Method for controlling a vapor compression system with a bypass valve

Abstract

A method for controlling a vapor compression system (1) is disclosed. The vapor compression system (1) includes a compressor unit (2), a heat exchanger (5), a receiver (7), an expansion device (8), and an evaporator (9) arranged in a refrigerant path. The compressor unit includes at least one main compressor (3) and at least one receiver compressor (4). The vapor compression system (1) further includes a bypass valve (12) that fluidly interconnects the gas outlet (10) of the receiver (7) with the main compressor / multiple main compressors (3). A first pressure setpoint and a second pressure setpoint for the current pressure in the receiver (7) are defined, the second pressure setpoint being higher than the first pressure setpoint. When the load demand of the vapor compression system (1) exceeds the maximum capacity of the receiver compressor / multiple receiver compressors (4), the opening of the bypass valve (12) is controlled to adjust the current pressure in the receiver (7) according to the first pressure setpoint, and to reach the first pressure setpoint. When the receiver compressor / multiple receiver compressors (4) is operating and the load demand of the vapor compression system (1) is lower than the maximum capacity of the receiver compressor / multiple receiver compressors (4), the opening of the bypass valve (12) is controlled to adjust the current pressure in the receiver (7) according to the second pressure setpoint, and to reach the second pressure setpoint.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for controlling a vapour compression system having a receiver in order to maintain a desired pressure in the receiver. In the method according to the invention, the prevailing pressure in the receiver is regulated by controlling a receiver compressor or by controlling the opening degree of a bypass valve. Furthermore, a proper regulation of the prevailing pressure in the receiver is obtained in an energy efficient manner, regardless of the load demand of the vapour compression system. BACKGROUND

[0002] A vapour compression system, such as a refrigeration system, an air conditioning system or a heat pump, typically comprises a compressor unit comprising one or more compressors, an evaporator, an expansion device and a heat rejecting heat exchanger arranged in a refrigerant path. Thereby, refrigerant flowing in the refrigerant path is compressed by the compressor(s) of the compressor unit before being supplied to the heat rejecting heat exchanger. When passing through the heat rejecting heat exchanger, the refrigerant exchanges heat with the surroundings or an auxiliary fluid flowing through the heat rejecting heat exchanger in such a way that heat is rejected from the refrigerant. The refrigerant is then passed through the expansion device, where the refrigerant is subjected to an expansion and is then supplied to the evaporator. The refrigerant supplied to the evaporator is in the form of a mixture of gaseous refrigerant and liquid refrigerant. When passing through the evaporator, the liquid portion of the refrigerant evaporates while the refrigerant exchanges heat with the surroundings or an auxiliary fluid stream flowing through the evaporator in such a way that heat is absorbed by the refrigerant.

[0003] In some vapour compression systems, a receiver is arranged in the refrigerant path between the heat rejecting heat exchanger and the expansion device. In the receiver, the refrigerant is separated into a gaseous portion and a liquid portion. The liquid portion of the refrigerant is supplied to the expansion device via a liquid outlet in the above described manner. The gaseous portion of the refrigerant can be supplied to the compressor unit via a gas outlet. In this case, the gaseous refrigerant can be supplied to a suction line interconnecting the outlet of the evaporator and the compressor unit via a bypass valve. Alternatively, the gaseous refrigerant can be directly supplied to a dedicated receiver compressor, which does not receive refrigerant from the evaporator. Supplying the gaseous refrigerant to the receiver compressor is more energy efficient compared to supplying the gaseous refrigerant to the suction line via the bypass valve or to the expansion device, since no pressure drop is introduced thereby, and thus less energy is required to compress the refrigerant to the desired pressure level.

[0004] The prevailing pressure in the receiver must be maintained at a certain level in order to ensure that the vapour compression system operates in an appropriate manner. The prevailing pressure in the receiver can be adjusted by operating the receiver compressor and / or the bypass valve at an appropriate opening. It is desirable to apply the receiver compressor whenever possible, because, as mentioned above, this is more energy efficient than applying the bypass valve. However, when the flow of gaseous refrigerant leaving the receiver is low, it can be insufficient to maintain the receiver compressor to operate stably, thereby causing the receiver compressor to be repeatedly stopped and started, resulting in excessive wear of the receiver compressor. In such a situation, it is more desirable to apply the bypass valve.

[0005] Furthermore, in order to allow the prevailing pressure in the receiver to be adjusted by operating the receiver compressor in case of an exceptionally high load on the vapour compression system, sometimes the capacity of the receiver compressor is determined such that the maximum capacity of the receiver compressor is used rarely or even never. This increases the cost of manufacturing the vapour compression system.

[0006] Furthermore, when the prevailing pressure in the receiver is controlled by means of the receiver compressor as well as by means of the bypass valve, there is a risk that controlling the receiver compressor and controlling the bypass valve can conflict with each other.

[0007] EP 2 999 932 B1 (corresponding to WO 2014 / 179442 A1) discloses a method for pressure control in a CO2 refrigeration system. A controller receives pressure measurements from a pressure sensor within a receiver tank and operates both a gas bypass valve and a parallel compressor in response to the pressure measurements to control the pressure within the receiver tank. The controller compares the pressure within the receiver tank to a first threshold pressure and a second threshold pressure. When the pressure within the receiver tank is between the first threshold pressure and the second threshold pressure, the controller controls the pressure within the receiver tank using only the gas bypass valve, and when the pressure within the receiver tank exceeds the second threshold pressure, the controller controls the pressure within the receiver tank using both the gas bypass valve and the parallel compressor. SUMMARY

[0008] It is an object of embodiments of the present invention to provide a method for controlling a vapour compression system which allows the prevailing pressure in the receiver to be adjusted appropriately without the manufacturing cost of the vapour compression system being excessively high.

[0009] It is a further object of embodiments of the present invention to provide a method for controlling a vapour compression system in which the risk of a conflict between controlling the receiver compressor and controlling the bypass valve is minimised.

[0010] The present invention provides a method for controlling a vapour compression system, the vapour compression system comprising a compressor unit arranged in a refrigerant path, a heat rejecting heat exchanger, a receiver, an expansion device and an evaporator, the compressor unit comprising at least two compressors, at least one of the compressors being a main compressor fluidly connected to an outlet of the evaporator and at least one of the compressors being a receiver compressor fluidly connected to a gas outlet of the receiver, the vapour compression system further comprising a bypass valve fluidly interconnecting the gas outlet of the receiver and the main compressor / compressors, the method comprising the steps of:

[0011] - defining a first pressure set point for an existing pressure in the receiver and a second pressure set point for the existing pressure in the receiver, the second pressure set point being higher than the first pressure set point, and

[0012] - controlling an opening degree of the bypass valve,

[0013] wherein, in case the load demand of the vapour compression system exceeds the maximum capacity of the receiver compressor / compressors, the opening degree of the bypass valve is controlled so as to regulate the existing pressure in the receiver according to the first pressure set point and so as to reach the first pressure set point; and wherein, in case the receiver compressor / compressors are operating and the load demand of the vapour compression system is below the maximum capacity of the receiver compressor / compressors, the opening degree of the bypass valve is controlled so as to regulate the existing pressure in the receiver according to the second pressure set point and so as to reach the second pressure set point.

[0014] Thus, the present invention provides a method for controlling a vapour compression system. In the context of the present invention, the term "vapour compression system" shall be interpreted to mean any system in which a fluid medium flow, such as a refrigerant, is circulated and alternately compressed and expanded, thereby providing a volume of refrigeration or heating. Thus, the vapour compression system can be a refrigeration system, an air conditioning system, a heat pump, etc.

[0015] Thus, the vapour compression system comprises a compressor unit arranged in a refrigerant path, a heat rejecting heat exchanger, a receiver, an expansion device and an evaporator. The expansion device is arranged to control a refrigerant supply to the evaporator. The compressor unit comprises at least two compressors. At least one of the compressors is a main compressor fluidly connected to an outlet of the evaporator and at least one of the compressors is a receiver compressor fluidly connected to a gas outlet of the receiver. The vapour compression system further comprises a bypass valve fluidly interconnecting the gas outlet of the receiver and the main compressor / compressors.

[0016] Thus, the refrigerant flowing in the refrigerant path is compressed by the compressor of the compressor unit before being supplied to the heat rejecting heat exchanger. When the refrigerant passes through the heat rejecting heat exchanger, a heat exchange takes place between the refrigerant and the surrounding environment or an auxiliary fluid stream flowing through the heat rejecting heat exchanger in such a way that heat is rejected from the refrigerant. The heat rejecting heat exchanger can be in the form of a condenser, in which case the refrigerant is at least partially condensed when passing through the heat rejecting heat exchanger. As an alternative, the heat rejecting heat exchanger can be in the form of a gas cooler, in which case the refrigerant passing through the heat rejecting heat exchanger is cooled, but remains in a gaseous or transcritical state.

[0017] The refrigerant leaving the heat rejecting heat exchanger can be supplied to the receiver via a high pressure valve or an ejector. In the receiver, the refrigerant is separated into a liquid portion and a gaseous portion. The liquid portion of the refrigerant leaves the receiver via a liquid outlet and is supplied to the evaporator via an expansion device. In the expansion device, the refrigerant undergoes an expansion and the refrigerant supplied to the evaporator is in a mixed state of gaseous refrigerant and liquid refrigerant. In the evaporator, the liquid portion of the refrigerant is at least partially evaporated while a heat exchange takes place between the refrigerant and the surrounding environment or an auxiliary fluid stream flowing through the evaporator in such a way that heat is absorbed by the refrigerant. Finally, the refrigerant leaving the evaporator is supplied to the main compressor(s).

[0018] The gaseous portion of the refrigerant in the receiver can leave the receiver via a gas outlet and be supplied directly to the receiver compressor(s) or to the main compressor(s) via a bypass valve.

[0019] In the method according to the present application, first a first pressure setpoint for the prevailing pressure in the receiver and a second pressure setpoint for the prevailing pressure in the receiver are defined. The second pressure setpoint is higher than the first pressure setpoint. In the context of the present application, the term "pressure setpoint" is to be interpreted as meaning a pressure value that can be applied in a setpoint control of the prevailing pressure in the receiver, i.e. each of the first and second pressure setpoints represents a pressure level that can be selected as a target value for the prevailing pressure in the receiver of a control loop.

[0020] The first pressure setpoint can advantageously represent a pressure level that is desired in the receiver under given operating conditions. The prevailing pressure in the receiver should be maintained at a level that ensures that the vapour compression system operates properly and efficiently, e.g. by ensuring that there is a sufficient pressure difference across the expansion device to ensure that there is sufficient refrigerant supplied to the evaporator. How to select a proper pressure setpoint for the receiver pressure is known per se.

[0021] The second pressure setpoint is higher than the first pressure setpoint and can in certain situations be applied, which will be described in further detail below. The second pressure setpoint can be defined based on the first pressure setpoint, e.g. by adding a fixed offset value to the first pressure setpoint.

[0022] The opening degree of the bypass valve is controlled in the following way.

[0023] In case the load demand of the vapour compression system exceeds the maximum capacity of the receiver compressor(s), the opening degree of the bypass valve is controlled so as to adjust the prevailing pressure in the receiver according to the first pressure setpoint, i.e. the lower pressure setpoint, and so as to reach the first pressure setpoint, i.e. to reach a pressure level in the receiver equal to the first pressure setpoint.

[0024] In the context of the present invention, the term "load demand of the vapour compression system" shall be interpreted to mean the current demand for cooling or heating provided by the vapour compression system. In order for the vapour compression system to be able to meet this demand, an amount of refrigerant must be compressed by the compressor of the vapour compression system and an amount of gaseous refrigerant must be removed from the receiver. In case the amount of gaseous refrigerant that needs to be removed from the receiver exceeds the amount of refrigerant that the receiver compressor(s) are able to remove, the load demand of the vapour compression system exceeds the maximum capacity of the receiver compressor(s). Thus, in these situations, the receiver compressor(s) cannot maintain the desired pressure level in the receiver even if they are operated at maximum capacity. Therefore, when this happens, the prevailing pressure in the receiver is adjusted by means of the bypass valve, i.e. the opening degree of the bypass valve is controlled according to the setpoint control strategy based on the first pressure setpoint. The first pressure setpoint represents the pressure level that is actually desired for the prevailing pressure in the receiver. Thereby, the desired pressure level in the receiver can be maintained also in abnormal or extreme operating conditions without the maximum capacity of the receiver compressor(s) needing to be designed too large.

[0025] The desired pressure level in the receiver can be, for example, a pressure level that ensures proper pressure drop across the expansion device, thereby ensuring sufficient flow of refrigerant through the evaporator.

[0026] In case the receiver compressor(s) are operating and the load demand of the vapour compression system is below the maximum capacity of the receiver compressor(s), the opening degree of the bypass valve is controlled so as to adjust the prevailing pressure in the receiver according to the second pressure setpoint, i.e. the higher pressure setpoint, and so as to reach the second pressure setpoint, i.e. to reach a pressure level in the receiver equal to the second pressure setpoint.

[0027] When the load demand of the vapour compression system is below the maximum capacity of the receiver compressor(s), the receiver compressor(s) are actually able to remove the required amount of gaseous refrigerant from the receiver. Thus, in this case, and assuming that at least one receiver compressor is operating, the prevailing pressure in the receiver is regulated mainly by means of the receiver compressor(s), i.e. the receiver compressor(s) are controlled according to a set point control strategy, preferably based on a first pressure set point. However, the opening degree of the bypass valve is also controlled according to a set point control strategy, but based on a second pressure set point, i.e. based on a pressure set point which is higher than the pressure level which is actually desired in the receiver. Thereby it is ensured that if the receiver compressor(s) react slowly to changes in the load demand of the vapour compression system and thereby cause an increase in the prevailing pressure in the receiver, the bypass valve temporarily assists the receiver compressor(s) in controlling the prevailing pressure in the receiver. The receiver compressor(s) can react slowly, for example, due to limitations in the frequency at which the compressor(s) are allowed to start and / or stop. However, by controlling the opening degree of the bypass valve according to the second pressure set point, i.e. the higher pressure set point, it is ensured that the prevailing pressure in the receiver is still controlled mainly by means of the receiver compressor(s), whereas if the receiver compressor(s) successfully reduce the prevailing pressure in the receiver to a pressure level which is below the second pressure set point, the bypass valve will gradually close and the pressure control will be handed over completely to the receiver compressor(s).

[0028] Thus, even in exceptional or extreme operating conditions of the vapour compression system with high load demands, the method according to the present application provides for a proper and energy-efficient regulation of the prevailing pressure in the receiver without the risk of having to design the maximum capacity of the receiver compressor(s) too large and without the risk of having a conflict between controlling the receiver compressor(s) and controlling the bypass valve.

[0029] The method can further comprise the steps of:

[0030] - stopping the receiver compressor(s) or preventing the receiver compressor(s) from starting, and

[0031] - controlling the opening degree of the bypass valve so as to regulate the prevailing pressure in the receiver according to the first pressure set point and so as to reach the first pressure set point.

[0032] If the load demand of the vapour compression system is very low, the amount of gaseous refrigerant that needs to be removed from the receiver can be insufficient to ensure stable operation of the receiver compressor(s). This can result in undesired repeated start-ups and stoppages of the receiver compressor(s). Therefore, in case the load demand of the vapour compression system is lower than the minimum capacity of the receiver compressor(s), the receiver compressor(s) are stopped if they are operating, or prevented from starting if they are not operating. Thus, when this situation occurs, the prevailing pressure in the receiver is regulated without using the receiver compressor(s) at all.

[0033] Instead, the prevailing pressure in the receiver is regulated by means of the bypass valve only. Therefore, the opening degree of the bypass valve is controlled so as to regulate the prevailing pressure in the receiver in accordance with the first pressure setpoint, i.e. in accordance with a pressure setpoint that represents the pressure level that is actually desired in the receiver. In other words, the opening degree of the bypass valve is controlled in accordance with the setpoint control strategy based on the first pressure setpoint.

[0034] This even further ensures that the prevailing pressure in the receiver is properly regulated under operating conditions in which the vapour compression system has a low load demand, without the risk of a conflict between controlling the receiver compressor(s) and controlling the bypass valve.

[0035] In the context of the present invention, the term "minimum capacity of the receiver compressor(s)" is to be interpreted as meaning the lowest operable capacity of the receiver compressor(s) that ensures stable operation of the receiver compressor(s) without the need for repeated stoppages and start-ups of the receiver compressor(s).

[0036] The method can further comprise the following steps: in case the load demand of the vapour compression system increases to a level above the minimum capacity of the receiver compressor / multiple receiver compressors,

[0037] - closing the bypass valve,

[0038] - starting the receiver compressor / multiple receiver compressors and controlling the capacity of the receiver compressor / multiple receiver compressors so as to regulate the prevailing pressure in the receiver in accordance with the first pressure setpoint, and so as to reach the first pressure setpoint, and

[0039] - keeping the bypass valve closed as long as the prevailing pressure in the receiver is below the second pressure setpoint.

[0040] According to this embodiment, in case the receiver compressor(s) have been stopped due to the low load demand on the vapour compression system as described above, the load demand on the vapour compression system is monitored. In case the load demand on the vapour compression system increases to a level above the minimum capacity of the receiver compressor(s), it can be assumed that the receiver compressor(s) can be operated in a stable manner. Therefore, it is again desired to regulate the prevailing pressure in the receiver by means of the receiver compressor(s).

[0041] Hence, when this occurs, the bypass valve is closed and at least one receiver compressor is started, the capacity of the receiver compressor(s) is controlled so as to regulate the prevailing pressure in the receiver according to the first pressure setpoint, and so as to reach the first pressure setpoint. In other words, the capacity of the receiver compressor(s) is controlled according to the setpoint control strategy based on the first pressure setpoint.

[0042] Furthermore, the bypass valve remains closed as long as the prevailing pressure in the receiver is below the second pressure setpoint. Thereby, it is ensured that the prevailing pressure in the receiver is regulated by means of the receiver compressor(s) only as long as it is below the second pressure setpoint, and thereby sufficiently close to the desired pressure level (i.e. the first pressure setpoint) at which it is assumed that the receiver compressor(s) are able to regulate the prevailing pressure in the receiver properly without any assistance from the bypass valve. Thereby, a conflict between controlling the receiver compressor(s) and controlling the bypass valve is effectively prevented.

[0043] The method can further comprise the steps of: in case the prevailing pressure in the receiver exceeds the second pressure setpoint, opening the bypass valve and subsequently controlling the opening of the bypass valve so as to regulate the prevailing pressure in the receiver according to the second pressure setpoint, and so as to reach the second pressure setpoint.

[0044] According to this embodiment, if the prevailing pressure in the receiver increases to a level above the second pressure setpoint, the bypass valve is enabled to assist the receiver compressor(s) in controlling the prevailing pressure in the receiver. Such an increase in the prevailing pressure in the receiver can for example be due to the receiver compressor(s) not being able to react to changes in operating conditions fast enough. Once the bypass valve is opened for this purpose, the opening of the bypass valve is controlled so as to regulate the prevailing pressure in the receiver compressor(s) according to the second pressure setpoint (i.e. the higher pressure setpoint), and so as to reach the second pressure setpoint.

[0045] Thus, the opening of the bypass valve is not controlled so as to always lower the prevailing pressure in the receiver to a level corresponding to the first pressure setpoint. Rather, when the prevailing pressure in the receiver approaches the second pressure setpoint, the bypass valve will gradually close, thereby leaving the regulation of the prevailing pressure in the receiver to the receiver compressor(s) to a greater extent. However, it is not excluded that the bypass valve remains open even if the prevailing pressure in the receiver is lowered to a level below the second pressure setpoint. But once the bypass valve has been closed, it will remain closed as long as the prevailing pressure in the receiver is below the second pressure setpoint.

[0046] Thus, according to this embodiment, the opening of the bypass valve is controlled according to a "half-side hysteresis" control strategy, which makes it possible to leave the regulation of the prevailing pressure in the receiver to the receiver compressor(s) to the largest possible extent. However, in case the receiver compressor(s) are unable to react to changes in operating conditions sufficiently fast, the bypass valve is enabled to assist the receiver compressor(s), but only to the extent necessary, thereby minimizing the risk of control conflicts between the receiver compressor(s) and the bypass valve.

[0047] The second pressure setpoint can be between 1 bar and 10 bar higher than the first pressure setpoint, such as between 1 bar and 8 bar, such as between 2 bar and 4 bar. A larger difference between the first pressure setpoint and the second pressure setpoint ensures that the receiver compressor is applied to a large extent, but allows the prevailing pressure in the receiver to deviate from the first pressure setpoint, and thereby from the desired pressure level, before the bypass valve is opened. On the other hand, a smaller difference between the first pressure setpoint and the second pressure setpoint will prevent the prevailing pressure in the receiver from deviating from the desired pressure level, but will cause the bypass valve to open further and more frequently, and thereby the receiver compressor will be utilized to a lesser extent. A difference between the first pressure setpoint and the second pressure setpoint within the interval of 1 bar to 10 bar provides a suitable balance between these two objectives.

[0048] Thus, the method can further comprise, if the bypass valve is closed, keeping the bypass valve closed as long as the prevailing pressure in the receiver is below the second pressure setpoint. This can be the case regardless of the reason why the bypass valve is closed.

[0049] The method can further comprise the steps of, in case the prevailing pressure in the receiver increases from a pressure level below the second pressure setpoint to a pressure level above the second pressure setpoint, opening the bypass valve and subsequently controlling the opening of the bypass valve so as to regulate the prevailing pressure in the receiver according to the second pressure setpoint, and so as to reach the second pressure setpoint.

[0050] According to this embodiment, once the bypass valve has been closed, the bypass valve remains closed as long as the prevailing pressure in the receiver is below the second pressure setpoint. If the prevailing pressure in the receiver increases to a level above the second pressure setpoint, the bypass valve is opened and controlled in the manner described above. Thus, the opening of the bypass valve is controlled according to a "half-side hysteresis" control strategy.

[0051] The method can further comprise the steps of: operating the receiver compressor / multiple receiver compressors at maximum capacity when the load demand of the vapour compression system exceeds the maximum capacity of the receiver compressor / multiple receiver compressors, and controlling the opening of the bypass valve so as to regulate the prevailing pressure in the receiver according to the first pressure setpoint and so as to reach the first pressure setpoint.

[0052] According to this embodiment, when the load demand of the vapour compression system is very high and exceeds the maximum capacity of the receiver compressor / multiple receiver compressors, the receiver compressor / multiple receiver compressors remain operated at maximum capacity, thereby ensuring that the receiver compressor / multiple receiver compressors are applied to the maximum possible extent. However, since the load demand of the vapour compression system exceeds the maximum capacity of the receiver compressor / multiple receiver compressors, this is not sufficient to maintain the prevailing pressure in the receiver at the desired pressure level. Therefore, the regulation of the prevailing pressure in the receiver is now handed over to the bypass valve. Thus, the opening of the bypass valve is controlled so as to regulate the prevailing pressure in the receiver according to the first pressure setpoint and so as to reach the first pressure setpoint.

[0053] Thus, according to this embodiment, it is possible to maintain the desired pressure level in the receiver even in extreme operating conditions without having to design the capacity of the receiver compressor / multiple receiver compressors to be too large, since in such cases the bypass valve acts as a "back-up" for the receiver compressor / multiple receiver compressors. BRIEF DESCRIPTION OF DRAWINGS

[0054] The application will now be described in further detail with reference to the accompanying drawings, in which:

[0055] Figure 1 is a diagrammatic view of a vapour compression system controlled in accordance with the method according to the application, and

[0056] Figure 2 is a set of curves illustrating the method according to an embodiment of the application. DETAILED DESCRIPTION

[0057] Figure 1is a diagrammatic view of a vapour compression system 1 controlled in accordance with a method according to an embodiment of the application. The vapour compression system 1 comprises a compressor unit 2 comprising at least two compressors 3, 4 (of which two are shown) arranged in a refrigerant path, a heat rejecting heat exchanger 5, a high pressure valve 6, a receiver 7, an expansion valve 8 and an evaporator 9. The compressor 3 is a main compressor fluidly connected to an outlet of the evaporator 9 and the compressor 4 is a receiver compressor fluidly connected to a gas outlet 10 of the receiver 7.

[0058] The refrigerant flowing in the refrigerant path is compressed by the compressors 3, 4 before being supplied to the heat rejecting heat exchanger 5. In the heat rejecting heat exchanger 5, heat exchange takes place between the refrigerant flowing through the heat rejecting heat exchanger 5 and the surrounding environment or an auxiliary fluid stream flowing through the heat rejecting heat exchanger 5 in such a way that heat is rejected from the refrigerant.

[0059] The refrigerant leaving the heat rejecting heat exchanger 5 passes through the high pressure valve 6, where the refrigerant undergoes expansion before being supplied to the receiver 7. In the receiver 7, the refrigerant is separated into a liquid portion and a gaseous portion. The liquid portion of the refrigerant leaves the receiver 7 via a liquid outlet 11 and is supplied to the expansion device 8, where the refrigerant undergoes expansion before being supplied to the evaporator 9. Thereby, the refrigerant supplied to the evaporator 9 is in a gas-liquid mixed state.

[0060] In the evaporator 9, heat exchange takes place between the refrigerant flowing through the evaporator 9 and the surrounding environment or an auxiliary fluid stream flowing through the evaporator 9 in such a way that heat is absorbed by the refrigerant while a liquid portion of the refrigerant at least partly evaporates. Finally, the refrigerant leaving the evaporator 9 is supplied to the main compressor 3.

[0061] The gaseous portion of the refrigerant in the receiver 7 can leave the receiver via the gas outlet 10. The gaseous refrigerant can be directly supplied to the receiver compressor 4 or the gaseous refrigerant can be supplied to the main compressor 3 via a bypass valve 12. Thereby, the prevailing pressure in the receiver 7 can be regulated by appropriately controlling the capacity of the receiver compressor 4 or by appropriately controlling the opening of the bypass valve 12.

[0062] When controlling the vapour compression system 1 of Figure 1 in accordance with a method according to an embodiment of the application, the capacity of the receiver compressor 4 and the opening of the bypass valve 12 are controlled respectively in such a way that an efficient regulation of the prevailing pressure in the receiver 7 is ensured while ensuring that the control of the receiver compressor 4 and the control of the bypass valve 12 do not conflict. This can be achieved for example in the way described below with reference to Figure 2 .

[0063] Figure 2is a set of curves illustrating a method for controlling a vapour compression system according to an embodiment of the application. The vapour compression system can for example be Figure 1 the vapour compression system illustrated in

[0064] The upper curve illustrates the opening degree of the bypass valve as a function of time. The middle curve illustrates the capacity of the receiver compressor as a function of time. The lower curve illustrates the receiver pressure as a function of time. The dashed line in the lower curve illustrates the variation of a pressure setpoint applied to control the opening degree of the bypass valve as a function of time. The pressure setpoint can vary between a first pressure setpoint corresponding to the continuous line 13 and a higher second pressure setpoint. This will be described in further detail below. The first pressure setpoint represents a desired pressure level in the receiver.

[0065] Between time to and time ti, the load demand of the vapour compression system is low and below the minimum capacity of the receiver compressor. Therefore, the amount of gaseous refrigerant that needs to be removed from the receiver is insufficient to allow the receiver compressor to operate in a stable manner. Therefore, the receiver compressor remains in a stopped state and the prevailing pressure in the receiver is instead regulated by means of the bypass valve. Therefore, the opening degree of the bypass valve is controlled so as to regulate the prevailing pressure in the receiver according to the low first pressure setpoint. It can be seen that this causes the opening degree of the bypass valve to gradually increase.

[0066] At time ti, the load demand of the vapour compression system increases to a level above the minimum capacity of the receiver compressor. Therefore, it is now possible to operate the receiver compressor in a stable manner. Therefore, the receiver compressor is started and the capacity of the receiver compressor is controlled so as to regulate the prevailing pressure in the receiver according to the first pressure setpoint. At the same time, the pressure setpoint for the opening degree of the bypass valve is changed from the first pressure setpoint to the high second pressure setpoint. Since the prevailing pressure in the receiver is significantly lower than the second pressure setpoint, this causes the bypass valve to close. Therefore, the prevailing pressure in the receiver is now regulated solely by means of the receiver compressor. It can be seen that the capacity of the receiver compressor subsequently increases as a function of time in an attempt to obtain a pressure in the receiver equal to the first pressure setpoint by means of the receiver compressor. It can also be seen that, despite this, the prevailing pressure in the receiver continues to increase. However, as long as the prevailing pressure in the receiver is below the second pressure setpoint, the bypass valve remains closed in order to ensure that the prevailing pressure in the receiver is regulated mainly by means of the receiver compressor and in order to avoid a conflict between controlling the receiver compressor and controlling the bypass valve.

[0067] At time t2, the prevailing pressure in the receiver has increased above the second pressure setpoint, although the receiver compressor is not yet operating at its maximum capacity and thus the receiver compressor should be able to adjust the prevailing pressure in the receiver to reach the first pressure setpoint. This can be for example because the receiver compressor reacts too slowly to changes in operating conditions. Therefore, to assist the receiver compressor, the bypass valve is opened and the opening of the bypass valve is controlled according to the second pressure setpoint. It can be seen that this causes the prevailing pressure in the receiver to decrease and as the prevailing pressure in the receiver approaches the second pressure setpoint, the bypass valve starts to gradually close until it is fully closed at time t3. Therefore, from time t3 to time t4, the prevailing pressure in the receiver is adjusted solely by means of the receiver compressor, because the bypass valve is not allowed to open as long as the prevailing pressure in the receiver is below the second pressure setpoint.

[0068] At time t4, the receiver compressor reaches maximum capacity, but the prevailing pressure in the receiver is much higher than the first pressure setpoint. Therefore, the load demand of the vapour compression system now exceeds the maximum capacity of the receiver compressor. Therefore, the pressure setpoint for the bypass valve is changed from the second pressure setpoint to the first pressure setpoint. Since the prevailing pressure in the receiver is higher than the first pressure setpoint, this causes the bypass valve to open and now the prevailing pressure in the receiver is effectively adjusted by means of the bypass valve, while the receiver compressor continues to operate at maximum capacity in order to ensure that the receiver compressor is applied to the largest extent possible. Therefore, in this case, the bypass valve provides a "back-up capacity" for the receiver compressor and can handle these extreme conditions without the need to design the maximum capacity of the receiver compressor to be too large.

[0069] At time t5, the prevailing pressure in the receiver has reached a level sufficiently below the first pressure setpoint to cause the bypass valve to close. In response to this, the pressure setpoint for the bypass valve is changed from the first pressure setpoint to the second pressure setpoint in order to ensure that the bypass valve remains closed as long as the prevailing pressure in the receiver is below the second pressure setpoint. Therefore, the receiver compressor takes over the responsibility of adjusting the prevailing pressure in the receiver again.

Claims

1. A method for controlling a vapour compression system (1) comprising a compressor unit (2) comprising at least two compressors (3, 4) arranged in a refrigerant path, a heat rejecting heat exchanger (5), a receiver (7), an expansion device (8) arranged to control the supply of refrigerant to an evaporator (9), at least one of the compressors being a main compressor (3) fluidly connected to an outlet of the evaporator (9) and at least one of the compressors being a receiver compressor (4) fluidly connected to a gas outlet (10) of the receiver (7), the vapour compression system (1) further comprising a bypass valve (12) fluidly interconnecting the gas outlet (10) of the receiver (7) with the main compressor / compressors (3), characterised in that, The method comprises the steps of: - defining a first pressure setpoint for the prevailing pressure in the receiver (7) and a second pressure setpoint for the prevailing pressure in the receiver (7), the second pressure setpoint being higher than the first pressure setpoint, and - controlling the opening of the bypass valve (12), wherein, in case the load demand of the vapour compression system (1) exceeds the maximum capacity of the receiver compressor(s) (4), the opening of the bypass valve (12) is controlled so as to adjust the prevailing pressure in the receiver (7) based on the first pressure setpoint according to a setpoint control strategy and so as to reach the first pressure setpoint; and wherein, in case the receiver compressor(s) (4) are operating and the load demand of the vapour compression system (1) is lower than the maximum capacity of the receiver compressor(s) (4), the opening of the bypass valve (12) is controlled so as to adjust the prevailing pressure in the receiver (7) based on the second pressure setpoint according to a setpoint control strategy and so as to reach the second pressure setpoint.

2. The method of claim 1, further comprising the step of: In case the load demand of the vapour compression system (1) is lower than the minimum capacity of the receiver compressor(s) (4), - stopping the receiver compressor(s) (4) or preventing the receiver compressor(s) (4) from starting, and - controlling the opening of the bypass valve (12) so as to adjust the prevailing pressure in the receiver (7) according to the first pressure setpoint and so as to reach the first pressure setpoint.

3. The method of claim 2, further comprising the step of: In case the load demand of the vapour compression system (1) increases to a level higher than the minimum capacity of the receiver compressor(s) (4), - closing the bypass valve (12), - starting the receiver compressor(s) (4) and controlling the capacity of the receiver compressor(s) (4) so as to adjust the prevailing pressure in the receiver (7) according to the first pressure setpoint and so as to reach the first pressure setpoint, and - keeping the bypass valve (12) closed as long as the prevailing pressure in the receiver (7) is lower than the second pressure setpoint.

4. The method of claim 3, further comprising the step of: In case the prevailing pressure in the receiver (7) exceeds the second pressure setpoint, opening the bypass valve (12) and subsequently controlling the opening of the bypass valve (12) so as to adjust the prevailing pressure in the receiver (7) according to the second pressure setpoint and so as to reach the second pressure setpoint.

5. The method according to any of the preceding claims, wherein, The second pressure setpoint is between 1 bar and 10 bar higher than the first pressure setpoint.

6. The method of any one of claims 1 to 4, wherein, If the bypass valve (12) is closed, keeping the bypass valve (12) closed as long as the prevailing pressure in the receiver (7) is lower than the second pressure setpoint.

7. The method of claim 6, further comprising the step of: In case the prevailing pressure in the receiver (7) increases from a pressure level below the second pressure setpoint to a pressure level above the second pressure setpoint, the bypass valve (12) is opened and subsequently the opening degree of the bypass valve (12) is controlled in order to adjust the prevailing pressure in the receiver (7) according to the second pressure setpoint and in order to reach the second pressure setpoint.

8. The method of any one of claims 1 to 4, further comprising the step of: In case the load demand of the vapour compression system (1) exceeds the maximum capacity of the receiver compressor / multiple receiver compressors (4), the receiver compressor / multiple receiver compressors (4) is / are operated with maximum capacity and the opening degree of the bypass valve (12) is controlled in order to adjust the prevailing pressure in the receiver (7) according to the first pressure setpoint and in order to reach the first pressure setpoint.

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