Post
by Thomc » Tue May 04, 2004 12:22 pm
Answering the original question, probably the best cooling is provided by using the fans for exhaust. Also, using a pair of fans (or three or more) in parallel provides more air flow than using the same fans in series. In other words, using two fans with both of them providing exhaust will provide more air flow than using the same two fans with one doing exhaust and the other doing intake.
Of course, this can be taken to illogical extremes - like mounting four or five case fans, with all of them (plus a high CFM power supply) providing exhaust. This type of ridiculous extreme would leave you with a high negative pressure area in the case, and the only intake available would be through small cracks and unused screw holes. This could result in very high speed air intakes, producing various interesting and amusing squeals and whistles that might be difficult to tune into anything approaching harmony. It would also turn your PC into a kind of relatively ineffective air filter - resulting in more filth being introduced to your system than you could reasonable expect to achieve with anything less than DSL or a cable modem. (Whoops - wrong kind of filth)
Of course, you could set up a hugely parallel exhaust fan system and try to seal all the little miscellaneous holes - which would probably result in sucking the sides of your case in against the motherboard. (Just kidding, no case fan produces the kind of air pressure needed to create this type of vacuum. In face, I'm not sure any fan does - they would probably just start to cavitate. To produce this type of vacuum, you would probably need a positive displacement device like an air pump. But perhaps I digress...)
If the computer is in a dusty area and you are interested in keeping the computer insides clean (and therefore alive), you may want to effectively filter the air coming into the computer. You can do this one of two ways. The easiest is to configure the system so you have intake fans providing more CFMs than the combined exhaust fans - and then make sure that the intake fans are filtered on the intake side. Dust should not come in to your system through the miscellaneous holes, because the air should be coming out through those holes instead of going in to them. You should be able to test this with smoke, although I have never tried it myself.
The other way to effectively filter the incoming air is to effective seal the case - get rid of all the miscellaneous holes - except for the exhaust fan ports and the area that you have designated as the intake port. Put a filter on the intake port, and let the exhaust fans pull the air in through the filter, and not in through any other spots. This is probably the most efficent and quietest way to cool your system. Be aware, however, that sealing your average PC case is a little like trying to plug the leaks in a sieve - only not as easy.
Finally, the question of hard drive operating temperatures. Sometimes, the answer is actually found by reading the manual - or barring that, to visit the web site. I checked five hard drive manufacturer's web sites - Fujitsu, IBM, Maxtor, Samsung and Seagate - and at each site I checked the technical specification document for multiple drives - 2.5" & 3.5" drives, 5400 & 7200 RPM UDMA/133 drives, 7200 RPM SATA drives, 10K & 15K SCSI drives. I checked current generation drives, and some drives going back as far as two or three generations. I didn't bother to check out any MFM, RLL or ESDI drives.
There were a few models (the Seagate 7200.7 drives smaller than 200GB, for instance) that listed their operating temperature range as 5-60°C. For the vast majority of drives, however (including the 200GB Seagate 7200.7 drives), the operating temperature range was listed as 5-55°C. This range (5-55°C) was so prevalent that I would not hesitate to say that is the range for ALL relatively modern 3.5" IDE or SCSI drives. For the sake of simplicity - and safety - I would simply ignore the few spec sheets that listed 60°C as the upper limit, and focus on 5-55°C as the recommended operating temperature range for modern 3.5" SCSI & IDE drives - and most (Fujitsu, Samsung, Seagate) 2.5" drives.
The HUGE exclusion to all of the above is IBM 2.5" drives. According to the data sheet for every IBM 2.5" drive I looked up, the maximum operating temp for these drives is 35°. Better keep your Thinkpad cool, Jack...
Of course, for any system you are working with, the best idea is to actually look up the specs for the hard drives in that system. For me, however, I'm going to use 5-55°C as the guide for my desktop drives for now.
All of that said, I have heard it said (or seen it written) that for every 10°C you can lower the operating temperature of an electronics component - within the device's recommended operating temperature - you double its projected life. So a 10,000 hour MTBF at 55°C would become a 20,000 hour MTBF at 45°C - and 40,000 hour MTBF at 35°C. I don't know if this is true, but it certainly has its proponents.
I do know that more something heats and cools, the more it expands and contracts - which is hard on strictly electroncs devices, let alone mechanical devices like drive motors and stepper motors. So I would say that as long as you don't cool the drives below ambient air temperature (you don't want moisture condensing on the drives), the cooler you can keep the drives (within their recommended operating range), the better.
Remember, though, when it comes to cooling the hard drive - or any other internal device - it is not necessary for the fans you mount to be intake or exhaust fans. If you are getting enough airflow to cool your system in general with your existing exhaust and power supplies fans, and you don't want to mount an intake fan (which either needs a filter or is going to bring in more dust) - you can mount a fan inside the case blowing on the hard drive. This will cool the drives - and if the fan is correctly mounted inside the system, it will be quieter than if it is mounted as an intake fan.
So for those of you still awake, thanks for coming to todays rambling rant, and be sure to drop in for our next epic length entry, tentatively titled "How to tell the difference between a duck - ice cream has no bones".
