EP2224180A1

EP2224180A1 - Multi-unit air conditioner, outdoor unit thereof and method of controlling refrigerant pressure

Info

Publication number: EP2224180A1
Application number: EP10154664A
Authority: EP
Inventors: Satoshi Watanabe; Shinichi Isozumi; Keisuke Mitoma; Tatsuhiro Yasuda
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Thermal Systems Ltd
Priority date: 2009-02-25
Filing date: 2010-02-25
Publication date: 2010-09-01
Anticipated expiration: 2030-02-25
Also published as: JP2010196985A; EP2224180B8; JP5570739B2; EP2224180B1

Abstract

The opening control of a pressure reducing expansion valve 40 is performed only when the refrigerant pressure on the high-pressure side in an outdoor unit 10 is high during a cooling operation. The opening of the valve 40 is determined on the basis of the driving frequency of a compressor 16 at that point in time, i.e., the volumetric flow rate of a refrigerant. A pressure reduction in the valve 40 is suppressed when the volumetric flow rate of the refrigerant is large and a pressure loss in a communication pipe 30 is large, and the pressure reduction in the valve 40 is increased when the volumetric flow rate of the refrigerant is small and a pressure loss in a communication pipe 30 is small, whereby the amount of pressure reduction of the refrigerant is adjusted in the total refrigerant circuit from the compressor 16 to an indoor expansion valve 22.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a multi-unit air conditioner in which a plurality of indoor units are provided for one outdoor unit, an outdoor unit thereof, and a method of controlling the multi-unit air conditioner.

Description of the Related Art

In buildings and the like, a multi-unit air conditioner in which a plurality of indoor units are provided for one outdoor unit is used.
An outdoor unit and each indoor unit of a multi-unit air conditioner are connected via refrigerant pipes and electrical wires.
In a building having a plurality of floors, an outdoor unit is often installed on the roof, and a pressure difference of a refrigerant, what is called a head difference due to a difference in installation height occurs between the outdoor unit and the indoor units installed in each floor of the building.
When the head difference increases in the case where the outdoor unit is installed above the indoor units, the refrigerant pressure acting on the high-pressure side of the indoor unit side, i.e., expansion valves of the indoor units increases. If the refrigerant pressure acting on the expansion valves becomes excessive, damage to the expansion valves and the like may occur. In order to avoid this, an upper limit to the refrigerant pressure in a refrigerant circuit is set and in the design stage a level difference between the outdoor unit and the indoor units is set on the basis of this upper limit.
Nevertheless, the refrigerant pressure acting on the high-pressure side of the indoor unit side, i.e., the expansion valves of the indoor units increases, particularly during cooling operations, under various conditions such as the operation load condition of a plurality of the indoor units and the outdoor air temperature. Particularly during cooling operations, the refrigerant pressure may exceed the upper limit set in the design stage.
Even when the refrigerant pressure does not exceed the upper limit, due to an increase in the head difference, the refrigerant pressure acting on the expansion valves of the indoor units increases more than in the case of installation on a flat place at ground-level. For this reason, the pressure difference between before and behind the expansion valves increases.
Then, when the opening of the expansion valves is changed in order to control the refrigerant flow rate of individual indoor units and for other purposes, even with the same amount of change of the opening, the larger the pressure difference between before and behind the expansion valves, the larger the amount of change of the refrigerant flow rate, with the result that the controllability of the refrigerant flow rate worsens.
In order to solve such problems, proposals have been made for the control of the opening of an expansion valve in the outdoor unit side so that the pressure of the refrigerant flowing into an expansion valve in the indoor unit in operation becomes not more than a predetermined reference value (refer to Japanese Patent Laid-Open No. 2008-185292 , for example).
In Japanese Patent Laid-Open No. 2008-185292 above, it is necessary to set a target value of a pressure loss of the refrigerant in the expansion valve of the outdoor unit in operation on the basis of an installation level difference between the outdoor unit and the indoor units. This is because during an operation, the opening of the expansion valve of the outdoor unit is controlled so that a pressure loss of the refrigerant in the expansion valve of the outdoor unit is maintained at a set target value.
However, in the case of such a technique, during an operation it is necessary to constantly calculate the circulating volume of the refrigerant in the refrigerant circuit, and the control load in a control section is large.
Because it is necessary to set a target value of the pressure of the refrigerant on the basis of an installation level difference between the outdoor unit and the indoor units, it becomes necessary to perform setting work and the like suited to the building in which the multi-unit air conditioner is installed, and this requires time and labor. Furthermore, if a mistake is made in setting work, it is impossible to perform pressure control as expected.
An upper limit to the refrigerant pressure in the refrigerant circuit is set in anticipation of such a high-load case where the outdoor air temperature is high and hence cooling operations are intensely performed in many of the indoor units. Thus, there is a limit to the level difference between the outdoor unit and the indoor units. Therefore, it is impossible to install the outdoor unit and the indoor units with a level difference greater than the limit.
The present invention has been made in view of such technical problems, and it is the object of the invention to provide a multi-unit air conditioner which is easily installed even when an installation level difference between an outdoor unit and indoor units is great, prevents the refrigerant pressure in a refrigerant circuit from becoming excessive without performing complicated control, and enables the outdoor unit and the indoor units to be installed with a level difference greater than conventionally, an outdoor unit thereof, and a method of controlling the multi-unit air conditioner.

SUMMARY OF THE INVENTION

The multi-unit air conditioner of the present invention having the above object is a multi-unit air conditioner comprising an outdoor unit provided with a compressor which compresses a refrigerant and a plurality of indoor units provided with an indoor heat exchanger. This multi-unit air conditioner comprises a pressure reducing expansion valve which reduces the pressure of the refrigerant discharged from the outdoor unit, and a control section which adjusts the opening of the pressure reducing expansion valve, during a cooling operation, on the basis of the rotation speed of the compressor.
As described above, the opening of the pressure reducing expansion valve is adjusted, during a cooling operation, on the basis of the rotation speed of the compressor and the pressure of the refrigerant discharged from the outdoor unit is reduced, whereby the refrigerant pressure is prevented from becoming excessive on the high-pressure side of the expansion valve of the indoor unit.
At this time, the control section adjusts the opening of the pressure reducing expansion valve on the basis of the rotation speed of the compressor, i.e., the volumetric flow rate of the refrigerant discharged from the compressor. Concretely, the control section adjusts the opening of the pressure reducing expansion valve by setting the opening of the pressure reducing expansion valve at a large value when the rotation speed of the compressor is high and by setting the opening of the pressure reducing expansion valve at a small value when the rotation speed of the compressor is low. The reason for this is that the amount of pressure reduction in the pressure reducing expansion valve is optimally adjusted in consideration of a pressure loss in a communication pipe which connects the outdoor unit and the indoor units.
The multi-unit air conditioner may further comprise a pressure sensor which detects the refrigerant pressure on the high-pressure side of the compressor of the outdoor unit or on the high-pressure side of an expansion valve of the indoor unit, and the control section may be adapted to adjust the opening of the pressure reducing expansion valve when the refrigerant pressure detected by the pressure sensor has exceeded a predetermined reference value. This enables the control for adjusting the opening of the pressure reducing expansion valve to be performed only when the refrigerant pressure detected by the pressure sensor has exceeded a predetermined reference value.
When the pressure sensor is provided on the high-pressure side of the expansion valve of the indoor unit, it is preferred that the pressure sensor be provided only in one indoor unit having the greatest head difference from the outdoor unit, because the cost would increase if pressure sensors are provided in all of the plurality of indoor units.
The outdoor unit may also comprise a supercooling heat exchanger which supercools a liquid refrigerant, and the pressure reducing expansion valve may be provided on the downstream side of the supercooling heat exchanger. The pressure reducing expansion valve may be provided on the upstream side of the supercooling heat exchanger.
The present invention may also provide an outdoor unit of a multi-unit air conditioner comprising a plurality of indoor units, which comprises a compressor which compresses a refrigerant supplied to the plurality of indoor units, a pressure reducing expansion valve which reduces the pressure of the refrigerant discharged from the outdoor unit, and a control section which adjusts the opening of the pressure reducing expansion valve, during a cooling operation, on the basis of the rotation speed of the compressor.
The present invention may also provide a method of controlling a multi-unit air conditioner comprising an outdoor unit provided with a compressor and a plurality of indoor units provided with an indoor heat exchanger. In this case, the control method comprises detecting the refrigerant pressure on the high-pressure side of the compressor of the outdoor unit or on the high-pressure side of an expansion valve of the indoor unit, and adjusting the opening of the pressure reducing expansion valve when the detected refrigerant pressure has exceeded a predetermined reference value. The adjusting of the opening of the pressure reducing expansion valve involves detecting the rotation speed of the compressor and adjusting the opening of the pressure reducing expansion valve for reducing the pressure of a refrigerant discharged from the outdoor unit on the basis of the detected rotation speed of the compressor.
According to the present invention, the opening of the pressure reducing expansion valve is adjusted, during a cooling operation, on the basis of the rotation speed of the compressor and the pressure of the refrigerant discharged from the outdoor unit is reduced, whereby the refrigerant pressure is prevented from becoming excessive on the high-pressure side of the expansion valve of the indoor unit. At this time, the opening adjustment of the pressure reducing expansion valve is performed on the basis of the rotation speed of the compressor, i.e., the volumetric flow rate of the refrigerant discharged from the compressor. As a result, the amount of pressure reduction in the pressure reducing expansion valve is appropriately adjusted by setting the opening of the pressure reducing expansion valve at a large value when the rotation speed of the compressor is high, that is, when the volumetric flow rate of the refrigerant discharged from the compressor is large and a pressure loss in the pipe is large and by setting the opening of the pressure reducing expansion valve at a small value contrastingly when the rotation speed of the compressor is low and a pressure loss in the pipe is small.
As a result, even when the installation height difference between the outdoor unit and the indoor units is great, the refrigerant pressure in the refrigerant circuit is prevented from becoming excessive and it is possible to install the outdoor unit and the indoor units with an unprecedentedly great level difference. Furthermore, it is possible to directly adjust the opening of the pressure reducing expansion valve by use of a predetermined correlation map (an opening map) and the like on the basis of the rotation speed of the compressor. This requires no control involving complicated calculations and thus enables to reduce the control load. In addition, it becomes unnecessary to perform prior setting work suited to a building where the multi-unit air conditioner is to be installed. Thus the installation becomes easy.
Because the opening of the pressure reducing expansion valve is adjusted when the refrigerant pressure on the high-pressure side of the compressor of the outdoor unit or the high-pressure side of the expansion valve of the indoor unit has exceeded a reference value, the intervention of the processing for the adjustment of the opening of the pressure reducing expansion valve does not occur so long as the refrigerant pressure does not exceed a reference value. Also in this respect, it is possible to reduce the control load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the configuration of a multi-unit air conditioner in an embodiment of the present invention;
FIG. 2 is a diagram showing the refrigerant circuit configuration of an outdoor unit;
FIG. 3 is a diagram showing the whole of the flow of processing for the adjustment of a pressure reducing expansion valve opening;
FIG. 4 is a diagram showing an example of an opening map used in the processing for the adjustment of a pressure reducing expansion valve opening, in which the relationship between the compressor driving frequency and the pressure reducing expansion valve opening is shown;
FIGS. 5A, 5B, 5C and 5D are diagrams showing the relationships between the compressor driving frequency and the high-pressure side refrigerant pressure, pressure loss, liquid-head load pressure and the refrigerant pressure just before the indoor expansion valve, respectively;
FIG. 6 is a Mollier diagram in this embodiment;
FIG. 7 is a Mollier diagram in a conventional configuration;
FIG. 8 is a diagram showing another example of the refrigerant circuit configuration of the outdoor unit in the multi-unit air conditioner of the present invention; and
FIG. 9 is a Mollier diagram in the configuration of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter the present invention will be described in detail on the basis of the embodiment shown in the accompanying drawings.
FIG. 1 is a diagram showing the configuration of a multi-unit air conditioner in this embodiment.
As shown in FIG. 1, the multi-unit air conditioner comprises a plurality of indoor units 20A, 20B, ... installed on each floor in a building 100 for an outdoor unit 10, which is installed mainly on the roof of the building 100.
The outdoor unit 10 and each of the indoor units 20A, 20B, ... are connected to each other by a refrigerant pipe which feeds a refrigerant and a communication pipe 30 which includes an electrical wire transmitting control signals and the like.
Each of the indoor units 20A, 20B, ... comprises an indoor heat exchanger 21, an indoor expansion valve 22, a sub-controller composed of a computer, and a room temperature sensor (not shown). The temperature in the rooms where the indoor units 20A, 20B, ... are installed is detected by the room temperature sensors, and the sub-controller controls the opening of the indoor expansion valve 22 so that the indoor temperature detected by the room temperature sensors becomes close to a target temperature set by the user, thereby adjusting the amount of heat exchange in the indoor heat exchanger 21.
As shown in FIG. 2, the outdoor unit 10 is composed mainly of a receiver 11, an outdoor expansion valve 12, an outdoor heat exchanger 13, a four-way valve 14, an accumulator 15, and a compressor 16. Downstream of the compressor 16, in a pipe between the compressor 16 and the four-way valve 14 there is provided a pressure sensor 17 which detects the high-pressure side pressure of the refrigerant whose pressure is raised by the compressor 16.
In the outdoor unit 10, a main controller (a control section) composed of a computer, which is not shown, controls the rotation speed of the compressor 16, the opening of the outdoor expansion valve 12 and the like on the basis of electrical information sent from the outdoor temperature sensor and each of the indoor units 20A, 20B, ... The main controller performs the switching between heating and cooling by use of the four-way valve 14. Since the receiver 11, the outdoor expansion valve 12, the outdoor heat exchanger 13, the four-way valve 14, the accumulator 15, and the compressor 16 are well-known elements the descriptions of the configurations and functions of these elements are omitted.
In this embodiment, the outdoor unit 10 comprises a supercooling heat exchanger 35 which supercools the refrigerant having passed through the receiver 11. The supercooling heat exchanger 35 is of a double-pipe construction provided with an inner pipe 35a and an outer pipe 35b. Part of the liquid refrigerant is divided at the outlet of the receiver 11 and is fed into the inner pipe 35a after being reduced in pressure by the expansion valve 35c. This pressure-reduced refrigerant is evaporated in the inner pipe 35a, whereby the refrigerant flowing in the outer pipe 35b is cooled in a supercooled state. The refrigerant supercooled in the outer pipe 35b of the supercooling heat exchanger 35 is fed to the indoor units 20A, 20B, ... and the refrigerant evaporated in the inner pipe 35a is fed into the accumulator 15.
Furthermore, the outdoor unit 10 comprises a pressure reducing expansion valve 40 which reduces the pressure of the condensed refrigerant having passed through the outdoor heat exchanger 13 and the receiver 11. This pressure reducing expansion valve 40 is juxtaposed with a check valve 41 and lets the refrigerant to flow into the pressure reducing expansion valve 40 only during a cooling operation. In this embodiment, during a cooling operation the pressure reducing expansion valve 40 is on the downstream side of the supercooling heat exchanger 35.
In this pressure reducing expansion valve 40, the opening thereof is controlled by the main controller of the outdoor unit 10.
Hereinafter details of the opening control of the pressure reducing expansion valve by the main controller will be described referring to FIG. 3. The main controller automatically performs the opening control of the pressure reducing expansion valve, which will be described below, when a processing section such as a CPU performs predetermined processing in conjunction with a storage section such as a memory on the basis of a computer program stored beforehand in the storage section.
The main controller performs the opening control of the pressure reducing expansion valve 40 only during a cooling operation. For this reason, as shown in FIG. 3, when the main controller starts a cooling operation (Step S101), it detects, at intervals of a predetermined given time, the high-pressure side pressure of the refrigerant whose pressure has been raised by the compressor 16, and which is outputted from the pressure sensor 17 (Step S102).
A judgment is made as to whether or not the detected refrigerant pressure exceeds a predetermined threshold value (a reference value), for example, 3.0 MPa (Step S103). As a result, when the detected refrigerant pressure does not exceed a predetermined threshold value, the flow of processing returns to Step S102 and the processing is repeated. On the other hand, when the detected refrigerant pressure exceeds the predetermined threshold value, the opening adjustment of the pressure reducing expansion valve 40 is performed as will be described in detail later (Step S104).
The flow of processing returns to Step S102 if the cooling operation is continued after the opening adjustment of the pressure reducing expansion valve, and a series of processing is finished unless the cooling operation is continued (Step S105).
Now the processing for the opening adjustment of the pressure reducing expansion valve 40 will be described in detail.
The processing for the opening adjustment of the pressure reducing expansion valve 40 is performed on the basis of an opening map as shown in FIG. 4, for example.
The main controller recognizes the driving frequency (rotation speed) of the compressor 16 for the purpose of operating the outdoor unit 10. The main controller also recognizes the opening of the pressure reducing expansion valve 40.
As shown in FIG. 4, information on an opening map in which the driving frequency of the compressor 16 and the opening of the pressure reducing expansion valve 40 are related to each other beforehand is stored in the main controller.
When the flow of processing has proceeded to the processing for the opening adjustment of the pressure reducing expansion valve 40 at Step S104, the main controller adjusts the opening of the pressure reducing expansion valve 40 to an opening suited to the driving frequency of the compressor 16 at that point in time on the basis of the information on the opening map.
It is possible to configure the opening of the pressure reducing expansion valve 40 in such a manner that as shown by a line indicated by the symbol (A) in FIG. 4, the opening of the pressure reducing expansion valve 40 is in a directly proportional relation to the driving frequency of the compressor 16 and hence when the driving frequency of the compressor 16 increases or decreases, also the opening of the pressure reducing expansion valve 40 increases or decreases continuously. With this configuration, however, the opening and closing actions of the pressure reducing expansion valve 40 are frequently performed in synchronization with changes in the driving frequency of the compressor 16. Therefore, as indicated by the symbol (B) in FIG. 4, it is preferable to adopt an opening map in which the opening of the pressure reducing expansion valve 40 is set in a staircase pattern with respect to the driving frequency of the compressor 16. That is, the range of the driving frequency of the compressor 16 is divided into a plurality of divisions, for example, frequencies F0 to F1, F1 to F2, F2 to F3, ... and in each division, the opening and the driving frequency are related to each other, with the openings W1, W2, ... of the pressure reducing expansion valve 40 kept constant. As a result, the opening of the pressure reducing expansion valve 40 is set at a plurality of stages according to the driving frequency of the compressor 16.
When the driving frequency of the compressor 16 is in the vicinity of a frequency at which the opening of the pressure reducing expansion valve 40 changes over to another stage, what is called the hunting phenomenon may sometimes occur, that is, the opening of the pressure reducing expansion valve 40 changes frequently in response to changes in the driving frequency of the compressor 16. For this reason, as indicated by the symbols (B) and (C) in FIG. 4, it is preferable to adopt what is called hysteresis control which involves adopting different values as the driving frequency of the compressor 16 at which the opening of the pressure reducing expansion valve 40 is changed for the case where the opening of the pressure reducing expansion valve 40 is increased and the case where the opening of the pressure reducing expansion valve 40 is decreased.
In the opening map of FIG. 4, the map configuration is such that the opening of the pressure reducing expansion valve 40 is increased when the driving frequency of the compressor 16 is large, whereas the opening of the pressure reducing expansion valve 40 is decreased when the driving frequency of the compressor 16 is small.
In the present invention, the pressure of the refrigerant discharged from the compressor 16 and fed into the indoor expansion valve 22 is reduced by the pressure reducing expansion valve 40. Also a pressure loss of the refrigerant in the communication pipe 30 has an effect on the pressure of the refrigerant discharged from the compressor 16 and fed into the indoor expansion valve 22. For this reason, the opening map of the pressure reducing expansion valve 40 is set in consideration of a pressure loss of the refrigerant in the communication pipe 30.
As shown in FIG. 5A, the refrigerant pressure on the high-pressure side of the compressor increases when the driving frequency of the compressor 16 increases. Also, as shown in FIG. 5B, a pressure loss in the communication pipe 30 increases as an absolute value with increasing driving frequency of the compressor 16. On the other hand, as shown in FIG. 5C, the liquid head load pressure due to a level difference between the outdoor unit 10 and the indoor units 20A, 20B, ... is almost constant regardless of the driving frequency of the compressor 16 so long as no great density change occurs. Therefore, all these considered, as shown in FIG. 5D, the refrigerant pressure just before the indoor expansion valve 22 increases with increasing driving frequency of the compressor 16.
That is, when the driving frequency of the compressor 16 is large, the flow rate (volumetric flow rate) of the refrigerant discharged from the compressor 16 is large and the refrigerant pressure on the high-pressure side of the compressor also increases. However, a pressure reduction is possible in the communication pipe 30 because of a large pressure loss in the pipe 30 itself. In addition, the degree of a pressure reduction in the pressure reducing expansion valve 40 is lowered by increasing the opening of the expansion valve 40 in order to prevent the insufficient capacity of the expansion valve 40 which might be caused by the resistance of the expansion valve 40 itself. On the other hand, when the driving frequency of the compressor 16 is small, the flow rate of the refrigerant discharged from the compressor 16 is small and the pressure loss in the communication pipe 30 is small. Therefore, the degree of a pressure reduction in the pressure reducing expansion valve 40 is raised by reducing the opening of the pressure reducing expansion valve 40.
It is possible for the pressure reducing expansion valve 40 to reduce the pressure of the refrigerant discharged from the outdoor unit 10 in the manner described above. FIGS. 6 and 7 are Mollier diagrams to make a comparison between the configuration of this embodiment and a conventional configuration in which the pressure reducing expansion valve 40 is not provided.
As shown in FIG. 7, in the conventional configuration, in the communication pipe between the outdoor unit exit X1 and the indoor unit inlet X2, the refrigerant pressure increases due to a head difference between the outdoor unit and the indoor unit. As a result, on the high-pressure side of the indoor unit, the refrigerant pressure may increase excessively.
On the other hand, as shown in FIG. 6, according to the configuration shown in this embodiment, it is possible for the pressure reducing expansion valve 40 (in the position of the symbol X3 in FIG. 6) to reduce the pressure of the refrigerant discharged from the outdoor unit 10. As a result, it is possible to prevent the refrigerant pressure from increasing excessively on the inlet side of the indoor unit even when the refrigerant pressure increases due to a head difference between the outdoor unit and the indoor unit in the communication pipe between the outdoor unit exit X1 and the indoor unit inlet X2.
The opening of the pressure reducing expansion valve 40 is adjusted in this manner, whereby it is possible to reduce the pressure of the refrigerant supplied to the indoor units 20A, 20B, ... This makes it possible to prevent an excessive refrigerant pressure from acting on the indoor expansion valve 22 in the indoor units 20A, 20B, ... As a result, it is possible not only to prevent damage to the indoor expansion valve 22, but also to suppress a differential pressure before and behind the indoor expansion valve 22.
Because the opening control of the pressure reducing expansion valve 40 is performed on the basis of the refrigerant pressure detected by the pressure sensor 17 and the driving frequency of the compressor 16, it is unnecessary to perform setting work for performing this opening adjustment for each building 100 where this multi-unit air conditioner is installed, and hence installation becomes easy.
Furthermore, the opening control of the pressure reducing expansion valve 40 is performed only when the refrigerant pressure on the high-pressure side in the outdoor unit 10 is high during a cooling operation. Therefore, unnecessary intervention of the opening control of the pressure reducing expansion valve 40 does not occur, and the opening control is performed only when the load is high as in a case where the outdoor temperature is high and a cooling operation is performed intensely in many of the indoor units 20A, 20B, ... Hence it is possible to suppress the control load.
In addition, in the opening control of the pressure reducing expansion valve 40, the opening of the pressure reducing expansion valve 40 is determined on the basis of the driving frequency of the compressor 16 at that point in time, i.e., the volumetric flow rate of the refrigerant. As a result, a pressure reduction in the pressure reducing expansion valve 40 is suppressed when the volumetric flow rate of the refrigerant is large and a pressure loss in the communication pipe 30 is large. A pressure reduction in the pressure reducing expansion valve 40 is increased when the volumetric flow rate of the refrigerant is small and a pressure loss in the communication pipe 30 is small. Thereby, it is possible to adjust the amount of pressure reduction of the refrigerant in the total refrigerant circuit from the compressor 16 to the indoor expansion valve 22. As a result, it is possible to constantly appropriately reduce the refrigerant pressure regardless of the operation condition, and it is possible to prevent an excessive refrigerant pressure from acting on the indoor expansion valve 22.
Thus, it becomes possible to install the multi-unit air conditioner of this embodiment even when the head difference, i.e., the level difference between the outdoor unit 10 and the indoor units 20A, 20B, ... is greater than conventionally.
Although in the above-described embodiment, the pressure reducing expansion valve 40 is arranged on the downstream side of the supercooling heat exchanger 35, the arrangement of the pressure reducing expansion valve 40 is not limited to this. For example, as shown in FIG. 8, the pressure reducing expansion valve 40 may be arranged on the upstream side of the supercooling heat exchanger 35. In this case, as shown in FIG. 9, after the pressure reduction of the refrigerant is performed by the pressure reducing expansion valve 40 provided before the outlet of the outdoor unit 10, the refrigerant flows through the outlet of the outdoor unit 10 and the refrigerant pressure increases under the loading of the liquid head in the communication pipe 30. Also in this arrangement, the same effect as in the above-described embodiment can be obtained.
In this case, because the refrigerant flows through the supercooling heat exchanger after a pressure reduction in the pressure reducing expansion valve 40, it is possible to perform supercooling to a great extent at the outlet of the outdoor unit 10. As a result, a two-phase flow is less apt to occur at the inlet of the expansion valve of each of the indoor units 20A, 20B, ... and thus the occurrence of non-cooling due to the choke of the indoor expansion valve 22 and the generation of flowing noises of the refrigerant are suppressed.
The processing for the opening adjustment of the pressure reducing expansion valve 40 is not limited to the configuration shown in the above-described embodiment. For example, it may open and close the pressure reducing expansion valve 40 only by a given opening when the opening adjustment of the pressure reducing expansion valve 40 is performed. For example, in a case where the opening of the pressure reducing expansion valve 40 is divided into 50 stages, in performing the opening control of the pressure reducing expansion valve 40, the opening of the pressure reducing expansion valve 40 is reduced only by an amount corresponding to two stages, for example. In this case, in order to ascertain whether or not the refrigerant pressure on the upstream side of the indoor expansion valve 22 has decreased due to the reduction of the opening of the pressure reducing expansion valve 40, it is preferable to perform what is called feedback control which involves monitoring the refrigerant pressure after the opening adjustment by use of the refrigerant pressure sensor provided just before the indoor expansion valve 22 or the pressure sensor 17 of the outdoor unit 10, and making a judgment as to whether or not repeating a further reduction of the opening of the pressure reducing expansion valve 40 on the basis of the result of the monitoring.
Needless to say, an opening map used in the processing for the opening adjustment of the pressure reducing expansion valve 40 is not limited to the opening map shown in FIG. 4. It is possible to appropriately adopt other opening maps, for example, an opening map in which the opening of the pressure reducing expansion valve 40 changes exponentially with respect to the driving frequency of the compressor 16.
Furthermore, in the above-described embodiment, the flow of processing proceeds to the processing for the opening adjustment of the pressure reducing expansion valve 40 on the basis of the refrigerant pressure on the high-pressure side of the compressor 16, which is detected by use of the pressure sensor 17 of the outdoor unit 10. However, it is also possible to adopt a configuration in which this is performed on the basis of the refrigerant pressure detected by use of the pressure sensor provided just before the indoor expansion valve 22. In this case, however, if the indoor expansion valve 22 is provided for all of the plurality of the indoor units 20A, 20B, ..., this leads to an increase in cost. Consequently, it is also possible to adopt a configuration in which a pressure sensor is installed just before the indoor expansion valve 22 only for one indoor unit having the greatest head difference from the outdoor unit 10 among the plurality of indoor units 20A, 20B, ..., and the flow of processing is caused to proceed to the processing for the opening adjustment of the pressure reducing expansion valve 40 on the basis of the refrigerant pressure detected by this pressure sensor.
In addition to this configuration, it is possible to make a choice from the configurations mentioned in the above-described embodiment and to make appropriate changes to other configurations without departing from the spirit and scope of the present invention.

Claims

A multi-unit air conditioner comprising an outdoor unit (10) provided with a compressor (16) which compresses a refrigerant and a plurality of indoor units (20A, 20B) provided with an indoor heat exchanger (21), characterized in that it comprises:
a pressure reducing expansion valve (40) which reduces the pressure of the refrigerant discharged from the outdoor unit; and

a control section which adjusts an opening of the pressure reducing expansion valve (12), during a cooling operation, on the basis of the rotation speed of the compressor.
The multi-unit air conditioner according to claim 1, further comprising a pressure sensor (17) which detects the refrigerant pressure on the high-pressure side of the compressor (16) of the outdoor unit (10) or on the high-pressure side of an expansion valve (22) of the indoor unit (20A, 20B) ,
wherein the control section adjusts the opening of the pressure reducing expansion valve (40) when the refrigerant pressure detected by the pressure sensor (17) has exceeded a predetermined reference value.
The multi-unit air conditioner according to claim 1 or 2, wherein the control section adjusts the opening of the pressure reducing expansion valve (40) by setting the opening of the pressure reducing expansion valve (40) at a large value when the rotation speed of the compressor (17) is high and by setting the opening of the pressure reducing expansion valve (40) at a small value when the rotation speed of the compressor (17) is low.
The multi-unit air conditioner according to any one of claims 1 to 3, wherein the outdoor unit (10) comprises a supercooling heat exchanger (35) which supercools a liquid refrigerant discharged from the outdoor unit and
wherein the pressure reducing expansion valve (40) is provided on the downstream side of the supercooling heat exchanger (35).
An outdoor unit of a multi-unit air conditioner comprising a plurality of indoor units (20A, 20B) provided with an indoor heat exchanger (21), comprising
a compressor (17) which compresses a refrigerant supplied to the plurality of indoor units,
characterized in that it comprises
a pressure reducing expansion valve (40) which reduces the pressure of the refrigerant discharged from the outdoor unit; and
a control section which adjusts the opening of the pressure reducing expansion valve (40), during a cooling operation, on the basis of the rotation speed of the compressor (17).
A method of controlling a multi-unit air conditioner comprising an outdoor unit (10) provided with a compressor (17) which compresses a refrigerant and a plurality of indoor units (20A, 20B) provided with an indoor heat exchanger (21), comprising the steps of:
detecting a refrigerant pressure on the high-pressure side of the compressor (17) of the outdoor unit (10) or on the high-pressure side of an expansion valve (22) of the indoor unit; and

adjusting an opening of the pressure reducing expansion valve (40) when the detected refrigerant pressure has exceeded a predetermined reference value,

wherein the adjusting of the opening of the pressure reducing expansion valve (40) involves detecting a rotation speed of the compressor (17) and adjusting the opening of the pressure reducing expansion valve (40) for reducing the pressure of a refrigerant discharged from the outdoor unit (10) on the basis of the detected rotation speed of the compressor (17).