(NATO science series 99) dr s kakaç (auth ), s kakaç, h f smirnov, m r avelino (eds ) low temperature and cryogenic refrigeration springer netherlands (2003) (4)

Low Temperature and Cryogenic Refrigeration NATO Science Series A Series presenting the results of scientific meetings supported under the NATO Science Programme The Series is published by lOS Press,.

Low Temperature and Cryogenic Refrigeration

NATO Science Series

A Series presenting the results of scientific meetings supported under the NATO Science Programme

The Series is published by lOS Press, Amsterdam, and Kluwer Academic Publishers in conjunction with the NATO Scientific Affairs Division

Sub-Series

I Life and Behavioural Sciences

II Mathematics, Physics and Chemistry

III Computer and Systems Science

IV Earth and Environmental Sciences

V Science and Technology Policy

lOS Press Kluwer Academic Publishers lOS Press

Kluwer Academic Publishers lOS Press

The NATO Science Series continues the series of books published formerly as the NATO ASI Series The NATO Science Programme offers support for collaboration in civil science between scientists of countries of the Euro-Atlantic Partnership Council The types of scientific meeting generally supported are "Advanced Study Institutes' and "Advanced Research Workshops", although other types of meeting are supported from time to time The NATO Science Series collects together the results of these meetings The meetings are co-organized bij scientists from NATO countries and scientists from NATO's Partner countries - countries of the CIS and Central and Eastern Europe

Advanced Study Institutes are high-level tutorial courses offering in-depth study of latest advances

Low Temperature and Cryogenic Refrigeration

State University of Estado,

Rio de Janeiro, Brazil

Springer Science+Business Media, B.V

Proceedings of the NATO Advanced Study Institute on

Low-Temperature and Cryogenic Refrigeration

Altin Yunus-Qesme, Izmir, Turkey

June 23 July 5, 2002

A C.I.P Catalogue record for this book is available from the Library of Congress

DOl 10.1007/978-94-010-0099-4

Printed on acid-free paper

All Rights Reserved

Originally published by Kluwer Academic Publishers in 2003

Softcover reprint of the hardcover 1 st edition 2003

No part of this work may be reproduced, stored in a retrieval system, or transmitted

in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception

of any material supplied specifically for the purpose of being entered

and executed on a computer system, for exclusive use by the purchaser of the work

Ion C Ionita, Ion V Ion, Ian K Smith, Nikola Stosic, Ahmed Kovacevic,

Christian Iosifescu and Viorel Popa

The Effect of Inlet Subcooling on Two-Phase Flow Dynamic Instabilities In-Tube

Sadik Kakaf, L Cao and Mila R Avelino

Transitional Processes and Crisis Phenomena in Boiling of Cryogenic Liquids 145

Alexander Pavlenko

Numerical Simulation of Heat and Mass Transfer in Heat Pump Working on

Alexander I Leontiev and Igor Derevich

Hydrodynamics and Heat Transfer in Boiling and Evaporation in Cryogenic Falling

Alexander Pavlenko

Modem Problems of Cryogenic Heat Transfer and its Enhancement (Generalization of

Experimental Results, Practical Recommendations and Different Applications) 201

G.A Dreitser

Heat Transfer in a Liquid Nitrogen at High Centrifugal Acceleration Fields 221

Vladimir Y Zhukov and Mark 0 Lutcet

vi

Overcooling Phenomenon by Symmetrical or Asymmetrical Collison of Thermal Waves

Shuichi Torii and Wen-Jei Yang

Alexander 1 Leontiev

Ram Devireddy and John Bischof

Wen-Jei Yang and S Mochizuki

Wen-Jei Yang and Sadanari Mochizuki

Cila Herman

Henry F Smirnov

Leonard L Vasiliev, D.A Mishkinis, A.A Antukh and L.L Vasiliev, Jr

Leonard L vasiliev, D.A Mishkinis, A.A Antukh, A G Kulakov and L.L vasiliev, Jr

Leonard L Vasiliev and A G Kulakov

Ray Radebaugh

Plate-Fin Heat Exchangers for Cryogenic Applications with Special Emphasis on

Vishwas V Wadekar

Refrigeration of Low-Temperature Superconducting Coils for Nuclear Fusion

Roberto Zanino and L Savoldi Richard

Modelling of Refrigeration Poultry Meat Processes

Gratiela-Maria Tarlea

Thermophysical Properties of Foods at Frozen State

I~mail H Tavman, S Tavman and S Kumcuoglu

This volume contains an archival record ofthe NATO Advanced Study Institute on Temperature and Cryogenic Refrigeration held in <;e~me-Izmir, Turkey June 27 - July 5,

Low-2002 The NATO ASIs are intended to be high-level teaching activity in scientific and technical areas of current concern Certainly, the subject of refrigeration needs no justification in this regard Cryogenics, the science and technology of extremely low temperatures, plays a major role in medical purposes, especially in Cryobiology and Cryosurgery, space technology and cooling of low temperature electronics On the other hand, in large-scale power and process refrigeration and air conditioning systems, the usual goals are to reduce the size of a device, to lower the process irreversibility and power consumption required for a specific cold duty and to upgrade the capacity of existing low temperature systems

In this volume, the reader may find interesting chapters on the basic refrigeration systems, non-compression refrigeration and cooling, and important topics related to global environmental issues, alternative refrigerants, optimum refrigerant selection, and cost-to-quality optimization of refrigerants, advanced thermodynamics of reverse cycle machine, applications in medicine, cryogenic technology, heat pipes, heat pipe applications, heat pipe technology in refrigeration and air conditioning, gas-solid sorption refrigerator, multi salt resorption heat pump, cryocoolers, thermoacoustic refrigeration, refrigeration systems for low temperature electronics, cryogenic heat transfer and enhancement and other related lectures with regard to theory, design and applications of the subject matter During the ten working days of the Institute, the invited lecturers covered fundamentals and applications of low-temperature and cryogenic refrigeration The sponsorship of the NATO Scientific Affairs Division is gratefully acknowledged; in person we are very thankful to Dr Fausto Pedrazzini director of ASI programs who continuously supported and encouraged us at every phase of our organization of this institute Our special gratitude goes out to Drs Niliifer Egrican, Hafit Yiincii, and Arif

Hepba~li for coordinating sessions and we are very thankful to Giilter Mat, Sernra Karamehmetoglu, Melda Koksal and Akif Murat Giiriin for their invaluable efforts in making the Institute a success A word of appreciation is also due to the members ofthe session chairmen for the r efforts in expediting the technical sessions We are very grateful to Mrs Annelies Kersbergen of Kluwer Academic publishers for her close collaboration in preparing this archival record of the Institute, and Dr F Arin<;, Secretary General of ICHMT for his guidance and help during the entire process of the organization

of this Institute

To organize such a NATO Institute on refrigeration was initiated by Profs Victor Mazur and Boris Kosoy; we are very thankful for their help and encouragement during the organization of the Institute

Finally our heartfelt thanks to all lecturers and authors, who provided the substance of the Institute, and to the participants for their attendance, questions and comments

vii

SadikKaka<; Henry F Smirnov Mila R A velino

INTRODUCTION TO THE INSTITUTE

Dr S Kakay

Department of Mechanical Engineering

University of Miami

Coral Gables, FL 33124

Introduction to the 10th NATO Advanced Study Institute

Refrigeration systems for food preservation and air conditioning play prominent roles

in our everyday lives Because of the new developments in technology, low temperature and cryogenic heat transfer found important applications as diverse as in aerospace, sensors, cryobiology, cryosurgery, and cryoelectronics All presently-available superconducting materials must be cooled to cryogenic temperatures for their operation

In recent years, environmental aspects have been an important issue in designing and development of low-temperature refrigeration systems

Cryogenics, the science and technology of extremely low temperatures, plays a major role in medical purposes, especially in cryobiology and cryosurgery, space technology and in the cooling of low-temperature electronics Current research efforts focus both on the development of alternative refrigeration and refrigerant mixes and on alternative technologies, such as Stirling engines, pulse tube refrigerators, thermoelectric refrigerators and thermoacoustic refrigerators One of the important components of these refrigerators is their heat exchanger In low temperature and especially in cryogenic refrigeration, the transfer of heat is one of the major concerns and requires a better understanding of the heat transfer dimensions

On the other hand, in large-scale power and process refrigerating and air conditioning systems, the usual goals are to reduce the size of a device, to lower the process irreversibilities and power consumption required for a specific cold duty and to upgrade the capacity of existing low-temperature systems Worldwide demand for efficient, reliable and economical cold generation and storage is accelerating rapidly, particularly

in large-scale power and process refrigeration and air conditioning systems There is increasing demand to miniaturize cryogenic refrigerators (cryocoolers)

Some efforts have been made to develop mesoscale cryocoolers, which are smaller than the macroscale, but larger than the micro scale cryocoolers, especially in the aerospace industry, no one has yet produced a fully-contained cryocooler on a length scale of, say, a few centimeters Ongoing work at several institutions, however, targets mesoscale cryocoolers based on a variety of working cycles, including Stirling, pulse-tube, sorption, reverse-Brayton, and the Joule-Thomson cycles The key challenges in most of these systems are the development of efficient heat exchangers, especially regenerators

S K~ et aI (elis.), Low Temperature and Cryogenic Refrigeration, 1-3

© 2003 Kluwer Academic Publishers

One approach for continuing the rapid advances in the performance of microelectronics is through cooling the devices, either to the subambient range or to cryogenic temperatures The only way this can be done is through the application of a refrigerator, either a macroscaled-sized refrigerator for cooling at the system level, or meso- or microscale refrigerators for cooling at the device level The only available microscale refrigerator, both now and probably for the near future, is the thermoelectric refrigerator, which is commercially available in packages as small as a few centimeters However, many other kinds of mesoscale refrigerators may become available during the next several years, including vapor-compression, Stirling, pulse-tube, reverse-Brayton, and Joule-Thomson refrigerators, which will open up new possibilities for refrigeration cooling of electronics All fundamental heat transfer processes -conduction, convection, and radiation - can be important in cryogenic systems Convection, including boiling heat transfer, is dominant in systems cooled by immersion

in liquid cryogens Radiation is key in all cryogenic devices, due to the ubiquitous presence of an evacuated thermal radiation shield between the cryogenic system and the ambient environment Conduction, especially thermal conduction across solid/solid interfaces, is of great concern due to the increasing use of mechanical cryocoolers

In this Institute we have the following directions:

1 The common problems, including the thermodynamics, global problems

2 The low-temperatures 4:ooling and refrigeration

3 Cryogenic heat transfer principles

4 The cryobiology and cryogenics in medical applications

5 The cryogenics fundamentals and practical applications

Our scientific program will start with an introduction to Cryogenic Heat Transfer, general review of the Basic Refrigeration Systems, a general review of non-Compression Refrigeration and Cooling, an important topic related to Global Environmental issues and Refrigeration Engineering, Alternative Refrigerants, Optimum Refrigerant Selection, and Cost-to-Quality Optimization of Refrigerants Thermodynamics and transfer in adsorption machines and advances in thermodynamics of inverse cycle machines will be discussed and fundamentals and application of the low temperature and cryogenic refrigeration machines will be introduced in detail We have interesting lectures on applications in medicine There are interesting lectures on low temperature and refrigeration cryobiology, cryogenic technology

In several lectures, on heat pipe, heat pipe applications, heat pipe technology in refrigeration and air conditioning Lectures also include gas-solid sorption refrigerator, multi salt resorption heat pump Pulse tube cryocoolers is another important topic on the subject of the Institute The lecture will review the development of the pulse tube refrigeration, which is the most efficient of all cryocoolers and can be used in space missions As other important applications of the subject matter are the low-temperature

3

applications of thermoacoustic refrigeration, and refrigeration systems for low temperature electronics which will be discussed

The modem trend toward miniaturization of devices requires a better understanding

of heat transfer phenomena in small dimensions Devices having dimensions of the order

of microns are being developed for use in cooling of integrated circuits, biochemical applications, and cryogenics In this respect, microchannel heat transfer has received much attention due to their ultimate potential for cooling high power microelectronic and application in biochemical and aerospace industries The lecture on microscale heat transfer at low temperatures discusses this problem: the fundamentals of heat transfer in small space and time domains There are fundamental and applied subjects on heat transfer and evaporation of cryogenics, over cooling phenomenon by symmetrical collisions of thermal waves in a thin film, cryogenic heat transfer and enhancement We have two lectures on nuclear fusion: one lecture will coverthe refrigeration of low critical temperature superconducting coils for nuclear fusion, the other one presents Sonoluminescence and bubble thermodynamic fusion in Chilled Liquid

SYSTEMS

B.Y KOSOY

Department of Engineering Thermodynamics - State Academy of Refrigeration,

113 Dvoryanskaya Street, 65026, Odessa, Ukraine

1 Introduction

Current paper briefly covers a historical background [28,32,33] and presents a general view of basic refrigeration equipment and systems with respect to the application of ther-modynamic analysis to mechanical compression and non-mechanical refrigeration cycles, the two most common methods of thermal energy transfer

re-Modem refrigeration systems are used in different settings to lower the temperature of

a substance below that of its surroundings, such as the storage of medicines, blood, and the most important application, the processing, storage and transportation of perishable foods Refrigeration systems are very commonly used in residential, commercial and industrial plant air conditioning systems where air temperature is controlled They are also used in specialized situations such as the chemical processing industry and refining industries to remove the heat produced by reactions used to make products and to separate components

by condensation, distillation or crystallization, and numerous other materials processing steps where air, liquid and solid temperatures must be regulated Low temperature or cryo-genic processes have also become popular, due in part to the increased demand for air liq-uefaction and separation capacity to make liquid products such as liquid oxygen and liquid nitrogen and also for the low temperature separation of natural gas and natural gas products Conventional mechanical refrigeration is performed by a mechanical system or appara-tus designed and constructed so that heat is transferred from one substance to another The greater the difference in temperature between the two substances, the faster the heat flow

As the temperature of the substances equalizes, the flow of heat slows and stops completely when the temperatures are equal This effect is used in refrigeration Three methods by which heat may be transferred from a warmer substance to a colder one are conduction, convection, and radiation [15)

The first research in refrigeration theory was done by Sir F Bacon in the 17th century and by M Lomonosov in the middle of the 18th century Thus, in [23] Lomonosov wrote:

"The lowest degree of heat or in other words the highest degree of cold is the beginning of the first and lowest temperature zone, which is not shown yet That zone ends at the freez-ing point of water" and "diluted with warm water in a suitable vessel saltpeter produces enough coldness to freeze even during the summer"

W Cullen who cooled water by drawing a vacuum over it in 1755 makes first known attempt at vapour compression refrigeration The term "refrigerator" was coined by a Mary-land engineer, T Moore, in 1800 This device would now be called an "ice box" - a cedar tub, insulated with rabbit fur, filled with ice, surrounding a sheetmetal container for trans-

5

S KaJau; et al (eds.), Low Temperature and Cryogenic Refrigeration, 5-22

© 2003 Kluwer Academic Publishers

6

porting butter from rural Maryland to Washington, DC In 1834, J Perkins obtained the first patent for the vapour-compression refrigeration system, which used ether in a vapour compression cycle O Evans in 1805 proposed, but did not build, a device that evaporated sulphuric acid absorbed by water The first absorption machine was designed by E Carre in

1850, using water and sulphuric acid His brother, F Carre, developed the first nia/water refrigeration machine in 1859 In 1851,1 Gorrie is granted a patent to build the first commercial machine in the world used for refrigeration and air conditioning [13] The world's first successful sealed vapour compression refrigeration system is presented at the First International Congress of Refrigeration in 1908 This accomplishment revolutionized refrigeration because it needed no additional refrigerant during its operating life

ammo-This progression continues today, when both mechanical and non-mechanical tion systems make transportation, storage, and use of refrigerated goods easy and practical

refrigera-2 Thermodynamics

First of all, we will take a closer look at the thermodynamic basics of refrigeration systems [4-6,9,11,17,18,26,27] Shown on Fig 1 is the cyclic refrigeration device operating be-tween two constant temperature reservoirs and the T -s diagram for the working fluid when the reversed Carnot cycle is used Recall that in the Camot cycle heat transfers take place at constant temperature The standard comparison of refrigeration cycles is the reversed Car-not cycle, Fig.l A refrigerator, which operation is based on the reversed Camot cycle is called a Carnot refrigerator, and their coefficients of performance, COPs are

temperatures While the work interactions for the cycle are not indicated on the figure, the work produced by the turbine helps supply some of the work required by the compressor from extemal sources It also should be noticed, that TH couldn't be less than the tempera-

3

4

Figure 2 The ideal vapour-compression cycle

ture of the environment to which heat is rejected, and T L cannot be greater than the perature of the cold region from which heat is removed

tem-The reversed Camot Cycle is not practical because easier to compress vapour only and not liquid-vapour mixture, and cheaper to have irreversible expansion through an expansion valve

The compression-compression refrigeration cycle is the principle upon which tional air conditioning systems, heat pumps, and refrigeration systems are able to cool and dehumidify air in a defined volume (e.g., a living space, a vehicle, a freezer, etc.) Some of the advantages of this method include: longer-lasting effects, no danger of toxic leakage, and fewer costs for producers and consumers The compression-compression cycle is made possible because the refrigerant is a condensable gas and exhibits specific properties when

conven-it is placed under varying pressures and temperatures It has four components: evaporator, compressor, condenser, and expansion (or throttle) valve Different temperature limits and temperature reducing capacities are attained depending on the system components and re-frigerant selected

In an ideal compression-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapour and is cooled to the saturated liquid state in the con-denser Then it is throttled to the evaporator pressure and vaporizes, absorbing heat from the refrigerated space

8

The ideal compression-compression cycle consists of four processes: isentropic pression (1-2), constant pressure heat rejection in the condenser (2-3), throttling in an ex-pansion valve (3-4), and constant pressure heat addition in the evaporator (4-1), Fig 2 The ordinary household refrigerator is a good example of the application ofthis cycle •

com-Results of First and Second Law Analysis for Steady-Flow are presented in Table!

Table I Results of analysis for ideal vapour-compression cycle

LlS>0, Wnet = 0, Qnet = 0

P=const

fftn = m(h2 - hi)

QH =m(h2 -h3) h3= h4

QL =m(hl -h4)

The efficiency of a refrigeration cycle is traditionally described by an energy-efficiency ratio (EER) It is defined as the ratio of the heat absorption from an evaporator to the work done by a compressor

fJ=~= h l -h 4 <fJmax

Deviation of the actual compression-compression

refrigeration cycle from the ideal cycle primarily are

due to (a) heat transfer to or from the surroundings

and (b) pressure drops associated with fluid flow The

actual cycle might approach the one shown in Fig 3:

State I might be in super heat region to ensure that the

compressor is always handling the vapour phase This

will increase the size of evaporator

Process 1-2 is not isentropic due to irreversibility

Heat transfer from compressor to/from surroundings

will also affect the compressor perfonnance

State 3, in general, is in the sub cooled region This is a beneficial effect, however, this will also increase the size of the condenser

The power cycles can be used as refrigeration cycles by simply reversing them One of these cycles is the reversed Brayton cycle known as the gas refrigeration cycle, Fig 4 It is used in Gas (Air-Standard) Refrigeration Systems to cool aircraft cabin (open cycle) and to obtain very low (cryogenic) temperatures after modification with regeneration The work output of the turbine can be used to reduce the work input requirements to the compressor Thus, the COP ofa gas refrigeration cycle is

(3)

Stirling coolers have been known for decades Briefly, a Stirling cycle cooler presses and expands a gas (typically helium) to produce cooling This gas shuttles back and forth through a regenerator bed to develop much larger temperature differentials than the simple compression and expansion process affords A Stirling cooler uses a displacer to force the gas back and forth through the regenerator bed and a piston to compress and ex-pand the gas The regenerator bed is a porous element with a large thennal inertia During operation, the regenerator bed develops a temperature gradient One end of the device be-comes hot and the other end becomes cold Stirling coolers are desirable because they are non-polluting, are efficient and have very few moving parts The use of Stirling coolers has been proposed for conventional refrigerators However, it has been recognized that the in-tegration of free-piston Stirling coolers into conventional refrigerated cabinets requires dif-ferent techniques than conventional compressor systems

com-Very low temperatures can be achieved by operating two or more compression systems in series such that the condenser of a low-temperature cycle provides the heat input to the evaporator of a high-temperature cycle, called cascading Nonnally a different refrigerant would be used in each separate cycle, in order to match the desired range of T & P Cascade refrigeration systems are known and commonly believed neces-sary to reach substantially reduced temperatures in the test chamber of a testing and condi-tioning apparatus In the cascade refrigeration system, a first stage system including a com-pressor and condenser cools a refrigerant This cooled refrigerant is then passed in a heat-exchange relationship with a refrigerant of the second stage The temperature of this second stage refrigerant is thus lowered before the refrigerant passes through coils in an evapora-tor The evaporator is positioned in the test chamber or is in fluid communication with the test chamber More specifically, the high temperature-side refrigerant circuit, on the one hand, comprises a closed circuit fonned by sequential connection, established by refrigerant piping, of a compressor, a heat source-side heat exchanger, an expansion valve, and an evaporation portion of a refrigerant heat exchanger On the other hand, the low tempera-ture-side refrigerant circuit comprises a closed circuit fonned by sequential connection, established by refrigerant piping, of a compressor, a condensation portion of the refrigerant heat exchanger, an expansion valve, and an application-side heat exchanger Such basic cascade refrigeration systems have been further modified with bypass circuits to enhance operating efficiencies

compression-10

T

Figure 5 Cascade vapour-compression system

The Ts diagram, Fig 5, shows an ideal double-cascade system using the same ant in each loop The COP of a refrigeration system also increases as a result of cascading The cascade refrigeration systems are very effective for providing a substantially reduced temperature in the test chamber of an environment testing and conditioning apparatus However, the need for two compressors in particular substantially increases the unit's initial cost

refriger-Such a two-stage cascade refrigerating cycle refrigeration system finds applications in refrigerating apparatus such as showcases for foods or the like installed at stores (e.g., super markets and convenience stores) Defined in such a showcase are a display space for frozen foods in the showcase chamber and an air passage for the circulation of air with the display space The application-side heat exchanger, which is disposed in the air passage, is able to provide a supply of air into the showcase chamber with the aid of an air blower

However, in such a conventional showcase constructed in the way described above, the operation will be brought into a stop when there occurs a failure in some equipment on the heat source side (e.g., the compressor), even though the application-side equipments are normally operating There are some possible means of coping with such stoppage, one of which is to transfer the goods to another showcase that remains in operation This, how-ever, results in an increase in the load of refrigerating/cooling, therefore producing the problem of making it impossible to maintain the quality of goods at a satisfactory level Particularly, in the case a freezing showcase stops, this produces the problem that the stored goods cannot be preserved at a satisfactory level of quality even when transferred into a cold storage showcase

To reduce the required compressor work input multi-stage compression refrigeration systems could be applied, Fig 6

The cascade-type system is representative of the usual "two fluid" approach to stage refrigeration in that the mechanical compression-compression refrigeration stage is the final, direct refrigeration step in the controlled cooling of the merchandiser evaporator coils for maintaining product zone temperatures, and the other or "secondary" coolant is circulated in heat exchange with the condensers of the refrigeration stage to enhance effi-ciency

multi-It is known, that for gas power cycles, the heat removed from the intercooler usually is transferred to the environment For refrigeration cycle, the sink for the energy can be the circulating refrigerant by itself, because in many sections of the cycle, the temperature of the refrigerant is below the environment temperature

Thus, the COP of such a cycle is

(4)

The term displacer refrigerator is to be understood as a Gifford McMahon, Stirling or similar refrigeration machine Single-stage refrigeration machines of this kind have a work-ing area with a displacer The working area is connected in alternating fashion to a high pressure and a low-pressure source of gas in such a manner that during the forced to-and-from motion of the displacer, a thermodynamic cycle is performed The working gas is cy-cled preferably through a regenerator (heat accumulator for precooling of the entering gas) located within the displacer During operation of the refrigeration machine, heat is removed

Figure 6 Multistage compression system

with regenerative cooling

Figure 7 Multipurpose refrigeration system

at one of the two ends of the working area With a single-stage refrigerator of this kind and with helium as the working gas, temperatures down to 10-30 K may be generated Dis-placer refrigerators offer the advantage of being relatively powerful and their theoretical fundamentals are well understood Their disadvantage is the generation of vibrations caused

by the mass of the displacer as it moves to-and-from

Also known are refrigeration machines, the operation of which is based on the principle

of the pulse tube These machines include an area with a fixed refrigerator in which flowing gas is cooled by exchanging the heat with the regenerator material, as well as a pulse tube into which the working gas from the area of the regenerator flows in and out periodically at one end (cold side) Connected to the other end (warm side) of the pulse tube, preferably through a constriction, is a sealed volume By suitably selecting this throt-tle, the phase relationship between mass flow and pressure variation in the area of the pulse tube may be influenced for the purpose of attaining optimum performance Besides the above approach detailed (referred to as a "Orifice Pulse Tube" design) other types of design such as ("Double Inlet", "4-valve") also exist that modifY the phase relationship The effi-ciency of refrigeration machines of these kinds is limited These designs offer an advantage however, in that they do not generate any vibrations since they do not contain any moving parts

in-Through a lecture on occasion of the "Cryogenic Engineering Conference" Columbus, Ohio, in July 1995 it is known to combine a displacement refrigerator with a pulse tube refrigerator The displacer refrigerator forms the first stage and, the pulse tube refrigerator forms the second stage of a multistage low-temperature refrigeration machine In order to ensure that the cold end of the displacer refrigerator and the warm end of the pulse tube refrigerator are at the same temperature, a thermal link made from a rigid copper panel is provided which is linked in a thermally well conducting manner to both ends of the refrig-erators Owing to this rigid link, vibrations produced by the displacer refrigerator spread to the pulse tube refrigerator Thus, this known type of combined refrigeration machine is not suited for cooling vibration sensitive objects

A refrigerator with a single compressor can provide refrigeration at several tures by throttling the refrigerant in stages in multipurpose refrigeration systems, Fig 7

tempera-A different form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of lOO to 2000 C is absorption refrigera-tion, which uses a solution and a refrigerant contained in the solution as working mediums The system includes a generator for heating the solution to change a portion of the refriger-ant into vapour; a condenser communicated with the generator, for condensing refrigerant vapour coming from the generator; a throttling device communicated with the condenser, for throttling condensed refrigerant coming from the condenser; an evaporator communi-cated with said throttling device, having water circulating therewith in, capable of effecting heat transfer between the refrigerant coming from the throttling device and the circulating water; an absorber communicated with the evaporator, having solution reservoired there within, for absorbing refrigerant coming from the evaporator by means of the reservoired solution; and a heat exchanger communicated with the absorber and the generator, for per-forming indirect heat exchange between solution coming from the generator and solution coming from the absorber, characterized in that a solution mixing device, disposed between

: - - - - _ - -I

I

Figure 8 Absorption refrigeration system

the heat exchanger and the generator, is provided for mixing solution leaving the generator and solution going to enter the generator The system disclosed is capable of increasing the performance of heat transfer in the generator and reducing the cost of the generator, and thermal stress will also be reduced The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the

14

transport medium Figure 8 shows that the low-pressure ammonia vapour is absorbed in water and the liquid solution is pumped to a high-pressure by a liquid pump The work in-put to the pump is usually very small, and the COP of absorption refrigeration systems is

Nowadays absorption refrigeration cycles (ARC) technology is growing rapidly and has became a real alternative to compression refrigeration cycles, due to the advantages of ARC as far as primary energy savings and respect for the environment are concerned The application of absorption refrigeration cycles to reduce energy costs requires the consump-tion of the industrial plant to be assessed The plants, which are most suitable for ARC in-tegration, are those that need chilled water and have a low temperature waste heat source This means: a source of waste heat is used; substitution of one technology by another, which is more respectful to the environment

Vortex tubes are well known Typical vortex tubes are designed to operate with condensable gas such as air A typical vortex tube turns compressed air into two air streams, one of relatively hot air and the other of relatively cold air A common application for prior vortex tubes is in air supply lines and other applications, which utilize gas under a high pressure A vortex tube does not have any moving parts A vortex tube operates by imparting a rotational vortex motion to an incoming compressed air stream; this is done by directing compressed air into an elongated channel in a tangential direction

non-3 Refrigeration Principles and Basic Equipment

Thus, refrigeration is the process of removing heat from an area or a substance and usually

is done by an artificial means of lowering the temperature These means include the use of mechanical compression technology, non- mechanical refrigeration, and ice [3,7,8,15, 22,24,25]

There are many disadvantages and constrains associated with current refrigeration signs and methods Many refrigeration systems are limited by the temperature of the refrig-erant For vapour compression systems, the lowest attainable temperature of the refrigerant

de-is typically determined by its composition, its associated properties and the selected ing pressure Ambient air conditions, including temperatures and relative humidity, may limit refrigeration produced by air coolers such as fin fans and cooling towers Absorption systems exhibit another disadvantage similar to that of the vapour compression systems limitations in that the selected operating pressure, e.g., the vacuum, coupled with the brine selection for the absorption system, indirectly result in temperature limits At higher vac-uums, the vaporization temperature drops for a given brine composition Once the vacuum level is determined, for a given brine composition, the vaporization temperature is estab-lished These temperature limits result in limited refrigeration available, condenser capacity limits and purity limits on products Accordingly, a continuing need exists for overcoming these temperature limitations or indirect temperature limitations as reflected by system pressure or ambient conditions

operat-3.1 REFRIGERANTS

Until 1930, engineers had only toxic or flammable refrigerants to choose from for their systems Sulphur dioxide, methyl chloride, ethyl chloride, or isobutane were used in virtu-ally all household systems until the General Motors Research Laboratory synthesized chlorofluorocarbon refrigerants The discovery was announced in 1930, and a new refriger-ant, called Freon, was sold to anyone who wanted to use it Nowadays various materials are known to the art, which can be used as heat transfer media in refrigeration systems [29] These materials include water, aqueous brines, alcohols, glycols, ammonia, hydrocarbons, ethers, and various halogen derivatives of these materials While many of these materials are effective as heat transfer media under certain conditions, practical considerations elimi-nate many of them from use in key commercial settings, such as in refrigeration systems in grocery stores In these applications, only a fraction of the class of known heat transfer agents is of commercial significance

One factor that eliminates many heat transfer media from consideration is their ronmental impact Many known heat transfer media are being phased out because of their environmental persistence The use of CFCs (chlorofluorocarbons) raises two important environmental concerns First, CFCs are stable enough to enter the stratosphere where they decompose to chlorine free radicals that catalyse the destruction of ozone This is unfortu-nate because ozone absorbs ultraviolet radiation, which damages DNA in plants and ani-mals Second, CFCs absorb infrared radiation, which contributes to global warming [10] The abovementioned environmental matters have resulted in national and international laws and regulations for the elimination and/or reduction in the production and use of such CFC chemicals The refrigeration industry in general has been a primary target for government regulation with the result that some refrigerants, such as R-502, previously in common use

envi-in commercial foodstore refrigeration for many years are now beenvi-ing replaced by newer CFC types of refrigerants However, such newer refrigerants are even more expensive than the more conventional CFC types, thereby raising basic cooling system installation and maintenance costs and creating higher loss risks in conventional backroom types of com-mercial systems having long refrigerant piping lines from the machine room to the store merchandisers Obviously, the refrigeration industry has been concerned over its role in the environmental crisis, and has been seeking new refrigeration systems, as well as new non-CFC chemicals, in an attempt to help control the CFC problem while maintaining high effi-ciency in food preservation technology [34]

non-Another factor that removes many heat transfer agents from consideration is their icity This is the case, for example, with ammonia and with many of the ethylene glycols The toxicity of these materials, by ingestion, inhalation, or transdermal absorption, makes them dangerous to handle and unsuitable for commercial food handling environments Still other heat transfer agents are disfavoured because of their flammability This is the case, for example, with most ethers and hydrocarbons The risk of flammability is particu-larly great where the heat transfer agent is subject to large positive pressures within the refrigeration cycle

tox-Other heat transfer agents are dis favoured because they are gases at normal operating temperatures An example of this type of refrigerant is ammonia Gaseous heat transfer media require special high pressure equipment, such as pressure regulators and reinforced tubing, that are not required for refrigerants that remain in a liquid state through most or all

of the operating cycle Furthermore, high-pressure systems are prone to leakage Thus, it

In this lecture we shall emphasize on mechanical refrigeration systems and basic equipment that should be used for the proper completion of mechanical refrigeration process [1,2,12,14]

Mechanical refrigeration systems are an arrangement of components in a system that puts the thermodynamic principles into practice to provide artificial cooling To do this, you must supply the following: (a) a metered supply of relatively cool refrigerant liquid under pressure; (b) a device in the location to be cooled that operates at reduced pressure so that when the cool, pressurized liquid enters, it will expand, evaporate, and take heat from the space to be cooled; ( c) a means of compressing the vapour; and (d) a means of condens-ing it back into a liquid, removing its superheat, latent heat of vaporization, and some of its sensible heat

Every mechanical refrigeration system operates at two different pressure levels The high-pressure side of the refrigeration system comprises all the components that operate at

or above condensing pressure These components are the discharge side of the compressor, the condenser, the receiver, and all interconnected tubing up to the metering device or ex-pansion valve The low-pressure side of a refrigeration system consists of all the compo-nents that operate at or below evaporating pressure These components comprise the low-pressure side of the expansion valve, the evaporator, and all the interconnecting tubing up

to and including the low side of the compressor

The refrigeration cycle of a mechanical refrigeration system may be illustrated by using figure 9 The pumping action of the compressor (I) draws vapour drawn from the evapora-tor (2) This action reduces the pressure in the evaporator, causing the liquid particles to evaporate As the liquid particles evaporate, the evaporator is cooled Both the liquid and vapour refrigerant tend to extract heat from the warmer objects in the insulated refrigerator cabinet The ability of the liquid to absorb heat as it vaporizes is very high in comparison to that of the vapour As the liquid refrigerant is vaporized, the low-pressure vapour is drawn into the suction line by the suction action of the compressor (1) The evaporation of the liquid refrigerant would soon remove the entire refrigerant from the evaporator if it were not replaced The replacement of the liquid refrigerant is usually controlled by a metering device or expansion valve (3) This device acts as a restrictor to the flow of the liquid re-frigerant in the liquid line Its function is to change the high-pressure, subcooled liquid re-frigerant to low-pressure, low-temperature liquid particles, which will continue the cycle by absorbing heat

The refrigerant low-pressure vapour drawn from the evaporator by the compressor through the suction line, in tum, is compressed by the compressor to a high-pressure va-pour, which is forced into the condenser (4) In the condenser, the high~pressure vapour condenses to a liquid under high pressure and gives up heat to the condenser The heat is

removed from the condenser by the cooling medium of air or water The condensed liquid refrigerant is then forced into the liquid receiver (5) and through the liquid line to the ex-pansion valve by pressure created by the compressor, making a complete cycle

Figure 9 Cycle of mechanical refrigeration system

The refrigeration system consists of four basic components - the compressor, the denser, the liquid receiver, the evaporator, and the control devices These components are essential for any system to operate on the principles previously discussed

con-Refrigeration compressors have but one purpose - to withdraw the heat-laden ant vapour from the evaporator and compress the gas to a pressure that will liquety in the condenser The designs of compressors vary, depending upon the application and type of refrigerant There are three types of compressors classified according to the principle of operation - reciprocating, rotary, and centrifugal

refriger-Many refrigerator compressors have components besides those normally found on compressors, such as unloaders, oil pumps, mufflers, and so on

The type of compressor used has a particular bearing on the refrigerant Reciprocating compressors are best adapted to low specific volume refrigerant, i.e., high pressure, while centrifugal compressors are best customized to high specific volume refrigerant, i.e., low pressure Rotary compressors are much utilized for the refrigerators, air conditioners, and the like, because of their compact size and simple structure

The condenser removes and dissipates heat from the compressed vapour to the rounding air or water to condense the refrigerant vapour to a liquid The liquid refrigerant then falls by gravity to a receiver (usually located below the condenser), where it is stored, and available for future use in the system

sur-There are three basic types of condensers- air-cooled, water-cooled, and evaporative The first two are the most common, but the evaporative types are used where low-quality water and its disposal make the use of circulating water-cooled types impractical

18

A typical air-cooled condenser comprises a single conduit formed into a serpentine-like shape so that pluralities of rows of conduit are formed parallel to each other Metal fins or other aids are usually attached to the outer surface of the serpentine conduit in order to in-crease the transfer of heat between the superheated refrigerant vapour passing through the condenser and the ambient air Heat is rejected from the superheated vapour as it passes through the condenser and the refrigerant exits the condenser as a saturated or subcooled liquid

Water-cooled condensers are of the mUltipass shell and tube type, with circulating ter flowing through the tubes The refrigerant vapour is admitted to the shell and condensed

wa-on the outer surfaces of the tubes

The capacity of the water-cooled condenser is affected by the temperature of the water, quantity of water circulated, and the temperature of the refrigerant gas The capacity of the condenser varies whenever the temperature difference between the refrigerant gas and the water is changed An increased temperature difference or greater flow of water increases the capacity of the condenser The use of colder water can cause the temperature difference

to increase

An evaporative condenser operates on the principle that heat can be removed from condensing coils by spraying them with water or letting water drip onto them and then forc-ing air through the coils by a fan This evaporation of the water cools the coils and con-denses the refrigerant within

The evaporator physically resembles the serpentine-shaped conduit of the condenser Ideally, the refrigerant completely evaporates by absorbing heat from the defined volume to

be cooled (e.g., the interior of a refrigerator) and leaves the evaporator as saturated vapour

at the suction pressure of the compressor and re-enters the compressor thereby completing the cycle

Evaporators are mainly of two types - dry or flooded The inside of a dry evaporator frigerant is fed to the coils only as fast as necessary to maintain the temperature wanted The coil is always filled with a mixture of liquid and vapour refrigerant At the inlet side of the coil, there is mostly liquid; the refrigerant flows through the coil (as required); it is va-porized until, at the end, there is nothing but vapour In a flooded evaporator, the evaporator

re-is always filled with liquid refrigerant A float maintains liquid refrigerant at a constant level As fast as the liquid refrigerant evaporates, the float admits more liquid, and, as a result, the entire inside of the evaporator is flooded with liquid refrigerant up to a certain level determined by the float

The two basic types of evaporators are further classified by their method of tion, either direct expanding or indirect expanding In the direct-expanding evaporator, heat

evapora-is transferred directly from the refrigerating space through the tubes and absorbed by the refrigerant In the indirect-expanding evaporator, the refrigerant in the evaporator is used to cool some secondary medium, other than air This secondary medium or refrigerant main-tains the desired temperature of the space Usually brine, a solution of calcium chloride is used as the secondary refrigerant

Natural convection or forced-air circulation is used to circulate air within a refrigerated space [15] Air around the evaporator must be moved to the stored food so that heat can be extracted, and the warmer air from the food returned to the evaporator Natural convection can be used by installing the evaporator in the uppermost portion of the space to be refrig-erated, so heavier cooled air will fall to the lower food storage and the lighter food-warmed air will rise to the evaporator The forced air type evaporator could be constructed of many

copper tubes, which conduct heat welL To further enhance heat transfer the pipes could have aluminium fins pressed onto them This vastly increases the surface area that is ex-posed to the air This type of evaporator could also have a fan motor pulling air through the fins

Significant lowering of the evaporator efficiency is linked to the icing process One proach that is employed to avoid evaporator icing is to simply run the device at higher re-frigerant temperatures This, however, limits the cooling capability of the device Further-more, if the goal is to remove water from the ambient air, it is preferred that the device be run as cold as possible so that the air is cooled to as close to the freezing point of water as possible since the colder the air, the less water it can retain Running the device at a higher refrigerant temperature is thus inefficient since it leaves water in the air

ap-To maintain correct operating conditions, and is usually used in large refrigerated spaces control process devices are needed in a refrigeration system

Metering devices, such as expansion valves and float valves, control the flow of liquid refrigerant between the high side and the low side of the system It is at the end of the line between the condenser and the evaporator These devices are of five different types: an automatic expansion valve (also known as a constant-pressure expansion valve), a thermo-static expansion valve, low-side and high-side float valves, and a capillary tube

The expansion device reduces the pressure of the liquid refrigerant thereby turning it into a saturated liquid-vapour mixture, which is throttled to the evaporator In order to re-duce manufacturing costs, the expansion device is typically a capillary tube, which consists

of a long tube of small diameter It acts as a constant throttle on the refrigerant The length and diameter of the tube are important; any restrictions cause trouble in the system It feeds refrigerant to the evaporator as fast as the condenser produces it When the quantity of re-frigerant in the system is correct or the charge is balanced, the flow of refrigerant from the condenser to the evaporator stops when the compressor unit stops When the condensing unit is running, the operating characteristics of the capillary tube equipped evaporator are the same as if it were equipped with a high-side float

The capillary tube is best suited for household boxes, such as freezers and window conditioners, where the refrigeration load is reasonably constant and small horsepower mo-tors are used

air-A known expansion valve comprises a prism-shaped valve body, the body being equipped with a valve chamber and a power element for operating a valve means formed within the valve chamber

This kind of expansion valve comprises two passages communicated to the valve chamber, and a passage through which the refrigerant returning from the evaporator to the compressor travels An operating shaft capable of communicating the movement of the power element to the valve means penetrates the passage through which the refrigerant re-turning from the evaporator to the compressor travels, and transmits the temperature infor-mation of the refrigerant to the power element

The low-side float expansion valve controls the liquid refrigerant flow where a flooded evaporator is used It consists of a ball float in either a chamber or the evaporator on the low-pressure side of the system

The float actuates a needle valve through a lever mechanism As the float lowers, frigerant enters through the open valve; when it rises, the valve closes

re-In a high-side float expansion valve, the valve float is in a liquid receiver or in an iary container on the high-pressure side of the system Refrigerant from the condenser

auxil-20

flows into the valve and immediately opens it, allowing refrigerant to expand and pass into the evaporator Refrigerant charge is critical An overcharge of the system floods back and damages the compressor An undercharge results in a capacity drop

3.3 REFRIGERATION EQUIPMENT

Refrigeration equipment can be classified as either self-contained or remote units contained equipment houses both the insulated storage compartments (refrigerated), in which the evaporator is located, and an uninsulated compartment (nonrefrigerated), in which the condensing unit is located, in the same cabinet Self-contained refrigerating equipment includes such equipment as domestic refrigerators and freezers, water coolers, reach-in and walk-in refrigerators, small cold-storage plants, and ice plants [16,19-21,30] Remote refrigerating equipment has the condensing unit installed in a remote location from the main unit These types of units are used where the heat liberated from the condenser cannot enter the space where the unit is installed or space is limited for installation

Self-Exterior finishes for reach-in refrigerators are usually of stainless steel, aluminium, or vinyl, while the interior finishes are usually metal or plastic, and the refrigerator cabinet is insulated with board or batten type polystyrene or urethane Reach-in refrigerators are nor-mally self-contained, with an air-cooled condenser, but in larger refrigerators, with remote condensers, water-cooled condensers are sometimes used Evaporators in reach-in refrigera-tors are generally the unit cooler type with dry coils In smaller-capacity refrigerators, ice-making coils, similar to those used in domestic refrigerators, are often used as well as straight gravity coils R-12 and R-502 are normally used in these units The evaporator is mounted in the centre of the upper portion of the food compartment In operation, warm air

is drawn by the fan into the upper part of the unit cooler, where it passes over the tor coils, is cooled, and then is discharged at the bottom of the cooler The air then passes

evapora-up through the interior and around the contents of the refrigerator The cycle is completed when the air again enters the evaporator The low-pressure control is set to operate the evaporator on a self-defrosting cycle, and temperature is thus control led Another type of control system uses both temperature and low-pressure control or defrost on each cycle The evaporator fan is wired for continuous operation within the cabinet

Walk-in refrigerators are normally larger than reach-in types and are either built-in or prefabricated sectional walk-in units The exteriors and interiors of these units are normally galvanized steel or aluminium Most walk-in refrigerators use rigid polyurethane board, batten, or foamed insulation between the inner and outer walls Most systems have the compressor and condenser outside the main structure and use either a wall-mounted forced-air or gravity-type evaporator that is separated from the main part of the cabinet interior by

a vertical baffle

Most domestic refrigerators are of two types - either a single door fresh food tor or a two-door refrigerator-freezer combination, with the freezer compartment on the top portion of the cabinet, or a vertically split cabinet (side-by-side), with the freezer compart-ment on the left side of the cabinet A single door fresh food refrigerator consists of an evaporator placed either across the top or in one of the upper comers of the cabinet The condenser is on the back of the cabinet or in the bottom of the cabinet below the hermetic compressor During operation, the cold air from the evaporator flows by natural circulation through the refrigerated space The shelves inside the cabinet are constructed so air can circulate freely past the ends and sides, eliminating the need for a fan This refrigerator has

refrigera-a mrefrigera-anurefrigera-al defrost, which requires threfrigera-at the refrigerrefrigera-ator be turned off periodicrefrigera-ally (usurefrigera-ally overnight) to enable the build-up of frost on the evaporator to melt The two-door refrigera-tor-freezer combination is the most popular type of refrigerator It is similar to the fresh food refrigerators in construction and the location of components except it sometimes has

an evaporator for both the freezer compartment and the refrigerator compartment Also, if it

is a frost-free unit, the evaporators are on the outside of the cabinet Because of the two separate compartments (refrigerator-freezer) and the larger capacity, these types of refrig-erators use forced air (fans) to circulate the air through the inside of both compartments The two-door refrigerator also has one of the following three types of evaporator defrost systems: manual defrost, automatic defrost, or frost-free

There are two types of automatic defrosting: the hot gas system or the electric heater system The hot gas system, through the use of solenoid valves, uses the heat in the vapour from the compressor discharge line and the condenser to defrost the evaporator The other system uses electric heaters to melt the ice on the evaporator surface

A frost-free refrigerator-freezer has the evaporator located outside the refrigerated partment On the running part of the cycle, air is drawn over the evaporator and is forced into the freezer and refrigerator compartments by a fan On the off part of the cycle, the evaporators automatically defrost

com-Water coolers provide water for drinking at a temperature under lOoe Two types of water coolers are instantaneous and storage The instantaneous type only cools water when

it is being drawn; the storage type maintains a reservoir of cooled water One instantaneous method used places coils in a flooded evaporator through which the water flows A second instantaneous method uses double coils with water flowing through the inner coil with re-frigerant flowing in the space between the inner coil and the outer coil A third instantane-ous method is to coil the tubing in a water storage tank This allows refrigerant to flow through it Most water coolers use a heat exchanger or precooler, which precools the fresh water line to the evaporator, reducing cooling requirements for the evaporator A thermo-stat, which is manually set and adjusted, is located in the cooler housing close to the evapo-rator

4 Conclusion

This paper has presented an attempt of overview of the principles involved in refrigeration theory throughout two hundred year development during one-hour lecture The author has benefited from his experience with undergraduate teaching for mechanical engineering stu-dents in writing this paper It is noteworthy that because of lack of time, many details are beyond the scope of the current presentation, as modem refrigeration is a powerful engi-neering field with a wide range of methods and a variety of equipment and designs

On the other hand, novel but well-developed tools of thermodynamic optimisation ing exergoeconomics and ecological costing could considerably improve application of the first and second law analysis for the refrigeration system design It is important to mention that the successful accomplishment of a refrigeration system design is a complex assign-ment, requiring principles not only from engineering thermodynamics, but also from fluid mechanics, heat and mass transfer, mechanical design, automatic controls, etc

us-22

References

1 ASHRAE (1985) Handbook Fundamentals American Society of Heating, Refrigerating and

Air-Conditioning Engineers, Inc

2 ASHRAE (1998) Handbook Refrigeration, American Society of Heating, Refrigerating and

Air-Conditioning Engineers, Inc

3 Bartlett, L., Woolrich, W R (1941) Refrigeration for Operating Engineers in Ice and Cold Storage Plants, University of Texas Bureau of Engineering Research, Austin

4 Bejan, A (1997) Advanced Engineering Thermodynamics, 2M ed., Wiley, New York

5 Bejan, A., Tsatsaronis, G., Moran, M (1996) Thermal Design and Optimization, John Wiley & Sons,

Inc., New York

6 Black, W.z., and J.G Hartley (1991) Thermodynamics, 2nd ed., Harper Collins, New York

7 Bougard, J., Afgan, N H (1987) Heat and Mass Transfer in Refrigeration and Cryogenics,

Springer-Verlag

8 Brown, S (1947) Air Conditioning and Elements of Refrigeration, McGraw-Hill, New York

9 Callen, H.B (1985) Thermodynamics, John Wiley & Sons, Inc., New York

10 Complying with the Refrigerant Recycling Rule, in Stratospheric Ozone Protection: Final Rule mary (1993), U.S Environmental Protection Agency, Washington

Sum-II Doolittle, J.S., Hale, F.J (1984) Thermodynamicsfor Engineers, John Wiley & Sons, Inc., New York

12 Dossat, T.J (1981) Principles of Refrigeration, 2nd edition, Wiley, New York

13 Gorrie, J (May 6, 1851) Improved Process for the Artificial Production of Ice, U.S Patent NQ 8080

14 Gosney, W.B (1982) Principles of Refrigeration, Cambridge Univ Press, 1982

15 Incropera, F.P., DeWitt, D.P (1996) Fundamentals of Heat and Mass Transfer, John Wiley & Sons,

Inc., New York

16 James, S.J., Schofield, 1 (1998) Modeling of food refrigeration systems, in V Gaukel and W.E.L Speiss (eds.), Proceedings of the 3rd Karlsruhe Nutrition Symposium, European Research towards Safer and Better Food, pp 293-301

Abbott, M.M., Vanness, H.C (1989) Thermodynamics, McGraw-Hill, New York

17 Klein, S.A (1992) Design considerations for refrigeration cycles, Int J Refrig., Vol 15,181-185

18 Kosoy, B.V (1997) Rational energy consumption in refrigerating engineering, Refrigerating ing and Technology 57, 36-38

Engineer-19 Kosoy, B.V., Lomovtsev, P.B (1999) Low-temperature systems automation design, Optimum Control Information Systems and Software, VoU, Part 2, 182-189

20 Kurilev, E.S., Gerasimov, N.A (1980) Refrigeration Systems, Mashinostroenie, Moscow

21 Langley, Billy C (1982) Refrigeration and Air Conditioning, Reston Publishing Company, Inc., Reston

Virginia

22 Levin, 1.1., Tkachev, A.G., Rozenfeld, L.M (1939) Refrigeration Machines, Pishepromizdat, Moscow

23 Lomonosov, M.V (1752) Considerations about the Reason of Heat and Cold, St.-Petersburg

24 Martinovskyy, V.S (1950) Refrigeration Machines, Pishepromizdat, Moscow

25 Martynovskyy, V.S (1952) Thermodynamic Parameters of Heat and Refrigeration Machines,

Go-senergoizdat, Moscow

26 Moran, M., Shapiro, H.N (1992) Fundamentals of Engineering Thermodynamics, John Wiley & Sons,

Inc., New York

27 Myers, G.E (1989) Engineering Thermodynamics, Prentice-Hall

28 O E Anderson, Jr (1953) Refrigeration in America, Princeton University Press, Princeton, New

Jer-sey

29 Platzer, B., Polt, A., Maurer, G (1990) Thermophysical Properties of Refrigerants, Springer-Verlag

30 Rose-Innes, A.C (1964) Low Temperature Techniques, English Universities Press, London

31 Stoecker, W.F (1989) Design of Thermal Systems, McGraw-Hill, New York

32 Thevenot, R (1997) A history of Refrigeration Throughout the World, International Institute

ofRefrig-eration, Paris

33 Woolrich, W R (1967) The Men Who Created Cold: A History of Refrigeration, Exposition Press,

New York

34 Zurer, P.S (1993) Looming Ban on Production ofCFCs, Halons Spurs Switch to Substitutes, Chemical

& Engineering News, 15, 12

sci-1999 The next meeting from this series (200 I) just improved such a statement

Nowadays a situation in the high-tech refrigeration is even more complicated, and only three methods presented in this paper are totally independent and self-sufficient one from each other Hopefully, both absorption, thermoelectric, and magnetic cooling processes will significantly contribute into progress of energy saving and environmentally friendly refrig-eration technologies in the nearest future

2 Absorption Refrigeration

Absorption refrigeration and cooling is today a well-proven technology [3,5,13,21,43] The absorption machines that are commercially available are powered by steam, by hot water or

by combustion gases Although a variety of applications may be proposed, the main market

in most countries is the production of chilled water in cooling of buildings [9,12,35] As economical conditions vary from country to country, absorption systems may be at the same time a small niche market in one country and the dominant technology in another country

23

S Ka~ et al (eds.), Low Temperature and CryoJ:enic R~friJ:eration, 23-37

© 2003 Kluwer Academic Publishers

24

Condenser

Heat Rejection

Chilled Water

Evaporator

Generator

I - : Heat in

Solution Heat Exchanger

Heat Rejection

Absorber

Figure 1 Schematic of a single-effect absorption refri~erator

The basic principle of an absorption cooling machine may be illustrated with Figure 1

In its simplest design the absorption machine consists of an evaporator (a vessel in which liquid refrigerant vaporizes by removing heat and thus, producing cooling), a condenser (a vessel in which refrigerant vapours are liquefied by removing heat), an absorber (a vessel in which the absorbent solution absorbs refrigerant vapours into solution), at least one genera-tor (a vessel in which refrigerant vapours are produced from the solution of the absorbent and the refrigerant by supplying heat), a heat exchanger (for exchanging heat between two fluid streams) and a solution pump

In a compression cycle chiller, cold is produced in the evaporator where the refrigerant

or working medium is vaporized and heat is rejected in the condenser where the refrigerant

is condensed In an absorption cycle, compressing the refrigerant vapour is effected by the absorber, the solution pump and the generator in combination, instead of a mechanical va-pour compressor Vapour generated in the evaporator is absorbed into a liquid absorbent in the absorber, [7] The absorbent that has taken up refrigerant, spent or weak absorbent is pumped to the generator where the refrigerant is released as a vapour, which vapour is to be condensed in the condenser The regenerated or strong absorbent is then led back to the absorber to pick up refrigerant vapour anew Heat is supplied to the generator at a compara-tively high temperature and rejected from the absorber at a comparatively low level, analo-gously to a heat engine The words "thermochemical compressor" have actually been used

in specialized literature to describe the function of the generator and absorber half of the absorption cycle

Refrigerant and absorbent in an absorption cycle form what is called a working pair [24,25,42] The refrigerant for an absorption system has to meet a few important require-ments:

• High solubility in absorbent solution at the absorber operating temperature

• Low solubility in absorbent solution at the desorber operating temperature

• Incapable of reacting irreversibly with absorbent within operating temperature range

Absorbent must have a low vapour pressure compared with the refrigerant and low heat capacity Many pairs have been proposed through the years but only two of them have been widely used: ammonia together with water as absorbent and water together with a solution

of lithium bromide in water as absorbent The ammonia-water pair is mostly found in frigeration applications, with low evaporation temperatures, below O°C The water-lithium bromide pair is widely used for air-cooling applications, where it is not necessary to cool below O°C The pressure levels in the ammonia-water machine are usually above atmos-pheric pressure while the water-lithium bromide machines generally operate in partial vac-uum

re-One major difference between the absorption and the vapour compression machines is the fluids used In most cases, a vapour compression machine uses a pure refrigerant In an absorption machine, the refrigerant can be pure, as in the lithium bromide-water or ammo-nia-sodium thiocyanate systems [39], or it can be a binary mixture, as in the ammonia-water system If the refrigerant in an absorption system is pure, then the refrigerant will follow the same behaviour in the evaporator and condenser If, on the other hand, the re-frigerant is binary, the difference in volatility of the different substances will cause the tem-perature to change along the length of the component For binary mixtures, the condensa-tion stage is a two-part process, where most of the absorbent is recovered in a rectifier and the refrigerant liquefied in the condenser Heat from the rectifier is rejected to the same sink

as for the condenser The refrigerant still contains a small amount of absorbent that has some effect on the operating temperature of the evaporator

For the absorbent-refrigerant pair to be acceptable, the refrigerant and absorbent must have a strong molecular attraction Since these forces cause the fluids to deviate from ideal fluid behaviour, the transport and thermodynamic properties of the refrigerant and solution have to be determined experimentally Consequently, it is difficult to find fluids that have better characteristics that what is currently being used

The heat flows in the basic cycle are the following:

• Heat is supplied, and cooling is produced, at a low temperature level

• Heat is rejected in the condenser at an intermediate temperature level

• Heat is rejected from the absorber, also at an intermediate level

• Heat is supplied to the generator at a high temperature level

The basic cycle illustrated in Fig 1 may be modified in several ways One is to utilize all possible opportunities for heat recovery within the cycle in order to improve the heat economy within the cycle For example, it is customary to heat exchange the streams of weak absorbent leaving the absorber with the regenerated or strong absorbent that is led back into the absorber When all heat recovery opportunities that can reasonably be used have been incorporated into the design of a machine, one obtains a cooling coefficient of

tem-The term "single effect" comes from the observation that an ideal machine would vide an amount of cooling equal to the heat provided to the generator Thermodynamic losses in the system dictate that the cooling capacity for a real system is always less than the heat input

pro-A number of different configurations can produce a double effect absorption chiller The two basic types are the double-condenser double effect and the double-absorber double effect [2] Their principle is based on the fact that the cooling capacity depends primarily

on the amount of refrigerant that is vaporized in the evaporator and that by reusing the waste heat from either the condensation or the absorption stages, more refrigerant can be desorbed from solution

A triple effect system has considerably more arrangement possibilities The simplest is

to use the heat from the absorber and condenser of one single effect cycle to fire another single effect cycle The high temperature cycle provides approximately one effect of cool-ing With respect to the low cycle, though, the high cycle operates as a heater, dumping the heat from the source and the high cycle evaporator into the low cycle desorber Since the low temperature cycle would ideally receive two units of heating for every one unit of heat supplied to the high cycle desorber, there would ideally be two units of cooling supplied by the low cycle The total would be three effects of cooling and four effects of heating for every unit of heat supplied to the high cycle desorber

3 The Ammonia-Water Hydrogen Cycle

Another class of thermo-chemically driven cycles requires no work or electricity because the entire cycle operates at a single pressure thus eliminating the need for a mechanical pump (work) Instead these more complex cycles use at least three working fluids, which achieve low temperature evaporation and high temperature condensation by varying the partial pressure of the refrigerant

The most familiar single pressure cycle is the ammonia-water-hydrogen cycle patented

by Platen and Munters (1928), [26,29,30,33,34] Refrigeration machines with this cycle have been used since then and are recently receiving new attention Single pressure cycle machines use three working fluids at a single pressure They achieve cooling by lowering the partial pressure of the refrigerant thus allowing it to evaporate In the ammonia-water-hydrogen cycle, ammonia is the refrigerant The cycle is shown in Figure 2 The ammonia

is driven (1) from its mixture with water (the absorbent) in the generator (5) by the tion of heat, QG Some of this heat drives the bubble pump, where the evaporated ammonia

applica-is used to lift the liquid mixture (now weak in refrigerant) back into the absorber (6), The nearly pure ammonia vapour enters the condenser (1), where it is condensed at its saturation temperature for the system's total pressure The condensed ammonia flows down into the evaporator (2)

~ Pure Ammonia ~ Ammonia-Water

(-;:·:·::·:·:.'.1 Pure Hydro gen

E$$l$~ Ammonia-Hydrogen

Figure 2 The ammonia- water hydrogen cycle

The evaporator is perhaps the most interesting component of the system Here the uid ammonia is exposed to gaseous hydrogen, which lowers the partial pressure on the liq-uid ammonia This reduction in the partial pressure allows evaporatIon at ammonia's satura-tion temperature for its partial pressure in the evaporator, a temperature lower than that of the condenser Thus, the evaporator is essentially equivalent to the expansion valve in a dual pressure cycle The cool vapor mixture of ammonia and hydrogen (3) falls into the absorber Here, water, bubble pumped from the generator (6), absorbs the vapor ammonia allowing the light hydrogen to rise back to the evaporator (4) Finally, the liquid ammonia-water mixture flows back into the generator (5) completing the cycle

liq-4 The Einstein Cycle

On November 11, 1930, Albert Einstein and Leo Szilard obtained U.S patent number 1,781,541 for a single pressure absorption refrigerator [15] Their cycle (see Fig 3) utilizes butane as its refrigerant, ammonia as a pressure equalizing fluid, and water as an absorbing fluid [1,11,18] The partial pressure of the butane is reduced by ammonia vapor and in-

(compo-Meanwhile, liquid water from the generator is sprayed into the condenser/absorber (35) With its great affinity for ammonia vapor, this sprayed water absorbs the vapor am-monia from the ammonia-butane mixture This absorption of the ammonia vapor increases the partial pressure on the butane vapor to nearly the total pressure, allowing it now to con-dense at butane's saturation temperature for the total pressure (higher than butane's satura-tion temperature at the partial pressure of the evaporator) The butane and the ammonia water separate due to their respective density differences and the fact that ammonia-water is immiscible with butane at the condenser/absorber's temperature and pressure Since liquid butane is less dense than liquid ammonia-water, it is the top liquid and is siphoned back to the evaporator In the interim, the ammonia-water mixture leaves from the bottom of the condenser/absorber (27) and enters the solution heat exchanger (component 28) Here, the mixture is pre-heated before entering the generator (component 29)

Inside the generator, heat is applied to the strong ammonia-water solution driving off ammonia vapor where it rises under the influence of pressure created by the liquid head, hJ, and is carried to the evaporator (1) The remaining weak ammonia-water solution is pumped up to a reservoir (component 35) via a bubble pump (component 36) [22,40,45-47] In the reservoir, any residual ammonia vapor from the bubble pump is sent to the con-denser/absorber The weak ammonia water solution falls to the solution heat exchanger where it gives off heat to the strong ammonia-water solution leaving the condenser Finally, the water is sprayed into the condenser/absorber (35)

Many conclusions can be gleaned from the cycle thermodynamic performance, tive working fluids performance, and bubble pump [8] performance results First, at a given system pressure, there is a minimum evaporator temperature and a maximum con-denser/absorber temperature The maximum condenser/absorber temperature is the satura-tion temperature of the refrigerant at the system pressure The minimum evaporator tem-perature depends upon the choice of pressure equalizing fluid and refrigerant fluid mixture For the case of the ammonia-butane mixture, the minimum evaporator temperature corre-sponds to the three phase flash temperature at the system pressure The maximum lift may

alterna-be adjusted by changing either the refrigerant fluid or the pressure equalizing fluid

5 Thermoelectric Cooling

The term "Thermoelectricity" is generally restricted to the irreversible conversion of tricity into heat described by the English physicist James P Joule and to three reversible effects named for Seebeck, Peltier, and Thomson, their respective discoverers According to Joule's law, a conductor carrying a current generates heat at a rate proportional to the prod-uct of the resistance (R) of the conductor and the square of the current (I) The German

elec-physicist Thomas J Seebeck discovered in the 1820s that if a closed loop is formed by ing the ends of two strips of dissimilar metals and the two junctions of the metals are at different temperatures, an electromotive force, or voltage, arises that is proportional to the temperature difference between the junctions A circuit of this type is called a thermocou-ple; a number of thermocouples connected in series are called a thermopile In 1834 the French physicist Jean C A Peltier discovered an effect inverse to the Seebeck effect: If a current passes through a thermocouple, the temperature of one junction increases and the temperature of the other decreases, so that heat is transferred from one junction to the other

Peters-Cold Side Ceramic

Figure 4 Thermoelectric cooler schematic

tions to junctions Unfortunately, the poor thermoelectric properties of known materials made them unsuitable for use in a practical refrigerating device As discussed by Nolas et

al [32], from the mid-1950s to the present the major thermoelectric material design proach was that introduced by A.V Ioffe, leading to semiconducting compounds such as Bi2 Te3, which is currently used in thermoelectric coolers These materials made possible the development of practical thermoelectric devices for attaining temperatures below ambient without the use of vapour-compression refrigeration [20,31,37]

ap-In recent years there has been increased interest in the application of thermoelectrics to electronic cooling, accompanied by efforts to improve their performance through the devel-opment of new bulk materials and thin film microcoolers [10,38,41] The modem commer-cial TEe consists of a number of p- and n- type semiconductor couples connected electri-cally in series and thermally in parallel These couples are sandwiched between two ther-mally conductive and electrically insulated substrates The heat pumping direction can be

changed by altering the polarity of the charging DC current A TEC schematic is illustrated

in Figure 4 Figure 5 depicts various sizes ofTECs

Figure 5 Various sizes ofTECs

The usefulness of thermoelectric materials for refrigeration is often characterized by the dimensionless product, ZT, of the thermoelectric figure of merit _Z and temperature T (in K) The value of the thermoelectric figure of merit is given by

de-The heat pumping capacity, Qp, ofa thermoelectric cooling module is given by

(2)

where N is the number of couples, G is the ratio of cross-sectional area/length of each

ther-moelectric element, I is the electrical current, and Tc is the cold side temperature in K, and

by relatively low heat flux A TEC can be used in different applications where cooling or temperature control of an object is required In general, TECs are most often used when an object:

I Needs to be cooled below the ambient temperature, or

2 Requires to be maintained at a constant temperature under a fluctuating ambient ture

tempera-TECs are perfect for cooling a small, low heat load object Due to the low COP pared to compressor cooling, TECs lose their advantage if the cooling load is higher than

com-200 watts However, because TECs have no moving parts, they are lightweight and reliable, they create no electrical noise, and can be operated in any orientation or environment In some instances, TECs are used to cool kilowatts of heat TECs are exceptionally suitable for precision temperature control of an object such as a laser diode, CCD or other small objects Paired with a DC power supply and an electronics proportional/integral (PI) con-troller packaged in a single chip device, TECs are able to control the temperature of an ob-ject to +/- 0.1 0 C accuracy Today, no other cooling method yet can provide such precise, simple and convenient temperature control

Thermoelectric cooler is a promising device for CPU cooling in personal computer, not only to remove the heat generated from CPU but also to reduce the operating temperature

of CPU in order to increase the CPU speed Actually, the feasibility of utilizing tric cooler is dependent of the available technology in heat sink and thermoelectric cooler efficiency, and the amount of heat generation of CPU

thermoelec-The COP of a thermoelectric cooler is usually smaller than 1.0 for commercial moelectric module presently available in market [27,28] In other word every Watt of heat generated in CPU that needs to be pumped to the ambient will require more than one watt

ther-of electrical energy input to the thermoelectric module Besides dissipating the heat ated in CPU, the heat sink needs to dissipate an additional amount of energy (the electrical energy input to the thermoelectric module) Hence, the utilization ofTE cooler will increase the energy dissipation load of a heat sink, unless the heat generated in CPU is not very large

gener-or the thermal resistance of the heat sink is low enough

Due to a rapid technology breakthrough in design and manufacturing techniques, the thermal resistance of heat sink (from heat source to ambient) has been reduced from 1.0 K./W for a conventional design using extruded aluminium fins to 0.6 K./W recently for a modern design using all-copper structure with large fin surface area and light weight (ap-proximately 230g) Under this circumstance, it is rather confusing whether one can benefit from using the thermoelectric device for CPU cooling, especially for high-speed CPU such

as AMD Athlon Series that has a heat load higher than 50W at 1 GHz clock speed

6 Magnetic Refrigeration

Magnetic refrigeration is a method of refrigeration based on the magnetocaloric effect [4,6,23,48] This effect, discovered in 1881, is defined as the response ofa solid to an ap-plied magnetic field, which appears as a change in its temperature This effect is obeyed by all transition metals and lanthanide-series elements When a magnetic field is applied, these metals, known as ferromagnets, tend to heat up As heat is applied, the magnetic moments align When the field is removed, the ferromagnet cools down as the magnetic moments become randomly oriented Gadolinium, a rare-earth metal, exhibits one of the largest known magnetocaloric effects It was used as the refrigerant for many of the early magnetic refrigeration designs

The problem with using pure gadolinium as the refrigerant material is that it does not exhibit a strong magnetocaloric effect at room temperature More recently, however, it has been discovered that arc-melted alloys of gadolinium, silicon, and germanium are more efficient at room temperature

34

Using the magnetocaloric effect for refrigeration purposes was first investigated in the mid-1920's but is just now nearing a point where it could be useful on a commercial scale The main difference associated with this process is the absence of a compressor The com-pressor is the most inefficient and expensive part of the conventional gas compression sys-tem In place of the compressor are small beds containing the magnetocaloric material, a small pump to circulate the heat transfer fluid, and a drive shaft to move the beds in and out

of the magnetic field The heat transfer fluid used in this process is water mixed with nol instead of the traditional refrigerants that pose threats to the environment

etha-A mixture of water and ethanol serves as the heat transfer fluid for the system The fluid first passes through the hot heat exchanger, which uses air to transfer heat to the at-mosphere The fluid then passes through the copper plates attached to the non-magnetized cooler magnetocaloric beds and loses heat A fan blows air past this cold fluid into the freezer to keep the freezer temperature at approximately -18° C The heat transfer fluid then gets heated up to 27° C as it passes through the copper plates adjoined by the magnetized warmer magneto caloric beds, where it continues to cycle around the loop However, the magnetocaloric beds simultaneously move up and down, into and out of the magnetic field Figure 6 shows how the cold air from the freezer is blown into the refrigerator by the freezer fan The temperature of the refrigerator section is kept around 4° C

The ultimate goal of magnetic refrigeration technology would be to develop a standard refrigerator for home use The usage of magnetic refrigeration has the potential to reduce operating cost and maintenance cost when compared to the conventional method of com-pressor-based refrigeration By eliminating the high capital cost of the compressor and the high cost of electricity to operate the compressor, magnetic refrigeration can efficiently and economically replace compressor-based refrigeration The major advantages of the mag-netic refrigeration technology over compressor-based refrigeration are the design technol-ogy, environmental impact, and operating cost savings

7 Conclusion Remarks

Vapour compression systems require work to power the compressor, which is generally provided by electricity from thermally driven (usually combustion) central power plants In

contrast, direct thermally driven cycles eliminate the need for a central power plant Instead

of converting heat into work and then using work to pump heat through a temperature lift, they use heat to pump heat directly

Nowadays absorption refrigeration cycles technology is growing rapidly and has came a real alternative to compression refrigeration cycles The application of absorption refrigeration cycles to reduce energy costs requires the consumption of the industrial plant

be-to be assessed The plants, which are most suitable for absorption refrigeration cycles gration, are those that need chilled water and have a low temperature waste heat source This means:

inte-• considerable economic savings as a result of the drastic reduction in the mary energy consumption caused by a mechanical compression refrigeration system,

pri-• a source of waste heat is used,