Am I the only one that finds the design of 120mm fan PSU's ineffective?

S

Shaman

Limp Gawd
Joined
Sep 17, 2006
Messages
501
The idea is that a big 120mm fan sucks air from inside the case and exhausts it through the grill at the back of the PSU, cooling both the case and the PSU in the process. IMHO this isn't true because:


- A PSU is very cramped inside, no way all the air it sucks in can go out through the grill. What happens is alot of hot air gets trapped inside it.

- Many 120mm PSU's have opening at the back (inside of the case), which means some of the hot air gets dumped back inside the case and is continually recirculated.

- The PSU sits right on top of the cpu area, the CPU heat and possibly the GPU heat gets dumped right into the PSU causing it to run hotter, lose efficiency and make the fan ramp up which increases noise.

- If the fan at the back of the case is 80mm, the PSU fan will overpower it and get the hot air that should have been exhausted by the case fan.

- If the fan at the back of the case is 120mm in size, they'll both be 'struggling for air'. Add a CPU fan blowing down at the heatsink and you have 3 fans in very close proximity blowing air in 3 opposite directions.


Most of what I said only applies to 'standard' case layouts. The Antec cases with a separate compartment for the PSU are an exception, and a brilliant one at that! I don't think a PSU should have a big part in cooling the case, it's main purpose should be to take care of cooling itself and mind it's own business. But if a PSU on top of the CPU must be used (which is the majority of cases), then a single 80mm fan PSU, preferably with no vents at the bottom should be used instead, because the only way a PSU can completly exhaust it's hot air is with an 80mm fan at the back of the case.

I only came up with these theories after getting a 120mm PSU myself. :p I thought the 120mm fan on a PSU was cool and it would keep case temps down and run quiet, but found the opposite to be true. No more 120mm PSU's for me, thank you.

:cool:
 
Allow me to help kind of clear some things up.

Shaman said:
- A PSU is very cramped inside, no way all the air it sucks in can go out through the grill. What happens is alot of hot air gets trapped inside it.
Click to expand...

Not really true. The airflow from the fan goes up into the PSU, and since hot air tends to rise, it follows the path of least resistance out the back of the case. Sitting in one spot is not really something that hot air does, especially when a continuous flow of air is heading towards it and coaxing it to move so that new, cooler air can replace it.

- Many 120mm PSU's have opening at the back (inside of the case), which means some of the hot air gets dumped back inside the case and is continually recirculated.
Click to expand...

Again, path of least resistance. I don't know about everyone's PSU, but the Corsair HX series has no openings except for the modular connectors, and airflow to those is blocked by the PCB and a piece of plastic. However, if you pour water into a bucket, and one side has a hole the size of a dime while the other side has a hole the size of a grapefruit, you can be sure the water will pour out of that grapefruit sized hole first, because it's much easier. Similar theory for the airflow.

- The PSU sits right on top of the cpu area, the CPU heat and possibly the GPU heat gets dumped right into the PSU causing it to run hotter, lose efficiency and make the fan ramp up which increases noise.
Click to expand...

The ATX specification requires a power supply to have an exhaust fan due to the fact that it was designed to help clear hot air through the case. Most cases have an exhaust fan right beneath the PSU and next to the CPU to exhaust the heat that the CPU and GPUs produce. The ambient temperature of the PSU does depend heavily on how much heat is being produced inside the case by the components nearby, but this doesn't really change regardless of having a 120mm fan underneath or two 80mm fans in a push-pull config. The interior temp is always going to be about the same, as heat rises to the top of the case.

- If the fan at the back of the case is 80mm, the PSU fan will overpower it and get the hot air that should have been exhausted by the case fan.
Click to expand...

Not true. The size of the fan is only a part of the factor. The real issue is CFM, or cubic feet per minute. This changes by RPM and fan blade design, as well as fan size. It's possible (and even common) to find high RPM 80mm fans that outdo lower RPM 120mm fans. The key is using the right combination so that you create a positive airflow through the PSU. In fact, since most 120mm fan PSUs have only that single fan inside, and allow the airflow to naturally progress out the back through clever PCB and heatsink design and plastic ducting placed inside, any additional exhaust fan is just gravy, as it's going to have more airflow than the same design would have without it.

- If the fan at the back of the case is 120mm in size, they'll both be 'struggling for air'. Add a CPU fan blowing down at the heatsink and you have 3 fans in very close proximity blowing air in 3 opposite directions.
Click to expand...

There's no real "struggling for air", because this isn't a vacuum. A case isn't airtight, there are openings and crevices all over that allow air to fill the case. It makes a small difference in efficiency of cooling design, but you'll notice that most new heatsinks have fans that blow horizontally across them, with the exhaust either being out that back of the case or up into the PSU. Either of which are preferable to the impinging airflow downwards onto the heatsink, but even that doesn't have much effect, really. There's plenty of air inside your case for your fans to exhaust. Even if you don't have any intake fans, your case will still be sucking in air and exhausting it sufficiently in most cases.

I only came up with these theories after getting a 120mm PSU myself. :p I thought the 120mm fan on a PSU was cool and it would keep case temps down and run quiet, but found the opposite to be true. No more 120mm PSU's for me, thank you.
Click to expand...

That's not really fair, because there's a huge variety in the quality and performance of 120mm and 80mm PSUs. I could go buy a 120mm PSU that is much noisier than the 80mm fan used in a PC Power & Cooling Turbocool, or I could buy one that's much less noisy.

Another thing to remember is high efficiency plays a big part. When your power supply is pumping out, say, 400W of power, if it's only 70% efficient, you're using about 570W of power from the wall, meaning that you're producing 170W of heat.

However, if that same 400W is being produced by a PSU that's 80% efficient, you're only drawing 500W, producing 100W of heat, which drops your heat production by a full 70W.

Regardless of fan design, that's preferable. Also, one thing about 120mm fan designs is that they can produce equal CFM to 80mm fan designs at much lower RPM, and thus lower noise. If your power supply needs, say, 25CFM to move through it in order to cool it appropriately, and you need a fan that can do that, a 120mm fan will do that 25CFM more quietly than an 80mm fan will, barring some really bad fan designs.
 
Redbeard said:
Allow me to help kind of clear some things up.
Click to expand...
Yep, you did a good job clearing some things up, thank you. :)

Redbeard said:
The airflow from the fan goes up into the PSU, and since hot air tends to rise, it follows the path of least resistance out the back of the case. Sitting in one spot is not really something that hot air does, especially when a continuous flow of air is heading towards it and coaxing it to move so that new, cooler air can replace it.
Click to expand...
True, maybe the word 'trapped' was badly chosen, takes longer to get out of the PSU is more correct. I believe that in a PSU with an 80mm exhaust fan and no vents at the bottom, the hot air goes out quicker, instead of having to find it's own way out - it goes out in a straight line instead of having to take a 90º turn.. Unless you prove me wrong again.

Redbeard said:
Again, path of least resistance. I don't know about everyone's PSU, but the Corsair HX series has no openings except for the modular connectors, and airflow to those is blocked by the PCB and a piece of plastic. However, if you pour water into a bucket, and one side has a hole the size of a dime while the other side has a hole the size of a grapefruit, you can be sure the water will pour out of that grapefruit sized hole first, because it's much easier. Similar theory for the airflow.
Click to expand...
Yes I was refering mostly to Seasonic PSU's and some others I've seen that have vents on the inside of the case. It's true that most of the hot air will go out through the honeycomb grill at the back of the case, that's why I said *some*. I always thought the vents on the inside of the case where there to help some of that 'trapped' air to find a quicker exit, is this correct? Correct or not, *some* air gets dumped back into the case, which is why I tapped those vents on my PSU.

Redbeard said:
Also, one thing about 120mm fan designs is that they can produce equal CFM to 80mm fan designs at much lower RPM, and thus lower noise. If your power supply needs, say, 25CFM to move through it in order to cool it appropriately, and you need a fan that can do that, a 120mm fan will do that 25CFM more quietly than an 80mm fan will, barring some really bad fan designs.
Click to expand...
I think that if a case has a 120mm fan at the back right next to the CPU, a PSU with an 80mm fan and no vents at the bottom will receive less direct heat than a 120mm PSU would. Thus the PSU would require less CFM to keep cool, and the 80mm could possible run as quiet as the 120mm would because it wouldn't need to move as many CFM.

Are 120mm PSU's really necessary? The feeling I have is that there's so many 120mm PSU nowadays only because they look cool and are trendy. The only real reason I can think off is lower noise levels, a 120mm fan is quieter than an 80mm fan when moving the same CFM, no doubt about that but if the PSU didn't have a 120mm fan to begin with, it wouldn't play such a big part in cooling the case and wouldn't get hit with so much hot air from the components right bellow it.

But what do I know, I'm no PSU tech. :D
 
No, the case is not a vacuum, but every fan has an optimal working CFM which requires a certain amount of space in front of the fan so the vortex can develop (otherwise CFM goes down the noise goes up).

Generally this is nearly two inches of just empty free space in front of the fan. You can test this yourself. Have a fan sitting out and just slowly move a piece of cardboard closer and closer to it and see when the noise starts getting louded and it's struggling. So for the most effective airflow, using cramped PSUs is not possible. Having both a bottom fan and a back fan (80 or 90mm) is preferable since you get an active directed airflow. Additionally, back to the vacuum thing, no the case i not a vacuum but the different air pressures going in different directions do create dead spots and the fans do fight against each other. What CPUs need is a diagonal heatsink so the air can go in through the bottom corner and equally output right to the rear fan and PSU intake.

ATX design was really horrible in that sense. The old AT made much more sense since the CPU was sometimes on the bottom right corner of the board and the case intake fan at the front blew over the CPU and then the air was properly exhausted up the back to the PSU. BTX cases are a little better at this with the airflow going straight through.
 
Acutally I find it defective, not ineffective.

a) I watercool
b) provide dedicated airflow to the VR's and memory sticks and ensure the case has good overall airflow using both a front and rear fan(s).
c) mount the power supply upside down (not all cases support this easily) and cut a hole in the top of the case, make a gasket out of weatherstripping foam rubber so what was the bottom fan is now the top fan pulling cool ambient air thru the supply. The use of a nice filter/grill on the hole in the case top makes it look like it was designed to work that way.
 
BillParrish said:
Acutally I find it defective, not ineffective.

a) I watercool
b) provide dedicated airflow to the VR's and memory sticks and ensure the case has good overall airflow using both a front and rear fan(s).
c) mount the power supply upside down (not all cases support this easily) and cut a hole in the top of the case, make a gasket out of weatherstripping foam rubber so what was the bottom fan is now the top fan pulling cool ambient air thru the supply. The use of a nice filter/grill on the hole in the case top makes it look like it was designed to work that way.
Click to expand...
I thought about cutting a hole like that once to make the PSU only cool itself, but I didn't want to mess my lian li up any more. Seeing as hot air rises, I'm not sure how good for the health of a 120mm PSU is running it upside down though, unless you put the fan blowing up?

Another good option for single 80mm fan PSU's if you have a blowhole at the top of the case (between the PSU and optical drive) is to make a duct to feed fresh air to the PSU vents inside the case. I made an 80mm top blowhole in my case once which is now unused, but I never went ahead with it because a) I didn't want to go buy another PSU and b) I got fed up with modding.

I just slapped a papst 120mm fan in the PSU running at a constant 900rpm and didn't bother with it again.
 
Shaman said:
Seeing as hot air rises,
Click to expand...

Hot air will rise in a calm environment, but with a fan blowing, it will move where the fan wants it to go.

If you have a top fan in the case already, an open top and a 120mm fan sucking will just draw that hot air back into the PSU.

Interior intake for a PSU is a legacy of the original AT cases, when CPUs didn't even have heat sinks. They were the main source of cooling for the whole case. The whole ATX case standard would have to change to support a PSU without an interior intake as someone has noted.

I don't think that it's so bad an idea if:
  1. The fan has more than enough capacity to move air for both the case and the PSU
  2. The parts in the PSU are designed to operate at the resulting temperatures.

This won't change until the industry decides that the PSU ventilation is more of a problem than it is.
 
Seeing as hot air rises, I'm not sure how good for the health of a 120mm PSU is running it upside down though, unless you put the fan blowing up?
Click to expand...

A good concern, but convection (heat rises) is not going to overcome the postive airflow put out by the fan, even a low speed one. All the heat will get blown out the back. Actually thats a pretty good idea, the fans usually can be flipped over but it depends on the mounting and fan design. I have see some inexpensive power supply fans with mounting flages only on one side. They could be replaced tho. Dont know why I didnt think of flipping the fans, it cant hurt and in theory better unless a lot of heat at the back of the pc from other fans etc. , hmmm where is that screwdriver ?
 
BillParrish said:
A good concern, but convection (heat rises) is not going to overcome the postive airflow put out by the fan, even a low speed one. All the heat will get blown out the back. Actually thats a pretty good idea, the fans usually can be flipped over but it depends on the mounting and fan design. I have see some inexpensive power supply fans with mounting flages only on one side. They could be replaced tho. Dont know why I didnt think of flipping the fans, it cant hurt and in theory better unless a lot of heat at the back of the pc from other fans etc. , hmmm where is that screwdriver ?
Click to expand...
Oh I thought you had flipped the fan. :) It's worth a try. I think it'll be better to bring in fresh air through the honeycomb grill and blow it out through the top of the case. The PSU should runner cooler than if the fan was blowing down pushing the hot air back against the heatsinks/pcb.

There's the possibility that the hot air being exhausted by the case fan bellow can go into the PSU, but I don't think it should be too much of a problem.
 
Shaman said:
Oh I thought you had flipped the fan. :) It's worth a try. I think it'll be better to bring in fresh air through the honeycomb grill and blow it out through the top of the case. The PSU should runner cooler than if the fan was blowing down pushing the hot air back against the heatsinks/pcb.

There's the possibility that the hot air being exhausted by the case fan bellow can go into the PSU, but I don't think it should be too much of a problem.
Click to expand...

LOL after I checked, its on my old school NF2 system, I had flipped the fans, was only 4-5 years ago, I really think either way you do it, anything is better than pulling already warmed up case air thru the PS.
 
jstnomega said:
Tho case and PS specific, this is in the spirit of what the OP was getting at - a "clean" supply of frsh air for the PS, as opposed to the case-internal recycled stuff.

http://www.silentpcreview.com/article66-page2.html
Click to expand...
Yes I read that article when it came out. :) Actually it was some of SPCR's staff that helped design the Antec P180, which was the first ever case with a separate compartment for the PSU. Antec then did the same for the NSK2400/Fusion and the NSK3300. Antec is at the top of the game when it comes to silence computing.
 
You must log in or register to reply here.
Back
Top