| blah blah Cutaway of a heat pipe showing the wick lining the pipe. |
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In modern computers, cooling is a critical factor. CPUs generate a lot of heat that must be disposed of. Running a CPU without cooling, even for a few seconds, can destroy the chip on some systems. Modern CPUs have internal temperature sensors, and some motherboards have thermistors in the CPU socket to sense the CPU's temperature. The system can be configured to shut down if the CPU overheats.[1]
Head disposal starts with the microprocessor itself. Some are designed to contact a heat sink directly, but others have a heat spreader cemented to the chip or chip package. The heat spreader efficiently distributes heat before contact with a heat sink.
A typical heat sink is a block of aluminum with cooling fins machined or molded into its structure. The fins provide a large surface area to transfer heat to the air. Newer heat sinks have thin aluminum fins mounted around heat pipes (see below).
There are several varieties of heat sink attachments, all of which provide firm pressure to ensure good thermal contact with the CPU. Intel's newer heat sinks snap into place. The pins that hold these heat sinks in place can be rotated to remove the heat sink.
There is always some thermally-conductive material between the heat sink and the CPU. This material prevents an air gap between the CPU and the heat sink.[2] This may be a thermally-conductive adhesive in some cases, but it is more often a rubber-like pad or a paste. When CPUs became powerful enough to require heat management, this paste was typically a mixture of silicone grease and zinc oxide (white thermal grease). Zinc oxide has been replaced with other metals with better thermal conductivity.
In 2012, hardwaresecrets.com tested about 75
different compounds for thermal efficiency under actual operating conditions.
They concluded that chocolate was worse than no thermal compound at all, whereas
mustard performed better than some commercial compounds. However, there was
little difference in performance between the commercial products tested; the
spread was about 6 degrees Celsius.
Many heat sinks come with pre-applied thermal pads. These usually must be
replaced if the heat sink is removed from the CPU.
Thermal paste may dry out over time and lose its effectiveness. There is no recommended time when thermal paste should be replaced, so it should be checked. Should the CPU temperature become a problem, the condition of the thermal paste should be checked.
Some CPU coolers use heat pipes. Heat pipes are lined with a porous material, called a wick, that is saturated with a coolant fluid.
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Heat from the CPU vaporizes the fluid, which causes it to absorb heat (evaporating water feels cool because it absorbs heat, taking heat away from you and making you feel cooler). The vapor travels along the inner pipe to the opposite end, where it is usually attached to a heat sink. The fluid cools and returns to its liquid state, losing heat that is radiated away by the heat sink. The coolant then travels through the porous material back to the CPU.
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Heat pipes can convey heat from the cramped area around the CPU to a more-open area elsewhere on the motherboard, allowing a larger heat sink than can be mounted directly on the CPU. Heat pipes are better than bulk aluminum at moving heat, so heat sinks can be made more efficient by using heat pipes.
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Some motherboards use heat pipes to conduct heat from the support chips (northbridge, southbridge, etc.) to heat sinks near the CPU. This allows the CPU fan to cool the support chips.
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Some people insert aluminum nails into potatoes before baking them. Theoretically, the aluminum nail conducts heat to the center of the potato, cooking it from the inside out. Laboratory tests published in Scientific American magazine showed that it didn't work. The aluminum alone doesn't conduct heat well enough to make much difference. However, sticking heat pipes into the potatoes, with large heat sinks on the outside, significantly shortens the cooking time.
Some high-performance computers use liquid cooling. Much like an automobile cooling system, these pump a liquid coolant (often water) from the CPU and other chips to a large radiator on the computer. You can recognize a liquid cooling system by the plastic tubes, the radiator, and the lack of traditional heat sinks.
A Peltier device is a solid-state cooling component. Simply activating a Peltier device with a voltage will conduct heat from one side to the other. i.e., one side will get hot, and the other will get cold. Some high-performance coolers use a Peltier device between the CPU and the heat sink to help move heat away from the CPU.
Some low-power computers have no fan at all. However, most CPUs generate enough heat to require substantial heat sinks with fans. CPU fans usually come attached to a matching heat sink. When a CPU comes with a substantial warranty, it may come with a matching fan and heat sink that must be used to keep the warranty valid.
Motherboards have one or more connectors to plug fans into. Modern computers can control the fan speed based on the CPU temperature.
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To reduce noise, fans and heat sinks are being made larger. A large fan can move the same amount of air as a small fan while turning at a slower speed, thus making less noise. (Rack-mounted servers often use tiny high-speed fans that make a terrible racket.)
The CPU fan may have a connector with two, three, or four pins. Older fans with two-pin connectors had no speed control; they ran at full speed all the time. A fan with a three-pin connector has a speed sensor. The speed is controlled by varying the voltage applied to the fan. A fan with a four-pin connector has a speed sensor, and the speed is controlled via the fourth wire. The pins are usually arranged, and the motherboards are designed such that three and four-wire fans can be used on motherboards designed for either type. However, if the motherboard doesn't support interchanging three and four-pin fans, speed control may not work if the fan connector doesn't match the motherboard connector.
1. The best cooling is achieved by blowing air out of the case. This causes air to be drawn into the case from many places, evenly cooling the computer.
NCR tested this myth in the 1970s. They found that cooling was much more effective if the air was blown into the equipment case rather than exhausted out of the case. However, you will find that most manufacturers still mount fans to exhaust hot air rather than blowing in cool air.
2. You must ensure that all unused expansion slots are covered for proper cooling. Otherwise, air will not be circulated evenly in the case.
This myth died an ignominious death when manufacturers started poking more holes in a computer case than a colander.
3. The cooling effectiveness of a case is related to the case form factor.
This is like
saying your engine cooling system's effectiveness is related to your car's size.
Sure, you could argue that a large car can have a larger radiator, but when was
the last time you heard someone say, "I want an SUV so I can have a cooler
engine." The form factor of a computer case tells you the maximum size of
motherboard you can put in the case. We're talking apples and oranges here.
Intel developed a form factor called BTX, which was touted to have better
cooling than ATX but was only on the market for about a year, starting in 2005.
Memory heat has become an issue in recent years. Many memory modules come with built-in heat spreaders and heatsinks and rely on existing air circulation in the case for cooling. Hobbyists and gamers may push their systems beyond the manufacturer's design limits by overclocking the CPU. This not only requires extra cooling for the CPU but may also require extra cooling for the memory. Memory cooling fans clip to the socket edges and blow air directly on the memory modules.
1. What must you do if you remove a heat sink from a CPU?
a. Replace the thermal pad or thermal compound.
2. What will happen if you run a computer without a heat sink on the CPU?
a. The CPU will probably be destroyed in seconds.
3. What should you do if your computer shuts down unexpectedly?
a. Check and clean the dust out of the CPU heat sink.
4. What should you do if the CPU fan connector has a different number of pins than the motherboard?
a. Obtain a fan with a matching connector if possible. If not, use the existing fan, aligning pin one of the fan connector with pin one of the motherboard connector.
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