Control Of Refrigerant Injection Into A Compressor In An Economized Refrigeration Cycle

Abstract

A method of controlling injection into a compressor in a refrigeration cycle is described. A refrigeration cycle may comprise at least an economizer heat exchanger, a heat rejection heat exchanger, a first expansion device, and a compressor. A discharge port of the compressor is connected to the heat rejection heat exchanger via a discharge line and an injection port of the compressor is connected to the means for compressing. The economizer heat exchanger comprises a first path having an input connected to the heat rejection heat exchanger and an output connected to the first expansion device, and a second path having an input connected to the heat rejection heat exchanger via an economizer valve and an output connected to the injection port of the compressor via an injection line. The economizer valve is regulated based on a superheat level of the refrigerant in the economizer heat exchanger.

Claims

1. A method of controlling injection into a compressor in a refrigeration cycle, wherein the method is performed in a refrigeration cycle, which comprises at least a flash tank configured for receiving a refrigerant and separating liquid refrigerant and vapour refrigerant, and a compressor configured for compressing the refrigerant, wherein the compressor comprises a means for compressing, a suction port and an injection port, which is connected to the means for compressing for at least a time instance of the refrigeration cycle, wherein the flash tank is connected to the injection port of the compressor via an injection valve, the method comprising: determining a pressure in the flash tank; controlling the injection valve based on the determined pressure in the flash tank.

2. The method according to claim 1, wherein controlling the injection valve comprises: if the determined pressure in the flash tank is lower than a first threshold, closing the injection valve.

3. The method according to claim 2, wherein controlling the injection valve comprises: if the determined pressure in the flash tank is equal to or greater than the first threshold and lower than a second threshold, at least partially opening the injection valve.

4. The method according to claim 3, wherein opening the injection valve comprises: determining, by a proportional integral derivative, PID, controller, a value for an opening degree of the injection valve based on the determined flash tank pressure; and setting the opening degree of the injection valve to the determined value.

5. The method according to claim 3, further comprising: determining whether the compressor is operating; and wherein opening the injection valve is only carried out, if it is determined that the compressor is operating.

6. The method according to claim 3, wherein controlling the injection valve comprises: if the determined pressure in the flash tank is greater than the second threshold, closing the injection valve.

7. The method according to claim 1, the method further comprising: determining a pressure at the suction port of the compressor; determining whether the pressure at the suction port is lower than a third threshold; and if it is determined that the pressure at the suction port is lower than the third threshold: closing the injection valve; and turning off the compressor.

8. The method according to claim 1, the method further comprising: controlling the compressor based on the determined pressure in the flash tank.

9. The method according to claim 8, wherein controlling the compressor comprises: if the determined flash tank pressure is lower than a fourth threshold, determining, by a PID controller, an operating speed for the compressor and setting the operating speed to the determined operating speed.

10. The method according to claim 9, wherein controlling the compressor comprises: if the determined flash tank pressure is equal to or greater than the fourth threshold and lower than a fifth threshold, unloading the compressor; and if the determined flash tank pressure is greater than the fifth threshold, stopping operation of the compressor.

11. The method according to claim 1, wherein the compressor comprises a discharge port and wherein the refrigeration cycle further comprises a heat rejection heat exchanger, which is connected to the discharge port of the compressor, and an expansion device disposed between the heat rejection heat exchanger and the flash tank, wherein the method further comprises: if the determined flash tank pressure is lower than a sixth threshold, setting an opening degree of the expansion device to a predetermined value; if the determined flash tank pressure is equal to or greater than the sixth threshold and lower than a seventh, setting the opening degree of the expansion device to a value determined by a PID controller based on a first heat rejection heat exchanger pressure mode; determining that the pressure in the flash tank is equal to or greater than the seventh and lower than an eighth threshold and setting the opening degree of the expansion device to a value determined by the PID controller based on a second heat rejection heat exchanger pressure mode; or determining that the pressure in the flash tank is equal to or greater than the eighth threshold and lower than a ninth threshold and controlling the opening degree of the expansion device based on fuzzy regulation; or determining that the pressure in the flash tank is equal to or greater than the ninth threshold and lower than a tenth threshold and controlling the opening degree of the expansion device based on a flash tank pressure regulation mode; or determining whether the pressure in the flash tank is equal to or greater than the tenth threshold and closing the expansion device.

12. The method according to claim 11, wherein the first heat rejection heat exchanger pressure mode comprises controlling the expansion device based on the temperature of the refrigerant in the heat rejection heat exchanger.

13. The method according to claim 11, wherein the second heat rejection heat exchanger pressure mode (HRHE_mode2) comprises controlling the expansion device based on the temperature of the refrigerant in the heat rejection heat exchanger and the pressure of the refrigerant in the flash tank.

14. The method according to claim 11, wherein the flash tank pressure regulation mode comprises controlling the expansion device based on the pressure if the refrigerant in the flash tank.

15. The method according to claim 1, wherein the refrigerant cycle comprises a by-pass line connected between the flash tank and the suction port of the compressor, wherein the by-pass line comprises a by-pass valve, and wherein the method further comprises: determining that the pressure of the flash tank is lower than a eleventh threshold and closing the by-pass valve; or determining that the pressure of the flash tank is equal to or greater than the eleventh threshold and lower than a twelfth threshold, and determining, by a PID controller, a value for an opening degree of the by-pass valve based on the determined flash tank pressure; or determining that the pressure of the flash tank is equal to or greater than the twelfth threshold and lower than a thirteenth threshold, and opening the by-pass valve completely; or determining that the pressure of the flash tank is equal to or greater than the thirteenth threshold and setting an opening degree of the by-pass valve to a predetermined value.

Description

DRAWINGS

[0056] In the drawings, like reference characters generally refer to the same parts throughout the different drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[0057] In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0058] FIG. 1 shows a schematic of an exemplary refrigeration system control of refrigerant injection into a compressor in an economized refrigeration cycle;

[0059] FIG. 2 shows a diagram of the influence of refrigerant injection on the optimum heat rejection heat exchanger pressure;

[0060] FIG. 3 shows a discharge temperature over injection pressure diagram for exemplary embodiments of the current invention;

[0061] FIG. 4a, 4b show block diagrams of the inputs and outputs of controllers as may be used in connection with the current invention;

[0062] FIG. 5 shows a flow diagram of a method of controlling the injection into a compressor according to an embodiment of the current invention;

[0063] FIG. 6 shows a flow diagram of an alternative method of controlling the injection into a compressor according to another embodiment of the current invention;

[0064] FIG. 7 shows a decision diagram of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to regulating the amount of injection into the compressor;

[0065] FIG. 8 shows a decision diagram, which further specifies step 310 of FIG. 7;

[0066] FIG. 9 shows a decision diagram, which further specifies step 314 of FIG. 7; and

[0067] FIG. 10 shows a decision diagram, which further specifies step 318 of FIG. 7.

DETAILED DESCRIPTION

[0068] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced.

[0069] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0070] FIG. 1 shows a schematic of a refrigeration system 1 for economizer-based control of refrigerant injection into a compressor 2 of the refrigeration system 1. The refrigeration system 1 comprises a compressor 2, which comprises a suction port 2a, a discharge port 2b, and an injection port 2c, a heat rejection heat exchanger 3 downstream of the compressor 2, a first expansion device 4 downstream of the heat rejection heat exchanger 3, and a heat accepting heat exchanger 7 downstream of the first expansion device 4 and upstream of the compressor 2.

[0071] Further, the refrigeration system 1 comprises a second expansion device 6 and a flash tank 5. The flash tank 5 and the second expansion device 6 are disposed between the first expansion device 4 and the heat accepting heat exchanger 7. In detail, the flash tank 5 is disposed downstream of the first expansion device 4 and upstream of the second expansion device 6, which is disposed upstream of the heat accepting heat exchanger 7. Thereby, the pressure and the temperature of the refrigerant could be reduced.

[0072] In the refrigeration system 1 depicted in FIG. 1, the flash tank 5 comprises two separation chambers 5a, 5b. However, it would also be possible that the flash tank separates the liquid refrigerant and the vapour refrigerant in the same volume.

[0073] The two separation chambers 5a, 5b include a chamber 5a used for collecting vapour or flash gas and a chamber 5b for collecting liquid. Liquid collecting chamber 5b comprises at least one outlet. The connection between the flash tank 5 and the second expansion device 6 is established via at least one of the at least one outlets of the liquid collecting chamber 5b of the flash tank 5.

[0074] The vapour collecting chamber 5a of the flash tank 5 comprises at least one outlet. The at least one outlet of the vapour collecting chamber 5a is connected to the suction port of the compressor 2 via a by-pass path 8 and a by-pass valve 9.

[0075] Although a flash tank 5 and a by-pass line 8 are depicted in FIG. 1, the person skilled in the art will appreciate that the flash tank 5 is not necessary for the refrigeration system. In at least some embodiments, no flash tank is used or a flash tank 5 is used without a by-pass line.

[0076] The refrigeration system 1 comprises an economizer heat exchanger 11. The economizer heat exchanger comprises two path—a first path 11a, which is connected to the heat rejection heat exchanger 3 and the first expansion device 4, and a second path 11b, which is connected to the heat rejection heat exchanger 3 via an economizer valve 13 and is connected to the injection port 2c of the compressor 2 via an injection line 12. In the example depicted in FIG. 1, the first path and the second path of the economizer have counter-wise flow directions.

[0077] In the economizer heat exchanger 11 depicted in FIG. 1, the first path 11a and the second path 11b are in near proximity to each other, such that heat exchange is possible between both paths. Because the refrigerant in the second path 11b is expanded by the economizer valve 13, the refrigerant in the second path 11b has a lower temperature than the refrigerant in the first path 11a. Therefore, heat is exchanged from the refrigerant of the first path 11a to the refrigerant of the second path 11b. This process is a subcooling process, which decreases the amount of heat of the refrigerant in the first path 11a and may thereby also reduce the temperature of the refrigerant in the first path 11a.

[0078] Further, the refrigeration system 1 comprises a controller 10, which is used for regulating at least the economizer valve 13. Additionally, the controller 10 may also be used to control any of the first expansion device 4, the flash tank 5, the second expansion device 6, the by-pass valve 9, and the compressor 2. The operation of the controller 10 is based on the superheat level of the refrigerant in the economizer heat exchanger 11. Additionally, the controller 10 may also use system parameters like the pressure of the refrigerant in the injection line 12 or the temperature of the refrigerant, which is discharged from the compressor 2.

[0079] FIG. 1 indicates the connection for exchanging control signals by ease of dashed lines. Although FIG. 1 shows dashed lines between the controller 10 and the economizer valve 13, the first expansion device 4, the second expansion device 6, the compressor 2, and the flash tank 5, the person skilled in the art will appreciate that these dashed lines are shown for illustration purposes only. The controller 10 may be connected to any subset of the aforementioned components of the refrigeration cycle. With respect to the connection between the controller 10 and the flash tank 5, it is to be noted that the controller 10 may be connected to a sensor within the flash tank 5, wherein the sensor may be a pressure sensor. Furthermore, in some examples, multiple controllers may be employed in the refrigeration system. Each of these multiple controllers may control any subset of the expansion devices, the compressor, and the flash tank as is described before with respect to controller 10.

[0080] FIG. 2 shows a diagram of the influence of refrigerant injection on the optimum heat rejection heat exchanger pressure. In detail, FIG. 2 depicts the coefficient of performance (COP) depending on the pressure of the refrigerant in the heat rejection heat exchanger (p.sub.c). Thereby, solid line 50 represents the curve of the COP for a refrigeration system with closed injection valve, i.e. without injection of refrigerant into the means for compressing of the compressor.

[0081] The dashed line 55 represents an exemplary curve of the COP for the same refrigeration system with an at least partially opened injection valve, i.e. with injection of refrigerant into the means for compressing of the compressor. The difference between the curves is shown for illustrative purpose.

[0082] In a refrigeration system, the operating conditions are controlled in order to achieve a higher COP. Without refrigerant injection, the COP depends on the temperature of the refrigerant in the heat rejection heat exchanger. However, refrigerant injection has a direct influence on the efficiency of the system. This influence depends on the injection conditions, like pressure of the injected refrigerant or temperature of the injected refrigerant. As can be seen, injection does not only improve the overall COP. Injection also shifts the maximum of the COP to a lower pressure of the refrigerant in the heat rejection heat exchanger. The maximum of the respective curve represents the optimum heat rejection heat exchanger pressure. This optimum pressure is lower when injection of refrigerant into the compressor is used.

[0083] FIG. 3 shows a discharge temperature over injection pressure diagram for exemplary embodiments of the current invention. The pressure p corresponds to the pressure at which the refrigerant is injected into the injection port of the compressor. This pressure may be measured in the second path of the economizer or in the injection line and may be referred to as injection pressure. The temperature T corresponds to the temperature of the refrigerant, which is discharged from the discharge port of the compressor. This temperature may be measured at the discharge port or in the connection line between the discharge port and the heat rejection heat exchanger and may be referred to as discharge line temperature, DLT.

[0084] In the temperature-pressure-diagram, different areas 70, 71, 72, 73, 74, 75 are depicted. These areas are based on particular pressure and temperature thresholds and indicate the pressure and temperature ranges for each of the three operation modes or combinations thereof.

[0085] Below an injection pressure of p.sub.0, no injection into the compressor is performed. In this case, the injection pressure would be too low for an efficient injection. Instead, the pressure may be so low that the refrigerant from the injection line would not be injected into the compressor, but that undesired reverse flow from the compressor through the discharge line may occur. P.sub.0 may be referred to as minimum pressure for injection.

[0086] Also, there will be no injection performed for a pressure higher than p.sub.max. If the pressure would exceed p.sub.max, the refrigerant would be injected at such a high pressure that the compressor may be damaged or the efficiency of the refrigeration cycle would be reduced. Similarly, no injection will be performed for temperatures exceeding a maximum temperature value of T.sub.max.

[0087] Between the pressure stages p.sub.0 and p.sub.max, injection is performed based on the three operation modes or combinations thereof. Thereby, the first operation mode is denoted as superheat control mode. The first operation mode is performed for pressure ranges from p.sub.0 up to p.sub.1 and temperature ranges below T.sub.1. The corresponding area in the temperature-pressure-diagram is area 70.

[0088] The second operation mode is denoted as discharge line temperature control mode and is performed for pressure ranges from p.sub.0 up to p.sub.1 and temperature ranges between T.sub.2 and T.sub.max. The corresponding area in the temperature-pressure-diagram is area 72.

[0089] At 71, for p.sub.0 to p.sub.1 and T.sub.1 to T.sub.2, a combination of the superheat control mode and the discharge line temperature control mode is performed.

[0090] The third operation mode is denoted as injection pressure control mode and is performed for pressure ranges from p.sub.2 up to p.sub.max and temperature ranges below T.sub.1. The corresponding area in the temperature-pressure-diagram is area 74.

[0091] At 73, for p.sub.1 to p.sub.2 and discharge line temperatures below T.sub.1, a combination of the superheat control mode and injection pressure control mode is performed.

[0092] Further, for discharge line temperatures higher than T.sub.1 and injection pressures between p.sub.1 and p.sub.max, a combination of all three operation modes is performed in area 75.

[0093] The person skilled in the art will appreciate that the pressure stages p.sub.i and the temperature stages T.sub.i are for illustrative purposes. The particular values of these stages depend on the system to which the control operation is applied.

[0094] FIGS. 4a, 4b show block diagrams of the inputs and the outputs of controllers as may be used in connection with the current invention.

[0095] In FIG. 4a, the controller, which is represented by block “CTRL” receives the superheat value of the refrigerant in the second path of the economizer as input and controls at least the economizer valve, which is denoted as economizer heat exchanger valve “EHXV”. Additionally, the controller may also control the operation of the compressor CMP. In FIG. 4a, the output arrow of the compressor CMP is shown as dashed line in order to illustrate that the controller may perform economizer valve control, or both, the economizer valve control and the compressor control.

[0096] In FIG. 4b, the controller receives the superheat value as input and controls the economizer valve and optionally the compressor. Further, the controller receives the pressure of the refrigerant in the injection line (denoted as economizer heat exchanger pressure “EHXP”) and the temperature of the refrigerant in the discharge line (denoted as discharge line temperature “DLT”) as additional inputs.

[0097] FIG. 5 shows a flow diagram of a method 100 of controlling the injection into a compressor according to an embodiment of the invention. The method 100 may be performed by a controller in a refrigeration cycle, for example controller 10 as depicted in FIG. 1. Throughout the flow diagram, solid lines indicate steps, which are essential to the current invention, whereas dashed lines indicate steps, which are performed in preferred embodiments of the current invention.

[0098] The method 100 comprises the step of determining 102 a pressure of the refrigerant in the injection line 12. Determining a pressure of the refrigerant in the injection line may comprise determining a pressure in any part of the injection line 12, the second path of the economizer heat exchanger 11b, or at the outlet of the second path of the economizer heat exchanger 11b.

[0099] Further, the method 100 comprises the step of determining 104 a temperature of the refrigerant in the discharge line 14. Because of the similar temperature of the refrigerant at the discharge port 2b and the discharge line 14, determining the temperature of the refrigerant at the discharge port 2b of the compressor 2 also may be performed by measuring the temperature of the refrigerant in the discharge line 14.

[0100] Also, the method comprises regulating 106 the economizer valve 13 by using a first operation mode. The first operation mode may correspond to the superheat control mode. Regulating 106 the economizer valve 13 may comprise determining 108 whether to proceed with regulating the economizer valve by using the first operation mode or whether to perform one of a second and a third operation mode. Thereby, the first operation mode may establish a default operation of the controller. The second operation mode may correspond to the discharge line temperature control mode and the third operation mode may correspond to the injection pressure control mode.

[0101] Based on the determining 108, the method 100 may comprise proceeding 110 with regulating 106 the economizer valve by using the first operation mode, or regulating 112 the economizer valve by using the second operation mode, or regulating 114 the economizer valve by using the third operation mode.

[0102] FIG. 6 shows a flow diagram of the method 200 of controlling the injection into a compressor according to an alternative embodiment of the invention. The method 200 may be performed by a controller in a refrigeration cycle, for example controller 10 as depicted in FIG. 1.

[0103] The method 200 comprises the step of determining 202 a pressure of the refrigerant in the injection line 12. Determining a pressure of the refrigerant in the injection line may comprise determining a pressure in any part of the injection line 12, the second path of the economizer heat exchanger 11b, or at the outlet of the second path of the economizer heat exchanger 11b.

[0104] Further, the method 200 comprises the step of determining 204 a temperature of the refrigerant in the discharge line 14. Because of the similar temperature of the refrigerant at the discharge port 2b and in the discharge line 14, determining the temperature of the refrigerant at the discharge port of the compressor 2 also may be performed by measuring the temperature of the refrigerant in the discharge line 14.

[0105] Also, the method 200 comprises the step of selecting 206 one of a first operation mode, a second operation mode, and a third operation mode. Thereby, the first operation mode may correspond to the superheat control mode, the second operation mode may correspond to the discharge line temperature control mode, and the third operation mode may correspond to the injection pressure control mode.

[0106] Further, the method 200 comprises regulating 208 the economizer valve 13 by using the selected operation mode.

[0107] FIG. 7 shows a decision diagram 300 of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to regulating the amount of injection into the compressor. The amount of injection into the compressor is controlled by regulating the so-called economizer valve or economizer heat exchanger valve, which is referred to as EHXV (cf. reference sign 13 in FIG. 1). The decision may be carried out by a controller, for example controller 10.

[0108] The method starts at step 302 where the determined pressure of the refrigerant in the injection line is received. In FIG. 7, the pressure of the refrigerant in the injection line is referred to as p.

[0109] At step 304, it is determined whether the injection pressure p is below a first threshold. In case that the pressure is lower than the first threshold, the method continues at step 306 where the economizer valve EHXV is closed. Otherwise, the method continues at step 308.

[0110] At step 308, it is determined whether the injection pressure p is greater than or equal to the first threshold and lower than a second threshold. In case that the injection pressure is greater than or equal to the first threshold and lower than the second threshold, the method continues at step 310 where the economizer valve EHXV is opened. There, the opening degree of the economizer valve EHXV is calculated as a function of at least one of the superheat value, SH, of the refrigerant in the economizer heat exchanger or the temperature of the refrigerant in the discharge line, DLT. As will be described in more detail with respect to FIG. 8, the opening degree may be calculated based on the superheat value, the discharge line temperature, or a combination of both, depending on value of the discharge line temperature. In case that the injection pressure is not greater than or equal to the first threshold and lower than the second threshold, the method continues at step 312.

[0111] At step 312, it is determined whether the injection pressure p is greater than or equal to the second threshold and lower than a third threshold. In case that the injection pressure is greater than or equal to the second threshold and lower than the third threshold, the method continues at step 314 where the economizer valve EHXV is opened. There, the opening degree of the economizer valve EHXV is calculated as a function of at least the superheat value, SH, the injection pressure, p. As will be described in more detail with respect to FIG. 9, the opening degree may be calculated based on the injection pressure, the discharge line temperature, or a combination of both, depending on value of the discharge line temperature. Additionally, considering the superheat value for the calculation may also be possible. In case that the injection pressure is not greater than or equal to the second threshold and lower than the third threshold, the method continues at step 316.

[0112] At step 316, it is determined whether the injection pressure p is greater than or equal to the third threshold and lower than a fourth threshold. In case that the injection pressure is greater than or equal to the third threshold and lower than the fourth threshold, the method continues at step 318 where the economizer valve EHXV is opened. There, the opening degree of the economizer valve EHXV is calculated as a function of at least the injection pressure, p. As will be described in more detail with respect to FIG. 10, the discharge line temperature or the superheat value may also be considered for the calculation of the opening degree, depending on value of the discharge line temperature. In case that the injection pressure is not greater than or equal to the third threshold and lower than the fourth threshold, the method continues at step 320 where the economizer valve EHXV is closed and the compressor is turned off.

[0113] In case the method reaches either one of steps 306, 310, 314, 318, or 320, the method may again continue at step 302 by determining or receiving an injection pressure p. In this case, the method may determine or receive an updated value for the injection pressure p.

[0114] FIG. 8 shows a decision diagram, which describes a method 400 for determining the opening degree of the economizer valve EHXV based on step 310 of the decision diagram of FIG. 7 in more detail.

[0115] Following step 310, method 400 receives a determined value for the temperature of the refrigerant in the discharge line at step 402.

[0116] At step 404, it is determined whether the discharge line temperature DLT is below a fifth threshold. In case that the temperature is lower than the fifth threshold, the method continues at step 406 where the opening degree of the economizer valve EHXV is calculated as a function of the superheat level of the refrigerant in the economizer. This refers to the first operation mode, also called superheat control mode. With reference to FIG. 3, step 406 may refer to the operation performed for pressure and temperature being located in area 70. Otherwise, the method continues at step 408.

[0117] At step 408, it is determined whether the discharge line temperature DLT is greater than or equal to the fifth threshold and lower than a sixth threshold. In case that the temperature is greater than or equal to the fifth threshold and lower than the sixth threshold, the method continues at step 410 where the opening degree of the economizer valve is calculated as a function of the superheat level of the refrigerant in the economizer and the temperature of the refrigerant in the discharge line, DLT. Thereby, a combination of the superheat control and the discharge line control mode may be performed. With reference to FIG. 3, step 410 may refer to the operation performed for pressure and temperature being located in area 71. In case that the discharge line temperature is not greater than or equal to the fifth threshold and lower than the sixth threshold, the method continues at step 412.

[0118] At step 412, it is determined whether the discharge line temperature DLT is greater than or equal to the sixth threshold and lower than a seventh threshold. In case that the temperature is greater than or equal to the sixth threshold and lower than the seventh threshold, the method continues at step 414 where the opening degree of the economizer valve EHXV is calculated as a function of the temperature of the refrigerant in the discharge line, DLT. Thereby, the operation is performed based on the discharge line control mode. With reference to FIG. 3, step 414 may refer to the operation performed for pressure and temperature being located in area 72. In case that the discharge line temperature is not greater than or equal to the sixth threshold and lower than seventh threshold, the method continues at step 416 where the economizer valve EHXV is closed and the compressor is turned off.

[0119] In case the method reaches either one of steps 406, 410, 414, or 416, the method may again continue at step 402 by determining or receiving the discharge line temperature. In this case, the method may determine or receive an updated value for the discharge line temperature.

[0120] FIG. 9 shows a decision diagram, which describes a method 500 for determining the opening degree of the economizer valve EHXV based on step 314 of the decision diagram of FIG. 7 in more detail.

[0121] Following step 314, method 500 receives a determined value for the temperature of the refrigerant in the discharge line at step 502.

[0122] At step 504, it is determined whether the discharge line temperature DLT is below an eighth threshold. In case that the temperature is lower than the eighth threshold, the method continues at step 506 where the opening degree of the economizer valve EHXV is calculated as a function of the pressure of the refrigerant in the injection line and the superheat value. Thereby, a combination of the superheat control mode and the injection pressure control mode is performed. With reference to FIG. 3, step 506 may refer to the operation performed for pressure and temperature being located in area 73. Otherwise, the method continues at step 508.

[0123] At step 508, it is determined whether the discharge line temperature DLT is greater than or equal to the eighth threshold and lower than a ninth threshold. In case that the temperature is greater than or equal to the eighth threshold and lower than the ninth threshold, the method continues at step 510 where the opening degree of the economizer valve is calculated as a function of the superheat value, the pressure of the refrigerant in the injection line, and the temperature of the refrigerant in the discharge line, DLT. Thereby, a combination of all three operation modes is performed. With reference to FIG. 3, step 510 may refer to the operation performed for pressure and temperature being located in area 75. In case that the discharge line temperature is not greater than or equal to the eighth threshold and lower than the ninth threshold, the method continues at step 512, where the economizer valve EHXV is closed and the compressor is turned off.

[0124] In some embodiments, the eighth threshold is equal to the fifth threshold and the ninth threshold is equal to the seventh threshold.

[0125] In case the method reaches either one of steps 506, 510, or 512, the method may again continue at step 502 by determining or receiving the discharge line temperature. In this case, the method may determine or receive an updated value for the discharge line temperature.

[0126] FIG. 10 shows a decision diagram, which describes a method 600 for determining the opening degree of the economizer valve EHXV based on step 318 of the decision diagram of FIG. 7 in more detail.

[0127] Following step 318, method 600 receives a determined value for the temperature of the refrigerant in the discharge line at step 602.

[0128] At step 604, it is determined whether the discharge line temperature DLT is below a tenth threshold. In case that the temperature is lower than the tenth threshold, the method continues at step 606 where the opening degree of the economizer valve EHXV is calculated as a function of the pressure of the refrigerant in the injection line. Thereby, injection pressure control mode is performed. With reference to FIG. 3, step 606 may refer to the operation performed for pressure and temperature being located in area 74. Otherwise, the method continues at step 608.

[0129] At step 608, it is determined whether the discharge line temperature DLT is greater than or equal to the tenth threshold and lower than an eleventh threshold. In case that the temperature is greater than or equal to the tenth threshold and lower than the eleventh threshold, the method continues at step 610 where the opening degree of the economizer valve is calculated as a function of the superheat value, the pressure of the refrigerant in the injection line, and the temperature of the refrigerant in the discharge line, DLT. Thereby, a combination of all three operation modes is performed. With reference to FIG. 3, step 610 may refer to the operation performed for pressure and temperature being located in area 75. In case that the discharge line temperature is not greater than or equal to the tenth threshold and lower than the eleventh threshold, the method continues at step 612, where the economizer valve EHXV is closed and the compressor is turned off.

[0130] In some embodiments, the tenth threshold is equal to the fifth threshold and the eleventh threshold is equal to the seventh threshold.

[0131] In case the method reaches either one of steps 606, 610, or 612, the method may again continue at step 602 by determining or receiving the discharge line temperature. In this case, the method may determine or receive an updated value for the discharge line temperature.

[0132] In some embodiments, the operation of the control operation is performed on the basis of the interrelated methods described with respect to FIGS. 7 to 10. If, in such a case, the eighth threshold is equal to the fifth threshold and the ninth threshold is equal to the seventh threshold and the tenth threshold is equal to the fifth threshold and the eleventh threshold is equal to the seventh threshold, one arrives at the areas 70 to 75 described with respect to FIG. 2, wherein the first threshold corresponds to p.sub.0, the second threshold corresponds to p.sub.1, the third threshold corresponds to p.sub.2, the fourth threshold corresponds to p.sub.max, the fifth threshold corresponds to T.sub.1, the sixth threshold corresponds to T.sub.2, and the seventh threshold corresponds to T.sub.max. Accordingly, the first operation mode, which is the superheat control mode, is performed in area 70, the second operation mode, which is the discharge line temperature control mode, is performed in area 72, and the third operation mode, which is the injection pressure control mode, is performed in area 74, whereas a combination of the first and second control modes is performed in area 71, a combination of the first and the third operation modes is performed in area 73, and a combination of all three operation modes is performed in area 75, and the economizer expansion valve is closed, while the compressor is turned off outside of areas 70 to 75.

[0133] What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the scope of the appended claims.