Cooling
Content
Computer cooling is required to remove the waste heat produced by computer components, to keep components within permissible operating temperature limits. Components that are susceptible to temporary malfunction or permanent failure if overheated include integrated circuits such as CPUs, chipset, graphics cards, and hard disk drives. Components are often designed to generate as little heat as possible, and computers and operating systems may be designed to reduce power consumption and consequent heating according to workload, but more heat may still be produced than can be removed without attention to cooling. Use of heatsinks cooled by airflow reduces the temperature rise produced by a given amount of heat. Attention to patterns of airflow can prevent the development of hotspots. Computer fans are very widely used to reduce temperature by actively exhausting hot air. There are also more exotic and extreme techniques, such as liquid cooling. Many computers are designed to sound an alarm or switch off if certain critical internal temperatures exceed a specified limit. Cooling may be designed to reduce the ambient temperature within the case of a computer e.g. by exhausting hot air, or to cool a single component or small area (spot cooling). Components commonly individually cooled include the CPU, GPU and the northbridge chip.
Generators of unwanted heat
Integrated circuits (e.g., CPU and GPU) are the prime generators of heat in modern computers. Heat generation can be reduced by efficient design and selection of operating parameters such as voltage and frequency, but ultimately acceptable performance can often only be achieved by accepting significant heat generation.
The dust buildup on this laptop CPU heat sink after three years of use has made the laptop unusable due to frequent thermal shutdowns. In operation, the temperature of a computer's components will rise until the heat transferred to the surroundings is equal to the heat produced by the component, i.e., thermal equilibrium is reached. For reliable operation, the temperature must never exceed a specified maximum permissible value for each component. For semiconductors, instantaneous junction temperature, rather than component case, heatsink, or ambient temperature is critical. Cooling can be hindered by:
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Dust acting as a thermal insulator and impeding airflow, thereby reducing heat sink and fan performance. Poor airflow including turbulence due to friction against impeding components such as ribbon cables, or improper orientation of fans, can reduce the amount of air flowing through a case and even create localized whirlpools of hot air in the case. In some
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cases of equipment with bad thermal design, cooling air can easily flow out through "cooling" holes before passing over hot components; cooling in such cases can often be improved by blocking of selected holes. Poor heat transfer due to poor thermal contact between components to be cooled and cooling devices. This can be improved by the use of thermal compounds to even out surface imperfections, or even by lapping.
Damage prevention
Thermal sensors in some CPUs and GPUs can shut down the computer when high temperatures are detected, although this may not guarantee safe operation. An integrated circuit may also shut down parts of itself when maximal performance is not required, as when idling, or the clock speed may be reduced under low workloads or when temperature is high, reducing power consumption and hence heat generation.
Air cooling
Further information: Computer fan Fans are most commonly used for air cooling when natural convection is insufficient. Computer fans may be fitted to the computer case, and attached to CPUs, GPUs, chipset, PSU, hard drives and PCI cards. Common fan sizes include 40, 60, 80, 92, 120, and 140 mm square. As of 2011, 200mm to 230mm fans are sometimes used in high-performance computers.
In desktop computers
The airflow through a case optimized for desk-top use in a non-smoking environment is nonideal for use on the floor or at the desks of smokers. Desktop computers typically use one or more fans for cooling. 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[citation needed].
If fans are fitted to force air into the case more effectively than it is removed, the pressure inside becomes higher than outside, referred to as a "positive" airflow; a balanced or neutral airflow is more efficient[citation needed], although a slightly positive airflow results in less dust build up if dust filters are used. Negative pressure inside the case can create problems such as dirty air flowing in through optical drives, depositing dust.
In high density computing
Data centers typically contain many racks of thin, horizontally mounted 1U servers. Air is drawn in at the front of the rack and exhausted at the rear. Because data centers typically contain large numbers of computers and other power-dissipating devices, they risk equipment overheating; extensive HVAC systems are used to prevent this. 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. Another way of accommodating large numbers of systems in a small space is to use blade chassis, oriented vertically rather than horizontally, to facilitate convection. Air heated by the hot components tends to rise, creating a natural air flow along the boards (stack effect), cooling them. Some manufacturers take advantage of this effect.[1][2]
In laptop computing
A laptop computer's CPU and GPU heatsinks, and copper pipes transferring heat to an exhaust fan expelling hot air
The heat is expelled from a laptop by an exhaust fan Most laptops use forced air cooling in order to keep the CPU and other components within their operating temperature range. Because the fan's air is forced through a small port, the fan and heatsinks can be clogged by dust or be obstructed by objects placed near the air intake or exhaust. This can cause overheating, and can be a cause of malfunction (crashing), and ultimately component failure in laptops. The severity of this problem varies with mechanical
airflow design, use, and power dissipation of laptop computers. As of 2011 lower-power components are increasingly used, reducing heat generation[citation needed]. Some laptop components, such as hard drives and optical drives, are commonly cooled by contact with the computer's frame, to which they transfer heat which is then dissipated.
Liquid submersion cooling
An uncommon practice is to submerge the computer's components in a thermally, but not electrically, conductive liquid. Personal computers cooled in this manner do not generally require fans or pumps, and may be cooled exclusively by passive heat exchange between the computer's parts, the cooling fluid and the ambient air. Extreme component density supercomputers such as the Cray-2 and Cray T90 used additional liquid-to-chilled liquid heat exchangers for heat removal. The liquid used must have sufficiently low electrical conductivity not to interfere with the normal operation of the computer. 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.[3] For these reasons, it is preferred that the liquid be dielectric. The 3M company manufactures various liquids, such as Fluorinert, for this purpose. Various oils, including cooking, motor and silicone oils, have been successfully used for cooling personal computers. Evaporation can pose a problem, and the liquid may require either to be regularly refilled or sealed inside the computer's enclosure. According to one company that builds and sells mineral oil submersion kits, they initially found that oil would be lost through a wicking effect up cables that were submerged in the oil. This is no longer the case, as they modified the kit. [4]
Waste heat reduction
Where powerful computers with many features are not required, less powerful computers or ones with fewer features can be used. As of 2011 a VIA EPIA motherboard with CPU typically dissipates approximately 25 watts of heat, whereas a more capable Pentium 4 motherboard and CPU typically dissipates around 140 watts. Computers can be powered with direct current from an external power supply brick which does not generate heat inside the computer case. The replacement of cathode ray tube (CRT) displays by more efficient thinscreen liquid crystal display (LCD) ones in the early twenty-first century reduces power consumption significantly.
Heat-sinks
Main article: Heat sink
Passive heatsink on a Chipset. A component may be fitted in good thermal contact with a heatsink, a passive device with large thermal capacity and with a large surface area relative to its volume. Heatsinks are usually made of a metal with high thermal conductivity such as aluminium or copper[5], and incorporate fins to increase surface area. Heat from a relatively small component is transferred to the larger heatsink; the equilibrium temperature of the component plus heatsink is much lower than the component's alone would be. Heat is carried away from the heatsink by airflow, either due to natural convection or by forced-air fan airflow. Fan cooling is often used to cool processors and graphics cards of high electrical power. In a computer a typical heat-generating component may be manufactured with a flat surface; a block of metal with a corresponding flat surface and finned construction, sometimes with a fan attached, is clamped to the component. To fill poorly-conducting air gaps due to imperfectly flat and smooth surfaces, a thin skim of thermal grease, a thermal pad, or thermal adhesive may be interposed between the component and heatsink. Heat is removed from the heat-sink by convection, to some extent by radiation, and possibly by conduction if the heat-sink is in thermal contact with, say, the metal case. Inexpensive fancooled aluminium heat sinks are often used on standard desktop computers. Heat-sinks with copper base-plates, or made of copper, have better thermal characteristics than aluminium; a copper heat-sink is more effective than an aluminium one of the same size, which is relevant with high-power-consumption components used in high-performance computers. Passive heat sinks are commonly found on older CPUs, parts that do not dissipate much power, such as the chipset, computers with low-power processors, and equipment where silent operation is critical and fan noise unacceptable. Usually a heat-sink is clamped to the integrated heat spreader (IHS), a flat metal plate the size of the CPU package which is part of the CPU assembly and spreads the heat locally, with a thin skim of thermal paste between them to compensate for surfaces not perfectly flat. The spreader's primary purpose is to redistribute heat; the finned heat-sink removes it more efficiently.
Several DDR2 and DDR3 sticks of RAM of capacity 2GB or greater, and system memory, are fitted with a finned heatsink clipped onto the top edge of the memory stick. The same technique is used for video cards that use a finned passive heatsink over the GPU.
Active heat sink with a fan and heat pipes. Dust tends to build up in the crevices of finned heatsinks, particularly with the high airflow produced by fans. This keeps the air away from the hot component, reducing cooling effectiveness; removing the dust restores effectiveness.
Peltier (thermoelectric) cooling
Main article: Thermoelectric cooling In 1821 T. J. Seebeck discovered that different metals, connected at two different junctions, will develop a micro-voltage if the two junctions are held at different temperatures. This effect is known as the "Seebeck effect"; it is the basic theory behind the TEC (thermoelectric cooling). In 1834 Jean Peltier discovered the inverse of the Seebeck effect, now known as the "Peltier effect": applying a voltage to a thermocouple creates a temperature differential between two sides. This results in an effective, albeit extremely inefficient, heat pump. Modern TECs use several stacked units each composed of dozens or hundreds of thermocouples laid out next to each other, which allows for a substantial amount of heat transfer. A combination of bismuth and tellurium is most commonly used for the thermocouples. A Peltier-effect device could be considered to be a heat sink as it does sink heat, but is not usually described as such. As active heat pumps which consume power, TECs can produce temperatures below ambient, impossible with passive heatsinks, radiator-cooled fluid cooling, and heatpipe HSFs.
Fluid cooling
Main article: water cooling
DIY Water cooling setup showing 12v pump, CPU Waterblock and the typical application of a T-Line While originally limited to mainframe computers, fluid cooling has become a practice largely associated with overclocking in the form of either manufactured kits, or in the form of do-ityourself setups assembled from individually gathered parts. The past few years has seen fluid cooling increasing its popularity with pre-assembled, moderate to high performance, desktop computers. Fluids have the ability to dissipate more heat from the parts being cooled than the various types of metals used in heatsinks, making it suitable for overclocking and high performance computer applications.[6] Advantages of fluid cooling include the fact that a system is not limited to cooling one component, but can be set up to cool the central processing unit, graphics processing unit, and/or other components at the same time with the same system. As opposed to air cooling, fluid cooling is also influenced less by the ambient temperature. Fluid cooling's comparatively low noise-level compares favorably to that of active cooling, which can become quite noisy. One disadvantage to fluid cooling is the potential for a coolant leak. Leaked coolant can damage any electronic components with which it comes into contact. Another drawback to fluid cooling is the complexity of the system; an active heat sink is much simpler to build, install, and maintain than a water cooling solution.[7] The IBM Aquasar system uses hot water cooling to achieve energy efficiency, the water being used to heat buildings as well.[8][9]
Heat pipe
Main article: Heat pipe
A graphics card with a heatpipe cooler design. A heat pipe is a hollow tube containing a heat transfer liquid. The liquid absorbs heat and evaporates at one end of the pipe. The vapor travels to the other (cooler) end of the tube, where it condenses, giving up its latent heat. The liquid returns to the hot end of the tube by gravity or capillary action and repeats the cycle. Heat pipes have a much higher effective thermal conductivity than solid materials. For use in computers, the heat sink on the CPU is attached to a larger radiator heat sink. Both heat sinks 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 PCs and laptops, or where no fan noise can be tolerated, as in audio production. Because of the efficiency of this method of cooling, many desktop CPUs and GPUs, as well as high end chipsets, use heat pipes in addition to active fan-based cooling to remain within safe operating temperatures.
Phase-change cooling
Phase-change cooling is an extremely effective way to cool the processor. A vapor compression 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 of the same type as in a window air conditioner. The compressor compresses a gas (or mixture of gases) which compresses it into a liquid. Then, the liquid is pumped up to the processor, where it passes through a condenser (heat dissipation device) and then an expansion device to vaporize the fluid, the expansion device used can be a simple capillary tube to a more elaborate thermal expansion valve. The liquid evaporates (changing phase), absorbing the heat from the processor as it draws extra energy from its environment to accommodate this change (see latent heat). The evaporation can produce temperatures reaching around −15 to −150 degrees Celsius. 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 −150 degrees Celsius, depending on the load, wattage of the processor, the refrigeration system (see refrigeration) and the gas mixture used. This type of system suffers from a number of issues but, mainly, one must be concerned with dew point and the proper insulation of all sub-ambient surfaces that must be done (the pipes will sweat, dripping water on sensitive electronics). Alternately, a new breed of cooling system is being developed, inserting a pump into the thermo siphon loop. This adds another degree of flexibility for the design engineer, as the heat can now be effectively transported away from the heat source and either reclaimed or dissipated to ambient. Junction temperature can be tuned by adjusting the system pressure; higher pressure equals higher fluid saturation temperatures. This allows for smaller condensers, smaller fans, and/or the effective dissipation of heat in a high ambient environment. These systems are, in essence, the next generation fluid cooling paradigm, as they are approximately 10 times more efficient than single phase water. Since the system uses a dielectric as the heat transport media, leaks do not cause a catastrophic failure of the electric system. This type of cooling is seen as a more extreme way to cool components, since the units are relatively expensive compared to the average desktop. They also generate a significant amount of noise, since they are essentially refrigerators, however the compressor choice and air cooling system is the main determinant of this, allowing for flexibility for noise reduction based on the parts chosen.
Liquid nitrogen
Liquid nitrogen may be used to cool overclocked components. As liquid nitrogen boils at -196 °C, far below the freezing point of water, it is valuable as an extreme coolant for short overclocking sessions. In a typical installation of liquid nitrogen cooling, a copper or aluminum pipe is mounted on top of the processor or graphics card. After being heavily insulated against condensation, the liquid nitrogen is poured into the pipe, resulting in temperatures well below -100 °C. Evaporation devices ranging from cut out heat sinks with pipes attached to custom milled copper containers are used to hold the nitrogen as well as to prevent large temperature changes. However, after the nitrogen evaporates, it has to be refilled. In the realm of personal computers, this method of cooling is seldom used in contexts other than overclocking trialruns and record-setting attempts, as the CPU will usually expire within a relatively short period of time due to temperature stress caused by changes in internal temperature. Although liquid nitrogen is non-flammable, it can condense oxygen directly from air. Mixtures of liquid oxygen and flammable materials can be dangerously explosive.
Liquid helium
Liquid helium, colder than liquid nitrogen, has also been used for cooling. Liquid helium evaporates at -269 °C, and temperatures ranging from -230 to -240 °C have been measured from the heatsink.[10] However, liquid helium is more expensive and more difficult to store and use than liquid nitrogen. Also, extremely low temperatures can cause integrated circuits to stop functioning. Silicon-based semiconductors, for example, will freeze out at around -233 °C.[11]
Soft cooling
Soft cooling is the practice of utilizing software to take advantage of CPU power saving technologies to minimize energy use. This is done using halt instructions to turn off or put in standby state CPU subparts that aren't being used or by underclocking the CPU. While resulting in slower total speeds, this can be very useful if overclocking a CPU to improve user experience rather than increase raw processing power, since it can prevent the need for noisier cooling.
Undervolting
Undervolting is a practice of running the CPU or any other component with voltages below the device specifications. An undervolted component draws less power and thus produces less heat. The ability to do this varies by manufacturer, product line, and even different production runs of the same exact product (as well as that of other components in the system), but processors are often specified to use voltages higher than strictly necessary. This tolerance ensures that the processor will have a higher chance of performing correctly under sub-optimal conditions, such as a lower-quality motherboard or low power supply voltages. Below a certain limit the processor will not function correctly, although undervolting too far does not typically lead to permanent hardware damage. Undervolting is used for quiet systems, as less cooling is needed because of the reduction of heat production, allowing noisy fans to be omitted. It is also used when battery charge life must be maximised.
Integrated chip cooling techniques
Conventional cooling techniques all attach their “cooling” component to the outside of the computer chip, or via IHS and/or heat sinks. This “attaching” technique will always exhibit some thermal resistance, reducing its effectiveness. The heat can be more efficiently and quickly removed by directly cooling the local hot spots. At these locations, power dissipation of over 300W/cm2 (typical CPU are less than 100W/cm2, although future systems are expected to exceed 1000W/cm2 [12]) can occur. This form of local cooling is essential to developing high power density chips. This ideology has led to the investigation of integrating cooling elements into the computer chip. Currently there are two techniques: micro-channel heat sinks, and jet impingement cooling. In micro-channel heat sinks, channels are fabricated into the silicon chip (CPU), and coolant is pumped through them. The channels are designed with very large surface area which results in large heat transfers. Heat dissipation of 3000W/cm2 has been reported with this technique.[13] In comparison to the Sun power density of around 7400W/cm2. The heat dissipation can be further increased if two-phase flow cooling is applied. Unfortunately the system requires large pressure drops, due to the small channels, and the heat flux is lower with dielectric coolants used in electronic cooling. Another local chip cooling technique is jet impingement cooling. In this technique, a coolant is flown through a small orifice to form a jet. The jet is directed toward the surface of the CPU chip, and can effectively remove large heat fluxes. Heat dissipation of over 1000W/cm2has been reported.[14] The system can be operated at lower pressure in comparison to the micro-channel method. The heat transfer can be further increased using two-phase flow cooling and by integrating return flow channels (hybrid between micro-channel heat sinks and jet impingement cooling).
Optimisation of cooling
Cooling can be improved by several techniques which may involve additional expense or effort. These techniques are often used, in particular, by those who run parts of their computer (such as the CPU and GPU) at higher voltages and frequencies than specified by manufacturer (overclocking), which increases heat generation.
The installation of higher performance, non-stock cooling may also be considered modding. Many overclockers simply buy more efficient, and often, more expensive fan and heat sink combinations, while others resort to more exotic ways of computer cooling, such as liquid cooling, Peltier effect heatpumps, heat pipe or phase change cooling. There are also some related practices that have a positive impact in reducing system temperatures:
Use of thermal conductive compounds
Thermal compound is commonly used to enhance the thermal conductivity from the CPU or GPU to the heatsink cooler. (Left to right: Arctic MX-2, Arctic MX-4, Tuniq TX-4, Antec Formula 7, Noctua NT-H1 Perfectly flat surfaces in contact give optimal cooling, but perfect flatness and absence of microscopic air gaps is not practically possible, particularly in mass-produced equipment. A very thin skim of thermal compound, which is much more thermally conductive than air, though much less so than metal, can improve thermal contact and cooling by filling in the air gaps. If only a small amount of compound just sufficient to fill the gaps is used, best temperature reduction will be obtained. There is much debate about the merits of compounds, and overclockers often consider some compounds to be superior to others. The main consideration is to use the minimal amount of thermal compound required to even out surfaces, as the thermal conductivity of compound is typically 20 to 400 times worse than that of metal, though better than air[15]. Heat-conductive pads are also used, often fitted by manufacturers to heatsinks. They are less effective than properly applied thermal compound, but simpler to apply and, if fixed to the heatsink, cannot be omitted by users unaware of the importance of good thermal contact, or replaced by a thick and ineffective layer of compound. Unlike some techniques discussed here, the use of thermal compound or padding is almost universal when dissipating significant amounts of heat.
Heat sink lapping
Mass-produced CPU heat spreaders and heatsink bases are never perfectly flat or smooth; if these surfaces are placed in the best contact possible, there will be air gaps which reduce heat conduction. This can easily be mitigated by the use of thermal compound, but for the best possible results surfaces must be as flat as possible. This can be achieved by a laborious process known as lapping, which can reduce CPU temperature by typically 5 °C[16][17].
Use of rounded cables
Most older 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 crosstalk (signal carrying conductors inducing signals in nearby conductors), but there is no empirical evidence of rounding cables reducing performance. This may be because the length of the cable is short enough so that the effect of crosstalk is negligible, or may be due to the fact that 80 conductor cables are individually shielded to begin with. Problems usually arise when the cable is not electromagnetically protected and the length is considerable, a more frequent occurrence with older network cables. These computer cables can then be cable tied to the chassis or other cables to further increase airflow. This is less of a problem with new computers that use Serial ATA which has a much narrower cable.
Airflow optimization
The colder the cooling medium (the air), the more effective the cooling. Cooling air temperature can be improved with these guidelines:
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Supply cool air to the hot components as directly as possible. Examples are air snorkels and tunnels that feed outside air directly and exclusively to the CPU or GPU cooler. For example, the BTX case design prescribes a CPU air tunnel. Expel warm air as directly as possible. Examples are: Conventional PC (ATX) power supplies blow the warm air out the back of the case. Many dual-slot graphics card designs blow the warm air through the cover of the adjacent slot. There are also some aftermarket coolers that do this. Some CPU cooling designs blow the warm air directly towards the back of the case, where it can be ejected by a case fan. Air that has already been used to spot-cool a component should not be reused to spotcool a different component (this follows from the previous items). The ATX case design can be said to violate this rule, since the power supply gets its "cool" air from the inside of the case, where it has been warmed up already. The BTX case design also violates this rule, since it uses the CPU cooler's exhaust to cool the chipset and often the graphics card. Prefer cool intake air, avoid inhaling exhaust air (outside air above or near the exhausts). For example, a CPU cooling air duct at the back of a tower case would inhale warm air from a graphics card exhaust. Moving all exhausts to one side of the case, conventionally the back, helps to keep the intake air cool. Hiding cables behind motherboard tray or simply apply ziptie and tucking cables away to provide unhindered airflow.
Fewer fans but strategically placed will improve the airflow internally within the PC and thus lower the overall internal 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 by the fans while in operation.
There is little agreement on the effectiveness of different fan placement configurations, and little in the way of systematic testing has been done. For a rectangular PC (ATX) case, a fan in the front with a fan in the rear and one in the top has been found to be a suitable configuration. However, AMD's (somewhat outdated) system cooling guidelines notes that "A front cooling fan does not seem to be essential. In fact, in some extreme situations, testing showed these fans to be recirculating hot air rather than introducing cool air."[18] It may be that fans in the side panels could have a similar detrimental effect—possibly through disrupting the normal air flow through the case. However, this is unconfirmed and probably varies with the configuration.
Generators of unwanted heat
Integrated circuits (e.g., CPU and GPU) are the prime generators of heat in modern computers. Heat generation can be reduced by efficient design and selection of operating parameters such as voltage and frequency, but ultimately acceptable performance can often only be achieved by accepting significant heat generation.
The dust buildup on this laptop CPU heat sink after three years of use has made the laptop unusable due to frequent thermal shutdowns. In operation, the temperature of a computer's components will rise until the heat transferred to the surroundings is equal to the heat produced by the component, i.e., thermal equilibrium is reached. For reliable operation, the temperature must never exceed a specified maximum permissible value for each component. For semiconductors, instantaneous junction temperature, rather than component case, heatsink, or ambient temperature is critical. Cooling can be hindered by:
• •
Dust acting as a thermal insulator and impeding airflow, thereby reducing heat sink and fan performance. Poor airflow including turbulence due to friction against impeding components such as ribbon cables, or improper orientation of fans, can reduce the amount of air flowing through a case and even create localized whirlpools of hot air in the case. In some
•
cases of equipment with bad thermal design, cooling air can easily flow out through "cooling" holes before passing over hot components; cooling in such cases can often be improved by blocking of selected holes. Poor heat transfer due to poor thermal contact between components to be cooled and cooling devices. This can be improved by the use of thermal compounds to even out surface imperfections, or even by lapping.
Damage prevention
Thermal sensors in some CPUs and GPUs can shut down the computer when high temperatures are detected, although this may not guarantee safe operation. An integrated circuit may also shut down parts of itself when maximal performance is not required, as when idling, or the clock speed may be reduced under low workloads or when temperature is high, reducing power consumption and hence heat generation.
Air cooling
Further information: Computer fan Fans are most commonly used for air cooling when natural convection is insufficient. Computer fans may be fitted to the computer case, and attached to CPUs, GPUs, chipset, PSU, hard drives and PCI cards. Common fan sizes include 40, 60, 80, 92, 120, and 140 mm square. As of 2011, 200mm to 230mm fans are sometimes used in high-performance computers.
In desktop computers
The airflow through a case optimized for desk-top use in a non-smoking environment is nonideal for use on the floor or at the desks of smokers. Desktop computers typically use one or more fans for cooling. 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[citation needed].
If fans are fitted to force air into the case more effectively than it is removed, the pressure inside becomes higher than outside, referred to as a "positive" airflow; a balanced or neutral airflow is more efficient[citation needed], although a slightly positive airflow results in less dust build up if dust filters are used. Negative pressure inside the case can create problems such as dirty air flowing in through optical drives, depositing dust.
In high density computing
Data centers typically contain many racks of thin, horizontally mounted 1U servers. Air is drawn in at the front of the rack and exhausted at the rear. Because data centers typically contain large numbers of computers and other power-dissipating devices, they risk equipment overheating; extensive HVAC systems are used to prevent this. 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. Another way of accommodating large numbers of systems in a small space is to use blade chassis, oriented vertically rather than horizontally, to facilitate convection. Air heated by the hot components tends to rise, creating a natural air flow along the boards (stack effect), cooling them. Some manufacturers take advantage of this effect.[1][2]
In laptop computing
A laptop computer's CPU and GPU heatsinks, and copper pipes transferring heat to an exhaust fan expelling hot air
The heat is expelled from a laptop by an exhaust fan Most laptops use forced air cooling in order to keep the CPU and other components within their operating temperature range. Because the fan's air is forced through a small port, the fan and heatsinks can be clogged by dust or be obstructed by objects placed near the air intake or exhaust. This can cause overheating, and can be a cause of malfunction (crashing), and ultimately component failure in laptops. The severity of this problem varies with mechanical
airflow design, use, and power dissipation of laptop computers. As of 2011 lower-power components are increasingly used, reducing heat generation[citation needed]. Some laptop components, such as hard drives and optical drives, are commonly cooled by contact with the computer's frame, to which they transfer heat which is then dissipated.
Liquid submersion cooling
An uncommon practice is to submerge the computer's components in a thermally, but not electrically, conductive liquid. Personal computers cooled in this manner do not generally require fans or pumps, and may be cooled exclusively by passive heat exchange between the computer's parts, the cooling fluid and the ambient air. Extreme component density supercomputers such as the Cray-2 and Cray T90 used additional liquid-to-chilled liquid heat exchangers for heat removal. The liquid used must have sufficiently low electrical conductivity not to interfere with the normal operation of the computer. 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.[3] For these reasons, it is preferred that the liquid be dielectric. The 3M company manufactures various liquids, such as Fluorinert, for this purpose. Various oils, including cooking, motor and silicone oils, have been successfully used for cooling personal computers. Evaporation can pose a problem, and the liquid may require either to be regularly refilled or sealed inside the computer's enclosure. According to one company that builds and sells mineral oil submersion kits, they initially found that oil would be lost through a wicking effect up cables that were submerged in the oil. This is no longer the case, as they modified the kit. [4]
Waste heat reduction
Where powerful computers with many features are not required, less powerful computers or ones with fewer features can be used. As of 2011 a VIA EPIA motherboard with CPU typically dissipates approximately 25 watts of heat, whereas a more capable Pentium 4 motherboard and CPU typically dissipates around 140 watts. Computers can be powered with direct current from an external power supply brick which does not generate heat inside the computer case. The replacement of cathode ray tube (CRT) displays by more efficient thinscreen liquid crystal display (LCD) ones in the early twenty-first century reduces power consumption significantly.
Heat-sinks
Main article: Heat sink
Passive heatsink on a Chipset. A component may be fitted in good thermal contact with a heatsink, a passive device with large thermal capacity and with a large surface area relative to its volume. Heatsinks are usually made of a metal with high thermal conductivity such as aluminium or copper[5], and incorporate fins to increase surface area. Heat from a relatively small component is transferred to the larger heatsink; the equilibrium temperature of the component plus heatsink is much lower than the component's alone would be. Heat is carried away from the heatsink by airflow, either due to natural convection or by forced-air fan airflow. Fan cooling is often used to cool processors and graphics cards of high electrical power. In a computer a typical heat-generating component may be manufactured with a flat surface; a block of metal with a corresponding flat surface and finned construction, sometimes with a fan attached, is clamped to the component. To fill poorly-conducting air gaps due to imperfectly flat and smooth surfaces, a thin skim of thermal grease, a thermal pad, or thermal adhesive may be interposed between the component and heatsink. Heat is removed from the heat-sink by convection, to some extent by radiation, and possibly by conduction if the heat-sink is in thermal contact with, say, the metal case. Inexpensive fancooled aluminium heat sinks are often used on standard desktop computers. Heat-sinks with copper base-plates, or made of copper, have better thermal characteristics than aluminium; a copper heat-sink is more effective than an aluminium one of the same size, which is relevant with high-power-consumption components used in high-performance computers. Passive heat sinks are commonly found on older CPUs, parts that do not dissipate much power, such as the chipset, computers with low-power processors, and equipment where silent operation is critical and fan noise unacceptable. Usually a heat-sink is clamped to the integrated heat spreader (IHS), a flat metal plate the size of the CPU package which is part of the CPU assembly and spreads the heat locally, with a thin skim of thermal paste between them to compensate for surfaces not perfectly flat. The spreader's primary purpose is to redistribute heat; the finned heat-sink removes it more efficiently.
Several DDR2 and DDR3 sticks of RAM of capacity 2GB or greater, and system memory, are fitted with a finned heatsink clipped onto the top edge of the memory stick. The same technique is used for video cards that use a finned passive heatsink over the GPU.
Active heat sink with a fan and heat pipes. Dust tends to build up in the crevices of finned heatsinks, particularly with the high airflow produced by fans. This keeps the air away from the hot component, reducing cooling effectiveness; removing the dust restores effectiveness.
Peltier (thermoelectric) cooling
Main article: Thermoelectric cooling In 1821 T. J. Seebeck discovered that different metals, connected at two different junctions, will develop a micro-voltage if the two junctions are held at different temperatures. This effect is known as the "Seebeck effect"; it is the basic theory behind the TEC (thermoelectric cooling). In 1834 Jean Peltier discovered the inverse of the Seebeck effect, now known as the "Peltier effect": applying a voltage to a thermocouple creates a temperature differential between two sides. This results in an effective, albeit extremely inefficient, heat pump. Modern TECs use several stacked units each composed of dozens or hundreds of thermocouples laid out next to each other, which allows for a substantial amount of heat transfer. A combination of bismuth and tellurium is most commonly used for the thermocouples. A Peltier-effect device could be considered to be a heat sink as it does sink heat, but is not usually described as such. As active heat pumps which consume power, TECs can produce temperatures below ambient, impossible with passive heatsinks, radiator-cooled fluid cooling, and heatpipe HSFs.
Fluid cooling
Main article: water cooling
DIY Water cooling setup showing 12v pump, CPU Waterblock and the typical application of a T-Line While originally limited to mainframe computers, fluid cooling has become a practice largely associated with overclocking in the form of either manufactured kits, or in the form of do-ityourself setups assembled from individually gathered parts. The past few years has seen fluid cooling increasing its popularity with pre-assembled, moderate to high performance, desktop computers. Fluids have the ability to dissipate more heat from the parts being cooled than the various types of metals used in heatsinks, making it suitable for overclocking and high performance computer applications.[6] Advantages of fluid cooling include the fact that a system is not limited to cooling one component, but can be set up to cool the central processing unit, graphics processing unit, and/or other components at the same time with the same system. As opposed to air cooling, fluid cooling is also influenced less by the ambient temperature. Fluid cooling's comparatively low noise-level compares favorably to that of active cooling, which can become quite noisy. One disadvantage to fluid cooling is the potential for a coolant leak. Leaked coolant can damage any electronic components with which it comes into contact. Another drawback to fluid cooling is the complexity of the system; an active heat sink is much simpler to build, install, and maintain than a water cooling solution.[7] The IBM Aquasar system uses hot water cooling to achieve energy efficiency, the water being used to heat buildings as well.[8][9]
Heat pipe
Main article: Heat pipe
A graphics card with a heatpipe cooler design. A heat pipe is a hollow tube containing a heat transfer liquid. The liquid absorbs heat and evaporates at one end of the pipe. The vapor travels to the other (cooler) end of the tube, where it condenses, giving up its latent heat. The liquid returns to the hot end of the tube by gravity or capillary action and repeats the cycle. Heat pipes have a much higher effective thermal conductivity than solid materials. For use in computers, the heat sink on the CPU is attached to a larger radiator heat sink. Both heat sinks 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 PCs and laptops, or where no fan noise can be tolerated, as in audio production. Because of the efficiency of this method of cooling, many desktop CPUs and GPUs, as well as high end chipsets, use heat pipes in addition to active fan-based cooling to remain within safe operating temperatures.
Phase-change cooling
Phase-change cooling is an extremely effective way to cool the processor. A vapor compression 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 of the same type as in a window air conditioner. The compressor compresses a gas (or mixture of gases) which compresses it into a liquid. Then, the liquid is pumped up to the processor, where it passes through a condenser (heat dissipation device) and then an expansion device to vaporize the fluid, the expansion device used can be a simple capillary tube to a more elaborate thermal expansion valve. The liquid evaporates (changing phase), absorbing the heat from the processor as it draws extra energy from its environment to accommodate this change (see latent heat). The evaporation can produce temperatures reaching around −15 to −150 degrees Celsius. 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 −150 degrees Celsius, depending on the load, wattage of the processor, the refrigeration system (see refrigeration) and the gas mixture used. This type of system suffers from a number of issues but, mainly, one must be concerned with dew point and the proper insulation of all sub-ambient surfaces that must be done (the pipes will sweat, dripping water on sensitive electronics). Alternately, a new breed of cooling system is being developed, inserting a pump into the thermo siphon loop. This adds another degree of flexibility for the design engineer, as the heat can now be effectively transported away from the heat source and either reclaimed or dissipated to ambient. Junction temperature can be tuned by adjusting the system pressure; higher pressure equals higher fluid saturation temperatures. This allows for smaller condensers, smaller fans, and/or the effective dissipation of heat in a high ambient environment. These systems are, in essence, the next generation fluid cooling paradigm, as they are approximately 10 times more efficient than single phase water. Since the system uses a dielectric as the heat transport media, leaks do not cause a catastrophic failure of the electric system. This type of cooling is seen as a more extreme way to cool components, since the units are relatively expensive compared to the average desktop. They also generate a significant amount of noise, since they are essentially refrigerators, however the compressor choice and air cooling system is the main determinant of this, allowing for flexibility for noise reduction based on the parts chosen.
Liquid nitrogen
Liquid nitrogen may be used to cool overclocked components. As liquid nitrogen boils at -196 °C, far below the freezing point of water, it is valuable as an extreme coolant for short overclocking sessions. In a typical installation of liquid nitrogen cooling, a copper or aluminum pipe is mounted on top of the processor or graphics card. After being heavily insulated against condensation, the liquid nitrogen is poured into the pipe, resulting in temperatures well below -100 °C. Evaporation devices ranging from cut out heat sinks with pipes attached to custom milled copper containers are used to hold the nitrogen as well as to prevent large temperature changes. However, after the nitrogen evaporates, it has to be refilled. In the realm of personal computers, this method of cooling is seldom used in contexts other than overclocking trialruns and record-setting attempts, as the CPU will usually expire within a relatively short period of time due to temperature stress caused by changes in internal temperature. Although liquid nitrogen is non-flammable, it can condense oxygen directly from air. Mixtures of liquid oxygen and flammable materials can be dangerously explosive.
Liquid helium
Liquid helium, colder than liquid nitrogen, has also been used for cooling. Liquid helium evaporates at -269 °C, and temperatures ranging from -230 to -240 °C have been measured from the heatsink.[10] However, liquid helium is more expensive and more difficult to store and use than liquid nitrogen. Also, extremely low temperatures can cause integrated circuits to stop functioning. Silicon-based semiconductors, for example, will freeze out at around -233 °C.[11]
Soft cooling
Soft cooling is the practice of utilizing software to take advantage of CPU power saving technologies to minimize energy use. This is done using halt instructions to turn off or put in standby state CPU subparts that aren't being used or by underclocking the CPU. While resulting in slower total speeds, this can be very useful if overclocking a CPU to improve user experience rather than increase raw processing power, since it can prevent the need for noisier cooling.
Undervolting
Undervolting is a practice of running the CPU or any other component with voltages below the device specifications. An undervolted component draws less power and thus produces less heat. The ability to do this varies by manufacturer, product line, and even different production runs of the same exact product (as well as that of other components in the system), but processors are often specified to use voltages higher than strictly necessary. This tolerance ensures that the processor will have a higher chance of performing correctly under sub-optimal conditions, such as a lower-quality motherboard or low power supply voltages. Below a certain limit the processor will not function correctly, although undervolting too far does not typically lead to permanent hardware damage. Undervolting is used for quiet systems, as less cooling is needed because of the reduction of heat production, allowing noisy fans to be omitted. It is also used when battery charge life must be maximised.
Integrated chip cooling techniques
Conventional cooling techniques all attach their “cooling” component to the outside of the computer chip, or via IHS and/or heat sinks. This “attaching” technique will always exhibit some thermal resistance, reducing its effectiveness. The heat can be more efficiently and quickly removed by directly cooling the local hot spots. At these locations, power dissipation of over 300W/cm2 (typical CPU are less than 100W/cm2, although future systems are expected to exceed 1000W/cm2 [12]) can occur. This form of local cooling is essential to developing high power density chips. This ideology has led to the investigation of integrating cooling elements into the computer chip. Currently there are two techniques: micro-channel heat sinks, and jet impingement cooling. In micro-channel heat sinks, channels are fabricated into the silicon chip (CPU), and coolant is pumped through them. The channels are designed with very large surface area which results in large heat transfers. Heat dissipation of 3000W/cm2 has been reported with this technique.[13] In comparison to the Sun power density of around 7400W/cm2. The heat dissipation can be further increased if two-phase flow cooling is applied. Unfortunately the system requires large pressure drops, due to the small channels, and the heat flux is lower with dielectric coolants used in electronic cooling. Another local chip cooling technique is jet impingement cooling. In this technique, a coolant is flown through a small orifice to form a jet. The jet is directed toward the surface of the CPU chip, and can effectively remove large heat fluxes. Heat dissipation of over 1000W/cm2has been reported.[14] The system can be operated at lower pressure in comparison to the micro-channel method. The heat transfer can be further increased using two-phase flow cooling and by integrating return flow channels (hybrid between micro-channel heat sinks and jet impingement cooling).
Optimisation of cooling
Cooling can be improved by several techniques which may involve additional expense or effort. These techniques are often used, in particular, by those who run parts of their computer (such as the CPU and GPU) at higher voltages and frequencies than specified by manufacturer (overclocking), which increases heat generation.
The installation of higher performance, non-stock cooling may also be considered modding. Many overclockers simply buy more efficient, and often, more expensive fan and heat sink combinations, while others resort to more exotic ways of computer cooling, such as liquid cooling, Peltier effect heatpumps, heat pipe or phase change cooling. There are also some related practices that have a positive impact in reducing system temperatures:
Use of thermal conductive compounds
Thermal compound is commonly used to enhance the thermal conductivity from the CPU or GPU to the heatsink cooler. (Left to right: Arctic MX-2, Arctic MX-4, Tuniq TX-4, Antec Formula 7, Noctua NT-H1 Perfectly flat surfaces in contact give optimal cooling, but perfect flatness and absence of microscopic air gaps is not practically possible, particularly in mass-produced equipment. A very thin skim of thermal compound, which is much more thermally conductive than air, though much less so than metal, can improve thermal contact and cooling by filling in the air gaps. If only a small amount of compound just sufficient to fill the gaps is used, best temperature reduction will be obtained. There is much debate about the merits of compounds, and overclockers often consider some compounds to be superior to others. The main consideration is to use the minimal amount of thermal compound required to even out surfaces, as the thermal conductivity of compound is typically 20 to 400 times worse than that of metal, though better than air[15]. Heat-conductive pads are also used, often fitted by manufacturers to heatsinks. They are less effective than properly applied thermal compound, but simpler to apply and, if fixed to the heatsink, cannot be omitted by users unaware of the importance of good thermal contact, or replaced by a thick and ineffective layer of compound. Unlike some techniques discussed here, the use of thermal compound or padding is almost universal when dissipating significant amounts of heat.
Heat sink lapping
Mass-produced CPU heat spreaders and heatsink bases are never perfectly flat or smooth; if these surfaces are placed in the best contact possible, there will be air gaps which reduce heat conduction. This can easily be mitigated by the use of thermal compound, but for the best possible results surfaces must be as flat as possible. This can be achieved by a laborious process known as lapping, which can reduce CPU temperature by typically 5 °C[16][17].
Use of rounded cables
Most older 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 crosstalk (signal carrying conductors inducing signals in nearby conductors), but there is no empirical evidence of rounding cables reducing performance. This may be because the length of the cable is short enough so that the effect of crosstalk is negligible, or may be due to the fact that 80 conductor cables are individually shielded to begin with. Problems usually arise when the cable is not electromagnetically protected and the length is considerable, a more frequent occurrence with older network cables. These computer cables can then be cable tied to the chassis or other cables to further increase airflow. This is less of a problem with new computers that use Serial ATA which has a much narrower cable.
Airflow optimization
The colder the cooling medium (the air), the more effective the cooling. Cooling air temperature can be improved with these guidelines:
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Supply cool air to the hot components as directly as possible. Examples are air snorkels and tunnels that feed outside air directly and exclusively to the CPU or GPU cooler. For example, the BTX case design prescribes a CPU air tunnel. Expel warm air as directly as possible. Examples are: Conventional PC (ATX) power supplies blow the warm air out the back of the case. Many dual-slot graphics card designs blow the warm air through the cover of the adjacent slot. There are also some aftermarket coolers that do this. Some CPU cooling designs blow the warm air directly towards the back of the case, where it can be ejected by a case fan. Air that has already been used to spot-cool a component should not be reused to spotcool a different component (this follows from the previous items). The ATX case design can be said to violate this rule, since the power supply gets its "cool" air from the inside of the case, where it has been warmed up already. The BTX case design also violates this rule, since it uses the CPU cooler's exhaust to cool the chipset and often the graphics card. Prefer cool intake air, avoid inhaling exhaust air (outside air above or near the exhausts). For example, a CPU cooling air duct at the back of a tower case would inhale warm air from a graphics card exhaust. Moving all exhausts to one side of the case, conventionally the back, helps to keep the intake air cool. Hiding cables behind motherboard tray or simply apply ziptie and tucking cables away to provide unhindered airflow.
Fewer fans but strategically placed will improve the airflow internally within the PC and thus lower the overall internal 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 by the fans while in operation.
There is little agreement on the effectiveness of different fan placement configurations, and little in the way of systematic testing has been done. For a rectangular PC (ATX) case, a fan in the front with a fan in the rear and one in the top has been found to be a suitable configuration. However, AMD's (somewhat outdated) system cooling guidelines notes that "A front cooling fan does not seem to be essential. In fact, in some extreme situations, testing showed these fans to be recirculating hot air rather than introducing cool air."[18] It may be that fans in the side panels could have a similar detrimental effect—possibly through disrupting the normal air flow through the case. However, this is unconfirmed and probably varies with the configuration.
