60052091-Heat-Pipes

Heat Pipes

CHAPTER 1: INTRODUCTION TO HEAT PIPES

1.1 PURPOSE

In any engineering field, the main problem faced by an engineer

is dissipation of heat, that is produced on site during operation of the

component. Over the years, various methods have been developed to dissipate

this heat effectively. These include various types of heat sinks, fins, fans, etc..

In fields like computer hardware, electronics, etc., limited space

is available. Dissipation of heat from an entire system like a CPU or an electronic

PCB; fans or heat sinks can be used. But for cooling of small electronic

components like resisters, transistors, semiconductor chips, etc. heat sinks or

fans cannot be used. In such cases, HEAT PIPES prove to be a useful device.

They are available in various shapes and sizes .Heat pipes are very effective in

dissipating heat from any component. New types of heat pipes are being

invented and constant research is being done. The report deals with concept,

types, components and construction of heat pipes along with applications in

electronic engineering field.

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Heat Pipes

1.2 HISTORY

The development of the heat pipe originally started with Angier

March Perkins who worked initially with the concept of the working fluid only in

one phase (he took out a patent in 1839 on the hermetic tube boiler which works

on this principle). Jacob Perkins (descendant of Angier March) patented the

Perkins Tube in 1936 and they became widespread for use in locomotive boilers

and baking ovens. The Perkins Tube was a system in which a long and twisted

tube passed over an evaporator and a condenser, which caused the water within

the tube to operate in two phases. Although these early designs for heat transfer

systems relied on gravity to return the liquid to the evaporator (later called a

thermosyphon), the Perkins Tube was the jumping off point for the development

of the modern heat pipe. The concept of the modern heat pipe, which relied on a

wicking system to transport the liquid against gravity and up to the condenser,

was put forward by R.S. Gaugler of the General Motors Corporation. According

to his patent in 1944, Gaugler described how his heat pipe would be applied to

refrigeration systems. Heat pipe research became popular after that and many

industries and labs including Los Alamos, RCA, the Joint Nuclear Research

Centre in Italy, began to apply heat pipe technology their fields. By 1969, there

was a vast amount of interest on the part of NASA, Hughes, the European Space

Agency, and other aircraft companies in regulating the temperature of a

spacecraft and how that could be done with the help of heat pipes. There has

been extensive research done to date regarding specific heat transfer

characteristics, in addition to the analysis of various material properties and

geometries.

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Heat Pipes

CHAPTER 2: CONCEPT AND PRINCIPLE

2.1 DEFINITION

A heat pipe has been defined in several different ways as follows:

- A heat pipe is a super thermal conductor that transmits thermal energy by

evaporation & condensation of the working fluid.

- A heat pipe is a synergistic engineering structure which is equivalent to a

material having thermal conductivity greatly exceeding that of any known metal.

Basically a heat pipe is a thermal energy absorbing & transferring

system, which can carry about one thousand times more heat energy than an

equivalent size of copper rod, for the same temperature gradient. In other words,

it has an effective thermal conductivity several hundred times more than an

equivalent size of copper. Due to their phenomenally high thermal conductivity,

heat pipes are virtually isothermal.

It consists of an evacuated sealed tube with a capillary mechanism, incorporated

for the return of the working fluid. Thus, it is a self contained, passive, energy

transferring device.

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Heat Pipes

2.2 OPERATING PRINCIPLE

The heat pipe makes use of latent heats of vaporization and

condensation, aided with the capillary pumping action, to transfer enormous heat

fluxes. The working fluid inside the heat pipe is in equilibrium with its own vapour,

as the container is sealed under vacuum. Thermal energy, applied to the external

surface of the heat pipe, causes the working fluid near the surface, to evaporate

instantaneously. The vapour thus formed, absorbs the latent heat of vaporisation.

Due to the pressure gradients, thus created within the heat pipe by the rapid

generation of vapour, the excess vapour is forced to a remote area within the

heat pipe, having low temperature and pressure. Here, the thermal energy is

removed, causing the vapour to condense into liquid, thereby, giving up the latent

heat of condensation. The condensed liquid then flows back to the high

temperature region, to be reused, thus completing a cycle.

Thus, the heat pipe works continuously in a closed loop evaporation

condensation cycle. Only a negligible quantity of heat transfers through the metal

body of the pipe, as almost all the heat is transferred through liquid vapour

transformation.

A heat pipe may be divided into three main regions :

1. Evaporator Section

2. Adiabatic Section

3. Condenser Section

2.2.1 EVAPORATOR SECTION :

This is the heat in section of the heat pipe. Due to very low

vapour pressure, as a result of evacuation, water in the wick boils at 50C, and

converts into water vapour. As water has very high latent heat of vaporisation

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Heat Pipes (about 2500 KJ/Kg), large quantity of heat is absorbed in the form of latent heat

of phase transformation.

2.2.2 ADIABATIC SECTION :

This section separates the evaporator and the condenser

regions. Heat is neither absorbed, nor rejected in this region. The water vapour,

during its flow from the evaporator to the condenser, undergoes a slight pressure

drop in this section.

2.2.3 CONDENSER SECTION :

This is the Heat out section of the heat pipe. Heat is removed

from this portion, using forced air, water or even by natural convection. This

causes the steam inside the pipe to condense, releasing large quantity of heat, in

the form of latent heat of condensation.

A peculiar characteristic of a heat pipe is that any portion of the heat pipe can be

used as an evaporator or a condenser, Hence, a heat pipe is totally reversible,

and it needs no external power for its operation. Also, it has no mechanically

moving parts.

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Heat Pipes

FIGURE 2.2 Operating Principle

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Heat Pipes

TABLE 1: COMPARISON OF OPERATING PARAMETERS

Specification Natural convection Forced convection

1. Capacity (W) 360 220 80 150 600 100 80

2. Temperature (0C) 110 55 30 80 110 40 40

3. Length (mm) 500 360 360 360 360 200 L-shaped

4. Diameter (mm) 16 16 16 16 16 11 6

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Heat Pipes

CHAPTER 3: CONSTRUCTION OF HEAT PIPE

3.1 COMPONENTS

The casing can be made out of a variety of different materials,

depending on the specifications and the working fluid (some combinations are

not compatible for material). A heat pipe has three different components:

1. The Casing,

2. The Working Fluid,

3. The Wick.

Most heat pipes currently used have copper, stainless steel, or

aluminum casings. The wick is often a woven wire mesh that is composed of

very small pores. Stainless steel is easiest to work with but copper is also

used. Aluminum on the other hand, is difficult to weave and therefore in using

this material it is difficult to achieve a small poresize . The pore size is

important because the wick operates under the principle of capillary action.

Capillary action describes how fluid in a very small tube will be forced up

through this tiny opening causing the fluid to rise. This fluid transport against

gravity is passive and can be attributed to the atmospheric pressure pushing

the through the small pores, and the surface tension felt between the

molecules of the fluid itself (thereby ensuring a continuos stream of fluid

moving up the wick). The wick is usually located against the inside walls of

the heat pipe.

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Heat Pipes

3.2 CONSTRUCTION :

A heat pipe is a thin walled metal tube, which is sealed from both

ends. The sealed container is in the form of an evacuated tube, usually of circular

cross section. The inner surface of this tube is lined with a wick, held tightly

against the container wall. The basic purpose of providing the wick is to transport

the working fluid inside the tube from one end of the tube (condenser) to the

other end (evaporator), by capillary action. A small quantity of the working fluid

(depending upon the operating temperature), is introduced in the heat pipe,

Thereafter, the system is evacuated and sealed.

TABLE 2: FLUIDS SUITABLE FOR HEAT PIPES

(Properties at atmospheric pressure)

Fluid Melting

Point (0K)

Boiling

Point (0K)

Density

(Kg/m3)

Latent Heat

(KJ/Kg)

Surface Tension

(mN/m)Ammonia 196 240 682 1370 41Water 273 373 1000 2250 76Cesium 302 978 1794 612 76Potassium 337 1033 819 2077 86Sodium 371 1156 929 4210 190Lithium 452 1590 509 19631 386Lead 600 2010 10492 585 470

CHAPTER 4: CLASSIFICATION AND TYPES OF HEAT PIPES

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Heat Pipes

4.1 CLASSIFICATION

Heat pipes can be classified in several different ways as follows:-

1. By Operating Temperature Range

Cryogenic

Ambient

Liquid metal

2. By Wicking Structure

Arterial

Composite

3. By Function

Rotating / Revolving heat pipes

Micro- heat pipes

Variable Conductance heat pipes

Thermal diodes

Some of these types are as follows:-

4.1.1ROTATING AND REVOLVING HEAT PIPES :

For the purpose of this discussion, a rotating heat pipe will be

defined as one that rotates longitudinally around its own central axis, such as the

shaft of an electric motor. The heat pipe case may be uniform in cross section or

may be tapered to promote return of the working fluid to the evaporator. By

comparison, a revolving heat pipe is one that rotates around an axis located

some distance from and parallel to the central axis of the heat pipe.

4.1.2 VARIABLE CONDUCTANCE HEAT PIPES :

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Heat Pipes

As is the case with most two-phase cycles, the presence of

noncondensible gases creates a problem due to the partial blockage of the

condensing area. Heat pipes are no exception. During normal operation, any

noncondensible gases present are carried to the condenser and remain there,

reducing the effective condenser area. This characteristic, although normally

undesirable, can be used to control both the direction and amount of heat

transferred.

Gas-loaded Heat Pipes :

In this type of device the thermal conductance of the heat pipe

varies as a function of the gas front position. As the heat available at the

evaporator varies, the vapor temperature varies and the gas contained within the

gas reservoir expands or contracts, moving the gas front. This in turn results in a

variation in the thermal conductance, that is, as the heat flux increases, the gas

front recedes and the thermal conductance increases due to the larger

condenser surface area. In this way, the temperature drop across the evaporator

and condenser can be maintained fairly constant even though the evaporator

heat flux may fluctuate. A gas-loaded heat pipe is shown in Fig.4.1.2(a).

A variable conductance heat pipe (VCHP) constructed from stainless steel with

ammonia as the working fluid, was designed to control the temperature and

dissipate heat in a cesium clock utilized at a ground station of the Global

Positioning System. The performance specifications allowed the removal of

approximately 5 W while maintaining a constant temperature bandwidth of 57 +

130C while the ambient conditions ranged from -25 to + 550C.

Excess-Liquid Heat Pipes :

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Heat Pipes Fig. 4.1.2(b) shows schematic of Excess-liquid heat pipe. They

operate in much the same manner as gas-loaded heat pipes, but utilize excess

working fluid to block portions of the pipe and control the condenser size or

prevent reversal of heat transfer.Figure illustrates the principle used in excess

liquid or liquid flow-modulated heat pipes. This type of heat pipe has two

separate wicking structures, one to transport liquid trap. As the temperature

gradient is reversed, the liquid moves into the trap and starves the evaporator of

fluid.

4.2 SPECIFIC TYPES OF HEAT PIPES

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Heat Pipes

4.2.1 FLAT PIPES :-

Many different types of heat pipes have been developed over

the years. Manufacturers are now able to make heat pipes in any geometry and

specifically tailored to the needs of the consumer. Several types of heat pipes

include heat pipes with thermal diodes or thermal switches (including variable

conductance heat pipes) and flat plate heat pipes. Flat heat pipes are just that;

the orientation of the wick structure is designed so that the liquid is more evenly

distributed to the top and the bottom of the plate. The wick structure in a flat plate

is a sintered metal; it is a metal powder that has been molded and heated until

the metal has fused, creating a structurally stable metal with small pores within.

Flat heat pipes produce a surface that has a relatively uniform temperature

distribution and large surface area. These would be useful in the case where one

needs to radiate heat uniformly instead of from a point source. The use of flat

plates as wall components could be one possible application for heat pipe

technology.

FIGURE 4.2.1 FLAT PIPE

4.2.2 THERMAL SWITCHES:-

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Heat Pipes Thermal switches in a heat pipe serve to prevent the pipe from

working in certain cases. This can be accomplished by introducing a blockage,

made possible in a variety of different ways. Methods would include freezing the

fluid, placing a magnetically operated vane within the pipe which would block the

vapor flow, or using a physical displacement block (which controls the amount of

fluid in the reservoir and in the heat pipe by blocking the fluid from being

transported by the wick).

FIGURE 4.2.2 THERMAL SWITCH

4.2.3 THERMAL DIODES:-

Another possible way to stop or control the heat transfer within the

pipe would be by limiting the acting surface of the condenser by using an inert

gas (this is the principle also behind variable conductance heat pipes). Thermal

diodes allow the heat pipe to only work in one direction. In one example of a heat

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Heat Pipes diode, if the location of the condenser and evaporator switch, the liquid becomes

trapped in a reservoir whose wicks are not connected to the rest of the pipe. This

makes it so that the liquid will not be able to travel down the length of the heat

pipe until the condenser and evaporator switch again to heat the liquid to the

gaseous phase so it can flow down the pipe once more.

FIGURE 4.2.3 THERMAL DIODE

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Heat Pipes

CHAPTER 5: APPLICATIONS IN THE ELECTRONICS INDUSTRY--

5.1 INTRODUCTION

The thermal control of electronic components has one principal

objective, to maintain relatively constant component temperature equal to or

below the manufacture's maximum specified service temperature. Investigations

have demonstrated that an increase in temperature of as little as 100C can

reduce the reliability of some systems by as much as 50%. For this reason, new

thermal control schemes must be capable of eliminating hot spots within the

electronic devices, removing heat from these devices, and dissipating this heat to

the surrounding environment.

Thermal control schemes to remove heat from individual devices

and systems include the traditional means of free and forced gaseous and liquid

convection as well as conduction and radiation or combinations thereof.

Although air cooling is the best understood and most frequently

used technique, it is limited in the heat removal rate by the convection coefficient.

Direct cooling methods are capable of attaining extremely high heat flux levels,

but they present problems with contamination and are extremely expensive.

Although an issue of considerable discussion, indirect cooling strategies appear

to be the best near-term solution for the thermal control of advanced computer

architectures.

Heat pipes, because of their high thermal conductivity, provide

an essentially isothermal environment with very small temperature gradients

between the individual components. Hence, they are an acceptable alternative to

the large, bulky aluminum or copper fin structures of complex geometries that are

currently the industry standard. The high heat transfer characteristics, the ability

to maintain constant evaporator temperatures under different heat flux levels,

and the diversity and variability of evaporator and condenser sizes make the heat

pipe an effective device for the thermal control of electronic components. This

review of applications includes recent advances and developments that affect the

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Heat Pipes implementation of heat pipes in the thermal control of electronic devices. The

applications fall into three categories: indirect, where the heat pipe is placed in

contact with the component or device is an integral part of the heat pipe and / or

is in direct contact with the working fluid; and system level heat pipes, where a

heat pipe is used to control the temperature in equipment cabinets or systems.

5.2 INDIRECT HEAT PIPE THERMAL CONTROL :-

Because of the high effective conductivity of heat pipes compared

to that of conventional heat sinks, heat pipes have been proposed and selected

for thermal control of individual components, series of components, and entire

printed wire boards. The simplest heat pipe heat sinks are cylindrical with a

copper or aluminum case and water, acetone, or methanol as the working fluid.

Using this configuration, heat can be removed from power transistors, thyristors,

or individual chips. These components are often mounted on the evaporator

portion of the pipe and attached mechanically. A series of fins attached to the

condenser end of the heat pipe provides the mechanism for heat rejection to a

coolant, either through free or forced convection to a gas or a liquid. Fig.5.2 heat

pipe heat sink for power transistors.

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Heat Pipes 5.2.1 Semiconductor Chip Cooling:-

In this conceptual design, it was proposed that the back of the

integrated circuit chip be bonded to the evaporator portion of a heat pipe

evaporator, which could be constructed from screen, sintered powder, or a series

of axial grooves. A porous wicking material lining the inside circumference of the

heat pipe would connect the evaporator and condenser, where heat would be

dissipated by free convection. Fig. 5.2.1 illustrates this design.

5.2.2 Micro Heat Pipe:-

Although all of the previously discussed applications are

relatively large compared to the size of most semiconductor devices, this need

not be the case. Micro- heat pipe concept, that is, a wickless heat pipe "so small

that the mean curvature of the vapor-liquid interface is necessarily comparable in

magnitude to the reciprocal of the hydraulic radius of the flow channel" can also

be used. Fig.5.2.2 shows a micro heat pipe.

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Heat Pipes

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Heat Pipes

5.3 DIRECT HEAT PIPE THERMAL CONTROL :-

Where the electrical power is high and the heat rejection

requirements large, it may be necessary to control the temperature by immersing

the devices in a dielectric fluid. Fluid near the saturation temperature typically

results in nucleate pool boiling and requires the use of a vapor space condenser.

This two-phase loop (i.e., the boiling of the liquid, the condensation of the vapor,

and the return of the condensate) is viewed as one form of a two-phase, closed

loop thermosyphon.

The generation of vapor bubble imposes several problems on the

thermal control of electronic devices. First and most important among these is

the critical heat flux, the maximum permissible level of the evaporator heat flux.

Beyond this level, the vapor completely blankets the heat source and results in

an increased temperature drop, leading to dryout and overheating. Second, the

formation and collapse of vapor bubbles may generate dynamic forces on the

chips and leads, creating high frequency mechanical vibration and subsequent

failure. Third and finally, the presence of vapor bubbles may decrease the

electric breakdown voltage of the dielectric fluid. Fig. 5.3 shows c\s of heat pipe

cooled MIC RF transistor.

While not as potentially damaging as film boiling, nucleate pool

boiling will result in an increased temperature drop. Two techniques have been

investigated to reduce this temperature drop. The first is to make the device an

integral part of the wick structure to ensure that fresh liquid always remains in

contact with the heat source. The second is the direct evaporation (with no

bubble nucleation) of a very thin liquid film.

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Heat Pipes

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Heat Pipes

5.4 COOLING OF PCBs:

A PCB has a high power dissipation and relatively large heat flow

path to the heat sink. The resulting temperature rise is excessive. Many

systems utilize air-cooled cold plates for the sidewalls of the chassis. Plug-

in PCBs are then used to support electronic components. These are

cooled by conducting heat, along metal strips laminated to the PCB. Fig.

5.4(a) illustrates this type.

Heat pipes can be added to back surface of PCB to sharply increase

heat transferred from centre to the edges. A high temperature rise may

still occur at the interface of PCB with chassis cold plate, unless a high

interface pressure device such as wedge clamp, is use, as shown in Fig.

5.4(b).

Heat pipes with 90 deg. bend to improve heat transfer from circuit

boards that do not plug in, as shown in Fig.5.4(c).Sometimes cooling fins

are extended to improve cooling. Heat pipes are used along length of the

fin.

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Heat Pipes

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Heat Pipes

CHAPTER 6: CONCLUSION

6.1 ADVANTAGES OF HEAT PIPES:

1. No moving parts & no external power required, implying high reliability (in

fact, a correctly designed system will last indefinitely).

2. Extremely light weight & miniatursed.

3. Heat transport capacity of 100-500 Watts/Cm2 and even much higher

capacity values can be obtained.

4. Extremely low thermal resistance, of the order of 0.2 C/ Watt or even

lower with fins.

5. Completely silent and reversible in operation.

6. Heat pipes used in electronics component cooling eliminate hot spots due

to isothermal nature.

7. Heat pipes are ruggedly built and can withstand a lot of abuse .

8. The absence of gravitational forces improves their performance in space

applications.

9. Finally , a heat pipe, as a transformer, allows the heat to be absorbed as

a high heat flux (i.e over small area) and transferred from the heat pipe with a

low heat flux (i.e. over a large area).

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Heat Pipes Thus, it can be seen that concept of heat pipes has benefited the Electronic Industry to a large extent. Limited space requirement is the main

cause of its use in electronics. Reversibility of heat pipes has made them further

flexible in use. Small semiconductor chips are now easily cooled using concept of

heat pipes. Apart from a few limitations, heat pipes have proved to be a boon to

electronic industry.

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Heat Pipes

REFERENCES 1) D.Chisolm

The Heat Pipes

Mills & Boon Publication Limited

1971 First Edition.

2) S.P.Sukhatme

Heat Transfer

Orient Longman Publication

1989, Third Edition.

3) P.Dunn & D.A.Reay

Heat Pipes

Pergamon Press

Third Edition.

4) G.P. Peterson

An Introduction To Heat Pipes

John Wiley & Sons, Inc.

5) www.electronics-cooling.com

www.heatpipes.ot.kr

The Internet

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CHAPTER 5: APPLICATIONS IN THE ELECTRONICS INDUSTRY--5.2 INDIRECT HEAT PIPE THERMAL CONTROL :-