and put a vent into it. Keep the original door inside to show them when they decide to complain about it if they ever do.

You can have sheet metal vent caps made in any shape or form so you could expand that 4 or 6" vent into a wide flat final vent to fit under or above a modified door and it would be rather stealth.

You could get creative and vent out the power feed conduit. They never have those things full.

Find an existing light on the back wall and covertly replace it with a vent.

Move to somewhere else that isnt owned by idiots.

How many walk-ins do you get daily? If you dont really need the storefront, put your rent into a very nice outbuilding in your backyard.

[Mike Null]

Move to somewhere else that isnt owned by idiots.

Or some place where they never heard of a fire insurance inspector.:rolleyes:

[Shawn Cavaretta]

James

i wish i could do one of those.

the back wall is concrete. and is below grade. there is a second floor above.

already tried the bathroom vent and it fills the shop next door with smoke.

i own a hobby shop and the laser is in the shop and i produce model railroad building kits.

so moving the laser wont work.

[Mark Winlund]

You could get creative and vent out the power feed conduit. They never have those things full.



If you do this, you are going to have lots of new friends that have uniforms and badges.....

Mark

[Dan Hintz]

James,

Those are some very scary suggestions...

Being in such close proximity to other vendors (and customers), it would most likely get you arrested for killing someone with a bad allergy if you exhaust anywhere near people without properly filtering first. That said, you have to most likely filter anyway, might as well make it a good one and vent back inside.

[Mike Null]

In a strip mall abiding by the rules is vital as the livlihood of others is at stake.

Many years back my next door neighbor in a strip mall had a fire in his store. It closed me down for two weeks and could have been worse. He wasn't breaking any rules except common sense rules. A solvent rag started the fire.

[Joe De Medeiros]

This does not compute.

My POS Harbor Fright 1HP blower is used by quite a few people here and they have no problem with runs well exceeding 15' total length (that also includes several bends)...

oops I had it set to 4" pipe.

the HF has 914cfm @ 1hp it's a little more powerful then the 750cfm.


here some numbers from the PSI site
• Up to 20 drops
• Up to 75ft run (this is total run)
• Shop size to 5600sq ft
• 1700 CFM (this is at 2.5" wg this is with almost no duct connected)
• 3.5HP Motor
• 14.25” Max Static Pressure

I also have not added system effect by pull vs push, pull has less system effect than pushing.

[Richard Rumancik]

The problem that I have with recirculating air is that there is no really easy way to be sure that the air is not contaminated. If you assume that if you can't smell it, then it is safe, it might get you into some trouble. There are many contaminants that you might not be able to smell. It is a bit of an open-loop system in that you have to assume it is working unless something suggests otherwise (eg you feel nauseous). Yes, you can weigh the charcoal but this is pretty indirect. If there was a simple instrument that could tell you air quality so you would know when to change filters then it might be more viable for laser users. But with all the different toxic gases generated from burning numerous plastics and organics, what chemical would you look for? I'm thinking a canary might be a good idea . . .

Shawn, are you cutting only wood? That might make it easier to set up a filtration system if you have a single material being processed.

Personally, that concrete wall seems like an attractive option to me. It is not that hard to punch a 4" hole in a concrete wall and run a snorkel exhaust up the back wall. And it would not be that expensive to close it off (if and when you ever leave.) Is the concrete wall in the back where it would not be that visible? You would want to go up as high as you can and away from any windows/intake vents. Maybe there is some way to make it less obtrusive by running it alongside some other "infrastructure" on the building.

Maybe there is a hidden location that you can run a pipe to the roof. Is there a vent pipe for the toilet? Perhaps you could run an exhaust pipe along side the vent pipe.

Without seeing your exact situation it is difficult to make good suggestions but if you are stumped (sounds like you have thought about it a lot and tried a few things) then I'd bring in a tradesperson who can give you some practical advice and might identify a solution that would work for you and keep the landlord happy.

[Dan Hintz]

oops I had it set to 4" pipe.

the HF has 914cfm @ 1hp it's a little more powerful then the 750cfm.
Still does not compute...

My entire run is 4" pipe, I have my custom charcoal/HEPA filtration system in there, etc., and at 90% blower speed the smoke closely hugs the substrate from the front of the machine to the back. When the vector table is installed, cardboard sheets are sucked quickly to the table and don't budge with me pushing on the top... I have to peel up a corner and reposition.

Whatever tool you're using to compute this stuff with is obviously not providing very valid answers for the real world. When I was first researching my DC needs, I spoke with a number of exhaust techs (from some of the big companies)... none of their suggestions proved out in the real world. They all said the HF units were entirely too underpowered for my needs, quoting me $4-5k blowers as a minimum to get the job done (some were talking 3-phase units here). Real-world experience has shown the HF units to be plenty powerful for laser exhaust.

[Joe De Medeiros]

Still does not compute...

My entire run is 4" pipe, I have my custom charcoal/HEPA filtration system in there, etc., and at 90% blower speed the smoke closely hugs the substrate from the front of the machine to the back. When the vector table is installed, cardboard sheets are sucked quickly to the table and don't budge with me pushing on the top... I have to peel up a corner and reposition.



your system is running as a vacuum systems, and the original poster was talking about pushing the air 75ft down a 6" duct, this is very different. The math is also different.

[Dan Hintz]

your system is running as a vacuum systems, and the original poster was talking about pushing the air 75ft down a 6" duct, this is very different. The math is also different.
But according to your post the unit would stall trying to push air down a 4" pipe a mere 15'. I may not get as strong of a reaction pushing, but I guarantee the motor won't stall if I swap which end I connect the blower to.

[Joe De Medeiros]

But according to your post the unit would stall trying to push air down a 4" pipe a mere 15'. I may not get as strong of a reaction pushing, but I guarantee the motor won't stall if I swap which end I connect the blower to.

The stall just means it's not pushing air, the motor would still be rotating, it's an aerodynamic stall, not a motor stall. When you look at the fan curve it tells you at what point you your CFM will drop off as you approach the max static pressure the fan can push. The smaller you make the duct the higher the static pressure will get. To maintain the same CFM the duct has to get larger the longer it gets.

[Dan Hintz]

Okay, I can live with the explanation of aerodynamic stall... I'll have to try it...

[Joe De Medeiros]

Okay, I can live with the explanation of aerodynamic stall... I'll have to try it...

The new fan your looking at the specs say run length 30 feet and it has a max SP of 9.5" wc, this is from the site, they even post a nice fan curve so you can see how the CFM's drop off with static pressure. So if I do a quick calculation at the max of 30 feet it would be at 5.6" wc, you would have to go to 51 feet before you would exceed 9.5" wc.

[James Terry]

How does larger pipe to maintain static pressure relate to velocity? For vapor only this may be a non-important concept, but if you are also moving dust or larger, then velocity is maybe more important than static pressure..?

I would think that making a pipe larger after the blower would slow down the moving air. Therefore you end up with more of a smoke stack instead of a blast stack. And if you were inclined to shrink the pipe down at the end in order to regain some velocity, then any debris would probably fall out inside of the pipe.

I dont know anything, I just get lost in my thoughts...

[Joe De Medeiros]

For debris you want the FPM to be greater than 3000fpm, as you increase pipe diameter you decrease FPM. Went designing a system you start by how much CFM, and velocity is required, and select a fan that will meet those specs, and still overcome the friction loss in the system.

The idea is to not lose to much CFM due to friction loss (loss in CFM = loss in velocity), and the longer you make a run the more friction loss there is. If you make the pipe too small you increase the friction loss, think of it like blowing through a straw, you have a harder time with a straw compared to a 3/4" hose. When designing a ventilation system you want it to draw, not push air. Dust collectors are designed to draw air, and have little to no duct on the push side.