Method for switching compressor capacity

Abstract

A method for operating a compressor unit (2) comprising one or more compressors (8, 9, 10) is disclosed, the compressor unit (2) being arranged in a vapour compression system (1). Two or more options for distributing the available compressor capacity of the compressor unit (2) between being connected to a high pressure suction line (11) and to a medium pressure suction line (13) are defined. For each option, an expected impact on one or more operating parameters of the vapour compression system (1), resulting from distributing the available compressor capacity according to the option, is predicted. An option is selected, based on the predicted expected impact for the options, and based on current operating demands of the vapour compression system (1), and the available compressor capacity is distributed according to the selected option, e.g. by means of settings of one or more valve arrangements (14, 15).

Claims

1. A method for operating a compressor unit comprising one or more compressors, the compressor unit being arranged in a vapour compression system, the vapour compression system further comprising a heat rejecting heat exchanger, a high pressure expansion device, a receiver and at least one evaporator unit, each evaporator unit comprising an evaporator and an expansion device controlling a supply of refrigerant to the evaporator, each compressor of the compressor unit being alternately connectable to a high pressure suction line or to a medium pressure suction line, the high pressure suction line interconnecting a gaseous outlet of the receiver and the compressor unit and the medium pressure suction line interconnecting an outlet of the evaporator unit(s) and the compressor unit, wherein the vapour compression system further comprises at least one valve arrangement, each valve arrangement of the at least one valve arrangement being arranged to selectively connect one compressor of the one or more compressors to the high pressure suction line or to the medium pressure suction line, the method comprising the steps of: defining two or more options for distributing the available compressor capacity of the compressor unit between compressors of the compressor unit being connected to the high pressure suction line and to the medium pressure suction line, for each option, predicting an expected impact on one or more operating parameters of the vapour compression system, resulting from distributing the available compressor capacity according to the option, selecting an option, based on the predicted expected impact for the options, and based on current operating demands of the vapour compression system, and distributing the available compressor capacity according to the selected option by switching one or more compressors from being connected to the medium pressure suction line to being connected to the high pressure suction line, or vice versa, by operating the at least one valve arrangement.

2. The method according to claim 1, wherein the step of switching one or more compressors is performed without stopping the compressor(s).

3. The method according to claim 2, wherein the step of distributing the available compressor capacity according to the selected option comprises switching one or more compressors of the compressor unit on or off.

4. The method according to claim 1, wherein the valve arrangement comprises a two-way valve arranged to connect the one compressor of the one or more compressors to the high pressure suction line and a non-return valve arranged to connect the one compressor of the one or more compressors to the medium pressure suction line.

5. The method according to claim 1, wherein the step of distributing the available compressor capacity according to the selected option comprises switching one or more compressors of the compressor unit on or off.

6. The method according to claim 1, wherein the one or more operating parameters of the vapour compression system comprises energy consumption, mass flow distribution, cooling capacity, heat recovery, number of starts or stops of compressors, runtime equalization of compressors, and/or oil return to the compressor unit.

7. The method according to claim 1, wherein the step of predicting an expected impact on one or more operating parameters of the vapour compression system is performed using a model based approach.

8. The method according to claim 1, wherein the step of selecting an option is further based on one or more expected future requirements for operating the vapour compression system, and wherein the step of distributing the available compressor capacity according to the selected option comprises switching a compressor which is currently not running from being connected to the high pressure suction line to being connected to the medium pressure suction line, or vice versa, in order to be able to meet the expected future requirements.

9. The method according to claim 1, wherein the vapour compression system further comprises a low temperature evaporator unit, a low temperature compressor unit having an inlet connected to an outlet of the low temperature evaporator unit, and a low temperature valve arrangement arranged to selectively interconnect an outlet of the low temperature compressor unit to the high pressure suction line or to the medium pressure suction line, wherein at least some of the options define settings for the low temperature valve arrangement.

10. The method according to claim 9, wherein the step of distributing the available compressor capacity comprises operating the low temperature valve arrangement.

11. The method according to claim 1, wherein the step of defining two or more options for distributing the available compressor capacity is performed on the basis of current and/or expected operating conditions of the vapour compression system.

12. The method according to claim 1, wherein the high pressure expansion device is an ejector having a primary inlet connected to an outlet of the heat rejecting heat exchanger, an outlet connected to the receiver and a secondary inlet connected to the medium pressure suction line, and wherein the method further comprises the step of monitoring oil return to the compressors.

13. The method according to claim 12, wherein the step of selecting an option comprises selecting an option in which at least one compressor is connected to the medium pressure suction line in the case that the oil returned to the compressors decreases below a predefined minimum level.

14. A vapour compression system comprising a compressor unit comprising a plurality of compressors, a heat rejecting heat exchanger, a high pressure expansion device, a receiver and at least one evaporator unit, each evaporator unit comprising an evaporator and an expansion device controlling a supply of refrigerant to the evaporator, each compressor of the compressor unit being alternately connectable to a high pressure suction line and to a medium pressure suction line, the high pressure suction line interconnecting a gaseous outlet of the receiver and the compressor unit and the medium pressure suction line interconnecting an outlet of the evaporator unit(s) and the compressor unit, wherein the vapour compression system further comprises at least one valve arrangement arranged to selectively connect one of the compressors to the high pressure suction line or to the medium pressure suction line, the valve arrangement comprising a two-way valve arranged to connect the compressor to the high pressure suction line and a non-return valve arranged to connect the compressor to the medium pressure suction line.

15. The vapour compression system according to claim 14, wherein the high pressure expansion device is an ejector having a primary inlet connected to an outlet of the heat rejecting heat exchanger, an outlet connected to the receiver and a secondary inlet connected to the medium pressure suction line.

16. The vapour compression system according to claim 14, further comprising a heat recovery heat exchanger arranged in the refrigerant path between an outlet of the compressor unit and an inlet of the heat rejecting heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a diagrammatic view of a vapour compression system according to an embodiment of the invention.

DETAILED DESCRIPTION

(3) FIG. 1 is a diagrammatic view of a vapour compression system 1 according to an embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2, two heat recovery heat exchangers 3a, 3b, a heat rejecting heat exchanger 4, an ejector 5, a high pressure valve 6, a receiver 7 and one or more evaporator units (not shown), arranged in a refrigerant path. Each evaporator unit comprises an evaporator and an expansion device arranged to control a refrigerant supply to the evaporator.

(4) The compressor unit 2 comprises a number of compressors 8, 9, 10, four of which are shown. One of the compressors 8 is permanently connected to a high pressure suction line 11 interconnecting a gaseous outlet 12 of the receiver 7 and the compressor unit 2. Another one of the compressors 9 is permanently connected to a medium pressure suction line 13 interconnecting an outlet of the evaporator units and the compressor unit 2. The last two compressors 10 are selectively connected to the high pressure suction line 11 or to the medium pressure suction line 13 via a valve arrangement 14, 15. One of the valve arrangements is in the form of a three-way valve 14, and the other valve arrangement 15 is in the form of a two-way valve 16 arranged to connect the compressor 10 to the high pressure suction line 11 and a non-return valve 17 arranged to connect the compressor 10 to the medium pressure suction line 13. When the two-way valve 16 is open, the compressor 10 is connected to the high pressure suction line 11, via the two-way valve 16. Simultaneously, the non-return valve 17 is closed, preventing that the compressor 10 receives refrigerant from the medium pressure suction line 13. When the two-way valve 16 is closed, a refrigerant supply to the compressor 10 from the high pressure suction line 11 is prevented. Instead, the non-return valve 17 is opened, thereby allowing the compressor 10 to receive refrigerant from the medium pressure suction line 13.

(5) Accordingly, the compressor capacity represented by the compressors 10 can be shifted between being applied for compressing refrigerant received from the gaseous outlet 12 of the receiver 7, via the high pressure suction line 11, and being applied for compressing refrigerant received from the outlet(s) of the evaporator unit(s), via the medium pressure suction line 13. Since the two-way valve 16 can be switched between an open and a closed position without having to stop the compressor 10, this valve arrangement 15 allows a part of the compressor capacity to be switched between being connected to the high pressure suction line 11 and the medium pressure suction line 13, without having to stop the compressor 10. This allows the compressor capacity to be shifted fast and without inducing unnecessary wear on the compressors 10.

(6) Refrigerant flowing in the refrigerant path is compressed by the compressors 8, 9, 10 of the compressor unit 2. Some of the refrigerant leaving the compressor unit 2 passes through high temperature heat recovery heat exchanger 3a as well as through low temperature heat recovery heat exchanger 3b before being supplied to the heat rejecting heat exchanger 4, and some of the refrigerant only passes through the low temperature heat recovery heat exchanger 3b before being supplied to the heat rejecting heat exchanger 4. The refrigerant passing through the high temperature heat recovery heat exchanger 3a is typically the refrigerant which was compressed by the compressors 9, 10 which are connected to the medium pressure suction line 13.

(7) In the heat recovery heat exchangers 3a, 3b, heat exchange takes place between the refrigerant and a heat recovery system (not shown), in such a manner that heat is rejected from the refrigerant, i.e. the refrigerant is cooled. The heat recovery system may, e.g., be used for providing heating of domestic water and/or for room heating.

(8) In the heat rejecting heat exchanger 4 heat exchange takes place between the refrigerant and the ambient, or with a secondary fluid flow across the heat rejecting heat exchanger 4, in such a manner that heat is rejected from the refrigerant. The heat rejecting heat exchanger 4 may be in the form of a condenser, in which case the refrigerant passing through the heat rejecting heat exchanger 4 is at least partly condensed. Alternatively, the heat rejecting heat exchanger 4 may be in the form of a gas cooler, in which case the refrigerant passing through the heat rejecting heat exchanger 4 is cooled, but remains in a gaseous or transcritical state.

(9) The refrigerant leaving the heat rejecting heat exchanger 4 passes through either the ejector 5, via a primary inlet 18 of the ejector 5, or through the high pressure valve 6, before being supplied to the receiver 7. The refrigerant undergoes expansion when passing through the ejector 5 or the high pressure valve 6, and the refrigerant supplied to the receiver 7 is in a mixed liquid and gaseous state. 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 unit(s), where the refrigerant is expanded in the expansion device(s) before being supplied to the evaporator(s). In the evaporator(s) the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient, or with a secondary fluid flow across the evaporator(s), in such a manner that heat is absorbed by the refrigerant. The refrigerant leaving the evaporator unit(s) is supplied to the medium pressure suction line 13.

(10) At least some of the refrigerant flowing in the medium pressure suction line 13 may be supplied to the compressors 9, 10 being connected thereto. Furthermore, at least some of the refrigerant flowing in the medium pressure suction line 13 may be supplied to a secondary inlet 19 of the ejector 5.

(11) The gaseous part of the refrigerant in the receiver 7 may be supplied to the high pressure suction line 11, via the gaseous outlet 12 of the receiver 7. The refrigerant flowing in the high pressure suction line 11 may be supplied to the compressors 8, 10 being connected thereto. Furthermore, the refrigerant flowing in the high pressure suction line 11 may be supplied to the medium pressure suction line 13, via a bypass valve 20

(12) The vapour compression system 1 further comprises a low temperature compressor unit 21, comprising a number of low temperature compressors 22, two of which are shown. The low temperature compressor unit 21 typically forms part of a refrigerant circuit which provides low temperature cooling, e.g. for one or more freezers.

(13) The outlets of the low temperature compressors 22 are selectively connectable to the high pressure suction line 11 or to the medium pressure suction line 13, via low temperature valve arrangements 23, 24. One of the low temperature valve arrangements is in the form of a three-way valve 23. The other one of the low temperature valve arrangement 24 comprises a two-way valve 25 and a non-return valve 26, similarly to the arrangement described above.

(14) According to an embodiment of the invention, a number of options for distributing the available compressor capacity of the compressor unit 2 between being connected to the high pressure suction line 11 and to the medium pressure suction line 13 may be defined. The options may advantageously include various combinations of settings of the valve arrangements 14, 15, 23, 24.

(15) For each of the options, an expected impact on one or more operating parameters of the vapour compression system 1, resulting from distributing the available compressor capacity according to the option, is predicted. For instance, the impact on energy efficiency of the vapour compression system 1, mass flow distribution in the vapour compression system 1, cooling capacity, wear on the compressors 8, 9, 10, oil return to the compressors 8, 9, 10, heat recovery, etc. may be taken into account, possibly in a prioritized manner.

(16) Based on the predicted expected impact for the options, and on current operating demands for the vapour compression system 1, one of the available options is selected. For instance, the most energy efficient of the options which provide a required cooling capacity could be selected.

(17) Finally, the available compressor capacity of the compressor unit 2 is distributed according to the selected option, i.e. the valve arrangements 14, 15, 23, 24 are set in accordance with the selected option. It should be noted that the settings of the low temperature valve arrangements 23, 24 distribute the discharge of the low temperature compressors 22 between the high temperature pressure suction line 11 and the medium pressure suction line 13. This may be used for ensuring that a sufficient refrigerant supply is available in each of these suction lines 11, 13.

(18) It should be noted that the present invention also covers embodiments in which some of the components illustrated in FIG. 1 are omitted. For instance, the vapour compression system 1 may comprise only an ejector 5, the high pressure valve 6 being omitted, or the vapour compression system 1 may comprise only a high pressure valve 6, the ejector 5 being omitted.

(19) Furthermore, none of the compressors 8, 9, 10 may be permanently connected to the high pressure suction line 11, and/or none of the compressors 8, 9, 10 may be permanently connected to the medium pressure suction line 13. Furthermore, all of the compressors 10 being selectively connected to the high pressure suction line 11 or to the medium pressure suction line 13 may be connected via three-way valves 14, or all of the compressors 10 may connected via valve arrangements 15 comprising a two-way valve 16 and a non-return valve 17.

(20) Furthermore, the low temperature compressor unit 21 and/or the heat recovery heat exchanger 3 may be omitted.

(21) 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.