PH L PS TECH N'CAL REVEW VOLUME 29, 1968, No. 7 The Philips helium liquefier G. J. Haarhuis The single-stage gas-refrigerating machine, described in this journal in 1954, provided a simple and efficient means of producing liquid air and nitrogen. With the advent of the two-stage machine, described in 1964, the liquefaction of hydrogen became a simple matter. Another step forward has now been taken with the construction of an efficient helium liquefier based on a pair of two-stage gas-refrigerating machines. This liquefier, the product of close cooperation between a design team in the ndustrial Equipment Division and a research team at the Philips Research Laboratories, is simple to operate and can process very impure helium gas. With the appearance in 1963 of a gas-refrigerating machine which gave high-efficiency refrigeration down to 20 "K, and could even reach 11 or 12 "K [11, the stage was set for the design of a helium liquefier based on a gas-refrigerating machine. The inversion temperature of the Joule- Thomson effect for helium lies between 40 OK and 50 OK, so that it must be possible to achieve the last refrigeration step (down to 4.2 OK) by means of an expansion (or throttling) process, just as in existing helium liquefiers. The strongly increasing demand for liquid helium for scientific and technical applications, and the advantages associated with the use of gas-refrigerating machines, prompted the decision to embark upon the development of a liquefier of this kind. Partly through the invention at Philips Research Laboratories of a new type of heat exchanger with a very high thermal efficiency [21 and the expansion ejector [31, and partly through the application of a new type 0f compressor equipped with pistons using a rolling diaphragm [41, a helium liquefier has emerged from this development work which possesses a number of very attractive features. The thermodynamic design is by r. G. Prast of the Philips Research Laboratories. The new liquefier system, which is very compact and r. G. J. Haarhuis is with the Philips ndustrial Equipment Division (PT), Eindhoven. (once started up) automatic in operation, delivers about 10 litres of liquid helium per hour, and if the helium contains 2 % of air it can still operate continuously for 100 hours without requiring cleaning. Even if the percentage of air is much higher the operation of the liquefier remains unimpaired, and an air content of as high as 10 % is in fact permissible for a limited time. f the helium used contains less than 2 % of air, the period of continuous operation is well over 100 hours. Once the liquefaction process has started, all the operator has to do is to ensure that the liquefier is regularly supplied with helium gas, and to replace full Dewar vessels by empty ones. The efficiency of the system is high: the production of one litre of liquid helium only uses up 2.8 kwh of electrical energy, including the energy required for removing the 2 % of air. Unlike other liquefiers, this one does not require a supply of liquid nitrogen for precooling or purification of the gas supply. [1] G. Prast, A gas-refrigerating machine for temperatures down to 20 "K and lower, Philips tech. Rev. 26, 1-11, 1965. [2] G. Vonk, Acompact heat exchanger of high thermal efficiency, Philips tech. Rev. 29, 158-162, 1968 (No. 5). [3] J. A. Rietdijk, The expansion ejector, a new cryogenic device, Philips tech. Rev. 28, 243-245, 1967 (No. 8). [4] For the roling diaphragm, see: J. A. Rietdijk, H. C. J. van Beukering, H. H. M. van der Aa and R. J. Meijer, A positive rod or piston seal for large pressure differences, Philips tech. Rev. 26, 287-296, 1965.
198 PHLTPS TECHNCAL REVEW VOLUME 29 Construction and operation Fig.il shows a simplified diagram of the liquefier. A compressor compresses the helium gas at room tem-: perature from 2.5 bar to 20 bar (gas flow rate 2.5 gis;. 1 bar is about 1 atm). The compressed gas passes the five gauze-type heat exchangers H to H5 and the precoolers R to R4 of a pair of two-stage gas-refrigerating machines. n the last heat exchanger a temperature of 7 "K is reached. The helium leaving the system in a liquid state is replaced by helium gas supplied through the tube S at a pressure of 20 bar. This gas flow first passes through all five heat exchangers before joining the main flow (at' Con). The cold gas expands in the expansion ejector EE to 2.5 bar and is cooled down further by the Joule-Thomson effect. A large proportion of this gas flows through the five heat exchangers H5 to H back to the compressor.,on its way it gives up nearly all its cold to the two other flows passing through the heat exchangers. The part of the cold gas that does not flow back undergoes a second expansion in the expansion valve JK, this time to a pressure of 1 bar, and a certain fraction liquefies during this expansion. Gas and liquid then flow to vessel C, and the gas then returns from C to the main flow via the expansion ejector. The whole process described here is continuous. During the period immediately after the system is switched on, no helium is supplied through S. Gas circulates then only in the middle and left-hand tubes shown in H to H5 in fig. 1. The system then gradually cools down. When the temperature between H2 and H3 has dropped to 80 "K, this starting period is considered to be over and the gas supply starts through S. The decrease of pressure in the system as a result of the cooling during the starting period is compensated by the supply of pure helium through S2. n the actual system the vessel C is a Dewar flask in which the liquid helium produced is collected and stored. The components inside the chain-dotted rectan- gle are contained in another Dewar flask, the cryostat. This is in communication with C by means of a specially constructed connecting system with coaxial feed tubes. Special features The chief feature that distinguishes the diagram of fig. 1 from that of any other liquefier is that it includes the expansion ejector EE (fig. 2). This works both as an expansion valve and as a pump of the water-jet type: S._.,_., -, H20, _J!,Hl ' 1 i He Fig. 1. Simplified diagram of the Philips helium liquefier. Comp compressor. R to R4 four precooling stages obtained from two two-stage gas-refrigerating machines. H to H5 gauze-type heat exchangers. EE expansion ejector. JK expansion valve. Ha heat exchanger. C vessel for collecting liquid helium. Fresh gas is regularly supplied to the system through tube S; it passes through the heat exchangers H to H5 and then joins the main stream at COli. mpurities in the gas supply are eliminated at the places denoted by the relevant chemical symbol. The part inside the chain-dotted line is contained in a special Dewar vessel. L 1bar, l..j',._._._._._._._,_j
1968, No. 7 HELUM LQUEFER 199 it draws the helium gas at 1 bar from the vessel C and raises.it toa pressure of 2.5 bar. This is at the same time the suction pressure of the compressor, which can therefore be very much smaller than if it had to draw the gas from C. This is important for efficiencyand also saves space. the cooling process. This ideal situation is much better approximated when cold is supplied at four different temperatures - here 100, 65, 30 and 15 "K - than when it is supplied at only one or two. n the second place, when cold is supplied at four temperatures it is possible to remove the impurities from the helium gas 100 OK 90 80 60 50 40 30, ' r--tl 100, \ 200 \ 300 W ~- 1-- \ \ \ r--_...!. --_, \ ' r--~=280w 1\ \ \ \ \ \ \ \ \ \, \, 1\ \ \, \ \ \ \ ~, _l.1 1 200 \ ~ \ 130- Fig. 2. Cross-section of the expansion ejector. The helium gas flowing from above (20 bar) expands to 2.5 bar, giving cooling through the Joule-Thomson effect. At the same time, through the opening on the left, helium gas of 1 bar is drawn in from the collector vessel (C in fig. ) and again compressed to 2.5 bar. 20 10 J ) j_ 0., 40 50 60 70 80 -Tt 80 The gas flowing from the ejector EE to the expansion valve JK is cooled in the heat exchanger H6 by the gas flowing back from the vessel C to the ejector. t therefore arrives at JK with a temperature of only 5.2 "K, and because of this ow temperature no less than 60 to 70% of the gas is liquefied during the expansion at JK. The pair of two-stage gas-refrigerating machines with which the liquefier is equipped work at different temperatures and thus drive four precooling stages. Two machines have to be used since the output yield with one machine ofthe conventional type (A20) is less than the amount considered desirable. The use of more than one machine has two other important advantages, however. n the first place, in cooling a flow of gas or liquid the highest efficiency is.in t~eory obtained when cold is supplied to the medium at each temperature in Fig. 3. The relation between the first-stage temperature Tl and the "head" temperature T2 of a Philips A20 two-stage gas refrigerating machine which is found when the refrigerating capacity P is varied while that of the head (P2) is held constant, and vice versa. (These results represent the average of measurements made on a number of machines.) The shape of the curves indicates that variation of P has hardly any effect on the refrigerating capacity delivered by the head at a particular temperature. (The dashed parts of the curves correspond to situations in which the head is less cold than the other stage; such situations do not of course arise when the machine is in normal use.) (mainly air) in a very effective way by adsorption. This will be discussed separately in the next section.. A feature of the two-stage machine which we have found very useful is that the cold production of the first stage - i.e. the one with the higher temperature - has hardlyany effect on that of the second stage (fig. 3). The 'system can thus be arranged so that a
200 PHLPS TECHNCAL REVEW VOLUME 29 large part of the cold is delivered at the higher temperature, which improves the 'efficiency. We have already mentioned the compressor with roling diaphragms, specially designed for this liquefier. This compressor, one of a type designed by r. H. J. Verbeek of the Philips ndustrial Equipment Division, will be the subject of a forthcoming article in this journal. The roling diaphragms form a hermetic seal between the spaces containing the gas for compression and the spaces containing oil. This is of particular importance: without an oil-free compressor a system like that described here would 110t be able to work continuously for very long. Purification of the gas supply At the temperature of liquid helium' all other substances are solids. mpurities in the gas supplied must therefore be removed before this temperature is reached, otherwise they can cause stoppages.' n the Philips helium liquefier this is done by adsorption, as we noted earlier. With one exception, the adsorbers are all located in the cryostat, each being placed at the coldest possible place consistent with the requirement that the impurity in question is to remain in the gas phase (cf. fig. ). The new helium liquefier differs in this respect from all existing ones, which require pre-purification of the helium supply to reduce the impurity content to about 0.01 % by volume. This pre-purification requires the use of liquid nitrogen, which is not required in the Philips liquefier. As can be seen from fig. 1, the adsorber that traps water vapour works at room temperature. Carbon dioxide gas is trapped by an adsorber situated between the first and second heat exchangers, where the prevailing temperature is about 125 OK. Both adsorbers contain "molecular sieves", and if the helium gas entering through S contains 2 % of air and is saturated with water vapour, they must be regenerated once every 100 hours.... Oxygen, nitrogen and argon are removed between the second heat exchanger and the third (temperature about 70 OK). This is done by means of two adsorbers consisting of activated charcoal which operate alternately for periods of 40 minutes. While one is working the other is being regenerated, which is done by raising the temperature to about 145 OK and pumping off the desorbed gàs. The switch-over is automatic. f the gas supply contains more than 2 % of air, part of it is liquefied in the second heat exchanger. This liquid is collected in a Dewar vessel and from time to time automatically removed from the system (see below). Finally, between the fourth heat exchangers and the fifth there is another small charcoal adsorber which traps hydrogen and neon; the quantity of these gases contained in the gas supply is usually very small. This adsorber can also work for 100 houts continuously with 2 % of air in the helium supply. - All these adsorbers are located in the supply line (S-Con) so that the gas circulatingin the startingperiod does not pass through them. For purifying this gas a separate adsorber is included between H2 and H3. As the temperature of the system falls, this adsorber traps an increasingly larger fraction of any impurities, and eventually the circulating helium is practically pure. Technical details The construction of the new liquefier is illustrated schematically in jig. 4. The photograph (fig. 5) gives a general view of the system. The cryostat, which contains the five gauze-type heat exchangers, the ejector, the expansion valve and the devices for purifying the gas (except the adsorber for water vapour) are contained inside an evacuated Dewar vessel (1-0.1 torr) of special design. This vessel is divided into three compartments by two refrigerated radiation shields RS and RS2, coated with silver on both sides; the shields extend into the high-vacuum space between the two walls of the Dewar vessel. Each shield is cooled by a freezer of one of the refrigerating machines; RS has a temperature of about 100 OK, and RS2 has a temperature of 30 OK. The use of these shields considerably improves the heat insulation of the Dewar vessel. The part of the shield situated between the walls of the Dewar vessel and the part in the open space are made separate as the equipment has to be demountable. The rim ofthe shield is increased in area by means of a special strip and the gap between the two parts is kept small (about 0.25 mm), thus ensuring sufficient heat transfer from conduction in the gas. The expansion valve JK is automatically controlled. At the top of the cryostat there is a double-bellows system, not shown in fig. 4, which is connected with the line containing the helium that has just passed the. expansion valve: The position of the bellows is determined by the difference between the pressure in the line (normally just above 1 bar) and the pressure of the outside air. When the pressure in the line increases, the bellows system closes the expansion valve tighter; when the pressure decreases, the valve opens wider. This simple control method proves eminently satisfactory in practice. At the centre of fig. 4, next to the air adsorbers, a component B can be seen which we have notyetmentioned. This starts to operate when the helium gas supplied to the liquefier is so strongly contaminated with air that air begins to condense in the heat exchanger-he. This liquid air is collected in B, and automatically blown out again when the liquid reaches a certainlevel.
1968, No. 7 HELlUM LQUEFER 201 Fig. 4. Complete constructional diagram ofphilips helium liquefier. Dew walls of Dewar vessel (cryostat). RSi and RS2 refrigerated radiation shields at 100 OK and 30 OK respectively. D delivery tube, containing one channel for removing the liquefied helium and one for the returning gas. B buffer vessel. Pi and P2 vacuum pumps. The various adsorbers for removing impurities are again denoted by the chemical symbol of the gas which they adsorb. B device for collecting and automatically blowing off liquid air; this comes into operation only when the impurity content of the helium gas is very high (see also fig. 6). The other letters have the same significanee as in fig..
202 PHLlPS TECHNCAL REVEW VOLUME 29 Fig. 5. The Philips helium liquefier. One Philips two-stage gas-refrigerating machine is to be seen on the left in the background and another can be seen at the far right. A Dewar vessel in which the liquid helium is collected and removed can be seen in the foreground. The compressor is behind the right-hand refrigerating machine. The cryostat is at the centre of the picture; the cabinet attached to it on the right contains meters for registering temperatures and pressures. Details of this automatic system are given in fig. 6, together with the air adsorbers and the corresponding valves, feeder tubes, etc. The vessel B contains a hollow pin, which is open at the bottom and is fitted inside with a reed relay. Floating on the liquid air around the pin is a ball F with a permanent magnet inside it. f the liquid level rises there comes a moment at which the magnet is level with the relay reeds, which are ferromagnetic. The
1968, No. 7 HELUM LlQUEFTER 203 reeds then spring together, the valve V 3 opens and remains open until F has gone down far enough for the relay to open again. Things are so arranged that no helium gas can be blown off. The air adsorbers Al and A2 are brought into the circuit alternately by means of the pneumatically operated double-acting valves V and V2. When an adsorber is being cleaned, the desorbed gas escapes through the tube shown in the drawing at the top, which leads to a vacuum pump. A 4 is the adsorber which purifies the gas circulating in the system during the period immediately after the system has been switched on. The last part of the new liquefier we shall discuss is the delivery tube, i.e. the tube (D in fig.4) through which the liquid helium leaves the cryostat and flows to the Dewar (fig. 7). n the sleeve 1, which can be seen in the foreground offig. 5, there are two coaxial tubes 2 and 3, which are surrounded in a part of the delivery 220 V r ----0 0-----, J L J Fig. 6. Apparatus for removing nitrogen, oxygen and argon from the helium gas supply. H2 and H3 gauze heat exchangers (see fig. ). Ai and A2 groups of two adsorbers connected in series, one of which is in operation while the other is being regenerated. The adsorbed gas escapes through the tubes shown at the top, which lead to a vacuum pump. hand V2 are double-acting pneumatically operated valves for switching over from one group of adsorbers to the other. A3 adsorber for use during the short transitional period after switching over. H7 heat exchanger. B collector vessel for the air condensed in H2, with a device (F, T) for automatically opening the valve V3. Adsorber A3 is also an air adsorber, which comes into operation immediately after Al has been switched over to A2 or vice versa. The adsorber just switched in is then still relatively warm, and during the first few seconds does not trap anywhere near all the air that passes. This air is adsorbed by A3, which is always sufficiently cold because the gas flow from A,2 (0.35 gis) first passes the heat exchanger H7 through which the main flow (2.5 gis) also passes. The time during which the liquefier can operate continuously is mainly determined by the capacity of A3. tube by a refrigerated radiation-shield 4, silvered on the outside. n the space between 3 and 1 there is a high vacuum. Gas and liquid flow through the inner coaxial tube to the collector vessel (in fig. 1 the part of the line between K and C), and gas flows back to the cryostat (line between C and EE in fig. 1) through the outer tube (shaded). The radiation shield 4 is connected with one of the radiation shields of the cryostat, i.e. the shield at about 100 "K. The end of the delivery tube projecting into the collector vessel (on the right in the figure) contains valves 6 and 7, which can be pneumatically
204 PHLlPS TECHNCAL REVEW VOLUME 29 Fig. 7. a) Delivery tube;the end at the right reaches into the vessel where the liquid product is collected, the other end into the cryostat. 1 outside wall. 2 central tube for the removal of liquid and gas.3tubethrough which gas flows back to thecryostat, coaxial with 2. 4 radiation shield (loo OK) coaxialwith 2 and 3. 5 bellows system for closing the tube, working pressure 2.5 bar; the pin 6 closes tube 2,.the ring 7 closes the holes in block 8 through which the gas can enter tube 3. 9 compression spring. b) Connection of the tube to the cryostat. As long as the tube is not in the lowest possible position, a ball valve, with balll0 and spring 11, prevents the escape of helium gas and liquefied helium coming from the expansion valve (K in fig. and fig. 6). When the tube enters the cryostat to the fullest possible extent, it presses the ball 10 downwards and the helium then has access through holes 12 to the central tube 2. The gas returning to the cryostat can flow to the expansion ejector (EE in fig. and fig. 6) via hole 13 and tube 14.15 rubber bellows; this enables the tube to be raised for changing the container, without resulting in an open connection between the ejector and the outside air. operated by means of the bellows system 5. The other end of the tube reaching into the cryostat can easily be taken out, which automatically closes the line which supplies the liquid and gas to the central tube. A special method of manufacture is used for making the complex assembly of coaxial walls 1, 2, 3 and 4: the structure is fabricated in rectilinear form, all the spaces are filled with water, and it is then cooled in a suitable way to the temperature of liquid nitrogen and bent to shape. - 8 7 9 Summary. The article describes an easily operated, highly automatic helium liquefier which makes use of Joule-Thomson effect cooling and has a pair of two-stage gas-refrigerating machines for precooling. The installation produces about 10 litres of liquid helium an hour at a high efficiency (power consumption 2.8 kwh per litre). The helium gas does not have to be carefully purified beforehand. f the gas contains 2 % of air, the system can work for 100 hours continuously; a much higher air content can be tolerated for short periods of operation. Factors contributing to the high efficiency are the availability of four precooling temperatures (approx. 100, 65, 30 and 15 OK) and the use of special gauze-type heat exchangers and an expansion ejector. The expansion ejector permits the use of a relatively small compressor (suction pressure 2.5 instead of bar, and compression 20 bar). This compressor is equipped with rolling diaphragms. All impurities are removed by adsorption, each at the most appropriate temperature. The connection between the cryostat and the vessel in which the liquid helium is collected is formed by a specially designed delivery tube, containing coaxial feeders for the liquid product and the returning gas.."