Abstract:
A refrigeration system includes a refrigerant vapor compression unit including a compressor and one or more current sensors to determine a total system current draw. One or more pressure sensors are positioned to determine a refrigerant pressure in the refrigerant vapor compression unit. A controller compares the current draw on the system and the refrigerant pressure to selected acceptable ranges and determines a level of refrigerant charge in the refrigerant vapor compression unit. A method of determining a level of refrigerant charge in a refrigeration system includes measuring a refrigerant pressure in the refrigeration system and sensing a current draw of the refrigeration system. A level of refrigerant charge in the refrigeration system is calculated from the refrigerant pressure and the current draw.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to refrigeration systems. More specifically, the subject matter disclosed herein relates to determination and monitoring of refrigerant levels in refrigeration systems. 
         [0002]    Fruits, vegetables and other perishable items, including meat, poultry and fish, fresh or frozen, are commonly transported in the cargo box of a truck or trailer, or in an intermodal container. Accordingly, it is customary to provide a transportation refrigeration system in operative association with the cargo box for cooling the atmosphere within the cargo box. The transport refrigeration system includes a refrigerant vapor compression system, also referred to as a transport refrigeration unit, and an on-board power unit. The refrigerant vapor compression system typically includes a compressor, a condenser, an expansion device and an evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The power unit includes an engine, typically diesel powered. 
         [0003]    The system is charged with a selected volume of refrigerant, which is held in a receiver, or holding tank, sufficient to allow for a desired amount of refrigeration over a wide range of operating temperatures. In a typical system, the receiver includes one or more sight glasses to allow an operator to visually assess the refrigerant charge level present in the receiver. When adding refrigerant in the field (when refrigerant charge in the system cannot be weighed), sight glasses are used to verify a correct amount of refrigerant added to the receiver to result in an acceptable level of refrigerant in the receiver. Sight glasses are also used to periodically determine if the system is over charged or under charged. Accuracy in determining the level of refrigerant charge via the sight glasses can be unreliable, as the method is dependent on the conditions under which the system is operating when the refrigerant level is checked. At certain operating conditions, the system can appear to be undercharged or overcharged based on the sight glass observation, but may in fact be properly charged. 
       BRIEF SUMMARY 
       [0004]    In one embodiment, a refrigeration system includes a refrigerant vapor compression unit including a compressor and one or more current sensors to determine a total system current draw. One or more pressure sensors are positioned to determine a refrigerant pressure in the refrigerant vapor compression unit. A controller compares the current draw on the system and the refrigerant pressure to selected acceptable ranges and determines a level of refrigerant charge in the refrigerant vapor compression unit. 
         [0005]    In another embodiment, a method of determining a level of refrigerant charge in a refrigeration system includes measuring a refrigerant pressure in the refrigeration system and sensing a current draw of the refrigeration system. A level of refrigerant charge in the refrigeration system is calculated from the refrigerant pressure and the current draw. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic view of an embodiment of a transport refrigeration system; 
           [0009]      FIG. 2  is a schematic view of another embodiment of a transport refrigeration system; and 
           [0010]      FIG. 3  is a schematic view of an embodiment of a receiver for a transport refrigeration system. 
       
    
    
       [0011]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
       DETAILED DESCRIPTION 
       [0012]    Referring initially to  FIGS. 1 and 2 , there are depicted exemplary embodiments of transport refrigeration systems for cooling the atmosphere within the cargo box of a truck, trailer, container, intermodal container or similar cargo transportation unit. The transportation refrigeration system  10  includes a transport refrigeration unit  12  including a compressor  14 , a refrigerant heat rejecting heat exchanger  16  (e.g., condenser or gas cooler), an expansion device  18 , a refrigerant evaporator heat exchanger  20  and a suction modulation valve  22  connected in a closed loop refrigeration circuit including refrigerant lines  24 ,  26  and  28  and arranged in a conventional refrigeration cycle. The transport refrigeration system  10  further includes an electronic system controller  30 , a diesel engine  32  and an engine controller  34 . The transport refrigeration system  10  is mounted as in conventional practice to an exterior wall of the truck, trailer or container with the compressor  14  and the heat rejecting heat exchanger  16  with its associated condenser fan(s) (not shown) and diesel engine  32  disposed externally of the refrigerated cargo box. 
         [0013]    As is conventional practice, when the transport refrigeration unit  12  is operating in a cooling mode, low temperature, low pressure refrigerant vapor is compressed by the compressor  14  to a high pressure, high temperature refrigerant vapor and passed from the discharge outlet of the compressor  14  into refrigerant line  24 . The refrigerant circulates through the refrigerant circuit via refrigerant line  24  to and through the heat exchange tube coil or tube bank of the heat rejecting heat exchanger  16 , wherein the refrigerant vapor condenses to a liquid, thence through the receiver  36 , which provides storage for excess liquid refrigerant, and thence through the subcooler coil  38  of the heat exchanger. The subcooled liquid refrigerant then passes through refrigerant line  24  through a first refrigerant pass of the refrigerant-to-refrigerant heat exchanger  40 , and thence traverses the expansion device  18  before passing through the evaporator heat exchanger  20 . In traversing the expansion device  18 , which may be an electronic expansion valve (EXV) as depicted in  FIG. 1  or a mechanical thermostatic expansion valve (TXV) as depicted in  FIG. 2 , the liquid refrigerant is expanded to a lower temperature and lower pressure prior to passing to the evaporator heat exchanger  20 . 
         [0014]    In flowing through the heat exchange tube coil or tube bank of the evaporator heat exchanger  20 , the refrigerant evaporates, and is typically superheated, as it passes in heat exchange relationship return air drawn from the cargo box passing through the airside pass of the evaporator heat exchanger  20 . The refrigerant vapor thence passes through the refrigerant line  26 , the refrigerant vapor traverses a second refrigerant pass of the refrigerant-to refrigerant heat exchanger  40  in heat exchange relationship with the liquid refrigerant passing through the first refrigerant pass thereof. Before entering the suction inlet of the compressor  14 , the refrigerant vapor passes through the suction modulation valve  22  disposed in refrigerant line  26  downstream with respect to refrigerant flow of the refrigerant-to-refrigerant heat exchanger  40  and upstream with respect to refrigerant flow of the compressor  14 . By selectively reducing the open flow area through the suction modulation valve  22 , the controller  30  can selectively restrict the flow of refrigerant vapor supplied to the compressor  14 , thereby reducing the capacity output of the transportation refrigeration unit  12  and in turn reducing the power demand imposed on the engine  32 . 
         [0015]    Air drawn from within the cargo box by the evaporator fan(s) (not shown) associated with the evaporator heat exchanger  20 , is passed over the external heat transfer surface of the heat exchange tube coil or tube bank of the evaporator heat exchanger  20  and circulated back into the interior space of the cargo box. The air drawn from the cargo box is referred to as “return air” and the air circulated back into the cargo box is referred to as “supply air”. It is to be understood that the term “air” as used herein includes mixtures of air and other gases, such as for example, but not limited to nitrogen or carbon dioxide, sometimes introduced into a refrigerated cargo box for transport of perishable product such as produce. 
         [0016]    Although the particular type of evaporator heat exchanger  20  used is not limiting of the invention, the evaporator heat exchanger  20  may, for example, comprise one or more heat exchange tube coils, as depicted in the drawing, or one or more tube banks formed of a plurality of tubes extending between respective inlet and outlet manifolds. The tubes may be round tubes or flat tubes and may be finned or un-finned. One or more pressure sensors  44  are located in the refrigeration unit  12 , and one or more current sensors  46  are located at, for example, the compressor  14 . During operation of the refrigeration system  10 , the pressure sensors  44  and the current sensors  46  monitor refrigerant pressure and/or current draw at the compressor  14 . The information collected by the pressure sensors  44  and current sensors  46  is transmitted to the controller  30 , where the controller  30  compares the information to preselected acceptable ranges of pressure and current, based on the conditions under which the refrigeration unit  12  is operating during measurement, such as open area percent of the suction modulation valve  22 . It is to be appreciated that in other embodiments, other valves, such as an electronic expansion valve (not shown) may be utilized. In some embodiments, the open area is determined by measuring air or refrigerant temperatures in the transport refrigeration system. If the refrigerant pressure and/or compressor  14  current draw is outside of the acceptable ranges, the controller  30  will trigger an alarm, such as an audible alarm or a light or message displayed at a control panel to indicate that a refrigerant charge in the refrigeration unit  12  is outside of the acceptable range, either higher or lower than desired. For example, if a current draw at the compressor  14  is higher than allowed by the selected range, it may be indicative of an excess of refrigerant charge. Similarly, if a refrigerant pressure exceeds an upper limit of the acceptable range, it may indicate an excess of refrigerant charge in the refrigeration unit  12 . 
         [0017]    The controller  30  may continuously monitor the pressure and current, or such measurements may be collected by the controller at selected time intervals, or when the refrigeration system  10  is initially started after being idle. Further, the pressure and temperature sensors may be utilized to verify a correct amount of refrigerant was added during maintenance of the refrigeration system  10 . 
         [0018]    Referring to  FIG. 3 , the pressure sensors  44  and current sensors  46  may be used in with one or more sight glasses  48  located at the receiver  36 . An operator may use the sight glass  48  to visually verify a correct level of the refrigerant in the receiver, especially that the refrigerant level or charge is not too low. 
         [0019]    While the above disclosure is related to transport refrigeration systems, one skilled in the art will recognize that elements of the present disclosure are also applicable to other vaport compression refrigeration systems. 
         [0020]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments 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.