Expansion device

Abstract

To provide an expansion device which controls high pressure-side pressure such that the pressure does not exceed a predetermined pressure in terms of absolute pressure, while operating in response to differential pressure between pressure at an inlet thereof and pressure at an outlet thereof. An expansion device comprises an orifice for expanding refrigerant, a valve element disposed on a downstream side of a valve hole and urged by a shape-memory alloy spring in a valve-opening direction, a shaft disposed to extend through the valve hole and having one end thereof rigidly fixed to the valve element, and a spring provided at the other end of the shaft, for receiving load generated by differential pressure between pressure of refrigerant at an inlet of the device and pressure of refrigerant at an outlet thereof, in a valve-opening direction. The shape-memory alloy spring senses the temperature of refrigerant on the downstream side to perform temperature-dependent correction of a set value of the differential pressure for opening the valve element. Therefore, pressure on an upstream side of the valve hole is controlled as if by absolute pressure. Further, when the pressure on the upstream side exceeds a predetermined pressure, the spring is bent to open the expansion valve, and hence the pressure is prevented from rising above the predetermined pressure.

Description

CROSS REFERENCE TO RELATED APPLICATION, IF ANY

[0001] This application claims priority of Japanese Application No. 2006-29557 filed on Feb. 7, 2006, entitled "Expansion Device", and No. 2006-254254 filed on Sep. 20, 2006, entitled "Expansion Device".

BACKGROUND OF THE INVENTION

[0002] (1) Field of the Invention

[0003] The present invention relates to an expansion device, and more particularly to an expansion device for expanding refrigerant in a refrigeration cycle for an automotive air conditioner.

[0004] (2) Background Art

[0005] A refrigeration cycle for an automotive air conditioner has been proposed from the viewpoint of global environmental problems, which uses carbon dioxide or the like as refrigerant for use in the refrigeration cycle. In the refrigeration cycle using carbon dioxide as refrigerant, component elements have pressure resistant structures such that they can withstand high pressure, since the operating pressure of the refrigeration cycle using carbon dioxide is high. Further, in a compressor for compressing the refrigerant and an expansion device for expanding the compressed refrigerant, when the high pressure of the refrigerant enters a dangerous region from a pressure-withstanding viewpoint, control for reducing the pressure is carried out, or the compressor and the expansion device are configured to be capable of lowering the pressure.

[0006] For example, an expansion device is known which is configured such that a high pressure-side pressure of refrigerant at an inlet of the refrigerant is compared with the atmospheric pressure, and when the high pressure-side pressure exceeds a predetermined pressure, the expansion device opens the valve to lower the high pressure-side pressure (see e.g. Japanese Unexamined Patent Publication No. 2004-142701 (FIGS. 2 and 5)). This expansion device comprises a bellows which externally receives the pressure of refrigerant introduced into the refrigerant inlet of the expansion device, for contracting as the pressure of refrigerant rises, and has an inside thereof open to the atmosphere, and a valve mechanism which opens as the bellows contracts. With this arrangement, the bellows compares the high pressure-side pressure of refrigerant introduced into the refrigerant inlet of the expansion device with the atmospheric pressure, and when the pressure of refrigerant exceeds predetermined pressure, which is considered to be dangerous to the refrigeration cycle from the pressure-withstanding viewpoint, the bellows contracts, and the valve mechanism proportionally opens in response to the contraction of the bellows to lower the pressure. Thus, the bellows senses the high pressure-side pressure of refrigerant at the refrigerant inlet in terms of absolute pressure to control the valve mechanism, whereby prevention of the high pressure-side pressure of refrigerant at the refrigerant inlet from becoming higher than predetermined pressure can be effected to some extent.

[0007] Further, Japanese Unexamined Patent Publication No. 2004-142701 discloses another expansion device having the structure of a differential pressure control valve which does not sense the high pressure-side pressure in terms of absolute pressure, but operates in response to differential pressure between pressure at a refrigerant inlet and pressure at a refrigerant outlet. The expansion device is configured such that when the differential pressure between the pressure at the refrigerant inlet and the pressure at the refrigerant outlet exceeds a predetermined pressure, the expansion device opens the differential pressure control valve to lower the pressure at the refrigerant inlet.

[0008] As described above, since the expansion device is configured to limit pressure on the high-pressure side, there is no fear that the pressure on the high-pressure side becomes abnormally high. Further, the pressure on the high-pressure side is high when a high cooling power is demanded of the refrigeration cycle, and therefore in such a case, even if the compressor is operating with its maximum displacement, and the pressure on the high-pressure side exceeds the predetermined pressure, there is no need to control the discharge pressure such that it is lowered, on the compressor side, which makes it possible to operate the compressor efficiently at high discharge pressure, thereby enabling the refrigeration cycle to maintain a high cooling power.

[0009] However, in the conventional expansion devices, although the expansion device using the bellows can sense high pressure-side pressure in terms of absolute pressure for control, it is necessary to take into account the pressure-withstanding property of the bellows that directly receives the high pressure, whereas in the case of the expansion device having the structure of a differential pressure control valve, high pressure-side pressure is represented by a value obtained by adding low pressure side-pressure to the differential pressure between the pressure at the refrigerant inlet and the pressure at the refrigerant outlet, so that if the low pressure side-pressure undergoes a change, the high pressure-side pressure is directly influenced by the change, which makes it impossible to control the high pressure-side pressure in terms of absolute pressure.

SUMMARY OF THE INVENTION

[0010] The present invention has been made in view of the above points, and an object thereof is to provide an expansion device which operates in response to differential pressure between pressure of refrigerant at an inlet and pressure of refrigerant at an outlet, and when pressure on a high-pressure side exceeds a predetermined pressure in terms of absolute pressure, functions as a pressure relief valve.

[0011] To solve the above problem, the present invention provides an expansion device for expanding refrigerant circulating through a refrigeration cycle, comprising a differential pressure control valve for being opened by differential pressure between pressure of the refrigerant on an upstream side and pressure of the refrigerant on a downstream side, a spring disposed such that the spring urges the differential pressure control valve in a valve-closing direction, for opening the differential pressure control valve when the differential pressure becomes a value not lower than a predetermined value, and an actuator disposed on the downstream side, for correcting the predetermined value of the differential pressure at which the differential pressure control valve opens, according to a change in temperature or pressure of the refrigerant on the downstream side.

[0012] The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a system diagram showing a refrigeration cycle to which an expansion device according to a first embodiment is applied.

[0014] FIG. 2 is a diagram showing a Mollier chart of carbon dioxide.

[0015] FIG. 3 is a central longitudinal cross-sectional view of the arrangement of the expansion device according to the first embodiment.

[0016] FIG. 4 is a diagram showing a valve-opening characteristic of the expansion device according to the first embodiment.

[0017] FIG. 5 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a second embodiment.

[0018] FIG. 6 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a third embodiment.

[0019] FIG. 7 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a fourth embodiment.

[0020] FIG. 8 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a fifth embodiment.

[0021] FIG. 9 is a diagram showing a valve-opening characteristic of the expansion device according to the fifth embodiment.

[0022] FIG. 10 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a sixth embodiment.

[0023] FIG. 11 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a seventh embodiment.

[0024] FIG. 12 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to an eighth embodiment.

[0025] FIG. 13 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a ninth embodiment.

[0026] FIG. 14 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a tenth embodiment.

[0027] FIG. 15 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to an eleventh embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereafter, embodiments of the present invention will be described in detail with reference to the drawings showing expansion devices applied to a refrigeration cycle using carbon dioxide as refrigerant, by way of example.

[0029] FIG. 1 is a system diagram showing a refrigeration cycle to which an expansion device according to a first embodiment is applied. FIG. 2 is a diagram showing a Mollier chart of carbon dioxide. FIG. 3 is a central longitudinal cross-sectional view of the arrangement of the expansion device according to the first embodiment. FIG. 4 is a diagram showing a valve-opening characteristic of the expansion device according to the first embodiment.

[0030] As shown in FIG. 1, the refrigeration cycle comprises a compressor 1 for compressing refrigerant, a gas cooler 2 for cooling the compressed refrigerant, an expansion device 3 for throttling and expanding the cooled refrigerant, an evaporator 4 for evaporating the expanded refrigerant, an accumulator 5 for storing surplus refrigerant in the refrigeration cycle and separating refrigerant in a gaseous phase from the evaporated refrigerant to send the separated refrigerant to the compressor 1, and an internal heat exchanger 6 for performing heat exchange between refrigerant flowing from the gas cooler 2 to the expansion device 3 and refrigerant flowing from the accumulator 5 to the compressor 1. In FIG. 1, arrows indicate flows of refrigerant.

[0031] As indicated by A-B-C-D-A in FIG. 2, the refrigeration cycle operates such that refrigerant in a gaseous phase is compressed into high-temperature, high-pressure refrigerant by the compressor 1 (A-B); the high-temperature, high-pressure refrigerant is cooled by the gas cooler 2 (B-C); the cooled refrigerant is throttled and expanded to be changed into low-temperature, low-pressure refrigerant by the expansion device 3 (C-D); and the low-temperature, low-pressure refrigerant is evaporated by the evaporator 4 (D-A). In the process in which refrigerant is expanded by the expansion device 3, when the pressure of the refrigerant becomes lower than the saturated vapor line SL, the refrigerant is changed into a two-phase gas-liquid state, and when the refrigerant in the two-phase gas-liquid state is evaporated by the evaporator 4, it cools air in the vehicle compartment by depriving the air of latent heat of vaporization.

[0032] Further, in the refrigeration cycle using carbon dioxide as refrigerant, it is a common practice to dispose the internal heat exchanger 6 that performs heat exchange between refrigerant at an outlet port of the gas cooler 2 and refrigerant at the outlet of the evaporator 4, so as to lower the enthalpy of refrigerant at the inlet of the evaporator 4 to thereby enhance the cooling power of the refrigeration cycle.

[0033] The internal heat exchanger 6 is formed therein with a high-pressure passage for allowing high-pressure refrigerant introduced from the gas cooler 2 to flow therethrough, and a low-pressure passage for allowing low-pressure refrigerant introduced from the accumulator 5 to flow therethrough, and the expansion device 3 is disposed at the outlet of the high-pressure passage.

[0034] More specifically, as shown in FIG. 3, a body 11 of the internal heat exchanger 6 is formed therein with the high-pressure passage 12 for allowing the high-pressure refrigerant introduced from the gas cooler 2 to flow therethrough to the outlet thereof, and has a mounting hole 13 formed at an end of the high-pressure passage 12, for having the expansion device 3 mounted therein. In a state of the expansion device 3 being mounted in the mounting hole 13, a pipe 14 communicating with the evaporator 14 is fitted to an open end of the mounting hole 13 with locking screws 10 which are screwed into the body 11. The pipe 14 is formed to have an inner diameter slightly smaller than the outer diameter of the expansion device 3 such that the expansion device 3 is prevented from being removed from the mounting hole 13 by the high-pressure refrigerant.

[0035] The expansion device 3 disposed in the internal heat exchanger 6 has a body 21. A central portion of the body 21 has a refrigerant-introducing groove 22 circumferentially formed in an outer periphery thereof, for introducing refrigerant from the high-pressure passage 12. The refrigerant-introducing groove 22 is formed with a refrigerant inlet 23 which extends toward the center of the body 21. Further, in the center of a lower portion of the body 21, there is axially formed a valve hole 24, and the upstream side of the valve hole 24 is communicated with the refrigerant inlet 23. Further, a valve element 25 is axially movably disposed on the downstream side of the valve hole 24, for opening and closing the valve hole 24. The valve element 25 has an outer diameter larger than the inner diameter of the valve hole 24 such that the pressure of refrigerant introduced into the refrigerant inlet 23 is received in the valve-opening direction, and is urged in the valve-opening direction by a shape-memory alloy spring 26 forming a temperature-sensing section. It should be noted that the setting of the spring load of the shape-memory alloy spring 26 is adjusted by axially adjusting the position of a hollow cylindrical spring-receiving member 27 externally fixedly fitted on the valve element 25, with respect to the valve element 25. Furthermore, an orifice 28 is formed in the body 11 in a manner bypassing the valve hole 24.

[0036] Further, the body 11 axially movably holds a shaft 29 extending along the axis thereof. The shaft 29 has a lower end, as viewed in FIG. 3, which extends through the valve hole 24 and is rigidly press-fitted in the valve element 25, and an upper end, as viewed in the figure, which is integrally formed with a large-diameter engaging portion for engagement with a spring-receiving member 30. The shaft 29 is urged in the valve-closing direction via the spring-receiving member 30 by a spring 31. Thus, the expansion device 3 forms a differential pressure control valve which opens and closes by the differential pressure between pressure on the upstream side of the valve hole 24 and pressure on the downstream side thereof. It should be noted that the spring 31 is set to such a spring load as will bend to open the differential pressure control valve when the pressure on the inlet side of the expansion device 3 exceeds an upper limit of a control range of the spring load, e.g. 13 MPa. The setting of the spring load is adjusted by the amount of press-fitting of the shaft 29 into the valve element 25.

[0037] The shape-memory alloy spring 26 disposed on the low-pressure side of the differential pressure control valve has a feature that a spring load thereof is reversibly changed with respect to the cycle of temperature, and has characteristics that its spring load is small at temperature lower than the transformation temperature, whereas at temperature higher than the transformation temperature, the spring load becomes larger in proportion to a change in temperature. Therefore, the shape-memory alloy spring 26 serves as a temperature-sensing actuator which gives a spring load corresponding to the temperature of refrigerant on the low-pressure side to thereby control pressure on the high-pressure side, forming a low temperature-side temperature-sensing section.

[0038] It should be noted that an O ring 32 is fitted on the outer periphery of the body 21 at a location below the refrigerant-introducing groove 22, as viewed in FIG. 3, for sealing between the high-pressure passage 12 and the pipe 14 when the expansion device 3 is mounted in the mounting hole 13. Similarly, an O ring 33 is disposed between the body 11 and the pipe 14 at a location inside the locking screws 15 screwed into the body 11 for fixing the pipe 14, for sealing the low-pressure side of the expansion device 3 from the atmosphere.

[0039] In the expansion device 3 configured as above, when the differential pressure between pressure on the inlet side and pressure on the outlet side is small, the spring 31 is not bent by the differential pressure, and hence the differential pressure control valve remains closed. At this time, the high-pressure refrigerant having passed through the internal heat exchanger 6 flows through the orifice 28, and when having flowed out from the orifice 28, the refrigerant is adiabatically expanded to be changed into the low-pressure, low-temperature refrigerant, and is sent into the evaporator 4 via the pipe 14.

[0040] Until the inlet pressure of refrigerant at the inlet of the expansion device 3 rises up to 13 MPa, which is the upper limit of the control range, as shown in FIG. 4, the expansion device 3 has a fixed restriction passage cross-sectional area determined by the cross-sectional area of the orifice 28. When the inlet pressure at the inlet of the expansion device 3 has reached 13 MPa, the differential pressure control valve overcomes the urging force of the spring 31 in the valve-closing direction, to open. The valve hole 24 of the differential pressure control valve has a sufficiently larger diameter than that of the orifice 28, and therefore when the inlet pressure at the inlet of the expansion device 3 exceeds a valve-opening point, the restriction passage cross-sectional area of the expansion device 3 suddenly increases. This causes the inlet pressure to be always held not higher than the valve-opening point.

[0041] On the other hand, the shape-memory alloy spring 26 disposed on the low-pressure side of the differential pressure control valve senses the outlet temperature of refrigerant having flowed out from the expansion device 3, and when the outlet temperature is high, the shape-memory alloy spring 26 acts on the differential pressure control valve in the valve-opening direction, whereas when the outlet temperature of refrigerant is low, it acts on the differential pressure control valve in the valve-closing direction. More specifically, when the outlet temperature of refrigerant is higher than martensitic transformation temperature of the shape-memory alloy spring 26, the shape-memory alloy spring 26 is changed into an austenite phase in which it has a characteristic that its spring load is largely changed according to temperature. As a result, the shape-memory alloy spring 26 has a spring load that changes according to a change in the outlet temperature of refrigerant, to thereby apply load corresponding to the outlet temperature to the valve element 25 in the valve-opening direction.

[0042] For example, when the outlet temperature of the expansion device 3 is 10.degree. C., according to the FIG. 2 Mollier chart, the low pressure side-pressure is approximately 4.6 MPa. Therefore, the shape-memory alloy spring 26 is configured to generate a spring load corresponding to the pressure when the temperature is 10.degree. C. At this time, the differential pressure control valve is so adjusted as to be opened by a differential pressure of 8.4 MPa. This causes the inlet pressure of the expansion device 3 to be specifically set to be 13 MPa, which is obtained by adding 8.4 MPa, which is a differential pressure as a relative value with respect to 4.6 MPa, which corresponds to the outlet temperature of refrigerant, to 4.6 MPa. At this time, pressure in the refrigeration cycle changes in the cycle of A-B-C-D-A.

[0043] Here, assuming that the outlet temperature of the expansion device 3 has risen to 20.degree. C., the spring load of the shape-memory alloy spring 26 is increased to act on the differential pressure control valve in the valve-opening direction. At this time, the differential pressure at which the differential pressure control valve opens is changed to approximately 7.15 MPa, as is apparent from FIG. 2. When the outlet temperature of the expansion device 3 is 20.degree. C., the pressure of refrigerant is approximately 5.85 MPa, and hence the inlet pressure of the expansion device 3 is set to 13 MPa. At this time, pressure in the refrigeration cycle changes in the cycle of A'-B-C-D'-A'.

[0044] As described above, when a high cooling power is demanded, and the compressor 1 is operating with its maximum displacement, the expansion device 3 senses the differential pressure between the inlet pressure and the outlet pressure, and the outlet temperature, and performs temperature-dependent correction of the differential pressure by adding the differential pressure to a pressure corresponding to the outlet temperature, whereby the expansion device 3 operates as if the inlet pressure is controlled by absolute pressure. Moreover, when the inlet pressure exceeds 13 MPa, the differential pressure control valve serves simply as a pressure relief valve to suddenly open, so that the inlet pressure is controlled to be held at 13 MPa, which prevents the inlet pressure from rising abnormally.

[0045] FIG. 5 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a second embodiment. In FIG. 5, component elements identical or equivalent to those shown in FIG. 3 are designated by the same reference numerals, and detailed description thereof is omitted.

[0046] The expansion device 3a according to the second embodiment is distinguished from the expansion device 3 according to the first embodiment in that it is configured to sense the temperature of refrigerant at the inlet of the expansion device 3 such that the refrigeration cycle can be operated efficiently.

[0047] More specifically, in the expansion device 3a, the body 21 has an upper portion, as viewed in FIG. 5, which has a tubular cylinder formed in one piece with therewith which accommodates the spring 31 that is bent by the inlet pressure exceeding 13 MPa to act on the differential pressure control valve to open the same, and a shape-memory alloy spring 41 for sensing the inlet temperature in a state arranged in series. A biasing spring 42 is arranged parallel with the shape-memory alloy spring 41, for adjusting the characteristic of the shape-memory alloy spring 41. More specifically, the shape-memory alloy spring 41 and the biasing spring 42 are arranged between the spring-receiving member 30 engaged with an engaging portion integrally formed with an upper end, as viewed in FIG. 5, of the shaft 29, and a spring-receiving member 43 through the center of which the shaft 29 loosely extends, and the spring 31 is disposed between the spring-receiving member 43 and the bottom of the tubular cylinder. An upward motion, as viewed in FIG. 6, of the spring-receiving member 43 is restricted by an adjustment member 44 press-fitted into the cylinder, and a downward motion thereof, as viewed in FIG. 6, is restricted by a stopper 45 rigidly fixed to the shaft 29.

[0048] The adjustment member 44 is press-fitted into the cylinder until it reaches a position where it is brought into abutment with the spring 31 fully extended to a no spring-load status. With this arrangement, even when a force of the inlet pressure in the valve-opening direction acts on the spring 31 via the shaft 29, and the shape-memory alloy spring 41 and the biasing spring 42, the spring 31 is not bent is so for as the inlet pressure is not higher than 13 MPa, and when the inlet pressure exceeds 13 MPa, the spring 31 is bent to quickly open the differential pressure control valve.

[0049] On the other hand, the shape-memory alloy spring 41 senses the inlet temperature. When the inlet temperature is low, the shape-memory alloy spring 41 has a small spring load, and therefore the synthetic load of the shape-memory alloy spring 41 and the spring 42 is small, and the setting differential pressure for opening the differential pressure control valve is set to a small value. As the inlet temperature becomes higher, the synthetic load of the shape-memory alloy spring 41 and the spring 42 becomes larger, and hence the setting differential pressure as well is set to a larger value, whereby the shape-memory alloy spring 41 is in a stiffened state between the spring-receiving member 30 and the spring-receiving member 43 when the inlet temperature is not lower than a predetermined temperature at which a change in the spring load of the shape-memory alloy spring 41 with respect to a change in the temperature is saturated.

[0050] According to the expansion device 3a, when the inlet temperature is low, the spring loads of the shape-memory alloy spring 41 and the biasing spring 42 which act on the differential pressure control valve in the valve-closing direction are small, and hence the differential pressure control valve is made open to a very small opening degree corresponding to the orifice 28 by the differential pressure between the inlet pressure and the outlet pressure, whereby refrigerant is allowed to flow, causing adiabatic expansion of refrigerant. At this time, similarly to the expansion device 3 according to the first embodiment, the pressure of refrigerant at the inlet of the expansion device 3a is controlled to a pressure which is determined by the differential pressure across the differential pressure control valve and a pressure corresponding to the outlet temperature.

[0051] On the other hand, the temperature of refrigerant at the inlet of the expansion device 3a is sensed by the shape-memory alloy spring 41 so as to shift a predetermined value of the differential pressure subjected to temperature-dependent correction by the shape-memory alloy spring 26, according to a change in the inlet temperature. This makes it possible to control the temperature and pressure of refrigerant at the inlet of the expansion device 3a, that is, a temperature and a pressure at a point C in FIG. 2, along a control line CL approximated to an optimal control line that is considered to be capable of enhancing the cooling power of the refrigeration cycle while holding a high performance coefficient of the refrigeration cycle.

[0052] Of course, also in the expansion device 3a, when the inlet pressure has risen to exceed 13 MPa, the spring 31 bends to suddenly open the differential pressure control valve, which prevents the inlet pressure from rising above the valve-opening point.

[0053] FIG. 6 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a third embodiment. In FIG. 6, component elements identical or equivalent to those shown in FIG. 5 are designated by the same reference numerals, and detailed description thereof is omitted.

[0054] The expansion device 3b according to the third embodiment is distinguished from the expansion device 3a according to the second embodiment in that the positional relationship between the spring 31, the shape-memory alloy spring 41, and the spring 42 is reversed.

[0055] According to the expansion device 3b, when the inlet temperature is low, the shape-memory alloy spring 41 has a small spring load, and a valve-opening force of the valve element 25, generated by the differential pressure between the inlet pressure and the outlet pressure is transmitted via the shaft 29, the spring-receiving member 30, the spring 31, and the spring-receiving member 43, to thereby bend the shape-memory alloy spring 41, which causes the differential pressure control valve to be made open to a very small opening degree. At this time, similarly to the expansion devices 3 and 3a according to the first and second embodiments, the pressure of refrigerant at the inlet of the expansion device 3b is controlled to a pressure which is determined by a pressure corresponding to the differential pressure across the differential pressure control valve and the outlet temperature. Further, the temperature of refrigerant at the inlet of the expansion device 3a is controlled by the shape-memory alloy spring 41 disposed at the inlet of the expansion device 3a along the control line CL approximated to the optimal control line. As to the inlet pressure of refrigerant at the inlet of the expansion device 3a, when the spring 31 senses pressure exceeding 13 MPa, it causes the differential pressure control valve to be suddenly opened.

[0056] FIG. 7 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a fourth embodiment. In FIG. 7, component elements identical or equivalent to those shown in FIG. 5 are designated by the same reference numerals, and detailed description thereof is omitted.

[0057] Although the expansion device 3c according to the fourth embodiment has the same basic construction as that of the expansion device 3a according to the second embodiment, the expansion device 3c is distinguished from the expansion device 3a in that it is configured such that the spring 31 for sensing high pressures, the shape-memory alloy spring 41, and the spring 42 can be easily assembled and adjusted.

[0058] More specifically, in the expansion device 3c, the spring-receiving member 43 through the center of which the shaft 29 loosely extends is integrally formed with a hollow cylindrical body accommodating the shape-memory alloy spring 41 and the spring 42, and a stopper 45 for adjusting the spring loads of the shape-memory alloy spring 41 and the spring 42 via the spring-receiving member 30 is press-fitted into the hollow cylindrical body. Further, the spring-receiving member 43 is placed on an upper portion, as viewed in FIG. 7, of the spring 31, and the adjustment member 44 is rigidly fixed to the body 21 such that the adjustment member 44 accommodates the spring 31 and the spring-receiving member 43.

[0059] When the expansion device 3c is assembled, first, the shape-memory alloy spring 41 and the spring 42 are assembled while adjusting the spring loads thereof in advance. More specifically, the shape-memory alloy spring 41 and the spring 42, and the spring-receiving member 30 are placed in the hollow cylindrical body of the spring receiving member 43 in the mentioned order, and the stopper 45 is press-fitted into the hollow cylindrical body until it reaches a predetermined position, to thereby adjust the spring loads of the shape-memory alloy spring 41 and the spring 42, whereby a high temperature-side temperature-sensing section is constructed. Then, the high temperature-side temperature-sensing section is placed on the spring 31 disposed in an upper space of the body 21, as viewed in FIG. 7, and the adjustment member 44 having a hollow cylindrical shape and having an engaging portion with an upper portion thereof, as viewed in the figure, bent inward, is placed from above to cover them. In a state in which an upper portion of the body 21 is partially press-fitted into a lower portion, as viewed in the figure, of the adjustment member 44, the adjustment member 44 is further pushed down, as viewed in the figure, until the engaging portion is brought into abutment with an upper end, as viewed in the figure, of the spring-receiving member 43, whereby the adjustment member 44 is fitted to the body 21. Furthermore, if the adjustment member 44 is pushed down, as viewed in the figure, as required, it is possible to adjust the spring load of the spring 31. Finally, the shaft 29 is inserted from above, as viewed in the figure, and further, the differential pressure control valve is press-fitted by a predetermined amount into the valve element 25 to which is applied the adjusted spring load of the shape-memory alloy spring 26, such that the differential pressure control valve is made open to a predetermined minimum opening degree by the shape-memory alloy spring 26. Thus, the expansion device 3c is assembled.

[0060] Since the expansion device 3c has the same basic construction as that of the expansion device 3a according to the second embodiment, the expansion device 3c operates quite the same way as the expansion device 3a.

[0061] FIG. 8 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a fifth embodiment. FIG. 9 is a diagram showing the valve-opening characteristic of the expansion according to the fifth embodiment. In FIG. 8, component elements identical or equivalent to those shown in FIG. 7 are designated by the same reference numerals, and detailed description thereof is omitted.

[0062] The expansion device 3d according to the fifth embodiment is distinguished from the expansion device 3c according to the fourth embodiment in that in place of the high temperature-side temperature-sensing section including the shape-memory alloy spring 41 and its biasing spring 42 of the expansion device 3c, it is provided with a spring 42a for opening the differential pressure control valve by a differential pressure lower than 13 MPa.

[0063] Referring to FIG. 9, the expansion device 3d is characterized in that it has two valve-opening points at which the expansion device 3d opens the differential pressure control valve in response to changes in the inlet pressure of refrigerant on the upstream side. More specifically, in a stage of low inlet pressure, the expansion device 3d is made open to the predetermined minimum opening degree, and has a fixed restriction passage cross-sectional area. When the inlet pressure becomes higher, and first exceeds a predetermined value set by the spring 42a, the spring 42a is bent to open the differential pressure control valve. As the inlet pressure becomes higher, the restriction passage cross-sectional area increases proportionally. When the inlet pressure further increases to exceed 13 MPa, which is set by the spring 31, the differential pressure control valve suddenly opens to lower the inlet pressure, thereby preventing the inlet pressure from rising above 13 MPa.

[0064] FIG. 10 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a sixth embodiment. In FIG. 10, component elements identical or equivalent to those shown in FIG. 5 are designated by the same reference numerals, and detailed description thereof is omitted.

[0065] Although the expansion device 3e according to the sixth embodiment has the same basic construction as that of the expansion device 3a according to the second embodiment, the expansion device 3e is distinguished from the expansion device 3a which has the high temperature-side temperature-sensing section that senses the temperature of refrigerant at the outlet of the internal heat exchanger 6 in that the expansion device 3e has a high temperature-side temperature-sensing section that senses the temperature of refrigerant at the inlet of the internal heat exchanger 6, that is, the temperature of refrigerant at the outlet of the gas cooler 2.

[0066] In the internal heat exchanger 6, in the high-pressure passage 12 formed in the body 11, a refrigerant inlet passage 46 into which high-pressure refrigerant is introduced from the gas cooler 2 is formed such that it passes in the vicinity of the mounting hole 13 to which the expansion device 3e is mounted. The mounting hole 13 is formed to extend to the refrigerant inlet passage 46 such that when the expansion device 3e is mounted to the mounting hole 13, the high temperature-side temperature-sensing section thereof is located within the refrigerant inlet passage 46. Further, in the expansion device 3e, an O ring 47 is circumferentially formed on the outer periphery of the body 21 so as to prevent refrigerant in the refrigerant inlet passage 46 from leaking into the refrigerant inlet 23 in a state of the expansion device 3e mounted to mounting hole 13.

[0067] Also in this construction of the expansion device 3e, the operation of the expansion device is the same as that of the expansion device 3a except that the high temperature-side temperature-sensing section senses the temperature of refrigerant at the outlet of the gas cooler 2 in the refrigerant inlet passage 46 of the internal heat exchanger 6.

[0068] FIG. 11 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a seventh embodiment. In FIG. 11, component elements identical or equivalent to those shown in FIG. 10 are designated by the same reference numerals, and detailed description thereof is omitted.

[0069] The expansion device 3f according to the seventh embodiment is distinguished from the expansion device 3e according to the sixth embodiment in that the construction of the high temperature-side temperature-sensing section that senses the temperature of refrigerant at the outlet of the gas cooler 2 is simplified. More specifically, the expansion device 3f is configured such that the stopper 45 included in the high temperature-side temperature-sensing section of the expansion device 3e is eliminated while arranging the shape-memory alloy spring 41 and the spring 42 in series with the spring 31 for sensing high pressure. As a result, when the temperature and the pressure of refrigerant at the outlet of the gas cooler 2 are high, the shape-memory alloy spring 41 acts in the direction of increasing the spring load of the high pressure-sensing spring 31, and hence the expansion device 3f has a characteristic that at a valve-opening point thereof, in response to changes in the inlet pressure, it opens the differential pressure control valve not sharply but a little more smoothly.

[0070] FIG. 12 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to an eighth embodiment. In FIG. 12, component elements identical or equivalent to those shown in FIG. 3 are designated by the same reference numerals, and detailed description thereof is omitted.

[0071] The expansion device 3g according to the eighth embodiment is distinguished from the expansion device 3 according to the first embodiment which has the high pressure-sensing spring 31 disposed on the upstream side in that the expansion device 3g has the spring 31 disposed on the downstream side.

[0072] More specifically, in the expansion device 3g, the valve element 25 is disposed on the downstream side of the valve hole 24 formed through the body 21, and the high pressure-sensing spring 31 is disposed to urge the piston 51 which is integrally formed with the valve element 25 and is axially and movably accommodated within the body 21, in the valve-closing direction, while the shape-memory alloy spring 26 of the low temperature-side temperature-sensing section is disposed to urge the piston 51 in the valve-opening direction. The high pressure-sensing spring 31 has a spring load thereof adjusted by an adjustment screw 52 screwed into the body 21. With this arrangement, the spring 31 is bent in response to the differential pressure between the pressure of refrigerant on the upstream side and the pressure of refrigerant on the downstream side, whereby a predetermined value of the differential pressure, required for opening the differential pressure control valve, is subjected to correction dependent on the outlet temperature of refrigerant on the downstream side, sensed by the shape-memory alloy spring 26, whereby when the pressure of refrigerant on the upstream side is high, the inlet pressure of refrigerant is always held at 13 MPa set by the spring 31.

[0073] Further, in the expansion device 3g, the orifice 28 is formed through the valve element 25, for allowing refrigerant to flow at a minimum flow rate when the differential pressure control valve is fully closed, and a strainer 53 is disposed on the upstream side of the valve hole thereof, for removing foreign matter from refrigerant.

[0074] FIG. 13 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a ninth embodiment. In FIG. 13, component elements identical or equivalent to those shown in FIG. 12 are designated by the same reference numerals, and detailed description thereof is omitted.

[0075] The expansion device 3h according to the ninth embodiment is distinguished from the expansion device 3g according to the eighth embodiment in that it is configured to incorporate a second differential pressure control valve in the differential pressure control valve (hereinafter referred to as "the first differential pressure control valve") of the expansion device 3g, such that two differential pressure control valves having different valve-opening points function in parallel.

[0076] More specifically, in the expansion device 3h, the orifice 28 formed in the valve element 25 of the first differential pressure control valve serves as a valve hole of the second differential pressure control valve, with a valve element 61 being disposed on the downstream side, for opening and closing the valve hole, and a piston 62 integrally formed with the valve element is axially movably accommodated in the piston 51 of the first differential pressure control valve. The piston 62 is urged by the spring 63 in the valve-closing direction, and the spring load of the spring 63 is adjusted by an adjustment screw 64 screwed into the piston 51. Further, in the valve element 61 as well, there is formed an orifice 65 for allowing refrigerant to flow at a minimum flow rate when the first and second differential pressure control valves are fully closed.

[0077] The expansion device 3h configured as above has a characteristic, as shown in FIG. 9, that it has two valve-opening points at which the expansion device 3h opens in response to changes in the inlet pressure of refrigerant on the upstream side. More specifically, in the expansion device 3h, the shape-memory alloy spring 26 senses the outlet temperature of refrigerant on the downstream side to correct the predetermined value of differential pressure required for opening the first differential pressure control valve, whereby the inlet pressure of refrigerant is sensed as a pseudo absolute pressure. Here, in a stage of low inlet pressure, the expansion device 3h has a fixed restriction passage cross-sectional area determined by the cross-sectional area of the orifice 65 of the second differential pressure control valve. When the inlet pressure becomes higher, and first, the differential pressure between the inlet pressure on the upstream side and the outlet pressure on the downstream side exceeds a pressure set by the spring 63, the second differential pressure control valve opens, and as the differential pressure becomes higher, the restriction passage cross-sectional area increases proportionally. After that, when the inlet pressure reaches 13 MPa, the first differential pressure control valve starts to open. Further, when the inlet pressure increases to exceed 13 MPa set by the spring 31, the first differential pressure control valve suddenly opens. This causes the inlet pressure to decrease, and hence prevents the same from increasing above 13 MPa.

[0078] In the aforementioned first to ninth embodiments, the low temperature-side temperature-sensing section is configured to correct the predetermined value of the valve-opening differential pressure for opening the differential pressure control valve according to changes in the temperature of refrigerant on the downstream side of the differential pressure control valve. However, the predetermined value of the valve-opening differential pressure can be corrected not only according to changes in the temperature of refrigerant on the downstream side of the differential pressure control valve but also according to changes in the pressure of refrigerant on the downstream side of the differential pressure control valve. This is because refrigerant is in a saturated liquid state at the outlet of the expansion device, and in this saturated liquid state, the temperature and the pressure of refrigerant are constant without undergoing any change, as shown by line D-A or D'-A' of the FIG. 2 Mollier chart, and therefore if the temperature is determined, the pressure is determined. As described above, in the evaporator 4 on the outlet side of the expansion device, the evaporation pressure of refrigerant is constant, and moreover the temperature and the pressure have a linear relation therebetween, so that it is possible to consider that sensing of the pressure of refrigerant at the outlet of the expansion device is equivalent to sensing of the temperature of refrigerant at the outlet of the expansion device. This makes it possible to cause an expansion device to have the same function as that of the expansion devices 3 to 3h according to the first to ninth embodiments by configuring the expansion device such that in place of the low temperature-side temperature-sensing section, a low temperature-side pressure-sensing section senses the pressure of refrigerant at the outlet of the expansion device, to correct the predetermined value of the valve-opening differential pressure according to changes in the pressure of refrigerant on the downstream side of the differential pressure control valve. Hereinafter, a detailed description will be given of constructions provided with such a low temperature-side pressure-sensing section.

[0079] FIG. 14 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to a tenth embodiment. In FIG. 14, component elements identical or equivalent to those shown in FIG. 12 are designated by the same reference numerals, and detailed description thereof is omitted.

[0080] The expansion device 3i according to the tenth embodiment is provided with a low temperature-side pressure-sensing section in place of the shape-memory alloy spring 26 as the low temperature-side temperature-sensing section of the expansion device 3g according to the eighth embodiment. More specifically, the expansion device 3i has a power element 71 fixed thereto by screwing an open end of a hollow cylindrical portion of the body 21 into the power element 71. When high pressure is sensed, the power element 71 acts in the direction of decreasing the spring load of the high pressure-sensing spring 31 which sets the valve-opening differential pressure for opening the differential pressure control valve, to thereby serve as a pressure-sensing actuator that corrects a predetermined value of the valve-opening differential pressure in a decreasing direction.

[0081] The power element 71 is formed by holding a diaphragm 74 made of a thin metal plate between an outer housing 72 having a center projected outward and an inner housing 73 having an opening in the center thereof and having a hub connected to the body 21, and welding all the outer peripheries of the housings 72 and 73 and the diaphragm 74 under high-pressure gas or vacuum atmosphere along the whole circumferences thereof. A hermetically sealed space formed by the outer housing 72 and the diaphragm 74 accommodates a disc spring 75, a spring 76, and a spring receiving member 77. The load of the disc spring 75 is adjusted by combining a plurality of disc springs (three in the illustrated example) having respective appropriate spring loads. The spring load of the spring 76 is adjusted by plastically inwardly deforming an end face of the outer housing 72 to change the position of the spring-receiving member 77 in the direction of compressing the spring 76. On a side of the diaphragm 74 opposite to a side thereof where the disc spring 75 is disposed, a displacement-transmitting member 78 is disposed for transmitting the displacement of the diaphragm 74 to the spring 31. A stopper 79 in the form of a step is formed on an inner wall of the housing 73, for restricting the motion of the displacement-transmitting member 78 in the direction of increasing the spring load of the spring 31. This inhibits the expansion device from correcting the predetermined value of the differential pressure when the compressor 1 is operating in a state in which the pressure of refrigerant on the downstream side of the differential pressure control valve is low.

[0082] It should be noted that although in the present embodiment, part of a screw thread of the body 21, which is screwed into the power element 71, is cut such that the pressure of refrigerant on the downstream side of the differential pressure control valve easily reaches the diaphragm 74, the cut part is not necessarily required since portions of the power element 71 and the body 21 screwed together are not completely hermetically sealed.

[0083] In the expansion device 3i constructed as above, when the differential pressure between the pressure on the inlet side and the pressure on the outlet side is small, the spring 31 is not bent by the differential pressure, so that the differential pressure control valve is closed. At this time, high-pressure refrigerant having passed through the internal heat exchanger 6 flows through the orifice 28, and when having flowed out from the orifice 28, the refrigerant is adiabatically expanded to be changed into low-pressure, low-temperature refrigerant, and is sent to the evaporator 4 via the pipe 14.

[0084] Until the inlet pressure of refrigerant at the inlet of the expansion device 3i rises up to 13 MPa, which is the upper limit of the control range, the expansion device 3i has a fixed restriction passage cross-sectional area determined by the cross-sectional area of the orifice 28. When the inlet pressure at the inlet of the expansion device 3i has reached 13 MPa, the differential pressure control valve overcomes the urging force of the spring 31 in the valve-closing direction, to open. The valve hole 24 of the differential pressure control valve has a sufficiently larger diameter than that of the orifice 28, and therefore when the inlet pressure at the inlet of the expansion device 3i exceeds a valve-opening point, the restriction passage cross-sectional area of the expansion device 3i suddenly increases. This causes the inlet pressure to be always held not higher than the valve-opening point.

[0085] On the other hand, the power element 71 disposed on the low-pressure side of the differential pressure control valve senses the outlet pressure of refrigerant having flowed out from the expansion device 3i, and when the outlet pressure is high, the shape of a central portion of the disc spring 75 that receives the pressure via the diaphragm 74 is changed to be made concave inward (downward, as viewed in FIG. 14) such that the disc spring 75 acts in the direction of decreasing the valve-opening differential pressure, whereas when the outlet pressure is low, the shape of the central portion of the disc spring 75 is changed to be inflated outward (upward, as viewed in FIG. 14) such that the disc spring 75 acts in the direction of increasing the valve-opening differential pressure. That is, the power element 71 corrects the predetermined value of the valve-opening differential pressure, by applying load corresponding to the outlet pressure of the differential pressure control value to the valve element 25 in the valve-opening direction.

[0086] With this arrangement, when a high cooling power is demanded and the compressor 1 is operating with its maximum displacement, the expansion device 3i senses the differential pressure between the inlet pressure and the outlet pressure, and the outlet pressure, and pressure correction is performed by adding the differential pressure to the outlet pressure, whereby the expansion device 3i operates as if it controlled the inlet pressure by absolute pressure. Moreover, when the inlet pressure exceeds 13 MPa, the differential pressure control valve suddenly opens, serving simply as a pressure relief valve, so that the inlet pressure is controlled to be held at 13 MPa, which prevents the inlet pressure from rising abnormally.

[0087] It should be noted that in the case where a chamber accommodating the disc spring 75 is under vacuum, the power element 71 can detect the outlet pressure of refrigerant at the outlet of the expansion device 3i as an absolute value, and therefore it is possible to accurately monitor the inlet pressure of refrigerant at the inlet of the expansion device 3i by the absolute pressure. Further, in the case where the chamber accommodating the disc spring 75 is filled with a high-pressure gas, it is possible to employ a disc spring having a small spring load as the disc spring 75 since the high-pressure gas acts as an air spring. In this case, the stopper 79 restricts the motion of the displacement-transmitting member 78 such that when the expansion device 3i is separately placed as a part, the high-pressure gas does not inflate the diaphragm 74 excessively toward the differential pressure control valve.

[0088] FIG. 15 is a central longitudinal cross-sectional view of the arrangement of an expansion device according to an eleventh embodiment. In FIG. 15, component elements identical or equivalent to those shown in FIG. 13 are designated by the same reference numerals, and detailed description thereof is omitted.

[0089] In the expansion device 3j according to the eleventh embodiment, the shape-memory alloy spring 26 of the FIG. 13 expansion device 3h according to the ninth embodiment is changed to the low temperature-side temperature-sensing section shown in FIG. 14. More specifically, in the expansion device 3j, the power element 71, which when sensing a high pressure, corrects the spring load of the spring 35 having been urging the first differential pressure control valve in the valve-closing direction, in the decreasing direction, is fitted to the hollow cylindrical portion of the body 21 by screwing the latter into the former. Further, the orifice 28 of the valve element 25 of the first differential pressure control valve is provided with the orifice 65 for allowing refrigerant to flow at the minimum flow rate when the first and second differential pressure control valves are fully closed.

[0090] According to the expansion device 3j constructed as above, the power element 71 senses the outlet pressure of refrigerant on the downstream side to correct the predetermined value of the differential pressure required for opening the first differential pressure control valve, whereby the inlet pressure of refrigerant is sensed as a pseudo absolute pressure. Here, in a stage of low inlet pressure, the expansion device 3j has a fixed restriction passage cross-sectional area determined by the cross-sectional area of the orifice 65 of the second differential pressure control valve. When the inlet pressure becomes higher, and first, the differential pressure between the inlet pressure on the upstream side and the outlet pressure on the downstream side exceeds a pressure set by the spring 63, the second differential pressure control valve opens, and as the differential pressure becomes higher, the restriction passage cross-sectional area increases proportionally. After that, when the inlet pressure reaches 13 MPa, the first differential pressure control valve starts to open. Further, when the inlet pressure increases to exceed 13 MPa set by the spring 31, the first differential pressure control valve suddenly opens. This causes the restriction passage cross-sectional area to suddenly increase, to lower the inlet pressure, and therefore the inlet pressure is prevented from rising above 13 MPa.

[0091] The expansion device according to the present invention is configured such that it corrects the predetermined value of the differential pressure at which the differential pressure control valve opens, according to a change in the temperature or pressure of the refrigerant on the downstream side, detected by the actuator, that is, it corrects the set differential pressure of the differential pressure control valve by using the temperature or pressure on the low-pressure side. This enables the differential pressure control valve to operate as if it sensed the inlet pressure on the high-pressure side in terms of absolute pressure, without being influenced by the pressure on the low pressure side in spite of the differential pressure control valve operating in response to the differential pressure.

[0092] Further, even if the inlet pressure exceeds the predetermined pressure depending on the operating condition of the compressor, when the inlet pressure exceeds the predetermined pressure, the spring is bent to suddenly open the differential pressure control valve to reduce the inlet pressure. As a consequence, the inlet pressure is held at the predetermined pressure, which makes it possible to positively avoid a state in which pressure on the high-pressure side becomes abnormally high.

[0093] The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.

Claims

1. An expansion device for expanding refrigerant circulating through a refrigeration cycle, comprising: a differential pressure control valve for being opened by differential pressure between pressure of the refrigerant on an upstream side and pressure of the refrigerant on a downstream side; a spring disposed such that said spring urges said differential pressure control valve in a valve-closing direction, for opening said differential pressure control valve when the differential pressure becomes a value not lower than a predetermined value; and an actuator disposed on the downstream side, for correcting the predetermined value of the differential pressure at which said differential pressure control valve opens, according to a change in temperature or pressure of the refrigerant on the downstream side.

2. The expansion device according to claim 1, wherein said actuator is a low temperature-side temperature-sensing section which is disposed on the downstream side, for urging a valve element of said differential pressure control valve in a valve-opening direction, and corrects the predetermined value such that the predetermined value is made lower according to a rise in the temperature of the refrigerant on the downstream side.

3. The expansion device according to claim 2, wherein said low temperature-side temperature-sensing section is a shape-memory alloy spring whose load for correcting the predetermined value of the differential pressure at which said differential pressure control valve opens, is changed according to a change in the temperature of the refrigerant on the downstream side within a predetermined temperature range.

4. The expansion device according to claim 1, comprising an orifice provided parallel with a valve hole of said differential pressure control valve, for bypassing said differential pressure control valve.

5. The expansion device according to claim 2, comprising a shaft disposed in a manner extending through a valve hole and having one end thereof rigidly fixed to said valve element disposed on a downstream side of the valve hole, for transmitting a force generated by the differential pressure in a direction of opening or closing said valve element, and wherein the other end of said shaft is engaged with said spring disposed on an upstream side of the valve hole in a direction in which said spring is further bent as the differential pressure becomes higher, said spring receiving a load in a direction in which said spring is further bent as the temperature of the refrigerant on the downstream side becomes higher, via said shaft, whereby temperature-dependent correction is performed by said low temperature-side temperature-sensing section.

6. The expansion device according to claim 5, comprising a high temperature-side temperature-sensing section disposed in series with said spring, in a manner urging said differential pressure control valve in the valve-closing direction from the upstream side thereof, for shifting the predetermined value corrected by said low temperature-side temperature-sensing section, according to a change in the temperature of the refrigerant on the upstream side.

7. The expansion device according to claim 6, wherein said high temperature-side temperature-sensing section is a shape-memory alloy spring whose spring load changes according to the change in the temperature of the refrigerant on the upstream side.

8. The expansion device according to claim 7, wherein a biasing spring is disposed parallel with said shape-memory alloy spring.

9. The expansion device according to claim 7, comprising a first spring-receiving member disposed between said spring and said high temperature-side temperature-sensing section, and a stopper rigidly fixed to said shaft, and wherein when said high temperature-side temperature-sensing section senses a temperature not lower than the predetermined value, said stopper restricts an increase in the spring load of said first spring-receiving member.

10. The expansion device according to claim 9, wherein said shape-memory alloy spring is accommodated in a bottomed hollow cylindrical body in a state in which a spring load thereof is adjusted via a second spring-receiving member.

11. The expansion device according to claim 10, comprising a shaft disposed in a manner extending through the valve hole and the hollow cylindrical body, and having one end thereof rigidly fixed to said valve element, for transmitting a force generated by the differential pressure in the direction of opening said valve element, and wherein the other end of said shaft is engaged with said second spring-receiving member within said hollow cylindrical body in a direction in which said shape-memory alloy spring is further bent as the differential pressure becomes higher.

12. The expansion device according to claim 6, wherein said differential pressure control valve which is opened by bending of said spring occurring when the differential pressure becomes higher than the predetermined value is set to a predetermined very small opening degree when the differential pressure is not higher than the predetermined value.

13. The expansion device according to claim 5, comprising another spring disposed in series with said spring in a manner urging said differential pressure control valve in the valve-closing direction from the upstream side thereof, for progressively opening said differential pressure control valve from a set differential pressure lower than the predetermined value.

14. The expansion device according to claim 2, comprising another differential pressure control valve disposed such that said another differential pressure functions in parallel with said differential pressure control valve, for being opened by differential pressure lower than the predetermined value at which said spring opens said differential pressure control valve.

15. The expansion device according to claim 1, wherein said actuator is a low temperature-side pressure-sensing section which is disposed such that said actuator receives, via said spring, a valve element of said differential pressure control valve which is moved on the downstream side in a valve-opening direction by the differential pressure, and acts in a direction of decreasing a spring load of said spring according to a rise in the pressure of the refrigerant on the downstream side to thereby correct the predetermined value in a decreasing direction.

16. The expansion device according to claim 15, wherein said low temperature-side pressure-sensing section is a power element in which a diaphragm is hermetically held between a first housing having a center projected outward and a second housing having an opening in a center, and a disc spring provided within said first housing, for supporting, from inside, said diaphragm displaced in the valve-opening direction of said differential pressure control valve by the pressure of the refrigerant on the downstream side.

17. The expansion device according to claim 16, wherein a chamber of said power element within which said disc spring is accommodated is held under vacuum.

18. The expansion device according to claim 16, wherein a chamber of said power element within which said disc spring is accommodated is filled with gas, and a stopper is formed on the second housing, for restricting inflation of said diaphragm.

19. The expansion device according to claim 15, comprising another differential pressure control valve disposed such that said another differential pressure functions in parallel with said differential pressure control valve, for being opened by differential pressure lower than the predetermined value at which said spring opens said differential pressure control valve.

20. An expansion device for expanding refrigerant circulating through a refrigeration cycle, comprising: an orifice provided between a refrigerant inlet and a refrigerant outlet; a differential pressure control valve disposed parallel with said orifice, for being opened by differential pressure between pressure of the refrigerant at the refrigerant inlet and pressure of the refrigerant at the refrigerant outlet; a spring disposed such that said spring urges said differential pressure control valve in a valve-closing direction, for opening said differential pressure control valve when the differential pressure becomes not lower than a predetermined value; and set differential pressure-correcting means disposed at the refrigerant outlet, for correcting a set differential pressure by changing a load of said spring according to a change in temperature or pressure of the refrigerant at the refrigerant outlet, such that a pressure on an upstream side at which said differential pressure control valve opens is not changed.

21. The expansion device according to claim 20, wherein said set differential pressure-correcting means corrects the set differential pressure in a decreasing direction as the temperature or the pressure of the refrigerant at the refrigerant outlet becomes higher.