Vapour compression system with at least two evaporator groups

Abstract

A method for controlling a vapour compression system in an energy efficient and stable manner, the vapour compression system (1) including at least two evaporator groups (5a, 5b, 5c), each evaporator group (5a, 5b, 5c) including an ejector unit (7a, 7b, 7c), at least one evaporator (9a, 9b, 9c) and a flow control device (8a, 8b, 8c) controlling a flow of refrigerant to the at least one evaporator (9a, 9b, 9c). For each evaporator group (5a, 5b, 5c) the outlet of the evaporator (9a, 9b, 9c) is connected to a secondary inlet (12a, 12b, 12c) of the corresponding ejector unit (7a, 7b, 7c).

Claims

1. A method for controlling a vapour compression system comprising a compressor group comprising one or more compressors, a heat rejecting heat exchanger, a receiver, and at least two evaporator groups, each evaporator group comprising an ejector unit, at least one evaporator and a flow control device controlling a flow of refrigerant to the at least one evaporator, wherein an outlet of the heat rejecting heat exchanger is connected to a primary inlet of the ejector unit of each of the evaporator groups, an outlet of each ejector unit is connected to an inlet of the receiver, and an outlet of the at least one evaporator of each evaporator group is connected to a secondary inlet of the ejector unit of the corresponding evaporator group, the method comprising the steps of: obtaining a pressure of a refrigerant leaving the heat rejecting heat exchanger, for at least one evaporator group, obtaining a value for an operating parameter related to that at least one evaporator group, and controlling at least one ejector unit of the at least two evaporator groups in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger or in accordance with the obtained value for the operating parameter, wherein the operating parameter of the at least one evaporator group of the least two evaporator groups is a parameter reflecting a quantity of refrigerant flowing through the at least one evaporator of the at least one evaporator group, which is not evaporated.

2. The method according to claim 1, wherein the step of controlling the at least one ejector unit comprises: controlling a first ejector unit of a first evaporator group of the at least two evaporator groups in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger, and controlling a second ejector unit of a second evaporator group of the at least two evaporator groups in accordance with the obtained value for the operating parameter related to the at least one evaporator group.

3. The method according to claim 2, further comprising the step of obtaining a temperature of refrigerant leaving the heat rejecting heat exchanger, and wherein the step of controlling the first ejector unit in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger 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 operating the first ejector unit on the basis of the comparison.

4. The method according to claim 1, wherein the step of controlling the at least one ejector unit comprises the steps of: determining whether the total capacity of the ejector units of the least two evaporator groups needs to be increased, decreased or maintained, based on the obtained pressure of refrigerant leaving the heat rejecting heat exchanger, when the total capacity of the ejector units of the least two evaporator groups needs to be increased or decreased, selecting at least one evaporator group of the least two evaporator groups, based on the obtained value for the operating parameter, and increasing or decreasing the capacity of the ejector unit of the selected at least one evaporator group.

5. The method according to claim 4, wherein the step of selecting at least one evaporator group of the least two evaporator groups comprises the steps of: comparing the obtained value for the operating parameter to a corresponding reference value, when the total capacity of the ejector units of the least two evaporator groups needs to be increased, selecting the evaporator group of the least two evaporator groups having the largest deviation between the obtained value for the operating parameter and the reference value, and when the total capacity of the ejector units of the least two evaporator groups needs to be decreased, selecting the evaporator group of the least two evaporator groups having the smallest deviation between the obtained value for the operating parameter and the reference value.

6. The method according to claim 5, further comprising the step of adjusting a pressure prevailing inside the receiver when the deviation between the obtained operating parameter and the reference value exceeds a predefined threshold value for one or more evaporator groups of the least two evaporator groups.

7. The method according to claim 5, further comprising the step of increasing the capacity of the ejector unit of a first evaporator group of the least two evaporator groups and decreasing the capacity of the ejector unit of a second evaporator group of the least two evaporator groups, when the deviation between the obtained operating parameter and the reference value for the first evaporator group is significantly larger than the deviation between the obtained operating parameter and the reference value of the second evaporator group.

8. The method according to claim 1, wherein the operating parameter for the at least one evaporator group of the least two evaporator groups includes a pressure prevailing inside the at least one evaporator of the at least one evaporator group.

9. The method according to claim 1, wherein the operating parameter for the at least one evaporator group of the least two evaporator groups includes a temperature of a secondary fluid medium flowing across the at least one evaporator of the at least one evaporator group.

10. A method for controlling a vapour compression system comprising a compressor group comprising one or more compressors, a heat rejecting heat exchanger, a receiver, and at least two evaporator groups, each evaporator group comprising an ejector unit, at least one evaporator and a flow control device controlling a flow of refrigerant to the at least one evaporator, wherein an outlet of the heat rejecting heat exchanger is connected to a primary inlet of the ejector unit of each of the evaporator groups, an outlet of each ejector unit is connected to an inlet of the receiver, and an outlet of the at least one evaporator of each evaporator group is connected to a secondary inlet of the ejector unit of the corresponding evaporator group, the method comprising the steps of: obtaining a pressure of a refrigerant leaving the heat rejecting heat exchanger, for at least one evaporator group, obtaining a value for an operating parameter related to that at least one evaporator group, and controlling at least one ejector unit of the at least two evaporator groups in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger or in accordance with the obtained value for the operating parameter, wherein the step of controlling the at least one ejector unit comprises the steps of: determining whether the total capacity of the ejector units of the least two evaporator groups needs to be increased, decreased or maintained, based on the obtained pressure of refrigerant leaving the heat rejecting heat exchanger, when the total capacity of the ejector units of the least two evaporator groups needs to be increased or decreased, selecting at least one evaporator group of the least two evaporator groups, based on the obtained value for the operating parameter, and increasing or decreasing the capacity of the ejector unit of the selected at least one evaporator group, and wherein the step of selecting at least one evaporator group of the least two evaporator groups comprises the steps of: comparing the obtained value for the operating parameter to a corresponding reference value, when the total capacity of the ejector units of the least two evaporator groups needs to be increased, selecting the evaporator group of the least two evaporator groups having the largest deviation between the obtained value for the operating parameter and the reference value, and when the total capacity of the ejector units of the least two evaporator groups needs to be decreased, selecting the evaporator group of the least two evaporator groups having the smallest deviation between the obtained value for the operating parameter and the reference value.

11. The method according to claim 10, further comprising the step of adjusting a pressure prevailing inside the receiver when the deviation between the obtained operating parameter and the reference value exceeds a predefined threshold value for one or more evaporator groups of the least two evaporator groups.

12. The method according to claim 10, further comprising the step of increasing the capacity of the ejector unit of a first evaporator group of the least two evaporator groups and decreasing the capacity of the ejector unit of a second evaporator group of the least two evaporator groups, when the deviation between the obtained operating parameter and the reference value for the first evaporator group is significantly larger than the deviation between the obtained operating parameter and the reference value of the second evaporator group.

13. The method according to claim 10, wherein the step of controlling the at least one ejector unit comprises: controlling a first ejector unit of a first evaporator group of the at least two evaporator groups in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger, and controlling a second ejector unit of a second evaporator group of the at least two evaporator groups in accordance with the obtained value for the operating parameter related to the at least one evaporator group.

14. The method according to claim 13, further comprising the step of obtaining a temperature of refrigerant leaving the heat rejecting heat exchanger, and wherein the step of controlling the first ejector unit in accordance with the obtained pressure of the refrigerant leaving the heat rejecting heat exchanger 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 operating the first ejector unit on the basis of the comparison.

15. The method according to claim 10, wherein the operating parameter for the at least one evaporator group of the least two evaporator groups is a pressure prevailing inside the at least one evaporator of the at least one evaporator group.

16. The method according to claim 10, wherein the operating parameter for the at least one evaporator group of the least two evaporator groups is a temperature of a secondary fluid medium flowing across the at least one evaporator of the at least one evaporator group.

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) FIGS. 1-6 are diagrammatic views of vapour compression systems according to various embodiments of the invention.

DETAILED DESCRIPTION

(3) FIG. 1 is a diagrammatic view of a vapour compression system 1 according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor group 2 comprising a number of compressors 3, two of which are shown, and a heat rejecting heat exchanger 4. The vapour compression system 1 further comprises two evaporator groups 5a, 5b. The first evaporator group 5a is arranged to provide cooling for a number of cooling entities or display cases, and the second evaporator group 5b is arranged to provide air condition for one or more rooms at the facility where the cooling entities or display cases are positioned. The vapour compression system 1 further comprises a receiver 6.

(4) The first evaporator group 5a comprises a first ejector unit 7a, a flow control device in the form of a first expansion valve 8a, and a first evaporator 9a. It should be noted that, even though the first evaporator 9a is shown as a single evaporator, it could in fact be two or more evaporators, arranged fluidly in parallel, each evaporator being arranged to provide cooling for a specific cooling entity or display case. In this case, each evaporator may be provided with a separate flow control valve, e.g. in the form of an expansion valve, controlling the flow of refrigerant to the evaporator.

(5) Similarly, the second evaporator group 5b comprises a second ejector unit 7b, a flow control device in the form of a second expansion valve 8b, and a second evaporator 9b. Also in this case, the second evaporator 9b could be two or more evaporators, each arranged to provide air conditioning for a separate room.

(6) Refrigerant flowing in the vapour compression system 1 is compressed by means of the compressors 3 of the compressor group 2. The compressed refrigerant is supplied to the heat rejecting heat exchanger 4, where heat exchange takes place with the ambient in such a manner that heat is rejected from the refrigerant to the ambient. In the case that the heat rejecting heat exchanger 4 is in the form of a condenser, the refrigerant passing through the heat rejecting heat exchanger 4 is at least partly condensed. In the case that the heat rejecting heat exchanger 4 is in the form of a gas cooler, the refrigerant passing through the heat rejecting heat exchanger 4 is cooled, but no phase change takes place.

(7) The refrigerant leaving the heat rejecting heat exchanger 4 is supplied to a primary inlet 10a of the first ejector unit 7a and to a primary inlet 10b of the second ejector unit 7b. Refrigerant leaving the ejector units 7a, 7b is supplied to the receiver 6, where the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant leaves the receiver 6 via liquid outlets 11a, 11b, and is supplied to the evaporator 9a of the first evaporator group 5a, via the first expansion valve 8a, as well as to the evaporator 9b of the second evaporator group 5b, via the second expansion valve 8b.

(8) The refrigerant leaving the first evaporator 9a is supplied either to the compressor group 2 or to a secondary inlet 12a of the first ejector unit 7a. The part of the refrigerant which is supplied to the compressor group 2 is supplied to a dedicated main compressor 3a which can only receive refrigerant from the first evaporator 9a. It is desirable that as large a fraction as possible of the refrigerant leaving the first evaporator 9a is supplied to the secondary inlet 12a of the first ejector unit 7a, because thereby the first evaporator group 5a is operated as energy efficient as possible. In fact, under ideal operating conditions, the main compressor 3a should not be operating at all. However, the main compressor 3a can be switched on when operating conditions are such that the first ejector 7a is not capable of sucking all of the refrigerant leaving the first evaporator 9a.

(9) All of the refrigerant leaving the second evaporator 9b is supplied to a secondary inlet 12b of the second ejector unit 7b. Thus, the outlet of the second evaporator 9b is not connected to the compressor group 2, and the refrigerant flow in the second evaporator group 5b is essentially determined by the ejector capacity of the second ejector unit 7b.

(10) Thus, the secondary inlet 12a of the first ejector unit 7a only receives refrigerant from the first evaporator 9a, and the secondary inlet 12b of the second ejector unit 7b only receives refrigerant from the second evaporator 9b. Accordingly, the first evaporator group 5a and the second evaporator group 5b are independent of each other, and can be controlled independently of each other by controlling the ejector capacities of the respective ejector units 7a, 7b.

(11) The gaseous part of the refrigerant in the receiver 6 is supplied to the compressor group 2, via a gaseous outlet 13 of the receiver 6. This refrigerant is supplied directly to a dedicated receiver compressor 3b. The refrigerant supplied from the gaseous outlet 13 of the receiver 6 to the receiver compressor 3b is at a pressure level which is higher than the pressure level of the refrigerant supplied from the first evaporator 9a to the main compressor 3a, because the refrigerant supplied from the gaseous outlet 13 of the receiver 6 does not undergo expansion in the first expansion valve 8a. Therefore, the energy required in order to compress the refrigerant received from the gaseous outlet 13 of the receiver 6 is lower than the energy required in order to compress the refrigerant received from the first evaporator 9a.

(12) According to one embodiment, the ejector capacity of the first ejector unit 7a may be controlled on the basis of the pressure of refrigerant leaving the heat rejecting heat exchanger 4, and in order to ensure that the pressure is maintained at an appropriate level. In this case the ejector capacity of the second ejector 7b may be controlled on the basis of an operating parameter related to the second evaporator group 5b, e.g. a pressure prevailing inside the second evaporator 9b, a temperature of a secondary fluid flow across the second evaporator 9b, or a parameter reflecting how much of the refrigerant circulating in the second evaporator group 5b is actually evaporated or not evaporated when passing through the second evaporator 9b.

(13) According to another embodiment, the pressure of refrigerant leaving the heat rejecting heat exchanger 4 may be used as a basis for determining whether the total ejector capacity of the ejector units 7a, 7b should be increased, decreased or maintained at the current level. If it is determined that the total ejector capacity should be increased or decreased, either the first evaporator group 5a or the second evaporator group 5b is selected, based on a measured operating parameter for each of the evaporator groups 5a, 5b, e.g. one of the operating parameters described above. In the case that the total ejector capacity should be increased, the evaporator group 5a, 5b being most in need of the additional ejector capacity is selected. Similarly, in the case that the total ejector capacity should be decreased, the evaporator group 5a, 5b which needs the ejector capacity least is selected. Finally, the ejector capacity of the ejector unit 7a, 7b of the selected evaporator group 5a, 5b is adjusted in order to provide the required increase or decrease of the total ejector capacity.

(14) FIG. 2 is a diagrammatic view of a vapour compression system 1 according to a second embodiment of the invention. The vapour compression system 1 of FIG. 2 is similar to the vapour compression system 1 of FIG. 1, and it will therefore not be described in detail here. In the vapour compression system 1 of FIG. 2, the compressor group 2 comprises a number of compressors 3, three of which are shown. Each of the compressors 3 is provided with a three way valve 14, allowing each of the compressors 3 to be connected to either the outlet of the first evaporator 9a or the gaseous outlet 13 of the receiver 6. Thus, the compressors 3 are not dedicated main compressors or dedicated receiver compressors, but each compressor 3 may operate as a main compressor or as a receiver compressor'. This allows the total available compressor capacity of the compressor group 2 to be shifted between main compressor capacity and receiver compressor capacity, according to the current requirements, by appropriately controlling the three way valves 14.

(15) FIG. 3 is a diagrammatic view of a vapour compression system 1 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. The vapour compression system 1 of FIG. 3 further comprises a high pressure valve 15 arranged in a part of the refrigerant path which interconnects the outlet of the heat rejecting heat exchanger 4 and the receiver 6. Thus, the high pressure valve 15 is arranged fluidly in parallel with the ejector units 7a, 7b. In the vapour compression system 1 of FIG. 3 it is therefore possible to select whether refrigerant leaving the heat rejecting heat exchanger 4 should pass through one of the ejector units 7a, 7b or through the high pressure valve 15.

(16) FIG. 4 is a diagrammatic view of a vapour compression system 1 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. 1, and it will therefore not be described in detail here. The vapour compression system 1 of FIG. 4 comprises a third evaporator group 5c, comprising a third ejector unit 7c, a third expansion valve 8c and a third evaporator 9c.

(17) The outlet of the third evaporator 9c is connected to the secondary inlet 12c of the third ejector unit 7c only, i.e. all of the refrigerant leaving the third evaporator 9c is supplied to the secondary inlet 12c of the third ejector unit 7c, similarly to the situation described above with reference to FIG. 1 and the second evaporator group 5b.

(18) The third evaporator 9c is in the form of a plate heat exchanger, e.g. a liquid to liquid heat exchanger. Thus, the third evaporator group 5c may, e.g., be used for providing air condition to a part of the building which is arranged remotely with respect to the compressor group 2 and the heat rejecting heat exchanger 4.

(19) FIG. 5 is a diagrammatic view of a vapour compression system 1 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. 4, and it will therefore not be described in detail here. In the vapour compression system 1 of FIG. 5 the compressors 3 of the compressor group 2 are all connected to the outlet of the first evaporator 9a as well as to the gaseous outlet 13 of the receiver 6, via respective three way valves 14. This has already been described above with reference to FIG. 2.

(20) FIG. 6 is a diagrammatic view of a vapour compression system 1 according to a sixth embodiment of the invention. The vapour compression system 1 of FIG. 6 is very similar to the vapour compression system 1 of FIG. 4, in the sense that the vapour compression system 1 comprises three evaporator groups 5a, 5b, 5c. However, in the vapour compression system 1 of FIG. 6, only the second evaporator group 5b and the third evaporator group 5c are provided with an ejector unit 7b, 7c. The first evaporator group 5a, on the other hand, is not provided with an ejector unit. Accordingly, all of the refrigerant leaving the first evaporator 9a is supplied to the main compressor 3a of the compressor group 2, all of the refrigerant leaving the second evaporator 9b is supplied to the secondary inlet 12b of the second ejector unit 7b, and all of the refrigerant leaving the third evaporator 9c is supplied to the secondary inlet 12c of the third ejector unit 7c.

(21) The vapour compression system 1 of FIG. 6 may, e.g., be suitable in situations where the total expansion capacity provided by the ejector units 7b, 7c can easily be utilised by the second evaporator group 5b and the third evaporator group 5c. In this case, adding a further ejector unit to the first evaporator group 5a will not improve the energy efficiency of the vapour compression system 1. Alternatively, the vapour compression system 1 of FIG. 6 may, e.g., be suitable in situations where the evaporating temperature of the first evaporator 9a is so low that an ejector unit arranged in the first evaporator group 5a will not be capable of lifting the pressure of the refrigerant leaving the first evaporator 9a.

(22) 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.