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21 mars 2011 1 21 /03 /mars /2011 06:31

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator (also a heat exchanger) in a car SONY VAIO VGN-FW21E Battery .

Heat sinks also help to cool electronic and optoelectronic devices, such as higher-power lasers and light emitting diodes (LEDs).

A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air Asus M52N Battery .

Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer CPU or graphics processors  Dell Latitude E6400 ATG Battery .

For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Thermal adhesive (also known as thermal grease) is added to the base of the heatsink to help its thermal performance  SONY VAIO VGN-FW21J Battery .

Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance.

Basic heat sink heat transfer principle

A heat sink is an object that transfers thermal energy from a higher temperature to a lower temperature fluid medium SONY VAIO VGN-FW21L Battery .

The fluid medium is frequently air, but can also be water or in the case of heat exchangers, refrigerants and oil. If the fluid medium is water, the 'heat sink' is frequently called a cold plate.

To understand the principle of a heat sink, consider Fourier's law of heat conduction  SONY VAIO VGN-FW21M Battery .

Joseph Fourier was a French mathematician who made important contributions to the analytical treatment of heat conduction.Fourier's law of heat conduction, simplified to a one-dimensional form in the x-direction, shows that when there is a temperature gradient in a body  SONY VAIO VGN-FW21Z Battery ,

heat will be transferred from the higher temperature region to the lower temperature region. The rate at which heat is transferred by conduction, qk, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred  SONY VAIO VGN-FW25T/B Battery .

Consider a heat sink in a duct, where air flows through the duct, as shown in Figure 2. It is assumed that the heat sink base is higher in temperature than the air. Applying the conservation of energy, for steady-state conditions, and Newton’s law of cooling to the temperature nodes shown in Figure 2 gives the following set of equations SONY VAIO VGN-FW26T/B Battery .

\dot{Q} = \dot{m}c_{p,in}(T_{air,out} - T_{air,in}) (1)
\dot{Q} = \frac{T_{hs} - T_{air,av}}{R_{hs}} (2)

where T_{air,av} = \frac{T_{air,out} + T_{air,in}}{2} (3) Dell Latitude E6400 battery,10400mAh,11.1V

Using the mean air temperature is an assumption that is valid for relatively short heat sinks. When compact heat exchangers are calculated, the logarithmic mean air temperature is used. \dot{m} is the air mass flow rate in kg/s Dell Latitude E6400 battery,10400mAh,11.1V .

The above equations show that

  • When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat sink base temperature  Dell Latitude E6500 battery,10400mAh,11.1V .
  • And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat sink base temperature.
    • The increase in heat sink thermal resistance with decrease in flow rate will be shown in later in this article SONY VAIO VGN-FW27/B Battery .
  • The inlet air temperature relates strongly with the heat sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat sink base temperature SONY VAIO VGN-FW27/W Battery .
  • Therefore, if there is no air or fluid flow around the heat sink, the energy dissipated to the air can not be transferred to the ambient air. Therefore, the heat sink functions poorly  SONY VAIO VGN-FW31ZJ Battery .
  • Furthermore, a heat sink is not a device with the "magical ability to absorb heat like a sponge and send it off to a parallel universe" .

Other examples of situations in which a heat sink has impaired efficiency SONY VAIO VGN-FW27T/H Battery :

  • Pin fins have a lot of surface area, but the pins are so close together that air has a hard time flowing through them.
  • Aligning a heat sink so that the fins are not in the direction of flow SONY VAIO VGN-FW29/B Battery .
  • Aligning the fins horizontally for a natural convection heat sink. Whilst a heat sink is stationary and there are no centrifugal forces and artificial gravity, air that is warmer than the ambient temperature always flows upward, given essentially-still-air surroundings; this is convective cooling SONY VAIO VGN-FW290JRB Battery .

Design factors which influence the thermal performance of a heat sink

Material

The most common heat sink material is aluminium. Chemically pure aluminium is not used in the manufacture of heat sinks, but rather aluminium alloys  SONY VAIO VGN-FW290JTB Battery .

Aluminium alloy 1050A has one of the higher thermal conductivity values at 229 W/m•K. However, it is not recommended for machining, since it is a relatively soft material. Aluminium alloys 6061 and 6063 are the more commonly used aluminium alloys, with thermal conductivity values of 166 and 201 W/m•K, respectively  SONY VAIO VGN-FW290JTH Battery .

The aforementioned values are dependent on the temper of the alloy.

Copper is also used since it has around twice the conductivity of aluminium, but is three times as heavy as aluminium . Copper is also around four to six times more expensive than aluminium, but this is market dependent   SONY VAIO VGN-FW33G/E1 Battery .

Copper and aluminium prices can be compared in figures 3 and 4, or on Internet websites, such as the London Metal Exchange . Aluminium has the added advantage that it is able to be extruded, while copper can not. Copper heat sinks are machined and skived. Another method of manufacture is to solder the fins into the heat sink base SONY VAIO VGN-FW33G/W Battery .

Another heat sink material that can be used is diamond. With a value of 2000 W/mK it exceeds that of copper by a factor of five . In contrast to metals, where heat is conducted by delocalized electrons, lattice vibrations are responsible for diamond's very high thermal conductivity SONY VAIO VGN-FW33GW Battery .

For thermal management applications, the outstanding thermal conductivity and diffusivity of diamond is an essential. Nowadays CVD diamond is used as submounts for high-power integrated circuits and laser diodes.

Composite materials can be used   (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

Examples are a copper-tungsten pseudoalloy, AlSiC (silicon carbide in aluminium matrix), Dymalloy (diamond in copper-silver alloy matrix), and E-Material (beryllium oxide in beryllium matrix). Such materials are often used as substrates for chips, as their thermal expansion coefficient can be matched to ceramics and semiconductors Dell Studio XPS 1640 Battery .

Fin efficiency

Fin efficiency is one of the parameters which makes a higher thermal conductivity material important. A fin of a heat sink may be considered to be a flat plate with heat flowing in one end and being dissipated into the surrounding fluid as it travels to the other Dell Vostro 1710 Battery .

As heat flows through the fin, the combination of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, therefore, the heat transfer to the fluid, will decrease from the base to the end of the finDell KM958 battery .

This factor is called the fin efficiency and is defined as the actual heat transferred by the fin, divided by the heat transfer were the fin to be isothermal (hypothetically the fin having infinite thermal conductivity). Equations 6 and 7 are applicable for straight fins Dell Studio 1555 battery .

\eta_f = \frac{tanh(mL_c)}{mL_c}
mL_c = \sqrt{\frac{2h_f}{k t_f}}L_f

Where:  Sony VGP-BPS13 battery

  • hf is the convection coefficient of the fin
  • k is the thermal conductivity of the fin material
    • Aluminum: 120 to 240 W/(m·K)
  • Lf is the fin height (m)
  • tf is the fin thickness (m)   Sony VGP-BPS13/S battery

To increase the fin efficiency of fins:

  • Decrease the fin aspect ratio, by:
  • Increase the thermal conductivity of the fins, (e.g. by using copper instead of aluminum)

Spreading resistance

Another parameter that concerns the thermal conductivity of the heat sink material is spreading resistance  Sony VGP-BPS13B/B battery .

Spreading resistance occurs when thermal energy is transferred from a small area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat sink base  Sony VGP-BPL9 battery .

The spreading resistance phenomenon is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform across the base of the heat sink  Sony VGP-BPS13B/B battery .

This nonuniformity increases the heat sink's effective thermal resistance.

To decrease the spreading resistance in the base of a heat sink  Sony VGP-BPL15 battery :

  • Increase the base thickness
  • Choose a different material with better thermal conductivity
  • Use a vapour chamber or heat pipe in the heat sink base   Dell Inspiron E1505 battery .

Fin arrangements

A pin fin heat sink is a heat sink that has pins that extend from its base. The pins can be cylindrical, elliptical or square. A pin is by far one of the more common heat sink types available on the market   Dell Latitude E6400 battery .

A second type of heat sink fin arrangement is the straight fin. These run the entire length of the heat sink. A variation on the straight fin heat sink is a cross cut heat sink. A straight fin heat sink is cut at regular intervals but at a coarser pitch than a pin fin type  HP Pavilion dv6000 Battery .

In general, the more surface area a heat sink has, the better it works . However, this is not always true. The concept of a pin fin heat sink is to try to pack as much surface area into a given volume as possible . As well, it works well in any orientation. Kordyan  has compared the performance of a pin fin and a straight fin heat sink of similar dimensions HP Pavilion dv8000 battery .

Although the pin fin has 194 cm2 surface area while the straight fin has 58 cm2, the temperature difference between the heat sink base and the ambient air for the pin fin is 50 °C. For the straight fin it was 44 °C or 6 °C better than the pin fin  HP PAVILION DV6000 Battery,  8800mAh, 10.8V .

Pin fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along the pins (see figure 17) rather than only tangentially across the pins   HP PAVILION DV8000 Battery,  8800mAh, 10.8V.

Another configuration is the flared fin heat sink; its fins are not parallel to each other, as shown in figure 5. Flaring the fins decreases flow resistance and makes more air go through the heat sink fin channel; otherwise, more air would bypass the fins. Slanting them keeps the overall dimensions the same, but offers longer fins. Forghan, et al  SONY VAIO VGN-FZ Battery .

have published data on tests conducted on pin fin, straight fin and flared fin heat sinks. They found that for low approach air velocity, typically around 1 m/s, the thermal performance is at least 20% better than straight fin heat sinks  HP Pavilion DV6000 battery - 8800mAh .

Lasance and Eggink also found that for the bypass configurations that they tested, the flared heat sink performed better than the other heat sinks tested.

Surface colour

The heat transfer from the heatsink is mediated by two effects: conduction via the coolant, and thermal radiation  HP Pavilion DV8000 battery - 7800mAh .

The surface of the heatsink influences its emissivity; shiny metal absorbs and radiates only a small amount of heat, while matte black radiates highly.

In coolant-mediated heat transfer, the contribution of radiation is generally small  HP Pavilion DV6000 battery - 8800mAh .

A layer of coating on the heatsink can then be counterproductive, as its thermal resistance can impair heat flow from the fins to the coolant. Finned heatsinks with convective or forced flow will not benefit significantly from being colored  HP Pavilion DV8000 battery - 6600mAh .

In situations with significant contribution of radiative cooling, e.g. in case of a flat non-finned panel acting as a heatsink with low airflow, the heatsink surface finish can play an important role. Matte-black surfaces will radiate much more efficiently than shiny bare metal  SONY VAIO VGN-FZ18 Battery .

The importance of radiative vs coolant-mediated heat transfer increases in situations with low ambient air pressure (e.g. high-altitude operations) or in vacuum (e.g. satellites in space).

Processor/Microprocessor cooling

Heat dissipation is an unavoidable by-product of all but micropower electronic devices and circuits SONY VAIO VGN-FZ21E Battery.

In general, the temperature of the device or component will depend on the thermal resistance from the component to the environment, and the heat dissipated by the component  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

To ensure that the component temperature does not overheat, a thermal engineer seeks to find an efficient heat transfer path from the device to the environment. The heat transfer path may be from the component to a printed circuit board (PCB), to a heat sink, to air flow provided by a fan, but in all instances, eventually to the environment SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Two additional design factors also influence the thermal/mechanical performance of the thermal design:

  1. The method by which the heat sink is mounted on a component or processor. This will be discussed under the section attachment methods Dell Latitude E6400 battery,10400mAh/7800mAh .
  2. For each interface between two objects in contact with each other, there will be a temperature drop across the interface. For such composite systems, the temperature drop across the interface may be appreciable Dell Latitude E6400 Battery,10400mAh/7800mAh .
  3. This temperature change may be attributed to what is known as the thermal contact resistance . Thermal interface materials(TIM) decrease the thermal contact resistance   Dell Latitude E6400 Battery,10400mAh/7800mAh .

Attachment methods for microprocessors and similar ICs

As power dissipation of components increases and component package size decreases, thermal engineers must innovate to ensure components won't overheat. Devices that run cooler last longer  Dell Latitude E6400 Battery,10400mAh/7800mAh .

A heat sink design must fulfill both its thermal as well as its mechanical requirements. Concerning the latter, the component must remain in thermal contact with its heat sink with reasonable shock and vibration. The heat sink could be the copper foil of a circuit board, or else a separate heat sink mounted onto the component or circuit board Dell Latitude E6500 battery,10400mAh/7800mAh .

Attachment methods include thermally conductive tape or epoxy, wire-form z clips, flat spring clips, standoff spacers, and push pins with ends that expand after installing  Dell Latitude E6500 battery,10400mAh/7800mAh .

  • Thermally conductive tape

Thermally conductive tape is one of the most cost-effective heat sink attachment materials . It is suitable for low-mass heat sinks and for components with low power dissipation   Dell Latitude E6500 battery,10400mAh/7800mAh .

It consists of a thermally conductive carrier material with a pressure-sensitive adhesive on each side.

This tape is applied to the base of the heat sink, which is then attached to the component. Following are factors that influence the performance of thermal tape   Dell Latitude E6500 battery,10400mAh/7800mAh :

  1. Surfaces of both the component and heat sink must be clean, with no residue such as a film of silicone grease.
  2. Preload pressure is essential to ensure good contact. Insufficient pressure results in areas of non-contact with trapped air, and results in higher-than-expected interface thermal resistance HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .
  3. Thicker tapes tend to provide better "wettability" with uneven component surfaces. "Wettability" is a term used to describe the percentage area of contact of a tape on a component. Thicker tapes, however, have a higher thermal resistance than thinner tapes  HP Pavilion DV7 battery - 7200mAh .
  4. From a design standpoint, it is best to strike a balance by selecting a tape thickness that provides maximum "wettablilty" with minimum thermal resistance.

Epoxy is more expensive than tape, but provides a greater mechanical bond between the heat sink and component, as well as improved thermal conductivity . The epoxy chosen must be formulated for this purpose  Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

Most epoxies are two-part liquid formulations that must be thoroughly mixed before being applied to the heat sink, and before the heat sink is placed on the component. The epoxy is then cured for a specified time, which can vary from 2 hours to 48 hours. Faster cure time can be achieved at higher temperatures Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

The surfaces to which the epoxy is applied must be clean and free of any residue.

The epoxy bond between the heat sink and component is semi-permanent/permanent. This makes re-work very difficult and at times impossible   SONY VAIO VGN-FW21E Battery .

The most typical damage caused by rework is the separation of the component die heat spreader from its package.

  • Wire form Z-clips

More expensive than tape and epoxy, wire form z-clips attach heat sinks mechanically   SONY VAIO VGN-NR11S/S Battery .

To use the z-clips, the printed circuit board must have anchors. Anchors can be either soldered onto the board, or pushed through. Either type requires holes to be designed into the board. The use of RoHS solder must be allowed for because such solder is mechanically weaker than traditional Pb/Sn solder  SONY VAIO VGN-NR11M/S Battery .

To assemble with a z-clip, attach one side of it to one of the anchors. Deflect the spring until the other side of the clip can be placed in the other anchor. The deflection develops a spring load on the component, which maintains very good contact  SONY VAIO VGN-NR11Z/S Battery .

In addition to the mechanical attachment that the z-clip provides, it also permits using higher-performance thermal interface materials, such as phase change types  SONY VAIO VGN-NR11Z/T Battery .

  • Clips

Available for processors and ball grid array (BGA) components, clips allow the attachment of a BGA heat sink directly to the component   Sony VAIO VGN-FZ21E Battery .

The clips make use of the gap created by the ball grid array (BGA) between the component underside and PCB top surface. The clips therefore require no holes in the PCB. They also allow for easy rework of components  Sony VAIO VGN-FW21E Battery .

Examples of commercially available clips are the maxiGRIPTM and superGRIPTM range from Advanced Thermal Solutions (ATS) and the Talon ClipTM from Malico. The three aforementioned clipping methods use plastic frames for the clips, but the ATS designs uses metal spring clips to provide the compression force  Sony VAIO VGN-NR11S/S Battery .

The Malico design uses the plastic "arm" to provide a mechanical load on the component. Depending on the product requirement, the clipping methods will have to meet shock and vibration standards, such as Telecordia GR-63-CORE, ETSI 300 019 and MIL-STD-810Sony VAIO VGN-NR11Z/S Battery .

  • Push pins with compression springs

For larger heat sinks and higher preloads, push pins with compression springs are very effective . The push pins, typically made of brass or plastic, have a flexible barb at the end that engages with a hole in the PCB; once installed, the barb retains the pin Sony VAIO VGN-NR11M/S Battery .

The compression spring holds the assembly together and maintains contact between the heat sink and component. Care is needed in selection of push pin size. Too great an insertion force can result in the die cracking and consequent component failure  Sony VAIO VGN-NR11Z/T Battery .

  • Threaded standoffs with compression springs

For very large heat sinks, there is no substitute for the threaded standoff and compression spring attachment method . A threaded standoff is essentially a hollow metal tube with internal threads  SONY VAIO VGN-FZ180E Battery .

One end is secured with a screw through a hole in the PCB. The other end accepts a screw which compresses the spring, completing the assembly. A typical heat sink assembly uses two to four standoffs, which tends to make this the most costly heat sink attachment design. Another disadvantage is the need for holes in the PCB  SONY VAIO VGN-FZ220E Battery .

Thermal interface materials

Thermal contact resistance occurs due to the voids created by surface roughness effects, defects and misalignment of the interface. The voids present in the interface are filled with air  SONY VAIO VGN-FZ340E Battery .

Heat transfer is therefore due to conduction across the actual contact area and to conduction (or natural convection) and radiation across the gaps . If the contact area is small, as it is for rough surfaces, the major contribution to the resistance is made by the gapsSONY VAIO VGN-FZ430E Battery .

To decrease the thermal contact resistance, the surface roughness can be decreased while the interface pressure is increased. However, these improving methods are not always practical or possible for electronic equipment. Thermal interface materials (TIM) are a common way to overcome these limitations  SONY VAIO VGN-FZ460E Battery .

Properly applied thermal interface materials displace the air that is present in the gaps between the two objects with a material that has a much-higher thermal conductivity. Air has a thermal conductivity of 0.022 W/m•K  while TIMs have conductivities of 0.3 W/m•K  and higher  SONY VAIO VGN-FZ480E Battery .

When selecting a TIM, care must be taken with the values supplied by the manufacturer. Most manufacturers give a value for the thermal conductivity of a material. However, the thermal conductivity does not take into account the interface resistances  SONY VAIO VGN-FZ4000 Battery .

Therefore, if a TIM has a high thermal conductivity, it does not necessarily mean that the interface resistance will be low.

Selection of a TIM is based on three parameters: the interface gap which the TIM must fill, the contact pressure, and the electrical resistivity of the TIM  SONY VAIO VGN-FZ31E Battery .

The contact pressure is the pressure applied to the interface between the two materials. The selection does not include the cost of the material. Electrical resistivity may, or may not, be important, depending upon electrical design details  SONY VAIO VGN-FZ31B Battery .

Light emitting diode lamps

Light emitting diode (LED) performance and lifetime are strong functions of their temperature . Effective cooling is therefore essential  SONY VAIO VGN-FZ31J Battery .

A case study of a LED based downlighter shows an example of the calculations done in order to calculate the required heat sink necessary for the effective cooling of lighting system. The article also shows that in order to get confidence in the results, multiple independent solutions are required that give similar results  SONY VAIO VGN-FZ31M Battery .

Specifically, results of the experimental, numerical and theoretical methods should all be within 10% of each other to give high confidence in the results.

A heat sink is rarely a desired thing in passive fire protection   SONY VAIO VGN-FZ31Z Battery .

Rather, it is usually a problem that must be overcome to maintain fire-resistance ratings. The proven ability to overcome heat sinks in construction is subject to building code and fire code regulations  SONY VAIO VGN-FZ38M Battery .

Firestopping

  • Problem – Metallic penetrants and sleeves, at a density of 7.9 kg/L are denser than common firestops or concrete SONY VGP-BPS8 Battery .
  • Consequently, during a fire, they will absorb more heat and conduct it to the unexposed side of a fire barrier (thus "cooling"the exposed side at the expense of the unexposed side), such as the cold side of a firewall. This is undesirable  SONY VGP-BPS13/S Battery .
  • Even if the fire is stopped by the barrier, one must keep the unexposed side cool to prevent autoignition of combustibles on the unexposed side of a fire barrier. The unexposed side may very well be an area of refuge, which must be safeguarded to comply with the building code  SONY VGP-BPS13A/B Battery .
  • Greater penetrant and sleeve conductivity leads to lower T-ratings. Higher density firestops, such as firestop mortars act as heat sinks to absorb heat away from small penetrants, such as cables, thus increasing T-ratings  SONY VGP-BPS13B/B Battery .
  • Benefit – a rare exception where heat sinks are beneficial in firestops is where intumescents must be activated, such as in a firestop containing a plasticpipe   SONY VGP-BPS13A/S Battery .
  • Heat sinks such as wire mesh and extra metallic sleeving may be used to carry heat to intumescents to activate expansion which should choke off a melting plastic pipe or melting pipe covering, such as foamed plastic or fibreglass SONY VGP-BPS13AS Battery .

Fireproofing

In fireproofing of structural steel as well as providing circuit integrity to cables, cable trays, junction boxes and electrical conduit, the metallic items that are protected by the fireproofing measures act as heat sinks  Dell Inspiron 1320n Battery .

Fireproofing methods are used to defeat the heat sink properties of the items they protect. In the case of circuit integrity measures, electrical services will fuse and short circuit above 140°C  Dell Inspiron 1464 Battery .

In soldering

Temporary heat sinks were sometimes used while soldering circuit boards, preventing excessive heat from damaging sensitive nearby electronics  Dell Inspiron 1564 Battery .

In the simplest case, this means partially gripping a component using a heavy metal crocodile clip, hemostat or similar clamp. Modern semiconductor devices, which are designed to be assembled by reflow soldering, can usually tolerate soldering temperatures without damage   Dell Inspiron 1764 Battery .

On the other hand, electrical components such as magnetic reed switches can malfunction if exposed to hotter soldering irons, so this practice is still very much in use   Dell Studio 1450 Battery .

Methods to determine heat sink thermal performance

In general, a heat sink performance is a function of material thermal conductivity, dimensions, fin type, heat transfer coefficient, air flow rate, duct size  Dell Studio 1457 Battery .

To determine the thermal performance of a heat sink, a theoretical model can be made. Alternatively, the thermal performance can be measured experimentally. Due to the complex nature of the highly 3D flow in present in applications, numerical methods or CFD can also be used  Dell Latitude D610 Battery .

This section will discuss the aforementioned methods for the determination of the heat sink thermal performance.

A heat transfer theoretical model

One of the methods to determine the performance of a heat sink is to use heat transfer and fluid dynamics theory Toshiba NB100 Battery .

One such method has been published by Jeggels, et al. , though this work is limited to ducted flow. Ducted flow is where the air is forced to flow through a channel which fits tightly over the heat sink. This makes sure that all the air goes through the channels formed by the fins of the heat sink  Toshiba Satellite M65 battery .

When the air flow is not ducted, a certain percentage of air flow will bypass the heat sink. Flow bypass was found to increase with increasing fin density and clearance, while remaining relatively insensitive to inlet duct velocity  Toshiba Satellite M60 battery .

The heat sink thermal resistance model consists of two resistances, namely the resistance in the heat sink base, Rb, and the resistance in the fins, Rf. The heat sink base thermal resistance, Rb, can be written as follows if the source is a uniformly applied the heat sink base   Dell Latitude D830 Battery .

If it is not, then the base resistance is primarily spreading resistance:

R_b = \frac{t_b}{kA_b} (4)

where tb is the heat sink base thickness, k is the heat sink material thermal conductivity and Ab is the area of the heat sink base Dell Latitude D620 Battery .

The thermal resistance from the base of the fins to the air, Rf, can be calculated by the following formulas.

The flow rate can be determined by the intersection of the heat sink system curve and the fan curve   Dell Inspiron Mini 10 Battery .

The heat sink system curve can be calculated by the flow resistance of the channels and inlet and outlet losses as done in standard fluid mechanics text books, such as Potter, et al.  and White .

Once the heat sink base and fin resistances are known, then the heat sink thermal resistance, Rhs can be calculated as Sony VGN-FW11S Battery :

Rhs = Rb + Rf (14)

Using the equations 5 to 13 and the dimensional data in , the thermal resistance for the fins was calculated for various air flow ratesSony VGN-FW11M Battery .

The data for the thermal resistance and heat transfer coefficient are shown in Figure 14. It shows that shows that for an increasing air flow rate, the thermal resistance of the heat sink decreases.

Experimental methods

Experimental tests are one of the more popular ways to determine the heat sink thermal performance Sony VGN-FW139E/H battery .

In order to determine the heat sink thermal resistance, the flow rate, input power, inlet air temperature and heat sink base temperature need to be known, as shown in figure 15. However, figure 15 shows a test setup for a ducted flow heat sink applicationDell Latitude E5400 Battery .

Vendor-supplied data is commonly provided for ducted test results . However, the results are optimistic and can give misleading data when heat sinks are used in an unducted application. Another problem with the set up, be it unducted or ducted, is the losses into the board  Dell Latitude E4200 Battery .

These must be taken into account. More details on heat sink testing methods and common oversights can be found in Azar, et al. .

Numerical methods

In industry, thermal analyses are often ignored in the design process or performed too late — when design changes are limited and become too costly  Dell Inspiron 300M Battery .

Of the three methods mentioned in this article, theoretical and numerical methods can be used to determine an estimate of the heat sink or component temperatures of products before a physical model has been made. A theoretical model is normally used as a first order estimate  Dell Vostro A840 Battery .

Numerical methods or computational fluid dynamics (CFD) provide a qualitative (and sometimes even quantitative) prediction of fluid flows.What this means is that it will give a visual or post-processed result of a simulation, like the images in figures 16 and 17, but the quantitative or absolute accuracy of the result is not guaranteed Dell Studio 1737 battery .

CFD can give an insight into flow patterns that are difficult, expensive or impossible to study using experimental methods . Experiments can give a quantitative description of flow phenomena using measurements for one quantity at a time, at a limited number of points and time instances  Dell Inspiron E1505 battery .

If a full scale model is not available or not practical, scale models or dummy models can be used. The experiments can have a limited range of problems and operating conditions. Simulations can give a prediction of flow phenomena using CFD software for all desired quantities  Dell RM791 battery ,

with high resolution in space and time and virtually any problem and realistic operating conditions. However, the results still need to be validated. Another problem with CFD is that the inputs need to be correct. It is the classic case of "Garbage in, garbage out  Dell XPS M1530 battery ."

A heat spreader is most often simply a copper plate, having high thermal conductivity. Functionally, it is a primary heat exchanger that moves heat between a heat source and a secondary heat exchanger. The secondary heat exchanger is always larger in cross sectional area, surface area and volume  Dell XPS M2010 battery .

By definition, the heat is "spread out", such that the secondary heat exchanger has a larger cross sectional area contacting the heat spreader than the heat source. The heat flow is the same in both heat exchangers, but the heat flux density is less in the secondary, so it can be made of a less expensive material such as aluminum  Toshiba Satellite P10 Battery ,

and is a better match to an air heat exchanger, since the low heat transfer coefficient for air convection is adequate for a low heat flux.

A heat spreader is generally used if and only if the heat source tends to have a high heat-flux density Acer Aspire One battery 10400mAh ,

(high heat flow per unit area), and for whatever reason, heat can not be conducted away effectively by the secondary heat exchanger. For instance, this may be because it is air-cooled, giving it a lower heat transfer coefficient than if it were liquid-cooled. A high enough heat exchanger transfer coefficient is often sufficient to avoid the need for a heat spreader SONY VGN-FZ210CE Battery .

A heat pipe is a heat transfer mechanism that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces  Dell Precision M70 Battery .

At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid in contact with a thermally conductive solid surface turns into avapor by absorbing heat from that surface. The vapor condenses back into a liquid at the cold interface, releasing the latent heat  Toshiba Satellite L305 Battery .

The liquid then returns to the hot interface through either capillary action or gravity action where it evaporates once more and repeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phase change depending on the demands of the working conditions of the thermally managed system  Toshiba Satellite T4900 Battery .

A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe  Toshiba PA3399U-2BRS battery ,

and then the pipe is filled with a fraction of a percent by volume of working fluid(or coolant) chosen to match the operating temperature. Examples of such fluids include water, ethanol, acetone, sodium, or mercury  Toshiba Satellite A200 Battery .

Due to the partial vacuum that is near or below the vapor pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase. The use of a vacuum eliminates the need for the working gas to diffuse through any other gas and so the bulk transfer of the vapor to the cold end of the heat pipe is at the speed of the moving molecules  Toshiba Satellite 1200 Battery .

In this sense, the only practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end.

Inside the pipe's walls, an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid  Toshiba Satellite M300 Battery .

This is typically a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end  SONY VGP-BPS13A/Q Battery .

The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.

A heat pipe is not a thermosiphon, because there is no siphon   SONY VGP-BPS13A/Q Battery .

Thermosiphons transfer heat by single-phase convection. (See also: Perkins tube, afterJacob Perkins.)

Heat pipes contain no mechanical moving parts and typically require no maintenance  SONY VGP-BPS13B/Q Battery ,

though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapour pressure is low  SONY VGP-BPS13B/Q Battery .

The materials chosen depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2–4 K) to mercury (523–923 K) & sodium (873–1473 K) and even indium (2000–3000 K) for extremely high temperatures  SONY VGP-BPS13/Q Battery .

The vast majority of heat pipes for low temperature applications use some combination of ammonia (213–373 K), alcohol (methanol (283–403 K) orethanol (273–403 K)) or water (303–473 K) as working fluid. Since the heat pipe contains a vacuum, the working fluid will boil and hence take up latent heat at well below its boiling point at atmospheric pressure SONY VGP-BPS13/Q Battery .

Water, for instance, will boil at just above 273 K (0 degrees Celsius) and so can start to effectively transfer latent heat at this low temperature.

The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat SONY VGP-BPS21A/B Battery .

They are a fundamentally better heat conductor than an equivalent cross-section of solid copper (a heat sink alone, though simpler in design and construction, does not take advantage of the principle of matter phase transition). Some heat pipes have demonstrated a heat flux of more than 230 MW/m², nearly four times the heat flux at the surface of the sun SONY VGP-BPS21/S Battery .

Active control of heat flux can be effected by adding a variable volume liquid reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section SONY VGP-BPS21B Battery .

Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the area available for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design SONY VGP-BPS21A Battery .

A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal diode. This heat pipe will transfer heat in one direction, acting as an insulator in the other  SONY VGP-BPS21 Battery .

Vapor Chamber or Flat heat pipes

Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These components are a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system Sony VGP-BPS21A/B Battery .

Compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate cross section for heat flow even with a very thin device. These thin planar heat pipes are finding their way into “height sensitive” applications, such as notebook computers, and surface mount circuit board cores  Sony VGP-BPS21/S Battery .

It is possible to produce flat heat pipes as thin as 0.5 mm (thinner than a credit card).

Heat transfer

Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant   Sony VGP-BPS21A Battery .

Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature (hence they tend to equalise the temperature within the pipe)  Sony VGP-BPS21 Battery .

When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe  Sony Vaio PCG-5G2L Battery .

The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe Sony Vaio PCG-5G3L Battery .

Non-condensing gases (caused by contamination for instance) in the vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low Sony Vaio PCG-5J1L Battery .

The velocity of molecules in a gas is approximately the speed of sound and in the absence of non condensing gases, this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end Sony Vaio PCG-5K2L Battery .

The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action  Sony Vaio PCG-5J2L Battery .

When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapour has purged the non condensing gases from the pipe and then seals the end  Sony Vaio PCG-5K1L Battery .

An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspected that a water charged heat pipe would only work when the hot end reached the boiling point (100 °C) and steam was transferred to the cold endSony Vaio PCG-6S2L Battery .

However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0 °C). The heat pipe will operate, therefore, when the hot end is just slightly warmer than the melting point of the working fluid. Similarly Sony Vaio PCG-6S3L Battery ,

a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid.

The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid  Sony Vaio PCG-6V1L Battery .

The heat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C Sony Vaio PCG-6W1L Battery

(hypothetically, if the water was under extremely high pressure so it didn't vaporize or freeze over this temperature range). Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts  

ref="http://www.hdd-shop.co.uk/sony-vaio-pcg-6w2l-battery.html">Sony Vaio PCG-6W2L Battery .

Origins and research in the United States

The general principle of heat pipes using gravity (commonly classified as two phase thermosiphons) dates back to the steam age Sony Vaio PCG-6W3L Battery .

The modern concept for a capillary driven heat pipe was first suggested by R.S. Gaugler of General Motors in 1942 who patented the idea. The benefits of employing capillary action were independently developed and first demonstrated by George Grover at Los Alamos National Laboratory in 1963 and subsequently published in the Journal of Applied Physics in 1964 Sony Vaio PCG-7111L Battery .

Grover noted in his notebook:

"Heat transfer via capillary movement of fluids. The "pumping" action of surface tension forces may be sufficient to move liquids from a cold temperature zone to a high temperature zone Sony Vaio PCG-7112L Battery

(with subsequent return in vapor form using as the driving force, the difference in vapor pressure at the two temperatures) to be of interest in transferring heat from the hot to the cold zone. Such a closed system, requiring no external pumps Sony Vaio PCG-7113L Battery ,

may be of particular interest in space reactors in moving heat from the reactor core to a radiating system. In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels Sony Vaio PCG-7133L Battery ."

Between 1964 and 1966, RCA was the first corporation to undertake research and development of heat pipes for commercial applications (though their work was mostly funded by the US government) Sony Vaio PCG-7Z2L Battery .

During the late 1960s NASA played a large role in heat pipe development by funding a significant amount of research on their applications and reliability in space flight following from Grover's suggestion. NASA’s attraction to heat pipe cooling systems was understandable given their low weight, high heat flux, and zero power draw Sony Vaio PCG-8Y1L Battery .

Their primary interest however was based on the fact that the system wouldn’t be adversely affected by operating in a zero gravity environment. The first application of heat pipes in the space program was in thermal equilibration of satellite transponders  Sony Vaio PCG-8Y2L Battery .

Assatellites orbit, one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed to the deep cold of outer space. This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transpondersSony Vaio PCG-8Z1L Battery .

The heat pipe cooling system designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. The developed cooling system was the first description and usage of variable conductance heat pipes to actively regulate heat flow or evaporator temperature  Sony Vaio PCG-8Z2L Battery .

Corporate R&D

Publications in 1967 and 1968 by Feldman, Eastman, & Katzoff first discussed applications of heat pipes to areas outside of government concern and that did not fall under the high temperature classification such as SONY VAIO VGN-FW31J Battery :

air conditioning, engine cooling, and electronics cooling. These papers also made the first mentions of flexible, arterial, and flat plate heat pipes. 1969 publications introduced the concepts of the rotational heat pipe with its applications to turbine blade cooling and the first discussions of heat pipe applications to cryogenic processes SONY VAIO VGN-FW31E Battery .

Starting in the 1980s Sony began incorporating heat pipes into the cooling schemes for some of its commercial electronic products in place of both forced convection and passive finned heat sinks. Initially they were used in tuners & amplifiers, soon spreading to other high heat flux electronics applications  SONY VAIO VGN-FW31B Battery .

During the late 1990s increasingly hot microcomputer CPUs spurred a threefold increase in the number of U.S. heat pipe patent applications. As heat pipes transferred from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U.S. to Asia SONY VAIO VGN-FW31M Battery .

Modern CPU heat pipes are typically made from copper and use water as the working fluid.

Applications

Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are found SONY VAIO VGN-FW31Z Battery .

Heat pipes have since been used extensively in spacecraft as a means for managing internal temperature conditions.

Heat pipes are extensively used in many modern computer systems, where increased power requirements and subsequent increases in heat emission have resulted in greater demands on cooling systems  SONY VAIO VGN-FW38M Battery .

Heat pipes are typically used to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment SONY VAIO VGN-AW41JF Battery .

Solar Thermal

Heat pipes are also being widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the sun SONY VAIO VGN-AW41JF/H Battery .

In solar thermal water heating applications, an evacuated tube collector can deliver up to 40% more efficiency compared to more traditional "flat plate" solar water heaters. Evacuated tube collectors eliminate the need for anti-freeze additives to be added as the vacuum helps prevent heat loss  SONY VAIO VGN-AW41MF Battery .

These types of solar thermal water heaters are frost protected down to more than -3 °C and are being used in Antarctica to heat water.

Pipelines over permafrost

Heat pipes are used to dissipate heat on the Trans-Alaska Pipeline System  SONY VAIO VGN-AW41MF/H Battery .

Without them residual ground heat remaining in the oil, as well as that produced by friction and turbulence in the moving oil would conduct down the pipe's support legs. This would likely melt the permafrost on which the supports are anchored   SONY VAIO VGN-FW30B Battery .

This would cause the pipeline to sink and possibly sustain damage. To prevent this each vertical support member has been mounted with 4 vertical heat pipes  SONY VAIO VGN-FW50B Battery .

Cooking

Heat pipes have been designed to speed the cooking of roasts. The pipe is poked through the roast. One end of the pipe extends into the oven where it draws heat to the middle of the roast  SONY VAIO VGN-FW51B/W Battery .

Ventilation heat recovery

In heating, ventilation and air-conditioning systems, HVAC, heat pipes are positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the heat energy SONY VAIO VGN-FW51MF Battery .

The device consists of a battery of multi-row finned heat pipe tubes located within both the supply and exhaust air streams. Within the exhaust air side of the heat pipe, the refrigerant evaporates, taking its heat from the extract air  SONY VAIO VGN-FW51MF/H Battery .

The refrigerant vapour moves towards the cooler end of the tube, within the supply air side of the device, where it condenses and gives up its heat. The condensed refrigerant returns by a combination of gravity and capillary action in the wick  SONY VAIO VGN-FW51ZF Battery .

Thus heat is transferred from the exhaust air stream through the tube wall to the refrigerant, and then from the refrigerant through the tube wall to the supply air stream  SONY VAIO VGN-FW51ZF/H Battery .

Because of the characteristics of the device, better efficiencies are obtained when the unit is positioned upright with the supply air side mounted over the exhaust air side, this allows the liquid refrigerant to flow quickly under gravity back to the evaporator  SONY VAIO VGN-FW52JB Battery .

Generally, gross heat transfer efficiencies of up to 75% are claimed by manufacturers.

Limitations

Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have an effect on the optimal temperatures in which heat pipes work  SONY VAIO VGN-FW54FB Battery .

When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensation process will cease to occur; in such conditions, the heat pipe's thermal conductivityis effectively reduced to the heat conduction properties of its solid metal casing alone  SONY VAIO VGN-FW70DB Battery .

As most heat pipes are constructed of copper (a metal with high heat conductivity), an overheated heatpipe will generally continue to conduct heat at around 1/80 of the original conductivity SONY VAIO VGN-FW71DB/W Battery .

In addition, below a certain temperature, the working fluid will not undergo phase change, and the thermal conductivity will be reduced to that of the solid metal casing. One of the key criteria for the selection of a working fluid is the desired operational temperature range of the application SONY VAIO VGN-FW72JGB Battery .

The lower temperature limit typically occurs a few degrees above the freezing point of the working fluid.

Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations (though 1.6mm thin sheets can be fabricated)  SONY VAIO VGN-FW73JGB Battery .

Experiments have been conducted with micro heat pipes, which use piping with sharp edges, such as triangular or rhombus-like tubing. In these cases, the sharp edges transfer the fluid through capillary action, and no wick is necessary SONY VAIO VGN-FW74FB Battery .

Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits. Various cooling methods help to improve processor performance or reduce the noise of cooling fans  SONY VAIO VGN-FW81HS Battery .

Components which produce heat and are susceptible to performance loss and damage include integrated circuits such as CPUs, chipset and graphics cards, along with hard drives (though excessive cooling of hard drives has been found to have negative effects)  SONY VAIO VGN-FW81NS Battery .

Overheated parts fail early and may give sporadic problems resulting in system freezes or crashes.

Both integral and peripheral means are used to keep the temperature of each component at a safe level. With regard to integral means, CPU and GPUsare designed with energy efficiency, including heat dissipation, in mind  SONY VAIO VGN-FW81S Battery ;

though improved efficiency may only allow increased performance instead of reduced heat. Peripheral means include heat sinks to increase the surface area which dissipates heat, fans to speed up the exchange of air heated by the computer parts for cooler ambient air, and in some cases softcooling, the throttling of computer parts in order to decrease heat generation SONY VAIO VGN-FW82DS Battery .

As a safety measure, many computers are designed to turn themselves off if the internal temperature exceeds a certain point. Alternatively, some have an option in their BIOS that allows the user to determine if the system emits an alarm beep or shuts itself down when the core temperature reaches the level set by the user SONY VAIO VGN-FW82JS Battery .

However, setting this incorrectly can result in hardware damage or erratic system behaviour.

The amount of heat generated by an integrated circuit (e.g., a CPU or GPU), the prime cause of heat build up in modern computersSONY VAIO VGN-FW82XS Battery ,

is a function of the efficiency of its design, the technology used in its construction and the frequency and voltage at which it operates.

In operation, the temperature of a computer's components will rise until the heat lost to the surroundings is equal to the heat produced by the component  SONY VAIO VGN-FW83DS Battery ,

and thus the temperature of the component reaches equilibrium. For reliable operation, the equilibrium temperature must be sufficiently low for the structure of the computer's circuits to survive.

Cooling can be hindered by:  SONY VAIO VGN-FW83JS Battery

  • 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 SONY VAIO VGN-FW83XS Battery ,
  • can reduce the amount of air flowing through a case and even create localized whirlpools of hot air in the case.
  • Poor heat transfer due to a lack of, or poor application of thermal compounds and sufficient surface area of heat sinks to radiate off the heat SONY VAIO VGN-FW90NS Battery .

Damage prevention

Thermal sensors in some CPUs and GPUs can shut down the computer when high temperatures are detected. However, reliance on such measures may not prevent repeated incidents from permanently damaging the integrated circuit SONY VAIO VGN-FW90S Battery .

An integrated circuit may also shut down parts of the circuit when it is idling, or to scale back the clock speed under low workloads or high temperatures, with the goal of reducing both power use and heat generation  SONY VAIO VGN-FW91NS Battery .

Air cooling

Fans are most commonly used for air cooling. A computer fan may be attached to the computer case, or attached to a CPU, GPU, chipset, PSU, hard drive or PCI slot. Common fan sizes include 40, 60, 80, 92, 120, and 140 mm  SONY VAIO VGN-FW91S Battery .

Recently, 200mm fans have begun to creep into the performance market, as well as even larger sizes such as 230 and 240mm.

In desktops

Desktop computers typically use one or more fans for cooling  SONY VAIO VGN-FW92DS Battery .

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 SONY VAIO VGN-FW92JS Battery .

If there is more air being forced into the system than is being pumped out (due to an imbalance in the number or strength of fans), this is referred to as a "positive" airflow, as the pressure inside the unit would be higher than outside. A balanced or neutral airflow is the most efficient SONY VAIO VGN-FW93DS Battery ,

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 clogged optical drives due to sucking in air (and dust).

In high density computing

Data centers typically contain many racks of flat 1U servers  SONY VAIO VGN-FW93JS Battery .

Air is drawn in at the front of the rack and exhausted at the rear. Because data centers typically contain such large numbers of computers and other power-consuming devices, they risk overheating of the various components if no additional measures are taken  SONY VAIO VGN-FW139E/H Battery .

Thus, extensive HVAC systems are used. Often a raised floor is used so the area under the floor may be used as a large plenum for cooled air and power cabling.

Another way of accommodating large numbers of systems in a small space are blade chassis  SONY VAIO VGN-FW145E/W Battery .

In contrast to the horizontal orientation of flat servers, blade chassis are often oriented vertically. This vertical orientation facilitates convection. When the air is heated by the hot components, it tends to flow to the top on its own, creating a natural air flow along the boards  SONY VAIO VGN-FW190EBH Battery .

This stack effect can help to achieve the desired air flow and cooling. Some manufacturers expressly take advantage of this effect.

In laptop computing

Most laptops use air cooling in order to keep the CPU and other components within their operating temperature range  SONY VAIO VGN-FW190EDH Battery .

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 port. This can cause overheating, and can be a cause of component failure in laptops. The severity of this problem varies with laptop design, its use and power dissipation   SONY VAIO VGN-FW21E Battery .

With recent reductions in CPU power dissipation, this problem can be anticipated to reduce in severity.

Liquid submersion cooling

An uncommon practice is to submerge the computer's components in a thermally conductive liquid  SONY VAIO VGN-FW21J Battery .

Personal computers that are cooled in this manner do not generally require any fans or pumps, and may be cooled exclusively by passive heat exchange between the computer's parts, the cooling fluid and the ambient air  SONY VAIO VGN-FW21L Battery .

Extreme component density supercomputers such as the Cray-2 and Cray T90used additional liquid to chilled liquid heat exchangers in order to facilitate heat removal SONY VAIO VGN-FW21M Battery .

The liquid used must have sufficiently low electrical conductivity in order for it not to interfere with the normal operation of the computer's components. If the liquid is somewhat electrically conductive, it may be necessary to insulate certain parts of components susceptible to electromagnetic interference, such as the CPU SONY VAIO VGN-FW21Z Battery .

For these reasons, it is preferred that the liquid be dielectric.

Liquids commonly used in this manner include various liquids invented and manufactured for this purpose by 3M, such as Fluorinert  SONY VAIO VGN-FW25T/B Battery .

Various oils, including but not limited to cooking, motor and silicone oils have all 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 enclosureSONY VAIO VGN-FW26T/B Battery .

Liquid may also slowly seep into and damage components, particularly capacitors, causing an initially functional computer to fail after hours or days immersed.

Waste heat reduction

Where full-power, full-featured modern computers are not required  SONY VAIO VGN-FW27/B Battery ,

some companies opt to use less powerful computers or computers with fewer features. For example: in an office setting, the ITdepartment may choose a thin client or a diskless workstation thus cutting out the heat-laden components such as hard drives and optical disks SONY VAIO VGN-FW27/W Battery .

These devices are also often powered with direct current from an external power supply brick which still wastes heat, but not inside the computer itself.

The components used can greatly affect the power consumption and hence waste heat  SONY VAIO VGN-FW27T/H Battery .

A VIA EPIA motherboard with CPU typically generates approximately 25 watts of heat whereas a Pentium 4 motherboard typically generates around 140 watts. While the former has considerably less computing power, both types are adequate and responsive for tasks such as word processing and spreadsheets  SONY VAIO VGN-FW29/B Battery .

Choosing a LCD monitor rather than a CRT can also reduce power consumption and excess room heat, as well as the added benefit of increasing available physical desk space SONY VAIO VGN-FW290JRB Battery .

Conductive and radiative cooling

Some laptop components, such as hard drives and optical drives, are commonly cooled by having them make contact with the computer's frame, increasing the surface area which can radiate and otherwise exchange heat SONY VAIO VGN-FW290JTB Battery .

Spot cooling

In addition to system cooling, various individual components usually have their own cooling systems in place. Components which are individually cooled include, but are not limited to, the CPU, GPUand the Northbridge chip SONY VAIO VGN-FW290JTH Battery .

Some cooling solutions employ one or more methods of cooling, and may also utilize logic and/or temperature sensors in order to vary the power used in active cooling components  SONY VAIO VGN-FW52JB Battery .

Passive heat-sink cooling

Passive heat-sink cooling involves attaching a block of machined or extruded metal to the part that needs cooling. A thermal adhesive may be used  SONY VAIO VGN-FW51B/W Battery .

More commonly for a personal-computer CPU, a clamp holds the heat sink directly over the chip, with a thermal grease or thermal pad spread between. This block usually has fins and ridges to increase its surface area. The heat conductivity of metal is much better than that of air SONY VAIO VGN-FW50B Battery ,

and it radiates heat better than does the component that it is protecting (usually an integrated circuit or CPU). Until recently, fan-cooled aluminium heat sinks were the norm for desktop computers. Today, many heat sinks feature copper base-plates or are entirely made of copper  SONY VAIO VGN-FW47GYH Battery .

Dust buildup between the metal fins of a heat sink gradually reduces efficiency, but can be countered with a gas duster by blowing away the dust along with any other unwanted excess material SONY VAIO VGN-FW465GJB Battery .

Passive heat sinks are commonly found on older CPUs, parts that do not get very hot (such as the chipset), and low-power computers.

Usually a heat-sink is attached to the integrated heat spreader (IHS), essentially a large SONY VAIO VGN-FW465GJ/BE1 Battery ,

flat plate attached to the CPU, with conduction paste layered between. This dissipates or spreads the heat locally. Unlike a heat sink, a spreader is meant to redistribute heat, not to remove it. In addition, the IHS protects the fragile CPU.

Passive cooling involves no fan noise SONY VAIO VGN-FW465J/B Battery .

Active heat-sink cooling

Active heat-sink cooling uses the same principle as passive, with the addition of a fan that blows over or through the heat sink. The air movement increases the rate at which the heat sink can exchange heat with the ambient air SONY VAIO VGN-FW45TJB Battery .

Active heat sinks are the primary method of cooling modern processors and graphics cards.

The buildup of dust is greatly increased with active heat-sink cooling, because the fan continually takes in the dust present in the surrounding air SONY VAIO VGN-FW45GJB Battery .

Peltier cooling or 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  SONY VAIO VGN-FWZJ/H Battery .

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". He found that applying a voltage to a thermocouple creates a temperature differential between two sides SONY VAIO VGN-FW41M/H Battery .

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 ofheat transfer SONY VAIO VGN-FW41J/H Battery .

A combination of bismuth and tellurium is most commonly used for thermocouples.

As active heat pumps, TECs can cool the surface of components below ambient temperatures. This is impossible with common radiator cooled water cooling systems and heatpipe HSFs SONY VAIO VGN-FW41E/H Battery .

Water cooling

While originally limited to mainframe computers, water 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 SONY VAIO VGN-FW37J Battery .

The past few years has seen water cooling increasing its popularity with pre-assembled, moderate to high performance, desktop computers. Water has 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 SONY VAIO VGN-FW378J/B Battery .

Advantages to water 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, water cooling is also influenced less by the ambient temperature SONY VAIO VGN-FW378DH Battery .

Water cooling's comparatively low noise-level compares favorably to that of active cooling, which can become quite noisy. One disadvantage to water cooling is the potential for a coolant leak. Leaked coolant can damage any electronic components it comes in contact with SONY VAIO VGN-FW378J/B Battery .

Another drawback to water cooling is the complexity of the system; an active heat sink is much simpler to build, install, and maintain than a water cooling solution SONY VAIO VGN-FW37J Battery .

Computing folklore holds that users of the Sinclair ZX81, one of the first home computers, had to balance a carton of milk on top of the case to cool it down – perhaps an early form of water cooling SONY VAIO VGN-FW37J/B Battery .

Heat pipe

A heat pipe is a hollow tube containing a heat transfer liquid. As the liquid evaporates, it carries heat to the cool end, where it condenses and then returns to the hot end (under capillary action, or, in earlier implementations, under gravitation) SONY VAIO VGN-FW41E/H Battery .

Heat pipes thus 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 SONY VAIO VGN-FW41J/H Battery ,

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 absolute quiet is needed (such as in computers used in audio production studios during live recording) SONY VAIO VGN-FW41M/H Battery .

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 SONY VAIO VGN-FWZJ/H Battery .

A new design wrinkle is known as HDT for Heatpipe Direct Touch. In this usage, the heat pipe is in direct contact with the CPU chip skin. Heatpipe Direct Touch was first introduced in the Zaward ZikaRay ZIKA-01 heatsink on February 2007, using a patent obtained from Golden Sun News Techniques Corporation in Taiwan SONY VAIO VGN-FWZJ/H Battery .

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 SONY VAIO VGN-FW45GJB Battery .

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 condenses it into a liquid. Then, the liquid is pumped up to the processor, where it passes through an expansion device, this can be from a simple capillary tube to a more elaborate thermal expansion valve SONY VAIO VGN-FW45TJB Battery .

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 CelsiusSONY VAIO VGN-FW465J/B Battery .

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 SONY VAIO VGN-FW465GJ/BE1 Battery .

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) SONY VAIO VGN-FW465GJB Battery .

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 SONY VAIO VGN-FW47GYH Battery .

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 SONY VAIO VGN-FW50B Battery .

These systems are in essence the next generation liquid 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 SONY VAIO VGN-FW51B/W Battery .

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 SONY VAIO VGN-FW52JB Battery ,

allowing for flexibility for noise reduction based on the parts chosen.

Liquid nitrogen

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 http://www.laptopbattery-pack.co.uk/ .

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 http://www.battery-pack.co.uk/ .

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 refilledhttp://www.battery-mart.net/ .

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 temperaturestress caused by changes in internal temperature http://www.hdd-shop.co.uk/ .

Although liquid nitrogen is non-flammable, it can condense oxygen directly from air. Mixtures of liquid oxygen and flammable materials can be dangerously explosive Ibm THINKPAD X200 Battery 8800mAh, 11.1V Replacement .

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