Abstract:
The present invention provides a method and apparatus for using hot gas to defrost sequentially each refrigerated display case (evaporator) in a group of refrigerated display cases (evaporators). A unique cross-feed line connects the distributor and the evaporator suction line for each evaporator in the group of evaporators. Like a typical hot gas defrost system, the sequential hot gas defrost system may be time-initiated and time-terminated or time-initiated and temperature-terminated. Each evaporator in the group of evaporators is defrosted in turn while the remaining evaporators in the group continue to operate in the refrigeration mode. A unitary combination check valve and orifice simplifies the sequential hot gas defrost refrigeration system.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   Applicant&#39;s invention relates generally to the field of refrigeration, and more particularly but not by way of limitation, to a method and apparatus for sequentially defrosting a series of evaporators using hot refrigeration gas (also referred to herein as high pressure gas). 
   2. Discussion 
   Refrigerated display cases are common to grocery stores, convenience stores, and other purveyors of refrigerated or frozen foods. The display cases are frequently located in the same general location within the store. For ease of installation and convenience to shoppers, the display cases are commonly arranged to form a contiguous line or in series of cases. Adjacent display cases often share similar refrigeration demands. Within each case, a fan circulates cold air in a duct that encircles the case. The duct encloses an evaporator of a refrigeration system. 
   Grocery stores display frozen foods in open horizontal cases, closed horizontal cases wherein the frozen food products are accessible through glass doors, and closed vertical cases wherein the frozen food products are accessible through glass doors. 
   Grocery stores may display milk and related products in walk-in refrigerators. The food products are accessible through glass doors, and the shelves are re-stocked from within the walk-in refrigerator. Cheese, cold cuts, butter, juices, refrigerated desserts, and similar items may be available from tiered open-front cases. 
   In most instances, each case, whether used for refrigerated foods or frozen foods, contains a single evaporator. When appropriate, however, a cold case may contain two, three, or even more evaporators. Here, as in the refrigeration industry, the term “evaporator” may be used interchangeably with the term “evaporator coil” or “cooling coil,” from time to time, to mean the evaporator of a refrigeration system where environmental cooling occurs. The term “cold case,” as used herein, includes all types, styles, and configurations of refrigerated food cases. 
   Whether the cold case is an open horizontal frozen food display case or a walk-in dairy case, and whether the cold case has one or more cooling coils, every cold case faces a common problem. Over time, the circulating cold air entrains water vapor from the ambient air. The entrained water vapor condenses and freezes on the cold evaporator coil, thereby decreasing heat transfer efficiency between the refrigerant in the evaporator coil and the air in the duct. Each evaporator must be defrosted periodically to remove the frozen condensate. Thus, each refrigerated case has a refrigeration cycle of operation (also referred to herein as a refrigeration mode) and a defrost cycle of operation (also referred to sometimes herein as a defrost mode). During the refrigeration cycle, the refrigeration system cools the case. During the defrost cycle, a heat source melts frozen condensation which has collected on the evaporator coil. 
   A metering device introduces high pressure liquid refrigerant into a distributor which, in turn, distributes the refrigerant to the evaporator coils to cool the circulating air. The refrigerant within the evaporator absorbs heat from the circulating cold air used to cool the refrigerated display case. As the refrigerant absorbs heat, the refrigerant changes from a low pressure liquid to a low pressure vapor and then, on further absorption of heat, the temperature of the low pressure vapor increases. The terms “low pressure vapor” and “low pressure gas” are used interchangeably to describe the gaseous refrigerant as it leaves the evaporator after absorbing heat from the air duct. Low pressure vapor streams from two or more evaporator suction lines are combined in a low pressure vapor header (also referred to, interchangeably, as a “low pressure vapor suction header” or “suction header”) from which the refrigerant compressor takes suction. 
   The compressor compresses the low pressure vapor to a high pressure vapor, also referred to herein interchangeably as “high pressure vapor,” “high pressure gas” (HPG) or “hot gas”. A heat exchanger, normally referred to as a condenser because of its function, then cools the high pressure vapor sufficiently to change the high pressure vapor (HPV) refrigerant to a high pressure liquid (HPL). The high pressure liquid refrigerant is collected in a liquid receiver. From the liquid receiver, the high pressure liquid refrigerant is piped through a high pressure liquid refrigerant header to distributors. A metering device controls introduction of the high pressure liquid refrigerant into the distributor. Automatic expansion valves (commonly referred to as AEV or AXV valves), thermal expansion valves (commonly referred to as TEV or TXV valves), capillary tubing, and simple orifices are all known in the art as devices capable of metering the high pressure liquid refrigerant into the evaporator distributor. In some cases, high pressure liquid refrigerant is metered based on the temperature of the low pressure vapor leaving the evaporator. A single evaporator normally includes several branches which receive refrigerant from a common distributor. 
   A typical supermarket may have as many as 100 cold cases containing one or more evaporators within each case. The cold cases are typically arranged in groups. Long horizontal cases with open tops may be arranged end to end to permit access from both sides. Walk-in cases are often arranged side-by-side for shopping convenience. Whether electric heating or hot gas heating is used to defrost the cold cases, operators avoid defrosting all cases simultaneously. Instead, the cold cases are grouped based on the build-up of frost within the cases. Those cases which accumulate frost rapidly may be defrosted as many as four times in a 24-hour period. Other cases may require defrosting only three times per day, twice a day, or once a day. Some cases hold frozen foods, while other cases (e.g., dairy cases) require only moderate refrigeration. Still other cases (e.g., a cold case used to hold fresh flowers) my require only minimal refrigeration. The grouping of cases for defrosting may combine cases of differing refrigeration requirements. As used herein, the term “group” is used to mean at least two evaporators which share a common compressor suction header (also called the “evaporator suction line” herein) and a common high pressure liquid header from the condenser. 
   It is currently common practice to defrost the evaporator coils in a series of cases at the same time, in part because contiguous refrigerated display cases often share a common defrost timer. It is also common to defrost the evaporator coils every 6-8 hours. There are several notable problems with this approach to defrosting the evaporator coils of several cases at the same time. 
   One problem is that defroster units of the existing art generate a lot of water vapor during a defrost cycle. If a line of contiguous cases is defrosted at the same time, an undesirable layer of frost may accumulate within the case. 
   Another problem caused by defrosting the cases at the same time is the need for greater electrical power at the same time. Because the defroster unit wiring is often on the same circuit for a given series of cases, this in turn causes a need for larger wiring sizes to carry the high current demand required for the defrost cycle. Additionally, because the cost of power from public utilities is often based on peak demands, the cost of power may be greatly increased by defrosting all the cases at the same time. 
   Yet another problem with defrost control systems of the existing art is that many are highly complex with digital components and programmable controllers. This makes repairs difficult for repairmen of ordinary skill in the refrigeration art, who are often only familiar with non-digital electrical components. The term “non-digital” refers to relays, contactors, sensors, coils, switches and any other component that generally does not process digital information. 
   One of the most expensive aspects of the existing practice of defrosting a series of contiguous cases at the same time is that it often leads to food spoilage. By shutting down the refrigeration cycles of contiguous cases at the same time, there can be an increase in the temperature of the food product in the cases. Also, there is often a greater increase in the display section temperature of each case due to the combined effect of defrosting several contiguous cases at the same time. 
   The applicant recognized a need for an improved method and apparatus for defrosting refrigerated display cases to avoid the problems created when refrigerated display cases are simultaneously defrosted and also to avoid the problems of having complex digital components. Thus applicant obtained U.S. Pat. No. 6,629,422 for a Sequential Defrosting of Refrigerated Display Cases using electric heaters. In the &#39;422 patent, applicant disclosed and claimed a time-initiated, time-terminated defrost control method using electric heat defrosting. Electric heaters apply heat to the outside portions of the evaporator coils and to the heat transfer fins normally attached to the outside portions of the evaporator coils. 
   An alternative source of heat for defrosting refrigerated display cases is the compressed vapor (“hot gas”) from the compressor. Hot gas defrost utilizes the hot gas to apply heat directly to the inside of the evaporator. Most hot gas defrost systems use the latent heat of condensation of the compressed vapor as the heat source, but some use only sensible heat of highly super heated vapor. Most hot gas defrost systems introduce the hot gas at the distributor and bypass the metering device. A defrost time control will operate the compressor during the defrost cycle and shut off the circulating air duct fans. At the same time, the control will energize the hot gas solenoid valve and allow the hot gas to enter the evaporator coil via the distributor and warm the evaporator, thus removing the buildup of frost. The availability of a portion of the energy used for hot gas defrost within the refrigeration system makes hot gas defrost attractive from an energy-saving standpoint. 
   Hot gas defrost is also attractive from an energy-saving standpoint because the hot gas warms the evaporator coil from within. The warm air blown across the outsides of evaporator coils by electric defrost heaters also heats up the cold case. 
   Traditional hot gas defrost, while attractive, has many of the drawbacks of traditional electric heater defrost. Defrosting several cold cases at the same time requires more hot gas—hot gas supplied by a compressor powered by electricity. As the compressor continues to run, the cost of power may be greatly increased by defrosting all the cases at the same time. The hot gas must be produced by same compressors used to provide refrigeration to other evaporators. To ensure sufficient head pressure for proper operation of the refrigeration system as a whole, additional controls are sometimes required. 
   The existing practice of defrosting a series of cold cases at the same time often leads to food spoilage and the expenses associated therewith. Shutting down the refrigeration cycles of contiguous cases at the same time can result in an increase in the temperature of the food product in the cases. Moreover, the combined effect of defrosting several contiguous cases at the same time often results in a greater increase in the display section temperature of each case. 
   Method and apparatus for sequentially defrosting a group of evaporators using hot gas would reduce or eliminate momentary high demands on the refrigeration system (whether for simultaneous hot gas defrost or for simultaneous cooling of multiple cases), thereby producing a more nearly constant load on the refrigeration system, reducing spoilage related to excessive defrosting, save energy, and permit use of smaller refrigeration systems while ensuring sufficient cooling capacity. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and apparatus for using hot gas to defrost sequentially each refrigerated display case (evaporator) in a group of refrigerated display cases (evaporators). A unique cross-feed line connects the distributor and the evaporator suction line for each evaporator in the group of evaporators. Like a typical hot gas defrost system, the sequential hot gas defrost system may be time-initiated and time-terminated or time-initiated and temperature-terminated. Each evaporator in the group of evaporators is defrosted in turn while the remaining evaporators in the group continue to operate in the refrigeration mode. 
   The advantages and features of the present invention will be apparent from the following description when read in conjunction with the accompanying drawings and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of a portion of a display case refrigeration system according to the prior art. 
       FIG. 2  is a schematic representation of a portion of a display case refrigeration system according to the present invention wherein the display case has three evaporators. 
       FIG. 3  is a schematic representation of an evaporator according to the present invention wherein the evaporator is being defrosted by hot refrigeration gas. 
       FIG. 4  is a schematic representation of an evaporator according to the present invention wherein the evaporator is cooling the circulating air within the display case. 
       FIG. 5  is a schematic representation of the display case refrigeration system shown in  FIG. 2  wherein an evaporator in one portion of the display case is being defrosted while the other two evaporators within the same display case continue to cool. 
       FIG. 6  is a view of an integrated orifice and check valve included in applicant&#39;s invention. 
       FIG. 7  is another view of the combination orifice and check valve shown in  FIG. 6 . 
       FIG. 8  is another view of the combination orifice and check valve shown in  FIGS. 6-7 . 
       FIG. 9  is still another view of the other end of the combination orifice and check valve shown in  FIGS. 6-8 . 
       FIG. 10  is still another view of the combination orifice and check valve shown in  FIGS. 6-9 . 
       FIG. 11  is still another view of the combination orifice and check valve shown in  FIGS. 6-10 . 
       FIG. 12  is another view of the combination orifice and check valve shown in  FIGS. 6-11 . 
       FIG. 13  is another view of the other end of the combination orifice and check valve shown in  FIGS. 6-12 . 
       FIG. 14  is a cross-sectional view along  14 - 14  of the combination orifice and check valve shown in  FIG. 10 . 
       FIG. 15  is a cross-sectional view along  15 - 15  of the combination orifice and check valve shown in  FIG. 10 . 
       FIG. 16  illustrates applicant&#39;s method of sequentially defrosting each evaporator in a group of evaporators using hot refrigeration gas. 
   

   DETAILED DESCRIPTION 
   In the following description of the invention, like numerals and characters designate like elements throughout the figures of the drawings. 
   Referring to  FIG. 1 , a typical refrigeration system  20  receives low pressure vapor from a series of evaporators (not shown; see  FIG. 2 ) and returns cool refrigerant to the evaporators. Low pressure vapor (also called low pressure gas, LPV, or LPG) from evaporator discharge lines (also called evaporator suction lines)  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36  is combined in a low pressure vapor header  38  (also referred to herein as the evaporator suction header, the same as the compressor suction header). The low pressure vapor is pulled through compressor suction lines  40 ,  42 , and  44  to compressors  46 ,  48 , and  50 , respectively. The compressors  46 ,  48 ,  50  compress the low pressure vapor to a high pressure vapor (HPV) and discharge the high pressure vapor through compressor discharge lines  52 ,  54 , and  56  (also referred to as high pressure vapor refrigerant lines), respectively, to a high pressure vapor header  58  (also referred to as a high pressure gas header). A condenser  60  then cools the high pressure vapor to a high pressure liquid (HPL), which is carried through a condenser discharge line  62  (also referred to as a high pressure liquid line) and collected in a high pressure liquid receiver  64 . From the high pressure liquid receiver  64 , the high pressure liquid is carried through a high pressure liquid refrigerant supply header  66  through evaporator HPL refrigerant lines  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 , and  82  to distributors of individual evaporators (not shown). Within each evaporator, a metering device  122 ,  124 , 126  (see  FIG. 2 ) meters high pressure liquid refrigerant into the evaporator distributor based on a temperature measured at the evaporator outlet (also referred to as the evaporator suction line). 
   Still referring to  FIG. 1 , for systems utilizing traditional hot gas defrost, a hot gas main supply line  84  downstream of the compressor(s) supplies high pressure vapor to the distributors. A hot gas main supply line isolation valve  86  is opened during the defrost cycle and closed during the refrigeration cycle. 
   Referring now to  FIG. 2 , applicant&#39;s refrigeration system with sequential hot gas defrost  100  shows evaporators  102 ,  104 , and  106 . During the refrigeration cycle of operation, the evaporators  102 ,  104 ,  106  receive high pressure liquid refrigerant (HPL) from the HPL header  66  through evaporator HPL supply lines  68 ,  70 , and  72 , respectively, and return low pressure vapor (LPV) to the low pressure vapor header  38  through evaporator suction lines  22 ,  24 , and  26 , respectively. 
   Still referring to  FIG. 2 , wherein the refrigeration system with hot gas defrost  100  is operating in the refrigeration cycle, introduction of high pressure liquid refrigerant into the evaporators  102 ,  104 ,  106  through distributors  112 ,  114 ,  116  is controlled by metering devices  122 ,  124 ,  126 , respectively. As discussed above, an automatic expansion valve, a thermal expansion valve, capillary tubing, or an orifice may be used as the metering device. Some expansion valves include back-flow preventers. For purposes of illustration, but not as a limitation, the metering devices  122 ,  124 ,  126  of  FIG. 2  are represented as thermal expansion valves (TEVs or TXVs). The metering devices  122 ,  124 ,  126  open and close as needed to supply high pressure refrigerant liquid to the distributors  112 ,  114 ,  116 , respectively, in response to the temperature of low pressure vapor leaving the evaporators  102 ,  104 ,  106  as measured by temperature sensors  132 ,  134 ,  136 , respectively. 
   Still referring to  FIG. 2 , evaporator suction line isolation valves  142 ,  144 , and  146  in evaporator suction lines  22 ,  24 , and  26 , respectively, permit selective shutoff of flow of low pressure vapor to the low pressure vapor header  38 . Evaporator high pressure liquid refrigerant line isolation valves  152 ,  154 ,  156 , permit selective shutoff of supply of high pressure liquid refrigerant to distributors  112 ,  114 ,  116 , respectively. When the isolation valve  86  is opened, high pressure vapor can be selectively supplied to the distributors  112 ,  114 , 116 , and thence to the evaporators  102 , 104 ,  106 , using valves  172 ,  174 , and  176 , respectively. 
   It will be understood by one skilled in the art that the use of expansion valves with built-in back flow preventers will eliminate the need for the valves  152 , 154 , and  156  in HPL supply lines  68 ,  70 , and  72 , respectively. It will be further understood by one skilled in the art that prior art hot gas defrost methods and systems required only (1) shutting off the supply of high pressure liquid refrigerant to a group of evaporator distributors (such as  112 ,  114 ,  116 ) and (2) supply of high pressure vapor to the group of evaporator distributors. The process is typically either time-initiated/time-terminated or time-initiated/temperature-terminated without regard to unique characteristics of particular evaporators and the cases cooled by those evaporators. 
   Referring again to  FIG. 2 , the refrigeration system with sequential gas defrost  100  according to applicant&#39;s invention includes cross-feed lines  202 ,  204 , and  206 . Cross-feed line  202  connects the distributor  112  associated with the evaporator  102  to the evaporator suction line  22  at a location between the evaporator  102  and the isolation valve  142  in the evaporator suction line  22 . Cross-feed line  204  connects the distributor  114  associated with the evaporator  104  to the evaporator suction line  24  at a location between the evaporator  104  and the isolation valve  144  in the evaporator suction line  24 . Cross-feed line  206  connects the distributor  116  associated with the evaporator  106  to the evaporator suction line  26  at a location between the evaporator  106  and the isolation valve  146  in the evaporator suction line  26 . 
   Still referring to  FIG. 2 , cross-feed isolation check valves  212 ,  214 ,  216  are located in the cross-feed lines  202 ,  204 ,  206 , respectively, adjacent the evaporator suction lines  22 ,  24 ,  26 , respectively. Orifices  222 ,  224 ,  226  are located in the cross-feed lines  202 ,  204 ,  206 , respectively, adjacent the distributors  112 ,  114 ,  116 , respectively. Liquid injection check valves  232 ,  234 ,  236  are located in the cross-feed lines  202 ,  204 ,  206 , respectively, between the cross-feed isolation check valves  212 ,  214 ,  216  and the orifices  222 ,  224 ,  226 , respectively. A line  242  tees off the cross-feed line  202  at a point between the cross-feed isolation check valve  212  and the liquid injection check valve  232 . A line  244  tees off the cross-feed line  204  at a point between the cross-feed isolation check valve  214  and the liquid injection check valve  234 . A line  246  tees off the cross-feed line  206  at a point between the cross-feed isolation check valve  216  and the liquid injection check valve  236 . The lines  242 ,  244 , and  246  are connected to a common cross-feed header  248 . 
   Referring now to  FIG. 3 , the evaporator  104  according to applicant&#39;s refrigeration system with sequential hot gas defrost  100  is shown in the defrost cycle. The defrosting evaporator  104  contains evaporator coils  105 . In the defrost mode, the valve  144  in the evaporator suction line  24  and the valve  154  in the high pressure liquid supply line  70  are closed. The valve  174  in the high pressure vapor supply line is opened, thereby permitting high pressure vapor  250  (represented for purposes of illustration by circle symbols) to enter the distributor  114  and the coils  105  of the evaporator  104 . As the high pressure vapor  250  gives up heat to defrost the evaporator coils  105 , some of the high pressure vapor  250  condenses to become high pressure liquid  252  (represented for purposes of illustration by “x” symbols). Near the distributor  114 , the coils  105  contain only high pressure vapor  250 . Moving through the coils  105  toward the evaporator suction line  24 , the coils  105  contain a mixture of high pressure vapor  250  and high pressure liquid  252 . In the evaporator suction line  24 , the coils contain only high pressure liquid  252 . The condensation of the high pressure vapor  250  to high pressure liquid  252  is accompanied by release of the refrigerant&#39;s latent heat of vaporization. Thus a small amount of high pressure vapor  250  can provide a substantial amount of heating to defrost the coils  105  of the evaporator  104 . 
   Still referring to  FIG. 3 , the cross-feed line  204  connects the evaporator suction line  24  and the distributor  114 . The cross-feed isolation check valve  214  in the cross-feed line  204  is open because the pressure P 3  in the evaporator suction line  24  exceeds the pressure P 4  in the cross-feed line  204  where the line  244  connects the cross-feed line  204  to the common header  248  (See  FIG. 2 ). The liquid injection check valve  234  is closed because the pressure P 1  associated with the high pressure vapor  250  in the distributor  114  is greater than the pressure P 4  of the high pressure liquid  252  in the cross-feed line  204  between the cross-feed isolation check valve  214  and the liquid injection check valve  234 . The orifice  224  permits high pressure vapor  250  to move only as far as the high-pressure check valve  234 . The cross-feed line  204 , the cross-feed isolation check valve  214 , the liquid injection check valve  234 , the orifice  224 , and the line  244  teeing off the cross-feed line  204  form a cross-feed assembly which is replicated for each evaporator in the evaporator group. 
   Referring now to  FIG. 4 , the evaporator  102  of applicant&#39;s refrigeration system with sequential hot gas defrost  100  is shown in the refrigeration mode. The non-defrosting evaporator  102  contains evaporator coils  103 . In the refrigeration mode, the valve  142  in the evaporator suction line  22  and the valve  152  in the high pressure liquid supply line  68  are open. The valve  172  in the high pressure vapor supply line  162  (also referred to as evaporator hot gas supply line) is closed, thereby prohibiting high pressure vapor  250  from entering the distributor  112  and the coils  103  of the evaporator  102 . As the high pressure liquid  252  enters the evaporator coils  103 , the high pressure liquid  252  first absorbs heat through the evaporator coils  103  to become a mixture of low pressure vapor  254  and low pressure liquid  256 . As the mixture of low-pressure refrigerant liquid  256  and low pressure vapor  254  progresses through the evaporator coils  103  and pick up more heat, the mixture of low pressure liquid refrigerant  256  and low pressure vapor  254  becomes low pressure vapor  254 . By the time the refrigerant reaches the evaporator suction line  22 , the refrigerant consists completely of low pressure vapor  254 . The evaporation of the high pressure liquid  252  to low pressure vapor  254  is accompanied by absorption of the refrigerant&#39;s latent heat of vaporization. 
   For clarity, the high pressure vapor  250  (HPG) in the drawings is indicated by symbols in the shape of a circle. Low pressure liquid  256  (LPL) in the drawings is indicated by symbols in the shape of a star. Low pressure vapor  254  (LPV) in the drawings is indicated by symbols in the shape of a diamond. High pressure liquid  252  (HPL) in the drawings is indicated by symbols in the shape of an ex (“x”). 
   Still referring to  FIG. 4 , the cross-feed line  202  connects the evaporator suction line  22  and the distributor  112 . The cross-feed isolation check valve  212  in the cross-feed line  202  is closed because the pressure P 4  in the cross-feed line  202  between the cross-feed isolation check valve  212  and the liquid injection check valve  232  exceeds the pressure P 5  associated with the low pressure vapor  254  in the evaporator suction line  22 . Between the cross-feed isolation check valve  212  and the liquid injection check valve  232 , the cross-feed line  202  contains high pressure liquid  252  from condensed high pressure vapor  250  bleeding back into the common header  248 . The liquid injection check valve  232  is open because the pressure P 4  associated with the high pressure liquid  252  in the cross-feed line  202  is greater than the pressure P 6  associated with the mixture of low pressure vapor  254  and low pressure liquid  256  in the distributor  112 . The orifice  222  meters a prescribed quantity of high pressure liquid  252  (based on the size of the orifice) into the distributor  112 . Immediately downstream of the orifice  222  (a low pressure environment), the high pressure liquid  252  immediately becomes a mixture of low pressure vapor  254  and low pressure liquid  256 . The cross-feed line  202 , the cross-feed isolation check valve  212 , the liquid injection check valve  232 , the orifice  222 , and the line  242  teeing off the cross-feed line  202  form a cross-feed line assembly identical to the cross-feed line assembly in  FIG. 3 . 
   It will be understood by one skilled in the art that the high pressure liquid  252  metered into the distributor  112  derives from another evaporator (e.g., evaporator  104 , see  FIG. 3  and  FIG. 5 ) undergoing hot gas defrost. The common cross-feed header  248  provides high pressure liquid  252  to the common cross-feed header  248  whenever any evaporator in the group is in the defrost mode. Any high pressure liquid  252  in the common cross-feed header  248  is available to any evaporator operating in the refrigeration mode. 
   Referring now to  FIG. 5 , three grouped evaporators according to applicant&#39;s refrigeration system with sequential hot gas defrost  100  are shown in operation. The evaporators  102  and  106  are shown in the refrigeration mode, while the evaporator  104  is shown in the defrost cycle. The system conditions, with evaporators  102  and  106  in the refrigeration mode and evaporator  104  in the defrost mode, are as follows: 
                                               High pressure vapor header valve (86)   Open           High pressure vapor valves 172, 176   Closed           (Refrigeration)           High pressure vapor valve 174 (defrost)   Open           Valves 142, 146 (refrigeration)   Open           Valve 144 (defrost)   Closed           Valves 152, 156 (refrigeration)   Open           Valve 154 (defrost)   Closed           Cross-feed isolation check valves 212, 216   Closed           (refrigeration)           Cross-feed isolation check valve 214 (defrost)   Open           Liquid injection check valves 232, 236   Open           (refrigeration)           Liquid injection check valve 234 (defrost)   Closed           Contents of distributors 112 &amp; 116   Mixture of           (refrigeration)   LPV and LPL           Contents of distributor 114 (defrost)   HPG           Contents of evaporator coils near distributor   Mixture of           112 &amp; 116   LPV and LPL           Contents of evaporator coils near distributor   Mixture of           114 (Defrost)   HPG and HPL           Contents of evaporator coils near exit 22 &amp; 26   LPV           (refrigeration)           Contents of evaporator coils near exit 24   HPL           (Defrost)                        
It will be understood by one skilled in the art that the cross-feed line  206 , the cross-feed isolation check valve  216 , the liquid injection check valve  236 , the orifice  226 , and the line  246  teeing off the cross-feed line  206  form another cross-feed line assembly identical to those shown in  FIGS. 3 and 4 .
 
   Referring now to  FIG. 3-4  and  FIGS. 6-9 , a unitary check valve/orifice  300  (CVO) provides the functions of both a check valve and also an orifice. The CVO  300  is especially suited for use in applicant&#39;s refrigeration system with sequential hot gas defrost  100 . In  FIG. 3 , for example, wherein the evaporator  104  is shown in the defrost mode, the liquid injection check valve  234  is closed due to pressure differential as explained above and the orifice  224  is not in use. In  FIG. 4 , wherein the evaporator  102  is shown in the refrigeration mode, the liquid injection check valve  232  is open and the orifice  222  high pressure liquid  252  bleeds into the distributor  112  to supplement the introduction of high pressure liquid  252  into the distributor  112  through the metering device  122 . 
   Referring now to the CVO  300  shown in  FIGS. 6-9 , an elongated housing  302 , formed by the connection of male threaded housing member  309  to female threaded housing member  311 , has line connections  304 ,  306  connected to a generally cylindrical outer wall  308  by tapered portions  310 ,  312 . The elongated housing  302  encloses an elongated shuttle member  314 . The shuttle member  314  has conical ends  316 ,  318  which conform generally to interior conical surfaces  320 ,  322 , respectively, within the housing  302 . A compression seal  324  is disposed within a groove  326  on the conical end  316  of the elongated shuttle member  314 . Another compression seal  328  is disposed within a groove  330  on the other conical end  318  of the elongated shuttle member  314 . A dog-leg orifice  332  includes an axial portion  334  and an angular portion  336 . The axial portion  334  of the dog-leg orifice  332  extends from the center of the conical end  316  only part way toward the conical end  318 . Somewhere between the middle of the elongated shuttle member  314  and the conical end  318  of the elongated shuttle member  314 , the angular portion  336  of the dog-leg orifice  332  angles toward the conical end  318  of the elongated shuttle member  314 . Thus one end  338  of the dog-leg orifice  332 , associated with the axial portion  334 , is centered on the conical end  316  of the elongated shuttle member  314 . The other end  340  of the dog-leg orifice  332  is located on the conical end  318  of the elongated shuttle member  314  at a position between the groove  330  and the generally cylindrical outer wall  308  and opposite the interior conical surface  322  of the tapered portion  312  of the housing  302 . 
   Referring now to  FIGS. 8-9 , channels  342  along the outside of the elongated shuttle member  314  provide passageways for fluid flow when the unitary CVO  300  is not operating as a check valve. 
   In  FIG. 6 , the unitary CVO  300  is illustrated in a closed position wherein no fluid flow occurs. When the fluid pressure  344  within the pipe connection  304  exceeds the fluid pressure  346  within the pipe connection  306 , the elongated shuttle member  314  is forced in the direction of arrow  348  so the compression seal  328  located on the conical surface  318  of the elongated shuttle member  314  seals against the interior conical surface  322  of the tapered portion  312  of the housing  302 . 
   In  FIG. 7 , the unitary CVO  300  is illustrated in an open position wherein flow is from the pipe connection  306  end of the unitary CVO  300  to the pipe connection  304  end of the unitary CVO  300 . When the fluid pressure  346  within the pipe connection  306  exceeds the fluid pressure  344  within the pipe connection  304 , the elongated shuttle member  314  is forced in the direction of arrow  350  so the compression seal  328  located on the conical surface  318  of the elongated shuttle member  314  is no longer in contact with the interior conical surface  322  of the tapered portion  312  of the housing  302 . Instead, the compression seal  324  located on the conical surface  316  of the elongated shuttle member  314  seals against the interior conical surface  320  of the tapered portion  310  of the housing  302 . Although flow is permitted in the open position shown in  FIG. 7 , the flow is restricted by the sizing of the dog-leg orifice  332 . 
   Thus applicant&#39;s unitary CVO  300  prohibits flow completely in one direction, as illustrated in  FIG. 6 , and, in the other direction as illustrated in  FIG. 7 , meters flow based on the size of the dog-leg orifice  332 . 
   Referring once again to  FIGS. 2-5 , it will now be understood that applicant&#39;s unitary CVO  300  replaces the following two-part combinations of check valve and orifice: liquid injection check valve  232  and orifice  222  in evaporator  102 ; liquid injection check valve  234  and orifice  224  in evaporator  104 ; and liquid injection check valve  236  and orifice  226  in evaporator  106 . 
   Referring now to  FIGS. 10-15 , another unitary CVO  400  according to applicant&#39;s invention includes an elongated housing  402 , formed by the connection of male threaded housing member  409  to female threaded housing member  411 , with line connections  404 ,  406  connected to a generally cylindrical outer wall  408  by tapered portions  410 ,  412 . The elongated housing  402  encloses an elongated shuttle member  414 . The shuttle member  414  has conical ends  416 ,  418  which conform generally to interior conical surfaces  420 ,  422 , respectively, within the housing  402 . A compression seal  424  is disposed within a groove  426  on the interior conical surface  420  of the housing  402 . Another compression seal  428  is disposed within a groove  430  on the other interior conical surface  422  of the housing  402 . A dog-leg orifice  432  in the shuttle member  414  has an axial portion  434  and an angular portion  436 . The axial portion  434  of the dog-leg orifice  432  extends from the center of the conical end  416  only part way toward the conical end  418 . Somewhere between the middle of the elongated shuttle member  414  and the conical end  418  of the elongated shuttle member  414 , the angular portion  436  of the dog-leg orifice  432  angles toward the conical end  418  of the elongated shuttle member  414 . Thus one end  438  of the dog-leg orifice  432 , associated with the axial portion  434 , is centered on the conical end  416  of the elongated shuttle member  414 . The other end  440  of the dog-leg orifice  432  is located on the conical end  418  of the elongated shuttle member  414  at a position opposite the interior conical surface  422  between the compression seal  428  and the generally cylindrical outer wall  408  of the housing  402 . 
   In  FIG. 10 , the unitary CVO  400  is illustrated in a closed position wherein no fluid flow occurs. When the fluid pressure  444  within the pipe connection  404  exceeds the fluid pressure  446  within the pipe connection  406 , the elongated shuttle member  414  is forced in the direction of arrow  448  so the compression seal  428  located on the interior conical surface  422  of the elongated shuttle member  414  seals against the conical surface  418  of the elongated shuttle member  414 . 
   In  FIG. 11 , the unitary CVO  400  is illustrated in an open position wherein flow is from the pipe connection  406  end of the unitary CVO  400  to the pipe connection  404  end of the unitary CVO  400 . When the fluid pressure  446  within the pipe connection  406  exceeds the fluid pressure  444  within the pipe connection  404 , the elongated shuttle member  414  is forced in the direction of arrow  450  so the compression seal  428  located on the interior conical surface  422  of the housing  402  is no longer in contact with the conical surface  418  of the elongated shuttle member  414 . Instead, the compression seal  424  located on the interior conical surface  420  of the housing  402  seals against the conical surface  416  of the elongated shuttle member  414 . Although flow is permitted in the open position shown in  FIG. 11 , the flow is restricted (i.e., metered) based on the sizing of the dog-leg orifice  432 . 
   Thus applicant&#39;s unitary CVO  400  prohibits flow completely in one direction, as illustrated in  FIG. 11 , and restricts (i.e., meters) flow in the other direction based on the size of the dog-leg orifice  432 , as illustrated in  FIG. 12 . 
   Referring once again to  FIGS. 2-5 , it will now be understood that applicant&#39;s unitary CVO  400  can replace the following two-part combinations of check valve and orifice: liquid injection check valve  232  and orifice  222  in evaporator  102 ; liquid injection check valve  234  and orifice  224  in evaporator  104 ; and liquid injection check valve  236  and orifice  226  in evaporator  106 . 
   Referring now to  FIG. 14 , a cross-section along  14 - 14  in  FIG. 10  shows the compression seal  426  located on the interior conical surface  420  of the housing  402 . 
   In  FIG. 15 , a cross-section along  15 - 15  in  FIG. 10  shows the compression seal  428  located on the interior conical surface  422  of the housing  402 . 
   Applicant&#39;s unitary CVO  300  and unitary CVO  400  function as a check valve in one flow direction and as an orifice in the opposite flow direction, thus simplifying installations normally requiring both a check valve and an orifice. 
   Referring now to  FIG. 16  in conjunction with the structure shown in  FIGS. 2-5 , shown therein is a method of sequentially defrosting a group of evaporators using compressed hot gas as the heat source according to applicant&#39;s invention. The steps are as follows:
     1. Within each evaporator in the group of evaporators, provide a cross-feed isolation check line between the evaporator suction line and the distributor, wherein each cross-feed line contains a cross-feed isolation check valve adjacent the evaporator suction line, an orifice adjacent the distributor, a liquid injection check valve between the cross-feed isolation check valve and the liquid injection check valve, and a common cross-feed header connecting each cross-feed line at a location between the cross-feed isolation check valve and the liquid injection check valve; wherein each cross-feed isolation check valve permits fluid flow from the evaporator suction line to the common cross-feed header when the pressure in the evaporator suction line exceeds the pressure in the common cross-feed header; wherein each liquid injection check valve permits fluid flow from the common cross-feed header to the distributor when the pressure in the common cross-feed header exceeds the pressure in the distributor; and wherein each orifice meters flow of high pressure liquid refrigerant from the common cross-feed header into the line to the distributor based on the sizing of the orifice.   2. With the group of evaporators in the refrigeration mode with compressors running, shut off the high pressure liquid supply to the metering device of an evaporator selected for sequential hot gas defrost from the group of evaporators.   3. Shut off the valve in the evaporator suction line from the selected evaporator.   4. Supply high pressure vapor refrigerant to the distributor associated with the evaporator selected for sequential hot gas defrost.   5. After a predetermined time, return defrosted evaporator to service in refrigeration mode as follows:
       A. Shut off the high pressure vapor refrigerant supply to the distributor associated with the defrosted evaporator.   B. Open the valve in the evaporator suction line from the defrosted evaporator.   C. Open the high pressure liquid supply valve to the metering device of the defrosted evaporator.   
       6. Repeat steps 1 through 5 for each additional evaporator in the group of evaporators until each evaporator has been defrosted.   

   The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.