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The new hydrogen liquefier at the Clarendon Laboratory
THE NEW H Y D R O G E N LIQUEFIER AT THE CLARENDON LABORATORY A. J. CROFT Clarendon LaboratotT, University of Oxford, U.K. Received 8 February 1964
IN 1961, a Joule-Thomson hydrogen liquefier producing 30 l./hr was designed and built in the Clarendon Laboratory. Some explanation is necessary of the reasons for doing this in a low temperature laboratory where the chemical properties of hydrogen are not merely of no interest but might be thought an argument for not using it at all. We have three applications for liquid hydrogen in Oxford: as a precoolant for our helium liquefier, as a pre-coolant for apparatus subsequently used with liquid helium, and to a small extent as a cryogenic liquid per se. Our helium liquefier ~ was built in 1956. It uses externally produced liquid hydrogen and produces 12 1./hr internally. Our experience with this liquefier has been good: it has the reliability characteristic of liquefiers which have no moving parts at low temperature and the only significant maintenance required is annual overhaul of the compressor and vacuum pumps. We can produce up to 100 I. of liquid helium in one day's r u n - - o u r present consumption is just under 150 1. per week so that we have capacity in our plant for meeting increased demands. We felt, therefore, that we ought to provide for a continued supply of liquid hydrogen for many years to come. The hydrogen liquefier built in 19462 and largely rebuilt in 19543 was used with liquid air made by a condensation process in which the coolant was liquid oxygen. 4 In recent years it has become possible to procure bulk liquid nitrogen in Oxford. We therefore decided to change from liquid oxygen to liquid nitrogen as the primary source of cold and to abandon the use of liquid air. With two pumped liquid air baths, we were able to reach an average pre-cooling temperature of 59 ° K. Liquid nitrogen, however, has a triple point at 63-2 ° K and we found that when we used it the expected reduction in output to about 85 per cent in fact occurred. At this reduced output, the liquefier could not keep pace with the helium liquefier. We therefore had the alternatives of modifying the existing liquefier or building a new one of adequate output. The former would have been difficult and expensive, so we decided on the latter course. Our old liquefier is now in use in the University of Witwatersrand. CRYOGENICS
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1964
Basis of design Output: Capacity per run : Operating pressure: Pre-cooling:
Not less than 25 l./hr Not less than 200 1. About 60 atm (see below) Two stages of liquid nitrogen; 77 ° K and about 64 ° K ortho-para catalysis: To be frustrated rather than promoted (see below) Automatic control : Of liquid nitrogen levels and expansion valve and compressor bypass valve Principles of construction : Thermal insulation by means of large metal Dewar vessels enclosing 'stagnant' atmospheres (minimal use of high vacuum). Heatexchangers based on I.C.I. Integron finned tubing Layout: Operator to have separate room, apart from liquefier and machinery.
The reasons for working at a pressure lower than the optimum are entirely economic. The conventional approach would have been to work as near as possible to the pressure (approximately 120 atm) giving the optimum liquefaction coefficient for a pre-cooling temperature of 64 ° K. However, inspection of the curve of liquefaction coefficient against pressure shows that at 60 atm one need only increase the gas throughput by a factor of 1.4 to give the same rate of production of liquid. The financial advantage is shown in Table 1. Table
1
Pressure
Liquefaction coefficient (64 ° K)
Gas throughput (1~ o K)
Lit uid yi, 'ld
60 atrn 120 atm
0.20 0"28
120 m~/hr 85 ma/hr
27'7 I./hr 27.9 l./hr
Stages in I compressor 2 4
Cast £900 £3,500 approx
143
is in operation except for the drawing off of the liquid. As a general principle, techniques special to the low temperature laboratory have been avoided: the design is such that any well equipped concern in the scientific industry could have built the liquefier without difficulty. For example, the only parts calling for leak-tightness to high vacuum standards are the insulating spaces of the two large Dewar vessels and the four liquid transfer lines, and the two sealed thermometer systems. This approach has already borne fruit in that the Cryogenics and Radiation Division of Elliott Brothers (London) Limited are at present building a 500 W hydrogen refrigerator for the Herald reactor of the U.K.A.E.A. which is largely based on the design of this liquefier.
Since a major part of the cost of producing liquid hydrogen is in the depreciation of the capital equipment, the saving is not to be ignored even if capital is not scarce. A minor advantage is that less maintenance is required. Power and pre-coolant consumption are negligibly different. The main disadvantage is that some heat exchanger dimensions are greater--notably in the case of the exchanger in which the cold is extracted from the unliquefied fraction between 64 ° K and room temperature. Nonetheless, on balance the lower pressure alternative is well worth while economically. It can be shown that full conversion to the 20 ° K ortho-para equilibrium concentration is economically advantageous only if the liquid hydrogen is to be stored for about 10 days and worthwhile for the 50/50 mixture only if stored for about 4 days. When it is a matter of transporting liquid hydrogen other considerations naturally apply. Since nearly all the liquid hydrogen made in the Clarendon Laboratory is used on the day on which it is made, it is desirable to avoidconversion as far as possible. No conversion catalysts are used, therefore, and the catalytic removal of oxygen discourages conversion on the charcoal used for purification. The other basic features need little explanation. The inclusion of a bath of liquid nitrogen at atmospheric pressure greatly reduces the load on the pump and in fact a 7½ h.p. pump keeps the second bath at just above the triple point. One further heat exchanger is needed, but it is a relatively small one. The degree of automatic control applied was intended to relieve the operator of all manual activities once the liquefier
Circuit
The flow diagram of the equipment is shown in Figure 1. The compressor draws hydrogen from a 20 m 3 water-sealed outdoor gas-holder via a ballast tank. It leaves the compressor at about 60 ° C and is cooled in a separate heat exchanger to mains water temperature (see page 149). At this point there is a servo-operated bypass valve (compressed-air powered). Droplets of water and oil are removed in a 6 ft x 1 ft column flied with granite chips. This has a reservoir section in the bottom so that draining is necessary only intermittently. A similar column packed with 16/32 mesh activated alumina removes oil mist and vapour, oil cracking products, and nearly all the water vapour. A 3 in. x 24 in. vessel filled with Deoxo catalyst removes the oxygen impurity in the
l Atmosphere I V
;¢~4.f.; ¢%;.z .~ f ~ . . .
Liquid nitrogenL l
supp y
tank] leading to gas-holder
__•Ballast
; -..~ 5~/.5.,11
. . . .
r
I. . . .
P1--
-1--
i
'i
>'>1 >')1
Vacuum pump I
;gl
Liquid nitrogen (64 °K)
Liquid nitrogen (77 °K)'
Liquid hyclrogen~___O --_-_. . . . . .
XV
-,__-.:
Figure 1. Flow diagram ]44
CRYOGENICS
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1964
hydrogen. Naturally this introduces water, and a further drying column similar to the other is used to extract it. The hydrogen is cooled to about 0 ° C by a standard commercial Freon refrigerator unit before it passes into this final drying stage so as to reduce the water content to as low a value as possible. The hydrogen is generated in the laboratory by an electrolytic unit taking 18 V at 1,000 A from a germanium rectifier. The oxygen impurity is normally less than 0-2 per cent. The gas stream is then cooled to 77 ° K and further purified by charcoal (Ultrasorb grade SCII) in a 4 1. Vibrac steel bottle. After this, 20 per cent passes through a thermally insulated transfer line to the next stage and the remaining 80 per cent is returned to room temperature. This re-warming takes place in a triple-pass heat exchanger P l : the enthalpy difference of the dissimilar hydrogen streams is to some extent taken up by the nitrogen which evaporates from the liquid bath at slightly above atmospheric pressure. The room temperature stream leaving the first stage is divided at valves 3 and 4; the low temperature needle valve NV is preset appropriately. Nearly the whole of it goes through valve 3 to the exchanger Q1 where it is cooled by the returning unliquefied gas. A small fraction passes through valve 4 and this is cooled by the nitrogen vapour evaporating from the pumped bath at about 64 ° K in exchanger Q2. The setting of these three flow division valves is facilitated by a high pressure flowmeter in the path leading from valve 3 and monitored by thermometers at the room temperature ends of the three heat-exchangers (see below). These three streams unite and are cooled in a coil of Integron tubing lying in the pumped bath of liquid nitrogen, After final purification in a 2 1. vessel containing charcoal, the stream is again cooled in the liquid nitrogen bath so as to ensure a temperature as near as possible to that of the bath. It then passes into the final heat exchanger (Q3). As well as the usual expansion valve (XV) at the bottom of this exchanger, there is another (CDV) on a branch from the main high pressure path to the top of the final exchanger. This is used during that part of the initial cooling-down period when Joule-Thomson heating rather than cooling occurs. In our previous liquefier, it was found that blocks due to solid impurities could be cleared by passing warm clean hydrogen backwards through the system. A means of doing this has therefore been included. In the first two years' use of the liquefier there has been only one occasion for using this arrangement: it worked well and it was possible to resume liquefaction after a total blockage. The unliquefied fraction passes up exchangers Q3 and Ql and returns to the ballast tank, not to the gas-holder. By this arrangement, only the make-up gas--20 per cent of the total---is wet. Liquid nitrogen is supplied to the first stage from a , 1,200 gallon storage tank at 10 lb/in 2 pressure via a CRYOGENICS
• JUNE
1964
valve controlled by level (see below). From this Dewar it passes to the pumped tank by a similar automatic valve and another insulated line. Among the details not shown in Figure I are the pipes and valves involved in reactivating the charcoal purifiers, the line for drawing off the liquid hydrogen, all the safety devices, and many valves, etc. Constructional details and layout
The two Dewar vessels are constructed from an outer brass tube 15 in. diametei" by ~- in. thick by about 6 ft long with an inner Inconel tube 12 in. diameter by 1 mm thick. The bottom of each is closed by a dish-shaped spinning of the same metal. The inner and outer tubes are joined at the top by a U-section brass spinning, reinforced with radial webs. At the top of the outer tube there is a machined gunmetal flange with a groove for a synthetic rubber O-ring and keyhole slots which engage with mating pins on the top plate of the liquefier. The longitudinal seam of the Inconel inner tube was originally a softsoldered lap joint but in both Dewars leaks developed after some months of use. This is thought to have been due to disturbance of the seam when the joints at the ends were made. These seams were therefore remade by argon-arc butt-welding. All other joints are soft-soldered and have given no trouble. The gap is filled with Cabosil 5--a silica aerogel. Originally, this was mixed with aluminium dust to reduce radiative heat transfer but conducting bridges formed in both vessels after about a year and we have now reverted to plain aerogel. In any case, the heat loss across the gap is small compared with the longitudinal conduction down the Inconel liner. Great care has to be taken to avoid an over-pressure in these Dewars since the relatively thin liner will collapse under quite a small external pressure. A small vacuum gauge is therefore mounted on the front so that any substantial leakage can be detected in good time. Further, in the base there is a 3 in. diameter brass disc seating on an O-ring by atmospheric pressure only. A tube for level indication is fitted (see below). The first stage Dewar has to be lowered about every twelve hours of running time to allow the charcoal purifier to be re-activated. This is done by means of a pair of pneumatically powered piston and cylinder units by Lang Pneumatic Limited--a cheap way of doing this even apart from considerations of avoiding electrical gear. The heat exchangers are all of the type in which a helix of finned tubing is enclosed in the annular gap between two cylindrical sheets. The difference between this and most earlier types of similar principle, such as the Collins, is that this is of cylindrical rather than conical symmetry. It will become clear how the method of construction---developed in the Clarendon Laboratory in 1956--makes this possible. The tubing 145
used was copper Integron High-fin tubing produced by Imperial Chemical Industries Ltd. In this material, a helical fin is rolled up out of the thickness of the original tube. The four gas-gas exchangers and the two gas-liquid exchangers all embody tubing Code No. 09/1½/035 which has the following specification: Internal diameter: External diameter of tube: External diameter of fins: Fins per inch: Mean fin thickness: Ratio outside-inside surface: Thermal conductivity:
0-187 in. 0-258 in. 0"545-0"605 in. 9 0-024 in. 7"05 0"75 cal. cm -1 sec-1 deg.C -1 at 20°C
Two points must be made about the choice of tubing. The type of copper used is such that its thermal conductivity---especially at low temperature--is low enough not to compromise the performance of the exchanger by permitting undue longitudinal conduction. The heat flux down the thin Inconel confining walls is in fact greater than that along the axis of the copper helix. From the point of view of internal pressure drop, the ¼ in. i.d. size of Integron tubing would have been preferable, but this was subject to an extended delivery time and the less suitable ~6 in. i.d. size had to be used instead. The ends of the exchangers are closed by brass spinnings of U-section into which enough short lengths of ~- in. cupronickel tube (4 to 8) are hard-soldered to provide passages for the low pressure gas. These are manifolded together above the top plate. Annular baffles pierced with small holes serve to distribute the flow still further. The exchangers are made as follows. The inner cylinder o f Inconel sheet, 0.3 mm thick, is seamed on a mandrel by means of a soft-soldered lap-joint. The Integron tubing is then wound on tightly, with soft woven nylon cord filling the roughly triangular gap between adjacent fins. Several 20 ft lengths are necessary and hard-soldered sleeve joints are used between them. The mandrel is then rotated between centres on a lathe and a roller passed along so as to bend the tips of the fins slightly until all the fins would touch a cylindrical surface. In this way the irregularity in fin diameter is smoothed out without loss o f fin surface. The gap is again filled with nylon cord and another Inconel cylinder, seamed on another mandrel, is pushed over the Integron helix. The two end spinnings are soft-soldered on and the high pressure connections brought out through bushes in them. The exchanger in the first stage has three passes, two high and one low pressure. The helix is two-start and the two lengths of tubing are thermally bonded by tacking the tips of the fins with soft-solder. Exchanger QI is also two-start but the purpose here is merely to reduce pressure drop and the two 146
passages are manifolded at the ends. The dimensions are as follows: Gas-gas exchangers:
Exchanger
Overall external diameter
PI Q1 Q2 Q3
11 11 9 11
Overall length
in. in. in. in.
36 36 18 18
in. in. in. in.
Gas-liquid exchangers:
Exchanger P-stage Q-stage
First papt: Second part: First part: Second part
45 ft 18 ft 30 ft 10 ft
In the design of these exchangers, use was made of Parkinson's extension 5 to finned tubes of the work by Norris and Spofford 6 on heat transfer to arrays of pins attached to the outer surface of a tube. There is also a useful report by Bunn and Walling 7 who verified experimentally the results of calculations made on this basis. The two liquid nitrogen valves in the first stage have stainless steel plugs of 45 degrees semi-angle in ½ in. diameter square-edged nickel seats. They are actuated by 2 in. diameter bellows which form part of the automatic control system to be described later. A manual over-ride is provided. The three low temperature high pressure gas valves have stainless steel needles of 15 degrees semi-angle in in. diameter phosphor bronze square seats. The needles move longitudinally only: there is no rotation. The two manual valves are operated by a knob similar to that of a micrometer and similarly graduated. The pitch of the thread is 26 turns/in. The expansion valve is actuated by a pneumatic cylinder (see page 148) and a different design is necessary. The linkage can be instantly disconnected by a readily removable pin so as to allow manual control. The relative positions of the knob and the lever can be set positively to the nearest 12 degrees by a simple device involving pins and sockets working on the vernier principle. The thread is two-start and 8 turns/in, to provide enough longitudinal movement for the angular displacement possible--about 90 degrees. All connections to the liquefier pass through some 40 bushes, nearly all soft-soldered, in the two top plates to which the Dewars are bolted. These are machined from brass sheet and are ½ in. thick by 25 in. diameter. They are bolted to a rectangular aluminium alloy plate 61 in. by 38 in. by ~-~ in. thick. This rests on three brackets built into and bolted through a 14 in. brick wall (see Figure 2). There is a pit below to lower the Dewars into. On the other side of the wall is the control room (see Figure 3). In addition to the instruments, the high CRYOGENICS
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1964
Table 2 Parameter
Pressure
Reference
Type
Range
0-100 atm(gauge) 0-0-1 atm (gauge) 0-0-2 atm (gauge) 0-80 cm Hg (abs) Temperature Nitrogen vapour pressure: 0-20 cm Hg (abs) Helium gas: 0-80 T:ae cm Hg (abs) TP, T:-mQ,Tr~Q --10 to +30 = C Level Liq. N2 (P) 0-100 cm Iiq. Nil (all Hampson Liq. N2 (Q) 0-30 cm liq. N2 type) Liq. H., (Q) 0-70 cm liq. H~ HI, Hi, Ha Hi, Np NQ T,~
/
Parameter
Bourdon Diaphragm Diaphragm Diaphragm (compensated) Diaphragm (compensated) Diaphragm (compensated) Mereury-in-steel
Flow low pressure (also by observ- 0--175 mS/hr ing gas-holder) high pressure 0-0-6 atm Purity post-compressor (but precatalyst) pre-compressor
Eectro,t+c
ce,ls
'"";:,7_"<
~ ......... , ~
,-,..:++++..,:
+:g.o
Instrumentation Except where otherwise stated, the instruments are by Negretti and Zambra Ltd. Most of them have 9 in. dials. This may seem unnecessarily large but, provided there is space, generously sized dials are an advantage; an unusual indication or movement is more likely to be noticed, for example. In any case, the compensated and differential gauges, a total of six, are not made any smaller. The instruments are as listed in Table 2. The letters in the 'Reference' column refer to Figure 4.
Rotameter Budenberg Microvar differential pressure gauge across length of tubing
100-96 % hydrogen ',0-0.6 % oxygen
Differential diaphragm
pressure flow-dividing valves, handwheels to four valves whose bodies are on the liquefier side of the wall, and the drawing-off transfer line and valve are located here. Once the liquefier is running, the operator need leave the control room only occasionally to blow down the oil separator and to satisfy himself that all is well with the machinery.
Type
Range
~ II i,l
I
Cambridge Inst. Co. kat herometer instrument Home-made catalyst instrument
L'SOot
FI
~1.I
....
......
~°< . . . . ° . . . .
I
I +'::.. ,~ <-I '°" ~"' ........ "<+~J t t ~
,]"'~
I *'-----1 "IILo-J
_~ning ++ +"+
bottles,
1 O.o..,ng-o,~ II .o,o< +
-1200~ol liquid nitrogentonk
Figure 2. Plan riew o f la)'out
Figure 3. Control room CRYOGENICS
• JUNE
1964
147
Press~
(THo)
Tempe
¢T,) • ,--,-
(TN}
|
re (TI.I,)
(H 3)
(H L) __
Figure
4.
Automatic control The two liquid nitrogen levels are controlled by vapour pressure bulbs in direct communication with the 2 in. bellows units mentioned above. Oxygen is used for the liquid nitrogen bath at 77 ° K and nitrogen for the pumped bath at 63 ° K. Thick-walled copper tubing is used to provide the necessary heat flow to the bulb. It happens fortuitously that the P-stage valve opens when the Dewar is one-third full and closes when it is full again--this is quite advantageous since transfer losses are then minimal. The other takes up a floating position. This is a very cheap and reliable form of control but one has to be prepared for a little juggling to get the operation right. The expansion valve is operated by a 3-15 lb/in t signal from a 0-80 atm Negretti and Zambra pressure controller applied to a single pneumatic ram. A valve positioner is of course the right way of doing this job but this arrangement is cheaper and works satisfactorily. The opposing force is provided by trapping a suitable volume of air on the other side of the piston. The control pressure is taken from the same point as gauge H2. The operating pressure is set to the value, 54 atm, at which the liquefier will take the full output of the compressor at the maximum pressure. With the proportional band set at 2 per cent, the system is stable. Since the liquefaction coefficient is steeply dependent on pressure over the pressure range in~volved, this control is well worth while apart from its relieving the operator of having to adjust the expansion valve from time to time. A similar pressure controller opens a bypass valve if the input pressure to the liquefier reaches the value 148
m _
__°
bzstrurnentation
just below that at which the relief valve on the compressor operates. (These are intended only as safety valves and soon become damaged if they function more often than very occasionally.) When the air pressure to the bypass valve rises, the compressor pilot light is caused to flash on and off to warn the operator that something is amiss. Log book A loose-leaf log book with printed pages has been drawn up on the following basis: (1) During start-up, gas purities and times are noted. (2) At some time during the run, mean readings of the less important gauges are logged. (3) For a period of an hour at some point in each run, readings are taken every four minutes of gasholder level, return low pressure flow, pre-cooling temperature (TN), and pressure before expansion (H3). From these, the performance can be checked. (4) After the run, the total quantity of liquid hydrogen made, any unusual behaviour, and any adjustments or maintenance are noted down.
Compressor The compressor is the Reavell HCSA9, rated as follows: Output pressure: Suction capacity: Horse-power: Shaft speed:
1,000 lb/in 2 (68 atm) 65 ft3/min (110 m3/hr) 45 360 rev/min
It has two stages: the first, compressing to 100 lb/in 2, is 9 in. diameter by 8 in. stroke; the second 3¼ in. CRYOGENICS
•
JUNE
1964
diameter by 8 in. stroke. An intercooler and an aftercooler are integral with the compressor. It is desirable to run the compressor warm--about 40 ° C--to avoid the condensation of water. A further cooler has therefore been fitted to bring the high pressure gas stream down to mains water temperature and thus condense out as much as possible of the water before the two adsorptive drying stages. The following modifications were carried out by the manufacturers to make what is fundamentally an air compressor suitable for hydrogen. (1) A double grease-packed shaft seal. (2) A communication to the crankcase enabling it to be connected to the suction line. As wide a diameter as possible is used here so as to limit pulsations in the low pressure line to those occurring on the compression stroke: on the suction stroke the piston can draw into the cylinder the gas it displaces from the crankcase. (3) The two pressure relief valves are of the type that have discharge ports for piping up to the low pressure system. (4) Oil can be introduced into the crankcase without the escape of gas. (5) Lubrication of the first stage is by splash from the crankcase sump; a pump delivers oil to the second stage suction inlet. It so happens that the oil level in the crankcase stays constant. The compressor is driven via vee-belts by a motor in an adjoining room which houses the laboratory's smaller generators, etc. (see Figure 2). A sheet-metal casing surrounds the flywheel and is bolted to the shaft-seal housing and round a slot in the wall. In view of the good ventilation in both rooms, this provides adequate security against the possibility of hydrogen in combustible concentrations passing from the one to the other. The driving motor pulley incorporates a hydraulic coupling by Crofts of Bradford. This functions as a centrifugal clutch and enables one to use a squirrel cage rather than a slip-ring motor, an advantage on economic grounds alone. Further, should a fault develop in the compressor such that the power absorbed is greater than normal--though not enough to trip the starter--the oil in the coupling warms up and causes a fusible plug to melt. The oil then escapes, centrifugally, and the compressor comes to a standstill. In this way, the coupling gives a valuable degree of protection. Both the compressor and the nitrogen pump are protected against their being run without cooling water.
Nitrogen pump The liquid nitrogen bath is pumped by a Pulsometer 12X pump (now marketed by Alley Compressors Limited, Glasgow) which was used with the previous liquefier. Its specification is: CRYOGENICS
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1964
Swept volume: 123 ft3/min Shaft speed: 360 rev/min Horsepower: 7½ It is located in a hydrogen-free area (see Figure 2).
Operation The operating procedure, with representative times, is as follows:
Starting up 8.30 a.m. Fan turned on and ventilation checked. Oxygen content of gas-holder hydrogen determined (normal reading < 0"1 per cent). Compressor started (manual bypass open initially), automatic bypass checked. Dry air blown through nitrogen circuit. 8.45 a.m. Nitrogen pump started (not yet open to system). Flow of hydrogen gas through system started--momentarily through expansion valve, then through cool-down valve. (The system is still at room temperature.) Liquid hydrogen withdrawal line flushed out. Absence of hydrogen gas loss checked. 8.50 a.m. Dry.air supply stopped and pump opened to Q-stage liquid nitrogen tank. Check made for leakage into pumped system. Liquid nitrogen admitted to P-stage Dewar and both liquid nitrogen valves set to operate automatically. Cool-down valve adjusted from time to time to keep pressure in high pressure system roughly constant. 9.20 a.m. When THe reads 70 ° K, cool-down valve closed and J-T valve opened and connected to automatic control. Flow division valves set appropriately with aid of high pressure flowmeter. 9.30 a.m. Liquefaction should have started---checked by fall of gasholder, reduction of flow on Rotameter, and reading of THe.
Routine operation Every 20 min: Liquid hydrogen drawn off (or more frequently); oil separator blown down on hydrogen compressor and oil supply checked; compressor temperature checked. Every 60 min: Nitrogen pump checked. Every 12 hr: P-stage cleaner reactivated. (approx.) Every week: Small hydrogen leaks looked for with sniffer; compressor drive checked. Every2weeks: Frost-point taken of hydrogen gas after drying.
Shut-down 1 hrbefore:
Liquid nitrogen supply to P-stage Dewar shut off. 149
At shut-down: Vent from P-stage exchanger to atmosphere shut off. Q-stage liquid nitrogen tank isolated from pump. (Evaporating liquid nitrogen vents through blow-off in both cases.) Compressor shut down and held-up gas disposed of by means of separate 150 atm compressor and storage cylinders. Expansion valve disengaged from automatic control and shut. Nitrogen pump stopped and vented. Other items turned off. Emergency shut-down (This procedure is memorized by all operators.) Shut valve 1--it is never opened wider than necessary. Press emergency compressor stop button. Shut main nitrogen pump valve, stop pump, and vent. Reactivation P-stage Dewar is lowered by the pneumatic lift and an electric heater put round the charcoal bottle to bring it to 200 ° C. (The soft-soldered joint between the charcoal bottle and its plug is water-cooled.) Meanwhile a small rotary vacuum pump is opened to the bottle. After 1 hr the heater is taken off and when cool the system is refilled with hydrogen.
Safety In a hydrogen liquefier, the following three distinct sources of hazard are combined: (1) That found in all cryogenic apparatus: the possibility of increase in pressure when a closed system containing liquids and gases at low temperature warms up. (2) The high gas pressures associated with liquefiers. (3) The chemical properties of hydrogen. Although these present a wide field to be covered, this is the whole extent of the problem and a vigorous and logical approach will cover every hazard. This is the first line of defence. The second is to so design the plant that even if the unexpected does happen, the risk of damage to people and to equipment is minimal. Excess pressures from whatever cause can be covered by fitting relief valves so that no mistake on the part of the operator or mechanical failure of any part of the system can cause an unsafe pressure. Suppose, for instance, there is a high pressure valve which can be opened to let gas into a low pressure line. This line must be fitted with a blow-off so that if the high pressure valve is opened or if it develops a leak across the seating when it is closed, an excessive pressure will not be developed in the line should this ~aappen to be closed off at each end. A total of ten relief valves is fitted in various parts of the system and these cover every eventuality that it has been possible to foresee. The mechanical safety of the parts of the system at ]50
high pressure is established by an hydraulic test to twice the working pressure of the complete system. The water used incidentally removes the last trace of soldering flux. The tubing used has a bursting pressure of not less than five times the working pressure. The precautions against chemical explosion fall into two classes: those directed towards avoiding combustible mixtures and those directed towards avoiding igniting any that do occur. These include: Catalytic removal of oxygen from the hydrogen; Use of liquid nitrogen as a pre-coolant; Monitoring of gas purity before and during liquefaction; Monitoring for leakage from the system; Good ventilation; Attention to earthing of portable storage vessels which can get charged electrostatically. The Clarendon Laboratory's Code of Practice for the use of liquid hydrogen is given as an Appendix. The layout adopted is such as to concentrate as much as possible of the system away from the operator. The outside doors of the compressor and liquefier room are kept open while the plant is running. As will have been apparent, it has been possible to exclude driving motors and switchgear from the two rooms where hydrogen may be present in the atmosphere. At the time, there was no clear policy to be followed in the choice of electrical heating and lighting fittings, the compressor start and stop buttons, and the fan: hydrogen is among the gases specifically excluded from BS 229 : 1957 (and others) and the B.S.I. Code of Practice CP 1003. Since the concentration of hydrogen near electrical fittings is very unlikely to be high except for very short periods, it was decided to use flameproof fittings. At the present time, however, a second part to CP 1003 is in press and this will recommend pressurized fittings--already in use in some establishments. This will then become the accepted practice and steps will be taken to convert our present installation.
Performance Normal Liquefaction rate (internal): 31 l./hr Pressure before expansion: 52 atm (pre-cooling temp. 63.3 ° K mean) Compressor output: 125 m3/hr Liquefaction efficiency corresponding to yield of 31 1./hr at 15° C outside temperature: 0.204 Liquefaction efficiency calculated from enthalpy data s for 63 ° K and 52 arm: 0.214 Liquid nitrogen consumption: 1.2 litres/litre liq. H2 CRYOGENICS
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1964
With two compressors By using in addition to the two-stage Reavell compressor the four-stage Belliss and Morcom compressor (working at 65 atm) which has been mentioned above as being used for storing hydrogen gas at high pressure, we can increase the output at the expense of thermal efficiency: Liquefaction rate (internal): 40 1./hr Total compressor output: 180 m3/hr Corresponding liquefaction efficiency: 0.183 Liquid nitrogen consumption: 2-2 litres/litre liq. H2 (All that follows refers to the use of the liquefier with the single compressor only.)
Heat exchangers As is usually the case, the calculated and observed performances of the heat exchangers show some divergence, notably in the cases in which compromises had to be made in the design. Briefly, the performance of the exchangers is as follows: P1 Between the two high pressure streams: AT = 8 ° C Between incoming hydrogen and outgoing nitrogen: AT = 2 ° C QI ATat top = 15° C Q2 AT at top = 14° C Q3 Thermocouple measurements on this exchanger indicate that temperature equilibrium is achieved at a point about 70 per cent along its length: that is, it could have been about 12 in. long instead of 18 in. No measurements have been made of the temperature distribution along the gas-liquid exchangers. However, the temperature of the gas entering exchanger Q3 is within a few tenths of a degree of that of the final liquid nitrogen bath--that is, just above the triple point. The pressure drops across the low pressure side of the exchangers have proved gratifyingly low, viz. : P1 cooling down: 0.06 atm (0.9 lb/in 2) running: 0.002 atm Q2: 0-06 atm Q1 and Q3 (series): 0.025 atm (This pressure is too low to push the liquid out fast enough; by throttling at a valve in the low pressure line, we run with a pressure of 0.04 atm above the liquid.)
Purification The liquefier will produce over 400 I. between reactivations--the most it has been called on to produce in one run is about 250 I. On one occasion a suction leak through a valve cover on the compressor resulted in blockage; it was possible to clear it and resume liquefaction in about 15 min.
Summary The following aspects of the liquefier may be considered satisfactory: CRYOGENICS
• JUNE
1964
the liquefaction yield; the capacity of the purifying system; the instrumentation and automatic control. The following points call for comment. Although the overall thermal efficiency compares quite well with that of other, published liquefiers, it would have been better if the temperature differentials of heat exchangers P1 and Q2 had been lower. In the case of PI, it would have been better to divide its functions between two separate coaxial exchangers. Q2 could have been improved without increase in overall length by winding it so that the fins meshed with each other: this was unfortunately not thohght of at the time. The pressure drop of 9 atm across the high pressure path is excessive: had it been possible to use ¼ in. i.d. rather than ~-~ in. i.d. Integron, an increase in output of some 15 per cent could have been expected. Cost
Capital expenditure (1961) Raw materials and components: Compressor, hydraulic coupling, motor, and starter: Instruments and automatic control:
£1,200 £1,200 £850
Labour Liquefier, including instrument panel and installation but excluding building work: 2 man-years (over 9 month period) It is with great pleasure that I express my gratitude to Professor Bleaney and my colleagues for their ready agreement to this job's being undertaken, their support and forbearance while it was in progress, and their appreciation when it was completed; to Mr. Eric Tilbury, who was responsible for much of the detailed design, Mr. Ken Robinson who made a great deal of the liquefier, and the many other members of our staff who put all their customary skill and enthusiasm into the job, and special mention must be made of the contributions of Mr. Cyril Band, Mr. Archie Humphries, and our operator Mr. Alfred Hazel; to Mr. B. R. Bligh, a former colleague, for a most useful discussion in the early stages; and to the few firms who delivered the goods when they said they would and to the many who, though they did not, at least responded to cajolery just sufficiently not to dislocate our programme altogether.
REFERENCES
1. CROFT, A. J., and BLIGH, B. R. 'The Clarendon Laboratory Helium Liquefier.' Cryogenics 1, 231 (1961) 2. JONES, G. O., LARSEN, A. H., and SXMON,F. E. 'The Liquid Hydrogen Plant at the Clarendon Laboratory, Oxford.' Research, Lond. 1, 9 (1948) 3. CROFT, A. J., and SfMON, F. E. 'The New Oxford Hydrogen Liquefier.' Bull. int. Inst. Refi'ig. Annexe 1955-2
151
Wrua'E, G. K. Experimental Techniques in Low Temperature Physics (Clarendon, Oxford, 1959) Experimental Cryophysics (Eds. F. E. Hoare, L. C. Jackson, and N. Kurti) (Butterworth, London, 1961) SCOTT,R. B. Cryogenic Engineering (Van Nostrand, Princeton, N.J., 1959)
4. C'nov'r, A. J. 'A Laboratory Plant for Making Liquid Air from Liquid Oxygen.' J. sci. Instrum. 30, 74 (1953) 5. PARKINSON.D. H. Rep. Conf. de Physique des Basses Temperatures, Paris, 1955, p. 353 6. NORMS, R. H., and SPoFrORD, W. A. Trans. Amer. Soc. mech. Engrs. 42, 489 (1942) 7. BUNts, P. B., and WALUNG, J. C. 'A Description of the Hydrogen Liquefier at the MuUard Research Laboratories.' Mullard res. Lab. Rep. No. 2299 (1960) 8. WOOLLEV, H. W., SCOTT, R. B., and BR!CKWEDDE,F. G. 'Compilation of Thermal Properties of Hydrogen in its Various Isotopic and Ortho-Para Modifications.' d. Res. nat. Bur. Stand. 41, 379 (1948)
CROFT, A. J. 'Helium Liquefiers.' Progress in Cryogenics 3, p. 1 (Heywood, London, 1961) Technology and Uses of Liquid Hydrogen (Eds. R. B. Scott, W. H. Denton, and C. M. Nicholls) (Pergamon, Oxford) In press CROFr, A. J. CryogenicLaboratory Equipment (Plenum, New York). In preparation
APPENDIX: PRECAUTIONS TO BE OBSERVED IN T H E USE OF LIQUID H Y D R O G E N AT T H E C L A R E N D O N LABORATORY I. If you do not habitually wear SPECTACLES, yOU should wear a pair of the special ones obtainable from the Stores whenever you are handling liquid hydrogen or glass Dewar vessels not completely empty whatever they contain. 2. LIQUID NITROGEN only should be used for precooling apparatus which will later contain liquid hydrogen. 3. CELLULOSETAPE should not be used on parts o f apparatus which are immersed in liquid hydrogen - - i t gets charged up when the liquid hydrogen is removed and this leads to the generation of sparks. Cloth-based tapes are available from the Stores. Metal Dewars can become electrostatically charged: make sure that all metal equipment is earthed when handling liquid hydrogen. 4. The use of most types of ELECTRICALEQUIPMENT is dangerous where explosive concentrations of hydrogen may occur such as near the necks of Dewar vessels, etc. Among these are hot air blowers and other things embodying brush motors, most inspection lamps, and ordinary lighting fittings, spark coils, switches, and other electrical equipment generally which has not been specially
152
5.
6.
7.
8.
designed or fitted. (A hot air blower can sometimes be used safely if it is kept low and a long extension tube used.) DEWAR VESSELScontaining liquid hydrogen should not be left where they might get knocked over and they must on no account be left outside doors, in corridors, etc., nor in rooms where experiments are not in progress. Where liquid hydrogen is in use, there must be a fan in operation--the window or door should be open enough to allow a slight current Of air across the room. PIPES must be fitted to carry away evaporating hydrogen gas to the ventilating ducts. It is particularly important that these should take the large volume of gas boiled off during the initial filling. Everyone should know where the nearest and the next nearest FIRE EXTINGUISHERSare. The CARBON DIOXtOE type should be used except in rare cases where only a foam extinguisher will serve (notably for inaccessible or liquid fires). Use a water hose only in a very serious emergency. Do not hesitate to call the Fire Brigade. First-aid boxes (detachable from the wall) will be found in the two front halls.
CRYOGENICS • SUNE 1964
IN 1961, a Joule-Thomson hydrogen liquefier producing 30 l./hr was designed and built in the Clarendon Laboratory. Some explanation is necessary of the reasons for doing this in a low temperature laboratory where the chemical properties of hydrogen are not merely of no interest but might be thought an argument for not using it at all. We have three applications for liquid hydrogen in Oxford: as a precoolant for our helium liquefier, as a pre-coolant for apparatus subsequently used with liquid helium, and to a small extent as a cryogenic liquid per se. Our helium liquefier ~ was built in 1956. It uses externally produced liquid hydrogen and produces 12 1./hr internally. Our experience with this liquefier has been good: it has the reliability characteristic of liquefiers which have no moving parts at low temperature and the only significant maintenance required is annual overhaul of the compressor and vacuum pumps. We can produce up to 100 I. of liquid helium in one day's r u n - - o u r present consumption is just under 150 1. per week so that we have capacity in our plant for meeting increased demands. We felt, therefore, that we ought to provide for a continued supply of liquid hydrogen for many years to come. The hydrogen liquefier built in 19462 and largely rebuilt in 19543 was used with liquid air made by a condensation process in which the coolant was liquid oxygen. 4 In recent years it has become possible to procure bulk liquid nitrogen in Oxford. We therefore decided to change from liquid oxygen to liquid nitrogen as the primary source of cold and to abandon the use of liquid air. With two pumped liquid air baths, we were able to reach an average pre-cooling temperature of 59 ° K. Liquid nitrogen, however, has a triple point at 63-2 ° K and we found that when we used it the expected reduction in output to about 85 per cent in fact occurred. At this reduced output, the liquefier could not keep pace with the helium liquefier. We therefore had the alternatives of modifying the existing liquefier or building a new one of adequate output. The former would have been difficult and expensive, so we decided on the latter course. Our old liquefier is now in use in the University of Witwatersrand. CRYOGENICS
• JUNE
1964
Basis of design Output: Capacity per run : Operating pressure: Pre-cooling:
Not less than 25 l./hr Not less than 200 1. About 60 atm (see below) Two stages of liquid nitrogen; 77 ° K and about 64 ° K ortho-para catalysis: To be frustrated rather than promoted (see below) Automatic control : Of liquid nitrogen levels and expansion valve and compressor bypass valve Principles of construction : Thermal insulation by means of large metal Dewar vessels enclosing 'stagnant' atmospheres (minimal use of high vacuum). Heatexchangers based on I.C.I. Integron finned tubing Layout: Operator to have separate room, apart from liquefier and machinery.
The reasons for working at a pressure lower than the optimum are entirely economic. The conventional approach would have been to work as near as possible to the pressure (approximately 120 atm) giving the optimum liquefaction coefficient for a pre-cooling temperature of 64 ° K. However, inspection of the curve of liquefaction coefficient against pressure shows that at 60 atm one need only increase the gas throughput by a factor of 1.4 to give the same rate of production of liquid. The financial advantage is shown in Table 1. Table
1
Pressure
Liquefaction coefficient (64 ° K)
Gas throughput (1~ o K)
Lit uid yi, 'ld
60 atrn 120 atm
0.20 0"28
120 m~/hr 85 ma/hr
27'7 I./hr 27.9 l./hr
Stages in I compressor 2 4
Cast £900 £3,500 approx
143
is in operation except for the drawing off of the liquid. As a general principle, techniques special to the low temperature laboratory have been avoided: the design is such that any well equipped concern in the scientific industry could have built the liquefier without difficulty. For example, the only parts calling for leak-tightness to high vacuum standards are the insulating spaces of the two large Dewar vessels and the four liquid transfer lines, and the two sealed thermometer systems. This approach has already borne fruit in that the Cryogenics and Radiation Division of Elliott Brothers (London) Limited are at present building a 500 W hydrogen refrigerator for the Herald reactor of the U.K.A.E.A. which is largely based on the design of this liquefier.
Since a major part of the cost of producing liquid hydrogen is in the depreciation of the capital equipment, the saving is not to be ignored even if capital is not scarce. A minor advantage is that less maintenance is required. Power and pre-coolant consumption are negligibly different. The main disadvantage is that some heat exchanger dimensions are greater--notably in the case of the exchanger in which the cold is extracted from the unliquefied fraction between 64 ° K and room temperature. Nonetheless, on balance the lower pressure alternative is well worth while economically. It can be shown that full conversion to the 20 ° K ortho-para equilibrium concentration is economically advantageous only if the liquid hydrogen is to be stored for about 10 days and worthwhile for the 50/50 mixture only if stored for about 4 days. When it is a matter of transporting liquid hydrogen other considerations naturally apply. Since nearly all the liquid hydrogen made in the Clarendon Laboratory is used on the day on which it is made, it is desirable to avoidconversion as far as possible. No conversion catalysts are used, therefore, and the catalytic removal of oxygen discourages conversion on the charcoal used for purification. The other basic features need little explanation. The inclusion of a bath of liquid nitrogen at atmospheric pressure greatly reduces the load on the pump and in fact a 7½ h.p. pump keeps the second bath at just above the triple point. One further heat exchanger is needed, but it is a relatively small one. The degree of automatic control applied was intended to relieve the operator of all manual activities once the liquefier
Circuit
The flow diagram of the equipment is shown in Figure 1. The compressor draws hydrogen from a 20 m 3 water-sealed outdoor gas-holder via a ballast tank. It leaves the compressor at about 60 ° C and is cooled in a separate heat exchanger to mains water temperature (see page 149). At this point there is a servo-operated bypass valve (compressed-air powered). Droplets of water and oil are removed in a 6 ft x 1 ft column flied with granite chips. This has a reservoir section in the bottom so that draining is necessary only intermittently. A similar column packed with 16/32 mesh activated alumina removes oil mist and vapour, oil cracking products, and nearly all the water vapour. A 3 in. x 24 in. vessel filled with Deoxo catalyst removes the oxygen impurity in the
l Atmosphere I V
;¢~4.f.; ¢%;.z .~ f ~ . . .
Liquid nitrogenL l
supp y
tank] leading to gas-holder
__•Ballast
; -..~ 5~/.5.,11
. . . .
r
I. . . .
P1--
-1--
i
'i
>'>1 >')1
Vacuum pump I
;gl
Liquid nitrogen (64 °K)
Liquid nitrogen (77 °K)'
Liquid hyclrogen~___O --_-_. . . . . .
XV
-,__-.:
Figure 1. Flow diagram ]44
CRYOGENICS
- JUNE
1964
hydrogen. Naturally this introduces water, and a further drying column similar to the other is used to extract it. The hydrogen is cooled to about 0 ° C by a standard commercial Freon refrigerator unit before it passes into this final drying stage so as to reduce the water content to as low a value as possible. The hydrogen is generated in the laboratory by an electrolytic unit taking 18 V at 1,000 A from a germanium rectifier. The oxygen impurity is normally less than 0-2 per cent. The gas stream is then cooled to 77 ° K and further purified by charcoal (Ultrasorb grade SCII) in a 4 1. Vibrac steel bottle. After this, 20 per cent passes through a thermally insulated transfer line to the next stage and the remaining 80 per cent is returned to room temperature. This re-warming takes place in a triple-pass heat exchanger P l : the enthalpy difference of the dissimilar hydrogen streams is to some extent taken up by the nitrogen which evaporates from the liquid bath at slightly above atmospheric pressure. The room temperature stream leaving the first stage is divided at valves 3 and 4; the low temperature needle valve NV is preset appropriately. Nearly the whole of it goes through valve 3 to the exchanger Q1 where it is cooled by the returning unliquefied gas. A small fraction passes through valve 4 and this is cooled by the nitrogen vapour evaporating from the pumped bath at about 64 ° K in exchanger Q2. The setting of these three flow division valves is facilitated by a high pressure flowmeter in the path leading from valve 3 and monitored by thermometers at the room temperature ends of the three heat-exchangers (see below). These three streams unite and are cooled in a coil of Integron tubing lying in the pumped bath of liquid nitrogen, After final purification in a 2 1. vessel containing charcoal, the stream is again cooled in the liquid nitrogen bath so as to ensure a temperature as near as possible to that of the bath. It then passes into the final heat exchanger (Q3). As well as the usual expansion valve (XV) at the bottom of this exchanger, there is another (CDV) on a branch from the main high pressure path to the top of the final exchanger. This is used during that part of the initial cooling-down period when Joule-Thomson heating rather than cooling occurs. In our previous liquefier, it was found that blocks due to solid impurities could be cleared by passing warm clean hydrogen backwards through the system. A means of doing this has therefore been included. In the first two years' use of the liquefier there has been only one occasion for using this arrangement: it worked well and it was possible to resume liquefaction after a total blockage. The unliquefied fraction passes up exchangers Q3 and Ql and returns to the ballast tank, not to the gas-holder. By this arrangement, only the make-up gas--20 per cent of the total---is wet. Liquid nitrogen is supplied to the first stage from a , 1,200 gallon storage tank at 10 lb/in 2 pressure via a CRYOGENICS
• JUNE
1964
valve controlled by level (see below). From this Dewar it passes to the pumped tank by a similar automatic valve and another insulated line. Among the details not shown in Figure I are the pipes and valves involved in reactivating the charcoal purifiers, the line for drawing off the liquid hydrogen, all the safety devices, and many valves, etc. Constructional details and layout
The two Dewar vessels are constructed from an outer brass tube 15 in. diametei" by ~- in. thick by about 6 ft long with an inner Inconel tube 12 in. diameter by 1 mm thick. The bottom of each is closed by a dish-shaped spinning of the same metal. The inner and outer tubes are joined at the top by a U-section brass spinning, reinforced with radial webs. At the top of the outer tube there is a machined gunmetal flange with a groove for a synthetic rubber O-ring and keyhole slots which engage with mating pins on the top plate of the liquefier. The longitudinal seam of the Inconel inner tube was originally a softsoldered lap joint but in both Dewars leaks developed after some months of use. This is thought to have been due to disturbance of the seam when the joints at the ends were made. These seams were therefore remade by argon-arc butt-welding. All other joints are soft-soldered and have given no trouble. The gap is filled with Cabosil 5--a silica aerogel. Originally, this was mixed with aluminium dust to reduce radiative heat transfer but conducting bridges formed in both vessels after about a year and we have now reverted to plain aerogel. In any case, the heat loss across the gap is small compared with the longitudinal conduction down the Inconel liner. Great care has to be taken to avoid an over-pressure in these Dewars since the relatively thin liner will collapse under quite a small external pressure. A small vacuum gauge is therefore mounted on the front so that any substantial leakage can be detected in good time. Further, in the base there is a 3 in. diameter brass disc seating on an O-ring by atmospheric pressure only. A tube for level indication is fitted (see below). The first stage Dewar has to be lowered about every twelve hours of running time to allow the charcoal purifier to be re-activated. This is done by means of a pair of pneumatically powered piston and cylinder units by Lang Pneumatic Limited--a cheap way of doing this even apart from considerations of avoiding electrical gear. The heat exchangers are all of the type in which a helix of finned tubing is enclosed in the annular gap between two cylindrical sheets. The difference between this and most earlier types of similar principle, such as the Collins, is that this is of cylindrical rather than conical symmetry. It will become clear how the method of construction---developed in the Clarendon Laboratory in 1956--makes this possible. The tubing 145
used was copper Integron High-fin tubing produced by Imperial Chemical Industries Ltd. In this material, a helical fin is rolled up out of the thickness of the original tube. The four gas-gas exchangers and the two gas-liquid exchangers all embody tubing Code No. 09/1½/035 which has the following specification: Internal diameter: External diameter of tube: External diameter of fins: Fins per inch: Mean fin thickness: Ratio outside-inside surface: Thermal conductivity:
0-187 in. 0-258 in. 0"545-0"605 in. 9 0-024 in. 7"05 0"75 cal. cm -1 sec-1 deg.C -1 at 20°C
Two points must be made about the choice of tubing. The type of copper used is such that its thermal conductivity---especially at low temperature--is low enough not to compromise the performance of the exchanger by permitting undue longitudinal conduction. The heat flux down the thin Inconel confining walls is in fact greater than that along the axis of the copper helix. From the point of view of internal pressure drop, the ¼ in. i.d. size of Integron tubing would have been preferable, but this was subject to an extended delivery time and the less suitable ~6 in. i.d. size had to be used instead. The ends of the exchangers are closed by brass spinnings of U-section into which enough short lengths of ~- in. cupronickel tube (4 to 8) are hard-soldered to provide passages for the low pressure gas. These are manifolded together above the top plate. Annular baffles pierced with small holes serve to distribute the flow still further. The exchangers are made as follows. The inner cylinder o f Inconel sheet, 0.3 mm thick, is seamed on a mandrel by means of a soft-soldered lap-joint. The Integron tubing is then wound on tightly, with soft woven nylon cord filling the roughly triangular gap between adjacent fins. Several 20 ft lengths are necessary and hard-soldered sleeve joints are used between them. The mandrel is then rotated between centres on a lathe and a roller passed along so as to bend the tips of the fins slightly until all the fins would touch a cylindrical surface. In this way the irregularity in fin diameter is smoothed out without loss o f fin surface. The gap is again filled with nylon cord and another Inconel cylinder, seamed on another mandrel, is pushed over the Integron helix. The two end spinnings are soft-soldered on and the high pressure connections brought out through bushes in them. The exchanger in the first stage has three passes, two high and one low pressure. The helix is two-start and the two lengths of tubing are thermally bonded by tacking the tips of the fins with soft-solder. Exchanger QI is also two-start but the purpose here is merely to reduce pressure drop and the two 146
passages are manifolded at the ends. The dimensions are as follows: Gas-gas exchangers:
Exchanger
Overall external diameter
PI Q1 Q2 Q3
11 11 9 11
Overall length
in. in. in. in.
36 36 18 18
in. in. in. in.
Gas-liquid exchangers:
Exchanger P-stage Q-stage
First papt: Second part: First part: Second part
45 ft 18 ft 30 ft 10 ft
In the design of these exchangers, use was made of Parkinson's extension 5 to finned tubes of the work by Norris and Spofford 6 on heat transfer to arrays of pins attached to the outer surface of a tube. There is also a useful report by Bunn and Walling 7 who verified experimentally the results of calculations made on this basis. The two liquid nitrogen valves in the first stage have stainless steel plugs of 45 degrees semi-angle in ½ in. diameter square-edged nickel seats. They are actuated by 2 in. diameter bellows which form part of the automatic control system to be described later. A manual over-ride is provided. The three low temperature high pressure gas valves have stainless steel needles of 15 degrees semi-angle in in. diameter phosphor bronze square seats. The needles move longitudinally only: there is no rotation. The two manual valves are operated by a knob similar to that of a micrometer and similarly graduated. The pitch of the thread is 26 turns/in. The expansion valve is actuated by a pneumatic cylinder (see page 148) and a different design is necessary. The linkage can be instantly disconnected by a readily removable pin so as to allow manual control. The relative positions of the knob and the lever can be set positively to the nearest 12 degrees by a simple device involving pins and sockets working on the vernier principle. The thread is two-start and 8 turns/in, to provide enough longitudinal movement for the angular displacement possible--about 90 degrees. All connections to the liquefier pass through some 40 bushes, nearly all soft-soldered, in the two top plates to which the Dewars are bolted. These are machined from brass sheet and are ½ in. thick by 25 in. diameter. They are bolted to a rectangular aluminium alloy plate 61 in. by 38 in. by ~-~ in. thick. This rests on three brackets built into and bolted through a 14 in. brick wall (see Figure 2). There is a pit below to lower the Dewars into. On the other side of the wall is the control room (see Figure 3). In addition to the instruments, the high CRYOGENICS
• JUNE
1964
Table 2 Parameter
Pressure
Reference
Type
Range
0-100 atm(gauge) 0-0-1 atm (gauge) 0-0-2 atm (gauge) 0-80 cm Hg (abs) Temperature Nitrogen vapour pressure: 0-20 cm Hg (abs) Helium gas: 0-80 T:ae cm Hg (abs) TP, T:-mQ,Tr~Q --10 to +30 = C Level Liq. N2 (P) 0-100 cm Iiq. Nil (all Hampson Liq. N2 (Q) 0-30 cm liq. N2 type) Liq. H., (Q) 0-70 cm liq. H~ HI, Hi, Ha Hi, Np NQ T,~
/
Parameter
Bourdon Diaphragm Diaphragm Diaphragm (compensated) Diaphragm (compensated) Diaphragm (compensated) Mereury-in-steel
Flow low pressure (also by observ- 0--175 mS/hr ing gas-holder) high pressure 0-0-6 atm Purity post-compressor (but precatalyst) pre-compressor
Eectro,t+c
ce,ls
'"";:,7_"<
~ ......... , ~
,-,..:++++..,:
+:g.o
Instrumentation Except where otherwise stated, the instruments are by Negretti and Zambra Ltd. Most of them have 9 in. dials. This may seem unnecessarily large but, provided there is space, generously sized dials are an advantage; an unusual indication or movement is more likely to be noticed, for example. In any case, the compensated and differential gauges, a total of six, are not made any smaller. The instruments are as listed in Table 2. The letters in the 'Reference' column refer to Figure 4.
Rotameter Budenberg Microvar differential pressure gauge across length of tubing
100-96 % hydrogen ',0-0.6 % oxygen
Differential diaphragm
pressure flow-dividing valves, handwheels to four valves whose bodies are on the liquefier side of the wall, and the drawing-off transfer line and valve are located here. Once the liquefier is running, the operator need leave the control room only occasionally to blow down the oil separator and to satisfy himself that all is well with the machinery.
Type
Range
~ II i,l
I
Cambridge Inst. Co. kat herometer instrument Home-made catalyst instrument
L'SOot
FI
~1.I
....
......
~°< . . . . ° . . . .
I
I +'::.. ,~ <-I '°" ~"' ........ "<+~J t t ~
,]"'~
I *'-----1 "IILo-J
_~ning ++ +"+
bottles,
1 O.o..,ng-o,~ II .o,o< +
-1200~ol liquid nitrogentonk
Figure 2. Plan riew o f la)'out
Figure 3. Control room CRYOGENICS
• JUNE
1964
147
Press~
(THo)
Tempe
¢T,) • ,--,-
(TN}
|
re (TI.I,)
(H 3)
(H L) __
Figure
4.
Automatic control The two liquid nitrogen levels are controlled by vapour pressure bulbs in direct communication with the 2 in. bellows units mentioned above. Oxygen is used for the liquid nitrogen bath at 77 ° K and nitrogen for the pumped bath at 63 ° K. Thick-walled copper tubing is used to provide the necessary heat flow to the bulb. It happens fortuitously that the P-stage valve opens when the Dewar is one-third full and closes when it is full again--this is quite advantageous since transfer losses are then minimal. The other takes up a floating position. This is a very cheap and reliable form of control but one has to be prepared for a little juggling to get the operation right. The expansion valve is operated by a 3-15 lb/in t signal from a 0-80 atm Negretti and Zambra pressure controller applied to a single pneumatic ram. A valve positioner is of course the right way of doing this job but this arrangement is cheaper and works satisfactorily. The opposing force is provided by trapping a suitable volume of air on the other side of the piston. The control pressure is taken from the same point as gauge H2. The operating pressure is set to the value, 54 atm, at which the liquefier will take the full output of the compressor at the maximum pressure. With the proportional band set at 2 per cent, the system is stable. Since the liquefaction coefficient is steeply dependent on pressure over the pressure range in~volved, this control is well worth while apart from its relieving the operator of having to adjust the expansion valve from time to time. A similar pressure controller opens a bypass valve if the input pressure to the liquefier reaches the value 148
m _
__°
bzstrurnentation
just below that at which the relief valve on the compressor operates. (These are intended only as safety valves and soon become damaged if they function more often than very occasionally.) When the air pressure to the bypass valve rises, the compressor pilot light is caused to flash on and off to warn the operator that something is amiss. Log book A loose-leaf log book with printed pages has been drawn up on the following basis: (1) During start-up, gas purities and times are noted. (2) At some time during the run, mean readings of the less important gauges are logged. (3) For a period of an hour at some point in each run, readings are taken every four minutes of gasholder level, return low pressure flow, pre-cooling temperature (TN), and pressure before expansion (H3). From these, the performance can be checked. (4) After the run, the total quantity of liquid hydrogen made, any unusual behaviour, and any adjustments or maintenance are noted down.
Compressor The compressor is the Reavell HCSA9, rated as follows: Output pressure: Suction capacity: Horse-power: Shaft speed:
1,000 lb/in 2 (68 atm) 65 ft3/min (110 m3/hr) 45 360 rev/min
It has two stages: the first, compressing to 100 lb/in 2, is 9 in. diameter by 8 in. stroke; the second 3¼ in. CRYOGENICS
•
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1964
diameter by 8 in. stroke. An intercooler and an aftercooler are integral with the compressor. It is desirable to run the compressor warm--about 40 ° C--to avoid the condensation of water. A further cooler has therefore been fitted to bring the high pressure gas stream down to mains water temperature and thus condense out as much as possible of the water before the two adsorptive drying stages. The following modifications were carried out by the manufacturers to make what is fundamentally an air compressor suitable for hydrogen. (1) A double grease-packed shaft seal. (2) A communication to the crankcase enabling it to be connected to the suction line. As wide a diameter as possible is used here so as to limit pulsations in the low pressure line to those occurring on the compression stroke: on the suction stroke the piston can draw into the cylinder the gas it displaces from the crankcase. (3) The two pressure relief valves are of the type that have discharge ports for piping up to the low pressure system. (4) Oil can be introduced into the crankcase without the escape of gas. (5) Lubrication of the first stage is by splash from the crankcase sump; a pump delivers oil to the second stage suction inlet. It so happens that the oil level in the crankcase stays constant. The compressor is driven via vee-belts by a motor in an adjoining room which houses the laboratory's smaller generators, etc. (see Figure 2). A sheet-metal casing surrounds the flywheel and is bolted to the shaft-seal housing and round a slot in the wall. In view of the good ventilation in both rooms, this provides adequate security against the possibility of hydrogen in combustible concentrations passing from the one to the other. The driving motor pulley incorporates a hydraulic coupling by Crofts of Bradford. This functions as a centrifugal clutch and enables one to use a squirrel cage rather than a slip-ring motor, an advantage on economic grounds alone. Further, should a fault develop in the compressor such that the power absorbed is greater than normal--though not enough to trip the starter--the oil in the coupling warms up and causes a fusible plug to melt. The oil then escapes, centrifugally, and the compressor comes to a standstill. In this way, the coupling gives a valuable degree of protection. Both the compressor and the nitrogen pump are protected against their being run without cooling water.
Nitrogen pump The liquid nitrogen bath is pumped by a Pulsometer 12X pump (now marketed by Alley Compressors Limited, Glasgow) which was used with the previous liquefier. Its specification is: CRYOGENICS
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Swept volume: 123 ft3/min Shaft speed: 360 rev/min Horsepower: 7½ It is located in a hydrogen-free area (see Figure 2).
Operation The operating procedure, with representative times, is as follows:
Starting up 8.30 a.m. Fan turned on and ventilation checked. Oxygen content of gas-holder hydrogen determined (normal reading < 0"1 per cent). Compressor started (manual bypass open initially), automatic bypass checked. Dry air blown through nitrogen circuit. 8.45 a.m. Nitrogen pump started (not yet open to system). Flow of hydrogen gas through system started--momentarily through expansion valve, then through cool-down valve. (The system is still at room temperature.) Liquid hydrogen withdrawal line flushed out. Absence of hydrogen gas loss checked. 8.50 a.m. Dry.air supply stopped and pump opened to Q-stage liquid nitrogen tank. Check made for leakage into pumped system. Liquid nitrogen admitted to P-stage Dewar and both liquid nitrogen valves set to operate automatically. Cool-down valve adjusted from time to time to keep pressure in high pressure system roughly constant. 9.20 a.m. When THe reads 70 ° K, cool-down valve closed and J-T valve opened and connected to automatic control. Flow division valves set appropriately with aid of high pressure flowmeter. 9.30 a.m. Liquefaction should have started---checked by fall of gasholder, reduction of flow on Rotameter, and reading of THe.
Routine operation Every 20 min: Liquid hydrogen drawn off (or more frequently); oil separator blown down on hydrogen compressor and oil supply checked; compressor temperature checked. Every 60 min: Nitrogen pump checked. Every 12 hr: P-stage cleaner reactivated. (approx.) Every week: Small hydrogen leaks looked for with sniffer; compressor drive checked. Every2weeks: Frost-point taken of hydrogen gas after drying.
Shut-down 1 hrbefore:
Liquid nitrogen supply to P-stage Dewar shut off. 149
At shut-down: Vent from P-stage exchanger to atmosphere shut off. Q-stage liquid nitrogen tank isolated from pump. (Evaporating liquid nitrogen vents through blow-off in both cases.) Compressor shut down and held-up gas disposed of by means of separate 150 atm compressor and storage cylinders. Expansion valve disengaged from automatic control and shut. Nitrogen pump stopped and vented. Other items turned off. Emergency shut-down (This procedure is memorized by all operators.) Shut valve 1--it is never opened wider than necessary. Press emergency compressor stop button. Shut main nitrogen pump valve, stop pump, and vent. Reactivation P-stage Dewar is lowered by the pneumatic lift and an electric heater put round the charcoal bottle to bring it to 200 ° C. (The soft-soldered joint between the charcoal bottle and its plug is water-cooled.) Meanwhile a small rotary vacuum pump is opened to the bottle. After 1 hr the heater is taken off and when cool the system is refilled with hydrogen.
Safety In a hydrogen liquefier, the following three distinct sources of hazard are combined: (1) That found in all cryogenic apparatus: the possibility of increase in pressure when a closed system containing liquids and gases at low temperature warms up. (2) The high gas pressures associated with liquefiers. (3) The chemical properties of hydrogen. Although these present a wide field to be covered, this is the whole extent of the problem and a vigorous and logical approach will cover every hazard. This is the first line of defence. The second is to so design the plant that even if the unexpected does happen, the risk of damage to people and to equipment is minimal. Excess pressures from whatever cause can be covered by fitting relief valves so that no mistake on the part of the operator or mechanical failure of any part of the system can cause an unsafe pressure. Suppose, for instance, there is a high pressure valve which can be opened to let gas into a low pressure line. This line must be fitted with a blow-off so that if the high pressure valve is opened or if it develops a leak across the seating when it is closed, an excessive pressure will not be developed in the line should this ~aappen to be closed off at each end. A total of ten relief valves is fitted in various parts of the system and these cover every eventuality that it has been possible to foresee. The mechanical safety of the parts of the system at ]50
high pressure is established by an hydraulic test to twice the working pressure of the complete system. The water used incidentally removes the last trace of soldering flux. The tubing used has a bursting pressure of not less than five times the working pressure. The precautions against chemical explosion fall into two classes: those directed towards avoiding combustible mixtures and those directed towards avoiding igniting any that do occur. These include: Catalytic removal of oxygen from the hydrogen; Use of liquid nitrogen as a pre-coolant; Monitoring of gas purity before and during liquefaction; Monitoring for leakage from the system; Good ventilation; Attention to earthing of portable storage vessels which can get charged electrostatically. The Clarendon Laboratory's Code of Practice for the use of liquid hydrogen is given as an Appendix. The layout adopted is such as to concentrate as much as possible of the system away from the operator. The outside doors of the compressor and liquefier room are kept open while the plant is running. As will have been apparent, it has been possible to exclude driving motors and switchgear from the two rooms where hydrogen may be present in the atmosphere. At the time, there was no clear policy to be followed in the choice of electrical heating and lighting fittings, the compressor start and stop buttons, and the fan: hydrogen is among the gases specifically excluded from BS 229 : 1957 (and others) and the B.S.I. Code of Practice CP 1003. Since the concentration of hydrogen near electrical fittings is very unlikely to be high except for very short periods, it was decided to use flameproof fittings. At the present time, however, a second part to CP 1003 is in press and this will recommend pressurized fittings--already in use in some establishments. This will then become the accepted practice and steps will be taken to convert our present installation.
Performance Normal Liquefaction rate (internal): 31 l./hr Pressure before expansion: 52 atm (pre-cooling temp. 63.3 ° K mean) Compressor output: 125 m3/hr Liquefaction efficiency corresponding to yield of 31 1./hr at 15° C outside temperature: 0.204 Liquefaction efficiency calculated from enthalpy data s for 63 ° K and 52 arm: 0.214 Liquid nitrogen consumption: 1.2 litres/litre liq. H2 CRYOGENICS
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With two compressors By using in addition to the two-stage Reavell compressor the four-stage Belliss and Morcom compressor (working at 65 atm) which has been mentioned above as being used for storing hydrogen gas at high pressure, we can increase the output at the expense of thermal efficiency: Liquefaction rate (internal): 40 1./hr Total compressor output: 180 m3/hr Corresponding liquefaction efficiency: 0.183 Liquid nitrogen consumption: 2-2 litres/litre liq. H2 (All that follows refers to the use of the liquefier with the single compressor only.)
Heat exchangers As is usually the case, the calculated and observed performances of the heat exchangers show some divergence, notably in the cases in which compromises had to be made in the design. Briefly, the performance of the exchangers is as follows: P1 Between the two high pressure streams: AT = 8 ° C Between incoming hydrogen and outgoing nitrogen: AT = 2 ° C QI ATat top = 15° C Q2 AT at top = 14° C Q3 Thermocouple measurements on this exchanger indicate that temperature equilibrium is achieved at a point about 70 per cent along its length: that is, it could have been about 12 in. long instead of 18 in. No measurements have been made of the temperature distribution along the gas-liquid exchangers. However, the temperature of the gas entering exchanger Q3 is within a few tenths of a degree of that of the final liquid nitrogen bath--that is, just above the triple point. The pressure drops across the low pressure side of the exchangers have proved gratifyingly low, viz. : P1 cooling down: 0.06 atm (0.9 lb/in 2) running: 0.002 atm Q2: 0-06 atm Q1 and Q3 (series): 0.025 atm (This pressure is too low to push the liquid out fast enough; by throttling at a valve in the low pressure line, we run with a pressure of 0.04 atm above the liquid.)
Purification The liquefier will produce over 400 I. between reactivations--the most it has been called on to produce in one run is about 250 I. On one occasion a suction leak through a valve cover on the compressor resulted in blockage; it was possible to clear it and resume liquefaction in about 15 min.
Summary The following aspects of the liquefier may be considered satisfactory: CRYOGENICS
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the liquefaction yield; the capacity of the purifying system; the instrumentation and automatic control. The following points call for comment. Although the overall thermal efficiency compares quite well with that of other, published liquefiers, it would have been better if the temperature differentials of heat exchangers P1 and Q2 had been lower. In the case of PI, it would have been better to divide its functions between two separate coaxial exchangers. Q2 could have been improved without increase in overall length by winding it so that the fins meshed with each other: this was unfortunately not thohght of at the time. The pressure drop of 9 atm across the high pressure path is excessive: had it been possible to use ¼ in. i.d. rather than ~-~ in. i.d. Integron, an increase in output of some 15 per cent could have been expected. Cost
Capital expenditure (1961) Raw materials and components: Compressor, hydraulic coupling, motor, and starter: Instruments and automatic control:
£1,200 £1,200 £850
Labour Liquefier, including instrument panel and installation but excluding building work: 2 man-years (over 9 month period) It is with great pleasure that I express my gratitude to Professor Bleaney and my colleagues for their ready agreement to this job's being undertaken, their support and forbearance while it was in progress, and their appreciation when it was completed; to Mr. Eric Tilbury, who was responsible for much of the detailed design, Mr. Ken Robinson who made a great deal of the liquefier, and the many other members of our staff who put all their customary skill and enthusiasm into the job, and special mention must be made of the contributions of Mr. Cyril Band, Mr. Archie Humphries, and our operator Mr. Alfred Hazel; to Mr. B. R. Bligh, a former colleague, for a most useful discussion in the early stages; and to the few firms who delivered the goods when they said they would and to the many who, though they did not, at least responded to cajolery just sufficiently not to dislocate our programme altogether.
REFERENCES
1. CROFT, A. J., and BLIGH, B. R. 'The Clarendon Laboratory Helium Liquefier.' Cryogenics 1, 231 (1961) 2. JONES, G. O., LARSEN, A. H., and SXMON,F. E. 'The Liquid Hydrogen Plant at the Clarendon Laboratory, Oxford.' Research, Lond. 1, 9 (1948) 3. CROFT, A. J., and SfMON, F. E. 'The New Oxford Hydrogen Liquefier.' Bull. int. Inst. Refi'ig. Annexe 1955-2
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Wrua'E, G. K. Experimental Techniques in Low Temperature Physics (Clarendon, Oxford, 1959) Experimental Cryophysics (Eds. F. E. Hoare, L. C. Jackson, and N. Kurti) (Butterworth, London, 1961) SCOTT,R. B. Cryogenic Engineering (Van Nostrand, Princeton, N.J., 1959)
4. C'nov'r, A. J. 'A Laboratory Plant for Making Liquid Air from Liquid Oxygen.' J. sci. Instrum. 30, 74 (1953) 5. PARKINSON.D. H. Rep. Conf. de Physique des Basses Temperatures, Paris, 1955, p. 353 6. NORMS, R. H., and SPoFrORD, W. A. Trans. Amer. Soc. mech. Engrs. 42, 489 (1942) 7. BUNts, P. B., and WALUNG, J. C. 'A Description of the Hydrogen Liquefier at the MuUard Research Laboratories.' Mullard res. Lab. Rep. No. 2299 (1960) 8. WOOLLEV, H. W., SCOTT, R. B., and BR!CKWEDDE,F. G. 'Compilation of Thermal Properties of Hydrogen in its Various Isotopic and Ortho-Para Modifications.' d. Res. nat. Bur. Stand. 41, 379 (1948)
CROFT, A. J. 'Helium Liquefiers.' Progress in Cryogenics 3, p. 1 (Heywood, London, 1961) Technology and Uses of Liquid Hydrogen (Eds. R. B. Scott, W. H. Denton, and C. M. Nicholls) (Pergamon, Oxford) In press CROFr, A. J. CryogenicLaboratory Equipment (Plenum, New York). In preparation
APPENDIX: PRECAUTIONS TO BE OBSERVED IN T H E USE OF LIQUID H Y D R O G E N AT T H E C L A R E N D O N LABORATORY I. If you do not habitually wear SPECTACLES, yOU should wear a pair of the special ones obtainable from the Stores whenever you are handling liquid hydrogen or glass Dewar vessels not completely empty whatever they contain. 2. LIQUID NITROGEN only should be used for precooling apparatus which will later contain liquid hydrogen. 3. CELLULOSETAPE should not be used on parts o f apparatus which are immersed in liquid hydrogen - - i t gets charged up when the liquid hydrogen is removed and this leads to the generation of sparks. Cloth-based tapes are available from the Stores. Metal Dewars can become electrostatically charged: make sure that all metal equipment is earthed when handling liquid hydrogen. 4. The use of most types of ELECTRICALEQUIPMENT is dangerous where explosive concentrations of hydrogen may occur such as near the necks of Dewar vessels, etc. Among these are hot air blowers and other things embodying brush motors, most inspection lamps, and ordinary lighting fittings, spark coils, switches, and other electrical equipment generally which has not been specially
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designed or fitted. (A hot air blower can sometimes be used safely if it is kept low and a long extension tube used.) DEWAR VESSELScontaining liquid hydrogen should not be left where they might get knocked over and they must on no account be left outside doors, in corridors, etc., nor in rooms where experiments are not in progress. Where liquid hydrogen is in use, there must be a fan in operation--the window or door should be open enough to allow a slight current Of air across the room. PIPES must be fitted to carry away evaporating hydrogen gas to the ventilating ducts. It is particularly important that these should take the large volume of gas boiled off during the initial filling. Everyone should know where the nearest and the next nearest FIRE EXTINGUISHERSare. The CARBON DIOXtOE type should be used except in rare cases where only a foam extinguisher will serve (notably for inaccessible or liquid fires). Use a water hose only in a very serious emergency. Do not hesitate to call the Fire Brigade. First-aid boxes (detachable from the wall) will be found in the two front halls.
CRYOGENICS • SUNE 1964