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Beezerk
09-10-11, 11:01 AM
I've just ordered a new 110cfm 120mm fan for my pc case grow, the smell is getting bad so I need to take action and the fan I got seemed to have a decent cfm without being stupidly loud.
I already have a 120mm fan fitted on outtake so I was thinking of utilising both in a home made carbon filter.
So my question is, which fan would go first, high power or low power, so for instance...

case outtake > 50 cfm fan > 110 cfm fan

or

case outtake > 110 cfm fan > 50cfm fan

Get it?
I'm sure there's an equation to work it out but I'm ballsed if I can find it lol.
They will be integrated into a carbon filter which I'm yet to design but I need the basic set up right first.

Tabbatha
09-10-11, 11:12 AM
Highest impedence first as they push harder than they pull.

Further, if you double them up next to each other it has the effect of making them both run a bit faster because they take a bit of load off of each other. If it were me I'd have them both pushing air through the filter.

:)

Beezerk
09-10-11, 11:19 AM
Yeah thanks for that mate, the answer I was looking for, just had a little rethink, my initial idea was something like.

110 cfm fan > carbon filter > 50cfm fan or vice versa.
But alternatives are 110 cfm fan > 50 cfm fan > carbon filter or
carbon filter > 110 cfm fan > 50 cfm fan or
carbon filter > 110 cfm fan
50 cfm fan these two fans in parallel rather than in series if you get what I mean.

Fuck me it started off as a simple idea and now I need a fancy degree to work it out lol

I guess once the new fan arrives I can toy around with placement to see which works best.

Tabbatha
09-10-11, 11:24 AM
Basically if you work on the simple principal that PC fans push harder than they pull, and that a faster fan at the back will speed up the one in the front, but not the other way around.. the choice is straightforward.
You definitely want to be pushing air through the filter rather than pulling it as PC fans have pretty low impedence (the torque that they can use).

But, as you say.. getting them in your hands and fiddling around with the filter is usually the best way to see which order to put them in.


:)

Beezerk
09-10-11, 11:29 AM
Thanks mate, much appreciated.

Tabbatha
09-10-11, 11:31 AM
My pleasure fella. Can't wait to see the mod as I love all this sort of stuff :)

Anonymiss
09-10-11, 01:42 PM
In series is better in a high-impedance system.

Two fans in parallel will double the airflow at free delivery, but won't increase the pressure available, so it's most suitable for low impedance systems. Putting them in series will provide more pressure, and so more airflow in high impedance systems, but in a free-air system they won't shift any more than a single fan.

It looks something like this:
http://i52.tinypic.com/14smjk8.png

I'd put the higher powered fan on the intake side of the filter (blowing), and the lower-powered one on the exhaust side. Or, you could put them both in series on the intake side, but you'd probably want a flow straightener of some sort between them, or at least a reasonable length of duct. Simply stacking them close together won't make very much difference at all.

Anonymiss
09-10-11, 02:02 PM
Hmm... Thinking about this a bit more, putting them in series on the intake side is probably best. But using a flow straightener is important.

Fans work best with a laminar flow on their input side, but the air coming out of a fan is rotating (in the same direction as the fan) and turbulent, so the second one won't get as much of 'grip' on it compared to when it's coming in straight. It may increase the rotational velocity, but won't actually increase the pressure very much.

Which one is 'first' probably won't make much difference. A fan will produce a certain pressure at its output, but this is a pressure increase compared to its input, so the overall effect will be very much the same whichever way around they are.

Beezerk
09-10-11, 02:45 PM
Christ, I need a week to work all that lot out ;)

Cheers mate.

Anonymiss
09-10-11, 03:02 PM
Christ, I need a week to work all that lot out ;)

Hee hee! Sorry about that!

Short answer is: Best move in a high imepdance system (such as with a carbon filter) is probably to use them in series, on the intake side of the filter (blowing), with a flow straightener between the fans. If you don't use a flow straightener then any gain will probably be minimal, at best.

In a low impedance system then parallel may be better.

Tabbatha
09-10-11, 04:14 PM
Christ, I need a week to work all that lot out ;)

Cheers mate.

Just make it simple on yourself and double them up ;)

Beezerk
09-10-11, 04:31 PM
Lol ta.
Had a think while driving to cut down an auto with a touch of rot, will having the lower power fan second in the chain not restrict the power of the first or is my logic incorrect?
For some reason I'm convinced that with a higher powered fan second it will boost the first fan and make it easier to shift air.

Anyway, fuck this for now I'm going outside for some top notch Lowryder :smokie:

Tabbatha
09-10-11, 04:51 PM
My experience with PC fans agrees.

Just put them together when you get 5 mins and see for yourself. Listen carefully and put your hand in front each time and see what result you get. Then swap and repeat.

If they're doubled up right next to each other it's the same sort of effect as a double bladed fan (I think). Anon's idea seems to be focused on there being a gap in tbetween them, creating a situation where one fan struggles with the other, whereas mine slaps them both directly together and eliminates that issue.

What I know for sure is that I've built enough high-end PC's to know that doubling-up fans at the exhaust increases exhaust air and reduces the inside case temps compared to having one fan there on its own, so it must have an exponential effect somehow.

:)

Beezerk
10-10-11, 12:20 PM
Found this earlier...

http://www.physicsforums.com/showthread.php?t=258634

Looks like someone has the same idea as me, love the "can't really explain what I'm doing" comment lol.

Tabbatha
10-10-11, 12:28 PM
love the "can't really explain what I'm doing" comment lol.

Yeah it's pretty obvious what he's up to. ;)

Beezerk
10-10-11, 12:39 PM
I asked some guys on another forum I go on (non weed related) who are kinda clued up on this stuff, they seem to be in agreement that having two fans in parallel would shift more air, how the fook do I build a manifold for that then heh heh.
Actually just had an idea...

If I had the 110 cfm fitted to the pc case which exhausts into some kind of tube/manifold, then another fan fitted onto the side of the tube/manifold but facing towards the end where the carbon filter is. In effect you've got air from the pc case and fresh air coming into the manifold. Saves cutting out another hole in the case for the 2nd fan but I don't know if it's a stupid idea.
Sounds impressive though lol, where the fuck are Mythbusters when you need them!

Tabbatha
10-10-11, 12:47 PM
Parallel FTW. :)

Anonymiss
10-10-11, 12:49 PM
Anon's idea seems to be focused on there being a gap in tbetween them, creating a situation where one fan struggles with the other
My point is that the gap, and preferably a flow straightener, is important and will bring better results. Although pushing air straight through the second fan will result in it rotating, even without power, you don't want the rotational velocity of the air to drag (or push) the other fan around. Better results will be obtained if it's a laminar (straight, smooth, non-turbulent) flow.

In the absence of a flow straightener, a gap of (I think) three fan diameters is the minimum that the CIBSE recommend.

Fans that rotate in opposite directons are even better.


slaps them both directly together and eliminates that issue
Doing this is what creates that issue.


What I know for sure is that I've built enough high-end PC's to know that doubling-up fans at the exhaust increases exhaust air and reduces the inside case temps compared to having one fan there on its own, so it must have an exponential effect somehow
It will help, but not as much as if the flow is straightened first.


If they're doubled up right next to each other it's the same sort of effect as a double bladed fan (I think).
In those fans, the blades rotate in opposte directions. Someone (Delta?) makes PC fans with contra-rotating blades that work like this.

Anonymiss
10-10-11, 12:53 PM
they seem to be in agreement that having two fans in parallel would shift more air
They will, but only in low impedance systems. Using them in parallel won't increase the pressure.

Beezerk
10-10-11, 01:15 PM
Right, so I need higher pressure to force the air through the filter at the expense of cfm?
Something on those lines anyway ;0)
So back to one of the initial questions, high power first or second?

Anonymiss
10-10-11, 01:15 PM
Someone (Delta?) makes PC fans with contra-rotating blades that work like this.

Here's one (7,500 / 6,500 RPM):

http://www.youtube.com/watch?v=4GY8fj8lm-s

And another:
120x120x76mm
220.29 CFM @ 3,550 / 3,200 RPM
59.0 dBA
31.80 Watts
2.65 Amps
Pressure 15.04 mm H2O

http://www.youtube.com/watch?v=SCGJ42w02EU

Anonymiss
10-10-11, 01:28 PM
This is quite impressive! This pair will push themselves along a table, but damn they're loud!


http://youtube.com/watch?v=aNrPzJ7ykXY&t=1m36s

Beezerk
10-10-11, 03:04 PM
Done shit more reading up, so stacking them will give me increased air pressure which is what I'm after.
I've realised I have 3 fans lol, my final question is.

Would... fan>tube>fan>filter>fan work? or maybe just fan > filter > fan to save hassle.
So I'd be pushing air into the filter and an extra one pulling it through.
Is the fan on the outlet side of the filter a waste of time?

Beezerk
16-10-11, 01:23 PM
Ok I'm up and running, it's a fair old Heath Robinson set up but seems to be working fine and an added bonus is the temps in the pc case around the grow light have come down.

7650276503

I found that stacking the fans didn't really do anything and after trying lots of other configurations I decided on another fan blowing into the manifold to aid pushing the air from the case through the filter.
I dropped lucky with the manifold, a mate got it me on Friday from his factory.
Got the 110 cfm fan in the pc case blowing into the filter, smaller cfm fan blowing air from the outside both directed into the activated carbon which is trapped between a pair of the wife's tights ;0)
I used a couple of round covers from cd spindle cases to connect the fans to the manifold, I found them to be a perfect fit around the 120mm fans.
So far so good :leaf:

Tabbatha
16-10-11, 02:47 PM
Oh man.. I hadn't pictured it being like that at all!


Nice job, though. If it works it works :)

Beezerk
18-10-11, 07:15 PM
Got home tonight after working up in Scotland for a couple of days, not a hint of a smell in the house, I only really noticed it when I went right up to the case and even then if was very faint.
Looks to be working great so far, I may add another fan underneath the section where the carbon is sucking air through the filter later on in flowering, I imagine I'd be able to add more carbon then to fully ensure no stink.

Tabbatha
18-10-11, 08:31 PM
Gotta +1 you for the making of your own CF fella. Top mod.

Glad it works well, too. If it didn't you'd have noticed it the moment you walked in the room.

:)

Beezerk
26-10-11, 06:57 AM
Quick update...
There was definitely still a bit of a smell still coming from the case so I modified it slightly.
I removed the external fan which was blowing in fresh air into the manifold (I realised it wasn't working as I hoped it would) and fitted it to below the filter so it's now sucking air through it, I put a lot more activated carbon in the filter and also blanked off the inlet where the fan had been with gaffa tape.
The case temps rose quite quickly while the fans were down but dropped dramatically as soon as it was back online, the difference it makes it quite astounding.
Two days later and not a whiff, I'm very pleased with it, the plants are now well into flowering and are a smelly strain to say the least lol.