A METHOD FOR CONTROLLING A VAPOUR COMPRESSION SYSTEM AT A REDUCED SUCTION PRESSURE

Abstract

A method for controlling a vapour compression system (1) including a compressor unit (2) including one or more compressors (3, 12), a heat rejecting heat exchanger (4), a receiver (6), an expansion device (7) and an evaporator (8) arranged in a refrigerant path. A pressure value indicating a pressure prevailing inside the receiver (6) is obtained, and the obtained pressure value is compared to a first threshold pressure value. In the case that the obtained pressure value is below the first threshold pressure value, the compressor(s) (3, 12) of the compressor unit (2) are controlled in order to reduce a suction pressure of the vapour compression system (1).

Claims

1. A method for controlling a vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, a receiver, an expansion device and an evaporator arranged in a refrigerant path, the expansion device being arranged to control a supply of refrigerant to the evaporator, the method comprising the steps of: obtaining a pressure value indicating a pressure prevailing inside the receiver, comparing the obtained pressure value to a first threshold pressure value, and in the case that the obtained pressure value is below the first threshold pressure value, controlling the compressor(s) of the compressor unit in order to reduce a suction pressure of the vapour compression system.

2. The method according to claim 1, wherein the step of controlling the compressor(s) of the compressor unit comprises the steps of: reducing a suction pressure setpoint value from an initial suction pressure setpoint value, P.sub.0,set, to a reduced suction pressure setpoint value, P.sub.0,red, and controlling the compressor(s) of the compressor unit based on the reduced suction pressure setpoint value, P.sub.0,red.

3. The method according to claim 1, wherein the step of reducing the suction pressure comprises increasing the compressor capacity of the compressor unit.

4. The method according to claim 1, further comprising the step of adjusting a secondary fluid flow across the heat rejecting heat exchanger, based on the obtained pressure value.

5. The method according to claim 1, wherein the compressor unit comprises at least one main compressor being fluidly connected to an outlet of the evaporator and at least one receiver compressor being fluidly connected to a gaseous outlet of the receiver, and wherein the method further comprises the step of controlling the at least one receiver compressor based on the obtained pressure value.

6. The method according to claim 1, wherein the step of obtaining a pressure value comprises measuring the pressure prevailing inside the receiver.

7. The method according to claim 1, wherein the step of controlling the compressor(s) of the compressor unit comprises adjusting a compressor capacity of the compressor unit.

8. The method according to claim 7, wherein the step of adjusting a compressor capacity of the compressor unit comprises switching one or more compressors on or off.

9. The method according to claim 1, further comprising the steps of: after controlling the compressor(s) of the compressor unit in order to reduce the suction pressure of the vapour compression system, monitoring the pressure prevailing inside the receiver, comparing the monitored pressure prevailing inside the receiver to a second threshold pressure value, and in the case that the monitored pressure prevailing inside the receiver is above the second threshold pressure value, controlling the compressor(s) of the compressor unit in order to increase the suction pressure.

10. The method according to claim 1, further comprising the step of dynamically determining the first threshold pressure value.

11. The method according to claim 10, wherein the step of dynamically determining the first threshold pressure value comprises determining the first threshold value based on a varying initial suction pressure setpoint value, P.sub.0,set.

12. The method according to claim 11, wherein the step of dynamically determining the first threshold pressure value comprises adding a predefined pressure difference, ΔP, to the initial suction pressure setpoint value, P.sub.0,set, and applying the result as the first threshold pressure value, P.sub.thres=P.sub.0,set+ΔP.

13. The method according to claim 2, further comprising the step of adjusting a secondary fluid flow across the heat rejecting heat exchanger, based on the obtained pressure value.

14. The method according to claim 3, further comprising the step of adjusting a secondary fluid flow across the heat rejecting heat exchanger, based on the obtained pressure value.

15. The method according to claim 2, wherein the compressor unit comprises at least one main compressor being fluidly connected to an outlet of the evaporator and at least one receiver compressor being fluidly connected to a gaseous outlet of the receiver, and wherein the method further comprises the step of controlling the at least one receiver compressor based on the obtained pressure value.

16. The method according to claim 3, wherein the compressor unit comprises at least one main compressor being fluidly connected to an outlet of the evaporator and at least one receiver compressor being fluidly connected to a gaseous outlet of the receiver, and wherein the method further comprises the step of controlling the at least one receiver compressor based on the obtained pressure value.

17. The method according to claim 4, wherein the compressor unit comprises at least one main compressor being fluidly connected to an outlet of the evaporator and at least one receiver compressor being fluidly connected to a gaseous outlet of the receiver, and wherein the method further comprises the step of controlling the at least one receiver compressor based on the obtained pressure value.

18. The method according to claim 2, wherein the step of obtaining a pressure value comprises measuring the pressure prevailing inside the receiver.

19. The method according to claim 3, wherein the step of obtaining a pressure value comprises measuring the pressure prevailing inside the receiver.

20. The method according to claim 4, wherein the step of obtaining a pressure value comprises measuring the pressure prevailing inside the receiver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The invention will now be described in further detail with reference to the accompanying drawings in which

[0059] FIGS. 1-4 are diagrammatic views of four different vapour compression systems, each being controlled in accordance with a method according to an embodiment of the invention, and

[0060] FIG. 5 is a log(P)-h diagram illustrating control of a vapour compression system in accordance with a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0061] FIG. 1 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2 comprising one or more compressors 3, one of which is shown, a heat rejecting heat exchanger 4, a high pressure valve 5, a receiver 6, an expansion valve 7 and an evaporator 8 arranged in a refrigerant path.

[0062] Refrigerant flowing in the refrigerant path is compressed by the compressor 3 before being supplied to the heat rejecting heat exchanger 4. In the heat rejecting heat exchanger 4, heat exchange takes place between the refrigerant flowing through the heat rejecting heat exchanger 4 and the ambient or a secondary fluid flow across the heat rejecting heat exchanger 4, in such a manner that heat is rejected from the refrigerant. In the case that the heat rejecting heat exchanger 4 is in the form of a condenser, the refrigerant is thereby at least partly condensed. In the case that the heat rejecting heat exchanger 4 is in the form of a gas cooler, the refrigerant flowing through the heat rejecting heat exchanger 4 is cooled, but it remains in a gaseous or trans-critical state.

[0063] The refrigerant leaving the heat rejecting heat exchanger 4 is passed through the high pressure valve 5, where it undergoes expansion before being supplied to the receiver 6. In the receiver 6, the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant leaves the receiver 6 via a liquid outlet 9, and is supplied to the expansion device 7, where it undergoes expansion before being supplied to the evaporator 8. The refrigerant being supplied to the evaporator 8 is thereby in a mixed gaseous and liquid state.

[0064] In the evaporator 8, heat exchange takes place between the refrigerant flowing through the evaporator 8 and the ambient or a secondary fluid flow across the evaporator 8, 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 8 is once again supplied to the compressor 3.

[0065] The gaseous part of the refrigerant in the receiver 6 may be supplied directly to the compressor 3, via a gaseous outlet 10 and a bypass valve 11.

[0066] The vapour compression system 1 may be controlled in the following manner. A pressure value indicating a pressure prevailing inside the receiver 6 is obtained, e.g. by directly measuring the pressure by means of a pressure sensor arranged inside the receiver 6. The obtained pressure value is then compared to a first threshold pressure value. The first threshold pressure value may represent a pressure level inside the receiver 6, below which there is a risk that the vapour compression system 1 may not operate in an appropriate manner, because a low pressure inside the receiver 6 may lead to an insufficient supply of refrigerant to the evaporator 8.

[0067] In the case that the comparison reveals that the obtained pressure value is below the first threshold pressure value, the compressor 3 is operated in order to reduce the suction pressure of the vapour compression system 1, i.e. the pressure of refrigerant being supplied to the compressor 3. This may, e.g., be obtained by increasing the compressor capacity of the compressor unit 2, e.g. by increasing a speed of the compressor 3, or by switching on an additional compressor 3. Alternatively, the suction pressure may be reduced by reducing a suction pressure setpoint value from an initial suction pressure setpoint value, P.sub.0,set, to a reduced suction pressure setpoint value, P.sub.0,red, and then control the compressor 3 based on the reduced suction pressure setpoint value, P.sub.0,red.

[0068] In the case that it is subsequently revealed that the pressure prevailing inside the receiver 6 has increased to a level where there is no longer a risk that the vapour compression system 1 may not operate in an appropriate manner, the suction pressure may once again be increased. This may, e.g., be obtained by restoring the initial suction pressure setpoint value, P.sub.0,set, and then control the compressor 3 based on the restored, initial suction pressure setpoint value, P.sub.0,set.

[0069] FIG. 2 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a second embodiment of the invention. The vapour compression system 1 of FIG. 2 is very similar to the vapour compression system 1 of FIG. 1, and it will therefore not be described in detail here.

[0070] In the vapour compression system 1 of FIG. 2 the compressor unit 2 further comprises a receiver compressor 12 connected to the gaseous outlet 10 of the receiver 6. Thereby gaseous refrigerant from the receiver 6 may be supplied directly to the receiver compressor 12, and may therefore be compressed without having to be mixed with refrigerant leaving the evaporator 8, and thereby without affecting the suction pressure of the vapour compression system 1.

[0071] The vapour compression system 1 of FIG. 2 may be controlled essentially as described above with reference to FIG. 1. Furthermore, the pressure prevailing inside the receiver 6 may be controlled by controlling the receiver compressor 12.

[0072] FIG. 3 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a third embodiment of the invention. The vapour compression system 1 of FIG. 3 is very similar to the vapour compression system 1 of FIG. 2, and it will therefore not be described in detail here.

[0073] The vapour compression system 1 of FIG. 3 is not provided with a high pressure valve. Accordingly, the refrigerant leaving the heat rejecting heat exchanger 4 is supplied directly to the receiver 6 without undergoing expansion. The vapour compression system 1 of FIG. 3 may be controlled essentially as described above with reference to FIG. 1.

[0074] FIG. 4 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to a fourth embodiment of the invention. The vapour compression system 1 of FIG. 4 is very similar to the vapour compression system 1 of FIG. 2, and it will therefore not be described in detail here.

[0075] In the vapour compression system 1 of FIG. 4, an ejector 13 is arranged fluidly in parallel with the high pressure valve 5. Accordingly, refrigerant leaving the heat rejecting heat exchanger 4 may pass through the high pressure valve 5 or through the ejector 13. The ejector 13 further has its secondary inlet connected to the outlet of the evaporator 8. Accordingly, refrigerant leaving the evaporator 8 may either be supplied to the compressor 3 or to the ejector 13. The vapour compression system 1 of FIG. 4 may be controlled essentially as described above with reference to FIG. 1.

[0076] FIG. 5 is a log(P)-h diagram illustrating control of a vapour compression system in accordance with a method according to an embodiment of the invention. The vapour compression system being controlled could, e.g., be one of the vapour compression systems illustrated in FIGS. 1-4. From point 14 to point 15 the refrigerant is compressed in the compressor unit. Thereby the pressure as well as the enthalpy is increased. From point 15 to point 16 the refrigerant passes through the heat rejecting heat exchanger, where heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant. Thereby the enthalpy is decreased, while the pressure remains constant.

[0077] From point 16 to point 17 the refrigerant passes through a high pressure valve or an ejector, where the refrigerant undergoes expansion, and is received in the receiver. Thereby the pressure is decreased, while the enthalpy remains substantially constant.

[0078] In the receiver the refrigerant is separated into a liquid part and a gaseous part. Point 18 represents the liquid part of the refrigerant in the receiver, and point 19 represents the gaseous part of the refrigerant in the receiver. From point 18 to point 20 the liquid part of the refrigerant in the receiver is passed through the expansion device, where it undergoes expansion. Thereby the pressure is reduced while the enthalpy remains constant. From point 20 to point 14 the refrigerant passes through the evaporator, where heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the evaporator, in such a manner that heat is absorbed by the refrigerant. Thereby the enthalpy is increased, while the pressure remains constant.

[0079] From point 19 to point 14 the gaseous part of the refrigerant in the receiver is supplied to the suction line via a bypass valve, and is thereby mixed with refrigerant leaving the evaporator. Passing the refrigerant through the bypass valve causes the pressure to decrease while the enthalpy remains constant.

[0080] The position of the point 17 corresponds to the enthalpy of the refrigerant which leaves the heat rejecting heat exchanger and is supplied to the receiver. This enthalpy determines the liquid-vapour ratio of the refrigerant entering the receiver, and the liquid-vapour ratio of the refrigerant entering the receiver has an impact on the pressure prevailing in the receiver. Thus, when the enthalpy of the refrigerant entering the receiver is low, corresponding to the point 17 being arranged far to the left, a large portion of the refrigerant entering the receiver is liquid. Similarly, when the enthalpy of the refrigerant entering the receiver is high, corresponding to the point 17 being arranged far to the right, a large portion of the refrigerant entering the receiver is gaseous, i.e. in the form of vapour.

[0081] Accordingly, the liquid-vapour ratio of the refrigerant in the receiver, and thereby the pressure prevailing inside the receiver, can be adjusted by adjusting the enthalpy of the refrigerant leaving the heat rejecting heat exchanger. This may be done by adjusting a secondary fluid flow across the heat rejecting heat exchanger, e.g. by adjusting a fan speed of one or more fans driving this flow. Adjusting the secondary fluid flow has an impact on the heat transfer taking place in the heat rejecting heat exchanger, and this in turn affects the enthalpy of the refrigerant leaving the heat rejecting heat exchanger.

[0082] Thus, adjusting a secondary fluid flow across the heat rejecting heat exchanger is one way of controlling the pressure prevailing inside the receiver. Preferably, the liquid-vapour ratio of the refrigerant entering the receiver should be such that at least 5% of the refrigerant is in the form of vapour.

[0083] Furthermore, in order to ensure a sufficient refrigerant supply to the evaporator, a certain minimum pressure difference between the pressure prevailing inside the receiver and the suction pressure, i.e. the pressure difference across the expansion device, must be maintained. This pressure difference is represented by the difference between the pressure at point 19, representing the pressure prevailing inside the receiver, and the pressure at point 14, representing the suction pressure.

[0084] In the case that this pressure difference becomes too small, it may initially be attempted to increase the pressure prevailing inside the receiver, e.g. in the manner described above. If this is not sufficient to maintain the minimum pressure difference, the suction pressure may be reduced instead, thereby shifting point 14 downwards, i.e. towards a lower pressure value. This could, e.g., be done in the manner described above with reference to FIG. 1.

[0085] 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.