# Patent

## Title
Air conditioner and method for performing oil equalizing operation in the air conditioner

## Abstract

An air conditioner including an outdoor unit including a first compressor and a second compressor connected in parallel to the first compressor, an outdoor unit connected in parallel to the outdoor unit, the outdoor unit including a first compressor and a second compressor connected in parallel to the first compressor, the outdoor units being connected in parallel to indoor units, the compressors being connected by first-compressor oil equalizing tubes and second-compressor oil equalizing tubes to feed surplus oil in the compressors, the oil equalizing tubes being connected by an external oil equalizing tube, wherein oil equalization is performed by collecting lubricant oil in the first compressor of the outdoor unit, pressurizing the collected lubricant oil by a discharge pressure of another compressor connected in parallel to the first compressor in the same outdoor unit, that is, the second compressor, and feeding the pressurized lubricant oil into the first compressor or second compressor of the other outdoor unit. Accordingly, it is possible to reduce the oil equalizing operation time. Also, there is no restriction on the length of oil equalizing tubes. Accordingly, an enhancement in reliability is achieved.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Patent Application No. 2004-172560, filed on Jun. 10, 2004 in the Japanese Patent Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an air conditioner in which a plurality of outdoor units each including a plurality of low-pressure shell type compressors are connected, and a method for performing an oil equalizing operation in the air conditioner.
2. Description of the Related Art
An air conditioner, in which a plurality of outdoor units and a plurality of indoor units are connected in parallel to a refrigerant circuit, is well known. Each of the outdoor and indoor units may include, a plurality of compressors. In such an air conditioner, an oil reservoir is provided in each compressor. The oil reservoirs of the compressors are communicated via oil equalizing tubes so that an oil equalizing operation can be performed to prevent occurrence of a phenomenon that oil is not supplied to one or more of the compressors.
An example of such a structure will be described with reference to FIG. 21. In FIG. 21, “A” designates an outdoor unit of an air conditioner. The outdoor unit A is connected in parallel to another outdoor unit B while being connected in parallel to an indoor unit (not shown). The outdoor unit A includes a first compressor 32a and a second compressor 33a connected in parallel. The outdoor unit B includes a first compressor 32b and a second compressor 33b connected in parallel. Refrigerant discharge tubes 39a are connected to the compressors 32a and 33a, respectively. Refrigerant discharge tubes 39b are connected to the compressors 32b and 33b, respectively. The refrigerant discharge tubes 39a and 39b are joined and then connected to the indoor unit. A refrigerant suction tube extends from the indoor unit to the outdoor units. The refrigerant suction tube is branched into refrigerant suction tubes 40a, 41a, 40b, and 41b, which are connected to the compressors 32a, 33a, 32b, and 33b, respectively. Each of the compressors 32a, 33a, 32b, and 33b is a low-pressure shell type compressor, in which the internal pressure of a compressor shell thereof during operation of the compressor is lower than the internal pressure of the compressor shell in a stopped state of the compressor.
The first and second compressors 32a and 33a are connected by an oil equalizing tube 43a to feed surplus oil between the compressors 32a and 33a. The first and second compressors 32b and 33b are connected by an oil equalizing tube 43b to feed surplus oil between the compressors 32b and 33b. The oil equalizing tubes 43a and 43b are connected by a connecting tube 49.
Bypass tubes 59a and 59b are branched from respective discharge tubes 39a and 39b of the compressors 32a, 33a, 32b, and 33b. The bypass tubes 59a and 59b are connected to the suction tubes 40a, 41a, 40b, and 41b, respectively. Check valves 45a and 45b are arranged upstream from respective connections, each of which connects an associated one of the suction tubes 40a, 41a, 40b, and 41b and an associated one of the bypass tube 59a and 59b. 
Bypass opening/closing valves 48a and 48b are arranged at the bypass tubes 59a and 59b, respectively. The oil equalizing tubes 43a and 43b are provided with oil equalizing opening/closing valves 46a and 46b in association with the compressors, respectively.
When an oil equalizing operation is performed during operations of the compressors 32a, 33a, 32b, and 33b of the outdoor units A and B by opening, for example, only the bypass opening/closing valve 48a of the bypass tube 59a connected to the discharge tube 39a of the first compressor 32a of the outdoor unit A, the discharge pressure of the first compressor 32a is applied to the first compressor 32a, so that the oil reservoir of the first compressor 32a has a pressure higher than those of the remaining compressors. Accordingly, when all the oil equalizing opening/closing valves 46a and 46b are opened under this condition, lubricant oil in the first compressor 32a is supplied into the second compressor 33a of the outdoor unit A and the first and second compressors 32b and 33b of the outdoor unit B. On the other hand, when the bypass opening/closing valves 48a and 48b are sequentially opened, lubricant oil is supplied into all compressors 32a, 33a, 32b, and 33b in equal amounts (Korean Patent laid-open Publication No. 2000-337726).
In the above-mentioned conventional air conditioner, however, the pressure of each oil reservoir is hardly increased because the air conditioner has an arrangement in which an increase in the internal pressures of the compressors is achieved by operating the compressors in an opened state of the bypass opening/closing valves 48a and 48b to bypass the discharge pressure of each compressor into the same compressor. For this reason, there is a problem in that the oil equalizing operation must be performed for a prolonged period of time to move lubricant oil. Furthermore, there is a restriction that the length of each of the oil equalizing tubes must be short.
Moreover, it is necessary to install the bypass tubes 59a and 59b and bypass opening/closing valves 48a and 48b in the discharge tubes 39a and 39b of all compressors 32a, 33a, 32b, and 33b in the outdoor units A and B, respectively. It is also necessary to install the check valves 45a and 45b in the suction tubes 40a, 41a, 40b, and 41b of all of the compressors 32a, 33a, 32b, and 33b, respectively. For this reason, the overall arrangement is expensive. Also, there is a problem in that it is difficult to secure a desired reliability because an increased number of constituent elements are used.
SUMMARY OF THE INVENTION
Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.
The present invention has been made in view of the above-mentioned problems, and an aspect of the invention is to provide an air conditioner capable of achieving a reduction in oil equalizing operation time, and eliminating a restriction on the length of oil equalizing tubes, and thus, achieving an enhancement in system reliability and a reduction in costs, and to provide a method for performing an oil equalizing operation in the air conditioner.
In accordance with one aspect, the present invention provides an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, wherein each of the outdoor units further includes a check valve arranged at a suction tube connected to one of the compressors included in each outdoor unit, a bypass tube arranged at an outlet of at least one of the remaining compressors, and a bypass opening/closing valve arranged in the bypass tube, wherein the bypass tube is connected to the suction tube downstream from the check valve, wherein an oil equalizing tube opening/closing valve is arranged in the oil equalizing tube of each outdoor unit to cut off flow of lubricant oil through the oil equalizing tube, and wherein a connecting tube opening/closing valve is arranged in the connecting tube.
The bypass tube, bypass opening/closing valve, and check valve may be installed in only one of the compressors of each outdoor unit.
Each of the compressors may be a low-pressure shell type compressor in which an internal pressure of a shell of the compressor during an operation of the compressor is lower than an internal pressure of the shell in a stopped state of the compressor.
Accordingly, it is possible to feed lubricant oil from one compressor to another compressor connected in parallel to the one compressor by stopping the one compressor.
In accordance with another aspect, the present invention provides a method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, including: collecting lubricant oil in one compressor of one of the outdoor units; pressurizing the collected lubricant oil by a discharge pressure of another compressor, which is connected in parallel to said one compressor in the same outdoor unit; and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube to achieve oil equalization.
Accordingly, it is possible to achieve oil equalization by effectively using the discharge pressure of said another compressor.
In accordance with another aspect, the present invention provides a method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, comprising: collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit; pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube; and feeding lubricant oil among the compressors of the same outdoor unit.
Accordingly, it is possible to uniformly supply lubricant oil into all compressors.
Oil equalization may be performed by sequentially supplying lubricant oil into the compressors of the outdoor units by collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit, pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube, and feeding lubricant oil among the compressors of the same outdoor unit.
Accordingly, it is possible to achieve oil equalization using a simple operation.
The oil equalization may be performed by incorporating, in a controlled operation of the air conditioner, an oil equalizing operation to sequentially supply lubricant oil into the compressors, the oil equalizing operation including collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit, pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube, and feeding lubricant oil among the compressors of the same outdoor unit.
Accordingly, oil equalization can be achieved without the user being aware because it is unnecessary to use detectors during a normal controlled operation.
The oil equalization may be performed, starting from the collection of lubricant oil, when it is detected that an oil level in the oil reservoir of a particular one of the compressors is lower than a predetermined level.
Accordingly, it is possible to efficiently achieve oil equalization because it is possible to reliably supply lubricant oil into compressors, which preferentially require supply of lubricant oil.
In accordance with another aspect, the present invention provides a method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, each of the outdoor units further including a bypass tube connected to a discharge tube of the outdoor unit, the bypass tube communicating with a suction tube of one of the compressors in the outdoor unit only at an outlet of said one compressor, a bypass opening/closing valve arranged in the bypass tube, a check valve arranged at the suction tube upstream from a connection between the bypass tube and the suction tube, an oil equalizing tube opening/closing valve arranged in the oil equalizing tube of the outdoor unit to cut off flow of lubricant oil through the oil equalizing tube, and a connecting tube opening/closing valve arranged in the connecting tube, wherein oil equalization is performed by: collecting lubricant oil in the compressor, which includes the discharge tube connected with the bypass tube; applying, to the collected lubricant oil, a discharge pressure of another compressor, which is connected in parallel to said one compressor in the same outdoor unit, via the bypass tube opened by the bypass opening/closing valve, and the suction tube preventing a reverse flow therethrough by the check valve, thereby pressurizing the collected lubricant oil; and feeding the pressurized lubricant oil to one compressor of another outdoor unit via the oil equalizing tube opened by the oil equalizing tube opening/closing valve and the connecting tube opened by the connecting tube opening/closing valve.
Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:
FIG. 1 is a circuit diagram illustrating the entire configuration of an air conditioner according to an embodiment of the present invention;
FIG. 2 is a block diagram illustrating an oil equalization controller included in the air conditioner of FIG. 1;
FIG. 3 is a schematic view illustrating a part of FIG. 1;
FIG. 4 is a time chart according to a method for performing an oil equalizing operation in accordance with a first embodiment of the present invention;
FIG. 5 is a schematic view illustrating an oil equalizing operation;
FIG. 6 is a schematic view illustrating an oil equalizing operation;
FIG. 7 is a schematic view illustrating an oil equalizing operation;
FIG. 8 is a schematic view illustrating an oil equalizing operation;
FIG. 9 is a schematic view illustrating an oil equalizing operation;
FIG. 10 is a schematic view illustrating an oil equalizing operation;
FIG. 11 is a block diagram illustrating an oil equalization controller included in an air conditioner according to a second embodiment of the present invention;
FIG. 12 is a flow chart illustrating a method for performing an oil equalizing operation in accordance with a second embodiment of the present invention;
FIG. 13 is a flow chart illustrating the method for performing an oil equalizing operation in accordance with the second embodiment of the present invention;
FIG. 14 is a flow chart illustrating the method for performing an oil equalizing operation in accordance with the second embodiment of the present invention;
FIG. 15 is a flow chart illustrating the method for performing an oil equalizing operation in accordance with the second embodiment of the present invention;
FIG. 16 is a flow chart illustrating the method for performing an oil equalizing operation in accordance with the second embodiment of the present invention;
FIG. 17 is a schematic view illustrating an oil equalizing operation;
FIG. 18 is a schematic view illustrating an oil equalizing operation;
FIG. 19 is a schematic view illustrating an oil equalizing operation;
FIG. 20 is a schematic view illustrating an oil equalizing operation; and
FIG. 21 is a schematic view illustrating a part of a conventional air conditioner.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.
First, an air conditioner according to an embodiment of the present invention will be described with reference to FIGS. 1 to 10. The air conditioner according to this embodiment includes a refrigerant circuit formed by connecting a plurality of outdoor units 1a and 1b in parallel to an external liquid conduit 20 and an external gas conduit 21, and by connecting a plurality of indoor units 22 and 23 in parallel to the external liquid conduit 20 and external gas conduit 21. The number of the outdoor units 1a and 1b and the number of the indoor units 22 and 23 may be appropriately selected in accordance with the load to be air-conditioned.
The indoor unit 22 includes a heat exchanger 22a and an expansion valve 22b. The indoor unit 23 includes a heat exchanger 23a and an expansion valve 23b. As described above, the indoor units 22 and 23 are connected to the external liquid conduit 20 and external gas conduit 21.
Since the outdoor units 1a and 1b have the same configuration, the following description will be given mainly in conjunction with the outdoor unit 1a. Also, each constituent part of the outdoor unit 1b is designated by the same reference numeral as the corresponding constituent part of the outdoor unit 1a, but suffixed with a reference character “b”.
In the illustrated case, the outdoor unit 1a includes two compressors, that is, a first compressor 2a and a second compressor 3a. The first and second compressors 2a and 3a are connected, at outlets thereof, in parallel to a discharge tube 9a, which is, in turn, connected to a liquid separator 4a. The liquid separator 4a is connected, at an outlet thereof, to the external liquid conduit 20 via a four-way valve 5a, heat exchanger 6a, and a liquid collector 7a, in this order. Each of the compressors 2a and 3a is a low-pressure shell type compressor, in which the internal pressure of a compressor shell thereof during operation of the compressor is lower than the internal pressure of the compressor shell in a stopped state of the compressor. The four-way valve 5a is switched between a cooling mode position where refrigerant flows in a direction indicated by a solid-line arrow C (the state of FIG. 1) during a cooling operation and a heating mode position where refrigerant flows in a direction indicated by a dotted-line arrow H during a heating operation.
A liquid separator 8a is connected to the external gas conduit 21 through the four-way valve of the outdoor unit 1a. The branched suction tubes 10a and 11a of the first and second compressors 2a and 3a are connected to an outlet of the liquid separator 8a. The suction tube 10a of the first compressor 2a is connected to an inlet of the first compressor 2a. The suction tube 11a of the second compressor 3a is connected to an inlet of the second compressor 3a. The inlet of each of the first and second compressors 2a and 3a communicates with an oil reservoir provided in the associated compressor.
An oil return tube 14a is connected to the liquid separator 4a. The oil return tube 14a is also connected to an outlet of the liquid separator 8a via a pressure reducer 28a. 
A bypass tube 29a is connected to the oil return tube 14a to bypass oil from the oil return tube 14a to the suction tube 10a of the first compressor 2a. A third opening/closing valve 18a is arranged in the bypass tube 29a. 
A check valve 15a is arranged in the suction tube 10a of the first compressor 2a downstream from the connection between the bypass tube 29a and the suction tube 10a of the first compressor 2a. 
Thus, in the outdoor unit 1a, the check valve 15a, bypass tube 29a and third opening/closing valve 18a are installed only at the side of the first compressor 2a. These elements are not installed at the side of the second compressor 3a. Even in the case in which an increased number of compressors are used, the check valve 15a, bypass tube 29a and third opening/closing valve 18a are installed only at the side of the first compressor 2a. This arrangement is applied to the outdoor unit 1b in the same manner as the indoor unit 1a. That is, a check valve 15b, bypass tube 29b and third opening/closing valve 18b are installed only at the side of the first compressor 2b of the outdoor unit 1b. These elements are not installed in the other compressor of the outdoor unit 1b, that is, the second compressor 3b. 
The first and second compressors 2a and 3a are connected by an oil equalizing tube to feed surplus oil between the first and second compressors 2a and 3a. The oil equalizing tubes of the outdoor units 1a and 1b are connected by an external oil equalizing tube (connecting tube) 19. In detail, the first-compressor oil equalizing tube 12a connected to the first compressor 2a is connected with the second-compressor oil equalizing tube 13a connected to the second compressor 3a in the outdoor unit 1a. The first-compressor oil equalizing tube 12b connected to the first compressor 2b is connected with the second-compressor oil equalizing tube 13b connected to the second compressor 3b in the outdoor unit 1b. The external oil equalizing tube 19 is connected, at opposite ends thereof, to the connection between the first-compressor oil equalizing tube 12a and the second-compressor oil equalizing tube 13a in the outdoor unit 1a and the connection between the first-compressor oil equalizing tube 12b and the second-compressor oil equalizing tube 13b in the outdoor unit 1b. 
A first opening/closing valve 16a is arranged in the second-compressor oil equalizing tube 13a. A second opening/closing valve 17a is installed in the outdoor unit 1a near the connection between the second-compressor oil equalizing tube 13a and the external oil equalizing tube 19. In the outdoor unit 1b, a second opening/closing valve 17b is installed near the connection between a second-compressor oil equalizing tube 13b corresponding to the second compressor oil equalizing tube 13a and the external oil equalizing tube 19.
As shown in FIG. 2, oil equalization controller 24 includes a timer 25, an opening/closing controller 26 to control opening/closing of the first opening/closing valves (oil equalizing tube opening/closing valves) 16a and 16b, second opening/closing valves (connecting tube opening/closing valves) 17a and 17b, third opening/closing valves (bypass opening/closing valves) 18a and 18b, and a compressor controller 27 to control operations of the first compressors 2a and 2b and operations of the second compressors 3a and 3b. 
Hereinafter, a method for performing an oil equalizing operation in accordance with a control operation periodically carried out by the oil equalization controller 24 will be described with reference to the time chart of FIG. 4 and FIGS. 5 to 10. In accordance with this control operation, opening/closing of the first opening/closing valves 16a and 16b, second opening/closing valves 17a and 17b, and third opening/closing valves 18a and 18b are periodically controlled, and operations of the first compressors 2a and 2b and operations of the second compressors 3a and 3b are controlled, so that oil equalization is achieved in the first compressors 2a and 2b and second compressors 3a and 3b. 
The following description will be given with reference to FIG. 3, which is a simplified version of FIG. 1, for easy understanding of the periodic control operation. In FIG. 3, constituent elements respectively corresponding to those of FIG. 1 are designated by the same reference numerals. Although the bypass tubes 29a and 29b are branched from the oil return tubes 14a and 14b, respectively, in the case of FIG. 3, they are branched from the discharge tubes 9a and 9b of the first compressors 2a and 2b, respectively, in the case of FIG. 3. In FIG. 3, the oil separators 4a and 4b are omitted.
First, a control operation is carried out for a time T, as shown in the time chart of FIG. 4. Under this control operation, the first compressors 2a and 2b and second compressors 3a and 3b perform normal operations thereof, respectively. That is, the expansion valves 22b and 23b are adjusted in accordance with a load to be air-conditioned. Under this condition, the first compressors 2a and 2b and second compressors 3a and 3b are operated. Thus, an air-conditioning control operation is carried out. In this case, accordingly, the first opening/closing valves 16a and 16b, second opening/closing valves 17a and 17b, and third opening/closing valves 18a and 18b are maintained in a closed state.
Next, an oil equalizing operation is performed in six operations S1 to S6 for a time T for each of the six operations S1 to S6 by switching the operations of the first compressors 2a and 2b and the operations of the second compressors 3a and 3b, and simultaneously opening/closing the first opening/closing valves 16a and 16b, second opening valves 17a and 17b, and third opening/closing valves 18a and 18b at intervals of the time T. In detail, the operations of the first compressors 2a and 2b, the operations of the second compressors 3a and 3b, and the opening/closing of the first opening/closing valves 16a and 16b, second opening valves 17a and 17b, and third opening/closing valves 18a and 18b are controlled.














TABLE 









Operation
Operation
Operation
Operation
Operation
Operation



1
2
3
4
5
6


















Outdoor
First
Forced
Stop
Controlled
Controlled
Forced
Stop


Unit 1a
Compressor 2a
Operation

Operation
Operation
Operation



Second
Stop
Forced
Controlled
Controlled
Controlled
Forced



Compressor 3a

Operation
Operation
Operation
Operation
Operation



First
Opened
Closed
Closed
Closed
Closed
Opened



Opening/Closing



Valve 16a



Second
Closed
Opened
Closed
Closed
Opened
Closed



Opening/Closing



Valve 17a



Third
Closed
Opened
Closed
Closed
Closed
Opened



Opening/Closing



Valve 18a


Outdoor
First
Controlled
Forced
Stop
Forced
Stop
Controlled


Unit 1b
Compressor 2b
Operation
Operation

Operation

Operation



Second
Controlled
Controlled
Forced
Stop
Forced
Controlled



Compressor 3b
Operation
Operation
Operation

Operation
Operation



First
Closed
Closed
Opened
Opened
Closed
Closed



Opening/Closing



Valve 16b



Second
Closed
Opened
Closed
Closed
Opened
Closed



Opening/Closing



Valve 17b



Third
Closed
Closed
Opened
Closed
Opened
Closed



Opening/Closing



Valve 18b









In the oil equalizing operation, lubricant oil is collected in the first compressors 2a and 2b, which include respective check valves 15a and 15b, and respective third opening/closing valves 18a and 18b, and is then supplied from the first compressors 2a and 2b into other outdoor units. Upon supplying the lubricant oil, the first compressors 2a and 2b, in which the lubricant oil has been collected, are stopped. Under this condition, other compressors are forcibly operated to supply high-pressure gas into the first compressors 2a and 2b through the bypass tubes 29a and 29b, and thus, to sufficiently increase the internal pressure of the oil reservoirs of the first compressors 2a and 2b. 
During the oil equalizing operation, each compressor selectively operates, in addition to the above-described controlled operation mode, in particular operation modes such as a forced operation mode and a stop mode. Here, “forced operation” means to forcibly operate the compressors by desired power without using a normal compressor control method. Also, as is the definition of the word “stop”, “stop” used herein means to stop the operations of the compressors.
At operation S1 shown in FIG. 5, the first compressor 2a of the outdoor unit 1a is forcibly operated, whereas the second compressor 3a of the outdoor unit 1a is in a stopped state. Accordingly, the internal pressure of the first compressor 2a is lowered below the internal pressure of the shell of the stopped second compressor 3a, so that lubricant oil is fed from the second compressor 3a into the first compressor 2a via the second-compressor oil equalizing tube 13a and the first-compressor oil equalizing tube 12a (as indicated by a solid-line arrow), and is collected in the oil reservoir of the first compressor 2a. In this case, the first and second compressors 2b and 3b of the outdoor unit 1b are operated in a controlled operation mode. However, there is no flow of lubricant oil between the first and second compressors 2b and 3b because the first opening/closing valve 16b is in a closed state. Also, there is no flow of lubricant oil between the outdoor units 1a and 1b because the second opening/closing valves 17a and 17b are in a closed state.
At operation S2 shown in FIG. 6, the first compressor 2a of the outdoor unit 1a is in a stopped state, whereas the second compressor 3a of the outdoor unit 1a is forcibly operated. Accordingly, the gas pressure of the second compressor 3a is applied to the first compressor 2a via the second-compressor suction tube 11a, which prevents a reverse flow therethrough by the discharge tube 9a, bypass tube 29a, and check valve 15a. As a result, the oil reservoir of the first compressor 2a is pressurized, so that lubricant oil is collected in the first compressor 2b of the outdoor unit 1b via the first-compressor oil equalizing tube 12a, the external oil equalizing tube 19, and the first-compressor oil equalizing tube 12b of the outdoor unit 1b (as indicated by a solid-line arrow). In this case, the second compressor 3b is operated in a controlled operation mode. However, there is no adverse affect on the flow of lubricant oil by the controlled operation of the second compressor 3b because the first opening/closing valve 16b is in a closed state. Also, there is no adverse affect on the flow of lubricant oil by the forced operation of the second compressor 3a. 
At operation S3 shown in FIG. 7, the second compressor 3b of the outdoor unit 1b is forcibly operated, whereas the first compressor 2b of the outdoor unit 1b is in a stopped state. Accordingly, the internal pressure of the second compressor 3b is lowered below the internal pressure of the shell of the stopped first compressor 2b, so that lubricant oil is fed from the first compressor 2b into the second compressor 3b via the first-compressor oil equalizing tube 12b and second-compressor oil equalizing tube 13b (as indicated by a solid-line arrow), and is collected in the oil reservoir of the second compressor 3b. In this case, the first and second compressors 2a and 3a of the outdoor unit 1a are operated in a controlled operation mode. Also, the first opening/closing valve 16b is in a closed state. Accordingly, there is no flow of lubricant oil between the first and second compressors 2a and 3a. Also, there is no flow of lubricant oil between the outdoor units 1a and 1b because the second opening/closing valves 17a and 17b are in a closed state. Since the third opening/closing valve 18b is in a closed state in this case, the gas pressure of the second compressor 3b is applied to the oil reservoir of the first compressor 2b via the discharge tube 9b, bypass tube 29b, and first-compressor suction tube 10b. As a result, lubricant oil can be efficiently fed from the first compressor 2b into the second compressor 3b. 
At operation S4 shown in FIG. 8, the first compressor 2b of the outdoor unit 1b is forcibly operated, whereas the second compressor 3b of the outdoor unit 1b is in a stopped state. Accordingly, the internal pressure of the first compressor 2b is lowered below the internal pressure of the shell of the stopped second compressor 3b, so that lubricant oil is fed from the second compressor 3b into the first compressor 2b via the second-compressor oil equalizing tube 13b and first-compressor oil equalizing tube 12b (as indicated by a solid-line arrow), and is collected in the oil reservoir of the first compressor 2b. In this case, the first and second compressors 2a and 3a of the outdoor unit 1a are operated in a controlled operation mode. Also, the first opening/closing valve 16a is in a closed state. Accordingly, there is no flow of lubricant oil between the first and second compressors 2a and 3a. Also, there is no flow of lubricant oil between the outdoor units 1a and 1b because the second opening/closing valves 17a and 17b are in a closed state.
At operation S5 shown in FIG. 9, the second compressor 3b of the outdoor unit 1b is forcibly operated, whereas the first compressor 2b of the outdoor unit 1b is in a stopped state. Accordingly, the gas pressure of the second compressor 3b is applied to the first compressor 2b via the second-compressor suction tube 10b, which prevents a reverse flow therethrough by the discharge tube 9b, bypass tube 29b, and check valve 15b. As a result, the oil reservoir of the first compressor 2b is pressurized, so that lubricant oil is collected in the first compressor 2a of the outdoor unit 1a via the first-compressor oil equalizing tube 12b, the external oil equalizing tube 19, and the first-compressor oil equalizing tube 12a of the outdoor unit 1a (as indicated by a solid-line arrow). In this case, the second compressor 3a is operated in a controlled operation mode. However, there is no adverse affect on the flow of lubricant oil by the controlled operation of the second compressor 3a because the first opening/closing valve 16a is in a closed state. Also, there is no adverse affect on the flow of lubricant oil by the forced operation of the second compressor 3b. 
At operation S6 shown in FIG. 10, the second compressor 3a of the outdoor unit 1a is forcibly operated, whereas the first compressor 2a of the outdoor unit 1a is in a stopped state. Accordingly, the internal pressure of the second compressor 3a is lowered below the internal pressure of the shell of the stopped first compressor 2a, so that lubricant oil is fed from the first compressor 2a into the second compressor 3a via the first-compressor oil equalizing tube 12a and second-compressor oil equalizing tube 13a (as indicated by a solid-line arrow), and is collected in the oil reservoir of the second compressor 3a. In this case, the first and second compressors 2b and 3b of the outdoor unit 1b are operated in a controlled operation mode. Also, the first opening/closing valve 16b is in a closed state. Accordingly, there is no flow of lubricant oil between the first and second compressors 2b and 3b. Also, there is no flow of lubricant oil between the outdoor units 1a and 1b because the second opening/closing valves 17a and 17b are in a closed state. Since the third opening/closing valve 18a is in a closed state in this case, the gas pressure of the second compressor 3a is applied to the oil reservoir of the first compressor 2a via the discharge tube 9a, bypass tube 29a, and first-compressor suction tube 10a. As a result, lubricant oil can be efficiently fed from the first compressor 2a into the second compressor 3a. 
Thus, at operation S1 shown in FIG. 5 and operation S4 shown in FIG. 8, lubricant oil is collected in the first compressors 2a and 2b, which are provided with respective check valves 15a and 15b and respective bypass tubes 29a and 29b, in accordance with the characteristics of a low-pressure shell type compressor in which a reduction in pressure occurs during operation of the compressor. Accordingly, it is possible to reliably and inexpensively perform a desired oil equalizing operation, using a simple operation to stop a desired compressor.
At operation S2 shown in FIG. 6 and operation S5 shown in FIG. 9, it is possible to achieve an oil equalizing operation in which the lubricant oil from the stopped compressor of one outdoor unit is rapidly supplied into the other outdoor unit within a reduced period of time by efficiently using pressure generated in accordance with a forced operation of the other compressor of the one-compressor-stopped outdoor unit (that is, the second compressor 3a or 3b) during a normal controlled operation. Thus, it is possible to reduce the oil equalizing operation time, and to efficiently achieve oil equalization.
Also, at operation S3 shown in FIG. 7, oil equalization is performed between the first compressor 2b and the second compressor 3b in the outdoor unit 1b by feeding, into the second compressor 3b, the lubricant oil in the first compressor 2b supplied from the outdoor unit 1a at operation S2 shown in FIG. 6. At operation S6 shown in FIG. 10, oil equalization is performed between the first compressor 2a and the second compressor 3a in the outdoor unit 1a by feeding, into the second compressor 3a, the lubricant oil in the first compressor 2a supplied from the outdoor unit 1b at operation S5 shown in FIG. 9.
In accordance with the sequential feeding of lubricant oil in the above-described manner, the amounts of lubricant oil in all compressors 2a, 3a, 2b, and 3b are equalized within a reduced period of time. Thus, a reliable and efficient oil system is implemented. Accordingly, the restriction on the length and diameter of pipes is reduced. Also, there is no problem caused by different levels of the constituent elements of the air conditioner. Therefore, it is possible to achieve a widened freedom of the design including the installation of outdoor units. Moreover, it is basically necessary to install a check valve. (check valve 15a or 15b) and a bypass tube (bypass tube 29a or 29b) only in one compressor (first compressor 2a or 2b) of each outdoor unit (outdoor unit 1a or 1b). Accordingly, it is unnecessary to install such elements in all compressors, as in conventional cases. Thus, a reduction in costs is achieved in accordance with a reduction in the number of constituent elements used in the air conditioner. In addition, the costs may be further reduced because an improvement in the reliability of the system is achieved in accordance with a reduction in factors causing failure.
In particular, in accordance with the method for performing an oil equalizing operation, using a periodic control operation, as described above, oil equalization can be achieved, using simple operations. Accordingly, it is possible to easily manage the oil equalizing operation. Also, the oil equalization can be conveniently achieved because it is unnecessary to use detectors during a normal controlled operation.
Hereinafter, a method for performing an oil equalizing operation in accordance with a liquid level detection control operation carried out by the oil equalization controller 24 will be described with reference to the block diagram of FIG. 11, flow charts of FIGS. 12 to 16, and FIGS. 17 to 20. For the liquid level detection control operation, a compressor liquid level detector 30 is used in addition to the oil equalization controller 24 used for the above-described periodic control operation, as shown in FIG. 11. In accordance with the liquid level detection control operation, based on the detection results of the compressor liquid level detector 30, opening/closing of the first opening/closing valves 16a and 16b, second opening/closing valves 17a and 17b, and third opening/closing valves 18a and 18b are controlled, and operations of the first compressors 2a and 2b and operations of the second compressors 3a and 3b are controlled, so that oil equalization is performed in the first compressors 2a and 2b and second compressors 3a and 3b. The liquid level detector may be implemented using a flow switch.
Thus, the configuration of FIG. 11 is basically the same as that of FIG. 1, except for the addition of the compressor liquid level detector 30, and thus, description thereof will be omitted. Also, FIGS. 17 to 20, which are used in association with descriptions given by the flow charts, are simplified versions, as in FIGS. 5 to 10 used for the above-mentioned periodic control operation.
Also, in the flow charts shown in FIGS. 12 to 16, for simplification of description, the term “first” used in the first compressors 2a, 2b and first opening/closing valves 16a and 16b, the term “second” used in the second compressors 3a and 3b and second opening/closing valves 17a and 17b, and the term “third” used in the third opening/closing valves 18a and 18b are omitted. In addition, the forced operation in the operation mode of the compressors is simply referred to as “operation”, and the controlled operation is simply referred to as “normal control”.
At operation 10, a normal cooling/heating operation is performed, as shown in the flow chart of FIG. 12. In this case, accordingly, all opening/closing valves are closed so that all compressors are operated in a normal control mode at operation S11. During the operations of the compressors in the normal control mode, it is determined at operation S12 whether the oil level of the first compressor 2a is not higher than a predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S16 of FIG. 13. On the other hand, if the determination corresponds to “NO”, the procedure proceeds to operation S13.
At operation S13, it is determined whether the oil level of the second compressor 3a is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S23 of FIG. 14. On the other hand, if the determination corresponds to “NO”, the procedure proceeds to operation S14.
At operation S14, it is determined whether the oil level of the first compressor 2b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S32 of FIG. 15. On the other hand, if the determination corresponds to “NO”, the procedure proceeds to operation S15.
At operation S15, it is determined whether the oil level of the second compressor 3b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S39 of FIG. 16. On the other hand, if the determination corresponds to “NO”, the procedure proceeds to operation S10.
When it is determined at operation S12 that the oil level of the first compressor 2a is not higher than the predetermined level, operation S16 of FIG. 13 is executed to forcibly operate the first compressor 2a, to stop the second compressor 3a, and to operate the first and second compressors 2b and 3b in the controlled operation mode. In this case, only, the first opening/closing valve 16a is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S16 is maintained for a predetermined time at operation S17.
As a result, lubricant oil is moved from the second compressor 3a to the first compressor 2a, as indicated by arrow S16 in FIG. 17, thus increasing the oil level of the first compressor 2a. 
Thereafter, it is determined at operation S18 whether the oil level of the first compressor 2a is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S19. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S19, a control operation is executed to operate the first and second compressors 2a and 3a in the controlled operation mode, to forcibly operate the first compressor 2b, and to stop the second compressor 3b. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S19 is maintained for a predetermined time at operation S20.
As a result, lubricant oil is moved from the second compressor 3b to the first compressor 2b, as indicated by arrow S19 in FIG. 17, thus increasing the oil level of the first compressor 2b. 
At operation S21, a control operation is executed to operate the first and second compressors 2a and 3a in the controlled operation mode, to stop the first compressor 2b, and to forcibly operate the second compressor 3b. In this case, the first opening/closing valves 16a and 16b, and third opening/closing valve 18a are opened, whereas the second opening/closing valves 17a and 17b and third opening/closing valve 18b are closed. It is then determined at operation S22 whether the oil level of the first compressor 2a is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S21. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
As a result, lubricant oil is moved from the first compressor 2b to the first compressor 2a, as indicated by arrow S21 in FIG. 17, thus increasing the oil level of the first compressor 2a. 
When it is determined at operation S13 of FIG. 12 that the oil level of the second compressor 3a is not higher than the predetermined level, operation S23 of FIG. 14 is executed to forcibly operate the second compressor 3a, to stop the first compressor 2a, and to operate the first and second compressors 2b and 3b in the controlled operation mode. In this case, only the first opening/closing valve 16a is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S23 is maintained for a predetermined time at operation S24.
As a result, lubricant oil is moved from the first compressor 2a to the second compressor 3a, as indicated by arrow S23 in FIG. 18, thus increasing the oil level of the second compressor 3a. 
Thereafter, it is determined at operation S25 whether the oil level of the second compressor 3a is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S26. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S26, a control operation is executed to operate the first and second compressors 2a and 3a in the controlled operation mode, to forcibly operate the first compressor 2b, and to stop the second compressor 3b. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S26 is maintained for a predetermined time at operation S27.
As a result, lubricant oil is moved from the second compressor 3b to the first compressor 2b, as indicated by arrow S26 in FIG. 18, thus increasing the oil level of the first compressor 2b. 
At operation S28, a control operation is executed to stop the first compressors 2a and 2b and to forcibly operate the second compressors 3a and 3b. In this case, the first opening/closing valve 16b and third opening/closing valve 18a are closed, whereas the remaining opening/closing valves are opened. The condition established at operation S28 is maintained for a predetermined time at operation S29.
As a result, lubricant oil is moved from the first compressor 2b to the second compressor 3a, as indicated by arrow S28 in FIG. 18, thus increasing the oil level of the second compressor 3a. 
It is then determined at operation S30 whether the oil level of the second compressor 3a is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S31. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S31, a control operation is executed to stop the first compressor 2a, to forcibly operate the second compressor 3a, and to operate the first and second compressors 2b and 3b in the controlled operation mode. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state.
As a result, lubricant oil is moved from the first compressor 2a to the second compressor 3a, as indicated by arrow S31 in FIG. 18, thus increasing the oil level of the second compressor 3a. 
When it is determined at operation S12 that the oil level of the first compressor 2b is not higher than the predetermined level, operation S32 of FIG. 15 is executed to forcibly operate the first compressor 2b, to stop the second compressor 3b, and to operate the first and second compressors 2a and 3a in the controlled operation mode. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S32 is maintained for a predetermined time at operation S33.
As a result, lubricant oil is moved from the second compressor 3b to the first compressor 2b, as indicated by arrow S32 in FIG. 19, thus increasing the oil level of the first compressor 2b. 
Thereafter, it is determined at operation S34 whether the oil level of the first compressor 2b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S35. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S35, a control operation is executed to operate the first and second compressors 2b and 3b in the controlled operation mode, to forcibly operate the first compressor 2a, and to stop the second compressor 3a. In this case, only the first opening/closing valve 16a is opened, whereas the remaining opening/closing valves are maintained in a closed state.
As a result, lubricant oil is moved from the second compressor 3a to the first compressor 2a, as indicated by arrow S35 in FIG. 19, thus increasing the oil level of the first compressor 2a. 
At operation S37, a control operation is executed to operate the first and second compressors 2b and 3b in the controlled operation mode, to stop the first compressor 2a, and to forcibly operate the second compressor 3a. In this case, the first opening/closing valves 16a and 16b and third opening/closing valve 18b are closed, whereas the second opening/closing valves 17a and 17b and third opening/closing valve 18b are opened. Under this condition, it is determined at operation S38 whether the oil level of the first compressor 2b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S37. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
In accordance with the control operation executed at operation S37, lubricant oil is moved from the first compressor 2a to the first compressor 2b, as indicated by arrow S37 in FIG. 19, thus increasing the oil level of the first compressor 2b. 
When it is determined at operation S15 of FIG. 12 that the oil level of the second compressor 3b is not higher than the predetermined level, a control operation is executed at operation S39 of FIG. 16 to forcibly operate the second compressor 3b, to stop the first compressor 2b, and to operate the first and second compressors 2a and 3a in the controlled operation mode. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state. The condition established at operation S39 is maintained for a predetermined time at operation S40.
As a result, lubricant oil is moved from the first compressor 2b to the second compressor 3b, as indicated by arrow S39 in FIG. 20, thus increasing the oil level of the second compressor 3b. 
Thereafter, it is determined at operation S41 whether the oil level of the second compressor 3b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S42. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S42, a control operation is executed to operate the first and second compressors 2b and 3b in the controlled operation mode, to forcibly operate the first compressor 2a, and to stop the second compressor 3a. In this case, only the first opening/closing valve 16a is opened, whereas the remaining opening/closing valves are maintained in a closed state.
As a result, lubricant oil is moved from the second compressor 3a to the first compressor 2a, as indicated by arrow S42 in FIG. 20, thus increasing the oil level of the first compressor 2a. 
Operation S44 is then executed. At operation S44, a control operation is executed to stop the first compressors 2a and 2b and to forcibly operate the second compressors 3a and 3b. In this case, the first opening/closing valve 16a and third opening/closing valve 18b are closed, whereas the remaining opening/closing valves are opened. The condition established at operation S44 is maintained for a predetermined time at operation S45.
As a result, lubricant oil is moved from the first compressor 2a to the second compressor 3b, as indicated by arrow S44 in FIG. 20, thus increasing the oil level of the second compressor 3b. 
Thereafter, it is determined at operation S46 whether the oil level of the second compressor 3b is not higher than the predetermined level. If the determination corresponds to “YES”, the procedure proceeds to operation S47. On the other hand, if the determination corresponds to “NO”, the procedure returns to operation S10 of FIG. 12.
At operation S47, a control operation is executed to stop the first compressor 2b, to forcibly operate the second compressor 3b, and to operate the first and second compressors 2a and 3a in the controlled operation mode. In this case, only the first opening/closing valve 16b is opened, whereas the remaining opening/closing valves are maintained in a closed state.
As a result, lubricant oil is moved from the first compressor 2b to the second compressor 3b, as indicated by arrow S47 in FIG. 20, thus increasing the oil level of the second compressor 3b. 
Thus, even in the case of the method for performing an oil equalizing operation in accordance with the above-described liquid level detection control operation, lubricant oil is collected in the first compressors 2a and 2b, which are provided with respective check valves 15a and 15b and respective bypass tubes 29a and 29b, in accordance with the characteristics of a low-pressure shell type compressor in which a reduction in pressure occurs during operation of the compressor, through operations S16 and S19 of FIG. 17, operation S26 of FIG. 18, operations S32 and S35 of FIG. 19, and operation S42 of FIG. 20. Through operation S21 of FIG. 17, operation S28 of FIG. 18, operation S37 of FIG. 19, and operation S44 of FIG. 20, it is then possible to perform an oil equalizing operation in which the lubricant oil from the stopped compressor of one outdoor unit is rapidly supplied into the other outdoor unit within a reduced period of time by efficiently using pressure generated by forced operation of the other compressor in the outdoor unit having one compressor stopped.
Also, oil equalization is achieved by feeding lubricant oil from one of the compressors connected in parallel to the other compressor through operations S23 and S31 of FIG. 18, and operations S39 and S47 of FIG. 20.
In accordance with this embodiment, therefore, the amounts of lubricant oil in all compressors 2a, 3a, 2b, and 3b are equalized within a reduced period of time, similar to the previously described embodiment. Thus, a reliable and efficient oil system is implemented. Accordingly, the restriction on the length and diameter of pipes is reduced. Also, there is no problem caused by different levels of the constituent elements of the air conditioner. Therefore, it is possible to achieve a widened freedom of design including installation of outdoor units. Moreover, it is only necessary to install a check valve (check valve 15a or 15b) and a bypass tube (bypass tube 29a or 29b) in one compressor (first compressor 2a or 2b) of each outdoor unit (outdoor unit 1a or 1b). Accordingly, it is unnecessary to install such elements in all compressors, as in conventional air conditioning units. Thus, a reduction in costs is achieved in accordance with a reduction in the number of constituent elements used in the air conditioner.
In particular, in accordance with the method for performing an oil equalizing operation, using the liquid level detection control operation, it is possible to reliably supply lubricant oil into compressors, which preferably require a supply of lubricant oil due to the lowering of the liquid level during operation. Accordingly, there is an advantage in that efficient oil equalization can be achieved.
The present invention is not limited to the above-described embodiments. For example, connections, which are connected to the bypass tubes 29a and 29b of the air conditioner upstream from the bypass tubes 29a and 29b, are not limited to the oil return tubes 14a and 14b. These connections may be any of the sections through which high pressure gas or high pressure liquid passes, such as the discharge pipes 9a and 9b or the uppermost parts of the liquid collectors 7a and 7b, as long as the sections are at the side of the first compressors 2a and 2b. 
As is apparent from the above description, in accordance with one aspect of the present invention, it is basically necessary to install a bypass tube, a bypass opening/closing valve, and a check valve only in one compressor of each outdoor unit. Thus, a reduction in factors causing failure is achieved. Accordingly, the costs may be reduced because an improvement in the reliability of the system is achieved in accordance with the reduction in factors causing failure. When the bypass opening/closing valve is opened during operation of the other compressor of each outdoor unit, pressure from the other compressor is applied to the suction side of the one compressor via the bypass tube and suction tube. Thus, it is possible to pressurize lubricant oil in the oil reservoir of the one compressor, using the applied pressure, and thus, to feed the lubricant oil to another outdoor unit via a connecting tube. Therefore, it is possible to reduce the time taken to complete the oil equalizing operation. Also, there is no restriction on the length of oil equalizing pipes. Therefore, it is possible to achieve a wider freedom of design, and a reduction in costs.
In accordance with another aspect of the present invention, it is possible to feed lubricant oil from one of the compressors, connected in parallel, to the other compressor, using a simple operation to stop the one compressor. Thus, it is possible to reliably and inexpensively achieve oil equalization, using a simple operation to stop a desired compressor.
In accordance with another aspect of the present invention, it is possible to reliably and efficiently achieve oil equalization during a normal operation mode because the oil equalization can be performed by efficiently using the discharge pressure of the other one of the compressors connected in parallel.
In accordance with another aspect of the present invention, it is possible to uniformly supply all compressors, and thus, to more effectively perform an oil equalizing operation.
In accordance with another aspect of the present invention, it is possible to achieve oil equalization, using a simple operation, and thus, to easily manage the oil equalizing operation.
In accordance with another aspect of the present invention, the oil equalization can be achieved without the user being aware because it is unnecessary to use detectors during a normal controlled operation. Accordingly, it is possible to easily manage the oil equalizing operation by controlling the air conditioner such that the oil equalizing operation is appropriately performed prior to the normal controlled operation. Thus, reliable oil equalization can be achieved.
In accordance with another aspect of the present invention, it is possible to efficiently achieve oil equalization because it is possible to reliably supply lubricant oil into compressors, which preferentially require supply of lubricant oil.
Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

## Claims

1. An air conditioner comprising:
a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units comprising a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units,
wherein each of the outdoor units further comprises a check valve arranged at a suction tube connected to one of the compressors included in each outdoor unit, a bypass tube arranged at an outlet of at least one of the remaining compressors, and a bypass opening/closing valve arranged in the bypass tube,
wherein the bypass tube is connected to the suction tube downstream from the check valve,
wherein an oil equalizing tube opening/closing valve is arranged in the oil equalizing tube of each outdoor unit to cut off flow of lubricant oil through the oil equalizing tube, and
wherein a connecting tube opening/closing valve is arranged in the connecting tube.

2. The air conditioner according to claim 1, wherein each of the compressors is a low-pressure shell type compressor in which an internal pressure of a shell of the compressor during an operation of the compressor is lower than an internal pressure of the shell in a stopped state of the compressor.

3. The air conditioner according to claim 1, comprising an oil equalization controller.

4. The air conditioner according to claim 3, wherein the oil equalization controller comprises:
a timer;
an opening/closing controller to control opening/closing of the bypass opening/closing valve, the oil equalizing tube opening/closing valves, and the connecting tube opening/closing valves; and
a compressor controller to control operations of the compressors.

5. A method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, comprising:
collecting lubricant oil in one compressor of one of the outdoor units;
pressurizing the collected lubricant oil by a discharge pressure of another compressor, which is connected in parallel to said one compressor in the same outdoor unit; and
feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube to achieve oil equalization.

6. The method of claim 5, wherein each of the compressors is a low-pressure shell type compressor in which an internal pressure of a shell of the compressor during an operation of the compressor is lower than an internal pressure of the shell in a stopped state of the compressor.

7. A method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units including a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, comprising:
collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit;
pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube; and
feeding lubricant oil among the compressors of the same outdoor unit.

8. The method according to claim 7, wherein oil equalization is performed by sequentially supplying lubricant oil into the compressors of the outdoor units by collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit, pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube, and feeding lubricant oil among the compressors of the same outdoor unit.

9. The method according to claim 8, wherein oil equalization is performed by incorporating, in a controlled operation of the air conditioner, an oil equalizing operation to sequentially supply lubricant oil into the compressors, the oil equalizing operation comprising collecting lubricant oil in one of the compressors in one of the outdoor units, which can apply, to an oil reservoir in the compressor, a discharge pressure of another compressor in the same outdoor unit, pressurizing the collected lubricant oil by the discharge pressure of said another compressor in the same outdoor unit, and feeding the pressurized lubricant oil to one compressor of another outdoor unit via an oil equalizing tube and a connecting tube, and feeding lubricant oil among the compressors of the same outdoor unit.

10. The method according to claim 8, wherein the oil equalization is performed, starting from the collection of lubricant oil, when it is detected that an oil level in the oil reservoir of a particular one of the compressors is lower than a predetermined level.

11. A method for performing an oil equalizing operation in an air conditioner including a plurality of outdoor units connected in parallel with an indoor unit, each of the outdoor units comprising a plurality of compressors connected in parallel, the compressors being connected by an oil equalizing tube to feed surplus oil in each of the compressors to the remaining compressors, and a connecting tube to connect the oil equalizing tubes of the outdoor units, each of the outdoor units further including a bypass tube connected to a discharge tube of the outdoor unit, the bypass tube communicating with a suction tube of one of the compressors in the outdoor unit only at an outlet of said one compressor, a bypass opening/closing valve arranged in the bypass tube, a check valve arranged at the suction tube upstream from a connection between the bypass tube and the suction tube, an oil equalizing tube opening/closing valve arranged in the oil equalizing tube of the outdoor unit to cut off flow of lubricant oil through the oil equalizing tube, and a connecting tube opening/closing valve arranged in the connecting tube, wherein oil equalization is performed by:
collecting lubricant oil in the compressor, which includes the discharge tube connected with the bypass tube;
applying, to the collected lubricant oil, a discharge pressure of another compressor, which is connected in parallel to said one compressor in the same outdoor unit, via the bypass tube opened by the bypass opening/closing valve, and the suction tube preventing a reverse flow therethrough by the check valve, thereby pressurizing the collected lubricant oil; and
feeding the pressurized lubricant oil to one compressor of another outdoor unit via the oil equalizing tube opened by the oil equalizing tube opening/closing valve and the connecting tube opened by the connecting tube opening/closing valve.

12. The method of claim 11, wherein the outdoor units comprise an oil equalization controller.

13. The method of claim 12, wherein the oil equalization controller comprises:
a timer;
an opening/closing controller to control opening/closing of the bypass opening/closing valve, the oil equalizing tube opening/closing valves, and the connecting tube opening/closing valves; and
a compressor controller to control operations of the compressors.

14. The method of claim 12, wherein the oil equalization controller comprises a compressor liquid level detector.

15. The method of claim 12, comprising starting the oil equalization when the compressor liquid level detector detects that an oil level in the oil reservoir of a particular one of the compressors is lower than a predetermined level.

16. The method of claim 14, wherein the liquid level detector comprises a flow switch.

17. The method of claim 11, wherein each of the compressors is a low-pressure shell type compressor in which an internal pressure of a shell of the compressor during an operation of the compressor is lower than an internal pressure of the shell in a stopped state of the compressor.