Flash Tank-Based Control Of Refrigerant Injection Into A Compressor

Abstract

A method of controlling injection into a compressor in a refrigeration cycle is described 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 comprises determining a pressure in the flash tank and controlling the injection valve based on the determined pressure in the flash tank.

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

[0045] In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0046] FIG. 1a, 1b show schematics of exemplary refrigeration systems for flash tank-based control of refrigerant injection into a compressor;

[0047] FIG. 2 shows a diagram of the influence of refrigerant injection on the optimum heat rejection heat exchanger pressure;

[0048] FIG. 3a, 3b, 3c show block diagrams of the inputs and outputs of controllers as may be used in connection with the present invention;

[0049] FIG. 4 shows a flow diagram of the method of controlling the injection into a compressor according to an embodiment of the invention;

[0050] FIG. 5 shows a decision diagram of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling the amount of injection into the compressor;

[0051] FIG. 6 shows a decision diagram of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling the operation condition of the compressor;

[0052] 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 controlling a second expansion device, which is disposed between a heat rejection heat exchanger and the flash tank;

[0053] FIG. 8 shows a diagram representing the transition from second heat rejection heat exchanger pressure mode to flash tank pressure control mode for the second expansion device;

[0054] FIG. 9 shows a decision diagram of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling a by-pass valve for by-passing refrigerant from the flash tank to the suction port of the compressor.

DETAILED DESCRIPTION

[0055] 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.

[0056] 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.

[0057] FIG. 1a shows a schematic of a refrigeration system 1a for flash tank-based control of refrigerant injection into a compressor 2 of the refrigeration system 1a. The refrigeration system 1a comprises a compressor 2, which comprises a suction port, a discharge port, and an injection port, a heat rejection heat exchanger 3 downstream of the compressor 2, a first expansion device 6 downstream of the heat rejection heat exchanger 3, and a heat accepting heat exchanger 7 downstream of the first expansion device 6 and upstream of the compressor 2.

[0058] Further, the refrigeration system 1a comprises a second expansion device 4 and a flash tank 5. The second expansion device 4 is disposed downstream of the heat rejection heat exchanger 3 and upstream of the first expansion device 6. The second expansion device 4 is used to expand the refrigerant after it exits the heat rejection heat exchanger 3. Thereby the pressure and the temperature of the refrigerant could be reduced.

[0059] The flash tank 5 is connected downstream of the second expansion device 4 and upstream of the first expansion device 6. In the refrigeration system 1a 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.

[0060] 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 first 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.

[0061] 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 an injection path 8, which connects the at least one outlet of the vapour collecting chamber 5a to the injection port of the compressor 2. The injection path 8 comprises an injection valve 9.

[0062] Further, the refrigeration system 1a comprises a controller 10, which is used for controlling at least one of the injection valve 9 and the compressor 2 based on the determined pressure in the flash tank 5. Further, the controller 10 may also control the first expansion device 6 and/or the second expansion device 4. FIG. 1a indicates the connection for exchanging control signals by ease of dashed lines. Although FIG. 1a shows dashed lines between the controller 10 and the injection valve 9, the first expansion device 6, the second expansion device 4, 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.

[0063] FIG. 1b shows a schematic of a refrigeration system 1b for flash tank-based control of refrigerant injection into a compressor 2 of the refrigeration system 1b. The refrigeration system 1b differs from the refrigeration system 1a in that a by-pass path 11 connects the injection path 8 between the flash tank 5 and the injection valve 9 to the suction port of the compressor 2. The by-pass path 11 comprises a by-pass valve 12. In another example, which is not depicted in the Figures, it may also be possible that the by-pass path 11 is connected directly to the flash tank 5. As is depicted in FIG. 1b, the controller 10 may also be configured to control the by-pass valve 12.

[0064] With respect to the refrigeration systems 1a, 1b depicted in FIGS. 1a, 1b, it needs to be noted that elements of the refrigeration systems 1a, 1b, which are depicted in FIGS. 1a, 1b or any other Figure of this application as individual components, may be included in the same housing or may form a component of the cycle, which is capable of performing the operations of the individual components depicted in FIG. 1a, 1b. As an example, the second expansion device 4 may be integrated into the flash tank 5. As such, there are multiple different configurations of combining or configuring the individual components depicted in FIGS. 1a, 1b. Also, it is possible to include additional components, which are not depicted in the embodiment examples.

[0065] 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 (pc). Thereby, solid line 50 represents the curve of the COP for a refrigeration system with closed injection valve, whereas dashed line 55 represents the curve of the COP for the same refrigeration system with opened injection valve. In a refrigeration system, the operating conditions are controlled in order to achieve the highest 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.

[0066] FIGS. 3a, 3b, 3c show block diagrams of the inputs and the outputs of controllers as may be used in connection with the present invention.

[0067] In FIG. 3a, the controller, which is represented by block “CTRL” receives the flash tank pressure as input and controls at least one of the injection valve EVI and the compressor CMP. In FIG. 3a, the output arrow of the compressor CMP is shown as dashed line in order to illustrate that the controller may perform injection valve control, compressor control, or both.

[0068] In FIG. 3b, the controller receives the flash tank pressure as input and controls at least one of the injection valve EVI, the compressor CMP, the by-pass valve, or the second expansion device HPV. Similarly to FIG. 3a, dashed lines indicate that the controller may output any combination of the four output controls.

[0069] In FIG. 3c, the controller receives the flash tank pressure, the temperature at the heat rejection heat exchanger GCT, the pressure at the heat rejection heat exchanger GCP, and pressure at the suction port SCP as inputs and controls at least one of the injection valve EVI, the compressor CMP, the by-pass valve, or the second expansion device HPV. Similarly to FIGS. 3a, 3b, dashed lines indicate that the controller may output any combination of the four output controls.

[0070] FIG. 4 shows a flow diagram of the method 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 FIGS. 1a, 1b. The method 100 comprises the step of determining 102 a pressure in a flash tank 5. Determining a pressure in the flash tank 5 may comprise determining a pressure in a vapour collecting chamber 5a.

[0071] Further, the method 100 comprises the step of controlling 104 an injection valve 9 based on the determined pressure in the flash tank 5.

[0072] FIG. 5 shows a decision diagram 200 of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling the amount of injection into the compressor. The amount of injection into the compressor is controlled by controlling the injection valve, which is referred to as EVI. The decision may be carried out by a controller, for example controller 10.

[0073] The method starts at step 202 where the flash tank pressure is determined or determined flash tank pressure is received. In FIG. 5, the flash tank pressure is referred to as FTP.

[0074] At step 204, it is determined whether the flash tank pressure is lower than a first threshold. In case that the pressure is lower than the first threshold, the method continues at step 206 where the injection valve EVI is closed. Otherwise, the method continues at step 208.

[0075] At step 208, it is determined whether the flash tank pressure is greater than or equal to the first threshold and lower than a second threshold. In case that the flash tank pressure is greater than or equal to the first threshold and lower than the second threshold, the method continues at step 210 where the injection valve EVI is opened. In an example, the injection valve EVI may be fully opened at step 210. In case that the flash tank pressure is not greater than or equal to the first threshold and lower than a second threshold, the method continues at step 212 where the injection valve EVI is closed.

[0076] In case the method reaches either one of steps 206, 210, or 212, the method may again continue at step 202 by determining or receiving a flash tank pressure FTP. In this case, the method may determine or receive an updated value for the flash tank pressure FTP.

[0077] FIG. 6 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 controlling the operating condition of the compressor. The operating condition of the compressor is referred to as CMP in FIG. 6. The decision may be carried out by a controller, for example controller 10.

[0078] The method starts at step 302 where the flash tank pressure is determined or determined flash tank pressure is received. In FIG. 6, the flash tank pressure is referred to as FTP.

[0079] At step 304, it is determined whether the flash tank pressure is lower than a fourth threshold. In case that the pressure is lower than the fourth threshold, the method continues at step 306 where the operating condition of the compressor is calculated by a PID controller based on the pressure at the suction port. Otherwise, the method continues at step 308.

[0080] At step 308, it is determined whether the flash tank pressure is greater than or equal to the fourth threshold and lower than a fifth threshold. If this is the case, the method continues at step 310 where the compressor is unloaded. Otherwise, the method continues at step 312 where the compressor stops its operation.

[0081] In case the method reaches either one of steps 306, 310, or 312, the method may again continue at step 302 by determining or receiving a flash tank pressure FTP. In this case, the method may determine or receive an updated value for the flash tank pressure FTP.

[0082] FIG. 7 shows a decision diagram 400 of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling a second expansion device, which is disposed between a heat rejection heat exchanger and the flash tank. The second expansion device is referred to as high pressure valve HPV in FIG. 7. The decision may be carried out by a controller, for example controller 10.

[0083] The method starts at step 402 where the flash tank pressure is determined or determined flash tank pressure is received. In FIG. 7, the flash tank pressure is referred to as FTP.

[0084] At step 404, it is determined whether the flash tank pressure is lower than a sixth threshold. In case that the pressure is lower than the sixth threshold, the method continues at step 406 where the high pressure valve HPV is opened by a predetermined degree. The predetermined value may be determined based on the characteristics of the compressor, which is used in the refrigeration cycle. In one embodiment, the predetermined value may correspond to a fully opened second expansion device. The sixth threshold may be a minimum allowed flash tank pressure, which is necessary for proper operation of the flash tank. The predetermined value may be a value, which is known to provide acceptable performance of the refrigerant under standard conditions. Otherwise, the method continues at step 408.

[0085] At step 408, it is determined whether the flash tank pressure is greater than or equal to the sixth threshold and lower than a seventh threshold. In case that the flash tank pressure is greater than or equal to the sixth threshold and lower than the seventh threshold, the method continues at step 410 where the opening degree for the high pressure valve HPV is calculated by a PID controller based on a first heat rejection heat exchanger pressure mode (HRHE_mode1). The first heat rejection heat exchanger pressure mode represents a controlling of the second expansion device based on the temperature of the refrigerant in the heat rejection heat exchanger in order to reach optimum pressure in the heat rejection heat exchanger and thereby the optimum COP. In case that the flash tank pressure is not greater than or equal to the sixth threshold and lower than the seventh threshold, the method continues at step 412.

[0086] At step 412, it is determined whether the flash tank pressure is greater than or equal to the seventh threshold and lower than an eighth threshold. In case that the flash tank pressure is greater than or equal to the seventh threshold and lower than the eighth threshold, the method continues at step 414 where the opening degree for the high pressure valve HPV is calculated by a PID controller based on a second heat rejection heat exchanger pressure mode (HRHE_mode2). The second heat rejection heat exchanger pressure mode represents a controlling of the second 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. In case that the flash tank pressure is not greater than or equal to the seventh threshold and lower than the eighth threshold, the method continues at step 416.

[0087] At step 416, it is determined whether the flash tank pressure is greater than or equal to the eighth threshold and lower than a ninth threshold. In case that the flash tank pressure is greater than or equal to the eighth threshold and lower than the ninth threshold, the method continues at step 418 where the opening degree for the high pressure valve HPV is calculated by fuzzy regulation based on the pressure of the refrigerant in the heat rejection heat exchanger (HRHEP) and the flash tank pressure. Thereby, the pressure of the refrigerant in the heat rejection heat exchanger may be, for example, the pressure the refrigerant has at the outlet of the heat rejection heat exchanger or the pressure in the heat rejection heat exchanger. In case that the flash tank pressure is not greater than or equal to the eighth threshold and lower than the ninth threshold, the method continues at step 420.

[0088] At step 420, it is determined whether the flash tank pressure is greater than or equal to the ninth threshold and lower than a tenth threshold. In case that the flash tank pressure is greater than or equal to the ninth threshold and lower than the tenth threshold, the method continues at step 422 where the opening degree for the high pressure valve HPV is calculated based on a flash tank pressure regulation mode (FT_mode). The flash tank pressure regulation mode represents a controlling of the second expansion device based on the pressure of the refrigerant in the flash tank. In case that the flash tank pressure is not greater than or equal to the ninth threshold and lower than the tenth threshold, the method continues at step 424, where the high pressure valve HPV is closed.

[0089] In case the method reaches either one of steps 406, 410, 414, 418, 422, or 424, the method may again continue at step 402 by determining or receiving a flash tank pressure FTP. In this case, the method may determine or receive an updated value for the flash tank pressure FTP.

[0090] FIG. 8 shows a diagram representing an exemplary transition from the second heat rejection heat exchanger pressure mode to flash tank pressure control mode for the second expansion device. In detail, FIG. 8 depicts the regulation percentage between the second heat rejection heat exchanger pressure mode (HRHE_mode) and the flash tank pressure regulation mode (FT_mode). Thereby, solid curve 450 depicts the usage of the HRHE_mode2, whereas dashed curve 455 depicts the usage of the FT_mode. If the flash tank pressure is below the eighth threshold for the flash tank pressure, the control is based fully on the HRHE_mode2, according to step 414 of FIG. 7. Above the ninth threshold for the flash tank pressure, the control is based fully on the FT_mode, according to step 422 of FIG. 7.

[0091] If the flash tank pressure is between the eighth threshold and the ninth threshold, the control is performed based on a combination of the HRHE_mode2 and the FT_mode. This is represented by descending curve 450 and ascending curve 455. Thereby, the pressure stage between the eighth threshold and the ninth threshold corresponds to a transition zone from the HRHE_mode2 to the FT_mode. In this regard, it needs to be appreciated that the course of the curves 450, 455 in said transition zone is shown for illustrative purposes only. The course of the curves 450, 455 does not need to be linearly. Instead, the control may be performed based on fuzzy control, as is described with respect to step 418 of FIG. 7, which may lead to different courses of the curve 450, 455

[0092] FIG. 9 shows a decision diagram 500 of a preferred embodiment of a method of controlling the injection into a compressor, wherein the decision diagram relates to controlling a by-pass valve for by-passing refrigerant from the flash tank to the suction port of the compressor. The by-pass valve is referred to as BPV in FIG. 7. The decision may be carried out by a controller, for example controller 10.

[0093] The method starts at step 502 where the flash tank pressure is determined or determined flash tank pressure is received. In FIG. 7, the flash tank pressure is referred to as FTP.

[0094] At step 504, it is determined whether the flash tank pressure is lower than an eleventh threshold. In case that the pressure is lower than the eleventh threshold, the method continues at step 506 where the by-pass valve BPV is closed. Otherwise, the method continues at step 508.

[0095] At step 508, it is determined whether the flash tank pressure is greater than or equal to the eleventh threshold and lower than a twelfth threshold. In case that the flash tank pressure is greater than or equal to the eleventh threshold and lower than the twelfth threshold, the method continues at step 510 where the opening degree for the by-pass valve BPV is calculated by a PID controller based on the flash tank pressure FTP. Otherwise, the method continues at step 512.

[0096] At step 512, it is determined whether the flash tank pressure is greater than or equal to the twelfth threshold and lower than a thirteenth threshold. In case that the flash tank pressure is greater than or equal to the twelfth threshold and lower than the thirteenth threshold, the method continues at step 514 where the by-pass valve BPV is fully opened. Otherwise, the method continues at step 516, where the opening degree for the by-pass valve is set to a predetermined value. The predetermined value may be set by a user according to the characteristics of the refrigerant and the refrigerant system. In order to set a suitable value for the predetermined value, the predetermined value represents an opening degree for which the flash tank pressure is decreased without providing too high pressure to the suction port of the compressor.

[0097] In case the method reaches either one of steps 506, 510, 514, or 516, the method may again continue at step 502 by determining or receiving a flash tank pressure FTP. In this case, the method may determine or receive an updated value for the flash tank pressure FTP.

[0098] 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.