Method for controlling a vapour compression system with a variable receiver pressure setpoint

Abstract

A method for controlling a vapour compression system (1) is disclosed, the vapour compression system (1) comprising at least one expansion device (8) and at least one evaporator (9). For each expansion device (8), an opening degree of the expansion device (8) is obtained, and a representative opening degree, OD.sub.rep, is identified based on the obtained opening degree(s) of the expansion device(s) (8). The representative opening degree could be a maximum opening degree, OD.sub.max, being the largest among the obtained opening degrees. The representative opening degree, OD.sub.rep, is compared to a predefined target opening degree, OD.sub.target, and a minimum setpoint value, SP.sub.rec, for a pressure prevailing inside a receiver (7), is calculated or adjusted, based on the comparison. The vapour compression system (1) is controlled to obtain a pressure inside the receiver (7) which is equal to or higher than the calculated or adjusted minimum setpoint value, SP.sub.rec.

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, a receiver, at least one expansion device and at least one evaporator arranged in a refrigerant path, each expansion device of the at least one expansion device being arranged to control a supply of refrigerant to an evaporator of the at least one evaporator, the method comprising the steps of: obtaining an opening degree of each expansion device of the at least one expansion device, identifying a representative opening degree, OD.sub.rep, based on the obtained opening degree(s) of the at least one expansion device, comparing the representative opening degree, OD.sub.rep, to a predefined target opening degree, OD.sub.target, calculating or adjusting a minimum setpoint value, SP.sub.rec, for a pressure prevailing inside the receiver, based on the comparison, and controlling the vapour compression system to obtain a pressure inside the receiver which is equal to or higher than the calculated or adjusted minimum setpoint value, SP.sub.rec.

2. The method according to claim 1, wherein the step of identifying a representative opening degree, OD.sub.rep, comprises identifying a maximum opening degree, OD.sub.max, as the largest opening degree among the obtained opening degree(s) of the expansion device(s).

3. The method according to claim 1, wherein the step of calculating or adjusting a minimum setpoint value, SP.sub.rec, comprises reducing the minimum setpoint value, SP.sub.rec, in the case that the representative opening degree, OD.sub.rep, is smaller than the target opening degree, OD.sub.target.

4. The method according to claim 1, wherein the step of calculating or adjusting a minimum setpoint value, SP.sub.rec, comprises increasing the minimum setpoint value, SP.sub.rec, in the case that the representative opening degree, OD.sub.rep, is larger than the target opening degree, OD.sub.target.

5. The method according to claim 1, wherein a gaseous outlet of the receiver is connected to an inlet of the compressor unit via a bypass valve, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by operating the bypass valve.

6. The method according to claim 1, wherein the compressor unit comprises one or more main compressors connected between an outlet of the evaporator(s) and an inlet of the heat rejecting heat exchanger, and one or more receiver compressors connected between a gaseous outlet of the receiver and an inlet of the heat rejecting heat exchanger, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by controlling a refrigerant supply to the receiver compressor(s).

7. The method according to claim 1, wherein the vapour compression system further comprises an ejector, 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 the receiver, and an outlet of the at least one evaporator being connected to an inlet of the compressor unit and to a secondary inlet of the ejector.

8. The method according to claim 2, wherein the step of calculating or adjusting a minimum setpoint value, SP.sub.rec, comprises reducing the minimum setpoint value, SP.sub.rec, in the case that the representative opening degree, OD.sub.rep, is smaller than the target opening degree, OD.sub.target.

9. The method according to claim 2, wherein the step of calculating or adjusting a minimum setpoint value, SP.sub.rec, comprises increasing the minimum setpoint value, SP.sub.rec, in the case that the representative opening degree, OD.sub.rep, is larger than the target opening degree, OD.sub.target.

10. The method according to claim 3, wherein the step of calculating or adjusting a minimum setpoint value, SP.sub.rec, comprises increasing the minimum setpoint value, SP.sub.rec, in the case that the representative opening degree, OD.sub.rep, is larger than the target opening degree, OD.sub.target.

11. The method according to claim 2, wherein a gaseous outlet of the receiver is connected to an inlet of the compressor unit, via a bypass valve, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by operating the bypass valve.

12. The method according to claim 3, wherein a gaseous outlet of the receiver is connected to an inlet of the compressor unit, via a bypass valve, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by operating the bypass valve.

13. The method according to claim 4, wherein a gaseous outlet of the receiver is connected to an inlet of the compressor unit, via a bypass valve, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by operating the bypass valve.

14. The method according to claim 2, wherein the compressor unit comprises one or more main compressors connected between an outlet of the evaporator(s) and an inlet of the heat rejecting heat exchanger, and one or more receiver compressors connected between a gaseous outlet of the receiver and an inlet of the heat rejecting heat exchanger, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by controlling a refrigerant supply to the receiver compressor(s).

15. The method according to claim 3, wherein the compressor unit comprises one or more main compressors connected between an outlet of the evaporator(s) and an inlet of the heat rejecting heat exchanger, and one or more receiver compressors connected between a gaseous outlet of the receiver and an inlet of the heat rejecting heat exchanger, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by controlling a refrigerant supply to the receiver compressor(s).

16. The method according to claim 4, wherein the compressor unit comprises one or more main compressors connected between an outlet of the evaporator(s) and an inlet of the heat rejecting heat exchanger, and one or more receiver compressors connected between a gaseous outlet of the receiver and an inlet of the heat rejecting heat exchanger, and wherein the step of controlling the vapour compression system comprises controlling the pressure prevailing inside the receiver by controlling a refrigerant supply to the receiver compressor(s).

17. The method according to claim 2, wherein the vapour compression system further comprises an ejector, 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 the receiver, and an outlet of the evaporator(s) being connected to an inlet of the compressor unit and to a secondary inlet of the ejector.

18. The method according to claim 3, wherein the vapour compression system further comprises an ejector, 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 the receiver, and an outlet of the evaporator(s) being connected to an inlet of the compressor unit and to a secondary inlet of the ejector.

19. The method according to claim 4, wherein the vapour compression system further comprises an ejector, 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 the receiver, and an outlet of the evaporator(s) being connected to an inlet of the compressor unit and to a secondary inlet of the ejector.

20. The method according to claim 5, wherein the vapour compression system further comprises an ejector, 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 the receiver, and an outlet of the evaporator(s) being connected to an inlet of the compressor unit and to a secondary inlet of the ejector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further detail with reference to the accompanying drawings in which

(2) FIG. 1 is a diagrammatic view a vapour compression system being controlled in accordance with a method according to a first embodiment of the invention,

(3) FIG. 2 is a diagrammatic view a vapour compression system being controlled in accordance with a method according to a second embodiment of the invention,

(4) FIG. 3 is a diagrammatic view a vapour compression system being controlled in accordance with a method according to a third embodiment of the invention,

(5) FIG. 4 is a diagrammatic view a vapour compression system being controlled in accordance with a method according to a fourth embodiment of the invention,

(6) FIG. 5 illustrates control of the vapour compression system of FIG. 4,

(7) FIG. 6 is a block diagram illustrating a method according to an embodiment of the invention, and

(8) FIG. 7 is a block diagram illustrating a method according to an alternative embodiment of the invention.

DETAILED DESCRIPTION

(9) 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 a number of compressors 3, 4, three of which are shown, a heat rejecting heat exchanger 5, an ejector 6, a receiver 7, an expansion device 8, and an evaporator 9 arranged in a refrigerant path.

(10) Two of the shown compressors 3 are connected to an outlet of the evaporator 9. Accordingly, refrigerant leaving the evaporator 9 can be supplied to these compressors 3. The third compressor 4 is connected to a gaseous outlet 10 of the receiver 7. Accordingly, gaseous refrigerant can be supplied directly from the receiver 7 to this compressor 4.

(11) Refrigerant flowing in the refrigerant path is compressed by the compressors 3, 4 of the compressor unit 2. The compressed refrigerant is supplied to the heat rejecting heat exchanger 5, where heat exchange takes place in such a manner that heat is rejected from the refrigerant.

(12) The refrigerant leaving the heat rejecting heat exchanger 5 is supplied to a primary inlet 11 of the ejector 6, before being supplied to the receiver 7. When passing through the ejector 6 the refrigerant undergoes expansion. Thereby the pressure of the refrigerant is reduced, and the refrigerant being supplied to the receiver 7 is in a mixed liquid and gaseous state.

(13) In the receiver 7 the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant is supplied to the evaporator 9, via a liquid outlet 12 of the receiver 7 and the expansion device 8. In the evaporator 9, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place in such a manner that heat is absorbed by the refrigerant.

(14) The refrigerant leaving the evaporator 9 is either supplied to the compressors 3 of the compressor unit 2 or to a secondary inlet 13 of the ejector 6.

(15) The vapour compression system 1 of FIG. 1 is operated in the most energy efficient manner when all of the refrigerant leaving the evaporator 9 is supplied to the secondary inlet 13 of the ejector 6, and the compressor unit 2 only receives refrigerant from the gaseous outlet 10 of the receiver 7. In this case only compressor 4 of the compressor unit 2 is operating, while compressors 3 are switched off. It is therefore desirable to operate the vapour compression system 1 in this manner for as large a part of the total operating time as possible. When the pressure prevailing inside the receiver 7 is low, a large portion of the refrigerant in the receiver 7 is in a gaseous state, and thereby a large amount of gaseous refrigerant is available for being supplied to the compressor 4. Therefore a low pressure level inside the receiver 7 is in general desirable. The vapour compression system 1 is controlled in accordance with a setpoint value for the pressure prevailing inside the receiver 7, and in such a manner that this setpoint value is maintained within an appropriate range between a minimum setpoint value and a maximum setpoint value. In the method according to the invention, the minimum setpoint value, SP.sub.rec, is adjusted in order to allow the pressure inside the receiver 7 to decrease to a lower level when this is not disadvantageous with respect to other aspects of the control of the vapour compression system 1.

(16) A mass flow through the expansion device 8 is determined by the following equation:
{dot over (m)}=√{square root over (Δp)}.Math.k.Math.OD,
where {dot over (m)} is the mass flow through the expansion device 8, Δp is the pressure difference across the expansion device 8, i.e. p.sub.rec−p.sub.e, where p.sub.rec is the pressure prevailing inside the receiver 7 and p.sub.e is the evaporator pressure or the suction pressure, k is a constant relating to characteristics of the expansion device 8 and to the density of the refrigerant, and OD is the opening degree of the expansion device 8. Accordingly, when the pressure prevailing inside the receiver 7 is low, the pressure difference, Δp, across the expansion device 8 is small. Therefore, in order to obtain a given mass flow, {dot over (m)}, through the expansion device 8, it may be necessary to select a relatively large opening degree, OD, of the expansion device 8. If the opening degree, OD, is already close to the maximum opening degree of the expansion device 8, i.e. if the expansion device 8 is almost fully open, it will not be possible to increase the mass flow through the expansion device 8 by increasing the opening degree. Instead, the pressure difference, Δp, can be increased by increasing the pressure, p.sub.rec, prevailing inside the receiver. When this situation occurs, it may therefore be appropriate to increase the minimum setpoint value, SP.sub.rec.

(17) On the other hand, if the opening degree, OD, of the expansion device 8 is significantly lower than the maximum opening degree of the expansion device 8, it is possible to increase the opening degree, OD, in order to increase the mass flow through the expansion device 8, even if the pressure, p.sub.rec, prevailing inside the receiver 7, and thereby the pressure difference, Δp, across the expansion device 8, is reduced. Therefore, in this case it is safe to decrease the minimum setpoint value, SP.sub.rec, thereby allowing the pressure inside the receiver 7 to reach a lower level.

(18) Therefore, when controlling the vapour compression system 1 of FIG. 1, the opening degree, OD, of the expansion device 8 is obtained and compared to a target opening degree, OD.sub.target. The target opening degree, OD.sub.target, could advantageously be a relatively large opening degree, but sufficiently below the maximum opening degree of the expansion device 8 to allow the expansion device 8 to react to an increase in cooling demand by increasing the opening degree, OD, of the expansion device 8.

(19) Based on the comparison, the minimum setpoint value, SP.sub.rec, for the pressure prevailing inside the receiver 7 is calculated or adjusted, e.g. as described above. Subsequently, the vapour compression system 1 is controlled to obtain a pressure inside the receiver 7 which is equal to or higher than the calculated or adjusted minimum setpoint value, SP.sub.rec. The pressure prevailing inside the receiver 7 may, e.g., be adjusted by adjusting the compressor capacity of compressor 4.

(20) 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.

(21) In the vapour compression system 1 of FIG. 2, the gaseous outlet 10 of the receiver 7 is further connected to compressors 3, via a bypass valve 14. Thereby the pressure inside the receiver 7 may further be adjusted by operating the bypass valve 14, thereby controlling a refrigerant flow from the gaseous outlet 10 of the receiver 7 to the compressors 3.

(22) 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 systems 1 of FIGS. 1 and 2, and it will therefore not be described in detail here.

(23) In the vapour compression system 1 of FIG. 3 the ejector has been replaced by a high pressure valve 15. Thus, refrigerant leaving the heat rejecting heat exchanger 5 still undergoes expansion when passing through the high pressure valve 15, similarly to the situation described above with reference to FIG. 1. However, all of the refrigerant leaving the evaporator 9 is supplied to the compressor unit 2.

(24) In the compressor unit 2, one compressor 3 is shown as being connected to the outlet of the evaporator 9 and one compressor 4 is shown as being connected to the gaseous outlet 10 of the receiver 7. A third compressor 16 is shown as being provided with a three way valve 17 which allows the compressor 16 to be selectively connected to the outlet of the evaporator 9 or to the gaseous outlet 10 of the receiver 7. Thereby some of the compressor capacity of the compressor unit 2 can be shifted between ‘main compressor capacity’, i.e. when the compressor 16 is connected to the outlet of the evaporator 9, and ‘receiver compressor capacity’, i.e. when the compressor 16 is connected to the gaseous outlet 10 of the receiver 7. Thereby it is further possible to adjust the pressure prevailing inside the receiver 7 by operating the three way valve 17, thereby increasing or decreasing the amount of compressor capacity being available for compressing refrigerant received from the gaseous outlet 10 of the receiver 7.

(25) 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. 3, and it will therefore not be described in detail here.

(26) The vapour compression system 1 of FIG. 4 comprises three evaporators 9a, 9b, 9c arranged in parallel in the refrigerant path. Each evaporator 9a, 9b, 9c has an expansion device 8a, 8b, 8c associated therewith, each expansion device 8a, 8b, 8c thereby controlling a supply of refrigerant to one of the evaporators 9a, 9b, 9c. Each evaporator 9a, 9b, 9c may, e.g., be arranged to provide cooling for a separate volume, e.g. in the form of separate display cases in a supermarket.

(27) When controlling the vapour compression system 1 of FIG. 4 the opening degree of each of the expansion devices 8a, 8b, 8c is obtained. Then a representative opening degree, OD.sub.rep, is identified, based on the obtained opening degrees of the expansion devices 8a, 8b, 8c. The representative opening degree, OD.sub.rep, could, e.g., be a maximum opening degree, OD.sub.max, being the largest of the opening degrees of the expansion devices 8a, 8b, 8c.

(28) The representative opening degree, OD.sub.rep, is then compared to a target opening degree, OD.sub.target. Subsequently, the vapour compression system 1 is controlled essentially as described above with reference to FIG. 1.

(29) FIG. 5 illustrates control of the vapour compression system 1 of FIG. 4. It can be seen that an opening degree is communicated from each expansion device 8a, 8b, 8c to a controller 18. In response thereto, the controller 18 identifies a representative opening degree, OD.sub.rep, and compares the representative opening degree, OD.sub.rep, to a predefined target opening degree, OD.sub.target. Based on the comparison, the controller 18 calculates or adjusts a minimum setpoint value, SP.sub.rec, for a pressure prevailing inside the receiver 7, essentially as described above. The calculated or adjusted minimum setpoint value, SP.sub.rec, constitutes a lower limit for a setpoint value which is used for controlling the pressure prevailing inside the receiver 7.

(30) Furthermore, the controller 18 may set a setpoint value for the pressure inside the receiver 7 and control the vapour compression system 1 in accordance therewith. To this end the controller 18 receives measurements from a pressure sensor 19 arranged to measure the pressure prevailing inside the receiver 7. Based on the received measurements of the pressure prevailing inside the receiver 7, the controller 18 generates control signals for the compressor 4 which is connected to the gaseous outlet 10 of the receiver 7 and/or to the bypass valve 14. Thereby the controller 18 causes the pressure prevailing inside the receiver 7 to be controlled in order to reach the setpoint value.

(31) FIG. 6 is a block diagram illustrating a method according to an embodiment of the invention. Opening degrees, OD1, OD2, OD3, OD4, OD5 of five different expansion devices are provided to a first comparing block 20, where a maximum opening degree, OD.sub.max, being the largest among the opening degrees, OD1, OD2, OD3, OD4 and OD5, is identified. The maximum opening degree, OD.sub.max, is compared to a target opening degree, OD.sub.target, at a first comparator 21. An error signal is generated, based on this comparison, and supplied to a first PI controller 22. The output of the first PI controller 22 is supplied to a second comparing block 23. The second comparing block 23 further receives a signal, P_rec_SP, which represents a setpoint value for the pressure prevailing inside the receiver, and a signal, P_rec_min, which represents a minimum setpoint value, constituting a lower boundary for the setpoint value for the pressure inside the receiver.

(32) The second comparing block 23 selects the largest of the three received signals, and forwards this signal to a second comparator 24, where the signal is compared to a measured value, P_rec, of the pressure prevailing inside the receiver. The result of this comparison is supplied to a second PI controller 25, which in turn outputs a control signal in order to control the pressure prevailing inside the receiver.

(33) FIG. 7 is a block diagram illustrating a method according to an alternative embodiment of the invention. The method illustrated in FIG. 7 is very similar to the method illustrated in FIG. 6, and it will therefore not be described in detail here.

(34) In FIG. 7 it is illustrated that the setpoint, P_rec_SP for the pressure prevailing inside the receiver could be variable, e.g. on the basis of the prevailing operating conditions, such as the ambient temperature. It is further indicated that the last part of the process is simply a standard PI control of the pressure prevailing inside the receiver.

(35) 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.