GB2517346A

GB2517346A - Air conditioner and control method therefor

Info

Publication number: GB2517346A
Application number: GB1421070.2A
Authority: GB
Inventors: Satoru Yanachi; Yohei Kato
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2012-06-29
Filing date: 2012-06-29
Publication date: 2015-02-18
Anticipated expiration: 2032-06-29
Also published as: GB201421070D0; US20150153079A1; WO2014002145A1; JPWO2014002145A1; GB2517346B

Abstract

When operation commences in the air conditioner (100) of the present invention, an operation determination means (21) determines whether said operation is cooling operation or heating operation. When it is determined that the operation is not cooling operation, cooling is not performed at a discharge gas cooling device (2). When it is determined that the operation is cooling operation, cooling is performed at the discharge gas cooling device (2).

Description

DESCRIPTION

Title of Invention

AIR-CONDITIONING APPARATUS AND METHOD FOR CONTROLLING THE

SAME

Technical Field

[0001] The present invention relates to an air-conditioning apparatus and a method for controlling the apparatus, and particularly to an air-conditioning apparatus that performs a cooling operation and a heating operation and a method for controlling the air-conditioning apparatus.

Background Art

[0002] In an air-conditioning apparatus, a compressor that generates driving power for circulating refrigerant can be of a reciprocating type, a screw type, a scroll type, or a rotary type, for example. The compressor of any type encloses refrigerating machine oil in order to lubricate a sliding portion. To obtain reliability of the compressor, a predetermined amount of refrigerating machine oil having a predetermined concentration or more needs to be supplied to the compressor. In particular, in a situation (e.g., in start-up) in which the oil concentration is lowest with a decreased oil amount, refrigerating machine oil in a necessary amount or more can be enclosed in some cases for stable operation in order to continue the supply of the predetermined amount of refrigerating machine oil in the predetermined concentration or more.

[0003] The enclosure of the necessary amount or more of refrigerating machine oil for stable operation increases the oil-surface level in the compressor so that refrigerating machine oil in the compressor can be easily discharged. With an increase in amount of refrigerating machine oil with a high viscosity circulating in refrigerant (hereinafter referred to as an oil circulation rate), a pressure loss in pipes increases, resulting in a decrease in COP capacity disadvantageously.

[0004] To solve this problem, in some proposed methods, an oil separator is provided in a discharge part of the compressor in order to separate refrigerating machine oil from refrigerant, and the separated refrigerating machine oil returns to the compressor so that the concentration of oil circulating in a refrigeration cycle is reduced (see, for example, Patent Literatures 1 and 2). In another proposed method, compressor discharge gas is cooled to about a condensing temperature of refrigerant and refrigerating machine oil in a gaseous state is separated so that the separation efficiency is enhanced (see, for example, Patent Literature 3).

Citation List Patent Literature [0005] Patent Literature 1: Japanese Unexamined PatentApplication Publication No. 62-80473 Patent Literature 2: Japanese Unexamined Utility Model Registration Application Publication No. 2-1 31171 Patent Literature 3: Japanese Unexamined Patent Application Publication No. 62-981 70

Summary of Invention

Technical Problem [0006] There has been a need for an enhanced coefficient of performance (COP) in recent air-conditioning apparatuses. As described in Patent Literatures 1 to 3, a decrease in oil circulation rate by separating refrigerant and oil has been considered to enhance the COP. However, when focusing only on the oil circulation rate, the COP might decrease disadvantageously.

[0007] The present invention has been made to solve such a problem as described above, and is intended to provide an air-conditioning apparatus that can enhance a COP in both a cooling operation and a heating operation and a method for controlling the air-conditioning apparatus.

Solution to Problem [0008] An air-conditioning apparatus according to the present invention includes a refrigerant circuit in which a compressor, a discharge gas cooling unit, an oil separator, a four-way valve, an indoor heat exchanger, an expansion valve, and an outdoor heat exchanger are connected to each other in a loop and capable of performing a cooling operation and a heating operation by switching a channel with the four-way valve, and also includes: an operation determination means for determining whether an operating state is the cooling operation or the heating operation; and a heat rejection control means for controlling the discharge gas cooling unit such that the discharge gas cooling unit cools discharge gas from the compressor if the operation determination means determines that the operating state is the cooling operation and the discharge gas cooling unit does not cool the discharge gas if the operation determination means determines that the operating state is the heating operation.

Advantageous Effects of Invention [0009] In an oil separator and an air-conditioning apparatus according to the present invention, in a cooling operation, discharge gas is cooled by a discharge gas cooling unit and separation in the oil separator is promoted so that the oil circulation rate is reduced and the COP is enhanced, and in a heating operation, cooling of the discharge gas is stopped so that heat rejection in the discharge gas cooling unit is reduced and a decrease in COP is reduced.

Brief Description of Drawings

[0010] [Fig. 1] Fig. 1 schematically illustrates a configuration of an air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 2] Fig. 2 is a graph showing a p-h diagram of an operating state in a cooling operation of the air-conditioning apparatus illustrated in Fig. 1.

[Fig. 3] Fig. 3 is a graph showing a p-h diagram of an operating state in a heating operation of the air-conditioning apparatus illustrated in Fig. 1.

[Fig. 4] Fig. 4 is a graph showing a relationship between a pressure loss through a segment from an evaporator outlet to a compressor inlet and a COP in the cooling operation of the air-conditioning apparatus illustrated in Fig. 1.

[Fig. 5] Fig. 5 is a graph showing a relationship between an oil circulation rate and a percentage to COP in the air-conditioning apparatus illustrated in Fig. 1.

[Fig. 6] Fig. 6 is a graph showing a relationship between the amount of heat rejection and a percentage to COP in a discharge gas cooling unit in the heating operation of the air-conditioning apparatus illustrated in Fig. 1.

[Fig. 7] Fig. 7 is a flowchart showing a method for controlling an air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 8] Fig. 8 is a flowchart showing a method for controlling an air-conditioning apparatus according to Embodiment 2 of the present invention.

[Fig. 9] Fig. 9 schematically illustrates a configuration of an air-conditioning apparatus according to Embodiment 3 of the present invention.

[Fig. 10] Fig. 10 schematically illustrates a configuration of an air-conditioning apparatus according to Embodiment 4 of the present invention.

[Fig. 11] Fig. 11 schematically illustrates a configuration of an air-conditioning apparatus according to EmbodimentS of the present invention.

Description of Embodiments

[0011] Embodiment 1 A preferred embodiment of an air-conditioning apparatus of the present invention will be described with reference to the drawings. Fig. 1 schematically illustrates an air-conditioning apparatus according to Embodiment 1 of the present invention. An air-conditioning apparatus 100 illustrated in Fig. 1 performs both a cooling operation and a heating operation, and includes a refrigerant circuit in which a compressor 1, a discharge gas cooling unit 2, an oil separator 3, a four-way valve 4 serving as a flow switching device, an indoor heat exchanger 6, an expansion valve 8, and an outdoor heat exchanger 9 are connected to each other in a loop. Among these components, the compressor 1, the discharge gas cooling unit 2, the oil separator 3, the four-way valve 4, the expansion valve 8, and the outdoor heat exchanger 9 constitute an outdoor unit 10, and an indoor heat exchanger 6 constitutes an indoor unit 11. The outdoor unit 10 and the indoor unit 11 are connected to each other by a gas-side extension pipe 5 and a refrigerant-side extension pipe 7. Refrigerant circulates between the outdoor unit 10 and the indoor unit 11 via the gas-side extension pipe 5 and the refrigerant-side extension pipe 7.

[0012] The compressor 1 causes refrigerant from the four-way valve 4 to be a high-temperature high-pressure discharge gas. The compressor 1 may be of various types. The compressor 1 that generates driving power for circulating this refrigerant may be of various types such as a reciprocating type, a screw type, a scroll type, and a rotary type.

[0013] The discharge gas cooling unit 2 cools discharge gas from the compressor 1, and includes a pump 12, a heat rejecter 13, and a heat exchanger 14. The pump 12, the heat rejecter 13, and the heat exchanger 14 constitute a circulation circuit in which water or brine, for example, circulates. The pump 12 causes a circulation substance such as water or brine to circulate between the heat rejecter 13 and the heat exchanger 14. The heat rejecter 13 cools (rejects heat from) water or brine, for example, circulating in the circulation circuit. The heat exchanger 14 exchanges heat between the discharge gas and water or brine, for example, flowing in the circulation circuit. The heat exchanger 14 includes a refrigerant channel through which the discharge gas from the compressor 1, for example, flows and a cooling channel 14a through which the circulation substance flows. The circulation substance flowing in the cooling channel 14a takes the quantity of heat of the discharge gas flowing in the refrigerant channel, thereby cooling the discharge gas. The heat exchanger 14 may be of various known types such as a shell-and-tube type, a shell-and-coil type, a dual-tube type.

[0014] When the pump 12 starts operating, water or brine, for example, circulates in the circulation circuit, and the heat exchanger 14 exchanges heat between water, for example, and the discharge gas. Water, for example, subjected to the heat exchange is deprived of heat (i.e., cooled) by the heat rejecter 13. In this manner, the discharge gas is cooled, in other words, heat is radiated from the discharge gas. On the other hand, when the pump 12 stops, circulation of water, for example, in the circulation circuit stops and the heat exchanger 14 does not exchange heat. Thus, the discharge gas is not cooled. Operation of the pump 12 is controlled by a heat rejection control means 22. By using the control of on/off operation of the pump 12 by the heat rejection control means 22, it is determined whether the discharge gas by the discharge gas cooling unit 2 is cooled or not.

[0015] The oil separator 3 separates oil from the discharge gas, and returns the separated oil to the compressor 1. The oil separator 3, for example, includes an inflow pipe through which gas discharged from the discharge gas cooling unit 2 flows into a hollow container, a discharge pipe through which refrigerant gas is discharged to the four-way valve 4, and a pipe which is located at the bottom thereof and through which the separated oil returns to the compressor 1. When gas flows from the inflow pipe into the hollow container, oil is attached to the surface of the hollow container, flows down toward the bottom surface of the hollow container, and then returns to the compressor 1 through the pipe. On the other hand, refrigerant gas is discharged from the discharge pipe toward the four-way valve 4.

[0016] The higher the temperature of the refrigerant gas discharged from the compressor 1, the larger amount of oil is separated from refrigerant in the oil separator 3. Specifically, cooling of the discharge gas by the discharge gas cooling unit 2 before the discharge gas enters the discharge gas from the compressor 1 into the oil separator 3 also reduces the temperature of oil included in the discharge gas. The cooling of oil increases the viscosity and the density of the oil, and accordingly, the oil can be more easily attached to, and captured at, the surface of the hollow container in the oil separator 3. Consequently, cooling of the discharge gas by the discharge gas cooling unit 2 can promote separation between oil and refrigerant in the oil separator 3.

[0017] The four-way valve 4 switches the direction of a flow of refrigerant depending on the operation mode of the indoor unit 11. Specifically, in a cooling operation, the four-way valve 4 is switched to allow discharge gas to flow from the oil separator 3 to the outdoor heat exchanger 9. On the other hand, in a heating operation, the four-way valve 4 is switched to allow refrigerant to flow from the oil separator 3 to the indoor heat exchanger 6. The expansion valve (reducing valve) 8 narrows a refrigerant channel in order to adjust the amount of refrigerant flowing into the evaporator.

[0018] The indoor heat exchanger 6 exchanges heat between indoor air and refrigerant. The outdoor heat exchanger 9 exchanges heat between outdoor air and refrigerant. Specifically, in the cooling operation, the indoor heat exchanger 6 serves as an evaporator and the outdoor heat exchanger 9 serves as a condenser. In the indoor heat exchanger 6, refrigerant absorbs heat from indoor air and cold air is blown, whereas in the outdoor heat exchanger 9, refrigerant rejects heat by outdoor air and hot air is released. On the other hand, in the heating operation, the indoor heat exchanger 6 serves as a condenser and the outdoor heat exchanger 9 serves as an evaporator. In the indoor heat exchanger 6, refrigerant rejects heat to indoor air and hot air is released, whereas in the outdoor heat exchanger 9, outdoor air absorbs heat from refrigerant and cold air is released.

[0019] The above-described operation of the outdoor unit 10 is controlled by control unit 20. In particular, the control unit 20 includes an operation determination means 21 and a heat rejection control means 22 in order to switch operation of the discharge gas cooling unit 2 between a cooling operation and a heating operation. The operation determination means 21 determines whether the operating state is the cooling operation or the heating operation. For example, the operation determination means 21 performs the determination of the operating state on the basis of the switching state of the four-way valve 4.

[0020] The heat rejection control means 22 controls cooling in the discharge gas cooling unit 2 on the basis of the determination by the operation determination means 21. Specifically, the heat rejection control means 22 controls an on/off operation of the pump 12, thereby controlling heat rejection of refrigerant in the heat exchanger 14. If the operation determination means 21 determines that the operating state is the cooling operation, the heat rejection control means 22 controls the pump 12 so that cooling in the heat exchanger 14 is performed. If the operation determination means 21 determines that the operating state is the heating operation, the heat rejection control means 22 stops operation of the pump 12 so that no cooling is performed.

[0021] Fig. 2 is a graph showing an example of a p-h diagram in the cooling operation of the air-conditioning apparatus 100. Referring now to Figs. 1 and 2, an example operation in the cooling operation of the air-conditioning apparatus will be described. First, refrigerant in the state of low-pressure gas is sucked in the compressor 1 (state a), and the low-pressure gas is compressed by the compressor 1 to be a high-temperature high-pressure gas (state b).

Discharge gas from the compressor 1 is cooled by the discharge gas cooling unit 2 (state c), and is separated into refrigerant and refrigerating machine oil in the oil separator 3. The refrigerating machine oil obtained by the separation in the oil separator 3 returns to a suction port of the compressor 1. On the other hand, refrigerant is condensed by the outdoor heat exchanger 9 to be high-pressure liquid refrigerant (state d). The liquid refrigerant becomes low-pressure two-phase refrigerant by the expansion valve 8 (state e), passes through the refrigerant-side extension pipe 7, and becomes low-pressure gas in the indoor heat exchanger 6 (state f). Thereafter, the low-pressure gas passes through the gas-side extension pipe 5 and returns to the compressor 1 (state a).

[0022] Fig. 3 is a graph showing an example of a p-h diagram in the heating operation of the air-conditioning apparatus 100. Referring now to Figs. 1 and 3, an example operation in the heating operation of the air-conditioning apparatus will be described. First, low-pressure gas is sucked in the compressor 1 (state al 0), is compressed by the compressor 1, and then becomes high-temperature high-pressure gas (state blO). The discharge gas cooling unit 2 performs no heat rejection, and the high-temperature high-pressure gas is separated into refrigerant and refrigerating machine oil in the oil separator 3.

The refrigerating machine oil obtained by the separation in the oil separator 3 returns to an inlet of the compressor 1. On the other hand, refrigerant passes through the gas-side extension pipe 5, is condensed in the indoor heat exchanger 6, and becomes high-pressure liquid refrigerant (state dl 0). The high-pressure liquid refrigerant becomes low-pressure two-phase refrigerant in the expansion valve 8 (state elO). The resulting refrigerant passes through the refrigerant-side extension pipe 7, becomes low-pressure gas in the outdoor heat exchanger 9 (state fi 0), and returns to the compressor 1 (state al 0).

[0023] As shown in Fig. 2, cooling by the discharge gas cooling unit 2 in the cooling operation promotes separation of refrigerating machine oil from refrigerant. Thus, in the cooling operation, a pressure loss through a segment from an evaporator outlet to the compressor inlet can be reduced, and the GOP can be enhanced by performing heat rejection of refrigerant in the discharge gas cooling unit 2. On the other hand, as shown in Fig. 3, no cooling operation is performed in the discharge gas cooling unit 2 in the heating operation, and thus, an enthalpy difference in the outdoor heat exchanger (the condenser) can be reduced, thereby reducing a decrease in COP.

[0024] Fig. 4 is a graph showing a relationship between a pressure loss and a GOP from the evaporator outlet to the compressor inlet. In Fig. 4, the abscissa represents (pressure at evaporator outlet)/(pressure at compressor inlet) x 100, which is 100% when the pressure loss is 0 (zero). As the pressure loss increases (i.e., the distance increases), the percentage (%) increases. The ordinate represents the percentage to COP in a case where the pressure loss is O (at 100% on the abscissa). Fig. 4 shows that the COP decreases with an increase in the pressure loss through the segment from the evaporator outlet to the compressor inlet.

[0025] As described above, in the cooling operation, the indoor heat exchanger 6 serves as an evaporator and the outdoor heat exchanger 9 serves as a condenser. On the other hand, in the heating operation, the indoor heat exchanger 6 serves as a condenser and the outdoor heat exchanger 9 serves as an evaporator. Specifically, the distance from the evaporator outlet to the compressor 1 refers to a distance from the indoor heat exchanger 6 of the indoor unit 11 to the compressor 1 of the outdoor unit 10 in the cooling operation, and refers to a distance from the outdoor heat exchanger 9 of the outdoor unit 10 to the compressor 1 of the outdoor unit lOin the heating operation.

[0026] In the cooling operation, the outdoor unit 10 and the indoor unit 11 are connected to each other by using the long gas-side extension pipe 5. Thus, a significant pressure loss, which is a major cause of COP decrease for a refrigeration cycle, occurs through a segment from the outlet of the evaporator (the indoor heat exchanger 6) to the inlet of the compressor 1. On the other hand, in the heating operation, although the outdoor heat exchanger 9 serves as an evaporator, the outdoor heat exchanger 9 and the compressor 1 constitute the same outdoor unit 10, the pipe connecting the outdoor heat exchanger 9 and the compressor 1 is greatly shorter than the gas-side extension pipe 5 described above. That is, unlike the cooling operation, a pipe that is a cause of a significant pressure loss such as the gas-side extension pipe 5 is not present between the evaporator outlet and the compressor inlet. Thus, in the heating operation, a decrease in pressure loss through a segment from the outlet of the evaporator (the outdoor heat exchanger 9) to the inlet of the compressor 1 can be minimized.

[0027] Fig. 5 is a graph showing a relationship between the oil circulation rate (= {oil flow rate/(refrigerant flow rate + oil flow rate)} x 100) and the percentage to COP. In Fig. 5, as the oil circulation rate increases, the percentage to COP decreases. Specifically, as the amount of refrigerating machine oil separated by the oil separator 3 from the discharge gas in the compressor 1 increases, the pressure loss in the gas-side extension pipe 5 decreases so that the COP increases. In other words, the COP can be enhanced irrespective of the operating state by cooling the discharge gas in the discharge gas cooling unit 2 and promoting separation of oil in the oil separator 3. However, it was found that cooling by the discharge gas cooling unit 2 in the heating operation disadvantageously reduces the COP.

[0028] Fig. 6 is a graph showing a relationship between the amount of heat rejection and the COP in the discharge gas cooling unit 2 in the heating operation.

In Fig. 6, the abscissa represents the percentage (the amount of heat rejection in the discharge gas cooling unit 2/the amount of heat rejection in the entire apparatus} x 100) of the amount of heat rejection in the discharge gas cooling unit 2 with respect to the amount of heat rejection of the entire refrigerant in the heating operation. The ordinate represents the percentage to COP in a case where the amount of heat rejection in the discharge gas cooling unit 2 is 0 (zero).

As shown in Fig. 6, as the amount of heat rejection increases, the percentage to COP decreases.

[0029] Specifically, in the heating operation, cooling of discharge gas from the compressor 1 by the discharge gas cooling unit 2 means that the quantity of heat that is originally intended to be used for heating the room with the indoor heat exchanger 6 is radiated in the discharge gas cooling unit 2 located upstream of the indoor heat exchanger 6. To compensate for the capacity necessary for heating, operation of the compressor 1 needs to be accelerated, which might cause cooling by the discharge gas cooling unit 2 to reduce the percentage to COP.

[0030] In addition, as described above, even when the separation efficiency in the oil separator 3 is increased, the efficiency in increasing the COP is small because of an originally small pressure loss thorough a segment from the outlet of the outdoor heat exchanger (the evaporator) 9 to the inlet of the compressor 1 (see Fig. 4). In other words, a decrease in COP caused by acceleration of the compressor frequency for compensation for heat rejection in the discharge gas cooling unit 2 exceeds the increase in COP obtained by reducing the oil circulation rate with cooling of the discharge gas. As a result, the COP decreases as a whole.

[0031] In view of this, the heat rejection control means 22 performs control such that the discharge gas cooling unit 2 performs cooling in the cooling operation.

In this manner, the discharge gas cooling unit 2 performs heat rejection with a reduction of a pressure loss through the segment from the evaporator outlet to the compressor inlet in the cooling operation, thereby enhancing the COP. On the other hand, in the heating operation, the heat rejection control means 22 performs control such that the discharge gas cooling unit 2 does not perform cooling. In this manner, a decrease in enthalpy difference in the outdoor heat exchanger (the condenser) can be avoided, thereby reducing a COP decrease.

[0032] Specifically, in an air-conditioning apparatus that always performs a cooling operation, a heat exchanger is placed in a cabinet such as a refrigerator, an outdoor unit is placed outside the cabinet, and the heat exchanger in the cabinet is connected to the outdoor unit by an extension pipe. In such an air-conditioning apparatus, the long extension pipe increases a pressure loss through a segment from an evaporator outlet to a compressor inlet, which significantly affects the COP of the air-conditioning apparatus. Thus, the COP can be significantly enhanced by reducing the pressure loss through the segment from the evaporator outlet to the compressor inlet due to an increase in separation efficiency. In addition, the COP can also be enhanced by reducing the condensing temperature with cooling of compressor discharge gas.

[0033] On the other hand, in the air-conditioning apparatus 100, such as an air conditioner, which performs a cooling operation and a heating operation, the COP disadvantageously decreases in the heating operation. In the air-conditioning apparatus 100, such as an air conditioner, which performs cooling and heating, the indoor-side heat exchanger 6 is placed inside the room and the outdoor unit 10 is placed outside the room such that the indoor heat exchanger 6 and the outdoor unit 10 are connected together by the extension pipes 5 and 7.

This configuration is similar to those of, for example, a refrigerator Thus, in the cooling operation, cooling of gas discharged from the compressor reduces the condensing temperature. Since the long extension pipe connects the evaporator outlet to the compressor inlet, the COP is significantly enhanced by reducing the oil circulation rate.

[0034] However, in the heating operation in which the indoor heat exchanger serves as a condenser and the outdoor heat exchanger serves as an evaporator, cooling of the discharge gas in the compressor 1 leads to extraction of heat in an amount that is originally intended to be used for heating in the indoor heat exchanger 6. The pipe connecting the evaporator outlet and the inlet of the compressor 1 together is short because the evaporator and the compressor 1 are connected in the same outdoor unit. In addition, the effect of enhancing the COP obtained by reducing the oil circulation rate is very small. Thus, in the heating operation, cooling of discharge gas of the compressor disadvantageously reduces the COP.

[0035] In view of this, in the air-conditioning apparatus 100, the discharge gas cooling unit 2 performs cooling in the cooling operation. Thus, the COP can be enhanced by performing heat rejection in the discharge gas cooling unit 2 with a reduced pressure loss through the segment from the evaporator outlet to the compressor inlet in the cooling operation. On the other hand, in the heating operation, the discharge gas cooling unit 2 does not perform cooling (heat rejection). Thus, a decrease in COP can be reduced while avoiding a decrease in enthalpy difference in the outdoor heat exchanger (the condenser).

[0036] Fig. 7 is a flowchart showing a preferred embodiment of a method for controlling an air-conditioning apparatus according to the present invention.

Referring now to Figs. 1 and 7, a method for controlling an air-conditioning apparatus 100 will be described. First, when operation starts (step ST1), the operation determination means 21 determines whether the operation is a cooling operation or a heating operation (step S12). If the operation is determined not to be the cooling operation but the heating operation, the discharge gas cooling unit 2 performs cooling (step ST3). On the other hand, if the operation is determined to be the cooling operation, operation of the heat rejection control means 22 allows the discharge gas cooling unit 2 to cool discharge gas.

[0037] In this manner, in the cooling operation, a pressure loss through a segment from an evaporator outlet to a compressor inlet is reduced, and heat rejection of refrigerant is performed in the discharge gas cooling unit 2, thereby enhancing the COP On the other hand, since no cooling (heat rejection) is performed in the discharge gas cooling unit 2 in the heating operation, a decrease in enthalpy difference in the outdoor heat exchanger (the condenser) 9 is avoided, thereby reducing a COP decrease.

[0038] Embodiment 2 Fig. 8 schematically shows an air-conditioning apparatus according to Embodiment 2 of the present invention. Referring now to Figs. 1 and 8, the air- conditioning apparatus will be described. In the method for controlling an air-conditioning apparatus shown in Fig. 8, like reference signs designate the identical or corresponding process steps in the method for controlling an air-conditioning apparatus shown in Fig. 7, and detailed description will not be repeated. The method for controlling an air-conditioning apparatus shown in Fig. 8 differs from that for the air-conditioning apparatus 100 shown in Fig. 1 in that discharge gas is cooled by the discharge gas cooling unit 2 in a case where an operating state is a cooling operation and an operating frequency f is greater than or equal to an operating frequency fref.

[0039] Specifically, the heat rejection control means 22 illustrated in Fig. 1 has the function of determining whether the operating frequencyf of the compressor 1 is greater than or equal to a predetermined operating frequency fref. If the operating state is a cooling operation and the compressor 1 operates at an operating frequency greater than or equal to the predetermined operating frequency fref, the heat rejection control means 22 controls the discharge gas cooling unit 2 such that the discharge gas cooling unit 2 cools discharge gas.

On the other hand, even if the operating state is the cooling operation but the compressor 1 operates at an operating frequency less than or equal to the predetermined operating frequency fref, the discharge gas cooling unit 2 is controlled not to cool the discharge gas. The predetermined operating frequency fref is determined in advance in consideration of a reduction in pressure loss obtained by reducing the oil circulation rate and increasing the output power of, for example, a pump of the discharge gas cooling unit 2.

[0040] In this manner, it is possible to prevent the influence of a decrease in COP caused by driving of a power source such as a pump 12 of the discharge gas cooling unit 2 from increasing. Specifically, the operating frequency f of the compressor 1 and the oil circulation rate are linearly related, that is, as the operating frequency increases, the oil circulation rate also increases. Thus, in a case where the operating frequency f of the compressor 1 is low, the effect of enhancing the COP obtained by reducing the oil circulation rate with the use of power of, for example, the pump of the discharge gas cooling unit 2 might be smaller than the influence of a reduced COP caused by power of, for example, the pump of the discharge gas cooling unit 2. To prevent this situation, a threshold process of the operating frequency f can prevent the influence of a reduced COP caused by power of, for example, the pump of the discharge gas cooling unit 2 from increasing.

[0041] In the case of performing a cooling operation at an operating frequency f around the predetermined operating frequency fref, cooling in the discharge gas cooling unit 2 is switched between on and off so that operation might be unstable.

Thus, in a case where the operating frequency becomes the predetermined operating frequency or more even when a predetermined period in which the operating frequency is less than or equal to the predetermined operating frequency does not elapse, the heat rejection control means 22 may control the discharge gas cooling unit 2 such that the discharge gas cooling unit 2 promptly stops cooling of discharge gas. Thereafter, when the operating time in which the operating frequency is greater than or equal to the predetermined operating frequency fref continues for a predetermined period or longer, the heat rejection control means 22 may control the discharge gas cooling unit 2 such that the discharge gas cooling unit 2 starts cooling of the discharge gas. Alternatively, cooling operation of the discharge gas cooling unit 2 may not be promptly switched depending on the predetermined operating frequency fref by using, for example, a variation rate of the operating frequency f.

[0042] Embodiment 3 Fig. 9 schematically illustrates an air-conditioning apparatus according to Embodiment 3 of the present invention. Referring now to Fig. 9, an air-conditioning apparatus 200 will be described. In the air-conditioning apparatus illustrated in Fig. 9, like reference signs designate the identical or corresponding parts of the air-conditioning apparatus 100 shown in Fig. 1, and detailed description will not be repeated. The air-conditioning apparatus 200 illustrated in Fig. 9 differs from the air-conditioning apparatus 100 illustrated in Fig. 1 in the configuration of the discharge gas cooling unit.

[0043] A discharge gas cooling unit 202 illustrated in Fig. 9 includes a first three-way valve 215 and a heat exchanger 14. The first three-way valve 215 and the heat exchanger 14 constitute a circulation circuit. The first three-way valve 215 is connected to a suction side of a compressor 1, a four-way valve 4, and the heat exchanger 14. A pipe is branched such that suction gas flows into both the compressor 1 and a cooling channel 14a of the heat exchanger 14 at a side downstream of the three-way valve 215. Thus, the discharge gas cooling unit 202 has a configuration that can cool discharge gas flowing in a refrigerant circuit by circulating un-compressed suction gas in the cooling channel 14a of the heat exchanger 14.

[0044] A heat rejection control means 222 determines whether suction gas that is yet to enter the compressor 1 circulates in the cooling channel 14a of the heat exchanger 14 or not by switch control with the first three-way valve 215.

Specifically, the heat rejection control means 222 controls switching of the three-way valve 215 such that suction gas flows in the heat exchanger 14 in a cooling operation. Then, heat exchange is performed between suction gas and discharge gas from the compressor 1 in the heat exchanger 14, thereby cooling discharge gas. On the other hand, the heat rejection control means 222 controls switching of the three-way valve such that refrigerant that is yet to be compressed does not flow in the heat exchanger 14 in a heating operation.

Then, no suction gas flows in the heat exchanger 14, and thus, discharge gas from the compressor 1 is not cooled.

[0045] The above-described configuration of Embodiment 3 can also reduce a pressure loss through a segment from an evaporator outlet to a compressor inlet in the cooling operation and enhance the COP by performing heat rejection of refrigerant in the discharge gas cooling unit 202. On the other hand, since no cooling is performed in the discharge gas cooling unit 202 in the heating operation, a decrease in enthalpy difference in the outdoor heat exchanger (the condenser) 9 can be avoided, thereby reducing a decrease in COP In addition, in performing heat rejection in the cooling operation, refrigerant can be heated in the heat exchanger 14 before the refrigerant is sucked in the compressor 1, and it is possible to prevent liquid refrigerant from returning to the compressor 1 and damaging the compressor 1.

[0046] Embodiment 4 Fig. 10 schematically illustrates an air-conditioning apparatus according to Embodiment 4 of the present invention. Referring now to Fig. 10, an air- conditioning apparatus 300 will be described. In the description of the air-conditioning apparatus 300 illustrated in Fig. 10, like reference signs designate the identical or corresponding parts of the air-conditioning apparatus 100 shown in Fig. 1, and detailed description will not be repeated. The air-conditioning apparatus 300 illustrated in Fig. 10 differs from the air-conditioning apparatus 100 illustrated in Fig. 1 in the configuration of a discharge gas cooling unit 302.

[0047] The discharge gas cooling unit 302 illustrated in Fig. 10 includes a heat rejecter 13 connected between a discharge-side pipe of a compressor 1 and an oil separator 3 and a damper 311 that blocks or allows an air supply of part of air supplied from the air supply unit 16 to the heat rejecter 13. A duct for conducting air from the air supply unit 16 may be disposed between the air supply unit 16 and the heat rejecter 13. The damper 311 has the function of changing the wind direction between a direction in which air from the air supply unit 16 strikes the heat rejecter 13 and a direction in which air from the air supply unit 16 is blocked. Operation of the damper 311 is controlled by the heat rejection control means 322. The cooling effect can be enhanced by providing a fin to the heat rejecter 13 so that the surface area is increased.

[0048] The heat rejection control means 322 performs control such that the damper 311 is opened in a cooling operation such that air is sent to the heat rejecter 13. Then, the heat rejecter 13 cools discharge gas. On the other hand, in a heating operation, the heat rejection control means 322 closes the damper 311 so as to block an air flow to the heat rejecter 13. Then, the discharge gas cooling unit 2 does not cool discharge gas.

[0049] The above-described configuration of Embodiment 4 can also promote separation of refrigerating machine oil from refrigerant, and thus, a pressure loss through a segment from an evaporator outlet to a compressor inlet is reduced in the cooling operation, and the discharge gas cooling unit 302 performs heat rejection, thereby enhancing the COP. On the other hand, in the heating operation, the discharge gas cooling unit 302 does not perform cooling (heat rejection), and thus, a decrease in enthalpy difference in the outdoor heat exchanger (the condenser) 6 can be avoided, thereby reducing a decrease in COP. The configuration illustrated in Fig. 10 allows the adjustment of the amount of heat rejection by the heat rejecter 13 to be performed only in the outdoor unit 10.

[0050] Embodiment 5 Fig. 11 schematically illustrates an air-conditioning apparatus according to Embodiment 5 of the present invention. Referring now to Fig. 11, an air-conditioning apparatus 400 will be described. In the air-conditioning apparatus 400 illustrated in Fig. 11, like reference signs designate the identical or corresponding parts of the air-conditioning apparatus 100 shown in Fig. 1, and detailed description will not be repeated. The air-conditioning apparatus 400 illustrated in Fig. 11 differs from the air-conditioning apparatus 100 illustrated in Fig. 1 in the configuration of a discharge gas cooling unit.

[0051] Adischarge gas cooling unit 402 includes a pump 12, a heat rejecter 13 connected to the pump 12, a heat exchanger 14 connected to the heat rejecter 13 and configured to perform heat exchange between discharge gas from a compressor 1 and circulating water, for example, and a second three-way valve 418 having two ports connected to a discharge pipe of the compressor 1 and the heat exchanger 14 and the other port bypassing from the discharge pipe of the compressor 1 to an inlet of an oil separator 3. That is, the channel is switched between the channel in which discharge gas from the compressor 1 passes through the heat exchanger 14 and the channel in which the discharge gas bypasses the heat exchanger 14 and flows into the oil separator 3 by switching the second three-way valve. The operation of the second three-way valve 418 is controlled by the heat rejection control means 22.

[0052] The heat rejection control means 422 switches the second three-way valve 418 such that discharge gas from the compressor 1 passes through the heat exchanger 14 in a cooling operation. Then, the discharge gas is cooled in the heat exchanger 14. On the other hand, the heat rejection control means 422 switches the second three-way valve 418 such that the discharge gas bypasses the heat exchanger 14 that exchanges heat with a discharge pipe of the compressor 1 in a heating operation. Then, the discharge gas flows into the oil separator 3 without passing through the heat exchanger 14, and the discharge gas is not cooled.

[0053] The above-described configuration of Embodiment 5 can also promote separation of refrigerating machine oil from refrigerant, and thus, a pressure loss through a segment from an evaporator outlet to a compressor inlet is reduced in the cooling operation, and the discharge gas cooling unit 402 performs heat rejection of refrigerant, thereby enhancing the GOP On the other hand, in the heating operation, the discharge gas cooling unit 402 does not perform cooling (heat rejection), and thus, a decrease in enthalpy difference in an outdoor heat exchanger (a condenser) 6 can be avoided, thereby reducing a decrease in COP.

[0054] Embodiments of the present invention are not limited to those described above. For example, in the above description, the heat rejection control means 22, 222, 322, and 422 perform control such that heat rejection is performed in a cooling operation but is not performed in operations except the cooling operation.

Alternatively, the adjustment may be performed to increase or decrease the amount of heat rejection in the cooling operation. For example, as illustrated in Fig. 8, in a case where the operating state is a cooling operation and the operating frequency is less than the predetermined operating frequency fref, cooling may be performed with a reduced flow rate at the cooling channel 14a of the heat exchanger 14 or with a low cooling capacity by, for example, controlling the damper 311 so as to reduce the amount of air supplied to the heat rejecter 13 in Fig. 10.

[0055] In the above-described configuration of the oil separator 3, a hollow container is used as an example. Alternatively, various known techniques such as a technique of reducing the flow rate of refrigerant gas in order to drop fine oil particles by utilizing the dead weight thereof and a technique of providing a filter in an oil separator so as to collect fine oil particles may be employed.

Reference Signs List [0056] 1 compressor, 2, 202, 302, 402 discharge gas cooling unit, 3 oil separator, 4 four-way valve, 5 gas-side extension pipe, 6 indoor heat exchanger, 7 refrigerant-side extension pipe, 8 expansion valve, 9 outdoor heat exchanger, 10 outdoor unit, 11 indoor unit, 12 pump, 13 heat rejecter, 14 heat exchanger, 14a cooling channel, 16 air supply unit, 20 outdoor unit controller, 21 operation determination means, 22, 222, 322, 422 heat rejection control means, 100, 200, 300, 400 air-conditioning apparatus, 215 first three-way valve, 311 damper, 418 second three-way valve, f operating frequency, fref predetermined operating frequency.