Potential fume extractor. Please comment

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Potential fume extractor. Please comment

Postby dirktheeng » Fri Aug 19, 2011 12:49 am

All,

I am getting ready to purchase the fan/blower for fume extraction. I have seen others use big blowers, but found 1 post that said all we need is 400CFM. I would prefer not to use a big dust extractor blower becuase it is so loud. I found a few inline blowers that seem like they could be good. They are hydroponic blowers and are designed to be fairly quiet.

I don't have far to go for the piping and can get away without any elbows (it's going strait out a window). Here's a pic of what I was thinking:

potential fan.jpg
potential fan


It does 440cfm, it's galvanized and is listed as "quiet" whatever that means... but it is only 80 bucks.

What do you think?
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Another option

Postby dirktheeng » Fri Aug 19, 2011 1:51 am

here's another option

I found several of these kind of fans... they are 198cfm and I could mount up 3 of them in a housing to make a fairly powerful extractor. They are fairly quiet as well. 120VAC

quiet fan.jpg
quiet fan
quiet fan.jpg (20.81 KiB) Viewed 23989 times
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Re: Potential fume extractor. Please comment

Postby naPS » Fri Aug 19, 2011 2:11 am

That post said 400CFM @ 6" of static, which is an amazing amount of static pressure. I'm not sure about the first fan, but the second fan is definitely not up to the task if the pressure requirement is indeed that high.
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Question for Bart

Postby dirktheeng » Fri Aug 19, 2011 8:30 pm

Bart,

What are the requirements and/or recommendations for the flow rate of air through the 2.X Laser. I saw that 400CFM value at 6" of water. However, that pressure rating has to depend a LOT on the size and type of tubing and number of bends/ total length. If I need 400CFM, and try to push that volume of air through 2 inch pipe with 20 elbows, and 100ft of piping, my pressure drop may be several feet, but if I have a strait run of pipe only 12" long at a diameter of 8", my pressure drop may be only tenths of an inch

To establish this value, it is important to know what the purpose of the ventalation is. Are we just shooting for containing the off gasses or are we trying to sweep the cut surface? If we are trying to sweep the cut surface, what kind of velocity do we need to achieve? Basically, if we are just trying to contain the gasses, I can calculate the flow rate based on OSHA face velocity targets for fume hoods and the open area of gaps and such in the skins and I can test the effectiveness with a smoke pencil or candle/match. Basically, you just want to make sure you have a moderate inflow of gas at every gap.

If we need to sweep the surface, which we might to keep soot marks down, then do we have any estimate as to what value?

Once I can determine this value, I can estimate the pressure drop because I know my piping and size a fan.
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Question for Bart - partial answer

Postby dirktheeng » Fri Aug 19, 2011 9:56 pm

All,

I made some quick estimations:

I figured there would be 3 main sorces of "leaks" in the envelope of the laser: 1) the ventilation slots, 2) the rear slots where the air and water tubing comes in, and 3) the door gap.

There are 23 slots that mesure 4mmx60mm, 3 slots in the rear that measure 12mm x 36.5mm, and a 1/8" gap around the door that measures about 19" x 29". I calculate that the total open area is about 0.324ft^2. If I give a safety factor of 2, this comes to 0.647 ft^2.

The maximum recommended face velocity for extreme toxisity and carsinogenic work is 150 ft/min. For ordinary chemical work, it ranges from 80-100 ft/min.

Using the maximum face velocity and the open area of the laser envelope, one can calculate a necessary flow volume of 97 cfm. This should be a safe level of flow to ensure that no vapors escape the laser system during operation. The only thing to be concerned about is the air nozzle if it operates in close proximity to the vanes at the front of the laser.

Also, one should add the flow rate of the air inlet nozzle. At 70LPM, one has to add about 2.5cmf. For a rounded value, one should plan on a minimum of 100 cfm.

Also, I looked into the pressure drop accross the openings... I treated it as a mesh with a given open area.. the pressure drop was very low.. so low as to say it is neglegible. The entrance effect of the air going into the exit was much more significant, but still very low (less than about 0.1"wc). The only thing left to estimate is the drop to get through the pipe outside and the exit drop effects.

The other thing to ask yourself is how long do I need to let the system vent b/4 I open it to make sure I don't expose myself to bad stuff. Usually about 3 full air exchanges purge an area (though for very toxic things, you should really calculate the time dependant concentration profile to be sure you get below the PEL). I don't expect to have really toxic stuff generated in any kind of high concentration, so 3 air exchanges should be enough. The total volume of the laser box is around 6.67 cubic feet, so about 12 seconds are needed b/4 opening the lid
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Re: Potential fume extractor. Please comment

Postby bdring » Fri Aug 19, 2011 10:10 pm

The total flow has a lot less to do with personal safety than performance inside the laser. It does not take much to get a negative pressure in the laser.

Without a high flow rate the smoke swirls around the edges and takes a while to clear. The leaks and gaps help clear it out from the edges. The intent of the slots was to wash air over the surface. It comes down to personal performance preferences. Most people running these for a while tend to like the the bigger centrifugal blowers. They do very well with longer hose runs.

This is a good source of info too.

viewtopic.php?f=19&t=120&start=10#p628
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Re: Potential fume extractor. Please comment

Postby dirktheeng » Fri Aug 19, 2011 10:50 pm

Reading through that document, the CFM rating they give for less than 10 ft run is only 235CFM. I don't know what to make of their pressure rating though. It seems to me that they should have specified a flow rate and changed the pressure rating rather than the other way around. As I have desinged fume and dust extraction systems before for work, this is very odd. Further, they don't specify the number of allowable bends or the bend radiius to duct diameter needed. That makes a tremendous ammount of difference. For a single 90 deg bend, that is equivelent to about 3 ft of tube... so 3 bends puts you past the 10 ft gradient. That's just a very odd way of specifying stuff.

It is very deceptive to publish requirements like this. The required flow rate should not increase with distance (unless you have very leaky tubing which you can seal with mastic and/or tape)... the pressure drop does... I understand the fan curves will give a higher cfm in exchange for pressure drop but the curves are very specific to the make and type, so this isn't very informative.

I know what my minimum safe operation value is as stated before, and I guess we will have to see what the operational performance is as a result of that.
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Re: Potential fume extractor. Please comment

Postby dirktheeng » Fri Aug 19, 2011 11:09 pm

bdring wrote:Without a high flow rate the smoke swirls around the edges and takes a while to clear. The leaks and gaps help clear it out from the edges. The intent of the slots was to wash air over the surface. It comes down to personal performance preferences. Most people running these for a while tend to like the the bigger centrifugal blowers. They do very well with longer hose runs.


I think that I would use a blower if I could put it outside and would probably want one if I had a long run with a bunch of bends. However, I am literally going to have about 12-24 inches of pipe run and I'll probably just use flex hose for that

As far as the smoke clearing, I can either add some strategically placed holes or a circulation fan or two inside the case.

i can get an axial fan that's pretty strong rated for almost 200 CFM for only $10 or so. I think I am going to give that a shot. My wife would really appreciate a quiet fan, I think. According to the minimum value I calculated, I am really only going to need about 35 cfm to meet the minimum OSHA requirements to keep me safe during cutting. If I need more, I will think about a different solution.

Edit: I can also get 2 fans to increase the flow rate if needed.
Last edited by dirktheeng on Sat Aug 20, 2011 2:09 pm, edited 1 time in total.
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Re: Potential fume extractor. Please comment

Postby metalman » Fri Aug 19, 2011 11:23 pm

Dirk, In your situation, you will be running your laser right next to the window that you will be exhausting through. You shouldn't need a exhaust fan or blower with a large static pressure. Though, you might need to run the exhaust through a charcoal filter, if you don't want the neighbors to get a whiff of what your cutting...

The other issue is makeup air. Your apartment is probably pretty tight with respect to air infiltration. For every CFM out you need at least that much makeup air coming in. A partially open window in another room should provide enough.

Jay
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Re: Potential fume extractor. Please comment

Postby dirktheeng » Sat Aug 20, 2011 2:07 pm

metalman wrote:Dirk, In your situation, you will be running your laser right next to the window that you will be exhausting through. You shouldn't need a exhaust fan or blower with a large static pressure. Though, you might need to run the exhaust through a charcoal filter, if you don't want the neighbors to get a whiff of what your cutting...

The other issue is makeup air. Your apartment is probably pretty tight with respect to air infiltration. For every CFM out you need at least that much makeup air coming in. A partially open window in another room should provide enough.

Jay


Jay,

I am not so concerned about the smell. I only plan on cutting wood and acrylic for most all of my projects... wood is just going to smell like camp fire and acrylic has a sweetish smell. When we move, I'll concider doing something with other materials that may have a more pungent odor. Those two materials can't be any worse than people grilling with wood/charcoal which my neighbors do a lot. I'll test it out and see how "bad" it is with those materials. If it is bad, then I'll come up with something to mitigate that. I have also designed/sized odor and toxic gas filters. Some of the hoods you can buy take the fumes, put them through a filter and return them to the room. It can be done, but it isn't cheap and it is a continual matanance thing, plus you really should have gas sensors in place to monitor for failures if you have to rely on that and the filters really increases the pressure drop. Anyhow, neither of those materials produces toxic off gassing and getting it outside and diluted is probably the best thing.

The place isn't that tight... we have vents in the bathroom that go directly outside, but opening a window or door upwind will really help.
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