A METHOD FOR CONTROLLING SUCTION PRESSURE OF A VAPOUR COMPRESSION SYSTEM

Abstract

A method for controlling a vapour compression system (1) is disclosed. The vapour compression system (1) includes an ejector (4), and has a non-return valve (11) arranged in the refrigerant path between an outlet (12) of an evaporator (7) and an inlet (10) of a compressor unit (2), in such a manner that a refrigerant flow from the outlet (12) of the evaporator (7) towards the inlet (10) of the compressor unit (2) is allowed, while a fluid flow from the inlet (10) of the compressor unit (2) towards the outlet (12) of the evaporator (7) is prevented. A pressure, P.sub.0, of refrigerant leaving the evaporator (7) is measured and a value being representative for a pressure, P.sub.suc, of refrigerant entering the compressor unit (2) is obtained. The pressures, P.sub.0 and P.sub.suc, are compared to respective reference pressure values, P.sub.0,ref and P.sub.suc,ref. In the case that ε.sub.0>ε.sub.suc, where ε.sub.0=P.sub.0−P.sub.0,ref and ε.sub.suc=P.sub.suc−P.sub.suc,ref, the compressor unit (2) is controlled based on P.sub.0, and in the case that ε.sub.suc>ε.sub.0, the compressor unit (2) is controlled based on P.sub.suc.

Claims

1. A method for controlling a vapour compression system, the vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, an ejector, a receiver, at least one expansion device and at least one evaporator arranged in a refrigerant path, an outlet of the heat rejecting heat exchanger being connected to a primary inlet of the ejector, an outlet of the ejector being connected to an inlet of the receiver, and an outlet of the evaporator being connected to a secondary inlet of the ejector and to an inlet of the compressor unit, wherein the vapour compression system further comprises a non-return valve arranged in the refrigerant path between the outlet of the evaporator and the inlet of the compressor unit, in such a manner that a refrigerant flow from the outlet of the evaporator towards the inlet of the compressor unit is allowed, while a fluid flow from the inlet of the compressor unit towards the outlet of the evaporator is prevented, and wherein a gaseous outlet of the receiver is connected to the inlet of the compressor unit via a bypass valve, the method comprising the steps of: measuring a pressure, P.sub.0, of refrigerant leaving the evaporator, obtaining a value being representative for a pressure, P.sub.suc, of refrigerant entering the compressor unit, comparing the pressures, P.sub.0 and P.sub.suc, to respective reference pressure values, P.sub.0,ref and P.sub.suc,ref, in the case that ε.sub.0>ε.sub.suc, where ε.sub.0=P.sub.0−P.sub.0,ref and ε.sub.suc=P.sub.suc−P.sub.suc,ref, controlling the compressor unit based on P.sub.0, and in the case that ε.sub.suc>ε.sub.0, controlling the compressor unit based on P.sub.suc.

2. The method according to claim 1, wherein P.sub.suc,ref is selected in such a manner that P.sub.suc,ref=P.sub.0,ref+ΔP.sub.max, where ΔP.sub.max is a maximum attainable pressure lift provided by the ejector.

3. The method according to claim 1, wherein the vapour compression system comprises at least one medium temperature evaporator and at least one low temperature evaporator, and wherein the pressure, P.sub.0, is measured at an outlet of the medium temperature evaporator.

4. The method according to claim 3, wherein the vapour compression system further comprises a low temperature compressor unit, and wherein an outlet of the low temperature evaporator is connected to an inlet of the low temperature compressor unit, and an outlet of the low temperature compressor unit is connected to the inlet of the compressor unit.

5. The method according to claim 1, further comprising the step of controlling a pressure prevailing inside the receiver by adjusting an opening degree of the bypass valve.

6. The method according to claim 1, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring P.sub.suc.

7. The method according to claim 1, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring a pressure prevailing inside the receiver and deriving P.sub.suc from the pressure prevailing inside the receiver.

8. The method according to claim 1, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises deriving P.sub.suc from P.sub.0.

9. The method according to claim 1, wherein the step of controlling the compressor unit based on P.sub.0 comprises controlling the compressor unit in order to obtain that P.sub.0=P.sub.0,ref, and/or the step of controlling the compressor unit based on P.sub.suc comprises controlling the compressor unit in order to obtain that P.sub.suc=P.sub.suc,ref.

10. The method according to claim 2, further comprising the step of controlling a pressure prevailing inside the receiver by adjusting an opening degree of the bypass valve.

11. The method according to claim 3, further comprising the step of controlling a pressure prevailing inside the receiver by adjusting an opening degree of the bypass valve.

12. The method according to claim 4, further comprising the step of controlling a pressure prevailing inside the receiver by adjusting an opening degree of the bypass valve.

13. The method according to claim 2, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring P.sub.suc.

14. The method according to claim 3, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring P.sub.suc.

15. The method according to claim 4, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring P.sub.suc.

16. The method according to claim 5, wherein the step of obtaining a value being representative for the pressure, P.sub.suc, comprises measuring P.sub.suc.

17. The method according to claim 2, wherein the step of controlling the compressor unit based on P.sub.0 comprises controlling the compressor unit in order to obtain that P.sub.0=P.sub.0,ref, and/or the step of controlling the compressor unit based on P.sub.suc comprises controlling the compressor unit in order to obtain that P.sub.suc=P.sub.suc,ref.

18. The method according to claim 3, wherein the step of controlling the compressor unit based on P.sub.0 comprises controlling the compressor unit in order to obtain that P.sub.0=P.sub.0,ref, and/or the step of controlling the compressor unit based on P.sub.suc comprises controlling the compressor unit in order to obtain that P.sub.suc=P.sub.suc,ref.

19. The method according to claim 3, wherein the step of controlling the compressor unit based on P.sub.0 comprises controlling the compressor unit in order to obtain that P.sub.0=P.sub.0,ref, and/or the step of controlling the compressor unit based on P.sub.suc comprises controlling the compressor unit in order to obtain that P.sub.suc.sup.=P.sub.suc,ref.

20. The method according to claim 4, wherein the step of controlling the compressor unit based on P.sub.0 comprises controlling the compressor unit in order to obtain that P.sub.0=P.sub.0,ref, and/or the step of controlling the compressor unit based on P.sub.suc comprises controlling the compressor unit in order to obtain that P.sub.suc=P.sub.suc,ref.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The invention will now be described in further detail with reference to the accompanying drawings in which

[0053] FIG. 1 is a diagrammatic view of a vapour compression system being operated according to a method according to a first embodiment of the invention,

[0054] FIG. 2 is a diagrammatic view of a vapour compression system being operated according to a method according to a second embodiment of the invention,

[0055] FIG. 3 is a diagrammatic view of a vapour compression system being operated according to a method according to a third embodiment of the invention,

[0056] FIG. 4 is a graph illustrating pressure conditions in a vapour compression system being operated in accordance with a method according to an embodiment of the invention, and

[0057] FIG. 5 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0058] FIG. 1 is a diagrammatic view of a vapour compression system 1 being operated in accordance with a method according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2, a heat rejecting heat exchanger 3, an ejector 4, a receiver 5, three expansion devices 6 and three evaporators 7 arranged in a refrigerant path. The evaporators 7 are arranged fluidly in parallel, and each of the expansion devices 6 supplies refrigerant to one of the evaporators 7. A bypass valve 8 interconnects a gaseous outlet 9 of the receiver 5 and an inlet 10 of the compressor unit 2. A non-return valve 11 is arranged in the refrigerant path between an outlet 12 of the evaporators 7 and the inlet 10 of the compressor unit 2.

[0059] Refrigerant flowing in the refrigerant path is compressed by the compressor unit 2. The compressed refrigerant is supplied to the heat rejecting heat exchanger 3, where heat exchange takes place with the ambient in such a manner that heat is rejected from the refrigerant. The refrigerant leaving the heat rejecting heat exchanger 3 is supplied to a primary inlet 13 of the ejector 4. In the ejector 4, the refrigerant undergoes expansion, and is supplied to the receiver 5. In the receiver 5, the liquid part of the refrigerant is separated from the gaseous part of the refrigerant.

[0060] The liquid part of the refrigerant in the receiver 5 is supplied to the expansion devices 6, where it undergoes expansion before being supplied to the respective evaporators 7. In the evaporators 7, heat exchange takes place between the refrigerant and the ambient in such a manner that heat is absorbed by the refrigerant, while the liquid part of the refrigerant is at least partly evaporated.

[0061] The refrigerant leaving the evaporators 7 may either be supplied to the inlet 10 of the compressor unit 2, via the non-return valve 11, or it may be supplied to a secondary inlet 14 of the ejector 4.

[0062] When performing the method according to the invention, a pressure, P.sub.0, of refrigerant leaving the evaporators 7 is measured by means of sensor 15, and a pressure, P.sub.suc, of refrigerant entering the compressor unit 2 is measured by means of sensor 16. As an alternative, P.sub.suc could be obtained in an alternative manner, e.g. by deriving P.sub.suc from one or more other measured parameters, e.g. P.sub.0 or a pressure prevailing inside the receiver 5.

[0063] P.sub.0 and P.sub.suc are then compared to respective reference pressure values, P.sub.0,ref and P.sub.suc,ref, and it is investigated whether ε.sub.0>ε.sub.suc or ε.sub.suc>ε.sub.0, where ε=P.sub.0−P.sub.0,ref and ε=P.sub.sucP.sub.suc,ref. ε.sub.0 and ε.sub.suc mey be referred to as error values.

[0064] If it turns out that ε.sub.0>ε.sub.suc, then P.sub.suc is closer to P.sub.suc,ref than P.sub.0 is to P.sub.0,ref. This indicates that the pressure prevailing in the part of the refrigerant path between the non-return valve 11 and the inlet 10 of the compressor unit 2, i.e. P.sub.suc, is under control. On the other hand, it is very desirable to ensure that P.sub.0 is very close to P.sub.0,ref, because thereby it is ensured that the performance of the evaporators 7 is optimised. Therefore, when ε.sub.0>ε.sub.suc the compressor unit 2 is controlled based on P.sub.0. More particularly, the capacity of the compressor unit 2 is adjusted in order ensure a refrigerant supply to the evaporators 7 which results in P.sub.0 being as close to P.sub.0,ref as possible, i.e. minimising go.

[0065] If it turns out that ε.sub.suc>ε.sub.0, then P.sub.0 is closer to P.sub.0,ref than P.sub.suc is to P.sub.suc,ref. This indicates that the pressure prevailing in the part of the refrigerant path between the non-return valve 11 and the inlet 10 of the compressor unit 2, i.e. P.sub.suc, might be increasing towards an undesirable level. For instance, if the non-return valve 11 is closed, and all of the refrigerant which leaves the evaporators 7 is therefore supplied to the secondary inlet 14 of the ejector 4, this may lead to a situation where P.sub.suc increases while P.sub.0 remains steady. This is particularly the case if the bypass valve 8 is also fully open. If the compressor unit 2 is controlled based on P.sub.0 under these circumstances, there is a risk that P.sub.suc reaches an unacceptable level. Therefore, when this occurs, the compressor unit 2 is controlled based on P.sub.suc.

[0066] FIG. 2 is a diagrammatic view of a vapour compression system 1 being operated in accordance with a method according to a second embodiment of the invention. The vapour compression system 1 is very similar to the vapour compression system 1 of FIG. 1, and it will therefore not be described in detail here.

[0067] The vapour compression system 1 of FIG. 2 comprises three medium temperature evaporators 7a, corresponding to the evaporators 7 illustrated in FIG. 1, and three low temperature evaporators 7b, each receiving refrigerant from a separate expansion device 6b. The low temperature evaporators 7b are designed to provide a lower cooling temperature than the medium temperature evaporators 7a. As a consequence, the pressure prevailing in the low temperature evaporators 7b is also lower than the pressure prevailing in the medium temperature evaporators 7a. Therefore the refrigerant leaving the low temperature evaporators 7b is supplied to a low temperature compressor unit 17, in order to increase the pressure of the refrigerant before it reaches the compressor unit 2.

[0068] The refrigerant leaving the low temperature compressor unit 17 is supplied to the refrigerant path between the non-return valve 11 and the inlet 10 of the compressor unit 2. Thereby this part of the refrigerant path receives a refrigerant supply which is completely independent of the refrigerant flow out of the medium temperature evaporators 7a, and thereby completely decoupled from P.sub.0. Therefore, in this embodiment there is a particular risk that P.sub.suc increases while P.sub.0 remains steady, and the method described above with reference to FIG. 1 is therefore particularly relevant here.

[0069] FIG. 3 is a diagrammatic view of a vapour compression system 1 being operated in accordance with a method according to a third embodiment of the invention. The vapour compression system 1 is very similar to the vapour compression system 1 of FIG. 2, and it will therefore not be described in detail here.

[0070] In the vapour compression system 1 of FIG. 3 the refrigerant leaving the low temperature compressor unit 17 is supplied to the refrigerant path between the outlet 12 of the medium temperature evaporators 7b and the non-return valve 11. Thereby this supply of refrigerant directly affects P.sub.0, but only indirectly affects P.sub.suc.

[0071] FIG. 4 is a graph illustrating pressure conditions in a vapour compression system being operated in accordance with a method according to an embodiment of the invention. The vapour compression system could, e.g., be one of the vapour compression system shown in FIGS. 1-3.

[0072] Reference pressure values, P.sub.0,ref and P.sub.suc,ref, are shown. It can be seen that P.sub.suc,ref has been selected in such a manner that P.sub.suc,ref.sup.=P.sub.0,ref+ΔP.sub.max, where ΔP.sub.EX is a maximum attainable pressure lift provided by an ejector forming part of the vapour compression system.

[0073] Actual pressure values, P.sub.0 and P.sub.suc, have been measured and plotted as a function of time. It can be seen that initially P.sub.suc is well below the corresponding reference pressure value, P.sub.suc,ref, thereby indicating that P.sub.suc is within an acceptable range. The compressor unit of the vapour compression system is therefore controlled based on P.sub.0, resulting in P.sub.0 performing small variation around the corresponding reference pressure value, P.sub.0,ref.

[0074] At a certain point in time, P.sub.suc starts increasing, eventually to a level above P.sub.suc,ref. This introduces a risk that the pressure in the part of the refrigerant path which is connected to the inlet of the compressor unit may reach an unacceptable level. Therefore, when ε.sub.suc=P.sub.suc−P.sub.suc,ref reaches a level where it becomes larger than ε.sub.0=P.sub.0−P.sub.0,ref, the compressor unit is instead controlled based on P.sub.suc, in order to decrease P.sub.suc to a level corresponding to P.sub.suc,ref, or lower.

[0075] FIG. 5 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 18. At step 19, a pressure, P.sub.0, of refrigerant leaving the evaporator and a pressure, P.sub.suc, of refrigerant entering the compressor unit are measured. It should be noted that P.sub.suc, or another value being representative for P.sub.suc, could be obtained in another manner than by direct measurement, as described in detail above.

[0076] At step 20, error values, ε.sub.0 and ε.sub.suc, are derived as ε.sub.0=P.sub.0−P.sub.0,ref and ε.sub.suc=P.sub.suc−P.sub.suc,ref, where P.sub.0,ref and P.sub.suc,ref are reference pressure values corresponding to P.sub.0 and P.sub.suc, respectively.

[0077] At step 21 it is investigated whether 60>ε.sub.suc. If this is the case, the process is forwarded to step 22, where the compressor unit is controlled based on P.sub.0. In the case that step 21 reveals that ε.sub.0 is not larger than ε.sub.suc, the process is instead forwarded to step 23, where the compressor unit is controlled based on P.sub.suc. From step 22 as well as from step 23, the process is returned to step 19 for new measurements of P.sub.0 and P.sub.suc.

[0078] It should be noted that the error values, ε.sub.0 and ε.sub.suc, need not be expressly derived at step 20, as long as it is possible to perform the investigation of step 21.

[0079] 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.