Methods for extra bed cooling?

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Farr0wn3d
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Re: Methods for extra bed cooling?

Post by Farr0wn3d » Mon Apr 27, 2015 4:34 am

I'm sorry to contradict you rsilvers. I have serious respect for your skills, as I use your print configuration files and it has made a large improvement on my print quality. However, in this case I think you might be mistaken.

If you're not using a duct, then a fans static pressure rating isnt as relevant as you might think. As part of my industrial mechanic trade certification, part of the course was devoted to fan types, designs, and applications.

A fans static pressure rating is based on the fan being dead-headed on the discharge end, and measures the amount of pressure that the fan can produce, and hold back, when totally blocked.
(hence the term "static pressure" because there is no movement of the air in this test)

The reason you need a fan with a higher static pressure rating for a ducted application is because the fan needs to be able to overcome more of a restriction (reduced outlet size). However when the fan is free flowing, its static pressure rating isn't of much consequence because as long as the air is getting to the where it needs to be, and in an adequate amount, that's what matters. The amount of force with which it arrives at the nozzle really doesnt matter because once its there, a given volume of air can only absorb so much heat energy as it passes by, which is why flow matters more than pressure in a non ducted application. If anything, a fan in a non ducted setting benefits more from being as close to where you want to cool as possible.

Jsc, if you absolutely must retain your current duct, then yes it might be a good idea to get a fan with a higher in h20 rating, or you can keep your 50mm fan (which produces an adequate amount of air in my opinion) if you just redesign your duct to simply "guide" the air to the nozzle while keeping the air path as close to the fans outlet size as possible, which will utilize all your available CFM and your lack of static pressure wont be such a concern.

Just my 2 cents

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Farr0wn3d
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Re: Methods for extra bed cooling?

Post by Farr0wn3d » Mon Apr 27, 2015 4:39 am

I just had a look at that fan that you posted JSC, thats a radial flow fan, sometime referred to as a "squirrel cage fan" or a blower. It will draw air in through the center and discharge tangentially (to the side, for all intents and purposes) just an FYI before you commit to ordering it, because it will require a different design than the axial flow fans that come with the M2 and the one you replaced it with.

jsc
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Re: Methods for extra bed cooling?

Post by jsc » Mon Apr 27, 2015 6:23 am

Ah, that's interesting., I hadn't noticed that. Well, in any case my current duct design is working well. It's pretty much as free flowing as it's going to get given the space constraints, with a large exit end; had to be, since I'm using it to blow over dual nozzles.

Thanks for the lesson on fan stats, that was very clear.

rsilvers
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Re: Methods for extra bed cooling?

Post by rsilvers » Mon Apr 27, 2015 3:56 pm

wrenches wrote:The reason you need a fan with a higher static pressure rating for a ducted application is because the fan needs to be able to overcome more of a restriction (reduced outlet size). However when the fan is free flowing, its static pressure rating isn't of much consequence because as long as the air is getting to the where it needs to be, and in an adequate amount, that's what matters. The amount of force with which it arrives at the nozzle really doesnt matter because once its there, a given volume of air can only absorb so much heat energy as it passes by, which is why flow matters more than pressure in a non ducted application. If anything, a fan in a non ducted setting benefits more from being as close to where you want to cool as possible.
What you say is true about needing lots of static pressure for more duct restriction. That is what is taught to people about how to use the static pressure value for fan selection, but I never disagreed with that.

I am proposing that more air hitting the nozzle area is better for removing heat than less air. I think we can agree on that. I am also saying that CFM only says how much air is moved in total rather than on one target area. So if we have two fans, both having the same CFM, but one is 200x200mm and one is 20x20mm, it is safe to say that the smaller one will be blowing faster and harder and have more force of wind. I think we can all agree on that. So how do you describe this force of wind? It is called pressure - specifically total pressure. Without knowing the actual total pressure, you can just compare fans using the static pressure value in the specs.

I don't love quoting Wikipedia, but:

http://en.wikipedia.org/wiki/Static_pressure

"In fluid dynamics, many authors use the term static pressure in preference to just pressure to avoid ambiguity. Often however, the word ‘static’ may be dropped and in that usage pressure is the same as static pressure at a nominated point in a fluid."

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Farr0wn3d
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Re: Methods for extra bed cooling?

Post by Farr0wn3d » Tue Apr 28, 2015 1:01 am

I had originally posted a reply but after more consideration I think I see what you're getting at. Either way, if you can get your hands on a 40mm fan with adequate flow and pressure ratings, it would leave all options open, not to mention you could mount it closer to the nozzle with more freedom to aim it best. I think I might see whats out there for 40mm options and perhaps try something out.

Thanks for your input Rsilvers.
Jeremy.

rsilvers
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Re: Methods for extra bed cooling?

Post by rsilvers » Tue Apr 28, 2015 2:37 am

One way to compare two fans is to heat up the nozzle to 250C, turn it off, and then graph the temp fall rate.

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Farr0wn3d
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Re: Methods for extra bed cooling?

Post by Farr0wn3d » Thu Jul 30, 2015 2:05 am

after trying to duct a higher CFM fan and losing most of my airflow because the fan simply cannot overcome the restriction of a duct, I'm interested in trying your fan idea rsilvers. Only thing is, arrow no longer has them. do you have any other fans that you have your eye on that are high static pressure?

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Re: Methods for extra bed cooling?

Post by jsc » Thu Jul 30, 2015 2:35 am

The one I linked to originally is about as good as you can find for a 50mm 24V fan on Digikey.

Go to DC Fans and start filtering. If you select on voltage and dimensions (24V, 50x50mm) that will cut down a lot. Then sort by static pressure and it is tied for second, behind another Delta model with slightly more static pressure but slightly less CFM.

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Re: Methods for extra bed cooling?

Post by Farr0wn3d » Thu Jul 30, 2015 2:51 am

Thats the one I'm currently running, but when paired with just about any duct, including ones I've designed myself to limit restriction but still guide the air, just lead to the pressure ahead of the fan forcing past the blade tips and out the back. I'm ready to give a 40mm fan a shot and see if a smaller one with better static pressure rating will do a better job. the 50mm fan you suggested definitely moves plenty of air, but that air blows all over the place rather than specifically where I need it.

There seems to be plenty of 12v fans out there with really attractive specs, however the same fan in 24v has poor ones.. go figure

CCVirginia
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Re: Methods for extra bed cooling?

Post by CCVirginia » Thu Jul 30, 2015 5:15 am

I was wondering about cooling as well. As anyone considered/tried putting a thin/wide fixed duct on the bottom of the top support bar, facing forward across the entire printer? You could mount a couple of 12v 120mm fans in series and move quite a bit of air (probably be annoying, blowing in your face - but it would also clean the bed). This would also remove the weight of the extruder mounted fan.
It sounds like a few of you have real knowledge about such things, I'm just guessing.

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