Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Heat Pipes 381

Heat

addition

Heat

rejection

Evaporation

zone

Wick (capillary-porous)

Condensation

zone

Sealed case

Adiabatic zone

(b)

Heat

rejection

Heat

addition

Evaporation

zone

Condensation

zone

Sealed case

Gravitation

(a)

Figure 7.1 Two basic heat pipe conﬁgurations: (a) thermosiphon and (b) capillary driven.

Heating coil

Evaporator

Vapor

Wick

Liquid flow

Container

Condenser

Figure 7.2 A cutaway view of a cylindrical heat pipe (Courtesy of Los Alamos National Laboratory.

A heat pipe is a synergistic engineering structure which, within certain limitations on the manner

of use, is equivalent to a material having a thermal conductivity greatly exceeding that of any

known metal. Shown in Figure 7.2 is a cutaway view of a cylindrical heat pipe with a homogeneous

screen wick. Working ﬂuid is vaporized in the evaporator section and ﬂows toward the condenser

section where it deposits its heat by condensation. Capillary forces in the porous wick return

the condensed working ﬂuid to the evaporator section. Heat transfer occurs through the capillary

movement of ﬂuids. The “pumping” action of surface tension forces may be sufﬁcient to move

382 Refrigeration Systems and Applications

liquids from a low temperature zone to a high temperature zone (with subsequent return in vapor

form using as the driving force the difference in vapor pressure at the two temperatures). Such

a closed system, requiring no external pumps, 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.

Note that in a heat pipe assembly, the coil supporting rod and the induction coil are assembled as

one integral unit and they do not rotate. Instead, only the outer shell, or jacket, rotates on the heavy

duty inner bearings mounted on each end of the nonrotating coil support rod. This construction

eliminates the need for rotary joints. When an AC voltage of commercial frequency is supplied,

the induction coil generates ﬂux lines whose direction alternates with the power supply frequency.

And, since the roll shell is mounted on the same axis as the induction coil, the shell functions as one

complete turn of a secondary coil. Therefore, the coil, which receives the power, does not heat up;

rather, the shell heats up, following Faraday’s law. Thus, the roll shell itself is the heat source, not

some remotely located heater or boiler. It is well known that the electromagnetic induction method

is almost 100% efﬁcient in converting electrical energy into heat. The shell has several gun-drilled

holes running the full width of the roll, called jacket chambers, the number of which will vary with

roll speciﬁcations. In each of the chambers, a small amount of thermal medium is placed, after

which, each chamber is sealed and evacuated. So, we have thermal medium in a vacuum. When

the roll is operating, the heat from the induction principle causes this thermal medium to vaporize.

Since the pressure of vaporization is greater than the pressure of condensation, the vapor must move

to any cooler area within the jacket chamber and it then condenses, giving off to the shell surface

the latent heat of vaporization. Thus, there is a continuous cycle of vaporization and condensation

taking place in the vacuum of each jacket chamber, which is the phenomenon known as the heat

pipe principle. These heat pipes have an extremely rapid rate of heat transmission (almost the speed

of sound) and each heat pipe contains a very large amount of latent heat. The heat pipe action is

what maintains the highly accurate roll surface temperature because it responds so rapidly, and

automatically, to any slight change of thermal load. So, with a temperature correction device, the

accurate surface temperature is maintained not only in the cross direction but also in the machine

direction. Since no oil ﬂows through the journals of the rolls, the temperature of the journal, where

the support bearing for the frame is mounted, is about one half of the roll surface temperature. This

means that the external bearings should last much longer and that high temperature bearings are

not always needed. So, with no rotary joints, no seals, no oil leaks, and cooler-running bearings,

the maintenance of the rolls is noticeably and signiﬁcantly less than that of conventional rolls.

More importantly, environmental concerns that are normally associated with oil and heat rolls

are eliminated.

7.2.1 Heat Pipe Use

In heat pipe utilization, there are three primary objectives that we mainly expect from heat pipes:

• To act as a primary heat conductive path. When a heat source and heat sink need to be placed

apart, a heat pipe can be a very effective heat conduction path for transporting heat from the

heat source to the heat sink.

• To aid heat conduction of a solid. Heat pipes can add to the efﬁciency and transport capacity

of a thermal shunt.

• To aid heat spreading of a plane. Heat pipes can be used to increase the heat spreading across

a large heat sink base, thereby effectively increasing the base thermal conductivity. The effect of

this is the decrease of the temperature gradient across the base (increase the efﬁciency), thereby

lowering the heat source temperature.

Heat Pipes 383

7.3 Heat Pipe Applications

Heat pipes are used for a wide variety of heat-transfer applications covering the complete spectrum

of thermal applications. Heat pipes are ideal for any application where heat must be transferred

with a minimum thermal gradient either to increase the size of heat sink, to relocate the sink to

a remote location, or where isothermal surfaces are required. Some typical heat pipe applications

include

• cooling of electronic devices and computers,

• cooling of high-heat-load optical components,

• cooling of milling machine spindles,

• cooling of injection molds,

• cryogenic systems,

• aircraft thermal control systems,

• cooling of engine components in conventional aircraft,

• spacecraft systems,

• heat exchangers,

• waste heat recovery systems,

• various industrial processes, for example, metallurgical, chemical, pharmaceutical, food, oil reﬁn-

ing, power generation, transportation, communication, electronics, and so on, and

• solar energy conversion and power generation systems.

Heat pipes are used in a wide range of products like air conditioners, refrigerators, heat exchang-

ers, transistors, capacitors, and so on. Heat pipes are also used in laptops to reduce the working

temperature for better efﬁciency. Their application in the ﬁeld of cryogenics is very signiﬁcant,

especially in the development of space technology, particularly for spacecraft temperature equal-

ization, component cooling, temperature control, and radiator design in satellites, and moderator

cooling, removal of heat from the reactor at emitter temperature, and elimination of troublesome

thermal gradients along the emitter and collector in spacecraft.

Heat pipes are extremely effective in transferring heat from one location to another. A common

spaceﬂight heat pipe has an effective thermal conductivity many thousands of times that of copper.

Although there are many ground applications for heat pipes, in a space-borne environment radiation

and conduction are the sole means of heat transfer, so heat pipes are a fundamental aspect of a

satellite thermal and structural subsystem design.

Figure 7.3 shows two conﬁgurations of heat pipes in laptops: (a) the heat pipe connected to

the keyboard setup and (b) the setup of the heat pipe connected to the back screen. In the past

Xie et al. (2001) have conducted comprehensive case studies on the above conﬁgurations and their

applications in notebook computers.

7.3.1 Heat Pipe Coolers

Although heat pipes have been known in their basic working principles since the nineteenth century,

it is only since the 1960s that they have been extensively used in several industrial ﬁelds. Owing

to their relatively simple structure and improved thermal characteristics, heat pipe coolers show

substantial gain in weight and size reduction. In addition, they are totally maintenance free, with a

proven operating reliability in excess of 30 years. Heat pipe coolers (Figure 7.4) can be designed

and manufactured with an electrical insulation up to 12 kV, while the two parts of the thermal

circuit, the condensing and evaporating sections, respectively, can be physically separated to avoid

hazardous contacts and dust accumulation. The versatility of these systems allows great freedom

in designing customized thermal solutions.

384 Refrigeration Systems and Applications

Screen

Keyboard

Heat dissipating

panel

Heat pipe

(b)

PCB (printed circuit board)

TCP (tape carrier package)

Screen

Al block

Keyboard

Heat

pipe

(a)

PCB (printed circuit board)

TCP (tape carrier package)

Figure 7.3 (a) The heat pipe connected to the keyboard setup. (b) The setup of the heat pipe connected to

the back screen (Adapted from Xie et al., 2001).

(a) (b) (c)

Figure 7.4 (a) A customized heat pipe unit, especially developed for cooling with natural air (for a total power

of 1 kW, with 7.5 kV Al

insulators, being used on onboard equipment in a subway train). (b) A stack of

7.5 kV insulated heat pipe coolers (with Al

insulators) designed for natural air cooling of a pair of thyristors

for onboard equipment in a subway train. Light construction with aluminum evaporator and ﬁns. (c) A stack of

noninsulated heat pipe sinks designed to cool a pair of thyristors with forced ventilation, mounted with special

clamping equipment. The equipment is intended for use in conjunction with an AC mill drive (Courtesy of

Bosari Thermal Management s.r.l .).

7.3.2 Insulated Water Coolers

Manufactured with the most advanced technologies, water cooled and insulated heat sinks provide

electrical separation of the electronic component from the water circuit by means of high thermal

conductivity ceramic components (Figure 7.5). They are further designed with oversized superﬁcial

discharged paths and feature very low total thermal resistance. They now offer a reliable, efﬁcient,

economic, and ecologically favorable alternative to the conventional cooling and treated water

coolant systems.

Heat Pipes 385

(a)

(

)

Figure 7.5 (a) An insulated water cooler. (b) A heat exchanger designed for the natural air cooling of a sealed

container (Courtesy of Bosari Thermal Management s.r.l.).

7.3.3 Heat Exchanger Coolers

This is considered a new solution for the cooling of sealed containers. On the basis of the heat pipe

technology, the heat exchangers have wider radiating surfaces, thus saving weight and dimensions,

when compared with the conventional cooling systems. In this regard, Figure 7.5b shows a heat

exchanger designed for the natural air cooling of a sealed container, designed for onboard equipment

of a tramway (dissipating surface ∼10 m

and total weight 25 kg).

7.4 Heat Pipes for Electronics Cooling

All electronic components, from microprocessors to high-end power converters, generate heat,

and rejection of this heat is necessary for their optimum and reliable operation. As electronic

design allows higher throughput in smaller packages, dissipating the heat load becomes a critical

design factor. Many of today’s electronic devices require cooling beyond the capability of standard

metallic heat sinks. The heat pipe is meeting this need and is rapidly becoming a mainstream

thermal management tool. In fact, heat pipes have been commercially available since the mid

1960s. Only in the past few years, however, has the electronics industry adopted heat pipes as

reliable, cost-effective solutions for high-end cooling applications.

As mentioned earlier, a heat pipe is essentially a passive heat-transfer device with an extremely

high effective thermal conductivity. The two-phase heat-transfer mechanism results in heat-transfer

capabilities from one hundred to several thousand times that of an equivalent piece of copper. The

heat pipe, in its simplest conﬁguration, is a closed, evacuated, cylindrical vessel with the internal

walls lined with a capillary structure or wick that is saturated with a working ﬂuid. Since the heat

pipe is evacuated and then charged with the working ﬂuid prior to being sealed, the internal pressure

is set by the vapor pressure of the ﬂuid. As heat is input at the evaporator, ﬂuid is vaporized, creating

a pressure gradient in the pipe. This pressure gradient forces the vapor to ﬂow along the pipe to

a cooler section where it condenses, giving up its latent heat of vaporization. The working ﬂuid is

then returned to the evaporator by the capillary forces developed in the wick structure.

Heat pipes can be designed to operate over a very broad range of temperatures from cryogenic

(<–243

◦

C) applications utilizing titanium alloy/nitrogen heat pipes to high temperature applications

(>2000

◦

C) using tungsten/silver heat pipes. In electronic cooling applications where it is desirable

to maintain junction temperatures below 125−150

◦

C, copper–water heat pipes are typically used.

386 Refrigeration Systems and Applications

Copper–methanol heat pipes are used if the application requires heat pipe operation below 0

◦

(Garner, 1996).

Perhaps the best way to demonstrate the heat pipe’s application to electronics cooling is to

present a few of the more common examples. Currently, one of the highest volume applications

for heat pipes is cooling the Pentium processors in notebook computers. Because of the limited

space and power available in notebook computers, heat pipes are ideally suited for cooling the high

power chips (see Figure 7.3).

Fan-assisted heat sinks require electrical power and reduce battery life. Standard metallic heat

sinks capable of dissipating the heat load are too large to be incorporated into the notebook package.

Heat pipes, on the other hand, offer a high-efﬁciency, passive, compact heat-transfer solution. Three

or four millimeter diameter heat pipes can effectively remove the high ﬂux heat from the processor.

The heat pipe spreads the heat load over a relatively large-area heat sink, where the heat ﬂux is so

low that it can be effectively dissipated through the notebook case to ambient air. The heat sink

can be the existing components of the notebook, from electromagnetic interference shielding under

the key pad to metal structural components.

Typical thermal resistances for such applications at 6−8 W heat loads are 4–6

◦

C/W. High power

mainframe, mini-mainframe, server, and workstation chips may also employ heat pipe heat sinks.

High-end chips dissipating up to 100 W are outside the capabilities of conventional heat sinks. Heat

pipes are used to transfer heat from the chip to a ﬁn stack large enough to convect the heat to the

supplied air stream. The heat pipe isothermalizes the ﬁns, eliminating the large conductive losses

associated with standard sinks. The heat pipe heat sinks dissipate loads in the 75−100 W range

with resistances from 0.2 to 0.4

◦

C/W, depending on the available airﬂow.

In addition, other high-power electronics including silicon controlled rectiﬁers, insulated gate

bipolar transistors, and thyristors often utilize heat pipe heat sinks. In fact, heat pipe heat sinks

are capable of cooling several devices with total heat loads up to 5 kW. These heat sinks are also

available in an electrically isolated version, where the ﬁn stack can be at ground potential with the

evaporator operating at device potentials of up to 10 kV. Typical thermal resistances for the high-

power heat sinks range from 0.05 to 0.1

◦

C/W. Again, the resistance is predominately controlled

by the available ﬁn volume and airﬂow (for details, see Garner, 1996).

Example 7.1

Consider a heat pipe used as a heat sink for electronic cooling. The heat load is 16 W and the

thermal resistance of the heat pipe is 4

◦

C/W. What is the temperature difference involved in the

dissipation of this heat load?

Solution

The rate of heat dissipation is the temperature difference divided by the thermal resistance. Then,

Q =

T

−→ T =

QR = (16 W)(4

◦

C/W) = 64

◦

7.5 Types of Heat Pipes

Various innovative applications of heat pipes demand a complete and thorough understanding of the

physical phenomena occurring in a heat pipe. In this regard, efforts have been directed toward more

detailed numerical and analytical modeling of conventional heat pipes, thermosiphons, rotating heat

pipes, and micro heat pipes as well as capillary pump loops (Faghri, 1996).

Heat Pipes 387

Since the 1960s, various types of heat pipe heat exchangers have been developed, which are as

follows:

• capillary pumped loop heat pipes,

• gas loaded heat pipes,

• variable conductance heat pipes,

• micro and miniature heat pipes (particularly for microelectronics cooling),

• coaxial heat pipes,

• rotating heat pipes,

• pulsating heat pipes,

• osmotic heat pipes,

• chemical heat pipes,

• gravity-driven geothermal heat pumps,

• thermosiphon heat pipes (here, the word “thermosiphon” is used to describe both single-phase

and evaporative gravity-assisted heat transport devices),

• low temperature and cryogenic heat pipes, and

• alkali metal heat pipes.

7.5.1 Micro Heat Pipes

Cotter (1984) ﬁrst introduced the concept of micro heat pipes incorporated into semiconductor

devices to provide more uniform temperature distribution and better heat transfer. The primary

operating principles of micro heat pipes are essentially the same as those occurring in larger, more

conventional heat pipes. Heat applied to one end of the heat pipe vaporizes the liquid in that

region and forces it to move to the cooler end where it condenses and gives up the latent heat of

vaporization. This vaporization and condensation process causes the liquid–vapor interface in the

liquid arteries to change continually along the pipe, as illustrated in Figure 7.6, and results in a

capillary pressure difference between the evaporator and condenser regions. This capillary pressure

difference promotes the ﬂow of the working ﬂuid from the condenser back to the evaporator through

the triangular-shaped corner regions. These corner regions serve as liquid arteries; thus, no wicking

structure is required.

7.5.2 Cryogenic Heat Pipes

Cryogenic heat pipes operate between 4 and 200 K. Typical working ﬂuids include helium, argon,

oxygen, and krypton. The amount of heat that can be transferred for cryogenic heat pipes is quite

low because of the small heats of vaporization, high viscosities, and small surface tensions of the

working ﬂuids.

7.6 Heat Pipe Components

In general, a traditional heat pipe structure is of a hollow cylindrical container ﬁlled with a vapor-

izable liquid as working ﬂuid as shown in Figure 7.7.

A heat pipe typically consists of a sealed container lined with a wicking material. The container

is evacuated and backﬁlled with just enough liquid to fully saturate the wick. Because heat pipes

operate on a closed two-phase cycle and only pure liquid and vapor are present within the container,

the working ﬂuid will remain at saturation conditions as long as the operating temperature is between

the triple point and the critical state. As illustrated in Figure 7.7, a heat pipe consists of three distinct

388 Refrigeration Systems and Applications

Liquid

Heat outputHeat input

Side

view

End

view

Vapor

20 mm

120 mm

Evaporator

Condenser

Figure 7.6 A micro heat pump (Peterson, 1994).

A. Heat is absorbed in the evaporating section.

B. Fluid boils to vapor phase.

C. Heat is released from the upper part of cylinder to th

environment; vapor condenses to liquid phase.

D. Liquid returns by gravity to the lower part of cylinder

(evaporating section).

Condenser

section

Evaporator

section

Figure 7.7 A heat pipe structure (Courtesy of Heat Pipe Technology, Inc.).

regions: an evaporator or heat addition region, a condenser or heat rejection region, and an adiabatic

or isothermal region. When heat is added to the evaporator region of the container, the working ﬂuid

present in the wicking structure is heated until it vaporizes. The high temperature and corresponding

high pressure in this region cause the vapor to ﬂow to the cooler condenser region, where the vapor

condenses, giving up its latent heat of vaporization. The capillary forces existing in the wicking

structure then pump the liquid back to the evaporator.

Heat Pipes 389

Basically a heat pipe consists of three major components, namely,

• the container, which can be constructed from glass, ceramics, or metals;

• a working ﬂuid, which can vary from nitrogen or helium for low-temperature (cryogenic) heat

pipes to lithium, potassium, or sodium for high-temperature (liquid metal) heat pipes; and

• a wicking structure or capillary structure, constructed from woven ﬁberglass, sintered metal

powders, screens, wire meshes, or grooves.

Each of these three components is equally important, with careful consideration given to the

material type, thermophysical properties, and compatibility. For example, the container material

must be compatible with both the working ﬂuid and the wicking structure, strong enough to with-

stand pressures associated with the saturation temperatures encountered during storage and normal

operation, and must have a high thermal conductivity. In addition to these characteristics, which are

primarily concerned with the internal effects, the container material must be resistant to corrosion

resulting from interaction with the environment and must be malleable enough to be formed into

the appropriate size and shape.

7.6.1 Container

Basic requirements of the heat pipe case include a container capable of maintaining a leak-proof seal

and structural integrity throughout the entire pressure range to which the heat pipe will be exposed.

Therefore, the function of the container is to isolate the working ﬂuid from the outside environment.

It has to therefore be leak proof, maintain the pressure differential across its walls, and enable trans-

fer of heat to take place from and into the working ﬂuid. Possible materials include pure metal alloys

such as aluminum, stainless steel, or copper; composite materials, either metal or carbon composite;

or for higher temperature applications, refractory materials or linings to prevent corrosion.

Careful consideration should be given to the selection of the container or case material for heat

pipes. Various factors that should be considered include the following (Peterson, 1994):

• compatibility with the wicking structure and working ﬂuid,

• operating temperature range of the proposed device,

• evaporator and condenser sizes and shapes,

• applicability,

• reliability,

• strength to weight ratio,

• internal operating pressure,

• thermal conductivity,

• ease of fabrication (including welding, machinability, and ductility),

• possibility of external corrosion,

• porosity, and

• wettability.

In addition to the compatibility problem associated with heat pipes, it must be remembered that

heat pipes and thermosiphons are in fact “unﬁred pressure vessels” and as a result must be designed

to meet the appropriate pressure vessel codes.

7.6.2 Working Fluid

A ﬁrst consideration in the identiﬁcation of a suitable working ﬂuid is the operating vapor temper-

ature range. Within the approximate temperature band, several possible working ﬂuids may exist,

390 Refrigeration Systems and Applications

and a variety of characteristics must be examined in order to determine the most acceptable of

these ﬂuids for the application considered. The prime requirements are as follows:

• compatibility with wick and wall materials,

• good thermal stability,

• wettability of wick and wall materials,

• vapor pressure not too high or low over the operating temperature range,

• high latent heat,

• high thermal conductivity,

• low liquid and vapor viscosities,

• high surface tension, and

• acceptable freezing or pour point.

The selection of the working ﬂuid must also be based on thermodynamic considerations which

are concerned with the various limitations to heat ﬂow occurring within the heat pipe like, viscous,

sonic, capillary, entrainment, and nucleate boiling levels. These will be explained later.

In heat pipe design, a high value of surface tension is desirable in order to enable the heat pipe

to operate against gravity and to generate a high capillary driving force. In addition to high surface

tension, it is necessary for the working ﬂuid to wet the wick and the container material, that is, the

contact angle should be zero or very small. The vapor pressure over the operating temperature range

must be sufﬁciently high to avoid high vapor velocities, which tend to set up large temperature

gradient and cause ﬂow instabilities.

A high latent heat of vaporization is desirable in order to transfer large amounts of heat with

minimum ﬂuid ﬂow, and hence to maintain low pressure drops within the heat pipe. The thermal

conductivity of the working ﬂuid should preferably be high in order to minimize the radial tem-

perature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The

resistance to ﬂuid ﬂow will be minimized by choosing ﬂuids with low values of vapor and liquid

viscosities. Table 7.1 lists a few media with their useful temperature ranges.

Heat pipe working ﬂuids range from helium and nitrogen for cryogenic temperatures, to liquid

metals like sodium and potassium for high temperature applications. Some of the more common heat

pipe ﬂuids used for electronics cooling applications are ammonia, water, acetone, and methanol.

Although many working ﬂuids are used in various heat pipe models, the most common are water

and methanol. Water is thermodynamically superior to methanol under most conditions and becomes

the ﬂuid of choice where it is applicable. The useful range for water is generally 50−200

◦

C. The

useful range for methanol is slightly lower, generally 20−120

◦

C. Since methanol freezes at a very

low temperature, −97

◦

C, it is useful in gravity-aided, pool-boiling applications where water heat

pipes would be subject to freezing.

Heat pipe working ﬂuids including water maintain the normal freezing point. Properly designed

heat pipes, however, will not be damaged by the freezing and thawing of the working ﬂuid. Heat

pipes will not operate until the temperature rises above the freezing temperature of the ﬂuid.

Water at atmospheric pressure boils at 100

◦

C. Inside a heat pipe, the working ﬂuid (e.g., water)

is not at atmospheric pressure. The internal pressure of the heat pipe is the saturation pressure of

the ﬂuid at the corresponding ﬂuid temperature. As such, the ﬂuid in a heat pipe will boil at any

temperature above its freezing point. Therefore, at room temperature (e.g., 20

◦

C), a water heat pipe

is under partial vacuum, and the heat pipe will boil as soon as heat is input.

Owing to the vaporization and condensation of the working ﬂuid that takes place in a heat pipe

(as a basis for the operation), the selection of an appropriate working ﬂuid is a crucial factor in

the design and manufacture of heat pipes. It is important to ensure that the operating temperature

range is adequate for the application. While most applications involving the use of heat pipes

in the thermal control of electronic devices and systems require the use of a working ﬂuid with