Patent Publication Number: US-2022235972-A1

Title: Refrigeration cycle apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Application No. PCT/JP2019/034210 filed on Aug. 30, 2019, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a refrigeration cycle apparatus. 
     BACKGROUND 
     As one of representative refrigeration cycle apparatuses, there is an air-conditioning apparatus. Some air-conditioning apparatus is provided with an accumulator formed to store excess refrigerant produced, for example, by a difference in operating condition between cooling operation and heating operation. 
     For example, an air-conditioning apparatus described in Patent Literature 1 includes an accumulator having a tank, an inlet pipe, and an outlet pipe. The tank stores excess refrigerant. The inlet pipe is connected to an upper part of the tank and through which evaporated refrigerant is guided into the tank. The outlet pipe has a U-shaped bent portion. In the outlet pipe, gas refrigerant is sucked through a gas suction opening and, through the outlet pipe, the gas refrigerant is delivered toward a compressor. Further, the outlet pipe has an oil return hole bored through the bent portion of the outlet pipe and through which refrigerating machine oil accumulated at the bottom of the tank is guided back to the compressor. 
     In the air-conditioning apparatus described in Patent Literature 1, a large amount of liquid refrigerant is temporarily stored in the tank of the accumulator during start-up of heating or during return after completion of defrosting in a case in which the outside air temperature is very low. 
     In a state in which a large amount of liquid refrigerant is stored in the tank of the accumulator, a large amount of liquid refrigerant is sucked into the compressor through the oil return hole, with the result that there is an increase in the amount of liquid return to the compressor. This is undesirable in terms of reliability of the compressor. The term “liquid return” means the suction of refrigerant into the compressor in the form of a liquid. 
     Further, activating the compressor in a state in which a large amount of liquid refrigerant is stored in the tank of the accumulator causes many oil contents to be released from the compressor into a refrigerant circuit, with the result that these oil contents are retained again in the accumulator together with liquid refrigerant. At this time, specific gravity brings about a state of commonly-called two-layer separation in which a layer of refrigerating machine oil is formed on top of a layer of liquid refrigerant accumulated in a lower part of the tank of the accumulator. In a state of occurrence of two-layer separation, the liquid refrigerant or the refrigerating machine oil is sucked into the compressor through the oil return hole, which is followed by the degree of flow differential pressure. Especially at low temperatures, there is a decrease in efficiency of oil return to the compressor, as there is an increase in viscosity of the refrigerating machine oil. 
     For this reason, the air-conditioning apparatus described in Patent Literature 1 is provided with a bypass through which a portion of hot gas discharged from the compressor is introduced into the bottom of the tank of the accumulator. Hot gas guided from the compressor into the bypass is injected into the bottom of the tank and blown up from the bottom toward the inside of the tank. In this manner, the layer of liquid refrigerant and the layer of refrigerating machine oil in the tank are efficiently stirred by the hot gas thus injected, quickly evaporate and gasify, and are guided into the compressor via the outlet pipe. 
     Thus, in the air-conditioning apparatus described in Patent Literature 1, providing the bypass through which a portion of hot gas discharged from the compressor is introduced brings about improvement in efficiency of oil return to the compressor by mixing together liquid refrigerant and refrigerating machine oil even in a state of occurrence of two-layer separation. This reduces liquid return to the compressor and brings about improvement in reliability of the compressor. 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 4295530 
       
    
     However, introducing hot gas through a direct bypass into the bottom of the tank of the accumulator as in the case of the air-conditioning apparatus described in Patent Literature 1 undesirably makes the tank of the accumulator complex in structure and makes the installation of pipes or other acts complex. 
     SUMMARY 
     The present disclosure has been made to solve such a problem and has an object to provide a refrigeration cycle apparatus configured to bring about improvement in reliability of a compressor without making an accumulator complex in structure. 
     A refrigeration cycle apparatus according to an embodiment of the present disclosure includes a refrigerant circuit in which a compressor, a first heat exchanger, a first expansion device, a second expansion device, and a second heat exchanger are connected by refrigerant pipes and through which refrigerant circulates, an accumulator provided to the refrigerant circuit, formed to store liquid refrigerant, and from which gas refrigerant is caused to be sucked into the compressor, a liquid-level sensing device provided to the accumulator and configured to sense a liquid level of the liquid refrigerant stored in the accumulator, and a controller configured to, in a case in which the liquid level sensed by the liquid-level sensing device is higher than a threshold, perform a limiting operation of reducing an amount of suction of the gas refrigerant that is sucked from the accumulator into the compressor and, in a case in which the liquid level is lower than or equal to the threshold, perform a normal operation. 
     A refrigeration cycle apparatus according to an embodiment of the present disclosure makes it possible to bring about improvement in reliability of a compressor without making an accumulator complex in structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 1. 
         FIG. 2  is a block diagram showing a configuration of a controller of an air-conditioning apparatus according to Embodiment 1. 
         FIG. 3  is a plan view showing an example of a structure of a liquid-level sensing device of  FIG. 1 . 
         FIG. 4  is a schematic view for explaining a shape of the liquid-level sensing device of  FIG. 3  to be attached to an accumulator. 
         FIG. 5  is a perspective view showing a state in which the liquid-level sensing device of  FIG. 3  is attached to the accumulator. 
         FIG. 6  is a cross-sectional view schematically showing the state in which the liquid-level sensing device of  FIG. 3  is attached to the accumulator. 
         FIG. 7  is a diagram showing a graph for explaining a relationship between surface temperatures of the accumulator and the heights of temperature sensors. 
         FIG. 8  is a flow chart showing an example of a sequence of actions making up a liquid-level sensing process in the air-conditioning apparatus according to Embodiment 1. 
         FIG. 9  is a flow chart showing a sequence of actions making up a frequency control process of the air-conditioning apparatus according to Embodiment 1. 
         FIG. 10  is a block diagram showing a configuration of a controller of an air-conditioning apparatus according to Embodiment 2. 
         FIG. 11  is a flow chart showing a sequence of actions making up a process of controlling the opening degree of an expansion device of the air-conditioning apparatus according to Embodiment 2. 
         FIG. 12  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 3. 
         FIG. 13  is a block diagram showing a configuration of a controller of an air-conditioning apparatus according to Embodiment 3. 
         FIG. 14  is a flow chart showing a sequence of actions making up a process of controlling the opening and closing of a hot-gas bypass valve of the air-conditioning apparatus according to Embodiment 3. 
         FIG. 15  is a block diagram showing a configuration of a controller of an air-conditioning apparatus according to Embodiment 4. 
         FIG. 16  is a flow chart showing a sequence of actions making up a control process of the air-conditioning apparatus according to Embodiment 4. 
         FIG. 17  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 5. 
         FIG. 18  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 6. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, refrigeration cycle apparatuses according to embodiments of the present disclosure are described with reference to the drawings. The present disclosure is not limited to the following embodiments, but may be variously changed without departing from the scope of the present disclosure. Further, the present disclosure encompasses all combinations of combinable ones of constituent elements shown in the following embodiments. Further, constituent elements given identical reference signs in each drawing are identical or equivalent to each other, and these reference signs are common throughout the full text of the description. Pieces of equipment or other devices of the same kind that are for example differentiated by subscripts such as A and B may be described with an omission of subscripts in a case in which they do not particularly need to be differentiated or identified. Furthermore, in each drawing, relative relationships in dimension between constituent elements, the shapes of the constituent elements, or other features of the constituent elements may be different from actual ones. 
     Embodiment 1 
     In the following, a refrigeration cycle apparatus according to Embodiment 1 is described.  FIG. 1  is a schematic diagram showing a configuration of the refrigeration cycle apparatus according to Embodiment 1.  FIG. 1  shows an air-conditioning apparatus  1000  as an example of the refrigeration cycle apparatus. As shown in  FIG. 1 , the air-conditioning apparatus  1000  includes an outdoor unit  1 , two indoor units  2 A and  2 B, and a controller  3 . The two indoor units  2 A and  2 B are connected in parallel to each other, and are identical in configuration to each other. The outdoor unit  1  and each of the indoor units  2 A and  2 B are connected by refrigerant pipes to form a refrigerant circuit. Although, in the example shown in  FIG. 1 , two indoor units  2 A and  2 B are connected to one outdoor unit  1 , this is not intended to impose any limitation, and one indoor unit  2  or three or more indoor units  2  may be connected. Further, a plurality of outdoor units  1  may be connected. 
     The air-conditioning apparatus  1000  according to Embodiment 1 includes a liquid-level sensing device  15 . During start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the position of a liquid surface of refrigerant stored in an accumulator  14  installed in the outdoor unit  1 . A state in which the position of the liquid surface is higher than a preset threshold Th is hereinafter referred to as a state in which “the liquid level is higher than the threshold Th”. A state in which the liquid level in the accumulator  14  is higher than the threshold Th means that the amount of refrigerant that is contained in the refrigerant circuit excluding the accumulator  14  is small. In this case, there is an increase in gas flow rate of refrigerant flowing through the refrigerant circuit, so that the compressor  11  easily sucks low-pressure gas refrigerant. As a result, there is a possibility that there may occur liquid return from the accumulator  14  to the compressor  11 . When liquid return occurs, liquid refrigerant accumulates at the bottom of the compressor  11 , so that there is a deterioration of start-up characteristics during heating. For this reason, in Embodiment 1, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  performs a limiting operation of reducing the amount of suction of refrigerant that is sucked from the accumulator  14  into the compressor  11 . Specifically, the controller  3  makes the frequency of the compressor  11  installed in the outdoor unit  1  lower than or equal to a preset first specified value Sp 1 . This makes it possible to reduce the occurrence of liquid return to the compressor  11 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. In the following, the air-conditioning apparatus  1000  according to Embodiment 1 is described in detail. 
     [Configuration of Air-Conditioning Apparatus  1000 ] 
     (Outdoor Unit  1 ) 
     The outdoor unit  1  is installed outdoors. The outdoor unit  1  includes the compressor  11 , a refrigerant flow switching device  12 , an outdoor heat exchanger  13 , the accumulator  14 , and an expansion device  20 . 
     The compressor  11  sucks in low-pressure gas refrigerant, compresses the gas refrigerant thus sucked in, and discharges resultant high-temperature and high-pressure gas refrigerant. Examples of the compressor  11  include an inverter compressor whose frequency is varied and whose capacity, which is an amount of refrigerant that is sent out per unit time, is thus controlled. The frequency of the compressor  11  is controlled by the controller  3 . 
     The refrigerant flow switching device  12  is for example a four-way valve, and performs switching between cooling operation and heating operation by switching the directions in which refrigerant flows. During cooling operation, the refrigerant flow switching device  12  switches such that a discharge side of the compressor  11  is connected to the outdoor heat exchanger  13  as indicated by a solid line in  FIG. 1 . Further, during heating operation, the refrigerant flow switching device  12  switches such that the discharge side of the compressor  11  is connected to indoor heat exchangers  22 A and  22 B of the indoor units  2 A and  2 B as indicated by a dashed line in  FIG. 1 . Switching between flow passages in the refrigerant flow switching device  12  is controlled by the controller  3 . 
     The outdoor heat exchanger  13  exchanges heat between outdoor air supplied by a fan (not illustrated) or other devices and refrigerant flowing through the refrigerant circuit. The outdoor heat exchanger  13  is a first heat exchanger. During cooling operation, the outdoor heat exchanger  13  operates as a condenser formed to condense the refrigerant by transferring the heat of the refrigerant to the outdoor air. Further, during heating operation, the outdoor heat exchanger  13  operates as an evaporator formed to evaporate the refrigerant and cool the outdoor air by the heat of vaporization. 
     The accumulator  14  is provided on a low-pressure side that is a suction side of the compressor  11 . The accumulator  14  includes a hermetic container  141 , an inlet pipe  142  through which refrigerant is introduced into the hermetic container  141 , and a U-shaped outlet pipe  143  through which gas refrigerant is delivered from inside. 
     The accumulator  14  is formed such that excess refrigerant produced by a difference in operating condition between cooling operation and heating operation or excess refrigerant produced by a transient change in operation is introduced through the inlet pipe  142  into the hermetic container  141 . In the hermetic container  141  of the accumulator  14 , the excess refrigerant thus introduced is separated into gas refrigerant and liquid refrigerant. The gas refrigerant thus separated is sucked from the accumulator  14  via the outlet pipe  143  into the compressor  11 . Meanwhile, the liquid refrigerant thus separated is stored in the accumulator  14 . The liquid refrigerant thus separated either is sucked in small amounts into the compressor  11  through an oil return hole  143   a  or evaporates and gasifies with the passage of time and is sucked into the compressor  11 . 
     The expansion device  20  has a variable opening degree and adjusts the flow rate of refrigerant. The expansion device  20  is connected between the outdoor heat exchanger  13  and the indoor heat exchangers  22 A and  22 B. The expansion device  20  is, for example, a valve, such as an electronic expansion valve, whose opening degree is controllable. The opening degree of the expansion device  20  is controlled by the controller  3 . The expansion device  20  is a first expansion device. 
     Further, the outdoor unit  1  includes a refrigerant temperature sensor  18  and an outside air temperature sensor  19 . The refrigerant temperature sensor  18  is provided at a refrigerant inlet of the accumulator  14  and senses the temperature of refrigerant flowing into the accumulator  14 . The outside air temperature sensor  19  senses the temperature of outside air. 
     (Indoor Unit  2 A) 
     The indoor unit  2 A is installed indoors. The indoor unit  2 A includes an expansion device  21 A and the indoor heat exchanger  22 A. 
     The expansion device  21 A has a variable opening degree and adjusts the flow rate of refrigerant. The expansion device  21 A is connected between the indoor heat exchanger  22 A and the expansion device  20 . The expansion device  21 A is, for example, a valve, such as an electronic expansion valve, whose opening degree is controllable. The opening degree of the expansion device  21 A is controlled by the controller  3 . 
     The indoor heat exchanger  22 A exchanges heat between air supplied by a fan (not illustrated) or other devices and refrigerant flowing through the refrigerant circuit. This produces heating air or cooling air that is supplied to an indoor space. The indoor heat exchanger  22 A operates as an evaporator during cooling operation to perform cooling by cooling air in an air-conditioned space. Further, the indoor heat exchanger  22 A operates as a condenser during heating operation to perform heating by heating air in an air-conditioned space. 
     (Indoor Unit  2 B) 
     The indoor unit  2 B is installed indoors. The indoor unit  2 B includes an expansion device  21 B and the indoor heat exchanger  22 B. 
     The expansion device  21 B has a variable opening degree and adjusts the flow rate of refrigerant. The expansion device  21 B is connected between the indoor heat exchanger  22 B and the expansion device  20 . The expansion device  21 B is, for example, a valve, such as an electronic expansion valve, whose opening degree is controllable. The opening degree of the expansion device  21 B is controlled by the controller  3 . 
     The indoor heat exchanger  22 B exchanges heat between air supplied by a fan (not illustrated) or other devices and refrigerant flowing through the refrigerant circuit. This produces heating air or cooling air that is supplied to an indoor space. The indoor heat exchanger  22 B operates as an evaporator during cooling operation to perform cooling by cooling air in an air-conditioned space. Further, the indoor heat exchanger  22 B operates as a condenser during heating operation to perform heating by heating air in an air-conditioned space. 
     The expansion devices  21 A and  21 B are each a second expansion device. 
     Further, the indoor heat exchangers  22 A and  22 B are each a second heat exchanger. 
     Further, the accumulator  14 , the compressor  11 , the outdoor heat exchanger  13 , the expansion device  20 , the expansion devices  21 A and  21 B, and the indoor heat exchangers  22 A and  22 B are connected by the refrigerant pipes to form the refrigerant circuit, through which refrigerant circulates. 
     (Controller  3 ) 
     The controller  3  exercises overall control of the outdoor unit  1  and the indoor units  2 A and  2 B. Further, the controller  3  controls the frequency of the compressor  11  on the basis of a result of sensing of the liquid surface of refrigerant in the accumulator  14 . The controller  3  has an arithmetic device such as a microcomputer, and achieves various functions by executing software such as a program stored in a memory. Alternatively, the controller  3  is dedicated hardware, such as a circuit device, that achieves various functions. Although, in  FIG. 1 , the controller  3  is provided outside the outdoor unit  1  and the indoor units  2 A and  2 B, this is not intended to impose any limitation, and the controller  3  may be provided inside any of the outdoor unit  1  and the indoor units  2 A and  2 B. 
     In Embodiment 1, as mentioned above, the accumulator  14  is provided with the liquid-level sensing device  15  configured to sense the position of the liquid surface of refrigerant stored in the accumulator  14 . The liquid-level sensing device  15  includes heaters  16  and a plurality of temperature sensors  17   a  to  17   c . In this example, three temperature sensors  17   a  to  17   c  are provided. However, this is not intended to limit the number of temperature sensors. That is, two or more temperature sensors need only be provided, as the liquid-level sensing device  15  needs only be able to sense the position of a liquid surface in the hermetic container  141  of the accumulator  14  on a scale of one to two or more. 
     Under control of the controller  3 , the heaters  16  heat a surface of the accumulator  14  uniformly in a height direction of the accumulator  14 . The height direction of the accumulator  14  is hereinafter referred to as “Z direction”. The plurality of temperature sensors  17   a  to  17   c  are disposed at different heights of the accumulator  14 , and sense surface temperatures of the accumulator  14  at the heights at which they are disposed. The temperature sensor  17   a  senses a surface temperature Ta of a lower part of the accumulator  14 . The temperature sensor  17   b  senses a surface temperature Tb of a middle part of the accumulator  14 . The temperature sensor  17   c  senses a surface temperature Tc of an upper part of the accumulator  14 . 
     The controller  3  controls, on the basis of temperatures sensed by the temperature sensors  17   a  to  17   c  of the liquid-level sensing device  15 , the refrigerant temperature sensor  18 , the outside air temperature sensor  19 , or other sensors, a liquid-level sensing operation that is performed by the liquid-level sensing device  15 . Further, the controller  3  determines the liquid level in the accumulator  14  on the basis of a result of the liquid-level sensing. Furthermore, the controller  3  controls the frequency of the compressor  11  on the basis of the liquid level in the accumulator  14 . 
       FIG. 2  is a block diagram showing a configuration of the controller  3  of an air-conditioning apparatus  1000  according to Embodiment 1. As shown in  FIG. 2 , the controller  3  includes a temperature difference calculating unit  31 , a liquid-level determining unit  32 , an output unit  33 , a heater control unit  34 , a storage unit  35 , and a frequency control unit  36 . 
     The temperature difference calculating unit  31  calculates a temperature difference ΔT high  by subtracting the surface temperature Ta of the accumulator  14  as sensed by the temperature sensor  17   a  from the surface temperature Tc of the accumulator  14  as sensed by the temperature sensor  17   c . Further, the temperature difference calculating unit  31  calculates a temperature difference ΔT middle  by subtracting the surface temperature Ta of the accumulator  14  as sensed by the temperature sensor  17   a  from the surface temperature Tb of the accumulator  14  as sensed by the temperature sensor  17   b.    
     The liquid-level determining unit  32  reads out a set value T 1  stored in the storage unit  35  and compares the set value T 1  with the temperature differences ΔT high  and ΔT middle  calculated by the temperature difference calculating unit  31 . Moreover, the liquid-level determining unit  32  determines, on the basis of a result of the comparison, the position of a liquid surface of liquid refrigerant in the accumulator  14 . 
     The output unit  33  outputs information regarding the liquid level in the accumulator  14  on the basis of a result of the determination made by the liquid-level determining unit  32 . Usable examples of the output unit  33  include a display, a light-emitting diode (LED), and a speaker. In a case in which the output unit  33  is a display, the information regarding the liquid level is displayed, for example, as characters or figures. In a case in which the output unit  33  is an LED, the information regarding the liquid level is displayed, for example, such that the LED is turned on, blinks, and is turned off. In a case in which the output unit  33  is a speaker, the information regarding the liquid level is notified by sounds. 
     The heater control unit  34  controls the turning on and turning off of the heaters  16  on the basis of various temperatures sensed by the temperature sensors  17   a  to  17   c , the refrigerant temperature sensor  18 , and the outside air temperature sensor  19 . The heater control unit  34  sends the heaters  16  a control signal for controlling the turning on and turning off of the heaters  16 . 
     The storage unit  35  stores in its inside various types of information that are used in performing a process in each unit of the controller  3 . The storage unit  35  has stored in advance in its inside the set value T 1 , which is used by the liquid-level determining unit  32 . Further, the storage unit  35  has stored in its inside set temperatures T 2 , T 3 , and T 4  that are used by the heater control unit  34 . 
     The frequency control unit  36  determines, on the basis of a result of the determination made by the liquid-level determining unit  32 , the frequency of the compressor  11  with reference to frequency information stored in the storage unit  35 . The frequency control unit  36  sends the compressor  11  a frequency control signal for controlling the frequency of the compressor  11 . 
     [Structure of Liquid-Level Sensing Device  15 ] 
     A structure of the liquid-level sensing device  15  is described.  FIG. 3  is a plan view showing an example of a structure of the liquid-level sensing device  15  of  FIG. 1 . As shown in  FIG. 3 , the liquid-level sensing device  15  includes a belt unit  151 , a heat insulating material  152 , the heaters  16 , and the temperature sensors  17   a  to  17   c.    
     The belt unit  151  is formed by a metallic component such as an elongated aluminum tape. The belt unit  151  has a length corresponding to the shape and size of the accumulator  14  to which it is attached, and is wound around the accumulator  14  in the height direction of the accumulator  14 , that is, the Z direction. 
     The heat insulating material  152  is provided on a surface of the belt unit  151 . The heat insulating material  152  is formed to extend in a length direction of the belt unit  151 . The heaters  16  are provided on the surface of the belt unit  151 . The heaters  16  are for example bendable belt heaters, and are provided along both widthwise ends of the heat insulating material  152 . 
     The length of each of the heaters  16  may be shorter than the entire length of the belt unit  151  in the length direction of the belt unit  151 , and is determined to correspond to the size of the accumulator  14 . For example, it is preferable that the length of each of the heaters  16  be about equal to the entire length of the accumulator  14  in the height direction of the accumulator  14 , that is, the Z direction with the liquid-level sensing device  15  attached to the accumulator  14 . It is not always the case that the plurality of heaters  16  are provided. For example, only one heater  16  may be provided, as long as the accumulator  14  is sufficiently heated. 
     The temperature sensors  17   a  to  17   c  are provided on the heat insulating material  152 . That is, in a case in which the liquid-level sensing device  15  is provided with the plurality of heaters  16 , the temperature sensors  17   a  to  17   c  are provided between the plurality of heaters  16 . The temperature sensors  17   a  to  17   c  are sequentially arranged in different positions in the height direction of the accumulator  14 , that is, the Z direction. The respective locations of the temperature sensors  17   a  to  17   c  are determined to correspond to the heights at which they sense the surface temperatures of the accumulator  14  with the liquid-level sensing device  15  attached to the accumulator  14 . 
     A purpose for which the heaters  16  are thus provided at both widthwise ends of the heat insulating material  152  and on the accumulator  14  in the height direction of the accumulator  14 , that is, the Z direction is to heat the accumulator  14  evenly along the height of the accumulator  14  when the accumulator  14  is heated by the heaters  16 . Further, a purpose for which the temperature sensors  17   a  to  17   c  are provided on the heat insulating material  152  is to prevent, for example, the heat of the heaters  16  and external heat from being transmitted to the temperature sensors  17   a  to  17   c  when the surface temperatures of the accumulator  14  are sensed by the temperature sensors  17   a  to  17   c . Furthermore, a purpose for which the temperature sensors  17   a  to  17   c  are provided between the plurality of heaters  16  is to accurately sense the surface temperatures of the accumulator  14 . 
       FIG. 4  is a schematic view for explaining a shape of the liquid-level sensing device  15  of  FIG. 3  to be attached to the accumulator  14 . In the liquid-level sensing device  15  shown in  FIG. 3 , the belt unit  151  and the heaters  16  are bendable. This makes it possible to bend the liquid-level sensing device  15  to correspond to the shape of the accumulator  14  as shown in  FIG. 4 . 
     [Attachment of Liquid-Level Sensing Device  15 ] 
       FIG. 5  is a perspective view showing a state in which the liquid-level sensing device  15  of  FIG. 3  is attached to the accumulator  14 . As shown in  FIG. 5 , the liquid-level sensing device  15  is attached to the accumulator  14  such that the liquid-level sensing device  15  is wound around the accumulator  14  and the length direction of the belt unit  151  corresponds to the Z direction. In  FIG. 5 , a width direction of the accumulator  14  is referred to as “Y direction”, and a depth direction of the accumulator  14  is referred to as “X direction”. 
     At this time, the belt unit  151  is bent such that an upper surface of the belt unit  151  on which the heaters  16  and the temperature sensors  17   a  to  17   c  are provided is an inner circumferential surface. Then, the liquid-level sensing device  15  is attached such that the heaters  16  and the temperature sensors  17   a  to  17   c  make contact with the surface of the accumulator  14 . 
       FIG. 6  is a cross-sectional view schematically showing the state in which the liquid-level sensing device  15  of  FIG. 3  is attached to the accumulator  14 . As shown in  FIG. 6 , the liquid-level sensing device  15  is attached to the accumulator  14  such that the temperature sensors  17   a  to  17   c  are located at the predetermined heights. In  FIG. 6 , the Z direction and the Y direction correspond to the Z direction and the Y direction of  FIG. 5 , respectively. 
     In the example shown in  FIG. 6 , the accumulator  14  includes a hermetic container  141 , an inlet pipe  142  through which refrigerant is introduced into the hermetic container  141 , and a U-shaped outlet pipe  143  through which gas refrigerant is supplied from inside to the compressor  11 . The outlet pipe  143  has an oil return hole  143   a  through which liquid refrigerant flows in and a gas suction opening  143   b  through which gas refrigerant is sucked in. 
     In the accumulator  14 , excess refrigerant introduced into the hermetic container  141  through the inlet pipe  142  is separated into gas refrigerant and liquid refrigerant. The gas refrigerant thus separated is sucked from the accumulator  14  via the gas suction opening  143   b  and the outlet pipe  143  into the compressor  11 . Meanwhile, the liquid refrigerant thus separated is stored in the accumulator  14 . The liquid refrigerant thus separated either is sucked in small amounts into the compressor  11  through the oil return hole  143   a  or evaporates and gasifies with the passage of time and is sucked in through the gas suction opening  143   b  and sucked into the compressor  11 . 
     In particular, the temperature sensor  17   a  is in a place where it is allowed to sense the surface temperature of the lower part of the accumulator  14 . Specifically, the temperature sensor  17   a , which is provided in the lowermost part, is located below the oil return hole  143   a  of the outlet pipe  143  of the accumulator  14 . This is intended for the temperature sensor  17   a  to be allowed to sense the surface temperature of the accumulator  14  in a place where liquid refrigerant is surely present. 
     Further, the temperature sensor  17   c  is in a place where it is allowed to sense the surface temperature of the upper part of the accumulator  14 . Specifically, the temperature sensor  17   c , which is provided in the uppermost part, is located below the gas suction opening  143   b  of the outlet pipe  143  of the accumulator  14 . This is intended to prevent a liquid surface  140  of liquid refrigerant from reaching an upper side of the gas suction opening  143   b  during liquid-level sensing. 
     The temperature sensor  17   b  may be in a place at any height between the temperature sensor  17   a  and the temperature sensor  17   c . Specifically, it is preferable that the temperature sensor  17   b  be in a place where the liquid surface  140  is desired to be sensed. 
     In a case in which the temperature sensors  17   a  to  17   c  are positioned as noted above, an area from an upper surface of the accumulator  14  to the temperature sensor  17   c  is hereinafter referred to as “area A”. Further, an area from the temperature sensor  17   c  to the temperature sensor  17   b  is referred to as “area B”, and an area from the temperature sensor  17   b  to the temperature sensor  17   a  is referred to as “area C”. It should be noted that an area from the temperature sensor  17   a  to the bottom is an area in which liquid refrigerant is surely present. A reason for this is that the temperature sensor  17   a  is located below the oil return hole  143   a  and liquid refrigerant stored below the oil return hole  143   a  remains without being sucked into the outlet pipe  143 . In Embodiment 1, the threshold Th is defined to be a boundary between the area A and the area B. That is, the threshold Th is defined to be a height position of the temperature sensor  17   c , that is, an installation position of the temperature sensor  17   c  in the Z direction. 
     [Liquid-level Sensing Process] 
     A method for sensing the liquid surface  140  of liquid refrigerant in the accumulator  14  according to Embodiment 1 is described.  FIG. 7  is a diagram showing a graph for explaining a relationship between the surface temperatures of the accumulator  14  and the heights of the temperature sensors  17   a  to  17   c.    
       FIG. 7  shows surface temperatures in the respective positions of the temperature sensors  17   a  to  17   c  in a case in which the liquid surface  140  of liquid refrigerant in the accumulator  14  is present in the area B as shown in  FIG. 6 . 
     As shown in  FIG. 7 , there is a difference between the surface temperature of the accumulator  14  as sensed by the temperature sensor  17   c  located above the liquid surface  140  of liquid refrigerant present in the area B, and the surface temperature of the accumulator  14  as sensed by the temperature sensors  17   a  and  17   b  located below the liquid surface  140  of the liquid refrigerant. Specifically, the surface temperature Tc sensed by the temperature sensor  17   c  is higher than the surface temperature Ta sensed by the temperature sensor  17   a  and the surface temperature Tb sensed by the temperature sensor  17   b . A reason for this is that the surface temperatures of the accumulator  14  having been heated vary as the thermal conductivity of liquid refrigerant and the thermal conductivity of gas are different. 
     In Embodiment 1, the controller  3  senses the liquid surface  140  in the accumulator  14  on the basis of the surface temperatures Ta to Tc of the accumulator  14  having been heated as sensed by the temperature sensors  17   a  to  17   c  of the liquid-level sensing device  15 . 
     In Embodiment 1, the temperature sensor  17   a  is provided in a place where liquid refrigerant is surely present. For this reason, as the temperature sensor  17   a  senses a surface temperature of the accumulator  14  in a liquid area where liquid refrigerant is always present, the temperature thus sensed is defined as a reference temperature. 
     As mentioned above, in a case in which the temperature sensors  17   b  and  17   c  are located below the liquid surface  140  of the liquid refrigerant and have sensed surface temperatures of the accumulator  14  in liquid areas, the surface temperatures sensed by the temperature sensors  17   b  and  17   c  are substantially equal to the surface temperature sensed by the temperature sensor  17   a . On the other hand, in a case in which the temperature sensors  17   b  and  17   c  are located above the liquid surface  140  of the liquid refrigerant and have sensed surface temperatures of the accumulator  14  in a gas area, the surface temperatures sensed by the temperature sensors  17   b  and  17   c  are higher than the surface temperature sensed by the temperature sensor  17   a.    
     That is, it is possible to, by separately calculating a temperature difference between each of the surface temperatures sensed by the temperature sensors  17   b  and  17   c  and the surface temperature sensed by the temperature sensor  17   a  and comparing the temperature difference with the preset set value T 1 , determine in which of the areas A to C the liquid surface  140  is present. Specifically, in a case in which the temperature difference between the surface temperature sensed by the temperature sensor  17   b  and the surface temperature sensed by the temperature sensor  17   a  is greater than or equal to the set value T 1 , the position of the liquid surface  140  is determined to be present in an area below the temperature sensor  17   b . Similarly, in a case in which the temperature difference between the surface temperature sensed by the temperature sensor  17   c  and the surface temperature sensed by the temperature sensor  17   a  is greater than or equal to the set value T 1 , the position of the liquid surface  140  is determined to be present in an area below the temperature sensor  17   c.    
     It should be noted that temperature differences between a surface temperature of the accumulator  14  in a liquid area and a surface temperature of the accumulator  14  in a gas area vary depending on the heating capacity or other characteristics of the heaters  16  configured to heat the accumulator  14 . Therefore, a set value serving as a threshold is determined in advance to correspond to the heating capacity or other characteristics of the heaters  16 . 
     The liquid-level sensing process is performed after the accumulator  14  has been heated by turning on the heaters  16 . When the heaters  16  are turned on, the safety of the air-conditioning apparatus  1000  is considered. 
     In Embodiment 1, in a case in which the surface temperatures Ta to Tc of the accumulator  14  as sensed by the temperature sensors  17   a  to  17   c  are each lower than or equal to a set temperature T 2 , the heater control unit  34  controls the heaters  16  such that the heaters  16  are turned on. The set temperature T 2  is a guaranteed outside air temperature at which the operation of the air-conditioning apparatus  1000  is guaranteed or a temperature that is slightly higher than the guaranteed outside air temperature, and is determined in advance. This is intended to prevent the liquid-level sensing process from being performed when an outside air temperature is a temperature at which the operation of the air-conditioning apparatus  1000  is not guaranteed. 
     Further, in a case in which a temperature difference between the surface temperature sensed by the temperature sensor  17   a  and the temperature of refrigerant at the inlet of the accumulator  14  as sensed by the refrigerant temperature sensor  18  is less than or equal to a set temperature T 3 , the heater control unit  34  controls the heaters  16  such that the heaters  16  are turned on. The set temperature T 3  is set such that liquid refrigerant stored in the accumulator  14  does not evaporate. This is intended to prevent liquid refrigerant in the accumulator  14  from evaporating into gas refrigerant when the accumulator  14  is heated by the heaters  16 . 
     Furthermore, in a case in which the outside air temperature sensed by the outside air temperature sensor  19  is lower than or equal to a preset set temperature T 4 , the heater control unit  34  may control the heaters  16  such that the heaters  16  are turned on. Further, the control of the turning on and turning off of the heaters  16  is not limited to such a case in which safety is considered, but for example, the turning on and turning off may be repeated every set period of time. 
       FIG. 8  is a flow chart showing an example of a sequence of actions making up a liquid-level sensing process in the air-conditioning apparatus  1000  according to Embodiment 1. 
     In step S 1 , the heater control unit  34  controls the heaters  16  such that the heaters  16  are turned on. Consequently, the accumulator  14  is heated. 
     In step S 2 , after the set period of time has elapsed after the heaters  16  are turned on, the temperature sensors  17   a  to  17   c  sense the respective surface temperatures Ta to Tc of the accumulator  14 . 
     In step S 3 , the temperature difference calculating unit  31  calculates the temperature difference ΔT middle  and the temperature difference ΔT high  on the basis of the surface temperatures Ta to Tc sensed by the temperature sensors  17   a  to  17   c . The temperature difference ΔT middle  between the surface temperature Tb sensed by the temperature sensor  17   b  and the surface temperature Ta sensed as a reference temperature by the temperature sensor  17   a  is calculated on the basis of Formula (1). Further, the temperature difference ΔT high  between the surface temperature Tc sensed by the temperature sensor  17   c  and the surface temperature Ta sensed by the temperature sensor  17   a  is calculated on the basis of Formula (2). 
     
       
         
           
             
               
                 
                   
                     Temperature 
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                     Difference 
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                     Δ 
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                     Tb 
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                     Ta 
                   
                 
               
               
                 
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                   1 
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                     Temperature 
                     ⁢ 
                         
                     Difference 
                     ⁢ 
                         
                     Δ 
                     ⁢ 
                     
                       T 
                       
                         h 
                         ⁢ 
                         i 
                         ⁢ 
                         g 
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                         h 
                       
                     
                   
                   = 
                   
                     
                       T 
                       ⁢ 
                       c 
                     
                     - 
                     
                       T 
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                       a 
                     
                   
                 
               
               
                 
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     Next, the liquid-level determining unit  32  reads out the set value T 1  from the storage unit  35  for the temperature difference ΔT middle  and the temperature difference ΔT high . Then, the liquid-level determining unit  32  compares, with each of the temperature differences ΔT middle  and ΔT high  calculated in step S 3 , the set value T 1  read out from the storage unit  35 . 
     In step S 4 , the liquid-level determining unit  32  determines whether the temperature difference ΔT middle  is greater than or equal to the set value T 1  and the temperature difference ΔT high  is greater than or equal to the set value T 1 . Positive determination on the basis of these criteria means that the temperature sensor  17   b  and the temperature sensor  17   c  are located in gas areas, where no liquid refrigerant is present. 
     In the case of YES in step S 4 , the liquid-level determining unit  32  proceeds to step S 5 , in which the liquid-level determining unit  32  determines that the area A and the area B are gas areas and the liquid surface  140  is present in the area C. On the other hand, in the case of NO in step S 4 , the process shifts to step S 6 . 
     In step S 6 , the liquid-level determining unit  32  determines whether the temperature difference ΔT high  is greater than or equal to the set value T 1  and the temperature difference ΔT middle  is less than the set value T 1 . Positive determination on the basis of these criteria means that the temperature sensor  17   b  is located in a gas area and the temperature sensor  17   c  is located in a liquid area, where liquid refrigerant is present. 
     In the case of YES in step S 6 , the liquid-level determining unit  32  proceeds to step S 7 , in which the liquid-level determining unit  32  determines that the area A is a gas area and the liquid surface  140  of liquid refrigerant is present in the area B. On the other hand, in the case of NO in step S 6 , the process shifts to step S 8 . 
     In step S 8 , the liquid-level determining unit  32  determines whether the temperature difference ΔT high  is less than the set value T 1  and the temperature difference ΔT middle  is less than the set value T 1 . Positive determination on the basis of these criteria means that the temperature sensor  17   b  and the temperature sensor  17   c  are located in liquid areas. 
     In the case of YES in step S 8 , the liquid-level determining unit  32  proceeds to step S 9 , in which the liquid-level determining unit  32  determines that the liquid surface  140  of liquid refrigerant is present in the area A. On the other hand, in the case of NO in step S 8 , the liquid-level determining unit  32  proceeds to step S 10 , in which the liquid-level determining unit  32  determines that it is unclear in which area the liquid surface  140  of liquid refrigerant is present. 
     In step S 11 , the liquid-level determining unit  32  outputs a result of the determination made in step S 5 , S 7 , S 9 , or S 10 . In a case in which it has been determined as a result of step S 5  that the liquid surface  140  of liquid refrigerant is present in the area C, the liquid-level determining unit  32  outputs a result that the liquid level is a level c. In a case in which it has been determined as a result of step S 7  that the liquid surface  140  of liquid refrigerant is present in the area B, the liquid-level determining unit  32  outputs a result that the liquid level is a level b. In a case in which it has been determined as a result of step S 9  that the liquid surface  140  of liquid refrigerant is present in the area A, the liquid-level determining unit  32  outputs a result that the liquid level is a level a. Further, in a case in which it has been determined in step S 10  that it is unclear in which area the liquid surface  140  of liquid refrigerant is present, the liquid-level determining unit  32  outputs a result that the liquid level is unclear. 
     In Embodiment 1, as mentioned above, the liquid-level sensing method for performing liquid-level sensing with the liquid-level sensing device  15  has been described. However, this is merely an example and is not intended to impose any limitation. Examples of other liquid-level sensing methods include the following method, which is executed by the controller  3 . 
     Temperature detecting devices such as thermistors are installed at the inlet and an outlet of the accumulator  14 , and in a case in which a temperature difference between the temperature of refrigerant at the inlet of the accumulator  14  and the temperature of refrigerant at the outlet of the accumulator  14  is greater than a preset set value T 5 , the controller  3  determines that the liquid level is higher than the threshold Th. It should be noted that the refrigerant temperature sensor  18  may be used as a temperature detecting device at the inlet of the accumulator  14 . 
     Alternatively, a pressure sensor and a thermistor may be installed on a high-pressure side of the compressor  11 , and the controller  3  may be configured to calculate a degree of superheat of discharge from a detected pressure and a detected temperature and determine, on the basis of the degree of superheat of discharge, whether the liquid level is higher than the threshold Th or normal. It should be noted that the degree of superheat is a temperature difference between a superheated vapor temperature and a saturation temperature at a certain pressure. Thus, when the compressor  11  sucks in a small amount of gas refrigerant, the degree of superheat rises. On the other hand, when the compressor  11  sucks in a large amount of gas refrigerant, the degree of superheat drops. Therefore, when the degree of superheat is lower than a preset set value T 6 , it is determined that the liquid level is higher than the threshold Th. 
     Thus, even in a case in which no liquid-level sensing device  15  is provided, it is possible to sense the liquid level in the accumulator  14 . It should be noted that an advantage of the use of the liquid-level sensing device  15  is that the liquid level is detected with high accuracy. On the other hand, an advantage of the omission of the liquid-level sensing device  15  is low cost. 
     Thus, the air-conditioning apparatus  1000  according to Embodiment 1 has the liquid-level sensing device  15 . During start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  1 . In a case in which the liquid level in the accumulator  14  is higher than the preset threshold Th, the controller  3  makes the frequency of the compressor  11  installed in the outdoor unit  1  lower than or equal to the preset first specified value Sp 1 . This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000 . The following describes how the controller  3  operates. 
     [Process of Controlling Frequency of Compressor  11 ] 
       FIG. 9  is a flow chart showing a sequence of actions making up a frequency control process of the air-conditioning apparatus  1000  according to Embodiment 1. 
     In step S 21 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  11  is stopped. 
     In step S 22 , the frequency control unit  36  of the controller  3  determines, on the basis of the liquid level sensed in step S 21 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the frequency control unit  36  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 23 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the frequency control unit  36  determines that the liquid level is normal, and ends the process of  FIG. 9 . 
     In step S 23 , the frequency control unit  36  of the controller  3  sets the frequency of the compressor  11  to the preset first specified value Sp 1  or lower. 
     Thus, in Embodiment 1, the frequency control unit  36  controls the frequency of the compressor  11  on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the frequency control unit  36  sets the frequency of the compressor  11  to the first specified value Sp 1  or lower. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11  upon start-up of the compressor  11 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the frequency control unit  36  controls the frequency of the compressor  11  by a control method for normal operation. That is, the frequency control unit  36  controls the frequency of the compressor  11  such that a target condensing temperature or a target discharge temperature is achieved. 
     In Embodiment 1, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  1 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  makes the frequency of the compressor  11  installed in the outdoor unit  1  lower than or equal to the preset first specified value Sp 1 . This inhibits the rise in gas flow rate of refrigerant flowing through the refrigerant circuit, making it possible to reduce the suction of low-pressure refrigerant into the compressor  11 . As a result, this makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  11  without making the accumulator  14  complex in structure. 
     Modification of Embodiment 1 
     In a modification of Embodiment 1, the frequency control unit  36  sets the frequency of the compressor  11  to the first specified value Sp 1  or lower in step S 23  of  FIG. 9  and then determines an amount of increase in the frequency of the compressor  11  to correspond to an amount of change in the liquid level in the accumulator  14 . 
     Specifically, the liquid-level sensing process of  FIG. 8  is performed twice or more. The frequency control unit  36  calculates a difference between the previous liquid level and the latest liquid level. The difference serves as the amount of change in the liquid level in the accumulator  14 . The storage unit  35  has stored in its inside a table on which amounts of increase in the frequency of the compressor  11  are predetermined for respective amounts of change in liquid level. The frequency control unit  36  refers to the table and determines an amount of increase in the frequency of the compressor  11  on the basis of the amount of change in the liquid level in the accumulator  14 . This makes it possible to gradually increase the frequency of the compressor  11  to correspond to the amount of change in liquid level. 
     In Embodiment 1, the liquid-level sensing device  15  has three temperature sensors  17  such that there are three categories of liquid levels. However, in the modification of Embodiment 1, the liquid-level sensing device  15  may have four or more temperature sensors  17 . In this case, it is possible to more finely sense amounts of change in liquid level. 
     In the modification of Embodiment 1, as noted above, the frequency control unit  36  determines an amount of increase in the frequency of the compressor  11  to correspond to an amount of change in the liquid level in the accumulator  14 . This makes it possible to increase the frequency of the compressor  11  while reducing the suction of low-pressure refrigerant into the compressor  11 , thus making it possible to bring about further improvement in start-up characteristics of the air-conditioning apparatus  1000  than that of Embodiment 1. This also makes it possible to bring about improvement in reliability of the compressor  11  without making the accumulator  14  complex in structure. 
     Embodiment 2 
       FIG. 10  is a block diagram showing a configuration of a controller  3  of an air-conditioning apparatus  1000  according to Embodiment 2. As shown in  FIG. 10 , the controller  3  includes a temperature difference calculating unit  31 , a liquid-level determining unit  32 , an output unit  33 , a heater control unit  34 , a storage unit  35 , and an expansion control unit  37 . 
     A difference from Embodiment 1 is that as shown in  FIG. 10 , the controller  3  includes the expansion control unit  37  instead of the frequency control unit  36  shown in  FIG. 2 . As other components and actions of the controller  3  are identical to those of Embodiment 1, a description of such components and actions is omitted here. 
     Further, also in Embodiment 2, the air-conditioning apparatus  1000  is described as an example of a refrigeration cycle apparatus. As the air-conditioning apparatus  1000  is identical in overall configuration to that shown in  FIG. 1 , a description of the air-conditioning apparatus  1000  is omitted here. 
     The expansion control unit  37  determines the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B with reference to opening degree information stored in the storage unit  35  on the basis of a result of the determination made by the liquid-level determining unit  32 . The expansion control unit  37  sends at least one of the expansion devices  20 ,  21 A, and  21 B an opening degree control signal for controlling the opening degree of at least a corresponding one of the expansion devices  20 ,  21 A, and  21 B. 
     [Process of Controlling Opening Degrees of Expansion Devices  20 ,  21 A, and  21 B] 
       FIG. 11  is a flow chart showing a sequence of actions making up a process of controlling the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B of the air-conditioning apparatus  1000  according to Embodiment 2. 
     In step S 31 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  11  is stopped. 
     In step S 32 , the expansion control unit  37  of the controller  3  determines, on the basis of the liquid level sensed in step S 31 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the expansion control unit  37  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 33 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the expansion control unit  37  determines that the liquid level is normal, and ends the process of  FIG. 11 . 
     In step S 33 , the expansion control unit  37  of the controller  3  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the preset second specified value Sp 2  or higher. 
     Thus, in Embodiment 2, the expansion control unit  37  controls the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the expansion control unit  37  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the second specified value Sp 2  or higher. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11  upon start-up of the compressor  11 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the expansion control unit  37  controls the opening degrees of the expansion devices  20 ,  21 A, and  21 B by the control method for normal operation. That is, the expansion control unit  37  controls the opening degrees of the expansion devices  20 ,  21 A, and  21 B such that a degree of subcooling SC at an outlet of the outdoor heat exchanger  13  or degrees of subcooling SC at outlets of the indoor heat exchangers  22 A and  22 B reach a target degree of subcooling. 
     In Embodiment 2, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  1 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the second specified value Sp 2  or higher for the limiting operation. This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  11 , thus making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. Further, as in Embodiment 1, also in Embodiment 2, the gas flow rate of refrigerant flowing through the refrigerant circuit is inhibited, thus the occurrence of liquid return is also reduced. This also makes it possible to bring about improvement in reliability of the compressor  11  without making the accumulator  14  complex in structure. 
     Modification of Embodiment 2 
     In a modification of Embodiment 2, the expansion control unit  37  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the second specified value Sp 2  or higher in step S 33  of  FIG. 11  and then determines an amount of decrease in the opening degree of at least a corresponding one of the expansion devices  20 ,  21 A, and  21 B to correspond to an amount of change in the liquid level in the accumulator  14 . 
     Specifically, the liquid-level sensing process of  FIG. 8  is performed twice or more. The expansion control unit  37  calculates a difference between the previous liquid level and the latest liquid level. The difference serves as the amount of change in the liquid level in the accumulator  14 . The storage unit  35  has stored in its inside a table on which amounts of decrease in the opening degrees of the expansion devices  20 ,  21 A, and  21 B are predetermined for respective amounts of change in liquid level. The expansion control unit  37  refers to the table and determines an amount of decrease in the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B on the basis of the amount of change in the liquid level in the accumulator  14 . This makes it possible to gradually decrease the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to correspond to the amount of change in liquid level. 
     In Embodiment 2, the liquid-level sensing device  15  has three temperature sensors  17  such that there are three categories of liquid levels. However, in the modification of Embodiment 2, the liquid-level sensing device  15  may have four or more temperature sensors  17 . In this case, it is possible to more finely sense amounts of change in liquid level. 
     In the modification of Embodiment 2, as noted above, the expansion control unit  37  determines an amount of decrease in the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to correspond to an amount of change in the liquid level in the accumulator  14 . This makes it possible to gradually decrease the opening degree of an expansion device while reducing the suction of low-pressure refrigerant into the compressor  11 , thus making it possible to accelerate the condensation of refrigerant in the indoor heat exchangers  22 A and  22 B, which operate as condensers. As a result, this makes it possible to bring about further improvement in start-up characteristics of the air-conditioning apparatus  1000  than that of Embodiment 2. 
     Embodiment 3 
       FIG. 12  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 3.  FIG. 12  shows an air-conditioning apparatus  1000  as an example of the refrigeration cycle apparatus. A difference between  FIG. 12  and  FIG. 1  is that a bypass  51  and a hot-gas bypass valve  52  are provided in  FIG. 12 . As other components of the air-conditioning apparatus  1000  are identical to those shown in  FIG. 1 , a description of such components is omitted here. 
     The bypass  51  is provided between a high-pressure side that is a refrigerant discharge side of the compressor  11  and a low-pressure side that is an inflow side of the accumulator  14 . The bypass  51  serves as a bypass through which high-temperature gas refrigerant discharged from the compressor  11  flows to the inflow side of the accumulator  14 . 
     The hot-gas bypass valve  52  is provided to the bypass  51 . The hot-gas bypass valve  52  is, for example, a solenoid valve. The hot-gas bypass valve  52  opens and closes to circulate or intercept the gas refrigerant flowing through the bypass  51 . Specifically, in an open state, the hot-gas bypass valve  52  causes high-temperature gas refrigerant flowing from the discharge side of the compressor  11  into the bypass  51  to flow out to the inflow side of the accumulator  14 . On the other hand, when in a closed state, the hot-gas bypass valve  52  intercepts the flow of gas refrigerant from the discharge side of the compressor  11  to the inflow side of the accumulator  14 . The opening and closing of the hot-gas bypass valve  52  are controlled by the controller  3 . 
       FIG. 13  is a block diagram showing a configuration of the controller  3  of the air-conditioning apparatus  1000  according to Embodiment 3. As shown in  FIG. 13 , the controller  3  includes a temperature difference calculating unit  31 , a liquid-level determining unit  32 , an output unit  33 , a heater control unit  34 , a storage unit  35 , and a bypass valve control unit  38 . 
     A difference from Embodiment 1 is that as shown in  FIG. 13 , the controller  3  includes the bypass valve control unit  38  instead of the frequency control unit  36  shown in  FIG. 2 . As other components and actions of the controller  3  are identical to those of Embodiment 1, a description of such components and actions is omitted here. 
     The bypass valve control unit  38  determines the opening and closing of the hot-gas bypass valve  52  on the basis of a result of the determination made by the liquid-level determining unit  32 . The bypass valve control unit  38  sends the hot-gas bypass valve  52  an opening-and-closing control signal for controlling the opening and closing of the hot-gas bypass valve  52 . 
     [Process of Controlling Opening and Closing of Hot-Gas Bypass Valve  52 ] 
       FIG. 14  is a flow chart showing a sequence of actions making up a process of controlling the opening and closing of the hot-gas bypass valve  52  of the air-conditioning apparatus  1000  according to Embodiment 3. 
     In step S 41 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  11  is stopped. 
     In step S 42 , the bypass valve control unit  38  of the controller  3  determines, on the basis of the liquid level sensed in step S 41 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the bypass valve control unit  38  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 43 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the bypass valve control unit  38  determines that the liquid level is normal, and ends the process of  FIG. 14 . 
     In step S 43 , the bypass valve control unit  38  of the controller  3  switches the hot-gas bypass valve  52  from a closed state to an open state. 
     Thus, in Embodiment 3, the bypass valve control unit  38  controls the opening and closing of the hot-gas bypass valve  52  on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the bypass valve control unit  38  switches the hot-gas bypass valve  52  from a closed state to an open state for the limiting operation. This causes high-temperature gas discharged from the compressor  11  to be introduced into a suction pipe of the accumulator  14  via the bypass  51  upon start-up of the compressor  11 . This inflow of the high-temperature gas into the accumulator  14  causes liquid refrigerant in the accumulator  14  to evaporate. This results in delivery of the refrigerant from the accumulator  14  to the refrigerant circuit. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the bypass valve control unit  38  switches the hot-gas bypass valve  52  from an open state to a closed state. This causes the air-conditioning apparatus  1000  to perform a normal operation. 
     In Embodiment 3, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  1 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  switches the hot-gas bypass valve  52  from a closed state to an open state. This causes hot gas to be introduced from the compressor  11  into the inlet pipe  142  of the accumulator  14 , causing the high-temperature hot gas to flow into the accumulator  14 . This causes liquid refrigerant of the accumulator  14  to evaporate and be delivered to the refrigerant circuit excluding the accumulator  14 . As a result, this makes it possible to reduce the suction of low-pressure refrigerant into the compressor  11 , thus making it possible to reduce the occurrence of liquid return to the compressor  11 . This makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. Thus, in Embodiment 3, it is possible to bring about improvement in evaporation characteristics of the accumulator  14  with hot gas and bring about improvement in start-up characteristics during heating. It is also possible to bring about improvement in reliability of the compressor  11  without making the accumulator  14  complex in structure. 
     Embodiment 4 
       FIG. 15  is a block diagram showing a configuration of a controller  3  of an air-conditioning apparatus  1000  according to Embodiment 4. As shown in  FIG. 15 , the controller  3  includes a temperature difference calculating unit  31 , a liquid-level determining unit  32 , an output unit  33 , a heater control unit  34 , a storage unit  35 , a frequency control unit  36 , an expansion control unit  37 , and a bypass valve control unit  38 . 
     A difference from Embodiment 1 is that as shown in  FIG. 15 , the controller  3  further includes the expansion control unit  37  and the bypass valve control unit  38 . As other components and actions of the controller  3  are identical to those of Embodiment 1, a description of such components and actions is omitted here. Further, the expansion control unit  37  is identical to that described in Embodiment 2. The bypass valve control unit  38  is identical to that described in Embodiment 3. 
     Further, also in Embodiment 4, the air-conditioning apparatus  1000  is described as an example of a refrigeration cycle apparatus. As the air-conditioning apparatus  1000  is identical in overall configuration to that shown in  FIG. 12 , a description of the air-conditioning apparatus  1000  is omitted here. 
     Thus, Embodiment 4 is a combination of Embodiments 1 to 3. 
     [Control Process] 
       FIG. 16  is a flow chart showing a sequence of actions making up a control process of the air-conditioning apparatus  1000  according to Embodiment 4. 
     In step S 51 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  11  is stopped. 
     In step S 52 , the frequency control unit  36  of the controller  3  determines, on the basis of the liquid level sensed in step S 51 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the frequency control unit  36  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 53 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the frequency control unit  36  determines that the liquid level is normal, and ends the process of  FIG. 16 . 
     In step S 53 , the frequency control unit  36  of the controller  3  sets the frequency of the compressor  11  to the preset first specified value Sp 1  or lower. 
     Next, in step S 54 , the expansion control unit  37  of the controller  3  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the preset second specified value Sp 2  or higher. 
     Next, in step S 55 , the bypass valve control unit  38  of the controller  3  switches the hot-gas bypass valve  52  from a closed state to an open state. 
     Thus, in Embodiment 4, the frequency control unit  36  sets the frequency of the compressor  11  to the first specified value Sp 1  or lower for a first limiting operation on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Furthermore, the expansion control unit  37  sets the opening degree of at least one of the expansion devices  20 ,  21 A, and  21 B to the second specified value Sp 2  or higher for a second limiting operation. Furthermore, the bypass valve control unit  38  switches the hot-gas bypass valve  52  from a closed state to an open state for a third limiting operation. This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  11  upon start-up of the compressor  11 , thus making it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  11 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the frequency control unit  36  ends the process of  FIG. 16  to perform a normal operation. 
     As noted above, as Embodiment 4 is a combination of Embodiments 1 to 3, in Embodiment, it is possible to bring about effects that are similar to those of Embodiments 1 to 3. 
     Although, in Embodiment 4, the frequency control unit  36  performs the liquid-level determining process of step S 52 , this is not intended to impose any limitation. For example, the expansion control unit  37  or the bypass valve control unit  38  may perform the liquid-level determining process of step S 52 . 
     Further, although, in Embodiments 1 to 4, the frequency control unit  36 , the expansion control unit  37 , or the bypass valve control unit  38  performs the liquid-level determining process, this is not intended to impose any limitation. For example, the liquid-level determining unit  32  may perform the liquid-level determining process. 
     Embodiment 5 
       FIG. 17  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 5.  FIG. 17  shows an air-conditioning apparatus  1000  as an example of the refrigeration cycle apparatus. 
     As shown in  FIG. 17 , the air-conditioning apparatus  1000  includes an outdoor unit  100 , a plurality of indoor units  300 A and  300 B, a relay unit  200 , and a controller  3 . As in the case of  FIG. 1 , the controller  3  is provided outside the outdoor unit  100 , the relay unit  200 , and the indoor units  300 A and  300 B. However, this is not intended to impose any limitation, and the controller  3  may be provided inside any of the outdoor unit  100 , the relay unit  200 , and the indoor units  300 A and  300 B. Although in Embodiment 5, a case is illustrated in which two indoor units  300 A and  300 B are connected to one outdoor unit  100 , two or more outdoor units  100  may be provided. Further, three or more indoor units  300 A and  300 B may be connected. 
     As shown in  FIG. 17 , the outdoor unit  100 , the indoor units  300 A and  300 B, and the relay unit  200  are connected by refrigerant pipes to form a refrigerant circuit. The outdoor unit  100  is configured to supply the two indoor units  300 A and  300 B with heating energy or cooling energy. The two indoor units  300 A and  300 B are connected in parallel to each other, and are identical in configuration to each other. The indoor units  300 A and  300 B are configured to cool or heat an air-conditioned space such as the inside of a room with heating energy or cooling energy supplied from the outdoor unit  100 . The relay unit  200  is interposed between the outdoor unit  100  and the indoor units  300 A and  300 B, and is configured to switch, upon request of the indoor units  300 A and  300 B, the flows of refrigerant supplied from the outdoor unit  100 . 
     Further, the air-conditioning apparatus  1000  includes a load capacity detecting unit  220  configured to detect cooling and heating load capacities of the plurality of indoor units  300 A and  300 B. The cooling and heating load capacities here are cooling load capacities and heating load capacities of the plurality of indoor units  300 A and  300 B. The load capacity detecting unit  220  includes liquid pipe temperature detecting units  303 A and  303 B and gas pipe temperature detecting units  304 A and  304 B. 
     Note here that the outdoor unit  100  and the relay unit  200  are connected on a high-pressure side by a high-pressure pipe  402  through which high-pressure refrigerant flows and are connected on a low-pressure side by a low-pressure pipe  401  through which low-pressure refrigerant flows. Further, the relay unit  200  and the indoor unit  300 A are connected by a gas branch pipe  403 A, and the relay unit  200  and the indoor unit  300 B are connected by a gas branch pipe  403 B. Gaseous refrigerant flows mostly through the gas branch pipes  403 A and  403 B. Further, the relay unit  200  and the indoor unit  300 A are connected by a liquid branch pipe  404 A, and the relay unit  200  and the indoor unit  300 B are connected by a liquid branch pipe  404 B. Liquid refrigerant flows mostly through the liquid branch pipes  404 A and  404 B. 
     (Outdoor Unit  100 ) 
     The outdoor unit  100  includes a capacity-variable compressor  111 , a refrigerant flow switching device  112 , a heat exchange unit  150 , an accumulator  14 , and a flow control unit  130 . The flow control unit  130  limits the directions in which refrigerant flows. The refrigerant flow switching device  112  switches the directions in which refrigerant flows through the outdoor unit  100 . Although the refrigerant flow switching device  112  is a four-way valve in the illustrated case, the refrigerant flow switching device  112  may be a combination of two-way valves, three-way valves, or other devices. 
     The heat exchange unit  150  includes a main pipe  114 , an air-sending device  115 , an outdoor heat exchanger  113  serving as a first heat exchanger, and an expansion device  120  serving as a first expansion device. 
     The outdoor heat exchanger  113  operates as an evaporator or a condenser. In a case in which the outdoor heat exchanger  113  is an air-cooled heat exchanger, the outdoor heat exchanger  113  is formed to exchange heat between refrigerant and outdoor air, and in a case in which the outdoor heat exchanger  113  is an water-cooled heat exchanger, the outdoor heat exchanger  113  is formed to exchange heat between refrigerant and water, brine, or other substances. The air-sending device  115  is configured to control a heat exchange capacity by varying the amount of air that is sent to the outdoor heat exchanger  113 . The main pipe  114  has a first end connected to the refrigerant flow switching device  112  and a second end connected to the high-pressure pipe  402 , and is provided with the outdoor heat exchanger  113  and the expansion device  120 . 
     The expansion device  120  is connected in series to the outdoor heat exchanger  113  through the main pipe  114 , and is configured to adjust the flow rate of refrigerant flowing through the main pipe  114 . The expansion device  120  is, for example, an opening-degree-variable electronic expansion valve or other devices. The opening degree of the expansion device  120  is controlled by the controller  3 . 
     The flow control unit  130  includes a third check valve  105 , a fourth check valve  106 , a fifth check valve  107 , and a sixth check valve  108 . The third check valve  105  is provided to a pipe connecting the heat exchange unit  150  with the high-pressure pipe  402 , and allows passage of refrigerant from the heat exchange unit  150  toward the high-pressure pipe  402 . The fourth check valve  106  is provided to a pipe connecting the refrigerant flow switching device  112  of the outdoor unit  100  with the low-pressure pipe  401 , and allows passage of refrigerant from the low-pressure pipe  401  toward the refrigerant flow switching device  112 . The fifth check valve  107  is provided to a pipe connecting the refrigerant flow switching device  112  of the outdoor unit  100  with the high-pressure pipe  402 , and allows passage of refrigerant from the refrigerant flow switching device  112  toward the high-pressure pipe  402 . The sixth check valve  108  is provided to a pipe connecting the heat exchange unit  150  with the low-pressure pipe  401 , and allows passage of refrigerant from the low-pressure pipe  401  toward the heat exchange unit  150 . 
     Further, the outdoor unit  100  is provided with a discharge pressure detecting unit  126 . The discharge pressure detecting unit  126  is provided to a pipe connecting the refrigerant flow switching device  112  with a discharge side of the compressor  111 , and is configured to detect a discharge pressure of the compressor  111 . The discharge pressure detecting unit  126  is, for example, a sensor or other devices, and sends a signal representing a detected discharge pressure to the controller  3 . The discharge pressure detecting unit  126  may include a storage device or other devices. In this case, the discharge pressure detecting unit  126  accumulates data representing detected discharge pressures in the storage device or other devices for a preset period of time and sends a signal containing data representing detected discharge pressures to the controller  3  in each set cycle. 
     Moreover, the outdoor unit  100  is provided with a suction pressure detecting unit  127 . The suction pressure detecting unit  127  is provided to a pipe connecting the refrigerant flow switching device  112  with the accumulator  14 , and is configured to detect a suction pressure of the compressor  111 . The suction pressure detecting unit  127  is, for example, a sensor or other devices, and sends a signal representing a detected suction pressure to the controller  3 . The suction pressure detecting unit  127  may include a storage device or other devices. In this case, the suction pressure detecting unit  127  accumulates data representing detected suction pressures in the storage device or other devices for a preset period of time and sends a signal containing data representing detected suction pressures to the controller  3  in each set cycle. 
     (Indoor Units  300 A and  300 B) 
     The indoor units  300 A and  300 B include indoor heat exchangers  322 A and  322 B serving as second heat exchangers and expansion devices  321 A and  321 B serving as second expansion devices, respectively. The indoor heat exchangers  322 A and  322 B operate as condensers or evaporators. The expansion devices  321 A and  321 B adjust the flow rates of refrigerant circulating through the indoor units  300 A and  300 B. Each of the indoor units  300 A and  300 B is configured to cool or heat an air-conditioned space such as the inside of a room with heating energy or cooling energy supplied from the outdoor unit  100 . The expansion devices  321 A and  321 B are, for example, opening-degree-variable electronic expansion valves or other devices. 
     The indoor units  300 A and  300 B are provided with the gas pipe temperature detecting units  304 A and  304 B and the liquid pipe temperature detecting units  303 A and  303 B, respectively. The gas pipe temperature detecting unit  304 A is provided between the indoor heat exchanger  322 A and the relay unit  200 , and is configured to detect the temperature of refrigerant flowing through the gas branch pipe  403 A connecting the indoor heat exchanger  322 A with the relay unit  200 . The gas pipe temperature detecting unit  304 B is provided between the indoor heat exchanger  322 B and the relay unit  200 , and is configured to detect the temperature of refrigerant flowing through the gas branch pipe  403 B connecting the indoor heat exchanger  322 B with the relay unit  200 . The gas pipe temperature detecting units  304 A and  304 B are, for example, thermistors or other devices, and send signals representing detected temperatures to the controller  3 . The gas pipe temperature detecting units  304 A and  304 B may include storage devices or other devices. In this case, the gas pipe temperature detecting units  304 A and  304 B accumulate data representing detected temperatures in the storage devices or other devices for a preset period of time and send signals containing data representing detected temperatures to the controller  3  in each set cycle. 
     The liquid pipe temperature detecting unit  303 A is provided between the indoor heat exchanger  322 A and the expansion device  321 A, and the liquid pipe temperature detecting unit  303 B is provided between the indoor heat exchanger  322 B and the expansion device  321 B. The liquid pipe temperature detecting unit  303 A detects the temperature of refrigerant flowing through the liquid branch pipe  404 A connecting the indoor heat exchanger  322 A with the expansion device  321 A, and the liquid pipe temperature detecting unit  303 B detects the temperature of refrigerant flowing through the liquid branch pipe  404 B connecting the indoor heat exchanger  322 B with the expansion device  321 B. The liquid pipe temperature detecting units  303 A and  303 B are, for example, thermistors or other devices, and send signals representing detected temperatures to the controller  3 . The liquid pipe temperature detecting units  303 A and  303 B may include storage devices or other devices. In this case, the liquid pipe temperature detecting units  303 A and  303 B accumulate data representing detected temperatures in the storage devices or other devices for a preset period of time and send signals containing data representing detected temperatures to the controller  3  in each set cycle. 
     (Relay Unit  200 ) 
     The relay unit  200  includes a first branching unit  240 , a second branching unit  250 , a gas-liquid separator  201 , a relay bypass pipe  209 , a liquid outflow control valve  204 , a heat exchange unit  260 , and a relay bypass flow control valve  205 . The relay unit  200  is provided between the outdoor unit  100  and the indoor units  300 A and  300 B. The relay unit  200  switches, upon request of the indoor units  300 A and  300 B, the flows of refrigerant supplied from the outdoor unit  100 , and distributes, to the plurality of indoor units  300 A and  300 B, the refrigerant supplied from the outdoor unit  100 . 
     The first branching unit  240  has first ends connected to the gas branch pipes  403 A and  403 B and second ends connected to the low-pressure pipe  401  and the high-pressure pipe  402 , and is formed to allow refrigerant to circulate in one direction during cooling operation and allow refrigerant to circulate in another direction during heating operation. The first branching unit  240  includes heating solenoid valves  202 A and  202 B and cooling solenoid valves  203 A and  203 B. The heating solenoid valves  202 A and  202 B have their respective first ends connected to the gas branch pipes  403 A and  403 B, have their respective second ends connected to the high-pressure pipe  402 , and are configured to be opened during heating operation and closed during cooling operation. The cooling solenoid valves  203 A and  203 B have their respective first ends connected to the gas branch pipes  403 A and  403 B, have their respective second ends connected to the low-pressure pipe  401 , and are configured to be opened during cooling operation and closed during heating operation. 
     The second branching unit  250  has first ends connected to the liquid branch pipes  404 A and  404 B and second ends connected to the low-pressure pipe  401  and the high-pressure pipe  402 , and is formed to allow refrigerant to circulate in one direction during cooling operation and allow refrigerant to circulate in another direction during heating operation. The second branching unit  250  includes first check valves  210 A and  210 B and second check valves  211 A and  211 B. 
     The first check valves  210 A and  210 B have their respective first ends connected to the liquid branch pipes  404 A and  404 B, have their respective second ends connected to the high-pressure pipe  402 , and allow passage of refrigerant from the high-pressure pipe  402  toward the liquid branch pipes  404 A and  404 B. 
     The first check valves  211 A and  211 B have their respective first ends connected to the liquid branch pipes  404 A and  404 B, have their respective second ends connected to the low-pressure pipe  401 , and allow passage of refrigerant from the liquid branch pipes  404 A and  404 B toward the low-pressure pipe  401 . 
     The gas-liquid separator  201  is configured to separate refrigerant into gaseous refrigerant and liquid refrigerant, and has an inflow side connected to the high-pressure pipe  402 , a gas outflow side connected to the first branching unit  240 , and a liquid outflow side connected to the second branching unit  250 . The relay bypass pipe  209  is formed to connect the second branching unit  250  with the low-pressure pipe  401 . The liquid outflow control valve  204  is connected to the liquid outflow side of the gas-liquid separator  201 , and is, for example, an opening-degree-variable electronic expansion valve or other devices. The liquid outflow control valve  204  is configured to adjust the flow rate of liquid refrigerant flowing out from the gas-liquid separator  201 . 
     The heat exchange unit  260  is composed of a first heat exchange unit  206  and a second heat exchange unit  207 . The first heat exchange unit  206  is provided between the liquid outflow side of the gas-liquid separator  201  and the liquid outflow control valve  204  and to the relay bypass pipe  209 . The first heat exchange unit  206  is formed to exchange heat between the liquid refrigerant flowing out from the gas-liquid separator  201  and refrigerant flowing through the relay bypass pipe  209 . The second heat exchange unit  207  is provided downstream of the liquid outflow control valve  204  and to the relay bypass pipe  209 . The second heat exchange unit  207  is formed to exchange heat between refrigerant flowing out from the liquid outflow control valve  204  and the refrigerant flowing through the relay bypass pipe  209 . 
     The relay bypass flow control valve  205  is connected to an upstream side of the second heat exchange unit  207  in the relay bypass pipe  209 , and is, for example, an opening-degree-variable electronic expansion valve or other devices. The relay bypass flow control valve  205  is configured to adjust the flow rate of a portion of refrigerant flowing out from the second heat exchange unit  207  that has flowed into the relay bypass pipe  209 . 
     Note here that the first check valves  210 A and  2106  have their upstream sides connected to a downstream side of the second heat exchange unit  207  and the relay bypass pipe  209 . Thus, refrigerant having flowed out from the second heat exchange unit  207  divides into refrigerant flowing toward the first check valves  210 A and  2106  and refrigerant flowing into the relay bypass pipe  209 . Further, the second check valves  211 A and  211 B have their downstream sides connected between the liquid outflow control valve  204  and the upstream side of the second heat exchange unit  207 . That is, refrigerant having flowed out from the second check valves  211 A and  211 B flows into the second heat exchange unit  207 , exchanges heat, and then divides into refrigerant flowing toward the first check valves  210 A and  210 B and refrigerant flowing into the relay bypass pipe  209 . 
     Further, the relay unit  200  is provided with a liquid outflow pressure detecting unit  231 , a downstream liquid outflow pressure detecting unit  232 , and a relay bypass temperature detecting unit  208 . The liquid outflow pressure detecting unit  231  is provided between the first heat exchange unit  206  and an upstream side of the liquid outflow control valve  204 , and is configured to detect a pressure of refrigerant at the liquid outflow side of the gas-liquid separator  201 . The liquid outflow pressure detecting unit  231  is, for example, a sensor or other devices, and sends a signal representing a detected pressure to the controller  3 . The liquid outflow pressure detecting unit  231  may include a storage device or other devices. In this case, the liquid outflow pressure detecting unit  231  accumulates data representing detected pressures in the storage device or other devices for a preset period of time and sends a signal containing data representing detected pressures to the controller  3  in each set cycle. 
     The downstream liquid outflow pressure detecting unit  232  is provided between a downstream side of the liquid outflow control valve  204  and the second heat exchange unit  207 , and is configured to detect a pressure of refrigerant having flowed out from the liquid outflow control valve  204 . The downstream liquid outflow pressure detecting unit  232  is, for example, a sensor or other devices, and sends a signal representing a detected pressure to the controller  3 . The downstream liquid outflow pressure detecting unit  232  may include a storage device or other devices. In this case, the downstream liquid outflow pressure detecting unit  232  accumulates data representing detected pressures in the storage device or other devices for a preset period of time and sends a signal containing data representing detected pressures to the controller  3  in each set cycle. Note here that the liquid outflow control valve  204  has its opening degree adjusted by the controller  3  such that a difference between a pressure detected by the liquid outflow pressure detecting unit  231  and a pressure detected by the downstream liquid outflow pressure detecting unit  232  is constant. 
     The relay bypass temperature detecting unit  208  is provided to the relay bypass pipe  209 , and is configured to detect a pressure of refrigerant flowing through the relay bypass pipe  209 . The relay bypass temperature detecting unit  208  is, for example, a thermistor or other devices, and sends a signal representing a detected temperature to the controller  3 . The relay bypass temperature detecting unit  208  may include a storage device or other devices. In this case, the relay bypass temperature detecting unit  208  accumulates data representing detected temperatures in the storage device or other devices for a preset period of time and sends a signal containing data representing detected temperatures to the controller  3  in each set cycle. Note here that the controller  3  adjusts the opening degree of the relay bypass flow control valve  205  on the basis of at least one of a pressure detected by the liquid outflow pressure detecting unit  231 , a pressure detected by the downstream liquid outflow pressure detecting unit  232 , and a temperature detected by the relay bypass temperature detecting unit  208 . 
     As shown in  FIG. 17 , also in Embodiment 5, a liquid-level sensing device  15  is attached to the accumulator  14 , as in the case of Embodiments 1 to 4. As the accumulator  14  and the liquid-level sensing device  15  are identical in configuration and operation to those of Embodiment 1, a description of the configurations and operations of the accumulator  14  and the liquid-level sensing device  15  is omitted here. 
     Also in Embodiment 5, as in the case of Embodiment 1, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  100 . In a case in which the liquid level in the accumulator  14  is higher than the preset threshold Th, the controller  3  makes the frequency of the compressor  111  installed in the outdoor unit  100  lower than or equal to the preset first specified value Sp 1  for the limiting operation. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  111 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000 . The following describes how the controller  3  operates. 
     [Process of Controlling Frequency of Compressor  111 ] The controller  3  operates in the same manner as in the flow chart of  FIG. 9  described above. Therefore, how the controller  3  operates is described with reference to  FIG. 9 . It should be noted that the controller  3  is configured in the same manner as shown in  FIG. 2 . 
     In step S 21 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  111  is stopped. 
     In step S 22 , the frequency control unit  36  of the controller  3  determines, on the basis of the liquid level sensed in step S 21 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the frequency control unit  36  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 23 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the frequency control unit  36  determines that the liquid level is normal, and ends the process of  FIG. 9 . 
     In step S 23 , the frequency control unit  36  of the controller  3  sets the frequency of the compressor  111  to the preset first specified value Sp 1  or lower. 
     Thus, in Embodiment 5, the frequency control unit  36  controls the frequency of the compressor  111  on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the frequency control unit  36  sets the frequency of the compressor  111  to the first specified value Sp 1  or lower. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  111  upon start-up of the compressor  111 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the frequency control unit  36  controls the frequency of the compressor  111  by a control method for normal operation. That is, the frequency control unit  36  controls the frequency of the compressor  111  such that a target condensing temperature or a target discharge temperature is achieved. 
     In Embodiment 5, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  100 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  makes the frequency of the compressor  111  installed in the outdoor unit  100  lower than or equal to the preset first specified value Sp 1 . This inhibits the rise in gas flow rate of refrigerant flowing through the refrigerant circuit, making it possible to reduce the suction of low-pressure refrigerant into the compressor  111 . This makes it possible to reduce the occurrence of liquid return to the compressor  111 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  111  without making the accumulator  14  complex in structure. 
     Also in Embodiment 5, as in the case of the modification of Embodiment 1, an amount of increase in the frequency of the compressor  111  may be determined to correspond to an amount of change in the liquid level in the accumulator  14  after the frequency of the compressor  111  has been set to the first specified value Sp 1  or lower. In this case, effects that are similar to those of the modification of Embodiment 1 are brought about. 
     Modification 1 of Embodiment 5 
     The controller  3  may be configured as shown in  FIG. 10 , as in the case of Embodiment 2. In this case, the controller  3  performs the following process in accordance with the flow chart of  FIG. 11  described above. 
     In step S 31 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  111  is stopped. 
     In step S 32 , the expansion control unit  37  of the controller  3  determines, on the basis of the liquid level sensed in step S 31 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the expansion control unit  37  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 33 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the expansion control unit  37  determines that the liquid level is normal, and ends the process of  FIG. 11 . 
     In step S 33 , the expansion control unit  37  of the controller  3  sets the opening degree of at least one of the expansion devices  120 ,  321 A, and  321 B to the preset second specified value Sp 2  or higher. 
     Thus, in Modification 1 of Embodiment 5, the expansion control unit  37  controls the opening degree of at least one of the expansion devices  120 ,  321 A, and  321 B on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the expansion control unit  37  sets the opening degree of at least one of the expansion devices  120 ,  321 A, and  321 B to the second specified value Sp 2  or higher for the limiting operation. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  111  upon start-up of the compressor  111 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the expansion control unit  37  controls the opening degree of the expansion device  120 ,  321 A, or  321 B by the control method for normal operation. 
     That is, the expansion control unit  37  controls the opening degree of the expansion device  120 ,  321 A, or  321 B such that a degree of subcooling SC at an outlet of the outdoor heat exchanger  113  or the indoor heat exchanger  322 A or  322 B reaches a target degree of subcooling. 
     In Modification 1 of Embodiment 5, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  100 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  sets the opening degree of at least one of the expansion devices  120 ,  321 A, and  321 B to the second specified value Sp 2  or higher. This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  111 , thus making it possible to reduce the occurrence of liquid return to the compressor  111  and making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  111  without making the accumulator  14  complex in structure. 
     Also in Modification 1 of Embodiment 5, as in the case of the modification of Embodiment 2, an amount of decrease in the opening degree of at least one of the expansion devices  120 ,  321 A, and  321 B may be determined to correspond to an amount of change in the liquid level in the accumulator  14  after the opening degree of at least a corresponding one of the expansion devices  120 ,  321 A, and  321 B has been set to the second specified value Sp 2  or higher. In this case, effects that are similar to those of the modification of Embodiment 2 are brought about. 
     Modification 2 of Embodiment 5 
     Also in Embodiment 5, as in the case of Embodiment 3, as shown in  FIG. 12 , the bypass  51  may be provided between a high-pressure side that is a refrigerant discharge side of the compressor  111  and a low-pressure side that is an inflow side of the accumulator  14 . Further, as shown in  FIG. 12 , the bypass  51  may be provided with the hot-gas bypass valve  52 . As the bypass  51  and the hot-gas bypass valve  52  are identical in configuration and operation to those of Embodiment 3, a description of the configurations and operations of the bypass  51  and the hot-gas bypass valve  52  is omitted here. 
     In this case, the controller  3  is configured as shown in  FIG. 13  described above. Further, the controller  3  controls the opening and closing of the hot-gas bypass valve  52  in accordance with the flow chart of  FIG. 14  described above. 
     In Modification 2 of Embodiment 5, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  100 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  switches the hot-gas bypass valve  52  from a closed state to an open state for the limiting operation. This causes high-temperature hot gas to flow into the accumulator  14 , thus causing liquid refrigerant in the accumulator  14  to evaporate and be delivered to the refrigerant circuit. As a result, this makes it possible to reduce the suction of low-pressure refrigerant into the compressor  111 , thus making it possible to reduce the occurrence of liquid return to the compressor  111  and making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. 
     Modification 3 of Embodiment 5 
     Also in Embodiment 5, as in the case of Embodiment 4, the controller  3  may be configured as shown in  FIG. 15  described above. 
     In this case, the controller  3  performs a process in accordance with the flow chart of  FIG. 16  described above. As this process is equivalent to a combination of Embodiment 5, Modification 1 of Embodiment 5, and Modification 2 of Embodiment 5, a description of the process is omitted here. 
     As noted above, as Modification 3 of Embodiment 5 is a combination of Embodiment 5, Modification 1 of Embodiment 5, and Modification 2 of Embodiment 5, Modification 3 of Embodiment 5 brings about effects that are similar to those of Embodiment 5, Modification 1 of Embodiment 5, and Modification 2 of Embodiment 5. 
     Embodiment 6 
       FIG. 18  is a schematic diagram showing a configuration of a refrigeration cycle apparatus according to Embodiment 6.  FIG. 18  shows an air-conditioning apparatus  1000  as an example of the refrigeration cycle apparatus. 
     As shown in  FIG. 18 , the air-conditioning apparatus  1000  includes a refrigerant circuit in which an outdoor unit  400  and a plurality of indoor units  500 A,  500 B,  500 C, and  500 D are connected by refrigerant pipes and through which refrigerant circulates. The four indoor units  500 A,  500 B,  500 C, and  500 D are connected in parallel to one another, and are identical in configuration to one another. Although the air-conditioning apparatus  1000  of the present embodiment includes four indoor units  500  here, the number of indoor units  500  that are installed is not limited to four. Pieces of equipment or other devices of the same kind that are for example differentiated by subscripts such as A, B, C, and D may be described with an omission of subscripts in a case in which they do not particularly need to be differentiated or identified. Moreover, how high or low temperatures, pressures, or other parameters are is not determined in relation to absolute values but relatively determined in terms of states, actions, or other conditions in systems, apparatuses, or other devices. 
     Further, as in the case of Embodiments 1 to 5, the air-conditioning apparatus  1000  includes a controller  3 . As in the case of  FIG. 1 , the controller  3  is provided outside the outdoor unit  400  and the indoor units  500 A,  500 B,  500 C, and  500 D. However, this is not intended to impose any limitation, and the controller  3  may be provided inside any of the outdoor unit  400  and the indoor units  500 A,  500 B,  500 C, and  500 D. 
     In the present embodiment, as shown in  FIG. 18 , the outdoor unit  400  includes a compressor  411 , a refrigerant flow switching device  412 , an outdoor heat exchanger  413 , an outdoor fan  404 , an accumulator  14 , an expansion device  420 , a double pipe  407 , and a bypass expansion valve  408 . 
     The compressor  411  sucks in, compresses, and discharges refrigerant. The compressor  411  includes an inverter device or other devices and, by arbitrarily changing its frequency, is configured to finely change the amount of refrigerant that the compressor  411  sends out per unit time. 
     The refrigerant flow switching device  412 , such as a four-way valve, switches, for example, between the flow of refrigerant during cooling operation and the flow of refrigerant during heating operation in accordance with an instruction from an outdoor control device  110 . Further, the outdoor heat exchanger  413  exchanges heat between refrigerant and outdoor air. For example, the outdoor heat exchanger  413  operates as an evaporator during heating operation to exchange heat between low-pressure refrigerant having flowed in via the expansion device  420  and air, evaporate, and gasify the refrigerant. Further, the outdoor heat exchanger  413  operates as a condenser during cooling operation to exchange heat between refrigerant that has flowed into the compressor  411  from the refrigerant flow switching device  412  and has compressed in the compressor  411  and air, condense, and liquefy the refrigerant. The outdoor heat exchanger  413  is provided with the outdoor fan  404 , which is an air-sending device, for efficient heat exchange between refrigerant and air. A fan motor  405  is a motor configured to drive the outdoor fan  404 . A rotation speed of the outdoor fan  404  is also finely changed by arbitrarily changing the drive frequency of the fan motor  405  with an inverter device. Note here that the direction in which the outdoor fan  404  is rotated by the fan motor  405  is a positive rotation. 
     The double pipe  407 , which serves as an inter-refrigerant heat exchanger, exchanges heat between refrigerant flowing through a main flow passage of the refrigerant circuit and refrigerant branching off from the main flow passage and having has its flow rate adjusted by the bypass expansion valve  408 . In particular, the double pipe  407  is formed to, in a case in which subcooling of refrigerant is needed during cooling operation, subcool refrigerant and supply the indoor units  500 A,  500 B,  500 C, and  500 D with the refrigerant thus subcooled. Liquid flowing via the bypass expansion valve  408  is returned to the accumulator  14  via a bypass pipe. The accumulator  14  is connected to a low-pressure side of the compressor  411  and stores excess refrigerant. 
     Further, the outdoor unit  400  of the present embodiment includes a high-pressure pressure sensor  415  and a low-pressure pressure sensor  416 . The high-pressure pressure sensor  415  detects a pressure in a discharge pipe of the compressor  411 . The low-pressure pressure sensor  416  detects a pressure in an inlet pipe of the accumulator  14 . 
     The indoor units  500 A,  500 B,  500 C, and  500 D include indoor heat exchangers  522 A,  522 B,  522 C, and  522 D and expansion devices  521 A,  521 B,  521 C, and  521 D, respectively. Each of the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D exchanges heat between refrigerant and air in an air-conditioned space. Each of the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D operates as a condenser during heating operation. That is, each of the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D exchanges heat between refrigerant having flowed in from a pipe through which gas refrigerant flows and air, condenses the refrigerant into liquid or two-phase gas-liquid refrigerant, and lets out the liquid or two-phase gas-liquid refrigerant. Meanwhile, each of the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D operates as an evaporator during cooling operation. That is, each of the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D exchanges heat between refrigerant brought into a low-pressure state by a corresponding one of the expansion devices  521 A,  521 B,  521 C, and  521 D and air, gasifies the refrigerant by causing the refrigerant to remove heat from the air, and lets out the refrigerant thus gasified. Further, the indoor units  500 A,  500 B,  500 C, and  500 D are provided with indoor fans  517 A,  517 B,  517 C, and  517 D configured to adjust the flow of air with which heat is exchanged. The driving speeds, that is, air volumes of these indoor fans  517 A,  517 B,  517 C, and  517 D are determined, for example, by settings configured by a user, although this is not intended to impose any particular limitation. The expansion devices  521 A,  521 B,  521 C, and  521 D are provided, so that the pressures of refrigerant in the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D are adjusted by changing the opening degrees of the expansion devices  521 A,  521 B,  521 C, and  521 D. 
     As shown in  FIG. 18 , also in Embodiment 6, a liquid-level sensing device  15  is attached to the accumulator  14 , as in the case of Embodiments 1 to 5. As the accumulator  14  and the liquid-level sensing device  15  are identical in configuration and operation to those of Embodiment 1, a description of the configurations and operations of the accumulator  14  and the liquid-level sensing device  15  is omitted here. 
     Also in Embodiment 6, as in the case of Embodiment 1, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  400 . In a case in which the liquid level in the accumulator  14  is higher than the preset threshold Th, the controller  3  makes the frequency of the compressor  411  installed in the outdoor unit  400  lower than or equal to the preset first specified value Sp 1  for the limiting operation. This makes it possible to reduce the occurrence of liquid return to the compressor  411 , making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000 . The following describes how the controller  3  operates. 
     In Embodiment 6, the outdoor heat exchanger  413  is a first heat exchanger, and the indoor heat exchangers  522 A,  522 B,  522 C, and  522 D are each a second heat exchanger. Further, the expansion device  420  is a first expansion device, and the expansion devices  521 A,  521 B,  521 C, and  521 D are each a second expansion device. 
     [Process of Controlling Frequency of Compressor  411 ] 
     The controller  3  operates in the same manner as in the flow chart of  FIG. 9  described above. Therefore, how the controller  3  operates is described with reference to  FIG. 9 . It should be noted that the controller  3  is configured in the same manner as shown in  FIG. 2 . 
     In step S 21 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  411  is stopped. 
     In step S 22 , the frequency control unit  36  of the controller  3  determines, on the basis of the liquid level sensed in step S 21 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the frequency control unit  36  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 23 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the frequency control unit  36  determines that the liquid level is normal, and ends the process of  FIG. 9 . 
     In step S 23 , the frequency control unit  36  of the controller  3  sets the frequency of the compressor  411  to the preset first specified value Sp 1  or lower. 
     Thus, in Embodiment 6, the frequency control unit  36  controls the frequency of the compressor  411  on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the frequency control unit  36  sets the frequency of the compressor  411  to the first specified value Sp 1  or lower. This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  411  upon start-up of the compressor  411 , thus making it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  411 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the frequency control unit  36  controls the frequency of the compressor  411  by a control method for normal operation. That is, the frequency control unit  36  controls the frequency of the compressor  411  such that a target condensing temperature or a target discharge temperature is achieved. 
     In Embodiment 6, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  400 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  makes the frequency of the compressor  411  installed in the outdoor unit  400  lower than or equal to the preset first specified value Sp 1 . This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  411  upon start-up of the compressor  411 , thus making it possible to reduce the occurrence of liquid return to the compressor  411  and making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  411  without making the accumulator  14  complex in structure. 
     Also in Embodiment 6, as in the case of the modification of Embodiment 1, an amount of increase in the frequency of the compressor  411  may be determined to correspond to an amount of change in the liquid level in the accumulator  14  after the frequency of the compressor  411  has been set to the first specified value Sp 1  or lower. In this case, effects that are similar to those of the modification of Embodiment 1 are brought about. 
     Modification 1 of Embodiment 6 
     The controller  3  may be configured as shown in  FIG. 10 , as in the case of Embodiment 2. In this case, the controller  3  performs the following process in accordance with the flow chart of  FIG. 11  described above. 
     In step S 31 , the liquid level in the accumulator  14  is sensed by performing the process of the flow chart of  FIG. 8  in a state in which the operation of the compressor  411  is stopped. 
     In step S 32 , the expansion control unit  37  of the controller  3  determines, on the basis of the liquid level sensed in step S 31 , whether the liquid level in the accumulator  14  is higher than the threshold Th. Specifically, in a case in which the liquid level in the accumulator  14  is the level a, the expansion control unit  37  determines that the liquid level in the accumulator  14  is higher than the threshold Th, and proceeds to step S 33 . On the other hand, in a case in which the liquid level in the accumulator  14  is the level b or the level c, the expansion control unit  37  determines that the liquid level is normal, and ends the process of  FIG. 11 . 
     In step S 33 , the expansion control unit  37  of the controller  3  sets the opening degree of at least one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D to the preset second specified value Sp 2  or higher. 
     Thus, in Modification 1 of Embodiment 6, the expansion control unit  37  controls the opening degree of at least one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D on the basis of the liquid level in the accumulator  14  as sensed by the liquid-level determining unit  32 . Specifically, in a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the expansion control unit  37  sets the opening degree of at least one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D to the second specified value Sp 2  or higher for the limiting operation. This makes it possible to reduce the occurrence of liquid return from the accumulator  14  to the compressor  411  upon start-up of the compressor  411 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 . 
     On the other hand, in a case in which the liquid level in the accumulator  14  is determined to be normal, the expansion control unit  37  controls the opening degree of the expansion device  420 ,  521 A,  521 B,  521 C, or  521 D by the control method for normal operation. That is, the expansion control unit  37  controls the opening degree of the expansion device  420 ,  521 A,  521 B,  521 C, or  521 D such that a degree of subcooling SC at an outlet of the outdoor heat exchanger  413  or the indoor heat exchanger  522 A,  522 B,  522 C, or  522 D reaches a target degree of subcooling. 
     In Modification 1 of Embodiment 6, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  400 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  sets the opening degree of at least one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D to the second specified value Sp 2  or higher. This makes it possible to reduce the suction of low-pressure refrigerant into the compressor  411  upon start-up of the compressor  411 , thus making it possible to reduce the occurrence of liquid return to the compressor  411  and making it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  411  without making the accumulator  14  complex in structure. 
     Also in Modification 1 of Embodiment 6, as in the case of the modification of Embodiment 2, an amount of decrease in the opening degree of at least one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D may be determined to correspond to an amount of change in the liquid level in the accumulator  14  after the opening degree of at least a corresponding one of the expansion devices  420 ,  521 A,  521 B,  521 C, and  521 D has been set to the second specified value Sp 2  or higher. In this case, effects that are similar to those of the modification of Embodiment 2 are brought about. 
     Modification 2 of Embodiment 6 
     Also in Embodiment 6, as in the case of Embodiment 3, as shown in  FIG. 12 , the bypass  51  may be provided between a high-pressure side that is a refrigerant discharge side of the compressor  411  and a low-pressure side that is an inflow side of the accumulator  14 . Further, as shown in  FIG. 12 , the bypass  51  may be provided with the hot-gas bypass valve  52 . As the bypass  51  and the hot-gas bypass valve  52  are identical in configuration and operation to those of Embodiment 3, a description of the configurations and operations of the bypass  51  and the hot-gas bypass valve  52  is omitted here. 
     In this case, the controller  3  is configured as shown in  FIG. 13  described above. Further, the controller  3  controls the opening and closing operations of the hot-gas bypass valve  52  in accordance with the flow chart of  FIG. 14  described above. 
     In Modification 2 of Embodiment 6, as noted above, during start-up of heating or during return to heating after completion of defrosting operation of the air-conditioning apparatus  1000 , the liquid-level sensing device  15  senses the liquid level in the accumulator  14  installed in the outdoor unit  400 . In a case in which the liquid level in the accumulator  14  is higher than the threshold Th, the controller  3  switches the hot-gas bypass valve  52  from a closed state to an open state for the limiting operation. This causes high-temperature hot gas to flow into the accumulator  14 , thus causing liquid refrigerant in the accumulator  14  to evaporate and be delivered to the refrigerant circuit. As a result, this makes it possible to reduce the suction of low-pressure refrigerant into the compressor  411 , thus making it possible to reduce the occurrence of liquid return to the compressor  411 . As a result, this makes it possible to bring about improvement in start-up characteristics of the air-conditioning apparatus  1000  for heating. This also makes it possible to bring about improvement in reliability of the compressor  411  without making the accumulator  14  complex in structure. 
     Modification 3 of Embodiment 6 
     Also in Embodiment 6, as in the case of Embodiment 4, the controller  3  may be configured as shown in  FIG. 15  described above. 
     In this case, the controller  3  performs a process in accordance with the flow chart of  FIG. 16  described above. As this process is equivalent to a combination of Embodiment 6, Modification 1 of Embodiment 6, and Modification 2 of Embodiment 6, a description of the process is omitted here. 
     As noted above, as Modification 3 of Embodiment 6 is a combination of Embodiment 6, Modification 1 of Embodiment 6, and Modification 2 of Embodiment 6, Modification 3 of Embodiment 6 brings about effects that are similar to those of Embodiment 6, Modification 1 of Embodiment 6, and Modification 2 of Embodiment 6. 
     Further, although air-conditioning apparatuses  1000  are described in Embodiments 1 to 6 as examples of refrigeration cycle apparatuses, this is not intended to impose any limitation, and Embodiments 1 to 6 are also applicable to other refrigeration cycle apparatuses such as hot-water supply apparatuses, freezers, refrigerators, and automatic vending machines.