Refrigerating device

In order to achieve an improvement in the efficiency of a refrigerating cycle and achieve quick and precise response to changes in the environment or the operating state while using carbon dioxide as a coolant, the refrigerating cycle according to the present invention is provided with a first expander and a second expander and further with a vapor-liquid separator provided between the first and second expanders. The components are arranged such that pressure of a vapor-phase coolant at high pressure, compressed by a compressor and cooled by a radiator, is reduced to an intermediate pressure level in a vapor-liquid two-phase range by the first expander. Then the coolant in a condition of a vapor-liquid mix is separated into a vapor-phase coolant by the vapor-liquid separator, so that only the liquid-phase coolant is expanded by the second expander, so that the vapor-phase coolant is taken into the intake side of the compressor while maintaining the intermediate pressure level. Therefore, no unnecessary energy is expended for compressing the vapor-phase coolant, and as a result, efficiency of the cycle may be improved.

BACKGROUND
 The present invention relates to a supercritical refrigerating cycle that
 utilizes carbon dioxide as a coolant.
 An example of a refrigerating cycle utilizing carbon dioxide (CO.sub.2) as
 a coolant, which is disclosed in Japanese Examined Patent Publication No.
 H 7-18602, comprises a compressor, a radiator, a counter-flow heat
 exchanger, a means for expansion, an evaporator, an accumulator and the
 like.
 In this structure, coolant is compressed by the compressor to be a
 vapor-phase coolant with a high pressure, and then it is cooled at the
 radiator to reduce its enthalpy. During this process, since the
 high-pressure vapor-phase coolant is at a temperature equal to or higher
 than a supercritical temperature (in a supercritical range) of the
 coolant, it is not condensed and does not become a liquid phase state at
 the radiator. In this point, the refrigerating cycle is different from
 prior refrigerating cycles employing Freon. Then, the high pressure
 coolant with the reduced enthalpy travels through the expansion valve so
 that its pressure is reduced down to a vapor-liquid mix range, and thus,
 the liquid-phase component is increased for the first time in the coolant
 in this stage. Subsequently, the liquid-phase component in the coolant
 absorbs heat of a medium traveling through the evaporator to be evaporated
 and then it is taken into the compressor.
 In the refrigerating cycle described above, the counter-flow heat exchanger
 achieves heat exchange between the low temperature vapor-phase coolant
 taken into the compressor and the high-pressure vapor-phase coolant after
 passing through the radiator, and since the low pressure vapor-phase
 coolant is heated and at the same time the high-pressure vapor-phase
 coolant is cooled at the counter-flow heat exchanger, the efficiency of
 the refrigerating cycle is improved.
 However, as it is a known fact that there is an optimal heat exchanging
 capacity in a refrigerating cycle employing a counter-flow heat exchanger
 depending upon the environment in which it is operated or the operating
 state. It is another known fact that if the environment or the operating
 state changes, the optimal heat exchanging capacity also changes, and
 therefore the optimal heat exchanging capacity must be adjusted in order
 to achieve improved efficiency under varying conditions. However, if the
 optimal heat exchanging capacity is changed, a problem arises such that
 the degree of superheat of the coolant in an intake side of the compressor
 becomes excessive, resulting in a high discharge temperature.
 The temperature of the air entering the radiator changes constantly (due to
 changes in the external air temperature, during idling or high speed
 operation and the like). Furthermore, the force to drive the compressor is
 derived from the running engine so that the rotating state of the
 compressor changes in conformance to the running state. As such, when a
 refrigerating cycle as described above is employed in an air conditioning
 system for vehicles, problems arise because the environment or the
 operating state changes frequently.
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to provide a
 refrigerating cycle that utilizes carbon dioxide as a coolant to achieve
 an improvement in the efficiency of the refrigerating cycle and to respond
 quickly and precisely to changes in the environment or the operating
 state.
 In order to achieve the object described above, the refrigerating cycle
 according to the present invention, which comprises, at least, a
 compressor for compressing a vapor-phase coolant to a supercritical range,
 a radiator for radiating heat from the vapor-phase coolant in the
 supercritical range discharged from the compressor, a means for expansion
 for lowering pressure of the vapor-phase coolant in the supercritical
 range after passing through the radiator down to a vapor-liquid two-phase
 range and an evaporator for evaporating a liquid-phase component in the
 coolant with pressure reduced by the means for expansion, characterized in
 that the means for expansion is constituted of a first means for expansion
 and a second means for expansion, that a means for vapor-liquid separation
 is provided between the first means for expansion and the second means for
 expansion to separate the coolant with pressure reduced to the
 vapor-liquid two-phase range by the first means for expansion into a
 vapor-phase coolant to be returned to the compressor and a liquid-phase
 coolant to be delivered to the second means for expansion, and that a
 means for oil separation is provided on an upstream side of the second
 means for expansion to separate oil from the coolant and return the
 separated oil to the compressor.
 Thus, according to the present invention, because the first and second
 means for expansion are provided and the means for vapor-liquid separation
 is provided between the first and second means for expansion, the pressure
 of the high-pressure vapor-phase coolant compressed by the compressor and
 cooled by the radiator is reduced to an intermediate pressure and the
 vapor-liquid two-phase range by the first means for expansion, the coolant
 with a vapor-liquid mix substance is separated into a vapor-phase coolant
 and a liquid-phase coolant by the means for vapor-liquid separation, only
 the liquid-phase coolant is expanded by the second means for expansion and
 the vapor-phase coolant is taken into the intake side of the compressor
 while maintaining the intermediate pressure, so that unnecessary energy
 for compressing the vapor-phase coolant may be controlled to achieve an
 improvement in the cycle efficiency.
 In addition, because the means for oil separation is provided on the
 upstream side of the second means for expansion to separate the oil
 component from the liquid-phase coolant traveling to the second means for
 expansion and the evaporator, any reduction in the heat exchanging
 capability attributable to oil adhering in coolant passages in the
 evaporator can be prevented. Furthermore, since the separated oil at a low
 temperature is directly returned to the drive portion of the compressor,
 the efficiency of the compressor may be improved.
 Moreover, in the present invention, it is preferred that a threephase phase
 separator integrating the means for oil separation and the means for
 vapor-liquid separation is provided between the first means for expansion
 and the second means for expansion. Thus, the structure of the
 refrigerating cycle may be simplified.
 In addition, in the present invention, it is desirable that the means for
 oil separation is provided on the upstream side of the first means for
 expansion. Thus, the first means for expansion c an reduce the pressure of
 only the pure coolant from which oil is separated to assure a reduction in
 the pressure of the coolant to the vapor-liquid mix range with a high
 degree of reliability.
 Alternatively, in the present invention, it is preferred that a three-phase
 separator integrating the means for oil separation, the means for
 vapor-liquid separation and a first means for expansion communicating
 between the means for oil separation and the means for vapor-liquid
 separation is provided between the radiator and the second means for
 expansion. Thus, the structure of the refrigerating cycle may be
 simplified.
 In addition, in the present invention, it is desirable that the means for
 oil separation is provided on the upstream side of the radiator. Since
 carbon dioxide utilized as the coolant remains in the vapor phase state
 until it reaches the first means for expansion, oil solubility to the
 coolant is low, so that the oil adheres to the passage walls in the
 radiator and it causes reduction in the heat exchanging capability, as a
 result, it is desirable that the means for oil separation is provided on
 the upstream side of the radiator.
 Furthermore, in the present invention, it is desirable that the first means
 for expansion is an orifice tube and the s econd means for expansion is an
 automatic expansion valve which is controlled so as to maintain a degree
 of superheat thereof constantly. Alternatively, the first means for
 expansion may be an automatic expansion valve which is controlled so as to
 maintain a degree of superheat thereof constantly, and the second means
 for expansion may be an orifice tube. As a further alternative, the first
 means for expansion may be an electrically-controlled expansion valve
 which is controlled by an external signal and the second means for
 expansion may be an automatic expansion valve which is controlled so as to
 maintain a degree of superheat thereof. Or, both the first and second
 means for expansion may comprise an electrically-controlled expansion
 valve which is controlled by an external signal.
 Thus, since the refrigerating cycle is controlled to maintain a degree of
 superheat in the outlet side of the evaporator, it can respond to abrupt
 changes in the load attributable to external factors such as the
 environment or the operating state. In addition, since intermediate
 pressure control is executed by the first means for expansion, finer
 control of the refrigerating cycle is achieved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The following is an explanation of the preferred embodiments of the present
 invention given in reference to the drawings.
 A refrigerating cycle 1 in the first embodiment of the present invention
 illustrated in FIG. 1 utilizes carbon dioxide as its coolant and comprises
 a compressor 2 interlocked with a running engine (not shown) via a pulley
 21, a radiator 3 cooling the coolant discharged from the compressor 2, an
 oil separator 4 provided on a downstream side of the radiator 3, an
 orifice tube 5 as a first means for expansion provided on a downstream
 side of the oil separator 4, a vapor-liquid separator 6 connected to a
 downstream side of the orifice tube 5, an automatic expansion valve 7 as a
 second means for expansion to which a liquid-phase coolant separated by
 the vapor-liquid separator 6 is supplied and an evaporator 8 provided on
 the downstream side of the automatic expansion valve 7.
 In the refrigerating cycle 1 in the first embodiment, a vapor-phase coolant
 at low pressure Ps taken into the compressor 2 is first compressed by the
 compressor 2 to achieve a pressure Pd in the supercritical range for the
 coolant at the compressor 2 (a-b in the Mollier chart in FIG.9). Then, the
 vapor-phase coolant at the high pressure Pd is cooled by the radiator 3 to
 radiate heat of the coolant into the air passing through the radiator
 (b-c). The vapor-phase coolant cooled by the radiator 3 is sent to the oil
 separator 4 where the oil dissolved in the coolant or carried by the
 coolant is separated. The oil thus separated is returned to a drive
 portion of the compressor 2, i.e., a seal portion between a shaft and a
 case or a crank chamber, via oil return piping 10, and in this embodiment,
 a valve 11 for opening and closing (i.e., a shut-off valve) the oil return
 piping 10 is provided.
 The pressure of the vapor-phase coolant from which the oil is separated by
 the oil separator 4 is reduced to an intermediate pressure Pm by the
 orifice tube 5 as the first means for expansion (c-d). This intermediate
 pressure Pm is a specific level of pressure within the coolant
 vapor-liquid mix range, and the coolant to be sent out to the vapor-liquid
 separator 6 is in a state which the vapor phase coolant and the liquid
 phase coolant are mixed together. Then, the coolant, which is a vapor
 phase and liquid phase mixed substance, is separated into a vapor-phase
 coolant and liquid-phase coolant by the vapor-liquid separator 6, and the
 separated vapor-phase coolant directly returns to the to the intake side
 of the compressor 2 via vapor-phase coolant return piping 12. Thus, since
 the vapor-phase coolant which does not greatly affect the endothermic
 effect achieved in the evaporator 8 bypasses the evaporator 8 and is
 directly returned to the intake side of the compressor 2, an improvement
 is achieved in the heat exchanging efficiency in the evaporator 8, and
 because the unnecessary expenditure of energy for compressing the
 vapor-phase coolant is eliminated, the efficiency of the cycle may be
 improved.
 Then, the liquid-phase coolant separated by the vapor-liquid separator 6 is
 delivered to the automatic expansion valve 7 as the second means for
 expansion, and its pressure is reduced to a low level Ps (d-e). The
 automatic expansion valve 7, which is the type specifically referred to as
 a temperature-actuated expansion valve, is provided with a temperature
 sensing tube 9 placed in contact with piping in a discharge side of the
 evaporator 8, so that the degree of openness of the automatic expansion
 valve 7 is adjusted by that coolant sealed inside the temperature sensing
 tube 9 expanding or contracting as the temperature on an outlet side of
 the evaporator 8 fluctuates, and the quantity of the coolant passing
 inside the evaporator 8 and the low pressure Ps of the coolant is changed
 so as to maintain a temperature (a degree of superheat) on the outlet side
 of the evaporator 8 (f-a) constantly. Consequently, it becomes possible to
 respond to any abrupt changes in the load attributable to external
 factors.
 The liquid-phase coolant expanded at the automatic expansion valve 7
 absorbs heat from air passing through the evaporator 8 and evaporates to
 become a vapor-phase coolant to be taken into the compressor 2 (e-a).
 Through the process described above, a refrigerating cycle such that heat
 is absorbed at the evaporator 8 and the heat is discharged at the radiator
 3 is completed.
 The following is an explanation of other embodiments of the present
 invention, and the same reference numbers are assigned to identical
 members and members having identical functions to preclude the necessity
 for repeated explanation thereof.
 A refrigerating cycle 1A in the second embodiment illustrated in FIG. 2 is
 characterized in that the oil separator 4 is provided on an upstream side
 of the radiator 3. Thus, since the oil component is removed from the
 vapor-phase coolant before passing through the radiator 3, the coolant
 heat exchanging capability at the radiator 3 is improved.
 In a refrigerating cycle 1B in the third embodiment illustrated in FIG. 3,
 the first means for expansion is an automatic expansion valve 5A provided
 with a heat sensing tube 9 for detecting temperature on an outlet side of
 the evaporator 8 and the second means for expansion is an orifice tube 7A
 functioning as a fixed constrictor. In this structure, the temperature on
 the outlet side of the evaporator 8 is used to adjust the automatic
 expansion valve 5A as the first means for expansion, so that adjustment of
 the intermediate pressure Pm is achieved.
 In a refrigerating cycle 1C in the fourth embodiment illustrated in FIG. 4,
 an electrically-controlled expansion valve 5B (e.g., an electromagnetic
 expansion valve, an expansion valve adopting the actuator drive system or
 the like) controlled by a control unit (C/U) 14 is provided to constitute
 the first means for expansion. In the fourth embodiment, for detecting the
 intermediate pressure Pm, a sensor 13 such as a thermosensor for detecting
 temperature inside the vapor-liquid separator 6 or a pressure sensor
 directly to detect the intermediate pressure Pm is provided in the
 vapor-liquid separator 6, and the signal detected by the sensor 13 is
 input to the control unit (C/U) 14, where it undergoes arithmetic
 processing in conformance to a specific program, so that the expansion
 valve 5B is driven to achieve the correct intermediate pressure Pm. While
 this embodiment requires a higher production cost compared to the
 embodiments explained earlier, it achieves even finer control.
 In a refrigerating cycle ID in the fifth embodiment illustrated in FIG. 5,
 which is. provided with a sensor 13A (identical to the sensor 13 explained
 above) for detecting the intermediate pressure Pm at the vapor-liquid
 separator 6 and a sensor 9A for detecting the temperature on an outlet
 side of the evaporator 8, signals from the sensors 9A and 13A are input to
 a control unit (C/U) 14A, where they undergo arithmetic processing and are
 output as control signals to an electrically-controlled expansion valve 5B
 as the first means for expansion and an electrically-controlled expansion
 valve 7B as the second means for expansion. Thus, the appropriate
 intermediate pressure Pm and the desired low pressure Ps may be gained.
 A refrigerating cycle 1E in the sixth embodiment illustrated in FIG. 6 is
 provided with a three-phase separator 70 integrating an oil separator 4A
 and a vapor-liquid separator 6A between the orifice tube 5 as the first
 means for expansion and the automatic expansion valve 7 as the second
 means for expansion. While it is necessary to specially provide the
 three-phase separator 70 in this embodiment, the structure of the
 refrigerating cycle can be simplified while still achieving advantages
 similar to those achieved in the embodiments explained earlier.
 A refrigerating cycle 1F in the seventh embodiment illustrated in FIG. 7 is
 provided with a three-phase separator 71 integrating an oil separator 4B,
 a first means for expansion 5C and a vapor-liquid separator 6B. In this
 three-phase separator 71 which may be structured as illustrated in FIG. 8,
 for instance, the oil separator 4B and the vapor-liquid separator 6B are
 formed inside a case housing 72 and the oil separator 4B and the
 vapor-liquid separator 6B are communicated with each other by an orifice
 5C as the first means for expansion.
 The oil separator 4B is provided with an oil separation space 40
 communicating with a coolant induction port 73 and coolant induced into
 the oil separation space 40 collides against an inner wall portion 41
 facing opposite the coolant induction port 73 to separate oil and further
 oil is separated by passing through an oil separation filter 42. Thus, the
 oil separated by colliding against the inner wall portion 41 drips into an
 oil reservoir 44 along the inner wall portion 41, and the oil separated by
 the oil separation filter 42 drips down into the oil reservoir 44 via an
 oil guide 43. The oil collected in the oil reservoir 44 is returned to the
 compressor 2 via the oil return piping 10 connected to an oil delivery
 port 74.
 In addition, the coolant reaching a vapor-liquid separation space 60 of the
 vapor-liquid separator 6B from the oil separation space 40 via the orifice
 5C, whose pressure is reduced to the intermediate level Pm by the orifice
 5C until it achieves a mixed state in which a vapor-phase coolant and a
 liquid-phase coolant are mixed together, is discharged from the orifice 5C
 to collide against an inner wall portion 61 of the vapor-liquid separation
 space 60, and the liquid-phase coolant drips down into a liquid reservoir
 62 in a lower portion of the vapor-liquid separation space 60. Thus, the
 vapor-phase coolant is returned to the compressor 2 via the vapor-phase
 coolant return piping 12 connected to a vapor-phase coolant delivery port
 75 and the liquid coolant is delivered to the automatic expansion valve 7
 as the second means for expansion connected to a liquid-phase coolant
 delivery port 76. Thus, an added advantage of simplification in the
 circuit structure is achieved while still achieving advantages similar to
 those achieved in the embodiments explained earlier.
 Furthermore, a vapor-liquid separation filter may be provided inside the
 vapor-liquid separation space 60 to further promote vapor-liquid
 separation, or an electrically-controlled expansion valve may be provided
 in place of the orifice.5C in the seventh embodiment.
 As has been explained, according to the present invention, the first means
 for expansion is employed to reduce the pressure of the coolant to an
 intermediate pressure in a vapor-liquid mix range and only the
 liquid-phase coolant obtained through the process of vapor-liquid
 separation is delivered to the second means for expansion and the
 evaporator, so that the heat exchanging efficiency at the evaporator is
 improved, as a result, an improvement is achieved in the refrigerating
 efficiency in the refrigerating cycle utilizing a supercritical coolant.
 Thus, since the heat exchanging efficiency in a cycle utilizing a
 supercritical coolant such as carbon dioxide as an alternative to Freon
 can be improved in a simple structure, an environment-friendly and
 efficient refrigerating cycle is achieved.
 In addition, since the control of the degree of superheat is achieved by
 the first and/or second means for expansion according to the present
 invention, quick response can be achieved to any fluctuation in the
 cooling load resulting from changes in the environment and/or the
 operating state, which makes for a refrigerating cycle ideal for
 application in air conditioning systems for vehicles.
 Although the invention has been described in its preferred form 20 with a
 certain degree of particularity, it is understood that the present
 disclosure of the preferred form may be changed in the details of
 construction and in the combination and arrangement of parts without
 departing from the spirit and scope of the invention as hereinafter
 claimed.