EP4675209A1

EP4675209A1 - System and method for controlling a refrigeration unit with multi-compressor configuration

Info

Publication number: EP4675209A1
Application number: EP25178066.4A
Authority: EP
Inventors: Michal KOSTKOWSKI
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2024-07-02
Filing date: 2025-05-21
Publication date: 2026-01-07

Abstract

Described herein is a system and a method for controlling a refrigeration unit (100A) comprising a plurality of compressors (102). The system comprises a controller (114) connected to the plurality of compressors (102), where the controller (114) comprises a processor (114-1) with access to a memory (114-2) storing instructions executable by the processor (114-2), which causes the controller (114) to issue a first control signal to operate a first set of compressors among the plurality of compressors (102) at a variable speed, wherein the variable speed is selected based on a selected temperature to be maintained in a space (200) by the refrigeration unit (100A), and issue a second control signal to operate a second set of compressors among the plurality of compressors (102) at a predetermined speed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional Patent Application No. 63/666,766 filed on July 02, 2024 , which is incorporated by reference herein in its entirety.

BACKGROUND

Embodiments described herein relate to the field of tampering detection systems and, more particularly, to a system and method for controlling a refrigeration unit comprising a plurality of compressors.

SUMMARY

According to a first aspect of the invention there is provided a system for controlling a refrigeration unit comprising a plurality of compressors. The system comprises a controller connected to the plurality of compressors, where the controller comprises a processor with access to a memory storing instructions executable by the processor, which causes the controller to issue a first control signal to operate a first set of compressors among the plurality of compressors at a variable speed, wherein the variable speed is selected based on a selected temperature to be maintained in a space by the refrigeration unit, and issue a second control signal to operate a second set of compressors among the plurality of compressors at a predetermined speed.
In one or more embodiments, the controller is configured to select the predetermined speed for each of the second set of compressors based on a speed and efficiency characteristics or coefficient of performance of each of the second set of compressors.
In one or more embodiments, the controller is configured to select the first of compressors and the second of compressors among the plurality of compressors, based on one or more of a real-time cooling capacity of the refrigeration unit, a real-time cooling load on the refrigeration unit, and the selected temperature to be maintained in the space.
In one or more embodiments, upon detecting a measured temperature in the space to be equal to the selected temperature and the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, the controller is configured to operate the first set of compressors at the variable speed and operate the second set of compressors at the predetermined speed.
In one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be greater than the real-time cooling load on the refrigeration unit, the controller is configured to issue a control signal to stop the operation of remaining compressors among the plurality of compressors while operating the first and second sets of compressors.
In one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, the controller is configured to operate each of the plurality of compressors at the predetermined speed.
In one or more embodiments, the controller is configured to switch between the plurality of compressors and correspondingly select and update the first set of compressors and the second set of compressors among the plurality of compressors after a predefined operating cycle or a predefined time.
In one or more embodiments, the controller is configured to switch between the plurality of compressors operating at the variable speed and the predetermined speed based on an operating time of the corresponding compressors at the variable speed.
In one or more embodiments, at least one compressor among the plurality of compressors is a variable speed compressor that is connected to the controller via a variable speed drive.
In one or more embodiments, at least one compressor among the second set of compressors is a fixed-speed compressor.
In one or more embodiments, the refrigeration unit is associated with a refrigerated display cabinet or a chiller.
In one or more embodiments, the controller is configured to monitor, using a temperature sensor, real-time temperature in the space and correspondingly issue the first control signal to operate the first set of the compressors among the plurality of compressors at the variable speed to maintain the selected temperature in the space.
According to a second aspect of the invention there is provided a method for controlling a refrigeration unit comprising a plurality of compressors. The method comprises issuing, by a controller, a first control signal to operate a first set of compressors among the plurality of compressors at a variable speed, wherein the variable speed is selected based on a selected temperature to be maintained in a space by the refrigeration unit, and issuing, by the controller, a second control signal to operate a second set of compressors among the plurality of compressors at a predetermined speed.
In one or more embodiments, the method comprises selecting, by the controller, the predetermined speed for each of the second set of compressors based on a speed and efficiency characteristics or coefficient of performance of each of the second set of compressors.
In one or more embodiments, the method comprises selecting, by the controller, the first of compressors and the second of compressors among the plurality of compressors, based on one or more of a real-time cooling capacity of the refrigeration unit, a real-time cooling load on the refrigeration unit, and the selected temperature to be maintained in the space.
In one or more embodiments, upon detecting a measured temperature in the space to be equal to the selected temperature and the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, the method comprises operating, by the controller, the first set of compressors at the variable speed and operating, by the controller, the second set of compressors at the predetermined speed.
In one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be greater than the real-time cooling load on the refrigeration unit, the method comprises issuing, by the controller, a control signal to stop the operation of remaining compressors among the plurality of compressors while operating the first and second sets of compressors.
In one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, the method comprises operating, by the controller, each of the plurality of compressors at the predetermined speed.
In one or more embodiments, the method comprises switching between the plurality of compressors and updating the first of compressors and the second of compressors among the plurality of compressors after a predefined operating cycle or a predefined time.
In one or more embodiments, wherein the method comprises switching between the plurality of compressors operating at the variable speed and the predetermined speed based on a predefined operating cycle or operating time of the corresponding compressors at the variable speed.
The preceding summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, features, and techniques of the subject disclosure will become more apparent from the following description in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the subject disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the subject disclosure and, together with the description, serve to explain the principles of the subject disclosure.
In the drawings, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label, irrespective of the second reference label.

FIG. 1 illustrates an exemplary representation of a system for controlling the operation of a refrigeration unit having a multi-compressor configuration.
FIG. 2 illustrates an exemplary detailed block diagram of the system of FIG. 1.
FIG. 3 illustrates exemplary steps involved in a method for controlling the operation of a refrigeration unit having a multi-compressor configuration.
FIG. 4 illustrates an exemplary plot depicting a compressor speed vs coefficient of performance characteristics associated with the compressors of the refrigeration unit of FIG. 1.

DETAILED DESCRIPTION

The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
Various terms are used herein. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
In the specification, reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components, as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the subject disclosure, the components of this disclosure. Described herein may be positioned in any desired orientation. Thus, the use of terms such as "above," "below," "upper," "lower," "first," "second," or other like terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components.
Refrigeration systems are implemented in a wide range of applications, from household and commercial refrigerators and refrigerated cabinets to industrial cooling systems. These systems may employ multiple circuits with variable-speed compressors operating in parallel to maintain the desired temperature within a specified range. The efficiency and effectiveness of such systems are governed by their performance, energy consumption, and operational costs.
Traditionally, refrigeration systems with multiple compressors have employed control strategies that keep each of the running compressors operating at the same speed. This common speed may be adjusted to meet the cooling power based on the desired temperature. While this approach simplifies the control mechanism and ensures uniform operation of the compressors, it may present significant limitations concerning overall system efficiency.
In conventional control schemes, the primary focus may be on maintaining the controlled temperature, often at the expense of compressor efficiency. The restriction to operate all compressors at a common speed may result in uniform efficiency across the compressors at any given moment. However, this uniformity may not necessarily align with the highest possible efficiency for the system as a whole. Each compressor may have a different optimal operating speed where it performs with maximum efficiency, depending on various factors such as load conditions, environmental factors, and the specific characteristics of each compressor.
As a result, the inability to operate each compressor at its individual optimal speed leads to a scenario where the cooling power needed may be achieved, but not with the highest possible efficiency. The system's efficiency may become a product of the common speed command multiplied by the number of compressors running at that speed, rather than an aggregate of the individual optimal efficiencies of each compressor. This inefficiency may lead to higher energy consumption and increased operational costs, which may particularly be important in large-scale or industrial refrigeration applications.
Therefore, there is a need for an improved and efficient control strategy that may independently adjust the speed of each compressor to optimize the overall system efficiency while still meeting the desired temperature requirements.
Referring to FIGs. 1 and 2, a system 100 for controlling the operation of a refrigeration unit 100A having a multi-compressor configuration is disclosed. The refrigeration unit 100A may include a plurality of compressors 102-1 to 102-N (collectively referred to as compressors 102, herein), each driven by a corresponding motor 104-1 to 104-N (collectively referred to as motors 104, herein). These compressors 102 may be configured into at least one refrigerant circuit associated with the refrigeration unit 100A. Further, each refrigerant circuit may include at least one compressor from the plurality of compressors 102, a condenser 108, an evaporator 110, and an expansion device 112 connected in a closed loop. In one or more embodiments, the refrigeration unit 100A may be associated with one or more of a chiller system, a heating, ventilation, and air conditioning (HVAC) system, or a refrigerated cabinet, but not limited to the like.
The compressors 102 may be configured to compress a refrigerant vapor and deliver it to the condenser 108. The compressors 102 may be connected in separate refrigeration circuits, such that the refrigerant output by each compressor 102 may not be mixed and may travel in separate circuits through the refrigeration unit 100A before reentering the compressors 102 to begin another cycle. In one or more embodiments, the separate refrigeration circuits may use a single condenser 108 and a single evaporator 110 for the corresponding heat exchanges. In other embodiments, the refrigerant output (refrigerant vapor) by the compressors 102 may also be combined into a single refrigerant circuit to travel through the refrigeration unit 100A before being separated to reenter the compressors 102. Further, the refrigerant vapor delivered to the condenser 108 in the individual refrigeration circuits may enter into a heat exchange relationship with a fluid, such as air or water, and undergo a phase change to a refrigerant liquid due to this heat exchange. The condensed liquid refrigerant from the condenser 108 may then flow through the corresponding expansion devices 112 and supply to the evaporator 110 in the individual refrigeration circuits. The evaporator 110 may be further thermally connected to a load 202 present at the space or area of interest (AOI) 200 that is to be cooled or conditioned by the refrigeration unit 100A.
In one or more embodiments, each of the compressors 102 may be a variable-speed compressor. However, in other embodiments, some of the compressors may be a variable-speed compressor and the remaining compressors may be a fixed-speed compressor. In one or more embodiments, the compressors 102 may be screw compressors or centrifugal compressors, however, the compressors 102 may be of any suitable type of compressor including but not limited to reciprocating compressors, scroll compressors, and rotary compressors.
In one or more embodiments, the refrigeration unit 100A may include a plurality of variable speed drives 106-1 to 106-N (collectively referred to as VSD 106, herein), where one of the VSDs 106 may be connected to the motor 104 associated with each of the compressors 102 to power the corresponding compressors 102. However, the VSD 106 may not be employed in the embodiments where a fixed-speed compressor is employed in the refrigeration unit 100A. The VSD 106 may control the attributes of electrical power being supplied from a power source 116 to the compressor, thereby adjusting the speed of the respective compressors 102.
The system 100 for controlling the operation of the refrigeration unit 100A may include a controller 114 connected to the plurality of compressors 102. In embodiments where a variable-speed compressor is employed, the controller 114 may be connected to the VSD 106 associated with the corresponding compressors 102. Further, in embodiments where a fixed-speed compressor is employed, the controller 114 may be directly connected to the corresponding compressors 102. However, in one or more embodiments, the variable-speed compressors 102 may also be operated as a fixed-speed compressor, and in such embodiments, the controller 114 may be connected to the VSD 106 associated with the variable-speed compressors 102, where the VSD 106 may further drive the same compressor either at a fixed-speed or a variable speed. In one or more embodiments, the controller 114 may include a processor 114-1 with access to a memory 114-2 storing instructions executable by the processor 114-1, which may cause the controller 114 to perform one or more designated operations.
In one or more embodiments, the system 100 may further include one or more temperature sensors to monitor real-time temperature in a space 200 where the refrigeration unit 100A is installed to cool or condition the corresponding space 200. Further, the system 100 may additionally include humidity sensors to monitor real-time humidity in the space 200. However, in one or more embodiments, the system 100 may involve a thermostat 204 as shown in FIG. 2 that may replace the additional temperature and humidity sensors. The thermostat 204 may enable one or more users to select a temperature and humidity to be maintained in the space 200 by the refrigeration unit 100A and also monitor the real-time temperature and humidity within the space 200. In one or more embodiments, the temperature sensors, the humidity sensors, and/or the thermostat 204 may be part of the refrigeration unit 100A, which may be further connected to the controller 114. However, in other embodiments, the temperature sensors, the humidity sensors, and/or the thermostat 204 may also be part of the system 100 as well.
In one or more embodiments, the controller 114 may be configured to issue a first control signal to operate a first set of compressors among the plurality of compressors 102 at a variable speed that may be selected based on a selected temperature to be maintained in the space 200 by the refrigeration unit 100A. Simultaneously, the controller 114 may be configured to issue a second control signal to operate a second set of compressors among the plurality of compressors 102 at a predetermined speed. The controller 114 may select the predetermined speed for each of the second set of compressors based on the speed and efficiency characteristics or coefficient of performance of each of the second set of compressors.
Accordingly, the selected predetermined speed may allow the second set of compressors to operate at their optimal or sub-optimal efficiency, while the first set of compressors operating at the variable speed may facilitate maintaining the selected temperature in the space 200, thereby optimizing the overall system 100 efficiency while still meeting the desired temperature requirements for the space 200. In such embodiments, the controller 114 may select the first set of compressors and the second set of compressors among the plurality of compressors 102, based on one or more of the real-time cooling capacity of the refrigeration unit 100A, a real-time cooling load on the refrigeration unit 100A, and the selected temperature to be maintained in the space 200.
An exemplary compressor speed vs efficiency characteristics or coefficient of performance (COP) plot for a compressor has been shown in FIG. 4. For instance, in a non-limiting example, if the maximum COP of the compressor is determined to be maximum (say 2.4) while operating at a speed of 70 Hz or 4200 rotation per minute( rpm), the compressor may be operated at a fixed predetermined speed of 70 Hz or 4200 rpm. Similarly, all the second set of compressors may be individually operated at their optimal speed selected based on the speed vs efficiency characteristics or coefficient of performance plot of the respective compressors 102.
In one or more embodiments, upon detecting the real-time measured temperature in the space 200 to be equal to the selected temperature (to be maintained in the space 200) and the real-time cooling capacity of the refrigeration unit 100A to be equal to or less than the real-time cooling load on the refrigeration unit 100A, the controller 114 may operate the first set of compressors at the variable speed and simultaneously operate the second set of compressors at the predetermined speed with a tolerance of ±10%. For instance, in a non-limiting example, compressors 102-1, and 102-2 may be selected as the first set of compressors and may be operated at their optimal (predetermined) speeds. Further, compressor 102-N may be selected as the second set of compressors and may be operated at a variable speed to meet the cooling demand and maintain the selected temperature.
Further, in one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit 100A to be greater than the real-time cooling load on the refrigeration unit 100A, the controller 114 may be configured to issue a control signal to stop operation of remaining compressors 102 among the plurality of compressors 102 while operating the first and second sets of compressors 102 only, as all the compressors may not be required to be operated at the same time. For instance, in a non-limiting example, compressor 102-1 may be selected as the first set of compressors and may be operated at their optimal (predetermined) speeds. Further, the compressor 102-N may be selected as the second set of compressors and may be operated at a variable speed to meet the cooling demand and maintain the selected temperature. However, the operation of the remaining compressor 102-2 may be stopped. This allows the system 100 to efficiently select an optimum number of compressors among the plurality of compressors 102-1 to 102-N at a time while giving rest to the remaining compressors.
Further, in one or more embodiments, the controller 114 may switch between the plurality of compressors 102 and correspondingly select and update the first set of compressors (operating at variable speed), the second set of compressors (operating at predetermined speed), and/or the compressors 102 to be rested among the plurality of compressors 102 after a predefined operating cycle or a predefined time, based on an operating time of the corresponding compressors 102 at the variable speed. For instance, in a non-limiting example, initially, compressor 102-1 may be selected as the first set of compressors, and compressors 102-2 and 102-N may be selected as the second set of compressors. Later, after the predefined operating cycle or predefined time, compressor 102-2 may be selected as the first set of compressors, and compressors 102-1 and 102-N may be selected as the second set of compressors. Again, after the predefined operating cycle or predefined time the compressor 102-N may be selected as the first set of compressors, and compressors 102-1 and 102-2 may be selected as the second set of compressors. This may help lower the load and wearing of a specific set of compressors, thereby improving the operating life and performance of all the compressors 102.
Furthermore, in one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit 100A to be equal to or less than the real-time cooling load on the refrigeration unit 100A, the controller 114 may be configured to operate each of the plurality of compressors 102-1 to 102-N at the predetermined speed, as the priority may be to meet the cooling demand over efficiency. Thus, in such a condition, the system 100 may enable the refrigeration unit 100A to meet the cooling demand based on the selected temperature.
In one or more embodiments, the controller 114 may determine or monitor the cooling capacity of the refrigeration unit 100A, typically measured in BTUs per hour or Tons of Refrigeration (TR), based on one or more of the flow rate of the refrigerant within the refrigeration unit 100A, a temperature difference between an inlet and an outlet of the evaporator 110, an enthalpy change in the refrigeration unit 100A, and power consumption by the compressor 102 (which can help in determining the efficiency and performance of the system 100). The flow rate of the refrigerant or chilled fluid may be monitored using a flow meter and the temperature difference between the inlet and outlet of the evaporator 110 may be monitored using thermometers. In addition, pressure gauges may facilitate monitoring the pressure of the refrigerant, which may help in determining the enthalpy change. Further, a power meter may facilitate measuring the electrical consumption by the refrigeration unit 100A, aiding in performance and efficiency assessment.
In one or more embodiments, the controller 114 may determine or monitor the real-time cooling load on the refrigeration unit 100A based on one or more of the environmental conditions of the space 200 to be conditioned, rate of heat transfer through a boundary (surfaces) separating the space 200 and external environment, and power consumption of the refrigeration unit 100A, but not limited to the like. The temperature and humidity sensors or the thermostat 204 installed in the space 200 may facilitate in monitoring the environmental conditions, while heat flux sensors and power meters may facilitate in monitoring heat transfer and power consumption. This data may be logged and analyzed by the controller 114, enabling continuous monitoring of the cooling load on the refrigeration unit 100A.
Referring to FIG. 3, method 300 for controlling the operation of a refrigeration unit comprising a plurality of compressors is disclosed. Method 300 may involve the refrigeration unit 100A and its corresponding components and the controller 114 associated with the system 100 of FIGs 1 and 2.
Method 300 may include step 302 of issuing, by the controller, a first control signal to operate a first set of compressors among the plurality of compressors at a variable speed that may be selected based on a selected temperature to be maintained in the space by the refrigeration unit. Method 300 may include step 304 of issuing a second control signal to operate a second set of compressors among the plurality of compressors at a predetermined speed. The controller may select the predetermined speed for each of the second set of compressors based on the speed and efficiency characteristics or coefficient of performance of each of the second set of compressors. In one or more embodiments, steps 302 and 304 may be executed simultaneously, however, there may be a slight delay between the execution of these steps.
Thus, the selected predetermined speed may allow the second set of compressors to operate at their optimal or sub-optimal efficiency, while the first set of compressors operating at the variable speed may facilitate maintaining the selected temperature in the space, thereby optimizing the overall system efficiency while still meeting the desired temperature requirements for the space. At steps 302 and 304, the controller may select the first set of compressors and the second set of compressors among the plurality of compressors, based on one or more of the real-time cooling capacity of the refrigeration unit, a real-time cooling load on the refrigeration unit, and the selected temperature to be maintained in the space.
In one or more embodiments, upon detecting the real-time measured temperature in the space to be equal to the selected temperature (to be maintained in the space) and the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, method 300 may execute steps 302 and 304 of operating the first set of compressors at the variable speed and simultaneously operating the second set of compressors at the predetermined speed with a tolerance of ±10%. For example, if the predetermined speed is 4000 rpm, the controller may operate the compressors within a speed range of 3600 rpm to 4400 rpm.
Further, in one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be greater than the real-time cooling load on the refrigeration unit, method 300 may include step 306 of issuing, by the controller, a control signal to stop operation of remaining compressors among the plurality of compressors while operating the first and second sets of compressors only, as all the compressor may not be required to be operated at the same time. This allows the system to efficiently select an optimum number of compressors among the plurality of compressors at a time while giving rest to the remaining compressors.
Further, in one or more embodiments, method 300 may include the steps switching between the plurality of compressors and correspondingly selecting and updating the first of compressors (operating at variable speed), the second of compressors (operating at predetermined speed), and/or the compressors to be rested among the plurality of compressors after a predefined operating cycle or a predefined time. This switching among the plurality of compressors may be executed, based on an operating time of the corresponding compressors at the variable speed. This may help lower the load and wearing of a specific set of compressors, thereby improving the operating life and performance of all the compressors.
Furthermore, in one or more embodiments, upon detecting the real-time cooling capacity of the refrigeration unit to be equal to or less than the real-time cooling load on the refrigeration unit, method 300 may include step 308 of issuing, by the controller, operating each of the plurality of compressors at the predetermined speed, as the priority may be to meet the cooling demand over efficiency.
Thus, this disclosure provides a solution to the limitations and shortcomings associated with existing multi-compressor refrigeration units by providing an improved and efficient control strategy that may independently adjust the speed of each compressor to optimize the overall system efficiency while still meeting the desired temperature requirements. Moreover, the controlled switching of operating roles among the plurality of compressors based on an operating time of the corresponding compressors at the variable speed, may help lower the load and wearing of a specific set of compressors, thereby improving the operating life and performance of all the compressors.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined by the appended claims. Modifications may be made to adopt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention includes all embodiments falling within the scope of the invention as defined by the appended claims.
In interpreting the specification, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ....and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.

Claims

A system (100) for controlling a refrigeration unit (100A) comprising a plurality of compressors (102), the system comprising:
a controller (114) connected to the plurality of compressors, the controller comprising a processor (114-1) with access to a memory (114-2) storing instructions executable by the processor, which causes the controller to:
issue a first control signal to operate a first set of compressors among the plurality of compressors at a variable speed, wherein the variable speed is selected based on a selected temperature to be maintained in a space (200) by the refrigeration unit; and

issue a second control signal to operate a second set of compressors among the plurality of compressors at a predetermined speed.
The system of claim 1, wherein the controller (114) is configured to select the predetermined speed for each of the second set of compressors based on a speed and efficiency characteristics or coefficient of performance of each of the second set of compressors.
The system of claims 1 or 2, wherein the controller (114) is configured to select the first set of compressors and the second set of compressors among the plurality of compressors (102), based on one or more of a real-time cooling capacity of the refrigeration unit (100A), a real-time cooling load on the refrigeration unit, and the selected temperature to be maintained in the space (200).
The system of claim 3, wherein upon detecting a measured temperature in the space (200) to be equal to the selected temperature and the real-time cooling capacity of the refrigeration unit (100A) to be equal to or less than the real-time cooling load on the refrigeration unit, the controller (114) is configured to operate the first set of compressors at the variable speed and operate the second set of compressors at the predetermined speed.
The system of claims 3 or 4, wherein upon detecting the real-time cooling capacity of the refrigeration unit (100A) to be greater than the real-time cooling load on the refrigeration unit, the controller (114) is configured to issue a control signal to stop the operation of remaining compressors among the plurality of compressors (102) while operating the first and second sets of compressors; and/or
wherein upon detecting the real-time cooling capacity of the refrigeration unit (100A) to be equal to or less than the real-time cooling load on the refrigeration unit, the controller (114) is configured to operate each of the plurality of compressors (102) at the predetermined speed.
The system of any preceding claim, wherein the controller (114) is configured to switch between the plurality of compressors (102) and correspondingly select and update the first set of compressors and the second set of compressors among the plurality of compressors after a predefined operating cycle or a predefined time; and/or
wherein the controller (114) is configured to switch between the plurality of compressors (102) operating at the variable speed and the predetermined speed based on an operating time of the corresponding compressors at the variable speed.
The system of any preceding claim, wherein at least one compressor among the plurality of compressors (102) is a variable speed compressor that is connected to the controller via a variable speed drive (106), and/or wherein at least one compressor among the second set of compressors is a fixed-speed compressor.
The system of any preceding claim, wherein the refrigeration unit (100A) is associated with a refrigerated display cabinet or a chiller.
The system of any preceding claim, wherein the controller (114) is configured to monitor, using a temperature sensor, real-time temperature in the space (200) and correspondingly issue the first control signal to operate the first set of the compressors among the plurality of compressors (102) at the variable speed to maintain the selected temperature in the space.
A method (300) for controlling a refrigeration unit (100A) comprising a plurality of compressors (102), the method comprising:
issuing (302), by a controller (114), a first control signal to operate a first set of compressors among the plurality of compressors at a variable speed, wherein the variable speed is selected based on a selected temperature to be maintained in a space (200) by the refrigeration unit; and

issuing (304), by the controller, a second control signal to operate a second set of compressors among the plurality of compressors at a predetermined speed.
The method of claim 10, wherein the method comprises selecting, by the controller (114), the predetermined speed for each of the second set of compressors based on a speed and efficiency characteristics or coefficient of performance of each of the second set of compressors.
The method of claims 10 or 11, wherein the method comprises selecting, by the controller (114), the first set of compressors and the second set of compressors among the plurality of compressors (102), based on one or more of a real-time cooling capacity of the refrigeration unit (100A), a real-time cooling load on the refrigeration unit, and the selected temperature to be maintained in the space (200).
The method of claim 12, wherein upon detecting a measured temperature in the space (200) to be equal to the selected temperature and the real-time cooling capacity of the refrigeration unit (100A) to be equal to or less than the real-time cooling load on the refrigeration unit, the method comprises operating, by the controller (114), the first set of compressors at the variable speed and operating, by the controller, the second set of compressors at the predetermined speed.
The method of claims 12 or 13, wherein upon detecting the real-time cooling capacity of the refrigeration unit (100A) to be greater than the real-time cooling load on the refrigeration unit, the method comprises issuing (306), by the controller (114), a control signal to stop the operation of remaining compressors among the plurality of compressors (102) while operating the first and second sets of compressors; and/or wherein upon detecting the real-time cooling capacity of the refrigeration unit (100A) to be equal to or less than the real-time cooling load on the refrigeration unit, the method comprises operating (308), by the controller (114), each of the plurality of compressors (102) at the predetermined speed.
The method according to any of claims 10 to 14, wherein the method comprises switching between the plurality of compressors (102) and updating the first set of compressors and the second set of compressors among the plurality of compressors after a predefined operating cycle or a predefined time; and/or
wherein the method comprises switching between the plurality of compressors operating at the variable speed and the predetermined speed based on a predefined operating cycle or operating time of the corresponding compressors at the variable speed.