There's a lot of duct solutions, actually.
I made my first one out of cardboard and duct tape. It worked fine. Being so light, there was no problem with it slipping. The basic plan was one rectangular box for the rear fan, and another box for the CPU, and cutting away where they joined. And lots of duct tape.
The one I have now is a section of flexible aluminum 4" laundry dryer duct from Home Depot. 4" diameter duct can be easily coerced to fit fairly snugly on an 80mm fan. It's light, but then again you have to be careful with aluminum that it doesn't touch exposed mobo electrical components. I wrapped the ends in duct tape to prevent this.
Some people build their own out of plexiglass and plastic glue (from Home Depot again.)
You can buy a prebuilt duct solution with fan adapters and so on for $20 or less. "Badong" makes one.
this page has a couple of ducting solutions:
http://directron.com/systemcoolers.html
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How to attach your CPU air to the outside world with a duct?
2 ways:
1) CPU fan in normal position, and a duct from an intake fan to the CPU.
Advantages:
- Guaranteed ambient-temperature air hitting CPU.
Disadvantages:
- An intake fan in the rear can draw in some warm air from PS exhaust. Badong provides a "chimney" to prevent this.
- Case temperature is not helped by this (or hurt.)
2) CPU fan reversed so it pulls air thru heatsink rather than vice versa. Duct joins rear exhaust fan to the CPU/HSF.
Advantages:
- CPU heat dumped in the environment, so your case stays very cool. Good for whatever else in your case needs to stay cool.
Disadvantages:
- The air that the CPU breathes may have been warmed slightly by other components in the case. But this is a small effect. My case temp is less than 3C above ambient (how much less I am not sure.)
- Flipped fan may (or may not) be less efficient than a fan in the other position. You may also have to put in a little spacer so the fan is quiet and works properly upside down. However, the less efficient fan may likely be compensated for by the fact that the motherboard no longer receives hot air on it and therefore your CPU can lose more heat through the socket. (Don't laugh, it's probably a few % through the socket.)
You do not have to have the duct fitting snugly to provide maximum benefit.
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Anyhow, a lot of people choose (1) because it's a little easier, but I chose (2) because it's more elegant, preserves the original airflow and is nice for all your other components as well. In my current setup, I have 1 80 mm 2600 RPM Panaflo fan to exhaust (with duct) in back, and 1 80 mm 2600 RPM Panaflo fan in front for intake. Other case ventilation is the PS exhaust and whatever additional push the CPU HSF provides. It's quieter than the k6-2 500 system I donated to my wife.
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Fan brands:
A lot of people like Panaflo for a nice quiet fan at reasonable cost. The best case-type fans are probably Papst, but they are more costly.
the wesson