Method for controlling suction pressure based on a most loaded cooling entity

Abstract

A method for controlling suction pressure in a vapour compression system including one or more cooling entities is disclosed. For each cooling entity, a maximum required suction pressure and/or a required change in suction pressure for maintaining a target temperature in the refrigerated volume is obtained. A most loaded cooling entity among the one or more cooling entities is identified, based on the maximum required suction pressures and/or the required changes in suction pressure. The suction pressure of the vapour compression system is controlled in accordance with the maximum required suction pressure and/or required change in suction pressure for the identified most loaded cooling entity.

Claims

1. A method for controlling suction pressure in a vapour compression system, the vapour compression system comprising a compressor unit, a heat rejecting heat exchanger and two or more cooling entities arranged in a refrigerant path, each cooling entity comprising an expansion device and an evaporator arranged in thermal contact with a respective refrigerated volume, the method comprising the steps of: for each cooling entity, obtaining a maximum required suction pressure and/or a required change in suction pressure for maintaining a target temperature in the respective refrigerated volume, identifying a most loaded cooling entity among the two or more cooling entities, based on the maximum required suction pressures and/or the required changes in suction pressure, and controlling the suction pressure of the vapour compression system in accordance with the maximum required suction pressure and/or required change in suction pressure for the identified most loaded cooling entity.

2. The method according to claim 1, wherein the step of identifying a most loaded cooling entity comprises the steps of: comparing the maximum required suction pressures obtained for each of the cooling entities, and identifying the cooling entity having the lowest maximum required suction pressure as the most loaded cooling entity.

3. The method according to claim 1, wherein the step of controlling the suction pressure comprises the steps of: defining a setpoint value, P.sub.0, for the suction pressure, the setpoint value, P.sub.0, being the maximum required suction pressure for the most loaded cooling entity, and controlling a compressor capacity of the compressor unit in accordance with the defined setpoint pressure, P.sub.0, and in order to obtain a suction pressure which is equal to the setpoint pressure, P.sub.0.

4. The method according to claim 1, wherein the step of controlling the suction pressure comprises the steps of: defining a suction pressure adjustment, ΔP, for the suction pressure, the suction pressure adjustment, ΔP, being the required change in suction pressure for the most loaded cooling entity, and controlling a compressor capacity of the compressor unit in accordance with the defined suction pressure adjustment, ΔP, and in order to obtain an adjustment of the current suction pressure which is equal to the defined suction pressure adjustment, ΔP.

5. The method according to claim 1, wherein the step of obtaining a maximum required suction pressure and/or a required change in suction pressure for a given cooling entity is performed by a cooling entity controller arranged to control a supply of refrigerant to that cooling entity.

6. The method according to claim 1, further comprising the step of deriving performance information relating to the cooling entities and/or relating to the vapour compression system based on the obtained maximum required suction pressures.

7. The method according to claim 6, further comprising the step of identifying one or more cooling entities with degraded performance, based on the derived performance information.

8. The method according to claim 1, further comprising the step of, for each cooling entity, obtaining a maximum required evaporating pressure for maintaining a target temperature in the respective refrigerated volume, and wherein the step of obtaining a maximum required suction pressure and/or change in suction pressure for a given cooling entity is based on the maximum required evaporating pressure for that cooling entity.

9. The method according to claim 8, wherein the step of identifying a most loaded cooling entity is further based on the maximum required evaporating pressures.

10. The method according to claim 2, wherein the step of controlling the suction pressure comprises the steps of: defining a setpoint value, P.sub.0, for the suction pressure, the setpoint value, P.sub.0, being the maximum required suction pressure for the most loaded cooling entity, and controlling a compressor capacity of the compressor unit in accordance with the defined setpoint pressure, P.sub.0, and in order to obtain a suction pressure which is equal to the setpoint pressure, P.sub.0.

11. The method according to claim 2, wherein the step of controlling the suction pressure comprises the steps of: defining a suction pressure adjustment, ΔP, for the suction pressure, the suction pressure adjustment, ΔP, being the required change in suction pressure for the most loaded cooling entity, and controlling a compressor capacity of the compressor unit in accordance with the defined suction pressure adjustment, ΔP, and in order to obtain an adjustment of the current suction pressure which is equal to the defined suction pressure adjustment, ΔP.

12. The method according to claim 2, wherein the step of obtaining a maximum required suction pressure and/or a required change in suction pressure for a given cooling entity is performed by a cooling entity controller arranged to control a supply of refrigerant to that cooling entity.

13. The method according to claim 3, wherein the step of obtaining a maximum required suction pressure and/or a required change in suction pressure for a given cooling entity is performed by a cooling entity controller arranged to control a supply of refrigerant to that cooling entity.

14. The method according to claim 4, wherein the step of obtaining a maximum required suction pressure and/or a required change in suction pressure for a given cooling entity is performed by a cooling entity controller arranged to control a supply of refrigerant to that cooling entity.

15. The method according to claim 2, further comprising the step of deriving performance information relating to the cooling entities and/or relating to the vapour compression system based on the obtained maximum required suction pressures.

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 of a vapour compression system being controlled by means of a method according to an embodiment of the invention,

(3) FIG. 2 is a control diagram illustrating a method according to an embodiment of the invention,

(4) FIG. 3 is a flow chart illustrating a method according to a first embodiment of the invention, and

(5) FIG. 4 is a flow chart illustrating a method according to a second embodiment of the invention.

DETAILED DESCRIPTION

(6) FIG. 1 is a diagrammatic view of a vapour compression system 1 being controlled by means of a method according to an embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2 comprising a number of compressors 3, three of which are shown, a heat rejecting heat exchanger 4 and two cooling entities 5 arranged in a refrigerant path. It should be noted that it is not ruled out that the vapour compression system 1 comprises further cooling entities 5.

(7) Each cooling entity 5 comprises an expansion device 6, in the form of an expansion valve, and an evaporator 7. The evaporators 7 are each arranged in thermal contact with a refrigerated volume, e.g. in the form of a display case. The expansion devices 6 each control the supply of refrigerant to the corresponding evaporator 7.

(8) The pressure of refrigerant entering the compressor unit 2 is referred to as the suction pressure. This pressure level is controlled in accordance with a method according to an embodiment of the invention.

(9) Initially, for each of the cooling entities 5, a maximum required suction pressure and/or a required change in suction pressure for maintaining a target temperature in the corresponding refrigerated volume is obtained. The target temperature is typically an air temperature inside the refrigerated volume which is required in order to maintain the quality of products stored in the refrigerated volume.

(10) Obtaining the maximum required suction pressure and/or a required change in suction pressure for a given cooling entity 5 could, e.g., include determining the highest possible evaporating temperature which would ensure a sufficient heat transfer between the refrigerant flowing through the evaporator 7 and the air inside the refrigerated volume to maintain the target temperature inside the refrigerated volume. As an alternative to determining an absolute value of the evaporating temperature, a required change in evaporating temperature could be determined.

(11) From the determined evaporating temperature, or change in evaporating temperature, a corresponding evaporating pressure, or change in evaporating pressure, can be derived, based on characteristics of the refrigerant. Based on this derived evaporating pressure, or change in evaporating pressure, a corresponding suction pressure, or change in suction pressure, can be derived, with due consideration to any pressure drop which might take place between the evaporator 7 and the inlet of the compressor unit 2.

(12) Next, the most loaded cooling entity 5 is identified, based on the maximum required suction pressures and/or the required changes in suction pressure. This could, e.g., include comparing the obtained maximum required suction pressures and/or required changes in suction pressure and selecting the cooling entity 5 which requires the lowest suction pressure in order to be able to maintain the required target temperature in its refrigerated volume. In general, the lower the suction pressure, the lower the pressure levels in the evaporators 7 will be. A low pressure level in an evaporator 7 provides a low evaporating temperature, and thereby a large temperature difference between the evaporating temperature and the target temperature inside the refrigerated volume. This, in turn, provides a good heat transfer between the refrigerant flowing through the evaporator 7 and the air inside the refrigerated volume. Accordingly, the lower the suction pressure, the easier it will be for the cooling entities 5 to maintain the target temperatures inside the refrigerated volumes. It can therefore be assumed that if the suction pressure is controlled to be at a level which corresponds to the highest possible suction pressure which enables the most loaded cooling entity 5 to maintain the target temperature in its refrigerated volume, then the suction pressure will also be sufficiently low to enable all of the other cooling entities 5 to maintain the target temperature in their respective refrigerated volumes.

(13) Accordingly, the suction pressure is subsequently controlled in accordance with the maximum required suction pressure and/or required change in suction pressure for the identified most loaded cooling entity 5. Thereby it is ensured that the suction pressure is sufficiently low to enable all of the cooling entities 5 to maintain the target temperatures in the refrigerated volumes, but it is not excessively low. This is an advantage because a low suction pressure requires a large pressure increase to be provided by the compressors 3 of the compressor unit 2, and this is energy consuming. Therefore, keeping the suction pressure at a level which only just meets the requirements of each of the cooling entities 5 minimises the energy consumption.

(14) FIG. 2 is a control diagram illustrating a method according to an embodiment of the invention. A requested change in suction temperature, ΔT.sub.e,req,i, is obtained for each cooling entity of the vapour compression system. The most loaded cooling entity is then identified as the cooling entity requesting the largest change in suction temperature, i.e.
ΔT.sub.e,req,MLC=max.sub.i∈N(ΔT.sub.e,req,i),

(15) where ‘MLC’ denotes the most loaded cabinet, i.e. the most loaded cooling entity. ‘i’ is the numbering of the cooling entities, and N is the number of cooling entities in the suction group. The requested change in suction temperature of the most loaded cooling entity is supplied to an adder 8 which also receives a user defined suction temperature setpoint value, T.sub.suc,SP.

(16) In adder 8 the requested change in suction temperature of the most loaded cooling entity, ΔT.sub.e,req,MLC, and the user defined suction temperature setpoint value, T.sub.suc,SP, are added. The output from adder 8 is a suction temperature reference, T.sub.suc,ref, which is supplied to a second adder 10.

(17) In the second adder 10 the current suction temperature, T.sub.suc,current, is subtracted from the suction temperature reference, T.sub.suc,ref. The output of the second adder 10 is supplied to a suction pressure controller 12 which controls the suction pressure of the vapour compression system in accordance therewith, e.g. applying a proportional integral (PI) control strategy.

(18) FIG. 3 is a flow chart illustrating a method for controlling suction pressure of a vapour compression system according to a first embodiment of the invention. The vapour compression system could, e.g., be of the kind illustrated in FIG. 1.

(19) The process is started at step 13. At step 14 one of the cooling entities of the vapour compression system is selected. At step 15 a maximum required suction pressure for maintaining a target temperature in the refrigerated volume of the selected cooling entity is obtained. Thus, the maximum required suction pressure is the highest suction pressure which results in a heat transfer between refrigerant flowing through the evaporator and the air inside the refrigerated volume being sufficient to maintain the target temperature inside the refrigerated volume.

(20) At step 16 it is investigated whether or not further cooling entities exist, i.e. whether or not a maximum required suction pressure has been obtained for all of the cooling entities of the vapour compression system. If further cooling entities exist, the process is returned to step 14, and a new cooling entity is selected.

(21) In the case that step 16 reveals that there are no further cooling entities, i.e. that a maximum required suction pressure has been obtained for all of the cooling entities of the vapour compression system, the process is forwarded to step 17. At step 17 the cooling entity with the lowest maximum required suction pressure is identified. This could, e.g., include comparing the maximum required suction pressures obtained for each of the cooling entities. The identified cooling entity is regarded as the most loaded cooling entity.

(22) At step 18 a suction pressure setpoint, P.sub.0, is defined as the maximum required suction pressure of the most loaded cooling entity, which was identified in step 17. Since the most loaded cooling entity is the one which requires the lowest suction pressure in order to be able to maintain the target temperature inside its refrigerated volume, controlling the suction pressure to this level will ensure that the suction pressure is sufficiently low to ensure that all of the cooling entities are able to maintain the target temperature inside their respective refrigerated volumes.

(23) Finally, at step 19 the compressor capacity of the vapour compression system is controlled according to this suction pressure setpoint, P.sub.0. Thereby it is ensured that all of the cooling entities are able to maintain the target temperature inside their respective refrigerated volumes, while ensuring that the suction pressure is not excessively low.

(24) FIG. 4 is a flow chart illustrating a method for controlling suction pressure of a vapour compression system according to a second embodiment of the invention. The vapour compression system could, e.g., be of the kind illustrated in FIG. 1.

(25) The process is started at step 20. At step 21 one of the cooling entities of the vapour compression system is selected. At step 22 a required change in suction pressure for maintaining a target temperature in the refrigerated volume of the selected cooling entity is obtained. Thus, the required change in suction pressure is the minimum change with respect to the current suction pressure, which is required in order to ensure a heat transfer between refrigerant flowing through the evaporator and the air inside the refrigerated volume being sufficient to maintain the target temperature inside the refrigerated volume.

(26) At step 23 it is investigated whether or not further cooling entities exist, i.e. whether or not a required change in suction pressure has been obtained for all of the cooling entities of the vapour compression system. If further cooling entities exist, the process is returned to step 21, and a new cooling entity is selected.

(27) In the case that step 23 reveals that there are no further cooling entities, i.e. that a required change in suction pressure has been obtained for all of the cooling entities of the vapour compression system, the process is forwarded to step 24. At step 24 the most loaded cooling entity is identified, based on the required changes in suction pressure. More specifically, the most loaded cooling entity is the one where the required change in suction pressure results in the lowest suction pressure.

(28) At step 25 a suction pressure adjustment, ΔP, is defined as the required change in suction pressure for the most loaded cooling entity, which was identified in step 24. Since the most loaded cooling entity is the one which requires the lowest suction pressure in order to be able to maintain the target temperature inside its refrigerated volume, adjusting the suction pressure by this required change in suction pressure will ensure that the suction pressure is sufficiently low to ensure that all of the cooling entities are able to maintain the target temperature inside their respective refrigerated volumes.

(29) Finally, at step 26 the compressor capacity of the vapour compression system is controlled according to this suction pressure adjustment, ΔP. This could, e.g., include adjusting a suction pressure setpoint value by the suction pressure adjustment, ΔP, and subsequently control the compressor capacity in accordance with the adjusted setpoint value. Thereby it is ensured that all of the cooling entities are able to maintain the target temperature inside their respective refrigerated volumes, while ensuring that the suction pressure is not excessively low.

(30) 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.