SE1650785A1

SE1650785A1 - Air conditioner

Info

Publication number: SE1650785A1
Application number: SE1650785A
Authority: SE
Inventors: Rokkaku Yuichi; Taketa Atsushi; Ueno Madoka; IIO Kazushi; Nishimura Tatsuo
Original assignee: Sharp Kabushiiki Kaisha
Priority date: 2013-12-06
Filing date: 2014-08-08
Publication date: 2016-06-03
Also published as: MY174179A; JP2015111020A; CN105814373B; WO2015083399A1; SE542540C2; CN105814373A; JP6321363B2

Abstract

To provide an air conditioner including a refrigerant circuit formed by connecting a compressora condenser, a restriction device, and an evaporator to each other. When the amount of refrigerant circulating through the refrigerant circuit is large, the air conditioner performs suction superheat degree control which operates the restriction device on the basis of the suction temperature of the refrigerant sucked by the compressorWhen the amount of refrigerants circulating through the refrigerant circuit is small, the air conditioner performs suction superheat degree control until the refrigeration cycle is stabilized. After the refrigeration cycle is stabilized, the air conditioner performs discharge superheat degree control which operates the restriction device on the basis of the discharge temperature of the refrigerant discharged from compressorThereby, the air conditioner can perform efficient air conditioning operation irrespective of the amount of refrigerant circulating through the refrigerant circuit.

Description

Description Title of Invention: AIR CONDITIONER Technical Field[0001] The present invention relates to an air conditionercapable of performing stable air conditioning operation by adjusting the amount of circulating refrigerant.

Background Art[0002] In an air conditioner, a compressor, a condenser, arestriction device, and an evaporator are connected toeach other to form a refrigerant circuit. A refrigerantcirculates through the refrigerant circuit, and therebyair conditioning operation, such as cooling, heating ordehumidification, is performed. Here, an opening degreeof an expansion valve as the restriction device iscontrolled according to a suction superheat degreeobtained from the temperature of the evaporator and theoutlet temperature of the evaporator. Thereby, theamount of circulating refrigerant is properly adjusted,so that the air conditioning operation is efficientlyperformed.

[0003] As another control of the expansion valve, in PatentLiterature l, the difference between the dischargetemperature of the compressor, as a discharge superheatdegree, and the temperature of the condenser iscalculated, and then the opening degree of the expansionvalve is controlled on the basis of the calculatedtemperature difference and a target discharge temperaturedifference. The target discharge temperature differenceis calculated from the number of revolutions of thecompressor. The amount of circulating refrigerant isdetermined according to the number of revolutions of thecompressor. The opening degree of the expansion valve iscontrolled on the basis of the discharge temperaturedifference determined according to the number ofrevolutions of the compressor, and thereby the amount of circulating refrigerant is properly adjusted.

Citation List Patent Literature

[0004] Patent Literature l: Japanese Patent Laid-Open No. 2011- l22756 Summary of InventionTechnical Problem

[0005] In the control of the expansion valve based on thedischarge superheat degree, when the operation is started,the discharge temperature is not stabilized until thecompressor is warm. Therefore, it takes time until thedischarge temperature is stabilized, and hence thecontrol of the expansion valve cannot be performed duringthis time period, during which the air conditioner may beinefficiently operated.

[0006] On the other hand, when the expansion valve iscontrolled on the basis of the suction superheat degree,the discharge temperature can be stabilized in a shorttime. However, in the case where, when the amount ofcirculating refrigerant is small, the air conditioner isoperated most efficiently, the temperature differencebetween the temperature of the evaporator, and thesuction temperature of the refrigerant sucked into thecompressor is small. Therefore, the suction superheatdegree is small, so that it is difficult to control theexpansion valve.

[0007] In view of the above, it is an object of the presentinvention to provide an air conditioner capable ofquickly performing efficient air conditioning operationregardless of large or small of the amount of circulating refrigerant.

Solution to Problem[0008] An air conditioner of the present invention isprovided with a refrigerant circuit which is formed byconnecting a compressor, a condenser, a restrictiondevice, and an evaporator to each other, and a controlapparatus which controls the operation of the restrictiondevice according to the amount of refrigerant circulatingthrough the refrigerant circuit. The control apparatusperforms optimum air conditioning operation bydetermining the operating state and performing switchingbetween suction superheat degree control which operatesthe restriction device on the basis of the suctiontemperature of the refrigerant sucked into the compressor,and discharge superheat degree control which operates therestriction device on the basis of the dischargetemperature of the refrigerant discharged from thecompressor.

[0009] That is, when the amount of circulating refrigerantis large, the control apparatus performs the suctionsuperheat degree control which operates the restrictiondevice on the basis of the suction temperature of therefrigerant sucked into the compressor. When the amountof circulating refrigerant is small, the controlapparatus performs discharge superheat degree control which operates the restriction device on the basis of the discharge temperature of the refrigerant discharged fromthe compressor.[0010] The control apparatus determines the operating stateon the basis of whether the amount of circulatingrefrigerant is large or small. The control apparatusperforms switching between the superheat degree controlson the basis of the amount of refrigerant circulatingthrough the refrigerant circuit. When the amount ofcirculating refrigerant is large, the control apparatusperforms the suction superheat degree control, to quicklystabilize the refrigeration cycle. When the amount ofcirculating refrigerant is small, the difference betweenthe suction temperature and the evaporation temperatureis small, and hence it is difficult that the controlapparatus performs the superheat degree control on thebasis of the suction temperature. Therefore, when theamount of circulating refrigerant is small, the controlapparatus performs the discharge superheat degree control,to surely perform the superheat degree control. As aresult, efficient air conditioning operation can beperformed regardless of large or small of the amount ofcirculating refrigerant.

[0011] It is preferred that, when the amount of circulating refrigerant is small, the control apparatus performs the suction superheat degree control until the refrigeration cycle is stabilized, and that, after the refrigerationcycle is stabilized, the control apparatus performs thedischarge superheat degree control. Thereby, when airconditioning operation is started, the suction superheatdegree control is first performed, and hence therefrigeration cycle is quickly stabilized. The dischargesuperheat degree control is performed in the state wherethe refrigeration cycle is stabilized, and henceefficient air conditioning operation can be performedeven when the amount of circulating refrigerant is small.[0012] It is preferred that, when the refrigeration cyclebecomes unstable in the state where the control apparatusperforms the discharge superheat degree control, thecontrol apparatus performs the suction superheat degreecontrol. Thereby, in the state where the refrigerationcycle is unstable, when the discharge superheat degreecontrol is continued, the time until the refrigerationcycle is stabilized is increased. At this time, byswitching to the suction superheat degree control, therefrigeration cycle is quickly stabilized.

[0013] It is preferred that the control apparatus comparesa target suction superheat degree with a real suctionsuperheat degree, and determines whether or not therefrigeration cycle is unstable. Thereby, when the refrigeration cycle becomes unstable, the fluctuation of the suction superheat degree becomes remarkable, andhence it is possible to quickly detect that therefrigeration cycle is unstable.

[0014] It is preferred that, when the real suctionsuperheat degree fluctuates around the target suctionsuperheat degree, the control apparatus determines thatthe refrigeration cycle is stabilized. Thereby, when thehunting state of the real suction superheat degree iscontinued, it is forever difficult for the controlapparatus to determine whether or not the refrigerationcycle is stabilized. Therefore, in this state, when thecontrol apparatus determines that the refrigeration cycleis stable, the control apparatus can quickly switch fromthe suction superheat degree control to the dischargesuperheat degree control.

[0015] It is preferred that, as the superheat degreecontrol at the time when the amount of circulatingrefrigerant is large, the control apparatus performs thedischarge temperature control which operates therestriction device so that the discharge temperatureapproaches a set temperature, and determines theoperating state on the basis of the discharge temperature,and performs switching between the discharge temperaturecontrol and the suction superheat degree control according to the discharge temperature. Thereby, it is possible to prevent that the discharge temperaturebecomes high when the discharge temperature control isperformed.

[0016] It is preferred that, when the discharge temperatureis high, the control apparatus performs the dischargetemperature control, and that, when the dischargetemperature is low, the control apparatus performs thesuction superheat degree control. Thereby, when thedischarge temperature is low, and when the controlapparatus performs the suction superheat degree control,the suction superheat degree can be made to approach thetarget suction superheat degree, so that efficient airconditioning operation can be performed. In this state,when the discharge temperature is high, the controlapparatus performs the discharge temperature control.When the superheat degree control is switched in this way,the discharge temperature can be prevented from becominghigh, while efficient air conditioning operation isperformed.

[0017] It is preferred that, when the difference betweenthe target suction superheat degree and the real suctionsuperheat degree is small, the control apparatus performsthe discharge temperature control, and that, when thedifference between the target suction superheat degree and the real suction superheat degree is large, the control apparatus performs the suction superheat degreecontrol. Thereby, if the suction superheat degreecontrol is performed when the difference between thetarget suction superheat degree and the real suctionsuperheat degree is large, the difference becomes smaller,and the discharge temperature control is performed.Thereby, while the real suction superheat degree ismaintained close to the target suction superheat degree,air conditioning operation can be performed so that thedischarge temperature does not become high.

[0018] It is preferred that the control apparatusdetermines a discharge temperature determinationexpression on the basis of the evaporation temperature,the condensation temperature, and the suction superheatdegree, and that, on the basis of a discharge temperaturecalculated by the determination expression, the controlapparatus performs the discharge temperature control sothat the discharge temperature does not exceed an upperlimit value. In this way, the discharge temperaturecontrol is performed on the basis of the dischargetemperature obtained by the determination expression, andthereby the discharge temperature can be controlled sothat the real discharge temperature does not exceed theupper limit value of the discharge temperature.

[OOl9] It is preferred that, when the discharge temperaturecalculated by the determination expression is high, thecontrol apparatus performs the discharge temperaturecontrol, and that when the discharge temperaturecalculated by the determination expression is low, thecontrol apparatus performs the suction superheat degreecontrol. In this way, when the discharge temperature ishigh, the discharge temperature control is performed, andthereby the discharge temperature is prevented frombecoming high.

[0020] In order to reduce the air conditioning capacity atthe time when the compressor is operated at the minimumnumber of revolutions, it is preferred that the controlapparatus performs setting so that the target dischargesuperheat degree corresponding to the minimum number ofrevolutions of the compressor is lower than the usualtarget discharge superheat degree, and that the control apparatus performs the discharge superheat degree control on the basis of the set target discharge superheat degree.

[0021] In this case, the discharge superheat degree controlis performed so that the discharge superheat degreereaches the target discharge superheat degree, but thetarget discharge superheat degree is set low. Therefore,when the compressor is operated at the minimum number of revolutions, it is difficult to reach the target discharge superheat degree. For this reason, when thecompressor is operated at the minimum number of revolutions, the air conditioning capacity is reduced.

Advantageous Effects of Invention[0022] With the present invention, the degree of superheatcan be brought close to the target degree of superheatregardless of large or small of the amount of refrigerantcirculating through the refrigerant circuit, and henceefficient air conditioning operation can be always performed.

Brief Description of Drawings

[0023] [Figure l] Figure l is a view showing a schematicconfiguration of a refrigeration cycle of an airconditioner of the present invention.

[Figure 2] Figure 2 is a control block diagram of the airconditioner.

[Figure 3] Figure 3 is a flowchart of the airconditioning operation by superheat degree control of afirst embodiment.

[Figure 4] Figure 4 is a flowchart of determinationprocessing for suction superheat degree control and discharge superheat degree control.

[Figure 5] Figure 5 is a flowchart of the suctionsuperheat degree control.

[Figure 6] Figure 6 is a flowchart of the dischargesuperheat degree control.

[Figure 7] Figure 7 is a flowchart of air conditioningoperation by discharge superheat degree control of asecond embodiment.

[Figure 8] Figure 8 is a flowchart for determining thestability of a refrigeration cycle.

[Figure 9] Figure 9 is a flowchart of a third embodimentfor determining the stability of refrigeration cycle whenhunting is caused.

[Figure 10] Figure 10 is a flowchart when therefrigeration cycle is unstable while discharge superheatdegree control of a fourth embodiment is performed.[Figure 11] Figure 11 is a flowchart for determining thesuperheat degree control on the basis of the amount ofcirculating refrigerant in a fifth embodiment.

[Figure 12] Figure 12 is a flowchart of determinationprocessing for the suction superheat degree control anddischarge temperature control.

[Figure 13] Figure 13 is a flowchart of the dischargetemperature control.

[Figure 14] Figure 14 is a flowchart of determinationprocessing of a sixth embodiment for the suctionsuperheat degree control and the discharge temperature control.

[Figure 15]temperature[Figure 16]temperature[Figure 17]temperature[Figure 18]temperature[Figure 19]temperature[Figure 20] temperature Figure 15 is a flowchart of dischargecontrol of a seventh embodiment.Figure 16 is a flowchart of dischargecontrol of an eighth embodiment.Figure 17 is a flowchart of dischargecontrol of a ninth embodiment.

Figure 18 is a flowchart of dischargecontrol of a tenth embodiment.

Figure 19 is a flowchart of dischargecontrol of an eleventh embodiment.Figure 20 is a flowchart of discharge control of a twelfth embodiment.

[Figure 21] Figure 21 is a view showing a conventionalrelationship between the number of revolutions of acompressor and a discharge superheat degree.

[Figure 22] Figure 22 is a view showing a relationshipbetween the number of revolutions of the compressor andthe discharge superheat degree according to a thirteenth embodiment.

Description of Embodiments[0024] Figure 1 shows an air conditioner of a firstembodiment. The air conditioner is configured byconnecting an outdoor unit 1 to an indoor unit 2 by pipesand wirings.

The outdoor unit 1 includes a compressor 3, a four-way valve 4, an outdoor heat exchanger 5, an expansion valve 6, and an outdoor fan 7. The indoor unit 2 includes an indoor heat exchanger 8 and an indoor fan 9.

The compressor 3, the four-way valve 4, the outdoor heatexchanger 5, the expansion valve 6, and the indoor heatexchanger 8 are connected to each other by pipes to forma refrigerant circuit. In order to connect the pipe ofthe indoor unit 2 to the pipe of the outdoor unit l, theoutdoor unit is provided with a two-way valve l0 and athree-way valve ll. The expansion valve 6 is connectedto the indoor heat exchanger 8 by the pipe via the two-way valve 10. The four-way valve 4 is connected to theindoor heat exchanger 8 by the pipe via the three-wayvalve ll.

[0025] When the compressor 3 is driven, a refrigerantcirculates through the refrigerant circuit. Theexpansion valve 6 is adjusted to open in stages by beingdriven by a stepping motor, and to perform functions, asa restriction device, for adjusting reduction of and theamount of the circulating refrigerant. The refrigerantcirculates through the refrigerant circuit, and therebythe refrigeration cycle is formed. In the refrigerationcycle at the time of cooling operation, the refrigerantcirculates through the compressor 3, the four-way valve 4,the outdoor heat exchanger 5, the expansion valve 6, andthe indoor heat exchanger 8 in this order. The outdoor heat exchanger 5 functions as a condenser, and the indoor heat exchanger 8 functions as an evaporator. In therefrigeration cycle at the time of heating operation, therefrigerant circulates through the compressor 3, thefour-way valve 4, the indoor heat exchanger 8, theexpansion valve 6, and the outdoor heat exchanger 5 inthis order. The indoor heat exchanger 8 functions as acondenser, and the outdoor heat exchanger 5 functions asan evaporator. It should be noted that, instead of theexpansion valve 6, a capillary tube, and the like, can beused as the restriction device, and the amount ofcirculating refrigerant can be adjusted by changing thecombination of the plurality of capillary tubes.

[0026] In addition, as shown in Figure 2, the airconditioner includes a control apparatus 12 whichperforms air conditioning operations, such as cooling,heating and dehumidification, by controlling therefrigeration cycle. Further, the air conditionerincludes a room temperature detector 13, an outdoortemperature detector 14, a discharge temperature detector15 detecting the discharge temperature of the refrigerantdischarged from the compressor 3, a suction temperaturedetector 16 detecting the intake temperature (suctiontemperature) of the refrigerant sucked into thecompressor 3, a first temperature detector 17 detectingthe temperature of the outdoor heat exchanger 5, and a second temperature detector 18 detecting the temperature of the indoor heat exchanger 8. A temperature sensor,such as thermistor, is used in each of the temperaturedetectors 13 to 18.

[0027] It should be noted that a temperature detectordetecting the temperature of the refrigerant flowingthrough the pipe between the expansion valve 6 and theindoor heat exchanger 8 is used as the second temperaturedetector 18. The second temperature detector 18 isarranged between the expansion valve 6 and the two-wayvalve 10, and all the other temperature detectors exceptthe room temperature detector 13 are provided in theoutdoor unit 1. The temperature of the refrigerantflowing through the indoor heat exchanger 8 at the timeof cooling operation is the same as the temperature ofthe refrigerant passing through the expansion valve 6.Therefore, instead of providing the temperature detectordetecting the temperature of the indoor heat exchanger 8,the temperature of the indoor heat exchanger 8 can bedetected by detecting the temperature of the refrigerantby a temperature detector provided between the expansionvalve 6 and the indoor heat exchanger 8.

[0028] According to an instructed operation mode and on thebasis of the temperature detected by each of thetemperature detectors 13 to 18, the control apparatus 12 controls each of the number of revolutions (operation frequency) of the compressor 3, the opening degree of theexpansion valve 6, the number of revolutions of theoutdoor fan 7, and the number of revolutions of theindoor fan 9. It should be noted that the controlapparatus 12 is configured by an indoor control sectionprovided in the indoor unit 2, and an outdoor controlsection provided in the outdoor unit 1. The indoorcontrol section and the outdoor control section areconnected to be communicable to each other. The indoorcontrol section and the outdoor control section cooperatewith each other to control the operations of the indoorunit 2 and the outdoor unit 1. The outdoor controlsection collects the detection signals received from theplurality of temperature detectors 14 to 18 to transmitthe collected signals to the indoor control section, andthe indoor control section manages the detectedtemperature information.

[0029] The operation frequency of the compressor 3 is controlled in stages on the basis of frequency codes (FD).

The frequency codes are allocated to a plurality ofstages respectively correspond to operation frequencies.The higher frequency code corresponds to the higheroperation frequency. Each of the frequency codescorresponds to each of the number of revolutions. Thecontrol apparatus 12 selects a frequency code according to a control temperature determined on the basis of a room temperature and a set temperature, and then outputsthe frequency code to a driver of the compressor 3. Thedriver drives the compressor 3 at the operation frequencycorresponding to the frequency code. When the controlapparatus 12 outputs a command to the expansion valve 6,the expansion valve 6 is opened to the command openingdegree. The amount of refrigerant passing through theexpansion valve 6 is changed according to the openingdegree.

[0030] The control apparatus 12 determines the number ofrevolutions of the compressor 3 on the basis of a load ata set temperature set by a user, or a load at a settemperature set beforehand in an automatic operation mode,or a load at a detected temperature such as a roomtemperature and an outside air temperature. Then, thecontrol apparatus 12 determines the number of revolutionsof the indoor fan 9 corresponds to the number ofrevolutions of the compressor 3. The control apparatus12 controls the compressor 3 at the determined number ofrevolutions, and changes the number of revolutions of thecompressor 3 according to the room temperature. Also,the control apparatus 12 controls the indoor fan 9 on thebasis of the number of revolutions according to thenumber of revolutions of the compressor 3. Further, the control apparatus 12 determines the opening degree of the expansion valve 6 according to the determined number ofrevolutions of the compressor 3.[0031] When the air conditioning operation is performed,the refrigerant, whose amount corresponds to the numberof revolutions of the compressor 3, circulates throughthe refrigerant circuit. At the time of coolingoperation, the first temperature detector 17 detects thetemperature of the outdoor heat exchanger 5 as thecondenser, that is, the condensation temperature, whilethe second temperature detector 18 detects thetemperature of the indoor heat exchanger 8 as theevaporator, that is, the evaporation temperature. At thetime of heating operation, the first temperature detector17 detects the evaporation temperature, and the secondtemperature detector 18 detects the condensationtemperature.

[0032] In order to realize an efficient refrigeration cycle,the air conditioner controls the refrigeration cycle sothat the degree of superheat of the refrigerant becomes atarget degree of superheat according to the amount ofrefrigerant circulating through the refrigerant circuit.That is, the control apparatus 12 adjusts the amount ofcirculating refrigerants by controlling the opening degree of the expansion valve 6 according to the degree of superheat, and thereby the degree of superheat becomesthe target degree of superheat.[0033] Here, the control apparatus 12 determines theoperating state, and then performs air-conditioningoperation by switching between suction superheat degreecontrol for operating the expansion valve 6 on the basisof the suction temperature of the refrigerant sucked intothe compressor 3, and discharge superheat degree controlfor operating the expansion valve 6 on the basis of thedischarge temperature of the refrigerant breathed fromthe compressor 3. In the suction superheat degreecontrol, the control apparatus 12 controls the openingdegree of the expansion valve 6 so that the temperaturedifference between the suction temperature and theevaporation temperature approaches a predetermined value.In the discharge superheat degree control, the controlapparatus 12 controls the opening degree of the expansionvalve 6 so that the temperature difference between thedischarge temperature and the condensation temperatureapproaches a predetermined value.

[0034] The suction superheat degree is the differencebetween the suction temperature and the evaporationtemperature, and the target suction superheat degree isthe suction superheat degree set according to the amount of circulating refrigerant. When the temperature difference between the suction temperature and theevaporation temperature approaches the predeterminedvalue, the suction superheat degree calculated during theair conditioning operation approaches the target suctionsuperheat degree. The discharge superheat degree is thedifference between the discharge temperature and thecondensation temperature, and the target dischargesuperheat degree is the discharge superheat degree setaccording to the amount of circulating refrigerant. Whenthe temperature difference between the dischargetemperature and the condensation temperature approachesthe predetermined value, the discharge superheat degreecalculated during the air conditioning operationapproaches the target discharge superheat degree.

[0035] Then, on the basis of the amount of refrigerantcirculating through the refrigerant circuit, the controlapparatus 12 determines the operating state, and switchesthe superheat degree control according to the amount ofthe circulating refrigerants. The suction superheatdegree control can immediately respond to a change of therefrigeration cycle and quickly stabilizes therefrigeration cycle. On the other hand, since, when theamount of the circulating refrigerant is small, thechange of the temperature of refrigerant is small, it isdifficult that the refrigeration cycle is controlled by the suction superheat degree control. That is, when the amount of circulating refrigerant is large, the suctionsuperheat degree control is suitable. When the amount ofcirculating refrigerant is small, the discharge superheatdegree control is suitable.

[0036] As shown in Figure 3, when air conditioningoperation is performed, the control apparatus l2determines the number of revolutions of the compressor 3on the basis of the difference between the settemperature and the room temperature, and a load, such asthe outside air temperature (Sl). The number ofrevolutions of the compressor 3 corresponds to the amountof refrigerant circulating through the refrigerantcircuit. As the number of revolutions of the compressor3 is increased, the amount of circulating refrigerant isincreased, while as the number of revolutions of thecompressor 3 is reduced, the amount of circulatingrefrigerant is reduced. On the basis of the determinednumber of revolutions of the compressor 3, the controlapparatus l2 performs determination processing fordetermining the superheat degree control to be performed(S2). When the number of revolutions of the compressor 3is high, that is, when the amount of circulatingrefrigerant is large, the control apparatus l2 performsthe suction superheat degree control. (S3). When thenumber of revolutions of the compressor 3 is low, that is, when the amount of circulating refrigerant is small, the control apparatus 12 performs the discharge superheatdegree control (S4).[0037] In the determination processing, as shown in Figure4, the control apparatus 12 checks whether or not thedetermined number of revolutions of the compressor 3 isnot smaller than a preset first number of revolutions(S5). That is, the control apparatus 12 checks whetheror not the amount of circulating refrigerant is not lessthan a first preset value. When the number ofrevolutions is not smaller than the first number ofrevolutions (the amount of circulating refrigerant is notless than first set value), the control apparatus 12selects the suction superheat degree control (S6). Whenthe number of revolutions is smaller than the firstnumber of revolutions (the amount of circulatingrefrigerant is less than the first set value), thecontrol apparatus 12 selects the discharge superheatdegree control (S7). It should be noted that the firstnumber of revolutions (the first set value) isexperimentally predetermined and is set for each airconditioner.[0038] When starting the air conditioning operation, thecontrol apparatus 12 performs, for a fixed period of time,initial operation for driving the compressor 3 at the number of revolutions lower than the set number of revolutions. Thereby, the refrigerant circuit is filledwith the refrigerant, so that the operation of thecompressor 3 can be quickly stabilized. After theinitial operation, the control apparatus 12 drives thecompressor 3 at the number of revolutions determined onthe basis of the load, and also sets the opening degreeof the expansion valve 6 to a determined opening degree.[0039] When performing the air conditioning operation bythe suction superheat degree control, during the initialoperation, the control apparatus 12 determines, as shownin Figure 5, the target suction superheat degree on thebasis of the number of revolutions of the compressor 3corresponding to the amount of circulating refrigerant(S11). The target suction superheat degree isexperimentally obtained in advance, and, the targetsuction superheat degree for each number of revolutionsof the compressor 3 is stored in a nonvolatile memoryprovided in the control apparatus 12. It should be notedthat the target discharge superheat degree for eachnumber of revolutions of the compressor 3 is also storedin the nonvolatile memory. The control apparatus 12reads, from the memory, the target suction superheatdegree according to the determined number of revolutionsof the compressor 3.

[0040] When driving the compressor 3 at the determinednumber of revolutions, the control apparatus 12 acquiresthe real suction superheat degree (S12). The controlapparatus 12 calculates the real suction superheat degreeby subtracting the evaporation temperature detected bythe first temperature detector 17 or the secondtemperature detector 18 from the real suction temperaturedetected by the suction temperature detector 16. Whenperforming cooling operation, the control apparatus 12uses, as the evaporation temperature, the temperaturedetected by the second temperature detector 18. Whenperforming heating operation, the control apparatus 12uses, as the evaporation temperature, the temperaturedetected by the first temperature detector 17.

[0041] The control apparatus 12 compares the target suctionsuperheat degree with the real suction superheat degreeacquired (S13). When the real suction superheat degreeis larger than the target suction superheat degree, thecontrol apparatus 12 performs control to increase theopening degree of the expansion valve 6 (S14). Theopening degree of the expansion valve 6 is increased fromthe present opening degree by the predetermined openingdegree. When the opening degree of the expansion valve 6is increased, the refrigerant passing through theexpansion valve 6 is increased, and thereby the liquid refrigerant evaporated in the evaporator is increased, so that the temperature of the refrigerant flowing from theevaporator is decreased. As a result, the suctiontemperature is decreased, and thereby the real suctionsuperheat degree is decreased, so that the real suctionsuperheat degree approaches the target suction superheatdegree.

[0042] When the real suction superheat degree is smallerthan the target suction superheat degree, the controlapparatus 12 performs control to decrease the openingdegree of the expansion valve 6 (S15). The expansionvalve 6 is closed from the present opening degree by thepredetermined opening degree. When the opening degree ofthe expansion valve 6 is decreased, the refrigerantpassing through the expansion valve 6 is reduced, andthereby the liquid refrigerant evaporated in theevaporator is reduced, so that the temperature of therefrigerant flowing from the evaporator is reduced. As aresult, the suction temperature is increased, and therebythe real suction superheat degree is increased, so thatthe real suction superheat degree approaches the targetsuction superheat degree. It should be noted that, whenthe real suction superheat degree is equal to the targetsuction superheat degree, the control apparatus 12 doesnot change the opening degree of the expansion valve 6.

[0043] When performing the air conditioning operation bythe discharge superheat degree control, the controlapparatus l2 determines, as shown in Figure 6, the targetdischarge superheat degree on the basis of the number ofrevolutions of the compressor 3 corresponds to the amountof circulating refrigerants (S2l). When driving thecompressor 3 at the determined number of revolutions, thecontrol apparatus l2 acquires the real dischargesuperheat degree from the detected discharge temperatureand the detected condensation temperature (S22).

[0044] The control apparatus l2 compares the targetdischarge superheat degree with the real dischargesuperheat degree acquired (S23). When the real dischargesuperheat degree is larger than the target dischargesuperheat degree, the control apparatus l2 performscontrol to increase the opening degree of the expansionvalve 6 (S24). The opening degree of the expansion valve6 is increased by the predetermined opening degree. Whenthe opening degree of the expansion valve 6 is increased,the refrigerant passing through the expansion valve 6 isincreased, and thereby the liquid refrigerant evaporatedin the evaporator is increased, so that the temperatureof the refrigerant sucked into the compressor 3 isreduced. As a result, the discharge temperature of therefrigerant discharged from the compressor 3 is reduced, and thereby the real discharge superheat degree is reduced, so that the real discharge superheat degreeapproaches the target discharge superheat degree.[0045] When the real discharge superheat degree is smallerthan the target discharge superheat degree, the controlapparatus 12 performs control to reduce the openingdegree of the expansion valve 6 (S25). The openingdegree of the expansion valve 6 is reduced by thepredetermined opening degree. When the opening degree ofthe expansion valve 6 is reduced, the refrigerant passingthrough the expansion valve 6 is reduced, and thereby theliquid refrigerant evaporated in the evaporator isreduced. As a result, the temperature of the refrigerantsucked into the compressor 3 is reduced to increase thedischarge temperature. Thereby, the real dischargesuperheat degree is increased, so that the real dischargesuperheat degree approaches the target dischargesuperheat degree.

[0046] As described above, when air conditioning operationis performed by the superheat degree control, the roomtemperature is changed. When the room temperatureapproaches the set temperature, the number of revolutionsof the compressor 3 is reduced, and thereby the amount ofcirculating refrigerant is changed. On the bases of thechanged number of revolutions of the compressor 3, the control apparatus 12 selects one of the superheat degree control which is to be performed, and then performs theselected superheat degree control. Until stopping theair conditioning operation, the control apparatus 12performs one of the selected superheat degree controls.[0047] As described above, at the time of air conditioningoperation in which the amount of circulating refrigerantis large, the suction superheat degree control isperformed. Thereby, the refrigeration cycle can bequickly stabilized, so that efficient air conditioningoperation can be performed. However, at the time of airconditioning operation in which the amount of thecirculating refrigerant is small, it is difficult toadjust the amount of refrigerant in the suction superheatdegree control. Therefore, at the time of airconditioning operation in which the amount of thecirculating refrigerant is small, the discharge superheatdegree control is performed. Thereby, the amount ofrefrigerant can be appropriately adjusted, so thatefficient air conditioning operation is performed.Therefore, efficient air conditioning operation can bealways performed regardless of large or small of theamount of circulating refrigerant.

[0048] Meanwhile, for a while after the start of air conditioning operation, the compressor 3 is not warm, and hence the discharge temperature is not stabilized. When the opening degree of the expansion valve 6 is controlledin the discharge superheat degree control, the openingdegree of the expansion valve 6 is frequently changedaccording to the discharge temperature, and thereby therefrigeration cycle is not stabilized. For this reason,it is necessary that the expansion valve 6 is slowlyoperated, and hence it takes a long time until therefrigeration cycle is stabilized.

[0049] Therefore, in an air conditioner of a secondembodiment, when air conditioning operation is performedin the state where the amount of circulating refrigerantis small, the superheat degree control is switched, sothat the suction superheat degree control is firstperformed, and then the discharge superheat degreecontrol is performed. The suction superheat degreecontrol can immediately respond to changes in thetemperature of the refrigerant, and hence is suitable forthe superheat degree control at the operation startingtime when the temperature rise of the refrigerant islarge. Therefore, when the amount of circulatingrefrigerant is small, the control apparatus 12 performsthe suction superheat degree control until therefrigeration cycle is stabilized. After therefrigeration cycle is stabilized, the control apparatus 12 performs the discharge superheat degree control. It should be noted that the other configurations are thesame as those in the first embodiment.[0050] As shown in Figure 7, when starting air conditioningoperation, the control apparatus 12 determines the numberof revolutions of the compressor 3 on the basis of thedifference between the set temperature and the roomtemperature, and on the basis of a load, such as theoutside air temperature. At this time, the number ofrevolutions of the compressor 3 is set to a number ofrevolutions lower than a first number of revolutions.Therefore, on the basis of the determined number ofrevolutions of the compressor 3, the control apparatus 12determines the discharge superheat degree control as thesuperheat degree control to be performed (S31).

[0051] The control apparatus 12 first performs the suctionsuperheat degree control (S32), and then determineswhether or not the refrigeration cycle is stabilized(S33). When determining that the refrigeration cycle isstabilized, the control apparatus 12 switches from thesuction superheat degree control to the dischargesuperheat degree control and performs the dischargesuperheat degree control (S34).

[0052]The determination whether or not the refrigeration cycle is stabilized is performed on the basis of the suction temperature. As shown in Figure 8, whileperforming the suction superheat degree control, thecontrol apparatus 12 confirms that the number ofrevolutions of the compressor 3 is low, and checkswhether or not the detected real suction superheat degreeapproaches the target suction superheat degree (S35).When the number of revolutions of the compressor 3 isless than the first number of revolutions, and when thestate where the absolute value of the difference betweenthe target suction superheat degree and the real suctionsuperheat degree is less than a stability determinationvalue is continued for a fixed time period, that is, whenthe state where the expression: |target suction superheatdegree - real suction superheat degree| < A is satisfiedcontinuously for the fixed time period, the controlapparatus 12 determines that the refrigeration cycle isstabilized (S36). It should be noted that the stabilitydetermination value A is determined in advance by a testor the like.

[0053] When the number of revolutions of the compressor 3is less than the first number of revolutions, and whenthe absolute value of the difference between the targetsuction superheat degree and the real suction superheatdegree is not less than the stability determination value,or when the state where the absolute value of the difference between the target suction superheat degree and the real suction superheat degree is less than thestability determination value is not continued for thefixed time period, the control apparatus 12 determinesthat the refrigeration cycle is not yet stabilized. Atthis time, the control apparatus 12 continues the suctionsuperheat degree control until the refrigeration cycle isstabilized. It should be noted that, when the number ofrevolutions of the compressor 3 is not less than thefirst number of revolutions, the control apparatus 12continues the suction superheat degree control withoutperforming the discharge superheat degree control.

[0054] As described above, in the state where the suctionsuperheat degree control is first performed, even whenthe amount of the circulating refrigerant is small, thecontrol apparatus 12 can quickly respond to changes inthe temperature of refrigerant to stabilize therefrigeration cycle in a short time. Further, after therefrigeration cycle is stabilized, the control apparatus12 switches from the suction superheat degree control tothe discharge superheat degree control. Thereby, whenthe amount of circulating refrigerant is small, thecontrol apparatus 12 can quickly perform efficient airconditioning operation.

[0055]Here, the suction superheat degree control is performed at the time of starting operation, and thereby the real Suction Superheat degree approaches the targetSuction Superheat degree. For example, when the targetSuction Superheat degree iS low, the real SuctionSuperheat degree approacheS the target degree ofSuperheat. However, when the real Suction Superheatdegree iS not Stabilized to generate hunting, there iS acaSe where the difference between the degreeS ofSuperheat fluctuateS to repeatedly increaSe and decreaSe.In thiS caSe, it iS not poSSible to determine whether ornot the refrigeration cycle iS Stabilized.

[0056] Therefore, aS another embodiment for determiningwhether or not the refrigeration cycle iS Stabilized, anair conditioner of a third embodiment iS configured Suchthat, when hunting of the real Suction Superheat degreeiS generated in the vicinity of the target degree ofSuperheat, it iS regarded that the refrigeration cycle iSStabilized. That iS, aS in the caSe where the realSuction Superheat degree iS varied to be more or leSSthan the target Suction Superheat degree, when the realSuction Superheat degree fluctuateS up and down aroundthe target Suction Superheat degree, the controlapparatuS 12 determined that the refrigeration cycle iSStabilized. It Should be noted that the otherconfigurationS are the Same aS thoSe in the firSt andSecond embodimentS.

[0057] As shown in Figure 9, when starting air conditioningoperation in the state where the amount of circulatingrefrigerant is small, the control apparatus 12 performsthe suction superheat degree control (S41). The controlapparatus 12 confirms that the number of revolutions ofthe compressor 3 is low, and checks whether or not thedetected real suction superheat degree approaches thetarget suction superheat degree (S42). When the absolutevalue of the difference between the target suctionsuperheat degree and the real suction superheat degree isnot less than the stability determination value, thecontrol apparatus 12 continues the suction superheatdegree control, and again checks whether or not thedetected real suction superheat degree approaches thetarget suction superheat degree.

[0058] Here, when detecting that the absolute value of thedifference between the target suction superheat degreeand the real suction superheat degree is less than thestability determination value, the control apparatus 12confirms whether or not hunting of the real suctionsuperheat degree is generated in the vicinity of thetarget suction superheat degree (S43). When hunting ofthe real suction superheat degree is continued for afixed time, the control apparatus 12 determines that therefrigeration cycle is stabilized (S44), and switches from the suction superheat degree control to the discharge superheat degree control, to perform thedischarge superheat degree control (S45).[0059] When hunting of the real suction superheat degree isnot generated, or when hunting of the real suctionsuperheat degree is generated but is not continued forthe fixed time, the control apparatus 12 confirms whetheror not the state where the absolute value of thedifference between the target suction superheat degreeand the real suction superheat degree is less than thestability determination value is continued for the fixedtime period (S46). When the state is continued for thefixed time period, the control apparatus 12 determinesthat the refrigeration cycle is stabilized (S44), and thecontrol apparatus 12 performs the discharge superheatdegree control (S45). When the state where the absolutevalue of the difference between the target suctionsuperheat degree and the real suction superheat degree isless than the stability determination value is notcontinued for the fixed time period, the controlapparatus 12 determines that the refrigeration cycle isnot stabilized, and the control apparatus 12 continuesthe suction superheat degree control until therefrigeration cycle is stabilized.

[0060]As described above, in the state where the unstable state is continued in spite that the real suction superheat degree approaches the target suction superheatdegree, when the control apparatus 12 determines that therefrigeration cycle is stabilized, the control apparatus12 can quickly switch from the suction superheat degreecontrol to the discharge superheat degree control.Therefore, when the amount of circulating refrigerant issmall, the control apparatus 12 can quickly performefficient air conditioning operation.

[0061] Next, in the state where air conditioning operationis performed by the discharge superheat degree control,the operating state is changed when, according to changesin the outside air temperature, the room temperature, andthe like, the number of revolutions of the compressor 3is changed or the numbers of revolutions of the outsideand interior fans are changed. In the state where theamount of circulating refrigerant is small, when these changes are caused, the degree of superheat is greatly changed, so that the refrigeration cycle becomes unstable.

When the discharge superheat degree control is continued,it takes a long time until the refrigeration cycle isstabilized, and hence inefficient air conditioningoperation is performed for the long time. In order toquickly eliminate the instability of the refrigerationcycle, an air conditioner of a fourth embodiment is configured such that, when the refrigeration cycle becomes unstable, the superheat degree control isswitched.[0062] That is, in the state where air conditioningoperation is performed by the discharge superheat degreecontrol, when the refrigeration cycle becomes unstable,the control apparatus 12 switches from the dischargesuperheat degree control to the suction superheat degreecontrol, to perform the suction superheat degree control.When the refrigeration cycle is stable, the controlapparatus 12 continues the discharge superheat degreecontrol. The other configurations are the same as thoseof the first to third embodiments.

[0063] As shown in Figure 10, when air conditioningoperation is performed by the discharge superheat degreecontrol, the control apparatus 12 determines the targetdischarge superheat degree and the target suctionsuperheat degree on the basis of the amount of circulating refrigerant, that is, the number of revolutions of the compressor 3 at the present time (S51).

It should be noted that the target discharge superheatdegree and the target suction superheat degree aredetermined each time when the number of revolutions ofthe compressor 3 is changed.

[0064] Then, the control apparatus 12 acquires the realdischarge superheat degree and the real suction superheatdegree (S52). From the discharge temperature and thecondensation temperature respectively detected by thetemperature detectors, the control apparatus 12calculates the real discharge superheat degree. Further,from the suction temperature and the evaporationtemperature, the control apparatus 12 calculates the realsuction superheat degree.

[0065] The control apparatus 12 compares the target suctionsuperheat degree with the acquired real suction superheatdegree (S53). When the difference between the targetsuction superheat degree and the real suction superheatdegree is not more than the preset stabilizationreference value, the control apparatus 12 determines thatthe refrigeration cycle is stable, and continues thedischarge superheat degree control. In this case, thecontrol apparatus 12 compares the target dischargesuperheat degree with the acquired real dischargesuperheat degree (S54).

[0066] When the real discharge superheat degree is largerthan the target discharge superheat degree, the controlapparatus 12 performs control to increase the openingdegree of the expansion valve 6 (S55). The expansion valve 6 is opened to the predetermined opening degree, and the amount of refrigerant passing through theexpansion valve 6 is increased. Thereby, the amount ofliquid refrigerant evaporated in the evaporator isincreased, so that the temperature of the refrigerantsucked into the compressor 3 is reduced. As a result,the discharge temperature of the refrigerant dischargedfrom the compressor 3 is reduced, and the deal dischargesuperheat degree is reduced. Thereby, the real dischargesuperheat degree is reduced and approaches the targetdischarge superheat degree.

[0067] When the real discharge superheat degree is lessthan the target discharge superheat degree, the controlapparatus 12 performs control to reduce the openingdegree of the expansion valve 6 (S56). The openingdegree of the expansion valve 6 is reduced by thepredetermined opening degree, and thereby the amount ofrefrigerant passing through the expansion valve 6 isreduced, so that the amount of liquid refrigerantevaporated in the evaporator is reduced. As a result,the suction temperature is increased, and the dischargetemperature is also increased. Thereby, the realdischarge superheat degree is increased and approachesthe target discharge superheat degree.

[0068]When the operating state is changed due to environmental changes, the difference between the target suction superheat degree and the real suction superheatdegree becomes larger than the stabilization referencevalue in S53. At this time, the control apparatus 12determines that the refrigeration cycle is unstable, andswitches from the discharge superheat degree control tothe suction superheat degree control and performs thesuction superheat degree control (S57).

[0069] After shifting to the suction superheat degreecontrol, the control apparatus 12 compares the targetsuction superheat degree with the real suction superheatdegree, and checks whether or not the refrigeration cycleis stabilized (S58). When the difference between thetarget suction superheat degree and the real suctionsuperheat degree is larger than the stabilizationreference value, the control apparatus 12 determines thatthe refrigeration cycle is unstable, and continues thesuction superheat degree control. When the differencebetween the target suction superheat degree and the realsuction superheat degree is not larger than thestabilization reference value, the control apparatus 12determines that the refrigeration cycle is stable, andswitches from the suction superheat degree control to thedischarge superheat degree control, and performs thedischarge superheat degree control (S59).

[0070] As described above, in the state where airconditioning operation is performed by the dischargesuperheat degree control, when the refrigeration cyclebecomes unstable, the refrigeration cycle is quicklystabilized by switching from the discharge superheatdegree control to the suction superheat degree control.Thereby, it is prevented that the inefficient airconditioning operation is continued for a long time.[0071] Here, examples of refrigerants used in airconditioners include R407C, R4l0A, and the like. Further,examples of refrigerants having global Warming potentialsmaller than these refrigerants include R32 and Rl234yf.When, in the air conditioner using such refrigerant, thesuperheat degree control is performed so that the degreeof superheat approaches the target degree of superheat asdescribed above, the discharge temperature is increasedto a high temperature, for example, ll0 to l20°C. Such high temperature causes a failure of components, such as the compressor 3, which configure the refrigeration cycle.

Especially, when the refrigerant, such as R32, is used,the discharge temperature is easily increased to a hightemperature, and hence components having high heatresistance need to be used.

[0072] Therefore, in an air conditioner of a fifth embodiment, the discharge temperature control is performed so that the discharge temperature is notincreased to a high temperature. The dischargetemperature control is performed when the dischargetemperature is high. According to the real dischargetemperature, the control apparatus 12 controls therefrigeration cycle so that the discharge temperaturedoes not exceed a predetermined temperature. Further,the control apparatus 12 performs switching between thedischarge temperature control and the suction superheatdegree control. That is, the control apparatus 12performs the discharge temperature control when theamount of circulating refrigerant is large. It should benoted that the other configurations are the same as thosein the first to fourth embodiments.

[0073] When air conditioning operation is started, thecontrol apparatus 12 performs the suction superheatdegree control until the refrigeration cycle isstabilized. As shown in Figure 11, when therefrigeration cycle is stabilized, the control apparatus12 determines the number of revolutions of the compressor3 on the basis of the present load (S61). Then, on thebasis of the determined number of revolutions of thedecided compressor 3, that is, the amount of circulatingrefrigerant, the control apparatus 12 determines controlto be performed among the discharge superheat degree control, the suction superheat degree control, and the discharge temperature control (S62). Here, the controlapparatus 12 determines the operating state on the basisof the amount of circulating refrigerant and thedischarge temperature, and selects the superheat degreecontrol to be performed.

[0074] When the number of revolutions of the compressor 3is lower than a first number of revolutions, the controlapparatus 12 selects the discharge superheat degreecontrol (S63). When the number of revolutions of thecompressor 3 is not lower than the first number ofrevolutions, the control apparatus 12 selects the suctionsuperheat degree control or the discharge temperaturecontrol (S64, S65).

[0075] As shown in Figure 12, as a process to determinewhich of the suction superheat degree control and thedischarge temperature control should be used, the controlapparatus 12 confirms whether or not the number ofrevolutions of the compressor 3 is not smaller than asecond number of revolutions set beforehand according tothe present load (S66). The second number of revolutionsis larger than the first number of revolutions.

[0076] When the number of revolutions of the compressor 3 is smaller than the second number of revolutions, the control apparatus 12 performs the suction superheat degree control (S67). When the number of revolutions ofthe compressor 3 is not smaller than the second number ofrevolutions, the control apparatus 12 compares thedetected discharge temperature (Td) with a determinationdischarge temperature (Ta) (S68). When the detecteddischarge temperature is higher than the determinationdischarge temperature, the control apparatus 12 performsthe discharge temperature control (S69). For example, inthe state where the suction superheat degree control isperformed, when the discharge temperature becomes high,the discharge temperature control is performed.

[0077] When the detected discharge temperature is nothigher than the determination discharge temperature, thecontrol apparatus 12 performs the suction superheatdegree control (S68). When, although the number ofrevolutions of the compressor 3 is high, that is, theamount of circulating refrigerant is large, the dischargetemperature is not high, there is little possibility thatthe discharge temperature becomes high during the airconditioning operation. Therefore, even when performingthe usual suction superheat degree control, the controlapparatus 12 can perform the air conditioning operationso that the discharge temperature does not become high.[0078] When performing air conditioning operation by the discharge temperature control, the control apparatus 12 controls the opening degree of the expansion valve 6 onthe basis of the detected discharge temperature so thatthe discharge temperature does not exceed a maximumdischarge temperature. As shown in Figure 13, thecontrol apparatus 12 determines a target dischargetemperature (Taim) on the basis of the number ofrevolutions of the compressor 3 (S71). The targetdischarge temperature is set according to the refrigerantwhich is used. When air conditioning operation isstarted, the discharge temperature detector detects thedischarge temperature (Td) (S72). The control apparatus12 compares the target discharge temperature with thereal discharge temperature (S73).

[0079] When the real discharge temperature is higher thanthe target discharge temperature (Td > Taim), the controlapparatus 12 increases the opening degree of theexpansion valve 6 (S74). The amount of refrigerantpassing through the expansion valve 6 is increased, and the degree of superheat is reduced, so that the ged temperature of the refrigerant discharged fromthe compressor 3 is lowered. Thereby, the real dischargetemperature approaches the target discharge temperature.[0080]When the real discharge temperature is lower than the target discharge temperature (Td < Taim), the control apparatus 12 reduces the opening degree of the expansion valve 6 (S75). The amount of refrigerant passing throughthe expansion valve 6 is reduced, and the degree ofsuperheat is increased, so that the discharge temperatureis increased. Thereby, the real discharge temperatureapproaches the target discharge temperature.

[0081] When the real discharge temperature is equal to thetarget discharge temperature (Td = Taim), the controlapparatus 12 does not change the opening degree of theexpansion valve 6 (S76). The real discharge temperatureis maintained at the target discharge temperature.

[0082] As described above, by performing the superheatdegree control so that the discharge temperatureapproaches the target discharge temperature, it ispossible to prevent that the real discharge temperatureexceeds the maximum discharge temperature. Thereby, evenwhen R32 whose temperature is easily raised is used asthe refrigerant, it is possible to prevent that thetemperature of the refrigerant becomes high. As a result,since failure of components, such as the compressor 3,can be reduced, it is not necessary to use heat resistantcomponents.

[0083] In an air conditioner of a sixth embodiment which performs another form of determination processing of the suction superheat degree control and the discharge temperature control, one of the suction superheat degreecontrol and the discharge temperature control is selectedby the real suction superheat degree in addition to thenumber of revolutions of the compressor 3 and thedischarge temperature. Here, the operating state isdetermined on the basis of the suction superheat degreein addition to the amount of circulating refrigerant andthe discharge temperature. The other configurations arethe same as those in the first to fifth embodiments.[0084] As shown in Figure l4, when air conditioningoperation is performed, and thereby the refrigerationcycle is stabilized, the control apparatus 12 confirmswhether or not the number of revolutions of thecompressor 3, which is determined according to thepresent load, is not lower than the second number ofrevolutions (S66), and confirms whether or not thedetected discharge temperature is higher than thedetermination discharge temperature (S68). When thedetected discharge temperature is higher than thedetermination discharge temperature, the controlapparatus 12 compares the target suction superheat degreewith the real suction superheat degree (S70).

[0085] When the difference between the target suction superheat degree and the real suction superheat degree is large, that is, when the absolute value of the difference between the target suction superheat degree and the realsuction superheat degree is not less than the presetdetermination reference value, the control apparatus 12performs the suction superheat degree control (S67).When the suction superheat degree control is performed,the real suction superheat degree approaches the targetsuction superheat degree.

[0086] When the difference between the target suctionsuperheat degree and the real suction superheat degree issmall, that is, when the absolute value of the differencebetween the target suction superheat degree and the realsuction superheat degree is less than the determinationreference value, the control apparatus 12 performs thedischarge temperature control (S69). When the differencebetween the real suction superheat degree and the targetsuction superheat degree is small, the real suctionsuperheat degree is close to the target suction superheatdegree. It is not necessary that the suction superheatdegree is set as a control target. Therefore, thecontrol apparatus 12 sets the discharge temperature as acontrol target, and controls the refrigeration cycle sothat the discharge temperature is not excessivelyincreased.

[0087]When performing the discharge temperature control, the control apparatus 12 monitors the suction superheat degree, and checks the difference between the targetsuction superheat degree and the real suction superheatdegree. When the difference between the target suctionsuperheat degree and the real suction superheat degree islarge, the control apparatus 12 switches from thedischarge temperature control to the suction superheatdegree control. Then, when the real suction superheatdegree is brought close to the target suction superheatdegree by the suction superheat degree control, thecontrol apparatus 12 switches from the suction superheatdegree control to the discharge temperature control andperforms the discharge temperature control. In this way,while efficient air conditioning operation is performed,the discharge temperature can be prevented from becominghigh.

[0088] While the discharge temperature control is performed,the load is changed by the change of the room temperatureand the like. According to this change, the number ofrevolutions of the compressor 3 is changed, and theamount of refrigerant circulating through the refrigerantcircuit is also changed, so that the degree of superheatis changed. In an air conditioner of a seventhembodiment, in order to quickly respond to the change ofthe degree of superheat, the switching between thedischarge temperature control and the suction superheat degree control is performed according to the amount of circulating refrigerant. It should be noted that theother configurations are the same as those in the firstto sixth embodiments.

[0089] In the state where performing the dischargetemperature control, when changing the number ofrevolutions of the compressor 3 on the basis of the load,the control apparatus 12 selects one of the dischargetemperature control and the suction superheat degreecontrol according to the number of revolutions of thecompressor 3. As shown in Figure 15, during the airconditioning operation, the control apparatus 12 detects the number of revolutions of the compressor 3 and checks whether or not the number of revolutions is changed (S77).

[0090] When the number of revolutions of the compressor 3is not changed, the control apparatus 12 continues thedischarge temperature control (S71 to S76). When thenumber of revolutions of the compressor 3 is changed, thecontrol apparatus 12 confirms whether or not the numberof revolutions is reduced. When the number ofrevolutions is reduced, the control apparatus 12switches from the discharge temperature control to thesuction superheat degree control (S78). When the numberof revolutions of the compressor 3 is reduced, the amountof circulating refrigerant is reduced, and the suction superheat degree is increased. When the difference between the real suction superheat degree and the targetsuction superheat degree is large, the control apparatus12 performs the suction superheat degree control, andthereby the real suction superheat degree can be broughtcloser to the target suction superheat degree.

[0091] While performing the suction superheat degreecontrol, the control apparatus 12 check whether or not itis possible to switch from the suction superheat degreecontrol to the discharge temperature control (S79). Thatis, the control apparatus 12 confirms the number ofrevolutions of the compressor 3 and checks the differencebetween the target suction superheat degree and thedischarge temperature or the real suction superheatdegree. As a result, when the operation statecorresponds to any of the following cases, namely, whenthe number of revolutions of the compressor 3 isincreased, or when the discharge temperature is higherthan the determination discharge temperature, or wherethe absolute value of the difference between the realsuction superheat degree and the target suction superheatdegree is less than the determination reference value,the control apparatus 12 switches from the suctionsuperheat degree control to the discharge temperaturecontrol and performs the discharge temperature control.

[0092] It should be noted that, when the number ofrevolutions of the compressor 3 is increased in S77, thecontrol apparatus 12 does not perform switching of thesuperheat degree control and continues the dischargetemperature control. When the number of revolutions ofthe compressor 3 is increased, the discharge temperaturemay be increased. For this reason, when this changeoccurs, the discharge temperature control is performed sothat the discharge temperature does not become high.[0093] As another form of the discharge temperature controlperformed so that the discharge temperature does notbecome high, an air conditioner of an eighth embodimentperforms the discharge temperature control so that thedischarge temperature is maintained within apredetermined temperature range. As the dischargetemperature control, the control apparatus 12 controlsthe expansion valve 6 according to the dischargetemperature. It should be noted that the otherconfigurations are the same as those in the first toseventh embodiments.

[0094] As shown in Figure 16, when performing the dischargetemperature control, the control apparatus 12 determinesthe target discharge temperature and the target suctionsuperheat degree on the basis of the present number of revolutions of the compressor 3 (S81). The control apparatus 12 acquires the detected discharge temperatureand the detected real suction superheat degree (S82).The maximum discharge temperature Th and the minimumdischarge temperature Tl are set in advance. For example,the maximum discharge temperature is set to 100°C, andthe minimum discharge temperature is set to 95°C. Thecontrol apparatus 12 determines the detected dischargetemperature (S83). That is, the control apparatus 12compares the real discharge temperature Td with the setdischarge temperature.

[0095] When the discharge temperature Td is higher than themaximum discharge temperature Th (Td > Th), the controlapparatus 12 increases the opening degree of theexpansion valve 6 (S84). The opening degree of theexpansion valve 6 is increased from the present openingdegree by a predetermined opening degree. When theexpansion valve 6 is opened, the degree of superheat isreduced, and the discharge temperature is also reduced.Thereby, the discharge temperature is maintained withinthe predetermined temperature range.

[0096] When the discharge temperature Td is lower than theminimum discharge temperature Tl (Td < Tl), the controlapparatus 12 reduces the opening degree of the expansionvalve 6 (S85). The opening degree of the expansion valve 6 is reduced from the present opening degree by the predetermined opening degree. When the expansion valve 6is closed, the degree of superheat is increased, and thedischarge temperature is also increased. Thereby, thedischarge temperature is maintained within thepredetermined temperature range.

[0097] When the discharge temperature Td is between themaximum discharge temperature Th and the minimumdischarge temperature Tl, (Tlš¶kEšTh), the controlapparatus 12 compares the real suction superheat degreewith the target suction superheat degree (S86). When thereal suction superheat degree is not larger than thetarget suction superheat degree, the control apparatus 12does not change the opening degree of the expansion valve6 (S87). The discharge temperature is maintained withinthe predetermined temperature range.

[0098] When the real suction superheat degree is largerthan the target suction superheat degree, the controlapparatus 12 switches from the discharge temperaturecontrol to the suction superheat degree control andperforms the suction superheat degree control (S88).

When in the suction superheat degree control, the realsuction superheat degree is larger than the targetsuction superheat degree, the control apparatus 12increases the opening degree of the expansion valve 6.

When the opening degree of the expansion valve 6 increased from the present opening degree, thetemperature of the refrigerant flowing from theevaporator is lowered, and the suction temperature islowered, so that the real suction superheat degree isreduced. Thereby, while the discharge temperature ismaintained within the predetermined temperature range,the real suction superheat degree approaches the targetsuction superheat degree.

[0099] In each of the above-described embodiments, when theexpansion valve 6 is controlled in the dischargetemperature control, the discharge temperature iscontrolled so that the discharge temperature does notbecome high and further is maintained within thepredetermined temperature range. In an air conditionerof a ninth embodiment, the compressor 3 is controlled sothat the discharge temperature does not become high. Itshould be noted that the other configurations are thesame as those in the first to seventh embodiments.

[0100] The control apparatus 12 controls the number ofrevolutions of the compressor 3 on the basis of thedischarge temperature. As shown in Figure 17, whenperforming the discharge temperature control, the controlapparatus 12 determines the target discharge temperature(Taim) on the basis of the number of revolutions of the compressor 3 (S71). When air conditioning operation is started, the discharge temperature detector detects thedischarge temperature (Td) (S72). The control apparatus12 compares the target discharge temperature with thereal discharge temperature (S73).

[0101] When the real discharge temperature is higher thanthe target discharge temperature (Td > Taim), the controlapparatus 12 reduces the number of revolutions of thecompressor 3 (S91). The number of revolutions of thecompressor 3 is reduced from the present number ofrevolutions by a predetermined number of revolutions.Although the amount of refrigerant circulating throughthe refrigerant circuit is reduced, the dischargepressure of the compressor 3 is reduced, and hence thedischarge temperature is reduced. Thereby, the realdischarge temperature approaches the target dischargetemperature.

[0102] When the real discharge temperature is lower thanthe target discharge temperature (Td < Taim), the controlapparatus 12 increases the number of revolutions of thecompressor 3 (S92). The number of revolutions of thecompressor 3 is increased from the present number ofrevolutions by the predetermined number of revolutions.Although the amount of circulating refrigerant isincreased, the discharge pressure from the compressor 3 is increased, and hence the discharge temperature is increased. Thereby, the real discharge temperatureapproaches the target discharge temperature.[0103] When the real discharge temperature is equal to thetarget discharge temperature (Td = Taim), the controlapparatus 12 does not change the number of revolutions ofthe compressor 3 (S93). Thereby, the real dischargetemperature is maintained at the target dischargetemperature.

[0104] Further, in an air conditioner of a tenth embodiment,the indoor and outdoor fans 7 and 9 are controlled sothat the discharge temperature does not become high. Itshould be noted that the other configurations are thesame as those in the first to seventh embodiments.

[0105] The control apparatus 12 controls the drive of theindoor fan 9 or the outdoor fan 7 on the basis of thedischarge temperature. As shown in Figure 18, whenperforming the discharge temperature control, the controlapparatus 12 determines the target discharge temperature(Taim)on the basis of the number of revolutions of thecompressor 3 (S71). When air conditioning operation isstarted, the discharge temperature detector 15 detectsthe discharge temperature (Td) (S72). The controlapparatus 12 compares the target discharge temperature with the real discharge temperature (S73).

[0106] When the real discharge temperature is higher thanthe target discharge temperature (Td >Taim), the controlapparatus 12 increases the number of revolutions of theindoor fan 9 or the outdoor fan 7 (S94). It is preferredto increase the number of revolutions of the fan for theheat exchanger serving as the condenser in the airconditioning operation. That is, when cooling operationis performed, the number of revolutions of the outdoorfan 7 is increased, while when heating operation isperformed, the number of revolutions of the indoor fan 9is increased. The number of revolutions of the fan isincreased from the present number of revolutions by apredetermined number of revolutions. The heat exchangein the condenser is promoted, and thereby the amount ofliquid refrigerant flowing from the condenser isincreased, so that the amount of circulating refrigerantis increased. Thereby, the discharge temperature islowered, and the real discharge temperature approachesthe target discharge temperature.

[0107] When the real discharge temperature is lower thanthe target discharge temperature (Td < Taim), the controlapparatus 12 reduces the number of revolutions of theindoor fan 9 or the outdoor fan 7 (S95). It is preferredto reduce the number of revolutions of the fan for the heat exchanger serving as the condenser in air conditioning operation. That is, when cooling operationis performed, the number of revolutions of the outdoorfan 7 is reduced, while when heating operation isperformed, the number of revolutions of the indoor fan 9is reduced. The number of revolutions of the fan isreduced from the present number of revolutions by thepredetermined number of revolutions. The heat exchangein the condenser is suppressed, and thereby the amount ofthe liquid refrigerant flowing from the condenser isreduced, so that the amount of circulating refrigerant isreduced. The amount of refrigerant evaporated in theevaporator is increased, and hence the suction superheatdegree is increased. Thereby, the discharge temperatureis increased, so that the real discharge temperatureapproaches the target discharge temperature.

[0108] When the real discharge temperature is equal to thetarget discharge temperature (Td = Taim), the controlapparatus 12 does not change the number of revolutions ofeach of the outdoor and indoor fans 7 and 9 (S96).Thereby, the real discharge temperature is maintained atthe target discharge temperature.

[0109] The number of revolutions of the fan for thecondenser may be changed, and the number of revolutionsof the fan for the evaporator may also be changed. That is, the number of revolutions of the indoor fan 9 and the number of revolutions of the outdoor fan 7 may be bothchanged. When increasing the number of revolutions ofthe fan for the condenser, the control apparatus 12reduces the number of revolutions of the fan for theevaporator. When reducing the number of revolutions ofthe fan for the condenser, the control apparatus 12increases the number of revolutions of the fan for theevaporator.

[0110] Further, in an air conditioner of an eleventhembodiment, at least one of the compressor 3, theexpansion valve 6, the outdoor fan 7, and the indoor fan9 is controlled so that the discharge temperature doesnot become high. It should be noted that the otherconfigurations are the same as those in the first totenth embodiments.

[0111] On the basis of the discharge temperature, thecontrol apparatus 12 controls at least one of the numberof revolutions of the compressor 3, the opening degree ofthe expansion valve 6, the number of revolutions of theindoor fan 9, and the number of revolutions of theoutdoor fan 7. As shown in Figure 19, when performingthe discharge temperature control, the control apparatus12 determines the target discharge temperature (Taim) onthe basis of the number of revolutions of the compressor 3 (S71). When air conditioning operation is started, the discharge temperature detector detects the dischargetemperature (Td) (S72). The control apparatus 12compares the target discharge temperature with the realdischarge temperature (S73).

[0112] When the real discharge temperature is higher thanthe target discharge temperature (Td > Taim), the controlapparatus 12 performs one or a combination of a pluralityof reducing the number of revolutions of the compressor 3,increasing the opening degree of the expansion valve 6,changing the number of revolutions of the indoor fan 9,and changing the number of revolutions of the outdoor fan7 (S97). It should be noted that each of the fans 7 and9 is controlled so that the number of revolutions of thefan for the heat exchanger serving as the condenser isincreased. Thereby, the discharge temperature is lowered,and the real discharge temperature approaches the targetdischarge temperature.

[0113] When the real discharge temperature is lower thanthe target discharge temperature (Td < Taim), the controlapparatus 12 performs one or a combination of a pluralityof increasing the number of revolutions of the compressor3, reducing the opening degree of the expansion valve 6,changing the number of revolutions of the indoor fan 9,and changing the number of revolutions of the outdoor fan 7 (S98). It should be noted that each of the fans 7 and 9 is controlled so that the number of revolutions of thefan for the heat exchanger serving as the condenser isincreased. Thereby, the discharge temperature isincreased, and the real discharge temperature approachesthe target discharge temperature.

[0114] When the real discharge temperature is equal to thetarget discharge temperature (Td = Taim), the controlapparatus 12 does not change the operation of each of thecompressor 3, the expansion valve 6, the outdoor fan 7,and the indoor fan 9, and maintains the present state ofoperation (S99). Thereby, the real discharge temperatureis maintained at the target discharge temperature.

[0115] Here, the discharge temperature is most influenced by the compressor 3, and the levels of influence of the expansion valve 6, the outdoor fan 7, and the indoor fan 9 on the discharge temperature become lower in this order.

For this reason, when the discharge temperature suddenly becomes high, the control by the compressor 3 is selected.

Also, when the discharge temperature is slowly changed,the control by the fans 7 and 9 is selected. Further,this control may be performed in combination with theother controls. However, the compressor 3, the expansionvalve 6, the outdoor fan 7, and the indoor fan 9 can bearbitrarily combined with each other.

[0116] In an air conditioner of a twelfth embodiment asanother form of the discharge temperature control inwhich the discharge temperature is prevented frombecoming high, a determination expression relating to thedischarge temperature is used, and thereby the superheatdegree control is performed so that the dischargetemperature does not become high and is maintained withina predetermined temperature range. It should be notedthat the other configurations are the same as those inthe first to the eleventh embodiments.

[0117] On the basis of the condensation temperature, theevaporation temperature, and the suction superheat degree,the determination expression relating to the dischargetemperature is created as follows.

Td = dTc + ß - Te + SHWhere Td: discharge temperature, Tc: condensationtemperature, Te: evaporation temperature, SH: suctionsuperheat degree. 1

[0118] The discharge temperature obtained by thedetermination expression becomes substantially equal tothe theoretical value, and hence, the discharge temperature control can be performed by using the determination expression. Therefore, on the basis of thedischarge temperature obtained by the determinationexpression, the control apparatus 12 performs thesuperheat degree control so that the dischargetemperature does not become high. As the superheatdegree control, the control apparatus 12 controls atleast one of the number of revolutions of the compressor3, the opening degree of the expansion valve 6, thenumber of revolutions of the indoor fan 9, and the numberof revolutions of the outdoor fan 7.

[0119] When the discharge temperature calculated by thedetermination expression is high, the control apparatus12 performs the discharge temperature control, while whenthe discharge temperature calculated by the determinationexpression is low, the control apparatus 12 performs thesuction superheat degree control. That is, the controlapparatus 12 compares the detected discharge temperature(Td) with the determination discharge temperature (Ta).When the detected discharge temperature is higher thanthe determination discharge temperature, the controlapparatus 12 performs the discharge temperature control.For example, in the state where performing the suctionsuperheat degree control, when the discharge temperaturebecomes high, the control apparatus 12 performs thedischarge temperature control. When the detected discharge temperature is not higher than the determination discharge temperature, the controlapparatus 12 performs the suction superheat degreecontrol.

[0120] As shown in Figure 20, when performing the dischargetemperature control, the control apparatus 12 determinesthe target suction superheat degree on the basis of thepresent number of revolutions of the compressor 3 (S101).On the basis of the detected condensation temperature andthe detected evaporation temperature, the controlapparatus 12 acquires the real suction superheat degree,and further calculates the discharge temperature by thedetermination expression (S102). The control apparatus12 determines the calculated discharge temperature on thebasis of a preset maximum discharge temperature and apreset minimum discharge temperature (S103). For example,the maximum discharge temperature Th is set to 100°C, andthe minimum discharge temperature Tl is set to 95°C.

That is, the control apparatus 12 compares the dischargetemperature Td obtained by the determination expressionwith the set discharge temperature.

[0121] When the discharge temperature Td is higher than themaximum discharge temperature Th (Td > Th), the controlapparatus 12 performs one or a combination of a pluralityof reducing the number of revolutions of the compressor 3, increasing the opening degree of the expansion valve 6, changing the number of revolutions of the indoor fan 9,and changing the number of revolutions of the outdoor fan7 (S104). Thereby, the discharge temperature is reduced,so that the real discharge temperature becomes lower thanthe maximum discharge temperature to be maintained withinthe predetermined temperature range.

[0122] When the discharge temperature Td is lower than theminimum discharge temperature Tl (Td < Tl), the controlapparatus 12 performs one or a combination of a pluralityof increasing the number of revolutions of the compressor3, reducing the opening degree of the expansion valve 6,changing the number of revolutions of the indoor fan 9,and changing the number of revolutions of outdoor fan 7(S105). Thereby, the discharge temperature is increased,so that the real discharge temperature is higher than theminimum discharge temperature to be maintained within thepredetermined temperature range.

[0123] When the discharge temperature Td is between themaximum discharge temperature Th and the minimumdischarge temperature Tl (Tl S Td S Th), the controlapparatus 12 compares the real suction superheat degreewith the target suction superheat degree (S106). Whenthe real suction superheat degree is not larger than thetarget suction superheat degree, the control apparatus 12 does not change the operation of each of the compressor 3, the expansion valve 6, the outdoor fan 7, and the indoorfan 9, to maintain the present state (S107). Thereby,the real discharge temperature is maintained within thepredetermined temperature range.

[0124] When the real suction superheat degree is largerthan the target suction superheat degree, the controlapparatus 12 switches from the discharge temperaturecontrol to the suction superheat degree control, andperforms the suction superheat degree control (S108).When the real suction superheat degree is larger than thetarget suction superheat degree in the suction superheatdegree control, the control apparatus 12 performs one ora combination of a plurality of reducing the number ofrevolutions of compressor 3, increasing the openingdegree of the expansion valve 6, changing the number ofrevolutions of the indoor fan 9, and changing the numberof revolutions of the outdoor fan 7. Thereby, while thereal discharge temperature is maintained within thepredetermined temperature range, the real suctionsuperheat degree approaches the target suction superheatdegree.

[0125] When the difference between the real dischargetemperature and the discharge temperature obtained by thedetermination expression is large, the real discharge temperature approaches, with the lapse of time, the discharge temperature obtained by the determinationexpression. That is, the change of the dischargetemperature can be predicted. For example, when the realdischarge temperature is lower than the dischargetemperature obtained by the determination expression, itcan be predicted that the discharge temperature isfurther increased. In such a case, when the dischargetemperature control is performed on the basis of thedischarge temperature obtained by the determinationexpression, it is possible to prevent the dischargetemperature from becoming excessively high.

[0126] When R32 is used as a refrigerant in the airconditioner of each of the above-described embodiments,the refrigerant has characteristics of having a lowerpressure loss and a larger heat exchange amount per massflow rate as compared with the other refrigerants, suchas R4lO. For this reason, the minimum air conditioningcapacity is larger than that of the other refrigerants.Meanwhile, although when the amount of refrigerantcirculating through the refrigerant circuit is reduced,the discharge superheat degree control is performed, thedischarge superheat degree is reduced as the number ofrevolutions of the compressor 3 is reduced. Therefore,when the compressor 3 is driven at the minimum number ofrevolutions, the air conditioner is operated at the minimum air conditioning capacity. However, in the case where R32 is used as the refrigerant, even when thecompressor 3 is driven at the minimum number ofrevolutions, the air conditioning capacity is high, andhence the operating temperature range is narrowed.[0127] Therefore, in an air conditioner of a thirteenthembodiment, the air conditioning capacity at the timewhen the compressor 3 is driven at the minimum number ofrevolutions is reduced. To this end, the targetdischarge superheat degree is reduced as the number ofrevolutions of the compressor 3 is reduced, and thecontrol apparatus 12 performs the discharge superheatdegree control on the basis of the reduced targetdischarge superheat degree. It should be noted that theother configurations are the same as those in the firstto twelfth embodiments.

[0128] The minimum number of revolutions of the compressor3 is lower than the number of revolutions at a frequencycode FD1. Further, the target discharge superheat degreecorresponding to the minimum number of revolutions is setlower than the target discharge superheat degreecorresponding to FD1. As shown in Figure 21, the usualtarget discharge superheat degree corresponding to theminimum number of revolutions is set on the lineconnecting the target discharge superheat degree corresponding to the frequency code FD1 to the target discharge superheat degree corresponding to anintermediate number of revolutions.[0129] On the other hand, as shown in Figure 22, the targetdischarge superheat degree, which corresponds to theminimum number of revolutions when the dischargesuperheat degree control is performed, is set to a degreeof superheat which is lower than the usual targetdischarge superheat degree. The target dischargesuperheat degree corresponding to the minimum number ofrevolutions is set to a degree of superheat which islower than the line connecting the target dischargesuperheat degree corresponding to FD1 to the targetdischarge superheat degree corresponding to theintermediate number of revolutions.

[0130] When performing the discharge superheat degreecontrol, the control apparatus 12 performs the dischargetemperature control with the aim of the target dischargesuperheat degree. At this time, the control apparatus 12controls the opening degree of the expansion valve 6according to the difference between the target dischargesuperheat degree and the real discharge superheat degree.When the real discharge superheat degree is larger thanthe target discharge superheat degree, the controlapparatus 12 sets the target discharge superheat degree to a lower value so that the difference between the target discharge superheat degree and the real dischargesuperheat degree is increased. The control apparatus 12sets the opening degree of the expansion valve 6 to anopening degree larger than the usual opening degree.[0131] When the compressor 3 is operated at the minimumnumber of revolutions, the opening degree of theexpansion valve 6 is set to an opening degree larger thanthe usual opening degree. Thereby, it is difficult toreach the target discharge superheat degree, so that theair conditioning capacity is reduced. The airconditioning capacity at the time of the minimum numberof revolutions is reduced, and thereby the load appliedto the compressor 3 is reduced, so that it is possible toprevent the compressor 3 from being stopped. Thereby,since, even when the compressor 3 is operated at a lownumber of revolutions, the compressor 3 is not stopped,the air conditioning operation can be continued, andhence the user's comfort is not impaired. In this way,since the air conditioning capacity can be reduced, airconditioning operation can be performed at a lowoperating temperature, and thereby it is possible torealize an air conditioner which can operate in a widetemperature range.

[0132]As described above, the air conditioner of the present invention is provided with the refrigerant circuit which is formed by connecting the compressor 3,the condenser, the restriction device, and the evaporatorto each other, and the control apparatus 12 whichcontrols the operation of the restriction deviceaccording to the amount of refrigerant circulatingthrough the refrigerant circuit. When the amount ofcirculating refrigerant is large, the control apparatus12 performs the suction superheat degree control byoperating the restriction device on the basis of thesuction temperature of the refrigerant sucked by thecompressor 3. When the amount of circulating refrigerantis small, the control apparatus 12 performs the dischargesuperheat degree control by operating the restrictiondevice on the basis of the discharge temperature of therefrigerant discharged from the compressor 3.

[0133] When the amount of circulating refrigerant is large,the suction superheat degree control, which has goodresponsiveness to a change of the degree of superheat, issuitable, and when the amount of circulating refrigerantis small, the discharge superheat degree control, whichcan respond to even small changes of the degree ofsuperheat, is suitable. The suitable superheat degreecontrol can be performed according to the amount ofcirculating refrigerant, and hence efficient airconditioning operation can be performed.

[0134] When the amount of circulating refrigerant is small,the control apparatus 12 performs the suction superheatdegree control until the refrigeration cycle isstabilized. After the refrigeration cycle is stabilized,the control apparatus 12 performs the discharge superheatdegree control.

[0135] The refrigeration cycle can be quickly stabilized bythe suction superheat degree control. Therefore, whenthe suction superheat degree control and the dischargesuperheat degree control are performed in order,efficient air conditioning operation can be quicklyperformed.

[0136] In the suction superheat degree control, the controlapparatus 12 controls the opening degree of therestriction device so that the difference between thesuction temperature and the evaporation temperatureapproaches a predetermined value. In the dischargesuperheat degree control, the control apparatus 12controls the opening degree of the restriction device sothat the difference between the discharge temperature andthe condensation temperature approaches a predeterminedvalue.

[0137]When performing the discharge superheat degree control, and when the refrigeration cycle becomes unstable, the control apparatus 12 performs the suctionsuperheat degree control. In this way, when, in theunstable state of the refrigeration cycle, the dischargesuperheat degree control is switched to the suctionsuperheat degree control, the refrigeration cycle isquickly stabilized. Therefore, it is possible to shortenthe time period until the discharge superheat degreecontrol is resumed.

[0138] The control apparatus 12 compares the target suctionsuperheat degree with the real suction superheat degreeand determines whether or not the refrigeration cycle isunstable. That is, when the difference between thetarget suction superheat degree and the real suctionsuperheat degree is large, the control apparatus 12determines that the refrigeration cycle is unstable.

When the difference between the target suction superheatdegree and the real suction superheat degree is small,the control apparatus 12 determines that therefrigeration cycle is stable.

[0139] When the refrigeration cycle is unstable, the changeof the suction superheat degree is larger than the changeof the discharge superheat degree. Therefore, bydetermining on the basis of the suction superheat degree,the control apparatus 12 can quickly detect that the refrigeration cycle is unstable.

[0140] When the real suction superheat degree fluctuatesabout the target suction superheat degree, the controlapparatus 12 determines that the refrigeration cycle isstabilized. Thereby, the control apparatus 12 canquickly perform switching to the suitable superheatdegree control.

[0141] Further, the air conditioner is provided with therefrigerant circuit which is formed by connecting thecompressor 3, the condenser, the restriction device, andthe evaporator to each other, and the control apparatus12 which controls the operation of the restriction deviceaccording to the amount of refrigerant circulatingthrough the refrigerant circuit. The control apparatusdetermines the operating state and performs optimal airconditioning by performing switching between the suctionsuperheat degree control which controls the restrictiondevice on the basis of the suction temperature of therefrigerant sucked by the compressor, and the superheatdegree control which controls the restriction device onthe basis of the discharge temperature of the refrigerantdischarged from the compressor.

[0142] That is, according to the operating state determined on the basis of the amount of circulating refrigerant, and the like, the control apparatus 12 performs one of the suction superheat degree control which operates therestriction device on the basis of the suctiontemperature of the refrigerant sucked by the compressor 3,the discharge superheat degree control which operates therestriction device on the basis of the dischargetemperature of the refrigerant discharged from thecompressor 3, and the discharge temperature control whichoperates the restriction device so that the dischargetemperature approaches the set temperature.

[0143] According to the amount of circulating refrigerant,the discharge temperature, and the suction superheatdegree, the control apparatus 12 can select one suitablesuperheat degree control among the three superheat degreecontrols, and perform optimum air conditioning operation.[0144] When the amount of circulating refrigerant is large,the control apparatus 12 performs the dischargetemperature control which operates the restriction deviceso that the discharge temperature approaches the settemperature, and also performs switching between thedischarge temperature control and the suction superheatdegree control according to the discharge temperature.

By performing the discharge temperature control, thedischarge temperature is made not to exceed thepredetermined temperature.

[0145] When the discharge temperature is high, the controlapparatus 12 performs the discharge temperature control.When the discharge temperature is low, the controlapparatus 12 performs the suction superheat degreecontrol. In the state where the suction superheat degreeapproaches the target suction superheat degree, when thedischarge temperature becomes high, it is possible toprevent the discharge temperature from exceeding thepredetermined temperature.

[0146] When the difference between the target suctionsuperheat degree and the real suction superheat degree issmall, the control apparatus 12 performs the dischargetemperature control. When the difference between thetarget suction superheat degree and the real suctionsuperheat degree is large, the control apparatus 12performs the suction superheat degree control.

[0147] By performing switching between the suctionsuperheat degree control and the discharge temperaturecontrol according to the difference between the targetsuction superheat degree and the real suction superheatdegree, it is possible to perform efficient airconditioning operation and prevent the dischargetemperature from becoming high.

[0148] The determination expression of the dischargetemperature is determined on the basis of the evaporationtemperature, the condensation temperature, and thesuction superheat degree. Then, the control apparatus 12performs the discharge temperature control to prevent thedischarge temperature from exceeding the predeterminedtemperature on the basis of the discharge temperaturecalculated by the determination expression.

[0149] The change of the discharge temperature can bepredicted by the determination expression, and hence thedischarge temperature can be controlled not to exceed thepredetermined temperature.

[0150] When the discharge temperature calculated by thedetermination expression is high, the control apparatus12 performs the discharge temperature control. When thedischarge temperature calculated by the determinationexpression is low, the control apparatus 12 performs thesuction superheat degree control.

[0151] Thereby, it is possible that the suction superheatdegree is made to approach the target suction superheatdegree, and also the discharge temperature is preventedfrom exceeding the predetermined temperature.

[0152] According to the target value of the dischargetemperature and the calculated discharge temperature, thecontrol apparatus 12 controls at least one of the numberof revolutions of the compressor 3, the opening degree ofthe restriction device, and the number of revolutions ofthe fan for the heat exchanger.

[0153] According to the difference between the dischargetemperatures, the three control objects are suitablycombined, and thereby the discharge temperature can besurely prevented from exceeding the predeterminedtemperature and maintained within the predeterminedtemperature range.

[0154] When performing the discharge temperature control,in order to prevent the discharge temperature fromexceeding the predetermined temperature, and according tothe discharge temperature, the control apparatus 12performs one or a combination of a plurality ofincreasing the opening degree of the restriction device,reducing the number of revolutions of the compressor 3,increasing the number of revolutions of the fan for theheat exchanger, and especially increasing the number ofrevolutions of the fan for the condenser.

[0155]In order to reduce the air conditioning capacity at the time when the compressor 3 is operated at the minimum number of revolutions, the target discharge superheatdegree corresponding to the minimum number of revolutionsof the compressor 3 is set to be lower than the usualtarget discharge superheat degree, and the controlapparatus 12 performs the discharge superheat degreecontrol on the basis of the set target dischargesuperheat degree.

[0156] The air conditioning capacity is reduced at the timewhen the compressor 3 is operated at the minimum numberof revolutions, and hence the compressor 3 can beoperated even at a low operating temperature withoutbeing stopped. Thereby, air conditioning operation canbe performed in a wide operating temperature range.

[0157] On the basis of the number of revolutions of thecompressor 3, the control apparatus 12 determines whetherthe amount of circulating refrigerant is large or small.When the number of revolutions of the compressor 3 ishigher than a predetermined number, the control apparatus12 determines that the amount of circulating refrigerantis large. When the number of revolutions of thecompressor 3 is lower than the predetermined number, thecontrol apparatus 12 determines that the amount ofcirculating refrigerant is small. When, during airconditioning operation, the number of revolutions of the compressor 3 is changed according to the load, the control apparatus 12 performs switching of the superheatdegree control.[0158] It should be noted that the present invention is notlimited to the above described embodiments, but numerousmodifications and changes can be obviously made thereinwithout departing from the spirit and scope of thepresent invention. In the suction superheat degreecontrol and the discharge superheat degree control, notonly the control of the expansion valve 6, but also thecontrol of the compressor 3 and the control of the indoorand outdoor fans 7 and 9 may also performed.

[0159] As the second temperature detector 18, a temperaturedetector for the indoor heat exchanger 8 may also beprovided. The second temperature detector 18 directly detects the temperature of the indoor heat exchanger 8.

Reference Signs List

[0160]1 Outdoor unit2 Indoor unit3 Compressor4 Four-way valve5 Outdoor heat exchanger6 Expansion valve 7 Outdoor fan 8 Indoor heat exchanger 9 Indoor fan 10 Two-way valve 12 Control apparatus 13 Room temperature detector 14 Outdoor temperature detector15 Discharge temperature detector16 Suction temperature detector17 First temperature detector 18 Second temperature detector

Claims

1. [Claim l] An air conditioner including: a refrigerant circuitwhich is formed by connecting a compressor, a condenser,a restriction device, and an evaporator to each other;and a control apparatus which controls the operation ofthe restriction device according to the amount ofrefrigerant circulating through the refrigerant circuit,wherein the control apparatus determines the operatingstate and performs optimal air conditioning operation byperforming switching between suction superheat degreecontrol which operates the restriction device on thebasis of the suction temperature of the refrigerantsucked by the compressor, and superheat degree controlwhich operates the restriction device on the basis of thedischarge temperature of the refrigerant discharged fromthe compressor.

2. [Claim 2] The air conditioner according to claim l, wherein,when the amount of circulating refrigerant is large, thecontrol apparatus performs the suction superheat degreecontrol which operates the restriction device on thebasis of the suction temperature of the refrigerantsucked by the compressor, and when the amount ofcirculating refrigerant is small, the control apparatus performs discharge superheat degree control which operates the restriction device on the basis of thedischarge temperature of the refrigerant discharged fromthe compressor.

3. [Claim 3] The air conditioner according to one of claim l andclaim 2, wherein, when the amount of circulatingrefrigerant is small, the control apparatus performs thesuction superheat degree control until the refrigerationcycle is stabilized, and after the refrigeration cycle isstabilized, the control apparatus performs the dischargesuperheat degree control.

4. [Claim 4] The air conditioner according to one of claim l toclaim 3, wherein, when the refrigeration cycle isunstable during operation of the discharge superheatdegree control, the control apparatus performs thesuction superheat degree control.

5. [Claim 5] The air conditioner according to one of claim 3 andclaim 4, wherein the control apparatus compares a targetsuction superheat degree with a real suction superheatdegree, and determines whether or not the refrigerationcycle is unstable.

6. [Claim 6] The air conditioner according to one of claims 3 to claims 5, wherein, when the real suction superheat degree fluctuates around the target suction superheat degree, the control apparatus determines that the refrigerationcycle is stabilized.[

7. Claim 7] The air conditioner according to one of claim l toclaim 6, wherein, when the amount of circulatingrefrigerant is large, the control apparatus performsdischarge temperature control which operates therestriction device so that the discharge temperatureapproaches a set temperature, and performs switchingbetween the discharge temperature control and the suctionsuperheat degree control according to the dischargetemperature.

8. [Claim 8] The air conditioner according to claim 7, wherein,when the discharge temperature is high, the controlapparatus performs the discharge temperature control, andwhen the discharge temperature is low, the controlapparatus performs the suction superheat degree control.

9. [Claim 9] The air conditioner according to one of claim 7 andclaim 8, wherein, when the difference between the targetsuction superheat degree and the real suction superheatdegree is small, the control apparatus performs thedischarge temperature control, and when the differencebetween the target suction superheat degree and the realsuction superheat degree is large, the control apparatus performs the suction superheat degree control. [Claim lO]The air conditioner according to one of claim l to claim 9, wherein a discharge temperature determination expression is determined by an evaporation temperature, a condensation temperature, and the suction superheatdegree, and on the basis of a discharge temperaturecalculated by the determination expression, the controlapparatus performs the discharge temperature control sothat the discharge temperature does not exceed apredetermined temperature. [Claim ll] The air conditioner according to claim 10, wherein,when the discharge temperature calculated by thedetermination expression is high, the control apparatusperforms the discharge temperature control, and when thedischarge temperature calculated by the determinationexpression is low, the control apparatus performs thesuction superheat degree control. [Claim l2] The air conditioner according to one of claim l toclaim ll, wherein the control apparatus operates suchthat, in order to reduce air conditioning capacity at thetime when the compressor is operated at a minimum numberof revolutions, a target discharge superheat degreecorresponding to the minimum number of revolutions of thecompressor is set lower than a usual target discharge superheat degree, and the control apparatus performs the discharge superheat degree control on the basis of the set target discharge superheat degree.