Abstract:
A high-speed evaporator defrost system is described. It comprises a defrost conduit circuit having valves for directing hot high pressure refrigerant gas from a discharge line of a compressor and through a refrigeration coil of an evaporator of a refrigeration system during a defrost cycle thereof, and back to a suction header of the compressor through a reservoir of the refrigeration system to remove any liquid refrigerant contained in the refrigerant gas prior to returning to the suction header. The reservoir has an internal pressure which is generally at the same pressure as that of a suction header of the compressor thereby creating a pressure differential across the refrigeration coil sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil. The reservoir is repressurized after the defrost cycle for using the reservoir in a refrigeration cycle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a high-speed evaporator defrost system for defrosting refrigeration coils of evaporators in a short period of time without having to increase compressor head pressure. 
     BACKGROUND OF THE INVENTION 
     In refrigeration systems found in the food industry to refrigerate fresh and frozen foods, it is necessary to defrost the refrigeration coils of the evaporators periodically, as the refrigeration systems working below the freezing point of water are gradually covered by a thin layer of frost which reduces the efficiency of evaporators. The evaporators become clogged up by the build up of ice thereon during the refrigeration cycle, whereby the passage of air maintaining the foodstuff refrigerated is obstructed. Exposing foodstuff to temperature increases due to defrost cycles may have adverse effects on their freshness and quality. 
     One method known in the prior art for defrosting refrigeration coils uses an air defrost method wherein fans blow warm air against the clogged up refrigeration coils while refrigerant supply is momentarily stopped from circulating through the coils. The resulting defrost cycles may last up to about 40 minutes, thereby possibly fouling the foodstuff. 
     In another known method, gas is taken from the top of the reservoir of refrigerant at a temperature ranging from 80° F. to 90° F. and is passed through the refrigeration coils, whereby the latent heat of the gas is used to defrost the refrigeration coils. This also results in a fairly lengthy defrost cycle. 
     U.S. Pat. No. 5,673,567, issued on Oct. 7, 1997 to the present inventor, discloses a system wherein hot gas from the compressor discharge line is fed to the refrigerant coil by a valve circuit and back into the liquid manifold to mix with the refrigerant liquid. This method of defrost usually takes about 12 minutes for defrosting evaporators associated with meat display cases and about 22 minutes for defrosting frozen food enclosures. The compressors are affected by hot gas coming back through the suction header, thereby causing the compressors to overheat. Furthermore, the energy costs may increase with the compressor head pressure increase. 
     U.S. Pat. No. 6,089,033, published on Jul. 18, 2000 to the present inventor, introduces an evaporator defrost system operating at high speed (e.g. 1 to 2 minutes for refrigerated display cases, 4 to 6 minutes for frozen food enclosures) comprising a defrost conduit circuit connected to the discharge line of the compressors and back to the suction header through an auxiliary reservoir capable of storing the entire refrigerant load of the refrigeration system. The auxiliary reservoir is at low pressure and is automatically flushed into the main reservoir when liquid refrigerant accumulates to a predetermined level. The pressure difference between the low pressure auxiliary reservoir and the typical high pressure of the discharge of the compressor creates a rapid flow of hot gas through the evaporator coils, thereby ensuring a quick defrost of the refrigeration coils. Furthermore, the suction header is fed with low pressure gas, whereby preventing the adverse effects of hot gas and high head pressure on the compressors. Although this patent is fully operational and provides many advantages, the use of two reservoirs as well as an automation system for flushing the auxiliary reservoir proves to be an expensive solution for smallscale systems, such as systems with only one evaporator and compressor. 
     DISCLOSURE OF THE INVENTION 
     It is a feature of the present invention to provide an alternative method of defrosting evaporators at high speed for small-scale systems. 
     It is a further feature of the present invention to use this alternative method simultaneously with refrigeration cycles for medium-scale systems. 
     It is a still further feature of the present invention to use this alternative method simultaneously with refrigeration cycles for large-scale systems. 
     SUMMARY OF THE INVENTION 
     According to the above aim of the present invention, and according to a broad aspect thereof, there is provided a high-speed evaporator defrost system comprising a defrost conduit circuit. The defrost conduit circuit has valve means for directing hot high pressure refrigerant gas from a discharge line of at least one compressor and through a refrigeration coil of at least one evaporator of a refrigeration system during a defrost cycle thereof, and back to a suction header of the compressor through a reservoir of the refrigeration system to remove any liquid refrigerant contained in the refrigerant gas prior to returning to the suction header. The reservoir has an internal pressure which is generally at the same pressure as that of a suction header of the compressor thereby creating a pressure differential across the refrigeration coil sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil. The reservoir is repressurized after the defrost cycle for using the reservoir in a refrigeration cycle. 
     According to a further broad aspect of the present invention there is provided a high-speed evaporator defrost system comprising a defrost conduit circuit. The defrost conduit circuit has valve means for directing hot high pressure refrigerant gas from a discharge line of at least one compressor and through a refrigeration coil of at least one evaporator of a refrigeration system during a defrost cycle thereof, and back to a suction header of the compressor through a reservoir of the refrigeration system to remove any liquid refrigerant contained in the refrigerant gas prior to returning to the suction header. The refrigeration system has at least another evaporator in a refrigeration cycle. The reservoir has an internal pressure which is generally at the same pressure as that of a suction header of the compressor thereby creating a pressure differential across the refrigeration coil sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil. The reservoir is repressurized after the defrost cycle for using the reservoir in the refrigeration cycle. 
     According to a still further broad aspect of the present invention there is provided a high-speed evaporator defrost system comprising a defrost conduit circuit. The defrost conduit circuit has valve means for directing hot high pressure refrigerant gas from a discharge line of at least one compressor and through a refrigeration coil of at least one evaporator of a refrigeration system during a defrost cycle thereof, and back to a suction header of the compressor through a principal reservoir of the refrigeration system to remove any liquid refrigerant contained in the refrigerant gas prior to returning to the suction header. The refrigeration system has at least another evaporator in a refrigeration cycle. The principal reservoir has an internal pressure which is generally at the same pressure as that of a suction header of the compressor thereby creating a pressure differential across the refrigeration coil sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil. The defrost system has a buffer reservoir for use in the refrigeration cycle for accumulating high pressure refrigerant liquid therein. The principal reservoir is repressurized after the defrost cycle for use in the refrigeration cycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention with reference to examples thereof will now be described in detail having reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a refrigeration system adapted for operating a defrost cycle according to the present invention; 
     FIG. 2 is a schematic diagram of a refrigeration system adapted to operate a defrost cycle simultaneously with a refrigeration cycle; and 
     FIG. 3 is a schematic diagram of a refrigeration system operating a defrost cycle simultaneously with a refrigeration cycle with a buffer reservoir. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown generally at  10  a refrigeration system for feeding a refrigerant to an evaporator associated with a refrigeration unit such as a refrigerated display case or a frozen food enclosure. The system is provided with a compressor  11 , a condenser  12 , a refrigerant reservoir  13 , an expansion valve  14  and an evaporator  15 . The system  10  contains a refrigerant which is used for its properties and which changes phases throughout refrigeration and defrost cycles. The refrigerant, in a high pressure hot gas state, is fed from the compressor  11  to the condenser  12  by a discharge line  16 , following arrows A, B and C depicted in FIG.  1 . After being cooled in the condenser  12  as known in the art, the refrigerant, now in the state of a high pressure liquid/gas mixture, conveys to the refrigerant reservoir  13  through condenser line  17 , following arrows D and E. High pressure liquid refrigerant then reaches the evaporator  15  through a liquid line  18 , in the direction of arrows F and G, wherein the expansion valve  14  substantially reduces the liquid refrigerant pressure. Low pressure liquid refrigerant is vaporized in an evaporator coil  19  within the evaporator  15 , whereon air is blown to cool a refrigerated display case or frozen food enclosure (not shown). The refrigerant, in a low pressure gas state, then conveys from the evaporator coil  19  to the compressor  11 , via a suction line  20 , and illustrated by arrows H and I. 
     The refrigeration cycle described above further comprises known in the art elements such as a dryer  21 , a sight glass  22  and a plurality of maintenance valves  23 . Furthermore, an accumulator  24  within the suction line  20  ensures that the refrigerant reaching the compressor is in a gaseous state. 
     In a defrost cycle, hot gas refrigerant discharged at high pressure from the compressor  11  is fed to the evaporator  15 , whereas it is fed to the condenser  12  in the refrigeration cycle. This is achieved by a hot gas line  25  diverging from the discharge line  16  to reach the suction line  20 . A three-way valve  26  conveys the high pressure hot gas refrigerant discharged from the compressor  11  to the hot gas line  25 , following arrows A, K and L. Other valve systems such as a solenoid three-way valve, a pair of two way valves or the like may be used for the hereinabove described purpose. A valve  27 , normally open on the suction line  20 , is closed to direct the high pressure hot gas refrigerant from the hot gas line  25  to the evaporator  15 , in a direction opposite arrow H. A pressure regulator  28  located on the hot gas line  25  and as known in the art, lowers the pressure of the hot gas refrigerant passing therethrough. The low pressure hot gas flows through the evaporator coil  19  in a direction opposite from that of the refrigeration cycle, thereby heating the coil  19  to defrost it from the ice build-up thereon. The pressure drop resulting from the pressure regulator  28  ensures a rapid flow of hot gas refrigerant through the coil  19 . 
     Simultaneously with the above described diversion of hot gas refrigerant toward the evaporator  15  by the three-way valve and by the closure of valve  27 , a valve  33  on the liquid line  18 , normally open during the refrigeration cycle, is closed for preventing the high pressure liquid refrigerant of the reservoir B to flow toward the evaporator  15 . Furthermore, a valve  31  on the condenser line  17 , also normally open during the refrigeration cycle, is shut, whereby to prevent the high pressure liquid/gas refrigerant to flow back to the condenser  12 . Instead, the reservoir  13  is connected to the suction line  20  by a depressurizing line  30 , wherein a valve  34 , normally closed during the refrigeration cycle, is opened in the defrost cycle to allow the flow of high pressure gas refrigerant to the suction line  20 , following arrow M. A pressure regulator  32 , located upstream of the compressor  11 , reduces the pressure of refrigerant, as known in the art, in a closed part of the system  10  defined by the portion of the liquid line  18  from the valve  33  to the reservoir  13 , the portion of the condenser line  17  from the reservoir  13  to the valve  31 , the reservoir  13 , the depressurizing line  30 , and the portion of the suction line  20  extending from the valve  27  to the pressure regulator  32 . The above defined closed part of the system consequently becomes the low pressure portion of the system  10 . 
     The refrigerant, in a low pressure liquid/gas state, may then flow from the evaporator  15  to the reservoir  13  in the liquid line  18 , in a direction opposite arrows G and F. The liquid encompasses the expansion valve  14 , the dryer  21  and the valve  23  by passing through the unidirectional by-pass valves  29 , to reach the refrigerant reservoir  13 , now containing a low pressure liquid-gas refrigerant mixture. Thereafter, the pressure drop at the compressor  11  inlet collects the gas from the refrigerant reservoir  13  by the depressurizing line  30 , thereby closing the defrost cycle loop. The pressure regulator further  32  ensures that the head pressure in the suction line  20  of the compressor  11  is kept low, while the accumulator  24  still prevents liquid from entering the compressor  11 . 
     Once the defrost cycle is over, the refrigeration system  10  returns to the refrigeration cycle, wherefore valves  27 ,  31  and  33  return to their normally open position and valve  34  is closed. The three-way valve  26  is actuated to direct the compressor discharge to the condenser  12 , whereby the reservoir is repressurized with high pressure refrigerant for the operation of the refrigeration cycle. 
     In keeping the refrigerant reservoir in low pressure during the defrost cycles, a high pressure differential is kept to accelerate the high pressure hot gas refrigerant flowing through the evaporators, thereby accelerating the defrost cycles. Furthermore, the compressors are supplied with gas refrigerant resulting from the depressurization of the refrigerant reservoir, whereby a sufficient amount of hot gas is supplied to the evaporator in the defrost cycle. Liquid return to the compressors is also prevented by a system of unidirectional valves and accumulators. 
     The defrost cycle for the refrigeration system  10  depicted in FIG. 1, utilizing depressurization and repressurization of the refrigerant reservoir  13  for switching from and to the refrigeration cycle, may be operated in parallel with the refrigeration cycle in systems comprising more than one evaporator, i.e. an evaporator may be defrosting while another is refrigerating. Referring thus to FIG. 2, there is generally shown at  50  a refrigeration system for feeding a refrigerant to evaporators associated with refrigerated display cases and/or frozen food enclosures. The system is provided with compressors  51 , a condenser  52 , a refrigerant reservoir  53 , expansion valves  54  and evaporators  55 . Refrigerant gas, in a high pressure hot gas state, is fed from the compressors  51  to the condenser  12  by a discharge line  56  and following arrows A, B and C, with an oil separator  57  located thereon separating the lubricant oil from the refrigerant and returning the lubricant oil to the compressors  11  through lubricant line  58 . After being cooled in the condenser  52  as known in the art, the refrigerant, now in a state of high pressure liquid/gas mixture, conveys through a condenser line  59  to the refrigerant reservoir  53  following arrows D and E, wherein the liquid and gas portion of the mixture are separated. High pressure liquid refrigerant then reaches the liquid header  60 , as shown within brackets in FIG. 2, by conveying through a liquid line  59 ′ and following arrows F and G. A first suction header  62  is connected to the liquid header  60  by evaporator circuits  61 , whereby liquid refrigerant is supplied to the evaporators  55 . 
     Each of the evaporator circuits  61  comprises an inlet line  63 , an outlet line  64  and, therebetween, the evaporator  55  comprising an evaporator coil  65 . Furthermore, the expansion valve  54  is located on the inlet line  63  and substantially reduces the pressure of the liquid refrigerant supplied to the evaporator coil  65 . Low pressure liquid refrigerant is vaporized in the evaporator coil  65  within the evaporator  55 , whereon air is blown to cool the refrigeration unit (not shown). The refrigerant, in a low pressure gas state, then conveys from the evaporator coil  65  to the suction header  62 , via the outlet line  64 . An inlet valve  66  and an outlet valve  67  normally open during the refrigeration cycle, are located on the inlet and outlet lines  63  and  64 , may be closed to isolate the evaporator  55  from the liquid and first suction header  60  and  62 , for instance when running a defrost cycle, as explained hereinafter. The refrigerant, still in a low pressure gas state, conveys from the first suction header  62  to the second suction header  68 , passing through suction line  69  following arrow H. The low pressure gas refrigerant then reaches the compressors  51  through compressor lines  70 , connected to the second suction header  68 . Herein seen the suction line  69  comprises an accumulator  71 , as known in the art, for ensuring the supply of refrigerant only in a gaseous state to the compressors  51 . The refrigeration cycle described above further comprises known in the art elements, which are not all identified nor shown in FIG. 2 to simplify the figure, such as maintenance valves, dryers, sight glass and the like. 
     One of the evaporators  55  may be put in a defrost cycle while the others are in the above described refrigeration cycle. This is achieved by a hot gas line  72  diverging from the discharge line  56  to reach a hot gas header  73  following arrows I, shown within brackets. A valve  74  located on the hot gas line  72 , normally closed when no defrost cycle is running on the refrigeration system  50 , is fully opened while a valve  75  located, on the discharge line  56 , between the hot gas line  72  junction and the condenser  52  is slightly closed to ensure hot gas refrigerant will reach the hot gas header  73 . The refrigeration cycle will continue in the manner explained above, with the exception that a three-way valve  76  on the condenser line  59  redirects the liquid/gas mixture of refrigerant, coming from the condenser  52 , to a bypass circuit  77  and following arrow Q, whereby the mixture of refrigerant bypasses the reservoir  53 . The bypass circuit is connected to the liquid line  59 ′, whereby the evaporators  55  are supplied with refrigerant, as explained hereinabove. A unidirectional valve  87  as known in the art prevents the refrigerant from entering the reservoir  53  upon reaching the liquid line  59 ′. 
     In order to supply one of the evaporators  55  with hot gas refrigerant for defrosting purposes, the inlet and outlet valves  66  and  67  are shut, thereby preventing flow of liquid refrigerant from the liquid header  60  or the first suction header  62 . Defrost lines  78  connect the hot gas header  73  to a portion of the outlet lines  64  of the evaporator circuits  61 , between the evaporator  65  and the outlet valves  67 . The defrost lines  78  further comprise valves  79  located thereon, specifically opened for the defrost cycle of an evaporator  55 . The valves  79  also serve the purpose of reducing the pressure of the hot gas refrigerant passing therethrough, as known in the art. Therefore, low pressure hot gas refrigerant flows through the evaporator coil  65  of the evaporator  55  being defrosted, thereby heating the evaporator coil  65  to defrost it from the ice build up thereon. The pressure drop resulting from the valve  79  ensures a rapid flow of hot gas refrigerant through the coil  65 . The refrigerant, in a fluid/gas mixture, then flows through the inlet line  63  and bypasses the expansion valve  54  by passing through a unidirectional bypass valve  80 . The fluid/gas refrigerant thereafter reaches a defrost return header  81 , as shown in brackets in FIG. 2. A defrost return line  82  connects the inlet line  63  to the defrost return header  81 . The defrost return line  82  also comprises a valve  83 , specifically opened for the defrost cycle. 
     Simultaneously with the above described diversion of hot gas refrigerant toward one of the evaporators  55  by the hot gas line  72 , a pressure regulator  85  reduces the pressure of refrigerant, as known in the art, in a closed part of the refrigeration system  50  defined by the reservoir  53  and a reservoir return line  86 , thereby depressurizing the reservoir  53 . This part of the system  10  is closed as unidirectional valves  87  and  88  and three-way valve  76  isolate the reservoir  53  from the rest of the system  50 . When the pressure in the reservoir  53  reaches a lower value than the pressure of the liquid/gas refrigerant within the defrost return header  81 , the liquid/gas refrigerant flows therefrom through the unidirectional valve  88 , in the direction shown by arrow L. Thereafter, the low pressure in the first suction header  62 , resulting from the connection of the first suction header to an inlet side of the compressor  51 , ensures a flow of gas refrigerant from the reservoir  53  to the first suction header  62  via the reservoir return line  86  and in the direction shown by arrows M and N. An accumulator  89 , known in the art, ensures that refrigerant only in a gaseous state reaches the first suction header  62 . 
     The defrost cycle for the refrigeration system  50  depicted in FIG. 2, activated simultaneously with the refrigeration cycle for a plurality of evaporators  55 , is shown in FIG. 3 with a buffer reservoir  100 , whereby ensuring a continuous supply of liquid refrigerant to the evaporators  55  in the refrigeration cycle. The refrigeration system depicted in FIG. 3 is identical to the refrigeration system  50  of FIG. 2 apart from a few differences, which will be described hereinafter. Thus, like numerals will determine like elements. Furthermore, only the main elements are numbered on FIG. 3 for the simplicity of the illustration. 
     The buffer reservoir  100  is added to the liquid line  59 ′ of the previous refrigeration system  50  of FIG.  2 . Thus, the line now connecting the refrigerant reservoir  53  to the buffer reservoir  100  will be referred to as the transfer line  101 . The transfer line  101  includes the unidirectional valve  87 , whereby ensuring that liquid refrigerant may only flow from the refrigerant reservoir  53  to the buffer reservoir  100 . A liquid line  102  thereafter connects the buffer reservoir  100  to the liquid header  60 . As shown, the bypass circuit  77  is upstream of the buffer reservoir  100 . 
     The refrigeration system of FIG. 3 operates in the same manner as the refrigeration system  50  of FIG. 2, with the difference being that the liquid/gas refrigerant mixture exiting from the condenser  52  and conveying through condenser line  59 , will accumulate in the buffer reservoir  100  through transfer line  101 . Once the buffer reservoir  100  is full, the refrigerant reservoir  53  will then be filled. When a defrost cycle is initiated, the three-way valve  76  will redirect the high pressure liquid/gas refrigerant mixture from the condenser  52  to the buffer reservoir  100  through the bypass circuit  77 . As explained for FIG. 2, the refrigerant reservoir  53  is depressurized to serve as a reservoir for low pressure liquid/gas refrigerant mixture exiting from the defrosting evaporators. The buffer reservoir  100  thus ensures the continuous supply of high pressure liquid refrigerant to the evaporators in the refrigeration cycle. 
     As herein shown, the refrigeration systems of the present invention use the main reservoir, i.e. refrigerant reservoir, to maintain a low pressure in the system during the defrost cycles. They also allow for the efficient defrosting of evaporators working at low and medium temperatures, such as frozen food enclosures and refrigerated display cases. An advantage of the present invention resides in the fact that evaporators can be defrosted on a refrigeration system having only one refrigeration circuit and one compressor. The refrigeration systems of the present invention operate at low compressor head pressure, which provides better energy efficiency. The refrigeration system of the present invention are enabled to be adapted to existing evaporators without modification. 
     It is within the ambit of the present invention to cover any obvious modifications of the embodiments described herein, provided such modifications fall within the scope of the appended claims.