A METHOD FOR CONTROLLING A SUPPLY OF REFRIGERANT TO AN EVAPORATOR INCLUDING CALCULATING A REFERENCE TEMPERATURE

Abstract

A method for controlling a supply of refrigerant to an evaporator (2) of a vapour compression system (1) is disclosed. During a system identification phase an opening degree (12) of the expansion valve (3) is alternatingly increased and decreased, and a maximum temperature difference, (S.sub.4−S.sub.2).sub.max, between temperature, S.sub.4, of air flowing away from the evaporator (2) and temperature, S.sub.2, of refrigerant leaving the evaporator (2) is determined. During normal operation, the supply of refrigerant to the evaporator (2) is controlled by calculating a reference temperature, S.sub.2,ref, based on the monitored temperature, S.sub.4, and the maximum temperature difference, (S.sub.4−S.sub.2).sub.max, determined during the system identification phase. The supply of refrigerant to the evaporator (2) is controlled in order to obtain a temperature, S.sub.2, of refrigerant leaving the evaporator (2) which is substantially equal to the calculated reference temperature, S.sub.2,ref.

Claims

1. A method for controlling a supply of refrigerant to an evaporator of a vapour compression system, the vapour compression system comprising at least one evaporator, at least one compressor, at least one condenser and at least one expansion valve arranged in a refrigerant path, the method comprising the steps of: initiating a system identification phase, in which an opening degree of the expansion valve is alternatingly increased and decreased, during the system identification phase, monitoring a temperature, S.sub.2, of refrigerant leaving the evaporator, and a temperature, S.sub.4, of air flowing across the evaporator, at a position where the air is flowing away from the evaporator, and determining a maximum temperature difference, (S.sub.4−S.sub.2).sub.max, between the monitored temperatures, upon completion of the system identification phase, controlling a supply of refrigerant to the evaporator by: monitoring the temperature, S.sub.2, of refrigerant leaving the evaporator, and the temperature, S.sub.4, of air flowing away from the evaporator, calculating a reference temperature, S.sub.2,ref, based on the monitored temperature, S.sub.4, of air flowing away from the evaporator and the maximum temperature difference, (S.sub.4−S.sub.2).sub.max, determined during the system identification phase, and controlling the supply of refrigerant to the evaporator, based on the calculated reference temperature, S.sub.2,ref, and in order to obtain a temperature, S.sub.2, of refrigerant leaving the evaporator which is substantially equal to the calculated reference temperature, S.sub.2,ref.

2. The method according to claim 1, wherein the step of calculating a reference temperature, S.sub.2,ref, comprises calculating a mean temperature, S.sub.4, of the temperature, S.sub.4, of air flowing away from the evaporator, during a predefined previous time interval.

3. The method according to claim 2, wherein the reference temperature, S.sub.2,ref, is calculated as:
S.sub.2,ref=S.sub.4−(S.sub.4−S.sub.2).sub.max+Δ, wherein Δ is a constant safety value.

4. The method according to claim 1, wherein the step of controlling the supply of refrigerant to the evaporator is performed using a proportional integral (PI) regulator.

5. The method according to claim 4, further comprising the steps of, during the system identification phase, determining one or more system dynamic parameters, supplying the system dynamic parameters to the proportional integral (PI) regulator, and designing the proportional integral (PI) regulator in accordance with the system dynamic parameters.

6. The method according to claim 1, wherein the step of controlling the supply of refrigerant to the evaporator further comprises the steps of: obtaining an air temperature, T.sub.air, of an air flow across the evaporator, comparing the obtained air temperature, T.sub.air, to a reference value, T.sub.air,ref, and controlling the supply of refrigerant to the evaporator on the basis of the comparing step, as well as on the basis of the calculated reference temperature, S.sub.2,ref.

7. The method according to claim 2, wherein the step of controlling the supply of refrigerant to the evaporator is performed using a proportional integral (PI) regulator.

8. The method according to claim 3 wherein the step of controlling the supply of refrigerant to the evaporator is performed using a proportional integral (PI) regulator.

9. The method according to claim 2, wherein the step of controlling the supply of refrigerant to the evaporator further comprises the steps of: obtaining an air temperature, T.sub.air, of an air flow across the evaporator, comparing the obtained air temperature, T.sub.air, to a reference value, T.sub.air,ref, and controlling the supply of refrigerant to the evaporator on the basis of the comparing step, as well as on the basis of the calculated reference temperature, S.sub.2,ref.

10. The method according to claim 3, wherein the step of controlling the supply of refrigerant to the evaporator further comprises the steps of: obtaining an air temperature, T.sub.air, of an air flow across the evaporator, comparing the obtained air temperature, T.sub.air, to a reference value, T.sub.air,ref, and controlling the supply of refrigerant to the evaporator on the basis of the comparing step, as well as on the basis of the calculated reference temperature, S.sub.2,ref.

11. The method according to claim 4, wherein the step of controlling the supply of refrigerant to the evaporator further comprises the steps of: obtaining an air temperature, T.sub.air, of an air flow across the evaporator, comparing the obtained airs, T.sub.air, to a reference value, T.sub.air,ref, and controlling the supply of refrigerant to the evaporator on the basis of the comparing step, as well as on the basis of the calculated reference temperature, S.sub.2,ref.

12. The method according to claim 5, wherein the step of controlling the supply of refrigerant to the evaporator further comprises the steps of: obtaining an air temperature, T.sub.air, of an air flow across the evaporator, comparing the obtained air temperature, T.sub.air, to a reference value, T.sub.air,ref, and controlling the supply of refrigerant to the evaporator on the basis of the comparing step, as well as on the basis of the calculated reference temperature, S.sub.2,ref.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention will now be described in further detail with reference to the accompanying drawings in which

[0045] FIG. 1 is a diagrammatic view of a part of a vapour compression system for performing a method according to an embodiment of the invention,

[0046] FIG. 2 is a graph illustrating opening degree of an expansion valve and various temperature measurements during operation of a vapour compression system using a method according to an embodiment of the invention, and

[0047] FIG. 3 is a block diagram illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0048] FIG. 1 is a diagrammatic view of a part of a vapour compression system 1 for performing a method according to an embodiment of the invention. The vapour compression system 1 comprises an evaporator 2 arranged in a refrigerant path along with one or more compressors (not shown) and one or more condensers (not shown). An expansion valve 3 is also arranged in the refrigerant path for controlling the supply of refrigerant to the evaporator 2.

[0049] The vapour compression system 1 further comprises a number of temperature sensors. A first temperature sensor 4 is arranged in the refrigerant path after the outlet of the evaporator 2. Accordingly, the first temperature sensor 4 measures a temperature signal, S.sub.2, which represents the temperature of refrigerant leaving the evaporator 2.

[0050] A second temperature sensor 5 is arranged in a secondary air flow across the evaporator 2, at a position before the air reaches the evaporator 2. Accordingly, the second temperature sensor 5 measures a temperature signal, S.sub.3, which represents the temperature of air flowing towards the evaporator 2. It should be noted that, for the purpose of performing the method according to the invention, the second temperature sensor 5 could be omitted.

[0051] A third temperature sensor 6 is arranged in the secondary air flow across the evaporator 2, at a position after the air has passed the evaporator 2. Accordingly, the third temperature sensor 6 measures a temperature signal, S.sub.4, which represents the temperature of air flowing away from the evaporator 2.

[0052] During a system identification phase, an opening degree of the expansion valve 3 is alternatingly increased and decreased. Simultaneously, the temperature signals, S.sub.2 and S.sub.4, are monitored and supplied to a reference temperature calculator 7. In the reference temperature calculator 7 a maximum temperature difference, (S.sub.4−S.sub.2).sub.max, is determined, based on the measured temperature signals, S.sub.2 and S.sub.4. This will be described in further detail below with reference to FIG. 2. The reference temperature calculator 7 may, e.g., be or form part of a controller or a micro processor.

[0053] When the system identification phase has been completed, the temperature signal, S.sub.4, is monitored and supplied to the reference temperature calculator 7. The reference temperature calculator 7 then calculates a reference temperature, S.sub.2,ref, based on the monitored temperature, S.sub.4, and the maximum temperature difference, (S.sub.4−S.sub.2).sub.max, which was determined during the system identification phase. The reference temperature, S.sub.2,ref, may, e.g., be calculated as:

S.sub.2,ref=S.sub.4−(S.sub.4−S.sub.2).sub.max+Δ,

where S.sub.4 is a mean value of S.sub.4 obtained during a predefined previous time interval, and A is a constant safety value. The reference temperature, S.sub.2,ref, calculated in this manner represents a temperature of refrigerant leaving the evaporator 2, which provides an optimum superheat of the refrigerant.

[0054] The calculated reference temperature, S.sub.2,ref, is supplied to a proportional integral (PI) regulator 8. Furthermore, the temperature signal, S.sub.2, is monitored and supplied to the PI regulator 8. Based thereon the PI regulator 8 generates a control signal for the expansion valve 3. In response to the control signal, the expansion valve 3 adjusts the opening degree, and thereby the supply of refrigerant to the evaporator 2, in order to obtain a temperature, S.sub.2, of refrigerant leaving the evaporator 2, which is substantially equal to the calculated reference temperature, S.sub.2,ref.

[0055] Furthermore, the temperature signals, S.sub.3 and S.sub.4, are supplied to a sensor selection unit 9. The sensor selection unit 9 selects whether to select one of the temperature signals, S.sub.3 and S.sub.4, as an air temperature, T.sub.air, being representative for a temperature prevailing inside a refrigerated volume, or to select a weighted value of the two temperature signals, S.sub.3 and S.sub.4. The selection may, e.g., be based on the availability of sensors 5 and 6, or on the choice of the installer. Based on the selection, a temperature signal, T.sub.air, is generated, and T.sub.air is supplied to an air tracking unit 10.

[0056] A reference air temperature, T.sub.air,ref, is also supplied to the air tracking unit 10. The reference air temperature, T.sub.air,ref, represents a reference or target temperature which is desired inside the refrigerated volume, i.e. in the air flowing across the evaporator 2.

[0057] The air tracking unit 10 compares the temperature signal, T.sub.air, to the reference air temperature, T.sub.air,ref, and generates a signal which is supplied to the PI regulator 8. The generated signal indicates how close the actual air temperature, T.sub.air, is to the reference air temperature, T.sub.air,ref. Thus, when the PI regulator 8 generates the control signal for the expansion valve 3, it takes into account that adjustments to the opening degree of the expansion valve 3 may also be required in order to obtain a desired temperature of the air flowing across the evaporator 2. Accordingly, the opening degree of the expansion valve 3, and thereby the supply of refrigerant to the evaporator 2, is controlled in order to obtain a reference temperature, S.sub.2,ref, of refrigerant leaving the evaporator 2, as well as in order to obtain a reference temperature, T.sub.air,ref, of air flowing across the evaporator 2, inside a refrigerated volume.

[0058] The vapour compression system 1 is further provided with safety logic 11 in order to detect possible errors or safety issues. For instance, the safety logic 11 may detect if there is a risk of flooding the evaporator 2, i.e. if the superheat value of refrigerant leaving the evaporator 2 is approaching zero.

[0059] FIG. 2 is a graph illustrating opening degree of an expansion valve and various temperature measurements during operation of a vapour compression system using a method according to an embodiment of the invention. The vapour compression system could, e.g., be the vapour compression system illustrated in FIG. 1.

[0060] The graph of FIG. 2 illustrates three different phases of control of the vapour compression system, i.e. a pull-down phase, a system identification phase, and a normal control phase.

[0061] Prior to the pull-down phase the vapour compression system has been inoperative for a period of time. Therefore all temperatures, S.sub.2, S.sub.3 and S.sub.4, prevailing in the vapour compression system have equalized at substantially the same temperature level.

[0062] At initiation of the pull-down phase, the opening degree 12 of the expansion valve is set to a maximum value, in order to fill the evaporator as quickly as possible, thereby driving the measured temperatures downwards. The temperature signal, S.sub.2, represents the temperature of refrigerant leaving the evaporator, the temperature signal, S.sub.3, represents the temperature of air flowing towards the evaporator, and the temperature, S.sub.4, represents the temperature of air flowing away from the evaporator, as described above with reference to FIG. 1.

[0063] It can be seen from FIG. 2 that, during the pull-down phase, the temperature, S.sub.4, of air flowing away from the evaporator initially decreases faster than the other measured temperature, S.sub.2 and S.sub.3. This is due to the fact that the evaporator is cooling the air flowing across the evaporator in an efficient manner. The temperature, S.sub.3, of air flowing towards the evaporator and the temperature, S.sub.2, of refrigerant leaving the evaporator also decrease, but at a slower rate.

[0064] At a certain point in time the temperature, S.sub.2, of refrigerant leaving the evaporator decreases drastically. This is an indication that liquid refrigerant is present throughout almost the entire length of the evaporator, and that a zero superheat situation is approaching. Accordingly, the pull-down phase is ended, and the system identification phase is initiated.

[0065] During the system identification phase the opening degree 12 of the expansion valve is alternatingly increased and decreased between a maximum value and a minimum value. Furthermore, the temperature signals, S.sub.2, S.sub.3 and S.sub.4, are monitored.

[0066] It can be seen from FIG. 2 that the alternating increases and decreases of the opening degree 12 of the expansion valve results in significant decreases and increases of the temperature, S.sub.2, of refrigerant leaving the evaporator. This is due to the fact that the evaporator is almost filled with liquid refrigerant, and variations in the opening degree 12 of the expansion valve, and thereby in the supply of refrigerant to the evaporator, thereby significantly affects the temperature, S.sub.2, of refrigerant leaving the evaporator.

[0067] The temperature, S.sub.4, of air flowing away from the evaporator also varies during the system identification phase, since the air is cooled by the evaporator, and the temperature, S.sub.4, of air flowing away from the evaporator thereby depends on the evaporating temperature. However, the variations in the temperature, S.sub.4, of air flowing away from the evaporator are smaller than the variations in the temperature, S.sub.2, of refrigerant leaving the evaporator.

[0068] Finally, the temperature, S.sub.3, of air flowing towards the evaporator simply continues to decrease during the system identification phase, with only insignificant variations.

[0069] For each period of increasing and decreasing the opening degree 12 of the expansion valve, the temperature difference, (S.sub.4−S.sub.2), between the temperature, S.sub.2, of refrigerant leaving the evaporator and the temperature, S.sub.4, of air flowing away from the evaporator is monitored, and a largest value of the temperature difference is determined. Upon completion of the system identification phase a maximum temperature difference, (S.sub.4−S.sub.2).sub.max, is determined as the largest of the temperature differences, (S.sub.4−S.sub.2), determined during the periods of increasing and decreasing the opening degree 12 of the expansion valve. The maximum temperature difference, (S.sub.4−S.sub.2).sub.max, is supplied to a reference temperature calculator, and used for calculating a reference temperature, S.sub.2,ref, in the manner described above.

[0070] The maximum temperature difference, (S.sub.4−S.sub.2).sub.max, provides information regarding how far the temperature, S.sub.2, of refrigerant leaving the evaporator is from an optimal superheat value, under the given circumstances, and thereby information regarding how much further the temperature, S.sub.2, can be lowered without risking flooding of the evaporator, i.e. that liquid refrigerant is allowed to pass through the evaporator. A reference temperature, S.sub.2,ref, for the refrigerant leaving the evaporator, which is calculated on the basis of the maximum temperature difference, (S.sub.4−S.sub.2).sub.max, thereby takes this information into account.

[0071] Finally, the normal control phase is initiated. During this phase the opening degree 12 of the expansion valve is controlled in order to obtain a temperature, S.sub.2, of refrigerant leaving the evaporator, which is substantially equal to the reference temperature, S.sub.2,ref. The reference temperature, S.sub.2,ref, is continuously calculated, on the basis of the maximum temperature difference, (S.sub.4−S.sub.2).sub.max, which was determined during the system identification phase, and on the basis of the monitored temperature, S.sub.4, of air flowing away from the evaporator.

[0072] FIG. 3 is a block diagram illustrating a method according to an embodiment of the invention, e.g. for controlling the vapour compression system of FIG. 1.

[0073] An air temperature, T.sub.air, and a reference air temperature, T.sub.air,ref, are supplied to a comparator 13 of an air tracking unit 10, e.g. in the manner described above with reference to FIG. 1. The output of the comparator 13 is passed through a filter 14, and a gain 15 is applied to the signal before it is supplied from the air tracking unit 10 to an integral part of a proportional integral (PI) regulator 8.

[0074] Furthermore, a reference temperature calculator 7 calculates a reference temperature, S.sub.2,ref, of the refrigerant leaving the evaporator. The reference temperature, S.sub.2,ref, is calculated essentially in the manner described above, and is supplied to a comparator 16, where it is compared to a measured temperature, S.sub.2, of refrigerant leaving the evaporator.

[0075] The output of the comparator 16 is supplied to the PI regulator 8. Based on the input received from the air tracking unit 10 and the reference temperature calculator 7, the PI regulator 8 generates a control signal for an expansion valve, in order to control the opening degree of the expansion valve in such a manner that the temperature, S.sub.2, of refrigerant leaving the evaporator is substantially equal to the calculated reference temperature, S.sub.2,ref, and in such a manner that an air temperature, T.sub.air, which is equal to the reference air temperature, T.sub.air,ref, is obtained. The input received from the air tracking unit 10 ensures that the opening degree of the expansion valve is controlled in a smooth manner.

[0076] 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.