A METHOD FOR CONTROLLING A VARIABLE CAPACITY EJECTOR UNIT

Abstract

A method for controlling a variable capacity ejector unit (7) arranged in a refrigeration system (1) is disclosed. An ejector control signal for the ejector unit (7) is generated, based on an obtained temperature and an obtained pressure of refrigerant leaving a heat rejecting heat exchanger (3), or on the basis of a high pressure valve control signal for controlling an opening degree of a high pressure valve (6) arranged fluidly in parallel with the ejector unit (7). The ejector control signal indicates whether the capacity of the ejector unit (7) should be increased, decreased or maintained. The capacity of the ejector unit (7) is controlled in accordance with the generated ejector control signal. The power consumption of the refrigeration system (1) is reduced, while the pressure of the refrigerant leaving the heat rejecting heat exchanger (3) is maintained at an acceptable level.

Claims

1. A method for controlling a variable capacity ejector unit arranged in a refrigeration system, said refrigeration system further comprising a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path, wherein the ejector unit is fluidly connected in the refrigerant path between the heat rejecting heat exchanger and the expansion device, the method comprising the steps of: obtaining a temperature and a pressure of refrigerant leaving the heat rejecting heat exchanger, generating an ejector control signal for the ejector unit, based on the obtained temperature and the obtained pressure, said ejector control signal indicating whether the capacity of the ejector unit should be increased, decreased or maintained, and controlling the capacity of the ejector unit in accordance with the generated ejector control signal.

2. The method according to claim 1, wherein the step of generating an ejector control signal comprises the steps of: calculating a reference pressure value on the basis of the obtained temperature, comparing the calculated reference pressure value to the obtained pressure, and generating the ejector control signal based on said comparison.

3. The method according to claim 1, wherein the refrigeration system further comprises a high pressure valve arranged in the refrigerant path, fluidly in parallel with the ejector unit, between the heat rejecting heat exchanger and the expansion device, and wherein the method further comprises the steps of: generating a high pressure valve control signal for the high pressure valve on the basis of the obtained temperature and the obtained pressure, and controlling an opening degree of the high pressure valve in accordance with the high pressure valve control signal, wherein the ejector control signal is generated on the basis of the high pressure valve control signal.

4. The method according to claim 3, wherein the step of generating the ejector control signal comprises comparing the high pressure valve control signal to an upper limit value and a lower limit value, the lower limit value being lower than the upper limit value, and increasing the capacity of the ejector unit in the case that the high pressure valve control signal is higher than the upper limit value, decreasing the capacity of the ejector unit in the case that the high pressure valve control signal is lower than the lower limit value, and maintaining the capacity of the ejector unit in the case that the high pressure valve control signal is higher than the lower limit value and lower than the upper limit value.

5. The method according to claim 4, wherein the capacity of the ejector unit is only increased or decreased if the high pressure valve control signal has been higher than the upper limit value or lower than the lower limit value for a predefined time interval.

6. The method according to claim 3, further comprising the steps of: generating a feed forward signal based on the ejector control signal, said feed forward signal indicating whether the capacity of the ejector unit has been increased, decreased or maintained, and adjusting the high pressure valve control signal on the basis of the feed forward signal.

7. The method according to claim 1, wherein the ejector unit comprises two or more ejectors arranged fluidly in parallel in the refrigerant path, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises activating or deactivating one or more of the ejectors.

8. The method according to claim 7, wherein the two or more ejectors are arranged in an ejector block.

9. The method according to claim 1, wherein the ejector unit comprises at least one variable capacity ejector, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises adjusting the capacity of the variable capacity ejector.

10. A method for controlling a variable capacity ejector unit arranged in a refrigeration system, said refrigeration system further comprising a compressor, a heat rejecting heat exchanger, a high pressure valve, an expansion device and an evaporator arranged in a refrigerant path, wherein the ejector unit is fluidly connected in the refrigerant path between the heat rejecting heat exchanger and the expansion device, fluidly in parallel with the high pressure valve, the method comprising the steps of: generating a high pressure valve control signal for the high pressure valve, and controlling an opening degree of the high pressure valve in accordance with the high pressure valve control signal, monitoring the high pressure valve control signal, generating an ejector control signal for the ejector unit, based on the high pressure valve control signal, said ejector control signal indicating whether the capacity of the ejector unit should be increased, decreased or maintained, and controlling the capacity of the ejector unit in accordance with the generated ejector control signal.

11. The method according to claim 10, wherein the step of generating the ejector control signal comprises comparing the high pressure valve control signal to an upper limit value and a lower limit value, the lower limit value being lower than the upper limit value, and increasing the capacity of the ejector unit in the case that the high pressure valve control signal is higher than the upper limit value, decreasing the capacity of the ejector unit in the case that the high pressure valve control signal is lower than the lower limit value, and maintaining the capacity of the ejector unit in the case that the high pressure valve control signal is higher than the lower limit value and lower than the upper limit value.

12. The method according to claim 11, wherein the capacity of the ejector unit is only increased or decreased if the high pressure valve control signal has been higher than the upper limit value or lower than the lower limit value for a predefined time interval.

13. The method according to claim 10, further comprising the steps of: generating a feed forward signal based on the ejector control signal, said feed forward signal indicating whether the capacity of the ejector unit has been increased, decreased or maintained, and adjusting the high pressure valve control signal on the basis of the feed forward signal.

14. The method according to claim 2, wherein the refrigeration system further comprises a high pressure valve arranged in the refrigerant path, fluidly in parallel with the ejector unit, between the heat rejecting heat exchanger and the expansion device, and wherein the method further comprises the steps of: generating a high pressure valve control signal for the high pressure valve on the basis of the obtained temperature and the obtained pressure, and controlling an opening degree of the high pressure valve in accordance with the high pressure valve control signal, wherein the ejector control signal is generated on the basis of the high pressure valve control signal.

15. The method according to claim 4, further comprising the steps of: generating a feed forward signal based on the ejector control signal, said feed forward signal indicating whether the capacity of the ejector unit has been increased, decreased or maintained, and adjusting the high pressure valve control signal on the basis of the feed forward signal.

16. The method according to claim 5, further comprising the steps of: generating a feed forward signal based on the ejector control signal, said feed forward signal indicating whether the capacity of the ejector unit has been increased, decreased or maintained, and adjusting the high pressure valve control signal on the basis of the feed forward signal.

17. The method according to claim 2, wherein the ejector unit comprises two or more ejectors arranged fluidly in parallel in the refrigerant path, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises activating or deactivating one or more of the ejectors.

18. The method according to claim 3, wherein the ejector unit comprises two or more ejectors arranged fluidly in parallel in the refrigerant path, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises activating or deactivating one or more of the ejectors.

19. The method according to claim 4, wherein the ejector unit comprises two or more ejectors arranged fluidly in parallel in the refrigerant path, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises activating or deactivating one or more of the ejectors.

20. The method according to claim 5, wherein the ejector unit comprises two or more ejectors arranged fluidly in parallel in the refrigerant path, and wherein the step of controlling the capacity of the ejector unit in accordance with the generated ejector control signal comprises activating or deactivating one or more of the ejectors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The invention will now be described in further detail with reference to the accompanying drawings in which

[0073] FIG. 1 is a diagrammatic view of a refrigeration system comprising a variable capacity ejector unit being controlled using a method according to an embodiment of the invention, and

[0074] FIG. 2 is a graph illustrating control of a variable capacity ejector unit in accordance with a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0075] FIG. 1 is a diagrammatic view of a refrigeration system 1. The refrigeration system 1 comprises a compressor 2, a heat rejecting heat exchanger 3, an expansion device 4, in the form of an expansion valve, and an evaporator 5 arranged in a refrigerant path. A high pressure valve 6 and an ejector unit 7 are arranged fluidly in parallel in the refrigerant path between the heat rejecting heat exchanger 3 and the expansion device 4. In FIG. 1 the ejector unit 7 is illustrated as comprising two ejectors arranged fluidly in parallel, each ejector having a valve, such as a solenoid valve, arranged in front of the ejector, and the ejectors are activated and deactivated by opening and closing the corresponding valves. However, the ejector unit 7 could, alternatively, be of a kind comprising a single ejector having a variable capacity. In any event, the capacity of the ejector unit 7 is variable. The compressor 2 comprises two compressors 2a, 2b arranged in parallel. This will be described in further detail below.

[0076] Refrigerant flowing in the refrigerant path is compressed in the compressor 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 flowing in the heat rejecting heat exchanger 3.

[0077] The refrigerant leaving the heat rejecting heat exchanger 3 passes through either the ejector unit 7 or the high pressure valve 6 to a receiver 8. From the receiver 8 the gaseous part of the refrigerant is supplied directly to compressor 2b, thereby bypassing the expansion device 4 and the evaporator 5. The refrigerant being supplied to compressor 2b thereby has a relatively high pressure, and the work required by the compressor 2b is minimised.

[0078] The liquid part of the refrigerant leaving the receiver 8 is supplied to the expansion device 4, where it is expanded before being supplied to the evaporator 5. In the evaporator 5, heat exchange takes place with the ambient in such a manner that heat is absorbed by the refrigerant flowing in the evaporator 5, while the liquid part of the refrigerant is at least partly evaporated.

[0079] Refrigerant leaving the evaporator 5 is supplied to a separator 9, where the refrigerant is separated into a liquid part and a gaseous part. The gaseous part of the refrigerant is supplied to compressor 2a, where it is once again compressed. The liquid part of the refrigerant is returned to the ejector unit 7, where it constitutes a suction fluid which is mixed with a motive fluid, in the form of the refrigerant supplied from the heat rejecting heat exchanger 3 to the ejector unit 7. The high pressure motive fluid sucks the suction fluid, having a lower pressure, through a suction nozzle in the ejector.

[0080] A temperature sensor 10 and a pressure sensor 11 are arranged to measure the temperature and the pressure, respectively, of refrigerant leaving the heat rejecting heat exchanger 3. The signals measured by the temperature sensor 10 and the pressure sensor 11 are supplied to a high pressure valve controller 12. Based on the received signals, the high pressure valve controller 12 generates a high pressure valve control signal, specifying an opening degree of the high pressure valve 6. The generated high pressure valve control signal is supplied to the high pressure valve 6, and the opening degree of the high pressure valve 6 is controlled in accordance therewith.

[0081] Since the high pressure control signal is generated on the basis of the measured temperature and pressure of the refrigerant leaving the heat rejecting heat exchanger 3, the opening degree of the high pressure valve 6 is controlled in accordance with these parameters, and thereby the opening degree of the high pressure valve 6 is controlled in such a manner that an appropriate pressure level of the refrigerant leaving the heat rejecting heat exchanger 3 is obtained. In particular, it is ensured that the pressure does not reach an undesired low level.

[0082] The high pressure valve control signal is further supplied to an ejector controller 13. Based on the received high pressure control signal, the ejector controller 13 generates an ejector control signal, specifying a capacity level of the ejector unit 7. The generated ejector control signal is supplied to the ejector unit 7, and the capacity of the ejector unit 7 is controlled in accordance therewith. In the embodiment illustrated in FIG. 1, the capacity of the ejector unit 7 is adjusted by activating or deactivating one of the ejectors of the ejector unit 7, e.g. by opening or closing one of the valves arranged in front of the ejector units.

[0083] In the case that the high pressure valve control signal indicates that the opening degree of the high pressure valve 6 is relatively high, this is an indication that a large amount of refrigerant needs to be passed through the high pressure valve 6, at the current capacity of the ejector unit 7, in order to obtain a desired pressure level of the refrigerant leaving the heat rejecting heat exchanger 3. It may therefore be concluded that a larger amount of refrigerant could be passed through the ejector unit 7, without risking that the pressure of the refrigerant leaving the heat rejecting heat exchanger 3 decreases to an undesired level. Therefore, in this situation an ejector control signal is generated which indicates that the capacity of the ejector unit 7 shall be increased.

[0084] In the case that the high pressure valve control signal indicates that the opening degree of the high pressure valve 6 is relatively low, this is an indication that, at the current capacity of the ejector unit 7, it is necessary to keep the refrigerant flow through the high pressure valve 6 at a very low level in order to obtain an acceptable pressure level of the refrigerant leaving the heat rejecting heat exchanger 3. It may therefore be concluded that the amount of refrigerant passing through the ejector unit 7 is too large. Therefore, in this situation an ejector control signal is generated which indicates that the capacity of the ejector unit 7 shall be decreased.

[0085] In the case that the high pressure valve control signal indicates that the opening degree of the high pressure valve 6 is within an acceptable, predefined range, this is an indication that an acceptable pressure level of the refrigerant leaving the heat rejecting heat exchanger 3 can be obtained, at the current capacity of the ejector unit 7, with a reasonable amount of refrigerant passing through the high pressure valve 6. Therefore, in this situation an ejector control signal is generated which indicates that the current capacity of the ejector unit 7 shall be maintained.

[0086] Thus, the capacity of the ejector unit 7 is controlled on the basis of the high pressure valve control signal. Furthermore, the capacity of the ejector unit 7 is controlled in such a manner that as large a portion as possible of the refrigerant is passed through the ejector unit 7, rather than through the high pressure valve 6, while ensuring that the pressure of the refrigerant leaving the heat rejecting heat exchanger 3 does not decrease to an undesried level. Accordingly, the power consumption of the refrigeration system is reduced.

[0087] FIG. 2 is a graph illustrating control of a variable capacity ejector unit in accordance with a method according to an embodiment of the invention. The variable capacity ejector unit may, e.g., be the variable capacity ejector unit illustrated in FIG. 1. In the method according to this embodiment, the capacity of the ejector unit is controlled on the basis of a high pressure valve control signal.

[0088] The curve represents the opening degree of the high pressure valve, and may be derived from the high pressure valve control signal. A lower limit value (Low lim) and an upper limit value (High lim) are shown. The lower limit value represents an opening degree of the high pressure valve, which is so low that there is a risk that the pressure of the refrigerant leaving the heat rejecting heat exchanger decreases to an undesirable level. The upper limit value represents an opening degree of the high pressure valve, which is sufficiently high to allow a larger portion of the refrigerant leaving the heat rejecting heat exchanger to pass through the ejector unit instead of through the high pressure valve.

[0089] The graph of FIG. 2 illustrates that when the opening degree of the high pressure valve reaches the upper limit value, then the capacity of the ejector unit is increased (stepup). This causes the pressure of the refrigerant leaving the heat rejecting heat exchanger to decrease, and in response thereto, the opening degree of the high pressure valve is also decreased.

[0090] When the opening degree of the high pressure valve reached the lower limit value, then the capacity of the ejector unit is decreased (stepdown). This causes the pressure of the refrigerant leaving the heat rejecting heat exchanger to increase, and in response thereto, the opening degree of the high pressure valve is also increased.

[0091] As long as the opening degree of the high pressure valve remains between the lower limit value and the upper limit value, the capacity of the ejector unit is maintained at the current level.

[0092] 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.