and also, what about the wattage of the extra fans? any brand I should get for the fans, or is it just the same for simple component such as fan? thanks
Some people tell me the Cougar Vortex 120x25mm ~74CFM fan is great. It's another "quiet" fan. I was impressed with the Akasa Vipers, as I said. You might want to look into the Gentle Typhoon fans in the lower to mid-speed range. I have a current thread here about the high-output AP-30 Gentle Typhoon. From what I can gauge from your original post and questions, you're going to want a simpler path to follow for this fan configuration.
You mentioned dust as a hazard. I've seen many systems clotted up with kruft for the owner failing to do the maintenance (cleaning). The most likely outcome of that sort of neglect might be higher temperatures for the CPU, but nothing some compressed air wouldn't fix. The only real hazard I can think of would be dust mixed with metal filings or other conductive material -- not likely unless you use your computer in your workshop.
SO.
It's for you to decide, but I've found a sense of accomplishment to get the same cooling effect from strategically chosen and thermally controlled fans in smaller number than what your case design might suggest.
Maybe here's a few thoughts:
-- Count the number of PWM, dual-PWM/3-pin and 3-pin fan ports on your motherboard -- excluding any port labeled "PWR-FAN." [AFIK, this latter port is less likely to be thermally controllable.]
-- Determine from your motherboard manual which ports can be thermally controlled with BIOS settings. A lot of decent motherboards are bundled with proprietary monitoring and control software, and this may include the ability to create "fan-profiles" for important fans or groups of them (if you can manage it, but see my note about PWM fans and splitter devices like the Swiftech 8W-PWM-SPL-ST (~$10 item).
-- If you attend toward pressurizing your case a bit, determine the CFM of your case exhaust port(s). Usually, it's probably optimal to have a single case exhaust -- but again -- this is a matter for you to decide.
-- Based on the expected, top-end CFM of your exhaust, choose your intake fans to exceed that by as much as you want.
-- For intake, use the largest fans that fit the ports in your case. They will require fewer RPM to achieve the same or higher CFM airflow, and therefore make less noise.
-- When you shop for fans, read the specs. Many here will say that specs are not that meaningful, but it's the only basis of comparison unless you search for online comparison reviews by test labs. The specs include such things as:
== static pressure
== CFM at top speed
== noise rating in dBA
== type of bearing [ ball bearing lasts longer but can be noisier; rifle bearing is "self-lubricating;" "hydro-dynamic" and other features are better than "sleeve bearing," etc.]
== The fan should have a third wire (for 3-pin) -- the tach wire for monitoring RPM
== MTBF, or life-expectancy. 30,000 hours is short; 50,000 is average; 100,000 is superb.
== Amperage. The only fans you want are 12V, DC fans. If you run them off the motherboard, you're usually limited to a maximum 1.0A per fan port. But most of these fans I've recommended so far draw 0.35A or less. Since all the fans of interest are 12V DC, the amperage is a comparable measure of wattage. In other words, you're looking for fans with wattage less than 12W.
-- You can buy fans of the 120x25mm variety for as little as $5 or $6, but you may be sorry for it later. Generally, decent fans will cost you between $10 and $20 each. Another reason to plan your project for maximum cooling, minimum noise, and a limited number of fans you can thermally control from the motherboard.
Hopefully, your CPU came with a heatsink and fan that you can simply plug into the CPU_FAN port of your motherboard. If you buy a better heatpipe cooler for the CPU, it may come bundled with a fan. It's up to you to decide whether you like it or wish to replace it. I've been known to replace almost every fan that came with a CPU cooler, but others regard the fan and cooler as a sort of "package deal."
Again -- on the number of fans. I've seen folks eager to put 10 fans in a computer case. I had a friend who had four 140's in the sidepanel, two 120's in the front, and an extra exhaust out the top besides the rear exhaust port. Instead of automatic thermal control, he wanted to play with the rheostat knobs on a $50 fan controller. He thought his computer was a "conversation piece," and I observed that it probably was -- if you could hear the conversation over the din of all the fans running.
And -- back to the dust factor. If you can thermally control all the fans in your case without spending money on a separate front-panel fan-controller, the lower RPMs when the system is in an idle state will mean less dust accumulating. That also means less of a need to use filters with the fans, which reduce airflow. And further, it means that you'll be able to sleep with a computer running in your bedroom.