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Some info about cfm calculations

PostPosted: Fri Jan 27, 2012 11:18 pm
by educa
Hi,

For my machine, I'm looking to make an exhaust to be 100% sure all air from inside the laser chamber is extracted through a hose (2m long) and out of my window


I calculated that that the total area of my machine (the part where parts are lasered) is 1200x800x400 mm big, so thats aproximately 384 liters of air



I found a 252 cfm pc fan (DELTA make)

252 cfm seems to convert to 119 liters per second.


Does that mean that theoretically this fan can move the comlete air content of my machine in about 3.2 seconds ?

Am I also right that this is theoretically because it assumes that there is no static pressure inside the machine, so the fan has no back pressure which makes it harder to suck up enough air ?



On the specs of the fan there is mentioned a maximum air pressure of 35.877 of H2O. What can I calculate with that ?

Can I somehow know how many holes are needed in the case so that enough fresh air can come in to be sucked away then by the fan ?



Any help would be very appreciated. I am not that familiar with these calculations.

Kind regards,
Bart

Re: Some info about cfm calculations

PostPosted: Sat Jan 28, 2012 1:08 am
by awesomenesser
educa wrote:Hi,
For my machine, I'm looking to make an exhaust to be 100% sure all air from inside the laser chamber is extracted through a hose (2m long) and out of my window
I calculated that that the total area of my machine (the part where parts are lasered) is 1200x800x400 mm big, so thats aproximately 384 liters of air
I found a 252 cfm pc fan (DELTA make)
252 cfm seems to convert to 119 liters per second.
Does that mean that theoretically this fan can move the comlete air content of my machine in about 3.2 seconds ?
Am I also right that this is theoretically because it assumes that there is no static pressure inside the machine, so the fan has no back pressure which makes it harder to suck up enough air ?
On the specs of the fan there is mentioned a maximum air pressure of 35.877 of H2O. What can I calculate with that ?
Can I somehow know how many holes are needed in the case so that enough fresh air can come in to be sucked away then by the fan ?
Any help would be very appreciated. I am not that familiar with these calculations.
Kind regards,
Bart


I am still very hesitant about using muffin/pc fans to exhaust the inside of the laser cutter. I would instead recommend an inline fine like this you can find them all over ebay. That is the fan that I am currently using to exhaust my standard size buildlog 2.x and it is overkill for me. I put a little fan speed controller on it that I got from my local hardware store. I can only run the fan at about 50% before my case starts whistling from the intense suction.

I think that fan might work out if you were using a very short length of ducting (like Dirk's laser) but if you are planning on running it 2 meters then you should get something made to run inline a long length of ducting (like above).

P1010850.JPG
Here is my current setup.

Re: Some info about cfm calculations

PostPosted: Sat Jan 28, 2012 1:12 am
by BenJackson
I think the key is to ensure that the outlet side of your fan is "outside" as much as possible. I think my setup actually leaks are on the positive pressure side, which puts fumes back into the room. Ideally I'd move the blower farther away but I think I'm going to have to cut a hole in the wall to achieve that.

Re: Some info about cfm calculations

PostPosted: Sat Jan 28, 2012 11:47 am
by educa
So if I understand you correctly, you would advise to use an inline fan instead of a pc fan,

I can find these inline fans too here locally (they seem to be called UFO fans and used in greenhouse gardening)

http://www.ebay.de/itm/IN-LINE-UFO-Luft ... 27c16b2a22

I just wonder, why would these be better? They have around the same CFM capacity, but will they maybe accept more static pressure ?

Re: Some info about cfm calculations

PostPosted: Sat Jan 28, 2012 2:01 pm
by mondo50m
Home depot carries a 4 inch inline fan. It moves 80 CFM and is 4" in diameter. The best part is, it sells for $14.95. Would this fan be sufficient for the 2.x build?
http://www.homedepot.com/buy/building-m ... 62960.html

Re: Some info about cfm calculations

PostPosted: Sat Jan 28, 2012 5:17 pm
by mattrsch
I would recommend looking for a centrifugal type in line fan. The axial type like you linked don't do as well with flow restrictions. For a point of reference, I use a 550 cfm centrifugal fan with about 20 feet of 4" ducting and the flow seems just about right.

Re: Some info about cfm calculations

PostPosted: Sun Jan 29, 2012 2:30 am
by awesomenesser
educa wrote:So if I understand you correctly, you would advise to use an inline fan instead of a pc fan,
I can find these inline fans too here locally (they seem to be called UFO fans and used in greenhouse gardening)
http://www.ebay.de/itm/IN-LINE-UFO-Luft ... 27c16b2a22
I just wonder, why would these be better? They have around the same CFM capacity, but will they maybe accept more static pressure ?


Yea that is what i would recommend I have not had any issues so far with an inline fan. The small PC fans will not work with a lot of restriction on either end (they work well for pc's but not ducting). The centrifugal fans like dust collectors might work too but I think you are suppose to put them at the end of the duct (They have good suction but don't like to push air a long distance) (Some people put them outside at the end of the line). The high speed inline fans (like the link you provided) are made to be in the middle of a long duct and they are designed to pull and push the air, so they are ideal for laser cutters.

Edit the "inline fan" I keep talking about is called a "centrifugal inline fan" which is what I would recommend.

I would not recommend one of these...
crappyfan.jpg
Do not use one of these.
crappyfan.jpg (8.54 KiB) Viewed 30926 times

These are designed to boost a home air conditioning system. Basically a PC fan in a tube.

Re: Some info about cfm calculations

PostPosted: Sun Jan 29, 2012 11:10 am
by educa
so if I take such a 252cfm pc fan and mount it in a tube (round tube, I can easily machine the holders on my cnc , then I also have an inline fan ?


I guess the only advantage I'll have with these bigger ones is that they use less amps and accept more static pressure ?

Re: Some info about cfm calculations

PostPosted: Sun Jan 29, 2012 4:00 pm
by awesomenesser
educa wrote:so if I take such a 252cfm pc fan and mount it in a tube (round tube, I can easily machine the holders on my cnc , then I also have an inline fan ?
I guess the only advantage I'll have with these bigger ones is that they use less amps and accept more static pressure ?


I am sorry I think I just confused the situation. I do not recommend getting a pc fan because they cannot handle restriction on either end.

I do recommend a centrifugal inline fan because their purpose is to blow through long ducts with restriction. Like this link you posted earlier Link

Re: Some info about cfm calculations

PostPosted: Tue Jan 31, 2012 12:37 am
by educa
ok so now the important question :)

My machine is about 800 liters of air content (1600mmx800mmx600mm) and I need to suck the air away + the smoke of course.

800 liters is 0.8m³

So how big should my centrifugal fan be then ? I mean in m³/h

I can for example buy 1 with 760m³/h, which means 447cfm.

The exhaust pipe from machine to window makes only 1 bend of 90° and total size of that pipe is under 4 foot

Any tips ? Would this be overkill? They also sell it in lower capacities.

Kind regards,
Bart