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| Thread ID: 50784 | 2004-11-01 10:50:00 | Fans: Blast air on to CPU? Or suck it away from CPU? | george12 (7) | Press F1 |
| Post ID | Timestamp | Content | User | ||
| 286819 | 2004-11-02 04:21:00 | The problem is that even what may seem to be elementary can be complex. Lets imagine the air in the room and in the case has a uniform density of dust particles floating in it . When the fan blows air into the heatsink the air velocity is high when it enters and much lower where it leaves. Wouldn't the slower air tend to drop out the particles at the edges of the fins due to stickiness or whatever causes it to adhere to the fins, is this the same as the slower air entering when the fan sucks?? |
Terry Porritt (14) | ||
| 286820 | 2004-11-02 04:24:00 | Precipitate, Graham, but not stick ?? :) | Terry Porritt (14) | ||
| 286821 | 2004-11-02 04:35:00 | I also forgot to mention boundary layer theory as an added complexity, the air next to the fins is at a very low speed compared to the flow at the centre of the fin gap. I just cant think straight at the moment :) |
Terry Porritt (14) | ||
| 286822 | 2004-11-02 04:39:00 | Dust. Is only an issue if you dont know what an air filter is. D. |
drb1 (4492) | ||
| 286823 | 2004-11-02 04:41:00 | It's only a problem if you open the box and see that the dust has stuck (precipitately). :D | Graham L (2) | ||
| 286824 | 2004-11-02 05:24:00 | Well, I have put the PSU in (external, taped into the right place), put a fan in the front blowing fresh, cool air into the system. But the temperatures are the same as before (even back at 2.4GHz :(). 33 degrees CPU (Idle) 30 degrees System (Idle) 48 degrees CPU (Under load) 30 degrees System (Under load) That's it, I'm getting a new case NOW. George |
george12 (7) | ||
| 286825 | 2004-11-02 09:28:00 | I couldn't wait to get home tonight so that I could reverse my CPU fan to check out the theories! CPU temp shot up by 12 - 14 degrees C! After an hour on the couch meditating (and defluffing my navel) I arrived at this conclusion: Sucking air up from the CPU may well move air as well as blowing down onto the CPU. However the CPu is surrounded by other componants which are getting hot, therefore I'm pulling hot air through the heatsink. If we consider that the air furtherest (is that a word?) from the motherboard is likely to be cooler than closer to it then is makes sense to blow the cooler air down onto the heatsink. Of course ducting and other mods would make a difference, I simply have a 60mm CPU fan with a 80mm exhaust fan slightly higher up. |
POTUS (5276) | ||
| 286826 | 2004-11-02 20:34:00 | i've only skip read most of this thread but it seems you guys have mossed one small point. as far as wheather the fans should suck or blow air across a heatsink, it all depends on the design of the heatsink. most are designed for air to be blown across them. the main exception is alpha heatsinks which are designed for air to be sucked away from them. at the end of the day, use whatever works the best :) |
tweak'e (174) | ||
| 286827 | 2004-11-02 20:54:00 | I mentioned the design of heatsink right back at the start Tweak'e but as you mentioned it does seem to have been ignored. If the HSF is used in well designed case e.g air comes in from the front and exhausted out the back then in theory a fan that is blowing should have access to fresh air as its pushed out the back. Where as a fan that is sucking air would only be able to push stagnant air thats been heated by motherboard componets. A fan that is sucking would also be detrimental to overall case airflow, all the cases ive worked with provide fresh air above the HSF so that it can blow it over the HS, i dont know of any cases that provide fresh air around the motherboard so that it can be sucked out. |
Pete O'Neil (250) | ||
| 286828 | 2004-11-02 20:55:00 | That's interesting Potus, so I thought I'd do the same. My set up is a full tower case, an Athlon 1.33MHz Thunderbird (not renowned for cool running), and a 120mm fan in the side blowing outside air direct onto the cpu area. With the cpu fan blowing into the heat sink, the reported temperature rise between system temperature and cpu temperature is usually in the range 14 - 17 deg C, so if system temperature is say 23 deg, the cpu says about 38deg +-. Reversing the fan so it sucks, now gives a differential of 22 - 24deg so the cpu is about 8 deg hotter. System temperature presumably is measured by a sensor somewhere on the motherboard, and it is usually a few degrees above room temperature. Now in this case the air flow from the 120mm fan is opposing the flow out from the cpu fan, the cpu fan is having to fight a bit against the dynamic head of the 120mm fan. Whether this would account for the higher cpu temperature I dont know, maybe sucking is inherently less efficient than blowing? The beauty of science is the theories can be twisted to fit the facts :) Maybe a fan, which is not ducted, and is not a positive displacement device could be likened to an amplifier, where a small signal at the input is amplified by the gain, but if added at the output has little effect. So if the input to the fan is restricted a bit by the heatsink, its flow output will be a bit less than if the same restriction was put on the outlet? That is we dont have true reciprocity, even though the flow resistance of the heatsink should not depend upon direction. {Same as you dont expect water flow to depend on which direction it flows through a pipe} |
Terry Porritt (14) | ||
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