Abstract:
The present disclosure relates to a method for controlling a level of superheat during a pump mode of operation of a refrigeration system, wherein the refrigeration system can operate in either the pump mode or a compressor mode, and has an electronically controlled expansion valve (EEV). A controller obtains a stored, predetermined pump differential pressure range able to be produced by a pump of the system. The controller also obtains a stored, predetermined superheat range, and detects a superheat level. When the detected superheat level is outside of the superheat temperature range, the controller commands adjusting at least one of the EEV and a speed of the pump based on whether the detected superheat level is above or below the superheat range, and whether a current pump differential pressure is above or below the predetermined pump differential pressure range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. provisional application Ser. No. 62/311,765, filed Mar. 22, 2016, the entire disclosure of which is hereby incorporated by reference into the present disclosure. 
     
    
     FIELD 
       [0002]    The present disclosure relates to refrigeration systems, and more particularly to a refrigeration system and a control method used in connection with the refrigeration system having a pumped refrigerant “economizer” mode of operation, and still more particularly to a system and method for monitoring and controlling differential pump pressure and superheat to prevent damage to a compressor of the system when operating in a pump mode (i.e., refrigerant economizer mode), and when the system switches from the pump mode to compressor mode. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    The assignee of the present disclosure is a leader in the field of environmental control system used in small, medium and large scale data centers. As opposed to the more conventional air side economizer, the system described in this application uses a pumped refrigerant economizer. 
         [0005]    A pumped refrigerant economizer system utilizes a liquid pump to circulate refrigerant instead of operating the compressor when the indoor &amp; outdoor temperature conditions are suitable to provide cooling without requiring a vapor compression cycle. 
         [0006]    A pumped refrigerant economizer system also typically uses a system of sensors to measure the outside and inside air conditions, and if the outside conditions are suitable to allow cooling without requiring compression of the refrigerant, a liquid pump is used in place of the compressor to provide refrigerant flow through the system. This mode of operation is known in the industry as the “pump mode”. Running in the pumped mode reduces or eliminates the need for the air conditioning system&#39;s compressor(s) to run. This results in a significant energy savings for cooling the space. In “compressor mode” of operation, the compressor(s) of the system is/are running to supply the cooling needs for the space. 
         [0007]    When running in pump mode, however, care must be taken to not incur damage to various components of the system. In particular, it is understood that the lack of superheat leaving the evaporator during the pump mode can eventually lead to excessive liquid refrigerant collecting in the crankcase of the compressor. This can have detrimental effects on the compressor if a sufficient quantity of liquid refrigerant has collected in the compressor crankcase during the pump mode, and then the compressor mode is started. Excessive liquid accumulating in the crankcase (i.e., sump) of the compressor may potentially cause various issues such as sump frosting and/or oil foaming, oil dilution and/or washout when the compressor attempts to start up during transitions from the pump mode to the compressor mode. 
         [0008]    While present day economizer systems adjust the system&#39;s EEV (Electronic Expansion Valve) to maintain appropriate pump differential pressure while operating in the pump mode, there is no control over the superheat leaving the evaporator when the system is running in the pump mode. Accordingly, some means to control and limit the liquid refrigerant build-up in the compressor during the pump mode of operation is needed. 
       SUMMARY 
       [0009]    In one aspect the present disclosure relates to a method for controlling a level of superheat during a pump mode of operation of a refrigeration system, wherein the refrigeration system is able to operate in either the pump mode or a compressor mode of operation and includes an electronically controlled expansion valve (EEV). The method may comprise using a controller to obtain a stored, predetermined pump differential pressure range able to be produced by a pump of the refrigeration system. The controller may also be used to obtain a stored, predetermined superheat range, and to detect a superheat level. When the detected superheat level is outside of the predetermined superheat temperature range, the controller may be used to command adjusting at least one of the EEV and a speed of the pump based on whether the detected superheat level is above or below the predetermined superheat range, and whether a current pump differential pressure is above or below the predetermined pump differential pressure range. 
         [0010]    In another aspect the present disclosure relates to a method for controlling a level of superheat during a pump mode of operation of a refrigeration system, wherein the refrigeration system is able to operate in either the pump mode or a compressor mode of operation, and includes an electronically controlled expansion valve (EEV). The method may comprise defining a fixed adjustment step for the EEV, defining a fixed adjustment step for a change in pump speed, and defining a time delay interval to be applied between successive adjustments of the EEV and the pump speed. The method may further comprise using a controller to obtain a stored, predetermined pump differential pressure range able to be produced by a pump of the refrigeration system, as well as using the controller to obtain a stored, predetermined superheat range. The method may further include detecting a superheat level, and when the detected superheat level is outside of the predetermined superheat temperature range, adjusting the EEV and a speed of the pump based on whether the detected superheat level is above or below the predetermined superheat range, and in accordance with the predetermined EEV and pump speed fixed adjustment steps. 
         [0011]    In still another aspect the present disclosure relates to a refrigeration system configured to control a level of superheat during a pump mode of operation. The refrigeration system is able to operate in either the pump mode or a compressor mode of operation, and comprises a pump, an electronically controlled expansion valve (EEV), and a controller for controlling adjustments of the EEV and the pump. The controller further may be configured to obtain a stored, predetermined pump differential pressure range able to be produced by the pump, to obtain a stored, predetermined superheat range, and to detect a superheat level during operation of the refrigeration system. When the detected superheat level is outside of the predetermined superheat temperature range, the controller adjusts the EEV and a speed of the pump based on whether the detected superheat level is above or below the predetermined superheat range. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0013]      FIG. 1  is a high level block diagram of one embodiment of a refrigeration system of the present disclosure that incorporates an electronic controller based subsystem for controlling an electronic expansion valve to regulate pump differential pressure and evaporator superheat during a pumped mode of operation of the system; 
           [0014]      FIG. 2  is a high level flowchart showing operations associated a superheat control algorithm implemented by the controller of the system; 
           [0015]      FIG. 3  is a flowchart of a low superheat adjustment routine implemented by the controller; and 
           [0016]      FIG. 4  is a table of various control adjustment parameters that the controller observes when applying adjustments during use of the superheat control algorithm. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0018]    The present system and method involves using an electronic expansion valve (EEV) is used to regulate pump differential pressure during the pumped mode of operation. In addition, the system and method involves applying a control routine to control superheat leaving the evaporator. The superheat leaving the evaporator is controlled in conjunction with the pump differential pressure, which eliminates or significantly reduces the possibility of liquid refrigerant collecting in the compressor crankcase (i.e. sump) and causing oil dilution during the pumped mode of operation. Importantly, the system and method does not negatively affect the overall efficiency of the system. 
         [0019]    Referring to  FIG. 1  a refrigeration system  10  in accordance with one embodiment of the present disclosure is shown. The system  10  in this example forms an economizer system and is therefore able to operate in both a “pump” mode and a “compressor” mode. In the pump mode, lower temperature outside air is able to condense refrigerant at a lower pressure and a liquid refrigerant pump circulates refrigerant to cool the refrigerant and an interior room or building, with the compressor(s) of the system turned off. In the compressor mode the compressor(s) is/are used to help cool the interior of the room or building. In this example the system  10  includes at least one compressor  14 , an evaporator  16 , a check valve  17 , an electronic expansion valve (“EEV”)  18 , a superheat temperature and pressure sensor  20 , and a pump differential pressure sensor  12 . The compressor  14  and pump differential pressure sensor  12  are in bidirectional communication with a unit controller  22 . The unit controller  22  includes a processor  22 a, in one example a microprocessor, which may run a compressor superheat control algorithm  100   a , a pump differential pressure control algorithm  100   b , and a pump mode superheat control algorithm  100   c , all stored in non-volatile RAM or ROM. It will be appreciated that the compressor superheat control algorithm  100   a  is a standard algorithm that has been used in the industry for controlling superheat coming out of the evaporator  16  when operating in the compressor mode. Details of the pump differential control algorithm  100   b  may be found in U.S. Pat. Nos. 9,316,424 and U.S. Pat. No. 8,881,541, the disclosures of both of which are hereby incorporated by reference into the present application. The pump mode superheat control algorithm  100   c , which is an important feature of the present disclosure, will be discussed in further detail in connection with  FIGS. 2 and 3 . 
         [0020]    The system  10  further includes a condenser  24  which is used to receive hot gas from the compressor  14  during compressor mode and to provide heat transfer for the refrigerant during pump mode. The liquid refrigerant flows to a conventional receiver  24 a which is able to hold a predetermined quantity of the refrigerant charge, if needed, during operation of the system  10 . The liquid refrigerant continues to flow through a pump box  25 , through a liquid line  27 , and back to the electronic expansion valve  18 , as is well known with air conditioning systems. The pump box  25  contains refrigerant pump(s)  25   b  and pump speed controls, including a pump speed controller  25   a , for operation during economization mode. Historically the pump speed controller  25   a  will modulate the pump speed to meet the cooling demand and the unit controller  22  will modulate the EEV  18  to maintain pump differential pressure. In conjunction, this maintains refrigerant flow to the evaporator  16  in a method such that the cooling load in the room or data center is satisfied. If the outdoor temperature increases such that the load cannot be maintained, the unit controller  22  will turn the pumps ‘off’ and turn the compressor(s) ‘on’ to ensure the room load is maintained. During compressor operation the unit controller  22  maintains superheat using the superheat temperature and pressure sensor  20  as inputs to the compressor mode superheat control algorithm  100   a.    
         [0021]    In a different embodiment, the unit controller  22  may also use the pump mode superheat control algorithm  100   c , shown in  FIG. 1 , to determine an EEV position based on the superheat temperature and pressure sensor  20 . In this embodiment the unit controller  22  evaluates the superheat and pump differential condition to determine an appropriate EEV  18  position to best satisfy both conditions as described below. Additionally the unit controller  22  will determine a maximum pump speed limit based on the EEV  18  position and communicate this to the pump speed controller  25   a  to limit pump speed adjustment. 
         [0022]    The system  10  shown in  FIG. 1  also makes use of a an indoor relative humidity temperature sensor  28 , an outdoor temperature (dry bulb) sensor  30  and an indoor (dry bulb) temperature sensor  32 . 
         [0023]    Referring to  FIG. 2 , the unit controller  22  implements the compressor superheat control algorithm  100   c  to monitor and control the superheat leaving the evaporator coil  16 , while also monitoring and controlling the pump differential pressure. Initially, when operating in pump mode, the pump differential pressure control algorithm  100   b  for the system  10  defines an allowable pressure range made up of allowable upper and lower default pump pressure thresholds. A PID (proportional integral/derivative) calculation is used by the unit controller  22  to adjust the EEV  18  position during operation outside the allowable range. The new pump mode superheat (“SH”) control algorithm  100   c  applies the following additional parameters when making EEV  18  position adjustments in conjunction with the pump differential control: 
         [0024]    SH Control EEV close step (a percentage step value for making one adjustment when opening the EEV  18 ); 
         [0025]    SH Control open step (a percentage step value for making one adjustment when closing the EEV  18 ); 
         [0026]    Lower SH threshold (a predefined lower SH temperature threshold value); 
         [0027]    Upper SH threshold (a predefined upper SH temperature threshold value); 
         [0028]    Adjustment time delay (time delay between making successive adjustments of the EEV  18 ); 
         [0029]    Adjusted maximum pump  25   b  pressure upper threshold; 
         [0030]    Pump pressure upper threshold adjustment step (a percentage value when making a single adjustment step to set the pump  25   b  pressure upper threshold); and 
         [0031]    Maximum allowable pump  25   b  speed setting based on the EEV  18  position. For the above parameters, “pump speed” and “pump pressure” refer to the motor speed of the pump  25   b  and the differential pressure across the pump  25   b.    
         [0032]    Referring specifically to  FIG. 2 , various operations that may be associated with the pump mode superheat control algorithm  100   c  are illustrated. At operation  102  the system  10  begins operation in the pump mode. At operation  104  the unit controller  22  obtains allowable pressure ranges for pump differential pressure control in the pump mode. The unit controller  22  then obtains various superheat control adjustment parameters that the unit controller will observe when applying adjustments, as indicated at operation  106 . These parameters are illustrated in the table shown in  FIG. 4 . 
         [0033]    At operation  108  a startup timer included in the unit controller  22 , and associated with the pump start up condition determined by the unit controller, is then started. The startup timer allows for system pressures and temperatures to stabilize after pump mode operation is started to prevent erratic reaction to a transitory superheat condition. At operation  110  a check is made by the unit controller  22  to determine if the startup time has timed out and (e.g., 5 minutes has elapsed after pump startup), if not, the startup time is repeatedly checked by the unit controller  22  until the startup timer is detected as having timed out. When this event occurs, the unit controller  22  resets the startup time, as indicated at operation  112 , and then checks to determine if a low superheat condition is detected, as indicated at operation  114 . As noted above, this condition is determined by having the unit controller  22  evaluate and compare the actual suction temperature and pressure to a saturated temperature calculated from the suction pressure, to determine if the superheat temperature is below the predetermined lower superheat threshold. More specifically, the unit controller  22  is looking at the temperature and pressure of the refrigerant leaving the evaporator  16  and calculating the difference between the actual temperature and the saturated refrigerant temperature at the measured pressure condition (superheat). If the superheat temperature is above the predetermined lower superheat threshold, then operation  114  is repeated. If the superheat temperature is below the predetermined lower superheat threshold, then a low superheat condition is detected and the unit controller  22  implements a low superheat adjustment routine  200 , which is a subportion of algorithm  100   c , and which is shown in detail in  FIG. 3 . 
         [0034]    The low superheat adjustment routine  200  involves using the unit controller  22  to initially adjust the maximum pump pressure upper threshold to prevent the differential pressure control routine from calculating a more open valve position when the valve is closed by the superheat control routine, as indicated at operation  202 . Then the EEV is closed by a predetermined amount (e.g., percentage), which comprises one adjustment “step” (i.e., “increment”) to reduce refrigerant flow to the evaporator and increasing superheat, as indicated at operation  204 . 
         [0035]    At operation  206  the unit controller  22  then starts an adjustment time delay timer. 
         [0036]    Referring further to  FIG. 3 , the unit controller  22  then begins checking, as indicated at operation  208 , if the adjustment time delay timer has timed out. If a check produces a “No” answer, then operation  208  is repeated by the unit controller  22 . If the check at operation  208  produces a “Yes” answer, then the adjustment time delay timer is reset by the unit controller  22 , as indicated at operation  210 , and the unit controller then evaluates the interaction of superheat and pump differential conditions, as indicated at operation  212 , per the conditions listed in Table  1  of  FIG. 4 . More specifically, the unit controller  22  examines the low superheat temperature and the pump differential pressure to determine which one of the conditions in Table  1  ( FIG. 4 ) is present. Assuming that the low superheat condition is still present , the unit controller  22  adjusts the EEV  18  based on the specific condition from Table  1  that is present, as indicated at operation  214 . At operation  216  the unit controller  22  then evaluates the pump speed compared to the EEV  18  position and limits pump speed to the maximum allowable speed based on the current EEV  18  position. It will be appreciated that the maximum allowable pump speed for each EEV  18  position will also vary depending on the EEV  18  size and pump  25   b  displacement. 
         [0037]    With continuing reference to  FIG. 3 , the unit controller  22  then makes a check using the pump differential pressure sensor  12  to determine if the pump differential pressure upper threshold is adjusted back to the normal default value and if the current EEV  18  position is equal to the pump differential pressure control requested position, as indicated at operation  218 . A check is then made by the unit controller  22  to determine if the pump mode has been exited, as indicated at operation  220 . If it has, then the low superheat adjustment routine  200  ends. If the pump mode has not been exited yet, then at operation  114  ( FIG. 1 ) the unit controller  22  begins checking again for the presence of a superheat condition. 
         [0038]    The system and method of the present disclosure thus enables operation of economizer systems in both the pump and compressor modes while eliminating, or at least substantially reducing, the chance of liquid refrigerant being supplied to the suction inlet of the compressor(s) of the system  10  when the system exits the pump mode and begins operating in the compressor mode. The system  10  and method of the present disclosure does not require the addition of significant new components to the system or otherwise significantly complicate the operation of the system or reduce its efficiency, or increase its cost. The system and method of the present disclosure can also be retrofitted to existing economizer systems with only minor modifications. 
         [0039]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.