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HS Computer Hardware Outline
Week #8
Computer Cooling
Generators of Unwanted Heat
CPU, GPU, Chipset
Cooling Can be Hindered by
Dust, Poor Air Flow, Poor Heat Transfer
Damage Prevention – Thermal Shutdown
Mainframes & Supercomputers – Investigation began into Integrated Circuit Cooling Methods
Air Cooling
Desktop Computers
Servers
High Density Computing
Laptop Computers
Liquid submersion cooling - An uncommon practice is to submerge the computer's components in a
thermally, but not electrically, conductive liquid.
Waste heat reduction - Where powerful computers with many features are not required, less powerful
computers or ones with fewer features can be used. 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 thin-screen liquid crystal display (LCD) ones in the early
twenty-first century reduces power consumption significantly.
Heat Sinks – Passive Cooling
Peltier (thermoelectric) cooling - applying a voltage to a thermocouple creates a temperature
differential between two sides. This results in an effective, albeit extremely inefficient, heat pump.
Fluid Cooling
Heat Pipe
Phase-Change Cooling – AC Compressor
Liquid Nitrogen – Liquid Helium
Soft Cooling - utilizing software to take advantage of CPU power saving technologies
Undervolting - running the CPU or any other component with voltages below the device specifications
Optimization of cooling
Use of thermal conductive compounds
Heat sink lapping – surfaces must be as flat as possible
Use of rounded cables
Airflow optimization
Online documents –
Homework Online-
Computer Cooling
Specify a CPU Cooler if you wish or if your system will be overclocked
Computer cooling
From Wikipedia, the free encyclopedia
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 heat sinks 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.
A finned heat sink and fan clipped onto a
microprocessor, with a smaller heat sink
without fan in the background.
A 3-fan heat sink mounted on a graphics card to maximize cooling
efficiency of the GPU and surrounding components.
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, heat sink, 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 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.
Mainframes and supercomputers
As electronic computers became larger and more complex, cooling of the active components became a critical factor
for reliable operation. Early vacuum-tube computers, with relatively large cabinets, could rely on natural or forced air
circulation for cooling. However, solid state devices were packed much more densely and had lower allowable
operating temperature.
Starting in 1965, IBM and other manufacturers of mainframe computers sponsored intensive research into the physics
of cooling densely-packed integrated circuits. Many air and liquid cooling systems were devised and investigated,
using methods such as natural and forced convection, direct air impingement, direct liquid immersion and forced
convection, pool boiling, falling films, flow boiling, and liquid jet impingement. Mathematical analysis was used to
predict temperature rises of components for each possible cooling system geometry.
IBM developed three generations of Thermal Conduction Module (TCM) which used a water-cooled cold plate in
direct thermal contact with integrated circuit packages. Each package had a thermally conductive pin pressed onto it,
and helium gas surrounded chips and heat conducting pins. The design could remove up to 27 watts from a chip and
up to 2000 watts per module, while maintaining chip package temperatures around 50 C. Systems using TCMs were
the 3081 family (1980), ES/3090 (1984) and some models of the ES/9000 (1990). In the IBM 3081 processor, TCMs
allowed up to 2700 watts on a single printed circuit board while maintaining chip temperature at 69 C. Thermal
conduction modules using water cooling were also used in mainframe systems manufactured by other companies
including Mitsubishi and Fujitsu.
The Cray-1 supercomputer designed in 1976 had a distinctive cooling system. The machine was only 77 inches
(1950 mm) in height and 56 1/2 inches (1430 mm) in diameter, and consumed up to 115 kilowatts; this is comparable
to the average power consumption of a few dozen Western homes. The integrated circuits used in the machine were
the fastest available at the time, using emitter-coupled logic; however, the speed was accompanied by high power
consumption compared to later CMOS devices.
Heat removal was critical. Liquid Freon was circulated through piping embedded in vertical cooling bars in twelve
columnar sections of the machine. Each of the 1662 printed circuit modules of the machine had a copper core and was
clamped to the cooling bar. The system was designed to maintain the cases of integrated circuits at no more than 54
degrees Celsius; with Freon coolant circulating at 21 C. Final heat rejection was through a Freon to water heat
exchanger. Piping, heat exchangers, and pumps for the cooling system were arranged in an upholstered bench seat
around the outside of the base of the computer. About 20 per cent of the machine's weight in operation was coolant.
The later Cray-2 used direct immersion of the printed cicuit board modules in liquid Freon, which was not electrically
conductive. Freon was circulated over the boards at a velocity of about 25 mm/second.
Performance per watt of modern systems has greatly improved; many more computations can be carried out with a
given power consumption than was possible with the integrated circuits of the 1980s and 1990s. Recent
supercomputer projects such as Blue Gene rely on air cooling, which reduces cost, complexity, and size of systems
compared to liquid cooling.
Air cooling
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
Illustration of the airflow of the cooling air in a computer case during computer cooling
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.
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 (the opposite case is called "negative" airflow). There are some claims that
positive airflow results less dust in the case. To solve the dust problem, some models are fitted with dust filters but
these must be periodically cleaned up.
The air flow inside the typical desktop case is usually not strong enough for a passive CPU heatsink. Most of desktop
heat sinks are active, including one or even multiple directly attached fans.
Server cooling
A server with seven fans in the middle of the chassis, between drives on the right
and main motherboard on the left.
Server cooling fans in (1 U) enclosures are usually located in the middle of the
enclosure, between the hard drives at the front and passive CPU heat sinks at the
rear. Larger (higher) enclosures also have exhaust fans, and from approximately 4U
they may have active heat sinks. Power supplies generally have their own rearfacing exhaust fans.
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.
In laptop computing
A laptop computer's CPU and GPU heat
sinks, and copper pipes transferring heat
to an exhaust fan expelling hot air
The heat is expelled from a laptop by an
exhaust centrifugal fan
Laptops present a difficult mechanical airflow design, power dissipation, and cooling challenge. Constraints specific
to laptops include: the device as a whole has to be as light as possible; the form factor has to be built around the
standard keyboard layout; users are very close, so noise must be kept to a low minimum, and the case exterior
temperature must be kept low enough to be used on a lap. Cooling generally uses forced air cooling but heat pipes and
the use of the metal chassis or case as a passive heat sink are also common. Solutions to reduce heat include using
lower power-consumption ARM or Intel Atom processors.
Liquid submersion cooling
An uncommon practice is to submerge the computer's components in a thermally, but not electrically, conductive
liquid. Although rarely used for the cooling of computers, liquid submersion is a routine method of cooling large
power distribution components such as transformers. 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. For these reasons, it is preferred that the
liquid be dielectric.
A wide variety of liquids exist for this purpose, the most suitable being transformer oils and other specialty electrical
cooling oils such as 3M Fluorinert. Non-purpose 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.
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 thin-screen liquid crystal display (LCD)
ones in the early twenty-first century reduces power consumption significantly.
Heat sinks
Passive heat sink on a Chipset.
A component may be fitted in good thermal contact with a heat sink, a passive device
with large thermal capacity and with a large surface area relative to its volume. Heat sinks
are usually made of a metal with high thermal conductivity such as aluminum or copper,
and incorporate fins to increase surface area. Heat from a relatively small component is
transferred to the larger heat sink; the equilibrium temperature of the component plus heat
sink is much lower than the components alone would be. Heat is carried away from the
heat sink 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 heat sink.
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 fan-cooled aluminum heat sinks are often used
on standard desktop computers. Heat-sinks with copper base-plates, or made of copper, have better thermal
characteristics than aluminum; a copper heat-sink is more effective than an aluminum 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 heat sink clipped onto the top edge of the memory stick. The same
technique is used for video cards that use a finned passive heat sink over the GPU.
Active heat sink with a fan and heat pipes.
Dust tends to build up in the crevices of finned heat sinks, 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
In 1821 T. J. Seebeck discovered that different metals, connected at two different junctions, will develop a microvoltage 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.
As active heat pumps which consume power, TECs can produce temperatures below ambient, impossible with passive
heatsinks, radiator-cooled fluid cooling, and heat pipe HSFs.
Fluid cooling
DIY Water cooling setup showing 12v pump, CPU Water block and the typical
application of a T-Line
Water cooling is a highly effective method of removing excess heat. The
advantages of using water cooling over air cooling include water's higher
specific heat capacity, density, and thermal conductivity. The principle used in
cooling computers is identical to that used in an automobile's internal combustion engine, with the water being
circulated by a water pump through a block mounted on the CPU and out to a heat exchanger, typically a radiator.
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-it-yourself 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 heat sinks, making it suitable for overclocking and high performance
computer applications.
The technology of liquid cooling has evolved by integrating air cooling
technology to maximize the cooling efficiency while compensate the
shortcomings of liquid cooling.
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 of the disadvantages 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 is the complexity of the system; an active heat sink
is much simpler to build, install, and maintain than a water cooling solution. In short, installation of liquid coolers is
relatively complicated comparing with air cooling solutions, at the same time, they take up more space within the PC
case. To compensate the drawbacks, a combined air and liquid cooling solution is introduced to make the best out of
the two with easier setup and less space requirement.
The IBM Aquasar system uses hot water cooling to achieve energy efficiency, the water being used to heat buildings
as well.
Heat pipe A graphics card with a heat pipe 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 trial-runs 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 heat sink. 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.
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 lower 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 maximized.
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) 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. 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. 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).
Optimization 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 heat pumps, 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.
Heat-conductive pads are also used, often fitted by manufacturers to heat sinks. They are less effective than properly
applied thermal compound, but simpler to apply and, if fixed to the heat sink, 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.
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 spot-cool 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." 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.