Application of HEat Pipe

8/2/2019 Application of HEat Pipe

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364 SPECIAL APPLICATIONS FO R TOUGH COOLING JOBS

The birth of the heat pipe dates to about 1942. Very little attention was paid tothis device until about 1963, when it was suggested for spacecraft applications.Since then, many different heat pipe applications have found their way into cook-ing food, heating homes, cooling motorcycle engines, and recovering heat in in-dustrial exhaust systems.Heat pipes can be made in many different shapes and sizes. The most commonshape is the hollow cylinder or tube. They are often made in flat shapes, with Sturns, and in spirals.A typical heat pipe consists of a sealed tube that has been partially evacuated,so that its internal pressure is below the standard atmosphere of 14.7 psia. Theinside walls of the tube are usually lined with a capillary wick structure and a smallamount of a fluid, which will vaporize. When heat is applied at one end of thetube, the fluid within the pipe vaporizes o r boils. This generates a force that drivesthe vapor to the opposite end of the tube, where the heat is removed. Removingthe heat forces the vapor to condense, and the wick draws the fluid back to thestarting point, w here the process is repeated [66-671.There is a small pressure drop between the heating (or evaporating) end and thecooling (or condensing) end of the pipe. Therefore, the boiling and condensingcycle takes place over a very narrow temperature band. As a result, there is onlya small temperature difference between the heat source and the heat sink. A 24 in(60.9 cm ) long pipe, 0.50 in (1.27 cm) in diameter can pump 220 watts of heat at212 F (lOO"C), with about a 2 .4 "F (1 .3" C ) temperature difference along the lengthof the pipe. A solid copper bar for the same length and power, but with a 100F(55.5 "C) temperature difference along its length, would require a cross-sectionalarea of about 10.8 in2 (6 9.7 cm 2), and would weigh about 75 lb (34,050 g ), com-pared to the heat pipe weight of about 0.75 lb (340 g).The wick is probably the most critical part of the heat pipe design. It determinesthe capillary sucking action available for drawing the condensed liquid back to theevaporator end. The porosity and the continuity of the internal passages determinethe fluid resistance along the wick. The wick must have sufficient capacity to sup-ply liquid to the heat input end. An inadequate fluid return will result in the dryingup of the wick at the heat input end, which results in a breakdown of the wickoperation [6 8, 691.Capillary action of the wick in the heat pipe permits it to operate in any ori-entation in a gravity or acceleration field. A typical wick on earth, which has anacceleration of 1 OG, must be capable of raising the fluid against gravity and witha small internal resistance to flow at the sam e time. A small pore size is requiredto draw the liquid up to a high level in a capillary tube. However, a small poresize increases the internal flow resistance. A balance must therefore be made toprovide good fluid flow over long d istances in the heat pipe wick design, dependingupon the orientation in a gravity or acceleration field. A typical section throughthe length of a heat pipe is shown in Figure 10.1.Heat pipe wicks are often made of porous ceramic or w oven stainless steel wiremesh. Sometimes the wick is an integral part of the tube housing, formed by ex-truding small grooves along the inner surface of the wall structure, as shown inFigure 10.2.


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10.3 DEGRADED PERFORMANCE IN HEAT PIPES 365

end - sectionI -

- -Liauid return

- end --. . . . .

Wick

in transport outFigure 70.1 Section through the length of a typical heat pipe.

Extrudedgrooves

Sealedend

Figure 10.2 Heat pipe wick designs.

10.3 DEGRADED PERFORMANCE IN HEAT PIPESHeat pipes appear to have the ability to solve many thermal problems involvinghigh heat densities o r the transport of heat over long distances. How ever, it shouldbe pointed out that many of the old heat pipe manufacturers have gone out of thebusiness, and only a handful1 are left. This po ints out how difficult it is to makegood heat pipes, with good quality control, which will give good performance fora long period of time, and which can be sold at a good price.One of the biggest problems with heat pipes is that many of them often exhibitdegraded performance after they have been in operation for about three to sixmonths. This degraded performance occurs slowly, so it may not be noticed in theoperating hardware. The operating temperatures of the equipment will increaseover a period of time, until malfunctions and failures occur. Since thermocouplesare not usually mounted in operating hardware, the gradual temperature increaseover a long time span usually goes unnoticed.The single greatest cause of this degraded performance appears to be contami-nation that affects the vapor pressure within the heat pipe. Contamination oftenresults from the type of operation that is used to seal the ends of the tube after itis assembled. Many sealing methods can be used effectively if the processes areproperly controlled, the parts are adequately cleaned, and the assembly takes placein a clean room. Electron beam welding is reported to be one of the best knownmethods for sealing heat pipes today. This technique appears to provide a good


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366 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSseal with very little contamination. Many other sealing methods are still used,because they are less expensive. If the processes are properly controlled, a goodseal can be obtained.Other sources of outgassing are often found in the wick. Since the wick mustutilize many small orifices to pum p the fluid from the condenser to the evaporator,this section can easily contain trapped gasses that may affect the vapor pressure.Wicks must therefore be manufactured and cleaned very carefully before they areinstalled in the heat pipe.A clean room should be used to assemble heat pipes, to minimize possiblecontamination. The maximum airborne particle contamination should be about100,000particles per cubic foot, when the particle size is around 0 .5 pm. Whenthe particle size is around 5.0 pm, the maximum airborne particle contaminationshould be limited to about 700 particles per cubic foot.Burn-in tests should be performed on all heat pipes, for at least 100 hr. Thebum-in should be performed with 100% of the required design load but in anambient temperature at least 20%higher than the highest specified operating tem-perature, to ensure reliable operation.The cleaning process is extremely important. One heat pipe manufacturer re-ported an attempt to reduce m anufacturing costs by using the extruded grooves inplace of the porous material for the wick, as shown in Figure 10.2. The fluid thatwas used in the extrusion process was very difficult to remove com pletely, and theresulting Contamination affected the internal vapor pressure. This reduced the per-formance of the heat pipe after several months of operation. Since the particularprogram required very precise temperature controls for very long time periods, itwas decided that the lower cost extruded grooves should not be used for this ap-plication.

10.4 TYPICAL HEAT PIPE PERFORMANCEA heat pipe can work in any orientation, but its performance may be degradedwhen it is forced to work against gravity. This condition occurs when the heatinput end (evaporator end) is higher than the heat output end (condenser end). Inthis position, the fluid in the wick is forced to move up the pipe, against the di-rection of gravity. Figure 10.3 shows approximately how the performance of a 48in (122 cm) long water heat pipe with a coarse, medium, and fine wick changeswith the angle of orientation (0).The coarse wick is capable of handling much more power while it is operatingin a horizontal position. However, when the condenser end is angled down slightly,its pumping capacity is sharply reduced. The fine wick cannot pump a s much heatin the horizontal position. How ever, its pumping capacity does not drop off as fastwhen the condenser end is angled down. The typical performance that can beexpected from various heat pipes is shown in Table 10.1.

A wide variety of fluids can be used with heat pipes. A few of these fluids andtheir typical operating ranges are given in Table 10.2.


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10.4 TYPICAL HEAT PIPE PERFORMANCE 367EvaDorator en d

B 4""0 ''I

Condenser end

01 I I I I I I I I I0 10 20 30 40 50 60 70 80 90Angle , 8

Figure 10.3 Changes of the heat pipe capability when the condensor end is below the evap-orator end.TABLE 10.1 Typical Heat PipePerformance

OutsideDiameter Length Power(in) (in) (watts)a 6 300

12 17518 1508 6 50012 3 7518 350i 6 70012 5 7518 550

TABLE 10.2 Operating Temperatures forSeveral Heat Pipe FluidsOperating TemperatureFluid Range (OF)

AmmoniaSulfur dioxideWaterFC 43MercuryCesiumSodiumLithium

-50- 12515-11040-450

250-430400-820750- 1830920-22001500-3000


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368 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBS10.5 HEAT PIPE APPLICATIONSHeat pipes are generally used to transport heat from the source directly to the sinkwithout any external power. Heat pipes can eliminate hot spots and can accept heatthat has a high power density. Most heat pipes are external to the device they arecooling. Some new applications are available where the heat pipe is an integralpart of the case on an electronic component.A typical application may involve a component or a subassembly, such as aPCB, which has a high power dissipation and a relatively long heat flow path tothe heat sink. T he resulting temperature rise from the electronic components to theheat sink is excessive, so forced-air cooling and then liquid cooling are examined.When there is no room for fans and ducts, and when the size, weight, and priceof a liquid system is evaluated, the price of one or two sm all heat pipes may lookmore attractive.Many compact airborne electronic systems utilize air-cooled cold plates for thesidewalls of the chassis. Plug-in PCBs are then used to support the electroniccomponents. T hese are cooled by conducting the heat, along metal strips laminatedto the PCB, to the sidewalls of the chassis, as shown in Figure 10.4.ATR size chassis, the cross-sectional dimensions willbe approximately as shown in Figure 10.4. A plug-in PCB with an aluminum coremight be capable of dissipating about 15 watts, based upon a maxim um componentsurface temperature of 212F (l0OOC) and a cooling air temperature of 131F(55"C) flowing through the sidewall cold plates.What happens when the same size PCB has a power dissipation of 70 watts? A

Considering a typical

Ln

Wire harness Plug in connector '\ Bo tt om coverFigure 10.4 Cross section through a chassis with cold plate sidewalls.


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10.5 HEAT PIPE APPLICATIONS 369metal heat sink core on the PCB would have to be so large and heavy that it issimply unacceptable. A hollow core air-cooled PCB, similar to those shown inFigure 6.21,might be used w ith a m ultiple-fin heat exchanger at the center. How -ever, even this type of construction is only capable of dissipating about 50 to 55watts in the same environment. A liquid cooling system could be used, and itwould do an excellent job. However, there would be a large increase in the sizeand weight of the system.Heat pipes are ideal for the conditions described above. They can be added tothe back surface of the PCB to sharply increase the heat transferred from the centerof the PCB to the edges. A high temperature rise may still occur at the interfaceof the PCB with the chassis cold plate, unless a high interface pressure device suchas a wedge clamp, shown in Figure 3.23d, is used. A luminum or copper heat sinksshould be used under the heat pipes on the PCB to improve the heat flow to theheat pipes, as shown in Figure 10.5.Heat pipes have been made with 90" bends to improve the heat transfer fromcircuit boards that do not plug in . When cold plate sidewalls are used in a chassis,the 90" bend permits the heat pipe to carry the heat directly from the PCB to thesidewall of the chassis, as shown in Figure 10.6. This sharply reduces the tem-perature rise across the interface from the PCB to the chassis.Sometimes cooling fins must be extended to improve the cooling by increasingthe effective surface area. The heat transfer efficiency of a long fin can be improvedsubstantially by using heat pipes along the length of the fin, as shown in Figure

Component mountingsurface

Flat heat pipes in> lurninusTn[CB heat' Plug in connector

Figure 10.5 Flat heat pipes in an aluminum composite plug-in PCB.


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370 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBS

Components onprinted circuits

r" I0000 I I 1 . QAluminum heat sink andheat pipe with 90" bend

Chassis coldplate side wall

Figure 70.6 Heat pipe with a 90 bend for cooling circuit boards.

10.7.The temperature gradient along the fin will be quite small with the heat pipe,producing an isothermal fin. The temperature difference between the fin and theambient air will be increased, so that the heat transfer from the fin is also increased.Cooling fins can be added to the condenser end of the heat pipe to improvenatural and forced convection cooling. The heat pipe can be bent, if necessary, toreach hot components located in remote areas of a chassis or console. Figure 10.8shows a tall console with finned heat pipes that extend through the rear panel toprovide the required cooling.Experimental models have been made with small heat pipes that are fabricatedinto the structure of the electronic component itself. Power transistors have beenbuilt with a porous dielectric wick that is placed in contact with the active chip

Figure 70.7 Using heat pipes in long cooling fins to improve cooling efficiency.


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10.6 DIRECT AND INDIRECT LIQUID COOLING 371High power e lec t ron ic component

Finned heat pipes

Tal lelectronicconsole

Rear panel

Figure 10.8 Section through a tall console showing heat pipes extending through the rearpanel to improve cooling.

substrate and the inside surfaces of a standard TO type of container. The unit isevacuated, the wick is saturated with a suitable fluid, and the assembly is hermet-ically sealed. The evaporation process of the heat pipe takes place on the transistorsubstrate, and the condensation takes place on the cooler surfaces of the transistorwalls. Capillary action in the wick brings the fluid back to the substrate, w here theevaporation process is repeated.

10.6 DIRECT AND INDIRECT LIQUID COOLINGLiquid cooling , in general, is much m ore effective for removing heat than air cool-ing. T herefore, when high power densities are involved, liquid cooling may be theonly practical method for maintaining reasonable component temperatures. Liquidcooling systems are basically classified as direct or indirect. In a direct coolingsystem, the liquid is in direct contact with the electronic components, which per-mits the coolant to pick up the heat and carry it away. In an indirect system, theliquid coolant does not come into direct contact with the components. Instead, heatis transferred from the hot com ponent to som e intermediate system and then to theliquid. The intermediate system conveys the heat to the liquid by conduction, con-vection, or radiation. Typical examples of intermediate systems are heat exchang-ers and fans [6] .Direct liquid cooling systems usually have the electronic components com-pletely immersed in a fluid that has no effect upon the electrical operation of thesystem. Heat is transferred directly to the fluid by conduction and convection, withvery little radiation. Sometimes a low pressure pump is added to circulate the fluid


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372 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSthrough the electronics to increase the effective cooling. Sometimes the pump isused to spray the liquid coolant directly on the electronic components to carry awaythe heat.Indirect liquid cooling systems usually shift the heat transfer problem from theelectronic components to another, more remote location. Since the heat must bedumped somewhere, it is often convenient to pump the coolant fluid to a remoteair-to-liquid heat exchanger. Large fans can then be used to drive cooling airthrough the heat exchanger, which will cool the fluid and heat the air. The coolerfluid is pumped back to the electronics to pick up more heat, and the hot air isexhausted to the surrounding ambient.indirect cooling systems are often used in missiles and satellites that must op-erate in the hard vacuum of outer space. The cooling liquid is usually circulatedthrough cold plates, to pick up heat from components m ounted on the cold plates.The liquid is then pumped to remote space radiators that are located on the surfaceof the spacecraft, facing away from the sun toward deep space, where the sinktemperature is absolute zero (-460"R or -273K). Heat from the liquid is pickedup by the space radiator and dumped into space. Th e cooler liquid is pumped backto the electronics to pick up more heat.The U.S.Navy makes extensive use of indirect cooling to remove heat fromelectronic systems. Electronic components are often mounted on cold plates thatuse fresh distilled deionized water as the coolant. Forced convection cooling withfresh water is much more effective than is forced convection cooling with air.Typical heat transfer coefficients for fresh water can easily be a s much as 100 timesgreater than typical heat transfer coefficients for air. In addition, water has a spe-cific heat that is more than four times greater than air. W ater can therefore absorbfour times as much heat as air for the same temperature rise and weight flow.The fresh water is cooled by circulating it through a remote heat exchanger,which has fresh water on one side and saltwater (seawater) on the opposite side.After the fresh water has been cooled, it is returned to the electronic cold plate.Saltwater is used to cool the fresh water. The salt water, which has picked up theheat from the fresh water, is dum ped back into the sea. Special care must be usedto prevent the fresh water from freezing during the winter, or when operating incold climates.

10.7 FORCED-LIQUID COOLING SYSTEMSHigh-pow er-dissipating electronic systems often m ake use of forced-liquid coolingtechniques to control hot spot temperatures. One very common type of coolingdevice is the cold plate, which provides cooling by conduction and by forced-liquidconvection. Electronic components are mounted on a metal plate, through whicha cooling liquid is circulated, to carry away the heat. The electronic componentsare fastened directly to the cold plate, which is made of aluminum or copper forhigh heat conduction. This provides a good heat conduction path from the com-ponents to the cooling liquid in the cold plate [ 6 ,70, 711.


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10.7 FO RCED-LIQUID COOLING SYSTEMS 373

The methods for analyzing the thermal characteristics of a liquid cooling systemare very similar to the methods shown in Chapter6 for analyzing forced-air coolingfor electronics. Many of the equations are the same, with only the values for suchparameters as density, viscosity, thermal conductivity, and specific heat changing.A pump replaces the fan in a liquid-cooled system. To select a suitable pump,it is necessary to be able to calculate the total pressure loss through any givensystem. Three major pressure drops are usually evaluated: (1 ) friction, which isdetermined by the liquid velocity and surface roughness within the pipe; (2) dif-ference in elevation; and (3) fitting losses due to elbows, tees, and transitions.These fittings are usually the major source of the pressure drop through an elec-tronic liquid cooling system.

As in the air-cooled systems, an exact calculation of the pressure drop is verydifficult to obtain. Therefore, it is convenient to use approximate methods that arewell documented. One of the most common methods makes use of an equivalentpipe length for various fittings such as elbows and tees. Since the pressure dropthrough straight pipes with various degrees of roughness is well documented, thecalculations are simplified. To obtain a common basis for calculating the equiva-lent pipe length for various fittings, the effective length is expressed as the numberof diameters of a round pipe. In this way, the equivalent length of many fittingscan be determined. Table 10.3shows the effective length of pipe for several fittingsexpressed in terms of their equivalent diameters. The values are for turbulent flowconditions, but they can also be used to approximate the pressure drop for laminarflow systems.Pressure losses will also occur at the pipe inlet, depending upon the geometry

TABLE 10.3 Effective Length for Various Pipe Fittings (3, 6,241

Type of FittingEffective Length

(Number of Diameters)45" elbow90"elbow miter, zero radius90" elbow, 1 O diameter radius90" elbow, 1.5 diameter radius90"elbow, 2.0 diameter radius180" bend, 1.5 diameter radius180" bend, 4-8 diameter radiusGlobe valve, fully openGate valve, fully openGate valve, f closedGate valve, 4 closedGate valve, 3 closedCoupling union

15603226205010

300740

200800

0


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374 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSTABLE 10.4 Effective Len gth

Type of Opening

am?y\kuz7 r\&Lj.7J7

Point-TPointcB Branch

for Various Pipe Entrances- ___ -Effective Length(Number of Diameters)escription

Square edge entry 20

Well-rounded entry 2

40rotruding entry(Borda's mouthpiece)

Teeelbow entering 60run a t point A

Teebranch at point Belbow entering 90

of the inlet. Some typical losses are shown in Table 10.4. The entry losses areshown as an effective pipe length, which is expressed in terms of the equivalentdiameter of a round pipe [3 , 6, 241.

10.8 PUMPS FOR LIQUID-COOLED SYSTEMSThe pum p must be capable of circulating the proper amoun t of coolant through thesystem against the total pressure drop through all the fittings, the heat exchanger,and the cold plate. The electric motor driving the pump will usually be matchedto the pump so that the motor cannot be overloaded.Many different types of pumps are available, such as gear, reciprocating, andcentrifugal. Gear type pumps do not become airbound very easily, as do centrif-ugal pumps. However, some centrifugal pumps have been developed to operate


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10.11 SIMPLE LlOUlD COOLING SYSTEM 375without rotating shaft seals, so that a long-term leakproof system can be obtained

A number of different safety devices are available to turn off the pump if theflow of the liquid is blocked or if the fluid leaks out. Protective devices are alsoavailable for preventing damage due to excessive pressure in a liquid cooling sys-tem.

[721.

10.9 STORAGE AND EXPANS ION TANKTotally enclosed cooling systems are often used to cool electronic equipment.Therefore, some provisions must be made to accommodate the thermal expansionof the fluid as the temperature increases. Also, air must be removed from thecoolant and some type of cushion should be provided to reduce pressure surges inthe system. This is all accomplished with an expansion tank, which also acts asthe storage tank for the liquid coolant.

10.10 LIQUID COOLANTSWater is probably the best cooling liquid available in terms of density, viscosity,thermal conductivity, and specific heat. For good long-term operation, distilledwater that has been deionized should be used. When the temperature is expectedto drop below freezing or when the surface temperature is expected to exceed212F (lOO"C), ethylene glycol should be added to the water. Ethylene glycol hasa lower thermal conductivity than water, so its addition will degrade the thermalperformance of the water. However, when both aluminum and copper are beingused in the cooling circuit, the addition of ethylene glycol will prevent corrosionof the passages.Many other liquids are available for cooling electronic systems, such as sili-cones, but they have a much lower thermal conductivity than water. Coolanol 45,a silicone ester made by Monsanto Chem ical Company, is a very effective coolantthat has a useful cooling range from -5 8 F (-5 0 C ) to 39 2 F (200 C). Its vis-cosity is substantially higher than that of water. This may increase the pressuredrop, which may require the use of a larger pump motor in a system where thewater is replaced by Coolanol [72].A liquid that has approximately the same viscosity as water is FC 75, a fluo-rochemical made by the 3M Company. This liquid has a specific heat about onefourth that of water. Tables 10.5 and 10.5A show the thermal properties of severaldifferent coolant fluids.10.1 1 SIMPLE LIQUID COOLING SYSTEMA simple liquid cooling system can be designed by combining the various itemsdescribed in the previous sections. A simplified schematic of an elementary liquidcooling system is shown in Figure 10.9.


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Q TABLE 10.5 Properties of Various Cooling Fluids: English UnitsFC 75 Coolanol 25 Coolanol 452 Water

77F 140F 77F 140F 77F 140F 77F 140F(25C) (60C) (25C) (60C) (25C) (60C) (25C) (60C)

Visco sity (Ibift hr ) 2.17 1.14 3.49 2.05 10.89 5.93 43.32 16.84Density (lb/ft3) 62.21 61.40 109.8 103.6 56.19 54.63 55.87 54.31The rma l con ducti vity 0.353 0.378 0.037 0.035 0.076 0.074 0.078 0.075(Btuihr ft "F)Specific heat (Btuilb OF) 0.998 0.998 0.248 0.263 0.45 0.48 0.45 0.49Boiling 212F (100C) 214F (101C ) 355F (179C) 355F (179C)Freezing 32F (0C) -171F (-113C) -125F (-87"C)* -88F (-6 TC )*Pour Point.

TABLE 10.5A Properties of Various Cooling Fluids: Metric Units

Viscosity (poise) (g/cm sec)Density (g/cm3)Thermal conductivity(cal/sec cm "C)Specific heat (cal/g "C)BoilingFreezing

Water FC 7525C 60C 25C 60C

(77F) (140F) (77F) (140F)0.00896 0.00470 0,01441 0.008470.995 0.982 1.757 1.6580.00146 0.00156 0.000153 0.0001440.998 0.998 0.248 0.263

100C (212F ) 101C (214F )0C (32F) - 13C ( - 171F)

Coolanol 25 Coolanol 4525C 60C 25C 60"C( 7 P F ) (140F) (77F) (140F)

0.045 0.0245 0.179 0.06960.900 0.875 0.895 0.8700.000314 0.000306 0.000322 0.0003110.45 0.48 0.45 0.49

179C (355F ) 179C (355F )-87C (-125"F)* -67C (-88"F)*

'Pour Point


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10.72 MOUNTING COMPONENTS FO R INDIRECT LIQUID COOLING 377

- Expansion and -storage tank

10.12 MOUNTING COMPONENTS FOR INDIRECT LIQUIDCOOLING

I \ \I

- ElectronicH h equipment-

Electronic components must always be mounted so that there is a low thermalresistance heat flow path to the heat sink. This is true for any electronic systemthat must provide reliable operation for steady state conditions. This may not benecessary for transient conditions, where the power is not on long enough for thetemperature to stabilize.Components mounted on liquid-cooled cold plates should be provided with flatsmooth surfaces, to improve the transfer of heat across the mounting interface.Power devices such as transistors, resistors, diodes, and transformers can be fas-tened to the cold plate w ith bolts that are capable of applying h igh forces. Boltedcomponents can be removed easily if they have to be replaced.Silicone grease can be applied at the interface of the component to the coldplate to further reduce the thermal resistance on high-power-dissipating devices.Silicone grease should not be used anywhere on a cold plate if there are ce-mented joints in the area, Silicone grease tends to migrate, so that it will contam-inate the cemented surfaces and reduce the bond strength. In a vibration or shockenvironment, the cemented interface can fracture at a very low stress level, pro-ducing a catastrophic failure.For very-high-power-dissipating components, it is often necessary to reduce theinterface temperature rise to an absolute minimum . Under these circumstances, thecomponent may be soldered directly to the cold plate mounting surface. This isdone by first pretinning both surfaces with a low temperature so lder. T he solder isthen reflowed using a high temperature for a short period of time to prevent heatfrom soaking into the component and damaging it. A heat sink may be used onthe body of the component to pull away some of the heat during the reflow process,to reduce the possibility of damage.When a large number of similar high-power components must be cooled, it isconvenient to mount them on a liquid-cooled cold plate, as shown in Figure 10.10,


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378 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSCold plate High power component

Additional methods for mounting high-power components are discussed in Sec-tions 4.4 through 4 .8 .Plug-in PCBs are often cooled with the use of liquid-cooled cold plates, whichare part of the side walls on an electronic chassis. Aluminum or copper heat sinksare used on the PCB to conduct the heat from the components to the sidewall coldplates, as shown in Figure 10.11. This technique is very similar to the air-cooledcold plates described in Sections 6 . 2 3 through 6.26, except that the cooling fluidis a liquid instead of air.

10.13 BASIC FORCED-LIQUID FLOW RELATIONSThe standard fluid flow equations can be used to solve heat transfer problems forliquids as well as for air. Most of the airflow equations can also be used for liquidflow, with some minor modifications. The symbols used for airflow are the sameas the sym bols used for liquid flow, except that the values are different for liquidsand gases.

High power comp Liquid cooled si

Aluminum core PCB

Figure 10.17 Liquid-cooled chassis with condu ction-cooled PCB.


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10.13 BASIC FORCED-LIQUID FLOW RELATIONS 379The two parameters most often evaluated in forced-air cooling, as well as inforced-liquid cooling, are the temperature rise and the pressure drop. The temper-

ature rise is measured in O F for English units and in "C for metric units. Thepressure drop in air-cooled systems is measured in inches or centimeters of water.How ever, w ith liquid cooled units the pressure drops are much higher, so that theyare normally expressed in lb/in2 o r in g/cm 2.The temperature rise in a liquid-cooled system can be determined from Eq . 6 . 7 .The sym bols remain the same as those defined by that equation , except that every-thing now relates to the flow of the liquid instead of the air flow. Consistent setsof units must be used for English and metric systems.A t =- (ref. Eq. 6 .7)WC,

where Q = power dissipationW = liquid coolant flow rateC = specific heat of the liquid coolantA convection film will develop in liquid cooled systems as well as in air-cooledsystems. Th is film clings to the heat transfer surface and restricts the flow of heat.The forced convection coefficient for liquid cooling is also shown as h, and itscharacteristics are defined in Eqs. 3.53 and 5 . 7 for English units and m etric units.The temperature rise across the forced convection film then becomes:

QhcAA t =- (ref. Eqs. 3.53 and 5.7)

where Q = power dissipationh, = forced convection coefficientA = surface area

The value of the liquid forced convection coefficient can be determined fromEq. 6 . 9 . The symbols are the same, except that they now must reflect the valuesof the liquid instead of the air.-213

h , = JC,G (g) (ref. Eq. 6 .9)/ - \ -213h , = JC,G (F,) (ref. Eq. 6 .9)where J = Colbum factor

C, = specific heatG = weight flow velocityK = thermal conductivityp = viscosity

The Reynolds number, which is shown in Eq. 6.11 must be obtained before theColburn J factor can be determined. The symbols are the same as for air; only thevalues change.


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(ref. Eq. 6.11)GDN R = - -DP --CL CL

For laminar flow conditions through smooth tubes with Reynolds numbers lessthan about 2000, the Colburn J factor can be determined from Eq. 10.1.1.6J = ( (~~)0.666 (10.1)

The forced convection coefficient h, has relatively little change in the laminarflow region, with low weight flow velocity ( G ) values. In this range the forcedconvection coefficient is really determined by the hydraulic diameter, as shown inFigure 10.12 . The forced convection coefficient increases rapidly as the hydraulicdiameter decreases. When the weight flow velocity increases in the turbulent flowrange, there is a corresponding increase in the convection coefficient [ 6 ] .For turbulent flow conditions through smooth pipes, where the Reynolds num-ber is greater than about 7000, the J factor can be determined from Eq. 10.2 [3 ,151.

Weight velocity, G (g /wc cm2)20 40 60 801004 6 8 1 0

800 -O 600-4 i2 4 0 0 - Tube diameterU

N

0.125 0.317

- 0.010- 0.008

I.t"

I I I I I I I I I104 2 4 6 8105 2 4 6 8 106Weight velocity, C (Ib/hr ft')

10.006r n

(10.2)

Figure 10.12 Forced convection coefficient for water in smooth pipes. (Ref: General ElectricHeat Transfer Data Book, 1975)


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10.73 BASIC FORCED-LIQUID FLOW RELATIONS 387

The relation above applies for low viscosity liquids where the Prandtl numberThe weight flow velocity G through pipes and ducts is determined from Eq.

( C , p l K ) is between values of about 1.5 to 20.6.12. The symbols for liquids are the same as those for air.

WG = -Awhere W = weight flow of liquid

(ref. Eq. 6.12)

A = cross section flow areaPressure drop relations for liquid flow through smooth pipes and ducts are de-

termined from the same Darcy flow equations for air. When the pressure loss is tobe expressed in the height of a column of water, Eq. 6.82 is used. The symbolsfor liquids are the same as those for air. The equation can be used for laminar orturbulent flow conditions.

HL = 4f ()() (ref. Eq. 6.82)For laminar flow conditions in round tubes, the Fanning friction factor is usedin Eq. 6.82, as shown in Table 6.7.

16f = -R (ref. Table 6 .7 )

Sometimes it is more convenient to write the head loss using the Hagen-Poi-seulle friction factor. The head loss relation is then shown by Eq. 10.3.

(10.3)For laminar flow in round tubes, the Hagen-Poiseulle friction factor is thenused, as shown in Eq. 10.4.*

64f = -N R (10.4)

Note that the friction constant in front of both Eq. 6.82 and Eq. 10.3 will haveFor turbulent flow conditions with Reynolds numbers up to lOO,OOO, the fric-a value of 64.tion factor to be used with Eq. 10.3 is shown in Eq. 10.5 [15].

*Do not become confused with the Darcy equationfo r laminarjlow in roundpipes, shown in Eq. 6.82.In the Darcy equation, the Fanning friction factor is used where f = 161NR.


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382 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBS0.316(Nk)0.25f=- (10.5)

For turbulent flow conditions with Reynolds numbers up to 300,000, the fric-tion factor to be used with Eq. 10.3 is shown in Eq. 10.6.(10.6)

Ducts with circular cross sections are not always used in liquid cooling systems.Therefore, it is convenient to use a hydraulic diameter for evaluating liquid sys-tems. The hydraulic diameter is defined as shown in Eq. 10.7.4 x cross-sectional area

wetted perimeterH = (10.7)For a circular cross section, this is simply the pipe diameter.

4(R D '14)RDH =

= D = pipe diameter

For a rectangular cross section with inside dimensions a X b, the hydraulicdiameter is4ab 2ab

2 a + 2 b a + bD - -=-H -Sometimes the hydraulic radius is required for some flow problems. The hy-draulic radius is defined in Eq. 10.8.

cross-sectional areawetted perimeterRH = (10.8)

For a circular cross section, the hydraulic radius is one fourth of the diameterof the pipe.7rDL/4 D

X D 4R - - = -H -

10.14 SAMPLE PROBLEM-TRANSISTORS ON AWATER-COOLED COLD PLATE

(10.9)

A water-cooled cold plate supports 16 stud mounted transistors, which dissipate37.5 watts each, for a total power dissipation of 600 watts. The coolant flow rateis 1.0 gal/min (62.8 glsec) with an inlet temperature of 95F (35"C), flowing


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10 .14 SAMPLE PROBLEM-TRANSISTORS ON A WATER-COOLED COLD PLATE 383

_ - 28 stud mounted transistors

Figure 10.73 Water-cooled cold plate with 16 transistors.

through a tube that has a 0.312 in (0.792 cm ) inside diameter, as shown in Figure10.13 . The maximum allowable component mounting surface temperature is 160 F(71"C). Determine the component surface temperature when silicone grease isused at the transistor moun ting interface. Also determine the pressure drop throughthe system resulting from a flow rate of 1 gal/min.

SOLUTION: COMPONENT SURFACE TEMPERATUREThe com ponent mounting surface temperature is determined first. This is obtainedby calculating the temperature rise along individual segments along the heat flowpath from the component to the coolant as follows:Ar, = temperature rise across the transistor mounting interface from the transistorcase to the heat sink surface, using silicone grease at the interfaceA r2 = temperature rise through the aluminum cold plate from the transistor to the

coolant tubingA t3 = temperature rise across the liquid coolant convection film from the w alls ofthe tube to the coolantAt4 = temperature rise of the coolant as it flows through the cold plate picking upheat from the transistorsThe physical properties of water are shown in Table 10.6 for English units and inTable 10.6A for metric units.


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384 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSTABLE 10.6 Properties of Water: English Unlts

CpCLKc, P K(OF) (g)&) (&) (dimensionless)

32405060708090

100I10120130140150160170180190200

1.0091.0051.0021 .Ooo0.9980.9980.9970.9970.9970.9970.9980.9980.9991 Ooo1 .0011.0021.003I .004

4.333.753.172.712.372.081.851.651.491.361.241.141.040.970.900.840.790.74

0.3270.3320.3380.3440.3490.3550.3600.3640.3680.3720.3750.3780.3810.3840.3860.3890.3900.392

13.411.39.47.96.85.85.14.54.03.63.33.02.72.52.32.22.11.9

Starting with A t , the temperature rise across the transistor interface is obtainedfrom Table 3 . 3 , using thermal grease at the mounting interface of the a-28 studmoun ted transistor.

(10.10)CwattA t , = 0.30- 37.5 watts) = 11.2"C (20.2"F )Symmetry is used to determine the temperature rise through the aluminum coldplate from one transistor to the coolant tube, as shown in Figure 10.14. Conver-sions for English units and metric units are made with Tables 1.1 through 1.11 ,The heat is conducted through the aluminum plate, so that the conduction heattransfer equation is used.

QL2 - K AA t -- (ref. Eq. 3.2)


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10.74 SAMPLE PROBLEM-TRANSISTORS ON A WATER-COOLED COLD PLATE 385TABLE 10.6A Properties of Water: M etric Units

CL K Q!Yncal(T) (s)(A ) (sec cm T) (dimensionless)04.4

10.015.621.126.732.237.843.348.954.460.065.671.176.782.287.893.3

1.0091.0051.0021 OOo0.9980.9980.9970.9970.9970.9970.9980.9980.9991.0001.0011.0021.0031.004

0.01790.01550.01310.01120.00980.00860.00760.00680.00610.00560.005 10.00470.00430.00400.00370.00350.00330.0031

0.001350.001370.001390.001420.001440.001470.001490.001500.001520.001540.001550.001560.001570.001580.001590.001610.001610.00162

13.411.39.47.96.85.85.14.54.03.63.33.02.72.52.32.22.11.9

Given Q = 37.5 watts = 128 Btu/hr = 8.96 cal/secL = 1.0 in = 0.0833 f t = 2.54 cmK = 100 Btu/hr ft "F = 0.413 cal/sec cm "C

In English units:= 15.4"F128)(0.0833 ft )(lOO)(O.O0694)At2 =

In metric units:(8.96)(2.54 cm)(0.413)(6.45)t2 = = 8.5"C

(10.11)

(10.1 la)The forced convection coefficient across the liquid film in the coolant tube can-not be obtained until it has been determined if the liquid flow is laminar or tur-bulent. Therefore, the Reynolds number must be determined.


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Transistor37.5 watts

.- c1.0 in

_ - - _ _ _ _ _ _ _

Figure 10.14 Heat transfer path from transistor to coolant tube.

GD (ref. Eq. 6.11)N R = cl

Given W = 1.0 gallmin = 8.3 lb/min = 62.8 g/secA = (~/4)(0.312)* 0.0764 in2 = 0.000531 ft2 = 0.493 cm2

W (8.3 lb/min)(60 min/hr) lbA 0.000531 ft2 hr ft262.8 g/sec gG = = 127.38-.493 cm2 sec cm2G = - = = 9.38 x 105-D = 0.312 in = 0.026 ft = 0.792 cmp = 1.75 lb/ft hr = 0.00722 g/cm sec[(ref. Table 10.6) at 95F (35"C)I

In English units:(9.38 x 105)(0.026 t)

1.75,q = = 1.39 X lo4 (dimensionless) (10.12)In metric units:

(127.38)(0.792)0.00722R = = 1.39 X lo4 (dimensionless) (10.12a)

Since the Reynolds number is well above 3000, the coolant flow is turbulent.The forced convection coefficient for the coolant is determined from Eq. 6.9. TheColburn J factor for the turbulent flow is determined from Eq. 10.2.Given h, = JC,G (ref. Eq. 6.9)

NR = 1.39 X'104 (ref. Eq. 10.12)


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10.14 SAMPLE PROBLEM-TRANSISTORS ON A WATER-COOLED COLD PLATE 387

0.025(NR)

J = - o,2 (ref. Eq. 10.2)0.025 = 0.00371 (dimensionless)(1.39 x 104)0.2=

Cp = 0.997 Btu/lb "F = 0.997 cal/g "CG = 9.38 x lo5 lb/hr ft2 = 127.38 g/sec cm2K

[ref. Table 10.6 at 95F (35"C)I

-- '-.8 [dimensionless; ref. Table 10.6 at 95F (35"C)ISubstitute into Eq. 6.9 for English units.

h, = (0.00371)(0.997)(9.38 x 105)(4.8)-0.666Btuh, = 1220 hr ft2"F

Substitute into Eq. 6.9 for metric units.h, = (0.00371)(0.997)(127.38)(4.8)-0~666

calsec cm20C, = 0.166

(10.13)

(10.13a)The temperature rise across the liquid coolant film in the tube is determinedfrom Eq. 5.7.

QhcA

At3 =- ref. Eq. 5.7)Given Q = 600 watts = 2047.8 Btu/hr = 143.4 cal/sech, = 1220 Btu/hr ft2 "F = 0.166 cal/sec cm2 "C

A = ?r DL inside surface area of 0.312 in diameter tube)A = n(0.312)(16 in length) = 15.7 in2A = 0.109 ft2 = 101.2 cm2

Substitute into Eq. 5.7 for English units.

2047.8 Btu/hrA t - = 15.4"F- (1220)(0.109 ft2)Substitute into Eq. 5.7 for metric units.

143.4 calhec(0.166)( 101.2 cm2)t3 = = 8.5"C

"(10.14)

(10.14a)


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The temperature rise of the coolant as it flows through the cold plate is deter-mined from Eq. 6.7.A t 4 =- ref. Eq. 6.7)WCPGiven Q = 600 watts = 2047.8 Btu/hr = 143.3 cal/sec

W = 1.0 gal/min = 8.3 lb/min = 498 lb/hr = 62.8 g/secC p = 0.997 Btu/lb "F = 0.997 cal/g "C (ref. Table 10.6)

Substitute into Eq. 6.7 for English units.2047.8 Btu/hr(498)(0.997)t4 = = 4.1"F

Substitute into Eq. 6.7 for metric units.143.4 cal/sec

- (62.8)(0.997)t - = 2.3"C

(10.15)

(10.15a)

The surface temperature of the transistor is determined by adding all of thetemperature rises along the heat flow path to the inlet temperature of the coolant.In English units:t, = 95 + A t , + At2 + At3 + At4t , = 95 + 20.2 + 15.4 + 15.4 + 4.1 = 150.1"F (10.16)

In metric units:t , = 35 + 11.2 + 8.5 + 8.5 + 2.3 = 65.5"C (10.16a)

Since the transistor surface temperature is less than 160F (71"C), the designis satisfactory.

SOLUTION: PRESSURE DROPThe pressure drop through the system is determined from Eq. 10.3. The flowthrough the cold plate is turbulent, as shown by Eq. 10.12. Therefore, the frictionfactor shown in Eq. 10.5 is used for the pressure drop calculations.

H, = f (L, (") (ref. Eq. 10.3)D 2g


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10.14 SAMPLE PROBLEM-TRANSISTORS ON A WATER-COOLED COLD PLATE 389Given NR = 1.39 x lo4 (ref. Eq. 10.12) (dimensionless)

0.316(NR)0.25f=- (ref. Eq. 10.5)(friction factor)

o,25 = 0.0291 (dimensionless).316= (1.39 x 10 )D = 0.312 in = 0.026 ft = 0.792 cm (diameter)g = 32.2 ft/sec2 = 980 cm/sec2 (gravity)W = 8.3 lb/min = 0.138 lb/sec = 62.8 g/sec (fluid flow)p = 62.4 lb/ft3 = 1.0 g/cm3 (density of water)A = (0.312)2 = 0.0764 in2 = 0.000531 ft2 = 0.493 cm2 (area)4W

P AV =- velocity of water in pipe)In English units:

0.138 lb/sec ftV = = 4.16-(62.4)(0.000531 t2) secIn metric units:

62.8 g/sec cm(1.0)(0.493 cm2) sec= = 127.4-

The length (L) f the coolant flow path is obtained from the length of the straighttubing plus the pipe fittings expressed in terms of equivalent pipe diameters, asshown in Table 10.3. The pipe diameter is 0.312 in (0.792 cm); see Table 10.7.Substitute into Eq. 10.3 for English units.(4.16 ft/sec)2HL (0.0291) (=) = 1.15 ft H20 (10.17)0.312 in (2)(32.2 ft/sec2)

TABLE 10.7 Diameter and Length of Various FittlngsNumber of Length

Fitting Type Diam eters in cm180" bend, Length = ~ ( 2 . 0 ) 6 .3 15 .990 " elbo w, 2 diameters radius 20 6 . 2 15.790" lbow, 2 diameters radius 20 6 . 2 15.7

6 8 . 6Total equivalent pipe length 4 5 . 7 115.9

Straight pipe,8 + 8 + 2 + 1 + 2 + 6 in -7 .0-


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390 SPECIAL APPLICATIONS FOR TOUGH COOLING JOBSSubstitute into Eq. 10.3 for metric units.

115.9 cm (127.4 cm/sec)20.792 cm (2)(980 cm/sec2) = 35.2 cm H 2 0 (10.17a)L = (0.0291)The head loss can also be expressed as a pressure loss by comparing the valuewith the standard atmosphere. This is where a standard atmosphere will support acolumn of water 34 ft high, so that 14.7 lb/in2 equals 34 ft of water.

1.15 lb34 inA P =- 14.7) = 0.503 (10.18)

For metric units, a standard atmosphere is 76 cm of mercury, which representsabout 1034 cm of water. This is then equivalent to a pressure of 1034 g/cm2.35.2 g1034 cmA P =- 1034) = 35.27 (10.18a)

Figure 10.15 shows a multiple-bay cabinet assembly for naval ships and sub-marines with engine instruments and an engine throttle control.

Figure 10.15 A mu/tip/e.bay cabinet assembly fo r naval ships and submarines with engineinstruments and an engine throttle control. (Courtesy of Litton Systems Inc.).


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10.15 SOLID STATE THERMOELECTRIC COOLING 39110.15 SOLID STATE THERMOELECTRIC COOLINGSolid state thermoelectrics are being used for cooling in a wide variety of appli-cations from refrigerators to inertial navigation systems. These devices are avail-able in many different sizes, shapes, operating currents, voltages, and pumpingcapacities, and they operate on direct current. When direct current is applied, itproduces a hot face and a cold face. Electronic components that must be cooledare mounted on the cold side. Heat must be removed from the hot side to preventits transfer to the cold side of the device. The hot side can be attached to a heatexchanger to improve the heat removal, which improves the performance of thecold side. The system operation can be reversed by simply switching the positiveand negative electrical connections, which will switch the hot and cold faces [73].A single-stage thermoelectric device can achieve a temperature differential ofabout 65C across the hot and cold faces. This can be increased by stacking onemodule on the top of another module, which is known as staging or cascading.When two m odules are stacked together, they can achieve temperature differencesof about 85C across their hot and cold faces.Thermoelectric modules are usually packaged between ceramic plates to pro-vide electrical insulation along with good thermal conduction and a high mechan-ical strength. Since ceramic is usually very brittle, special care must be used inthe mounting or the clamping methods to prevent the ceramic from cracking. Asoft interface material, such as indium fo il, is often used at the clamping interfacesto distribute the loads more uniformly to reduce high forces and stresses. Some-times silicone grease is used to reduce the thermal resistance across mounting in-terfaces, and sometimes a soft RTV adhesive is used for the same reason.Electrical power is required to operate a thermoelectric cooling device. Thisrepresents a heat load which must be removed. Any device that is mounted on thethermoelectric cooler will also dissipate heat, which must be rem oved. Therefore,there are two heat loads that must be considered in the application of a thermoe-lectric cooler; the electrical input power (or I 2 R), and the heat from the devicebeing cooled. The total heat load is the sum of these two heat sources.There are m any cases where the local ambient temperature is too high to provideadequate cooling for temperature-sensitive devices. Refrigeration systems or ther-moelectric cooling devices are then required. Where a small size, light weight,low noise, and low cost are important, thermoelectric systems can be used veryeffectively.Special manufacturing processes and som e types of test equipmen t often requirevery precise temperature controls. These requirements can often be met with ther-moelectric devices.

Application of HEat Pipe

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