SEMINAR REPORTON

“HEAT PIPEHEAT PIPE”

SUBMITED BY

Mr. Balaji M.Chavan

Under the guidance of

Prof. U. A. Shinde. Prof. R.G.Biradar.

Co-Guide Guide

Department of Mechanical Engineering

S. V. E. R.I.’s

COLLEGE OF ENGINEERING, PANDHARPUR

2003-2004

1

HEAT PIPE

S. V. E. R.I.’s

COLLEGE OF ENGINEERING, PANDHARPUR

CERTIFICATECERTIFICATE

This is to certify that the seminar report entitled

“ HEAT PIPE ”“ HEAT PIPE ”

has been carried out

By

Mr. BALAJI M.CHAVAN

of B.E. ( MECHANICAL ) class in partial fulfillment

for award of Bachelor’s Degree in Mechanical

Engineering as per curriculum laid down by the

SHIVAJI UNIVERSITY, KOLHAPUR during the

academic year 2003-2004.

(Prof. U. A. SHINDE) (Prof. R. G.BIRADAR) Co-Guide Guide

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(Prof. S. A.PATIL) (Prof. B. P. RONGE) H.O.D. Principal

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ABSTRACT

The transfer of the heat energy by conduction using solid

material is essentially limited by thermal conductivity of that material. As

the thermal conductivity increases cost of the material also increases

hence it become costly. Because of the thermal energy is being

transported by evaporation–condensation process rather than conduction.

The heat pipe can transfer the heat much more effectively than the solid

conductor of the same cross-section in practice conduction of heat by heat

pipe may be several hundred (500) times that the best available metal

conductor such as copper.

Heat pipe system provides the maximum effective heat sink

surface area with the minimum volume demand. A heat pipe heat sink is a

passive cooling device that requires no moving parts, and operates silently

and reliably. Additionally, heat pipe technology is emerging as a cost-

effective thermal design solution. This paper explains the operation of heat

pipes and its various applications.

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HEAT PIPE

INDEXINDEX

SR.NO. CONTENTS PAGE NO.

1 HISTORY 1

2 INTRODUCTION 2

3 WORKING 3

4 DESIGN CONSIDERATIONS 4

5 OPERATING LIMITATIONS 10

6 APPLICATIONS 13

7 ADVANTAGES 19

8 DISADVANTAGES 20

9 CONCLUSION 21

10 REFERENCES 22

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HEAT PIPE

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 on to the analysis

faddist various material properties and geometries

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HEAT PIPE

Introduction

What is a Heat Pipe?

A heat pipe is a simple device that can quickly transfer heat from

one Point to another. By means of evaporation & condensation of fluid in

a sealed system they are often referred to as the "superconductors" of

heat as they possess an extra ordinary heat transfer capacity & rate with

almost no heat loss.

It consists of a sealed aluminum or copper container whose inner

surface have a capillary wicking material. The working fluid is placed

inside it &it is highly evacuated. Because of that the working fluid is

virtually in a state of liquid-vapour equilibrium. consequently, a slight

increase in temperature will cause it to boil &evaporate The central

portion of it is heavily insulated on the outside. One end of pipe is known

as heating end (evaporator) where heat is absorbed & the other end is

known as cooling end (condenser) where heat is given out.

A heat pipe is similar to a thermosyphon. It differs from a

thermosyphon by Virtue obits ability to transport heat against gravity by

an evaporation –condensation cycle with the help of porous capillaries

that form the wick. The wick provides the capillary driving force to return

the condensate to the evaporator. The quality and type of wick usually

determines the performance of the heat pipe, for this is the heart of the

product. different types of wicks are used depending on the application

for which the heat pipes being used

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HEAT PIPE

Working

Inside the container is a liquid under its own pressure, that enters the

pores of the capillary material, wetting all internal surfaces. Applying

heat at any point along the surface of the heat pipe causes the liquid at

that point to boil and enter a vapor state. When that happens, the liquid

picks up the latent heat of vaporization. The gas, which then has a

higher pressure, moves inside the sealed container to a colder location

where it condenses. Thus, the gas gives up the latent heat of

vaporization and moves heat from the input to

Fig. Working of the heat Pipe

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HEAT PIPE

Design Considerations

The three basic components of a heat pipe are:

1. the container

2. the working fluid

3. the wick or capillary structure

The choice of each component has marked effect on the working

Performance of heat pipe and therefore proper selection of each

Component is very important in design of heat pipe. Following explanation

is given below

1. Container

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 transfer of heat to take

place from and into the working fluid.

Selection of the container material depends on many factors. These

are as follows:

Compatibility (both with working fluid and external environment)

Strength to weight ratio

Thermal conductivity

Ease of fabrication, including welding, machine ability and ductility

Porosity

Wettability

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Most of the above are self-explanatory. A high strength to weight ratio

is more important in spacecraft applications. The material should be non-

porous to prevent the diffusion of vapor. A high thermal conductivity

ensures minimum temperature drop between the heat source and the

wick.

Material used for heat pipe is stainless steel; copper; aluminum,

ceramic material, glass etc depending on temperature range Usually they

are in tubular form but it can be constructed in any shape such as Y, T, U,

etc. depending upon requirement Effect of length & diameter on The heat

transfer capacity of heat pipe is specified by the “axial power

rating”(APR)Which is energy moving axially along the pipe larger the

diameter; greater will be the APR for the given length ;a 5 mm dia.&15cm

long pipe has an APR of 75 watts which increases to 500 watts if dia. Is

increased to20mm. The physical size of heat pipe that have been

operated successfully range from 6 mm to 150mm in dia. & up to 5 miter

long in length

2. Working fluid

A first consideration in the identification of a suitable working fluid

is the Operating vapour temperature range. Within the approximate

temperature band, several possible working fluids may exist, 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:

compatibility with wick and wall materials

good thermal stability

wettability of wick and wall materials

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vapour 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

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. 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 i.e. contact angle should be zero or

very small. The vapor pressure over the operating temperature range

must be sufficiently great to avoid high vapor velocities, which tend to

setup 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

temperature gradient and to reduce the possibility of nucleate boiling at

the wick or wall surface..

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HEAT PIPE

Tabulated below are a few mediums with their useful

ranges of temperature.

MEDIUM MELTING

PT. ( C)

BOILING

PT. AT

ATM.

PRESSURE

( C)

USEFUL

RANGE

( C)

Helium

Nitrogen

Ammonia

Acetone

Methanol

FlutecPP2

Ethanol

Water

Toluene

Mercury

Sodium

Lithium

Silver

-271

-210

-78

-95

-98

-50

-112

0

-95

-39

98

179

960

-261

-196

-33

57

64

76

78

100

110

361

892

1340

2000

-271 to -269

-203 to -160

-60 to 100

0 to 120

10 to 130

10 to 160

0 to 130

30 to 200

50 to 200

250 to 650

600 to 1200

600 to 1200

1000 to 1800

1000 to 1800

1800 to 2300

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3. Wicks or Capillary Structure

It is a porous structure made of materials like steel, aluminum,

nickel or copper in various ranges of pore sizes. They are fabricated using

metal foams, and more particularly felts, the latter being more frequently

used. By varying the pressure on the felt during assembly, various pore

sizes can be produced. By incorporating removable metal mandrels, an

arterial structure can also be molded in the felt.

Fibrous materials, like ceramics, have also been used widely. They

generally have smaller pores. The main disadvantage of ceramic fibers is

that, they have little stiffness and usually require a continues support by a

metal mesh. Thus while the fiber itself may be chemically compatible with

the working fluids, the supporting materials may cause problems. More

recently, interest has turned to carbon fibers as a wick material. Carbon

fiber filaments have many fine longitudinal grooves on their surface, have

high capillary pressures and are chemically stable. A number of heat pipes

that have been successfully constructed using carbon fibre wicks seem to

show a greater heat transport capability.

The prime purpose of the wick is to generate capillary pressure to

transport the working fluid from the condenser to the evaporator. It must

also be able to distribute the liquid around the evaporator section to any

area where heat is likely to be received by the heat pipe. Often these two

functions require wicks of different forms. The selection of the wick for a

heat pipe depends on many factors, several of which are closely linked to

the properties of the working fluid.

The maximum capillary head generated by a wick increases with

decrease in pore size. The wick permeability increases with increasing

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HEAT PIPE

pore size. Another feature of the wick, which must be optimized, is its

thickness. The heat transport capability of the heat pipe is raised by

increasing the wick thickness. The overall thermal resistance at the

evaporator also depends on the conductivity of the working fluid in the

wick. Other necessary properties of the wick are compatibility with the

working fluid and wettability. The most common types of wicks that are

used are as follows:

Sintered Powder

This process will provide high power handling, low temperature

gradients and high capillary forces for anti-gravity applications. The

photograph shows a complex sintered wick with several vapor channels

and small arteries to increase the liquid flow rate. Very tight bends in the

heat pipe can be achieved with this type of structure.

Grooved Tube

The small capillary driving force generated by the axial grooves is

adequate for low power heat pipes when operated horizontally, or with

gravity assistance. The tube can be readily bent. When used in

conjunction with screen mesh the performance can be considerably

enhanced.

Screen Mesh

This type of wick is used in the majority of the products and

provides readily variable characteristics in terms of power transport and

orientation sensitivity, according to the number of layers and mesh counts

used.

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HEAT PIPE

Operating Limitations

Since the heat pipe benefits from the phase change of the working

fluid, the thermodynamics of the process are critical. The operation of the

heat pipe is limited by several operating phenomena. Each of these

limitations is dependant on the wick structure, working fluid, temperature,

orientation, and size of the heat pipe. Below is a brief description of each

of the limitations:

Capillary Limit

The wick structure of the heat pipe generates a capillary pressure,

which is dependent on the pore radius of the wick and the surface tension

of the working fluid. The capillary pressure generated by the wick must be

greater than the sum of the gravitational losses, liquid flow losses through

the wick, and vapor flow losses. The liquid and vapor pressure drops are a

function of the heat pipe and wick structure geometry (wick thickness,

effective length, vapor space diameter, etc) and the fluid properties (latent

heat, density, viscosity, etc). A critical heat flux exists that balances the

capillary pressure with the pressure drop associated with the fluid and

vapor circulation. For horizontal or against gravity (evaporator at a higher

elevation than the condenser), the capillary limit is the heat pipe limit. For

gravity-aided orientations, the capillary limitation may be neglected, and

the flooding limit may be used if the heat pipe can have an excess fluid

charge.

Boiling Limit

As more heat is applied to the heat pipe at the evaporator, bubbles

may be formed in the evaporator wick. The formation of vapor bubbles in

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the wick is undesirable because they can cause hot spots and obstruct the

circulation of the liquid.

As the heat flux is increased, more bubbles are formed. At a certain

heat flux limit, the bubble formation completely blocks the liquid flow. This

limitation is associated to a radial heat flux (heat is applied to the

perimeter of the heat pipe). The boiling limitation is typically a high

temperature phenomenon. Heat flux limitations for various wick structures

should be used for design criteria. Sintered powder metal wick structures

have significantly more surface area, and can therefore handle higher heat

fluxes. Conservative values are 50 W/cm2 for powder metal wicks, 10

W/cm2 for screen wicks, 5 W/cm2 for bare wall thermosyphons.

Sonic Limit

In a heat pipe of constant vapor space diameter, the vapor flow

accelerates and decelerates because of the vapor addition in the

evaporator and the vapor removal in the condenser. The changes in vapor

flow also change the pressures along the heat pipe. As more heat is

applied to the heat pipe, the vapor velocities generally increase. A choked

flow condition will eventually arise, where the flow becomes sonic. At this

point, the vapor velocities can not increase and a maximum heat transport

limitation is achieved. The heat flux that results in choked flow is

considered the sonic limit. The addition of more heat will result in an un

proportional increase in the heat pipe temperature delta by an increase in

the evaporation temperature. This phenomenon is self-correcting as the

heat pipe warms up. An additional benefit of the high vapor velocities is

the very quick response to heat input.

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HEAT PIPE

Entrainment Limit

Since the vapor and the liquid move in opposite directions in a heat

pipe, a shear force exists at the liquid-vapor interface. If the vapor velocity

is sufficiently high, a limit can be reached at which the liquid will be torn

from the pores of the wick and entrained in the vapor. When enough fluid

is entrained in the vapor that the condensate flow is stopped, abrupt dry-

out of the wick at the evaporator results. The corresponding heat flux that

results in this phenomenon is called the Entrainment Limit. The

Entrainment Limit is typically not the bounding value.

Flooding Limit

The flooding limit is only applicable to gravity aided orientations

with excess fluid. The wick structure is saturated and the excess fluid

results in a “puddle” flow on the surface of the wick structure. The flooding

limit, similar to the entrainment, occurs when high vapor velocities

preclude the fluid that is flowing on the surface of the wick to return to the

evaporator. The vapor shear hold up prevents the condensate from

returning to the evaporator and leads to a flooding condition in the

condenser section. This causes a partial dry-out of the evaporator, which

results in wall temperature excursions or in limiting the operation of the

system. By increasing the heat flux above the flooding limit, it is possible

to achieve liquid flow reversal leading to:

1) The accumulation of liquid in the condenser.

2) The accumulated liquid falling to the evaporator due to gravity.

3) The reestablishment of a flow reversal situation.

4) The repeat cycle of flooding and normal flow.

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Fig. Cooking pin

HEAT PIPE

Applications

Heat pipe has been, and is currently being, studied for a variety of

applications, covering almost the entire spectrum of temperatures

encountered in heat transfer processes. Heat pipes are used in a wide

range of products like air-conditioners, refrigerators, heat exchangers,

transistors, capacitors, etc. 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. We shall now discuss a brief account of the various

applications of heat pipe technology.

1.Heat pipe used for extracting solar energy

Solar radiations are focused on the heat pipe at the evaporator

side by a parabolic reflector. Heat pipe leading from the reflector could be

coupled to steam raising boilers or end of heat pipe directly used as

cooking plate.

A recent use of heat pipe in kitchen is a cooking pin. This is a

simply a heat pipe which when inserted into joint of meat cooking in an

oven, speedup the rate of heat flow. Thus saving the time &fuel &yielding

a more uniform roast. A particular type is shown below.

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HEAT PIPE

2. Heat dissipation device

In the electronic devices, various components (such as I.C.s,

capacitors etc.) generates the heat .The performance of these

components decreases with increase in temperature. Hence heat

dissipation is necessary. For this fan is directly placed over the device &

hence occupy most valuable real estate.

Here heat pipe play very imp. role in theses cases heat pipe is

employed to transfer the heat from small area available on the component

to a larger area where the heat is released to atmosphere. This is

achieved by keeping the evaporator of the heat pipe .In contact with the

electronic device &the condenser gives the heat to the large surface area.

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Chip which to be cooled

Heat Pipe

Fig .Heat Deceptions Device

HEAT PIPE

3.Temperature Control device

In some applications, it is required to maintain the temperature of

device to specific value, heat pipe can be used for the same application

with small modifications A reservoir containing a non-condensable gas is

connected to the heat removal end (the condenser) of the heat pipe. This

gas forms an interface with the vapor & “chokes off” part of condenser

area of heat pipe. As the temperature of the device increases the vapor

pressure inside the pipe & the non-condensable gas is forced back into

reservoir, thereby opening up additional condenser area to give more heat

flow. Thus finally reduces the temperature of the device& vice-versa

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Condenser Length

Fig . Temperature Control Device.

Non Condensable Gas

HEAT PIPE

4. Dry drilling

For the proper drilling the temperature of the drill must be low; for

this we are using coolant which Is very costly & goes waste after use .By

using heat pipe we can solve this problem .As here liquid is not used for

cooling, it is called dry drilling. Fig shows distribution of heat in drill & use

of heat pipe

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Distribution of heat in drill

Heat pipe used in drill shank

HEAT PIPE

5. Injection moulding and die casting

Fig. Heat pipe used for Injection Moulding &Die Casting

Heat Pipes are widely used for improving cooling efficiency of

injection moulds and Die casting dies all over the world. This method of

cooling has helped reduce cycle time, reduce rejection and improves

quality of product. Sketches given inside the folder describe various

applications where one can confidently use Heat Pipes. These are taken

from actual examples of moulds, which are in production all over India. In

conventional water-cooling, effectiveness of water cooling goes down due

to rusting, blocking of cooling channels. In case of Heat Pipes since water

is not circulated directly in the core, cooling efficiency remains the same

throughout the life of the mould.

Good Reasons To Use Heat Pipes:

1. Reduce cycle time 2. Eliminate hot spots

3. Reduce wastage 4. Improve product quality

5. Increase mould life 6. Eliminate core clogging

7. Cut mould and Moulding costs 8. Upgrade old moulds

9. Use damaged moulds 10. Eliminate hot spots

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HEAT PIPE

6. Heat Pipe Heat Exchanger

Typical finned air-to-air Heat Pipe heat exchangers comprise of

number of tubular gravity assist. Air to air Heat Pipe teed finned Heat

Pipes arranged in staggered pitch, depending upon the application. One of

the advantages of the Heat Pipe Heat Exchanger is its ability to operate

without cross contamination between the two gas streams. Use of Gravity

Assisted Heat Pipe complies orientation evaporator above condenser.

7. Application for I.C.engine

Heat pipe uses energy of exhaust gas for homogeneous vaporizing

the fuel, which is coming after carburettor. Therefore fuel consumption is

decreased.

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HEAT PIPE

Advantages of the heat pipes

Rate of heat transfer is very high than the solid material.

It has no moving parts hence maintains is not required

It can transmit heat over the appreciable distance without loss of

the heat (i. e isothermal). And thus permitting separation of the

heat source and sink

It require no power source to accomplish this function

It can transfer the heat where a very low temperature difference is

available in between source and sink.

It is ideal device for removing the heat from a concentrated heat

source such as thermocore.

It is rugged like any piece of pipe or tube and has no any wearing

part hence it has long life.

The absence of the gravity does not affect the operation of the heat

pipe determinately liquid flow does not depend upon gravity

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HEAT PIPE

Disadvantages

Like any other practical devices, heat pipe has also

disadvantages as listed below:

Undesired increase in point-to-point temperature differential along

the heat pipe can lead to damage to evaporator section

Length of heat pipe is limited

Design is complicated

The cost of a given heat pipe will tend to reach a minimum in the

temperature range of 70 to 120 degree C. But above &below this

range, total cost of heat pipe will be more

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HEAT PIPE

Conclusion

Now days we requires the transfer of heat from one place

(source) to other place (sink) very fatly, without loss of energy &

economically. These requirements are fulfilled by heat pipe. Presently it

plays very important role in thermal science .it is widely used all over the

world for improving efficiency & rate of heat transfer. It is presently used in

space technology, thermal power stations, home applications etc has. It

has very bright future.

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HEAT PIPE

References BOOKS

Heat & Mass Transfer………….S.C.Arora & S. Domkundwar

Heat Transfer…………………...Pavaskar.

WEBSITES

www.google.com

www.heat pipe .com

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