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Is the air flow 'optimum' in your computer case?

Discussion in 'PC Hardware' started by Christer, 2005/11/16.

  1. 2005/11/16
    Christer

    Christer Geek Member Staff Thread Starter

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    Hello all!

    When a 160 GB Seagate 7200.7+ PATA was given to me as a gift, I started testing it in the mobile rack. The temperature according to Everest Home Edition was considerably hotter than a 120 GB Hitachi 7K250 PATA. This triggered a discussion over at StorageReview.

    To make a long story short, my computer case (SuperPower Zephyr KS-201) is 4½ years old and by no means state of the art. It had the standard single case fan, front-intake, which was located at the bottom, blowing at nothing. An additional rear-exhaust fan was optional but there was only the standard fan in my KS-201. The temperatures below refer to two identical 120 GB Hitachi 7K250 PATA, #1 fixed and #2 in the mobile rack. The computer was started cold and measurements taken after 2-3 hours (temperatures stabilized) of very light load:

    When the Seagate substituted the Hitachi in the mobile rack, it was running at 48-49°C.

    The discussion prompted me to move the fan from the front-intake position to the rear-exhaust position and the reduction in temperatures was amazing:

    When the Seagate substituted the Hitachi in the mobile rack, it was running at 37-38°C.

    NDT (Net Drive Temperature = measured temperature - ambient temperature) has been lowered by 12°C for the Seagate compared to a 10°C reduction for the Hitachi. The fixed drive shows the least reduction in NDT, 7°C.

    It seems like having a look inside your computer case is a good idea. If there only is a front-intake fan at the bottom, then it is probably only drawing dust into the case. Well, hopefully it is increasing general air circulation but not much more.

    Moving the fan to the rear-exhaust position will draw fresh air through the front, cooling the drives. The CPU fan will also exhaust into this air flow and CPU/MB temperatures will also benefit, albeit to a lesser extent.

    More modern computer cases have a standard rear-exhaust fan and an additional front-intake fan is optional. Some have both fans as standard. (The front-intake fan is located in front of the 3.5" drive bays to direct the air flow over the drives.) This makes it less of an issue in more recent computer cases.

    I wonder ...... :confused: ...... what the designer of the KS-201 was thinking. Why not ship it with a standard rear-exhaust fan and an additional front-intake fan as an option. (I actually believe that fans in both positions would make the situation worse for the drives, compared to a single rear-exhaust fan.)

    Christer
     
    Christer,
    #1
  2. 2005/11/17
    mattman

    mattman Inactive Alumni

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    Hello Christer,

    Putting the case fan at the front may be considered "traditional ". Most workstation types also use the front fan as a CPU cooler (a large heatsink is put on the CPU and it is positioned behind the front fan).

    I would think that a front fan located at the bottom of the tower would provide a current of cool air directly across the motherboard and it's components, so cooling the RAM, the video card, other chips and finally removing the heated air from around the CPU. If an added fan is at the back, it will cause low pressure in the case and air will be taken in through ALL the air intakes (maybe more from the back intakes). This would be compared to a stream of air directed from the front fan to the PSU fan.

    Located at the back (and creating a general low pressure in the whole of the case) an extra fan may be drawing air in through the gaps around the drive bays and so making the drives cooler, BUT, as someone mentioned in a previous discussion, it may be pulling air through the cases of optical drives and the floppy drive (HDDs are airtight) and with that dragging dust as well.

    It may be to avoid dust. A "stream" may deposit dust on the motherboard components, not much problem to clean off. "Low pressure" may be depositing dust in the removable drives.

    I have not thought of it this way before. I have just convinced myself to leave the front fan where it is, but buy another fan to add at the back if I want more cooling :D

    Matt
     
    mattman,
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  4. 2005/11/17
    Christer

    Christer Geek Member Staff Thread Starter

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    Hello Matt!

    Moving the fan to the rear-exhaust position lowered MB and CPU temperatures by ~3°C. I have no idea of the RAM temperatures but they probably run cooler too.

    When the fan was in the front-intake position, assuming that the air flow is straight towards the back of the computer, it blew at absolutely nothing. What happens aerodynamically is this:

    Air pressure is constant ...... :eek: ...... total air pressure = static air pressure + dynamic air pressure. When a fan sets the air in motion, the dynamic pressure is increased which incures a drop in static pressure. This draws adjacent air into the air flow (and creates turbulence). Since the air flow is below most components, the air inside the computer case is drawn down into the air flow.

    The above is only regarding the air flow, a front-intake fan will increase the air pressure inside the computer case and the rear-exhaust fan will decrease it. One fan in each position will 'zero each other out'. If multiple fans are fitted, such as a fan supplying air to the CPU cooler via a duct or the small fan in a mobile rack, the total flow into the case should be balanced by an exhaust fan of the corresponding capacity.

    The fan in the power supply was the only exhaust fan in my case and it ventilates itself but not much more. The small fan in the mobile rack did not stand a chance against the bigger front-inlet fan and the HDD inside it ran 1°C hotter than the fixed HDD. After shifting the case fan to rear-exhaust, the fan in the mobile rack does a good job and the HDD inside it now runs 1°C cooler than the fixed HDD.

    You are probably right that the air flow was linked upwards from front to rear by the PSU fan but but that flow is contra acted by the dynamics described above. No matter what, the temperatures of my drives were lowered by ~8°C (average) and the MB/CPU by ~3°C.

    Yes but it can't be as serious as the front-intake fan vacuuming the floor!

    There is a deposit of dust in the mobile rack but it has a wide meshed intake grille. With the tray closed, the opticals are much tighter.

    If the front-intake fan would actually draw air past something at the front, then yes, it would improve cooling of those devices but in my case, I believe that it would make things worse.

    Christer
     
    Christer,
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  5. 2005/11/30
    Chiles4

    Chiles4 Inactive

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    So what exactly is the moral of the story? It was a bit hard to follow. :D Additional cooling to a case should be added in the form of rear fans as opposed to front fans?
     
    Chiles4,
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  6. 2005/11/30
    Whiskeyman Lifetime Subscription

    Whiskeyman Inactive Alumni

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    I would say the moral is try the fans at different locations and directions to see which works best for your case. I remove my side cover and stretch a bungie cord from front to back and tie a 120mm fan in the center blowing air directly at the northbridge and CPU.
     
    Whiskeyman,
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  7. 2005/12/02
    mattman

    mattman Inactive Alumni

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    I think Christer's (hello Christer :) ) experiments were quite persuavise. Then I wondered what would happen if it was taken to it's "limits ".
    You see nowadays that cases have many fans, so I imagined putting all those fans in the same area at the back of the case. There would be a very low pressure at the intake side of these fans and thus a "general" low pressure inside the case, causing air to rush in from all the air intakes (and also from any gaps). This may be very dependent on the way the case is built (regarding airflow) and may also depend on what "physical obstructions" there are inside the case, eg. large PCI or graphics cards, are cables tied back, are the drives in adjacent bays, etc?

    The way that you find cases are being made now is to direct flows/streams of air: *in from bottom-front; *in, over the CPU and other major components; *out, at the top-back; *out, from the top panel. This would seem logical to me.

    I live in a hot climate, of course, in winter my temps are good, in summer not so good. "Streams" of air may be more effective when the ambient temperatures are higher.

    It would be good to see results of experiments on different fan configurations, in different cases, and over various ambient temperatures.

    Thermaltake are making cases with different compartments that each have their own fan, so the drives have a different compartment from the other hardware. It will be interesting to see if this type of system becomes the "norm ".

    It may also depend on the quality of the case. I would expect that a high quality case would have a high quality "air flow" system (and I would suppose that expensive does not necessarily mean quality).

    Matt
     
    mattman,
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  8. 2005/12/02
    Christer

    Christer Geek Member Staff Thread Starter

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    The moral of the story might be to think twice before scrapping the old computer case instead of reusing it. In my case (pun and confusion intended ......:p ......) a single change of the position of a single fan made a huge difference at no cost whatsoever.

    If you are shopping for a new case, how could you make a $50 case work as good as a $150 case? Maybe by adding a fan or two increasing the cost by $25? That might leave some investment potential for a better power supply and at a total cost of $100 your case would actually be better!

    However, an Antec Sonata II with a 450W Antec Power Supply at $150 (converted from swedish currency) would be difficult to beat in terms of cost/benefit.

    What I found out earlier (with the original fan position at the front-intake) is that in the winter time with an ambient temperature of 22-23°C the Net Drive Temperatures were ~20°C but in the summer time with an ambient temperature of 31-32°C the Net Drive Temperatures were ~16°C. Did the cooling efficiency increase when it was warmer? What will happen next summer with the fan in the rear-exhaust position?

    Christer
     
    Christer,
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