Patent Publication Number: US-9410727-B1

Title: Systems and methods for defrosting an evaporator in a refrigeration system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of mechanical refrigeration which requires the periodic removal of frost from the evaporator heat transfer surfaces. 
     Methods which perform evaporator defrosting using refrigerant gas are well established by open-source technical publications. As stated by ASHRAE Handbook-Refrigeration-2010, Chapter 15: Retail Food Store and Equipment, compressor discharge gas or gas from the top of the warm receiver at saturated conditions flows to the evaporators requiring defrost. But during this process, the gas can condense to a liquid state and subsequently cause damage to the compressor. This persistent problem has been the attention of much patent activity but these efforts have lead to complex and ineffective solutions. Therefore the refrigeration industry still required a simple, reliable and cost-effective method of defrosting using refrigerant gas. 
     From a review of the technical literature and patent history, it appears two general concepts have been applied in an attempt to reduce compressor damage during gas defrost. One general concept has focused on methods for handling liquid refrigerant returning to the compressor, either by capture, diversion or re-evaporation. A clear example of this concept is presented by U.S. Pat. No. 4,318,277 to Cann et al which describes an accumulator for capturing liquid refrigerant returning to the compressor and then the utilization of hot gas from the compressor to vaporize the captured liquid refrigerant. And in similar fashion, U.S. Pat. No. 3,636,723 to Kramer explains the application of a heater for re-evaporating the captured liquid. 
     The second general concept has focused on methods for recirculating refrigerant vapor from the condenser to the evaporator while bypassing the expansion valve, thereby attempting to transfer heat from the ambient medium (typically the outside air) to the frost by using the compressor to recirculate refrigerant from the condenser to the evaporator. U.S. Pat. No. 2,069,201 to Allison describes the actuation of a bypass loop from the condenser to the evaporator but fails to assure that the loop contains only vapor prior to actuation. It is therefore believed that this method would result in substantial compressor damage. Likewise U.S. Pat. No. 5,065,584 to Byczynski explains an actuated recirculation loop from the condenser-to-evaporator-to-compressor but does not provide a means for sequestering liquid refrigerant that may reside within this loop prior to actuation. U.S. Pat. No. 2,688,850 to White and U.S. Pat. No. 3,098,363 to Shrader describe an alternate method of recirculating refrigerant vapor from the condenser to the evaporator which diverts a portion of the refrigerant flow from the condenser. This diversion, or condenser bypass, significantly reduces performance of the condenser and thus the condenser cannot achieve its full potential for heat transfer. 
     In summary, a review of technical literature and prior art shows that gas-defrost can be an effective means of removing frost from evaporators. Nevertheless, in its current form as shown by prior art, gas-defrost can still lead to compressor failure and there are opportunities for improving its effectiveness. Therefore, what is needed is a gas-defrost method which assures that liquid does not return to the compressor. What is further needed is a gas-defrost method which transfers the maximum amount of heat from the ambient medium to the frosted evaporator. In order to achieve commercial viability, what is yet further needed is an effective defrosting method which operates with minimal compressor power and can be easily and reliably implemented. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a refrigeration gas defrost method which will not damage the compressor due to liquid refrigerant returning to the compressor. A further object of present invention is to provide a refrigeration gas defrost method that fully transfers the heat available from the ambient medium to the frosted evaporator and thus the defrost process can be accomplished within a minimum amount of time. A further object of the present invention is to provide a gas defrost method which expends a minimum amount of compressor power and can be easily and reliably implemented, thus becoming commercially viable. 
     In order to achieve these objects, the present invention proposes a defrosting sequence of events which first sequesters the liquid refrigerant within a containment vessel and then recirculates refrigerant vapor between the compressor, condenser and the frosted evaporator. In this manner, heat from the ambient medium is transferred from the condenser to the frosted evaporator and thus used to melt the frost from the evaporator. The present invention assures that refrigerant returning to the compressor is always in a superheated state and therefore cannot damage the compressor with liquid refrigerant. The present invention strives to minimize the time required to defrost the evaporator by applying two features. First, the present invention assures that the refrigerant gas does not bypass the condenser and thus fully transfers the heat available from the ambient medium to the frosted evaporator. And second, the present invention maintains the refrigerant gas returning to the compressor at a pressure slightly below its saturation pressure and therefore maximizes the mass flowrate of refrigerant and subsequently maximizes the transfer heat from the ambient medium to the frosted evaporator. And further, the present invention strives to minimize the pressure differential across the compressor and thereby maintains low compressor power while defrosting the frosted evaporator. And further still, the present invention requires the application of only a minimal number of standard-practice components and therefore is anticipated to provide cost-effective, reliable service. 
     Due to the novelty of the stated method, the development of this invention required the construction and testing of prototype assemblies. The testing confirmed that the stated method provides a quick defrosting without damaging the compressor and therefore the present invention is considered commercially viable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic piping diagram of a conventional refrigeration system which has been embellished by the principle features of the invention. 
         FIG. 2  is a sequence-of-events table which identifies the sequential action required to implement the invention. 
     
    
    
     REFERENCE NUMERALS IN DRAWINGS 
     Reference numerals applied to all drawings 
     
         
         
           
               10  Compressor 
               11  Evaporator 
               12  Fan 
               13  Condenser 
               14  Fan 
               15  Pipe 
               16  Pipe 
               17  Pipe 
               18  Valve 
               19  Pipe 
               20  Receiver 
               21  Pipe 
               22  Valve 
               23  Pipe 
               24  Expansion valve 
               25  Pipe 
               26  Pipe 
               27  Valve 
               28  Pipe 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to the field of refrigeration utilizing a condenser which is exposed to an ambient medium warmer than the freezing point of water, and further to the field of refrigeration utilizing an evaporator operating at temperatures below the freezing point of water and finally to the field of refrigeration utilizing an evaporator which is exposed to moist air. 
     In  FIG. 1 , compressor  10  transfers refrigerant vapor from evaporator  11  to condenser  13 . Evaporator  11  is connected to compressor  10  with pipe  15 . Evaporator  11  is a heat exchanger which absorbs heat from the surrounding air. The surrounding air traverses evaporator  11  using fan  12 . Compressor  10  is connected to condenser  13  with pipe  16 . Condenser  13  is a heat exchanger which transfers heat either to or from the ambient medium. As shown by this embodiment, the ambient medium is air which traverses condenser  13  using fan  14 . But other forms of ambient medium can likewise be applied without departing from the scope of the invention. For example, the ambient medium could be water instead of air which could traverse condenser  13  using a pump instead of a fan. 
     Refrigerant is now transferred to evaporator  11  along two alternate paths, marked on  FIG. 1  as “A” and “B”. Along path “A”, condenser  13  is connected to valve  18  with pipe  17 . Valve  18  is connected to receiver  20  with pipe  19 . Valve  18  is of the two-position type (either open or closed) and can be actuated by any means (for example, manually or electrically actuated). As an alternate form, valve  18  could be a check-valve type, allowing flow to travel from pipe  17  to pipe  19  but prevent flow to travel from pipe  19  to pipe  17 . Receiver  20  is a storage vessel of sufficient size to store all of the liquid refrigerant within the refrigeration system. Receiver  20  is connected to valve  22  with pipe  21 . Valve  22  is of the two-position type (either open or closed) and can be actuated by any means (for example, manually or electrically actuated). Valve  22  is connected to expansion valve  24  with pipe  23 . Expansion valve  24  is connected to evaporator  11  with pipe  25 . In summary, a continuous path “A” is formed from condenser  13  to evaporator  11  by the sequential connection of parts  17 - 18 - 19 - 20 - 21 - 22 - 23 - 24 - 25 . Along path “B”, condenser  13  is connected to valve  27  with pipe  26 . Valve  27  is of the two-position type (either open or closed) and can be actuated by any means (for example, manually or electrically actuated). Valve  27  is connected to evaporator  11  with pipe  28 . In summary, an alternate continuous path “B” is formed from condenser  13  to evaporator  11  by the sequential connection of parts  26 - 27 - 28 . 
     The operation of the preferred embodiment is now described. During the process of refrigeration, compressor  10  pressurizes refrigerant vapor to a hot, high-pressure state. The hot, high-pressure vapor flows through pipe  16  to condenser  13 . The ambient medium transverses condenser  13  using energized fan  14 , causing heat to flow from the hot, high-pressure vapor to the ambient medium and subsequently causing the hot, high-pressure vapor to condense into a high-pressure liquid. Valve  18  and valve  22  are open and therefore the high pressure liquid is allowed to flow to evaporator  11  along path “A”. Valve  27  is closed and therefore flow is prevented along Path “B”. While flowing along path “A”, expansion valve  24  imparts a significant loss in pressure to the high-pressure liquid, causing the high-pressure liquid to expand to cold low-pressure mixture of liquid and vapor before entering evaporator  11 . 
     The surrounding air traverses evaporator  11  using energized fan  12 , causing heat to flow from the surrounding air to the cold low-pressure mixture of liquid and vapor, causing the mixture to transition to cold low-pressure vapor. The cold low-pressure vapor travels to compressor  10  through pipe  15 . The cold low-pressure vapor is then re-compressed to hot, high-pressure vapor to complete the refrigeration cycle. 
     It is now noted that compressor  10  is specifically design to receive only vapor and compressor  10  can be damaged if instead liquid is received. Therefore, the specific purpose of expansion valve  24  is to modulate the refrigerant flow to evaporator  11  to assure that the refrigerant traveling to compressor  10  is slightly superheated vapor, with the amount of superheat generally in the range of 5 F to 10 F. (Superheat is defined as the difference between the actual temperature of the vapor and the temperature at which the vapor will start to condense). To this end, expansion valve  24  reduces the refrigerant flow to evaporator  11  if a low superheat is sensed, and conversely, expansion valve  24  increases the refrigerant flow to evaporator  11  if a high superheat is sensed. 
     As heat is removed from evaporator  11 , frost can form on the outside surface of evaporator  11  if the outside surface of evaporator  11  is below the freezing point of water and the surrounding air contains water vapor. This formation of frost will eventually impede the surrounding air from traversing evaporator  11  and thus become an impediment to the transfer of heat. At this point in time, the frost must be removed from evaporator  11  with a process typically called “defrosting”. 
     An improved method for defrosting is the focus of the subject invention and is accomplished by implemented two distinct steps: 
     Step #1 is initiated by closing valve  22 . With the closing of valve  22 , high pressure liquid refrigerant is prevented from flowing to evaporator  11  and subsequently the residual liquid refrigerant within evaporator  11  is quickly transformed to a vapor and transferred by compressor  10  to condenser  13 . Within condenser  13 , the vapor condenses to a liquid state and the liquid travels through valve  18  to receiver  20 . Step #1 is terminated when all of the liquid refrigerant within the refrigeration system has been stored in receiver  20 . Thus at the termination of Step #1, evaporator  11  and condenser  13  contain only refrigerant vapor. It is now noted that the stated Step #1 procedure is in accordance with the common refrigeration practice termed “pump-down” as described by typical refrigeration references such as Refrigeration &amp; Air Conditioning Technology 6 th  Edition; Whitman, et al. As described by the stated reference, the termination of Step #1 is readily determined by sensing a substantial pressure reduction with evaporator  11 . 
     Step #1 is terminated and then Step #2 is initiated by closing valve  18 , opening valve  27  and de-energizing fan  12 . With valve  18  closed, liquid refrigerant stored in receiver  20  is not allowed to leave receiver  20 . With valve  27  open, refrigerant vapor can freely recirculate from condenser  13  to evaporator  11  to compressor  10  along Path “B”. Thus refrigerant vapor recirculating from condenser  13  to evaporator  11  to compressor  10  remains in a vapor state and compressor  10  is protected from damage due to receiving refrigerant in the liquid state. It is now also noted that the ambient medium traversing condenser  13  is warmer than evaporator  11  in its frosted state and therefore heat is transferred from the ambient medium to the refrigerant vapor as the refrigerant vapor flows through condenser  13  and then from the refrigerant vapor to evaporator  11  as the refrigerant vapor flows through evaporator  11 . When fan  12  is de-energized, the stated heat is not transferred to the surrounding air but instead is fully applied to the frost on the outside surfaces of evaporator  11  and sequentially the frost starts to convert to a liquid and drips off of evaporator  11  thus initiating the defrost process. It is further noted that Path “B” bypasses the flow restriction imposed by expansion valve  24  and therefore can maintain the refrigerant vapor flow from condenser  13  to evaporator  11  with minimal effort. It is also further noted that the refrigerant vapor flow through Path “B” is only from condenser  13 , without any refrigerant bypassing condenser  13  as described by the stated prior art, and therefore the heating effectiveness of condenser  13  is fully realized. 
     Step #2 can be solely used to fully defrost evaporator  11  but the period of time required to fully defrost evaporator  11  can be reduced by also opening valve  22 . With the opening of valve  22 , high pressure liquid refrigerant is allowed to flow to expansion valve  24  and subsequently expansion valve  24  introduces liquid refrigerant into the refrigerant vapor recirculating from condenser  13  to evaporator  11  to compressor  10 . Since the stated recirculating refrigerant vapor is in a superheated state, the liquid refrigerant introduced by expansion valve  24  is vaporized. By virtue of its purposeful design, expansion valve  24  introduces liquid refrigerant into the stated recirculating refrigerant vapor only as required to maintain the vapor traveling to compressor  10  in a slightly superheated state and thus compressor  10  remains protected from damage due to receiving refrigerant in the liquid state. It is now noted that heat transfer effectiveness of the vapor recirculating from condenser  13  to evaporator  11  to compressor  10  is highly dependent on the density (mass per unit volume) of the stated recirculating vapor, as explained by “Heat Transfer in Refrigerator Condensers and Evaporators”, Admiral, D. and Bullard, C., University of Illinois paper #ACRC TR-48; dated August 1993. And it is further noted that saturated refrigerant vapor is denser than superheated vapor and therefore the highest vapor density is achieved by maintaining the vapor as near to the saturated state as possible; that is, in a nearly saturated state. Therefore, with the opening of valve  22 , vapor recirculating from condenser  13  to evaporator  11  to compressor  10  is maintained at the highest practical density, thereby achieving the highest rate of heat transfer and subsequently minimizing the period of time required to defrost evaporator  11 . Step #2 is terminated when all of frost has been removed from evaporator  11 . Well-established refrigeration practices are available for sensing the point of complete frost removal. Again referring to Whitman, et al., the point of complete frost removal is commonly indicated by sensing when the surface of evaporator  11  has risen above the freezing point of water. At the termination of Step #2, the refrigeration system returns to its standard cooling mode by closing valve  27  and energizing fan  12 . 
       FIG. 2  delineates the sequence of events for the preferred embodiment in tabular form. Three distinct modes of operations are shown: normal refrigeration and the two steps of defrost per the subject invention. For normal refrigeration, compressor  10  is energized, fan  12  is energized, fan  14  is energized, valve  18  is open, valve  22  is open and valve  27  is closed. Normal refrigeration is terminated and Step #1 is initiated when excessive frost has accumulated on the outside surface of evaporator  11 . For Step #1, compressor  10  is energized, fan  12  is energized, fan  14  is energized, valve  18  is open, valve  22  is closed and valve  27  is closed. Step #1 is terminated and Step #2 is initiated when it is perceived that all of the liquid refrigerant is stored within receiver  20 . As per common “pump-down” refrigeration practice, this condition can be identified by a low refrigerant pressure within evaporator  11 . For Step #2, compressor  10  is energized, fan  12  is de-energized, fan  14  is energized, valve  18  is closed, valve  22  is open and valve  27  is open. Step #2 is terminated and the system returns to normal refrigeration when all of the frost has been removed from evaporator  11 , generally sensed when the temperature of evaporator  11  has risen above the freezing point of water. 
     In conclusion, the preferred embodiment of the present invention provides a method of a defrosting method which assures that only superheated vapor leaves the evaporator and thereby protects the compressor from damage due to the return of liquid refrigeration. In addition, the preferred embodiment of the present invention minimizes the amount of time required for defrosting by applying two features: 1) the full refrigerant flow is directed to the condenser, with no means of bypass, so that heat transfer capability of the condenser is fully realized and 2) the density of the superheated vapor leaving the evaporator is maintained at the highest practical density so that the heat transfer rates within the condenser and evaporator are maximized. Furthermore, the preferred embodiment of the present invention minimizes the pressure differential across the compressor and subsequently minimizes the compressor power required for defrosting. And further still, the preferred embodiment of the present invention requires only a few additional standard parts and therefore is deemed to be both practical and commercially viable. 
     It should be understood that the preferred embodiment is merely illustrative of the present invention. Numerous variations in design and use of the present invention may be contemplated in view of the following claims without straying from the intended scope and field of the invention disclosed herein.