Abstract:
A method of refrigeration control through a refrigeration system of a refrigerated transport container includes performing a defrost cycle on the refrigeration system by activating a heat source; and restarting the refrigeration system after the defrost cycle has completed, wherein restarting the refrigeration system includes performing a liquid slugging avoidance process including: initiating a compressor of the refrigeration system at a speed; opening a pressure equalization valve in parallel with the compressor in response to the initiating; opening a liquid valve in series between a condenser and an evaporator after opening of the pressure equalization valve; and closing the pressure equalization valve after a period of time.

Description:
FIELD OF INVENTION 
     The subject matter disclosed herein relates generally to the field of refrigeration, and more particularly to methods and systems of defrosting a transport container refrigeration system and avoiding compressor slugging. 
     DESCRIPTION OF RELATED ART 
     Generally, in direct drive transport refrigeration, a road compressor is directly coupled to a vehicle. The associated refrigerating system is relatively simple with few valves that do not protect the compressor against liquid flow back. Because these systems use hot gas defrost technology with a shunted condenser and expansion valve, the compressor may systematically suck liquid from the evaporator during and after the hot gas defrost. Accordingly, compressor damage and premature failure is possible. Furthermore, in direct drive applications subsequent defrost-timings are non-optimal. Thus, the art would well receive improvements in defrosting transport container refrigeration systems and avoiding compressor slugging in transport container refrigeration systems 
     BRIEF SUMMARY 
     According to an example embodiment of the present invention, a method of refrigeration control through a refrigeration system of a refrigerated transport container includes performing a defrost cycle on the refrigeration system by activating a heat source; and restarting the refrigeration system after the defrost cycle has completed, wherein restarting the refrigeration system includes performing a liquid slugging avoidance process including: initiating a compressor of the refrigeration system at a speed; opening a pressure equalization valve in parallel with the compressor in response to the initiating; opening a liquid valve in series between a condenser and an evaporator after opening of the pressure equalization valve; and closing the pressure equalization valve after a period of time. 
     According to another example embodiment of the present invention is a refrigeration system, comprising: a condenser; a compressor in serial communication with the condenser; an evaporator in serial communication with the condenser and the compressor; a heat source for thawing an evaporator coil during a defrost cycle; a pressure equalization valve arranged in parallel with the compressor; a liquid valve in series between the condenser and the evaporator; a controller for controlling the pressure equalization valve and the liquid valve to implement a liquid slugging avoidance process upon restart of the compressor after a defrost cycle. 
     Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates a refrigeration system, according to an example embodiment; 
         FIG. 2  illustrates a refrigeration system, according to an example embodiment; 
         FIG. 3  is a representation of a defrost cycle and a liquid slugging avoidance process, according to an example embodiment; 
         FIG. 4  is a representation of a relationship between defrost energy consumption versus time to the next defrost cycle, according to an example embodiment; 
         FIG. 5  illustrates a method of preventing liquid slugging in a refrigeration system, according to an example embodiment; and 
         FIG. 6  illustrates a method of refrigeration control, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of methods and systems of transport container refrigeration control and defrost operations are described herein in detail. Technical effects and benefits of such methods include limiting compressor liquid flow through novel starting sequences. 
     Turning to  FIG. 1 , a simplified representation of a refrigeration system  100  for transport containers is illustrated. Additional components included within the system  100  are omitted from  FIG. 1  for the purpose of clarity only, and are described in detail with reference to  FIG. 2 . The system  100  may be electrically powered. For example, the system  100  may be in electrical communication with a power medium  107 . The power medium  107  may be supplied through a vehicle powered generator, AC mains, three-phase power grid, or any other suitable power supply. 
     The system  100  includes inverter  105  in communication with the power medium  107 . The inverter  105  may convert power supplied through power medium  107  into power usable by the system  100 . 
     The system  100  further includes evaporator  102  in communication with the inverter  105 , and heater  120  in communication with the inverter  105  and the evaporator  102 . The heater  120  may be arranged or mechanically mounted within/upon the evaporator  102  for relatively efficient defrosting and heating operations. Heater  120  may be a resistive heater, or other known form of heater. 
     The system  100  further includes sensor  101  in communication with the evaporator  102 . The sensor  101  may be a temperature sensor coupled to the evaporator  102 , also referred to as a defrost temperature thermistor (DTT). The sensor  101  may signal the start and/or cessation of defrosting operations. 
     The system  100  further includes sensor  103  in communication with the evaporator  102 . The sensor  103  may be a Return Air Temperature (RAT) sensor, which may determine a temperature of a transport container being refrigerated. According to some example embodiments, the sensor  103  may also be a Supplied Air Temperature sensor. Further, a plurality of sensors may be used as sensor  103 , including any combination of applicable or desirable sensors configured to determine temperature of a container or unit to be refrigerated. 
     The system  100  further includes compressor  104  in communication with the inverter  105 . If compressor  104  is a variable speed compressor, the inverter  105  and controller  110  control the speed of the compressor  104 . Alternatively, the compressor  104  may be a fixed speed compressor in exemplary embodiments. The system  100  further includes condensing unit  106  and condenser fan  160  in communication with the inverter  105 . 
     Additionally, the system  100  includes a controller  110  in communication with one or more components of the system  100  to facilitate operation and control of the system  100 . For example, the controller  110  may include any appropriate processing means including a general-purpose computer processor, microcontroller, ASIC, FPGA, discrete electrical control loops, or any combination thereof; and further comprise any appropriate memory or storage means to enable storage of computer executable instructions that, when executed by the controller  110 , direct the controller  100  to perform any or all of the methods described herein. 
     Turning now to  FIG. 2 , a more detailed illustration of the refrigeration circuit of the system  100  is illustrated. As illustrated, the system  100  further includes pressure equalization valve (PEV)  201 . The PEV  201  is arranged in series between the condenser  106  and the evaporator  102 . Further, the PEV  201  is arranged in parallel across the compressor  104 . The PEV  201  may equalize pressure established through the compressor  104  to avoid slugging as described herein. 
     The system further includes expansion valve  207  upstream of the evaporator  102 . The expansion valve  207  is configured to allow expansion of refrigerant utilized in the system  100 . The system  100  further includes liquid valve (LV)  206  arranged in series with and between the condenser  106  and the expansion vale  207 . The system  100  further includes air cooled after cooler (ACC)  204 , or a liquid accumulator, in series between the condenser  106  and the liquid valve  206 . As described in further detail herein, the controller  110  controls the PEV  201  and liquid valve  206  during compressor restart to avoid liquid slugging. 
     Turning to  FIG. 3 , a representation of defrost conditions of the system  100  are illustrated, according to an example embodiment. As shown, temperature curve  310  represents the actual temperature value measured in the refrigeration system  100  or within the transport container. Temperature curve  310  may correspond to the temperature measured by the discharge temperature sensor  101  or sensor  103 . 
     An appropriate or predetermined temperature range denoted as the range between temperature limit values T 1  and T 2  is also illustrated. This temperature range between T 1  and T 2  may be the appropriate temperature range for items contained in a refrigerated unit or container, limits of temperature of the unit or container, limits determined through an international or local standard, or any combination thereof. As further illustrated, as the detected temperature is relatively close to or below the lower temperature limit T 2 , denoted at time value  301 , defrost operations are enabled. 
     The defrost cycle involves using a heater  120  (e.g., a resistor heater) to thaw ice from the evaporator coil. During the defrost cycle, defrost energy consumption of the system  100  is measured. The calculation of defrost energy consumption generally may be represented by power consumed (e.g., watts) by the heater  120  times the time to complete the defrost cycle (e.g., seconds). 
     Upon calculation of the defrost energy consumption, a next appropriate time delay between defrost cycles may be determined by controller  110 .  FIG. 4  illustrates a relationship between defrost energy consumption in Watts×hour (Wh) or Watts×seconds (Ws) versus time delay to the next defrost cycle in minutes (min) or second (s). As shown in  FIG. 4 , the defrost energy consumption and time delay have a generally inverse relationship such that a high defrost energy consumption results in a shorter time delay to the next defrost cycle.  FIG. 4  illustrates a linear relationship for a portion of the values. It is understood that other mathematical relationships may be used to determine the time delay as a function of the defrost energy consumption. 
     Referring back to  FIG. 3 , upon appropriate completion of defrosting, denoted by the temperature rising substantially close to temperature limit T 1 , refrigeration of the container may start. 
     The start-up sequence of refrigeration system  100  may be facilitated by the controller  110  through the methods described below.  FIG. 5  illustrates exemplary waveforms  500  in a method of preventing liquid slugging in refrigeration system  100  upon restart after defrost. 
     As illustrated, upon refrigeration restart the compressor  104  is activated at a low frequency or low power as shown at plot  501 . If the compressor  104  is fixed speed, the compressor will stay at the same speed throughout the process. Furthermore, upon refrigeration commencement, the PEV  201  is fully opened as shown in plot  503 . In response to the compressor operating at low frequency and the PEV  201  being opened for a predetermined or desired time V 1 , the liquid valve  206  is also opened as shown in plot  504  and the evaporator fan motor is driven to a higher speed as shown in plot  502 . 
     Upon expiration of a second predetermined time threshold V 2 , the PEV  201  is closed. Upon expiration of a third predetermined time threshold V 3 , compressor  104  speed is ramped up to about 75% or less of available speed (depending upon cooling demand) over the course of a fourth predetermined time threshold V 4 . Speed is maintained for a fifth predetermined time threshold V 5  at which point compressor  104  speed is driven to a maximum available or steady state operational speed. Alternatively, if the compressor is a fixed speed compressor, the compressor will stay at the same speed throughout the restart. 
     The predetermined time thresholds V 1 -V 5  are established based upon desired reduction in liquid slugging of the refrigeration circuit of the system  100 . For example, as the compressor superheat becomes positive and the compressor mass flow remaining low, threshold V 1  expires and liquid valve  206  is opened. In response to pressure equalization, the compressor  104  speed is ramped as noted by the ramp in plot  501 . 
     Upon reaching a desired percentage (e.g., 75%) or less (depending upon cooling demand) of available compressor speed, a short time delay V 5  is introduced to stabilize the refrigeration cycle before bringing compressor  104  to steady state operating speed. Again, if the compressor is a fixed speed unit, the compressor will stay at the same speed throughout the restart with valves  201  and  206  being controlled as described above. 
     Turning now to  FIG. 6 , a method of refrigeration control is illustrated. The method  600  includes monitoring a temperature of a unit or container being refrigerated at block  601 . The method  600  further includes determining if the temperature (at sensor  103  or sensor  101 ) is below a defrost threshold at block  602 . If the temperature is above the threshold, temperature monitoring continues at block  601 . If the temperature is below the threshold the method includes beginning a defrost cycle at block  603 . During the defrosting operation  603 , the method  600  includes determining if the temperature of the unit is above a predetermined value at block  604 . If the temperature has exceeded the threshold, heating/defrosting is disabled at block  605 , and the method  600  includes determining defrost energy consumption during the previous defrost cycle at block  606  as described above with reference to  FIG. 4 . 
     The method  600  further includes enabling refrigeration using the liquid slugging avoidance method of  FIG. 5 , at block  607  in response to the determination. Thereafter, temperature is monitored at blocks  608 - 609 , if the temperature of the unit is below the defrost threshold, the method includes defrosting the unit or container based upon the previous defrost cycle defrost energy consumption at block  610 . During the defrost operation, temperature is monitored at blocks  611 - 612 , and if the temperature of the unit or container is above the temperature threshold, heating is disabled at block  613 , defrost energy consumption for the preceding defrost cycle is determined at block  614 , and refrigeration using liquid slugging avoidance is initiated again at block  607 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiment of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.