Air conditioner

Abstract: An air conditioner in which, when a first compressor 1, 201 is being operated and a second compressor 2, 202 is stopped, liquid refrigerant does not flow into the second compressor even in a state of wet vapor suction, a decrease in lubricating oil in the second compressor or a decline in its concentration are not experienced, and breakage of the second compressor, due to faulty lubrication when the second compressor is started, does not occur. The air conditioner which does not lapse into a wasteful shortage of cooling and heating capabilities when the air conditioner is not in the state of wet vapor suction, by reliably detecting the presence or absence of the state of wet vapor suction. The first compressor and the second compressor are connected together by an equalizing pipe 3, 203. A bypass passage 28 which connects together a discharge pipe 6 and a suction pipe 8 of the second compressor 2 is provided.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an air conditioner in which two compressors are connected in parallel with a refrigerant circuit of one system. 2. Description of Relevant Art: As a conventional apparatus of this type, one shown in Fig. 53A (or 53B) is known. In the drawing, reference character A denotes a heat source unit, and B denotes an indoor unit. Reference numeral 1 (or 201) denotes a first compressor of a low-pressure shell type; 2 (or 202), a second compressor of the low-pressure shell type; and 3 (or 203), an equalizing pipe for connecting together the shell of the first compressor 1 and the shell of the second compressor 2, the equalizing pipe 3 being disposed at a position sufficiently higher than a minimum oil level for properly effecting the lubrication of the compressors. Numeral 4 denotes a discharge pipe of the first compressor 1; 5, a discharge pipe of the second compressor 2; 6, a common discharge pipe provided after the discharge pipes 4, 5 converge; 7 (or 212), a suction pipe of the first compressor 1; 8 (or 213), a suction pipe of the second compressor 2; 9, a common suction pipe before branching into the suction pipes 7, 8; 10 (or 204), an oil separator provided in the common discharge pipe 6 and having a shell 10a, an inlet pipe 10b, an outlet pipe 10c, and an oil return pipe 10d; 11 (or 205), a four-way changeover valve; 12 (or 206), a heat source unit-side heat exchanger; 15 (or 209), an accumulator provided in a branching portion in which the common suction pipe 9 branches into the suction pipes 7, 8; 22, an oil-returning bypass passage for connecting the oil return pipe 10d of the oil separator 10 and the common suction pipe 9; 23 (or 210), a solenoid on-off valve provided midway in the oil-returning bypass passage 22; and 24 (or 211), a capillary tube provided in parallel with the solenoid on-off valve 23. Numeral 16 denotes a U-pipe provided in the accumulator 15 and corresponding to the suction pipe 8, and numeral 17 denotes a U-pipe provided in the accumulator 15 and corresponding to the suction pipe 8. Numeral 18 denotes a bypass hole provided in the U-pipe 16 and designed to prevent the first compressor 1 from becoming damaged by temporarily sucking lubricating oil and a liquid refrigerant 25 accumulated in the U-pipe 16 at the time of the starting of the first compressor 1. Numeral 19 denotes a bypass hole provided in the U-pipe 17 and designed to prevent the second compressor 2 from becoming damaged as the second compressor 2 temporarily sucks lubricating oil and the liquid refrigerant 25 accumulated in the U-pipe 17 at the time of the starting of the second compressor 2. Numeral 20 denotes an oil return hole provided in the U-pipe 16 for gradually sucking the lubricating oil and the liquid refrigerant 25 accumulated in the bottom of the accumulator 15 and returning the same to the first compressor 1. Numeral 21 denotes an oil return hole provided in the U-pipe 17 for gradually sucking the lubricating oil and the liquid refrigerant 25 accumulated in the bottom of the accumulator 15 and returning the same to the second compressor 2. The heat source unit A is arranged as described above. Numeral 13 (or 207) denotes a throttling device; 14 (or 208), an indoor-side heat exchanger; and B, an indoor unit comprised of the aforementioned throttling device 13 and the indoor-side heat exchanger 14. Numeral 26 denotes a first connecting pipe having one end connected to the heat source unit A by the heat source unit-side heat exchanger 12 and the other end connected to the indoor unit B by the throttling device 13, while numeral 27 denotes a second connecting pipe having one end connected to the heat source unit A by the four-way changeover valve 11 and the other end connected to the indoor unit B by the indoor-side heat exchanger 14. In the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during heating operation. Next, a description will be given of the operation during cooling operation. The high-temperature, high-pressure gas refrigerant discharged from the first compressor 1 or the second compressor 2 passes through the oil separator 10 and the four-way changeover valve 11, and flows into the heat source unit-side heat exchanger 12 where the gas refrigerant radiates heat and condenses into a high-pressure liquid refrigerant. The pressure of this liquid refrigerant is reduced by the throttling device 13, and flows into the indoor-side heat exchanger 14 as a low-pressure gas-liquid two-phase refrigerant. By absorbing heat here, the refrigerant evaporates, flows into the accumulator 15 via the four-way changeover valve 11, passes thorough the U-pipes 16, 17 and the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2. At this time, as for the lubricating oil which has flowed out together with the refrigerant from the first compressor 1 or the second compressor 2, a major portion of it is separated by the oil separator 10, and is accumulated in the shell 10a of the oil separator 10. A portion of the accumulated lubricating oil, together with the gas refrigerant in the oil separator 10, is constantly sent to the accumulator 15 via the common suction pipe 9 by the capillary tube 24. The remaining lubricating oil in the shell 10a of the oil separator 10 is sent to the accumulator 15 via the common suction pipe 9 as the solenoid on-off valve 23 is opened. The lubricating oil which was not separated by the oil separator 10 is sent together with the refrigerant to the accumulator 15 via the four-way changeover valve 11, the heat source unit-side heat exchanger 12, the throttling device 13, the indoor-side heat exchanger 14, and the four-way changeover valve 11. The lubricating oil which has entered the accumulator 15 is accumulated in the bottom of the accumulator 15, and a portion of it flows into the U-pipes 16, 17 through the oil return holes 20, 21, passes through the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2. Since the first and second compressors 1, 2 are of the low-pressure shell type, the following relationships hold among the pressure P S0 at a branching portion where the common suction pipe 9 branches into the suction pipes 7, 8, the pressure P S1 within the shell of the first compressor 1, and the pressure P S2 within the shell of the second compressor 2: P S1 = P S0 - ΔP S1 P S2 = P S0 - ΔP S2 where ΔP S1 is a pressure loss from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 to the first compressor 1, while ΔP S2 is a pressure loss from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 to the second compressor 2, and the following relationships hold: ΔP S1 = z 1 r g V 12 /2 ΔP S2 = z 2 r g V 22 /2 r g : concentration of the gas refrigerant V 1 : flow rate of the gas refrigerant flowing through the suction pipe 7 V 2 : flow rate of the gas refrigerant flowing through the suction pipe 8 z 1 : constant representing channel resistance from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 up to the first compressor 1 z 2 : constant representing channel resistance from the branching portion where the common suction pipe 9 branches into the suction pipes 7, 8 up to the second compressor 2 Accordingly, a pressure difference ΔP S12 (P S1 - P S2 ), which is shown below, takes place in the shells of the first compressor 1 and the second compressor 2. ΔP S12 = (-z 1 V 12 + z 2 V 22 ) r g /2 Therefore, the pressure difference ΔP which occurs at both ends of the equalizing pipe 3 becomes as shown below. It should be noted, however, that it is assumed that both ends of the equalizing pipe are at substantially the same height. ΔP = ΔP S12 + r 1 g (h 1 - h 2 ) r 1 : concentration of a mixture of the lubricating oil and the liquid refrigerant g: gravitational acceleration h 1 : liquid level of the first compressor 1 with respect to the connecting portion between the shell of the first compressor 1 and the equalizing pipe 3 (when the liquid level is lower than the connecting portion, a setting is provided such that h 1 = 0) h 2 : liquid level of the second compressor 2 with respect to the connecting portion between the shell of the second compressor 2 and the equalizing pipe 3 (when the liquid level is lower than the connecting portion, a setting is provided such that h 2 = 0) That is, when ΔP > 0, the gas refrigerant and a mixed liquid (a mixed liquid of the lubricating oil and the liquid refrigerant) flow from the first compressor 1 to the second compressor 2 through the equalizing pipe 3. Meanwhile, when ΔP < 0, the gas refrigerant and the mixed liquid (the mixed liquid of the lubricating oil and the liquid refrigerant) flow from the second compressor 2 to the first compressor 1 through the equalizing pipe 3. In addition, when ΔP > 0, the liquid level of the mixed liquid (the mixed liquid of the lubricating oil and the liquid refrigerant) in the first compressor 1 drops until it reaches the position of the equalizing pipe 3, but it does not drop further below that position. Hence, if the concentration of the lubricating oil is high, the lubrication of the second compressor 2 is effected properly. Thus, a refrigeration cycle during cooling is formed. Next, a description will be given of the operation during heating operation. The high-temperature, high-pressure gas refrigerant discharged from the first compressor 1 or the second compressor 2 passes through the oil separator 10 and the four-way changeover valve 11, and flows into the indoor-side heat exchanger 14 where the gas refrigerant radiates heat and condenses into a high-pressure liquid refrigerant. The pressure of this liquid refrigerant is reduced by the throttling device 13, and flows into the heat source unit-side heat exchanger 12 as a low-pressure gas-liquid two-phase refrigerant. By absorbing heat here, the refrigerant evaporates, flows into the accumulator 15 via the four-way changeover valve 11, passes thorough the U-pipes 16, 17 and the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2. At this time, as for the lubricating oil which has flowed out together with the refrigerant from the first compressor 1 or the second compressor 2, a major portion of it is separated by the oil separator 10, and is accumulated in the shell 10a of the oil separator. A portion of the accumulated lubricating oil, together with the gas refrigerant in the shell 10a of the oil separator, is constantly sent to the accumulator 15 via the common suction pipe 9 by the capillary tube 24. The remaining lubricating oil in the shell 10a of the oil separator 10 is sent to the accumulator 15 via the common suction pipe 9 as the solenoid on-off valve 23 is opened. The lubricating oil which was not separated by the oil separator 10 is sent together with the refrigerant to the accumulator 15 via the four-way changeover valve 11, the indoor-side heat exchanger 14, the throttling device 13, the heat source unit-side heat exchanger 12, and the four-way changeover valve 11. The lubricating oil which has entered the accumulator 15 is accumulated in the bottom of the accumulator 15, and a portion of it flows into the U-pipes 16, 17 through the oil return holes 20, 21, passes through the suction pipes 7, 8, and returns to the first compressor 1 or the second compressor 2. Since the flow through the equalizing pipe 3 is utterly the same as during cooling, description thereof will be omitted here. Thus, a refrigeration cycle during heating is formed. The flow-rate of a mixed fluid flowing through the equalizing pipe 3 is calculated in simple form by the following formula: G 1 = a (▵p/r) ½ G 1 : flow rate of a mixed liquid of refrigerant and lubricating oil flowing through the equalizing pipe 3 α: flow coefficient ρ: concentration of the mixed liquid ▵p: differential pressure across the equalizing pipe (=difference between internal pressures of compressor shells) With such a conventional air conditioner, after the power supply of the air conditioner is turned on, in a state in which, of the two compressors 1 and 2, the compressor 1 is being operated and the compressor 2 is being stopped, when the compressor 2 is started after the running capacity of the compressor 1 has reached a certain level, the compressor 2 is started as it is. Then, a description will be given of the case where the liquid refrigerant is accumulated in the accumulator 15 and of the operation of the refrigerant system at that time. When the first compressor 1 is started in a state in which both the first compressor 1 and the second compressor 2 are being stopped, since the evaporation temperature of the refrigerant in the evaporator (the indoor-side heat exchanger 14 during cooling operation and the heat source unit-side heat exchanger 12 during heating operation) has not been sufficiently lowered, the unevaporated liquid refrigerant temporarily flows into the common suction pipe 9. However, since the accumulator 15 is provided, the liquid refrigerant which has been sucked in the state of wet vapor does not reach the first compressor 1 or the second compressor 2, is temporarily stored in the accumulator 15, flows into the U-pipe 16 through the oil return hole 20 together with the lubricating oil accumulated here, and returns gradually to the first compressor 1 via the suction pipe 7. For this reason, the first compressor 1 is prevented from being damaged by the temporary wet vapor suction at the time of starting. In addition, since the quantity of wet vapor sucked at that time is not very large, the liquid refrigerant in the accumulator 15 is removed in a relatively short time. In addition, when the first compressor 1 is started in a state in which the both the first and second compressors 1 and 2 have been stopped for a long time, the first compressor 1 is started in the state in which a large quantity of refrigerant lies inside the shells of the first and second compressors 1 and 2 as a liquid refrigerant. In this case, the liquid refrigerant held up inside the shell of the first compressor 1 is discharged in the form of a saturated gas or partially in the liquid state as it is, and the liquid refrigerant flows into the oil separator 10 via the discharge pipe 4 and the common discharge pipe 6. Since the discharge pipe 4, the common discharge pipe 6, and the oil separator 10 have become cool by being cooled by the outside air during stopping for a long time, the saturated gas refrigerant discharged from the first compressor 1 is cooled, condensed and liquefied. In addition, since the first compressor 1 is operated and the second compressor 2 remains stopped, the pressure within the shell of the first compressor 1 is lower than the pressure within the shell of the second compressor 2, and the liquid refrigerant held up inside the shell of the second compressor 2 is supplied to the first compressor 1 via the equalizing pipe 3. In the same way as the liquid refrigerant held up inside the shell of the first compressor 1, this liquid refrigerant is discharged in the form of a saturated gas or partially in the liquid state as it is, and this liquid refrigerant flows into the oil separator 10 via the discharge pipe 4 and the common discharge pipe 6, while the saturated gas refrigerant is cooled, condensed and liquefied. In the oil separator 10, a major portion of the liquid refrigerant is separated, and flows into the common suction pipe 9 via the solenoid on-off valve 23 since the solenoid on-off valve 23 is open for a fixed period of time during starting. However, since the accumulator 15 is provided, the liquid refrigerant which has been sucked in the state of wet vapor does not reach the first compressor 1 or the second compressor 2, is temporarily stored in the accumulator 15, flows into the U-pipe 16 through the oil return hole 20 together with the lubricating oil accumulated here, and returns gradually to the first compressor 1 via the suction pipe 7. For this reason, the first compressor 1 is prevented from being damaged by the temporary, but a large quantity of, wet vapor suction at the time of starting after stopping for a long time. In addition, since the quantity of wet vapor sucked at that time is very large, the liquid refrigerant in the accumulator 15 is removed after the lapse of a relatively long time. During the cooling operation the first connecting pipe 26 is in the high-pressure liquid single phase or in the gas-liquid two-phase state in which the dryness is very small, but during the heating operation the first connecting pipe 26 is in the gas-liquid two-phase state in which the dryness is 0.1 to 0.2. Hence, the average concentration of the refrigerant in the first connecting pipe 26 is much greater during the cooling operation than during the heating operation, so that the quantity of refrigerant distributed in the first connecting pipe 26 is larger during the cooling operation. Accordingly, in a case where the locations of installation of the heat source unit A and the indoor unit B are distant from each other and the first connecting pipe 26 is long, the total quantity of refrigerant required during the cooling operation becomes greater than the total quantity of refrigerant required during the heating operation. Since the quantity of refrigerant charged in the system is normally determined by the operation in which the total quantity of refrigerant required becomes maximum, excess refrigerant is produced during the heating operation by a portion in which the total quantity of refrigerant required is smaller than during the cooling operation. This excess refrigerant is distributed in the accumulator 15. In addition, if the first connecting pipe 26 is short, excess refrigerant is produced in the case of a chargeless system in which the refrigerant is not added when the setup work of the system is carried out by fixing the quantity of refrigerant charged in the system irrespective of the length of the first connecting pipe 26, i.e., by setting the quantity of refrigerant charged in the system to be the total quantity of refrigerant required when the length of the first connecting pipe 26 is the largest irrespective of the length of the first connecting pipe 26. This excess refrigerant is distributed in the accumulator 15. As described above, at the time when the power is turned on, there is a high possibility that a large quantity of refrigerant was held up in the compressors before then. When the compressor 1 is started, the lubricating oil is also liable to flow out together with the liquid refrigerant owing to foaming at the time of starting, so that the quantity of lubricating oil in the compressor 1 becomes small. When only the compressor 1 is being operated, since the internal pressure of the shell of the compressor 2 being stopped is higher than the internal pressure of the shell of the compressor 1 being operated, the lubricating oil in the compressor 2 is supplied together with the refrigerant to the shell in the compressor 1. In addition, since oil is not returned from the accumulator 15 to the compressor 2 being stopped, the level of the mixed liquid of the lubricating oil and the refrigerant in the compressor 2 drops to the vicinity of the height of the equalizing pipe 3. When the compressor 2 is started in such a stopped state, the refrigerant undergoes foaming in the compressor 2 due to a sudden drop in the internal pressure of the shell during starting, the lubricating oil in the compressor 2 is liable to be discharged together with the refrigerant, and the pressure difference across the equalizing pipe 3 becomes small or is reversed. Hence, the quantity of lubricating oil supplied to the compressor 1 through the equalizing pipe 3 decreases. In addition, an observation is made of the compressor 2 which is started, in a case where its running capacity is smaller than that of the compressor 1 already in operation, the relative magnitude of the pressure within the shell is higher in the case of the compressor 2. As a result, when the compressor 2 is started, the liquid level in the shell of the compressor 2 rises with foaming, and even though the liquid level may be lower than the equalizing pipe 3 during stopping, the liquid level rises above the height of the equalizing pipe 3 after starting, so that the mixed liquid of the refrigerant and the lubricating oil is liable to flow out from the equalizing pipe 3 toward the compressor 1. In a case where the running capacity of the compressor 2 which is started is higher than that of the compressor 1 being operated, the internal pressure of the shell becomes higher in the case of the compressor 1. Hence, oil is returned to the compressor 2 from the compressor 1 through the equalizing pipe 3, but when the concentration of the lubricating oil in the compressor 1 which is already in operation is low, such an oil-returning effect is small. In a state in which the liquid refrigerant is being accumulated in the accumulator 15, even when the compressor 1 is being operated and the compressor 2 is being stopped, since the interior of the shell of the compressor 1 and the interior of the shell of the compressor 2 communicates with each other via the equalizing pipe 3, the internal pressure of the shell of the compressor 2 being stopped also drops. Consequently, wet gas refrigerant moves from the accumulator 15 to the compressor 2, and condenses in the compressor 2, so that the concentration of the lubricating oil in the compressor 2 decreases gradually. For this reason, in a case where the compressor 2 is started for the first time after the power is turned on, there has been a risk that a bearing is damaged due to a shortage of the lubricating oil. In addition, in a case where excess liquid refrigerant is accumulated in the accumulator 15 when the compressor 2 being stopped is started, wet vapor suction has been liable to occur. In addition, in a case where the compressor 1 is started with the refrigerant being held up in both compressors, the concentration of the lubricating oil in the compressor 1 is low, and the lubricating oil is liable to flow out due to foaming during starting. Yet, when the compressor 2 is being stopped, the mixed liquid of the refrigerant and the lubricating oil in the compressor 2 is also supplied to the compressor 1 through the equalizing pipe 3. However, if the compressor 2 is started in a short time after the starting of the compressor 1, the lubricating oil in the compressor 2 is discharged together with the refrigerant due to foaming, and it becomes difficult for the oil to be returned to the compressor 1 from the equalizing pipe 3. Furthermore, since the quantity of lubricating oil in the compressor 1 is not sufficient, it is difficult to expect the return of oil from the compressor 1 through the equalizing pipe 3 to the compressor 2 which was started. Hence, there has occurred a problem of seizure of the bearings of the compressors due to the shortage of the lubricating oil in the compressor 1 and the compressor 2. Furthermore, since the conventional air conditioner is arranged as described above, in a case where the first compressor 1 is being operated and the second compressor 2 is being stopped, the refrigerant flows into the first compressor 1 not only via the U-pipe 16 and the suction pipe 7 but also via the U-pipe 17, the suction pipe 8, the shell of the second compressor 2, and the equalizing pipe 3. At this time, if the liquid refrigerant is accumulated in the accumulator 15, the liquid refrigerant is also accumulated inside the U-pipe 17 due to the presence of the oil return hole 21. Hence, since the flow rate of the refrigerant supplied from the bypass hole 19 is insufficient as the flow rate of the refrigerant supplied to the first compressor via the shell of the second compressor 2, and the liquid refrigerant accumulated inside the U-pipe 17 flows into the shell of the second compressor 2. As a result, the lubricating oil in the shell of the second compressor 2 is mixed with the liquid refrigerant, which has flowed in, and flows out to the shell of the first compressor 1 via the equalizing pipe 3 since the pressure within the shell of the first compressor 1 is lower than the pressure within the shell of the second compressor 2. Thus, the lubricating oil in the second compressor 2 declines in terms of its absolute quantity while the first compressor 1 is being operated and the second compressor 2 is being stopped, and the concentration of the lubricating oil also declines. Consequently, there has been a problem in that a shortage of lubricating oil or faulty lubrication due to such as the lack of viscosity of the lubricating oil occur when the second compressor is started possibly resulting in the breakage of the second compressor. What is desired is an air conditioner which overcomes or mitigates this problem. EP-A-0 410 570 discloses an air conditioner in accordance with the preamble of claim 1. The present invention provides an air conditioner as set forth in claim 1. Preferred features of the invention are set forth in the subsidiary claims. The invention will be described further, by way of example only with reference to the accompanying drawings, in which: Fig. 28 is a refrigerant circuit diagram centering on a refrigerant system of an air conditioner in accordance with a first embodiment of the present invention; Fig. 29 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with first to 10th embodiments of the present invention; Fig. 30 is a control block diagram of the air conditioner in accordance with the 3rd embodiment of the present invention; Fig. 31 is a control block diagram of the air conditioner in accordance with the 4th embodiment of the present invention; Fig. 32 is a control block diagram of the air conditioner in accordance with the 5th embodiment of the present invention; Fig. 33 is a control block diagram of the air conditioner in accordance with the 7th embodiment of the present invention; Fig. 34 is a control block diagram of the air conditioner in accordance with the 8th embodiment of the present invention; Fig. 35 is a control block diagram of the air conditioner in accordance with the 9th embodiment of the present invention; Fig. 36 is a control block diagram of the air conditioner in accordance with the 10th embodiment of the present invention; Fig. 37 is a flowchart illustrating details of control by a solenoid on-off valve controller of the air conditioner in accordance with the 3rd embodiment of the present invention; Fig. 38 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 4th embodiment of the present invention; Fig. 39 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 5th embodiment of the present invention; Fig. 40 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 7th embodiment of the present invention; Fig. 41 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 8th embodiment of the present invention; Fig. 42 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 9th embodiment of the present invention; Fig. 43 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 10th embodiment of the present invention; Fig. 44 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with a 11th embodiment of the present invention; Fig. 45 is a control block diagram of the air conditioner in accordance with the 11th embodiment of the present invention; Fig. 46 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with a 12th embodiment of the present invention; Fig. 47 is a control block diagram of the air conditioner in accordance with the 12th embodiment of the present invention; Fig. 48 is a flowchart illustrating details of control by the solenoid on-off valve controller of the air conditioner in accordance with the 12th embodiment of the present invention; Fig. 49 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with a 13th embodiment of the present invention; Fig. 50 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with the 13th embodiment of the present invention; Fig. 51 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with a 14th embodiment of the present invention; Fig. 52 is a refrigerant circuit diagram centering on the refrigerant system of an air conditioner in accordance with the 14th embodiment of the present invention; Fig. 53A is a refrigerant circuit diagram centering on the refrigerant system of a conventional air conditioner; and Fig. 53B is a refrigerant circuit diagram in accordance with a conventional example. First Embodiment Hereafter, a description will be given of an embodiment of the present invention. Fig. 28 is a refrigerant circuit diagram of an air conditioner in accordance with an embodiment of the present invention. In the drawing, same reference characters or numerals with that shown in Fig. 53A denote component parts that are similar to those of the conventional air conditioner shown in Fig. 53A, and description thereof will be omitted here. Numeral 28 denotes a bypass passage which branches off midway in the pipe between the four-way changeover valve 11 and the outlet pipe 10c of the oil separator 10, converges with the suction pipe 8 between the accumulator 15 and the second compressor 2, and has a certain channel resistance (a much greater channel resistance than that of the main flow to the indoor unit B). In addition, it is assumed that in a case where the load on the indoor unit B is small and it is unnecessary for both the first and second compressors 1 and 2 to be operated, and either one of them needs to be operated, the first compressor 1 is unfailingly started and the second compressor 2 is stopped, and that in a case where starting is effected in a state in which both units are stopped, the first compressor 1 is first started, and if the load on the indoor unit is large and both units need to be operated, the second compressor 2 is additionally started. It should be noted that, in the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during the cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during the heating operation. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the conventional air conditioner shown in Fig. 53A except for a portion concerning the bypass passage 28, description thereof will be omitted here, and a description will be given of the portion concerning the bypass passage 28. The high-temperature, high-pressure gas refrigerant discharged from the first compressor 1 or the second compressor 2 flows into the outlet pipe 10c of the oil separator 10 via the oil separator 10, and part of the gas refrigerant flows into the bypass passage 28 here. Since the bypass passage 28 is branched off midway in the pipe between the four-way changeover valve 11 and the outlet pipe 10c of the oil separator 10, the liquid is separated by the oil separator 10, so that the refrigerant which flows into the bypass passage 28 is a gas refrigerant which is always at a high temperature. The high-temperature gas refrigerant, which has flown into the bypass passage 28 and has a much greater channel resistance than that of the main flow to the indoor unit B, undergoes pressure reduction to a low level while flowing through the bypass passage 28, and flows into the suction pipe 8 in the form of low-pressure, high-temperature gas refrigerant. When the first compressor 1 is being operated and the second compressor is being stopped, the pressure within the suction pipe 8 rises, so that neither the liquid refrigerant nor the gas refrigerant flows into the suction pipe 8 from the accumulator 15. When the first compressor 1 is being operated and the second compressor 2 is being stopped, the internal pressure of the shell of the second compressor 2 is higher than the internal pressure of the shell of the first compressor 1, so that most of the low-pressure, high-temperature gas refrigerant which has flown into the suction pipe 8 flows into the first compressor 1 via the second compressor 2 and the equalizing pipe 3. At this time, the level of a mixed liquid (a mixture of the lubricating oil and the liquid refrigerant) in the second compressor 2 drops until it reaches the position of the equalizing pipe 3, but it does not drop further than that; since the high-temperature gas refrigerant passes through the shell of the second compressor 2, the concentration of the lubricating oil does not decline. If the refrigerant flowing into the bypass passage 28 is in excess, part of it flows into the first compressor 1 via the accumulator 15 and the suction pipe 7. Accordingly, even if the liquid refrigerant is accumulated in the accumulator 15 when the first compressor 1 is being operated and the second compressor 2 is being stopped, the gas refrigerant which is supplied from the bypass passage 28 is sufficient in terms of the flow rate of the refrigerant supplied to the first compressor via the shell of the second compressor 2, so that the liquid refrigerant in the accumulator 15 does not flow into the second compressor 2. Also, should the liquid refrigerant in the accumulator 15 flow into the suction pipe 8, the liquid refrigerant is evaporated by the high-temperature gas refrigerant supplied from the bypass passage 28, so that the liquid refrigerant is prevented from flowing into the second compressor 2. Thus, even if the liquid refrigerant is accumulated in the accumulator 15 when the first compressor 1 is being operated and the second compressor 2 is being stopped, the absolute quantity of the lubricating oil in the second compressor 2 does not decrease, and the concentration of the lubricating oil does not decline. Hence, it is possible to obtain a highly reliable air conditioner in which a shortage of the lubricating oil or faulty lubrication due to such as the lack of viscosity of the lubricating oil do not occur when the second compressor is started, which may otherwise result in the breakage of the second compressor. It should be noted, however, that since the refrigerant flowing in the bypass passage 28 flows neither to the heat source unit-side heat exchanger 12 nor to the indoor-side heat exchanger 14, the cooling and heating capabilities are undermined by the portion of the quantity of the refrigerant flowing in the bypass passage 28. 2nd Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 2nd embodiment of the present invention. In the drawing, reference characters or numerals A denote component parts that are similar to those of the air conditioner in accordance with the first embodiment shown in Fig. 28, and description thereof will be omitted here. Numeral 29 denotes a solenoid on-off valve disposed midway in the bypass passage 28. In addition, it is assumed that in a case where the load on the indoor unit B is small and it is unnecessary for both the first and second compressors 1 and 2 to be operated, and either one of them needs to be operated, the first compressor 1 is unfailingly started and the second compressor 2 is stopped, and that in a case where starting is effected in a state in which both units are stopped, the first compressor 1 is first started, and if the load on the indoor unit is large and both units need to be operated, the second compressor 2 is additionally started. It should be noted that, in the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during the cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during the heating operation. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the conventional air conditioner shown in Fig. 53A except for a portion concerning the solenoid on-off valve 29, description thereof will be omitted here. In addition, since the operation of the refrigerant in the bypass passage 28 with the solenoid on-off valve 29 opened is the same as that of the refrigerant in the bypass passage 28 in accordance with the 1st embodiment shown in Fig. 28, description thereof will be omitted here, and a description will be given of the portion concerning the solenoid on-off valve 29. The solenoid on-off valve 29 is opened only when the first compressor 1 is operated and the second compressor 2 is stopped, and the solenoid on-off valve 29 is closed at other times. Accordingly, when the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is opened, and the high-temperature gas refrigerant is supplied to the suction pipe 8. Thus, even if the liquid refrigerant is accumulated in the accumulator 15, the absolute quantity of the lubricating oil in the second compressor 2 does not decrease, and the concentration of the lubricating oil does not decline. Hence, it is possible to obtain a highly reliable air conditioner in which a shortage of the lubricating oil or faulty lubrication due to such as the lack of viscosity of the lubricating oil do not occur when the second compressor 2 is started, which may otherwise result in the breakage of the second compressor 2. On the other hand, in a case where both the first and second compressors 1 and 2 are operated, the solenoid on-off valve 29 is closed, so that the refrigerant does not flow to the bypass passage 28. Hence, the cooling and heating capabilities are not undermined when both the first and second compressors 1 and 2 are being operated. 3rd Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 3rd embodiment of the present invention. Fig. 30 is a control block diagram of the air conditioner in accordance with the 3rd embodiment of the present invention. In the drawing, reference numeral 35 denotes a compressors-continuous-stop-time measuring device for counting a time when both the first and second compressors 1 and 2 are being continuously stopped; 36, a compressor-continuous-operation-time measuring device which starts timing upon starting of the first compressor 1 for counting a time of continuous operation of the first compressor 1; and 37, a solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the time counted by the compressors-continuous-stop-time measuring device 35 and the time counted by the compressor-continuous-operation-time measuring device 36. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioner in accordance with the 2nd embodiment, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. In a system in which excess refrigerant does not occur (which is realizable by making the first connecting pipe 26 short, or by disposing a liquid reservoir between the heat source unit-side heat exchanger 12 and the first connecting pipe 26 or by disposing the second accumulator between the four-way changeover valve 11 and the accumulator 15, without adopting the chargeless system), the accumulation of the liquid refrigerant in the accumulator 15 takes place after the starting of the first compressor 1 in a state in which both the first and second compressors 1 and 2 have been stopped continuously for a long time, or after the starting of the first compressor 1 in a state in which both the first and second compressors 1 and 2 have been stopped although not for a very long time. The quantity of wet vapor sucked when the first compressor 1 is started in the state in which both the first and second compressors 1 and 2 have been stopped continuously for a long time is very large, so that a considerably long time is required until the liquid refrigerant in the accumulator 15 removed. On the other hand, the quantity of wet vapor sucked when the first compressor 1 is started in the state in which both the first and second compressors 1 and 2 have been stopped although not for a very long time is not very large, so that a very long time is not required until the liquid refrigerant in the accumulator 15 removed. Accordingly, if the opening and closing of the solenoid on-off valve 29 are controlled by the solenoid on-off valve controlling device 37 as described below, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened, and the high-temperature gas refrigerant is supplied to the suction pipe 8. Thus, even if the liquid refrigerant is accumulated in the accumulator 15, the absolute quantity of the lubricating oil in the second compressor 2 does not decrease, and the concentration of the lubricating oil does not decline. Hence, it is possible to obtain a highly reliable air conditioner in which a shortage of the lubricating oil or faulty lubrication due to such as the lack of viscosity of the lubricating oil do not occur when the second compressor 2 is started, which may otherwise result in the breakage of the second compressor 2. When the liquid refrigerant in the accumulator 15 is no longer present, the solenoid on-off valve 29 is closed and the liquid refrigerant does not flow through the bypass passage 28, so that there is no shortage in the cooling and heating capabilities. Referring now to the flowchart shown in Fig. 37, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 50 in Fig. 37, a determination is made as to whether or not it is the first starting after the turning on of the power. If it is the first starting, it is determined that it is the starting after stopping for a long time, and the operation proceeds to Step 53; if it is the second or subsequent starting, the operation proceeds to Step 51. If it is determined in Step 51 that a time t off counted by the compressors-continuous-stop-time measuring device 35 has reached a second set time t2 set in advance, it is judged that it is the starting after stopping for a long time, and the operation proceeds to Step 53. If t off has not reached the second set time t2, it is judged that it is the starting after stopping for a short time, and the operation proceeds to Step 52. If it is determined in Step 52 that a time t on counted by the compressor-continuous-operation-time measuring device 36 has reached a first set time t1 which has been set in advance to a relatively short time, though sufficient to overcome the accumulation of the liquid refrigerant in the accumulator 15 due to the wet vapor suction during starting after stopping for a short time, it is judged that the liquid refrigerant in the accumulator 15 has been removed, and the operation proceeds to Step 54 to close the solenoid on-off valve 29 so as to avoid the shortage of the cooling and heating capabilities. Meanwhile, if it is determined in Step 52 that t on has not reached t1, it is judged that the liquid refrigerant in the accumulator 15 has not been removed, and the operation proceeds to Step 55 so as to maintain the open state of the solenoid on-off valve 29 and supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. If it is determined in Step 53 that the time t on counted by the compressor-continuous-operation-time measuring device 36 has reached a third set time t3 set in advance to be longer than the first set time t1, it is judged that the liquid refrigerant in the accumulator 15 has been removed, and the operation proceeds to Step 54 to close the solenoid on-off valve 29 so as to avoid the shortage of the cooling and heating capabilities. Meanwhile, if it is determined in Step 53 that t on has not reached t3, it is judged that the liquid refrigerant in the accumulator 15 has not been removed, and the operation proceeds to Step 55 so as to maintain the open state of the solenoid on-off valve 29 and supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 4th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 4th embodiment of the present invention. In the drawing, reference numeral 30 denotes a discharge-temperature detecting device provided on the discharge pipe 4. Fig. 31 is a control block diagram of the air conditioner in accordance with the 4th embodiment of the present invention. In the drawing, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the temperature detected by the discharge-temperature detecting device 30. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 15th and 16th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large, so that the solenoid on-off valve 29 is opened. Since the liquid refrigerant flows into the first compressor 1 while the liquid refrigerant is accumulated in the accumulator 15, the discharge gas temperature is low, but when the liquid refrigerant is removed from the accumulator 15, the superheated gas refrigerant flows into the first compressor 1, so that the discharge gas temperature becomes high. Therefore, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when a temperature Td detected by the discharge-temperature detecting device 30 reaches a level greater than or equal to a set value Tdl of a discharge-temperature upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when Td drops to a level less than or equal to a set value Td2 of a discharge-temperature lower limit set in advance, it is judged that the liquid refrigerant is accumulated again in the accumulator 15 due to the occurrence of excess refrigerant caused by a change in the operation mode (such as a change from the cooling operation to the heating operation) or the like. Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. Referring now to the flowchart shown in Fig. 38, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 60 in Fig. 38, a determination is made as to whether or not the temperature Td detected by the discharge-temperature detecting device 30 is at a level greater than or equal to the set value Td1 of the discharge-temperature upper limit set in advance, and if Td ≥ Td1, the operation proceeds to Step 61 to close the solenoid on-off valve 29, and then the operation proceeds to Step 62. Meanwhile, if Td < Td1, the operation proceeds directly to Step 62. In Step 62, a determination is made as to whether or not Td is less than or equal to the set value Td2 of the discharge-temperature lower limit set in advance such that Td2 < Td1. If Td ≤ Td2, the operation proceeds to Step 63 to open the solenoid on-off valve 29, and the operation returns to Step 60. Meanwhile, if Td > Td2, the operation returns directly to Step 60. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 5th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 5th embodiment of the present invention. In the drawing, reference numeral 31 denotes a first pressure detecting device provided in the common discharge pipe 6. In addition, it is assumed that in a case where the load on the indoor unit is small and it is unnecessary for both the first and second compressors 1 and 2 to be operated, and either one of them needs to be operated, the first compressor 1 is unfailingly started and the second compressor 2 is stopped, and that in a case where starting is effected in a state in which both units are stopped, the first compressor 1 is first started, and if the load on the indoor unit is large and both units need to be operated, the second compressor 2 is additionally started. It should be noted that, in the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during the cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during the heating operation. Fig. 32 is a control block diagram of the air conditioner in accordance with the 5th embodiment of the present invention. In the drawing, reference numeral 38 denotes a discharge-temperature superheat detecting device which is comprised of the discharge-temperature detecting device 30 and the first pressure detecting device 31, and calculates the degree of superheat in the discharge temperature on the basis of the temperature detected by the discharge-temperature detecting device 30 and the pressure detected by the first pressure detecting device 31. In addition, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the temperature detected by the discharge-temperature detecting device 30. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 4th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large, so that the solenoid on-off valve 29 is opened. Since the liquid refrigerant flows into the first compressor 1 while the liquid refrigerant is accumulated in the accumulator 15, the degree of superheat in the discharge gas temperature is low, but when the liquid refrigerant is removed from the accumulator 15, the superheated gas refrigerant flows into the first compressor 1, so that the degree of superheat in the discharge gas temperature becomes high. In most cases, whether or not the liquid refrigerant is accumulated in the accumulator 15 can be determined from the discharge gas temperature level; however, in cases such as when the high-pressure level is low, the liquid refrigerant is not present in the accumulator 15, and the degree of superheat in the discharge gas temperature is high, but the discharge gas temperature is low. As a result, the determination as to whether or not the liquid refrigerant is accumulated in the accumulator 15 can be made more accurately on the basis of the degree of superheat in the discharge gas temperature, although this determining process is complicated. Therefore, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when a degree of superheat SHd detected by the discharge-temperature superheat detecting device 38 reaches a level greater than or equal to a set value SHd1 of a discharge-temperature superheat upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when SHd drops to a level less than or equal to a set value SHd2 of a discharge-temperature superheat lower limit set in advance, it is judged that the liquid refrigerant is accumulated again in the accumulator 15 due to the occurrence of excess refrigerant caused by a change in the operation mode (such as a change from the cooling operation to the heating operation) or the like. Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. Referring now to the flowchart shown in Fig. 39, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 70 in Fig. 39, a determination is made as to whether or not the degree of superheat SHd detected by the discharge-temperature superheat detecting device 38 is at a level greater than or equal to the set value SHd1 of the discharge-temperature superheat upper limit set in advance, and if SHd ≥ SHd1, the operation proceeds to Step 71 to close the solenoid on-off valve 29, and then the operation proceeds to Step 72. Meanwhile, if SHd < SHd1, the operation proceeds directly to Step 72. In Step 72, a determination is made as to whether or not SHd is less than or equal to the set value SHd2 of the discharge-temperature superheat lower limit set in advance such that SHd2 < SHd1. If SHd ≤ SHd2, the operation proceeds to Step 73 to open the solenoid on-off valve 29, and the operation returns to Step 70. Meanwhile, if SHd > SHd2, the operation returns directly to Step 70. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 6th Embodiment Similar effects are obtained if the discharge-temperature detecting device 30 is provided on the common discharge pipe 6 in the 4th and 5th embodiments. In addition, similar effects are obtained if the first pressure detecting device 31 is provided in the common discharge pipe 6 or the discharge pipe 5 in the 4th and 5th embodiments. 7th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 7th embodiment of the present invention. In the drawing, reference numeral 32 denotes a first shell-temperature detecting device provided on the bottom of the shell of the first compressor 1. Fig. 33 is a control block diagram of the air conditioner in accordance with the 7th embodiment of the present invention. In the drawing, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the temperature detected by the first shell-temperature detecting device 32. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 6th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large, so that the solenoid on-off valve 29 is opened. Since the liquid refrigerant flows into the first compressor 1 while the liquid refrigerant is accumulated in the accumulator 15, the concentration of the lubricating oil in the shell of the first compressor 1 is low, but when the liquid refrigerant is removed from the accumulator 15, the superheated gas refrigerant flows into the first compressor 1, so that the concentration of the lubricating oil in the shell of the first compressor 1 becomes high. Meanwhile, a mixed liquid of the lubricating oil and the liquid refrigerant has a characteristic that, under the same conditions of pressure, the higher the concentration of the lubricating oil, the higher the temperature of the mixed liquid. Hence, it is possible to detect the temperature of the mixed liquid on the basis of the temperature of the bottom of the shell of the first compressor 1. Therefore, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when a temperature Tshell 1 detected by the first shell-temperature detecting device 32 reaches a level greater than or equal to a set value Tshell 1 of a shell-temperature upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when the detected temperature Tshell 1 drops to a level less than or equal to a set value Tshell 12 of a first-shell-temperature lower limit set in advance, it is judged that the liquid refrigerant is accumulated again in the accumulator 15 due to the occurrence of excess refrigerant caused by a change in the operation mode (such as a change from the cooling operation to the heating operation) or the like. Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. In addition, in the determination of the presence or absence of the accumulation of the liquid refrigerant in the accumulator 15, the state of wet vapor being sucked to the first compressor 1 can be detected more directly by the detection of the temperature of the bottom of the first compressor 1 rather than by the detection of the discharge gas temperature. Hence, the former detection method is more accurate although the method of mounting the first shell-temperature detecting device 32 is difficult. Referring now to the flowchart shown in Fig. 40, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 80 in Fig. 40, a determination is made as to whether or not the temperature Tshell 1 detected by the first shell-temperature detecting device 32 is at a level greater than or equal to the set value Tshell 11 of the first-shell-temperature upper limit set in advance, and if Tshell 1 ≥ Tshell 11 , the operation proceeds to Step 81 to close the solenoid on-off valve 29, and then the operation proceeds to Step 82. Meanwhile, if Tshell 1 < Tshell 11 , the operation proceeds directly to Step 82. In Step 82, a determination is made as to whether or not Td is less than or equal to the set value Tshell 12 of the first-shell-temperature lower limit set in advance such that Tshell 12 < Tshell 11 . If Tshell 1 ≤ Tshell 12 , the operation proceeds to Step 83 to open the solenoid on-off valve 29, and the operation returns to Step 80. Meanwhile, if Tshell 1 > Tshell 12 , the operation returns directly to Step 80. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 8th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with an 8th embodiment of the present invention. In the drawing, reference numeral 33 denotes a second shell-temperature detecting device provided on the bottom of the shell of the second compressor 2. Fig. 34 is a control block diagram of the air conditioner in accordance with the 8th embodiment of the present invention. In the drawing, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the temperature detected by the second shell-temperature detecting device 33. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 7th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. A mixed liquid of the lubricating oil and the liquid refrigerant has a characteristic that, under the same conditions of pressure, the higher the concentration of the lubricating oil, the higher the temperature of the mixed liquid. Hence, it is possible to detect the temperature of the mixed liquid on the basis of the temperature of the bottom of the shell of the second compressor 2. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the concentration of the lubricating oil in the shell of the second compressor 2 being low is large, and the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large. Hence, the influx of the liquid refrigerant from the accumulator 15 into the second compressor 2 can be suppressed by opening the solenoid on-off valve 29 and supplying the high-temperature gas refrigerant from the bypass passage 28 to the suction pipe 8. At the same time, the liquid refrigerant in the shell of the second compressor 2 is evaporated by the high-temperature gas refrigerant supplied from the bypass passage 28, thereby making it possible to increase the concentration of the lubricating oil in the shell of the second compressor 2. The increase in the concentration of the lubricating oil in the second compressor 2, in turn, increases the temperature of the mixed liquid in the shell of the second compressor 2, and the temperature of the bottom of the shell of the second compressor 2 rises. As a result, when the temperature of the bottom of the shell of the second compressor 2 rises, it is determined that the concentration of the lubricating oil in the shell of the second compressor 2 has increased, so that the solenoid on-off valve 29 is opened, thereby avoiding the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. If, at that time, the accumulation of the liquid refrigerant in the accumulator 15 has not yet been overcome, the liquid refrigerant flows into the second compressor 2 from the accumulator 15, which causes a decline in the concentration of the lubricating oil in the mixed liquid in the shell of the second compressor 2, and the temperature of the mixed liquid in the shell of the second compressor 2 drops, so that the temperature of the bottom of the shell of the second compressor 2 also drops. If the drop in the temperature of the bottom of the shell of the second compressor 2, by opening the solenoid on-off valve 29 again, it becomes possible again to suppress the influx of the liquid refrigerant from the accumulator 15 into the second compressor 2 and to increase the concentration of the lubricating oil in the shell of the second compressor 2. That is, when the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the solenoid on-off valve 29 is opened. In the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when a temperature Tshell 2 detected by the shell-temperature detecting device 33 of the second compressor 2 reaches a level greater than or equal to a set value Tshell 2 of a shell-temperature upper limit of the second compressor 2 set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when the detected temperature Tshell 2 drops to a level less than or equal to a set value Tshell 22 of a shell-temperature lower limit of the second compressor 2 set in advance, it is judged that the liquid refrigerant is accumulated in the accumulator 15 (or the accumulation of the liquid refrigerant has not been overcome). Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. In addition, it is also possible to detect a case where the liquid refrigerant is not present in the accumulator 15 but the concentration of the lubricating oil in the second compressor 2 is low, and the concentration can be increased by opening the solenoid on-off valve 29. Referring now to the flowchart shown in Fig. 41, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 90 in Fig. 41, a determination is made as to whether or not the temperature Tshell 2 detected by the second shell-temperature detecting device 33 is at a level greater than or equal to the set value Tshell 21 of the shell-temperature upper limit of the second compressor 2 set in advance, and if Tshell 2 ≥ Tshell 21 , the operation proceeds to Step 91 to close the solenoid on-off valve 29, and then the operation proceeds to Step 92. Meanwhile, if Tshell 2 < Tshell 21 , the operation proceeds directly to Step 92. In Step 92, a determination is made as to whether or not Tshell 2 is less than or equal to the set value Tshell 22 of the shell-temperature lower limit of the second compressor 2 set in advance such that Tshell 22 < Tshell 21 . If Tshell 2 ≤ Tshell 22 , the operation proceeds to Step 93 to open the solenoid on-off valve 29, and the operation returns to Step 90. Meanwhile, if Tshell 2 > Tshell 22 , the operation returns directly to Step 90. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. Also, in a case where the liquid refrigerant is not present in the accumulator 15 but the concentration of the lubricating oil in the second compressor is low, it is possible to increase the concentration of the lubricating oil in the second compressor. 9th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 9th embodiment of the present invention. In the drawing, reference numeral 34 denotes a second pressure detecting device provided in the common suction pipe 9. Fig. 35 is a control block diagram of the air conditioner in accordance with the 9th embodiment of the present invention. In the drawing, reference numeral 39 denotes a first shell-temperature superheat detecting device which is comprised of the first shell-temperature detecting device 32 and the second pressure detecting device 34 and calculates the degree of superheat of the first shell temperature on the basis of the temperature detected by the first shell-temperature detecting device 32 and the pressure detected by the second pressure detecting device 34. In addition, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the degree of superheat detected by the first shell-temperature superheat detecting device 39. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 8th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large, so that the solenoid on-off valve 29 is opened. Since the liquid refrigerant flows into the first compressor 1 while the liquid refrigerant is accumulated in the accumulator 15, the concentration of the lubricating oil in the shell of the first compressor 1 is low, but when the liquid refrigerant is removed from the accumulator 15, the superheated gas refrigerant flows into the first compressor 1, so that the concentration of the lubricating oil in the shell of the first compressor 1 becomes high. Namely, there is a characteristic that the higher the concentration of the lubricating oil, the higher the degree of superheat in the temperature of the mixed liquid. Hence, it is possible to detect the degree of superheat in the temperature of the mixed liquid on the basis of the degree of superheat in the temperature of the bottom of the shell of the first compressor 1. Here, the degree of superheat in the temperature of the mixed liquid referred to device a temperature difference between the temperature of the mixed liquid and the saturation temperature of the refrigerant under a pressure persisting at a time when the concentration of the lubricating oil in the mixed liquid is 0%. The degree of superheat in the temperature of the bottom of the shell device the temperature difference between the temperature of the bottom of the shell and the saturation temperature of the refrigerant under that pressure. Therefore, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when the degree of superheat SHshell 1 detected by the first shell-temperature superheat detecting device 39 reaches a level greater than or equal to a set value SHshell 11 of a first-shell-temperature superheat upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when SHshell 1 drops to a level less than or equal to a set value SHshell 12 of a first-shell-temperature superheat lower limit set in advance, it is judged that the liquid refrigerant is accumulated again in the accumulator 15 due to the occurrence of excess refrigerant caused by a change in the operation mode (such as a change from the cooling operation to the heating operation) or the like. Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. In addition, in the determination of the presence or absence of the accumulation of the liquid refrigerant in the accumulator 15, the state of wet vapor being sucked to the first compressor 1 can be detected more directly by the detection of the temperature of the bottom of the first compressor 1 rather than by the detection of the discharge gas temperature. Hence, the former detection method is more accurate although the method of mounting the first shell-temperature detecting device 32 is difficult. In addition, in the detection of the state of wet vapor suction, the detection based on the degree of superheat is complicated but is more accurate than the detection based on the temperature, since correction based on pressure is added. Referring now to the flowchart shown in Fig. 42, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 100 in Fig. 42, a determination is made as to whether or not the temperature SHshell 1 detected by the first shell-temperature superheat detecting device 39 is at a level greater than or equal to the set value SHshell 11 of the first-shell-temperature superheat upper limit set in advance, and if SHshell 1 ≥ SHshell 11 , the operation proceeds to Step 101 to close the solenoid on-off valve 29, and then the operation proceeds to Step 102. Meanwhile, if SHshell 1 < SHshell 11 , the operation proceeds directly to Step 102. In Step 102, a determination is made as to whether or not the detected temperature SHshell 1 is less than or equal to the set value SHshell 12 of the first-shell-temperature superheat lower limit set in advance such that SHshell 12 < SHshell 11 . If SHshell 1 ≤ SHshell 12 , the operation proceeds to Step 103 to open the solenoid on-off valve 29, and the operation returns to Step 100. Meanwhile, if SHshell 1 > SHshell 12 , the operation returns directly to Step 100. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 10th Embodiment Fig. 29 is a refrigerant circuit diagram of an air conditioner in accordance with a 10th embodiment of the present invention. Fig. 36 is a control block diagram of the air conditioner in accordance with the 10th embodiment of the present invention. In the drawing, reference numeral 40 denotes a second shell-temperature superheat detecting device which is comprised of the second shell-temperature detecting device 33 and the second pressure detecting device 34 and calculates the degree of superheat of the second shell temperature on the basis of the temperature detected by the second shell-temperature detecting device 33 and the pressure detected by the second pressure detecting device 34. In addition, reference numeral 37 denotes the solenoid on-off valve controlling device for controlling the opening and closing of the solenoid on-off valve 29 on the basis of the degree of superheat detected by the second shell-temperature superheat detecting device 40. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 9th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. A mixed liquid of the lubricating oil and the liquid refrigerant has a characteristic that, under the same conditions of pressure, the higher the concentration of the lubricating oil, the higher the temperature of the mixed liquid, i.e., the higher the concentration of the lubricating oil, the higher the degree of superheat in the temperature of the mixed liquid. Hence, it is possible to detect the degree of super heat in the temperature of the mixed liquid on the basis of the degree of superheat in the temperature of the bottom of the shell of the second compressor 2. Here, the definitions of the degree of superheat in the temperature of the mixed liquid and the degree of superheat in the temperature of the bottom of the shell are the same as those given in the 22th embodiment. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the concentration of the lubricating oil in the shell of the second compressor 2 being low is large, and the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large. Hence, the influx of the liquid refrigerant from the accumulator 15 into the second compressor 2 can be suppressed by opening the solenoid on-off valve 29 and supplying the high-temperature gas refrigerant from the bypass passage 28 to the suction pipe 8. At the same time, the liquid refrigerant in the shell of the second compressor 2 is evaporated by the high-temperature gas refrigerant supplied from the bypass passage 28, thereby making it possible to increase the concentration of the lubricating oil in the shell of the second compressor 2. The increase in the concentration of the lubricating oil in the second compressor 2, in turn, increases the degree of superheat in the mixed liquid in the shell of the second compressor 2, and the degree of superheat in the temperature of the bottom of the shell of the second compressor 2 rises. As a result, when the degree of superheat in the temperature of the bottom of the shell of the second compressor 2 rises, it is determined that the concentration of the lubricating oil in the shell of the second compressor 2 has increased, so that the solenoid on-off valve 29 is opened, thereby avoiding the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. If, at that time, the accumulation of the liquid refrigerant in the accumulator 15 has not yet been overcome, the liquid refrigerant flows into the second compressor 2 from the accumulator 15, which causes a decline in the concentration of the lubricating oil in the mixed liquid in the shell of the second compressor 2, and the degree of superheat in the temperature of the mixed liquid in the shell of the second compressor 2 drops, so that the degree of superheat in the temperature of the bottom of the shell of the second compressor 2 also drops. If the drop in the degree of superheat in the temperature of the bottom of the shell of the second compressor 2, by opening the solenoid on-off valve 29 again, it becomes possible again to suppress the influx of the liquid refrigerant from the accumulator 15 into the second compressor 2 and to increase the concentration of the lubricating oil in the shell of the second compressor 2. That is, when the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the solenoid on-off valve 29 is opened. In the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when the degree of superheat Tshell 2 detected by the second shell-temperature superheat detecting device 40 reaches a level greater than or equal to a set value Tshell 21 of a second-shell-temperature superheat upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when SHshell 2 drops to a level less than or equal to a set value SHshell 22 of a second-shell-temperature superheat lower limit set in advance, it is judged that the liquid refrigerant is accumulated in the accumulator 15 (or the accumulation of the liquid refrigerant has not been overcome). Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. In addition, it is also possible to detect a case where the liquid refrigerant is not present in the accumulator 15 but the concentration of the lubricating oil in the second compressor 2 is low, and the concentration can be increased by opening the solenoid on-off valve 29. In addition, in the detection of the low concentration of the lubricating oil in the second compressor 2, the detection based on the degree of superheat is complicated but is more accurate than the detection based on the temperature, since correction based on pressure is added. Referring now to the flowchart shown in Fig. 43, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 110 in Fig. 43, a determination is made as to whether or not the degree of superheat SHshell 2 detected by the second shell-temperature superheat detecting device 40 is at a level greater than or equal to the set value SHshell 21 of the second-shell-temperature superheat upper limit set in advance, and if SHshell 2 ≥ SHshell 21 , the operation proceeds to Step 111 to close the solenoid on-off valve 29, and then the operation proceeds to Step 112. Meanwhile, if SHshell 2 < SHshell 21 , the operation proceeds directly to Step 112. In Step 112, a determination is made as to whether or not SHshell 2 is less than or equal to the set value SHshell 22 of the second-shell-temperature superheat lower limit set in advance such that SHshell 22 < SHshell 21 . If SHshell 2 ≤ SHshell 22 , the operation proceeds to Step 113 to open the solenoid on-off valve 29, and the operation returns to Step 110. Meanwhile, if SHshell 2 > SHshell 22 , the operation returns directly to Step 110. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. Also, in a case where the liquid refrigerant is not present in the accumulator 15 but the concentration of the lubricating oil in the second compressor 2 is low, it is possible to increase the concentration of the lubricating oil in the second compressor 2. 11th Embodiment Fig. 44 is a refrigerant circuit diagram of an air conditioner in accordance with a 11th embodiment of the present invention. In the drawing, reference numeral 41 denotes a flow-rate controlling device provided midway in the pipe of the bypass passage 28. It is assumed that the first compressor 1 is a compressor whose flow rate is controllable. It should be noted that, in the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during the cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during the heating operation. Fig. 45 is a control block diagram of the air conditioner in accordance with the 11th embodiment of the present invention. In the drawing, reference numeral 42 denotes a compressor-running-capacity determining device for determining the running capacity of the first compressor 1; and numeral 43 denotes a flow-rate-controlling-device controlling device for controlling the opening of the flow-rate controlling device 41 on the basis of the running capacity of the first compressor 1 determined by the compressor-running-capacity determining device 42 and the pressure detected by the first pressure detecting device 31. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioner in accordance with the first embodiment, description thereof will be omitted here, and a description will be given of the details of control by the flow-rate-controlling-device controlling device 43. Here, it is assumed that, with respect to the flow-rate controlling device 41, the following relationship holds between a channel cross-sectional area S and the opening x of the flow-rate controlling device 41 (k1 is a constant): S = k1 x In addition, since the refrigerant flowing through the bypass passage 28 can be regarded as a compressive fluid, the following relationship holds between the channel cross-sectional area S and the primary pressure of the flow-rate controlling device 41, i.e., the high-pressure level Ph and a refrigerant flow rate Gb in the bypass passage 28 (k2 is a constant): Gb = k2 Ph S Namely, the following relationship holds among Gb, x, and Ph: Gb = k1 k2 x Ph If it is assumed that the flow rate of the refrigerant in the first compressor 1 is G1, that the low rate of the refrigerant in the bypass passage 28 necessary and sufficient for preventing the liquid refrigerant from flowing in from the accumulator 15 is Gb0, that the pressure loss from the accumulator 15 up to the first compressor 1 is ΔPs1, and that the pressure loss from a converging portion of the bypass passage 28 and the suction pipe 8 up to the first compressor 1 via the shell of the second compressor 2 and the equalizing pipe 3 is ΔPs2, the following relations substantially hold (k3, k4, and n are constants): ΔPs1 = k3 G1 n ΔPs2 = k4 Gb0 n ΔPs1 = ΔPs2 Accordingly, the following relationship holds between G1 and Gb0 (k5 is a constant): Gb0 = k5 G1 In addition, the following relationship substantially holds between G1 and the running capacity Q of the first compressor 1 (k6 is a constant): Q = k6 G1 Namely, the following relationship holds between Q and Gb0: Gb0 = (k5/k6) Q Now, if Gb > Gb0, the cooling and heating capabilities are lost more than necessary by the portion of Gb - Gb0. On the other hand, if Gb < Gb0, the liquid refrigerant flows into the second compressor 2 from the accumulator 15. That is, if channel resistance is added by a solenoid on-off valve capillary tube, an orifice and the like without providing the bypass passage 28 with the flow-rate controlling device capable of controlling the flow rate of the passing refrigerant, then unfailingly Gb > Gb0 or Gb < Gb0 depending on the running capacity of the first compressor 1 or the high-pressure level. In this case, since the channel resistance is selected so that Gb > Gb0 by placing priority on the protection of the compressor, so that the cooling and heating capabilities are undermined more than is necessary. From the above, to ensure that Gb = Gb0, it suffices if the opening x of the flow-rate controlling device 41 is set as follows (where, k = k5/(k1 k2 k6)): x = k Q/Ph The details of control by the flow-rate-controlling-device controlling device 43 will be described hereafter in concrete terms. The opening of the flow-rate controlling device 41 is set to an opening which is calculated by x = k Q/Pd on the basis of the running capacity Q of the first compressor 1 determined by the compressor-running-capacity determining device 42 and the pressure Pd detected by the first pressure detecting device 31. Since the flow-rate controlling device 41 is controlled in this manner, in a case where the first compressor 1 is being operated and the second compressor 2 is being stopped, a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof can be controlled by supplying a necessary and sufficient quantity of high-temperature gas refrigerant to the suction pipe 8 without lapsing into a shorting of the cooling and heating capacities more than is necessary. 12th Embodiment Fig. 46 is a refrigerant circuit diagram of an air conditioner in accordance with a 12th embodiment of the present invention. In the drawing, reference numeral 44 denotes a liquid-level detecting circuit having one end communicating with a lower end inside the accumulator 15 and the other end connected to the suction pipe 7; 45, a heating device disposed in contact with the liquid-level detecting circuit, adapted to heat the liquid-level detecting circuit and having a heating capacity for heating the liquid-level detecting circuit 44 so as to produce superheat vapor when wet vapor or saturated vapor flows through the liquid-level detecting circuit 44, or wet vapor or saturated vapor when the liquid refrigerant flows therethrough; and 46, a liquid-level-detecting temperature detecting device provided at an outlet of the liquid-level detecting circuit 44. In addition, it is assumed that in a case where the load on the indoor unit is small and it is unnecessary for both the first and second compressors 1 and 2 to be operated, and either one of them needs to be operated, the first compressor 1 is unfailingly started and the second compressor 2 is stopped, and that in a case where starting is effected in a state in which both units are stopped, the first compressor 1 is first started, and if the load on the indoor unit is large and both units need to be operated, the second compressor 2 is additionally started. It should be noted that, in the drawing, the solid-line arrows indicate the direction of flow of the refrigerant during the cooling operation, while the broken-line arrows indicate the direction of flow of the refrigerant during the heating operation. Fig. 47 is a control block diagram of the air conditioner in accordance with the 12th embodiment of the present invention. In the drawing, reference numeral 37 denotes the solenoid on-off valve controlling device for calculating the degree of superheat for liquid-level detection on the basis of the temperature detected by the liquid-level-detecting temperature detecting device 46 and the pressure detected by the second pressure detecting device 34, and for controlling the opening and closing of the solenoid on-off valve 29 on the basis of that result. Since the operation of the refrigerant (including lubricating oil) during the cooling and heating operations is utterly the same as that of the air conditioners in accordance with the 2nd to 10th embodiments, description thereof will be omitted here, and a description will be given of the details of control by the solenoid on-off valve controlling device 37. When the first compressor 1 is started with both the first and second compressors 1 and 2 being stopped, the possibility of the liquid refrigerant becoming accumulated in the accumulator 15 is large, so that the solenoid on-off valve 29 is opened. While the liquid refrigerant is accumulated in the accumulator 15, the liquid level of the accumulator 15 is above one end of the liquid-level detecting circuit 44 connected to the accumulator 15, and the liquid refrigerant flows through the liquid-level detecting circuit 44. As a result, even if the liquid refrigerant flowing through the liquid-level detecting circuit 44 is heated by the heating device, the liquid refrigerant passes through the outlet portion of the liquid-level detecting circuit 44 in the form of wet vapor or saturated vapor. Hence, the degree of superheat for liquid-level detection, which is calculated from the temperature detected by the temperature detected by the liquid-level-detecting temperature detecting device 46 and the pressure detected by the second pressure detecting device 34, is low. In a case where the liquid refrigerant is not present in the accumulator 15, since the vapor refrigerant, which flows through the liquid-level detecting circuit 44 while being heated by the heating device, passes through the outlet portion of the liquid-level detecting circuit 44 in the superheated state. Hence, the degree of superheat for liquid-level detection, which is calculated from the temperature detected by the temperature detected by the liquid-level-detecting temperature detecting device 46 and the pressure detected by the second pressure detecting device, is high. Therefore, in the case where the first compressor 1 is being operated and the second compressor 2 is being stopped, when the degree of superheat SHL for liquid-level detection reaches a level greater than or equal to a set value SHL 1 of a liquid-level-detection superheat upper limit set in advance, it is judged that the liquid refrigerant has been removed from the accumulator 15, so that the solenoid on-off valve 29 is closed, making it possible to avoid the shortage of the cooling and heating capabilities caused by the bypassing of the refrigerant to the bypass passage 28. In addition, when SHL drops to a level less than or equal to a set value SHL 2 of a liquid-level-detection superheat lower limit set in advance, it is judged that the liquid refrigerant is accumulated again in the accumulator 15 due to the occurrence of excess refrigerant caused by a change in the operation mode (such as a change from the cooling operation to the heating operation) or the like. Hence, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8 from the bypass passage 28, so as to control a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. In addition, in the determination of the presence or absence of the accumulation of the liquid refrigerant in the accumulator 15, since the determination is made directly by device of the liquid-level detecting circuit 44, so that the determination is accurate. Referring now to the flowchart shown in Fig. 48, the details of control by the solenoid on-off valve controlling device 37 will be described in concrete terms. First, when the first compressor 1 is started with both the first and second compressors being stopped, the solenoid on-off valve 29 is opened. Then in Step 120 in Fig. 48, a determination is made as to whether or not the degree of superheat SHL for liquid-level detection is at a level greater than or equal to the set value SHL 1 of the liquid-level-detection superheat upper limit set in advance, and if SHL ≥ SHL 1 , the operation proceeds to Step 121 to close the solenoid on-off valve 29, and then the operation proceeds to Step 122. Meanwhile, if SHL < SHL 1 , the operation proceeds directly to Step 122. In Step 122, a determination is made as to whether or not the degree of superheat SHL for liquid-level detection is less than or equal to the set value SHL 2 of the liquid-level-detection superheat lower limit set in advance such that SHL 2 < SHL 1 . If SHL ≤ SHL 2 , the operation proceeds to Step 123 to open the solenoid on-off valve 29, and the operation returns to Step 120. Meanwhile, if SHL > SHL 2 , the operation returns directly to Step 120. Since the solenoid on-off valve 29 is controlled in the above-described manner, in a case where the first compressor 1 is operated and the second compressor 2 is stopped, the solenoid on-off valve 29 is prevented from being opened when the liquid refrigerant is accumulated in the accumulator 15, which could unnecessarily result in the shortage of the cooling and heating capabilities. When the liquid refrigerant is accumulated in the accumulator 15, the solenoid on-off valve 29 is opened to supply the high-temperature gas refrigerant to the suction pipe 8, thereby controlling a decrease in the absolute quantity of the lubricating oil in the second compressor 2 and a decline in the concentration thereof. 13th Embodiment Similar effects are obtained if the accumulator 15 is provided midway in the common suction pipe 9 in the first to 12th embodiments, as shown in Fig. 49. In addition, similar effects are obtained if one accumulator 15 is provided midway in each of the suction pipe 7 and the suction pipe 8 in the first to 12th embodiments, as shown in Fig. 50. 14th Embodiment Similar effects are obtained if, as shown in Fig. 51, the oil separator 10 is provided at a converging portion of the discharge pipe 4 and the discharge pipe 5 in the first to 13th embodiments. In addition, similar effects are obtained if, as shown in Fig. 52, one oil separator 10 is provided midway in each of the discharge pipe 4 and the discharge pipe 5 in the first to 13th embodiments.

Claims: An air conditioner including a refrigeration circuit having a first low-pressure shell type compressor (1), a second low-pressure shell type compressor (2) which is only operated when the first compressor (1) is operated, the first and second compressor (1,2) being connected in parallel, an equalizing pipe (3) connecting together shells of the first and second compressors (1,2), a heat-source side heat exchanger (12), a throttling device (13), and an indoor-side heat exchanger (14), characterised in that: a bypass passage (28) branches off from a discharge pipe of the first compressor (1), a converging portion of discharge pipes of the first and second compressors (1,2), or a common discharging pipe located after convergence of the discharge pipes of the first and second compressors (1,2), and is connected to a suction pipe (8) of the second compressor (2). An air conditioner according to claim 1, further comprising an on-off valve (29) in the bypass passage (28). An air conditioner according to claim 2, wherein the on-off valve (29) is opened only when the first compressor (1) is operated and the second compressor (2) is stopped, and the on-off valve (29) is closed at other times. An air conditioner according to claim 2, further comprising an oil separator (10) in the discharge pipe of the first compressor (1), the converging portion of the discharge pipes of the first and second compressors (1,2), or the common discharge pipe located after convergence of the discharge pipes of the first and second compressors (1,2), the oil separator (10) having an inlet pipe (10b), an outlet pipe (10c), and an oil return pipe (10d). An air conditioner according to claim 4, further comprising: operation-time measuring means (36) which starts timing upon starting of the first compressor (1) for counting a time of continuous operation of the first compressor, wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (20) is opened at the time of starting the first compressor (1), and the on-off valve (29) is closed when the time counted by the operation-time measuring means (36) reaches a pre-set time. An air conditioner according to claim 4, further comprising: operation-time measuring means (36) which starts timing upon starting of the first compressor (1) for counting a time of continuous operation of the first compressor; and stop-time measuring means (35) for counting a time when the first and second compressors (1,2) are both continuously stopped; wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (29) is opened at the time of starting the first compressor (1), the on-off valve (29) is closed when the time (t on ) counted by the operation-time measuring means (36) reaches a first pre-set time (t 1 ) in a case where the time (t off ) counted by the stop-time measuring means (35) has not reached a second pre-set time (t 2 ), and the on-off valve (29) is closed when the time (t on ) counted by the operation-time measuring means (36) reaches a third pre-set time (t 3 ) longer than the first pre-set time (t 1 ) in a case where the starting of the first compressor (t) is a first starting after the turning on of the power or the time (t off ) counted by the stop-time measuring means (35) has reached the second pre-set time (t 2 ). An air conditioner according to claim 4, further comprising: a discharge-temperature detecting means (30) disposed on the discharge pipe (4) of the first compressor (1) or the common discharge pipe (6) or the converging portion of the discharge pipes (4,5) of the first and second compressors (1,2), wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off vale (29) is opened at the time of starting the first compressor (1), the on-off valve (29) is closed when the temperature (Td) detected by the discharge-temperature detecting means (30) reaches a level greater than or equal to a pre-set upper limit value (Td1), and the on-off valve (29) is opened when the said detected temperature (Td) drops to a level less than or equal to a pre-set lower limit value (Td2) which is lower than the upper limit value (Td1). An air conditioner according to claim 4, further comprising: discharge-temperature superheat detecting means (38) which comprises discharge-temperature detecting means (30) disposed on the discharge pipe (4) of the first compressor (1) or the common discharge pipe (6) or the converging portion of the discharge pipes (4,5) of the first and second compressors (1,2) and pressure detecting means (31) disposed in the refrigerant circuit on the discharge side of the first and second compressors (1,2), wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (29) is opened at the time of starting the first compressor (1), the on-off valve (29) is closed when a degree of superheat (SHd) detected by the discharge-temperature superheat detecting means (38) reaches a level greater than or equal to a pre-set upper limit value (SHd1), and the on-off valve (29) is opened when the detected degree of superheat (SHd) drops to a level less than or equal to a pre-set lower limit value (SHd2) which is lower than the upper limit value. An air conditioner according to claim 4, further comprising: shell-temperature detecting means (32 or 33) disposed on a shell of the first or second compressor (1 or 2), wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (29) is opened at the time of starting the first compressor (1), the on-off valve (29) is closed when the shell temperature detected by the detecting means (32 or 33) reaches a level greater than or equal to a pre-set upper limit value, and the non-off valve (29) is opened when the detected shell temperature drops to a level less than or equal to a pre-set lower limit value which is lower than the upper limit value. An air conditioner according to claim 4, further comprising: shell-temperature superheat detecting means (39 or 40) which comprises shell-temperature detecting means (32 or 33) disposed on a shell of the first or second compressor (1 or 2) and pressure detecting means (34) disposed in the refrigerant circuit on the suction side of the first and second compressors (1,2), wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (29) is opened at the time of starting the first compressor (1), the on-off valve (29) is closed when the degree of superheat detected by the detecting means (39 or 40) reaches a level greater than or equal to a pre-set upper limit value, and the on-off valve (29) is opened when the detected degree of superheat drops to a level less than or equal to a pre-set lower limit value which is lower than the upper limit value. An air conditioner according to claim 1, further comprising a flow-rate controlling device (41) in the bypass passage. An air conditioner according to claim 11, further comprising high-pressure detecting means (31) disposed in the discharge pipe (5) of the first compressor (1) or the common discharge pipe (6), the flow-rate controlling device (41) being controlled in accordance with the pressure detected by the high-pressure detecting means. An air conditioner according to claim 11, wherein the first compressor (1) is a compressor whose running capacity is controllable and the flow-rate controlling device (41) is controlled in accordance with the running capacity of the first compressor. An air conditioner according to claim 8, further comprising: an accumulator (15) in the refrigeration circuit; a liquid-level detecting circuit (44) having one end communicating with a lower part of the interior of the accumulator (15) and another end connected to a discharge pipe (7) of the accumulator (15); heating means (45) for heating the detecting circuit (44) and having a heating capacity falling within a range for heating the detecting circuit (44) so as to produce superheat vapor when wet vapor or saturated vapor flows through the detecting circuit (44) or to produce wet vapor or saturated vapor when liquid refrigerant flows therethrough; a liquid-level-detecting temperature detecting means (45) provided at an outlet part of the detecting circuit (44), for detecting liquid level; and low-pressure detecting means (34) disposed in a suction pipe of the first compressor (1), a suction pipe of the second compressor (2), or a common suction pipe of the first and second compressors (1,2); wherein when the first compressor (1) is operated and the second compressor (2) is stopped, the on-off valve (29) is closed when a degree of superheat (SHL) for liquid-level detecting calculated from the temperature detected by the liquid-level-detecting temperature detecting means (46) and the pressure detected by the low-pressure detecting means (34) is greater than a pre-set upper limit value (SHL 1 ), and the on-off valve (29) is opened when the said degree of superheat (SHL) is less than a pre-set lower limit value (SHL 2 ) which is lower than the upper limit value (SHL 1 ).

Inventors: KASAI TOMOHIKO, NAKAMURA TAKASHI, ONO TATSUO, KASAI, TOMOHIKO, C/O MITSUBISHI DENKI K. K., NAKAMURA, TAKASHI, C/O MITSUBISHI DENKI K. K., ONO, TATSUO, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: F24F1
IPC codes: F25B49, F25B31, F24F3, F25B13, F24F11