HEATING MEDIUM COMPRESSION APPARATUS, AIR CONDITIONER, AND HEATING MEDIUM COMPRESSION METHOD

Abstract

A heating medium compression apparatus includes: first and second compressors compressing a heating medium; suction side and discharge side pipings connecting the first and second compressors to a heat exchanger; a connection piping connecting a discharge side of the first compressor and a suction side of the second compressor in series; and a control unit controlling a flow rate of the heating medium flowing in the suction side piping, the discharge side piping, and the connection piping. The control unit alternatively connects the first or second compressor to the suction side and discharge side pipings, or connects the first and second compressors in series between the suction side and discharge side pipings and performs control such that the flow rate of the heating medium suctioned into the second compressor connected in series becomes higher than that of the heating medium discharged from the first compressor.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-136718, filed Aug. 13, 2020, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a heating medium compression apparatus, an air conditioner, and a heating medium compression method.

BACKGROUND ART

In a server room accommodating electronic equipment such as a server, an air conditioning apparatus (which will hereinafter be referred to as an air conditioner) absorbing heat generated from the electronic equipment and maintaining an appropriate room temperature is provided. Regarding a technology related to this air conditioner, a technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-139205 (which will hereinafter be referred to as Patent Document 1) is known.

As disclosed in paragraph [0010] of Patent Document 1, this air conditioner has a configuration in which an efficient variable capacity compressor having a low compression ratio and compressing a suctioned refrigerant to an intermediate pressure and a variable capacity compressor having a high compression ratio, compressing a suctioned refrigerant at an intermediate pressure to a high pressure, and discharging the refrigerant are connected in series. In addition, the air conditioner is configured to operate for either cooling or heating by switching pipings between the compressors and heat radiation/absorption units.

In addition, in the same manner as in Patent Document 1, Japanese Unexamined Patent Application, First Publication No. 2003-148824 (which will hereinafter be referred to as Patent Document 2) discloses an air conditioner in which two compressors are connected in series.

SUMMARY

By the way, when a low-pressure heating medium (e.g., a fluorine compound gas having a vapor pressure of 1 MPa or lower when being stored and transported in an ordinary environment) is used in an air conditioner, a quantity of heat per unit volume which the heating medium can move is smaller than that of a high-pressure heating medium. For this reason, there is a need to move a large amount of heating medium, and thus a compressor having a large discharge flow rate is required. In addition, in consideration of energy consumed to cool a server room, it is desired to efficiently operate a compressor in accordance with change in a heat generation quantity of a server and temperature change depending on a season and a time period.

An example object of the present invention is to efficiently operate a compression apparatus provided in an air conditioner.

In order to solve the foregoing problems, a first example aspect of the present invention is a heating medium compression apparatus that includes: a first compressor and a second compressor that compress a heating medium; a suction side piping and a discharge side piping that connect the first compressor and the second compressor to a heat exchanger; a connection piping that connects a discharge side of the first compressor and a suction side of the second compressor in series; and a control unit that controls a flow rate of the heating medium flowing in the suction side piping, the discharge side piping, and the connection piping, and the control unit alternatively connects the first compressor or the second compressor to the suction side piping and the discharge side piping or connects the first compressor and the second compressor in series between the suction side piping and the discharge side piping and performs control such that the flow rate of the heating medium suctioned into the second compressor connected in series becomes higher than the flow rate of the heating medium discharged from the first compressor.

A second example aspect of the present invention is a heating medium compression method for controlling a flow rate of a heating medium to a heat exchanger by connecting a first compressor and a second compressor compressing the heating medium to the heat exchanger alternatively or in series, and the heating medium compression method includes: performing control such that the flow rate of the heating medium suctioned into the second compressor becomes higher than the flow rate of the heating medium discharged from the first compressor in a state in which the first compressor and the second compressor are connected in series between a suction side piping and a discharge side piping.

The present invention can efficiently operate a compression apparatus that is provided with a plurality of compressors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a heating medium compression apparatus according to an example of a minimum configuration of a first example aspect of the present invention.

FIG. 2 is a piping system diagram of an air conditioner in which the heating medium compression apparatus according to a first example embodiment of the present invention is applied.

FIG. 3 is a piping system diagram of the heating medium compression apparatus according to the first example embodiment of the present invention.

FIG. 4 is a diagram explaining a flow of a heating medium in a first use mode of the apparatus illustrated in FIG. 3.

FIG. 5 is a diagram explaining a flow of a heating medium in a second use mode of the apparatus illustrated in FIG. 3.

FIG. 6 is a diagram explaining a flow of a heating medium in a third use mode of the apparatus illustrated in FIG. 3.

FIG. 7 is a diagram explaining a flow of a heating medium in a fourth use mode of the apparatus illustrated in FIG. 3.

FIG. 8 is a diagram illustrating change in compression ratio in each season explaining operation of the first example embodiment of the present invention.

EXAMPLE EMBODIMENT

A heating medium compression apparatus according to an example of a minimum configuration of a first example aspect of the present invention will be described with reference to FIG. 1.

This heating medium compression apparatus includes a first compressor 1, a second compressor 2, a suction side piping 4 and a discharge side piping 5 that connect the first compressor 1 and the second compressor 2 to a heat exchanger (an evaporator or a heat radiator) in parallel, a connection piping 6 that connects a discharge side of the first compressor 1 and a suction side of the second compressor 2 in series, and a control unit 3 that controls the flow rate of a heating medium flowing in the suction side piping 4, the discharge side piping 5, and the connection piping 6. The maximum capacity of the second compressor 2 is at least greater than or equal to the maximum capacity of the first compressor 1. The control unit 3 alternatively connects the first compressor 1 or the second compressor 2 to the suction side piping 4 and the discharge side piping 5. Alternatively, the control unit 11 connects the first compressor 1 and the second compressor 2 in series between the suction side piping 4 and the discharge side piping 5 and performs control such that the flow rate of a heating medium suctioned into the second compressor 2 connected in series becomes higher than the flow rate of a heating medium discharged from the first compressor 1.

In the heating medium compression apparatus having the foregoing configuration, the flow rate of a heating medium suctioned into the second compressor 2 is made higher than the flow rate of a heating medium discharged from the first compressor 1. Therefore, even when a compression ability of the second compressor 2 is the same as or greater than that of the first compressor 1, sufficient heating medium is suctioned into the second compressor 2, and thus an efficient operation can be performed.

A heating medium compression method according to an example of a minimum configuration of a second example aspect of the present invention will be described with reference to FIG. 1.

This heating medium compression method controls the flow rate of a heating medium to the heat exchanger by connecting the first compressor 1 and the second compressor 2 compressing a heating medium to the heat exchanger in parallel and/or in series. The heating medium compression method performs control such that the flow rate of a heating medium suctioned into the second compressor 2 becomes higher than the flow rate of a heating medium discharged from the first compressor 1 in a state in which the first compressor 1 and the second compressor 2 are connected in series between the suction side piping 4 and the discharge side piping 5.

In the foregoing heating medium compression method, the flow rate of a heating medium suctioned into the second compressor 2 is made higher than the flow rate of a heating medium discharged from the first compressor 1. Therefore, even when the compression ability of the second compressor 2 is the same as or greater than that of the first compressor 1, sufficient heating medium is suctioned into the second compressor 2, and thus an efficient operation can be performed.

A configuration according to a first example embodiment of the present invention, which concretizes FIG. 1, will be described with reference to FIGS. 2 to 8.

First, an air conditioner including the heating medium compression apparatus according to the first example embodiment will be described with reference to FIG. 2.

A heating medium compression apparatus 100 according to the present example embodiment suctions and compresses a heating medium subjected to heat absorption using a heat receiver 200. Specifically, for example, in air conditioning of a server room, air suctioned by the heat receiver 200 (air of which the temperature has risen due to heat discharge of a server or the like installed in the server room) is cooled through evaporation (decrease in pressure) of a heating medium as indicated by the arrow A in FIG. 1 and is released into the atmospheric air as indicated by the arrow B. The heating medium compression apparatus 100 suctions and compresses a heating medium of which the pressure has decreased through heat exchange and then circulates the heating medium to the heat receiver 200 via a heat radiator 300 and an expansion valve 400.

For example, the heat radiator 300 cools members such as a casing of a compressor 100a configuring the heating medium compression apparatus 100 or a heating medium compressed and discharged by the compressor 100a. For example, a system for radiating heat by means of circulation of cooling water and heat radiation in a cooling tower or a configuration for radiating heat in the atmospheric air using heat radiation fins is employed as the heat radiator 300. The expansion valve 400 supplies a low-temperature heating medium (in a liquid phase or a gas-liquid mixed phase) to the heat receiver 200 by adjusting the flow rate of the heating medium which has been compressed via the heating medium compression apparatus 100 and the heat radiator 300. It should be noted that in the air conditioner of FIG. 2, a low-pressure heating medium (e.g., a fluorine compound gas having a vapor pressure of 1 MPa or lower when being stored and transported in an ordinary environment) is used.

A configuration of the heating medium compression apparatus 100 will be described with reference to FIG. 3.

The reference sign 1 indicates a first compressor which compresses and discharges a suctioned heating medium. In the present first example embodiment, for example, a turbo compressor having a smaller compression ratio than a positive displacement compressor and a high discharge flow rate (volumetric flow rate) is employed.

The first compressor 1 compresses a heating medium supplied from the suction side piping 4 in a predetermined compression ratio, discharges the heating medium to the connection piping 6, and supplies the discharged heating medium to the suction side of the second compressor 2. In the present first example embodiment, a turbo compressor having the same specification as the first compressor 1 (at least the rated capacity identified on the basis of the compression ratio and the volumetric flow rate per unit time is the same) is employed as the second compressor 2.

The (first) discharge side piping 5 is connected to the discharge side of the second compressor 2. In addition, a (second) discharge side piping 7 is connected to the discharge side of the second compressor 2 in parallel with the (first) discharge side piping 5. Moreover, a bypass piping 8 and a return piping 9 are connected to the (second) discharge side piping 7.

A first valve 10 for adjusting the flow rate of a heating medium suctioned into the first compressor 1 is provided in the suction side piping 4. A second valve 20 for adjusting the flow rate of a heating medium flowing in the return piping 9 is provided in the return piping 9. A third valve 30 for adjusting the flow rate of a heating medium flowing in the bypass piping 8 is provided in the bypass piping 8. A fourth valve 40 for adjusting the flow rate of a heating medium flowing in the second discharge side piping 7 is provided in the second discharge side piping 7. A connection location between the second discharge side piping 7 and the bypass piping 8 is connected to the connection piping 6 via the return piping 9.

The first valve 10, the second valve 20, the third valve 30, and the fourth valve 40 are automatic valves operated using an electric motor or an air pressure, for example, and are controlled to be opened or closed or have an intermediate opening degree therebetween by a control unit not illustrated in FIGS. 2 to 8. However, some or all of the valves may be adjusted through a manual operation in accordance with an operation status of the air conditioner.

The heating medium compression apparatus 100 switches between operation modes of operation examples 1 to 4 (which will be described below) and compresses a heating medium through operations of the first valve 10, the second valve 20, the third valve 30, and the fourth valve 40.

Table 1 shows combinations of opening/closing statuses of the first valve 10, the second valve 20, the third valve 30, and the fourth valve 40 in the operation examples 1 to 4.

	TABLE 1
		First	Second	Third	Fourth
		valve 10	valve 20	valve 30	valve 40
	Operation	Opened	Adjusted	Closed	Adjusted
	example 1
	Operation	Opened	Opened	Adjusted	Opened
	example 2			(closed)	(adjusted)
	Operation	Closed	Opened	Opened	Closed
	example 3				(adjusted)
	Operation	Closed	Closed	Opened	Opened
	example 4

The operation examples 1 to 4 shown in Table 1 will be described with reference to FIGS. 4 to 7.

It should be noted that in FIGS. 4 to 7, solid lines indicate pipings in which a heating medium flows, dotted lines indicate pipings in which the flow rate of a heating medium is adjusted, and two-dot dashed lines indicate pipings in which physical pipes are present but no heating medium is flowing.

FIG. 4 illustrates the operation example 1.

This operation example 1 is an operation in a case in which a heat load of the air conditioner is significant. The first valve 10 is fully opened, the third valve 30 is fully closed, and the opening degrees of the second valve 20 and the fourth valve 40 are adjusted in accordance with the operation status.

That is, in this operation example 1, the first valve 10 is fully opened, and thus a heating medium is suctioned into the first compressor 1 from the suction side piping 4, compressed in a predetermined compression ratio, and discharged to the connection piping 6. Here, the flow rate of a heating medium discharged from the first compressor 1 becomes lower than the flow rate of a heating medium suctioned into the first compressor 1 in accordance with the compression ratio of the first compressor 1. A heating medium discharged from the first compressor 1 to the connection piping 6 is suctioned into the second compressor 2, compressed in a predetermined compression ratio, and discharged to the first discharge side piping 5 and the second discharge side piping 7. It should be noted that the third valve 30 is fully closed, and thus there is no chance for a heating medium to flow to the bypass piping 8, to circulate to the first compressor 1, and to be compressed again.

In addition, a heating medium compressed by the second compressor 2 returns to the connection piping 6 via the second discharge side piping 7 and the return piping 9 in accordance with the opening degrees of the second valve 20 and the fourth valve 40 and suctioned into the second compressor 2 again. Here, the opening degrees of the second valve 20 and the fourth valve 40 are adjusted such that the flow rate of a circulating heating medium which has returned through the return piping 9 becomes a suctioning flow rate close to the rated capacity allowing an efficient operation of the second compressor 2. Then, a heating medium compressed by the second compressor 2 is supplied to a heat receiver via the discharge side piping 5, is subjected to heat exchange with the air, and cools the air in the server room.

In this manner, in the operation example 1, part of a heating medium compressed by the second compressor 2 is caused to return to the suction side of the second compressor 2 via the second discharge side piping 7, the return piping 9, and the connection piping 6 and circulate, and thus the suctioning amount of the second compressor 2 can be larger than the discharging amount of the first compressor 1. Therefore, even when the second compressor 2 has a discharging capacity comparable to the first compressor 1, it is possible to prevent surging or deterioration in compression efficiency caused by an insufficient suctioning amount in a turbo compressor of which an output is not easily adjusted. That is, in addition to a heating medium having an amount smaller than the rated amount that is discharged from the first compressor 1, a heating medium circulating via the return piping 9 is suctioned into the second compressor 2 having the same compression ratio and discharge flow rate as those of the first compressor 1, and thus the second compressor 2 can be operated under conditions close to those of efficient rated suctioning amount and discharging amount.

FIG. 5 illustrates the operation example 2.

This operation example 2 is an operation in a case in which only the first compressor 1 is operated when the load of the server is small and the quantity of heat discharged inside the server room is small or when the outdoor temperature is low during the fall season, the winter season, or the like and the heat load of the air conditioner is small. In this operation example 2, the first valve 10, the second valve 20, and the fourth valve 40 are opened, and the opening degree of the third valve 30 is adjusted in accordance with the operation status.

That is, in this operation example 2, the first valve 10 is fully opened, and thus a heating medium is suctioned into the first compressor 1 through the suction side piping 4, compressed in a predetermined compression ratio, and discharged to the connection piping 6. Because the second valve 20 and the fourth valve 40 are fully opened, a heating medium which has flowed to the connection piping 6 is discharged through the discharge side piping 5, is supplied to the heat receiver, and cools the air in the server room.

In addition, the flow rate of a heating medium flowing in the first compressor 1 can be maintained and a heating medium at a necessary flow rate in accordance with the heat load can be supplied by adjusting the opening degree of the third valve 30. It should be noted that even when the rated capacity of the first compressor 1 is larger than the required flow rate of a heating medium, the first compressor 1 can be efficiently operated with a capacity close to the rated capacity by opening and closing the third valve 30 to adjust the flow rate of a heating medium circulating in the first compressor 1.

In this manner, in the operation example 2, the air conditioner can be operated with a suitable consumption of energy in accordance with the heat load by operating only the first compressor 1. It should be noted that in the conduit line of FIG. 5, the resistance of the conduit line reaching the first discharge side piping 5 via the first compressor 1, the return piping 9, and the second discharge side piping 7 is smaller than the resistance of the conduit line reaching the first discharge side piping 5 via the inside of the second compressor 2, and thus the flow rate of a heating medium flowing via the second compressor 2 is low.

It should be noted that if a configuration in which the second compressor 2 can be detached from the conduit line for a heating medium by providing valves on the suction side and the discharge side of the second compressor 2 (not illustrated in FIG. 5) and fully closing these valves is employed, the second compressor 2 can be detached from the conduit line for a heating medium for maintenance, inspection, or replacement with a spare compressor even during operation of the first compressor 1.

FIG. 6 illustrates the operation example 3.

In the same manner as the operation example 2 described above, this operation example 3 is an operation in a case in which only the second compressor 2 is operated when the load of the server is small and the quantity of heat discharged inside the server room is small or when the outdoor temperature is low during the fall season, the winter season, or the like and the heat load of the air conditioner is small. In this operation example 3, the first valve 10 is closed, the second valve 20 and the third valve 30 are opened, and the opening degree of the fourth valve 40 is adjusted in accordance with the operation status.

That is, in the operation example 3, the first valve 10 is fully closed, and thus a heating medium is not suctioned into the first compressor 1 through the suction side piping 4, and the second valve 20 and the third valve 30 are fully opened, and thus a heating medium is supplied to the suction side of the second compressor 2 via the bypass piping 8, the return piping 9, and the connection piping 6. A heating medium which has been suctioned into the second compressor 2 and compressed is compressed in a predetermined compression ratio and sent out to the evaporator through the discharge side piping 5. In addition, the second valve 20 is fully opened, and thus a heating medium which has been suctioned into the second compressor 2 and compressed is suctioned into the second compressor 2 again at a flow rate corresponding to the opening degree of the fourth valve 40 and compressed. That is, a heating medium depending on the heat load can be supplied to the evaporator through the discharge side piping 5 by adjusting the flow rate of a heating medium circulating in accordance with the opening degree of the fourth valve 40. It should be noted that in the operation example 3, because an uncompressed heating medium is suctioned into the second compressor 2, the flow rate of a heating medium is higher than that in a case in which a heating medium is compressed by the first compressor 1 and the volume thereof is reduced as in the operation example 1, and thus an operation status in which the fourth valve 40 is fully closed and circulation is halted may also occur.

In this manner, in the operation example 3, the air conditioner can be operated with a suitable consumption of energy in accordance with the heat load by operating only the second compressor 2. It should be noted that if a valve is provided on the discharge side (upstream from a connection point at which the bypass piping 8 branches from the connection piping 6) in addition to the first valve 10 on the suction side of the first compressor 1 and these valves are fully closed, the first compressor 1 can be detached from the conduit line for a heating medium for maintenance, inspection, or replacement with a spare compressor even during operation of the second compressor 2.

FIG. 7 illustrates the operation example 4.

This operation example 4 is an operation in a case in which the heat generation quantity from the server is extremely small, in a case in which the outdoor temperature is low during the fall season, the winter season, or the like and the heat load of the air conditioner is extremely small, or in a case in which the server is not energized due to a shutdown or the like. In this operation example 4, the first valve 10 and the second valve 20 are fully closed, and the third valve 30 and the fourth valve 40 are fully opened. In addition, both the first compressor 1 and the second compressor 2 are stopped.

That is, in the operation example 4, a heating medium circulates via the suction side piping 4, the bypass piping 8, the second discharge side piping 7, and the first discharge side piping 5.

It should be noted that if a valve is provided on the discharge side of the second compressor 2, both the first compressor 1 and the second compressor 2 can be detached from the conduit line for a heating medium for maintenance, inspection, or replacement with a spare compressor.

In the air conditioners configured as described above, as illustrated in FIG. 8, the first compressor 1 and the second compressor 2 can be operated or stopped and necessary power required to cool the server room can be reduced in accordance with seasonal change in outdoor temperature (load of the air conditioner).

Specifically, when the heat load is the smallest as in the winter season, the compressors are not operated so that cooling is performed through natural circulation of a heating medium (a state in which a heating medium evaporates due to heat absorption and is cooled through natural cooling while circulating in the conduit line). In addition, in a case of an intermediate heat load as in the spring season and the fall season, a low-compressed heating medium is supplied to the evaporator and cooling is performed by operating only one compressor and compressing the heating medium in a low compression ratio. In addition, when the heat load is significant as in the summer season, a highly-compressed heating medium is supplied to the evaporator and cooling is performed by operating two compressors and compressing the heating medium in a high compression ratio.

That is, in the first example embodiment, the same rated capacity (the compression ratio and the flow rate are the same) is employed for the first compressor 1 and the second compressor 2, and thus it is possible to achieve a cooling ability necessary for any of operation modes such as a case in which both the compressors are stopped, a case in which one compressor is operated, or a case in which two compressors are operated.

In addition, if compressors having rated capacities different from each other are employed as the first compressor 1 and the second compressor, the capacity can be finely adjusted in accordance with the heat load by selectively operating one compressor alone.

It should be noted that the compressors used in the heating medium compression apparatus according to the present example embodiment are not limited to turbo compressors. That is, even when a compressor of a different type other than a turbo type is applied, the compressor can be operated under efficient operation conditions, for example, operation conditions of the suctioning amount and the discharging amount close to the rated capacity, and reduction in power consumption can be achieved by adjusting the amount of a heating medium circulating in the compressor.

In addition, in order to prevent a phenomenon in which a heating medium flows in a direction different from an intended direction due to unexpected variation in pressure balance in the conduit line, or in order to more finely perform adjustment of the flow rate and the pressure or perform maintenance, inspection, and replacement of various kinds of equipment configuring the conduit line, an opening/closing valve or a non-return valve may be further added to the conduit line illustrated in FIG. 3. Moreover, in order to supply a heating medium having a low compression ratio and a high flow rate (for two compressors) to the heat exchanger, a conduit line for connecting the first compressor and the second compressor in parallel with respect to a suction side piping and a discharge side piping may be provided.

Hereinabove, an example embodiment of the present invention has been described in detail with reference to the drawings, but specific configurations are not limited to this example embodiment, and may also include design change and the like within a range not departing from the gist of the present invention.

The present invention can be utilized in a heating medium compression apparatus, an air conditioner, and a heating medium compression method.

Claims

1. A heating medium compression apparatus comprising: a first compressor and a second compressor that compress a heating medium;a suction side piping and a discharge side piping that connect the first compressor and the second compressor to a heat exchanger;a connection piping that connects a discharge side of the first compressor and a suction side of the second compressor in series; anda control unit that controls a flow rate of the heating medium flowing in the suction side piping, the discharge side piping, and the connection piping,wherein the control unit alternatively connects the first compressor or the second compressor to the suction side piping and the discharge side piping or connects the first compressor and the second compressor in series between the suction side piping and the discharge side piping and performs control such that the flow rate of the heating medium suctioned into the second compressor connected in series becomes higher than the flow rate of the heating medium discharged from the first compressor.

2. The heating medium compression apparatus according to claim 1, wherein a maximum capacity of the second compressor is at least greater than or equal to a maximum capacity of the first compressor.

3. The heating medium compression apparatus according to claim 1, further comprising a return piping that connects a discharge side and the suction side of the second compressor to each other, wherein the control unit further controls the flow rate of the heating medium flowing in the return piping.

4. The heating medium compression apparatus according to claim 1, further comprising a bypass piping that connects the suction side piping and the connection piping to each other, wherein the control unit further performs control such that the bypass piping is opened or closed.

5. The heating medium compression apparatus according to claim 3, wherein the suction side piping, the discharge side piping, and the return piping have valves for adjusting the flow rate, and the control unit controls opening degrees of the valves.

6. The heating medium compression apparatus according to claim 4, wherein each of the suction side piping, the discharge side piping, and the bypass piping have valves for adjusting the flow rate, and the control unit controls opening degrees of the valves.

7. The heating medium compression apparatus according to claim 1, wherein the first compressor and the second compressor are turbo compressors.

8. An air conditioner comprising: the heating medium compression apparatus according to claim 1; anda heat exchanger that performs heat exchange between the heating medium supplied from the heating medium compression apparatus and atmospheric air.

9. A heating medium compression method for controlling a flow rate of a heating medium to a heat exchanger by connecting a first compressor and a second compressor compressing the heating medium to the heat exchanger alternatively or in series, the heating medium compression method comprising: performing control such that the flow rate of the heating medium suctioned into the second compressor becomes higher than the flow rate of the heating medium discharged from the first compressor in a state in which the first compressor and the second compressor are connected in series between a suction side piping and a discharge side piping.