Why does case fan suck tissue on both sides?

[rasmasyean]

I bought this fan to replace a broken intake fan on my case.

Scythe SY1225SL12L 120mm "Slipstream" Case Fan
https://www.newegg.com/Product/Product.aspx?Item=9SIA9ZH3T20572

When I put a tissue on either side, it sucks it onto the fan. The only thing I see that's different is that the intake grate is not "holed", but rather "slotted", where the slots are formed by the metal bend out to form little "ramps".

Does this have anything to do with it? Where does the air actually go as it doesn't seem to make sense. Or does placing the tissue next to it alter it's impeller performance direction? What's the difference between this type of slot and the normal holes. Do I have a defective fan?

 

[k1114]

I don't know what you mean by holes and slots but the side where you see the wires and fan label is the back where the air goes out of. You can also tell by the fan blades. The concave side is exhaust. At lower fan speeds, there is more turbulence near the fan. Also if it's not mounted you get air circulating around it. Mount it, turn up the speed and try.

 

[rasmasyean]

"holed"
http://c773974.r74.cf2.rackcdn.com/0391504_546770.jpg

"slotted"
http://i.ebayimg.com/images/i/321071637426-0-1/s-l1000.jpg


It's mounted just like the previous fan was. It's mounted to a removable side panel so I was able to test it outside of the case to rule out the possibility that air is being blown at the fan from the inside to stick the tissue onto it with more power than the fan theoretically is pushing the tissue away.

But the phenomenon I'm observing is that the tissue both sticks to the external of the grate, as well as the exhaust side of the fan itself!

 

[rasmasyean]

I just tried to increase the fan speed to it's max 800 RPM and it still does the same thing, except the suction is stronger on both sides. I even tried a 2 tissue test, and when I cover both sides...for whatever reason it overdrives it to like 1000 and it scared me so I took the tissues off. lol

 

[k1114]

Possibly too much static pressure? It can't be suction everywhere. Are you covering it? You have to use small strands to see where air is going. String might be better.

 

[rasmasyean]

I cut a strand of tissue instead and moved it across the fan. It looks like off to the sides it's blowing it away. As soon as I start going toward the middle, it starts to suck the strand into itself. It's not as strong as my other chassis fan with the holed grate, but at least that one is consistent on it's intake and exhaust. Also, the slotted grate fan doesn't seem to suck that hard compared to the holed grate fan.

Hmm...I wonder what effect is that grate supposed to do? Since it's on the side and not the rear or front, perhaps it's designed to make the air "circulate around" in the center? I mean, it reminds me of those air conditioner vanes where you'd adjust to make the wind blow higher. But this one just "points" to the rear. So as originally set up with an intake fan, it's not blowing air to the rear, but sucking it in from the rear (which wouldn't make any sense). So maybe the geometry actually causes this type of fan to "exchange" sort of by making some circular wind or something?

 

[k1114]

The issue is too much static pressure. That is an airflow fan and you have it gasping for air. You need a static pressure fan as the slot grill is very restrictive. The fan is designed for a holed grill which is not restrictive although the one you pictured is old with small holes so still isn't good.

Slotted grills are just a poor design and very old, and I mean decades ago. They would have been more common back then but they are just so terrible that no one has used them for years. Wherever you got this case it's either ancient or someone who didn't have a brain designed it.

 

[rasmasyean]

This was the old one. It doesn't seem to specialize as one of the distinctive types (SP / AF), but can you tell by looking at it?
https://www.amazon.com/Cooler-Master-A12025-12CB-3BN-F1-DF1202512SELN-0-16A/dp/B01E43FI82

Anyways, I had 2 so I mounted a good one on the slotted-grill. It seems to exhibit somewhat similar characteristics as the new AF Fan, but not as strong of a suction on the exhaust side as the tissue strip doesn't pull as hard into the exhaust (i.e. it flaps around a bit more at most positions).

I also tried to flip the fan (from intake to exhaust) and it seems to solve the "double sucking" phenomenon. Though I don't think the exhaust is as strong as the holed-grill fan. Although perhaps it's blowing "sideways" more than directly perpendicular. Do you think it's a good idea to leave it like this?


The case IS pretty old (from 2008). It's this:

COOLER MASTER Centurion 534 RC-534-KKN2-GP Black Aluminum & Mesh bezel / SECC Chassis ATX Mid Tower Computer Case
https://www.newegg.com/Product/Product.aspx?Item=N82E16811119106

Here are some pictures of the grill and stuff.
https://imgur.com/a/DZ5I7

If you're wondering what that side holed-grill is for, there was a tube that mounted to it. It looked like was supposed to be for a hypothetical CPU positioned at that location. I removed it because it doesn't fit with my Heat Sink, not that it was ever truly aligned to any CPU's I ever had in there anyway.

 

[k1114]

You can tell by how big the gaps are between the fan blades. The closer they are, the better they are for sp.

Either way you put the fan, it won't make much difference for being restricted.

The side slotted grill was for exhaust. It's pointed like that to follow the common design to exhaust to the rear. Side fans are usually intake and it looks like they designed the other holed side fan to be intake. It's poorly designed. The cpu duct is also an old practice that no one does anymore.

 

[rasmasyean]

How exactly is it supposed to work? Does the high pressure from fat blades make more turbulence close up or something? While the thin blades make it more lamellar so that air flows more smoothly close by, causing less "erratic back pressure" to slow net air flow? I presume that comparable strength fans are supposed to be at the same RPM.


Hmm...perhaps that side panel was a reused part that originally mated to a case with no rear fan? I can definitely see someone deciding to reduce tooling costs and just make up some scheme to mount some right-size CPU-duct that is "near" the CPU, while turning an exhaust port into an intake just make it still "a modern standard".

 

[k1114]

You can still have thin fan blades for sp fans as long as they are close together. Many case fans tend to be general usage like the coolers master you have. They aren't close together for sp but not far apart for af. I'm no fan engineer but I would think thinner blades would have more turbulence near them and is causing a lower suction.

A rear fan isn't going to change the use of a side fan. Most side fans nowadays are lower to accommodate the gpu. Gpus weren't the major heat producers as they are now as more things are gpu accelerated. Cpu heatsinks were all mostly downfacing so would be more likely to work with a cpu duct. It's not like that anymore with most going with tower coolers.

 

[dudio]

I believe the fan is fine.
Best test would be to unmount it, and test it outside of the case.
My guess is that while in the case, it is seeing misdirected airflow blowing back into it's exhaust side.
Check all you fans, and set up a 'flow thru" direction.
(PS: I personally don't believe in side fans. They disrupt the airflow.
the ideal is front to back, intaking low and exhausting high)

 

[rasmasyean]

The fan wire is a decent enough length that I can run it mounted while opening the panel out so it's not near the other stuff.

It looks like in the original INTAKE configuration, the first and last slot blow air out instead of sucking in as it should.

So just based on "strand of tissue test", it looks like on both sides, it sucks in the middle and blows at the ends.


However, in the EXHAUST configuration, all the slots blow air out.


Yeah it would make sense to have a "wind tunnel" blowing tangentially across all the boards. But I'm guessing most manufacturers try to sell Drive Bays, Buttons, and USB ports and such in the front of the case. Prolly will charge "enthusiasts" more for front fan cases, I'm guessing.

 

[k1114]

Being an exhaust on the slotted grill isn't starving it for air but it's not effective when it's an af fan.

Many cases no longer have 5.25" bays in front and replace them with fans. They don't cost more than other cases. Even your case has a front fan so I don't get why you say a front fan case. It's always been normal for front intake and rear exhaust.

 

[rasmasyean]

I tested both directions and it appears there's not really any noticeable temperature difference at load.

My case actually doesn't have a front fan. But I can see how we don't need as many bays (if any) compared to before. Do they have drive bay fans? Are they any good? I don't think I'm running that hot. i7-7700K @ 65-68 C. If I get a fan, will it go significantly lower? Or is there a point if I'm not manual Overclocking?

 

[k1114]

You just don't have a fan in front but it can have one in front. They do have adapters to fit fans in the bays but it's not worth it. If adding more fans doesn't change temp then you have enough airflow and don't need more fans. You see lots of people with tons of fans and it doesn't help. It just adds more noise. Getting a better cooler will lower temps more if that's the stock heatsink but temps are fine.

 

[rasmasyean]

That's a heat pipe + fins + fan heat sink that I bought. The i7-7700k didn't come with a cooler.

I tested it while unplugging the fans and it went from 65-68 to 68-72. The "real world application" of encoding a video, if that's a good enough measure. Does this look right? 4 degrees looks insignificant. Maybe I should just disconnect them to reduce noise. Maybe it would be more significant when it gets warmer perhaps?

 

[k1114]

You didn't post your specs in this thread so I don't know what you have. You unplugged all fans and tested vs all plugged in? Both time and ambient temp matter. Heatsinks dissipate heat as long as there is a temperature difference. The bigger the difference the more heat is expelled. The fans keep a flow of cool air so that the air around the heatsink is always cool. Hot air around it (even with airflow) means less heat out and your components run hotter. With no airflow the heatsinks start building up heat and the hot air builds up in the case and doesn't exhaust out. Then it's a hot box.

 

[rasmasyean]

Oh sorry I wasn't specific. I meant I disconnected all the (2) case fans I have. The test was done for around 20 minutes which I guessed the temperatures would have reached a plateau. I mean, I was still in a T-shirt, but wasn't sweating like in some Summer days.

Also, the MB Temperature read 36 / 38 (fans / no fans), which I didn't think was significant to mention because it doesn't seem to be high compared to the CPU.

 

[k1114]

I don't think you have anything that produces a lot of heat so the air in the case was probably still cool. The temps plateau relative to the ambient temps so if that wasn't hot yet then they won't get much hotter.

 

[rasmasyean]

Now that you mention it about it, I don't think I ran anything that would stress the graphics card (GTX 1060). I suppose that can be a good source of heat as well if I ran one of those new games and stuff. It's not really powerful but suppose that potentially can raise the case temperature as well.

I suppose the real question is whether the noise of the case fans would be noticeable when the CPU/GPU fans rev up. Probably not, right?

 

[k1114]

Yes but at the same time all the sound adds up. You should have case fans on anyways but keeping them on low is sufficient.