Computer cooling
From Free net encyclopedia
Many components in a computer system unit produce large amount of heat during operation, including, but not limited to CPU, chipset, graphics card, and hard drives. This heat must be dissipated in order to keep these components within their safe operating temperatures. Overheated parts have a shorter life and may give sporadic problems resulting in system freezes or crashes. This is done mainly using heatsinks (to increase surface area) and fans (to move air).
Contents |
System Cooling
Air cooling in Desktops
Desktop computers typically use one or more fans for heat management. Almost all desktop power supplies have at least one fan to exhaust air from the case. Most manufacturers recommend bringing cool, fresh air in at the bottom front of the case, and exhausting warm air from the top rear.
If there is more air being forced into the system than being pumped out (due to an imbalance in the number of fans), this is referred to as a 'positive' airflow, as the pressure inside the unit would be higher than outside. A balanced or neutral airflow is the most efficient, although a slightly positive airflow results in less dust build up if dust filters are used. Template:Citation needed
Air Cooling in High Density Computing
Datacentres typically contain many racks of flat 1U servers. Air is drawn in at the front of the rack and exhausted at the rear. Because there are so many computers and other power-consuming devices in the datacentre, the room will quickly overheat without intervention. Thus, extensive HVAC systems are used. Often a raised floor is used so the area under the floor may be used as a large plenum for cooled air and power cabling.
Liquid Submersion Cooling
An uncommon practice is to submerse the computer's components in a thermally conductive liquid. Personal computers that are cooled in this manner do not generally require any fans or pumps, and may be cooled exclusively by passive heat exchange between the cooling fluid and the ambient air. Extreme density computers such as the Cray-2 may use additional radiators in order to facilitate heat exchange.
The liquid used must have sufficiently low electrical conductivity in order for it not to interfere with the normal operation of the computer's components. If the liquid is somewhat electrically conductive, it may be necessary to insulate certain parts of components susceptible to electromagnetic interference, such as the CPU[1]. For these reasons, it is preferred that the liquid be dielectric.
Liquids commonly used for this purpose include various liquids invented and manufactured for this purpose by 3M, such as Fluorinert. Cooking and motor oil have also been successfully used for cooling personal computers[2].
Evaporation can pose a problem, and the liquid may require either to be regularly refilled, or sealed inside the computer's enclosure.
Waste Heat Reduction
Where full-power, full-featured modern computers are not required, some companies opt to use less powerful computers or computers with fewer features. E.g. in an office setting, the IT department may opt for a thin client or a diskless workstation thus cutting out the heat of components such as hard drives and optical disks. These devices are also often powered with direct current from an external power supply brick (which still wastes heat, but not inside the computer).
The components used can greatly effect the power consumption and hence waste heat. A VIA EPIA motherboard with CPU typically radiates approximately 25 watts of heat whereas a Pentium 4 motherboard typically radiates around 140 watts. While the former has considerably less computing power, both types are adequate and responsive for tasks such as word processing and spreadsheets. Opting for an LCD monitor rather than a CRT can also reduce power consumption and excess room heat.
Conductive/Radiative Cooling
Some laptop components, such as hard drives and optical drives, are commonly cooled by having them make contact with the computer's frame, increasing the surface are which can radiate and otherwise exchange heat.
Spot Cooling
In addition to system cooling, various individual components usually have their own cooling systems in place. Components which are individually cooled include, but are not limited to, the CPU, GPU, hard drive and Northbridge.
Passive Heatsink Cooling
This involves attaching a block of machined metal to the part that needs cooling. An adhesive may be used, or more commonly for a CPU, a clamp is used to affix the heatsink tight over the chip, with a thermally conductive pad or gel spread in-between. This block usually has fins and ridges to increase its surface area. The heat conductivity of metal is much better than that of air, and its ability to radiate heat is better than that of the component part it is protecting (usually an integrated circuit/CPU). Commonly found on older CPUs, parts that do not get very hot (such as the chipset), and low-power computers.
Active Heatsink Cooling
This uses the same principle as a passive heatsink cooler, only now, a fan is blowing over or through the heatsink. This results in more air being blown through the heatsink, increasing the rate at which the heatsink can exchange heat with the air. Active heatsinks are the primary method of cooling a modern day processor or graphics card.
Peltier Cooling/TEC
This involves a Peltier effect heat pump which comes in direct contact with the component. The Peltier pump forces heat transfer at a higher rate than a normal heatsink. One side of the Peltier will cool down and the other side will heat up. The big disadvantage is that a Peltier is not an actual cooler, it can only transport heat from one point to the other. A processor with a peltier on top requires far more cooling than a processor without a Peltier.
Liquid (Water) Cooling
Main article: Liquid cooling for computers
Heat Pipe
A heat pipe is a hollow tube containing a heat transfer liquid. As the liquid evaporates, it carries heat to the cool end, where it condenses to the hot end (under capillary force). Heat pipes thus have a much higher effective thermal conductivity than solid materials. In computers, the heatsink on the CPU is attached to a larger radiator heatsink. Both heatsinks are hollow as is the attachment between them, creating one large heat pipe that transfers heat from the CPU to the radiator, which is then cooled using some conventional method. This method is expensive and usually used when space is tight (as in small form-factor PC's), or absolute quiet is needed (computers used in audio production studios during live recording).
Phase-change cooling
A more extreme way to cool the processor. A phase-change cooler is a unit which usually sits underneath the PC, with a tube leading to the processor. Inside the unit is a compressor, the same type that cools a freezer. The compressor compresses a gas into a liquid, and by cooling it from the outside (usually with fans), the liquid reaches temperatures around -30 degrees. Then, the liquid is pumped up to the processor, which heats it, causing the liquid to evaporate, and absorb the heat from the processor. The gas flows down to the compressor and the cycle begins over again. This way, the processor can be cooled to temperatures ranging from -15 to -30 degrees Centigrade, depending on the load, type and speed of the processor.
Liquid nitrogen
By welding an open pipe onto a heatsink, and insulating the pipe, it is possible to cool the processor either with liquid nitrogen (-199 degrees Centigrade) or dry ice. However, after the nitrogen evaporates, it has to be refilled. This method of cooling is only used for extreme overclocking trial runs and record-setting attempts.
CPU cooling
Image:CPU fan and heatsink.jpg Image:CPU copper heat sink.jpg Image:Laptop dust.jpg A CPU generates heat while operating. In operation the temperature of the CPU will thus rise until the temperature gradient between the CPU and its surroundings is such that the heat flow matches the input and the CPU temperature reaches equilibrium. The heat generated by a CPU is a function of the efficiency of its design, the technology used in its construction and the frequency and voltage at which it operates.
For reliable operation, the equilibrium temperature must be sufficiently low for the structure of the CPU to survive. It is common practice to include thermal sensors in the design of CPUs and internal logic that shuts down the CPU if reasonable bounds are exceeded. It is unwise to rely on this, however, as it is not universally implemented, and even if implemented is intended as a damage limitation feature and may not prevent the CPU temperature from reaching dangerous levels such that repeated incidents will cause premature failure of the CPU.
Some CPU designs are specifically tailored to minimise the energy dissipated in the CPU, and thus the heat flow out of the standard chip packaging is sufficient to maintain the CPU at an acceptable temperature. The design of the CPU may also incorporate features to shut down parts of the CPU when it is idling, or to scale back the clock speed under low workloads, all aimed at reducing the power dissipated in the CPU.
This can be done passively by a heatsink which improves the thermal coupling between the CPU and the surrounding air. This may be combined with positive airflow through the computer case driven by the fans in the power supply, or for still higher power systems by actively cooling the heatsink with one or more integral fans circulating air through the heatsink (see aircooling).
Maintaining effective heat transfer across the interface between the CPU and the heatsink is critical. This interface is usually formed by a contact pad on the heat sink clamped against the face of the CPU. The efficiency of this interface can be significantly increased by applying a thin layer of thermal grease between the two surfaces. In extreme cases lapping the surface of the heatsink and the CPU togeather to improve their contact area and thus the heat flow, will further improve this interface.
For thermal output beyond what air cooling can cope with, watercooling or possibly even phase change cooling of the CPU becomes necessary. These technologies used to be limited to mainframe computers, but in the relentless search for more power in desktop computers they are coming into use in desktop computers, notably Apple's Power Macintosh G5, which uses watercooling .
Today, high-end desktop processors are dissipating over 100 watts of heat over a surface area of less than 120 mm².
Until recently, fan cooled aluminium heatsinks were the norm for desktop computers. Today many heatsinks feature copper baseplates or are entirely made of copper, and mount fans of considerable size and power.
To extract the maximum performance from a CPU, a minority of users are using watercooling, Peltier cooling and heat pipe cooling. This relatively expensive technology is especially prevalent amongst overclockers.
The noise and unreliability of fan cooled heatsinks has also spawned another special interest group dedicated to finding technology to reduce the noise generated by computer systems, which comes mainly from the cooling fans in the CPU and power supply. In this field heat pipe cooling shows particular promise.
Besides using equipment to cool the CPU, users can also help to keep CPU temperatures down by preventing the accumulation of dust on the heatsink, which reduces the efficiency with which the heatsink transfers heat to the air, and by removing the build up of fluff and lint that accumulates between the heatsink fins, which impedes the free flow of air through the heat sink. It is recommended that the heatsink be inspected regularly and any contamination blown out with a gas duster.
The picture to right shows significant lint built up between the fins of the heatsinks, sufficient to warrant removal. The best method for removing lint is to use a can of compressed air and simply blow away the lint.
Causes of Heat Build Up
- Dust - Dust acts as a thermal insulator, and reduces the performance of heatsinks and fans.
- Poor Airflow (Turbulence) - Components and cabling cause friction (drag) and turbulent flow that reduce the amount of air flowing through a case, possibly causing stable whirlpools of hot air in certain areas. This could also be caused by a poorly designed computer case.
Cooling and Overclocking
Extra cooling is usually required by people who run parts of their computer (such as the CPU and graphics card) faster than manufacturer specifications, called overclocking. Increasing performance by this modification of settings results in a greater amount of heat generated. The installation of better, non-stock cooling may also be considered modding. Many overclockers simply buy more efficient, more expensive fan/heatsink combinations. A greater investment is involved in liquid cooling, but this is seen by many to be the ultimate in consumer level cooling.
There are also some related practices that have a positive impact in reducing system temperatures:
Heatsink Lapping
This is the smoothing and polishing of the contact (bottom) part of a heatsink to increase its heat transfer efficiency. Even a mediocre heatsink can be improved greatly by doing so. This is because, although smooth to the naked eye, most heatsinks made by machine tools have quite a rough surface characterized by ridges, which create regions of trapped insulating air. Polishing the surface using a combination of fine sandpaper and abrasive polishing liquids can produce a mirror-like shine that is the main indicator of a very flat metal surface.
Use of Exotic Thermal Conductive Compounds
Some overclockers use specialty thermal compounds whose manufacturers claim to have a much higher efficiency than stock thermal pads. Heatsinks clean of any grease or other thermal transfer compounds have a very thin layer of these products applied, and then are placed normally over the CPU. Many of these compounds have a high proportion of silver as their main ingredient. The resulting difference in the temperature of the CPU is measurable, and percentage-wise, the heat transfer does appear to be much superior to stock compounds, but this often works out to a difference of less than a degree.
Use of Rounded Cables
Most PCs use flat ribbon cables to connect storage drives (IDE or SCSI). These large flat cables greatly impede airflow by causing drag and turbulence. Overclockers and modders often replace these with rounded cables, with the conductive wires bunched together tightly, to reduce surface area. Theoretically, the parallel strands of conductors in a ribbon cable serve to reduce cross-talk (signal carrying conductors inducing signals in nearby conductors), but there is no anecdotal evidence of rounding cables reducing performance.
Airflow optimization
Many overclockers subscribe to the belief that less is more. In this case, fewer fans strategically placed will improve the airflow internally within the PC and thus lower the overall case temperature in relation to ambient conditions. The use of larger fans also improves efficiency and lowers the amount of waste heat along with the amount of noise generated while in operation.
For a rectangular PC case, a fan in the front with a fan in the rear and one in the top has been found to be the optimum configuration. Template:Citation needed Fans in the side panels tend to add turbulence to the internal airflow precluding excess heat evacuating from the case.
External links
- Coolers reviewed on DansData - Comparison and testing of around a hundred CPU coolers
- Overclockers Club
- The Power Mac G5's "Liquid Cooling" System
- Silent PC Review