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Cryogenic difficulties
GRYOGENIGS An International Journal of Low Temperature Engineering and Research
VOL.
3
NO. 2
JUNE
1963
CRYOGENIC DIFFICULTIES A. J. C R O F T Clarendon Laboratory, Oxford, U.K. Received 12 March 1963
IN his mission to create order out of chaos, the low temperature physicist must expect to meet with tougher opposition than that offered by the laws of thermodynamics. There are quite a number of sources of practical difficulty special to the field of cryogenics, both physical and technological. In the next few pages we shall look them in the face and discuss ways of keeping them at bay.
Primary sources of cold In the low temperature physicist's promised land, liquid helium is for ever on tap. In this world, however, taps run dry and drought is one of the most discouraging difficulties of all. There are generally the alternatives of operating on the 'do-it-yourself' principle or of buying one's cold from outside. Very often this choice will be determined by economic or logistic considerations. However, we will ignore these and concentrate on reliability as our only criterion. Liquid nitrogen and liquid oxygen are now industrial commodities on an increasingly large scale and consequently the reliability of supply is high in industrially developed parts of the world. On the other hand, there are no reports of laboratory scale liquid nitrogen plants which will continue to operate for many years without needing a good deal of attention. From the standpoint of reliability, it will nearly always be the case that it is better to buy than to make. Outside the United States, the case is different for liquid hydrogen and liquid helium. Though produced commercially in this country, the scale is still modest CRYOGENICS
• JUNE
|963
enough to leave it an open question whether or not the do-it-yourself principle is worth while for reliability. At the lowest level, there is a lot to be said for being able to pour invective straight into a human ear rather than down a telephone.
Liquefaction systems Once a liquefaction system is established, two categories of trouble are most likely to compromise its reliability, i.e. impurities in the gas stream and mechanical disorders. Certain other sources of difficulty-notably leaks--are common to most cryogenic equipment and will be dealt with later. Blocks. Like the human body, a liquefaction system is often put out of order by thrombosis. In a helium liquefier, for example, enough of any other substance will block it up. Dust should not be forgotten, especially in a new system. Copper tubing often contains cuprous oxide dust which can do considerable damage to valves. Purifying agents such as activated alumina and charcoal should be freed from dust before use by trapping a quantity between two sieves and blowing a jet of compressed air through. Oil from compressors, in the forms of liquid droplets, vapour, and cracking products, can get a surprisingly long way through a liquefaction system unless restrained assiduously. (The efficient extraction of oil from gas streams is sufficiently straightforward for there to be no case for using special types of compressor in ordinary liquefaction systems: the fire is only too likely to be hotter than the
65
frying pan.) The larger oil droplets can be removed in a column packed with lumps of almost any substance which will present enough surface for deposition combined with adequate channels for drainage. (That is, the lumps should not be much smaller than about 1 cm.) Oil mist, vapour, and cracking products are effectively removed by activated alumina, but the particle size must be small (e.g. 16/32 mesh). Pressure drop is not a consideration in a column of ordinary diameter. Water will also be removed by two such columns. Two points, however, are worth noting. The gas emerges from most compressors at a temperature considerably above that of the incoming cooling water however much water is used and even if an after-cooler is fitted. In any case, compressors should be run warm (about 40 ° C) to prevent condensation. The quantity of water which will keep the compressor at the right temperature may not be enough to cool the outcoming gas to the temperature of the incoming water, even if an efficient after-cooler is fitted. It is therefore usually worth while to fit an 'after-after-cooler'. Not only will more of the water content of the gas be deposited in' the first column in the liquid state, but the performance of the drying agent will be much improved. (It may even be worth while using a refrigeration system to get down t o about 0 ° C.) It is necessary to take some care in reactivating drying columns. Only if all parts of the adsorbent rise to the proper temperature--250 ° C for activated alumina --will the maximum dryness be achieved. The only efficient way of doing this is to blow hot air through. (Oil contaminated adsorbent cannot be reactivated and has to be replaced.) Although more sophisticated instruments are on the way, the best method of determining the dryness of a gas stream is still by frost point determination, e.g. by means of the Dobson hygrometer (Casella Ltd). An efficient drying column should give a frost point of about - 6 0 ° C. When it is becoming saturated, the frost point rises quite slowly; it may be reactivated when it rises to about _40°-C. Oxygen and nitrogen are commonly removed by charcoal or alumina at liquid oxygen or liquid nitrogen temperatures, and there is rarely trouble about this provided the input concentration is not too high. Since l"esidual water is also taken up, these cleaners have to be reactivated by heating to about 200 ° C under vacuum. Standard industrial thermal conductivity purity-measuring instruments (katharometers) are suitable for monitoring. Helium gas from commercial sources usually contains a trace of neon and this may build up in a system if it is not removed by means of a charcoal or alumina cleaner--which need only be of modest size--working at about 20 ° K. (This must, of course, be vented to atmosphere as it warms up.) Detection is not difficult spectroscopically but a quantitative mass 66
spectrometric analysis may be necessary if trouble from this cause is suspected. In laboratories where liquid hydrogen is used for precooling, it is all too easy to allow hydrogen into the helium gas recovery system. Extracting this is very tedious and every possible discouragement from letting this mixture occur is justified. The thermal conductivities of helium and hydrogen are too close to use this as a basis for analysis. The best way is to add a small amount of air and pass the mixture through the type of catalyst instrument commonly used to detect the presence of oxygen in hydrogen, or vice versa. Sometimes a liquefier may become blocked not because the incoming gas is more than usually impure but because the operator has allowed a purifier to become too warm, for example, by allowing a liquid bath to run low. The purifier may then release what it has taken up and pass it on to a point at a lower temperature. The situation can be retrieved if it is possible to warm up the relevant parts of the system. The hydrogen liquefier built in the Clarendon Laboratory in 1961 is fitted with a means of passing clean room temperature hydrogen backwards through the system from a point close to the expansion valve. It is not known how well this arrangement would work as there has not yet been an occasion to use it. An unusual case of blockage occurred recently. A laboratory purchased a vacuum-insulated transfer tube from a commercial source. The inner tube proved to be totally blocked, although it was possible to push a wire up either of the two legs. It was of the type in which two coaxial tubes are bent; no elbows were used and there was therefore no question of blockage by solder. An X-ray photograph finally diagnosed the trouble. The inner tube had become flattened over about a centimetre of its length, presumably owing to an over-pressure in the vacuum space at some stage in its manufacture. The tra/amatic effects of this sort of experience do not bear thinking about. Machinery. Difficulties with liquefaction systems which are not due to blockages or to leaks are likely to concern the machinery of the system. Such troubles are usually only too easy to diagnose, are often expensive and time consuming to correct, but are relatively easy to avoid. We shall therefore concentrate on how to keep out of trouble and will deal only with room temperature machinery for two reasons. First, the maintenance of expansion engines and the like is highly specialized to each type, and second, the author has had no experience in this field. Ideally one must choose good machinery in the first place. Some of the cheaper types of compressor are not intended for continuous duty in any case and may have been designed with more of an eye on the price than on the performance. In general, one gets what one pays for and economy in machinery is very rarely worth while. However, one should not buy any machine in the dark and the knowledge that the firm of CRYOGENICS
- JUNE
1963
one's choice has at least one satisfied client is a useful insurance. One important feature to look for in the design is whether valves can be replaced in a matter of minutes: these are the most likely source of minor trouble and should be easily accessible. Before acceptance, a careful test should be carried out on the gas concerned. Hydrogen has a lower viscosity than air, and helium a higher heat of compression : a test on air is therefore not enough. A full set of spare valves and spare belts should be ordered with the machine. Other spares that will be needed when overhauls are carried out include piston rings, oil seals, bearings, etc. After the first few hours of use, one must clean the machine out thoroughly, possibly change the oil, and check all nuts and bolts for the correct tightness. Much the same considerations apply to vacuum pumps. In this case, spare blades and springs should be kept. Once good machinery has been installed and is running, reliability is largely a question of good maintenance. This is entirely a matter of putting the right man on the job. A simple means of assessing candidates is suggested. Ask them 'What would you do if a big-end seized up in your car ?' The man who says 'Take it to the nearest garage' can be eliminated at once. Good marks can be given to the man who always carries a spare set of bearing shells and would get under the car and put them in on the road. However, the man to look for is the one who knows how long bearings last on his particular model and is slightly offended at any suggestion that he would allow himself to get into such a situation. A few random points to conclude: (1) It is a simple matter to include a pressure-sensitive switch so that a machine cannot be started up without cooling water; (2) If a large vacuum pump stops unexpectedly, a great quantity of oil can get a long way in a short time-a volume large enough to contain all the oil should be installed in an appropriate part of the line; (3) Water-sealed gas holders need immersion heaters with thermostats set to about 1° C. (Jokes about low temperature work being halted by cold weather are not funny for long.) Leaks Causes. In many fields of experimental work, leaks are a principal cause of long faces. Cryogenics brings special sources of trouble of this sort. An obvious one is the superfluid property of liquid helium-II: below the lambda point, a minute leak becomes a large one. However, apart from this, there is a strong tendency for leaks to increase in magnitude with decreasing temperature, and this can make detection difficult. Furthermore, repeated low temperature cycling can cause leaks to appear in materials which would stay leak-free indefinitely at room temperature. (A remarkable case of this occurred with the 1947 Oxford hydrogen liquefier. Certain heat exchanger tubes started to CRYOGENICS
• JUNE
1963
become porous after the liquefier had been in use for five years. These, however, were of stainless steel of dubious quality, and this isolated experience should not be taken to detract from the usefulness of modern stainless steels in cryogenic work.) Flaws sometimes occur in metals and alloys which are inherently suitable for low temperature work. The best insurance against such trouble is to buy material of the highest quality and inspect it carefully on receipt. It is very important to be able to identify the remainder of a piece of raw material which has been proved to contain flaws, so that it can be scrapped. A frequent cause of trouble is the use of brass containing lead. Brasses intended for machining contain up to 3 per cent of lead, which promotes 'free cutting'. This is insoluble and, if the brass is not porous as found, it is likely to become so on heating, especially if subjected to the temperature required for hard soldering. Most of the brass commonly sold in bar form is of this type: sheet brass is usually substantially lead-free. If material is ordered by the correct specification number, one knows what one is getting. Furthermore, some suppliers analyse every batch that passes through their hands and will provide copies of the result. (The Copper Development Association in Great Britain publishes useful handbooks on brass and copper.) Some materials which are perfectly reliable if handled properly may become porous if misused. German silver tubing can be drawn to 0.1 mm wall thickness; a touch with a flame from a hand t o r c h - resulting in the green colour characteristic of copper-can cause porosity. Leaks often occur at joints. Silver brazing alloys may become porous if the temperature is allowed to become too high or if they become contaminated with traces of other solders. Tin-lead solder is very reliable if the eutectic is used and if no other metals are present in the alloy, provided a proper routine is followed, i.e. clean, tin, wash, inspect, assemble, and clean. Wood's metal is somewhat less easy to use but rarely gives trouble. In both cases, rapid cooling is advisable since it discourages the growth of large crystals. If the appropriate method is carefully applied to a properly designed joint, leaks can be avoided altogether. Changing conditions of pressure can sometimes cause small distortions which may result in a puzzling variation in magnitude of a leak in a joint. A source of trouble which can give rise to an apparent leak is occluded hydrogen in metals. This may result in the deterioration of the vacuum in a permanently sealed-off component which is, in fact, free of leaks. Thorough heating during pumping out is probably the best way of avoiding the difficulty. Detection. There are two distinct fields of leak detection in cryogenics which involve widely different orders of magnitude: leaks-into the atmosphere from a system at higher than atmospheric pressure and leaks into what should be a high vacuum enclosure. 67
For the former, relatively cheap and crude methods will suffice. Soap and water and a sharp eye will detect leaks down to about 10-4 cm3/sec (10 -6 cm3/sec = 7.6 x 10-4 I~-l./sec), and the katharometric sniffing instruments will do somewhat better at greater convenience where hydrogen and helium are involved. Mass spectrometer leak detection instruments have now increased in sensitivity (to 10-~2 cm3/sec) and decreased in price (to about £1,000) and must be considered a necessity rather than a luxury in any laboratory where cryogenic apparatus is made. The purchase of one of these instruments is one of the most decisive steps that can be taken in the battle against trouble. (Students of Parkinson's Law will note that the price falls within the optimum range for fund raising.) Components of complicated cryostats can be tested before assembly; usually, a test at liquid nitrogen temperature is adequate and this can be done quite rapidly. Raw material that is known to give trouble occasionally--such as thin-walled tubing---can be tested before use.
Where a mass spectrometer is not available the familiar high vacuum techniques are especially easy to apply since the partial pressure of condensible vapours will be negligible. The type of Penning gauge in which the discharge is visible can be useful since the gas present can be identified by its colour at quite low pressures. Cure. Naturally, the most satisfactory course of action once a leak has been found is to replace the faulty part; this, however, may sometimes be difficult. Some porous materials can be made permanently leak-tight by 'tinning' over with tin-lead solder. Similarly, silver solder can be used to patch porous argonarc welding. Attempts to use plastics for this type of first aid are doomed to failure but treatment with glycerol, though messy and only to be used in desperate cases, is often successful. It is important to know when to cut one's losses. A very small leak may take a long time to find even with the best equipment. In one's determination not to be beaten, it is easy to lose sight of the saving of time and temper that remaking part of the apparatus might bring. Glass. Glass presents a special problem, since it is porous to helium. This is a trouble that one must live with if one is not to throw away the many advantages of this material. Glasses low in boron content have a relatively low permeability; Monax is a much-used proprietary glass of this sort. To give a rough indication, the Dewar vessels used continuously in the Clarendon Laboratory for transporting liquid helium to cryostats are repumped every three months, but a Dewar forming part of a cryostat used by the author needed repumping as often as every third run. Helium loss. The economic pressure to conserve helium gas is considerable outside the United States 68
and is appreciable even there. In Great Britain, a laboratory using 5,000 l./yr of liquid helium and recovering 95 per cent will spend over £400 annually on helium replacement. This recovery rate is, in fact, very difficult to achieve. The author believes the loss rate to rise as the square of the number of experiments: the number of valves left open can be taken to increase linearly with the number of experiments, but the amount of helium lost on each occasion rises linearly with the quantity of helium in play, and this is proportional to the number of experiments. One way of reducing financial trouble from this source has been in practice for over a decade in one well known laboratory: a fine is levied for losses over a certain minimum per experiment. (The proceeds are devoted to the purchase of liquid refreshment for the staff. This gives pleasure to everybody: some enjoy the party and others enjoy criticizing the ethics of the arrangement.) The weak point in this scheme is that positive identification of losers is not easy and the income is consequently small. Heat leaks. Some sources of unwanted heat input to low temperature systems are obvious: room temperature radiation, too high a pressure in a vacuum space, nickel tubing in place of cupronickel, eddy currents from magnet current ripple, etc. There are, however, two somewhat esoteric sources of heating for which it is useful to be prepared. It sometimes happens that a relatively very large evaporation is noticed from a liquid helium Dewar or cryostat, the thermal insulation of which is known not to be at fault. Often, but not always, a deep musical note is heard and this gives a hint about what is going on. In certain conditions, self-maintaining oscillations can occur in gas columns; this problem was first treated by Lord Rayleigh. The conditions are a sufficiently steep temperature gradient and the right degree of thermal exchange with the wall of the tube containing the column. This occurs only for a relatively narrow range of tube sizes, which unfortunately includes those most often used. It is not usually difficult to cure these oscillations empirically. For example, they have been stopped in a built-in liquid helium transfer tube by pushing a few inches of pipe cleaner into the closedoff room temperature end after the tube has been used for filling the cryostat, t Another source of heating is of importance in work below i ° K when heat inputs of less than the order of 100 ergs/sec are important. Depending on the means adopted for mounting the specimen, heat inputs of maay thousand ergs per second can result from mechanical vibration; presumably some form of frictional hysteresis provides the necessary 'loss'. The sources may be vibration transmitted through the laboratory building structure or from such local sources as a bumping diffusion pump or a rotary pump. Apart from improvements in the mounting of the specimen, the only cure is isolation by the usual means. CRYOGENICS
• JUNE
1963
Serious trouble
She died, because she never knew These simple little rules and f e w . . . BELLOC
While it is not yet the case that life insurance proposal forms include the question 'Have you any intention of engaging in cryogenics ?', it is unhappily true that there are many opportunities in cryogenics for putting oneself and one's apparatus in peril. To conclude on a sombre note, we will enlarge upon some of the ways of avoiding the worst species of trouble. Hydrogen is of course undoubtedly a dangerous substance to work with. However, its hazardous feature lies solely in its chemical affinity for oxygen and, with care, the possibility of combustion can be eliminated. Chemical engineers regard many other substances-hydrocarbons, for example--as much more tricky to handle. It is just as important not to overrate the hazards of using hydrogen in cryogenics as it is not to underrate them, because hydrogen is a most useful substance. Nothing else bridges the gap between helium and the common warmer cryogenic liquids except neon--and this has a range of only three degrees between its boiling point at atmospheric pressure and its triple point. The most important precautions can be summarized as follows. (I) Keep the part of the system inside the laboratory as closed as possible (this is likely to involve running a vent pipe to the outside). (2) Use only liquid nitrogen, not liquid air or liquid oxygen. (3) Where combustible concentrations may occur, exclude rigorously any possibility of sparking. (Apart from obvious sources in electrical equipment, sparks can arise from unexpected electrostatic charges.) To avoid the possibility of physical explosion, one must look at a piece of apparatus from two points of view: (1) Is.there an adequate factor between normal working pressure and bursting pressure ?
CRYOGENICS • JUNE 1963
(2) Can the normal working pressure be exceeded in any circumstances ? The first point can be covered satisfactorily in only one way: the entire assembly should be tested hydraulically to twice the working pressure. No amount of calculation, over-dimensioning, reluctance to fill with water, lack of time, etc., must be allowed to stand in the way. Secondly, some entirely reliable means of releasing excessive pressure must be included. This may be a spring-loaded blow-off valve, a weighted disk seating on an O-ring, or a bursting disk. In a system so protected, no harm will result if an excessive pressure does occur accidentally. However, it may be as well to list some of the classical ways o f producing one. (1) Allowing atmospheric air to condense in the neck of a liquid helium or hydrogen vessel. (2) Allowing a system containing gas or liquid at low temperature to warm up at constant volume (includingcases where unexpected condensations may have occurred). (3) Allowing gas to leak past a faulty high pressure valve into a closed low pressure system. A not uncommon hazard which is difficult to anticipate is the metal Dewar vessel which develops a leak such that gas or liquid accumulates in a vacuum space. When the vessel warms up the gas may not be able to escape fast enough and the Dewar may blow up. Manufacturers do not often include bursting disks and the only means of avoiding trouble from this source is to keep a careful eye on the performance of Dewars and take out of service any that show signs of deterioration. This paper will have fulfilled its purpose if newcomers to cryogenics are neither completely discouraged nor lulled into a state of dreamy confidence. REFERENCES
1. Experimental Cryophysics (Eds. Hoare, Kurti, and Jackson) (Butterworths, London, 1961) 2. WroTE, G. K. Experimental Techniques in Low Temperature Physics (Clarendon, Oxford, 1959)
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VOL.
3
NO. 2
JUNE
1963
CRYOGENIC DIFFICULTIES A. J. C R O F T Clarendon Laboratory, Oxford, U.K. Received 12 March 1963
IN his mission to create order out of chaos, the low temperature physicist must expect to meet with tougher opposition than that offered by the laws of thermodynamics. There are quite a number of sources of practical difficulty special to the field of cryogenics, both physical and technological. In the next few pages we shall look them in the face and discuss ways of keeping them at bay.
Primary sources of cold In the low temperature physicist's promised land, liquid helium is for ever on tap. In this world, however, taps run dry and drought is one of the most discouraging difficulties of all. There are generally the alternatives of operating on the 'do-it-yourself' principle or of buying one's cold from outside. Very often this choice will be determined by economic or logistic considerations. However, we will ignore these and concentrate on reliability as our only criterion. Liquid nitrogen and liquid oxygen are now industrial commodities on an increasingly large scale and consequently the reliability of supply is high in industrially developed parts of the world. On the other hand, there are no reports of laboratory scale liquid nitrogen plants which will continue to operate for many years without needing a good deal of attention. From the standpoint of reliability, it will nearly always be the case that it is better to buy than to make. Outside the United States, the case is different for liquid hydrogen and liquid helium. Though produced commercially in this country, the scale is still modest CRYOGENICS
• JUNE
|963
enough to leave it an open question whether or not the do-it-yourself principle is worth while for reliability. At the lowest level, there is a lot to be said for being able to pour invective straight into a human ear rather than down a telephone.
Liquefaction systems Once a liquefaction system is established, two categories of trouble are most likely to compromise its reliability, i.e. impurities in the gas stream and mechanical disorders. Certain other sources of difficulty-notably leaks--are common to most cryogenic equipment and will be dealt with later. Blocks. Like the human body, a liquefaction system is often put out of order by thrombosis. In a helium liquefier, for example, enough of any other substance will block it up. Dust should not be forgotten, especially in a new system. Copper tubing often contains cuprous oxide dust which can do considerable damage to valves. Purifying agents such as activated alumina and charcoal should be freed from dust before use by trapping a quantity between two sieves and blowing a jet of compressed air through. Oil from compressors, in the forms of liquid droplets, vapour, and cracking products, can get a surprisingly long way through a liquefaction system unless restrained assiduously. (The efficient extraction of oil from gas streams is sufficiently straightforward for there to be no case for using special types of compressor in ordinary liquefaction systems: the fire is only too likely to be hotter than the
65
frying pan.) The larger oil droplets can be removed in a column packed with lumps of almost any substance which will present enough surface for deposition combined with adequate channels for drainage. (That is, the lumps should not be much smaller than about 1 cm.) Oil mist, vapour, and cracking products are effectively removed by activated alumina, but the particle size must be small (e.g. 16/32 mesh). Pressure drop is not a consideration in a column of ordinary diameter. Water will also be removed by two such columns. Two points, however, are worth noting. The gas emerges from most compressors at a temperature considerably above that of the incoming cooling water however much water is used and even if an after-cooler is fitted. In any case, compressors should be run warm (about 40 ° C) to prevent condensation. The quantity of water which will keep the compressor at the right temperature may not be enough to cool the outcoming gas to the temperature of the incoming water, even if an efficient after-cooler is fitted. It is therefore usually worth while to fit an 'after-after-cooler'. Not only will more of the water content of the gas be deposited in' the first column in the liquid state, but the performance of the drying agent will be much improved. (It may even be worth while using a refrigeration system to get down t o about 0 ° C.) It is necessary to take some care in reactivating drying columns. Only if all parts of the adsorbent rise to the proper temperature--250 ° C for activated alumina --will the maximum dryness be achieved. The only efficient way of doing this is to blow hot air through. (Oil contaminated adsorbent cannot be reactivated and has to be replaced.) Although more sophisticated instruments are on the way, the best method of determining the dryness of a gas stream is still by frost point determination, e.g. by means of the Dobson hygrometer (Casella Ltd). An efficient drying column should give a frost point of about - 6 0 ° C. When it is becoming saturated, the frost point rises quite slowly; it may be reactivated when it rises to about _40°-C. Oxygen and nitrogen are commonly removed by charcoal or alumina at liquid oxygen or liquid nitrogen temperatures, and there is rarely trouble about this provided the input concentration is not too high. Since l"esidual water is also taken up, these cleaners have to be reactivated by heating to about 200 ° C under vacuum. Standard industrial thermal conductivity purity-measuring instruments (katharometers) are suitable for monitoring. Helium gas from commercial sources usually contains a trace of neon and this may build up in a system if it is not removed by means of a charcoal or alumina cleaner--which need only be of modest size--working at about 20 ° K. (This must, of course, be vented to atmosphere as it warms up.) Detection is not difficult spectroscopically but a quantitative mass 66
spectrometric analysis may be necessary if trouble from this cause is suspected. In laboratories where liquid hydrogen is used for precooling, it is all too easy to allow hydrogen into the helium gas recovery system. Extracting this is very tedious and every possible discouragement from letting this mixture occur is justified. The thermal conductivities of helium and hydrogen are too close to use this as a basis for analysis. The best way is to add a small amount of air and pass the mixture through the type of catalyst instrument commonly used to detect the presence of oxygen in hydrogen, or vice versa. Sometimes a liquefier may become blocked not because the incoming gas is more than usually impure but because the operator has allowed a purifier to become too warm, for example, by allowing a liquid bath to run low. The purifier may then release what it has taken up and pass it on to a point at a lower temperature. The situation can be retrieved if it is possible to warm up the relevant parts of the system. The hydrogen liquefier built in the Clarendon Laboratory in 1961 is fitted with a means of passing clean room temperature hydrogen backwards through the system from a point close to the expansion valve. It is not known how well this arrangement would work as there has not yet been an occasion to use it. An unusual case of blockage occurred recently. A laboratory purchased a vacuum-insulated transfer tube from a commercial source. The inner tube proved to be totally blocked, although it was possible to push a wire up either of the two legs. It was of the type in which two coaxial tubes are bent; no elbows were used and there was therefore no question of blockage by solder. An X-ray photograph finally diagnosed the trouble. The inner tube had become flattened over about a centimetre of its length, presumably owing to an over-pressure in the vacuum space at some stage in its manufacture. The tra/amatic effects of this sort of experience do not bear thinking about. Machinery. Difficulties with liquefaction systems which are not due to blockages or to leaks are likely to concern the machinery of the system. Such troubles are usually only too easy to diagnose, are often expensive and time consuming to correct, but are relatively easy to avoid. We shall therefore concentrate on how to keep out of trouble and will deal only with room temperature machinery for two reasons. First, the maintenance of expansion engines and the like is highly specialized to each type, and second, the author has had no experience in this field. Ideally one must choose good machinery in the first place. Some of the cheaper types of compressor are not intended for continuous duty in any case and may have been designed with more of an eye on the price than on the performance. In general, one gets what one pays for and economy in machinery is very rarely worth while. However, one should not buy any machine in the dark and the knowledge that the firm of CRYOGENICS
- JUNE
1963
one's choice has at least one satisfied client is a useful insurance. One important feature to look for in the design is whether valves can be replaced in a matter of minutes: these are the most likely source of minor trouble and should be easily accessible. Before acceptance, a careful test should be carried out on the gas concerned. Hydrogen has a lower viscosity than air, and helium a higher heat of compression : a test on air is therefore not enough. A full set of spare valves and spare belts should be ordered with the machine. Other spares that will be needed when overhauls are carried out include piston rings, oil seals, bearings, etc. After the first few hours of use, one must clean the machine out thoroughly, possibly change the oil, and check all nuts and bolts for the correct tightness. Much the same considerations apply to vacuum pumps. In this case, spare blades and springs should be kept. Once good machinery has been installed and is running, reliability is largely a question of good maintenance. This is entirely a matter of putting the right man on the job. A simple means of assessing candidates is suggested. Ask them 'What would you do if a big-end seized up in your car ?' The man who says 'Take it to the nearest garage' can be eliminated at once. Good marks can be given to the man who always carries a spare set of bearing shells and would get under the car and put them in on the road. However, the man to look for is the one who knows how long bearings last on his particular model and is slightly offended at any suggestion that he would allow himself to get into such a situation. A few random points to conclude: (1) It is a simple matter to include a pressure-sensitive switch so that a machine cannot be started up without cooling water; (2) If a large vacuum pump stops unexpectedly, a great quantity of oil can get a long way in a short time-a volume large enough to contain all the oil should be installed in an appropriate part of the line; (3) Water-sealed gas holders need immersion heaters with thermostats set to about 1° C. (Jokes about low temperature work being halted by cold weather are not funny for long.) Leaks Causes. In many fields of experimental work, leaks are a principal cause of long faces. Cryogenics brings special sources of trouble of this sort. An obvious one is the superfluid property of liquid helium-II: below the lambda point, a minute leak becomes a large one. However, apart from this, there is a strong tendency for leaks to increase in magnitude with decreasing temperature, and this can make detection difficult. Furthermore, repeated low temperature cycling can cause leaks to appear in materials which would stay leak-free indefinitely at room temperature. (A remarkable case of this occurred with the 1947 Oxford hydrogen liquefier. Certain heat exchanger tubes started to CRYOGENICS
• JUNE
1963
become porous after the liquefier had been in use for five years. These, however, were of stainless steel of dubious quality, and this isolated experience should not be taken to detract from the usefulness of modern stainless steels in cryogenic work.) Flaws sometimes occur in metals and alloys which are inherently suitable for low temperature work. The best insurance against such trouble is to buy material of the highest quality and inspect it carefully on receipt. It is very important to be able to identify the remainder of a piece of raw material which has been proved to contain flaws, so that it can be scrapped. A frequent cause of trouble is the use of brass containing lead. Brasses intended for machining contain up to 3 per cent of lead, which promotes 'free cutting'. This is insoluble and, if the brass is not porous as found, it is likely to become so on heating, especially if subjected to the temperature required for hard soldering. Most of the brass commonly sold in bar form is of this type: sheet brass is usually substantially lead-free. If material is ordered by the correct specification number, one knows what one is getting. Furthermore, some suppliers analyse every batch that passes through their hands and will provide copies of the result. (The Copper Development Association in Great Britain publishes useful handbooks on brass and copper.) Some materials which are perfectly reliable if handled properly may become porous if misused. German silver tubing can be drawn to 0.1 mm wall thickness; a touch with a flame from a hand t o r c h - resulting in the green colour characteristic of copper-can cause porosity. Leaks often occur at joints. Silver brazing alloys may become porous if the temperature is allowed to become too high or if they become contaminated with traces of other solders. Tin-lead solder is very reliable if the eutectic is used and if no other metals are present in the alloy, provided a proper routine is followed, i.e. clean, tin, wash, inspect, assemble, and clean. Wood's metal is somewhat less easy to use but rarely gives trouble. In both cases, rapid cooling is advisable since it discourages the growth of large crystals. If the appropriate method is carefully applied to a properly designed joint, leaks can be avoided altogether. Changing conditions of pressure can sometimes cause small distortions which may result in a puzzling variation in magnitude of a leak in a joint. A source of trouble which can give rise to an apparent leak is occluded hydrogen in metals. This may result in the deterioration of the vacuum in a permanently sealed-off component which is, in fact, free of leaks. Thorough heating during pumping out is probably the best way of avoiding the difficulty. Detection. There are two distinct fields of leak detection in cryogenics which involve widely different orders of magnitude: leaks-into the atmosphere from a system at higher than atmospheric pressure and leaks into what should be a high vacuum enclosure. 67
For the former, relatively cheap and crude methods will suffice. Soap and water and a sharp eye will detect leaks down to about 10-4 cm3/sec (10 -6 cm3/sec = 7.6 x 10-4 I~-l./sec), and the katharometric sniffing instruments will do somewhat better at greater convenience where hydrogen and helium are involved. Mass spectrometer leak detection instruments have now increased in sensitivity (to 10-~2 cm3/sec) and decreased in price (to about £1,000) and must be considered a necessity rather than a luxury in any laboratory where cryogenic apparatus is made. The purchase of one of these instruments is one of the most decisive steps that can be taken in the battle against trouble. (Students of Parkinson's Law will note that the price falls within the optimum range for fund raising.) Components of complicated cryostats can be tested before assembly; usually, a test at liquid nitrogen temperature is adequate and this can be done quite rapidly. Raw material that is known to give trouble occasionally--such as thin-walled tubing---can be tested before use.
Where a mass spectrometer is not available the familiar high vacuum techniques are especially easy to apply since the partial pressure of condensible vapours will be negligible. The type of Penning gauge in which the discharge is visible can be useful since the gas present can be identified by its colour at quite low pressures. Cure. Naturally, the most satisfactory course of action once a leak has been found is to replace the faulty part; this, however, may sometimes be difficult. Some porous materials can be made permanently leak-tight by 'tinning' over with tin-lead solder. Similarly, silver solder can be used to patch porous argonarc welding. Attempts to use plastics for this type of first aid are doomed to failure but treatment with glycerol, though messy and only to be used in desperate cases, is often successful. It is important to know when to cut one's losses. A very small leak may take a long time to find even with the best equipment. In one's determination not to be beaten, it is easy to lose sight of the saving of time and temper that remaking part of the apparatus might bring. Glass. Glass presents a special problem, since it is porous to helium. This is a trouble that one must live with if one is not to throw away the many advantages of this material. Glasses low in boron content have a relatively low permeability; Monax is a much-used proprietary glass of this sort. To give a rough indication, the Dewar vessels used continuously in the Clarendon Laboratory for transporting liquid helium to cryostats are repumped every three months, but a Dewar forming part of a cryostat used by the author needed repumping as often as every third run. Helium loss. The economic pressure to conserve helium gas is considerable outside the United States 68
and is appreciable even there. In Great Britain, a laboratory using 5,000 l./yr of liquid helium and recovering 95 per cent will spend over £400 annually on helium replacement. This recovery rate is, in fact, very difficult to achieve. The author believes the loss rate to rise as the square of the number of experiments: the number of valves left open can be taken to increase linearly with the number of experiments, but the amount of helium lost on each occasion rises linearly with the quantity of helium in play, and this is proportional to the number of experiments. One way of reducing financial trouble from this source has been in practice for over a decade in one well known laboratory: a fine is levied for losses over a certain minimum per experiment. (The proceeds are devoted to the purchase of liquid refreshment for the staff. This gives pleasure to everybody: some enjoy the party and others enjoy criticizing the ethics of the arrangement.) The weak point in this scheme is that positive identification of losers is not easy and the income is consequently small. Heat leaks. Some sources of unwanted heat input to low temperature systems are obvious: room temperature radiation, too high a pressure in a vacuum space, nickel tubing in place of cupronickel, eddy currents from magnet current ripple, etc. There are, however, two somewhat esoteric sources of heating for which it is useful to be prepared. It sometimes happens that a relatively very large evaporation is noticed from a liquid helium Dewar or cryostat, the thermal insulation of which is known not to be at fault. Often, but not always, a deep musical note is heard and this gives a hint about what is going on. In certain conditions, self-maintaining oscillations can occur in gas columns; this problem was first treated by Lord Rayleigh. The conditions are a sufficiently steep temperature gradient and the right degree of thermal exchange with the wall of the tube containing the column. This occurs only for a relatively narrow range of tube sizes, which unfortunately includes those most often used. It is not usually difficult to cure these oscillations empirically. For example, they have been stopped in a built-in liquid helium transfer tube by pushing a few inches of pipe cleaner into the closedoff room temperature end after the tube has been used for filling the cryostat, t Another source of heating is of importance in work below i ° K when heat inputs of less than the order of 100 ergs/sec are important. Depending on the means adopted for mounting the specimen, heat inputs of maay thousand ergs per second can result from mechanical vibration; presumably some form of frictional hysteresis provides the necessary 'loss'. The sources may be vibration transmitted through the laboratory building structure or from such local sources as a bumping diffusion pump or a rotary pump. Apart from improvements in the mounting of the specimen, the only cure is isolation by the usual means. CRYOGENICS
• JUNE
1963
Serious trouble
She died, because she never knew These simple little rules and f e w . . . BELLOC
While it is not yet the case that life insurance proposal forms include the question 'Have you any intention of engaging in cryogenics ?', it is unhappily true that there are many opportunities in cryogenics for putting oneself and one's apparatus in peril. To conclude on a sombre note, we will enlarge upon some of the ways of avoiding the worst species of trouble. Hydrogen is of course undoubtedly a dangerous substance to work with. However, its hazardous feature lies solely in its chemical affinity for oxygen and, with care, the possibility of combustion can be eliminated. Chemical engineers regard many other substances-hydrocarbons, for example--as much more tricky to handle. It is just as important not to overrate the hazards of using hydrogen in cryogenics as it is not to underrate them, because hydrogen is a most useful substance. Nothing else bridges the gap between helium and the common warmer cryogenic liquids except neon--and this has a range of only three degrees between its boiling point at atmospheric pressure and its triple point. The most important precautions can be summarized as follows. (I) Keep the part of the system inside the laboratory as closed as possible (this is likely to involve running a vent pipe to the outside). (2) Use only liquid nitrogen, not liquid air or liquid oxygen. (3) Where combustible concentrations may occur, exclude rigorously any possibility of sparking. (Apart from obvious sources in electrical equipment, sparks can arise from unexpected electrostatic charges.) To avoid the possibility of physical explosion, one must look at a piece of apparatus from two points of view: (1) Is.there an adequate factor between normal working pressure and bursting pressure ?
CRYOGENICS • JUNE 1963
(2) Can the normal working pressure be exceeded in any circumstances ? The first point can be covered satisfactorily in only one way: the entire assembly should be tested hydraulically to twice the working pressure. No amount of calculation, over-dimensioning, reluctance to fill with water, lack of time, etc., must be allowed to stand in the way. Secondly, some entirely reliable means of releasing excessive pressure must be included. This may be a spring-loaded blow-off valve, a weighted disk seating on an O-ring, or a bursting disk. In a system so protected, no harm will result if an excessive pressure does occur accidentally. However, it may be as well to list some of the classical ways o f producing one. (1) Allowing atmospheric air to condense in the neck of a liquid helium or hydrogen vessel. (2) Allowing a system containing gas or liquid at low temperature to warm up at constant volume (includingcases where unexpected condensations may have occurred). (3) Allowing gas to leak past a faulty high pressure valve into a closed low pressure system. A not uncommon hazard which is difficult to anticipate is the metal Dewar vessel which develops a leak such that gas or liquid accumulates in a vacuum space. When the vessel warms up the gas may not be able to escape fast enough and the Dewar may blow up. Manufacturers do not often include bursting disks and the only means of avoiding trouble from this source is to keep a careful eye on the performance of Dewars and take out of service any that show signs of deterioration. This paper will have fulfilled its purpose if newcomers to cryogenics are neither completely discouraged nor lulled into a state of dreamy confidence. REFERENCES
1. Experimental Cryophysics (Eds. Hoare, Kurti, and Jackson) (Butterworths, London, 1961) 2. WroTE, G. K. Experimental Techniques in Low Temperature Physics (Clarendon, Oxford, 1959)
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