Patent Publication Number: US-2015059373-A1

Title: Superheat and sub-cooling control of refrigeration system

Description:
CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS 
     This application claims priority to and the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 61/873,971, filed Sep. 5, 2013, which application is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure broadly pertains to the field of refrigerant vapor compression systems, and particularly to a system for dynamically optimizing capacity and efficiency during less than optimal operating conditions to provide enhanced performance. 
     Superheat is typically defined as the amount of heat added to a refrigerant vapor after a change of state occurring in the evaporator of a refrigeration system. This measurement is an indicator of the performance of the evaporator portion of the system and can have a direct influence on overall system efficiency. It is known to calculate the system superheat using a variety of methods, i.e. using pressure/temperature sensors located between the outlet of the evaporator coil and the compressor(s) or by using two temperature sensors, one located between the expansion valve and the evaporator coil and the other located between the evaporator and the compressor(s). Too much superheat indicates the evaporator coil is only partially flooded and portions of the available heat transfer surface of the evaporator coil are not utilized. Too little superheat indicates the evaporator coil is over-flooded and liquid refrigerant may be exiting the evaporator and entering the compressor with negative performance and reliability effects on the compressor performance. In a typical system, the expansion valve is designed to ensure an excess of superheat to, among other things, avoid any liquid from returning to the compressor. 
     It is known to modify the superheat setting based on system performance parameters. For example, as described in U.S. Pat. No. 8,156,750 to Butorac et al. (Butorac), a system superheat level is maintained by dynamically adjusting the individual superheat setting of a plurality of evaporator coils connected to the system. To achieve this, an electronic expansion valve is opened or closed during the refrigeration system operation to allow either more or less liquid refrigerant to flow into the evaporator coil to maintain a specified level of superheat for optimal utilization of the evaporator coil heat exchanger surface area based on control limits established for the particular system. Though this approach utilizes a dynamic superheat setpoint for the system, the system still fails to maximize system performance under certain conditions. 
     Another performance parameter in vapor compression refrigeration systems is the concept of sub-cooling, defined as the amount of heat removed from the liquid refrigerant after the change of state from high pressure vapor to high pressure liquid in the condensing coil. The amount of sub-cooling is important to the efficiency and capacity of the refrigeration system and can be problematic particularly when the sub-cooling is too low. Sub-cooling assures that the refrigerant is in a single phase, liquid state as it is delivered to the expansion valve. If the sub-cooling is too low and a two phase mixture of liquid and vapor is delivered to the expansion valve, the consequences are primarily two-fold:  1 ) the mass flow of the two-phase mixture is reduced as compared to single phase liquid and system capacity is reduced affecting efficiency, and  2 ) the two-phase mixture passing through the expansion valve can create cavitation potentially damaging the seat of the expansion valve. 
     In single speed systems where the compressor and condenser fans operate either on or off, the sub-cooling is controlled by system design and proper refrigerant charge. These single speed systems, due to the lack of adaptive controls, are limited in operation to relatively high outdoor ambient temperatures and specific evaporator loading. In variable speed systems, where the compressor is operated at variable rates, the resulting pressures in the system can result in less-than-optimal sub-cooling levels which are also compounded by variations in the ambient temperature and evaporator load. It is known in the art to vary the speed of the condenser fan in response to changes in outdoor ambient temperature to help balance the temperature and pressures in the system to maintain proper sub-cooling levels. 
     BRIEF DESCRIPTION 
     In variable speed refrigeration systems where modulation of compressors, indoor blowers, condenser fans with active control of superheat and sub-cooling is possible with appropriate sensors and direct feedback control loops, conditions can develop where full optimization of each and every control parameter independent of and without regard to interaction between these control parameters may cause less than optimal performance of the system. Reduced capacity and degraded efficiency may be observed in the varying conditions such systems will routinely encounter in the installed environment in which they are expected to operate. 
     In one aspect of the present disclosure, a refrigeration vapor compression system comprises an electronic expansion valve, a sensor or sensors to measure system superheat, a sensor or sensors to measure system sub-cooling and a controller operatively coupled to the electronic expansion valve, the superheat sensor(s) and the sub-cooling sensor(s). The controller is configured to control at least one parameter to maintain a desired superheat and/or subcooling setting. 
     In another aspect, a method of optimizing a refrigeration vapor compression system comprises sensing superheat and sub-cooling levels of the system, determining if the operating conditions as indicated by the superheat and sub-cooling levels demand a priority to maintaining the superheat level or a priority to maintain sub-cooling levels for optimal system performance, and adjusting the superheat level to an appropriate setting to maintain peak system performance. Additionally, monitoring how much adjustment of the superheat level to preserve subcooling is present can provide insight into system refrigerant charge levels. 
     In accordance with another aspect, a refrigerant vapor compression system comprises a compressor, first and second heat exchangers fluidly coupled to the compressor, a variable orifice expansion valve located between the first and second heat exchangers, at least one sensor for sensing operating conditions of the system related to a system superheat level and a system subcooling level, a controller operative to receive sensed data from the at least one sensor and, based at least in part thereon, control at least one system component to optimize system performance. The controller is configured to monitor the system superheat level to determine whether the system superheat level is within a prescribed range of a system superheat setpoint, monitor the system subcooling level to determine whether the system subcooling level is within a prescribed range of a system subcooling setpoint, and iteratively: adjust at least one system setting to restore the system subcooling level to within the prescribed range of the system subcooling setpoint while the system superheat level is within its prescribed range, increment the system superheat setpoint when either the system subcooling level or the system superheat level cannot be maintained within their respective prescribed ranges, and adjust at least one system setting to achieve the incremented superheat setpoint. 
     The at least one sensor for sensing operating conditions of the system related to a system superheat level and a system subcooling level can include at least one of a refrigerant temperature sensor, refrigerant pressure sensor or an ambient temperature sensor, the controller being configured to receive information from the at least one sensor and use the information for monitoring the system superheat level or system subcooling level. The controller can include a memory containing a look-up table, the look-up table including at least one of a superheat setpoint value and/or a subcooling setpoint value corresponding to at least one sensed operating condition, and the controller can be configured to determine at least one of the system superheat setpoint and/or system subcooling setpoint by looking up a superheat setpoint value and a subcooling setpoint value using the at least one sensed operating condition. 
     The system can further include a condenser fan, and the controller can be configured to adjust a speed of the condenser fan to maintain the system subcooling level within the prescribed range of the system subcooling setpoint. The controller can also be configured to optimize the condenser fan speed by reducing the speed of the condenser fan when the system subcooling level exceeds the system subcooling setpoint. The system can also include a variable output compressor, and the controller can be configured to adjust an output capacity of the compressor to maintain the system subcooling level within the prescribed range of the system subcooling setpoint. The controller can also be configured to adjust the expansion valve to maintain the system superheat level within a prescribed range of the system superheat setpoint. 
     In accordance with another aspect, a method of optimizing a refrigerant vapor compression system having a compressor, first and second heat exchangers fluidly coupled to the compressor, a variable orifice expansion valve located between the first and second heat exchangers, and at least one sensor for sensing operating conditions of the system related to a system superheat level and a system subcooling level, the method comprises monitoring the system superheat level to determine whether the system superheat level is within a prescribed range of a system superheat setpoint, monitoring the system subcooling level to determine whether the system subcooling level is within a prescribed range of a system subcooling setpoint, and, iteratively: adjusting at least one system setting to restore the system subcooling level to within the prescribed range of the system subcooling setpoint while the system superheat level is within its prescribed range, incrementing the system superheat setpoint when either the system subcooling level or the system superheat level cannot be maintained within their respective prescribed ranges, and adjusting at least one system setting to achieve the incremented superheat setpoint. 
     The monitoring of the system superheat or system subcooling can include using at least one sensor for sensing system superheat or system subcooling. The at least one sensor for sensing system superheat or system subcooling can include at least one of a refrigerant temperature sensor, refrigerant pressure sensor or an ambient temperature sensor. 
     The method can further include assigning an initial system superheat value and system subcooling value at system startup based at least in part on at least one of an ambient temperature, a refrigerant line temperature or a refrigerant line pressure sensed by the at least one sensor. The method can also include adjusting a speed of a variable speed condenser fan to maintain the system subcooling level within the prescribed range of the system subcooling setpoint and/or adjusting an output capacity of the compressor to maintain the system subcooling level within the prescribed range of the system subcooling setpoint. The method can include adjusting the expansion valve to maintain the system superheat within a prescribed range of the system superheat setpoint. 
     In accordance with another aspect, an electronic control unit for controlling an associated expansion valve of a refrigerant vapor compression system having a compressor, first and second heat exchangers fluidly coupled to the compressor, a condenser fan, a variable orifice expansion valve located between the first and second heat exchangers, and at least one sensor for sensing operating conditions of the system related to a system superheat level and a system subcooling level, the electronic control unit comprises an input for receiving data from the at least one sensor, an output for sending a control signal to at least one of the condenser fan, the expansion valve, or the compressor, a memory that stores computer-executable instructions, and a processor configured to execute the computer-executable instructions to generate the control signal, the instructions comprising: monitoring the system superheat level to determine whether the system superheat level is within a prescribed range of a system superheat setpoint, monitoring the system subcooling level to determine whether the system subcooling level is within a prescribed range of a system subcooling setpoint and, iteratively: adjusting at least one system setting to restore the system subcooling level to within the prescribed range of the system subcooling setpoint while the system superheat level is within its prescribed range, incrementing the system superheat setpoint when either the system subcooling level or the system superheat level cannot be maintained within their respective prescribed ranges, and adjusting at least one system setting to achieve the incremented superheat setpoint. 
     The processor can be further configured to assign an initial system superheat value and system subcooling value at system startup based at least in part on at least one of an ambient temperature, a refrigerant line temperature or a refrigerant line pressure sensed by the at least one sensor. The processor can be further configured to adjust a speed of the variable speed condenser fan to maintain the system subcooling level within the prescribed range of the system subcooling setpoint. The processor can be further configured to adjust an output capacity of the compressor to maintain the system subcooling level within the prescribed range of the system subcooling setpoint. The processor can also be further configured to adjust the expansion valve to maintain the system superheat within a prescribed range of the system superheat setpoint. 
     In accordance with another aspect, a refrigerant vapor compression system comprises a compressor, first and second heat exchangers fluidly coupled to the compressor, a variable orifice expansion valve located between the first and second heat exchangers, at least one sensor for sensing operating conditions of the system related to a system superheat level and a system subcooling level, and means to receive sensed data from the at least one sensor and, based at least in part thereon, control at least one of the compressor or variable orifice expansion valve to maintain system performance. The means configured to: monitor the system superheat level to determine whether the system superheat level is within a prescribed range of a system superheat setpoint, monitor the system subcooling level to determine whether the system subcooling level is within a prescribed range of a system subcooling setpoint and, iteratively: adjust at least one system setting to restore the system subcooling level to within the prescribed range of the system subcooling setpoint while the system superheat level is within its prescribed range, increment the system superheat setpoint when either the system subcooling level or the system superheat level cannot be maintained within their respective prescribed ranges, and adjust at least one system setting to achieve the incremented superheat setpoint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary system in accordance with the present disclosure; 
         FIG. 2  is a block diagram of an exemplary controller in accordance with the present disclosure; and 
         FIG. 3  is a flowchart of an exemplary method in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a variable capacity refrigeration system is identified generally by reference numeral  10 . It will be appreciated that the illustrated system is just one of a variety of refrigeration systems, and that aspects of the disclosure are applicable to a wide variety of such systems. The system  10  includes a compressor  12 , which can be a modulating variable speed compressor or a tandem compressor system, for example. The compressor  12  is fluidly connected to a condensing coil  14  (heat exchanger) by a compressor refrigerant discharge tubing  16 . The condensing coil  14  is located in close proximity to a condensing fan assembly  18 , which includes a fan blade and a fan motor. The condensing fan motor can be a fixed speed, multiple speed or variable speed design. The outlet of the condensing coil  14  is fluidly connected to an inlet of an expansion valve  20  by refrigerant liquid tubing  24 . The expansion valve  20  controls the flow of refrigerant into an evaporator coil  28  (heat exchanger) and creates a pressure drop within the refrigerant tubing. The outlet of the evaporator coil  28  is fluidly connected to an inlet of the compressor  12  by refrigerant (vapor) tubing  30 , thus completing the exemplary closed refrigeration circuit. 
     In operation, the compressor  12  receives refrigerant vapor from the evaporator  28  and mechanically compresses the vapor from a low pressure to a higher pressure. This high pressure vapor travels to the condensing coil  14  via tubing  16 . The condensing fan assembly passes ambient air across the condensing coil  14 , removing heat from the high pressure vapor coming from the compressor  12  and enabling the refrigerant vapor to change state to a high pressure liquid within the condensing coil  14 . The high pressure liquid refrigerant is further cooled in the last portion of the condenser coil  14 , then exits the condensing coil  14  into the liquid line  16  as a sub-cooled liquid. The level of sub-cooling is defined as the difference of the liquid refrigerant temperature and the saturation temperature of the refrigerant at the pressure within the liquid line. Typical sub-cooling levels might be approximately 10 degrees F. In the illustrated embodiment, the sub-cooling level in the liquid line is measured by a pressure/temperature sensor  35 , whereby the saturation temperature is calculated based on the sensed pressure and subtracted from the liquid temperature to give the sub-cooling level. 
     Once the refrigerant exits the condensing coil  14 , it moves through the refrigerant liquid tubing  24  to the expansion valve  20 . The expansion valve  20  meters the liquid refrigerant into the evaporator coil  28 , creating a pressure drop. The reduction of pressure in the refrigerant tubing causes the liquid refrigerant to begin boiling, absorbing heat as air is passed over the evaporator coil  28  by the indoor blower assembly  19 . The refrigerant continues to boil and absorb heat in the evaporator coil  28  until it becomes a single phase vapor at which point the vapor will continue to heat above the saturation temperature. This added heat is the superheat of the system and is defined by the difference between the vapor temperature and the saturation temperature correlating to the pressure inside the evaporator tubing. System efficiency is maximized in this embodiment by maintaining an optimum superheat value (e.g., 5 degrees F.) with an electronic expansion valve and superheat controller, which can monitor the superheat level by a pressure/temperature sensor  39  mounted in the refrigerant suction line  30 . The superheated refrigerant vapor continues to the inlet of the compressor  12  where the cycle begins again. 
     During normal operation, a system controller  41  monitors inputs (superheat, subcooling, compressor speed, ambient temperature, etc.) and controls one or more of the system components based on system design parameters. The system controller  41  is illustrated schematically in  FIG. 2 , and generally includes an input  46  for receiving one or more signals from various system components, an output  48  for outputting a control signal to one or more system components, a processor  50  and a memory  52 . Stored within memory  52  are one or more modules including instructions used by the processor for performing various control functions. For example, a superheat module  54  and a subcooling module  56  are provided for carrying out at least some of the controller functions described herein. A lookup table (LUT)  58  is also stored in the memory  52  and contains various superheat setpoint values and/or subcooling setpoint values for various system conditions. For example, the LUT  58  may contain superheat and/or subcooling setpoint values corresponding to one or more sensed operating conditions such as ambient temperature, refrigerant line temperature, refrigerant line pressure, etc. 
     As noted, the controller  41  controls one or more of the system components based on system design parameters. In some applications, this can include adjusting the flow through the variable electronic expansion valve  20  by adjusting the valve opening to maintain the superheat setpoint. In one exemplary embodiment, the superheat setpoint can be approximately 5 degrees F. for optimal system performance. The controller  41  also monitors sub-cooling and is capable of adjusting the condenser fan  18  speed to maintain the sub-cooling setpoint. In one exemplary embodiment, the subcooling setpoint can be approximately 10 degrees F. or greater. In normal operation, sub-cooling can be maintained above the target threshold and the controller  41  gives priority to maintaining the superheat level as close to the target level as possible, optimizing the usage of the evaporator coil  28  (heat exchanger) surface area and, thus, maximizing the capacity and efficiency of the system. Additionally, if return and supply duct air temperatures (not shown) are monitored, the superheat setpoint can be adjusted to optimize system performance as operating conditions vary. This can be considered a normal mode of operation (e.g., superheat priority mode). 
     If during normal operation the system controller  41  senses the sub-cooling level dropping below a defined minimum level (e.g., outside a prescribed range), for example 6 degrees F., and could not control the condenser fan  18  (or other sub-cooling control method) to increase the sub-cooling level above the minimum threshold, the controller  41  can be configured to change operation modes and enter an alternate mode referred to herein as the sub-cooling priority mode. In the sub-cooling priority mode, the system controller  41  would adjust (e.g., increment) the superheat level to enhance the sub-cooling capabilities of the system under the operating conditions encountered. These conditions might include low demand, low ambient temperature, or a combination of both. Such conditions might commonly occur during periods of cooler ambient temperatures (e.g., night time, winter, etc.) 
     When operating in the sub-cooling priority mode, the system controller  41  would increase the superheat setpoint and begin closing the electronic expansion valve to obtain the new level. By increasing the superheat level, system pressures would increase effectively raising the saturation temperature in the condensing coil  14  which would increase the temperature difference between the outside ambient temperature and refrigerant saturation temperature. Additionally, as the superheat level is increased the relative amount of liquid refrigerant in the evaporator  28  decreases and that liquid refrigerant must then move to the condenser coil  14 , enhancing the ability for the heat exchanger to subcool this liquid. The increased temperature difference and increased amount of liquid charge in the condenser  14  would effectively result in a higher sub-cooling level, insuring the refrigerant in the liquid line was indeed single phase liquid being delivered to the expansion valve  20 . Even though some system performance could be sacrificed by raising the superheat level during this mode, overall system performance would be optimized by maintaining the sub-cooling levels in the proper range. Additionally, by monitoring the amount of dynamic adjustment of superheat setpoint required to obtain desired subcooling levels, a change (reduction) in the total system refrigerant charge level can be recognized and a system diagnostic provided to the operator. 
     In an alternate configuration, the system can further include an outdoor ambient temperature sensor  43  that is monitored by the system controller  41 . The system controller  41  can use information from the liquid line sensor  35 , either pressure, temperature or a combination of both, compare the liquid line data to the outdoor ambient temperature, and switch to the sub-cooling priority mode based on this comparison. For example, if the liquid line temperature was measured to be greater than 15 degrees F. above the ambient temperature, this might indicate a situation in which the condenser fan speed must be increased in an effort to increase the airflow through the condenser to increase sub-cooling performance, or, if adjusting the condenser fan speed proved inadequate to correct the subcooling level, demanding a switch to sub-cooling priority mode and, thus increase the superheat setpoint. The system controller  41  continues to monitor the sub-cooling levels and associated system performance until conditions are such that both sub-cooling and superheat can be maintained within the respective setpoint targets. This relationship between the liquid line temperature and outdoor ambient temperature can also be used to indicate other system performance issues, such as excessive condenser coil airflow, in which case the system power consumption may be greater than necessary, adversely affecting system efficiency. For example, if the liquid line saturation temperature was measured to be less than 5 degrees F. above the ambient temperature, this might indicate excessive condenser airflow in which the condenser fan speed may be reduced to conserve energy without adversely affecting overall system performance. Additionally, if a trend towards higher and higher superheat settings for a given set of operating conditions is observed, this may indicate a reduced amount of refrigerant available to obtain the desired system balance and give the opportunity for the control, recognizing the trend, to alert system monitors of a potential leak in the refrigeration system. 
     Turning to  FIG. 3 , a flowchart of an exemplary method in accordance with the present disclosure is illustrated. The method  60  begins with process step  62  wherein the system initiates a cooling cycle with preset parameters based on initial demand signal. For example, the preset parameters can include a superheat setpoint of 5 degrees F. and a subcooling setpoint of 10 degrees F. In process step  64 , the subcooling of the system is monitored (e.g., using the temperature/pressure sensors as described above). In process step  66 , if the subcooling is within target parameters (e.g., between the upper and lower control limits, also referred to as a prescribed range, for example 14 degrees and 10 degrees, respectively), the method proceeds to process step  68  and remains in normal mode (e.g., superheat priority mode). In this mode, the system components (e.g. condenser fans, compressor capacity) are controlled to maintain the superheat level at a setpoint (e.g., 5 degrees F.). This includes, among other things, adjusting the expansion valve and/or regulating the indoor blower motor. 
     If the subcooling of the system is not within target setpoint tolerances in process step  66 , the method diverts to process step  70  where the system components are controlled to restore the subcooling level to the target setpoint. At process step  72 , if the subcooling level is restored (e.g., above an upper limit tolerance, greater than 10 degrees F. in this example), the method returns to process step  68  and the system is operated in superheat priority mode with the system superheat being monitored in process step  74 . If the subcooling level is not restored, then in process step  76 , if the system parameters have not been adjusted to max/min limits, the method evaluates system operating conditions and adjusts the sub-cooling setpoint as indicated by one or more system inputs (process step  77 ), the method loops back to process step  64  and the method continues to monitor/adjust the subcooling as described to bring it back into desired range. By way of example, system parameters are considered adjusted to max/min limits when no further adjustment can be made to restore the subcooling level, such as adjusting the condenser fan speed, without disturbing/altering the system superheat setpoint. 
     Accordingly, if in process step  76  it is determined that the system parameters have been adjusted to max/min limits, the method proceeds to process step  78 , where the system enters a subcooling priority mode. This mode gives priority to restoring system subcooling and allows system superheat to temporarily deviate from target levels. In process step  80 , the subcooling is monitored, and in process step  82  if the subcooling level is restored, the method reverts to process step  64  and the method loops to monitor/control system subcooling. 
     Otherwise, if at process step  82  the system subcooling is not restored, then the method proceeds to process step  84  where the system superheat setpoint is incremented. For example, the system superheat setpoint of 5 degrees may be adjusted to 7 degrees, then revert back to process step  74  to stabilize the superheat at the new setpoint. Once the system superheat is within target tolerances in process step  86 , then the method reverts back to process step  64  and the method repeats. Otherwise, the method proceeds to process step  88  and the system parameters are adjusted to restore the superheat setpoint. If in process step  90  the system parameters are adjusted to their max/min limits, then the method reverts to process step  64  and the method repeats. If in process step  90  the system parameters have not been adjusted to max/min limits, then the method loops back to process step  74  and until the superheat value is restored. 
     The exemplary embodiment has been described with reference to the preferred embodiments. Modifications and alterations can occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.