WO2013019072A2

WO2013019072A2 - Air conditioner and method of controlling the same

Info

Publication number: WO2013019072A2
Application number: PCT/KR2012/006141
Authority: WO
Inventors: Junseong Park; Yongcheol Sa; Pilhyun Yoon
Original assignee: Lg Electronics Inc.
Priority date: 2011-08-01
Filing date: 2012-08-01
Publication date: 2013-02-07
Also published as: WO2013019072A3; KR20130014740A; KR101270816B1

Abstract

Provided are an air conditioner and a method of controlling the same. According to the embodiments, refrigerant can be sub-cooled while the air conditioner is operated in cooling mode so as to improve the cooling ability of the air conditioner, and refrigerant can be injected into to compressor while the air conditioner is operated in heating mode so as to increase the amount of refrigerant flowing in the compressor and thus to improve the heating ability of the air conditioner.

Description

AIR CONDITIONER AND METHOD OF CONTROLLING THE SAME

The present disclosure relates to an air conditioner and a method of controlling the air conditioner.

Air conditioners are used to maintain indoor air at predetermined states according to desired purposes and preferences. For example, air conditioners are used to keep indoor air cool in summer and warm in winter. In addition, air conditioners are used to adjust the humidity of indoor air for providing pleasant and clean environments. In the refrigeration cycle of an air conditioner, a refrigerant may be compressed, condensed, expanded, and evaporated for operation in cooling or heating mode.

Air conditioners can be classified into: split air conditioners in which indoor and outdoor units are separated; and one-boy air conditioners in which indoor and outdoor units are integrated.

In the case of a split air conditioner, an outdoor unit includes a compressor configured to compress a refrigerant, an outdoor heat exchanger configured to change heat with outdoor air, and an outdoor expansion device in which the refrigerant is decompressed, and an indoor unit includes an indoor heat exchanger configured to exchange heat with indoor air and an indoor expansion device in which the refrigerant is decompressed.

If an air conditioner is operated in heating mode, the amount of a refrigerant circulating in refrigeration cycle is large. In this case, a large amount of work is necessary for sufficient heating. That is, a compressor has to take a large amount of a refrigerant.

However, air conditioners of the related art are configured such that the same amount of a refrigerant is circulated in any mode. That is, the same amount of a refrigerant is circulated in cooling mode and heating mode. Therefore, when an air conditioner is operated in heating mode, the amount of a refrigerant circulating in the air conditioner is insufficient due to the limited system ability (capacity). Particularly, a compressor sucks an insufficient amount of a refrigerant as compared with a necessary amount.

Thus, air conditioners may not satisfy required heating conditions (such as an increase in the blowing rate of hot air, and heating at an extremely low temperature). That is, sufficient heating may not be obtained using air conditioners of the related art.

In addition, according to heating conditions and refrigeration cycle conditions, the temperature of a refrigerant at the outlet of a compressor may be excessively high to cause the compressor to be overloaded and thus to decrease heating efficiency.

In addition, if a refrigerant is not sufficiently condensed after passing through a condenser (that is, if a refrigerant is not sufficiently sub-cooled), cooling efficiency is low.

Embodiments provide an air conditioner including a multi-step compression unit for improving the cooling or heating ability.

In one embodiment, an air conditioner includes: an outdoor unit disposed in an outdoor area, the outdoor unit including a first compressor configured to compress a refrigerant and an outdoor heat exchanger configured to exchange heat with outdoor air; an indoor unit disposed in an indoor area, the indoor unit including an indoor heat exchanger configured to exchange heat with indoor air; a multi-step compression unit connecting the outdoor unit and the indoor unit; an inlet tube through which the refrigerant discharged from the outdoor unit or the indoor unit is introduced into the multi-step compression unit; and an outlet tube through which the refrigerant is discharged to the outdoor unit or the indoor unit after passing through the multi-step compression unit, wherein the multi-step compression unit includes: a second compressor configured to additionally compress the refrigerant after the refrigerant is discharged from the first compressor; and a sub-cooling heat exchanger at which the refrigerant condensed at the outdoor heat exchanger or the indoor heat exchanger is sub-cooled.

In another embodiment, there is provided a method of controlling an air conditioner including an outdoor unit and an indoor unit, the outdoor unit including a first compressor and an outdoor heat exchanger, the indoor unit including an indoor heat exchanger, the method including: allowing a refrigerant compressed by the first compressor to condense at the outdoor heat exchanger or the indoor heat exchanger; introducing the refrigerant into a multi-step compression unit after the refrigerant is discharged from the outdoor heat exchanger or the indoor heat exchanger; expanding at least a portion of the refrigerant introduced into the multi-step compression unit; and allowing the expanded portion of the refrigerant to exchange heat at a sub-cooling heat exchanger of the multi-step compression unit.

According to the embodiments, when the air conditioner is operated in cooling mode, a refrigerant discharged from the outdoor unit can be sub-cooled at a heat exchanger, and thus the cooling ability of the air conditioner can be improved.

In addition, when the air conditioner is operated in heating mode, a refrigerant discharged from the indoor unit can be bypassed and injected into a compressor to increase the total amount of refrigerant supplied to the compressor, and thus the heating ability of the air conditioner can be improved even under bad heating conditions.

In addition, if the temperature of the refrigerant at an outlet of the compressor is increased, at least a portion of the refrigerant discharged from the indoor unit can be expanded and injected into the compressor to prevent the temperature of the refrigerant from excessively increasing at the outlet of the compressor, thereby improving the heating ability of the air conditioner.

In the way, the heating/cooling efficiency of the air conditioner can be improved, and thus the air conditioner can be reliably used with less power consumption.

Fig. 1 is a block diagram illustrating an air conditioner according to an embodiment.

Fig. 2 is a systematic diagram illustrating a multi-step compression unit according to an embodiment.

Fig. 3 is a schematic diagram illustrating flows of a refrigerant when the air conditioner is operated in cooling mode according to an embodiment.

Fig. 4 is a schematic diagram illustrating flows of a refrigerant when the air conditioner is operated in heating mode according to an embodiment.

Fig. 5 is a block diagram illustrating a control configuration of an air conditioner according to an embodiment.

Fig. 6 is a flowchart for explaining a method of controlling the air conditioner in cooling mode according to an embodiment.

Fig. 7 is a flowchart for explaining a method of controlling the air conditioner in heating mode according to an embodiment.

Hereinafter, embodiments will now be described with reference to the accompanying drawings. However, the spirit and scope set forth in the present disclosure are not limited to the embodiments. Those of ordinary skill in the art will easily propose other embodiments within the spirit and scope.

Referring to Fig. 1, in the air conditioner of the embodiment, a refrigerant is circulated according to a refrigeration cycle. The air conditioner may be operated in cooling mode or heating mode according to the circulation direction of the refrigerant. An exemplary configuration of the air conditioner will now be described based on the case where the air conditioner is operated in cooling mode.

For operation in refrigeration cycle, the air conditioner includes an outdoor unit 20, an indoor unit 30, and a multi-step compression unit 100 connecting the outdoor unit 20 and the indoor unit 30.

The outdoor unit 20 includes a first compressor 21 configured to compress a refrigerant, an outdoor heat exchanger 23 at which the refrigerant compressed by the first compressor 21 changes heat with indoor air (and is condensed), and an outdoor expansion device 25 at which the refrigerant condensed at the outdoor heat exchanger 23 is expanded. The indoor unit 30 includes: an indoor expansion device 35 at which the refrigerant introduced into the indoor unit 30 is expanded; and an indoor heat exchanger 33 at which the refrigerant changes heat with indoor air (and is evaporated) after the refrigerant passes through the indoor expansion device 35.

In heating mode, the refrigerant is discharged from the first compressor 21 to the indoor heat exchanger 33, and then the refrigerant passes through the outdoor heat exchanger 23 after passing through the indoor expansion device 35 and the outdoor expansion device 25. At this time, the indoor heat exchanger 33 functions as a condenser, and the outdoor heat exchanger 23 functions as an evaporator.

The refrigerant discharged from one of the outdoor unit 20 and the indoor unit 30 is at least sub-cooled by the multi-step compression unit 100, and is then supplied to the other of the outdoor unit 20 and the indoor unit 30.

Hereinafter, the structure of the multi-step compression unit 100 will be described in detail with reference to the accompanying drawing.

Fig. 2 is a systematic diagram illustrating the multi-step compression unit 100 according to an embodiment.

Referring to Fig. 2, the multi-step compression unit 100 of the current embodiment includes refrigerant tubes 110 as main tubes through which a refrigerant discharged from the outdoor unit 20 or the indoor unit 30 is introduced to the indoor unit 30 or the outdoor unit 20.

The refrigerant tubes 110 include a first tube 111 and a second tube 112 through which the refrigerant is discharged from or introduced into the outdoor unit 20. In addition, the refrigerant tubes 110 further include a third tube 113 and a forth tube 114 through which the refrigerant is discharged from or introduced into the indoor unit 30.

When the air conditioner is operated in cooling mode, the first tube 111 may function as an “inlet tube” through which the refrigerant discharged from the outdoor unit 20 is introduced into the multi-step compression unit 100, and the second tube 112 may function as an “outlet tube” through which the refrigerant is discharged from the multi-step compression unit 100 to the outdoor unit 20. In addition, the third tube 113 may function as an “outlet tube” through which the refrigerant is discharged from the multi-step compression unit 100 to the indoor unit 30, and the fourth tube 114 may function as an “inlet tube” through which the refrigerant discharged from the indoor unit 30 is introduced into the multi-step compression unit 100.

On the other hand, when the air conditioner is operated in heating mode, the first tube 111 may function as an “outlet tube” through which the refrigerant is discharged to the outdoor unit 20, and the second tube 112 may function as an “inlet tube” through which the refrigerant is introduced into the multi-step compression unit 100. In addition, the third tube 113 may function as an “inlet tube” through which the refrigerant is introduced into the multi-step compression unit 100, and the fourth tube 114 may function as an “outlet tube” through which the refrigerant is discharged to the indoor unit 30.

The multi-step compression unit 100 includes: a sub-cooling heat exchanger 120; an expansion device 125 at which at least a portion of the refrigerant is expanded before the refrigerant is introduced into the sub-cooling heat exchanger 120; and a branch tube 123 through which at least the portion of the refrigerant is bypassed from the refrigerant tube 110 to the expansion device 125. The sub-cooling heat exchanger 120 may be understood as a structure at which a first refrigerant flowing in the refrigerant tube 110 exchanges heat with at least a bypassed portion of the first refrigerant (second refrigerant).

The sub-cooling heat exchanger 120 includes: a first heat exchanger 120a through which the refrigerant discharged from the outdoor unit 20 or the indoor unit 30 flows; and a second heat exchanger 120b through which at least the portion of the refrigerant bypassed to the branch tube 123 flows.

The first heat exchanger 120a and the second heat exchanger 120b may be considered as parts of the refrigerant tube 110 and the branch tube 123 or as additional heat exchanger members connected to the refrigerant tube 110 and the branch tube 123.

The multi-step compression unit 100 includes: a connection tube 118 through which a flow of the refrigerant is returned to the refrigerant tube 110 after passing through the second heat exchanger 120b; and a first valve 141 that can be opened to allow a flow of the refrigerant through the connection tube 118. If the first valve 141 is opened, a portion of the refrigerant passing through the second heat exchanger 120b can flow to the indoor unit 30 through the connection tube 118 and the first valve 141.

The multi-step compression unit 100 includes a second compressor 130, and in heating mode, the refrigerant discharged from the outdoor unit 20 is compressed at the second compressor 130. The second compressor 130 may be considered as a “high-pressure compressor” at which the refrigerant compressed at the first compressor 21 is additionally compressed. On the other hand, the first compressor 21 may be considered as a “low-pressure compressor.”

The multi-step compression unit 100 includes: an injection tube 117 through which a portion of the refrigerant is introduced (injected) into the second compressor 130 after passing through the second heat exchanger 120b; and a second valve 142 that can be opened to allow a flow of the refrigerant through the injection tube 117. The second compressor 130 includes an injection port 133 for receiving the refrigerant. The injection tube 117 may extend from the branch tube 123 to the second compressor 130.

In heating mode, at least a portion of the refrigerant flowing through the refrigerant tube 110 may be bypassed to the second compressor 130 to increase the amount of the refrigerant flowing in the second compressor 130 and thus to increase heating ability. In addition, since the refrigerant decreased in pressure or temperature at the expansion device 125 can be introduced into the second compressor 130, the temperature of the refrigerant at an outlet of the second compressor 130 can be decreased.

The multi-step compression unit 100 further includes: a suction tube 115 through which the refrigerant discharged from the outdoor unit 20 can be sucked into the second compressor 130; and a third valve 143 that can be opened to allow a flow of the refrigerant through the suction tube 115. The second compressor 130 includes a suction port 131 for receiving the refrigerant from the suction tube 115.

The second compressor 130 further includes a discharge port 132 to guide the refrigerant when the refrigerant is discharged from the second compressor 130. A fourth valve 144 is provided at a side of the discharge port 132 to guide the refrigerant from the second compressor 130 to the indoor unit 30. The fourth valve 144 may be a check valve configured to prevent a flow of the refrigerant from the indoor unit 30 to the discharge port 132.

The multi-step compression unit 100 further includes: an outdoor unit introduction tube 116 configured to guide to guide the refrigerant from the indoor unit 30 to the outdoor unit 20; and a fifth valve 145 configured to allow a flow of the refrigerant through the outdoor unit introduction tube 116.

When the air conditioner is operated in cooling mode, if the fifth valve 145 is opened, the refrigerant can flow from the indoor unit 30 to the outdoor unit 20 through the outdoor unit introduction tube 116.

The

tubes

111, 112, 113, and 114, and the

tubes

115, 116, 117, 118, and 123 may be considered as parts of the refrigerant tubes 110.

Hereinafter, with reference to the accompanying drawings, an explanation will be given on flows of a refrigerant in the multi-step compression unit 100 when the air conditioner is operated in cooling or heating mode.

Fig. 3 is a schematic diagram illustrating flows of a refrigerant when the air conditioner is operated in cooling mode according to an embodiment, and Fig. 4 is a schematic diagram illustrating flows of a refrigerant when the air conditioner is operated in heating mode according to an embodiment.

Referring to Fig. 3, when the air conditioner is operated in cooling mode, a refrigerant flows through the first compressor 21 and the outdoor heat exchanger 23 of the outdoor unit 20, and then the refrigerant flows into the sub-cooling heat exchanger 120 through the first tube 111.

In detail, at least a portion of the refrigerant branches off from the first tube 111 to the branch tube 123 and flows to the second heat exchanger 120b through the expansion device 125. The rest of the refrigerant flows to the first heat exchanger 120a.

Heat is exchanged at the first heat exchanger 120a and the second heat exchanger 120b. Since the refrigerant flowing in the branch tube 123 is decreased in pressure (temperature) at the expansion device 125, the refrigerant takes heat from the first heat exchanger 120a.

After passing through the second heat exchanger 120b, the refrigerant flows along the connection tube 118 and then meets the refrigerant flowing in the refrigerant tube 110. At this time, the first valve 141 may be opened, and the second valve 142 may be closed. As described above, before the refrigerant is introduced into the indoor unit 30, the refrigerant can be additionally cooled by the sub-cooling heat exchanger 120, cooling ability can be improved.

Thereafter, the combined refrigerant may be introduced into the indoor unit 30 through the third tube 113. The refrigerant flows through the indoor heat exchanger 33 of the indoor unit 30, and is then introduced into the multi-step compression unit 100 through the forth tube 114.

Then, the refrigerant is discharged from the multi-step compression unit 100 to the outdoor unit 20 through the outdoor unit introduction tube 116 and the second tube 112. At this time, the fifth valve 145 may be opened, and the third valve 143 may be closed.

Referring to Fig. 4, when the air conditioner is operated in heating mode, the refrigerant flows through the first compressor 21 of the outdoor unit 20, and then the refrigerant flows into the sub-cooling heat exchanger 100 through the second tube 112.

The refrigerant flows along the second tube 112 and the suction tube 115, and then the refrigerant is introduced into the second compressor 130 through the suction port 131. At this time, the third valve 143 may be opened, and the fifth valve 145 may be closed.

The refrigerant introduced into the second compressor 130 is additionally compressed and is then discharged to the indoor unit 30 through the discharge port 132 and the forth tube 114. Since the refrigerant is additionally compressed by the second compressor 130, the heating ability of the air conditioner can be improved.

The refrigerant introduced into the indoor unit 30 flows through the indoor heat exchanger 33 and the indoor expansion device 35, and then the refrigerant is discharged to the multi-step compression unit 100 through the third tube 113. Then, the refrigerant flows to the first heat exchanger 120a through the refrigerant tube 110. At this time, the first valve 141 may be closed to prevent the refrigerant from flowing to the connection tube 118.

After the refrigerant passes through the sub-cooling heat exchanger 120, at least a portion of the refrigerant is bypassed to the branch tube 123. The bypassed refrigerant is introduced into the second heat exchanger 120b through the expansion device 125.

The refrigerant flowing in the second heat exchanger 120b changes heat with the refrigerant flowing in the first heat exchanger 120a. As a result, the refrigerant flowing in the first heat exchanger 120a is sub-cooled, and the refrigerant flowing in the second heat exchanger 120b is decreased in temperature. The refrigerant passing through the first heat exchanger 120a may be introduced into the outdoor unit 20 through the first tube 111, and in the outdoor unit 20, the refrigerant may flow through the outdoor expansion device 25 and the outdoor heat exchanger 23.

The refrigerant passing through the second heat exchanger 120b may flow in the injection tube 117 and may then be injected into the second compressor 130 through the injection port 133. At this time, the second valve 142 may be opened, and the first valve 141 may be closed, so as to prevent the refrigerant passing through the second heat exchanger 120b from flowing into the connection tube 118 while allowing the refrigerant to flow to the injection port 133.

As described above, since at least a portion of the refrigerant flowing in the refrigerant tube 110 can be bypassed to the second compressor 130, a more amount of the refrigerant can be supplied to the second compressor 130 to improve the heating ability of the air conditioner.

The temperature of the refrigerant may abnormally be high after the refrigerant passes through the second compressor 130. However, since a refrigerant decompressed at the expansion device 125 can be introduced into the second compressor 130, the temperature of the refrigerant at the discharge port 132 of the second compressor 130 can be reduced.

The refrigerant injected into the second compressor 130 through the injection port 133 is mixed with the refrigerant introduced into the second compressor 130 through the suction port 131, and the mixed refrigerant is compressed and discharged to the indoor unit 30 through the discharge port 132. At this time, the fourth valve 144 may be opened.

Fig. 5 is a block diagram illustrating a control configuration of the air conditioner according to an embodiment, Fig. 6 is a flowchart for explaining a method of controlling the air conditioner in cooling mode according to an embodiment, and Fig. 7 is a flowchart for explaining a method of controlling the air conditioner in heating mode according to an embodiment.

Referring to Fig. 5, according to the current embodiment, the air conditioner includes: an input unit 51 through which an operation command can be input; a discharge temperature sensor 52 configured to detect the temperature of a refrigerant at the discharge port 132 of the second compressor 130; an outdoor temperature sensor 53 configured to detect the temperature of outdoor air; and a control unit 50 configured to control the above-listed units.

When the air conditioner is operated in heating mode, the required heating performance of the air conditioner may be varied according to outdoor air conditions (heating conditions). For example, if the outdoor air is at an extremely low temperature, the heating performance of the air conditioner may be low as compared with the case where outdoor dir is at a relatively high temperature. Therefore, if the temperature of outdoor air is equal to or lower than a set temperature, refrigerant may be injected to the second compressor 130 to increase the total amount of refrigerant supplied to the second compressor 130.

In addition, if the temperature (discharge temperature) of refrigerant at the discharge port 132 of the second compressor 130 is equal to or higher than a set temperature, the discharge temperature of the refrigerant can be reduced by injecting relatively cooler refrigerant into the second compressor 130 (that is, by injecting refrigerant passing through the outdoor expansion device 25 to the second compressor 130).

The amount of refrigerant to be injected to the second compressor 130 can be controlled by adjusting the opening degree of the expansion device 125. If heating conditions are bad (for example, if the temperature of outdoor air is equal to or lower than a first set temperature or the discharge temperature of refrigerant is equal to or higher than a second set temperature), the opening degree of the expansion device 125 can be increased to increase the amount of refrigerant to be injected to the second compressor 130.

According to the operation mode (cooling or heating mode) of the air conditioner determined by an operation command input through the input unit 51, the circulation direction of refrigerant may be controlled by opening or closing the first to fifth valves 141 to 145.

With reference to Fig. 6, an explanation will now be given on a method of controlling the air conditioner in cooling mode.

The air conditioner is turned on, and cooling mode is selected through the input unit 51 (S11).

In accordance with the cooling mode, flow rate control units such as the valves 141 to 145 may be on-off controlled. As described above, the first valve 141 and the fifth valve 145 may be opened, and the second valve 142, the third valve 143, and the fourth valve 144 may be closed (S12).

In addition, the expansion device 125 may be turned on. Refrigerant bypassed to the branch tube 123 is decompressed as the refrigerant passes through the expansion device 125, and then the refrigerant passes through the sub-cooling heat exchanger 120.

Refrigerant flowing in the refrigerant tube 110 and the refrigerant flowing in the branch tube 123 may exchange heat with each other and then meet each other. Thereafter, the refrigerant may be introduced into the indoor unit 30. That is, the refrigerant is sub-cooled while passing through the sub-cooling heat exchanger 120 and is then introduced into the indoor heat exchanger 33 (S13 and S14) so that the cooling ability of the air conditioner can be improved.

With reference to Fig. 7, an explanation will now be given on a method of controlling the air conditioner in heating mode.

The air conditioner is turned on, and heating mode is selected through the input unit 51 (S21).

In accordance with the heating mode, flow rate control units such as the valves 141 to 145 may be on-off controlled. As described above, the second valve 142, the third valve 143, and the fourth valve 144 may be opened, and the first valve 141 and the fifth valve 145 may be opened (S22).

A heating condition (outdoor air condition) may be detected using the outdoor temperature sensor 53, and the discharge temperature of refrigerant of the second compressor 130 may be detected using the discharge temperature sensor 52. Based on the heating condition or the discharge temperature, the amount of refrigerant to be injected into the second compressor 130 may be determined.

In addition, the expansion device 125 may be on-off controlled based on the detected heating condition or discharge temperature. Refrigerant decompressed at the expansion device 125 may be injected into the second compressor 130 by opening the second valve 142.

In detail, if the expansion device 125 is turned on, refrigerant bypassed to the branch tube 123 is decompressed while passing through the expansion device 125 and is then supplied to the sub-cooling heat exchanger 120.

After passing through the sub-cooling heat exchanger 120, the refrigerant is injected into the second compressor 130 so that the total amount of refrigerant passing through the second compressor 130 can be increased. In this way, the heating ability of the air conditioner can be improved, or the discharge temperature of refrigerant at the second compressor 130 can be reduced.

The amount of refrigerant to be injected to the second compressor 130 can be controlled by adjusting the opening degree of the expansion device 125. If heating conditions are bad (for example, if the temperature of outdoor is extremely low or the discharge temperature of refrigerant is equal to or higher than a set temperature), the opening degree of the expansion device 125 can be increased to increase the amount of refrigerant to be injected to the second compressor 130.

In this way, when the air conditioner is operated in heating mode, a portion of refrigerant circulating in a refrigerant system may be bypassed to inject the portion to the second compressor so as to increase the total amount of refrigerant flowing in the second compressor. As a result, the heating ability of the air conditioner can be improved, and the discharge temperature at the second compressor can be decreased.

In addition, since refrigerant introduced into the second compressor 130 through the suction tube 115 for being additionally compressed, the systematic ability of the air conditioner can be improved (S23, S24, and S25).

According to the embodiments, refrigerant can be sub-cooled while the air conditioner is operated in cooling mode so as to improve the cooling ability of the air conditioner, and refrigerant can be injected into to compressor while the air conditioner is operated in heating mode so as to increase the amount of refrigerant flowing in the compressor and thus to improve the heating ability of the air conditioner. Therefore, the industrial applicability of the air conditioner is high.

Claims

An air conditioner comprising:

an outdoor unit disposed in an outdoor area, the outdoor unit comprising a first compressor configured to compress a refrigerant and an outdoor heat exchanger configured to exchange heat with outdoor air;

an indoor unit disposed in an indoor area, the indoor unit comprising an indoor heat exchanger configured to exchange heat with indoor air;

a multi-step compression unit connecting the outdoor unit and the indoor unit;

an inlet tube through which the refrigerant discharged from the outdoor unit or the indoor unit is introduced into the multi-step compression unit; and

an outlet tube through which the refrigerant is discharged to the outdoor unit or the indoor unit after passing through the multi-step compression unit,

wherein the multi-step compression unit comprises:

a second compressor configured to compress the refrigerant after the refrigerant is discharged from the first compressor; and

a sub-cooling heat exchanger at which the refrigerant condensed at the outdoor heat exchanger or the indoor heat exchanger is sub-cooled.
The air conditioner according to claim 1, wherein the multi-step compression unit further comprises:

a main tube in which the refrigerant flows after passing through the inlet tube;

a branch tube branching off from the main tube; and

an expansion device configured to expand the refrigerant flowing in the branch tube and direct the refrigerant to the sub-cooling heat exchanger.
The air conditioner according to claim 2, wherein the multi-step compression unit further comprises:

an injection tube extending from the sub-cooling heat exchanger to the second compressor so as to inject the refrigerant to the second compressor after the refrigerant passes through the sub-cooling heat exchanger; and

a second valve configured to be selectively opened so as to selectively allow the refrigerant to flow in the injection tube after passing through the sub-cooling heat exchanger.
The air conditioner according to claim 3, wherein the refrigerant is injected to the second compressor through an injection port after the refrigerant passes through the sub-cooling heat exchanger if the air conditioner is operated in heating mode.
The air conditioner according to claim 1, wherein the multi-step compression unit further comprises:

a suction tube through which the refrigerant is sucked into the second compressor after being compressed by the first compressor; and

a third valve configured to be opened so as to selectively allow the refrigerant to flow in the suction tube.
The air conditioner according to claim 5, wherein the refrigerant is additionally compressed at the second compressor after being compressed at the first compressor if the air conditioner is operated in heating mode.
The air conditioner according to claim 6, wherein the multi-step compression unit further comprises a fourth valve disposed at an outlet side of the second compressor so as to introduce the refrigerant additionally compressed at the second compressor into the indoor unit.
The air conditioner according to claim 2, wherein the multi-step compression unit further comprises:

a connection tube through which a portion of the refrigerant passing through the sub-cooling heat exchanger is mixed with a portion of the refrigerant flowing in the main tube; and

a first valve configured to be selectively opened so as to introduce the portion of the refrigerant into the connection tube.
The air conditioner according to claim 8, wherein the portions of the refrigerant mixed through the connection tube are introduced into the indoor unit through the outlet tube if the air conditioner is operated in cooling mode.
The air conditioner according to claim 2, further comprising:

a discharge temperature sensor configured to detect a discharge temperature of the refrigerant at an outlet side of the second compressor; and

an outdoor temperature sensor configured to detect a temperature of outdoor air,

wherein an opening degree of the expansion device is controlled based on temperatures detected by the discharge temperature sensor or the outdoor temperature sensor.
A method of controlling an air conditioner comprising an outdoor unit and an indoor unit, the outdoor unit comprising a first compressor and an outdoor heat exchanger, the indoor unit comprising an indoor heat exchanger, the method comprising:

allowing a refrigerant compressed by the first compressor to condense at the outdoor heat exchanger or the indoor heat exchanger;

introducing the refrigerant into a multi-step compression unit after the refrigerant is discharged from the outdoor heat exchanger or the indoor heat exchanger;

expanding at least a portion of the refrigerant introduced into the multi-step compression unit; and

allowing the expanded portion of the refrigerant to exchange heat at a sub-cooling heat exchanger of the multi-step compression unit.
The method according to claim 11, wherein the expanded portion of the refrigerant exchanges heat at the sub-cooling heat exchanger with the rest of the refrigerant introduced into the multi-step compression unit if the air conditioner is operated in cooling mode.
The method according to claim 12, wherein the expanded portion of the refrigerant is mixed with the rest of the refrigerant after the expanded portion of the refrigerant exchanges heat at the sub-cooling heat exchanger.
The method according to claim 11, wherein the method further comprises injecting the refrigerant into a second compressor of the multi-step compression unit after the refrigerant changes heat at the sub-cooling heat exchanger if the air conditioner is operated in heating mode.
The method according to claim 11, wherein the refrigerant is additionally compressed at the second compressor after being compressed at the first compressor if the air conditioner is operated in heating mode.