A Precision Cryostat for the Range -35" to +25" c. EDW I S E. HOPER, Chemical Laboratories of Hanard U n i v e r s i t y , Cambridge. .\Ias+.
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EITERAL new features have been incorporated in ail automatic cryostat, the operation of which has proved to be exceedingly satisfactory. The apparatus requires no attention once it is set in operation, it contains a relatively
trul) of expanded cold liquid phase to enter the evaporator coil, which is iinniersed in a stirrecl liquid b:ith constituting the constant-temperature region. I n the suction line (gas phase or lon--side) leading from the evaporator to the compressor crankcase is placed a constant-pressure valve which maintains the pressure of t'he vaporizing refrigerant in the evaporator a t a predetermined value, depending upon t'he manual setting of the valve. K i t h proper adjustments of the expansion and constantpressure yalves, a n equilibrium temperature is soon reached in the bath, the rate of attainment being det,ermined by the setting of the expansion valve orifice and the ultimate value by the setting of both valves. This ultiniat'e (called the cooling-equilibrium) temperature should be somewhat greater than the temperature as determiiieil by the evaporator pressure translated into temperature through the intermediary of t,he vapor pressure-temperature relationship of the refrigerant (a table or graph, the latter being preferable, of the vapor pressure of the refrigerant i n units the same as that of the gages, is almost a necessity). A continuous electrical energy input is then admitted t o the bath until a temperature of a few degrees above the cooling-equilibrium temperature is reached; at this point, the continuous energy input is reduced in magnitude until it does not quite balance the energy withdrawn by the refrigeration system. The on-off intermittent energy input is then placed in operation, along with the modulator. The latter serves to vary the continuous energy input slowly, the amount of variation being a function of the length of time that the intermit,tent heater is on a,nd is off; such a type of modulation tends to counteract t,he effect of any slow, external changes upon the system. It is important that the evaporator coil be rim almost drythat is, that only a small amount' of evaporating liquid refrigerant spray be present at any one time. I:n this manner, the coil n.ill be kept a t a given temperature irrespective of the bath temperature, in contradistinction to the wet method of operation in which the evaporator is kept filled (generally automatically to a predetermined amount) with liquid phase; under such circumstances the refrigeration system will attempt to keep the bath (and the coil) at a constant temperature. With the n e t method of operation, the system of control and modulatioii here developed is not applicable, since no great amount of electrical energy input may be added to the bath vithout causing increased rate of evaporation which tends to counteract (and generally widely overshoots) the
large const'ant-temperature space, and i t maintains constsnt temperatures to within a few thousandt'hs of a degree for long periods of continuous operation. Since the temperatures obtainable with a conipressor (and auxiliary apparatus) such as that' contained in commercial household refrigerators were well within the required range, it proved feasible to utilize a machine of this type. After preliminary investigations, the following scheme was found sui table. The coinpressor is kept in constant operation; a n espansioii valve in the liquid refrigerallt line allon-s various quantities (depending upon the manual setting of the valve orifice con-
0 DETAILS O F CRYOSTAT 1:NO.T D R A ~ V TO N SCALE) A . dlumiiiuni shell base for Dewar tube B. Copper shell container ior Dewar tube C . Terminal connections for bath heaters, H D. Pyrex glass Deu-ar tube E . Transite-board end pieces for shell, B F . Points of support of evaporator coil, AT H . Bath beaters K. Point corresponding t o K in Figure 2, wheie evapurator coil connectlons may be broken t o move cryostat from place t o place. Copper tubing goes t o liquid refrigerant line from compressor L . Surface of b a t h liquid M. Expansion valve h'. Evaporator coil 0. Point corresponding t o 0 in Figure 2. Game function as K. COPper tubing goes t o constant-presaure valve (Q,Figure 2) S. Stirrer assembly of two right-angle shafts, each supported i n two ball hearings and connected by 45' gears T. Thermoregulator
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heating effect; if such a heating operation is attempted, various phenomena will result, depending upon the type and characteristics of the expansion valve employed, and under any conditions will give flooding of the low-pressure line, excess cooling, or a dry coil. To change the bath from one operating temperature to another, excess refrigeration or excess electrical energy input is employed to attain approximately the desired temperature; since it is considerably easier t o perform minute changes by means of the electrical energy input than by means of the refrigerating effect, the final adjustment is made by the former means. By proper manipulation of the two valves on either side of the evaporator in conjunction with pressure gages and a sensitive temperature-measuring device, it is simple, especially after familiarity with the characteristics of the system has been attained, to realize a condition of the correct dryness in the evaporator coil.
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perature in one instrument was satisfactorily performed, before sealing, by a weighing method. An ordinary open type of mercury thermoregulator was employed in some preliminary work and gave satisfactory service (providing access of atmospheric moisture was prevented) over a period of 20 to 80 hours, cleansing and refilling then being necessary to reattain the previous sensitivity. Simplicity of adjustment is not compatible with high sensitivity and reproducibility over long periods of time, especially at temperatures \There icing from the atmosphere can take place. The operation of these hermetically sealed, single-valued thermoregulators is startling, containing as they do but about 50 cc. of mercury, when they are contrasted with those containing 20 times as much filling fluid having a coefficient of expansion some ten times that of mercury. Such instruments exhibited but slight drift (less than 0.002" at -28" C., less than 0.001" at -11' C.) over periods of many days of constant operation. The disposition of the equipment in and on the cryostat is shown in Figure 1. It was desirable that the liquid utilized for the bath fluid be nonflammable and possess a relatively low vapor pressure and it was imperative that it have a lox freezing point and be noncorrosive. h 50 weight per cent aqueous solution of ethanol was satisfactory, although it is somewhat viscous at -30" C.
The Bath Proper The container for the bath liquid was a silvered Dei7 ar tube of Refrigeration Apparatus Pyrex glass (specially fabricated by the American Thermos Bottle Co., Norwich, Conn.), 17.8 cm. (7 inches) in diameter and .4 compressor (manufactured by the Serve1 Corp., New York, 45.7 cm. (18 inches) deep, inside dimensions. When the evaporaN. Y,) of 250-watt (0.33-horsepor~er)rating was employed, with tor coil, heating units, stirrer, and thermoregulator were in place a refrigerant filling of Freon-12 (dichlorodifluoromethane). It and the Dewar was filled to within 8 cm. of the rim, a constantwas found necessary to install an efficient oil trap in the comtemperature cylindrical space approximately 11 X 36 cm. was pressed gas line leading from the compressor to the condenser; available. until this was done, constant cooling could not be secured, probably because chilled oil slowly clogged the expansion valve The heating units were constructed by flattening a piece of thin-walled copper tubing over a doubled length (7.3 meters, 26 orifice. Shutoff valves were installed in the two copper tubing ohms) of No. 22 asbestos-insulated Chrome1 wire,'and brazing shut the bottom end of the tube. The heating units were wound into a helix which fitted snugly inside the eva orator coil. One such unit mas employed for m o h a t e d continuous and another for intermittent controlled heating. The evaporator coil was fabricated from 9 . 6 mm. (0.375-inch) diameter refrigeration grade copper tubing (0.875-mm., 0.035-inch wall) ; about 13 meters were wound into a helix of 24.5 turns which were slightly spaced one from the other and the bottom end was brought back through the spiral t o form the low-side connection. To the top end of the evaporator was connected an expansion valve [No. 672 automatic expansion valve, 1.98-mm. (0.078-inch) orifice, manufactured by the Detroit Lubricator Co., Detroit, Mich.] . (Standard refrigeration fittings were used throughout.) The exposed portions of the evaporator fittings were thermally insulated. The thermoregulator lyas of the mercuryfilled type. [ A thermoregulator of this type precludes the use of temperatures belov -33' C. Recently, there have been described control circuits that x-ould be applicable for lower temperatures, the modulation being inherent in the type of control (1, d).] It was found t,hat in order to secure reproducibility in this instrument, it was necessary to seal it hermetically. Excellent performance was realized b r using more than ordinary care in cleansing, filling (about 50 cc. of mercury were utilized), evacuating (to FIGURE2. SCHEhfATIC DIAGRAM O F REFRIGERITIOS CY( LE ( N O T DRAT? S T O ca. 0.2 mm.), and sealing off and by the presSCALE) ence of only pure, very clean mercury and clean .-1. Compressor tungsten wire inside the Pyrex glass container. B. High-pressure line froin compressor, 0.375-inch tubing C. Air condenser One type of hermetically sealed thermoregD. Oil filter in compressed gas line ulator was constructed which had provision for E. Oil return line t o crankcase F. Filter in condensed liquid line. From here, line is of 0.23-inch tubing adjusting the point of regulation by transferring G. Sight glass mercury t o or from a side-arm reservoir; it \vas If. Dehvdrator If. Dehydrator tube difficult to adjust this instrument to better than J. Liqiid Liquid line shutoff valve leading t o trap-refrigerating apparatus a few tent,hs of a degree to a predetermined K. Liquid line shutoff valve, leading t o cryostat unit L. Gage for high-pressure side of compressor, 1 t o 14.6 atmospheies value, but with more attention to the design of .M. Expansion valve mounted above cryostat the side arm, this could undoubtedly be imA-. Evaporator coil in cryostat of 0.375-inch copper tubing. From here all l o w I)ressur? line is of 0.375-inch size proved. To obtain a series of temperatures that, P. Cryostat evaporator gage, 0 t o 5.1 atmospheres compound type could be exactly reproduced at future times, Q. Constant-pressure valve, range 660 t o 2830 mm. nonadjustable, single-valued hermetically sealed R . Talve t o permit evaporator between K and S t o be exhausted by Iiuviliary vacuum thermoregulators were made up to nominal pump when evaporator system is broken open a t I< and 0 S. Suction line shutoff valve for oryostat unit values, in this specific case to -3O", -20", T. Suction line shutoff valve leading t o trap-refrigerating apparatus -loo, +lo", and +20° C. The adjustment of C. Suction line gage, 0 t o 12.9 atmospheres, compound type the proper amount of mercury for a given temV . Suction line leading t o compressor compressor
FEBRUARY 15, 1940
ANALYTICAL EDITION
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FIGURE 3. ELECTRICAL COSTROLCIRCUIT
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B. Rectifier-filter output T1. TvDe 45 amdifier electron tube M1. Aficroammeier i n thermoregulator ciicuir T. Thermoregulator connections T2. Type FG-57 Thyratron t u b e E l . Grid leak resistor, 0.6 megohm R 2 . Resistor, 0.25 megohm R3. Grid leak resistor, 2.0 megohm M2. Milliammeter in mechanical relay circui:, F1. Filament connections for T1. 2.5 volts F 2 . Heater connections for T2. 5.0 volts R 4 . hlechanical relay, 11,000 ohms, contact a r m in pusitiJn shown f c o o l ~ u g part of cycle) when relay is not energized ,V1. ?;eon glow lamp across intermittent heater leads, installed directly above thermometer-galvanometer scale X 2 . Neon glow lamp across Telechron coil, C 2 N 3 . Neon glow lamp across Telechron coil, C'3 M3. Ammeter i n intermittent heater leads C 2 . Telechron stator coil, energized on heating part of cycle to rotate m o v @le a r m E (on Variac) clocknise, increasing energy :supplied t o heater, H1 C 3 . Telechron stator coil, energized on cooling part of cycle t o rotate E counterclockwise, decreasing energy supplied t o H 1 5'1, S2. Safety contact devices, normally closed a s shown, t o prevent E from passing beyond end of normal path D. llechanical coupling between Telechron rotor and E V . Yariac. variable autotransformer R5. Variable resist.or in continuous heater circuit M4. Ammeter in continuoun heater circuit R6. Variable resis-tor in intermittentheater circuit H 1 . B a t h heater coil, 26 ohms, for continuous energy input t o bath H 2 . B a t h heater coil, 26 ohms, for intermittent energy input
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leads to the cryostat evaporator, so that it could be removed without releasing the Freon in the compressor side of the system. Three pressure gages proved to be indispensable in setting the apparatus in operation. In the low-side of the evaporator line leading to the compressor was placed a constant-pressure valve (Type CP 35H, manufactured by the Fedders Mfg. Co., Inc., Buffalo, N. Y.) to control the evaporator pressure. The schematic diagram of Figure 2 illustrates the complete syatem. Parallel to the copper tubing lines leading to the cryostat unit, another pair ( J and R, Figure 2 ) was taken off to cool a trap to various temperatures; the trap evaporator coil (not shown in Figure 2) was connected to a thermostatically controlled (by Freon vapor pressure), constant-superheat expansion valve and to a second constant-pressure valve. The two cooling systems both operating from the same compressor interacted to a negligible degree, except when the temperature of one or the other vias deliberately changed by many degrees. 2
one of the type mentioned by Kistiakorvsky et al. ( 6 ) , and since utilized by Gucker et al. ( 5 ) . In principle, the continuous heating current supplied to the bath is sloir-ly increased during the time that the intermittent heater is on and slowly decreased during the time that the heater is off. This arrangement tends to give heating and cooling periods of equal time intervals. Any external agency acting to influence the operation of the bath will be counteracted, providing the agency changes a t a rate comparable to or lees than the rate of counteraction. In practice, this principle was applied by means of a variable autotransformer, t,he Variac (manufactured by the General Radio Co., Cambridge, Mass.), the output of which is varied by means of a reversible, synchronous electric motor, the Telechron [Model 43M 536, Type C2M, 12-watt, manufactured by the Warren Telechron Co., Ashland, Mass. One stator coil (6-watt) was removed and so replaced that it gave opposite direction of rotation d!en energized], geared to the movable arm of the transformer. ,This method of the Telechron-Variac was suggested to the author by Professor Kistiakowsky of these laboratories as being somewhat simpler than the magnetic clutch mentioned in (@.I The shaft of the motor revolved at 60 r. p. h.; this was geared down to 0.417 r. p. h. and at 1-ampere transformer output this rate of rotation changed the output about 0.013 ampere per minute. To control the direction of rotation of the Telechron, an electron tube relay circuit was constructed, with its grid tied in t o the Thyratron grid, so that when the thermoregulator made contact both grids were given a negative charge. [It was necessary to construct a recti-
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Electrical Control Circuit A Thyratron (FG-57) tube served as a relay to translate the minute (+Omicroampere) thermoregulator impulses into intermittent energy input to the bath. It was found necessary to modulate the continuous energy input to the bath, since external variables such as room temperature, barometric pressure, and power line voltage were influencing the operation to an appreciable extent. -4 modulating device, which had been constructed in connection viith another problem, was added to the control system. This c o m p l i c a t e d the electrical circuit (mainly because of the inclusion of the Variac transformer in the modulator) but the resultant excellent operation of the control served to justify the addition. The modulating device is
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INDUSTRIAL AND EhGINEERIhG (XEMISTRY
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FIGURE 5. TYPICAL TEnrpemrLm
FLUCTUATIONS Of B A r l i OPERkPI.\
