AIR CONDITIONER

Abstract

 This air conditioner using a refrigerant of R32 controls the opening degree of an expansion valve so as to stably control the temperature of the refrigerant discharged from a compressor. The air conditioner (10) includes a refrigerant circuit (11) in which a compressor (12), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) are connected together, and through which HFC32 circulates as the refrigerant, and a controller (30) which performs an opening degree control every predetermined sampling time t to change the opening degree of the expansion valve (15) to a predetermined extent such that a discharge pipe temperature Tp of the compressor (12) reaches a target discharge pipe temperature Tpa. The sampling time t is set to be longer in a range where the opening degree of the expansion valve (15) is less than a predetermined value than in a range where the opening degree is equal to or more than the predetermined value.

Description

TECHNICAL FIELD

[0001] The present invention relates to an air conditioner using R32 as a refrigerant, and particularly relates to controlling the opening degree of an expansion valve.

BACKGROUND ART

[0002] A conventional air conditioner including a refrigerant circuit through which a refrigerant circulates to perform a vapor compression refrigeration cycle controls the opening degree of an expansion valve and controls the temperature of a refrigerant discharged from a compressor, thereby indirectly adjusting the degree of superheat of the refrigerant sucked into the compressor. In an air conditioner of this type, the opening degree of its expansion valve is feedback-controlled every predetermined period, as disclosed in Patent Document 1, for example.

[0003] Also, as disclosed in Patent Document 2, much attention has recently been focused on an air conditioner using, as a refrigerant, R32 (HFC32) having a small global warming potential (GWP) to reduce the influence on global warming.

CITATION LIST

PATENT DOCUMENT

[0004] 

PATENT DOCUMENT 1: Japanese Unexamined Utility Model Publication No. H03-34564

PATENT DOCUMENT 2: Japanese Unexamined Patent Publication No. 2012-122677

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0005] If the air conditioner using R32 as a refrigerant controls the opening degree of an expansion valve every predetermined period, as described above, it may be difficult to stably control the temperature of the refrigerant discharged from the compressor in a low load range where the amount of the refrigerant circulating decreases, which is a problem.

[0006] R32 has a relatively high refrigerating capacity per unit volume among various other refrigerants. The use of R32 can thus reduce the necessary amount of the refrigerant to circulate through a refrigerant circuit, and further decreases the amount of the refrigerant circulating in the low load range. Even if the opening degree of the expansion valve is changed in the low load range, the amount of the refrigerant circulating there is too small to allow the temperature of the refrigerant discharged to reach a target temperature immediately. Thus, when the opening degree is controlled next time, a determination will be made that there is still so much difference between the temperature of the refrigerant discharged and the target temperature that the opening degree of the expansion valve needs to be further changed, even though the opening degree has actually been controlled into an appropriate one. Continuously performing such a control of the opening degree of the expansion valve causes the temperature of the refrigerant discharged to exceed or fall short of the target temperature over and over again, i.e., causes so-called "hunching". As a result, it becomes difficult to stably control the temperature of the refrigerant discharged.

[0007] In view of these problems, the present invention was made to provide a technique for controlling the opening degree of an expansion valve such that an air conditioner using R32 as a refrigerant can control stably the temperature of the refrigerant discharged from its compressor.

SOLUTION TO THE PROBLEM

[0008] A first aspect of the invention is directed to an air conditioner including: a refrigerant circuit (11) in which a compressor (12), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) are connected together, and through which HFC32 circulates as a refrigerant to perform a refrigeration cycle; and a controller (30) which performs an opening degree control every predetermined period to change the opening degree of the expansion valve (15) to a predetermined extent such that the temperature of the refrigerant discharged from the compressor (12) reaches a target temperature. The controller (30) is configured such that the predetermined period is longer in a range where the opening degree of the expansion valve (15) is less than a predetermined value than in a range where the opening degree is equal to or more than the predetermined value.

[0009] According to the first aspect of the invention, one period of the opening degree control extends in the range where the opening degree of the expansion valve (15) is small, i.e., where a relatively small amount of refrigerant circulates through the refrigerant circuit (11). The temperature of the refrigerant discharged reaches or approaches the target temperature in the interval after a change of the opening degree of the expansion valve (15) and before the start of the next control of the opening degree. That is, it is not until the temperature of the refrigerant discharged is stabilized that the next control of the opening degree is started.

[0010] A second aspect of the invention is directed to an air conditioner including: a refrigerant circuit (11) in which a compressor (12), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) are connected together, and through which HFC32 circulates as a refrigerant to perform a refrigeration cycle; and a controller (30) which performs an opening degree control every predetermined period to change the opening degree of the expansion valve (15) to a predetermined extent such that the temperature of the refrigerant discharged from the compressor (12) reaches a target temperature. The controller (30) is configured such that the predetermined extent is smaller in a range where the opening degree of the expansion valve (15) is less than a predetermined value than in a range where the opening degree is equal to or more than the predetermined value.

[0011] According to the second aspect of the invention, the magnitude of change in the opening degree of the expansion valve (15) decreases in the range where the opening degree is small. This reduces the magnitude of variation in the temperature of the refrigerant discharged every time the opening degree is controlled. As a result, the temperature of the refrigerant discharged no longer rises or falls significantly, thus preventing the temperature of the refrigerant discharged from exceeding or falling short of the target temperature.

[0012] A third aspect of the invention is an embodiment of the first aspect of the invention. In the third aspect, the controller (30) is configured such that the predetermined extent is smaller in the range where the opening degree of the expansion valve (15) is less than the predetermined value than in the range where the opening degree is equal to or more than the predetermined value.

[0013] According to the third aspect of the invention, the predetermined period extends, and the magnitude of change in the opening degree of the expansion valve (15) decreases in the range where the opening degree is small. As a result, the temperature of the refrigerant discharged can be stabilized more easily by the start of the next control of the opening degree, and can also have the magnitude of its variation reduced every time the opening degree is controlled. This thus ensures that the temperature of the refrigerant discharged is prevented from exceeding or falling short of the target temperature.

[0014] A fourth aspect of the invention is an embodiment of the first or third aspect of the invention. In the fourth aspect of the invention, the controller (30) is configured such that in the range where the opening degree of the expansion valve (15) is less than the predetermined value, as the opening degree decreases, the predetermined period extends gradually.

[0015] According to the fourth aspect of the invention, the smaller the opening degree of the expansion valve (15) is, the less the amount of the refrigerant circulating through the refrigerant circuit (11) is, and the longer it takes for the temperature of the refrigerant discharged to reach the target temperature. However, as the opening degree decreases, the predetermined period extends gradually. This ensures that the temperature of the refrigerant discharged reaches the target temperature by the start of the next control of the opening degree.

[0016] A fifth aspect of the invention is an embodiment of any one of the first to fourth aspects of the invention. In the fifth aspect of the invention, once the opening degree of the expansion value (15) has become less than the predetermined value, the flow rate of the refrigerant flowing through the expansion valve (15) varies less steeply with respect to a change of the same magnitude in the opening degree of the expansion valve (15).

[0017] According to the fifth aspect of the invention, in the range where the opening degree of the expansion valve (15) is less than the predetermined value, the flow rate of the refrigerant circulating does not vary so significantly, considering the magnitude of change in the opening degree. Therefore, the amount of the refrigerant circulating through the refrigerant circuit (11) does not vary so significantly, either. It thus takes even a longer time for the temperature of the refrigerant discharged to reach the target temperature. However, the predetermined period extends or the magnitude of change in the opening degree decreases in the range where the opening degree is less than the predetermined value. This effectively prevents the temperature of the refrigerant discharged from exceeding or falling short of the target temperature.

ADVANTAGES OF THE INVENTION

[0018] As can be seen from the foregoing description, according to the present invention, one period of the opening degree control is set to be longer in the range where the opening degree of the expansion valve (15) is less than the predetermined value than in the range where the opening degree is equal to or more than the predetermined value. This allows the temperature of the refrigerant discharged to reach or approach the target temperature in the interval after a change of the opening degree of the expansion valve (15) and before the start of the next control of the opening degree, even if a relatively small amount of refrigerant circulates through the refrigerant circuit (11). That is, it is not until the temperature of the refrigerant discharged is stabilized that the next control of the opening degree may be started. The next control of the opening degree thus allows for setting the magnitude of change in the opening degree appropriately. This can prevent the temperature of the refrigerant discharged from exceeding or falling short of the target temperature. As a result, the temperature of the refrigerant discharged can be prevented from causing hunching, and can be controlled stably.

[0019] According to the second aspect of the invention, the magnitude of change in the opening degree of the expansion valve (15) is set to be smaller in the range where the opening degree is less than the predetermined value than in the range where the opening degree is equal to or more than the predetermined value. This can reduce the magnitude of variation in the temperature of the refrigerant discharged every time the opening degree is controlled, when a relatively small amount of refrigerant circulates through the refrigerant circuit (11). As a result, the temperature of the refrigerant discharged no longer rises or falls significantly, thus preventing the temperature of the refrigerant discharged from exceeding or falling short of the target temperature. Consequently, the temperature of the refrigerant discharged can be prevented from causing hunching, and can be controlled stably.

[0020] According to the third aspect of the invention, one period of the opening degree control is extended, and the magnitude of change in the opening degree is reduced, in the range where the opening degree of the expansion valve (15) is less than the predetermined value than in the range where the opening degree is equal to or more than the predetermined value. This thus ensures that the temperature of the refrigerant discharged never exceeds or falls short of the target temperature. Consequently, the temperature of the refrigerant discharged can be stably controlled just as intended.

[0021] According to the fourth aspect of the invention, in the range where the opening degree of the expansion valve (15) is less than the predetermined value, as the opening degree decreases, one period of the opening degree control is extended gradually. This ensures that the temperature of the refrigerant discharged reaches the target temperature by the start of the next control of the opening degree. Consequently, the temperature of the refrigerant discharged can be stably controlled highly successfully.

[0022] According to the fifth aspect of the invention, in the range where the opening degree of the expansion valve (15) is less than the predetermined value, the amount of the refrigerant circulating through the refrigerant circuit (11) does not vary so significantly, considering the magnitude of change in the opening degree. It thus takes even a longer time for the temperature of the refrigerant discharged to reach the target temperature. However, one period of the opening degree control is extended, or the magnitude of change in the opening degree is reduced, adaptively to the range where the opening degree is less than the predetermined value. This effectively prevents the temperature of the refrigerant discharged from exceeding or falling short of the target temperature. Therefore, the temperature of the refrigerant discharged can be effectively prevented from causing hunching.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 

[FIG. 1] FIG. 1 is a piping system diagram showing a configuration for an air conditioner according to a first embodiment.

[FIG. 2] FIG. 2 is a flowchart showing how to control the opening degree of an expansion valve according to the first embodiment.

[FIG. 3] FIG. 3 is a table showing a relationship between the opening range of the expansion valve and a sampling time.

[FIG. 4] FIG. 4 is a graph showing a relationship between the opening degree of the expansion valve and the rate of a refrigerant flowing through it.

[FIG. 5] FIG. 5 is a flowchart showing how to control the opening degree of an expansion valve according to a second embodiment.

[FIG. 6] FIG. 6 is a table showing a relationship between the opening range of the expansion valve and the magnitude of change in the opening degree.

DESCRIPTION OF EMBODIMENTS

[0024] Embodiments of the present invention will now be described in detail with reference to the drawings. The following embodiments are merely a preferred example in nature, and are not intended to limit the scope, applications, and use of the invention.

«First Embodiment»

[0025] A first embodiment of the present invention will be described. As shown in FIG. 1, an air conditioner (10) according to this embodiment includes a refrigerant circuit (11), and switches its modes of operation between a cooling operation and a heating operation. The refrigerant circuit (11) is implemented as a closed circuit by connecting a compressor (12), a four-way switching valve (13), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) together. The refrigerant circuit (11) is filled with R32 (HFC32, i.e., difluoromethane) as a refrigerant, and is configured to perform a vapor compression refrigeration cycle by allowing the refrigerant to circulate through itself.

[0026] In the refrigerant circuit (11), the four-way switching valve (13) has its fourth port connected to a discharge pipe of the compressor (12), its second port connected to a suction pipe of the compressor (12), its first port connected to an end of the outdoor heat exchanger (14), and its third port connected to an end of the indoor heat exchanger (16). The four-way switching valve (13) is configured to make a switch between a first state where the first and fourth ports communicate with each other and the second and third ports communicate with each other (i.e., the state indicated by the solid curves in FIG. 1) and a second state where the first and second ports communicate with each other and the third and fourth ports communicate with each other (i.e., the state indicated by the broken curves in FIG. 1).

[0027] In the refrigerant circuit (11), if the four-way switching valve (13) is switched to the first state, the refrigerant circulates in a cooling cycle in which the outdoor heat exchanger (14) serves as a condenser and the indoor heat exchanger (16) serves as an evaporator. In the refrigerant circuit (11), if the four-way switching valve (13) is switched to the second state, the refrigerant circulates in a heating cycle in which the indoor heat exchanger (16) serves as a condenser and the outdoor heat exchanger (14) serves as an evaporator. That is, the four-way switching valve (13) is an implementation of a switching mechanism that changes the circulating direction of the refrigerant in the refrigerant circuit (11).

[0028] The compressor (12) is implemented as a variable displacement compressor, of which the operating frequency is adjusted by an inverter circuit. The expansion valve (15) is configured such that its opening degree can be adjusted by a pulse motor. The outdoor heat exchanger (14) is configured to exchange heat between the refrigerant and the outdoor air, and the indoor heat exchanger (16) is configured to exchange heat between the refrigerant and the indoor air.

<Sensor, Controller>

[0029] The air conditioner (10) is provided with various sensors, and a controller (30) that controls the operating frequency of the compressor (12) and the opening degree of the expansion valve (15).

[0030] As shown in FIG. 1, the refrigerant circuit (11) is provided with a discharge pipe temperature sensor (21), an outdoor heat exchanger temperature sensor (22), and an indoor heat exchanger temperature sensor (23). The discharge pipe temperature sensor (21) detects the temperature of the discharge pipe of the compressor (12) (hereinafter referred to as a "discharge pipe temperature Tp"). The discharge pipe temperature Tp corresponds to the temperature of the refrigerant discharged from the compressor (12). The outdoor heat exchanger temperature sensor (22) detects the temperature of the refrigerant in the outdoor heat exchanger (14), and the indoor heat exchanger temperature sensor (23) detects the temperature of the refrigerant in the indoor heat exchanger (16). The temperature detected by the outdoor heat exchanger temperature sensor (22) corresponds to the condensing temperature Tc of the refrigerant during the cooling operation, and the evaporating temperature Te of the refrigerant during the heating operation, respectively. The temperature detected by the indoor heat exchanger temperature sensor (23) corresponds to the evaporating temperature Te of the refrigerant during the cooling operation, and the condensing temperature Tc of the refrigerant during the heating operation, respectively.

[0031] The controller (30) controls the opening degree of the expansion valve (15) every predetermined period (hereinafter referred to as a "sampling time t") during the cooling and heating operations such that the discharge pipe temperature Tp of the compressor (12) reaches a target discharge pipe temperature Tpa. The controller (30) is configured to change the sampling time t according to the present opening range of the expansion valve (15). Such a control of the opening degree will be described in detail later.

-Operation-

[0032] The operation of the air conditioner (10) will be described.

[0033] To start a cooling operation, the four-way switching valve (13) is switched to the first state in the refrigerant circuit (11). During the cooling operation, the refrigerant discharged from the compressor (12) dissipates heat into the outdoor air in the outdoor heat exchanger (14) to condense. The refrigerant condensed has its pressure reduced (i.e., the refrigerant expands) when passing through the expansion valve (15). The refrigerant with such a reduced pressure absorbs heat from the indoor air in the indoor heat exchanger (16) to evaporate, so that the indoor air is cooled and supplied to the room. This allows for cooling the room. The refrigerant evaporated in the indoor heat exchanger (16) is compressed by the compressor (12) and then discharged again.

[0034] To start a heating operation, the four-way switching valve (13) is switched to the second state in the refrigerant circuit (11). During the heating operation, the refrigerant discharged from the compressor (12) dissipates heat into the indoor air in the indoor heat exchanger (16) to condense. As a result, the indoor air is heated. This allows for heating the room. The refrigerant condensed has its pressure reduced (i.e., the refrigerant expands) when passing through the expansion valve (15). The refrigerant with the reduced pressure absorbs heat from the outdoor air in the outdoor heat exchanger (14) to evaporate. The refrigerant evaporated is compressed by the compressor (12) and discharged again.

<Operation of Controller>

[0035] The controller (30) controls the opening degree of the expansion valve (15) every predetermined sampling time t (sec) during the cooling and heating operations such that the discharge pipe temperature Tp of the compressor (12) reaches the target discharge pipe temperature Tpa. Specifically, the controller (30) performs a feedback control on the opening degree of the expansion valve (15) in accordance with the flowchart shown in FIG. 2.

[0036] In Step ST1, a determination is made whether or not the predetermined sampling time t has passed since the expansion valve (15) was driven (i.e., the opening degree of the expansion valve (15) was changed) last time. If the predetermined sampling time t has passed, the process proceeds to Step ST2.

[0037] In Step ST2, the target discharge pipe temperature Tpa is set. According to this embodiment, the target discharge pipe temperature Tpa is set to be such a value that makes the degree of superheat of the refrigerant sucked into the compressor (12) (i.e., the degree of superheat of the refrigerant at outlets of the heat exchangers (14, 16) each serving as an evaporator) a predetermined value. That is, according to this embodiment, controlling the discharge pipe temperature Tp indirectly controls the degree of superheat of the sucked refrigerant.

[0038] Specifically, the controller (30) sets the target discharge pipe temperature Tpa based on the condensing temperature Tc and the evaporating temperature Te which are respectively detected by the outdoor heat exchanger temperature sensor (22) and the indoor heat exchanger temperature sensor (23). For example, the target discharge pipe temperature Tpa is obtained by the following expression:


Where α, β, and γ are predefined coefficients.

[0039] Once the target discharge pipe temperature Tpa has been set in this manner, the process proceeds to Step ST3, in which the controller (30) receives the present discharge pipe temperature Tp measured by the discharge pipe temperature sensor (21).

[0040] In the subsequent step ST4, the magnitude ΔP (pulse) of change in the opening degree of the expansion valve (15) is set to allow the present discharge pipe temperature Tp that has been input to reach or approach the target discharge pipe temperature Tpa. As the opening degree of the expansion valve (15) increases, the amount of the refrigerant circulating in the heat exchangers (14, 16) each serving as an evaporator increases, and therefore, the degree of superheat of the outlet refrigerant decreases. This lowers the discharge pipe temperature Tp. As the opening degree of the expansion valve (15) decreases, the amount of the refrigerant circulating in the heat exchangers (14, 16) each serving as an evaporator decreases, and therefore, the degree of superheat of the outlet refrigerant increases. This raises the discharge pipe temperature Tp.

[0041] Specifically, the controller (30) is provided with a table (fuzzy table) to set the magnitude of change ΔP in opening degree in advance. According to the fuzzy table, the magnitude of change ΔP in opening degree is set according to the deviation of the discharge pipe temperature Tp from the target discharge pipe temperature Tpa, and the variation in discharge pipe temperature Tp per unit time. The controller (30) thus calculates not only the deviation but also the magnitude of variation per the unit time based on the discharge pipe temperature Tp obtained last time during the previous opening degree control and the discharge pipe temperature Tp obtained this time. The controller (30) sets the magnitude of change ΔP in opening degree based on the deviation and the magnitude of variation thus calculated.

[0042] After setting the magnitude of change ΔP in opening degree, the controller (30) drives the expansion valve (15) in Step ST5 such that the opening degree of the expansion valve (15) increases or decreases by the magnitude of change ΔP in opening degree.

[0043] In the next step ST6, a sampling time t is newly set. That is to say, the sampling time t is either maintained or changed. As shown in FIG. 3, the sampling time t is set to be a value that varies according to the size of the opening degree of the expansion valve (15). According to this embodiment, the opening degrees of the expansion valve (15) from the minimum one to the maximum one are classified into three opening degree ranges (namely, a large opening degree range, a medium opening degree range, and a small opening degree range) as shown in FIG. 4. The large opening degree range is a range in which the opening degree is equal to or more than a first predetermined value Px and equal to or less than the maximum opening degree, the middle opening degree range is a range in which the opening degree is equal to or more than a second predetermined value Py and less than the first predetermined value Px, and the small opening degree range is a range in which the opening degree is equal to or more than the minimum opening degree and less than the second predetermined value Py.

[0044] In Step ST6, the sampling time t is set to be "ta (sec)" if the present opening degree P of the expansion valve (15) falls within the large opening degree range. The sampling time t is set to be "tb (sec)" if the present opening degree P falls within the medium opening degree range. The sampling time t is set to be "tc (sec)" if the present opening degree P falls within the small opening degree range. The present opening degree P of the expansion valve (15) refers herein to the opening degree of the expansion valve (15) that has already been driven in Step ST5 (after the opening degree P has been changed). The magnitudes of ta, tb, and tc satisfy ta < tb < tc.

[0045] As can be seen, in performing the opening degree control on the expansion valve (15) according to this embodiment, the sampling time t becomes longer in the range where the opening degree P of the expansion valve (15) is less than the first predetermined value Px than in the range where the opening degree P is equal to or more than the first predetermined value Px. Furthermore, according to this embodiment, as the opening degree P of the expansion valve (15) decreases within the range where the opening degree P is less than the first predetermined value Px, the sampling time t gradually extends. That is, according to this embodiment, the smaller the opening degree P of the expansion valve (15) is, the longer the sampling time t is set to be.

[0046] Also, as shown in FIG. 4, the expansion valve (15) of this embodiment has such a characteristic that once its opening degree P has become less than the first predetermined value Px, the flow rate of the refrigerant flowing through the expansion valve (15) varies less steeply with respect to a change of the same magnitude in the opening degree P. That is, in the medium and small opening degree ranges, even a change in the opening degree P of the expansion valve (15) by the same magnitude ΔP causes a smaller variation in the flow rate of the refrigerant. Speaking more specifically, in performing the opening degree control on the expansion valve (15) according to this embodiment, the opening degree of the expansion valve (15) at which the relationship between the opening degree P and the flow rate of the refrigerant changes is set to be the first predetermined value Px.

[0047] If a sampling time t is newly set in Step ST6, the process goes back to Step ST1 to start the next process of opening degree control. Specifically, a determination is made in Step ST1 whether or not the sampling time t newly set has passed since the expansion valve (15) was driven. If the answer is YES, the process proceeds to Step ST2 and the same series of steps will be performed all over again.

[0048] If the opening degree P of the expansion valve (15) falls within one of the smaller opening degree ranges (namely, either the medium opening degree range or the small opening degree range), the rate of the refrigerant flowing through the expansion valve (15) decreases, and eventually, the amount of the refrigerant circulating through the refrigerant circuit (11) decreases. According to this embodiment, the use of R32 as the refrigerant causes a significant decrease in the amount of the refrigerant circulating in the range where the open degree P of the expansion valve (15) is small. In the opening degree range where a small amount of the refrigerant circulates, even a change in the opening degree P of the expansion valve (15) does not cause the discharge pipe temperature Tp to rise or fall so smoothly, and it takes some time for the discharge pipe temperature Tp to reach the target discharge pipe temperature Tpa. In that case, if the opening degree is controlled for the same sampling time t as in the range where the opening degree P of the expansion valve (15) is large (i.e., the large opening degree range), the following phenomenon occurs. Specifically, even though actually the opening degree P of the expansion valve (15) has already been controlled into an appropriate one, a determination is made that there is still a significant difference between the discharge pipe temperature Tp and the target discharge pipe temperature Tpa, and the opening degree P of the expansion valve (15) is further changed unnecessarily. That is, the next control of the opening degree is performed during a transitional period in which the discharge pipe temperature Tp is changing toward the target discharge pipe temperature Tpa. This causes the discharge pipe temperature Tp to exceed or falls short of the target discharge pipe temperature Tpa over and over again, i.e., causes hunching.

[0049] On the other hand, with the opening degree control of the expansion valve (15) according to this embodiment, the smaller the opening degree P of the expansion valve (15), the longer the sampling time t. This allows for performing the next control of the opening degree after the opening degree P of the expansion valve (15) has been changed to make the discharge pipe temperature Tp reach (or approach) the target discharge pipe temperature Tpa. That is, the discharge pipe temperature Tp can be made to reach (or approach) the target discharge pipe temperature Tpa and get stabilized by the start of the next control of the opening degree.

-Advantages of Embodiment-

[0050] As can be seen from the foregoing description, according to this embodiment, the sampling time t for the opening degree control (one period of the opening degree control) is set to be longer in the range where the opening degree P of the expansion valve (15) is less than the predetermined value (i.e., the first predetermined value Px) than in the range where the opening degree P is equal to or more than the predetermined value (i.e., the first predetermined value Px). Therefore, even if a relatively small amount of the refrigerant circulates through the refrigerant circuit (11), the discharge pipe temperature Tp can still be made to reach (or approach) the target discharge pipe temperature Tpa in the interval after a change of the opening degree of the expansion valve (15) and before the start of the next control of the opening degree. That is, it is not until the discharge pipe temperature Tp is stabilized that the next control of the opening degree may be started. Accordingly, the next control of the opening degree allows for appropriately detecting the deviation of the discharge pipe temperature Tp from the target discharge pipe temperature Tpa, and allows for appropriately setting the magnitude of change ΔP in opening degree. This can prevent the discharge pipe temperature Tp from exceeding or falling short of the target discharge pipe temperature Tpa. As a result, the discharge pipe temperature Tp can be prevented from causing hunching, and can be controlled stably.

[0051] Furthermore, according to this embodiment, the sampling time t is set to be even longer in the opening degree range in which the opening degree is less than the second predetermined value Py that is smaller than first predetermined value Px. That is to say, in the range where the opening degree P of the expansion valve (15) is less than the predetermined value (i.e., the first predetermined value Px), as the opening degree P decreases, the sampling time t for the opening degree control extends gradually. That is why even if the amount of the refrigerant circulating is approaching the lowest level, the discharge pipe temperature Tp can be made to reach (or approach) the target discharge pipe temperature Tpa just as intended by the start of the next control of the opening degree. Consequently, the discharge pipe temperature Tp can be controlled with good stability and high reliability.

[0052] In the range where the opening degree P of the expansion valve (15) is less than the predetermined value (i.e., the first predetermined value Px), the amount of the refrigerant circulating through the refrigerant circuit (11) does not vary so significantly (see FIG. 4) due to the characteristic of the expansion valve (15), considering the magnitude of change ΔP in the opening degree. Thus, in the range where the opening degree P of the expansion valve (15) is less than the predetermined value (i.e., the first predetermined value Px), it takes even a longer time for the discharge pipe temperature Tp to reach (or approach) the target discharge pipe temperature Tpa. With the opening degree control according to this embodiment, the sampling time t for the opening degree control is extended adaptively to the range where the opening degree is less than the predetermined value (i.e., the first predetermined value Px). This effectively prevents the discharge pipe temperature Tp from exceeding or falling short of the target discharge pipe temperature Tpa. Therefore, the discharge pipe temperature Tp can be effectively prevented from causing hunching.

«Second Embodiment»

[0053] A second embodiment of the present invention will be described. This embodiment is a modification of the air conditioner (10) of the first embodiment. Specifically, according to this embodiment, the opening degree control of the expansion valve (15) is carried out differently from in the first embodiment. Specifically, although the sampling time t is set to be longer in the range where the opening degree P of the expansion valve (15) is less than the predetermined value in the first embodiment, the sampling time t is constant in that range but the magnitude of change ΔP in opening degree is decreased in this embodiment.

[0054] The controller (30) of this embodiment controls the opening degree of the expansion valve (15) in accordance with the flowchart shown in FIG. 5. The control operations in Steps ST1-ST3 are performed in the same or similar way as their counterparts of the first embodiment.

[0055] In Step ST4, the magnitude of change ΔP (pulse) in the opening degree of the expansion valve (15) is set to allow the present discharge pipe temperature Tp to reach (or approach) the target discharge pipe temperature Tpa as in the first embodiment described above. The controller (30) is provided with a fuzzy table in advance in which the magnitude of change ΔP in opening degree is set according to the deviation of the discharge pipe temperature Tp from the target discharge pipe temperature Tpa and the variation in discharge pipe temperature Tp per unit time.

[0056] In the fuzzy table according to this embodiment, the magnitude of change ΔP in opening degree has a value that varies according to the opening degree range of the expansion valve (15), as shown in FIG. 6. The opening degrees of the expansion valve (15) are classified into the three opening degree ranges, namely, a large opening degree range, a medium opening degree range, and a small opening degree range, as in the first embodiment. In Step ST4, the magnitude of change ΔP in opening degree is set to be "ΔPa (pulse)" if the present opening degree P of the expansion valve (15) falls within the large opening degree range. The magnitude of change ΔP in opening degree is set to be "ΔPb (pulse)" if the present opening degree P falls within the medium opening degree range. The magnitude of change ΔP in opening degree is set to be "ΔPc (pulse)" if the present opening degree P falls within the small opening degree range. These magnitudes ΔPa, ΔPb, and ΔP satisfy ΔPa > ΔPb > ΔPc.

[0057] As can be seen, in performing the opening degree control on the expansion valve (15) according to this embodiment, the magnitude of change ΔP in opening degree becomes smaller in the range where the opening degree P of the expansion valve (15) is less than the first predetermined value Px than in the range where the opening degree P is equal to or more than the first predetermined value Px. Furthermore, according to this embodiment, as the opening degree P of the expansion valve (15) decreases within the range where the opening degree P is less than the first predetermined value Px, the magnitude of change ΔP in opening degree decreases gradually. That is, according to this embodiment, the smaller the opening degree P of the expansion valve (15) is, the smaller the magnitude of change ΔP in opening degree is set to be. Therefore, according to this embodiment, even if the deviation of the discharge pipe temperature Tp from the target discharge pipe temperature Tpa and the variation in discharge pipe temperature Tp per unit time remain unchanged, the smaller the opening degree P of the expansion valve (15), the smaller the magnitude of change ΔP in opening degree.

[0058] After setting the magnitude of change ΔP in opening degree in Step ST4, the controller (30) drives the expansion valve (15) in Step ST5 such that the opening degree of the expansion valve (15) increases or decreases by the magnitude of change ΔP in opening degree. When the expansion valve (15) is driven, the process goes back to Step ST1 to perform the next control of the opening degree.

[0059] With the opening degree control of the expansion valve (15) according to the embodiment, the magnitude of change ΔP in opening degree is set to be smaller in the range where the opening degree P of the expansion valve (15) is less than the predetermined value (i.e., the first predetermined value Px) than in the range where the opening degree P of the expansion valve (15) is equal to or more than the predetermined value (i.e., the first predetermined value Px). Thus, if a relatively small amount of the refrigerant circulates through the refrigerant circuit (11), the variation in discharge pipe temperature Tp per opening degree control can be reduced. This can prevent the discharge pipe temperature Tp from exceeding or falling short of the target discharge pipe temperature Tpa, since the discharge pipe temperature Tp never rises or falls significantly. As a result, the discharge pipe temperature Tp can be prevented from causing hunching, and can be controlled stably. Other functions and effects of this embodiment are the same as or similar to those of the first embodiment.

«Other Embodiments»

[0060] Each of the embodiments according to the present invention may be modified in the following manner.

[0061] For example, in the first embodiment, in addition to controlling the opening degree such that the sampling time t increases as the opening degree P of the expansion valve (15) decreases, the opening degree may also be controlled such that the magnitude of change ΔP in opening degree decreases as the opening degree P of the expansion valve (15) decreases as in the second embodiment. This can prevent the discharge pipe temperature Tp from exceeding or falling short of the target discharge pipe temperature Tpa just as intended. Accordingly, the discharge pipe temperature Tp can be controlled with more stability.

[0062] According to each of the embodiments described above, the opening degrees of the expansion valve (15) are supposed to be classified into three opening degree ranges. Alternatively, the opening degrees may also be classified into two ranges or four or more ranges. If the opening degrees are classified into two ranges, it is preferable to omit the second predetermined value Py from the first predetermined value Px and the second predetermined value Py, considering the characteristic of the expansion valve (15) (i.e., the relationship between the opening degree and the flow rate of the refrigerant).

[0063] The air conditioner (10) according to each of the embodiments described above may be capable of performing only one of the cooling and heating operations.

INDUSTRIAL APPLICABILITY

[0064] As can be seen from the foregoing description, the present invention is useful as an air conditioner including a refrigerant circuit through which R32 circulates as a refrigerant and which performs a vapor compression refrigeration cycle.

DESCRIPTION OF REFERENCE CHARACTERS

[0065] 

10

air conditioner

11

refrigerant circuit

12

compressor

14

outdoor heat exchanger

15

expansion valve

16

indoor heat exchanger

30

controller

Claims

1. An air conditioner comprising:

a refrigerant circuit (11) in which a compressor (12), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) are connected together, and through which HFC32 circulates as a refrigerant to perform a refrigeration cycle; and

a controller (30) which performs an opening degree control every predetermined period to change the opening degree of the expansion valve (15) to a predetermined extent such that the temperature of the refrigerant discharged from the compressor (12) reaches a target temperature, wherein

the controller (30) is configured such that the predetermined period is longer in a range where the opening degree of the expansion valve (15) is less than a predetermined value than in a range where the opening degree is equal to or more than the predetermined value.

2. An air conditioner, comprising:

a refrigerant circuit (11) in which a compressor (12), an outdoor heat exchanger (14), an expansion valve (15), and an indoor heat exchanger (16) are connected together, and through which HFC32 circulates as a refrigerant to perform a refrigeration cycle; and

a controller (30) which performs an opening degree control every predetermined period to change the opening degree of the expansion valve (15) to a predetermined extent such that the temperature of the refrigerant discharged from the compressor (12) reaches a target temperature, wherein

the controller (30) is configured such that the predetermined extent is smaller in a range where the opening degree of the expansion valve (15) is less than a predetermined value than in a range where the opening degree is equal to or more than the predetermined value.

3. The air conditioner of claim 1, wherein
the controller (30) is configured such that the predetermined extent is smaller in the range where the opening degree of the expansion value (15) is less than the predetermined value than in the range where the opening degree is equal to or more than the predetermined value.

4. The air conditioner of claim 1 or 3, wherein
the controller (30) is configured such that in the range where the opening degree of the expansion valve (15) is less than the predetermined value, as the opening degree decreases, the predetermined period extends gradually.

5. The air conditioner of any one of claims 1-4, wherein
once the opening degree of the expansion value (15) has become less than the predetermined value, the rate at which the refrigerant flows through the expansion valve (15) varies less steeply with respect to a change of the same magnitude in the opening degree of the expansion valve (15).

Drawing