Patent Publication Number: US-11022346-B2

Title: Method for detecting a loss of refrigerant charge of a refrigeration system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application of PCT/US2016/062458, filed Nov. 17, 2016, which claims the benefit of U.S. Provisional Application No. 62/256,557, filed Nov. 17, 2015, both of which are incorporated by reference in their entirety herein. 
    
    
     BACKGROUND 
     The present disclosure relates to refrigeration systems and, more particularly, to a method of detecting a loss of refrigerant charge. 
     In a typical refrigeration system, a refrigerant flows through a compressor and exits at a high pressure. The pressurized refrigerant may then flow through a condenser where the refrigerant may condense from a vapor and into a liquid, thus dispensing heat. From the condenser, the refrigerant in liquid form flows through an expansion valve where it experiences a pressure drop. From the expansion valve the refrigerant flows through an evaporator where it draws heat from the evaporator and returns to a vapor form. 
     Different types of refrigeration systems may utilize different refrigerants and operate at different pressures. One type of system is a trancritical refrigeration system that may use CO2 as a refrigerant. Such systems typically operate at high pressures which may range from 1000 psia to 1800 psia. Unfortunately, the higher the operating pressure the higher may be the risk of a refrigerant leak. Moreover, all refrigeration systems are sensitive toward loss of refrigerant charge and may lose operating efficiency or cease operating altogether. Improvements in the detection of such a refrigerant charge loss is desirable. 
     SUMMARY 
     A method of determining charge loss of a refrigeration system including inputting a supply/return air temperature, ambient temperature, a box temperature, and a compressor speed into an electronic controller of the refrigeration system; calculating a real-time air side temperature difference across an evaporator; calculating a first air side temperature difference across the evaporator by applying an algorithm having a first T-Map representative of normal operating conditions; confirming a detection prerequisite is satisfied; calculating a second air side temperature difference across the evaporator by applying the algorithm having a second T-Map representative of a loss of refrigerant charge; taking an action if the real-time air side temperature difference is less than the first air side temperature difference; and taking an action if the real-time air side temperature difference is less than the second air side temperature difference. 
     Additionally to the foregoing embodiment, the method includes inputting an evaporator multi-speed fan speed. 
     In the alternative or additionally thereto, in the forgoing embodiment, the algorithm applies a polynomial. 
     In the alternative or additionally thereto, in the forgoing embodiment, the first and second T-Maps are pre-programmed into the controller and provide a curve fit of a plurality of constants versus compressor speed. 
     In the alternative or additionally thereto, in the forgoing embodiment, the plurality of constants are six constants applied to ambient temperature and box temperature variables as part of the polynomial. 
     In the alternative or additionally thereto, in the forgoing embodiment, the detection prerequisite is a measured compressor speed being greater than a predefined compressor speed. 
     In the alternative or additionally thereto, in the forgoing embodiment, the detection prerequisite is the first air side temperature difference being greater than a predefined temperature difference. 
     In the alternative or additionally thereto, in the forgoing embodiment, the detection prerequisite is that the first air side temperature difference is determined after a predefined time span from initial system startup and initial pulldown. 
     In the alternative or additionally thereto, in the forgoing embodiment, the detection prerequisite is one of a plurality of detection prerequisites and at least includes a measured compressor speed being greater than a predefined compressor speed, the first air side temperature difference being greater than a predefined temperature difference, and the first air side temperature difference is determined after a predefined time span from initial system startup and initial pulldown. 
     In the alternative or additionally thereto, in the forgoing embodiment, the first and second T-Maps are representative of evaporator air side temperature difference versus ambient temperature, box temperature, compressor speed and refrigerant charge. 
     In the alternative or additionally thereto, in the forgoing embodiment, the refrigeration system is a transcritical refrigeration system. 
     In the alternative or additionally thereto, in the forgoing embodiment, the method includes inputting an evaporator variable speed fan speed. 
     A refrigeration system according to another, non-limiting, embodiment includes an electronic controller including, pre-programmed first and second T-Maps both representative of evaporator air side temperature difference versus ambient temperature, box temperature, compressor speed and refrigerant charge operating conditions, and wherein the first T-Map is representative of normal operating conditions and the second T-Map is representative of a loss of refrigerant charge, and pre-programmed prerequisites configured to be met prior to initiating an action based on a loss of refrigerant charge; and wherein the electronic controller is configured to calculate first and second evaporator air side temperatures based on the respective first and second T-maps and initiates an action if the first air side temperature difference is less than the second air side temperature difference. 
     Additionally to the foregoing embodiment, the refrigeration system is a transcritical refrigeration system. 
     In the alternative or additionally thereto, in the forgoing embodiment, the refrigerant is CO2. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. However, it should be understood that the following description and drawings are intended to be exemplary in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiments. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG. 1  is a perspective view of a refrigerated container utilizing a transport refrigeration unit as one, non-limiting, exemplary embodiment of the present disclosure; 
         FIG. 2  is a schematic of a refrigeration system of the transport refrigeration unit; 
         FIG. 3  is a table of T-Map Normal and T-Map Charge Loss data; and 
         FIG. 4  is a flow chart of a method of determining charge loss of the refrigeration system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an exemplary embodiment of a refrigerated container  10  having a temperature controlled cargo space  12  the atmosphere of which is refrigerated by operation of a transport refrigeration unit  14  associated with the cargo space  12 . In the depicted embodiment of the refrigerated container  10 , the transport refrigeration unit  14  is mounted in a wall of the refrigerated container  10 , typically in the front wall  18  in conventional practice. However, the refrigeration unit  14  may be mounted in the roof, floor or other walls of the refrigerated container  10 . Additionally, the refrigerated container  10  has at least one access door  16  through which perishable goods, such as, for example, fresh or frozen food products, may be loaded into and removed from the cargo space  12  of the refrigerated container  10 . 
     Referring now to  FIG. 2 , there is depicted schematically an embodiment of a refrigeration system  20  suitable for use in the transport refrigeration unit  14  for refrigerating air drawn from and supplied back to the temperature controlled cargo space  12 . Although the refrigeration system  20  will be described herein in connection with a refrigerated container  10  of the type commonly used for transporting perishable goods by ship, by rail, by land or intermodally, it is to be understood that the refrigeration system  20  may also be used in transport refrigeration units for refrigerating the cargo space of a truck, a trailer or the like for transporting perishable fresh or frozen goods. The refrigeration system  20  is also suitable for use in conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. The refrigeration system  20  could also be employed in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable and frozen product storage areas in commercial establishments. 
     The refrigeration system  20  may include a compressor  30  that may be multi-stage, a heat rejector  40  that may be a heat exchanger that rejects heat, a flash tank  60 , an evaporator  50  that may be a heat exchanger that absorbs refrigerant heat, and refrigerant lines  22 ,  24  and  26  connecting the aforementioned components in serial refrigerant flow order in a primary refrigerant circuit. A high pressure expansion device (HPXV)  45 , such as for example an electronic expansion valve, is disposed in refrigerant line  24  upstream of the flash tank  60  and downstream of the heat rejector  40 . An evaporator expansion device (EVXV)  55 , such as for example an electronic expansion valve, operatively associated with the evaporator  50 , is disposed in refrigerant line  24  downstream of the flash tank  60  and upstream of the evaporator  50 . 
     The compressor  30  functions to compress the refrigerant and to circulate refrigerant through the primary refrigerant circuit, and may be a single, multiple-stage refrigerant compressor (e.g., a reciprocating compressor or a scroll compressor) having a first compression stage  30   a  and a second stage  30   b , wherein the refrigerant discharging from the first compression stage  30   a  passes to the second compression stage  30   b  for further compression. Alternatively, the compressor  30  may comprise a pair of individual compressors, one of which constitutes the first compression stage  30   a  and other of which constitutes the second compression stage  30   b , connected in series refrigerant flow relationship in the primary refrigerant circuit via a refrigerant line connecting the discharge outlet port of the compressor constituting the first compression stage  30   a  in refrigerant flow communication with the suction inlet port of the compressor constituting the second compression stage  30   b  for further compression. In a two compressor embodiment, the compressors may be scroll compressors, screw compressors, reciprocating compressors, rotary compressors or any other type of compressor or a combination of any such compressors. In both embodiments, in the first compression stage  30   a , the refrigerant vapor is compressed from a lower pressure to an intermediate pressure and in the second compression stage  30   b , the refrigerant vapor is compressed from an intermediate pressure to higher pressure. 
     The compressor  30  may be driven by a variable speed motor  32  powered by electric current delivered through a variable frequency drive  34 . The electric current may be supplied to the variable speed drive  34  from an external power source (not shown), such as for example a ship board power plant, or from a fuel-powered engine drawn generator unit, such as a diesel engine driven generator set, attached to the front of the container. The speed of the variable speed compressor  30  may be varied by varying the frequency of the current output by the variable frequency drive  34  to the compressor drive motor  32 . It is to be understood, however, that the compressor  30  could in other embodiments comprise a fixed speed compressor. 
     The heat rejector  40  may comprise a finned tube heat exchanger  42  through which hot, high pressure refrigerant discharged from the second compression stage  30   b  (i.e. the final compression charge) passes in heat exchange relationship with a secondary fluid, most commonly ambient air drawn through the heat exchanger  42  by the fan(s)  44 . The finned tube heat exchanger  42  may comprise, for example, a fin and round tube heat exchange coil or a fin and flat mini-channel tube heat exchanger. In the depicted embodiment, a variable speed motor  46  powered by a variable frequency drive  48  drives the fan(s)  44  associated with the heat rejection heat exchanger  40 . 
     When the refrigeration system  20  operates in a transcritical cycle, the pressure of the refrigerant discharging from the second compression stage  30   b  and passing through the heat rejector  40 , referred to herein as the high side pressure, exceeds the critical point of the refrigerant, and the heat rejector  40  functions as a gas cooler. However, it should be understood that if the refrigeration system  20  operates solely in the subcritical cycle, the pressure of the refrigerant discharging from the compressor and passing through the heat rejector  40  is below the critical point of the refrigerant, and the heat rejector  40  functions as a condenser. As the method of operation disclosed herein pertains to operation of the refrigeration system  20  in a transcritical cycle, the heat rejector will also be referred to herein as gas cooler  40 . 
     The evaporator  50  may also comprise a finned tube coil heat exchanger  52 , such as a fin and round tube heat exchanger or a fin and flat, mini-channel tube heat exchanger. Whether the refrigeration system is operating in a transcritical cycle or a subcritical cycle, the evaporator  50  functions as a refrigerant evaporator. Before entering the evaporator  50 , the refrigerant passing through refrigerant line  24  traverses the evaporator expansion valve  55 , such as, for example, an electronic expansion valve or a thermostatic expansion valve, and expands to a lower pressure and a lower temperature to enter heat exchanger  52 . As the liquid refrigerant traverses the heat exchanger  52 , the liquid refrigerant passes in heat exchange relationship with a heating fluid whereby the liquid refrigerant is evaporated and typically superheated to a desired degree. The low pressure vapor refrigerant leaving heat exchanger  52  passes through refrigerant line  26  to the suction inlet of the first compression stage  30   a . The heating fluid may be air drawn by an associated fan(s)  54  from a climate controlled environment, such as a perishable/frozen cargo storage zone associated with a transport refrigeration unit, or a food display or storage area of a commercial establishment, or a building comfort zone associated with an air conditioning system, to be cooled, and generally also dehumidified, and thence returned to a climate controlled environment. 
     The flash tank  60 , which is disposed in refrigerant line  24  between the gas cooler  40  and the evaporator  50 , upstream of the evaporator expansion valve  55  and downstream of the high pressure expansion valve  45 , functions as an economizer and a receiver. The flash tank  60  defines a chamber  62  into which expanded refrigerant having traversed the high pressure expansion device  45  enters and separates into a liquid refrigerant portion and a vapor refrigerant portion. The liquid refrigerant collects in the chamber  62  and is metered therefrom through the downstream leg of refrigerant line  24  by the evaporator expansion valve  55  to flow through the evaporator  50 . 
     The vapor refrigerant collects in the chamber  62  above the liquid refrigerant and may pass therefrom through economizer vapor line  64  for injection of refrigerant vapor into an intermediate stage of the compression process. An economizer flow control device or valve  65 , such as, for example, a solenoid valve (ESV) having an open position and a closed position, is interposed in the economizer vapor line  64 . When the refrigeration system  20  is operating in an economized mode, the economizer flow control device  65  is opened thereby allowing refrigerant vapor to pass through the economizer vapor line  64  from the flash tank  60  into an intermediate stage of the compression process. When the refrigeration system  20  is operating in a standard, non-economized mode, the economizer flow control device  65  is closed thereby preventing refrigerant vapor to pass through the economizer vapor line  64  from the flash tank  60  into an intermediate stage of the compression process. 
     In an embodiment where the compressor  30  has two compressors connected in serial flow relationship by a refrigerant line, one being a first compression stage  30   a  and the other being a second compression stage  30   b , the vapor injection line  64  communicates with refrigerant line interconnecting the outlet of the first compression stage  30   a  to the inlet of the second compression stage  30   b . In an embodiment where the compressor  30  comprises a single compressor having a first compression stage  30   a  feeding a second compression stage  30   b , the refrigerant vapor injection line  64  may open directly into an intermediate stage of the compression process through a dedicated port opening into the compression chamber. 
     The refrigeration system  20  also includes a controller  100  operatively associated with the plurality of flow control valves  45 ,  55  and  65  interdisposed in various refrigerant lines as previously described. As in conventional practice, in addition to monitoring ambient air temperature (T amb ), supply box air (T SBAIR ), and return box air (T RBAIR ), the controller  100  also monitors various pressures and temperatures and operating parameters by means of various sensors operatively associated with the controller  100  and disposed at selected locations throughout the refrigeration system  20 . For example, a pressure sensor  102  may be disposed in association with the compressor  30  for measuring pressure discharge (P d ), or may be disposed in association with the gas cooler  40  to sense the pressure of the refrigerant at the outlet of the heat exchanger coil  42  of the gas cooler  40 , which pressure is equivalent to (P d ); a temperature sensor  104  may be disposed in association with the gas cooler  40  to measure the temperature (T gc ) of the refrigerant leaving the heat exchange coil  42  of the gas cooler  40 ; a temperature sensor  106  may be disposed in association with the evaporator  50  to sense the temperature (T EVAPout ) of the refrigerant leaving the heat exchanger  52  of the evaporator  50 ; and a pressure sensor  108  may be disposed in association with the suction inlet of the first compression stage  30   a  to sense the pressure (P s ) of the refrigerant feeding to the first compression stage  30   a . The pressure sensors  102  and  108  may be conventional pressure sensors, such as for example, pressure transducers, and the temperature sensors  104  and  106  may be conventional temperature sensors, such as for example, thermocouples or thermistors. 
     The term “controller” as used herein refers to any method or system for controlling and should be understood to encompass microprocessors, microcontrollers, programmed digital signal processors, integrated circuits, computer hardware, computer software, electrical circuits, application specific integrated circuits, programmable logic devices, programmable gate arrays, programmable array logic, personal computers, chips, and any other combination of discrete analog, digital, or programmable components, or other devices capable of providing processing functions. 
     The controller  100  is configured to control operation of the refrigeration system  20  in various operational modes, including several capacity modes. A capacity mode is a system operating mode wherein a refrigeration load is imposed on the system requiring the compressor to run in a loaded condition to meet the cooling demand. In an unloaded mode, the cooling demand imposed upon the system is so low that sufficient cooling capacity may be generated to meet the cooling demand with the compressor  30  running in an unloaded condition. The controller  100  is also configured to control the variable speed drive  34  to vary the frequency of electric current delivered to the compressor drive motor so as to vary the speed of the compressor  30  in response to capacity demand. 
     As noted previously, in transport refrigeration applicants, the refrigeration system  20  must be capable of operating at high capacity to rapidly pulldown the temperature within the cargo box upon loading and must be capable of operating at extremely low capacity during maintenance of the box temperature within a very narrow band, such as for example as little as +/−0.25° C. (+/−0.45° F.), during transport. Depending upon the particular cargo being shipped, the required box air temperature may range from as low as −34.4° C. (−30° F.) up to 30° C. (86° F.). Thus, the controller  100  will selectively operate the refrigeration system in response to a cooling capacity demand, such as during initial pulldown and recovery pulldowns, in an economized perishable mode or a standard non-economized perishable mode for non-frozen perishable products, and in an economized frozen mode or a standard non-economized frozen mode for frozen products. 
     The controller  100  may also selectively operate the refrigeration system  20  in an unload mode when maintaining the box temperature in a narrow band around a set point box temperature. Typically, the box temperature is controlled indirectly through monitoring and set point control of one or both of the temperature (T SBAIR ), of the supply box air, (i.e., the air exiting the evaporator  50 ), and the temperature (T RBAIR ), of the return box air (i.e., the air entering the evaporator  50 ). 
     Although not illustrated, the refrigeration system  20  may further include an intercooler as part of the air cooler  40  and which is disposed in the primary refrigerant circuit between the discharge outlet of the first compression stage  30   a  and the inlet to the second compression stage  30   b  whereby the partially compressed (intermediate pressure) refrigerant vapor (gas) passing from the discharge outlet of the first compression stage  30   a  to the inlet to the second compression stage  30   b  passes in heat exchange relationship with a flow of cooling media, such as, for example, but not limited to the cooling air flow generated by the gas cooler fan  44 . 
     Because transcritical refrigeration systems  20  operate at high pressures often ranging from about 1000 psia to 1800 psia for significant amounts of time, the risk of refrigerant leakage may be higher than low pressure refrigeration systems. A loss of refrigerant may cause a loss of cooling which could increase the risk of cargo damage. The present disclosure provides a method to detect a loss of charge (i.e., refrigerant leakage) before the refrigeration system suffers significant cooling loss, thus providing time to correct the condition before damage to cargo results. 
     A real-time air side temperature difference (dT a ) (i.e., T RBAIR −T SBAIR ) across the evaporator  50  may be determined by several variables and parameters as set forth below and regardless of system operating mode:
 
 dT   a   =f ( T   amb   ,T   box ,rpm_comp,rpm_evapfan, M charge)  (1)
 
     Where (T amb ) is the ambient temperature, (T box ) is cargo box temperature, (rpm_comp) is the compressor speed, (rpm_evapfan) is the evaporator fan speed, (M charge ) is refrigerant charge. 
     The air side temperature difference (dT a ) may thus generally be expressed as a function of the ambient temperature (T amb ), the box temperature (T box ), the compressor speed (rpm_comp), evaporator fan speed (rpm_evapfan), and the refrigerant charge (M charge ). Because of difficulties, time and expense in establishing an equation form through purely theoretical analysis, a method curve fit may be applied. A number of simulation runs make it possible to employ more efficient theoretical mathematical models to do such an optimization, compared to realizing the equation form purely by means of extensive experimental tests. 
     The model is then run at various conditions selected so as to cover the typical operation range of the refrigeration product. By running at the prescribed conditions, the air side temperature difference (dT a ) as well as the ambient temperature (T amb ), the box temperature (T box ), the compressor speed (rpm), evaporator fan speed (rpm_evapfan), and refrigerant charge (M charge ) can be determined for each condition. When all conditions are completed, a map (i.e., T-Map) of air side temperature difference versus ambient temperature, box temperature, compressor speed, evaporator fan speed and refrigerant charge may be created. A curve-fit may then be established based on the map to obtain a correlation of the air side temperature difference. Such a correlation may be a second order polynomial equation. 
     For example, two T-Maps may be generated, see  FIG. 3 . The first T-Map may be representative of normal refrigeration system  20  operation (T-Map Normal). The second T-Map may be representative of a loss of charge condition (T-Map Charge Loss). For both conditions, a second order polynomial equation may be sufficiently accurate to estimate the air side temperature difference (dT a ) at each compressor speed correction (from minimal frequency to maximal frequency), then air side temperature difference (dT a ) at any other speeds may be obtained through interpolation. The second order polynomial equation may be:
 
 dT   a   =CF   evapfan *[ a   0   +a   1 ( T   amb )+ a   2 ( T   box )+ a   3 ( T   amb ) 2   +a   4 ( T   box ) 2   +a   5 ( T   amb   ×T   box )]  (2)
 
     Where (CF evapfan ) is the correction factor based on the evaporator fan speed, is the function of evaporator fan speed ratio (Evaporator Fan Speed/Maximal Evaporator Fan Speed). Where a 0 , a 1 , a 2 , a 4 , a 5  are constants. 
     To establish an acceptable level of confidence, conditions under which a loss of charge can be detected may be established, thus avoiding false detections. Generally, a loss of charge may be detected with a higher level of confidence during high capacity operation conditions of the refrigeration system  20 , rather than low operation conditions. Moreover, simulations have shown that T-Map prediction has higher accuracy in high capacity operation as well. Thus to define the detection time window when the loss of charge detection is triggered a few rules may be established. Such rules may include: 
     a) Compressor speed or VFD: Higher compressor speed represents higher cooling capacity. To trigger on the loss of charge detection, the compressor  30  speed may need to be larger than a predefined speed. 
     b) Air side temperature difference under normal charge condition (i.e., T-Map Normal): To trigger on the loss of charge detection, the air side temperature difference calculated by T-Map Normal should be more than a predefined value. 
     c) Time Pulldown is run: The T-Map functions are curve-fit based steady state simulation results, thus are not applicable or inaccurate for startup and initial pulldown period when system is operated under high dynamics. The loss of charge detection should be started on certain time after startup and initial pulldown. 
     Referring to  FIG. 4 , a loss of charge detection algorithm may be preprogrammed into the controller  100  utilizing the T-Maps as previously discussed. For example, a loss of charge detection method may include the controller  100  receiving the measured variables such as: the box temperature (T box ), the compressor speed (rpm), evaporator fan speed (rpm_evapfan), and refrigerant charge (M charge ), charge), as step  200 . For step  202 , the controller  100  may calculate a air side temperature difference (dT a ) based on measured supply/return air temperature, As step  204 , the controller may check if detection prerequisites are satisfied. If “No,” the method returns to step  200 , if “Yes,” the method advances to step  206 . As step  206 , the controller calculate the first air side temperature difference (dT 1 ) based on pre-programed T-Map Normal and equation (1). As step  208 , the controller  100  compares the measured air side temperature difference (dT) and the first calculated air side temperature difference (dT 1 ). If the measured air side temperature difference is not less than the first air side temperature difference multiply a correction factor k, for example 0.9, the method returns to step  200 . Otherwise the method moves to step  210  to trigger a charge check alarm. As step  212 , the controller calculates a second air side temperature difference based on a pre-programed T-Map Charge Loss and equation (1). As step  214 , the controller  100  compares the measured air side temperature difference and second air side temperature differences. If the measured air side temperature difference is not less than the second air side temperature difference the method returns to step  212 . If the measured air side temperature difference is less than the second air side temperature difference the method advance to step  216 . As step  216 , the controller  100  may initiate an alarm signifying a loss of charge. 
     While the present disclosure is 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 without departing from the spirit and scope of the present disclosure. In addition, various modifications may be applied to adapt the teachings of the present disclosure to particular situations, applications, and/or materials, without departing from the essential scope thereof. The present disclosure is thus not limited to the particular examples disclosed herein, but includes all embodiments falling within the scope of the appended claims.