Method for controlling a vapour compression system having a receiver compressor

Abstract

A method for controlling a vapour compression system (1) is disclosed. The vapour compression system (1) comprises a compressor unit (2) comprising at least one main compressor (3) and at least one receiver compressor (4), a heat rejecting heat exchanger (5), a receiver (7), an expansion device (8) and an evaporator (9) arranged in a refrigerant path. The vapour compression system (1) further comprises a bypass valve (12) fluidly interconnecting a gas outlet (10) of the receiver (7) with the main compressor / multiple main compressors (3). A pressure difference across the bypass valve (12) is measured or derived and a mass flow rate of refrigerant through the bypass valve (12) is derived based on at least the pressure difference across the bypass valve (12) and using a fluid model. A minimum mass flow rate of refrigerant required to operate the receiver compressor (4) is derived based on a minimum displacement volume of the receiver compressor (4) and using a fluid model taking into account prevailing operating conditions. In case the derived mass flow rate of refrigerant through the bypass valve (12) exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor (4), the receiver compressor (4) is started and the bypass valve (12) is closed.

Description

Technical Field

[0001] This invention relates to a method for controlling a vapor compression system having a receiver and a receiver compressor, the receiver compressor being directly fluidly connected to the gas outlet of the receiver. The method according to the invention effectively ensures that the receiver compressor stops and starts at the optimal time regardless of the current operating conditions. Background Technology

[0002] Vapor compression systems, such as refrigeration systems, air conditioning systems, or heat pumps, typically include a compressor unit, a heat exchanger, an expander, and an evaporator arranged in a refrigerant path. The compressor unit includes one or more compressors. Thus, the refrigerant flowing in the refrigerant path is compressed by the compressor(s) of the compressor unit before being supplied to the heat exchanger. As it passes through the heat exchanger, the refrigerant exchanges heat with the surrounding environment or an auxiliary fluid flowing through it, thereby dissipating heat from the refrigerant. The refrigerant then passes through the expander, where it expands before being supplied to the evaporator. The refrigerant supplied to the evaporator is in the form of a mixture of gaseous and liquid refrigerant. As it passes through the evaporator, the liquid portion of the refrigerant evaporates, and heat exchanges occur between the refrigerant and the surrounding environment or an auxiliary fluid flowing through it, thereby absorbing heat from the refrigerant.

[0003] In some vapor compression systems, a receiver is arranged in the refrigerant path between the heat exchanger and the expansion unit. 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 unit via a liquid outlet in the manner described above. 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 via a bypass valve to the suction line that interconnects the evaporator outlet with the compressor unit. Alternatively, the gaseous refrigerant can be supplied directly to a dedicated receiver compressor that does not receive refrigerant from the evaporator. Supplying the gaseous refrigerant to the receiver compressor is more energy efficient than supplying it via a bypass valve to the suction line or to the expansion unit because it does not introduce a pressure drop, thus requiring less energy to compress the refrigerant to the desired pressure level. Therefore, it is desirable to supply the gaseous refrigerant from the receiver to the receiver compressor as much as possible.

[0004] However, when the flow rate of gaseous refrigerant leaving the receiver is low, it may be insufficient to maintain stable operation of the receiver compressor. This causes the receiver compressor to repeatedly stop and start, leading to excessive wear. In such cases, a bypass valve is preferable.

[0005] Therefore, when the flow rate of gaseous refrigerant leaving the receiver is low and thus the receiver compressor stops, a decision should be made to start the receiver compressor if it can be assumed that the flow rate of gaseous refrigerant leaving the receiver has increased to a level sufficient to ensure stable operation of the receiver compressor. If the receiver compressor starts but the flow rate of gaseous refrigerant leaving the receiver is still too low, this will result in undesirable repeated starting and stopping of the receiver compressor. On the other hand, if the receiver compressor remains stopped even though the flow rate of gaseous refrigerant leaving the receiver is actually sufficient to ensure stable operation, the vapor compression system will operate in a manner below achievable energy efficiency.

[0006] It may be difficult to establish the exact point in time when the flow rate of gaseous refrigerant leaving the receiver is sufficient to ensure stable operation of the receiver compressor, and the measurable parameters indicating that exact point in time depend on variable environmental operating conditions, such as ambient temperature and pressure. Therefore, the receiver compressor typically does not start until it is undeniably certain that it can operate stably. Consequently, there will be periods where, even though the receiver compressor could have been used and the vapor compression system could therefore have operated more energy-efficiently, the receiver compressor stops and gaseous refrigerant from the receiver is supplied to the bypass valve.

[0007] EP3581 860A1 (corresponding to US 2019 / 0376728A1) discloses a refrigeration system including a receiver, a gas bypass valve, a parallel compressor, and a controller. The controller is configured to switch from operating the gas bypass valve to operating the parallel compressor in response to a process variable value crossing a switching setpoint to control the pressure of gaseous refrigerant in the receiver, the switching setpoint depending on the amount of gaseous refrigerant produced by the refrigeration system.

[0008] Invention Description

[0009] The purpose of embodiments of the present invention is to provide a method for controlling a vapor compression system that allows for accurate determination of the appropriate switching point for starting or stopping the receiver compressor, regardless of current operating conditions.

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

[0011] - Measure or derive the pressure difference across the bypass valve.

[0012] - At least based on the pressure difference across the bypass valve and using a fluid model, derive the mass flow rate of the refrigerant through the bypass valve.

[0013] -Based on the minimum displacement volume of the receiver compressor and using a fluid model that takes into account current operating conditions, derive the minimum refrigerant mass flow rate required to operate the receiver compressor.

[0014] - Compare the derived refrigerant mass flow rate through the bypass valve with the derived minimum refrigerant mass flow rate required to operate the receiver compressor, and

[0015] - If the mass flow rate of refrigerant exported through the bypass valve exceeds the minimum mass flow rate of refrigerant required for the operation of the receiver compressor, start the receiver compressor and close the bypass valve.

[0016] Therefore, the present invention provides a method for controlling a vapor compression system. In the context of this invention, the term "vapor compression system" should be interpreted as any system in which a fluid medium (such as a refrigerant) circulates and is alternately compressed and expanded, thereby providing a certain volume of refrigeration or heating. Thus, a vapor compression system can be a refrigeration system, an air conditioning system, a heat pump, etc.

[0017] Therefore, the vapor compression system includes a compressor unit, a heat exchanger, a receiver, an expander, and an evaporator arranged in a refrigerant path. The expander is arranged to control the refrigerant supply to the evaporator. The compressor unit includes at least two compressors. At least one of these compressors is a main compressor fluidly connected to the outlet of the evaporator, and at least one of these compressors is a receiver compressor fluidly connected to the gas outlet of the receiver. The vapor compression system further includes a bypass valve that fluidly interconnects the gas outlet of the receiver with the main compressor / multiple main compressors.

[0018] Therefore, the refrigerant flowing in the refrigerant path is compressed by the compressor unit before being supplied to the heat exchanger. As the refrigerant passes through the heat exchanger, heat exchange occurs between the refrigerant and the surrounding environment or the auxiliary fluid flow passing through the heat exchanger, in a manner that allows heat to be dissipated from the refrigerant. The heat exchanger can be in the form of a condenser, in which case the refrigerant is at least partially condensed as it passes through the heat exchanger. Alternatively, the heat exchanger can be in the form of a gas cooler, in which case the refrigerant passing through the heat exchanger is cooled but remains in a gaseous or transcritical state.

[0019] Refrigerant leaving the heat exchanger may be supplied to the receiver via a high-pressure valve or ejector. In the receiver, the refrigerant is separated into a liquid 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 expansion, and the refrigerant supplied to the evaporator is in a mixed state of gaseous and liquid refrigerant. In the evaporator, the liquid portion of the refrigerant evaporates at least partially, while heat exchange occurs between the refrigerant and the surrounding environment or the auxiliary fluid flowing through the evaporator, by means of heat absorption by the refrigerant. Finally, the refrigerant leaving the evaporator is supplied to the main compressor / multiple main compressors.

[0020] The gaseous portion of the refrigerant in the receiver can leave the receiver via the gas outlet and be supplied directly to the receiver compressor / multiple receiver compressors or via a bypass valve to the main compressor / multiple main compressors.

[0021] In the method according to the invention, the pressure difference across the bypass valve is measured or derived at the beginning. This pressure difference can be measured directly, for example, by measuring the current pressure in the receiver and the suction pressure (i.e., the current pressure at the inlet of the main compressor / multiple main compressors). Alternatively, only the current pressure in the receiver can be measured, and the pressure difference across the bypass valve can be derived based on the measured receiver pressure and a reference pressure value of the suction pressure. As another alternative, the pressure difference can be derived separately from the current pressure in the receiver and the reference pressure value of the suction pressure.

[0022] Next, the mass flow rate of refrigerant through the bypass valve is derived, at least based on the pressure difference across the bypass valve, using a fluid model. The mass flow rate of refrigerant through the bypass valve is an accurate measurement of the amount of gaseous refrigerant that needs to be removed from the receiver. Therefore, the mass flow rate of refrigerant through the bypass valve is an appropriate parameter for determining whether the amount of gaseous refrigerant leaving the receiver is sufficient to ensure stable operation of the receiver compressor.

[0023] Advantageously, a fluid model is used to derive the mass flow rate of the refrigerant through the bypass valve because this takes into account the refrigerant's behavior under current operating conditions (including the pressure difference across the bypass valve). Thus, an accurate value for the mass flow rate of the refrigerant through the bypass valve can be obtained regardless of the current operating conditions.

[0024] In the context of this invention, the term "fluid model" should be interpreted as meaning a model that describes the behavior of a refrigerant as a function of relevant operating conditions (such as ambient temperature, pressure, etc.). A fluid model can, for example, specify multiple different relevant properties of the refrigerant (such as density, pressure, temperature, etc.) as functions of relevant environmental operating conditions.

[0025] Next, based on the minimum displacement volume of the receiver compressor, the minimum refrigerant mass flow rate required to operate the receiver compressor is derived using a fluid model that takes into account the current operating conditions.

[0026] Compressors are typically designed with a minimum capacity beyond which they can operate stably. The minimum capacity is defined as the minimum displacement volume of a gaseous medium that is insensitive to changes in environmental conditions. However, the mass flow rate discharged by a compressor at a given displacement volume depends on several properties of the gaseous medium, such as its density. These properties may vary in response to changes in operating conditions (e.g., ambient temperature, pressure, etc.). Therefore, the mass flow rate corresponding to a given displacement volume (e.g., the compressor's minimum displacement volume) also depends on the current operating conditions. However, by using a fluid model that takes into account the current operating conditions, the minimum mass flow rate of the refrigerant corresponding to the minimum displacement volume of the receiver compressor under the current operating conditions can be derived. This derived minimum mass flow rate indicates the lowest mass flow rate that ensures the receiver compressor can operate stably.

[0027] Next, the derived mass flow rate of refrigerant through the bypass valve is compared with the derived minimum mass flow rate of refrigerant required to operate the receiver compressor. Since both the derived mass flow rate through the bypass valve and the derived minimum mass flow rate of refrigerant required to operate the receiver compressor are mass flow rates taking into account current operating conditions, they can be directly compared. Therefore, if refrigerant were supplied to the receiver compressor instead of the bypass valve, the comparison results can easily reveal whether the current mass flow rate through the bypass valve is sufficient to ensure stable operation of the receiver compressor.

[0028] Finally, if the mass flow rate of refrigerant exported through the bypass valve exceeds the minimum mass flow rate of refrigerant required to operate the receiver compressor, the receiver compressor is started and the bypass valve is closed. Therefore, if the current mass flow rate of refrigerant through the bypass valve is actually sufficient to ensure stable operation of the receiver compressor, gaseous refrigerant is supplied to the receiver compressor instead of the bypass valve.

[0029] Since the decisions to start the receiver compressor and close the bypass valve are based on a comparison of the derived mass flow rate values ​​(as described above), an accurate basis for the decision is provided, taking into account the current operating conditions. This ensures that the receiver compressor is applied as soon as it is appropriate, regardless of the current operating conditions.

[0030] The method may further include the following steps: keeping the receiver compressor stopped and allowing the bypass valve to open if the mass flow rate of the refrigerant exported through the bypass valve is lower than the minimum mass flow rate of refrigerant required to operate the receiver compressor.

[0031] According to this embodiment, if the comparison results reveal that the amount of gaseous refrigerant to be removed from the receiver is insufficient to ensure stable operation of the receiver compressor, the receiver compressor does not start, and the bypass valve is allowed to remain open, meaning that gaseous refrigerant from the receiver continues to be supplied to the bypass valve instead of the receiver compressor. This effectively prevents undesirable repeated starting and stopping of the receiver compressor.

[0032] The method may further include the following steps: controlling the current pressure in the receiver by operating the receiver compressor when the mass flow rate of the refrigerant exported through the bypass valve exceeds the minimum mass flow rate of the refrigerant required by the operating receiver compressor; and controlling the current pressure in the receiver by operating the opening degree of the bypass valve when the mass flow rate of the refrigerant exported through the bypass valve is lower than the minimum mass flow rate of the refrigerant required by the operating receiver compressor.

[0033] According to this embodiment, the current pressure in the receiver is controlled, for example, by means of a setpoint control strategy, either by appropriately operating the receiver compressor or by appropriately operating the opening of the bypass valve. When the mass flow rate of the gaseous refrigerant leaving the receiver is sufficient to ensure stable operation of the receiver compressor, the current pressure in the receiver is controlled by means of the receiver compressor, for example, by controlling the capacity of the receiver compressor. On the other hand, when the mass flow rate of the gaseous refrigerant leaving the receiver is insufficient to ensure stable operation of the receiver compressor, the current pressure in the receiver is controlled by means of the bypass valve, for example, by appropriately adjusting the opening of the bypass valve.

[0034] The step of deriving the mass flow rate of the refrigerant through the bypass valve can be further based on the opening degree of the bypass valve.

[0035] According to this embodiment, the mass flow rate of refrigerant passing through the bypass valve is derived based on the pressure difference across the bypass valve and the opening degree of the bypass valve. A larger bypass valve opening results in a higher mass flow rate through the bypass valve. Therefore, the bypass valve opening is an important parameter when deriving the mass flow rate through the bypass valve.

[0036] The step of deriving the mass flow rate of the refrigerant through the bypass valve may include modeling the density of the refrigerant under current operating conditions.

[0037] According to this embodiment, the fluid model applied when deriving the mass flow rate of the refrigerant through the bypass valve includes a model that treats the refrigerant density as a function of relevant operating conditions (e.g., ambient temperature, pressure conditions, etc.). The refrigerant density is relatively sensitive to environmental conditions such as ambient temperature. Furthermore, the refrigerant density is important when deriving the mass flow rate from the volumetric flow rate; therefore, an accurate estimate of the refrigerant density under given operating conditions is crucial when deriving the mass flow rate of the refrigerant through the bypass valve.

[0038] For example, the mass flow rate through the bypass valve can be derived using an equation of the following form:

[0039]

[0040] in, The mass flow rate K is obtained through the bypass valve. v These are valve-specific tuning parameters, OD 旁通阀 ρ is the opening degree of the bypass valve, ρ is the density of the refrigerant, and ΔP is the pressure difference across the bypass valve. At least the density ρ can be modeled using a fluid model.

[0041] Similarly, the step of deriving the minimum refrigerant mass flow rate required to operate the receiver compressor may include modeling the density of the refrigerant under current operating conditions. This is similar to the embodiment described above.

[0042] For example, the minimum refrigerant mass flow rate required to operate the receiver compressor can be derived using an equation of the following form:

[0043]

[0044] in, V is the minimum mass flow rate of refrigerant required to operate the receiver compressor. min It is the compressor's specific minimum displacement volume, ρ 入口 It is the density of the refrigerant at the inlet of the receiver compressor, and eff vol This refers to the volumetric efficiency of the receiver compressor. At least the density ρ 入口 Fluid models can be used for modeling.

[0045] The fluid model used to derive the mass flow rate of refrigerant through the bypass valve can be the same as the fluid model used to derive the minimum mass flow rate of refrigerant required to operate the receiver compressor.

[0046] The step of deriving the minimum mass flow rate of refrigerant required to operate the receiver compressor may include deriving the mass flow rate corresponding to the displacement volume of the receiver compressor, such that the expected duty cycle of the receiver compressor is between 50% and 150%, for example between 60% and 120%, for example approximately 80%.

[0047] In the context of this invention, the term "duty cycle of the receiver compressor" should be interpreted as meaning the fraction of time the receiver compressor operates during operation.

[0048] Therefore, according to this embodiment, if the expected mass flow rate of refrigerant through the bypass valve is sufficient to allow the receiver compressor to operate for at least 50% of the time, it is assumed that the mass flow rate of refrigerant through the bypass valve is sufficient to ensure stable operation of the receiver compressor. The expected duty cycle is a suitable metric for the expected start and stop of the receiver compressor, and therefore it is appropriate to determine whether to start the receiver compressor based on this parameter.

[0049] It should be noted that, in the context of this invention, a duty cycle of exactly 100% for the receiver should be interpreted as meaning that the duty cycle exactly matches the physical minimum that the compressor can achieve without stopping. Therefore, a duty cycle higher than 100% should be interpreted as meaning a duty cycle correspondingly higher than that physical minimum. For example, a 120% duty cycle means that a load drop below 0% on the compressor will not cause the compressor to stop.

[0050] Fluid models can define the correlation between refrigerant pressure, temperature, specific volume, and / or density. In particular, fluid models can define such correlations at the refrigerant's dew point line. These parameters are important when converting volumetric flow rate to mass flow rate (and vice versa). Therefore, fluid models that define the correlations between these parameters are suitable for deriving the accurate switching point between operating a bypass valve and operating a receiver compressor.

[0051] Current operating conditions may include ambient temperature. Ambient temperature has a significant impact on refrigerants, particularly on their temperature, pressure, and density at various locations along their path. Therefore, it is important to consider ambient temperature when deriving the minimum refrigerant mass flow rate required to operate the receiver compressor.

[0052] For example, the pressure required in the receiver to ensure proper operation of the vapor compression system is closely related to the ambient temperature. The current pressure in the receiver also affects the refrigerant density and the pressure difference across the bypass valve, so the ambient temperature also indirectly affects these parameters.

[0053] In addition, ambient temperature affects the amount of vapor entering the receiver, which in turn indirectly affects the mass flow rate of the gas outlet from the receiver.

[0054] Alternatively or additionally, when deriving the mass flow rate through the bypass valve and / or when deriving the minimum mass flow rate of refrigerant required to operate the receiver compressor(s), the suction pressure, i.e., the pressure of the refrigerant entering the main compressor(s), can be taken into account. For example, the suction pressure can be applied as an input parameter value to the fluid model used for this purpose. Attached Figure Description

[0055] The invention will now be described in further detail with reference to the accompanying drawings, in which:

[0056] Figure 1 This is a schematic view of a vapor compression system controlled according to an embodiment of the present invention, and

[0057] Figure 2 This is a flowchart illustrating a method according to an embodiment of the present invention. Detailed Implementation

[0058] Figure 1 This is a schematic view of a vapor compression system 1 controlled according to a method according to an embodiment of the present invention. The vapor compression system 1 includes a compressor unit, a heat exchanger 5, a high-pressure valve 6, a receiver 7, an expansion valve 8, and an evaporator 9 arranged in a refrigerant path. The compressor unit includes at least two compressors 3 and 4 (two of which are shown). Compressor 3 is a main compressor fluidly connected to the outlet of the evaporator 9, and compressor 4 is a receiver compressor fluidly connected to the gas outlet 10 of the receiver 7.

[0059] The refrigerant flowing in the refrigerant path is compressed by compressors 3 and 4 before being supplied to the heat exchanger 5. In the heat exchanger 5, heat exchange occurs between the refrigerant flowing through the heat exchanger 5 and the surrounding environment or the auxiliary fluid flow flowing through the heat exchanger 5, in a manner that allows heat to be discharged from the refrigerant.

[0060] The refrigerant leaving the heat exchanger 5 passes through the high-pressure valve 6, where it expands and is then 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 the liquid outlet 11 and is supplied to the expansion device 8, where it expands and is then supplied to the evaporator 9. Thus, the refrigerant supplied to the evaporator 9 is in a gas-liquid mixed state.

[0061] In evaporator 9, the refrigerant flowing through evaporator 9 exchanges heat with the surrounding environment or the auxiliary fluid flowing through evaporator 9 in such a way that heat is absorbed by the refrigerant while at least part of the liquid portion of the refrigerant evaporates. Finally, the refrigerant leaving evaporator 9 is supplied to main compressor 3.

[0062] The gaseous portion of the refrigerant in receiver 7 can exit the receiver via gas outlet 10. The gaseous refrigerant can be supplied directly to receiver compressor 4, or it can be supplied to main compressor 3 via bypass valve 12. Thus, the current pressure in receiver 7 can be regulated by appropriately controlling the capacity of receiver compressor 4 or by appropriately controlling the opening of bypass valve 12.

[0063] When controlled according to the method of the embodiment of the present invention Figure 1 In the vapor compression system 1, it is ensured that the receiver compressor 4 operates only when the available amount of gaseous refrigerant in the receiver 7 is sufficient to ensure stable operation of the receiver compressor 4. Furthermore, the decision to switch between operating the bypass valve 12 and operating the receiver compressor 4 is based on accurate criteria, taking into account current operating conditions. This ensures that the receiver compressor 4 is used whenever appropriate. This can be seen, for example, in the following reference: Figure 2 The described method is implemented.

[0064] Figure 2 This is a flowchart illustrating a method according to an embodiment of the present invention. The process begins at step 3. In step 14, the pressure difference ΔP across the bypass valve is obtained, for example, by direct measurement or by deriving the pressure difference from one or more other measurement parameters.

[0065] In step 15, the mass flow rate of the refrigerant through the bypass valve is derived. The mass flow rate is derived based on the obtained pressure difference across the bypass valve and may also be derived based on other relevant parameters such as the opening degree of the bypass valve. Furthermore, the mass flow rate is derived using a fluid model, thereby taking into account the expected behavior of the refrigerant under given operating conditions. Therefore, the derived mass flow rate of the refrigerant through the bypass valve is highly accurate and provides an accurate measurement of the available amount of gaseous refrigerant.

[0066] In step 16, the minimum mass flow rate of refrigerant required to operate the receiver compressor is derived. The minimum mass flow rate is derived based on the minimum displacement volume of the receiver compressor, i.e., the minimum volume that the receiver compressor must discharge to operate stably without excessive start-ups and shutdowns. Furthermore, the minimum mass flow rate is derived using a fluid model that takes into account the current operating conditions. Thus, the derived minimum mass flow rate provides a very accurate measurement of the available mass flow rate to ensure stable operation of the receiver compressor under current operating conditions.

[0067] In step 17, the derived mass flow rate of refrigerant through the bypass valve is compared with the derived minimum mass flow rate of refrigerant required to operate the receiver compressor in order to determine whether the amount of gaseous refrigerant currently available is sufficient to ensure stable operation of the receiver compressor.

[0068] Therefore, if step 17 reveals that the mass flow rate of refrigerant flowing through the bypass valve exceeds the minimum mass flow rate of refrigerant required to operate the receiver compressor, it can be deduced that the available amount of gaseous refrigerant is sufficient to ensure stable operation of the receiver compressor. Therefore, in this case, the process proceeds to step 18, where the bypass valve is closed and the receiver compressor is started. As a result, the refrigerant leaving the receiver is supplied to the receiver compressor instead of the bypass valve, and the vapor compression system operates in an energy-efficient manner.

[0069] If the comparison results in step 17 reveal that the derived mass flow rate of refrigerant through the bypass valve does not exceed the minimum mass flow rate of refrigerant required to operate the receiver compressor, it can be inferred that the available amount of gaseous refrigerant is insufficient to ensure stable operation of the receiver compressor. Therefore, in this case, the process proceeds to step 19, in which the bypass valve remains open and the receiver compressor remains stopped. Thus, refrigerant leaving the receiver is supplied to the bypass valve instead of the receiver compressor, preventing repeated stops and starts of the receiver compressor due to insufficient available gaseous refrigerant.

[0070] Finally, in step 20, based on the comparison results of step 17, the current pressure in the receiver is controlled by appropriately controlling the capacity of the receiver compressor or by appropriately controlling the opening of the bypass valve.

Claims

1. A method for controlling a vapor compression system (1), the vapor compression system (1) comprising 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 comprising at least two compressors (3, 4), the expansion device (8) being arranged to control the refrigerant supply to the 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 vapor compression system (1) further comprising a bypass valve (12) fluidly interconnecting the gas outlet (10) of the receiver (7) with the main compressor (3), the method comprising the steps of: -Measure or derive the pressure difference across the bypass valve (12), -At least based on the pressure difference across the bypass valve (12) and using a fluid model that takes into account the current operating conditions, the mass flow rate of the refrigerant through the bypass valve (12) is derived. -Based on the minimum displacement volume of the receiver compressor (4) and using a fluid model that takes into account the current operating conditions, derive the minimum refrigerant mass flow rate required to operate the receiver compressor (4). - Compare the mass flow rate of the refrigerant exported through the bypass valve (12) with the minimum mass flow rate of refrigerant required to operate the receiver compressor (4), and - If the mass flow rate of the refrigerant exported through the bypass valve (12) exceeds the minimum mass flow rate of the refrigerant required to operate the receiver compressor (4), start the receiver compressor (4) and close the bypass valve (12). The current operating conditions include at least one of ambient temperature and pressure conditions.

2. The method according to claim 1, further comprising the following steps: If the mass flow rate of the refrigerant discharged through the bypass valve (12) is lower than the minimum mass flow rate of the refrigerant required to operate the receiver compressor (4), the receiver compressor (4) is kept stopped and the bypass valve (12) is allowed to open.

3. The method according to claim 1 or 2, further comprising the following steps: If the mass flow rate of the refrigerant discharged through the bypass valve (12) exceeds the minimum mass flow rate of the refrigerant required to operate the receiver compressor (4), the current pressure in the receiver (7) is controlled by operating the receiver compressor (4); and if the mass flow rate of the refrigerant discharged through the bypass valve (12) is lower than the minimum mass flow rate of the refrigerant required to operate the receiver compressor (4), the current pressure in the receiver (7) is controlled by operating the opening of the bypass valve (12).

4. The method according to claim 1 or 2, wherein, The step of deriving the mass flow rate of the refrigerant through the bypass valve (12) is further based on the opening degree of the bypass valve (12).

5. The method according to claim 1 or 2, wherein, The step of deriving the mass flow rate of the refrigerant through the bypass valve (12) includes modeling the density of the refrigerant under these current operating conditions.

6. The method according to claim 1 or 2, wherein, The steps to derive the minimum mass flow rate of the refrigerant required to operate the receiver compressor (4) include modeling the density of the refrigerant under these current operating conditions.

7. The method according to claim 1 or 2, wherein, The step of deriving the minimum mass flow rate of refrigerant required to operate the receiver compressor (4) includes deriving the mass flow rate corresponding to the displacement volume of the receiver compressor (4) such that the expected duty cycle of the receiver compressor (4) is between 50% and 150%.

8. The method according to claim 1 or 2, wherein, The fluid model defines the correlation between the pressure, temperature, specific volume, and / or density of the refrigerant.

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