A METHOD FOR CONTROLLING A VAPOUR COMPRESSION SYSTEM WITH A RECEIVER COMPRESSOR

Abstract

A vapour compression system (1) including a compressor unit (2) having 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) being arranged in a refrigerant path. The vapour compression system (1) further includes a bypass valve (12) fluidly interconnecting the gaseous outlet (10) of the receiver (7) and the main compressor(s) (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 at least on 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 prevailing operating conditions into account. In the case that 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.

Claims

1. A method for controlling a vapour compression system, the vapour compression system comprising a compressor unit comprising at least two compressors, a heat rejecting heat exchanger, a receiver, an expansion device and an evaporator being arranged in a refrigerant path, the expansion device being arranged to control a supply of refrigerant to the evaporator, at least one of the compressors being a main compressor being fluidly connected to an outlet of the evaporator and at least one of the compressors being a receiver compressor being fluidly connected to a gaseous outlet of the receiver, the vapour compression system further comprising a bypass valve fluidly interconnecting the gaseous outlet of the receiver and the main compressor(s), the method comprising the steps of: measuring or deriving a pressure difference across the bypass valve, deriving a mass flow rate of refrigerant through the bypass valve, based at least on the pressure difference across the bypass valve, and using a fluid model, deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor, based on a minimum displacement volume of the receiver compressor and using a fluid model taking prevailing operating conditions into account, comparing the derived mass flow rate of refrigerant through the bypass valve and the derived minimum mass flow rate of refrigerant required to operate the receiver compressor, and starting the receiver compressor and closing the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor.

2. The method according to claim 1, further comprising the step of keeping the receiver compressor stopped and allowing the bypass valve to be open in the case that the derived mass flow rate of refrigerant through the bypass valve is lower than the derived minimum mass flow rate of refrigerant required to operate the receiver compressor.

3. The method according to claim 1, further comprising the step of controlling a pressure prevailing in the receiver by operating the receiver compressor in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor, and controlling the pressure prevailing in the receiver by operating an opening degree of the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve is lower than the derived minimum mass flow rate of refrigerant required to operate the receiver compressor.

4. The method according to claim 1, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve is further based on an opening degree of the bypass valve.

5. The method according to claim 1, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve comprises modelling a density of the refrigerant under the prevailing operating conditions.

6. The method according to claim 1, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises modelling a density of the refrigerant under the prevailing operating conditions.

7. The method according to claim 1, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises deriving a mass flow rate corresponding to a displacement volume of the receiver compressor which results in an expected duty cycle of the receiver compressor of between 50% and 150%.

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

9. The method according to claim 1, wherein the prevailing operating conditions include ambient temperature.

10. The method according to claim 2, further comprising the step of controlling a pressure prevailing in the receiver by operating the receiver compressor in the case that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor, and controlling the pressure prevailing in the receiver by operating an opening degree of the bypass valve in the case that the derived mass flow rate of refrigerant through the bypass valve is lower than the derived minimum mass flow rate of refrigerant required to operate the receiver compressor.

11. The method according to claim 2, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve is further based on an opening degree of the bypass valve.

12. The method according to claim 3, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve is further based on an opening degree of the bypass valve.

13. The method according to claim 2, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve comprises modelling a density of the refrigerant under the prevailing operating conditions.

14. The method according to claim 3, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve comprises modelling a density of the refrigerant under the prevailing operating conditions.

15. The method according to claim 4, wherein the step of deriving a mass flow rate of refrigerant through the bypass valve comprises modelling a density of the refrigerant under the prevailing operating conditions.

16. The method according to claim 2, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises modelling a density of the refrigerant under the prevailing operating conditions.

17. The method according to claim 3, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises modelling a density of the refrigerant under the prevailing operating conditions.

18. The method according to claim 4, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises modelling a density of the refrigerant under the prevailing operating conditions.

19. The method according to claim 5, wherein the step of deriving a minimum mass flow rate of refrigerant required to operate the receiver compressor comprises modelling a density of the refrigerant under the prevailing operating conditions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

[0055] FIG. 1 is a diagrammatic view of a vapour compression system being controlled in accordance with a method according to an embodiment of the invention, and

[0056] FIG. 2 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION

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

[0058] 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 ambient or a secondary fluid flow across the heat rejecting heat exchanger 5, in such a manner that heat is rejected from the refrigerant.

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

[0060] In the evaporator 9, heat exchange takes place between the refrigerant flowing through the evaporator 9 and the ambient or a secondary fluid flow across the evaporator 9, in such a manner that heat is absorbed by the refrigerant, while the liquid part of the refrigerant is at least partly evaporated. Finally, the refrigerant leaving the evaporator 9 is supplied to the main compressor 3.

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

[0062] When the vapour compression system 1 of FIG. 1 is controlled in accordance with a method according to an embodiment of the invention, it is ensured that the receiver compressor 4 is only operated 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 an accurate foundation, taking the prevailing operating conditions into account. Thereby it is ensured that the receiver compressor 4 is applied whenever this is appropriate. This may, e.g., be obtained in the manner described below with reference to FIG. 2.

[0063] FIG. 2 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 13. At step 14 a pressure difference, ?P, across the bypass valve is obtained, e.g. by direct measurement or by deriving the pressure difference from one or more other measured parameters.

[0064] At step 15 a mass flow rate of refrigerant through the bypass valve is derived. The mass flow rate is derived based on the obtained pressure difference across the bypass valve, and possibly on further relevant parameters, such as an opening degree of the bypass valve. Furthermore, the mass flow rate is derived using a fluid model, and thereby expected behaviour of the refrigerant, under the given operating conditions, is taken into account. The derived mass flow rate of refrigerant through the bypass valve is thereby very accurate, and provides an accurate measure for the available amount of gaseous refrigerant.

[0065] At step 16 a minimum mass flow rate of refrigerant required to operate the receiver compressor is derived. The minimum mass flow rate is derived based on a minimum displacement volume of the receiver compressor, i.e. on the minimum volume which the receiver compressor must displace in order to operate in a stable manner and without too many starts and stops. Furthermore, the minimum mass flow rate is derived using a fluid model which takes the prevailing operating conditions into account. Thereby the derived minimum mass flow rate provides a very accurate measure for the mass flow rate which needs to be available in order to ensure stable operation of the receiver compressor, under the prevailing operating conditions.

[0066] At step 17 the derived mass flow rate of refrigerant through the bypass valve and the derived minimum mass flow rate of refrigerant required to operate the receiver compressor are compared in order to determine whether or not the currently available amount of gaseous refrigerant is sufficient to ensure stable operation of the receiver compressor.

[0067] Thus, in the case that step 17 reveals that the derived mass flow rate of refrigerant through the bypass valve exceeds the derived minimum mass flow rate of refrigerant required to operate the receiver compressor, it can be concluded that the available amount of gaseous refrigerant is sufficient to ensure stable operation of the receiver compressor. Therefore, when this is the case, the process is forwarded to step 18, where the bypass valve is closed and the receiver compressor is started. Thereby, the refrigerant leaving the receiver is supplied to the receiver compressor, rather than to the bypass valve, and the vapour compression system is operated in an energy efficient manner.

[0068] In the case that the comparison of step 17 reveals that the derived mass flow rate of refrigerant through the bypass valve does not exceed the derived minimum mass flow rate of refrigerant required to operate the receiver compressor, it can be concluded that the available amount of gaseous refrigerant is not sufficient to ensure stable operation of the receiver compressor. Therefore, when this is the case, the process is forwarded to step 19, where the bypass valve is kept open and the receiver compressor is kept in a stopped state. Thereby, the refrigerant leaving the receiver is supplied to the bypass valve, rather than to the receiver compressor, and repeated stops and starts of the receiver compressor, due to the insufficient amount of available gaseous refrigerant, are prevented.

[0069] Finally, at step 20, the pressure prevailing in the receiver is controlled by appropriately controlling the capacity of the receiver compressor, or by appropriately controlling the opening degree of the bypass valve, depending on the outcome of the comparison of step 17.

[0070] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.