Abstract:
In a refrigeration system having a pressurizer, a condenser, an expansion device and an evaporator, with the evaporator having an inlet header, an outlet header, and a plurality of channels therebetween, the outlet header has a liquid outlet and a vapor outlet and provision is made for separation of refrigerant liquid from refrigerant vapor. The liquid refrigerant is passed through a superheating heat exchanger to obtain complete evaporation and superheating prior to passing to the pressurizer. Various other features are provided to enhance the system operation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of U.S. Provisioanl Patent Application Ser. No. 60/587,793, filed Jul. 14, 2004, and entitled REFRIGERATION SYSTEM, which application is incorporated herein by this reference. 

   TECHNICAL FIELD 
   The invention relates generally to refrigeration systems and, more particularly to evaporators with parallel tubes requiring distribution of two-phase refrigerant. 
   The non-uniform distribution of two phase refrigerant in parallel tubes, for example in mini- or micro-channel heat exchangers, can significantly reduce heat exchanger efficiency. This is called maldistribution and is a common problem in heat exchangers with parallel refrigerant paths. Two-phase maldistribution problems are caused by the difference in density of the vapor and liquid phases. 
   In addition to the reduction of efficiency, two phase maldistribution may result in damage to the compressor because of liquid slugging through the evaporator. 
   DISCLOSURE OF THE INVENTION 
   The purpose of the current invention is to eliminate the evaporator deficiency associated with the maldistribution of two-phase refrigerant and to eliminate any harmful effect associated with liquid slugging through the evaporator. At the same time the invention avoids increased sizes and costs associated with additional components, such as, a superheating heat exchanger handling excessive thermal loads. 
   The present invention provides a closed loop refrigeration system comprising at least the following components: a suction line, a pressurizing means, a condenser, a liquid line, a superheating heat exchanger an expansion device, and an evaporator for cooling fluid. The evaporator has an inlet header, an outlet header, and refrigerant channels between the headers. External surfaces of the refrigerant channels are thermally exposed to the chilled or cooled fluid. The evaporator outlet header has a liquid outlet, a vapor outlet, and a means for liquid separation. The superheating heat exchanger has a high-pressure side and a low-pressure side. The high-pressure side carries liquid refrigerant from the liquid line. The low-pressure side carries refrigerant from the liquid outlet of the outlet header. The superheating heat exchanger is sized for complete evaporation of the non-evaporated liquid portion and provides a superheat at its low-pressure side outlet as required at evaporators outlets in each particular application. 
   Another major aspect of the invention is based on the inclusion of a liquid separator, which has a liquid outlet feeding the evaporator inlet header and a vapor outlet connected to the suction line at the outlet from the vapor outlet of the outlet header. 
   In the current invention the means for liquid separation in the evaporator outlet header is based on the gravity. The liquid outlet is placed in accordance with the direction of the gravity force and carries the non-evaporated liquid portion of two-phase refrigerant stream as it appears at the outlets from the channels of the evaporator. The vapor outlet is placed in accordance with the opposite direction of the gravity force and carries the vapor portion of two-phase refrigerant stream from the evaporator to the suction line. The diameters of the outlet header and of the liquid outlet are sized to provide adequate mass fluxes from the vapor and liquid outlets of the outlet header. The vapor outlet from the outlet header may have a restriction to compensate for pressure drop in the low-pressure side of the superheating heat exchanger. Also, the vapor outlet from the liquid separator may have a restriction to compensate for pressure drop in the evaporator. The pressuring means for vapor compression systems is a compressor. The pressurizing means for absorption systems consists of at least an absorber, a pump, and a generator. Air cooling evaporators use air as fluid; however, in other applications various secondary refrigerants are applicable. The expansion device may be used as a thermal expansion valve with a sensing bulb attached to the vapor outlet of the vapor header. When the liquid separator is applied, the sensing bulb is attached to the vapor outlet of the header downstream in respect to connection of the vapor outlet from the liquid separator. The expansion device, the liquid separator (if applied), the evaporator, and the superheating heat exchanger may be arranged as a common evaporator unit. There is an option to have a liquid-to-suction heat exchanger, which provides thermal contact liquid refrigerant outgoing from the condenser and vapor refrigerant outgoing from the low- pressure side of the superheating heat exchanger. The liquid line may consist of two parallel lines: a main liquid line with a main expansion device; and an additional line with the high-pressure side of the superheating heat exchanger and an additional expansion device. If the additional expansion device is a thermal expansion valve, then a sensing bulb may be attached to a vapor outlet of the superheating heat exchanger. If the additional expansion device is a capillary tube and the superheating heat exchanger is a shell-tube heat exchanger, then the capillary tube may be applied at the high-pressure side of the superheating heat exchanger inside the shell of the heat exchanger. 
   In the current invention the superheating heat exchanger is sized for complete evaporation of the non- evaporated liquid portion and provides a superheat at its low-pressure side outlet as required at evaporators outlets in each particular application. Since a superheating zone is removed from the evaporator, the evaporator capacity is substantially enhanced. Also, the reduced vapor quality at the evaporator inlet leads to improvement of the evaporator capacity. Since in the current invention the superheating heat exchanger involves just a portion of the entire mass flux provided by the compressor, costs and dimensions of the superheating heat exchanger are reduced as well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are illustrative of a mini-channel heat exchanger in accordance with the present invention. 
       FIG. 2  is pressure enthalpy diagram thereof. 
       FIG. 3  is a schematic illustration of a refrigeration system with a superheating heat exchanger in accordance with one aspect of the present invention. 
       FIG. 4  is a schematic illustration of an evaporator with a superheating heat exchanger and a liquid-to-suction heat exchanger in accordance with one aspect of the present invention. 
       FIG. 5  is a schematic illustration of the present invention employing a liquid separator. 
       FIG. 6  is a schematic illustration of the present invention employing two split liquid lines with two expansion devices. 
       FIG. 7  is a schematic illustration of the present invention employing two split liquid lines with two expansion valves. 
       FIG. 8  is a schematic illustration of the present invention employing two split liquid lines and a capillary tube inside the shell of a superheating heat exchanger. 
       FIG. 9  is a schematic illustration of the present invention employing two split liquid lines and a liquid separator. 
       FIG. 10  is a schematic illustration of vapor-compression refrigeration system operating in a cooling mode in accordance with one aspect of the invention. 
       FIG. 11  is a schematic illustration of vapor-compression refrigeration system operating in a heating mode in accordance with one aspect of the invention. 
       FIG. 12  is a schematic illustration of an absorption refrigeration system in accordance with one aspect of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a mini-channel or micro-channel heat exchanger with inlet header  1 , outlet header  2 , and tubes  3  interlaced with fins  4  externally exposed to a fluid to be chilled or cooled in the heat exchanger. As shown on the cross-sectional view, each tube  3  consists of a number of channels  5  to carry evaporating refrigerant. In the inlet to the inlet header  1  two-phase refrigerant is delivered to each tube and to each channel of tubes. Fluid inlet  6  faces first channels  7  of each tube and fluid outlet  8  faces last channels  9  of each tube. Obviously, this arrangement is a cross flow one. 
   The first challenge is to distribute equal amount of liquid and vapor portions of two-phase refrigerant between each tube. The second challenge is to distribute equal liquid and vapor portions of two-phase refrigerant between each channel of each tube. Refrigerant distributors have been useful to resolve the first challenge, but, the second challenge has remained unsolved. For example, air conditioners may have fluid temperature at inlet  5  equal to 80° F. and fluid temperature at outlet  6  equal to 58° F.; evaporating temperature is 45° F. In such cases loading temperature difference on the first channel is 80−45=35° R, but loading temperature difference on the last channel is 58−45=13° R, that is, 37% in respect to the loading temperature difference and thermal load on the first channel. If the first channel is properly fed and fully loaded, then the last channel is not fully loaded, liquid in the last channel is not fully evaporated and slugs through the evaporator, and the heat exchanger efficiency is equal to (100+37)/2=68.5% approximately. If the last channel is properly fed and fully loaded, then the first channel is overloaded, refrigerant in the first channel is substantially superheated and the heat exchanger deficiency is significant. 
   Effect of the maldistributed refrigerant is shown in  FIG. 2 . If no maldistribution exists, the regular vapor compression cycle for a compressor, a condenser, an expansion device, and an evaporator, is shaped as  1 - 2 - 3 - 4 - 1 , where  1 —is the compressor suction,  2 —is the compressor discharge,  3 —is the condenser outlet/expansion device inlet,  4 —is the evaporator inlet. If maldistribution of refrigerant takes place, some circuits of evaporators may be fed mostly by vapor and some circuits may be fed mostly by liquid. As a result, some circuits may have superheated vapor and some circuits may have liquid at their outlets. Appearance of liquid at the outlet, re-shapes the above-mentioned cycle to a shape  1 ′- 2 ′- 3 - 4 - 1 ′ and the compression process  1 ′- 2 ′ is moved to the two-phase zone. The non-evaporated liquid portion does not contribute in cooling of the fluid pumped through the evaporator and, as a result, the evaporator capacity is reduced. In addition, a compressor may be damaged if the non-evaporated liquid reaches its suction port. An attempt to design an evaporator operating with excessive refrigerant superheat to ensure no liquid at the evaporator outlet would result in further reduction of the evaporator capacity and COP. 
   The current invention is intended to complete evaporation, accomplish slight superheating in a superheating heat exchanger and to provide the cycle  1 - 2 - 3 - 3 ′- 4 ′- 1 ′- 1 , where  1 ,- 1  is superheating of vapor in the superheating heat exchanger;  3 - 3 ′ is sub-cooling of liquid in the superheating heat exchanger; and  4 ′- 1 ′ is cooling effect. Enthalpy difference of the process  4 ′- 1 ′ is equal to enthalpy difference of the process  4 - 1  of the regular vapor compression cycle. 
   In accordance with  FIG. 3  a refrigeration system consists of a closed loop with a compressor  10 , a condenser  11 , a liquid line  12 , an expansion device  13 , an evaporator  14  for cooling a fluid, superheating heat exchanger  15  and a suction line  16 . 
   The evaporator  14  has the inlet header  1  and the outlet header  2 . The outlet header  2  has a liquid outlet  17 , a vapor outlet  18 , and a means for liquid separation. The means for liquid separation are based on the gravity. The liquid outlet  17  is placed in accordance with the direction of the gravity force and the vapor outlet  18  is placed in accordance with the opposite direction of the gravity force. The liquid outlet  17  carries liquid and lubricant and the vapor outlet  18  carries vapor. The cross-sectional area of the vapor outlet header  2  and the cross-sectional area of the liquid outlet  17  are sized to provide adequate refrigerant mass fluxes from the outlets  17  and  18 . 
   The superheating heat exchanger  15  provides thermal contact between a high-pressure side  15   a  and a low-pressure side  15   b . The high-pressure side  15   a  carries liquid refrigerant from the liquid line  12  at the inlet to the expansion device  13 . The low-pressure side  15   b  carries liquid refrigerant mixed with lubricant outgoing from the liquid outlet  17 . The heat exchanger  15  is sized to provide complete evaporation of liquid refrigerant appeared in the outlet header  2  of the evaporator  14  and to accomplish some superheat at its low pressure outlet, recuperating heat to liquid refrigerant flowing through the liquid line  12 . The superheat at the outlet from the low-pressure side  15   b  of the superheated heat exchanger  15  should be the same as required at evaporators outlets in each particular application. It is important to note that the more substantial the two-phase refrigerant maldistribution is, the higher thermal loads are to be maintained, and the bigger sizes of the superheating heat exchanger  15  are required. Therefore, any efforts reducing the maldistribution should be considered and might be beneficial. 
   The vapor outlet  18  may have a restrictor  18   a  to compensate for pressure drop in the low-pressure side  15   b  of the superheating heat exchanger  15 . 
   Alternatively, the vapor outlet  18  may be connected to the driving side of an ejector pump  18   b  with the vapor outlet of the superheating heat exchanger connected to the driven side of the ejector pump  18   b  to compensate for pressure drip in the low-pressure side  15   b  of the superheating heat exchanger  15 . 
   The expansion device  13 , the evaporator  14 , and superheating heat exchanger  15  may be incorporated in one evaporator unit. 
   The expansion device  13  may be implemented as a capillary tube or as an orifice. If the expansion device  13  is an expansion valve, then a sensing bulb  19  of the valve should be located at outlet from the vapor outlet  18 . 
     FIG. 4  illustrates the difference between the traditional liquid-to-suction heat exchanger and the superheating heat exchanger  15 .  FIG. 4  shows a refrigeration system with a liquid-to-suction heat exchanger  20  providing thermal contact between a high-pressure side  20   a  and a low-pressure side  20   a . The high-pressure side  20   a  carries liquid refrigerant from the liquid line  12  prior to the inlet to the superheating heat exchanger  15 . The low-pressure side  20   b  carries vapor from the superheating heat exchanger  15  to the compressor  10 . The liquid-to suction heat exchanger  20  is not intended for the completion of the evaporation process as the superheating heat exchanger  15  is intended for. The function of the liquid-to-suction heat exchanger is to substantially increase superheat in the suction line  16  and to substantially increase a sub-cooling in the liquid line  12 . 
     FIG. 5  presents employment of a liquid separator  21 . The liquid separator  21  has two outlets: liquid outlet  22  and vapor outlet  23 . The liquid outlet  22  feeds the inlet header  1  of the evaporator  14 . The vapor outlet  23  is connected to the suction line  16  outgoing from the vapor outlet  18  of the outlet header  2 . The vapor outlet  23  may have a restrictor  23   a  as a compensator for refrigerant pressure drop in the evaporator  14  and its headers  1  and  2 . 
   The expansion device  13 , the evaporator  14 , the superheating heat exchanger  15 , and the liquid separator  21  may be incorporated in one evaporator unit. 
   The expansion device  13  may be implemented as a capillary tube or as an orifice. If the expansion device  13  is an expansion valve, then the sensing bulb  19  of the valve should be located at outlet from the vapor outlet  18  after a line connecting the vapor outlet  23  and the suction line  16 . 
     FIG. 6  illustrates a refrigeration system with the liquid line  12  split into two parts. The first part carries a major part of liquid refrigerant mass flux, and has the expansion device  13  attached to the inlet header  1 . The second part, which carries the remainder of the mass flux, includes the high-pressure side  15   a  of the superheating heat exchanger  15  and an additional expansion device  24  attached to the inlet header  1  as well. 
   If the expansion device  13  is an expansion valve, then the sensing bulb  19  of the valve should be located at outlet from the vapor outlet  18 . 
   It the expansion device  24  is an expansion valve, then a sensing bulb  25  of the valve should be located at outlet from the low-pressure refrigerant of the superheating heat exchanger  15  as per  FIG. 7 . In this case the expansion valve  24  operates on a reversed principle: it opens its orifice when the superheat is decreased, and it closes its orifice when superheat is increased. 
   If the expansion device  24  is a capillary tube, the capillary tube may be used as the high-pressure side  15   a  of the superheating heat exchanger  15  (i.e. within the superheating heat exchanger  15 ) as shown on  FIG. 8 . When, as a result of maldistribution, the amount of liquid in the outlet header  2  is increased, then the cooling effect on the capillary tube is increased as well, and the capillary tube capacity is increased as well. Thus, the increased refrigerant mass flow rate through the high-pressure side handles the increased amount of liquid in the outlet header  2 . 
     FIG. 9  adds the liquid separator  21  to the schematic of  FIG. 6 . Refrigerant expanded in the expansion device  13  and in the expansion device  24  feeds the liquid separator  21 . The liquid outlet  22  feeds the inlet header  1  of the evaporator  14 . The vapor outlet  23  is connected to the suction line  16  outgoing from the vapor outlet  18  of the outlet header  2 . All components on  FIG. 9  may be incorporated in one evaporator unit. 
   A liquid-to-suction heat exchanger is applicable to systems accommodating arrangements in  FIG. 5 ,  FIG. 6 ,  FIG. 7 ,  FIG. 8 , and  FIG. 9  in the same way as the liquid-to- suction heat exchanger shown on  FIG. 4 . 
     FIG. 10  and  FIG. 11  show a refrigerating system based on  FIG. 8 , but designed to operate in respective cooling and heating modes utilizing components shown in  FIG. 9 .  FIG. 10  relates to the cooling mode and  FIG. 11  relates to the heating mode. To enable the heating mode the refrigeration system has a fourway valve  25  and a suction accumulator  26  to handle refrigerant charge imbalance in the heating and cooling modes. Also, the system is equipped with check valves  27  and  28  in order to disable undesirable refrigerant streams when the operating mode is reversed from the cooling mode to the heating mode. Expansion devices  13  and  24  are by-directional-flow devices. During the heating mode the evaporator  14  functions as a condenser, the liquid separator  21  as a receiver, the condenser  11  as an evaporator, and the superheating heat exchanger  15  does not recuperate any thermal loads. 
   The expansion device  13 , the evaporator  14 , the superheating heat exchanger  15 , the liquid separator  21 , the additional expansion device  24 , and the check valves  27  and  28  may be fabricated as a separate evaporator unit  29 . 
   The liquid separator  21  and two split liquid lines introduced in  FIG. 6  are optional. 
   The condenser  11  may be a base for a condenser unit having the same component structure as the evaporator unit  29 .  FIG. 11  is a good illustration of this case: the unit condenser unit has a condenser, which is the evaporator  14 , a receiver, which is the liquid separator  21 , the expansion devices  13  and  24 , and the disabled superheating heat exchanger  15 . Again, the liquid separator  21  and two split liquid lines introduced in  FIG. 6  are optional for the condenser unit. 
     FIG. 12  shows an absorption system with evaporator concept shown in  FIG. 9 . In addition to components in  FIG. 9  the absorption system has a pressurizing means  30 , which includes a closed loop with the following components of absorption systems: an absorber  31 , a pump  32 , a heat exchanger  33 , a generator  34 , and a condenser  11 . As it was mentioned above the liquid separator  21  and two split liquid lines introduced in  FIG. 6  are optional. As well, a liquid-to-suction heat exchanger is optionally applicable in the same way as the liquid-to-suction heat exchanger shown on  FIG. 4 . 
   While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications in its structure may be adopted without departing from the spirit of the invention or the scope of the following claims.