Would air from 2 fans negatively interact & reduce flow?

[CaptRobertApril]

Now, correct me if I'm wrong (and I know you will) butt... and it's a big butt...

I'm designing a killer system. The sucka is gonna generate enough heat to get to be #1 on Al Gore's hitlist. Since the case is in the next room and I wouldn't hear it if was running a rocket engine, I wanna take both the sidecovers off the huge tower and plunk a monster three-foot wide industrial fan next to it. Crank the fan up and watch Dorothy fly through the room looking for Toto.

The question is, should I take out all the fans that are currently in the system? From what (little) I understand about fluid dynamics, it would seem to me that the smaller fans would negatively interact with the flow created by the monster fan and actually provide less airflow to the components.

Let's not discuss the PSU since that's a box and it's rather sheltered from the massive airflow. Let's just concentrate on stuff like the HSF and GPU.

This should be interesting... 8)

 

[rwaritsdario]

You are somewhat right, but the problem is that if you remove those fans youll be removing concentrated airflow to those parts, and CPU and GPU need a fan of their own... feel free to remove every other case fan tough because youll kinda have enough airflow...

 

[CaptRobertApril]

You are somewhat right, but the problem is that if you remove those fans youll be removing concentrated airflow to those parts, and CPU and GPU need a fan of their own... feel free to remove every other case fan tough because youll kinda have enough airflow...

Fair dinkum, but let's say that I go with some monster air CPU cooler like the Scythe Infinity I could definitely run that in passive mode. Sticking a 120mm on that would be overkill when it's four inches away from a three foot fan, right?

 

[still_life]

Why don't you buy an air cooler an stick that next to your case.

 

[CaptRobertApril]

Why don't you buy an air cooler an stick that next to your case.

I'd love to but the room the case is in has no windows. That would be great if I could make it work, but I can't.

 

[HotFoot]

Hello,

I would suggest that you might be disappointed with the results coming from pushing massive quantities of air in from the side of your computer. As the flow approaches the flat plate of the motherboard, it will divert and flow out the sides/top. There may not actually be very good flow rates close to the surface of the motherboard, and most importantly the cpu, gpu, and ram. On my computer, all of these components are passively cooled, and I can get away with this because I have a <100 watt CPU and very good case ventilation.

The current trend with computer air-cooling is to design the case so that air flows from front to back in a sort of "wind-tunnel". I'd suggest you look up how the BTX form factor has been designed for thermal considerations. In many cases, modern cases achieve better cooling with the side panel on rather than off. If you really want your industrial-sized fan to provide effective cooling, I would recommend fitting it with a custom funnel to push air into the bottom half of the front of your case, and leave the side panel on so that the air is forced to flow out through the hot parts of the computer. In this case, you should make sure to remove the intake fan by the hard drives (replaced by the huge fan and the exhaust fans at the rear. I'd leave the PSU alone, but you might find the forced flow through the fans causes annoying vibrations. Leave the cpu and gpu coolers as they are designed, so if they came with fans, leave those on for strong local airflow.

And finally, if you are planning to stick with the side-injection airflow, make sure you don't have a front air intake fan fighting the flow. Your huge fan will be the only air intake for the system.

 

[scorch]

One drawback to your cooling solution is that dust, (and if you have pets) pet hair will get thrown into the case. With my old 486 system I cleaned it out like every other year because there was only the power supply fan, with my current system with 3 fans (rear, processor, powersupply) im cleaning it out at least 3 times a year.

 

[CaptRobertApril]

If I go with the "closed case" option then I could use a "nautilus squirrel cage" type of fan that is going to have an even higher CFM rating than the 3' wide industrial fan. The windspeed inside that case would be inconceivable. I wouldn't be surprised to see 30mph. As long as any vibes it sets up generate noise but don't damage the components I don't care as I couldn't hear anything that's going on in that room. I also don't have to worry too much about dust as that little room is sealed, I have no pets and there is no carpet, etc. Well, you guys got me thinking... and that is a dangerous thing! :lol:

 

[rexter]

These guys have valid points.

I was just wondering if you would not mind to add water colling with that assembly then you might have a better cooling than just air power. You still need to redirect air flow though.

Best airflow design is if you have the same amount of air coming in as it and out. Like HOTFOOT said check the BTX design.

Proper location of components is important for optimize airflow.

 

[CaptRobertApril]

These guys have valid points.

I was just wondering if you would not mind to add water colling with that assembly then you might have a better cooling than just air power. You still need to redirect air flow though.

Best airflow design is if you have the same amount of air coming in as it and out. Like HOTFOOT said check the BTX design.

Proper location of components is important for optimize airflow.

I've used WC in quite a few builds, but that was mostly in the bad old days when we were using aquarium pumps. The build I'm considering now is a dual quadcore with 5 internal HDs and top-end DX10 cards and I just don't wanna run plumbing to all this stuff. What I've found in some WC builds is that the plumbing is set up properly for the CPU and GPU and since the guy is getting good temps on the readout he doesn't concern himself with the air cooling on the rest of the equipment in the case, usually ending up with searing hot HDs, RAM, Northbridge, etc. The setup I have with the case in another room really begs for a killer air system and I'm pretty well stuck on having it that way.

 

[Guest]

I think that one of your best bet would be to get a good case like this one:
Tj09 and stick super high CFM fan in there. With 4 high CFM 120mm fans I wouldn't worry about cooling!

Neat thing is the dedicated fan for video card, One 120mm on top of each quad core, dedicated fan for the HD. I am sure it would cool it properly

Imgaging sticking 3 of Those as exhaust!


BTW I think this case is set to have 25mm fans but You can probably make it hold 38mm no problem!

 

[JMecc]

If you want to put a great big fan to blow on the components I've got a good layout for you - see these pictures:

http://img209.imageshack.us/my.php?image=mobocutawaylv2.jpg

http://img209.imageshack.us/my.php?image=hdcutawayzo5.jpg

http://img184.imageshack.us/my.php?image=straightonyl7.jpg

http://img208.imageshack.us/img208/1156/moboxx8.jpg

Just need some sheet metal, a bender and a welder. Airflow should not be a problem with the 12" fan at the front.

Jo

 

[CaptRobertApril]

@labbbby

Those are definitely killer fans. The main reason I really wanted to go with an external single fan is to plug directly into the mains and bypass the PSU that is already gonna be feeding two quadcores and DX10 cards, etc. PSUs that can handle that job no problem start costing $500 and up and that's just too much damn money for something that is little more than a fancya$$ transformer. :lol:

@JMecc

That looks great! Kinda like a wind tunnel for your components. Great airflow too. It would be fun to put that into an acrylic case with a few LEDs!!!

 

[GrimReaperGuy]

I would have to advise against the one monster fan; we're trying to cool specific hot-spots here, not blow a gale through a prairie.
I'd recommend simply a good air cooler (such as the Tuniq Tower), with some >200 CFM Delta fans for cooling.
As Rexter said, however, I'd have to side with some extreme water cooling. With no noise limits, you could do some crazy things; industrial pumps, quad radiators with Delta's... but there's always the price issue.

 

[Cirtes]

Bad idea for several reasons.

First some principles of forced air convection:

1. Efficiency of forced air convection is proportional to the velocity of the air, to a point of diminishing returns.

2. Forced air will follow the path of least resistance.

3. Metal fins, pins, ribs etc typical of heatsinks provide significant resistance.

4. Velocity of air produced by a fan is the greatest at the fan blade and diminishes gratly with distance unless restricted by a constant cross section duct.

Some of the many reasons why a large 12" fan will not cool as well as several small fans attached to the heatsinks:

1. A large CFM, large diameter fan will likely produce lower air velocity than a small diameter high RPM fan.

2. Large fan will be far away from hetsinks producing significantly lower air velocity than a small fan attached directly to the heatsink

3. Large fan, if not ducted via venturi such that all of the air volume is forced into the heatsink fins; air will travel path of least resitance, over and around the hetsink, not through the fins, due to increased resistance in the heatsink.

You are much better off with using typical cooling heatsinks + fans and replacing the fans with same diameter high RPM units. Adding the big fan can help feed the small fans with fresh cool air but cannot be a substitute for spot cooling where necessary.

 

[CaptRobertApril]

Thanks for the info, Cirtes and Grim! I'm glad I prefaced this thread with "From what (little) I understand about fluid dynamics"

Ok, so can we have a consensus on the following:

a) Huge airflow through a case is not of paramount importance.
b) Individual heatsink fans do not negatively interact with huge airflow.
c) You're better off very efficiently cooling the known hotspots than trying to flatten Saskatchewan.

Now I also have questions about ambient temps. It seems to me (from a recent thread) that someone in a 90F room is not going to get the low component temps with WC than if he were in a 68F room. After all, no matter how efficient the rad, it's gonna be quite difficult to get the water much below ambient air temp (unless TEC or other cooling is applied.)

Now the room where I have my case is a small, windowless room which is not air conditioned. In the summer, ambient temps in that room can easily exceed 100F, and that's without a system running and adding even more degrees to the mix. The room where I sit is wonderfully AC'd but I can't duct that AC to the little room where the case lives.

Would the hot (!?!) setup be to get a monster triple, quad or whatever rad and set it up in my room, with the hoses running through the small hole in the wall where the cables now pass? The prob is that I emphatically do not want fans of any kind in my room, even the "silent" ones make an unacceptable amount of noise. My room is used for a lot of video viewing and the last thing I need in here is BUZZZZZZZZZZZZZZZZZZZZZZZZzzzzzzzzzzzzzzzzzzzzz.

If I wanted to go with a truly passive, massive rad for the WC system, any suggestions?

 

[gomerpile]

Saw the picks of the kind of fan you want and if the proper methods are not taken you will have real serious vibration this is my pick of the build, great care has been taken to produce a reduction of vibration. The fan cooling the rad is mounted on a stick of wood with rubber hosing under the stick of wood and same as the back one but it was a bit tricky to get the vibration away from the case. I found a method that allows the fan to slightly move, this reducing the vibration of the case.

 

[CaptRobertApril]

Nice build! Cosmetically it resembles the electrical panel in the basement of my friend's 1920s farmhouse, :lol: but since function is the key element, it's really great!

Now I don't think that even in this setup you're reaching that kind of windspeed but wouldn't there be some sort of low-end inertial cavitation near the fanblades of the expeller fan? That's basically what I meant by 2 fans negatively interacting. Or would the effect be so minor as to be irrelevant?

 

[gomerpile]

Not in this build, one blowing in the other blowing out, the holes in the back fan reduce the back splach some what more. I can say this, even with the psu out of the case the air blowing out is quite warm. You should see when the back fan is on speed 3 wind tunnel for sure and the case temp go's from 22 to 18 while system is oc'ed that is with the psu not in the case.

 

[CaptRobertApril]

Yes, I realize that the airflow in the case is from left of the photo to right. Both fans are pointing towards the right. What I meant by inertial cavitation is the effect that you get when you have two fans blowing in the same direction in a line. Let's call the fan on the left of the photo Fan A and the other Fan B. I understand that if Fan B is generating, say, 500 CFM by itself, if you turn on Fan A and have it generate 500 CFM of its own, blowing on the back of Fan B, the airflow will interfere with the exit volume of Fan B, bringing it from 500 CFM to, say, 400 CFM. Now I don't know whether the difference would be from 500 to 499 or from 500 to 300, but that's my understanding anyway. I very well may be dead wrong.

 

[iroc]

http://www.electronics-cooling.com/Resources/EC_Articles/MAY96/may96_01.htm

 

[CaptRobertApril]

Great link. Thanks!

"In both series and parallel operation, particularly with multiple fans (5, 6, 7, etc.), certain areas of the combined performance curve will be unstable and should be avoided. This instability is unpredictable and is a function of the fan and motor construction and the operating point."

Is that the "cavitation" effect I was discussing?

"It is also important to consider fan placement in the enclosure. Pressurizing the enclosure is the preferred method, since incoming air can be readily filtered. In addition, a pressurized enclosure will prevent dust entering through cracks or crevices. The fan is also handling cooler, denser air, and it will therefore have a slightly higher pressure capability (this may be a very slight advantage for low heat dissipating systems). An important feature of a pressurized system is that the fan life and reliability are increased due to the fan ambient temperature being lower. The disadvantage of pressurization is that heat generated by the fan is dissipated into the enclosure."

Now what extent of pressurization are we talking about? Is this the conventional pressurization of a fan blowing in more air than is easily let out, or actual pressurization of a noticeable amount which would require compressed air or ???

Regardless it was a very informative article!

 

[Guest]

Hehe nice thread you ended up making =), informative!

I would say the wind tunnel is really the best bet, but that pressurization thing is cool!

One more advice, I would suggest a Dual PSU configuration for this type of rig. One powerful one to feed the CPU/GPUS and nothing else and a ~300W or even lower for the remaining peripheral and fans.

Or two moderate one, using one for the CPU's and the other for GPU's... There's a few great DUAL PSU case out there, the upcoming Antec P190 might end up pretty good.

Anyway just another thread for you to explore, I am sure you can get by with ~300$ or so of PSU!

Good luck with your project =)

 

[CaptRobertApril]

Hehe nice thread you ended up making =), informative!

I would say the wind tunnel is really the best bet, but that pressurization thing is cool!

One more advice, I would suggest a Dual PSU configuration for this type of rig. One powerful one to feed the CPU/GPUS and nothing else and a ~300W or even lower for the remaining peripheral and fans.

Or two moderate one, using one for the CPU's and the other for GPU's... There's a few great DUAL PSU case out there, the upcoming Antec P190 might end up pretty good.

Anyway just another thread for you to explore, I am sure you can get by with ~300$ or so of PSU!

Good luck with your project =)

Thanks, labbbby! I've got this tower case that stands close to three feet high and I can hide Snoop Dogg's entire stash in there. I could put 4 PSUs and still have room for my surfboard! :lol:

Dual PSUs sound like fun since most of the really heavy duty PSUs I've seen are north of 5 Cnotes. However, that Tagan 1.1KW has caught my eye. Just over 3 bills. Hell, if my system needs more than 1100W I couldn't afford the electric bill anyway!

 

[JMecc]

Maybe I should have scaled the fan to still show some on the vid card... You still need individual component fans to force air through the fins like Cirtes said, but a large airflow through the case like I showed would keep the case internal temperature to almost room temperature rather than 10C more and therefore would help cooling (and having a cool-looking unique case, which you have to admit is part of the point).

4. Velocity of air produced by a fan is the greatest at the fan blade and diminishes gratly with distance unless restricted by a constant cross section duct.

Uh - what I have IS a duct, and the components REDUCE the x-section, ACCELERATING the flow at those places. Although the components are not aerodynamically shaped, the large fan will still provide much more CFM than 120mm fans trying the same thing in normal cases.

In most cases, even expensive ones, the vid card(s) have an almost static air supply - they suck back in much hotter air than room temperature. The heat below the card stays there (feel below your vid card - the heat sink is not cool like your processor's heatsink). The top of the vid card (and the back of the mobo at certain points) get hot and are never cooled (possibly causing the iPod charging killing mobos issue). In my design, these spots do get cooled and the large airflow mostly decouples the cooling of the individual components (i.e. vid card being hot doen't provide a hotter intake for the cpu & power supply).

So, you DO still need to cool the hot spots with local fans. BUT the huge front fan keeps the ambient temp lower, cooling the medium spots that unpredictably get hot and lowers the intake temperature of the local coolers. One large fan probably costs less than two 120mm's and will blow way, way more and to places you never thought to cool. It is also easy to deflect the flow to where you want it with angled pieces of sheet metal.

Jo