This is only slightly related, but why do all the cooling solutions i've seen blow air into a heat sink?
When blowing air it is compressed slightly and raises the temperature. When sucking the air off a heatsink the air is expanded slightly creating more cooling effect. It is possible to drop the temperature of a heatsink to below freezing when in an ambient temperature of 20degrees C, just buy reversing the fan.
Is there a reason this isn't done with computer cooling solutions?
The ability to remove heat is impacted by the density of the air moving across the heat source. Less dense air is less effective at removing heat. Living more than a mile above sea level, the cooling effectiveness in our systems is reduced by a factor of about .90 (1.0 is sea level). We have to cope with this loss in heat convection by increasing air speed to reach an equivalent heat transfer ratio.
Having a fan that pulled air across a heatsink will have a similar effect by reducing the air density. This would deteriorate performance.
I think there's also another, very practical reason for this.
Since the air inside a PC is very often charged with static electricity, it also carries dust, which easily collects on fins. I might be wrong but it's more efficient to blow air into the fins to disloge dust than it is to pull air.
nope, not much of a mathmatician. But I can show you a device that does it. If you buy a portable can cooler such as this [url]http://www.dse.co.nz/cgi-bin/dse.storefront/47292b... d/Product/View/M4500[/url] and pull it apart, you will find this is how it operates.
I have on eand if I put water in it and run it for a few minutes, the water starts to freeze.
I believe the device you link to is in effect a mini-refrigerator. It uses a very small condenser and compressor system that changes the phase (gas -> liquid, liquid -> gas) of a refrigerant to exploit latent heat and provide cooling. This effect is used in computers via rather expensive systems that product sub-freezing conditions and cool processors very well. It is not used very extensively because there is an inherent condensation risk, an enemy to the delicate electrical components. I am no expert, but I would assume the unit you linked to there is not powerful enough to cool a processor, which produces an absurd amount of heat continuously. The one you've got there is good at cooling hot stuff down, or even freezing other stuff, but it's probably not the greatest and continuously cooling something that is very hot. I don't know if this is even right or if it makes sense, but there is a possible explanation for you.
ahh yes I am an idiot.. I see it says right there that it is a TEC. TECs are used in CPU cooling applications, but not extensively and because they are very inefficient. Actually one of the better coolers in Anandtech's CPU testing charts, the Monsoon II from Vigor Gaming, uses controlled TEC technology.
That's a TEC Thermoelectric Cooler. Sometimes referred to as a Peltier. (http://en.wikipedia.org/wiki/Peltier-Seebeck_effec...">Link) They work well in extreme cooling but are horribly inefficient. To cool 100w of heat it often takes like 200w of energy, and thusly they produce 200w of heat. To run that cooler you need a 12v 5amp powersource.
Right you are, my mistake. I pulled one of these things apart years ago and didn't look close enough obviously... at least I won't continue to think this is how they work now :)
Sorry for writing useless rubbish up here.
At a pressure rise of 2mm of water, the temperature rise due to PdV work is negligible.
The more important design reason for mounting the fan on the blowing side is that it produces a better flow across the heat sink. If you want, start a fan in your room, then see how far away you can feel the airflow across your hand. It's much further on the output side than the intake side. The same principle applies in a fan on a heat sink. With the fan pushing air onto the cooler, you get the majority of the air covering the middle section of the cooler, whereas in a pulling configuration, a high fraction of the air flow would be coming from the edges near the fan, relatively cool portions of the heat sink.
True, but assuming the fan is ducted correctly, i.e the heat sink has some 'walls' forcing the air to be sucked in from the opposite side? Cooling the edges of a heatsink well should still produce the desired result, similar to the way a heat pipe configuration works.
I am aware that it is much more efficient to push a fluid/gas than 'pull' it for any reasonable distance, But on something as small as a heatsink would this still be the case?... and besides the movement of the cooling medium (air) is not the goal, rather the cooling effect on the heat sink.
Could anyone point me at some research with regard to PC cooling that would explain this?
If the fan is ducted well, you will get a similar heat transfer. However, ducting adds material and thus cost. A buck of material for ducting may not seem like much, but the multiplier to get to retail price is on the order of 3-5. So for 2 similar performing heatsinks, one an open push, the other a ducted pull, the ducted pull configuration will cost about $4 more, for no gain. On a $40 heatsink...
As for your second question, a 3" long heatsink is still "macroscale". The fluid properties aren't changing drastically, so the scaling relationships and equations would still be valid.
Also, "movement of the cooling medium" is a crucial aspect of a heatsink. Coeffiecients of heat transfer (h) are strongly dependent on the Reynolds number of the fluid flow. In a given fluid, the easiest way to up the Reynolds number is to raise the velocity term.
One thing I always wonder about these heatsink tests. How are they tested? On a test platform with the motherboard laying flat? Or, like most users have it? In an upright motherboard with the heatsink parallel to the ground?
With the advent of all these heatpipes that makes a big difference. If they are being tested on a testbed laying flat on a table the cooling results may be markedly better a real life setup in a tower case with a vertical motherboard. With the heatpipes lying sideways in a tower setup they would not seem to work like they were designed to since the fluid that is supposed to cool off and flow down to the top of the heatsink would lay trapped in the tubes that are laying on their sides..
quote: All cooling tests are run with the components mounted in the standard mid-tower case. The idle and stress temperature tests are run with the case closed and standing as it would in most home setups. Room temperature is measured before beginning the cooler tests and is maintained in the 20 to 22C (68F to 72F) range for all testing.
IIRC the heatpipes supposedly contain a mesh inside designed to help the fluid return to the base through capillary action.
Start with two enclosed metal buildings consisting of a single room.
In both, you put a a space heater operating on HIGH to represent the heat coming off the CPU. That's all a CPU is really, generating waste heat the equivalent of a light bulb, a space heater in effect.
You are in one building, and you put a fan blowing directly blowing on the space heater. No air is being sucked in from the outside, it is simply a fan blowing straight on the heater. There are various little holes around the building, but no appreciable net amount of air is traveling into or out of them.
I am in the second building. The same fan is turned around backwards, sucking the heat into the fan at the base of the heater, instead of blowing at it, and through a venturi duct this hot air is being shot out and shunted to the outside of the building. Other small holes around the building are allowing air pressure to come in because of the negative pressure caused by the fan sucking and venting air to the outside of the building at its heat source.
Further, if I were to break down one of the walls of my building (take off the side of the PC case and leave it off) I would stay even more cool, darn near close to the temperature outside the building even.
In the first building, the space heater will be cooler, because the violent air at the exit of the fan is more turbulent that air being sucked into the fan. But very quickly over time the heat in the building is going to rise and rise and rise, because all your fan is doing is blowing it right off the space heater and churning it up. The air being sucked into the fan is getting hotter and hotter and will lose its effect to cool the space heater down. You will end up dying of heat exhaustion, because the heat will continue to rise and stay at a high equiliberium level, only limited by the metal buildings ability to shed heat.
In the second building, the air around me won't be violently turbulent, but most of the heat off the space heater is being sucked into the fan and shunted out the building. I can sit there all day and watch the space heater and stay nice and cool.
If you don't believe me, set up two PCs and try it. One has enclosed case blowing the fan on CPU, stirring up the heat. The other has fan reversed, with venturi on it sucking like a vacuum the heat at the source of the heater, and directing it out of the case. Leave off the side panel.
Put two thermometers in both.
After a few hours running on a hot summer day, open up both cases and look at the thermometers, and feel with your hand. The enclosed case with the fan blowing on the CPU is going to be hot as hell, the open case will be near room temperature.
And when you're using nothing but air cooling, being as close to air temperature as possible is as good as you are going to get. You can't go below that just by blowing a fan at soemthing.
You see super expensive cases with tons of fans blowing out, etc. And a fan inside blowing on the CPU pushing the heat around. You're using lots of electricity, making more noise, etc. Much better to do away with them all, and just leave the side panel off, period.
I repeat, I repeat, the only thing the side panel is there for is to keep RATS (and cats and children and idiots) out of the PC, that is it, period. It contributes nothing to proper air suction or flow for fans designed to suck air out. RATS love to chew on IDE ribbon cables, it keeps their teeth from growing to long, they have to do it like hamsters, cockateils, and other critters.
I see a lot of PCs built with this idiot idea, esp. the minitower cases. One fan inside blowing on CPU, everything locked up as tight as a ship for air flow, And only the power supply fan to suck any air out. And the hot air its sucking into the PC power supply is from inside the case.
If these manufacturers would reverse the cpu fanon the CPU heatsink, and put a duct straight out of a hole in the middle of the side panel, you'd have two fans working to suck heat out right at the source like vacuum cleaners. Air will come in via ll the little openings all around the case.
In short, take the side panel off your case, and leave it off, if you have no kid, rat, or idiot problems.
Reverse the fan on your CPU heatsink (assuming its a traditional one).
Make a duct out of paper and tape to put around the exit of the fan like a tube, so that the CPU fan is sucking air in at the CPU heatsink like a vacuum cleaner, and shooting it outward past the boundaries of the case. If you don't fully understand what I'm talking about for making a venturi duct outward (even though its simple), just skip this step. Your cpu fan will still be shooting the heat outwards towards the missing side panel and out of your PC case.
There is one fatal flaw in your example above. In your suction building you are venting the heat outside the building. and in the blower building you are not. otherwise your examples are correct. In theory the amount of heat transfer is the same for blowing or sucking the air across a heat source with all variables being equal. As one of the previous posters stated to get the variables equal IE the exact amount of air flow.It cost more on the suction set up. Needing a shroud to focus the air flow. This added cost is why manufactures blow the air instead of sucking the air. After all if cost was not one of the driving factors in your system build we would all have decked out Intel hexacore overclocked beast with liquid nitrogen cooling.
We’ve updated our terms. By continuing to use the site and/or by logging into your account, you agree to the Site’s updated Terms of Use and Privacy Policy.
21 Comments
Back to Article
pc007 - Wednesday, October 31, 2007 - link
This is only slightly related, but why do all the cooling solutions i've seen blow air into a heat sink?When blowing air it is compressed slightly and raises the temperature. When sucking the air off a heatsink the air is expanded slightly creating more cooling effect. It is possible to drop the temperature of a heatsink to below freezing when in an ambient temperature of 20degrees C, just buy reversing the fan.
Is there a reason this isn't done with computer cooling solutions?
gmchenry - Wednesday, November 28, 2007 - link
The ability to remove heat is impacted by the density of the air moving across the heat source. Less dense air is less effective at removing heat. Living more than a mile above sea level, the cooling effectiveness in our systems is reduced by a factor of about .90 (1.0 is sea level). We have to cope with this loss in heat convection by increasing air speed to reach an equivalent heat transfer ratio.Having a fan that pulled air across a heatsink will have a similar effect by reducing the air density. This would deteriorate performance.
ObiWanCeleri - Saturday, March 15, 2008 - link
I think there's also another, very practical reason for this.Since the air inside a PC is very often charged with static electricity, it also carries dust, which easily collects on fins. I might be wrong but it's more efficient to blow air into the fins to disloge dust than it is to pull air.
Howard - Wednesday, October 31, 2007 - link
Below freezing? Can you show me the math?pc007 - Wednesday, October 31, 2007 - link
nope, not much of a mathmatician. But I can show you a device that does it. If you buy a portable can cooler such as this [url]http://www.dse.co.nz/cgi-bin/dse.storefront/47292b...d/Product/View/M4500[/url] and pull it apart, you will find this is how it operates.
I have on eand if I put water in it and run it for a few minutes, the water starts to freeze.
oopyseohs - Thursday, November 1, 2007 - link
I believe the device you link to is in effect a mini-refrigerator. It uses a very small condenser and compressor system that changes the phase (gas -> liquid, liquid -> gas) of a refrigerant to exploit latent heat and provide cooling. This effect is used in computers via rather expensive systems that product sub-freezing conditions and cool processors very well. It is not used very extensively because there is an inherent condensation risk, an enemy to the delicate electrical components. I am no expert, but I would assume the unit you linked to there is not powerful enough to cool a processor, which produces an absurd amount of heat continuously. The one you've got there is good at cooling hot stuff down, or even freezing other stuff, but it's probably not the greatest and continuously cooling something that is very hot. I don't know if this is even right or if it makes sense, but there is a possible explanation for you.oopyseohs - Thursday, November 1, 2007 - link
ahh yes I am an idiot.. I see it says right there that it is a TEC. TECs are used in CPU cooling applications, but not extensively and because they are very inefficient. Actually one of the better coolers in Anandtech's CPU testing charts, the Monsoon II from Vigor Gaming, uses controlled TEC technology.Schmide - Thursday, November 1, 2007 - link
That's a TEC Thermoelectric Cooler. Sometimes referred to as a Peltier. (http://en.wikipedia.org/wiki/Peltier-Seebeck_effec...">Link) They work well in extreme cooling but are horribly inefficient. To cool 100w of heat it often takes like 200w of energy, and thusly they produce 200w of heat. To run that cooler you need a 12v 5amp powersource.pc007 - Tuesday, November 6, 2007 - link
Right you are, my mistake. I pulled one of these things apart years ago and didn't look close enough obviously... at least I won't continue to think this is how they work now :)Sorry for writing useless rubbish up here.
Chuckles - Wednesday, October 31, 2007 - link
At a pressure rise of 2mm of water, the temperature rise due to PdV work is negligible.The more important design reason for mounting the fan on the blowing side is that it produces a better flow across the heat sink. If you want, start a fan in your room, then see how far away you can feel the airflow across your hand. It's much further on the output side than the intake side. The same principle applies in a fan on a heat sink. With the fan pushing air onto the cooler, you get the majority of the air covering the middle section of the cooler, whereas in a pulling configuration, a high fraction of the air flow would be coming from the edges near the fan, relatively cool portions of the heat sink.
pc007 - Wednesday, October 31, 2007 - link
True, but assuming the fan is ducted correctly, i.e the heat sink has some 'walls' forcing the air to be sucked in from the opposite side? Cooling the edges of a heatsink well should still produce the desired result, similar to the way a heat pipe configuration works.I am aware that it is much more efficient to push a fluid/gas than 'pull' it for any reasonable distance, But on something as small as a heatsink would this still be the case?... and besides the movement of the cooling medium (air) is not the goal, rather the cooling effect on the heat sink.
Could anyone point me at some research with regard to PC cooling that would explain this?
Chuckles - Friday, November 2, 2007 - link
If the fan is ducted well, you will get a similar heat transfer. However, ducting adds material and thus cost. A buck of material for ducting may not seem like much, but the multiplier to get to retail price is on the order of 3-5. So for 2 similar performing heatsinks, one an open push, the other a ducted pull, the ducted pull configuration will cost about $4 more, for no gain. On a $40 heatsink...As for your second question, a 3" long heatsink is still "macroscale". The fluid properties aren't changing drastically, so the scaling relationships and equations would still be valid.
Also, "movement of the cooling medium" is a crucial aspect of a heatsink. Coeffiecients of heat transfer (h) are strongly dependent on the Reynolds number of the fluid flow. In a given fluid, the easiest way to up the Reynolds number is to raise the velocity term.
assafb - Wednesday, October 31, 2007 - link
Thanks!EODetroit - Wednesday, October 31, 2007 - link
See title.Also you said you would review the new Razor mouse. Haven't seen that yet either.
Wesley Fink - Wednesday, October 31, 2007 - link
We are working on several NDAs right now, but we do plan a dual-radiator water cooling review in the near future.Margalus - Wednesday, October 31, 2007 - link
One thing I always wonder about these heatsink tests. How are they tested? On a test platform with the motherboard laying flat? Or, like most users have it? In an upright motherboard with the heatsink parallel to the ground?With the advent of all these heatpipes that makes a big difference. If they are being tested on a testbed laying flat on a table the cooling results may be markedly better a real life setup in a tower case with a vertical motherboard. With the heatpipes lying sideways in a tower setup they would not seem to work like they were designed to since the fluid that is supposed to cool off and flow down to the top of the heatsink would lay trapped in the tubes that are laying on their sides..
strikeback03 - Wednesday, October 31, 2007 - link
from page 3:IIRC the heatpipes supposedly contain a mesh inside designed to help the fluid return to the base through capillary action.
CrystalBay - Wednesday, October 31, 2007 - link
They stil rule in casesThankx for the review Wes,
Eight years ago I paid 300 for the first ACTS aluminum removable MB Tray.IT came with 4x80 CM Fans . It was good but too loud for my P3 700 @ 966...
IT is for sale ...inquire within...
choppergirl - Sunday, April 4, 2010 - link
I respectfully TOTALLY disagree. :-)Compare these two examples.
Start with two enclosed metal buildings consisting of a single room.
In both, you put a a space heater operating on HIGH to represent the heat coming off the CPU. That's all a CPU is really, generating waste heat the equivalent of a light bulb, a space heater in effect.
You are in one building, and you put a fan blowing directly blowing on the space heater. No air is being sucked in from the outside, it is simply a fan blowing straight on the heater. There are various little holes around the building, but no appreciable net amount of air is traveling into or out of them.
I am in the second building. The same fan is turned around backwards, sucking the heat into the fan at the base of the heater, instead of blowing at it, and through a venturi duct this hot air is being shot out and shunted to the outside of the building. Other small holes around the building are allowing air pressure to come in because of the negative pressure caused by the fan sucking and venting air to the outside of the building at its heat source.
Further, if I were to break down one of the walls of my building (take off the side of the PC case and leave it off) I would stay even more cool, darn near close to the temperature outside the building even.
In the first building, the space heater will be cooler, because the violent air at the exit of the fan is more turbulent that air being sucked into the fan. But very quickly over time the heat in the building is going to rise and rise and rise, because all your fan is doing is blowing it right off the space heater and churning it up. The air being sucked into the fan is getting hotter and hotter and will lose its effect to cool the space heater down. You will end up dying of heat exhaustion, because the heat will continue to rise and stay at a high equiliberium level, only limited by the metal buildings ability to shed heat.
In the second building, the air around me won't be violently turbulent, but most of the heat off the space heater is being sucked into the fan and shunted out the building. I can sit there all day and watch the space heater and stay nice and cool.
If you don't believe me, set up two PCs and try it. One has enclosed case blowing the fan on CPU, stirring up the heat. The other has fan reversed, with venturi on it sucking like a vacuum the heat at the source of the heater, and directing it out of the case. Leave off the side panel.
Put two thermometers in both.
After a few hours running on a hot summer day, open up both cases and look at the thermometers, and feel with your hand. The enclosed case with the fan blowing on the CPU is going to be hot as hell, the open case will be near room temperature.
And when you're using nothing but air cooling, being as close to air temperature as possible is as good as you are going to get. You can't go below that just by blowing a fan at soemthing.
You see super expensive cases with tons of fans blowing out, etc. And a fan inside blowing on the CPU pushing the heat around. You're using lots of electricity, making more noise, etc. Much better to do away with them all, and just leave the side panel off, period.
I repeat, I repeat, the only thing the side panel is there for is to keep RATS (and cats and children and idiots) out of the PC, that is it, period. It contributes nothing to proper air suction or flow for fans designed to suck air out. RATS love to chew on IDE ribbon cables, it keeps their teeth from growing to long, they have to do it like hamsters, cockateils, and other critters.
I see a lot of PCs built with this idiot idea, esp. the minitower cases. One fan inside blowing on CPU, everything locked up as tight as a ship for air flow, And only the power supply fan to suck any air out. And the hot air its sucking into the PC power supply is from inside the case.
If these manufacturers would reverse the cpu fanon the CPU heatsink, and put a duct straight out of a hole in the middle of the side panel, you'd have two fans working to suck heat out right at the source like vacuum cleaners. Air will come in via ll the little openings all around the case.
CHOPPERGIRL
http://choppergirl.air-war.org
choppergirl - Sunday, April 4, 2010 - link
In short, take the side panel off your case, and leave it off, if you have no kid, rat, or idiot problems.Reverse the fan on your CPU heatsink (assuming its a traditional one).
Make a duct out of paper and tape to put around the exit of the fan like a tube, so that the CPU fan is sucking air in at the CPU heatsink like a vacuum cleaner, and shooting it outward past the boundaries of the case. If you don't fully understand what I'm talking about for making a venturi duct outward (even though its simple), just skip this step. Your cpu fan will still be shooting the heat outwards towards the missing side panel and out of your PC case.
thoth@cheerful.com - Saturday, September 25, 2010 - link
choppergirl;There is one fatal flaw in your example above. In your suction building you are venting the heat outside the building. and in the blower building you are not. otherwise your examples are correct. In theory the amount of heat transfer is the same for blowing or sucking the air across a heat source with all variables being equal. As one of the previous posters stated to get the variables equal IE the exact amount of air flow.It cost more on the suction set up. Needing a shroud to focus the air flow. This added cost is why manufactures blow the air instead of sucking the air. After all if cost was not one of the driving factors in your system build we would all have decked out Intel hexacore overclocked beast with liquid nitrogen cooling.