Low-temperature expansion machine with gas equilibrium

Abstract

Expansion machine (100) operating with the Brayton cycle, wherein the expansion machine (100) operates with a gas supplied by a compressor at high and low pressures (Ph, Pl) to generate cooling at low temperatures, and comprises: a piston (1) in a cylinder (6), the piston (1) comprising: a drive shaft (2) at a hot end with low pressure (Pl) acting upon it, an inlet and an outlet valve (10, 11) at a cold end (9) of the cylinder (6) which inlet a high-pressure gas (Ph) when the piston (1) is near the cold end (9) of the cylinder (6) and while moving towards the hot end, and which outlet gas at low pressure (Pl) when the piston (1) is near the hot end of the cylinder (6) while moving towards the cold end (9); and a device for maintaining pressure on the hot end of the piston (1) outside a region of the drive shaft (2) at approximately the same pressure as on the cold end (9) of the piston (1) while the piston (1) is moving, the device comprising: a line (38) with an inlet check valve (13) in series with a throttle valve (15), which is connected between the hot end and a high-pressure line (30) from the compressor, and a line (37) with an outlet check valve (12) in series with a throttle valve (14), which is connected between the hot end and the high-pressure line (30) from the compressor, wherein the pressure on the warm end of the piston (1) is increased from the low pressure (Pl) to the high pressure (Ph) by means of an active valve (16) or a passive valve (17) in a line (39) which is connected between the high pressure line (30) from the compressor and the warm end of the piston (1), while the piston (1) is near the cold end (9).

Description

[0001] This invention relates to an expansion machine that uses the Brayton cycle to produce cooling at low or cryogenic temperatures.

[0002] A system using the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, from which gas is admitted through an inlet valve into an expansion chamber that expands the gas adiabatically, allows the expanded gas (which is colder) to exit through an outlet valve, circulates the cold gas through a load being cooled, and then returns the gas through the counterflow heat exchanger to the compressor.

[0003] US Patent No. 2,607,322 to S.C. Collins, a pioneer in the field, describes the design of an early expansion engine that was widely used to liquefy helium. The expansion piston is driven in a reciprocating motion by a crank mechanism connected to a flywheel and a generator / motor. The inlet valve is opened with the piston at the bottom of its stroke (minimal cold volume), and high-pressure gas drives the piston upward, causing the flywheel speed to increase and powering the generator. The inlet valve is closed before the piston reaches the top, and the pressure and temperature of the gas in the expansion chamber drop. At the top of its stroke, the exhaust valve opens, and gas flows out, while the piston is driven downward by the flywheel as it decelerates.Depending on the size of the flywheel, it can continue to drive the generator / motor to output power or it can consume power while acting as a motor.

[0004] The intake and exhaust valves are typically driven by cams connected to the flywheel, as shown in US patent US 3,438,220 A to S.C. Collins. This patent describes a mechanism different from the earlier patent that couples the piston to the flywheel in a manner that does not apply lateral forces to the seals at the hot end of the piston.

[0005] US Patent US 5 355 679 A to JG Pierce describes an alternative design of intake and exhaust valves which are similar to the '220' valves in that they are cam driven at room temperature and have seals.

[0006] US Patent No. 5,092,131 to H. Hattori et al. describes a Scotch-Yoke (cross-loop crank) drive mechanism and cold inlet and exhaust valves actuated by the reciprocating piston. All of these machines have atmospheric air acting on the warm end of the piston and were primarily designed to liquefy helium, hydrogen, and air. Recirculated gas is at near atmospheric pressure, and the supply pressure is approximately 10 to 15 atmospheres. Compressor input power is typically in the range of 15 to 50 kW.

[0007] Lower-capacity chillers operate with Gifford-McMahon (GM), pulse-tube, or Stirling cycles. Higher-capacity chillers typically operate with Brayton or Claude cycles using turbo-expanders. US Patent US 3,045,436 A to W.E. Gifford and H.O. McMahon describes the GM cycle. Lower-capacity chillers use recovery heat exchangers, in which the gas flows back and forth through a packed bed, never leaving the cold end of the expander. This is in contrast to Brayton cycle chillers, which can distribute cold gas to a distant load.

[0008] The amount of energy recovered by the generator in the Collins machine described in US Pat. No. 3,438,220 A is small relative to the compressor input power, so in many applications, mechanical simplicity is often more important than efficiency. US Pat. No. 6,202,421 B1 by J.F. Maguire et al. describes a machine that eliminates the flywheel and generator / motor by using a hydraulic drive mechanism for the piston. The intake valve is actuated by an electromagnet, and the exhaust valve is actuated by a solenoid / pneumatic combination. The motivation for the hydraulically driven machine is to provide a small and lightweight machine that can be removably connected to a superconducting magnet for cooling. The claims cover the removable connection.

[0009] US Patent No. 6,205,791 B1 by J.L. Smith describes an expansion engine having a freely suspended piston with working gas (helium) around the piston. The gas pressure above the piston, the warm end, is controlled by valves connected to two buffer volumes, one at a pressure approximately 75% of the difference between the high and low pressures, and the other at approximately 25% of the pressure difference. The electrically activated inlet, outlet, and buffer valves are timed to open and close so that the piston is driven up and down with a small pressure difference above and below the piston, so very little gas flows through the small clearance between the piston and the cylinder. A position sensor in the piston provides a signal that is used to control the timing of the opening and closing of the four valves.

[0010] If one thinks of a pulse tube as replacing a solid piston with a gas piston, then the same “two-buffer volume control” can be seen in US patent US 5 481 878 A by Zhu Shaowei. Fig. 3 of this patent shows the timing of the opening and closing of the four control valves, and Fig. Figure 3 of US 6 205 791 B1 shows the favorable PV diagram that can be achieved by good timing of the relationship between the piston position and the opening and closing of the control valves. The area of ​​the PV diagram is the work generated, and the maximum efficiency is achieved by controlling the amount of gas entering the expansion space between points 1 and 3 of the Fig. 3 of US 6 205 791 B1 is minimized relative to the PV work (which is equal to the cooling produced).

[0011] The timing of the opening and closing of the inlet and exhaust valves relative to the position of the piston is important to achieve good efficiency. Most of the machines built for helium liquefaction used cam-operated valves similar to those in the '220 Collins patent. US Pat. Nos. 6,205,791 B1 and 6,202,421 B1 show electrically operated valves. Other mechanisms include a rotary valve on the end of a cross-loop crankshaft, as shown in US Pat. No. 5,361,588 to H. Asami et al., and a shuttle valve actuated by the piston drive shaft, as shown in US Pat. No. 4,372,128 to Sarcia.

[0012] An example of a multi-port rotary valve similar to those described in the present invention can be found in U.S. patent application US 2007 / 0 119 188 A1 by M. Xu et al. U.S. patent US 6,256,997 B1 by RC Longsworth describes the use of "O" rings to reduce the vibrations associated with the pneumatically actuated piston hitting the ends of its stroke.

[0013] Provisional patent application S / N 61 / 313,868, filed March 15, 2010, by R.C. Longsworth, describes a reciprocating expander using a Brayton cycle in which the piston has a drive shaft at the hot end driven by a mechanical actuator or gas pressure alternating between high and low pressures, and wherein the pressure at the hot end of the piston in the area around the drive shaft is substantially the same as the pressure at the cold end of the piston while the piston is moving. Tests of the pneumatically actuated version of this concept have shown that it is not necessary to alternate the pressure on the shaft between high and low to cause the piston to reciprocate, but that it is possible to maintain the pressure on the shaft at a low pressure.This simplifies the construction of the machine because it is now only necessary to operate the cold high and low pressure valves.

[0014] Provisional patent application S / N 61 / 391 207, filed October 8, 2010, by RC Longsworth describes the control of a reciprocating expansion machine using a Brayton cycle as described in the previous application, which allows the time required to cool a mass to cryogenic temperatures to be minimized.

[0015] US 3 188 819 A relates to a method and apparatus for generating cooling in a system in which part of the work obtained from compressing, cooling, and expanding a fluid is in the form of heat energy, the fluid leaving the system having a higher temperature than that at which it enters the system. US 2003 / 0 168 618 A1 relates to a passive valve assembly for controlling flow into or out of a chamber through an opening. The valve assembly comprises a valve element arranged to open in the direction of flow through the opening. The valve element has a piston that can be moved back and forth within a cylinder containing gas. Upon opening of the valve, the gas in a first chamber of the cylinder is compressed, and this compression energy is used to reverse the direction of the valve element and return it to its seat.

[0016] US 3,613,385 A relates to a device suitable for developing cryogenic temperatures. Incoming high-pressure fluid is expanded and further cooled after first being cooled by heat exchange with returning low-pressure fluid. At least part of the work developed in the expander is extracted by compressing a fluid circulating back and forth between the expansion chamber and a compensation chamber where compression takes place. Means are provided for removing the heat of compression by heat exchange.

[0017] DE 11 2011 100 912 T5 relates to an expansion machine operated by gas supplied by a compressor for generating cooling at cryogenic temperatures. The expansion machine has a piston arranged in a cylinder, each having a hot end and a cold end. The piston has a drive spindle at its hot end, on which a pneumatic or mechanical force acts to cause a reciprocating movement of the piston. Furthermore, the expansion machine has inlet and outlet valves arranged at the cold end of the cylinder, through which a high-pressure gas is supplied when the piston is near the cold end of the cylinder and gas is expelled at a low pressure when the piston is near the warm end of the cylinder.The expansion machine further comprises means for maintaining the pressure in a volume displaced at the warm end of the piston outside the region of the drive spindle at approximately the same pressure as in a volume displaced at the cold end of the piston while the piston is moving, the means for maintaining the pressures approximately equal comprising shut-off valves or active valves connected between the volume displaced at the warm end and a supply and a return line to and from the compressor, respectively, and the opening and closing action of the active valves being coordinated with the position of the piston.

[0018] The present invention combines features of previous designs in new ways to achieve high efficiency. It provides a simplification of the basic design concept of our provisional patent application S / N 61 / 313,868, which features a piston with a drive shaft that creates a small pressure differential between the hot end, around the drive shaft, and the cold end of the piston as it moves.

[0019] The drive shaft is connected to the low-pressure line going to the compressor. The warm displacement volume is connected to the high-pressure line from the compressor by two lines, each with a check valve and a fixed or adjustable valve. The piston moves from the cold end to the warm end when the cold inlet valve is open, and moves to the cold end when the cold outlet valve is open. Adjustable valves in the two lines from the compressor high-pressure line to the warm displacement volume allow the circuit to be optimized over a wide range of speeds (and temperatures). A third line is added between the high-pressure line from the compressor and the warm displacement volume, which has an active or passive valve that opens while the piston is at the cold end. Fig. Figure 1 shows an engine 100 having a piston, shown in cross-section, in a cylinder with a drive shaft at the hot end, and schematic representations of the valves and heat exchangers. The schematic shows a line connected with an active valve between the hot displacement volume and the compressor low-pressure line. Fig. Figure 2 shows the engine 200, which has a piston shown in cross-section in a cylinder with a drive shaft at the warm end, and schematic representations of the valves and heat exchangers. The schematic shows a line connected with a passive valve in the drive shaft between the warm displacement volume and the compressor low-pressure line. Fig. 3 shows a pressure-volume diagram for the Fig. 1 and Fig. 2 machines shown. Fig. 4 shows valve opening and closing sequences for the Fig. 1 and Fig. 2 machines shown.

[0020] The two embodiments of this invention described in Fig. 1 and Fig. 2 use the same number and schematic to identify equivalent parts. Since expansion engines are typically oriented cold-end down to minimize convection losses in the heat exchanger, the movement of the piston from the cold end toward the warm end is often referred to as moving upwards, thus the piston moves up and down.

[0021] Fig. Figure 1 is a cross-sectional / schematic view of the engine assembly 100. The piston 1 reciprocates within the cylinder 6, which has a cold end cap 9, a warm mounting flange 7, and a warm cylinder head 8. The drive shaft 2 is attached to the piston 1 and reciprocates within the drive shaft cylinder 69. The cold-end displacement volume DVc 3 is separated from the hot-end displacement volume DVw 4 by the piston 1 and seal 50. The displacement volume above the drive shaft DVs 5 is separated from DVw by seal 51. Line 32 connects DVs 5 to the low pressure Pl in the low-pressure return line 31. Line 38 connects the high pressure in line 30 to DVw 4 through the adjustable valve Vwi 15 and the check valve CVi 13 (inlet check valve).Line 37 connects DVw 4 to the high pressure in line 30 through the check valve CVo 12 (outlet check valve) and the adjustable valve Vwo 14. The warm-end heat exchanger 42 is also in this line. Machine 100 differs from machine 200 by the active valve Va 16, which allows gas from Ph in line 30 to flow through line 39, when open, to DVw 4.

[0022] Cooling is produced when the inlet valve Vi 10 is opened with DVc 3 at a minimum, pushing piston 1 upwards with DVc at Ph against a balance pressure DVw, then closing Vi, opening Vo 11, expanding the gas in DVc as it flows out to Pl and cooling as it expands. Gas at Pl is forced out of DVc as piston 1 moves back towards the cold end 9. Cold gas flowing out through Vo passes through line 35 to heat exchanger 41 where it is heated by the load being cooled, then flows through line 36 to counterflow heat exchanger 40 where it cools incoming gas at Ph before the high pressure gas flows through line 34 to Vi 10.

[0023] Before Vi 10 opens, the pressure in VDw 4 is at Ph thanks to Va 16, which was open while piston 1 was stationary at the cold end. When Vi is opened, the pressure in DVc 3 and DVw 4 is close to Ph, but the pressure in DVs 5 is Pl, creating a force imbalance that drives piston 1 toward the hot end. Gas at a pressure slightly above the pressure in line 30 flows out through CVo 12 and the adjustable valve Vwo 14. The speed at which piston 1 moves toward the hot end is determined by setting Vwo 14. When DVw is at a minimum, Vi 10 is closed and Vo 11 is opened. Gas from line 30 at Ph flows through the adjustable valve Vwi 15 and CVi 13 through line 38 into DVw 4, pushing piston 1 toward the cold end. The speed at which piston 1 moves toward the cold end is determined by setting Vw 15.The process of increasing the pressure of DVw 4 when Va 16 is open causes the gas to become hot, i.e. the reverse of the process at the cold end.

[0024] This heat is removed in the heat exchanger 42 when gas is forced out through the line 37.

[0025] The force imbalance with gas at Pl on drive shaft 2 and gas at Ph in DVc 3 and DVw 4 is needed to overcome the pressure drop in Ph as gas flows through line 37, heat exchanger 40, and inlet valve Vi 10. The force imbalance also overcomes the friction in seals 50 and 51. In a real engine, the area of ​​drive shaft 2 is typically between 5% and 15% of the area of ​​the cold end of piston 1 and depends on how fast the engine is to be operated.

[0026] Fig. Figure 2 is a cross-sectional / schematic view of the engine assembly 200, which differs from the engine assembly 100 in that the active valve Va 16 has been replaced by the passive valve Vp 17. The passive valve Vp 17 is most conveniently installed in the drive shaft 2 such that gas with Ph is admitted into DVw 4 when the piston 1 comes close to the cold end. In the Fig. 2, Vp 17 consists of the annular groove 18 in the cylinder head 8 around the drive shaft 2, the sealing collar 19 which has a sliding fit on the drive shaft 2 and an "O" ring seal 20 on the outside and is held in place by a retaining ring 21, and transverse channels 22 and 24 connected by the channel 23 in the drive shaft 2. Gas at Ph connects from line 30 through line 33 to the annular groove 18. It is admitted through Vp 17 to DVw 4 when the piston 1 is near the cold end. The admission of high pressure gas into DVw pushes the piston 1 towards the cold end Vo 11, which is still open.

[0027] Provisional patent application S / N 61 / 313 868 describes a preferred construction of the inlet valve Vi 10 and the outlet valve Vo 11, both of which are pneumatically actuated at room temperature by gas circulated by a multi-way rotary valve.

[0028] If an engine is to be used to cool down a load and one wishes to maintain a constant output of work from the compressor, it is necessary to start at a maximum engine speed at room temperature and reduce the engine speed as it gets colder. This is done by reducing the speed of the multi-way rotary valve and the adjusting valves VWo 14 and VWi 15 so that piston 1 makes a full stroke but does not stay long at the warm end, moving from the cold end towards the warm end once DVw is at Ph. Alternatively, it is possible to operate at a constant speed, with the valves Vwi 15 and Vwo 14 in fixed positions for operation at the minimum temperature. If the speed is fixed, some gas is bypassed while the compressor cools down.

[0029] Fig. 3 shows the pressure-volume diagram and Fig. 4 shows the opening and closing sequences for the Fig. 1 and Fig. 2. The status point numbers on the PV diagrams correspond to the Fig. 4. The timing of valve opening and closing is not shown, only the sequence. Point 1 on the PV diagram represents piston 1 at the end of its stroke, minimum DVc, DVw at Ph, DVs at Pl. Vi opens, admitting gas at Ph into VDc. VDc increases as gas in DVw is forced out through line 37. At point 2, Vi is closed, then Vo is opened, point 3, so the pressure in DVc drops to Pl. Piston 1 moves a small amount toward the cold end to point 4 because gas at Ph is expanding in the clearance volume above the piston as DVc drops to Pl. Gas flows through line 38 into DVw and its pressure drops from Ph to Pl as it flows through Vwi 15 until VDc is minimal, point 5. At this point, Va 16 or Vp 17 opens, admitting gas with Ph into VDw 4. Point 6 is the point at which Vo 11 is closed.

[0030] Table 1 provides an example of the cooling capacities calculated for pressures of Vi of 2.2 MPa and Vo of 0.8 MPa. The helium flow rate is 6.0 g / s and includes the flow to the valve actuators Vi and Vo and gas to account for void volume. The heat exchanger efficiency is assumed to be 98%.

[0031] The engine is assumed to be variable speed, with a piston speed control mechanism and valve timing to provide a full stroke with only a short dwell time at the warm end of the stroke and sufficient dwell time at the bottom to fully pressurize DVw 4. The engine was sized to cool a mass from room temperature to approximately 30 K, assuming a maximum velocity of 6 Hz when warm. The optimal velocity is nearly proportional to the absolute temperature.

[0032] The engine uses the assumed flow rate at the assumed pressures for most of the cooldown. The cooling capacity Q and operating speed N are listed for temperatures T at Vi of 200 K and 60 K, respectively. It is obvious that an engine can be designed to operate at a fixed speed over a narrow temperature range, such as 120 K, to cool a cryopump to capture water vapor. The engine efficiency relative to Carnot increases as it cools and the engine slows down because a smaller fraction of the warm-end gas is used. The efficiency is maximum at about 80 K and then drops because heat exchanger losses predominate. Table 1 Calculated performance Maschine 100, 200 Dp - mm 101,4 S-mm 25,4 P-V Fig. 3 Tc -K 200 N-Hz 5,5 Q-W 1250 Wirk - % 8,7 Tc -K 60 N - Hz 2,0 Q-W 310 Wirk - % 16,6

[0033] Other embodiments are within the scope of the following claims. For example, the line 38 with Vwi 15 and CVi 16, along with CVo 12, can be eliminated if Vwo 14 is designed to have other essential properties for flowing into DVw 4 than flowing out. Va 16 and Vp 17 can be eliminated if Vwo 14 can be opened for a short period of time without CVo 12 if Va or Vp would have been opened. The circuit still generates a lot of cooling if the cycle time control is not ideal.

Claims

[1] Expansion machine (100) operating with the Brayton cycle, the expansion machine (100) operating with a gas supplied by a compressor at high and low pressures (Ph, Pl) to produce refrigeration at low temperatures, and comprising: a piston (1) in a cylinder (6), the piston (1) having: a drive shaft (2) at a warm end with low pressure (Pl) acting thereon, an inlet and an outlet valve (10, 11) at a cold end (9) of the cylinder (6) which admit a high-pressure gas (Ph) when the piston (1) is near the cold end (9) of the cylinder (6) and as it moves towards the warm end, and discharge gas at low pressure (Pl) when the piston (1) is near the warm end of the cylinder (6) as it moves towards the cold end (9); and means for maintaining a pressure on the warm end of the piston (1) outside a region of the drive shaft (2) at approximately the same pressure as on the cold end (9) of the piston (1) while the piston (1) is moving, the means comprising: a line (38) with an inlet check valve (13) in series with a throttle valve (15) connected between the warm end and a high pressure line (30) from the compressor, and a line (37) with an outlet check valve (12) in series with a throttle valve (14) connected between the warm end and the high pressure line (30) from the compressor, wherein the pressure on the warm end of the piston (1) is increased from the low pressure (Pl) to the high pressure (Ph) by means of an active valve (16) or a passive valve (17) in a line (39) connected between the high pressure line (30) from the compressor and the warm end of the piston (1), while the piston (1) is close to the cold end (9). [2] Expansion machine (100) according to claim 1, wherein the passive valve (17) is contained in the drive shaft (2). [3] Expansion machine (100) according to claim 1 or 2, wherein a speed at which the piston (1) moves between the hot and cold ends is changed by changing the settings of the throttle valves (14, 15). [4] An expansion machine (100) according to any preceding claim, wherein heat is removed from the warm end by means of a cooler (42) in the line (37) having the outlet check valve (12).

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