Cleaning case fans with compressed air...
While it is important to allow electronics to run at room temperature or somewhat higher, how high that temperature can safely go depends on the particular product.
Excessive heat is the enemy of most electronics, and will reduce useful life. Radios and televisions can last many years without ever being cleaned or cooled in any manner, but they would last longer if cleaned and cooled. Computers, on the other hand, must have functional cooling fans to reduce the temperature of key components like CPUs, video cards and power supplies or they can begin to make "mistakes" - manifested in program crashes, or screen artifacts.
Typically a computer will have a fan blowing air through the power supply, and another fan (or fans) blowing air around the inside of the case, and another fan cooling the CPU as part of an assembly which will include a "heat sink". "Heat sink" in the sense that the component drains heat away from the object it is cooling. Usually the heat sink is a mass of metal with a lot of surface area pressed hard onto the CPU to encourage heat from the CPU to transfer. The heat sink is often in the shape of fins that forced air can pass over or through, so the heat sink itself can be cooled.
Modern CPUs get hot instantly. If you try to run a computer without a heat sink on the CPU, it may take only seconds for the CPU to become too hot to touch. A decent motherboard will sense this and shut the computer down.
Sometimes extra fans are needed to keep certain types of computers cool - these might include computers that run constantly (like servers), or computers in confined spaces, or computers that are being forced to run faster than they are designed for (over-clocked). In other instances, extra fans may be used for cooling other components like hard drives or the surface chips on a motherboard.
In every instance, computer cases must be cleaned, with special attention paid to the cooling fans, so the components can perform as designed.
With the case cover removed, compressed air can be used to blow the dust, hair and other impurities out of the case. Sometimes it takes more pressure than a can of compressed air can deliver, so using "shop air" may be necessary.
This is a good job to do outside. Give the case a few short blasts across the opening to disperse the lighter materials before you go to work inside the case. Stand back after each blast so you don't breathe the stuff in. As you get into the case, refrain from blowing on the fans at first - get at the dust everywhere else. For the fans, hold a finger on them to keep them from spinning from the airflow. (If the fans are let to spin freely, they can be damaged at high rotation rates.) For the fan inside the power supply, push a thin stick-like object carefully in between the blades to keep it from spinning. Blow from both directions. If the CPU's heat sink has a tunnel component, remove it before cleaning so you can get the full force of the air onto the heat sink. Work your way back out of the case and blow the dust off the exterior.
For machines in smoky environments, cleaning may be impossible. Tar sticks to every suface and dust sticks on the tar. Compressed air will not remove the tar, you can only try to use something like Windex to wipe each and every surface you can access. It is quite common to have to replace fans in such machines (roughly $10-$20 each). You can usually notice these machines if one is brought into a smoke-free environment and started - the smell goes everywhere, instantly.
Here are some pics associated with dust on a heat sink.
Click on any picture to enlarge.
Here you can see the heat sink and fan assembly. If you look through the fan blades, you can see the matted dust making a blanket-like covering overtop of the heat sink fins making it impossible for air moved by the fan to reach into the fins and cool them.
This pic is overexposed but does show the same blanketing effect - look just below the leftmost fan blade to see a little bit of the heat sink fins.
This is the kind of dust you will find in a non-smoker's PC.
This is the CPU ZIF socket (zero insertion force) on the motherboard. In trying to remove the heat sink, the CPU has been pulled right out of the mount because it is stuck onto the heat sink (see below). The lever that normally releases the CPU is shown open, but when the heat sink is on top, you cannot release the CPU - you must pull the CPU out against the locking force. Excessive heat may be the cause of the sticking. Fortunately, no damage to the CPU pins occurred.
In this picture, you can see what the fins are supposed to be like. When clean, the top side should look like this bottom side, in the sense that the moving air can pass through the heat sink to cool it. (That's the inverted CPU stuck on the bottom.)
This picture shows a heat sink from a different computer, clogged right up.
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