METHOD FOR CONTROLLING EJECTOR CAPACITY IN A VAPOUR COMPRESSION SYSTEM

Abstract

A method for controlling ejector capacity in a vapour compression system (1) is disclosed. A parameter value being representative for a flow rate of liquid refrigerant from the evaporator(s) (8, 10) and into a return pipe (12, 13) is obtained, and the capacity of the ejector(s) (6) is adjusted based on the obtained parameter value. Ejector capacity may be shifted between low pressure ejectors (liquid ejectors) (6a, 6b, 6c, 6d) and high pressure ejectors (gas ejectors) (6e, 6f).

Claims

1. A method for controlling at least one ejector in a vapour compression system, the vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, at least one ejector, a receiver, at least one expansion device and at least one evaporator arranged in a refrigerant path, wherein each ejector is arranged in the refrigerant path with a primary inlet of the ejector connected to an outlet of the heat rejecting heat exchanger, an outlet of the ejector connected to the receiver and a secondary inlet of the ejector connected to a part of a return pipe receiving refrigerant from outlets of the evaporator(s), and wherein at least one of the ejector(s) is of a first, low pressure, kind, the method comprising the steps of: obtaining a pressure value of refrigerant leaving the heat rejecting heat exchanger, and/or a temperature value of refrigerant leaving the heat rejecting heat exchanger, and/or an ambient temperature value, and controlling at least the low pressure ejector(s) based on the obtained pressure value and/or temperature value.

2. The method according to claim 1, wherein the step of controlling at least the low pressure ejector(s) comprises preventing a flow of refrigerant from the outlet of the heat rejecting heat exchanger to the primary inlet of at least one low pressure ejector in the case that the pressure of refrigerant leaving the heat rejecting heat exchanger is above a predefined pressure threshold level and/or the temperature of refrigerant leaving the heat rejecting heat exchanger is above a predefined temperature threshold level.

3. The method according to claim 1, wherein the step of controlling at least the low pressure ejector(s) comprises allowing a flow of refrigerant from the outlet of the heat rejecting heat exchanger to the primary inlet of at least one low pressure ejector in the case that the pressure of refrigerant leaving the heat rejecting heat exchanger is below a predefined pressure threshold level and/or the temperature of refrigerant leaving the heat rejecting heat exchanger is below a predefined temperature threshold level.

4. The method according to claim 1, further comprising the step of obtaining a refrigerant pressure at an outlet of the ejector(s), and wherein the step of controlling at least the low pressure ejector(s) is further based on a pressure difference and/or a pressure ratio between the refrigerant pressure at the primary inlet of the ejector(s) and the refrigerant pressure at the outlet of the ejector(s).

5. The method according to claim 4, wherein the step of controlling at least the low pressure ejector(s) comprises: preventing a flow of refrigerant from the outlet of the heat rejecting heat exchanger to the primary inlet of at least one low pressure ejector in the case that the pressure difference and/or pressure ratio is above a predefined threshold level, and allowing a flow of refrigerant from the outlet of the heat rejecting heat exchanger to the primary inlet of at least one low pressure ejector in the case that the pressure difference and/or pressure ratio is below the predefined threshold level.

6. The method according to claim 1, further comprising the step of obtaining a refrigerant pressure at the secondary inlet of the ejector(s) and a refrigerant pressure at an outlet of the ejector(s), and wherein the step of controlling at least the low pressure ejector(s) is further based on a pressure difference and/or a pressure ratio between the refrigerant pressure at the secondary inlet of the ejector(s) and the refrigerant pressure at the outlet of the ejector(s).

7. The method according to claim 1, further comprising the step of calculating a pressure ratio: $\frac{P_{\mathrm{primary}}-P_{\mathrm{outlet}}}{P_{\mathrm{secondary}}-P_{\mathrm{outlet}}},$ where P.sub.primary is a pressure prevailing at the primary inlet of the ejector(s), P.sub.outlet is a pressure prevailing at the outlet of the ejector(s) and P.sub.secondary is a pressure prevailing at the secondary inlet of the ejector(s), and wherein the step of controlling at least the low pressure ejector(s) is further performed on the basis of the calculated pressure ratio.

7. The method according to claim 12, wherein the step of controlling at least the low pressure ejector(s) comprises increasing a capacity of the low pressure ejector(s) in the case that the calculated pressure ratio is below a predefined threshold level.

8. The method according to claim 2, wherein the step of controlling at least the low pressure ejector(s) comprises allowing a flow of refrigerant from the outlet of the heat rejecting heat exchanger to the primary inlet of at least one low pressure ejector in the case that the pressure of refrigerant leaving the heat rejecting heat exchanger is below a predefined pressure threshold level and/or the temperature of refrigerant leaving the heat rejecting heat exchanger is below a predefined temperature threshold level.

9. The method according claim 2, further comprising the step of obtaining a refrigerant pressure at an outlet of the ejector(s), and wherein the step of controlling at least the low pressure ejector(s) is further based on a pressure difference and/or a pressure ratio between the refrigerant pressure at the primary inlet of the ejector(s) and the refrigerant pressure at the outlet of the ejector(s).

10. The method according claim 3, further comprising the step of obtaining a refrigerant pressure at an outlet of the ejector(s), and wherein the step of controlling at least the low pressure ejector(s) is further based on a pressure difference and/or a pressure ratio between the refrigerant pressure at the primary inlet of the ejector(s) and the refrigerant pressure at the outlet of the ejector(s).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The invention will now be described in further detail with reference to the accompanying drawings in which

[0068] FIGS. 1-5 are diagrammatic views of vapour compression systems for performing methods according to various embodiments of the invention.

DETAILED DESCRIPTION

[0069] FIG. 1 is a diagrammatic view of a vapour compression system 1 for performing a method according to a first embodiment of the invention. The vapour compression system 1 comprises an MT compressor unit 2 and an LT compressor unit 3, each comprising a number of compressors. The vapour compression system 1 further comprises a heat rejecting heat exchanger 4, a high pressure valve 5, and ejector 6 and a receiver 7. A liquid outlet of the receiver 7 is connected to an MT evaporator 8 via an MT expansion valve 9 and to an LT evaporator 10 via an LT expansion valve 11. The evaporators 8, 10 are connected to an inlet of the MT compressor unit 2 via respective return pipes 12, 13.

[0070] The vapour compression system 1 of FIG. 1 may be operated in the following manner. Refrigerant is compressed by the compressors of the MT compressor unit 2 and 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.

[0071] The refrigerant leaving the heat rejecting heat exchanger 4 passes through high pressure valve 5 or through the ejector 6, via a primary inlet of the ejector 6, before being supplied to the receiver 7. The refrigerant passing through the high pressure valve 5 or the ejector 6, respectively, undergoes expansion, and the refrigerant being supplied to the receiver 7 is therefore in a mixed gaseous and liquid state.

[0072] In the receiver 7, the refrigerant is separated into a liquid part and a gaseous part. The gaseous part of the refrigerant may be supplied to a liquid separator 14 forming part of return pipe 12, via a gas bypass valve 15. The liquid part of the refrigerant is supplied to the evaporators 9, 10, via the expansion valves 9, 11.

[0073] In the evaporators 8, 10, heat exchange takes place between the refrigerant flowing through the respective evaporator 8, 10 and the ambient or a secondary fluid flow across the evaporator 8, 10, in such a manner that heat is absorbed by the refrigerant, thereby providing cooling. The MT evaporator 8 is arranged to provide cooling within a first temperature range, and the LT evaporator 10 is arranged to provide cooling within a second temperature range, the second temperature range being lower than the first temperature range. For instance, the MT evaporator 8 may be applied for providing cooling to chilled display cases in which a temperature of approximately 5° C. is required, while the LT evaporator 10 may be applied for providing cooling to freezer display cases in which a temperature of approximately −18° C. is required. The refrigerant leaving the LT evaporator 10 will normally be at a lower pressure level than the refrigerant leaving the MT evaporator 8. It is noted that, even though only one MT evaporator 8 and one LT evaporator 10 are shown in FIG. 1, it is not ruled out that the vapour compression system 1 could comprise two or more MT evaporators 8 and/or two or more LT evaporators 10, e.g. arranged fluidly in parallel.

[0074] The refrigerant leaving the LT evaporator 10 is supplied to the LT compressor unit 3, where the refrigerant is compressed, thereby increasing the pressure, before the refrigerant is supplied to the MT compressor unit 2.

[0075] The refrigerant leaving the MT evaporator 10 is supplied to the liquid separator 14. In the case that the refrigerant leaving the MT evaporator 10 contains a liquid part, the liquid part of the refrigerant is separated from the gaseous part of the refrigerant in the liquid separator 14. Thereby it is prevented that liquid refrigerant reaches the MT compressor unit 2.

[0076] At least part of the gaseous part of the refrigerant in the liquid separator 14 is supplied to the MT compressor unit 2. The liquid part of the refrigerant in the liquid separator 14, and possibly part of the gaseous part of the refrigerant, is supplied to the secondary inlet of the ejector 6.

[0077] The ejector 6 may be operated in the following manner. A parameter being representative for a flow rate of liquid refrigerant from the MT evaporator 8 and into the return pipe 12 is obtained. The parameter could, e.g., be in the form of a compressor capacity of the MT compressor unit 2, a number of flooded MT evaporators 8, an estimated or measured value for the flow rate of liquid refrigerant in the return pipe 12 and/or a flow rate of refrigerant at the outlet of the heat rejecting heat exchanger 4. This has been described in detail above.

[0078] Since the obtained parameter is representative for a flow rate of liquid refrigerant from the MT evaporator 8 and into the return pipe 12, the parameter reflects the current need for removing liquid refrigerant from the return pipe 12, in order to prevent liquid refrigerant from reaching the MT compressor unit 2.

[0079] Thus, in the case that the obtained parameter indicates that the current capacity of the ejector 6 is insufficient to meet the current requirements with respect to removal of liquid refrigerant from the return pipe 12, the capacity of the ejector 6 is increased. Similarly, in the case that the obtained parameter indicates that the current capacity of the ejector 6 is higher than required, the capacity of the ejector 6 may be reduced.

[0080] As an alternative, the ejector 6 may be operated in the following manner. The pressure of refrigerant leaving the heat rejecting heat exchanger 4 may be obtained, e.g. by direct measurement. Alternatively, the temperature of refrigerant leaving the heat rejecting heat exchanger 4 or an ambient temperature may be measured. Based thereon, the ejector 6 may be controlled. For instance, the capacity of the ejector 6 may be decreased in the case that the pressure of refrigerant leaving the heat rejecting heat exchanger 4 is above a predefined threshold value, and the capacity of the ejector 6 may be increased in the case that the pressure of refrigerant leaving the heat rejecting heat exchanger 4 is below the predefined threshold value.

[0081] The capacity of the ejector 6 may, e.g., be adjusted by adjusting the supply of refrigerant from the outlet of the heat rejecting heat exchanger 4 to the primary inlet of the ejector 6. For instance, a valve controlling this refrigerant flow may be opened or closed, or an opening degree of such a valve may be adjusted. Alternatively or additionally, an opening degree of the high pressure valve 5 may be adjusted in order to increase or decrease the fraction of refrigerant flowing via the high pressure valve 5, thereby decreasing or increasing the fraction of refrigerant flowing via the ejector 6 correspondingly.

[0082] FIG. 2 is a diagrammatic view of a vapour compression system 1 for performing 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.

[0083] In the vapour compression system 1 of FIG. 2, the refrigerant leaving the MT evaporator 8 as well as the refrigerant leaving the LT compressor unit 3 is supplied to a common return pipe 12. No liquid separator is arranged in the return pipe 12.

[0084] A receiver compressor 16 is connected directly to the gaseous outlet of the receiver 7. Thereby gaseous refrigerant can be supplied directly from the receiver 7 to the receiver compressor 16, thereby avoiding the pressure drop introduced in the expansion valves 9, 11 or in the gas bypass valve 15.

[0085] The vapour compression system 1 comprises four ejectors 6a, 6b, 6c, 6d arranged in parallel between the outlet of the heat rejecting heat exchanger 4 and the receiver 7. The ejectors 6a, 6b, 6c, 6d each has a capacity which varies from the capacity of each of the other ejectors 6a, 6b, 6c, 6d. Thus, ejector 6a has the highest capacity and ejector 6d has the lowest capacity. Ejector 6b has a capacity which is lower than the capacity of ejector 6a, but higher than the capacity of ejectors 6c and 6d, and ejector 6c has a capacity which is lower than the capacity of ejectors 6a and 6b, but higher than the capacity of ejector 6d.

[0086] Accordingly, by appropriately selecting which of the ejectors 6a, 6b, 6c, 6d should be switched on, i.e. receive refrigerant via its primary inlet, and which of the ejectors 6a, 6b, 6c, 6d should be switched off, i.e. not receive refrigerant via its primary inlet, the total capacity of the ejectors 6a, 6b, 6c, 6d can be adjusted.

[0087] FIG. 3 is a diagrammatic view of a vapour compression system 1 for performing 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 systems 1 of FIGS. 1 and 2, and it will therefore not be described in detail here.

[0088] The vapour compression system 1 of FIG. 3 comprises six ejectors 6a, 6b, 6c, 6d, 6e, 6f arranged in parallel between the outlet of the heat rejecting heat exchanger 4 and the receiver 7. The ejectors 6a, 6b, 6c, 6d, 6e, 6f have various capacities, similarly to the situation described above with reference to FIG. 2.

[0089] Four of the ejectors 6a, 6b, 6c, 6d are in the form of low pressure ejectors (or liquid ejectors) and two of the ejectors 6e, 6f are in the form of high pressure ejectors (or gas ejectors). As described above, low pressure ejectors 6a, 6b, 6c, 6d normally operate efficiently when the pressure of refrigerant leaving the heat rejecting heat exchanger 4 is low, and when the pressure difference between the primary inlet of the ejector 6a, 6b, 6c, 6d and the outlet of the ejector 6a, 6b, 6c, 6d is therefore small. For instance, low pressure ejectors 6a, 6b, 6c, 6d are capable of providing a high pressure lift for the refrigerant supplied to the secondary inlet of the ejector 6a, 6b, 6c, 6d under these circumstances.

[0090] On the other hand, high pressure ejectors 6e, 6f often require a somewhat larger pressure difference between the primary inlet of the ejector 6e, 6f and the outlet of the ejector 6e, 6f in order to provide a high pressure lift for the refrigerant supplied to the secondary inlet of the ejector 6e, 6f. However, under these circumstances, high pressure ejectors 6e, 6f normally operate more efficiently than low pressure ejectors 6a, 6b, 6c, 6d.

[0091] When controlling the ejectors 6a, 6b, 6c, 6d, 6e, 6f, e.g. essentially as described above with reference to FIGS. 1 and 2, the control may include shifting ejector capacity between the low pressure ejectors 6a, 6b, 6c, 6d and the high pressure ejectors 6e, 6f. For instance, in the case that the obtained parameter being representative for a flow rate of liquid refrigerant from the MT evaporator 8 and into the return pipe 12 reveals that it is required that a relatively large amount of liquid refrigerant needs to be removed from the return pipe 12, then the capacity of the ejectors 6a, 6b, 6c, 6d, 6e, 6f may be adjusted in such a manner that the total capacity of the low pressure ejectors 6a, 6b, 6c, 6d is increased while the total capacity of the high pressure ejectors 6e, 6f is decreased. Thereby it is ensured that the ejectors 6a, 6b, 6c, 6d, 6e, 6f which are actually operating are capable of handling the liquid refrigerant flow towards the return pipe 12.

[0092] Similarly, in the case that it is revealed that the current operating conditions are such that the high pressure ejectors 6e, 6f are expected to operate more efficiently than the low pressure ejectors 6a, 6b, 6c, 6d, then the capacity of the ejectors 6a, 6b, 6c, 6d, 6e, 6f may be adjusted in such a manner that the total capacity of the low pressure ejectors 6a, 6b, 6c, 6d is decreased while the total capacity of the high pressure ejectors 6e, 6f is increased. Thereby it is ensured that the vapour compression system 1 is operated as efficiently as possible.

[0093] FIG. 4 is a diagrammatic view of a vapour compression system 1 for performing 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.

[0094] The vapour compression system 1 of FIG. 4 only comprises an MT compressor unit 2 and an MT evaporator 8, i.e. the LT compressor unit and the LT evaporator of the vapour compression system 1 of FIG. 2 are not present in the vapour compression system 1 of FIG. 4. The ejectors 6a, 6b, 6c, 6d of the vapour compression system 1 of FIG. 4 are controlled essentially as described above with reference to FIG. 2.

[0095] FIG. 5 is a diagrammatic view of a vapour compression system 1 for performing a method according to a fifth embodiment of the invention. The vapour compression system 1 of FIG. 5 is very similar to the vapour compression system 1 of FIG. 3, and it will therefore not be described in detail here.

[0096] The vapour compression system 1 of FIG. 5 only comprises an MT compressor unit 2 and an MT evaporator 8, i.e. the LT compressor unit and the LT evaporator of the vapour compression system 1 of FIG. 3 are not present in the vapour compression system 1 of FIG. 5. The ejectors 6a, 6b, 6c, 6d, 6e, 6f of the vapour compression system 1 of FIG. 5 are controlled essentially as described above with reference to FIG. 3.

[0097] 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.