AIR CONDITIONER

Abstract

An air conditioner may be provided that includes a plurality of compressors compressing a refrigerant, a first heat exchanger for condensing the refrigerant compressed in the compressors, a first expansion valve for expanding the condensed refrigerant, a second expansion valve for expanding the refrigerant emerging from the first expansion valve, and a second heat exchanger for evaporating the refrigerant emerging from the second expansion valve. The refrigerant from the first expansion valve may be guided such that a portion of the refrigerant is introduced into one of the compressors after bypassing the second expansion valve and the second heat exchanger, and a remaining portion of the refrigerant may be introduced into another one of the compressors after passing through the second expansion valve and second heat exchanger.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Korean Patent Application No. 10-2010-0110417, filed Nov. 8, 2010, the subject matter of which is incorporated herein by reference.

BACKGROUND

1. Field

Embodiments of the present invention may relate to an air conditioner having a plurality of compressors.

2. Background

An air conditioner is an appliance for cooling/heating a room or purifying air in the room by using a refrigeration cycle for a refrigerant. The refrigeration cycle may include a compressor, a condenser, an expansion device, and an evaporator.

An air conditioner may include a plurality of compressors for one outdoor unit. One or more compressors may selectively operate in accordance with a load. The plurality of compressors may include first and second compressors connected in parallel via a refrigerant suction line and a refrigerant discharge line.

At a low load, only one of the first compressor and the second compressor may operate. On the other hand, at a high load, the first compressor and the second compressor may operate simultaneously.

When the first and second compressors operate simultaneously, the refrigerant compressed in the first compressor and the refrigerant compressed in the second compressor may sequentially pass through an indoor heat exchanger, an expansion device, and an outdoor heat exchanger, and are then distributed to the first and second compressors. The resultant refrigerant may be sucked into the first and second compressors in a low-temperature and low-pressure state.

In such an air conditioner, a high electric power consumption and a large refrigerant circulation amount may be required because a low-temperature and low-pressure refrigerant is sucked into the first and second compressors.

BRIEF DESCRIPTION OF THE DRAWINGS

Arrangements and embodiments may be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein:

FIG. 1 is a diagram of a refrigeration cycle in an air conditioner according to an exemplary embodiment of the present invention;

FIG. 2 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment (of FIG. 1) when a second compressor operates alone;

FIG. 3 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner (of FIG. 1) when a first compressor operates alone;

FIG. 4 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner of (FIG. 1) when a first compressor and a second compressor operate simultaneously;

FIG. 5 is a P-h diagram according to operation modes of a plurality of compressors in an embodiment (of FIG. 1);

FIG. 6 is a diagram of a refrigeration cycle in an air conditioner according to an exemplary embodiment of the present invention;

FIG. 7 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment (of FIG. 6) when a second compressor operates alone;

FIG. 8 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment of (FIG. 6) when a first compressor operates alone;

FIG. 9 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment of (FIG. 6) when a first compressor and a second compressor operate simultaneously; and

FIG. 10 is a P-h diagram according to operation modes of a plurality of compressors in an embodiment of (FIG. 6).

DETAILED DESCRIPTION

FIG. 1 is a diagram of a refrigeration cycle in an air conditioner according to an exemplary embodiment of the present invention. FIG. 2 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when a second compressor operates alone. FIG. 3 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when a first compressor operates alone. FIG. 4 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when a first compressor and a second compressor operate simultaneously. FIG. 5 is a P-h diagram according to operation modes of a plurality of compressors in an embodiment. Other embodiments and configurations may also be provided.

The air conditioner may include a plurality of compressors 2, 8 for compressing a refrigerant, a first heat exchanger 10 for condensing the refrigerant that emerges from the compressors 2, 8, a first expansion valve 40 for expanding the refrigerant condensed by the first heat exchanger 10, a second expansion valve 42 for expanding the refrigerant that emerges from the first expansion valve 40, a second heat exchanger 12 for evaporating the refrigerant that emerges from the second expansion valve 42, and a variable refrigerant line 46 for guiding the refrigerant that emerges from the first expansion valve 40 to pass though the second expansion valve 42 and the second heat exchanger 12, or to bypass the second expansion valve 42 and the second heat exchanger 12.

The variable refrigerant line 46 may guide the refrigerant that emerges from the first expansion valve 40 such that a first portion of the refrigerant is introduced into one of the plurality of compressors, such as the compressor 8 after bypassing the second expansion valve 42 and the second heat exchanger 12, with a second portion (or remaining portion) of the refrigerant being introduced into another one of the plurality of compressors, such as the compressor 2, or the variable refrigerant line 46 to guide the refrigerant that emerges from the first expansion valve 40 such that an entirety of the refrigerant that emerges from the first expansion valve 40 is distributed to the plurality of compressors (such as the compressors 2 and 8) after passing through the second expansion valve 42 and the second heat exchanger 12.

The variable refrigerant line 46 may also guide the refrigerant that emerges from the first expansion valve 40 such that an entirety of the refrigerant is introduced into one of the plurality of compressors (such as the compressor 8) after passing through the second expansion valve 42 and the second heat exchanger 12, such that an entirety of the refrigerant is introduced into another one of the plurality of compressors (such as the compressor 2) after passing through the second expansion valve 42 and the second heat exchanger 12, or such that an entirety of the refrigerant is distributed to the plurality of compressors 2 and 8 after passing through the second expansion valve 42 and the second heat exchanger 12.

The plurality of compressors 2 and 8 may include two or three compressors. The following description may be provided in conjunction with an example in which the plurality of compressors include two compressors, namely the first compressor 2, and the second compressor 8 connected to the first compressor 2 in parallel via a refrigerant suction line 4 and a refrigerant discharge line 6.

In accordance with operating conditions, such as a load, the first and second compressors 2 and 8 may operate simultaneously or only one of the first and second compressors 2 and 8 may operate.

The first and second compressors 2 and 8 may have a same displacement or may have different displacements.

Where the first and second compressors 2 and 8 have different displacements, only the compressor 8 having a smaller displacement may operate, only the compressor 2 having a greater displacement may operate, and/or the compressors 2 and 8 may simultaneously operate, in accordance with a load. Thus, the first and second compressors 2 and 8 may have different displacements.

The air conditioner may have a plurality of partial-load operation modes and a full-load operation mode to meet various operating conditions including an ambient temperature and a desired room temperature, for example.

The load of the air conditioner may include a first partial load acceptable by operation of a smaller-displacement of one of the first and second compressors 2 and 8 alone. The load of the air conditioner may also include a second partial load that can not be accepted by operation of the smaller-displacement one of the first and second compressors 2 and 8, but may be acceptable by operation of a greater-displacement one of the first and second compressors 2 and 8 alone. The load of the air conditioner may also include a full load that is acceptable by simultaneous operations of the first and second compressors 2 and 8, but can not be accepted by operation of the greater-displacement one of the first and second compressors 2 and 8 alone.

When the load of the air conditioner is not greater than the first partial load, a first partial-load operation mode may be carried out in which the smaller-displacement one of the first and second compressors 2 and 8 may operate alone (without the other one of the first and second compressors 2, 8). On the other hand, when the load of the air conditioner is greater than the first partial load, but is not greater than a second partial load, a second partial-load operation mode, in which the greater-displacement one of the first and second compressors 2 and 8 may operate alone (without the other one of the first and second compressors 2, 8). Additionally, when the load of the air conditioner is greater than the second partial load, a full-load operation mode may be carried out in which the first and second compressors 2 and 8 operate simultaneously.

Since a portion of the refrigerant that emerges from the first expansion valve 40 may be compressed in the second compressor 8 after bypassing the second expansion valve 42 and the second heat exchanger 12, the second compressor 8 may have a smaller displacement than the first compressor 2. The following description may be provided in conjunction with an example in which the second compressor 8 has a smaller displacement than the first compressor 2.

The first partial-load operation mode is a second compressor operation mode in which the second compressor 8 operates alone (without the first compressor 2), and the refrigerant that emerges from the first expansion valve 40 flows to the second compressor 8 after passing through the second expansion valve 42 and the second heat exchanger 12, as shown in FIG. 2.

The second partial-load operation mode is a first compressor operation mode in which the first compressor 2 operates alone (without the second compressor 8), and the refrigerant emerging from the first expansion valve 40 flows to the first compressor 2 after passing through the second expansion valve 42 and the second heat exchanger 12, as shown in FIG. 3.

The full-load operation mode is a first and second compressor simultaneous operation mode in which the first and second compressors 2 and 8 operate simultaneously, and a first portion of the refrigerant that emerges (or is output) from the first expansion valve 40 flows to the second compressor 8 after bypassing the second expansion valve 42 and the second heat exchanger 12, whereas a second portion (or remaining portion) of the refrigerant that emerges (or is output) from the first expansion valve 40 flows to the first compressor 2 after bypassing the second expansion valve 42 and the second heat exchanger 12, as shown in FIG. 4.

A cooling mode or a heating mode may be selectively performed when the air conditioner includes a cooling/heating switching valve 14. In the cooling mode, the cooling/heating switching valve 14 may guide the refrigerant compressed in at least one of the first and second compressors 2 and 8 to the second heat exchanger 12, and may guide the refrigerant that evaporated in the first heat exchanger 10 to the compressor that is in operation. In the heating mode, the cooling/heating switching valve 14 may guide the refrigerant compressed in at least one of the first and second compressors 2 and 8 to the first heat exchanger 10, and may guide the refrigerant evaporated in the second heat exchanger 12 to the compressor that is in operation.

The cooling/heating switching valve 14 may be connected to the first and second compressors 2 and 8 via the refrigerant suction line 4 and the refrigerant discharge line 6. The cooling/heating switching valve 14 may also be connected to the first heat exchanger 10 via a cooling/heating switching valve-first heat exchanger connecting line 16. Additionally, the cooling/heating switching valve 14 may be connected to the second heat exchanger 12 via a cooling/heating switching valve-second heat exchanger connecting line 18.

The refrigerant suction line 4 may have a branching point 22 where the refrigerant that emerges from the cooling/heating switching valve 14 is distributed to the first compressor 2 and the second compressor 8. The refrigerant suction line 4 may include a common suction line 24 connected to the cooling/heating switching valve 14 and the branching point 22, and suction lines 26 and 28 that branches from the common suction line 24. The suction line 26 may be a first compressor-side suction line for guiding the refrigerant that flows through the common suction line 24 to the first compressor 2. The suction line 28 may be a second compressor-side suction line for guiding the refrigerant that flows through the common suction line 24 to the second compressor 8.

The refrigerant discharge line 6 may have a joining point 32 where the refrigerant that emerges from the first compressor 2 may join the refrigerant that emerges from the second compressor 8. The refrigerant discharge line 6 may include a common discharge line 34 connected to the cooling/heating switching valve 14 and the joining point 32, and discharge lines 36 and 38 that join (or connect) to the common discharge line 34. The discharge line 36 may be a first compressor-side discharge line for guiding the refrigerant compressed by the first compressor 2 via the joining point 32 to the common discharge line 34. The discharge line 38 may be a second compressor-side discharge line for guiding the refrigerant compressed in the second compressor 8 via the joining point 32 to the common discharge line 34. A first discharge-side check valve 37 may be provided at the first compressor-side discharge line 36, to prevent the refrigerant compressed in the second compressor 8 from flowing to the first compressor 2. A second discharge-side check valve 39 may be provided at the second compressor-side discharge line 38 to prevent the refrigerant compressed in the first compressor 2 from flowing to the second compressor 8.

The first heat exchanger 10 may evaporate the refrigerant in the cooling mode, and may condense the refrigerant in the heating mode. The first expansion valve 40 may be provided between the first heat exchanger 10 and the second heat exchanger 12. In the heating mode, the first expansion valve 40 may expand the refrigerant that flows toward a bypass device 50 after being condensed in the first heat exchanger 10. The first expansion valve 40 may include an electronic expansion valve (EEV) or a linear expansion valve (LEV) in which an opening degree is adjustable. The bypass device 50 may be described below.

In the air conditioner, the first heat exchanger 10 may be provided in an indoor unit I, whereas the first compressor 2, the second compressor 8, the second heat exchanger 12, the second expansion valve 42, the first expansion valve 40, and the variable refrigerant line 46 may be provided in an outdoor unit O.

An indoor expansion valve 11 may be provided in the indoor unit I to expand the refrigerant flowing to the first heat exchanger 10 in the cooling mode. The indoor expansion valve 11 may include an EEV or a LEV in which an opening degree is adjustable. In the cooling mode, the indoor expansion valve 11 may expand the refrigerant that passes through the second expansion valve 42 and the first expansion valve 40. In the heating mode, the indoor expansion valve 11 may be fully opened to allow the refrigerant emerging (or output) from the first heat exchanger 10 to pass therethrough.

The bypass device 50 may be provided in the variable refrigerant line 46. In addition to the bypass device 50, the variable refrigerant line 46 may include a one-way valve 60. The bypass device 50 may guide a portion of the refrigerant emerging (or output) from the first expansion valve 40 in the heating mode to bypass the second expansion valve 42 and the second heat exchanger 12, and thus flow to a point between the branching point 22 of the refrigerant suction line 4 and the second compressor 8. The one-way valve 60 may prevent the refrigerant emerging from the bypass device 50 from flowing to the branching point 22 of the refrigerant suction line 4.

The bypass device 50 may be a gas injection device for injecting refrigerant into the second compressor 8 under a condition that the refrigerant is in a liquid phase. The bypass device 50 may be connected between the branching point 22 of the refrigerant suction line 4 and the second compressor 8. The bypass device 50 may allow the refrigerant to be introduced into the second compressor 8 in the heating mode under a condition that the refrigerant has low-temperature and low-pressure liquid-phase. The bypass device 50 may allow the refrigerant to be introduced into the second compressor 8 under a condition that the refrigerant has an intermediate pressure that is less than a condensation pressure of the first heat exchanger 10, but is greater than the evaporation pressure of the second heat exchanger 12.

The bypass device 50 may include an inner heat exchanger 53 having a first passage 51 for guiding the refrigerant to flow between the second expansion valve 42 and the first expansion valve 40, and a second passage 52 through which a refrigerant flows while exchanging heat with the refrigerant in the first passage 51. The bypass device 50 may include a first bypass passage 54 that has a first end connected between the first passage 51 (of the inner heat exchanger 53) and the second expansion valve 42, and a second end connected to the second passage 52 (of the inner heat exchanger 53).

The bypass device 50 may also include a second bypass passage 55 having a first end connected to the second passage 52, and a second end connected to the suction line 28 of the second compressor 8.

The inner heat exchanger 53 may be provided between the first expansion valve 40 and the second expansion valve 42 to allow the refrigerant that emerges from the first expansion valve 40 to flow to the second expansion valve 42, and to allow the refrigerant that emerges from the second expansion valve 42 to flow to the first expansion valve 40.

The bypass device 50 may further include a third expansion valve 56 provided at the first bypass passage 54. The third expansion valve 56 may include an LEV or an EEV in which an opening degree is adjustable. The third expansion valve 56 may have a smaller capacity than the first expansion valve 40 and the second expansion valve 42. When a capacity of the third expansion valve 56 is greater than or equal to a capacity of the first expansion valve 40 and the second expansion valve 42, there is a high possibility of the refrigerant, which is in a liquid phase, from being introduced into the second compressor 8. In this example, it may be difficult to finely adjust pressure and temperature of the refrigerant introduced into the first bypass passage 54. On the other hand, when the capacity of the third expansion valve 56 is less than the first expansion valve 40 and the second expansion valve 42, a possibility of the refrigerant, which is in a liquid phase, may be minimized from being introduced into the second compressor 8. In this example, pressure and temperature of the refrigerant introduced into the first bypass passage 54 may be more finely adjusted. The third expansion valve 56 may control the refrigerant introduced into the first bypass passage 54 to have a pressure less than a condensation pressure of the first heat exchanger 10, but greater than an evaporation pressure of the second heat exchanger 12. The third expansion valve 56 may be closed in the cooling mode, irrespective of the load of the air conditioner. In a full-load heating mode, the third expansion valve 56 may be opened to a predetermined opening degree in order to allow the refrigerant to be introduced into the second compressor 8 after passing through the bypass device 50. On the other hand, in a partial-load heating mode, the third expansion valve 56 may be closed to prevent the refrigerant from passing through the bypass device 50.

The one-way valve 60 may be provided between the branching point 22 of the refrigerant suction line 4 and a connecting point 58 of the bypass device 50 to the refrigerant suction line 4. The one-way valve 60 may include a check valve that guides the refrigerant passing through the branching point 22 of the refrigerant suction line 4 to flow to the second compressor 8, while preventing the refrigerant emerging from the bypass device 50 from flowing to the first compressor 2.

The air conditioner may diversely control the first compressor 2, the second compressor 8 and the third expansion valve 56, in accordance with a heating load.

When the air conditioner is in a heating mode, and the load thereof is a first-partial load or a second-partial load, the air conditioner may be controlled such that the second compressor 8 operates, the first compressor 2 is in a stopped state, and the third expansion valve 56 is closed.

On the other hand, when the air conditioner is in a heating mode, and the load thereof is the full load, the air conditioner may be controlled such that the first and second compressors 2 and 8 operate, and the third expansion valve 56 is opened.

Each of the first and second compressors 2 and 8 may be a constant speed compressor that compresses a refrigerant at constant speed. Each of the first and second compressors 2 and 8 may be a variable displacement compressor such as a variable displacement inverter compressor. Alternatively, one of the first and second compressors 2 and 8 may be a constant speed compressor, whereas the other one of the first and second compressors 2 and 8 may be a variable displacement compressor.

Where one of the first and second compressors 2 and 8 is a variable displacement compressor, and the other one of the first and second compressors 2 and 8 is a constant speed compressor, the air conditioner may more diversely cope with load.

Where the first compressor 2 is a variable displacement compressor, and the second compressor 8 is a constant speed compressor, one of the second compressor operation mode or the first and second compressor simultaneous operation mode may be selectively performed in accordance with the first partial load or full load without performing one of the second compressor operation mode, first compressor operation mode, and first and second compressor simultaneous operation mode in accordance with the first partial load, the second partial load, or the full load.

That is, at a partial load not greater than the first partial load, only the second compressor 8 may operate to cope with the partial load. At a load greater than the first partial load, but not greater than the full load, the second compressor 8 may operate, and the first compressor 2 may operate in a variable displacement mode to achieve load balancing. Accordingly, it may be possible to efficiently cope with a load greater than the first partial load as well as a load not greater than the first partial load, while minimizing an electric power consumption.

For example, where the first compressor 2 is a 5HP variable displacement compressor, and the second compressor 8 is a 2HP constant speed compressor, the second compressor 8 may operate alone at a partial load corresponding to 2HP or less (namely the second compressor operation mode). On the other hand, at a load corresponding to a displacement greater than 2HP, but not greater than 7HP (for example, 3H, 4H, 5H, 6H, or 7H), the second compressor 8 may operate (for 2H), and the first compressor 2 may operate in a variable displacement mode to achieve load balancing (1H, 2H, 3H, 4H, or 5H) (i.e., the first and second compressor simultaneous operation mode). In this example, the first and second compressors 2 and 8 may efficiently cope with a load up to 7H.

On the other hand, where the first compressor 2 is a constant speed compressor, and the second compressor 8 is a variable displacement compressor, one of the second compressor operation mode, the first compressor operation mode, and first and second compressor simultaneous operation mode may be selectively performed in accordance with the first partial load, the second partial load, or the full load.

For example, where the first compressor 2 is a 5HP constant speed compressor, and the second compressor 8 is a 2HP variable displacement compressor, the second compressor 8 may operate alone at a partial load corresponding to 2HP or lower (i.e., the second compressor operation mode). At a partial load corresponding to a displacement greater than 2HP, but not greater than 5HP, the first compressor 2 may operate alone, irrespective of the level of the load (i.e., the first compressor operation mode). On the other hand, at a load corresponding to a displacement greater than 5HP, but not greater than 7HP (e.g. 6H or 7H), the first compressor 2 may operate (for 5H), and the second compressor 8 may operate in a variable displacement mode to achieve load balancing (1H or 2H) (i.e., the first and second compressor simultaneous operation mode). In this example, the first and second compressors 2 and 8 may efficiently cope with a load of 0 to 2H and a load of 5 to 7H. Also, it may be possible to cope with a load corresponding to a displacement greater than 2HP, but not greater than 5HP.

Where the first compressor 2 is a variable displacement compressor, and the second compressor 8 is a constant speed compressor, an entirety of the load it may be efficiently coped with through the second compressor operation and the first and second compressor simultaneous operation. Accordingly, the first compressor 2 may be a variable displacement compressor having a greater displacement than the second compressor 8, and the second compressor 8 may be a constant speed compressor having a smaller displacement than the first compressor 2.

A volume ratio between the first and second compressors 2 and 8 may vary by adjusting an operation frequency of the first compressor 2. Through the volume ratio variation, one may control an intermediate pressure at which an optimal efficiency is obtained in accordance with operating conditions.

Where the first compressor 2 is a variable displacement compressor having a greater displacement than the second compressor 8, and the second compressor 8 is a constant speed compressor having a smaller displacement than the first compressor 2, a load acceptable by operation of the second compressor 8 alone (without the first compressor) may be set to the partial load, and a load acceptable by simultaneous operations of the first and second compressors 2 and 8 may be set to the full load. When the load of the air conditioner is a partial heating load, the second compressor 8 may operate, the first compressor 2 may be in a stopped state, and the third expansion valve 56 may be closed. On the other hand, when the load is a full heating load, the first and second compressors 2 and 8 may operate, and the third expansion valve 56 may be opened.

Functions of the air conditioner as described above may be described in more detail.

When the air conditioner is in the heating mode and in the second compressor operation mode in which the second compressor 8 operates alone (without the first compressor 2), as shown in FIG. 2 and “A” of FIG. 5, the refrigerant that is compressed in the second compressor 8 (a), may be condensed in the first heat exchanger 10 (b). The refrigerant may then be expanded in at least one of the first expansion valve 40 and the second expansion valve 42 while passing through the first expansion valve 40, the inner heat exchanger 53 and the second expansion valve 42 (c). Thereafter, the refrigerant may evaporate in the second heat exchanger 12 (d), and may then return to the second compressor 8 after passing through the one-way valve 60. The refrigerant may heat the first exchanger 10 while circulating through the second compressor 8, the first heat exchanger 10, the first expansion valve 40, the second expansion valve 42, the second heat exchanger 12, the one-way valve 60, and the second compressor 8.

Since the second compressor 8 has a smaller displacement than the first compressor 2, the second compressor operation mode, in which the second compressor 8 operates alone, may exhibit a lower compression work, a lower condensation pressure, and a higher evaporation pressure than the first compressor operation mode, in which the first compressor 2 operates alone, as shown in “A” and “B” of FIG. 5.

On the other hand, when the air conditioner is in the heating mode and in the first compressor operation mode, in which the first compressor 2 operates alone, as shown in FIG. 3 and “B” of FIG. 5, the refrigerant that is compressed in the first compressor 2 (e), may be condensed in the first heat exchanger 10 (f). The refrigerant may then expand in at least one of the first expansion valve 40 and the second expansion valve 42 while passing through the first expansion valve 40, the inner heat exchanger 53, and the second expansion valve 42 (g). Thereafter, the refrigerant may evaporate in the second heat exchanger 12 (h), and may then return to the first compressor 2. The refrigerant may heat the first exchanger 10 while circulating the first compressor 2, the first heat exchanger 10, the first expansion valve 40, the second expansion valve 42, and the second heat exchanger 12.

When the air conditioner is in the heating mode and in the first- and second-compressor simultaneous operation mode in which the first and second compressors 2 and 8 operate simultaneously, as shown in FIG. 4 and “C” and “D” of FIG. 5, the refrigerant that is compressed in the first compressor 2 (i) may be joined with the refrigerant that is compressed in the second compressor 8 (j). The joined refrigerant is condensed in the first heat exchanger 10 (k) and is then passed through the first expansion valve 40.

A portion of the refrigerant passing through the first expansion valve 40 flows to the first bypass passage 54, and then flows to the second bypass passage 55 after being expanded by the third expansion valve 56 (i), and passing through the second passage 52 of the inner heat exchanger 53 (m). The refrigerant flowing to the second bypass passage 55 is sucked into the second compressor 8 without being sucked into the first compressor 2 in accordance with a function of the one-way valve 60. The refrigerant is compressed in the second compressor 8 (j).

The remaining portion of the refrigerant passing through the first expansion valve 40, namely, the refrigerant that does not flow to the first bypass passage 54, may exchange heat with the refrigerant that passes through the second passage 52 (of the inner heat exchanger 53), while passing through the first passage 51 (of the inner heat exchanger 53). Thereafter, this refrigerant is expanded by the second expansion valve (n) and is then evaporated in the second heat exchanger 12 (o). The evaporated refrigerant is sucked into the first compressor 2 and is then compressed (i).

In this example, the refrigerant may heat the first heat exchanger 10 while not only circulating the first compressor 2, the first heat exchanger 10, the first expansion valve 40, the inner heat exchanger 53, the second expansion valve 42, the second heat exchanger 12, and the first compressor 2 (“C” of FIG. 5), but also circulating the second compressor 8, the first heat exchanger 10, the first expansion valve 40, the third expansion valve 56, the inner heat exchanger 53, and the second compressor 8 (“D” of FIG. 5).

It may be possible to introduce, into the first compressor 2, the refrigerant sequentially passing through the first expansion valve 40, the inner heat exchanger 53, the second expansion valve 42, and the second heat exchanger 12, and to introduce, into the second compressor 8, the refrigerant sequentially passing through the first expansion valve 40, the third expansion valve 56, and the inner heat exchanger 53 while simultaneously operating the first and second compressors 2 and 8. Additionally, electric power consumption may be reduced but also density of the refrigerant may be increased, thereby increasing an amount of the refrigerant circulating the air conditioner and enhancing capacity of the refrigerant, as compared to an example in which an entirety of a low-temperature and low-pressure refrigerant is compressed by a single compressor, by controlling pressure of the refrigerant such that pressure of the refrigerant introduced into the second compressor 8 is greater than pressure of the refrigerant introduced into the first compressor 2.

FIG. 6 is a diagram of a refrigeration cycle in an air conditioner according to an exemplary embodiment of the present invention. FIG. 7 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when a second compressor operates alone. FIG. 8 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when a first compressor operates alone. FIG. 9 is a diagram of a refrigerant flow in a refrigeration cycle in an air conditioner in an embodiment when the first and second compressors operate simultaneously. FIG. 10 is a P-h diagram according to operation modes of a plurality of compressors in an embodiment. Other embodiments and configurations may also be provided.

As shown in FIGS. 6 to 9, in an air conditioner according to an embodiment, the bypass device 50 may include a gas/liquid separator 62 provided between the first expansion valve 40 and the second expansion valve 42. The bypass device 50 may include a gas/liquid separator connecting passage 64 having a first connected to the gas/liquid separator 62, and a second end connected to the suction line 28, in order to guide a gas-phase refrigerant emerging from the gas/liquid separator 62 to the suction line 28 of the second compressor 8. Configurations and functions of an air conditioner according to an embodiment, except for the bypass device 50, may be identical or similar to those of previous embodiments. Accordingly, no further description may be provided of identical or similar configurations and functions.

The gas/liquid separator 62 may separate liquid phase refrigerant and gas phase refrigerant from the refrigerant expanded by the first expansion valve 40. The gas/liquid separator 62 may be connected to the first expansion valve 40 by a first expansion valve connecting line, and may be connected to the second expansion valve 42 by a second expansion valve connecting line.

The bypass device 50 may further include a third expansion valve 66 provided at the gas/liquid separator connecting passage 64. The third expansion valve 66 may adjust an amount of the refrigerant flowing from the gas/liquid separator 62 to the gas/liquid separator connecting passage 64. The third expansion valve 66 may include an EEV or a LEV in which an opening degree is adjustable. The third expansion valve 66 may be closed when the first compressor 2 operates alone (without the second compressor 8), or the second compressor 8 operates alone (without the first compressor 2), and may be opened only when the first and second compressors 2 and 8 operate simultaneously.

The third expansion valve 66 may have a smaller capacity than the first expansion valve 40 and the second expansion valve 42. When the capacity of the third expansion valve 66 is greater than or equal to the capacity of the first expansion valve 40 and the second expansion valve 42, there may be a high possibility of liquid phase refrigerant being introduced into the second compressor 8. In this example, it may be difficult to finely adjust pressure and temperature of the gas phase refrigerant that emerges from the gas/liquid separator 62. On the other hand, when the third expansion valve 66 has a smaller capacity than the first expansion valve 40 and the second expansion valve 42, the possibility of the liquid phase refrigerant being introduced into the second compressor 8 may be minimized, and pressure and temperature of the gas phase refrigerant flowing to the first bypass passage 54 may be more finely adjusted.

When the air conditioner is in a heating mode and in a second compressor operation mode, in which the second compressor 8 operates alone, as shown in FIG. 7 and “A” of FIG. 10, the refrigerant that is compressed in the second compressor 8 (a), may be condensed in the first heat exchanger 10 (b). The refrigerant may then expand in at least one of the first expansion valve 40 and the second expansion valve 42 while passing through the first expansion valve 40, the gas/liquid separator 62, and the second expansion valve 42 (c). Thereafter, the refrigerant may evaporate in the second heat exchanger 12 (d), and may then return to the second compressor 8 after passing through the one-way valve 60. The refrigerant may heat the first exchanger 10 while circulating the second compressor 8, the first heat exchanger 10, the first expansion valve 40, the gas/liquid separator 62, the second expansion valve 42, the second heat exchanger 12, the one-way valve 60, and the second compressor 8.

Since the second compressor 8 has a smaller displacement than the first compressor 2, the second compressor operation mode, in which the second compressor 8 operates alone (without the first compressor 2), may exhibit a lower compression work, a lower condensation pressure, and a higher evaporation pressure than a first compressor operation mode, in which the first compressor 2 operates alone (without the second compressor 8), as shown in “A” and “B” of FIG. 10.

On the other hand, when the air conditioner is in the heating mode and in the first compressor operation mode, in which the first compressor 2 operates alone (without the second compressor 8), as shown in FIG. 8 and “B” of FIG. 10, the refrigerant that is compressed in the first compressor 2 (e), may be condensed in the first heat exchanger 10 (f). The refrigerant may then expand in at least one of the first expansion valve 40 and the second expansion valve 42 while passing through the first expansion valve 40, the gas/liquid separator 62, and the second expansion valve 42 (g). Thereafter, the refrigerant may evaporate in the second heat exchanger 12 (h), and may then return to the first compressor 2. The refrigerant may heat the first exchanger 10 while circulating the first compressor 2, the first heat exchanger 10, the first expansion valve 40, the gas/liquid separator 62, the second expansion valve 42, and the second heat exchanger 12.

When the air conditioner is in the heating mode and in the first- and second-compressor simultaneous operation mode, in which the first and second compressors 2 and 8 operate simultaneously, as shown in FIG. 9 and “C” and “D” of FIG. 10, the refrigerant that is compressed in the first compressor 2 (p) may join with the refrigerant that is compressed in the second compressor 8 (q). The joined refrigerant may be condensed in the first heat exchanger 10 (r), and may then primarily expand while passing through the first expansion valve 40 (s). The refrigerant primarily expanded by the first expansion valve 40 may be introduced into the gas/liquid separator 62 which in turn separates the refrigerant into a gas phase refrigerant and a liquid phase refrigerant (t). The gas phase refrigerant from the gas/liquid separator 62 passes through the third expansion valve 66, and then flows to the suction line 28 of the second compressor 8. In the second compressor 8, the gas phase refrigerant is compressed (q). The liquid phase refrigerant from the gas/liquid separator 62 is secondarily expanded by the second expansion valve 42 (u), and is then evaporated in the second heat exchanger 12 (v). The evaporated refrigerant is sucked into the first compressor 2, and is then compressed (p).

In this example, the refrigerant may heat the first heat exchanger 10 while not only circulating the first compressor 2, the first heat exchanger 10, the first expansion valve 40, the gas/liquid separator 62, the second expansion valve 42, the second heat exchanger 12, and the first compressor 2 (“E” of FIG. 10), but also circulating the second compressor 8, the first heat exchanger 10, the first expansion valve 40, the gas/liquid separator 62, the third expansion valve 66, and the second compressor 8 (“F” of FIG. 10).

It may be possible to introduce, into the first compressor 2, the refrigerant sequentially passing through the first expansion valve 40, the gas/liquid separator 62, the second expansion valve 42, and the second heat exchanger 12, and to introduce into the second compressor 8, the refrigerant sequentially passing through the first expansion valve 40, the gas/liquid separator 62, and the third expansion valve 56 while simultaneously operating the first and second compressors 2 and 8. Additionally, electric power consumption may be reduced, but also density of the refrigerant may be increased, thereby increasing an amount of the refrigerant circulating the air conditioner and enhancing capacity of the refrigerant, as compared to an example in which an entirety of a low-temperature and low-pressure refrigerant is compressed by a single compressor, by controlling pressure of the refrigerant such that pressure of the refrigerant introduced into the second compressor 8 is greater than pressure of the refrigerant introduced into the first compressor 2.

Where the first compressor 2 is a variable displacement compressor having a greater displacement than the second compressor 8, and the second compressor 8 is a constant speed compressor having a smaller displacement than the first compressor 2, as in a previous embodiment, a load acceptable by operation of the second compressor 8 alone may be set to a partial load, and a load acceptable by simultaneous operations of the first and second compressors 2 and 8 may be set to a full load. When the load of the air conditioner is a partial heating load, the second compressor 8 may operate, the first compressor 2 may be in a stopped state, and the third expansion valve 56 may be closed. On the other hand, when the load is a full heating load, the first and second compressors 2 and 8 may operate, and the third expansion valve 56 may be opened.

The air conditioner according to embodiment(s) may have advantages.

An amount of electric power consumed in all compressors of the air conditioner in a heating mode may be minimized, as compared to an example in which an entirety of a low-temperature and low-pressure refrigerant is compressed by a single compressor, because the refrigerant compressed in the first compressor and the refrigerant compressed in the second compressor are sent to the first heat exchanger after being mixed, and a gas portion of the refrigerant primarily expanded by the first expansion valve is compressed in the second compressor after bypassing the second expansion valve and the second heat exchanger, whereas a liquid portion of the refrigerant primarily expanded by the first expansion valve is compressed in the first compressor after passing through the second expansion valve and the second heat exchanger.

Additionally, for a small heating load, the second compressor may operate alone to cope with the small heating load, whereas for a large heating load, the first and second compressors may operate simultaneously to cope with the large heating load.

A density of the refrigerant may be increased, and satisfy a required refrigerant capacity by the second compressor, which has a smaller displacement than the first compressor, as compared to an example in which the refrigerant is compressed in the second compressor under a condition that the refrigerant has a lower pressure than the refrigerant in a primarily expanded state, because only a gas portion of the primarily-expanded refrigerant is compressed in the second compressor.

Embodiments of the present invention may have been made in view of the above problems, and may provide an air conditioner capable of optimally coping with a heating load, reducing electric power consumption, and achieving an increase in heating capacity.

An air conditioner may include a first compressor, a second compressor connected to the first compressor in parallel by a refrigerant suction line and a refrigerant discharge line, a first heat exchanger for evaporating a refrigerant in a cooling mode, and condensing the refrigerant in a heating mode, a second heat exchanger for condensing the refrigerant in the cooling mode, and evaporating the refrigerant in the heating mode, a first expansion valve arranged between the first heat exchanger and the second heat exchanger, a second expansion valve arranged between the first expansion valve and the second heat exchanger, a bypass device connected between a branching point of the refrigerant suction line and the second compressor, to guide a portion of the refrigerant that emerges from the first expansion valve, to flow to a point between the branching point of the refrigerant suction line and the second compressor after bypassing the second expansion valve and the second heat exchanger, and a one-way valve arranged between the branching point of the refrigerant suction line and a connecting point of the bypass device to the refrigerant suction line to prevent the refrigerant that emerges from the bypass device from flowing to the branching point of the refrigerant suction line.

The second compressor may have a smaller displacement than the first compressor.

The first compressor may be a variable displacement compressor, and the second compressor may be a constant speed compressor.

The air conditioner may have a second compressor operation mode in which the second compressor operates alone, and the refrigerant that emerges from the first expansion valve may flow to the second compressor after passing through the second expansion valve and the second heat exchanger. The air conditioner may also have a first and second compressor simultaneous operation mode in which the first compressor and the second compressor operate simultaneously, and a portion of the refrigerant that emerges from the first expansion valve to flow to the second compressor after bypassing the second expansion valve and the second heat exchanger, whereas a remaining portion of the refrigerant that emerges from the first expansion valve to flow to the first compressor after passing through the second expansion valve and the second heat exchanger.

The air conditioner may selectively perform the second compressor operation mode and the first and second compressor simultaneous operation mode.

The air conditioner may further have a first compressor operation mode in which the first compressor operates alone, and the refrigerant that emerges from the first expansion valve flows to the first compressor after passing through the second expansion valve and the second heat exchanger.

The air conditioner may selectively perform the second compressor operation mode, the first and second compressor simultaneous operation mode, and the first compressor operation mode.

The bypass device may include an inner heat exchanger having a first passage for guiding the refrigerant to flow between the first expansion valve and the second expansion valve, and a second passage through which the refrigerant passes while exchanging heat with the refrigerant that passes through the first passage, a first bypass passage having an end connected between the first passage of the inner heat exchanger and the first expansion valve, and an opposite end connected to the second passage, and a second bypass passage having an end connected to the second passage, and an opposite end connected to a suction line of the second compressor.

The bypass device may further include a third expansion valve arranged in the first bypass passage.

The third expansion valve may have a smaller capacity than the first expansion valve and the second expansion valve.

At a partial heating load, the second compressor may operate, the first compressor may be in a stopped state, and the third expansion valve may be closed. At a full heating load, the first compressor and the second compressor may operate, and the third expansion valve may be opened.

The bypass device may include a gas/liquid separator arranged between the first expansion valve and the second expansion valve, and a gas/liquid separator connecting passage having an end connected to the gas/liquid separator, and an opposite end connected to a suction line of the second compressor to guide a gas portion of the refrigerant from the gas/liquid separator to flow to the suction line of the second compressor.

The bypass device may further include a third expansion valve arranged in the gas/liquid separator connecting passage.

The third expansion valve may have a smaller capacity than the first expansion valve and the second expansion valve.

At a partial heating load, the second compressor may operate, the first compressor may be in a stopped state, and the third expansion valve may be closed. At a full heating load, the first compressor and the second compressor may operate, and the third expansion valve may be opened.

An air conditioner may be provided that includes a plurality of compressors for compressing a refrigerant, a first heat exchanger for condensing the refrigerant compressed in the compressors, a first expansion valve for expanding the refrigerant condensed in the first heat exchanger, a second expansion valve for expanding the refrigerant emerging from the first expansion valve, and a second heat exchanger for evaporating the refrigerant emerging from the second expansion valve, wherein the refrigerant emerging from the first expansion valve is guided such that a portion of the refrigerant emerging from the first expansion valve is introduced into one of the plurality of compressors after bypassing the second expansion valve and the second heat exchanger, and a remaining portion of the refrigerant emerging from the first expansion valve is introduced into another one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger, or such that the entirety of the refrigerant emerging from the first expansion valve is introduced into one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger.

The air conditioner may further include at least one of a gas/liquid separator and an inner heat exchanger arranged between the first expansion valve and the second expansion valve, to guide only a gas portion of the refrigerant emerging from the first expansion valve to be introduced into the one of the plurality of compressors.

The one of the plurality of compressors may be a constant speed compressor, and the another one of the plurality of compressors may be a variable displacement compressor having a greater displacement than the constant speed compressor.

An operation frequency of the variable displacement compressor may be adjusted to vary a volume ratio between the variable displacement compressor and the constant speed compressor, thereby controlling an intermediate pressure at which an optimal efficiency is obtained in accordance with operating conditions.

An air conditioner may be provided that includes a plurality of compressors for compressing a refrigerant, a first heat exchanger for condensing the refrigerant compressed in the compressors, a first expansion valve for expanding the refrigerant condensed in the first heat exchanger, a second expansion valve for expanding the refrigerant emerging from the first expansion valve, and a second heat exchanger for evaporating the refrigerant emerging from the second expansion valve. The refrigerant emerging from the first expansion valve is guided such that a portion of the refrigerant emerging from the first expansion valve is introduced into one of the plurality of compressors after bypassing the second expansion valve and the second heat exchanger, and a remaining portion of the refrigerant emerging from the first expansion valve is introduced into another one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger, or such that the entirety of the refrigerant emerging from the first expansion valve is introduced into at least one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger.

Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to affect such feature, structure, or characteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.

Claims

1. An air conditioner comprising:

a first compressor;

a second compressor provided in parallel to the first compressor by a refrigerant suction line and a refrigerant discharge line;

a first heat exchanger to evaporate a refrigerant in a cooling mode, and to condense the refrigerant in a heating mode;

a second heat exchanger to condense the refrigerant in the cooling mode, and to evaporate the refrigerant in the heating mode;

a first expansion valve between the first heat exchanger and the second heat exchanger;

a second expansion valve between the first expansion valve and the second heat exchanger;

a bypass device connected between a branching point of the refrigerant suction line and the second compressor, to guide a portion of the refrigerant that is output from the first expansion valve to bypass the second expansion valve and the second heat exchanger and to flow to a point between the branching point of the refrigerant suction line and the second compressor; and

a one-way valve between the branching point of the refrigerant suction line and a connecting point of the bypass device to the refrigerant suction line, the one-way valve to prevent the refrigerant that is output from the bypass device from flowing to the branching point of the refrigerant suction line.

2. The air conditioner according to claim 1, wherein the second compressor has a smaller displacement than the first compressor.

3. The air conditioner according to claim 2, wherein the first compressor is a variable displacement compressor, and the second compressor is a constant speed compressor.

4. The air conditioner according to claim 1, wherein the air conditioner operates in a first and second compressor simultaneous operation mode and operates in a second compressor operation mode,

wherein the second compressor operation mode is a mode in which the second compressor operates alone without the first compressor, and the refrigerant that is output from the first expansion valve flows to the second compressor after passing through the second expansion valve and the second heat exchanger, and

wherein the first and second compressor simultaneous operation mode is a mode in which the first compressor and the second compressor operate simultaneously, and a first portion of the refrigerant that emerges from the first expansion valve bypass the second expansion valve and the second heat exchanger and flows to the second compressor, whereas a second portion of the refrigerant that emerges from the first expansion valve passing through the second expansion valve and the second heat exchanger and flows to the first compressor.

5. The air conditioner according to claim 4, wherein the air conditioner selectively performs the second compressor operation mode and the first and second compressor simultaneous operation mode.

6. The air conditioner according to claim 4, wherein the air conditioner further operates in a first compressor operation mode in which the first compressor operates alone without the second compressor, and the refrigerant that emerges from the first expansion valve passes through the second expansion valve and the second heat exchanger and flows to the first compressor.

7. The air conditioner according to claim 6, wherein the air conditioner selectively performs the second compressor operation mode, the first and second compressor simultaneous operation mode, and the first compressor operation mode.

8. The air conditioner according to claim 1, wherein the bypass device comprises:

an inner heat exchanger having a first passage for guiding the refrigerant to flow between the first expansion valve and the second expansion valve, and a second passage through which the refrigerant passes while exchanging heat with the refrigerant that passes through the first passage;

a first bypass passage having a first end connected between the first passage of the inner heat exchanger and the first expansion valve, and a second end connected to the second passage of the inner heat exchanger; and

a second bypass passage having a first end connected to the second passage, and a second end connected to a suction line of the second compressor.

9. The air conditioner according to claim 8, wherein the bypass device further comprises a third expansion valve provided at the first bypass passage.

10. The air conditioner according to claim 9, wherein the third expansion valve has a smaller capacity than the first expansion valve and the second expansion valve.

11. The air conditioner according to claim 9, wherein:

at a partial heating load, the second compressor operates, the first compressor is in a stopped state, and the third expansion valve is closed; and

at a full heating load, the first compressor and the second compressor operate, and the third expansion valve are opened.

12. The air conditioner according to claim 1, wherein the bypass device comprises:

a gas/liquid separator provided between the first expansion valve and the second expansion valve; and

a gas/liquid separator connecting passage having a first end connected to the gas/liquid separator, and a second end connected to a suction line of the second compressor, and the gas/liquid separator to guide a gas portion of the refrigerant from the gas/liquid separator to flow to the suction line of the second compressor.

13. The air conditioner according to claim 12, wherein the bypass device further comprises a third expansion valve provided at the gas/liquid separator connecting passage.

14. The air conditioner according to claim 13, wherein the third expansion valve has a smaller capacity than the first expansion valve and the second expansion valve.

15. The air conditioner according to claim 13, wherein:

at a partial heating load, the second compressor operates, the first compressor is in a stopped state, and the third expansion valve is closed; and

at a full heating load, the first compressor and the second compressor operate, and the third expansion valve are opened.

16. An air conditioner comprising:

a plurality of compressors each to compress a refrigerant;

a first heat exchanger to condense the refrigerant compressed by at least one of the compressors;

a first expansion valve to expand the refrigerant condensed by the first heat exchanger;

a second expansion valve to expand the refrigerant that is output from the first expansion valve; and

a second heat exchanger to evaporate the refrigerant that is output from the second expansion valve,

wherein a first portion of the refrigerant that is output from the first expansion valve is provided to a first one of the plurality of compressors after bypassing the second expansion valve and the second heat exchanger, and a second portion of the refrigerant that is output from the first expansion valve is provided to a second one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger, or an entirety of the refrigerant that is output from the first expansion valve is provided to one of the plurality of compressors after passing through the second expansion valve and the second heat exchanger.

17. The air conditioner according to claim 16, further comprising a bypass device to guide the first portion of the refrigerant and the second portion of the refrigerant.

18. The air conditioner according to claim 17, wherein the bypass device includes:

a gas/liquid separator between the first expansion device and the second expansion device;

a gas/liquid separator connection passage having a first end connected to the gas/liquid separator and a second end connected to a suction line of one of the compressors.

19. The air conditioner according to claim 18, wherein the gas/liquid separator receives the refrigerant expanded by the first expansion valve and separates liquid phase refrigerant and gas phase refrigerant.

20. The air conditioner according to claim 20, wherein the bypass device further includes a third expansion device provided at the gas/liquid separator connecting passage to adjust an amount of the refrigerant flowing through the gas/liquid separator connecting passage.

21. The air conditioner according to claim 20, wherein:

at a partial heating load, the second one of compressor operates, the first compressor is in a stopped state, and the third expansion valve is closed; and

at a full heating load, the first compressor and the second compressor operate, and the third expansion valve are opened.

22. The air conditioner according to claim 16, further comprising:

at least one of a gas/liquid separator and an inner heat exchanger provided between the first expansion valve and the second expansion valve, to guide only a gas portion of the refrigerant that is output from the first expansion valve to the first one of the plurality of compressors.

23. The air conditioner according to claim 16, wherein the first one of the plurality of compressors is a constant speed compressor, and the second one of the plurality of compressors is a variable displacement compressor having a greater displacement than the constant speed compressor.

24. The air conditioner according to claim 23, wherein an operation frequency of the variable displacement compressor is adjusted to vary a volume ratio between the variable displacement compressor and the constant speed compressor, an intermediate pressure is controlled at which an efficiency is obtained in accordance with operating conditions.