Patent Publication Number: US-6901763-B2

Title: Refrigeration system

Description:
FIELD OF THE INVENTION 
   This invention relates to refrigeration systems, and more particularly, to refrigeration systems that include components operating on the vapor compression cycle for cooling a refrigerant and which are provided with suction line heat exchangers. 
   BACKGROUND OF THE INVENTION 
   Refrigeration systems such as heat pump systems used for heating and cooling, air conditioning systems used for cooling air, refrigerators and freezers and the like most in use today operate on the so-called vapor compression principle. In these systems, a refrigerant is compressed by a compressor and then passed to a gas cooler (including condensers) to cool and/or condense the compressed refrigerant while at high pressure. The high pressure refrigerant is then passed to an expansion device such as a capillary or an expansion valve and then to an evaporator at a lower pressure where the refrigerant absorbs the latent heat vaporization of the refrigerant and/or sensible heat. 
   The refrigerant then exits the evaporator and is returned to the inlet of the compressor at low pressure to be compressed so that the cycle can be repeated continuously. 
   Most such systems include an accumulator somewhere in the path between the evaporator and the compressor which principally serves to contain excess refrigerant to assure that the system is always charged with sufficient refrigerant to operate. Many such systems, particularly those operating on a transcritical refrigerant such as CO 2  also include a so-called suction line heat exchanger. Such suction line heat exchangers (also sometimes referred to as internal heat exchangers) may also be found in very large systems employing more or less conventional refrigerants and in systems of more modest size operating with the refrigerant commonly known as R134 a.    
   A suction line heat exchanger includes two fluid flow paths in heat transfer relation with one another. One of the flow paths typically interconnects the gas cooler of the system with the evaporator at a location upstream of the expansion device and downstream of the gas cooler. The other flow path is located in the path of refrigerant flow between the evaporator and the inlet of the compressor. 
   In systems using more or less conventional refrigerants, the presence or absence of a suction line heat exchanger depends upon whether the added efficiency produced by the presence of the suction line heat exchanger is sufficient to offset the cost of the suction line heat exchanger itself and whether the system, when installed in its operating environment, can tolerate the bulk, both in terms of volume and in weight, of an additional heat exchanger. A system typical of the latter situation is one that may be employed in a vehicular application such as an automotive air conditioner. 
   On the other hand, when operating with transcritical refrigerants such as CO 2 , suction line heat exchangers are considered almost a virtual necessity in spite of their cost, weight or bulk because of the considerable improvement in efficiency that is obtained with them with such refrigerants. 
   Given modern day concerns for energy and the cost thereof, it is highly desirable that such a refrigeration system be as efficient as possible so as to minimize the expense of energy. The present invention is directed to improving the efficiency of a vapor compression refrigeration system including a suction line heat exchanger by obtaining even higher levels of efficiency than those obtainable with today&#39;s technology. 
   SUMMARY OF THE INVENTION 
   It is the principal object of the invention to provide a new and improved refrigeration system of the vapor compression type that employs a suction line heat exchanger by increasing the efficiency thereof. It is also a principal object of the invention to provide a new and improved method of operating a vapor compression refrigeration system of the type employing a suction line heat exchanger. 
   According to one object of the invention, there is provided a refrigeration system that includes a compressor having an inlet and an outlet, a gas cooler connected to the compressor outlet to cool compressed refrigerant received from the compressor and an evaporator connected to the gas cooler for receiving cooled, compressed refrigerant therefrom. The system includes a suction line heat exchanger which has a first refrigerant flow path interconnecting the gas cooler and the evaporator and a second refrigerant flow path in heat exchange relation with the first refrigerant flow path and interconnecting the evaporator and the inlet of the compressor. The system including the evaporator is constructed to deliver refrigerant from the evaporator to the second refrigerant flow path in the suction line heat exchanger at a quality less than 1 and to deliver refrigerant from the second flow path of the suction line heat exchanger to the inlet of the compressor at a quality substantially equal to 1 or in a super heated condition. 
   In one embodiment of the invention, an accumulator is located in the system and is located downstream of the second flow path and upstream of the inlet of the compressor. 
   In another embodiment of the invention, the system is provided with a compressor, a gas cooler and an evaporator as before. An accumulator is connected to the evaporator to receive refrigerant therefrom and a suction line heat exchanger is located in the system and has a first refrigerant flow path interconnecting the gas cooler and the evaporator and a second refrigerant flow path in heat exchange relation with the first refrigerant flow path and interconnecting the accumulator and the compressor inlet and receiving refrigerant from the accumulator at a quality less than 1 and delivering the refrigerant to the compressor inlet at a quality substantially equal to 1 or in a super heated condition. 
   According to the foregoing embodiment of the invention, the accumulator is a housing having an intended level of liquid refrigerant and a refrigerant vapor space above the intended level of liquid refrigerant. A first outlet from the accumulator is disposed above the intended level of liquid refrigerant and a second outlet from the accumulator is located below the intended level of liquid refrigerant. The first and second outlets are in fluid communication with each other and with the compressor inlet. 
   In a preferred embodiment, an accumulator such as mentioned before is constructed so that liquid refrigerant within the accumulator is entrained or educed into the refrigerant vapor. 
   One embodiment of the invention contemplates that the second outlet of the accumulator is disposed in a wall of the housing separate from the first outlet. 
   Preferably, the accumulator includes a tube within the housing and both the outlets comprise respective inlet ports in the tube. 
   In one embodiment, the inlet port defining the first outlet is upstream of the inlet port defining the second outlet to provide entrainment and/or eduction of the liquid refrigerant. 
   A highly preferred embodiment contemplates that the tube be a “U” or “J”-shaped tube having a first leg having the first inlet therein at a location above the intended level of liquid refrigerant and a second leg connected to the first leg by a bight and having the second outlet below the intended level of liquid refrigerant. 
   In such an embodiment, the accumulator may also include an intended level of system lubricant below the intended level of refrigerant liquid and the bight is located below the intended level of system lubricant and includes a system lubricant inlet port therein. According to this embodiment, lubricating oil from the system is also educed from the accumulator by the flow of refrigerant vapor therefrom. 
   According to another facet of the invention, there is provided a method of increasing the efficiency of a system including a vapor compression cooling cycle and having an evaporator with an inlet connected to an outlet of a gas cooler whose outlet in turn is connected to the inlet of a compressor. The compressor has an outlet connected to the inlet of the gas cooler and a suction line heat exchanger having two fluid flow paths in heat exchange relation with one another is provided. One of the flow paths is located between the evaporator inlet and the compressor outlet and the other flow path is located between the evaporator outlet and the compressor inlet. Refrigerant is located in the system and is of the type that may exist as a vapor, a liquid or a mixture of vapor and liquid whose quality at a given point is defined as the weight ratio of the mass of refrigerant vapor to the combined mass of refrigerant vapor and liquid refrigerant at the given point. The method includes the steps of (a) introducing refrigerant into the other flow path of the suction line heat exchanger at a quality less than 1; and (b) introducing refrigerant that has passed through the second flow path into the compressor inlet at a quality that is substantially equal to 1 or in a super heated condition. 
   According to the invention, a method of operating a refrigeration system having a vapor compression cooling cycle and of the type generally described previously includes the steps of (a) introducing refrigerant from an evaporator outlet into an accumulator; (b) discharging refrigerant having a quality less than 1 from the accumulator into the other flow path of the suction line heat exchanger; and (c) introducing refrigerant having a quality substantially equal to 1 or super heated vapor from the other flow path into the compressor inlet. 
   In one embodiment of the invention, the step of discharging refrigerant having a quality less than 1 from the accumulator into the other flow path of the suction line heat exchanger is performed by entraining or educing liquid refrigerant from the accumulator by refrigerant vapor exiting the accumulator to the compressor inlet. 
   In one embodiment, the latter step is performed within the accumulator while in another embodiment, the latter step is performed downstream of the accumulator. 
   Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of one form of vapor compression system made according to the invention; 
       FIG. 2  is a schematic of a modified embodiment of the refrigeration system; 
       FIG. 3  is a somewhat schematic, sectional view of one type of accumulator and educing system that may be employed in the invention; and 
       FIG. 4  is a somewhat schematic cross-sectional view of another form of accumulator and eduction system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Preferred embodiments of a refrigeration system made according to the invention and methods of operating the same will be described herein principally in the environment of so-called vehicular air conditioning systems. However, it is to be understood that the principles of the invention may be employed with efficacy in cooling cycles utilized in heat pumps, and in refrigeration systems generally, including refrigerators, freezers and other cooling devices as, for example, cooling systems for electronic components. Further, the invention is also useful in non-vehicular applications as well. Consequently, no limitation as to any particular type of refrigeration system or particular environment of use is intended except insofar as specifically stated in the appended claims. 
   Reference herein will be made to certain terms as, for example, the “quality of the refrigerant”. Quality is as conventionally defined, namely, the weight ratio of the mass of refrigerant in the vapor phase to the total mass of refrigerant, i.e., the combined mass of liquid refrigerant and refrigerant vapor, at a given point in the system. Thus, refrigerant wholly in the vapor phase will have a quality of 1 while refrigerant wholly in the liquid phase will have a quality of zero. Refrigerant that is both in the liquid and vaporous phase will have a quality greater than zero and less than 1, the exact number being determined by the ratio of refrigerant vapor to total refrigerant. 
   A quality “substantially equal to 1” is a refrigerant having a quality of 1 or possibly slightly less. The quality will be such that liquid refrigerant present, if any, will be insufficient to cause damage to the system compressor. The deviation from a quality of 1 that is tolerable will depend on both the compressor and refrigerant used in the system. 
   A quality of “less than 1” means a refrigerant that contains sufficient liquid refrigerant that, if passed to the system compressor, could damage the compressor. 
   The term “gas cooler” is intended to include condensers. 
   The terms “eduction” and “entrainment” are used interchangeably. 
   With the foregoing in mind, embodiments of the invention will be described. With reference to  FIG. 1 , the same is seen to include a compressor  10  for a refrigerant. The compressor  10  includes an outlet  12  and inlet  14 . The outlet  12  is connected to an air/gas or liquid heat exchanger in the form of a gas cooler  16 . Compressed refrigerant from the compressor flows in a line  18  to the gas cooler where is it cooled and/or condensed, typically by air, flowed through the gas cooler  16  by means of a fan  20  or the like. However, cooling for the refrigerant can be accomplished by other means as, for example, by using a liquid coolant. 
   From the gas cooler, the compressed refrigerant at high pressure passes in a line  22  to a first flow path  24  within a suction line heat exchanger  26 . The suction line heat exchanger also includes a second flow path  28  which is in heat exchange relation with the first flow path  24 . 
   From the first flow path  24 , the refrigerant is connected by a suitable conduit  30  to an expansion device  32  which may be in the form of an expansion valve, a capillary tube, or any other type of expansion device usable in refrigeration systems. The expansion device  32  reduces the pressure of a refrigerant which is then passed along a conduit  34  to the inlet  36  of an evaporator  38 . As shown, the evaporator  38  is an air/liquid heat exchanger and the liquid refrigerant, now at low pressure, is evaporated by means of an air stream passed through the evaporator  38  by a fan  40 . Within the evaporator  38 , the latent heat of evaporation as well as sensible heat is rejected to the air stream generated by the fan  40 . Of course, the latent heat and sensible heat of the refrigerant could be rejected to a liquid coolant, if desired. 
   Evaporated refrigerant emerges from the evaporator  38  at an outlet  40  and is conducted by a conduit  42  to the second flow path  28  of the suction line heat exchanger  26 . According to the invention, refrigerant emerging from the evaporator  38  at the outlet  40  and entering the second flow path  28  is at a quality less than one. Qualities as high as 0.9-0.95 provide increased efficiency of the system as will be described. However, increases in efficiency are increased for even lower qualities. The main point is that the quality be less than one as previously defined and have a lower limit that is sufficiently high that the desired heat rejection from the air to the refrigerant within the evaporator  38  occurs. 
   From the second flow path  28  of the suction line heat exchanger  26 , the refrigerant is now passed at a relatively high quality, not necessarily, but preferably, substantially equal to one as previously defined, by a conduit  44  to a conventional accumulator  46  which in turn discharges through a conduit  48  to the inlet  14  of the compressor  10 . Refrigerant leaving the accumulator  46  is at a quality that is substantially equal to one as previously defined. It is desirable, though not absolutely necessary, that the refrigerant entering the compressor inlet  14  be substantially at or slightly above its saturation temperature as opposed to a super heated temperature to reduce the heat loading on the compressor  10 . However, in some cases super heated vapor may be present and tolerable in the system. It is also desirable, as is well known, that the quality be substantially equal to one so that liquid refrigerant in a quantity that is sufficient to damage the compressor  10  during the compression process is not present. 
   In conventional systems of this sort, it has been typical to place the accumulator  46  upstream of the second flow path  28  of the suction line heat exchanger  26  and downstream of the evaporator outlet  40 . In such a conventional configuration, saturated refrigerant vapor enters the suction line heat exchanger  26  which then is superheated as a result of the heat exchange with the high pressure refrigerant stream exiting the gas cooler  16 . Superheated refrigerant vapor has a lesser density than saturated vapor and consequently reduces the efficiency of the compressor. Thus system efficiency is increased by locating the accumulator  40  between the compressor inlet  14  and the second flow path  28  of the suction line heat exchanger  26  as in this embodiment of the invention. 
   A further efficiency occurs through use of the invention in the configuration illustrated in FIG.  1 . With the suction line heat exchanger  26  located between the evaporator outlet  40  and the accumulator  46 , the fact that two phase refrigerant, i.e., refrigerant having a quality less than one, is present in heat exchange relation with high pressure refrigerant received from the gas cooler  16 , there is a greater reduction in the temperature of the compressed refrigerant as it exits the first flow path  24  because of a greater temperature drop along the first flow path  24 . This reduction has the effect of reducing the quality of the refrigerant entering the evaporator  38  which in turn has the effect of reducing possible flow maldistribution within the evaporator for greater efficiency. This in turn has the effect of improving evaporator capacity because the evaporator is used more effectively with fewer regions seeing superheated vapor as well as improving air side temperature distribution of air driven by the fan  40  through the evaporator  38 . 
   Furthermore, because the second flow path  28  receives two phase refrigerant, and refrigerant flow therein is two phase along at least part of its length, the second flow path  28  operates isothermally over much of its length. This means that the suction line heat exchanger is more effective since it does not materially contribute to refrigerant superheat entering the compressor  10  and has the beneficial effect of lowering the quality of the refrigerant entering the evaporator to provide improved evaporator capacity. 
   Turning now to  FIG. 2 , a highly preferred embodiment of the invention that provides a greater degree of control and regulation is described. Where like components are employed, like reference numerals are given. 
   In the embodiment illustrated in  FIG. 2 , the accumulator  46  is located between the evaporator outlet  40  and the second flow path  28  of the suction line heat exchanger  26 . The suction line heat exchanger  26 , and specifically, the second flow path  28  thereof, discharges into the inlet  14  of the compressor. 
   In this embodiment, refrigerant at a quality of less than 1 is placed in a conduit  50  that interconnects the outlet side of the accumulator  46  and the inlet side of the second flow path  28 . Within the suction line heat exchanger&#39;s second flow path  28 , any liquid phase refrigerant is evaporated so that refrigerant at a quality substantially equal to 1 or as a super heated vapor is flowed through a conduit  52  to the inlet  14  of the compressor  10 . The embodiment of  FIG. 2  is particularly useful in vehicular air conditioning systems. Such systems are typically optimized with the vehicle engine at idle speed. At idle speed, the mass flow rate of refrigerant through the vehicular air conditioning system is at a minimum and it is desired that it be sufficient so as to provide adequate cooling. At higher engine speeds, the mass flow rate of refrigerant is increased as compressor speed is increased and attaining the desired cooling is not a problem. Consequently, it is at an idle condition where greatest efficiency is required, i.e., it is at idle conditions where refrigerant in two phases, i.e., at a quality less than 1, is most required in the second flow path  28  of the suction line heat exchanger  26 . 
   In order to assure that refrigerant at the desired quality less than 1 is placed in the conduit  50 , the invention proposes certain modifications to the accumulator  46 . 
     FIG. 3  shows one such modification. 
   In the usual case, the accumulator  46  includes a housing  60 . Lines  62  and  64  within the housing, which in actual practice are imaginary, respectively designate the intended level of liquid refrigerant and the intended level of lubricant within the housing  60 . A U or J-shaped tube  66  is located within the housing  60  and includes a first leg  68  having an open end  70  which is located above the intended level of liquid refrigerant  62 . The tube  66  includes a second leg  72  which is connected to the first leg  68  by a bight  74 . It will be noted that bight  74  is located below the intended level of lubricant  64  within the housing  60 . The housing also includes an inlet (not shown). 
   The upper end of the leg  72  extends out of the housing  70  and is connected to a conduit  76  which extends to a tee  78 . The tee  78  is connected the line  50  and extends to the second flow path  28  of the suction line heat exchanger  26  (FIG.  2 ). 
   The accumulator housing  60  also includes an outlet  80  that is located below the intended level of liquid refrigerant  62  and above the intended level of lubricant  64 . The outlet  80  is also connected to the tee  50 . 
   Finally, a fluid flow restriction  84 , such as a valve, is located in the conduit  76  as illustrated in FIG.  3 . 
   In operation, refrigerant is discharged into the accumulator  48  and to the extent it is in two phases, it will separate into vapor which will occupy a vapor space  86  above the intended level of liquid refrigerant  62  and liquid refrigerant which will occupy the volume between the two lines  62  and  64 . Lubricant, conventionally carried by the refrigerant for purposes of lubricating the compressor  10  (FIGS.  1  and  2 ), settles to the bottom of the housing. 
   The bight  74  includes a small opening  88  below the intended level of lubricant  64 . 
   In any event, refrigerant vapor will enter the tube  66  through the open end  70  and pass downwardly past the port  88  where it will entrain or educt lubricant from the housing  70  in the flowing refrigerant vapor stream to be carried to the compressor  10  to lubricate the same. At the same time, liquid refrigerant will be urged out of the outlet  80  to the tee  78  where it will mix with the refrigerant vapor and entrained lubricant which exits the upper end of the leg  72 . The restriction  84  provides a desired regulation of the ratio of refrigerant vapor flow to liquid refrigerant flow to achieve the desired quality of refrigerant to be directed to the second flow path  28  of the suction line heat exchanger  26 . 
     FIG. 4  shows a modified embodiment of an accumulator. Where identical components are employed, they are given the same reference numerals and will not necessarily be redescribed in the interest of brevity. In this embodiment, the outlet  80  is omitted in favor of one or more ports in the leg  72 . The ports are given the reference numeral  92  and as can be appreciated from  FIG. 4 , are located below the intended level of liquid refrigerant  62  and above the intended level of lubricant  64 . The ports  92  are simply small holes, much like the port  88  for the lubricant. As a consequence, when refrigerant vapor from the space  86  enters the open end  70  of the tube  66  and passes therethrough to the conduit  50 , lubricant is entrained or educted at the port  88  and liquid phase refrigerant is educted or entrained into the vapor stream at the ports  92 . Consequently, a stream emerges from the accumulator shown in  FIG. 4  to the conduit  50  that has a quality less than 1. 
   The particular quality desired can be controlled by appropriate sizing of the ports  92  as well as by selection of the number of the ports  92 . 
   The embodiment of  FIG. 4  has the advantage over that shown in  FIG. 3  in that the flow restriction  84  can be omitted along with the outlet  80  and the tee  78  to accomplish the same results with a relatively minor addition to a conventional accumulator. The ports  92  can be simple holes or may be angled in the direction of refrigerant flow to provide a venturi-like action. 
   Most interestingly, modern day accumulators in refrigeration systems are conventionally designed to prevent any liquid refrigerant from exiting the accumulator in order to protect the compressor from damage. In the embodiments illustrated in  FIGS. 2 ,  3  and  4 , the desired operation is just the opposite, namely, that the accumulator is designed to intentionally cause liquid refrigerant to leave the accumulator to be directed to the second flow path  28  of the suction line heat exchanger as a result of being educted by or entrained in the exiting flow of saturated refrigerant vapor to the suction line heat exchanger  26 . The embodiments shown in  FIGS. 3 and 4  provide simple and inexpensive means of accomplishing this function with the embodiment of  FIG. 4  providing even greater simplicity than that of FIG.  3 . 
   As a consequence of the invention, in any of its embodiments, two phase refrigerant, that is, a refrigerant having a quality of less than 1, is directed to the second flow path  28  or low pressure side of the suction line heat exchanger  26  to improve the efficiency of operation of the same by lowering the quality of the compressed refrigerant on the high pressure side that is flowing to the evaporator  38 . Furthermore, because there is isothermal operation within the second flow path  28  over much of its length, refrigerant applied to the compressor inlet  14  is at a considerably lower temperature than in conventional systems. This provides advantages in terms of reducing the thermal load on the compressor  10  and is highly desirable in that thermal degradation of the lubricant typically contained in such systems is minimized or virtually eliminated altogether. Thus, not only is efficiency of operation of the entire system enhanced, but system Ion-gevity is increased as well.