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Refrigeration to below 20 K
The difficulties of cryogenic refrigeration are outlined and short descriptions are given of the refrigeration cycles which attain temperatures down to 0.03 K. Details of applications of available cryogenic equipment are presented. The refrigeration cycles are analysed on
Refrigeration W.
to below
the basis of the similarities of work (reversible or nearly reversible) and heat exchange mechanism. In conclusion, the author indicates that a considerable market is developing for cryogenic refrigerators in the range 2-20 K.
20 K
E. Gifford
Although liquid hydrogen was first liquefied in 1898 by Sir James Dewar, research activity at temperatures of 20 K or colder was not very great until after 1946. Since then, research has expanded immensely and many varied types of applications have appeared, the expectations are that many, many more will appear. One might expect even more interesting possibilities below 20 K than above 300 K, though one has only 20 K to go down and an infinite number to go up, because basic changes in matter are proportional to temperature ratios rather than temperature dfflerences. As many basic changes in matter in decreasing temperature are expected from 10 to 5 K as from I 000 to 500 K. If the temperature is increased about 10 times over room temperature, everything is a gas and is very difficult to work with. In lowering the temperature, one can achieve temperatures as low as 0.001 K, one twentythousandth of the 20 K. This interesting concept shows in the complexity of the refrigeration problem below 20 K. Whereas refrigeration to 20 K can be achieved with very simple methods-gas compression, heat exchange, and expansion-as one goes below this point, the problem becomes much more complex. Quite a few different methods have to be used and involve unusual concepts. Cryogenic
refrigeration
Achieving refrigeration at temperatures from room temperature down to near the absolute zero is achieved with thermo-mechanical devices for the most part. These devices must be built with conceptual components and tools to overcome difficulties. As one tries to achieve the lower temperatures, the conceptual components one can use vary and the possibility of using them decreases greatly. Also, as the temperature is lowered, new difficulties arise. It is interesting to see a compact list of the difficulties as they arise to see just what it is one has to work against. Difficulties of cryogenic refrigeration in the rough order that they appear as the temperature is lowered. The author is Professor of Mechanical and Aerospace Engineering, Syracuse University, One Smith Hall, Syracuse, New York 13210, USA and President of Cryomech Inc., 314 Ainsley Drive, Jamesville, New York 13076, USA. Received 5 December 1969. CRYOGENICS.
FEBRUARY
1970
I elastic room critical
qf l’ery elastic seals. Materials which have very properties of rubber do not exist much below temperature (250-270 K). Such seals are very in the design of cryogenic machinery.
Loss
2 Loss of lubricating oils. Fluids which have long polymer molecules and have properties of lubricating oils do not exist much below room temperature. Needless to say, lubricating oils are important in machinery to reduce friction and wear on rubbing metal parts, as well as to allow one to seal a piston and cylinder with a relatively large clearance. Attaining high reliability in non-oil-lubricated equipment is very difficult. 3
Single
vapour
pressure
curlIe becomes
insuficient.
As one tries to refrigerate over too wide a range from room temperature, vapour pressure curves simply do not give sufficient range, so one cannot use single stage vapour cycle refrigeration methods such as freon or ammonia systems. Multistage systems must be used and this introduces a considerable amount of undesirable complexity. 4 Some metals become very brittle. One is restricted in the metals one can use, as some metals become so brittle that they are very dangerous. 5 The thermal insulation must be sealed. Below the liquefaction point of air, the thermal insulation of refrigeration systems must be sealed or air will move through the insulation and condense, causing a very high heat leak. Thermal insulation costs must be greatly increased. 6 Air becomes CI plugging impurity. Once the temperature is lowered below the triple point of air, air will freeze out and plug the fine passages of heat exchangers, valves, or engines, possible working gases (only helium, hydrogen and neon) must be purified to a very high degree (a part or two per million, rather than per thousand)-not a small problem when one considers the desorption of vapours which might come from the oil in the compressor. 7
Hear
capacities
of’ normal
solids become very small.
This decrease in heat capacity makes it impossible to manufacture good thermal regenerator matrices out of 23
materials which are solid at room temperature, it limits the temperature one can operate to with thermal regenerators to 7 K. (Whereas this is a disadvantage in refrigerators using regenerators, it is advantageous in building a Simon type helium liquefier.8~9~10~15In such a system the heat capacity of metal would use up available refrigeration.) 8 Gaspressures are not available. Below I K, no gas has a significant vapour pressure. Therefore, one cannot expand gases for useful refrigeration purposes or even use them in gas bearings. The above list is rather formidable. However, the problems are not quite as bad as it suggests. As temperatures are lowered, very significant advantages also appear and help immensely in the job of attaining the low temperatures. The following is a list of the most significant of these advantages. I Some metals become This insulation: is important regions. Heat leak through be much less than at room
very good tkermal insulators. in reducing he&t leak to cold constructional materials can temperature.
2 Some metals become rery good thermal conductors. This conduction may be very important in the design of the heat exchangers necessary for refrigerators. 3 Most solids lose virtually all their heat capacity. This loss makes the very little refrigeration achieved in adiabatic demagnetization useful, as it can yield big temperature differences, also losses are cut in Simon type helium liquefaction by reducing the refrigeration required to cool the container. 4 Some metals become very strong. Some metals become much stronger at low temperatures. Also, other metals, when work-hardened at low temperatures, become even stronger. 5 Helium 3 dilution refrigerator becomes a possibility. Below the y-point for liquid helium, mixing varied concentrations of liquid 3He and 4He causes the 3He to act like an expanding gas and cool. 6 Adiabatic demagnetization becomes a possibility. As soon as the heat capacities of virtually all solids become very small, the small amount of reversible work which can be performed on paramagnetic salts can be useful in introducing large changes in the temperature of the materials. Many refrigeration methods have been used to attain temperatures below 20 K. Table 1 gives a list of all the useful ones and the temperature ranges where they are of interest. The first seven methods can be used on a continuous basis with only a room temperature heat sink. The last three must be coupled with one of the first seven to attain a low temperature, then they perform in a discontinuous manner and usually give refrigeration for relatively short periods. These refrigeration methods are quite different and it is, therefore, quite difficult to make generalizations about 24
Table
1. Useful
refrigeration
methods
for below
Method 1 2
Tempera& we,
Joule-Thomson method Brayton (Stirling) cycle 1,2.3.4 Claude cycle Expansion engines Expansion turbines Solvay cycle5 Gifford-McMahon cycle B-1 Simon Helium liquefaction 8~9*1”.11 Adiabatic demagnetization ‘I.‘* Helium 3 dilution and related methods
3
4 5 6
7 8 9 10
20 K K
2300 12300 l-150 1 o-1 50
1 o-1 50 1 o-1 50 7-l 50 4-l 0 o-1 o-0.03
w’~
them as there are so many ways that the divisions can be made-size, temperature. region of applicction, use of particular component, etc. Also, the divisions overlap a great deal. There are, however, two similarities between all the methods in Table I. The first similarity is work, frequently reversible or nearly reversible work, and the second is an efficient heat exchanger means. Work is performed on a material at a high temperature, giving a heating effect which is dissipated to the surroundings. Then the material is allowed to do work at a low temperature: cooling and the ability to adsorb heat results-to achieve refrigeration. For the first eight methods in Table I the material is always a gas-helium for temperatures below I5 K. The work is gas compression and expansion. It is necessary that the work be significant relative to the internal energy of the material. This necessity is, of course, true for gas compression or expansion and, therefore, substantial changes are achieved. If the work is very small relative to the internal energy, significant refrigeration cannot be achieved. Magnetization and electrification work cannot be useful at temperatures where CV = 3 R per molecule. The compression or expansion may occur in separate reciprocating or rotary compressors and expanders or together in one piece of machinery in different regions, such as in the Brayton or Stirling cycle ~JJJ equipment. Also, some of the compression and expansion may be in different parts of the same equipment, as in the GiffordMcMahon cycle.6*7 At the temperatures where no liquid lubricants are possible, reliable mechanical equipment is difficult to build. Ways of overcoming this difficulty are by rotating turbines on gas bearings and very simple, large clearance, slow moving devices, as are possible with the GiffordMcMahon cycle. Figure I shows a Cryomech Inc., Gifford-McMahon cycle 7.5 K refrigerator.
,;‘.f, . . .i ‘s’.
‘, : ,. .
Figure 1. Cryomech refrigerator
Inc.,
Gifford-McMahon
CRYOGENICS
cycle
. FEBRUARY
7.5 K
1970
At low enough temperatures where the lattice vibrational internal energy virtually no longer exists, the work of magnetization Wm can be much greater than the internal energy E. This phenomenon makes possible the achievement of very great temperature drop ratios in a one-cycle demagnetization. For gases, the work of compression WC and expansion We are only of the same order of magnitude as the internal energy. Therefore, only a drop in temperature of the order of half the initial temperature is possible in an adiabatic gas expansion. Cryogenic temperatures are not really possible in a one-cycle expansion. The other essentialcomponent of any low temperature refrigeration system, as mentioned above, is a very efficient heat exchange means for cooling the high pressure gas, prior to expansion, with the exhausting low pressure gas. This cooling is an absolutely essential element, as in expansion the gas only approximately drops to one-half its initial temperature, Ti. T, = 0.5 Ti
However, prior to expansion it must be cooled from room temperature, 300 K, to 7i. If Ti is to be 10 K, the heat exchange means must supply 290 K of cooling when the gas gives only 5 K temperature drop. As a result, the heat exchange means must have efficiencies of the order of 97*0-99.5%. Building heat exchangers such as this is a difficult job, especially if they have to be small. Heat exchange means may be divided into two classes -counterflow heat exchangers and thermal regenerators. In counterflow heat exchangers, the high pressure gas flows through separate channels from the low pressure gas. However, the two gases are in excellent thermal contact. In thermal regenerators, the high pressure and low pressure gases flow through the same channels at different times. The heat removed from the high pressure gas in cooling is stored in the matrix material of the regenerator. Later, the heat is restored to the samegas, now at low pressure, prior to being exhausted. Generally, when the gas expands it does work in a reciprocating compressor or turbine which must be withdrawn from the equipment by some meanssuch as a shaft. The Joule-Thomson method is special since the work done is against intermolecular forces and there is no need to take energy out by mechanical means.The JouleThomson method, however, is of course rather inefficient except when quite close to the boiling point of the refrigerant. The Simon method for liquefaction of heliurn8.9,*0~is consists of a single expansion of high pressure helium from 2,000 lb/in2 a to atmospheric pressure. If the initial temperature is about 10 K, the container will very nearly fill with liquid helium. Recently, the Simon method has also been operated conveniently from a GiffordMcMahon cycle 617refrigerator. There has not as yet been any real application (other than research) for temperatures lower than 1 K. Therefore, adiabatic demagnetization has only been used as a research tool. This usage may change soon with the highly expanded research activities. Adiabatic demagnetization is only useful at very low temperatures and precooling with one of the other methods is necessary. In adiabatic demagnetization, one is working only with solids and therefore cannot use fluid heat exchangers. CRYOGENICS.
FEBRUARY
1970
Fortunately, the internal energy is so small that, when a paramagnetic salt does work in expelling a magnetic field of the order of lo-20 kG, its temperature can drop from 1K to aslow asr& its initial temperature, 0.001K. Though the work of demagnetization is very small, the internal energy is much smaller and the small available refrigeration achieves a large temperature ratio. The newest method of cryogenic refrigeration to temperatures as low as 0.03 K is helium 3 dilution. A very brief description of the mechanism by which this is achieved is the following. Different solutions of 3He and 4He are cooled in separate passagesof a heat exchanger by an exhausting mixture of 3He and 4He. At the cold end of the heat exchanger when the two separate streams are mixed, the 4He is almost entirely superfluid and therefore the 3He expands as if the 4He was not there, does work, and cools. There has been a great deal of experimentation with this and related methods in many laboratories. The only use as yet is in research. Applications
and
available
equipment
A fairly good way to classify cryogenic refrigeration equipment is simply by size. As one reduces size, one finds the most advisable methods and heat exchange meanschange. The biggest application for cryogenic temperatures is the production of oxygen and liquid gasesfor convenient transport. The application includes liquid oxygen, methane, nitrogen, hydrogen. and others. The field is relatively old and involves typically large plants and equipment. There also have been some recent applications for unusual, large size refrigerators for research. These research applications include large cryopumping installations for space chambers, nuclear reactor cold chambers, and superconducting high Q cavities for a linear accelerator. The refrigeration methods are generally expansion engines and expansion turbines in the Claude cycle or various related forms of the Claude cycle. The equipment is really quite satisfactory as to cost, reliability and efficiency. Costsare such that the expenseof the refrigeration equipment does not make important applications impractical. The heat exchange means are sometimes counterflow heat exchangers and sometimes thermal regenerators, relative advantages are debatable, since both have advantages in special cases.There is always the effect of quality: a good turbine plant is better than a poor expansion engine plant, and vice versa! Large plants are sometimes augmented by the JouleThomson method. Some small plants are still built which use the Joule-Thomson method exclusively. Efficiency is not that important, frequently they operate at 78 K. Power costs are not a big part of the total cost. The newest large equipment has been the Philips Eindhoven’s introduction of Brayton (Stirling) cycle 1.2.3.4refrigerators into this field for some of the smaller loads. Their advantages for large units are debatable except in special cases. As a result of the very large increasein low temperature research since 1946, there have appeared several nonresearch applications for cryogenic temperatures-with many more coming. The refrigeration loads are typically very small. However, reliability requirements are very stringent. Small equipment is desired which will maintain low temperatures indefinitely without trouble. 25
Table
2. Applications
of small
load
cryogenic
Application Infra-red sensors Maser amplifiers Parametric amplifiers Lasers Small cryopumps High Q cavities Magnetometers Very sensitive voltmeters Transformers Motors Magnets Generators
refrigerators Temperature,
K
4, 30, 78 2-5 20, 50, 78
2-16
(possibilities based superconductivity)
on
I
Some of the applications are given in Table 2. In all of these applications small refrigerators will be required to maintain reliable low temperatures without carrying any very large load. The heat load is mostly heat leak by both conduction and radiation and may be only a fraction of a watt. The problems are that the refrigerators should be very reliable and small in size. The presently available equipment has the following characteristics. 1 The different items available are far too few (equipment for applications is a complicated function of many variables such as load, temperature, size, weight, cost and reliability). The applications will require different items of equipment. 2 The operation is not reliable enough. Unattended operating times of 5 000-20 000 h are desired. 3 The costs seem too high, because of the lack of volume sales in any one item. If there were a wide range of different items at reasonable cost, uses would be found. There are reasons for this shortage. The problems of small cryogenic refrigerators differ and, therefore, require different solutions. For example, making small enough turbine expanders is an exceedingly difficult task. Figure 2 shows a British Oxygen turbine which has a capacity of the order of 100-200 W at IO-20 K. Many of the small requirements are for only 0.5-2 W. The speeds are also enormous (200 000 to 400 000 r p m) and the tolerances very precise. It is obvious that it would be very difficult to reduce such a device one hundred times and achieve efficiency and low costs. When one tries to make cryogenic refrigerators small, one finds that thermal regenerators show immense advantages over counterflow heat exchangers. When an attempt is made to reduce the heat exchange means length from 3-10 ft, as it may be in large equipment, to less than one foot, thermal regenerators show the following four very important advantages. 1 They can be made much smaller and more efficient than counterflow heat exchangers. A regenerator 3-4 in. long can have an over-all efficiency of 99% or better. An efficiency of 98% in a 1 ft heat exchanger is very difficult to achieve. A 1% difference in heat exchange means efficiency can be very important in cryogenic refrigerators. 2 Their construction is much simpler and, therefore, they are much less costly to make. 3 They are almost completely insensitive to plugging by condensable impurities in the gas stream 26
Figure
2. British
Oxygen
Company
cryogenic
gas turbine
though their passages may be very much smaller. This is very important as the flow passages in counterflow heat exchangers get smaller with size and thus are very sensitive to plugging. The applications for small refrigerators typically call for very long reliabilities and therefore plugging would be more likely to occur. The reversing flow at varying pressures flushes the condensable impurities out of thermal regenerators. Refrigerators operating to 20 K using regenerators can function well when gas is saturated with water vapour. 4 The use of regenerators leads to the possibility of simplifying the construction of the refrigerator as in the Brayton cycle i,2,3.4 devices from which valves are eliminated, the Gifford-McMahon cycle 6.7 devices from which the high pressure cylinder seal is eliminated, and in which valves are simple and seals are at room temperature. When these advantages are considered, it should come as no surprise that a majority of the most successful small refrigerators use thermal regenerators. These are the following cycles. I Brayton (Stirling) cycle Manufactured by: Philips Eindhoven North American Philips Malaker Corporation (Figure 3 shows a small refrigerator of this type) 2 The Gifford-McMahon cycle Manufactured by: Cryogenic Technology Inc. The Welch Vacuum Pump Company Cryomech Inc. (Figure 1) 3 The Vuilleumier cycle Manufactured by: Hughes Aircraft Company There is one exception to the above comments about heat exchangers. One type of heat exchanger can be small and efficient. It is the tiny, finned capillary heat exchanger from which Joule-Thomson systems can be made. They are, however, limited in size and application. Only one tube must be used, or a very difficult balance problem develops. Also, the density and pressure on the two sides must be very different. Quite a number of American firms have made such CRYOGENICS
. FEBRUARY
1970
refrigerators for infra-red applications-Garrett Corporation, Santa Barbara Research, Air Products Company, as well as the Hymatic Engineering Company in England. typical Joule-Thomson refrigerator heat Some exchangers made by the Hymatic Engineering Co. are shown in Figure 4. The very high gas compression ratios make the compressor a difficult and expensive item to
Figure 3. Malaker refrigerator
Corporation
Brayton
(Stirling)
cycle
build. As a result, several firms sell only the heat exchanger to operate from a gas bottle. Two and threestage units can reach temperatures of 20-4.2 K. Conclusions There is a considerable market developing for cryogenic refrigerators in the range 2-20 K for many applications. For large capacities adequate equipment is available from quite a few sources. For small capacities (I-10 W) there are many developing applications. Also, considerable equipment has been developed and new models will be needed in the near future. The development of tiny high speed turbines seems to have been abandoned and Stirling cycle devices are having trouble meeting the needs for reliability and lack of vibration. The most successful models involve slow speed (72-l 50 r p m) refrigerators using regenerators and separate compressors and gas purification systems, such as the Gifford-McMahon cycle equipment and Vuilleumier cycle systems. Figure 1 shows a Cryomech Inc. twostage refrigerator which achieves 7.5 K. Uses for temperatures lower than 1 K are still only for basic research. Adiabatic demagnetization equipment is not available commercially. Helium 3 dilution equipment is available. However, most research workers prefer to build their own as the available systems are not really perfected or optimized. Many cryogenic research workers are trying to improve on the performance which has been attained so far.
REFERENCES
Figure 4. Joule-Thomson by the Hymatic Engineering
CRYOGENICS
. FEBRUARY
heat exchangers manufactured Company Limited
1970
1. KOHLER, J. W. L., and YONKERS, C. 0. Philips Techical Reoiew 16 (1954) J. W. L. Ptogress in Cryogenics, Vol. 2, 43, 2. KOHLER, t Heywood, London, 1960) 3. PRAST, G. Ciyogettics. 2, I56 (1963) 4. PRAST, G. Inrerrtalionnl Advances in Cryogenic Engineering 10, 40 (I 965) 5. COWANS, K. W., and WALSH, P. J. Advcrnces in Cryogenic D@neeriqy 10, 468 ( 1965) 6. GIFFORD, W. E., and MCMAHON, H. 0. ‘A low temperature heat pump,’ Proc. of 10th International Congress of Refrigeration (Copenhagen, Denmark, August 1959) 7. GIFFORD. W. E. Advances in Cryogenic Engineering 11 ( 1966) 8. SIMON, F. E. Zeirsges Kalre-Indtsrrie 39, 89 (1932) 9. SCOTT, R. B., and COOK, J. W. RSI 19, 889 (1948) IO. GIFFORD, W. E. Master’s Thesis, Catholic University, Washington DC ( 1950) II. DEKLERK, D., STEENLAND, M. J., and GORTER, C. J. Pllysicu 16. 861 (1950) 12. WHITE, G. K. Experimental Techniques in Low-Temperature Physics, 219 (Oxford University Press, NJ, 1959) 13. DAS, P., DEBRUYN OUBOTER, R., and TACONIS, K. W. Low Temperature Physics, LT9B. 1253 (Plenum Press, New York, 1965) 14. WHEATLEY, J. C. American Jorwnnl of Pl!)~sics 36, 181 (1968) 15. GIFFORD, W. E., KADAIKKAL, N., and ACHARYA. A. ‘A Simon helium liquefier with a Gitford-McMahon cycle for precooling’ (Presented at 1969 Cryogenic Engineering Conference, Univ. of California at Los Angeles, June, 1969) 16. DAUNT, J. G., and GOREE. W. S. ‘Miniature cryogenic refrigerators,’ (Study performed for Office of Naval Research with Stevens Institute of Technology and Stanford Research Institute)
21
Refrigeration W.
to below
the basis of the similarities of work (reversible or nearly reversible) and heat exchange mechanism. In conclusion, the author indicates that a considerable market is developing for cryogenic refrigerators in the range 2-20 K.
20 K
E. Gifford
Although liquid hydrogen was first liquefied in 1898 by Sir James Dewar, research activity at temperatures of 20 K or colder was not very great until after 1946. Since then, research has expanded immensely and many varied types of applications have appeared, the expectations are that many, many more will appear. One might expect even more interesting possibilities below 20 K than above 300 K, though one has only 20 K to go down and an infinite number to go up, because basic changes in matter are proportional to temperature ratios rather than temperature dfflerences. As many basic changes in matter in decreasing temperature are expected from 10 to 5 K as from I 000 to 500 K. If the temperature is increased about 10 times over room temperature, everything is a gas and is very difficult to work with. In lowering the temperature, one can achieve temperatures as low as 0.001 K, one twentythousandth of the 20 K. This interesting concept shows in the complexity of the refrigeration problem below 20 K. Whereas refrigeration to 20 K can be achieved with very simple methods-gas compression, heat exchange, and expansion-as one goes below this point, the problem becomes much more complex. Quite a few different methods have to be used and involve unusual concepts. Cryogenic
refrigeration
Achieving refrigeration at temperatures from room temperature down to near the absolute zero is achieved with thermo-mechanical devices for the most part. These devices must be built with conceptual components and tools to overcome difficulties. As one tries to achieve the lower temperatures, the conceptual components one can use vary and the possibility of using them decreases greatly. Also, as the temperature is lowered, new difficulties arise. It is interesting to see a compact list of the difficulties as they arise to see just what it is one has to work against. Difficulties of cryogenic refrigeration in the rough order that they appear as the temperature is lowered. The author is Professor of Mechanical and Aerospace Engineering, Syracuse University, One Smith Hall, Syracuse, New York 13210, USA and President of Cryomech Inc., 314 Ainsley Drive, Jamesville, New York 13076, USA. Received 5 December 1969. CRYOGENICS.
FEBRUARY
1970
I elastic room critical
qf l’ery elastic seals. Materials which have very properties of rubber do not exist much below temperature (250-270 K). Such seals are very in the design of cryogenic machinery.
Loss
2 Loss of lubricating oils. Fluids which have long polymer molecules and have properties of lubricating oils do not exist much below room temperature. Needless to say, lubricating oils are important in machinery to reduce friction and wear on rubbing metal parts, as well as to allow one to seal a piston and cylinder with a relatively large clearance. Attaining high reliability in non-oil-lubricated equipment is very difficult. 3
Single
vapour
pressure
curlIe becomes
insuficient.
As one tries to refrigerate over too wide a range from room temperature, vapour pressure curves simply do not give sufficient range, so one cannot use single stage vapour cycle refrigeration methods such as freon or ammonia systems. Multistage systems must be used and this introduces a considerable amount of undesirable complexity. 4 Some metals become very brittle. One is restricted in the metals one can use, as some metals become so brittle that they are very dangerous. 5 The thermal insulation must be sealed. Below the liquefaction point of air, the thermal insulation of refrigeration systems must be sealed or air will move through the insulation and condense, causing a very high heat leak. Thermal insulation costs must be greatly increased. 6 Air becomes CI plugging impurity. Once the temperature is lowered below the triple point of air, air will freeze out and plug the fine passages of heat exchangers, valves, or engines, possible working gases (only helium, hydrogen and neon) must be purified to a very high degree (a part or two per million, rather than per thousand)-not a small problem when one considers the desorption of vapours which might come from the oil in the compressor. 7
Hear
capacities
of’ normal
solids become very small.
This decrease in heat capacity makes it impossible to manufacture good thermal regenerator matrices out of 23
materials which are solid at room temperature, it limits the temperature one can operate to with thermal regenerators to 7 K. (Whereas this is a disadvantage in refrigerators using regenerators, it is advantageous in building a Simon type helium liquefier.8~9~10~15In such a system the heat capacity of metal would use up available refrigeration.) 8 Gaspressures are not available. Below I K, no gas has a significant vapour pressure. Therefore, one cannot expand gases for useful refrigeration purposes or even use them in gas bearings. The above list is rather formidable. However, the problems are not quite as bad as it suggests. As temperatures are lowered, very significant advantages also appear and help immensely in the job of attaining the low temperatures. The following is a list of the most significant of these advantages. I Some metals become This insulation: is important regions. Heat leak through be much less than at room
very good tkermal insulators. in reducing he&t leak to cold constructional materials can temperature.
2 Some metals become rery good thermal conductors. This conduction may be very important in the design of the heat exchangers necessary for refrigerators. 3 Most solids lose virtually all their heat capacity. This loss makes the very little refrigeration achieved in adiabatic demagnetization useful, as it can yield big temperature differences, also losses are cut in Simon type helium liquefaction by reducing the refrigeration required to cool the container. 4 Some metals become very strong. Some metals become much stronger at low temperatures. Also, other metals, when work-hardened at low temperatures, become even stronger. 5 Helium 3 dilution refrigerator becomes a possibility. Below the y-point for liquid helium, mixing varied concentrations of liquid 3He and 4He causes the 3He to act like an expanding gas and cool. 6 Adiabatic demagnetization becomes a possibility. As soon as the heat capacities of virtually all solids become very small, the small amount of reversible work which can be performed on paramagnetic salts can be useful in introducing large changes in the temperature of the materials. Many refrigeration methods have been used to attain temperatures below 20 K. Table 1 gives a list of all the useful ones and the temperature ranges where they are of interest. The first seven methods can be used on a continuous basis with only a room temperature heat sink. The last three must be coupled with one of the first seven to attain a low temperature, then they perform in a discontinuous manner and usually give refrigeration for relatively short periods. These refrigeration methods are quite different and it is, therefore, quite difficult to make generalizations about 24
Table
1. Useful
refrigeration
methods
for below
Method 1 2
Tempera& we,
Joule-Thomson method Brayton (Stirling) cycle 1,2.3.4 Claude cycle Expansion engines Expansion turbines Solvay cycle5 Gifford-McMahon cycle B-1 Simon Helium liquefaction 8~9*1”.11 Adiabatic demagnetization ‘I.‘* Helium 3 dilution and related methods
3
4 5 6
7 8 9 10
20 K K
2300 12300 l-150 1 o-1 50
1 o-1 50 1 o-1 50 7-l 50 4-l 0 o-1 o-0.03
w’~
them as there are so many ways that the divisions can be made-size, temperature. region of applicction, use of particular component, etc. Also, the divisions overlap a great deal. There are, however, two similarities between all the methods in Table I. The first similarity is work, frequently reversible or nearly reversible work, and the second is an efficient heat exchanger means. Work is performed on a material at a high temperature, giving a heating effect which is dissipated to the surroundings. Then the material is allowed to do work at a low temperature: cooling and the ability to adsorb heat results-to achieve refrigeration. For the first eight methods in Table I the material is always a gas-helium for temperatures below I5 K. The work is gas compression and expansion. It is necessary that the work be significant relative to the internal energy of the material. This necessity is, of course, true for gas compression or expansion and, therefore, substantial changes are achieved. If the work is very small relative to the internal energy, significant refrigeration cannot be achieved. Magnetization and electrification work cannot be useful at temperatures where CV = 3 R per molecule. The compression or expansion may occur in separate reciprocating or rotary compressors and expanders or together in one piece of machinery in different regions, such as in the Brayton or Stirling cycle ~JJJ equipment. Also, some of the compression and expansion may be in different parts of the same equipment, as in the GiffordMcMahon cycle.6*7 At the temperatures where no liquid lubricants are possible, reliable mechanical equipment is difficult to build. Ways of overcoming this difficulty are by rotating turbines on gas bearings and very simple, large clearance, slow moving devices, as are possible with the GiffordMcMahon cycle. Figure I shows a Cryomech Inc., Gifford-McMahon cycle 7.5 K refrigerator.
,;‘.f, . . .i ‘s’.
‘, : ,. .
Figure 1. Cryomech refrigerator
Inc.,
Gifford-McMahon
CRYOGENICS
cycle
. FEBRUARY
7.5 K
1970
At low enough temperatures where the lattice vibrational internal energy virtually no longer exists, the work of magnetization Wm can be much greater than the internal energy E. This phenomenon makes possible the achievement of very great temperature drop ratios in a one-cycle demagnetization. For gases, the work of compression WC and expansion We are only of the same order of magnitude as the internal energy. Therefore, only a drop in temperature of the order of half the initial temperature is possible in an adiabatic gas expansion. Cryogenic temperatures are not really possible in a one-cycle expansion. The other essentialcomponent of any low temperature refrigeration system, as mentioned above, is a very efficient heat exchange means for cooling the high pressure gas, prior to expansion, with the exhausting low pressure gas. This cooling is an absolutely essential element, as in expansion the gas only approximately drops to one-half its initial temperature, Ti. T, = 0.5 Ti
However, prior to expansion it must be cooled from room temperature, 300 K, to 7i. If Ti is to be 10 K, the heat exchange means must supply 290 K of cooling when the gas gives only 5 K temperature drop. As a result, the heat exchange means must have efficiencies of the order of 97*0-99.5%. Building heat exchangers such as this is a difficult job, especially if they have to be small. Heat exchange means may be divided into two classes -counterflow heat exchangers and thermal regenerators. In counterflow heat exchangers, the high pressure gas flows through separate channels from the low pressure gas. However, the two gases are in excellent thermal contact. In thermal regenerators, the high pressure and low pressure gases flow through the same channels at different times. The heat removed from the high pressure gas in cooling is stored in the matrix material of the regenerator. Later, the heat is restored to the samegas, now at low pressure, prior to being exhausted. Generally, when the gas expands it does work in a reciprocating compressor or turbine which must be withdrawn from the equipment by some meanssuch as a shaft. The Joule-Thomson method is special since the work done is against intermolecular forces and there is no need to take energy out by mechanical means.The JouleThomson method, however, is of course rather inefficient except when quite close to the boiling point of the refrigerant. The Simon method for liquefaction of heliurn8.9,*0~is consists of a single expansion of high pressure helium from 2,000 lb/in2 a to atmospheric pressure. If the initial temperature is about 10 K, the container will very nearly fill with liquid helium. Recently, the Simon method has also been operated conveniently from a GiffordMcMahon cycle 617refrigerator. There has not as yet been any real application (other than research) for temperatures lower than 1 K. Therefore, adiabatic demagnetization has only been used as a research tool. This usage may change soon with the highly expanded research activities. Adiabatic demagnetization is only useful at very low temperatures and precooling with one of the other methods is necessary. In adiabatic demagnetization, one is working only with solids and therefore cannot use fluid heat exchangers. CRYOGENICS.
FEBRUARY
1970
Fortunately, the internal energy is so small that, when a paramagnetic salt does work in expelling a magnetic field of the order of lo-20 kG, its temperature can drop from 1K to aslow asr& its initial temperature, 0.001K. Though the work of demagnetization is very small, the internal energy is much smaller and the small available refrigeration achieves a large temperature ratio. The newest method of cryogenic refrigeration to temperatures as low as 0.03 K is helium 3 dilution. A very brief description of the mechanism by which this is achieved is the following. Different solutions of 3He and 4He are cooled in separate passagesof a heat exchanger by an exhausting mixture of 3He and 4He. At the cold end of the heat exchanger when the two separate streams are mixed, the 4He is almost entirely superfluid and therefore the 3He expands as if the 4He was not there, does work, and cools. There has been a great deal of experimentation with this and related methods in many laboratories. The only use as yet is in research. Applications
and
available
equipment
A fairly good way to classify cryogenic refrigeration equipment is simply by size. As one reduces size, one finds the most advisable methods and heat exchange meanschange. The biggest application for cryogenic temperatures is the production of oxygen and liquid gasesfor convenient transport. The application includes liquid oxygen, methane, nitrogen, hydrogen. and others. The field is relatively old and involves typically large plants and equipment. There also have been some recent applications for unusual, large size refrigerators for research. These research applications include large cryopumping installations for space chambers, nuclear reactor cold chambers, and superconducting high Q cavities for a linear accelerator. The refrigeration methods are generally expansion engines and expansion turbines in the Claude cycle or various related forms of the Claude cycle. The equipment is really quite satisfactory as to cost, reliability and efficiency. Costsare such that the expenseof the refrigeration equipment does not make important applications impractical. The heat exchange means are sometimes counterflow heat exchangers and sometimes thermal regenerators, relative advantages are debatable, since both have advantages in special cases.There is always the effect of quality: a good turbine plant is better than a poor expansion engine plant, and vice versa! Large plants are sometimes augmented by the JouleThomson method. Some small plants are still built which use the Joule-Thomson method exclusively. Efficiency is not that important, frequently they operate at 78 K. Power costs are not a big part of the total cost. The newest large equipment has been the Philips Eindhoven’s introduction of Brayton (Stirling) cycle 1.2.3.4refrigerators into this field for some of the smaller loads. Their advantages for large units are debatable except in special cases. As a result of the very large increasein low temperature research since 1946, there have appeared several nonresearch applications for cryogenic temperatures-with many more coming. The refrigeration loads are typically very small. However, reliability requirements are very stringent. Small equipment is desired which will maintain low temperatures indefinitely without trouble. 25
Table
2. Applications
of small
load
cryogenic
Application Infra-red sensors Maser amplifiers Parametric amplifiers Lasers Small cryopumps High Q cavities Magnetometers Very sensitive voltmeters Transformers Motors Magnets Generators
refrigerators Temperature,
K
4, 30, 78 2-5 20, 50, 78
2-16
(possibilities based superconductivity)
on
I
Some of the applications are given in Table 2. In all of these applications small refrigerators will be required to maintain reliable low temperatures without carrying any very large load. The heat load is mostly heat leak by both conduction and radiation and may be only a fraction of a watt. The problems are that the refrigerators should be very reliable and small in size. The presently available equipment has the following characteristics. 1 The different items available are far too few (equipment for applications is a complicated function of many variables such as load, temperature, size, weight, cost and reliability). The applications will require different items of equipment. 2 The operation is not reliable enough. Unattended operating times of 5 000-20 000 h are desired. 3 The costs seem too high, because of the lack of volume sales in any one item. If there were a wide range of different items at reasonable cost, uses would be found. There are reasons for this shortage. The problems of small cryogenic refrigerators differ and, therefore, require different solutions. For example, making small enough turbine expanders is an exceedingly difficult task. Figure 2 shows a British Oxygen turbine which has a capacity of the order of 100-200 W at IO-20 K. Many of the small requirements are for only 0.5-2 W. The speeds are also enormous (200 000 to 400 000 r p m) and the tolerances very precise. It is obvious that it would be very difficult to reduce such a device one hundred times and achieve efficiency and low costs. When one tries to make cryogenic refrigerators small, one finds that thermal regenerators show immense advantages over counterflow heat exchangers. When an attempt is made to reduce the heat exchange means length from 3-10 ft, as it may be in large equipment, to less than one foot, thermal regenerators show the following four very important advantages. 1 They can be made much smaller and more efficient than counterflow heat exchangers. A regenerator 3-4 in. long can have an over-all efficiency of 99% or better. An efficiency of 98% in a 1 ft heat exchanger is very difficult to achieve. A 1% difference in heat exchange means efficiency can be very important in cryogenic refrigerators. 2 Their construction is much simpler and, therefore, they are much less costly to make. 3 They are almost completely insensitive to plugging by condensable impurities in the gas stream 26
Figure
2. British
Oxygen
Company
cryogenic
gas turbine
though their passages may be very much smaller. This is very important as the flow passages in counterflow heat exchangers get smaller with size and thus are very sensitive to plugging. The applications for small refrigerators typically call for very long reliabilities and therefore plugging would be more likely to occur. The reversing flow at varying pressures flushes the condensable impurities out of thermal regenerators. Refrigerators operating to 20 K using regenerators can function well when gas is saturated with water vapour. 4 The use of regenerators leads to the possibility of simplifying the construction of the refrigerator as in the Brayton cycle i,2,3.4 devices from which valves are eliminated, the Gifford-McMahon cycle 6.7 devices from which the high pressure cylinder seal is eliminated, and in which valves are simple and seals are at room temperature. When these advantages are considered, it should come as no surprise that a majority of the most successful small refrigerators use thermal regenerators. These are the following cycles. I Brayton (Stirling) cycle Manufactured by: Philips Eindhoven North American Philips Malaker Corporation (Figure 3 shows a small refrigerator of this type) 2 The Gifford-McMahon cycle Manufactured by: Cryogenic Technology Inc. The Welch Vacuum Pump Company Cryomech Inc. (Figure 1) 3 The Vuilleumier cycle Manufactured by: Hughes Aircraft Company There is one exception to the above comments about heat exchangers. One type of heat exchanger can be small and efficient. It is the tiny, finned capillary heat exchanger from which Joule-Thomson systems can be made. They are, however, limited in size and application. Only one tube must be used, or a very difficult balance problem develops. Also, the density and pressure on the two sides must be very different. Quite a number of American firms have made such CRYOGENICS
. FEBRUARY
1970
refrigerators for infra-red applications-Garrett Corporation, Santa Barbara Research, Air Products Company, as well as the Hymatic Engineering Company in England. typical Joule-Thomson refrigerator heat Some exchangers made by the Hymatic Engineering Co. are shown in Figure 4. The very high gas compression ratios make the compressor a difficult and expensive item to
Figure 3. Malaker refrigerator
Corporation
Brayton
(Stirling)
cycle
build. As a result, several firms sell only the heat exchanger to operate from a gas bottle. Two and threestage units can reach temperatures of 20-4.2 K. Conclusions There is a considerable market developing for cryogenic refrigerators in the range 2-20 K for many applications. For large capacities adequate equipment is available from quite a few sources. For small capacities (I-10 W) there are many developing applications. Also, considerable equipment has been developed and new models will be needed in the near future. The development of tiny high speed turbines seems to have been abandoned and Stirling cycle devices are having trouble meeting the needs for reliability and lack of vibration. The most successful models involve slow speed (72-l 50 r p m) refrigerators using regenerators and separate compressors and gas purification systems, such as the Gifford-McMahon cycle equipment and Vuilleumier cycle systems. Figure 1 shows a Cryomech Inc. twostage refrigerator which achieves 7.5 K. Uses for temperatures lower than 1 K are still only for basic research. Adiabatic demagnetization equipment is not available commercially. Helium 3 dilution equipment is available. However, most research workers prefer to build their own as the available systems are not really perfected or optimized. Many cryogenic research workers are trying to improve on the performance which has been attained so far.
REFERENCES
Figure 4. Joule-Thomson by the Hymatic Engineering
CRYOGENICS
. FEBRUARY
heat exchangers manufactured Company Limited
1970
1. KOHLER, J. W. L., and YONKERS, C. 0. Philips Techical Reoiew 16 (1954) J. W. L. Ptogress in Cryogenics, Vol. 2, 43, 2. KOHLER, t Heywood, London, 1960) 3. PRAST, G. Ciyogettics. 2, I56 (1963) 4. PRAST, G. Inrerrtalionnl Advances in Cryogenic Engineering 10, 40 (I 965) 5. COWANS, K. W., and WALSH, P. J. Advcrnces in Cryogenic D@neeriqy 10, 468 ( 1965) 6. GIFFORD, W. E., and MCMAHON, H. 0. ‘A low temperature heat pump,’ Proc. of 10th International Congress of Refrigeration (Copenhagen, Denmark, August 1959) 7. GIFFORD. W. E. Advances in Cryogenic Engineering 11 ( 1966) 8. SIMON, F. E. Zeirsges Kalre-Indtsrrie 39, 89 (1932) 9. SCOTT, R. B., and COOK, J. W. RSI 19, 889 (1948) IO. GIFFORD, W. E. Master’s Thesis, Catholic University, Washington DC ( 1950) II. DEKLERK, D., STEENLAND, M. J., and GORTER, C. J. Pllysicu 16. 861 (1950) 12. WHITE, G. K. Experimental Techniques in Low-Temperature Physics, 219 (Oxford University Press, NJ, 1959) 13. DAS, P., DEBRUYN OUBOTER, R., and TACONIS, K. W. Low Temperature Physics, LT9B. 1253 (Plenum Press, New York, 1965) 14. WHEATLEY, J. C. American Jorwnnl of Pl!)~sics 36, 181 (1968) 15. GIFFORD, W. E., KADAIKKAL, N., and ACHARYA. A. ‘A Simon helium liquefier with a Gitford-McMahon cycle for precooling’ (Presented at 1969 Cryogenic Engineering Conference, Univ. of California at Los Angeles, June, 1969) 16. DAUNT, J. G., and GOREE. W. S. ‘Miniature cryogenic refrigerators,’ (Study performed for Office of Naval Research with Stevens Institute of Technology and Stanford Research Institute)
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