Abstract:
Method for defrosting an evaporator in a refrigeration circuit ( 2 ) for circulating a refrigerant in a predetermined flow direction, the refrigeration circuit ( 2 ) comprising in flow direction a compressor unit ( 4 ), a heat-rejecting heat exchanger ( 6 ), an expansion device ( 12 ) and an evaporator ( 14 ), wherein the evaporator ( 14 ) comprises at least two refrigerant conduits ( 42; 44 ) and the method comprises the following steps: (a) operating the refrigeration circuit ( 2 ) in the normal cooling mode where the refrigerant exiting the heat-rejecting heat exchanger ( 6 ) flows through the expansion device ( 12 ) and through the evaporator ( 14 ) and towards the compressor unit ( 4 ); (b) terminating the cooling mode by interrupting the flow of the refrigerant exiting the heat-rejecting heat exchanger ( 6 ) into the evaporator ( 14 ); and (c) directing hot gas refrigerant through only a portion of the refrigerant conduits ( 42; 44 ) of the evaporator ( 14 ) for defrosting the evaporator ( 14 ).

Description:
FIELD 
     The present invention relates to a method for defrosting an evaporator in a refrigeration circuit for circulating a refrigerant in a predetermined flow direction, the refrigeration circuit comprising in flow direction a compressor unit, a heat-rejecting heat exchanger, an expansion device and an evaporator. The present invention further relates to a corresponding refrigeration circuit as well as an evaporator for use within such a refrigeration circuit and in combination with such method. 
     BACKGROUND 
     Icing of an evaporator in a refrigeration circuit is a common problem. Vapor from ambient air condenses and freezes on the heat exchanging surfaces of the evaporator in the conventional cooling mode and forms a continuously increasing ice layer over the time. It is known that such ice layer reduces the efficiency of the heat transfer through the evaporator resulting in loss of efficiency and increase of operational costs of the refrigeration system. 
     A conventional evaporator comprises at least one conduit for directing the refrigerant through the evaporator and typically fins for increasing the heat exchange surface of the evaporator. The conduit frequently is a serpentine tube with a plurality of passes through the evaporator and the fins are plate like elements having openings through which the individual passes or sections of the tube extend. The fins and tube sections are fixed to each other, for example by means of a force fit and provide each other the required structural stability. 
     It is conventional to remove the ice accumulation on the evaporator by way of defrosting the evaporator. A typical method for defrosting is interrupting the normal cooling operations and to defrost the evaporator. It is possible to speed up the defrosting cycle by providing heat to the evaporator. In many applications, the temperature in the environment of the evaporator is critical. If, for example, the refrigeration circuit is part of a supermarket refrigeration system, the evaporators are typically within the display cabinets and a sudden temperature increase of the nutrition within such display cabinet during defrost operation should be avoided under all circumstances. The defrost operation should, therefore, be completed within a very short time, which requires the supply of a substantial amount of heat within a short time period. On the other hand, due to space requirement and economical reasons, any additional defrost apparatus should be avoided. 
     SUMMARY 
     It is an object of the present invention to provide a method for defrosting an evaporator in a refrigeration circuit which is simple, which allows for the supply of a substantial amount of heat within a very short time, which avoids heating of the environment of the evaporator and which is not increasing the operational costs. 
     In accordance with an embodiment of the present invention a method for defrosting an evaporator in a refrigeration circuit is provided, which comprises the following steps:
         (a) operating the refrigeration circuit in the normal cooling mode where the refrigerant exiting the heat-rejecting heat exchanger flows through the expansion device and through the evaporator and towards the compressor;   (b) terminating the cooling mode by interrupting the flow of the refrigerant exiting the heat-rejecting heat exchanger into the evaporator; and   (c) directing hot gas refrigerant through only a portion of the refrigerant conduits of the evaporator for defrosting the evaporator.       

     The required heat is provided in the form of a hot gas refrigerant. It is possible to supply the hot gas refrigerant from the refrigeration circuit. Such hot gas refrigerant is directed to the evaporator in order to provide the heat for defrosting it. The hot gas refrigerant can be directed through the evaporator within a core portion thereof, i.e. a portion which is typically within the ice layer to be removed during the defrost cycle. The ice layer insulates the hot gas against the environment of the evaporator and avoids any major temperature variations. Best case, the flow of hot gas refrigerant to the evaporator is terminated once the ice is completely defrosted so that substantially no temperature increase is observed in the environment of the evaporator. 
     The hot gas used for defrosting can be directed from the exit or near the exit of the compressor unit of the refrigeration circuit. The gas leaving the compressor unit and entering the heat-rejecting heat exchanger, respectively is at high pressure and high temperature. 
     It is possible to direct the hot gas refrigerant through a refrigerant conduit of the evaporator. It is possible that the evaporator comprises two or more refrigerant conduit and it can be preferred to have refrigerant conduits of different properties, for example different strength, etc., in order to allow passing of the high temperature, high pressure gases refrigerant through the evaporator. The high pressure high temperature refrigerant will be passed through those conduits only during defrost operation which can sustain the high pressure, high temperature, etc. of the hot gas refrigerant. It is possible to pass the refrigerant exiting the heat rejection heat exchanger through all conduits, independent of their properties like strength, etc. during normal cooling mode. Thus, all the conduits within the evaporator are in use during the normal cooling mode, thus increasing the efficiency of the evaporator. It is also possible to provide all the conduits with the sufficient properties. One might also contemplate to pass the refrigerant exiting the heat-rejecting heat exchanger through only part of the conduits during normal cooling mode. 
     A sensor, or for example, a temperature sensor or the like, can be provided for sensing the icing condition of the evaporator. If a sensor is present, the method can include the steps of automatically initiating the defrost operation once a predetermined icing condition has been sensed and/or terminating the defrost operation once a predetermined defrost condition has been sensed. This allows for automatically surveying the icing condition of the evaporator and for automatically defrosting the evaporator once the system has determined the need for defrosting the evaporator. It is possible to provide a timing means for conducting defrosting operations at a particular time only, for example with supermarket refrigeration systems at night time only or at times where no or only a reduced number of customers is present. This might be preferred, since cooling requirements are typically less if no customers access the display cabinets so that undue increase of the temperature of the nutrition in the display cabinet during defrost mode is avoided. Such a timing of the defrost operation might further be advantageous in case of very high pressure of the hot gas refrigerant, for example with CO 2  refrigeration circuits. With such systems, the high pressure hot gas refrigerant in the customers area of a supermarket is sometimes regarded a risk which should be avoided. In such a situation, the flow of the hot gas refrigerant to the evaporator can be blocked outside the customers area of the supermarket, for example in the machine room of the refrigeration circuit and particularly next to the compressor unit itself. After termination of the defrost operation, the high pressure hot gas refrigerant in the defrost line can be drained, for example to any particular location in the refrigeration system. Accordingly, during opening hours of the supermarket no high pressure is present in the customers area. 
     The hot gas refrigerant exiting the evaporator during the defrost operation can be drained or returned to the liquid feed line of the refrigeration circuit. 
     In preparation of the defrosting operation, in particularly just in advance of letting the hot gas refrigerant into and through the evaporator, it might be advantageous to provide a step of evacuating the evaporator subsequent to the interruption of the normal flow of the refrigerant exiting the heat-rejecting heat exchanger to the evaporator. The evacuation of the evaporator can be performed by the compressor unit. Once the evacuation has been completed, the connection to the compressor unit can be closed and the compressor unit might even be shot down. The compressor unit may also be disconnected from the evaporator if no evacuation of the evaporator is performed. Also in this case, the compressor unit can be shut down. Alternatively, the compressor unit can continue to work, for example in case the defrost operation is performed only for a single or some evaporator out of a plurality of evaporators at a time. 
     After the defrost operation, the flow of hot gas refrigerant to the evaporator can be shut down. It is possible to evacuate the evaporator subsequent to terminating the flow of the hot gas refrigerant, before returning to the normal operation, i.e. in advance of letting refrigerant exiting the heat-rejecting heat exchanger flow through the evaporator. 
     The present invention further relates to a refrigeration circuit for circulating a refrigerant in a predetermined flow direction, comprising in flow direction a compressor unit, a heat-rejecting heat exchanger, an expansion device and an evaporator, wherein the refrigeration circuit further comprises a hot gas line leading to the evaporator and a defroster valve positioned in the hot gas line. The hot gas line can extend from an exit of the compressor unit to an entrance of the evaporator. The hot gas line may also extend from many other source of hot gas refrigerant to an entrance of the evaporator. The hot gas line may be connected to only one or only part of the evaporator&#39;s refrigerant conduits. It is possible to have the individual refrigerant conduits within the evaporator physically completely separate from each other. If there is a connection between the refrigerant conduits of the evaporator, a valve can be provided in such connection line or bridge line. The valve can be arranged, either physically or electronically, etc., with the defroster valve so that merely one of the defroster valve and such valve can be opened at a time. 
     An entrance bridge line can be provided connecting the entrances of two or more refrigerant conduits and comprising an entrance valve. There can be an exit bridge line connecting the exits of the two refrigerant conduits and comprising an exit valve. 
     The refrigeration circuit can be used for industrial cooling systems, supermarket refrigeration systems, etc. The refrigeration circuit can provide cooling at different temperature levels, like low temperature cooling for display cabinets for frozen food, medium temperature cooling for fish, milk products, etc. The hot gas for defrosting for example the low temperature circuit can be derived from the medium temperature circuit and vice versa. It is also possible to return the refrigerant after defrosting to the respectively other circuit. 
     The present invention further relates to an evaporator for a refrigeration circuit in accordance with any embodiment of the invention comprising two refrigerant conduits with one thereof being of higher strength than the other refrigerant conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the present invention are described in greater detail below with reference to the figures, wherein: 
         FIG. 1  shows a refrigeration circuit in accordance with the present invention; 
         FIG. 2  shows an evaporator in accordance with the present invention with its associated piping and valves in the normal cooling mode; 
         FIG. 3  shows the evaporator of  FIG. 2  in an interim mode between normal cooling mode and defrosting mode; 
         FIG. 4  shows the evaporator of  FIG. 2  in the defrosting mode; 
         FIG. 5  shows the evaporator of  FIG. 2  in an interim mode between defrost mode and normal cooling mode; 
         FIG. 6  shows an evaporator in accordance with the present invention with different piping; and 
         FIG. 7  shows an evaporator similar to that of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a refrigeration circuit  2  for circulating a refrigerant in a predetermined flow direction. The refrigeration circuit  2  comprises in flow direction a compressor unit  4 , a heat-rejecting heat exchanger  6 , a receiver  8 , at least one refrigeration consumer  10  comprising an expansion device  12  and an evaporator  14 . 
     The compressor unit can comprise one or a plurality of compressors  16  connected serially or in parallel with each other. 
     The heat-rejecting heat exchanger  6  can be a condenser if a conventional refrigerant is used. In case a “super critical” refrigerant, like CO 2 , etc., is used, i.e. if the refrigeration circuit  2  is operated in the super critical mode at least under particular operational conditions, the heat-rejecting heat exchanger  6  is of the type as termed a gascooler. 
     The receiver or liquid/fluid separator  8  receives the refrigerant exiting the heat-rejecting heat exchanger  6 . Liquid refrigerant collects in the lower portion  18  of the receiver  8  with gaseous refrigerant being present in the upper portion of the receiver  8 . A flash gas line  20  connects the upper portion of the receiver  8  with the compressor unit  4  and particularly a separate compressor  22  in case of the present embodiment. The separate compressor  22  can be controlled independently so that the step of compressing the flash gas can be optimized, particularly in respect of economic operation. 
     A high pressure line  24  connects the outlet  26  of the compressor unit  4  with the inlet  28  of the receiver  8 . In a typical application of the refrigeration circuit  2  in a supermarket refrigeration system for medium temperature cooling, i.e. where the refrigeration consumers  10  cool display cabinets for meat, milk products, fish, etc. to a temperature of slightly above 0° C., the pressure in the high pressure line  24  can be up to 120 bar and is typically approximately 85 bar in “summer mode” and approximately 45 bar in “winter mode”. The temperature of the refrigerant in the high pressure line  24  is approximately 120° C. 
     In the heat-rejecting heat exchanger, the temperature of the refrigerant is typically reduced to approximately 35° C., while the pressure of the refrigerant remains substantially unchanged. A high pressure connection line  30  connects the output  32  of the heat-rejecting heat exchanger  6  with the inlet  34  of the receiver  8 . An intermediate expansion device  36  is located in the high pressure connection line  30 . in the above example of medium temperature cooling the intermediate expansion device  36  reduces the pressure to between 30 and 40 bar and preferably  36  bar with such intermediate pressure being typically independent from “winter mode” and “summer mode”. A corresponding temperature subsequent to the intermediate expansion device  36  is approximately 0 to 5° C. 
     A liquid line  38  connects the liquid portion  18  of the receiver  8  with the refrigeration consumers  10 . An expansion device  12  of the refrigeration consumer  10  can reduce the pressure to typically between 20 and 30 bar and approximately 26 bar which results in a temperature of approximately −10° C. in the evaporator  14 . The refrigerant exiting the evaporator  14  is directed via suction line  40  to the compressor unit  4 . 
     As the evaporator  14  of each refrigeration consumer  10  is in contact with ambient air, it typically comprises surface extending means likes fins, etc. The contact with the ambient air during operation results in freezing of water from ambient air to the heat exchanger surfaces of the evaporator  14  with a resultant accumulation of ice over such surfaces. This icing of the evaporator results in a substantial drop of efficiency. For deicing purposes, the present invention provides for at least two refrigerant conduits  42 ,  44  in the evaporator, a hot gas refrigerant line  46  for supplying hot gases refrigerant for defrosting purposes and a defrost return line  48  for returning the refrigerant to the main portion of the refrigeration circuit  2 . 
     The piping of the evaporator  14  in the refrigeration circuit  2  is described with respect to  FIG. 2 . A defroster valve  50  is located in the hot gas line. A liquid feed valve  52  is position in the liquid line  38 , preferably in advance of the expansion device  12  in flow direction. The expansion device  12  is preferably a controllable expansion device in order to control the temperature and the refrigeration capacity, respectively of the evaporator. The liquid feed valve  52  and the expansion device  12  can be combined with each other or integrated with each other. 
     An entrance bridge line  54  connects the hot gas line  46  with the liquid line  38  and the different refrigerant conduits  42  and  44 , respectively, with each other. Similar, an exit bridge line  56  connects the suction line  40  with the return line  48  and the refrigerant conduits  42  and  44 , respectively, with each other. An entrance valve  58  can be present in the entrance bridge line  54  and an exit valve  60  can be located in the exit bridge line  56 . A return valve  62  can be located in the return line  48 . 
     The refrigerant conduits  42 ,  44  are of different characteristics. Particularly, the hot gas refrigerant conduit  44  has characteristics allowing to direct the hot pressure high temperature hot gas therethrough. Thus, the refrigerant conduit  44  is preferably of higher strength then the refrigerant conduit  42 , preferably having a higher wall thickness than the refrigerant conduit  42 . The refrigerant conduit  44  can also be made from a material with good thermal properties, allowing the contact with the hot gas and further for accommodating for the high temperature differences during the defrost operation. 
     The hot gas refrigerant conduit  44  and the refrigerant conduit  42  can be routed through the evaporator  14  in several passes with return portions  64  so that each refrigerant conduit  42 ,  44 , which preferably includes a plurality of tubes, goes back and forth through the evaporator  14 . Connected to the refrigerant  42 ,  44  are fins  66  as it is well-known in the art. 
     The arrangement of the hot gas refrigerant conduits  44  and the refrigerant conduits  42  within the evaporator  14  can be optimized for the particular application. Preferably, the distribution of the hot gas refrigerant conduit  44  within the evaporator  14  is such that the defrost operation can be performed evenly over the evaporator so that the defrost operation is completed at any place within the evaporator at approximately the same time. 
     A sensor  68  can be provided for sensing the icing condition of the evaporator. The sensor  68  can be a conventional temperature sensor, for example a thermal couple, etc. Any other types of sensors, for example optical sensors, physical sensors, etc. can be used for sensing the icing condition. The sensor information can be provided to a controller (not shown) which controls the defrost operation. The control may start the defrost mode once a certain time since the last defrost cycle has elapsed. Alternatively, the sensor also provides the information for starting the defrost mode. The control may alternatively stop the defrost operation after a certain predetermined time has elapsed. Alternatively, the control may stop the defrost cycle once the sensor signals a sufficient deicing condition. In case of a temperature sensor, a sufficient deicing condition can be stipulated if the temperature next to a heat exchanging surface of the evaporator  14  clearly exceeds the melting point, preferably at a temperature of between 5 and 20° C. and preferably a temperature of approximately 10 to 15° C. 
     As can be seen in  FIG. 1 , the hot gas line  46  can be connected to the exit  26  of the compressor unit  4 . The hot gas valve  50  can preferably be next to the compressor unit  4  so that not losses occur if no defrost cycle is running. A return line  48  preferably connects to the liquid line  38  but also can connect to the receiver  8 , etc. It is preferred to have a corresponding defrost system for each of the refrigeration consumers  10 . An individual defrost system can be provided for each of the refrigeration consumers  10 . It is, however, preferred to have a single hot gas line  46  and preferably also a single return line  48  connecting to the defrost systems of the respective refrigeration consumers  10 . Preferably, the defrost operation for each individual refrigeration consumer  10  can be performed independently from the other refrigeration consumers  10  so that only one or limited number of refrigeration consumers is defrosted at a time. To this effect, the hot gas line  46  and possibly also the return line  48  can provide respective branch lines leading to individual refrigeration consumers. Valves can be provided in the individual branch lines for connecting and disconnecting to the respective refrigeration consumer. A respective main hot gas valve and/or a respective main return valve can be provided for disconnecting the defrost system from all the refrigeration consumers  10 . 
     With respect to  FIG. 2 to 5  a method for defrosting the evaporator  14  is disclosed. In  FIG. 2  the operation in the normal cooling mode is shown. Particularly, as represented by the “X” within the valve, the hot gas valve  50  in line  46  is closed, while the liquid feed valve  52  in the liquid line  38  is open, as indicated by the line  38  leading through valve  52 . Thus, liquid reactant flows through the expansion device  52  and entrance bridge line  54  via the open entrance valve  58  into both refrigerant conduits  42 ,  44  and subsequently through exit bridge line  56  and the open exit valve  60  through suction line  40  to the compressor unit  4 . In course of switching over to defrost mode, liquid feed valve  52  and entrance valve  58  are closed as shown in  FIG. 3 . Vapor from both refrigerant conduits  42 ,  44  is sucked by the compressor unit  4  for a predetermined time. Subsequently, valve  60  is closed, thus isolating the refrigerant conduit  42  and the hot gas conduit  44  from each other. Thereafter, hot gas valve  50  and return valve  62  are opened. High pressure hot gas now enters the hot gas refrigerant conduit  44  and rapid defrost of the evaporator fins  66  begins ( FIG. 4 ). 
     At the end of the defrost cycle ( FIG. 5 ) which could be sensed in various conventional methods, for example by means of sensor  68 , hot gas valve  50  and return valve  48  are closed. Subsequently, exit valve  60  is opened to quickly reduce pressure in the hot gas refrigerant conduit  44 . 
     Then ( FIG. 2 ) liquid feed valve  52  and entrance valve  58  are opened to return to the conventional cooling mode. 
     The above referenced method and piping allows for using all the refrigerant conduits  42 ,  44  during normal cooling mode. The respective valves are either by means of the control or physically arranged so that the hot gas line  46  is connectable only to the hot gas refrigerant conduit  44 , but not to the refrigerant conduit  42 . 
     The embodiment of  FIG. 6  corresponds by and large to the embodiment as disclosed with respect to  FIG. 1 to 5 . The hot gas refrigerant conduit  44  and the refrigerant conduit  42  are, however, not connectable with each other. Correspondingly, the hot gas refrigerant conduit  44  serves for defrost purposes only but is not in use during conventional cooling operation. 
     The embodiment of  FIG. 7  is very similar to that of  FIG. 2 . The main difference resides in the fact that the entrance valve  58  is positioned in advance of the expansion devices  12  and  13  in flow direction. The advantage of such a construction is that a single-phase liquid refrigerant is always present at the entrance valve  58  in the embodiment of  FIG. 7 . In the embodiment of  FIG. 2 to 5 , also a two-phase refrigerant flow can be present at the entrance valve  58 . This requires high quality valves in order to avoid erosion of the valve with two-phase flow and resultant loss in sealing capability. The embodiment of  FIG. 7  has two separate expansion valves  12 ,  13  for low-pressure section and high-pressure section respectively and the entrance valve  58  is on the liquid line  38 . A skilled person will understand that the operation of the embodiment of  FIG. 7  is similar to that as disclosed in  FIG. 2 to 5 .