Improved split pulse tube connecting pipeline

Abstract

The connection line between the pressure wave generator and the cold head of the GM pulse tube refrigerator has reduced losses while maintaining or improving the desired characteristics of the standard corrugated hose connection line, including vibration isolation, separation distance and installation convenience. The basic approach is to reduce the internal void volume of the folds in the corrugated hose, combined with reducing the number of corrugations, adding fillers in the void volume, and adding vibration absorbing coatings.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application Serial No. 63 / 001,701, filed on March 30, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] Embodiments described herein are directed to minimizing the losses associated with the connecting lines separating the valve mechanism of a Gifford-McMahon (GM) type pulse tube refrigerator from the cold head that cools the device at cryogenic temperatures. Improvements to the connecting lines between the valve motor and the cold head increase the available cooling and reduce the transmission of vibrations. Background Art

[0004] Some devices in use today and new devices under development operate at 4K or below, requiring about 1W of cooling and low levels of vibration. GM-type pulse tube refrigerators are being used because they can have no moving parts in the cold head. GM-type pulse tube expanders produce refrigeration at low temperatures by cycling gas (usually helium) in and out of the cold head between high and low pressures (usually around 2.2 and 0.8 MPa, and between 1 and 2 Hz). A single-stage pulse tube cold head includes a regenerator having hot and cold ends, a cold end heat station, a pulse tube having hot and cold ends, and a pressure phase shifting mechanism connected to the hot end of the pulse tube. Gas circulates in and out of the hot end of the regenerator and flows through the cold end heat station as it pressurizes and depressurizes the gas in the pulse tube. Gas flowing into and out of the hot and cold ends of the pulse tube flows through a flow smoother, causing the gas to be stratified (not mixed). Gas entering the hot and cold ends of the pulse tube causes the gas remaining in the pulse tube to act as a compressible piston. The gas circulating in and out of the cold end can be thought of as acting on the end of a solid piston and moving about fifteen percent (15%) of the way into the pulse tube. The PV diagram is similar to a GM or Stirling cycle expander with a solid piston.

[0005] The pressure cycle in a GM-type pulse tube is provided by a continuous flow compressor having a low-pressure return line and a high-pressure supply line connected to a valve mechanism that circulates the gas to the cold head. In an integral expander, the valve mechanism and the cold head are coupled together. In a split pulse tube expander, the valve mechanism is separated from the cold head by a connecting line, which is sometimes flexible, called flex line, and sometimes a tube. Small diameter lines, with an inner diameter of less than about 3 mm, are usually made of tube, while larger diameter lines are usually made of corrugated hose.

[0006] The compressor and valve mechanism of a GM-type pulse tube is sometimes referred to as a pressure wave generator (PWG). The term is derived from the Stirling-type pulse tube, in which a reciprocating piston compressor is connected to a pulse tube coldhead by a connecting line or tube, and the compressor is called a pressure wave generator. Stirling-type pulse tubes typically operate at a high speed, such as 60 Hz, so that the energy of gas expansion in the coldhead can be recovered and stored as kinetic energy in the compressor piston. GM-type pulse tubes typically operate at a frequency of 1 to 2 Hz and do not recover expansion energy, but can be designed to be cooler than Stirling-type pulse tubes. On the other hand, Stirling-type pulse tubes can be much smaller and have higher efficiency at higher temperatures.

[0007] GM-type pulse tubes operating at very low temperatures (e.g. 4K) and low velocities rely on a vertical orientation, with the cold end of the pulse tube facing downward, to maintain stratification of the gas in the pulse tube. The connecting pipe between the PWG and the cold head in this application is usually fixed in place when the refrigerator is installed. Stirling-type pulse tube refrigerators operating at higher velocities and temperatures and with smaller diameter pulse tubes can be operated in other orientations, since the gas does not have time to stratify. They are widely used in spaces where there is no gravity to stratify the gas in the pulse tube.

[0008] In most applications, the main purpose of establishing a connecting line between the PWG and the pulse tube cold head is to isolate vibrations transmitted from the PWG to the pulse tube cold head. Other reasons are to consider installation constraints, isolation of electromagnetic interference from the PWG, or isolation of the PWG from magnetic fields. Using a connecting line with corrugated hose can isolate vibrations and provide flexibility in installing the valve mechanism in the GM type pulse tube.

[0009] Spoor's U.S. Patent No. 7,628,022 ("the '022 patent") describes a split Stirling-type pulse tube that uses a corrugated flexible hose with a braided cover (col. 4, line 32) to connect the PWG to the coldhead. The '022 patent describes the PWG as an "acoustic power source." The primary purpose of the connecting line is to separate the PWG from the coldhead, and the patent's main claim specifies a minimum length based on the frequency of operation.

[0010] There are two types of phase shift mechanisms currently used in GM-type pulse tubes operating near 4K. U.S. Patent No. 9,157,668 to Xu (the "'668 Patent") describes a basic dual inlet pulse tube with the addition of a drain line between the buffer volume and the compressor return line. Figure 1 , a basic dual inlet pulse tube includes a PWG component 12, V1 and V2, a connecting line 20, and the remaining components including a cold head. If no drainage line is added, only one line 20 is required to connect the PWG to the cold head. This is also true for a two-stage 4K pulse tube, such as the '668 patent. Figure 8, which shows line 220 as a connecting line between the PWG and the regenerator 242. U.S. Patent No. 10,066,855 to Xu (the "'855 Patent") describes a four-valve pulse tube. The name is derived from the phase shift mechanism that includes two valves in the PWG for circulating gas to one or a series of regenerators, and two valves for the first stage pulse tube. The '855 Patent describes flow control mechanisms for balancing the flow of gas to the second and third stage pulse tubes, each of which requires an additional pair of valves. Reference to the '855 Patent Figure 1 , the pulse tube includes the PWG components compressor 212, valves V1 to V6 and connecting pipes, connecting lines 220, 230 and 299, and the remaining components including the two-stage cold head. The connecting line 220 between the PWG and the regenerator delivers more than 90% of the flow to the cold head and is therefore much larger than the connecting lines 230 and 299. Relative to the double-inlet pulse tube, the two-stage four-valve pulse tube that requires three connecting lines seems to be at a disadvantage; however, the four-valve pulse tube does not require a buffer volume. The buffer volume of the double-inlet pulse tube may be relatively large and may have to be installed separately from the cold head, which requires a relatively small second connecting line.

[0011] One of the current applications of two-stage GM type pulses with separate PWGs and providing about 1W of cooling near 4K is for cooling superconducting magnets that produce high magnetic fields for nuclear magnetic resonance (NMR) studies. Three patents assigned to Bruker BioSpin GmbH address the problem of using a 4K pulse tube cold head mounted in the neck of an NMR cryostat, where the 4K pulse tube cold head cools the cold hood and condenses the helium vapor. U.S. Patent No. 9,995,510 to Wikus (the "'510 Patent") describes a novel connecting line that reduces the cyclic forces exerted on the cryostat due to the pressure cycling of the gas in the connecting line at 1 to 2 Hz. Figure 7 of the '510 Patent shows a standard corrugated hose, which typically has an SS braided sleeve on it to prevent it from stretching, and is described as isolating high frequency vibrations. U.S. Patent No. 9,982,840 to Wikus (the "'840 Patent") describes another aspect of isolating vibrations of a two-stage 4K GM type pulse with a PWG separated from the cold head by a corrugated hose. The cold head is mounted in the neck tube of the cryostat, and a flexible diaphragm is used to seal the heat flange of the pulse tube to the heat flange of the neck tube. The diaphragm seals the helium in the neck tube from the atmosphere, and the helium is at a pressure slightly above atmospheric pressure, so it tends to "float" the cold head. Wikus' U.S. Patent No. 10,401,447 (the "'447 Patent") adds a pivot bearing to the cold head of the '840 Patent, which supports the cold head in the neck tube so that the impulse force from the connecting tube allows it to swing back and forth a small amount, such as 1°, within the neck tube without transmitting force to the cryostat. The '447 Patent Figure 5 A connecting line is shown which comprises a straight pipe section in series with a corrugated hose section.

[0012] A 2012 paper by K. Nakano and MY Xu, titled “Development of a High-Efficiency 4K Two-Stage Pulse Tube Cryocooler with Split Valve Units,” published in Progress in Cryogenic Engineering, compared the performance of 1 m long corrugated connecting lines with different diameters. Photos of integral and split GM pulse expanders are shown. The split valve unit operated at higher temperatures with the same heat load due to losses caused by void volume and pressure drop in the connecting line between the valve unit and the cold head. For the split valve unit, the displacements of the cold ends of the first and second stages due to 1.7 Hz pressure cycling were measured to be + / -10.3 μm and + / -14.6 μm, respectively. A comparison of the vibrations of the integral and split valve motors at frequencies up to 200 Hz showed only a small difference at 1.7 Hz, but the vibrations of the split valve motors were reduced by up to 10 times at higher frequencies.

[0013] The current GM type pulse tube refrigerators that separate the PWG from the cold head use a commercially available corrugated SS hose with a braided SS cover for the connecting line. This hose provides flexibility for mounting the valve unit and reduces vibrations transmitted from the valve unit to the cold head. Summary of the invention

[0014] Embodiments of the present invention provide a reduction in losses in the connecting line between the pressure wave generator and the cold head of a GM type pulse tube refrigerator, while maintaining or improving the desired characteristics of the corrugated hose connecting line of the present application, including vibration isolation, separation distance, and installation convenience. The basic approach of the embodiment is to reduce the internal void volume of the folds in the corrugated hose, combined with reducing the number of corrugations, adding fillers in the void volume, and providing a vibration absorbing coating.

[0015] These advantages can be achieved, for example, by a pulse tube cryogenic refrigerator, comprising a connecting line between a pressure wave generator (PWG) and a pulse tube cold head, wherein the connecting line comprises a metal hose having a corrugation that allows the hose to bend at least ninety degrees (90°), the outer diameter (OD) of the corrugation being at least 40% greater than the inner diameter (ID), and wherein the internal volume of the hose is no greater than 33% of the core volume of the hose. In an embodiment, the outer convolutions forming the internal volume of the corrugation are more closely spaced than the inner convolutions forming the external volume of the corrugation. In an embodiment, the hose comprises a corrugated section having the corrugation and at least one straight tube section. The outer convolutions forming the internal volume of the metal hose are separated by straight tube sections having the same or smaller ID as the convolutions. In an embodiment, the internal volume of the corrugation defined by the outer convolutions is at least partially filled with an elastomer. In an embodiment, the exterior of the metal hose is coated with an elastomer. In an embodiment, the exterior of the metal hose is reinforced with a braided cover.

[0016] These advantages can be achieved, for example, by a pulse tube cryogenic refrigerator comprising a connecting line between a pressure wave generator (PWG) and a pulse tube cold head, wherein the connecting line comprises a bendable metal tube and a coupling having an elastomeric seal that prevents metal contact between the PWG and the cold head. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The embodiments are described in the following drawings, wherein the same or similar reference numerals represent the same or equivalent parts.

[0018] Figure 1 A schematic diagram of a dual-inlet GM type two-stage pulse tube refrigerator is shown, showing the relationship of the connecting pipelines to the PWG and the cold head.

[0019] Figure 2 A portion of a commercially available corrugated hose is shown.

[0020] Figure 3 A portion of a variable pitch corrugated hose according to the present invention is shown.

[0021] Figure 4 A portion of a connecting line combining a corrugated hose section with a straight pipe section is shown.

[0022] Figure 5 A portion of the connecting line joining the straight sections between the convolutions is shown.

[0023] Figure 6 A portion of a commercially available corrugated hose having a vibration absorbing coating is shown.

[0024] Fig. 7A A portion of a commercially available corrugated hose is shown in which an elastomer fills the interior volume of the corrugations.

[0025] Figure 7B A portion of a commercially available corrugated hose is shown, with the elastomer filling the interior volume of the corrugations and the tube lined to the ID.

[0026] Figure 8 A portion of the coupling is shown isolating the high frequency vibrations of the PWG from the coldhead via an elastomeric seal that prevents metal-to-metal contact.

[0027] Fig. 9 A portion of a connecting line is shown with an elastomeric bond between two sections of the connecting line that isolates the high frequency vibrations of the PWG from the coldhead. DETAILED DESCRIPTION

[0028] In this section, some embodiments of the present invention will be more fully described with reference to the accompanying drawings showing preferred embodiments of the present invention. However, the present invention can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided to make this disclosure thorough and complete and to convey the scope of the present invention to those skilled in the art. The same reference numerals refer to the same elements throughout, and apostrophes are used to represent similar elements in alternative embodiments.

[0029] Figure 1 A schematic diagram of a dual inlet GM type two-stage pulse tube refrigerator 500 is shown, showing the relationship of the connecting line 1 to the pressure wave generator 40 and the cold head 20. The PWG 40 consists of a compressor 41, a high pressure supply line 42, a high pressure valve 43, a low pressure valve 44 and a low pressure return line 45. The valves 43 and 44 are usually incorporated as ports in a rotary valve assembly driven by a valve motor. The valve motor is the main source of high frequency vibrations, which can be transmitted to the cold head through the connecting line 1. The supply and return lines 42 and 45 are usually flexible hoses that allow the compressor 41 to be separated from the valve motor assembly. The pulse tube cold head 20 is shown as being mounted on top of a cryostat 50, which contains the device being cooled. The cryostat 50 also provides the insulation required to minimize the heat load of the refrigerator. The components of the cold head 20 at ambient temperature are as follows: line 19, which connects the connecting line 1 to the hot end (R1) of the first stage heat accumulator 21, orifice 24 and orifice 31; line 25, which connects orifice 24 to the hot end of the first stage pulse tube 23 (PT1), to the first stage buffer volume 27 (B1) through orifice 26; and line 32, which connects orifice 31 to the hot end of the second stage pulse tube 30 (PT2), to the second stage buffer volume 34 (B2) through orifice 36. The cold components include the first stage heat station 22 at temperature T1, the second stage heat accumulator 28 (R2) and the second stage heat station 29 at temperature T2.

[0030] The above-mentioned paper cited by K.Nakano and MYXu describes a two-stage GM type pulse tube with a 1m long connecting pipeline. The pulse tube refrigerator described herein represents the improved use of the new connecting pipeline included in the present invention. It contains a compressor with a suction power of 6.4kW and a frequency of 50Hz, filled with 1.65MPa of helium, and the frequency of gas circulation to the cold head is 1.7Hz. The connecting pipeline is a standard braided corrugated hose, the ID of the hose connecting the heat accumulator is 9.5mm, and the ID of the hose connecting the first stage pulse tube is 4.0mm. The first stage temperature T1 is measured to be 45.4K, the heat load is 40W, and the second stage temperature T2 is measured to be 4.05K, and the heat load is 1.0W. In the case where the valve mechanism is integrated with the cold head, the temperatures under the same conditions are 37.0K and 3.86K respectively. Applications using the connecting pipeline according to the present invention will use a compressor that performs suction between about 1 and 15kW and operates between temperatures of about 3 and 80K. The connecting line will be less than about 1 m in length, have an inside diameter ("ID") between 3 and 15 mm, and typically have bends up to ninety degrees (90°). Note that the vibration measurements described above are made by sensor 51 (VS) on the coldhead opposite the connecting line connector 10.

[0031] Figure 2 A portion of a commercially available corrugated hose 100 is shown, currently used to connect pipelines. One manufacturer of such hoses, Hose Master, describes the manufacturing process of the hose as forming a metal (usually stainless steel SS) strip into a tube, seam welding it, and then mechanically forming the convolutions of the hose. Hose Master also manufactures corrugated hoses by forming a long strip of SS into a single convolution, coiling it into a spiral, and then seam welding the edges. The standard corrugated hose 2 has an SS braided cover 3 to prevent the thin-walled tube from stretching when under pressure. Without the braided cover, the typical pressure limit range of the hose is 0.1 to 0.3 MPa, while when a layer of braided cover is used, the pressure limit range is typically 5 to 10 MPa. The outer diameter (OD) of the hose is 1.4 times the inner diameter (ID). When the pressure in the pipeline cycles between high and low pressures, the outer convolution 4a partially surrounds the inner volume 4b, which represents the void volume. This void volume reduces the amount of gas reaching the cold head by the amount that pressurizes and depressurizes the inner volume 4b. The standard hose has an excess volume ratio (volume in convolutions / percentage of core volume) of about 50%, where the core volume is the volume in the ID times the length of the connected line. In other words, in this standard hose, the void volume is about 50% of the core volume. The inner convolutions 5a surround the outer convolution volume 5b. The convolutions act as a series of circular orifices that increase the pressure drop when the gas flows through the hose relative to a smooth wall tube with the same ID.

[0032] Figure 3A portion of a variable pitch corrugated hose according to the present invention is shown, wherein the distance between the inner convolutions 5a remains the same as that of a standard corrugated hose, but the radius of the outer convolutions 4a is smaller than the inner convolutions 5a. For a case where the radius of the inner convolutions 5a is six (6) times the radius of the outer convolutions 4a, the inner volume 4b is approximately 27% of the core volume, and the pressure drop is slightly reduced because of the larger radius of the inner convolutions 5a. For a case where the radius of the inner convolutions 5a is three (3) times the radius of the outer convolutions 4a, the inner volume 4b is approximately 33% of the core volume. The excess volume ratio of the corrugated hose of the present invention is not greater than 33%. Alternatively, the excess volume ratio of the corrugated hose of the present invention may be not greater than 27%.

[0033] Figure 4 A portion of a connecting line identified as "Straight / Corrugated Segment Hose" 300 is shown, which combines a segment of a corrugated hose 6 with a segment of a straight tube 7. The above-mentioned paper cited by K. Nakano and MY Xu uses a corrugated hose whose length at a ninety degree (90°) bend is two and a half times (2.5) the hose ID. The 9.5mm ID tube used has a length of less than 3mm at the bend, which accounts for three percent (3%) of its total length. Compared with a standard corrugated hose 2, multiple segments of the corrugated hose 6 spaced apart from the straight tube 7 will have a smaller internal volume and a lower pressure drop. However, the corrugated hose 6 spaced apart from the straight tube 7 may transmit more vibrations. It is worth noting that for an equivalent pressure drop, the ID of the corrugated hose is greater than the ID of the bendable tube. The increase in ID increases the void volume in the connecting line 1. The ID of the straight section 7 can be smaller than the ID of the corrugated section 6.

[0034] Figure 5 The configuration shown in FIG. 4 , identified as a "straight / corrugated hose" 400, shows a portion of a connecting line that incorporates straight sections 7 between convolutions 4a. As with hose 300, the corrugations 6 spaced apart from the straight sections 7 will have a smaller internal volume and lower pressure drop than a standard corrugated hose 2, but may transmit more vibration.

[0035] Figure 6 A portion of a commercially available standard corrugated hose 2 is shown with a vibration absorbing coating 8. Such a coating can be applied to any connecting line configuration to reduce vibrations transmitted from the PWG to the coldhead.

[0036] Fig. 7A A portion of a standard corrugated hose 2 is shown, with the elastomer 9 filling the inner volume of the corrugations. Such filling, even if only partially filling the outer convolutions, will reduce the void volume. If the elastomer 9 completely fills the outer convolutions 9 and provides a smooth bore, cooling losses due to void volume in the connecting lines are eliminated and losses due to pressure drop are reduced. The elastomer will also absorb vibrations. Figure 7BA liner 19 is added to the ID. In this case, the core volume is based on the ID of the tube. In either design, the ratio of the interior volume to the core volume may be 1.

[0037] Figure 8 A vibration isolating coupling 10a is shown which isolates the high frequency vibrations of the PWG from the coldhead via an elastomeric seal 11 which prevents metal to metal contact between a coupling tube 14 having a threaded lip and a coupling tube 13 having a lip for retaining a coupling ring 12 .

[0038] Fig. 9 A portion of a connecting line is shown with an elastomeric bond 15 between two sections of lines 17 and 18 to isolate the high frequency vibrations of the PWG from the cold head. The connecting line 1 can be made of a flexible copper or SS tube and incorporate an elastomeric bond coupling that will isolate high frequency vibrations and reduce low frequency vibrations associated with expansion and contraction of the line due to pressure cycling.

[0039] The terms and descriptions used herein are set forth by way of illustration only and are not meant to be limiting. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention and within the embodiments described herein.

Claims

1. A pulse tube cryogenic refrigerator, comprising: a pressure wave generator (PWG) comprising a compressor and a valve mechanism so that pressure circulation is provided by the compressor, the compressor having a low pressure return line and a high pressure supply line connected to the valve mechanism that circulates gas to the pulse tube coldhead; The connecting line between the valve mechanism and the pulse tube cold head; The connecting pipeline includes a metal hose having corrugations that allow the hose to bend at least ninety degrees (90°), the outer diameter (OD) of the corrugations being at least 40% larger than the inner diameter (ID) of the corrugations, and wherein the internal volume of the hose is no greater than 33% of the core volume of the hose.

2. The pulse tube cryogenic refrigerator of claim 1, wherein the radius of the outer folds forming the corrugated inner volume is smaller than the radius of the inner folds forming the corrugated outer volume.

3. The pulse tube cryogenic refrigerator of claim 1 , wherein the hose comprises a corrugated section having corrugations and at least one straight tube section, and the outer convolutions forming the inner volume of the corrugations are separated by straight tube sections having the same or smaller ID as the convolutions.

4. A pulse tube cryogenic refrigerator according to any one of claims 1 to 3, wherein the interior volume of the corrugations defined by the outer convolutions is at least partially filled with an elastomer.

5. The pulse tube cryogenic refrigerator according to any one of claims 1 to 3, wherein the exterior of the metal hose is coated with an elastomer.

6. The pulse tube cryogenic refrigerator according to any one of claims 1 to 3, wherein the exterior of the metal hose is reinforced by a braided cover.

7. A pulse tube cryogenic refrigerator, comprising: A connecting line between a pressure wave generator (PWG) and a pulse tube cold head, wherein the connecting line comprises a bendable metal tube and a coupling having an elastomeric seal to prevent metal contact between the PWG and the pulse tube cold head.

8. The pulse tube cryogenic refrigerator according to claim 7, wherein an elastomeric seal is provided between the coupling portions of the two sections of the metal tube.

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