ll 111111111111111 MIT 11/1i~iHi1 l111111111111111111
`
`3 9080 00040 3003
`
`SEC et al. v. MRI
`SEC Exhibit 1019.001
`IPR 2023-00199
`
`

`

`COOLING OF ELECTRONIC EQUIPMENT
`
`SCOTT
`
`0
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`
`SEC et al. v. MRI
`SEC Exhibit 1019.002
`IPR 2023-00199
`
`

`

`Cooling of
`Electronic
`Equipment
`
`SEC et al. v. MRI
`SEC Exhibit 1019.003
`IPR 2023-00199
`
`

`

`Cooling of
`Electronic
`Equipment
`
`ALLAN W. SCOTT
`Varian Associates
`Palo Alto, California
`
`A Wiley-Interscience Publication
`JOHN WILEY & SONS
`New York
`London
`Sydney
`
`Toronto
`
`SEC et al. v. MRI
`SEC Exhibit 1019.004
`IPR 2023-00199
`
`

`

`Copyright© 1974, by John Wiley & Sons, Inc.
`
`All rights reserved. Published simultaneously in Canada.
`
`No part of this book may be reproduced by any means,
`nor transmitted, nor translated into a machine language
`without the written permission of the publisher.
`
`Library of Congress Cataloging in Publication Data
`Scott, Allan W.
`Barker Engineering Library Cooling of electronic equipment.
`"A Wiley-lnterscience publication."
`·
`-
`ncludes bibliographical references .
`
`. Electronic apparatus and appliances-Cooling .
`. Title.
`621.381
`TK7870.S38
`ISBN 0-471-76780-8
`
`73-11154
`
`Printed in the United States of America
`10 9 8 7 6 5 4 3 2 1
`
`SEC et al. v. MRI
`SEC Exhibit 1019.005
`IPR 2023-00199
`
`

`

`Preface
`
`This book is written for electronic engineers, electronic technicians, and
`mechanical engineers who are concerned with the cooling of electronic
`equipment.
`All electronic equipment needs cooling, whether it uses only a few low
`power transistors or many high power tubes. In most equipment, the cooling
`design is as important as the electronic design itself.
`Unfortunately, no good textbooks are available on the cooling of elec(cid:173)
`tronic equipment. The many textbooks on heat transfer and cooling that are
`available are written' for furnace, boiler, and air conditioning designers.
`These standard textbooks concentrate on the derivation of the basic hydro(cid:173)
`dynamic equations and on such practical problems as heat exchanger design
`or the effect of scale in boilers.
`The formulas in the standard heat transfer and cooling textbooks are
`difficult for the electronic engineer to use, because they are expressed in the
`units of the heat exchanger or furnace designer. It is a formidable task for the
`electronic engineer to convert from the BTU per hour, °F, and square feet
`of these formulas to the watts, °C, and inches that are commonly used in
`designing electronic equipment. Even the tables in the standard textbooks on
`cooling are useless to the electronic engineer, who has no need to know the
`thermal conductivity of fire bricks or the thermal properties of carbon
`tetrachloride.
`This book is written to overcome these problems. It is intended for elec(cid:173)
`tronic engineers and is written with a terminology and approach they can
`readily understand.
`I am an electronic engineer and my main interest is in electronic design.
`At first, I tried to ignore the cooling problem, but when some of my finest
`designs "went up in smoke," I realized I had to face the cooling problem
`head-on.
`'!\
`My first step at trying to design cooling for electronic equipment was to
`get out my old college textbooks on hydrodynamics and heat transfer. It was
`easy enough to understand the theory of thermal radiation, forced convection
`of air and liquids, or ev'aporation cooling. Certainly any electronic engineer
`
`V
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`vi
`
`Preface
`
`can understand the concept of Reynolds number, laminar flow, turbulent
`flow, etc. The problem arises in converting the basic hydrodynamic formulas
`into a useful form and into an "electronic engineedng" system of units.
`I soon found that I was not alone among electronic engineers who suffered
`from a lack of literature on how to cool electronic equipment. This ~Q.9k has
`grown out of my need to have simple formulas for designing Jhe_cooling of
`my -own equipment. Soon· I· was distributing my notes to fellow engineers,
`so thafthey could design the cooling for their equipment. Then I was dis(cid:173)
`tributing these notes to our customers, so they could use our equipment
`without burning it up. I have presented the material of this book in university
`extension courses and in special industrial courses to electronic engineers who
`were, out of necessity, also fighting the cooling problem.
`This book presents the cooling formulas which can be found in any stan-
`/ r : mulas are all expressed in terms of power, temperature, dimensions, and the
`dard textbook on hydrodynamics or heat transfer in simple form. The for(cid:173)
`I \ thermal properties of materials .. '!J1e traditional normalizat!o_n. in terms. of
`' \ ·- Reynolds number, Prandtl number, or the other dimensionless constants
`of heat transfer theory has been eliminated. An electronic engineer need not
`·-learn heat transfer theory, ·although he probably already knows it, to be
`able to apply the simple formulas of this book.
`Perhaps most importantly, all the formulas in this book are expressed in
`units of the electronic engineer. Power is in watts, all dimensions are in inches,
`and temperature is in °C. Thermal properties of all materials commonly used
`in electronic equipment are tabulated in the same consistent set of units.
`Air flow and liquid flow are in units which are easily measured in electronic
`equipment. Finally, a complete appendix is provided with conversion tables
`to convert from the other systems of units used for heat transfer calculations
`to the " electronic engineering" systems of units of this book.
`A consistent set of symbols has been used throughout the book. All
`symbols are defined as they are introduced in each chapter, and a summary
`of symbols is presented in the appendix. Each symbol always represents the
`same quantity throughout the book, and is always in the same unit.
`All the important methods of cooling used in electronic equipment are
`covered in this book, including:
`• Conduction.
`• Radiation and natural convection.
`• Forced air cooling.
`• Forced liquid cooling.
`• Liquid evaporation.
`• Heat pipes.
`• Refrigeration and cryogenic cooling.
`Chapter 1 discusses the problem of cooling electronic equipment. The
`
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`Preface
`
`vii
`
`various cooling methods listed above are compared and the design problems
`that each cooling method presents are described.
`Chapters 2 through 6 discuss the basic methods of cooling: conduction,
`radiation and natural convection, forced air cooling, forced liquid cooling,
`and liquid evaporation, respectively. Each chapter begins with a discussion
`of a typical electronic equipment cooled by the method of that chapter.
`Then each chapter presents the necessary design formulas and the properties
`of materials commonly used in electronic equipment that are necessary for
`making calculations with this type of cooling. Where auxiliary equipment is
`required, such as fans or heat exchangers, available equipment and its
`capabilities are discussed. At the end of each chapter, sample calculations
`are given. The first of these sample calculations is usually for the actual
`electronic equipment used as the illustrative example at the beginning of the
`chapter. Finally, a list of references is presented in each chapter for further
`study.
`Chapter 7 discusses heat pipes. Chapter 8 describes refrigerated electronic
`equipment, including cryogenic coolers and thermoelectric cooling modules.
`Chapter 9 discusses transient cooling effects, such as occur in equipment
`warmup.
`The design information presented in this book gives only approximate
`answers. Therefore, after the cooling has been designed and the electronic
`equipment built, the thermal performance of the equipment must be measured.
`Techniques for making the necessary thermal measurements on electronic
`equipment are presented in Chapter 10.
`It would be nice if we electronic engineers could simply avoid the cooling
`problem and get on with our electronic design. Unfortunately, the cooling
`problem won't go away. This book permits the cooling problem to be faced
`and solved simply and quickly.
`I hope it will be of help to you.
`
`Palo Alto, California
`
`Allan W. Scott
`
`SEC et al. v. MRI
`SEC Exhibit 1019.008
`IPR 2023-00199
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`

`

`Contents
`
`J
`1
`Understanding the Cooling Problem
`
`1.1 The Basic Cooling Problem
`1.2 Heat Conduction
`1.3 Choice of Heat Transfer Method
`1.4 Radiation and Natural Convection
`1.5 Forced Air Cooling
`1.6 Forced Liquid Cooling
`1. 7 Cooling by Liquid Evaporation
`1.8 - Heat Pipes
`1.9 Refrigerated Equipment
`1.10 Transient Effects
`1.11 Thermal Measurement Techniques for Electronic Equipment
`1.12 Symbols, Units, and Conversion Factors
`1.13 References
`
`2
`Heat Conduction
`2.1 A Typical Heat Conduction Design
`2.2 The Basic Heat Conduction Formula
`2.3 The Materials of the Heat Sink
`2.4 The Geometry of the Heat Sink
`2.5 Mounting Interfaces
`2.6 Conduction of Heat Through the Cooling Fins
`2. 7 Examples
`2. 8 References
`
`'-, c'-"
`
`1
`
`2
`2
`3
`4
`5
`6
`7
`9
`10
`11
`11
`12
`12
`
`14
`
`14
`17
`18
`23
`25
`30
`35
`43
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`x Contents
`
`J 3
`
`Radiation and Natural Convection
`
`3.1 A Typical Cooling Design Using Radiation and Natural
`Convection
`3.2 Radiation
`3.3 Natural Convection
`3.4 Examples
`3.5 References
`
`J 4
`
`Forced Air Cooling
`4.1 A Typical Forced Air Cooling Design
`4.2 Temperature Rise of the Cooling Air
`4.3 Design Equations for Air Cooling Fins
`4.4 Forced Air Cooling of Racks and Cabinets
`4.5 The Effect of Altitude and Inlet Air Temperature
`4.6 Fans for Cooling Electronic Equipment
`4. 7 Examples
`4.8 References
`
`5
`Forced Liquid Cooling
`
`5. l A Typical Forced Liquid Cooling Design
`5.2 Design Equations for Forced Liquid Cooling
`5.3 Coolant Liquids
`5.4 Coolant Temperature
`5.5 Heat Exchangers, Coolant Pumps, and Auxiliary Equipment
`5.6 Examples
`5. 7 References
`
`44
`
`44
`46
`51
`55
`63
`
`64
`
`65
`69
`70
`75
`80
`86
`95
`106
`
`108
`
`109
`113
`125
`132
`137
`140
`152
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`Contents
`
`xi
`
`6
`Cooling by Liquid Evaporation
`6.1 Typical Evaporation Cooling Designs
`6.2 Design of Evaporation Cooling
`6.3 Cooling System Considerations for High Power Components
`6.4 Liquid Filled Electronics Packages
`6.5 Expendable Coolant Systems
`6.6 Examples
`6. 7 References
`
`7
`Heat Pipes
`
`7.1 A Typical Cooling Design Using Heat Pipes
`7.2 Heat Pipe Design and Fabrication
`7.3 Heat Pipe Performance Limits
`7.4 Example
`7.5 References
`
`8
`Refrigerated Equipment
`
`8.1 Refrigeration of Cooling Liquid or Air
`8.2 Refrigerated Heat Sinks
`8.3 Liquid Nitrogen Baths
`8.4 Thermoelectric Cooling
`8.5 Example
`8.6 References
`
`9
`Transient Effects
`
`9.1 Warmup Characteristics of a Typical Heat Sink
`9.2 Calculation of Transient Effects
`
`153
`
`155
`162
`174
`177
`182
`184
`189
`
`191
`
`193
`194
`196
`199
`203
`
`204
`
`207
`212
`214
`215
`224
`227
`
`228
`
`228
`232
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`xii Contents
`
`9.3 Examples
`9.4 Reference
`
`d- 10
`
`Thermal Measurement Techniques for Electronic Equipment
`
`10.1 Measurement of Temperature and Temperature Distribution
`10.2 Measurement of Air Flow and Air Pressure
`10.3 Measurement of Liquid Flow Rate and Liquid Pressure
`10.4 Thermal Mockups
`10.5 References
`
`Appendices
`
`A. Definition of Symbols
`B. Conversion of Units
`
`Index
`
`234
`240
`
`241
`
`242
`256
`262
`263
`264
`
`267
`
`268
`272
`
`281
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`

`

`1
`
`Understanding the
`Cooling Problem
`
`All electronic equipment needs cooling, whether it uses only a few low power
`transistors or many high power tubes. In most equipment, the cooling design
`is as important as the electronic design itself. A typical electronic cooling
`design is shown in Figure 1. 1. A 10 watt transistor is mounted on a finned
`aluminum heat sink. The transistor is 50 % efficient, so 20 W of electrical
`
`FIGURE 1.1
`Transistor mounted on an aluminum heat sink (Photo courtesy
`of Wakefield Engineering, Inc.)
`
`1
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`2
`
`Understanding the Cooling Problem
`
`input power are converted to 10 W of useful electrical output power and 10 W
`of heat. The 10 W of heat must be conducted from the transistor to the
`cooling fins, where it is then transferred to the surrounding environment.
`Heat always flows from hotter objects to cooler ones. Therefore, the
`transistor must be hotter than the cooling fins, and the cooling fins must be
`hotter than the surroundings. In any electronic equipment, the temperature
`of each component will rise until it is hot enough to transfer its internally
`generated heat to the surroundings. If the cooling design is not adequate,
`the component will get so hot, in an effort to transfer this heat, that it will
`destroy itself.
`
`1.1 THE BASIC COOLING PROBLEM
`
`The purpose of cooling electronic equipment is to keep the temperature
`of each component below its safe operating value.
`The electric component temperature is determined from:
`T (component)= AT (conduction)+ AT (transfer)+ T (surroundings)
`where:
`T (component)
`AT (conduction)
`
`(1.1)
`
`AT (transfer)
`
`is the temperature of the electronic component.
`is the temperature difference required to conduct the
`heat from the electronic component to the cooling
`fins.
`is the temperature rise of the fins above the sur(cid:173)
`roundings required to transfer the heat from the fins
`to the surrounding environment.
`T (surroundings) is the temperature of the surrounding environment.
`As Equation 1.1 shows, the cooling of electronic equipment consists of
`two parts:
`1. Conduction of heat from the electronic component to the cooling ~ns.
`2. Transfer of heat from the cooling fins to the environment.
`This book provides the necessary design information so that the tempera(cid:173)
`ture rises required for heat conduction and heat transfer can be calculated,
`so that the component temperature can be kept within acceptable limits.
`
`1.2 HEAT CONDUCTION
`
`The following factors must be considered in the conduction of heat
`from the electronic component to the cooling fins:
`1. The materials used for conducting heat.
`2. The geometry of the heat flow paths.
`
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`1.3 Choice of Heat Transfer Method
`
`3
`
`3. Thermal interfaces where parts are joined together.
`4. Conduction of heat through the cooling fins themselves.
`
`Complete design information which considers all the above factors, and
`which permits_.i( calculation of the conduction temperature rise, AT (con(cid:173)
`duction) of Equation 1.1, is presented in Chapter 2.
`
`1.3 CHOICE OF HEAT TRANSFER METHOD
`
`Once the heat has been conducted from the electronic component to
`the cooling fins, it must then be transferred to the surrounding environment
`by one of the following means:
`
`1. Radiation and natural convection.
`2. Forced air cooling.
`3. Forced liquid cooling.
`4. Liquid evaporation.
`
`The above list of heat transfer methods is arranged in order of increasing
`heat transfer effectiveness. For a given fin area, the least heat can be trans(cid:173)
`ferred by radiation and natural convection, more can be transferred by
`forced air cooling, even more can be transferred by forced liquid cooling,
`and the most can be transferred by liquid evaporation.
`The list is also arranged in order of increasing cooling system complexity.
`Heat transfer by radiation and natural convection requires no auxiliary
`equipment-just the cooling fins themselves-and is the simplest design.
`Forced air cooling requires a fan and fan controls and is more complicated.
`Forced liquid cooling requires a pump, coolant reservoir, cooling fluid, etc.,
`and is even more complicated.
`The effectiveness of each of the above methods of heat transfer is compared
`in Figure 1.2, which shows the heat that can be transferred per square inch
`of surface, when the fin surface is at I 00°C and the surroundings are at 20°C.
`For any one heat transfer method the amount of power that can be trans(cid:173)
`ferred varies over a wide range, depending on the details of the cooling
`design.
`Figure 1.2 also illustrates that for most applications more than one heat
`transfer method could be used. The best choice depends on a tradeoff between
`system simplicity, heat sink complexity, and fin area.
`The problems of the design of heat transfer by each of the four methods(cid:173)
`radiation and natural convection, forced air cooling, forced liquid cooling,
`and liquid evaporation-are summarized in the next four sections of this
`chapter. Complete design information on each heat transfer method is
`presented in Chapters 3 through 6 respectively. Each of these chapters begins
`
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`

`

`4
`
`Understanding the Cooling Problem
`
`RELATIVE EFFECTIVENESS OF HEAT TRANSFER METHODS
`
`Radiation and nat-
`ural convection
`t
`
`I
`
`Forced air
`
`I
`
`T (surface)
`T (surroundings)
`
`= 100°c
`= 20°c
`
`I
`
`Forced liquid
`
`t
`
`I
`
`I
`
`t
`
`Liquid evaporation
`
`.1
`
`10
`Heat transferred (watts per inch 2 )
`
`100
`
`FIGURE 1.2
`Relative effectiveness of heat transfer methods
`
`I
`
`1000
`
`1'
`
`with a typical example of heat transfer in electronic equipment, and the
`amount of heat transferred per square inch of surface in these examples is
`shown by the arrows in Figure 1.2.
`
`1.4 RADIATION AND NATURAL CONVECTION
`
`Radiation and natural convection are the simplest to use of all the
`heat transfer methods. No auxiliary equipment is required, just the cooling
`fins themselves. The hot fins radiate heat directly to the cooler sµrroundings.
`At the same time, the air near the hot fins is heated and rises and is replaced
`by cooler air. This convective air current provides additional heat transfer.
`A typical heat sink cooled by radiation and natural convection was shown
`in Figure I. I.
`In most electronic equipment, heat transfer occurs by radiation and
`natural convection simultaneously. However, the amount of heat transferred
`by each method depends on heat sink temperature, geometry, and orientation
`in different ways. For a particular electronic cooling design, the heat trans(cid:173)
`ferred by each method must be calculated separately and then combined.
`The amount of heat that can be transferred by radiation depends on:
`I. The temperature of the radiating surface.
`2. The temperature of the surroundings.
`3. Surface conditions of the fins.
`4. Shielding effects of adjacent fins.
`
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`1.5 Forced Air Cooling
`
`5
`
`The amount of heat that can be transferred by natural convection depends
`on the following factors:
`
`1. Temperature difference between the surface and the surrounding air.
`2. Dimensions of the surface.
`3. Orientation of the surface.
`4. Spacing between adjacent surfaces.
`5. Altitude (which determines air density).
`
`Complete design information on heat transfer by radiation and natural
`convection, which considers all the above factors, and which permits a
`calculation of the transfer temperature rise, !l.T (transfer) of Equation 1.1, is
`presented in Chapter 3.
`
`1.5 FORCED AIR COOLING
`
`As Figure 1.2 shows, the amount of heat that can be transferred from
`a given cooling fin area is increased by more than an order of magnitude by
`blowing air over the cooling fins, rather than relying on radiation and natural
`convection. Forced air cooling is more complicated to implement than
`cooling by radiation and natural convection, because a fan and the associated
`fan controls are required. Forced air cooling is, however, much simpler than
`forced liquid cooling, because a supply of cooling air is readily available and
`air does not have the freezing, boiling, or dripping problems of cooling
`liquids.
`A typical electronic cooling design using forced air cooling is shown in
`Figure 1.3. The design of air cooling poses two problems:
`
`1. Choice of the fan or blower.
`2. Design of the cooling fin geometry.
`
`FIGURE 1.3
`Forced air cooled heat sink (Photo
`courtesy of Wakefield Engineering)
`
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`

`6
`
`Understanding the Cooling Problem
`
`These two problems must be solved jointly. The amount of air flow that a
`particular fan can provide is determined by the pressure into which the fan
`must work. Both the amount of heat transfer that can be obtained from
`forced air cooling and the pressure required to force air through the cooling
`fins depends on air flow and fin geometry. Consequently, the choice of fan
`must be made in conjunction with the fin design.
`Complete design information which permits the most suitable fan and the
`most optimum cooling fin design to be chosen is presented in Chapter 4.
`Formulas are presented which permit a calculation of the heat transfer
`temperature rise, AT (transfer) of Equation 1.1, for the case of forced air
`cooling. The effects of high altitude operation on forced air cooling are
`included.
`
`1.6 FORCED LIQUID COOLING
`
`Forced air cooling, although simpler to implement than forced liquid
`cooling, does have several disadvantages. Forced air cooling may not be
`suitable for electronic equipment which must be operated at high altitudes
`where air density is low. The acoustic noise of the fan may be objectionable
`and the vibration of the fan may adversely affect the performance of the
`electronic equipment. The hot air ejected from the electronic package may
`also be objectionable.
`All of these disadvantages of forced air cooling are eliminated by the use of
`forced liquid cooling. The cooling pumps and coolant reservoir can be
`removed from the electronic package, so that quiet, vibration-free operation
`can be maintained. As shown in Figure 1.2, the use of forced liquid cooling
`provides an order of magnitude greater heat transfer per unit area than
`forced air cooling. For very high power electronic equipment, this greater
`heat transfer capability is a necessity, because the cooling fin area cannot be
`made great enough to transfer heat by forced air cooling. The use of liquid
`cooling also permits high density mounting of lower power electronic
`components. This high density mounting may not only be desirable from a
`packaging standpoint; it may be a necessity to achieve required electronic
`performance.
`A typical forced liquid cooling design of an electronic component is shown
`in Figure 1.4. The particular component is the liquid cooled collector of a
`high power microwave transmitter tube. The coolant inlet and outlet fittings
`and the multiple liquid cooling ducts can be seen in the cutaway view.
`The following factors must be considered in designing forced liquid
`cooling of electronic equipment:
`
`1. The design of the cooling ducts.
`
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`

`1.7 Cooling by Liquid Evaporation
`
`7
`
`FIGURE 1.4
`Forced liquid cooled collector of a high
`power microwave transmitter tube
`(Photo courtesy of Varian Associates)
`
`2. The type of cooling liquid.
`3. The effect of coolant inlet temperature.
`4. The coolant pump, heat exchanger, and other auxiliary equipment.
`
`Complete design information for forced liquid cooling, which considers
`all of the above factors, is given in Chapter 5. Formulas are presented which
`permit a calculation of the transfer temperature rise, AT (transfer) of Equation
`1.1, for forced liquid cooling.
`
`1.7 COOLING BY LIQUID EVAPORATION
`
`Evaporation cooling can be effectively used in the following different
`ways in electronic equipment:
`
`1. Cooling of high power components at high power densities.
`2. Cooling of all components in an electronic equipment by immersing the
`entire assembly in a package filled with dielectric oil.
`3. Maintaining a constant temperature bath for electronic components.
`4. Simple expendable cooling systems.
`
`SEC et al. v. MRI
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`

`8
`
`Understanding the Cooling Problem
`
`Figure 1.5 shows an electron tube cooled by evaporation. The anode of the
`tube, where heat is generated, is immersed in liquid. Heat is transferred from
`the tube by boiling the liquid. The vapor from the boiling process rises from
`
`FIGURE 1.5
`Cooling of a transmitter tube by liquid evaporation (Photo courtesy of ITT)
`
`the liquid bath, is condensed in a condenser, and is then returned to the
`cooling bath. The condenser heat transfer area can be made large enough,
`because it is separated from the electronic equipment, so that heat can be
`transferred from it to the surroundings by radiation and convection or by
`forced air cooling.
`No coolant pumps are needed for evaporation cooling. Consequently, it is
`an extremely simple cooling method. As shown in Figure 1.2, evaporation
`cooling provides greater heat transfer per unit surface area than any of the
`other methods.
`The disadvantages of cooling by evaporation are:
`
`1. The system can operate in one orientation only, with the liquid bath
`at the lowest point in the system.
`
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`1.8 Heat Pipes
`
`9
`
`2. Immediate destruction of the electronic component occurs if the
`maximum heat transfer rate is exceeded, because the temperature of the
`component increases very rapidly above the critical heat transfer rate.
`
`In designing cooling by evaporation, the following factors must be con-
`sidered:
`
`I. Choice of the cooling fluid.
`2. Design of the component surface where evaporation occurs.
`3. Design of the condenser.
`4. Pressure and pressure equalization.
`5. Equipment orientation.
`
`Complete design information for heat transfer by liquid evaporation,
`including consideration of all the above factors, is presented in Chapter 6.
`
`1.8 HEAT PIPES
`
`The effective cooling design of electronic equipment requires mm1-
`mizing both the temperature rise required for conducting heat fro.m the
`electronic component to the cooling fins and the temperature rise required
`for transferring heat from the cooling fins to the surroundings. As shown in
`Equation 1.1, both factors contribute to the operating temperature of the
`electronic component.
`In many designs sufficient space may be available for a large number of
`cooling fins, but the major problem is conducting the heat from the elec(cid:173)
`tronic component to the cooling fins. Even if the heat sink is made of copper,
`the temperature rise due to conduction may be excessive.
`Heat pipes offer a solution to this problem. A schematic drawing of a heat
`pipe is shown in Figure 1.6. The heat pipe consists of a hollow tube which
`
`section
`
`Radiator-
`
`r Evaporator ITran~port
`
`Heat in
`
`:)}/ii,su_l!l.ti~~. :: ::
`Capillary---~~~===~:==~~~~=~:=::~~D-i
`structure
`-
`-
`(wick)
`
`Heat in
`
`FIGURE 1.6
`Schematic drawing of a heat pipe
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`10 Understanding the Cooling Problem
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`has been evacuated and then filled with a coolant liquid. The incident heat
`evaporates the liquid at one end of the pipe and the vapor transports the
`heat to the other cooler end of the pipe. At· the cool end of the pipe the
`liquid condenses and transfers the heat. So far, this means of heat transfer
`is the same as evaporation cooling, discussed in the previous section. How(cid:173)
`ever, the heat pipe has an additional feature that permits it to be operated in
`any orientation. The inner surface of the heat pipe contains a capillary
`structure or "wick" which returns the condensed liquid to the hot evaporator
`end of the pipe by capillary action. The heat pipe can therefore even operate
`against gravity, that is, with its evaporator end upward.
`For a given temperature rise, a heat pipe can conduct several orders of
`magnitude more heat than a solid copper rod of the same diameter. Con(cid:173)
`sequently, heat pipes are finding increasing use for the conduction of heat in
`electronic equipment.
`Complete information on the use of heat pipes in electronic equipment is
`presented in Chapter 7.
`
`1.9 REFRIGERATED EQUIPMENT
`
`The purpose of cooling electronic equipment is to keep the temperature
`of the electronic components at some desired temperature. With the methods
`of cooling discussed in all the previous sections, the electronic component
`is always hotter than the temperature of the surroundings. Heat flows from
`hot to cold bodies, so the electronic component had to be the hottest element
`in order to transfer its heat to the surroundings.
`If necessary for achieving the desired electronic performance, the electronic
`component can be cooled to a temperature below the temperature of the
`surroundings by the use of refrigeration. In refrigerated equipment, heat
`does not flow from the electronic component, but is "pumped" by the
`refrigeration system from the cold component to the hot surroundings.
`Examples of electronic equipment where refrigeration is required are:
`
`• Infra-red detectors.
`• Masers.
`• Parametric amplifiers.
`Equipment which must work in surroundings which are at a higher
`temperature than the safe operating temperature of the electronic
`components.
`
`Refrigeration is not a substitute for good conventional cooling design. If
`the electronic component can be hotter than the surroundings, then by
`careful design, the heat can always be transferred from the component by
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`1.11 Thermal Measurement Techniques
`
`11
`
`conduction, radiation and natural convection, forced air cooling, forced
`liquid cooling, or evaporation cooling, and refrigeration is not necessary.
`Refrigeration systems for electronic equipment may be classified into the
`following types:
`
`• Refrigerated cooling air or cooling liquid.
`• Refrigerated heat sinks.
`• Liquid nitrogen baths.
`• Thermoelectric coolers.
`
`These refrigeration systems are discussed in detail in Chapter 8.
`
`1.10 TRANSIENT EFFECTS
`
`Chapters 2 through 8 all consider steady state conditions. Under steady
`state conditions, heat is transferred to the surroundings as fast as it is gener(cid:173)
`ated by the electronic component, and the temperatures of all elements-the
`component itself, the heat sink, the cooling fins, and the cooling air or liquid
`-remain constant with time.
`However, when electronic equipment is first turned on, or when the power
`input or the coolant flow rates are changed, the temperature of the electronic
`component and all other elements in the cooling system vary with time, until
`the equipment reaches its steady state temperature. Because many electronic
`components change their electrical characteristics when their temperature
`changes, the transient thermal characteristics of the equipment must be
`considered.
`Design information on transient thermal conditions in electronic equip(cid:173)
`ment is presented in Chapter 9.
`
`1.11 THERMAL MEASUREMENT TECHNIQUES FOR
`ELECTRONIC EQUIPMENT
`
`After the cooling of a particular electronic equipment has been designed,
`and the equipment has been built, the thermal performance of the equipment
`must be measured. All cooling design formulas give only approximate results,
`so experimental measurements are essential.
`The thermal measurements that must be made on electronic equipment
`include:
`
`1. Temperature of the electronic component, the heat sink, and the cooling
`fins.
`2. Temperature distribution through the various parts.
`3. Air flow rate.
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`12 Understanding the Cooling Problem
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`4. Air pressure.
`5. Liquid flow rate.
`6. Liquid pressure.
`
`Techniques for making the necessary thermal measurements on electronic
`equipment are presented in Chapter 10.
`
`1.12 SYMBOLS, UNITS, AND CONVERSION FACTORS
`
`A consistent set of units has been used throughout this book. Each
`symbol is defined as it is introduced in the text, and each symbol stands for
`the same quantity throughout the book. The units in which each symbol is
`measured are the same throughout the book. A listing of each symbol and
`the units in which it is measured is also presented in Appendix A.
`This book is intended for use by practicing engineers for the design of
`cooling of electronic equipment. For this reason the units of measurement
`used in all formulas have been chosen to be the easiest for electronic equip(cid:173)
`ment designers to use. For example, power or heat is expressed in watts,
`rather tl)!ln in British thermal unit