Patent GB-1217236-A (GB)
Title: Method and apparatus for continuously supplying refrigeration below 4.2°k
IPC: F28D7/04
Filed: 1968-01-18
Abstract: 1,217,236. Refrigerating; tubular heat exchangers. CRYOGENIC TECHNOLOGY Inc. 18 Jan., 1968 [31 Jan., 1967], No. 2865/68. Headings F4H and F4S. In a cryogenic refrigerator for maintaining temperatures below 4. 2 K., high pressure helium is passed through liquefaction means into a liquid helium reservoir, and vapour withdrawn from the reservoir by a vacuum pump is passed in heat exchange relationship with the high pressure stream. As illustrated in Fig. 1, helium from a compressor 26 is split into two streams, the first passing through a line 17 and a regulating valve 93 to be cooled in a heat exchanger 90, and the second passing through a refrigerator section 11 to be cooled to below 30 K. and then passing through a line 16 and a regulating valve 81 to join the first stream. The joint streams pass through heat exchangers 110, 111 and Joule Thomson valves 120, 121 to give liquid helium in a reservoir 125 which is used to cool a load. Subatmospheric pressure is maintained in the reservoir 125 by a vacuum pump 149 which withdraws gaseous helium from the reservoir via the heat exchangers 111, 110 and 90 and returns it to the compressor 26. The second stream of helium is cooled in the section 11 by indirect heat exchange with gas which is branched from the second stream and cooled in expansion engines 70 and 72 and with gas which is branched from the first stream at 65 and cooled in a heat exchanger 35 and expansion engine 66. The heat exchanger 35 and thermal shields 82 and 137 are cooled by liquid nitrogen from a source 57. Thermal insulation 87 and 136 surrounds the refrigerator section and the liquefaction section of the apparatus. In another embodiment, Fig. 2 (not shown), the first stream of helium is cooled in the liquid nitrogen heat exchanger, a branched stream then passing to the expansion engine 66. In another embodiment, Fig. 3 (not shown), part of the first stream is branched, cooled by liquid nitrogen, and admitted to an intermediate point of the higher pressure side of the heat exchanger 90. In addition, liquid nitrogen is used to cool the second helium stream itself. In another embodiment, Fig. 5 (not shown), part of the first stream is branched, cooled by liquid nitrogen, and then in part passed to the expansion engine 66 and in part passed through a regulating valve to an intermediate point of the higher pressure side of the heat exchanger 90. Additionally, helium is passed to one of the refrigerator expansion engines from a point further downstream in the higher pressure side of the heat exchanger 90. In a modification of the Fig. 5 embodiment, Fig. 4 (not shown), liquid nitrogen is not used, but its cooling function is accomplished by a branched part of the first helium stream which is passed through an expansion engine and subsequently returned to the compressor. In another embodiment, Fig. 6, the bulk of the liquid helium in the reservoir 125 is supplied through a valve 81 from a liquid helium source 11a. The pump 149 maintains subatmospheric pressure in the reservoir and the withdrawn gas, after compression, is returned to the reservoir through the heat exchanger 90 and expansion valve 118. The heat exchanger 90 may take the form shown in Fig. 9. The higher pressure side comprises a number of spirally wound layers of finned tubing, which layers decrease in diameter and spacing towards the cold end of the heat exchanger. Each layer may comprise tubes which provide a plurality of parallel fluid flow paths. The space between the finned tubes and a stepped casing 100 provides the lower pressure side of the heat exchanger. In a modification. Fig 10 (not shown), the heat exchanger casing is frusto-conical. It is stated that the systems of Figs. 1-5 may be operated with their Joule-Thomson valves fully open, liquefaction of the high pressure gas stream being accomplished solely by heat exchange with the cold vapour from the reservoir. In this operation, liquid helium from a separate source may be admitted to the reservoir if necessary.