Patent Publication Number: US-2013247593-A1

Title: Pulse tube refrigerator and method of operating thereof

Description:
RELATED APPLICATION 
     Priority is claimed to Japanese Priority Application No. 2012-063188, filed on Mar. 21, 2012, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The invention relates to a pulse tube refrigerator and a method of operation thereof. 
     2. Description of Related Art 
     For example, a pulse tube refrigerator has a compressor, a pulse tube, a regenerator, a phase control mechanism, and the like. High-pressure working gas generated in the compressor passes through the regenerator and the pulse tube, then flows into the phase control mechanism. 
     The phase control mechanism is configured with a buffer and an inertance tube which is disposed between the pulse tube and the buffer. The phase control mechanism generates a phase difference between varying pressure and varying flow of the working gas oscillating like sine waves, supplied from the compressor in the pulse tube. Thus, a cold thermal mass is generated between the pulse tube and the regenerator. 
     SUMMARY OF THE INVENTION 
     According to at least one embodiment of the present invention, a pulse tube refrigerator includes a compressor, a pulse tube including an internal space, a regenerator loaded with a regenerative material for exchanging heat with working gas, a buffer including an internal space with a predetermined capacity, a flow passage to connect an end of the pulse tube and the buffer, a temperature detecting section to detect temperature of the working gas, a flow adjusting section to adjust an amount of flow of the working gas flowing through the flow passage, and a controlling section to control the flow adjusting section in response to the temperature of the working gas detected at the temperature detecting section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a pulse tube refrigerator according to an embodiment of the present invention; 
         FIG. 2  is a flowchart showing a controlling procedure executed by a controller; 
         FIG. 3  is a graph showing a relationship between a characteristic of an inertance tube and cool-down time; and 
         FIG. 4  is a graph showing a relationship between a characteristic of an inertance tube and cool-down time. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     Characteristics of an inertance tube, which is a part of the phase control mechanism, are set to obtain the maximum refrigeration capacity at a specified refrigeration temperature of a pulse tube refrigerator. Therefore, even while a refrigerator is starting up, i.e., the temperature of the working gas is still high, an amount of flow of the working gas flowing through the inertance tube is set to the amount of flow corresponding with the specified refrigeration temperature of the refrigerator. Therefore, with a conventional pulse tube refrigerator, there is a problem that it takes a long cool-down time which is a time needed to reach the specified refrigeration temperature of the refrigerator from a normal temperature. 
       FIG. 1  shows a regenerative refrigerator according to the first embodiment of the present invention. In the present embodiment, a Stirling pulse-tube refrigerator  1  (called simply a “refrigerator”, hereafter) is taken as an example of a regenerative refrigerator to be explained. The refrigerator  1  has, on the whole, a compressor  2 , an extender  3 , and a phase control section  4 . 
     The compressor  2  is configured with a cylinder  6 , pistons  7 , linear motors  8 , plate spring units  15 , and the like in a housing  5 . 
     The cylinder  6  is disposed at the center of the housing  5 , extended in the horizontal direction in  FIG. 1 . In the cylinder  6 , a pair of pistons  7  are disposed, facing to each other. The pistons  7  in the cylinder  6  are configured to be capable of making a reciprocating motion in the axial direction (the horizontal direction in  FIG. 1 ). In between the pair of the pistons  7 , a compressing chamber  12  is formed. The compressing chamber  12  communicates with the expander  3  via a passage  13 . 
     A linear motor  8  is provided for each of the pistons  7 . The linear motor  8  drives the piston to make a reciprocating motion in the cylinder  6 . The linear motor  8  is configured with a permanent magnet  9 , an electromagnetic coil  10 , a yoke  11 , and a support holder  19 . 
     The permanent magnet  9  is fixed to the piston  7  by the support holder  19 . Therefore, the permanent magnet  9  moves in conjunction with the piston  7 . Also, the yoke  11  is fixed to the housing  5 . A ring-shaped concave section is formed on the yoke  11 , to make the permanent magnet  9  movable in the axial direction in the concave section. 
     The electromagnetic coil  10  is fixed at a position opposite to the permanent magnet  9  in the concave section of the yoke  11 . Alternating current oscillating with a prescribed frequency is supplied to the electromagnetic coil  10  from a power source (not shown). Once the alternating current is supplied to the electromagnetic coil  10 , a driving force is generated between the permanent magnet  9  and the electromagnetic coil  10  in the axial direction. As mentioned earlier, since the electromagnetic coil  10  is fixed on the yoke  11 , the piston  7  is driven in the cylinder  6  in the axial direction by the driving force generated with the linear motor  8 . 
     The plate spring unit  15  has its external circumference fixed to the housing  5  via the support member  14 , as well as having its internal circumference fixed to the piston  7 . The plate spring unit  15  has a function to support the piston  7  to make reciprocating motion in the compressor  2 . Therefore, when the piston  7  is driven in the axial direction by the linear motor  8 , the plate spring unit  15  allows the piston  7  to move in the axial direction, and after the piston  7  has moved, energizes the piston  7  with elastic repulsive force directed toward the opposite direction to the direction driven with the linear motor  8 . 
     Thus, each of the pistons  7  reciprocates in the axial direction in the cylinder  6 , to oscillate pressure of the working gas in the compressing chamber  12 . The varying pressure of the working gas in the compressing chamber  12  is supplied to the expander  3  via the passage  13 , to generate a cold thermal mass in the expander  3 . 
     The expander  3  has a regenerator  20 , a pulse tube  21 , a low-temperature heat exchanger  22 , and the like, to be configured in a pulse tube refrigerator. 
     The regenerator  20  is disposed in the middle of a flow passage of the working gas flowing from the compressor  2  to the pulse tube  21 . The regenerator  20  is configured to have loaded a regenerative material to accumulate cold thermal energy in its inner part of a cylindrical body. 
     The pulse tube  21  is a cylindrical tube, communicating with the regenerator  20  via a passage  22   a  provided in the low-temperature heat exchanger  22 . It is noted that although in the present embodiment, the regenerator  20  and the pulse tube  21  are connected with a folded connection, it is possible to adopt an in-line connection. 
     Next, operations of the pulse tube refrigerator  1  will be explained. Energy of the working gas supplied by the compressor  2  is transferred through the regenerator  20 , the low-temperature heat exchanger  22 , and the pulse tube  21  in order, to be consumed at the phase control section  4 . Between the regenerator  20  and the pulse tube  21 , an energy gap is generated due to work done when the working gas having the generated phase difference transitions from an isothermal state to an adiabatic state. To compensate for the gap, heat is absorbed at the low-temperature heat exchanger  22 , which generates a cold thermal mass. On the other hand, at a radiator  23  disposed at the higher temperature side of the pulse tube  21  (the lower end of the pulse tube in  FIG. 1 ), the heat absorbed at the low-temperature heat exchanger  22  is radiated. By repeating the series of operations, an object to be cooled, thermally connected to the low-temperature heat exchanger  22 , is cooled. 
     Next, the phase control mechanism  4  will be explained. The phase control mechanism  4  is configured with a flow passage  24 , a valve device  27 , a thermometer  28 , a buffer tank  29 , a controller  30 , and the like. 
     The flow passage  24  connects a high-temperature end of the pulse tube  21  and the buffer tank  29  having an internal space with a prescribed capacity. In the present embodiment, the flow passage  24  is configured with the inertance tube  25  (corresponding to a main flow passage in claims) and a bypass tube  26  (corresponding to a bypass flow passage in claims), which is separate from the inertance tube  25 . 
     The inertance tube  25  has an appropriate tube length and a diameter to flow the working gas at a specified refrigeration temperature of the refrigerator  1 , for example, 77 K. On the other hand, the bypass tube  26  has a smaller diameter than the pulse tube  21 . In the present embodiment, the inertance tube  25  and the bypass tube  26  has the same tube length. 
     The valve device  27  (corresponding to a flow adjusting section in the claims) is attached to the bypass tube  26 . The valve device  27  is an electromagnetic valve driven and controlled by the controller  30 , to control the amount of flow of the working gas in the bypass tube  26 . An amount of opening of the valve device  27  can be adjusted continuously. 
     The thermometer  28  (corresponding to a temperature detecting section in the claims) is attached to the low-temperature heat exchanger  22 . The thermometer  28  measures a temperature at a low-temperature end of the pulse tube  21 , which is equivalent to the temperature of the working gas flowing in the passage  22   a.  Therefore, it is possible to determine whether the temperature of the low-temperature heat exchanger  22  reaches the specified refrigeration temperature by measuring with the thermometer  28 . The temperature of working gas measured with the thermometer  28  is sent to the controller  30 . 
     The controller  30  controls the valve device  27  in response to the temperature of working gas measured with the thermometer  28 . Therefore, the amount of flow of the working gas flowing between the pulse tube  21  and the regenerator  20  is controlled by the controller  30 . Specifically, if the valve device  27  is closed completely, the amount of flow of the working gas flowing through the flow passage  24  flows only through the inertance tube  25 , which is the optimum flow in terms of maintaining the specified refrigeration temperature (for example, 77 K) of the refrigerator  1 . On the other hand, if the valve device  27  is fully opened, the working gas flowing through the flow passage  24  flows through both the inertance tube  25  and the bypass tube  26 , with which the amount of flow is larger than the amount obtained only with the inertance tube  25 . 
     The phase control mechanism  4  configured as above is to generate work of flow in the pulse tube  21 , and has a function to delay change of displacement of the working gas behind change of pressure of the working gas, where both the displacement and pressure are oscillating. This phase delay generated by the phase control mechanism generates the work of flow from the compressor  2  (source of oscillation), which generates a cold thermal mass at the low-temperature end of the pulse tube  21 . 
     The phase control mechanism  4  and the pulse tube  21  can be explained in terms of electrical circuitry, where the pulse tube  21  and the buffer tank  29  correspond to capacitor components, the flow passage  24  corresponds to an inductance component and a resistance component. Therefore, by adjusting characteristics of the flow passage  24 , in other words, by adjusting the amount of flow of the working gas flowing through the inertance tube  25  and the bypass tube  26 , it is possible to adjust the phase difference. 
       FIG. 2  is a flowchart showing a controlling procedure of the valve device  27  executed by the controller  30 , which is one of methods of operating the refrigerator  1 . The procedure shown in  FIG. 2  is executed routinely, for example, every predetermined period. 
     Once the procedure shown in  FIG. 2  starts, the controller  30  first determines whether the refrigerator  1  is in a start-up state at Step S 10 . 
     If it is determined that the refrigerator  1  is in the start-up state at Step S 10 , it means that the working gas has not yet been cooled down. Therefore if a positive determination is made at Step S 10  (YES), the procedure proceeds to Step S 11 , where the controller  30  opens the valve device  27  fully. After Step S 11 , the procedure proceeds to Step S 12 . On the other hand, if it is not determined that the refrigerator  1  is in the start-up state at Step S 10 , the procedure proceeds to Step S 12  without branching to Step S 11 . 
     At Step S 12 , the controller  30 , based on a signal sent from the thermometer  28 , reads a current temperature X of the working gas. The read temperature X of the working gas is sent to the controller  30 . 
     Next, at Step S 13 , the controller  30  determines an amount of opening of the valve device in response to the temperature X of the working gas read at Step S 12 . 
     Specifically, the controller  30  has a two dimensional map that has two parameters, where one is a changing temperature of the working gas and the other is an amount of opening of the valve device  27 , which is stored in the controller  30  beforehand. The controller  30  also stores a previous temperature that was obtained when the procedure was executed last time, which is compared with the current temperature X of the working gas to obtain a change in the temperature. 
     If the controller  30 , based on the obtained change, determines that the temperature of the working gas is rising, the valve device  27  is opened by a predetermined amount of opening. If the valve device  27  is already fully opened, it is maintained. With this operation, the diameter of the bypass tube is widened (flow resistance is reduced), which increases the amount of flow of the working gas through the bypass tube  26 . If the valve device  27  is already opened fully, the maximum amount of flow is kept flowing. 
     On the other hand, if the controller  30 , based on the obtained change, determines that the temperature of the working gas is falling, the controller  30  closes the valve device  27  by a predetermined amount. With this operation, the diameter of the bypass tube  26  is throttled down (flow resistance is increased), which reduces the amount of flow of the working gas through the bypass tube  26 . When the temperature X of the working gas reaches the specified refrigeration temperature, the valve device  27  becomes closed completely. 
     If the valve device  27  is closed completely, the working gas flows only through the inertance tube  25  between the pulse tube  21  and the buffer tank  29 . As described earlier, the inertance tube  25  is set to operate optimally at the specified refrigeration temperature. Therefore, at a joining section of the regenerator  20  and the pulse tube  21 , a cold thermal mass can be generated efficiently. 
     It is noted that an amount of opening of the valve device  27 , which is adjusted in response to a change of the temperature, depends on the specified refrigeration temperature of the refrigerator  1 , diameter of the bypass tube  26 , length of the flow passage  24 , or the like. The map stored in the controller  30  is generated with an experiment or other means which takes these factors into account. 
     In the refrigerator  1  according to the present embodiment, since the controller  30  controls the valve device  27  as above, the valve device  27  is opened widely if the working gas is not cooled down, including the start-up period. Therefore, a large amount of flow of the working gas flows between the pulse tube  21 . 
     With a large flow of the working gas between the pulse tube  21  flowing while the low-temperature end of the pulse tube  21  has a high temperature, an amount of work done by the working gas increases, which makes the temperature at the low-temperature end of the pulse tube  21  fall rapidly. Therefore, with the refrigerator  1  according to the present embodiment, it is possible to accelerate cool-down time compared to a conventional refrigerator provided only with an inertance tube capable of flowing an amount of flow only adapted to a specified refrigeration temperature. 
     However, if a large amount of flow of the working gas such as above is kept flowing, heat loss increases around the vicinity of the joining section of the regenerator  20  and the pulse tube  21 . Therefore, refrigeration efficiency of the working gas is reduced before reaching an operating temperature of the working gas. 
     In the refrigerator  1  according to the present embodiment, as above, the valve device  27  is closed in accordance with the falling temperature of the working gas, which gradually reduces the amount of flow of the working gas between the pulse tube  21  and the buffer tank  29 , to reduce the above heat loss gradually. Therefore, with the refrigerator  1  according to the present embodiment, it is possible to accelerate the cool-down time, as well as to securely cool the low-temperature heat exchanger  22  down to the specified refrigeration temperature. 
       FIG. 3  is a graph demonstrating that the cool-down time can be accelerated by changing the diameter of the flow passage disposed between the pulse tube  21  and the buffer tank  29 . In  FIG. 3 , the vertical axis shows the temperature of the low-temperature end of the pulse tube, and the horizontal axis shows cool-down time. 
     An arrow A in  FIG. 3  designates a characteristic of the refrigerator  1  according to the present embodiment, which is obtained with activating the above controlling procedure under conditions that a ratio of lengths, or an aspect ratio, between the inner diameter and the tube length of the flow passage  24  is set to 0.0055, where the inner diameter is the sum of the inertance tube  25  and the bypass tube  26 . 
     On the other hand, an arrow B in  FIG. 3  designates a characteristic of a conventional refrigerator provided only with an inertance tube with a fixed tube and a small aspect ratio of 0.0053. 
     As shown in  FIG. 3 , the refrigerator  1  according to the present embodiment (arrow A) has a shorter cool-down time than the conventional refrigerator (arrow B), which demonstrates that the refrigerator  1  according to the present embodiment 1 shortens cool-down time compared to the conventional refrigerator. 
     In the embodiment above, the flow passage is configured with the inertance tube  25  and bypass tube  26  to shorten cool-down time by controlling the amount of flow through the bypass tube  26 . However, the amount of flow of the working gas between the pulse tube  21  and the buffer tank  29  can be controlled also with flow resistance in the flow passage  24 . 
       FIG. 4  shows cool-down time with different aspect ratios (diameter/length) where the inner diameter of the inertance tube  25  is fixed and the tube length is changed. In  FIG. 4 , an arrow C designates a refrigerator with an aspect ratio of 0.0063, an arrow D designates a refrigerator with an aspect ratio of 0.0054, and an arrow E designates a refrigerator with an aspect ratio of 0.0059. 
     As shown in  FIG. 4 , the cool-down time varies with the length of the inertance tube. Therefore, these characteristics can be utilized to configure a refrigerator with shortened cool-down time. 
     As above, the present invention has been described in detail with reference to preferred embodiments thereof. Further, the present invention is not limited to these embodiments, examples and aspects, but various variations and modifications may be made without departing from the scope of the present invention.