United States Patent [1113,59U,597 Inventor David Neil Campbell [56] References {Cited} , England UNITED STATES PATENTS [21 1 Appl No. 847,917 [22] Filed Aug. 6, 1969 3,431,750 3/1969 Lefranc ...... 62/514 [451 Patented July/.6, 1971 Primary Examiner—-Meyer Perlin [73] Assignee The Hymatic Engineering Company Attorney—-Watson, Cole, Grindle & Watson Limited Redditch, England (32] Priority Aug. 6, 1968 ABSTRACT: The second stage in a miniature cryogenic two [33] Great Britain stage cooler, working on the Joule-Thomson principle, is sup [31] 37477/68 plied with gaseous which passes from a supply under pressure through one path of a tubular , _ to an expansion nozzle, after which the expanded gas returns [541 COOLING APPARATUS EMPLOYING THE J OULE through‘the other path‘of the heat exchanger to cool the in THOMSON EFFECT ‘ coming second stage gas. In the ?rst stage, a precooling 6 Claims, 1 Drawing Fig. refrigerant in liquid form is supplied to a metering nozzle which, havingno heat exchanger, is accommodated within the [52] US. Cl...... 62/514 heat exchanger of the second stage, and this refrigerant [51] Int. Cl ...... F25b 19/00 evaporates to precool the second stage refrigerant. The nozzle [50] Field of Search ...... 62/222 of each stage is automatically controlled to vary the flow of 5 14, 467 refrigerant in accordance with the demand for cooling.

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IN VE NTOR DAxnb NEH. CAMPBELL BY

424m ATTORNEY 3,590,597 ll 2 generally at atmospheric pressure, and in this case there can COOLING APPARATUS EMPLOYING THE JOlUlLlE not be any variation of temperature so long as the liquid is in THOMSON iElFh‘lElCT equilibrium with its vapor. Thus in the preferred arrangements This invention relates to cooling apparatus of the type em~ described, the control is not directly in accordance with the ploying the Joule-Thomson effect, that is to say in which cool temperature of the refrigerant but in accordance with the level ing is produced by expansion, through a nozzle, of a of the liquid refrigerant which comes into contact with a sen refrigerant in gaseous form under pressure, which before ex sor as the level rises, and controls the temperature of the sen pansion at the nozzle is at a temperature below its inversion temperature. ’ sor. Other arrangements may be employed in which the con The invention is particularly, though not exclusively, con trol is in accordance with some other or intermediate function cerned with such coolers which are of miniature size, for ex~ depending upon the supply and demand for cooling, for exam ample for cooling infrared detectors to the temperature of ple the sensor may have an extended tail affording a path for liquid nitrogen. In a typical construction the complete cooler the ?ow of heat to the liquid refrigerant, the length of which is largely situated within a cylindrical heat exchanger having a path varies with the level of refrigerant. Again the sensor may length of between 2 and 3 inches and a diameter of about a be in heat exchange relationship with parts of the apparatus third of an inch and wound with a helical coil of ?nned tubing above the pool of liquid refrigerant so that its temperature will of which the diameter over the fins is less than one-twentieth depend partly upon the level of liquid refrigerant and partly on of an inch. In general it is a requirement not only that the the temperature of the vapor escaping from the nozzle. cooler itself should be of extremely small size, but in addition In one arrangement in accordance with the present inven that the supply of compressed refrigerant required should also 20 tion the second stage expansion nozzle is arranged and con be as small and light as possible. trolled as described above and includes a needle valve con It is known that each refrigerant has a temperature, known trolled by a bellows the pressure upon which is determined by as its inversion temperature, at which no cooling is produced a sensor comprising a vapor bulb with an extended tail extend ing down to the pool of liquefied refrigerant. The ?rst stage is by the Joule-Thomson e?ect, that is to say by expanding the 25 refrigerant gas through an expansion nozzle. Above the inver generally similar but in this case the sensor bulb is secured to a sion temperature such expansion actually produces a rise of curved plate ?tting within the body of the heat exchanger at temperature, and it is only below it that it produces a cooling about the level of the metering nozzle so as to respond effect. Moreover at temperatures only slightly below the in primarily to the temperature of the heat exchanger at that part version temperature the cooling effect is correspondingly of the apparatus. Just below the metering nozzle a cup is slight and if a substantial cooling effect is required the expan secured to the inside of the body of the heat exchanger to col sion must be carried out from a temperature substantially lect liquid refrigerant formed by the first stage refrigerant, and below the inversion temperature. at the corresponding part of the length of the heat exchanger Thus in the case of Joule-Thomson coolers employing the helical finned tube is not finned but is soldered to the outer with an inversion temperature below atmospheric 35 surface of the heat exchanger tube so‘ as to transmit heat from temperature it is essential to employ more than one stage, so the second stage refrigerant within it to the liqui?ed first stage as to precool the refrigerant before expansion, but even in the refrigerant inpthe cup within the body. case of refrigerants, such as air or nitrogen, having an inver The invention may be put into practice in various ways but sion temperature substantially above atmospheric tempera one specific embodiment will be described by way of example tures, the efficiency can be considerably improved by employ 40 with reference to the accompanying drawing which shows a ing a precooling stage. This, however, has generally not been two-stage cooling device. done in coolers of the type with which the invention is con In this embodiment the invention is applied to a two-stage cerned since the space available has not been suf?cient to ac cooler of miniature size, perhaps 3 inches in length and one commodate a precooling stage. third inch in diameter comprising two stages. The ?rst, According to the present invention cooling apparatus in 45 precooling, stage is supplied with which is stored in cludes two stages in the second of which a second stage liquid form, under pressure but at atmospheric temperature, refrigerant from a supply in gaseous form is passed through and is admitted past a metering ori?ce to a space where it can one path of a heat exchanger, expanded through an expansion expand to atmospheric pressure, thereby effecting precooling. nozzle and passed back through the other path of the heat It operates with nitrogen in gaseous form under pressure exchanger to cool the incoming second stage refrigerant, and which is fed through the inner path of a helical tube heat a precooling ?rst stage in which a precooling refrigerant is exchanger to an expansion nozzle where it expands and cools supplied in liquid form under pressure through a metering due to the Joule-Thomson effect, producing a pool of liquid nozzle to a space at reduced pressure where it evaporates to nitrogen, while the remaining gas escapes by the outer path of cool the second stage refrigerant before the latter passes the heat exchanger, i.e. past the outside of the helical ?nned through the heat exchanger, the effective area of the nozzle of 55 tube, cooling the incoming nitrogen. each stage being automatically controlled in accordance with The cooling apparatus is of elongated form, and for pur a parameter depending upon the relationship between cooling poses of description it will be assumed that it is placed verti_ supplied and cooling demanded at that stage. cally with its cold end at the bottom. The apparatus includes In one form of the invention the apparatus is generally of tu an annular heat exchanger Mi comprising a tubular body 11 bular form, the heat exchanger comprising a ?nned tube around which is helically wound a ?nned coiled tube 12. An wound to helical form with the second stage refrigerant ?ow external coaxial tube 13, which may be the inner wall of a ing through it in one direction and past its outside in the op Dewar ?ask, is located around the ?nned coil and the space 14 posite direction, the second stage expansion nozzle being at between the body and the external tube provides a path for ex the cold end of the heat exchanger while the ?rst stage meter 65 haust gas ?owing past the fins to cool the incoming high-pres ing nozzle is enclosed within its warm end. sure working fluid within the helical ‘tube. The lower end of In the above-noted cooling apparatus designed, for exam~ the external tube is closed to provide a reservoir (not shown) ple, for cooling infrared detectors to the temperature of liquid in which the liquid working ?uid can accumulate. This lower nitrogen, the effective area of the nozzle of a Joule-Thomson end may be combined with the device to be cooled so that the cooler is automatically controlled, as by means of a needle 70 liquid nitrogen is in fact in contact with that device. The upper valve, in accordance with a parameter depending on the rela end of the helical ?nned tube 12 communicates with a lateral tionship between cooling supplied and cooling demanded. coupling W for connection to the supply of nitrogen under This may be in accordance with a temperature at a point in the pressure. Its lower end communicates with an expansion noz‘ cold end of the cooling apparatus. In general, however, the ap zle 22 which is arranged to be controlled by means ofa needle paratus will serve to produce a pool of liquid refrigerant, 75 valve 23 which is itself controlled by a bellows 2d. 3,590,597‘ 3 41 Thus at its lower end the tubular body of the heat exchanger in accordance with the temperature of the heat exchanger in has secured in it a stout metal ring 28. Secured to a point the region of the ?rst-stage ori?ce formed by the seating 54. under this ring, and thus offset from the axis of the heat Within the tubular body of the heat exchanger, just below exchanger, is the expansion nozzle 22, while a sensor bulb 30 the ?rst stage ori?ce, there is mounted a cup 7 5 the cylindrical depends from a diametrically opposite point. To support the wall of which is in heat exchange relationship with the body of expansion nozzle 22 a threaded socket 32 is secured to the the heat exchanger and which accommodates any Freon ring and receives the upper end of a screwed sleeve 33 on the which may be lique?ed by the ?rst stage. The cup contains a lower end of which is threaded a screwed plug 34 in which the perforated heat collector 76 of tubular form so as to assist in expansion nozzle 22 is formed. The screwed sleeve also serves transmitting heat from the body of the heat exchanger to any to retain a porous plug 35 or ?lter through which the working liquid Freon in the cup. ?uid is compelled to pass on its way to the expansion nozzle 22 The part of the helical tube 12 surrounding the cup is so as to condense and ?lter out any impurities which might deprived of its ?ns and wound in a close-pitched helix 77 otherwise block the expansion nozzle. round the body of the heat exchanger to which it is soldered so The bellows 24, situated coaxially within the lower part of asto provide effective heat exchange between it and the cup. the tubular body 11 of the heat exchanger, has its lower end 15 It is believed that the operation of the apparatus will be secured to the ring 22} while its upper end is closed by and clear from the description given above. In an apparatus having secured to a tubular operating member 25 which extends no automatic control it is of course necessary to provide a ?ow down through it and through the ring and at its lower end car of refrigerant suf?cient to provide for the least advantageous ries the moving part of a valve incorporating the ?oating nee 20 conditions, that is to say when the maximum cooling is dle valve 23. The upper end of the needle valve projects into required by the load and the ambient temperature is at a max the expansion ori?ce while its lower end is of conical form en imum. Under other conditions a much smaller ?ow of gaging a seating formed in a screwed plug 26 threaded into a refrigerant would suffice for requirements, and accordingly thimble 27 which is rigidly secured as by welding to the lower the amount of refrigerant that must be provided in the supply end of the tubular operating rod 25. The thimble has secured 25 is in fact far greater than is necessary. Thus considerable to it a shroud 39 in the form of a partial cage formed of wire economy is possible by providing automatic control even of a gauge which allows free escape of exhaust vapor while inter single stage Joule-Thomson cooler. As indicated above, how cepting any liquid droplets to prevent them from impinging on ever, the ef?ciency of a Joule-Thomson cooler is rather low if the sensor. The tubular operating rod 25 is provided with an S the refrigerant starts at a temperature only a little below the shaped guide blade 29 serving to keep it centered and prevent inversion temperature, and hence in these conditions waste of it from being tilted by the offset pressure on the needle valve. refrigerant can still occur even though there is automatic con The sensor bulb 30 communicates through a hole in the ring trol. The present arrangement, by providing precooling, in 28 with the interior of the tubular body of the heat exchanger creases the efficiency of the Joule-Thomson second stage and surrounding the bellows, so that the pressure in the vapor bulb thereby economizes in the supply of compressed nitrogen, of the sensor is communicatedto the outside of the bellows, 35 while adding little or nothing to the bulk or weight of the and as the pressure rises it tends to push the bellows down, cooler. In this case the automatic control is at least equally im withdrawing the needle valve 23 from the expansion ori?ce 22 portant in economizing the supply of liquid Freon. This is par and increasing the ?ow of refrigerant to increase the amount ticularly the case since in normal ambient temperatures the of cooling. The sensor bulb terminates a little below the level precooling may not be required, since the exhaust nitrogen of the ori?ce and is provided with an extended tail 31 of metal from the second stage may sufficiently precool the incoming projecting down some distance further. Thus as the level of nitrogen for that stage to give efficient operation of the Joule liquid refrigerant rises it ?rst comes into contact with the end Thomson expansion cooling. Thus the precooling by liquid of the extended tail so that heat ?owing from the sensor bulb Freon is primarily required when the apparatus is used at am has to flow through its whole‘ length. As the level of liquid bient temperatures where the efficiency of Joule-Thomson refrigerant rises the extent of cooling of the sensor bulb in 45 cooling would otherwise diminish in a single stage cooler. creases progressively, giving a smooth control of the needle What we claim as our invention and desire to secure by Let valve, and preventing hunting. ters Patent is: In accordance with the present invention the upper part of 1. Cooling apparatus of the type in which cooling is the heat exchanger contains a ?rst stage precooler supplied produced by expansion, through a nozzle, of a refrigerant in with Freon kept liquid under pressure. The ?rst stage is pro gaseous form under pressure, which before expansion at the vided with a control valve 53 which constructionally is very nozzle is at a temperature below its inversion temperature, in similar to that of the second stage, although it should be noted cluding two stages in the second of which a second stage that in this case it is merely a metering valve for liquid, and not refrigerant from a supply in gaseous form is passed through an expansion valve employing the Joule-Thomson effect as in 55 one path ofa heat exchanger, expanded through an expansion the second stage, The ?rst stage valve is situated coaxially with nozzle and passed back through the other path of the heat the heat exchanger and cooperates with a seating 54 carried exchanger to cool the incoming second stage refrigerant, a by the lower end of an admission tube 55 the upper end of precooling ?rst stage in which a precooling refrigerant is sup which is secured in a head 56 at the top of the exchanger and plied in liquid form under pressure through a metering nozzle communicates with a lateral coupling 57 through which liquid 60 to a space at reduced pressure where it evaporates to cool the Freon is supplied to it. As in the second stage a thimble 61 second stage refrigerant before the latter passes through the seating the lower end of the needle valve 53, is carried at the heat exchanger, and means for automatically controlling the lower end of a tubular operating member 62 the upper end of effective area of the nozzle of each stage so that the amount of which is secured to the upper end ofa bellows 63. In this case refrigerant flowing through each nozzle may be governed by the bellows is offset from the axis of the heat exchanger and is 65 the requirement of cooling at that stage. accommodated in a chamber surmounting the head 56 at the 2. Cooling apparatus as claimed in claim 1 in which the ap top of the heat exchanger. Thus the operating member ex paratus is generally of tubular form, the heat exchanger com tends up on one side of the tubular inlet pipe 55 carrying the prising a ?nned tube wound to helical form with the second seating 54 at its lower end, while at a diametrically opposite stage refrigerant ?owing through it in one direction and past position a tube 67 forming a sensor bulb 69 extends down its outside in the opposite direction, the second stage expan from an opening 70 in the head and has its lower end flattened sion nozzle being at the cold end of the heat exchanger while and secured to a curved shoe 72 engaging die inner face of the the ?rst stage metering nozzle is enclosed within its warm end. tubular body of the heat exchanger. The sensor bulb 69 con 3. Cooling apparatus as claimed in claim 2 in which the tains a volatile liquid, for example Freon, so that the outside of second stage expansion nozzle includes a needle valve con the bellows 63 is subjected to its vapor pressure which varies 75 trolled by a bellows the pressure upon which is determined by 3,590,597 5 6 said automatic control means which includes a sensor com‘ ’ secured to the inside of the body of the heat exchanger, just prising a vapor bulb with an extended tail extending down to a pool of lique?ed refrigerant. below the metering nozzle, to collect liquid refrigerant formed 4. Cooling apparatus as claimed in claim 3 in which the ?rst by the ?rst stage refrigerant, and at the corresponding part of the length of the heat exchanger the helical ?nned tube is not stage metering nozzle includes a needle valve controlled by a 5 bellows the pressure upon which is determined by said auto ?nned and is soldered to the outer surface of the heat matic control means which also includes a sensor bulb secured exchanger tube so as to transmit heat from the second stage to a curved plate ?tting within the body of the heat exchanger refrigerant in the cup within the body. at about the level of the metering nozzle so as to respond 6. Cooling apparatus as claimed in claim 5 wherein the primarily to the temperature of the heat exchanger at that part refrigerant is Freon in liquid form supplied under pressure and of the apparatus. at atmospheric temperature. 5. Cooling apparatus as claimed in claim 4 in which a cup is

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