Abstract:
A refrigeration defrost system including at least one frosted evaporator having an evaporator refrigerant vapor line and an evaporator refrigerant liquid line, comprises a first compressor having a suction inlet line and a discharge outlet line each connected to a discharge manifold, the discharge outlet being connected to said evaporator refrigerant vapor line. A first pressure regulator valve disposed in a refrigerant bypass passageway between the discharge manifold and the suction inlet line, feeds refrigerant vapor, when a defrost cycle is required, from the discharge manifold into the suction inlet line. The refrigerant vapor is fed from the first compressor into the discharge outlet line and into the frosted evaporator through the evaporator refrigerant vapor line, thereby defrosting said frosted evaporator. Also disclosed is a method of defrosting a frosted evaporator using a single, dedicated defrost compressor.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention concerns refrigeration systems, more particularly refrigeration defrost systems for defrosting a frosted evaporator.  
         BACKGROUND OF THE INVENTION  
         [0002]    Refrigeration systems are well known and widely used in supermarkets and warehouses to refrigerate, or maintain in a frozen state, perishable items, such as foodstuff.  
           [0003]    Conventionally, refrigeration systems include a network of refrigeration compressors and evaporators. Refrigeration compressors mechanically compress refrigerant vapors, which are fed from the evaporators, to increase their temperature and pressure. High temperature refrigerant vapors, under high-pressure, are fed to an outdoor air-cooled refrigerant condenser whereupon air, at ambient temperature, absorbs the latent heat from the vapors, as a result the refrigerant vapors liquefy. The liquefied refrigerant is fed through expansion valves, to reduce the temperature and pressure, to the evaporators whereupon the liquefied refrigerant evaporates by absorbing heat from the surrounding foodstuff.  
           [0004]    Since most evaporators operate at evaporating refrigerant temperatures that are lower than the freezing point of water (32° F., 0° C.), water vapor from ambient air freezes on the heat transfer surface of the evaporators, which creates a layer of frost on the surface. The frost layer decreases the efficiency of the heat transfer between the evaporator and the ambient air, which causes the temperature of the refrigerated space to increase above the required level. Maintaining the correct temperature of the refrigerated space is vitally important to maintain the quality of the stored food products. To do this, the evaporators must be defrosted regularly in order to re-establish their efficiency. During the defrosting period, the evaporator is out of service. It is therefore important to reduce the duration of the defrost period to avoid excessive rise of the refrigerated space temperature.  
           [0005]    Several patents exist that have tried to solve the problem of defrosting a frosted evaporator, including:  
           [0006]    U.S. Pat. No. 4,102,151, issued on Jul. 25, 1978, to Kramer et al, for “Hot Gas Defrost System with Dual Function Liquid Line”.  
           [0007]    U.S. Pat. No. 5,575,158, issued on Nov. 19, 1996, to Vogel for “Refrigeration Defrost Cycles”.  
           [0008]    U.S. Pat. No. 5,056,327, issued on Oct. 15, 1991, to Lammert for “Hot gas Defrost Refrigeration System”.  
           [0009]    U.S. Pat. No, 5,050,400, issued on Sep. 24, 1991 to Lammert for “Simplified Hot Gas Defrost Refrigeration System”.  
           [0010]    U.S. Pat. No. 6,286,322, issued on Sep. 11, 2001 to Vogel for “Hot gas Refrigeration System”.  
           [0011]    The above systems suffer from a number of significant drawbacks such as the use of complex systems of pipes, valves, water tanks, all of which may be difficult to maintain. Disadvantageously, some of the above systems require the addition of a superheater to appropriately route the refrigerant during the defrost cycle, thereby adding to the complexity and cost of the system.  
           [0012]    A common method for defrosting a frosted evaporator is the so-called hot refrigerant gas defrost method. Hot, high pressure refrigerant gas from a common discharge manifold or from an upper part of a refrigerant receiver, is fed backwards to the evaporator to be defrosted. The hot refrigerant gas is liquefied during its passage through the evaporator and its latent heat is used to melt the frost on the evaporator surface. The duration of the defrost period is directly proportional to the refrigerant mass flow. The higher the mass flow, the shorter the defrost period will be.  
           [0013]    Disadvantageously, the refrigerant mass flow during a defrost cycle depends solely on the condensing pressure of the refrigeration system which, especially during the colder periods of the year, when the possibility to operate with lower condensing pressures and therefore more efficiently is readily available, is economically unacceptable.  
           [0014]    Also, the liquid refrigerant obtained during the defrost is returned to the liquid line of the refrigeration system thus having a disruptive effect on the quality of the liquid refrigerant fed to the evaporators in normal operation, for example, so called “flash gas”, higher liquid temperature, and insufficient feeding of the most distant evaporators.  
           [0015]    Thus there is a need for a refrigeration system that is simple and inexpensive to operate, and which can be used simultaneously with the normal refrigeration cycle.  
         SUMMARY OF THE INVENTION  
         [0016]    The inventor has made a surprising and unexpected discovery that a single, dedicated compressor can be used to defrost a frosted evaporator in a refrigeration system. Moreover, during a defrost cycle, the single compressor operates with considerably higher suction pressure that the rest of the refrigeration compressor thus increasing efficiency and improving power consumption. Advantageously, the liquefied refrigerant is returned to the inlet of the refrigerant air cooled condenser, thus providing efficient cooling of the high pressure hot refrigerant gas before its entry into the refrigerant condenser, which increases the condenser efficiency during high ambient temperature periods of the year and reducing the condensing pressure. Another advantage is that during the cooler periods of the year, the refrigeration defrost system operates with low condensing pressures and provides efficient and rapid defrost cycle.  
           [0017]    Also, the compressor avoids the fluctuations of the refrigeration system pressures. During a defrost cycle, a high-pressure refrigerant gas is fed to the suction of the dedicated defrost compressor thus increasing its suction pressure, mass flow and power consumption efficiency. Also during the defrost cycle, the liquid refrigerant is fed through a desuperheating expansion valve to the suction of the dedicated defrost compressor to maintain acceptable suction temperature.  
           [0018]    In a first aspect of the present invention, there is provided a refrigeration defrost system including at least one frosted evaporator having an evaporator refrigerant vapor line and an evaporator refrigerant liquid line, said system comprising: a first compressor having a suction inlet line and a discharge outlet line each connected to a discharge manifold, said discharge outlet being connected to said evaporator refrigerant vapor line; and a first pressure regulator valve disposed in a refrigerant bypass passageway between said discharge manifold and said suction inlet line, for feeding refrigerant vapor, when a defrost cycle is required, from said discharge manifold into said suction inlet line, said refrigerant vapor being fed from said first compressor into said discharge outlet line and into said frosted evaporator through said evaporator refrigerant vapor line, thereby defrosting said frosted evaporator.  
           [0019]    In another aspect, a refrigeration defrost system, as described above, further includes a condenser having a condenser refrigerant vapor line and a condenser liquid refrigerant line, said condenser liquid refrigeration line being connected to said evaporator liquid refrigeration line, said first pressure regulator valve, during a refrigeration cycle, stops said refrigerant vapor from entering said suction inlet line, said condenser feeding liquid refrigerant into said evaporator liquid refrigerant line and said evaporator refrigerant vapor line feeding refrigerant vapor into said suction inlet line.  
           [0020]    In another aspect, a refrigeration defrost system as described above further includes a motorized ball valve disposed in a refrigerant defrost manifold between said discharge outlet line and said evaporator, in series connection with said first pressure regulator valve, for gradually feeding said refrigerant vapor into said evaporator refrigerant vapor line.  
           [0021]    In another aspect, a refrigeration defrost system, as described above further includes a first check valve in series connection with said pressure regulator valve for stopping low pressure refrigerant vapor from said evaporator refrigerant vapor line from feeding into said suction inlet line.  
           [0022]    Typically, in a refrigeration defrost system, as described above, a T-junction connects said refrigerant bypass passageway with said discharge manifold. The refrigerant bypass passageway further includes a solenoid valve and an expansion valve, in series connection between said suction inlet line and said condenser liquid refrigerant line, for feeding liquid refrigerant from said condenser liquid refrigerant line into said suction inlet line. The expansion valve is a desuperheating expansion valve.  
           [0023]    Typically, in a refrigeration defrost system, as described above, in which said condenser further includes a liquid refrigerant return inlet line connected to said evaporator refrigerant liquid line for feeding liquefied refrigerant into said condenser during said defrost cycle. A second check valve is connected between said evaporator refrigerant liquid line and said liquid refrigerant return inlet line.  
           [0024]    In another aspect, a refrigeration defrost system, as described above, further includes a second pressure regulator valve disposed in said discharge outlet line, said second pressure regulator valve regulating discharge outlet pressure during said defrost cycle.  
           [0025]    Typically, a refrigeration defrost system, as described above, further includes a liquid refrigerant receiver connected between said condenser and said evaporator.  
           [0026]    According to a second aspect of the present invention, the refrigeration defrost system further includes: first and second heat exchangers, said first heat exchanger being connected to said discharge manifold, said second heat exchanger being connected to said evaporator; a hot water tank connected to said first and second heat exchangers; and a three-way valve connected between said hot water tank and said first heat exchanger.  
           [0027]    Typically, a three-way motorized valve is connected between said first heat exchanger and said discharge manifold, said three-way valve being closed during said defrost cycle, hot water from said hot water tank flowing into said second heat exchanger and into said frosted evaporator to defrost said frosted evaporator.  
           [0028]    According to a third aspect of the present invention, there is provided a method of defrosting a frosted evaporator, said method comprising: feeding refrigerant vapor from a discharge manifold into a first compressor suction inlet line; and feeding said refrigerant vapor from said discharge outlet line into an evaporator suction inlet line, thereby defrosting said frosted evaporator.  
           [0029]    In another aspect, a method of defrosting a frosted evaporator, as described above, further includes: stopping low pressure refrigerant vapor from entering said compressor suction inlet line. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0030]    Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following Figures, wherein:  
         [0031]    [0031]FIG. 1 is a schematic diagram of an embodiment of a refrigeration defrost system having multiple evaporators and multiple compressors;  
         [0032]    [0032]FIG. 2 is a schematic diagram of the refrigeration defrost system of FIG. 1 showing a dedicated defrost compressor;  
         [0033]    [0033]FIG. 3 is a schematic diagram of a frosted evaporator from FIG. 2 connected to a dedicated compressor for defrosting; and  
         [0034]    [0034]FIG. 4 is a schematic diagram of another embodiment of the refrigeration defrost system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    With reference now to FIGS. 1 and 2, a refrigeration defrost system according to a first embodiment of the invention is generally illustrated at  10 . Broadly speaking, the defrost system  10  includes one or more compressors  12 , a refrigeration condenser  14 , one or more evaporators  16 , a liquid refrigerant receiver  18 , a liquid refrigerant pump  20 , one or more expansion valves  22 , and a network, shown generally at  24  that includes a variety of passageways (or conduits), valves and manifolds, through which the liquid refrigerant pump  20 , the evaporators  16 , the compressors  12 , and the condenser  14  are interconnected to circulate refrigeration fluid.  
         [0036]    During a refrigeration cycle (or non-defrost cycle), the compressors  12  compress low-pressure refrigerant vapors from the evaporators  16 . Each evaporator  16  includes an evaporator refrigerant vapor line  26  and an evaporator refrigerant liquid line  28 . The evaporator vapor line  26  feeds the low-pressure refrigerant vapors through a pressure-regulating valve  30  into a suction manifold  32  and then into the compressors  12 . The compressors  12  include a suction inlet line  34  and a discharge outlet line  36 . The suction inlet line  34  receives the low pressure refrigerant vapor from the suction manifold  32  and the compressor  12  compresses the low-pressure refrigerant vapor thereby increasing its pressure and temperature and producing hot, high pressure refrigerant vapor. The condenser  14  receives the hot, high pressure refrigerant vapor from the discharge outlet line  36  through an electrically open second pressure regulator valve  37 , disposed in the discharge outlet line  36 , though a discharge manifold  38  and a conduit  40  which connect the compressors  12  to the condenser  14 . The conduit  40  acts as a condenser refrigerant vapor line. In this embodiment, the condenser  14  is an outdoor air-cooled refrigeration condenser that is normally mounted on a roof of a building, although those skilled in the art will recognize that other types of condenser may be used to implement aspects of the invention. The condenser  14  condenses the hot, high pressure refrigerant vapors to produce high pressure liquid refrigerant that feeds through a condensate return conduit  42 , which acts as a condenser refrigerant liquid line, to the liquid refrigerant receiver  18 . A liquid refrigerant manifold  44  connects the liquid refrigerant pump  20  with the evaporators  16  through each expansion valve  22  and feeds the liquid refrigerant into evaporators  16  through the evaporator refrigerant liquid line  28 , thereafter the refrigerant vapor feeds from the evaporator vapor line  26  into the suction manifold  32 .  
         [0037]    Referring now to FIGS. 2 and 3, when a defrosting cycle is required to defrost a frosted evaporator a signal from a refrigeration control system (not shown) isolates and dedicates a single compressor  11  to defrost a frosted evaporator  13 , by energizing open a first pressure regulator valve  46 , normally electrically closed during the refrigeration cycle. The valve  46  is disposed in a refrigerant bypass passageway  48  that is connected between the suction inlet line  34  and the discharge manifold  38 . A T-junction  50  connects the bypass passageway  48  to the discharge manifold  38 . The second pressure regulator valve  37 , which is electrically open during the refrigeration cycle, now regulates the discharge outlet pressure. As best illustrated in FIG. 2, the open valve  46  feeds refrigerant vapor from the discharge manifold  38  (in the direction of the arrows) into the suction inlet line  34  along the bypass passageway  48 . The refrigerant vapors then feed from the compressor  11  into the discharge outlet line  36 . This increases the pressure to a level higher than the pressure in the suction manifold such that a first check valve  52 , in series connection with the pressure regulator valve  46 , closes to stop low pressure refrigerant vapor from the evaporator refrigerant vapor line  26  from feeding into the suction inlet line  34 . The signal from the refrigeration control system causes a motorized ball valve  54  that is disposed in a refrigerant defrost manifold  56  between the discharge outlet line  36  and the evaporator refrigerant vapor line  26 , to gradually open towards the manifold  56 . This gradual opening of valve  54 , in series connection with the valve  46  and the manifold  38 , gradually feeds refrigerant vapor from the discharge outlet line  36  towards the frosted evaporator  13  through the evaporator refrigerant vapor line  26 . The gradual opening of the valve  54  prevents the occurrence of thermal and mechanical stress in the evaporators during the defrost cycle. The increased suction pressure at the dedicated compressor  11  provides up to 70% higher mass flow, which ensures accelerated defrost cycles. The refrigerant defrost manifold  56  is in series connection with the pressure regulator valve  46  and the discharge outlet line  36 .  
         [0038]    As best illustrated in FIG. 3, the hot, high pressure refrigerant vapor feeds from the refrigerant defrost manifold  56  into the frosted evaporator  13  through a solenoid valve  58  and into the evaporator  13  through the evaporator vapor line  26 . Normally, during the refrigeration cycle, the evaporator vapor line  26  feeds low pressure vapor into the suction inlet line  34  via the suction manifold  32 . In the defrost cycle, the low pressure evaporator vapor line  26  receives the hot, high pressure refrigerant from the discharge outlet line  36 . The hot, high pressure refrigerant vapor defrosts the frosted evaporator  13  and converts the high pressure vapor into liquid refrigerant which exits the evaporator  13  through a check valve  59  and the evaporator liquid refrigerant line  28 .  
         [0039]    Referring to FIGS. 1 and 2, normally during the refrigeration cycle, the evaporator liquid refrigerant line  28  receives liquid refrigerant from the liquid refrigerant receiver  18  along the liquid refrigerant manifold  44 . During the defrost cycle, liquid condensate (liquid refrigerant) from the defrosted evaporator via the evaporator refrigerant liquid line  28  enters a defrost condensate return manifold  60  through a second solenoid valve  61  and into a liquid refrigerant return inlet line  62  with sufficient pressure to feed it into the condenser  14 .  
         [0040]    Referring to FIG. 2, when the refrigeration system control opens the valve  46 , a solenoid valve  64  opens and feeds liquid refrigerant from the liquid refrigerant manifold  44  into the suction inlet line  34  via an expansion valve  66 . The solenoid valve  64  and the expansion valve  66  are disposed in the refrigerant bypass passageway  48  and are in series connection between the suction inlet line  34  and the liquid refrigerant manifold  44 . The expansion valve  66  is a so-called desuperheating expansion valve and is used to maintain the temperature at an acceptable level at the suction inlet line  34  by allowing liquid refrigerant to mix with hot, high pressure refrigerant vapor at the suction inlet line  34  of the compressor  11  during the defrost cycle.  
         [0041]    After the frosted evaporator  13  is defrosted, the pressure regulator valve  46  closes to reestablish the compressor  11  as a non-defrost compressor  12  for normal refrigeration operation as described above.  
         [0042]    One skilled in the art will recognize that the single dedicated compressor  11  may be used to defrost more than one frosted evaporator. This can be achieved by controlling the hot, high pressure refrigerant&#39;s pathway from the refrigerant defrost manifold  56  into multiple frosted evaporators via each frosted evaporator&#39;s vapor line.  
         [0043]    In another embodiment, a source of heat may be used to increase the suction pressure of the single dedicated defrost compressor  11  during the defrost cycle. As best illustrated in FIG. 4, an additional circuit is added to the existing system  10  and includes a hot water tank  74 , a three-way motorized valve  68 , a pump  76  and two heat exchangers  72 ,  86 , all interconnected by a number of conduits  70 ,  80 ,  82 ,  84 , and  85 . During the normal refrigeration cycle, the hot, high pressure refrigerant vapors flow from the compressors  11  and  12  though the three way valve  68  along the conduit  70  to the first heat exchanger  72 . The pump  76  feeds water from the water tank  74  through a motorized valve  78  and along the conduit  80  to the heat exchanger  72 . The hot water from the first heat exchanger  72  is fed through the conduit  82  back to water tank  74 . The refrigerant leaving the heat exchanger  72  is fed through the conduits  38  and  40  to the external air-cooled condenser  14 . When the water temperature in the water tank  74  reaches a predetermined value, the three-way valve  68  closes the conduit  70  and opens the conduit  38 , which allows the hot, high pressure refrigerant vapors to flow to the air-cooled condenser  14 , thereby by-passing the first heat exchanger  72 .  
         [0044]    When a defrost is required, the refrigeration control system signals the motorized valve  78  to close the conduit  80  and open the conduit  84 , which allows the hot water to flow through the second heat exchanger  86 . At this point, the pressure-regulating valve  37  will be de-energized and will maintain the discharge pressure of compressor  11  at higher level than the pressure in the discharge manifold  38 . The motorized valve  54  will open the conduit  56  allowing the hot high-pressure refrigerant vapors from the compressor  11  to flow towards the refrigerant circuit and the evaporator to be defrosted. In this mode, the second heat exchanger  86  operates as an evaporator for the compressor  11 , such that the heat from the hot water will be absorbed by the second heat exchanger  86  and then used to defrost the frosted evaporator. The amount of water in the water tank  74  and the temperature at which the water should be maintained will depend on the amount of heat required to defrost the frosted evaporator.