Patent Publication Number: US-7721569-B2

Title: Method and apparatus for control of carbon dioxide gas cooler pressure by use of a capillary tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/755,947, entitled METHOD AND APPARATUS FOR CONTROL OF CARBON DIOXIDE GAS COOLER PRESSURE BY USE OF A CAPILLARY TUBE, filed on Jan. 13, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to vapor compression systems and, more particularly, to a transcritical vapor compression system in which the efficiency and capacity of the system can be adjusted. 
   2. Description of the Related Art 
   Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Such systems typically employ working fluids, or refrigerants, that remain below their critical pressure throughout the entire vapor compression cycle. Some vapor compression systems, however, such as those employing carbon dioxide as the refrigerant, typically operate as transcritical systems wherein the refrigerant is compressed to a pressure exceeding its critical pressure and wherein the suction pressure of the refrigerant is less than the critical pressure of the refrigerant, i.e., is a subcritical pressure. The basic structure of such a system includes a compressor for compressing the refrigerant to a pressure that exceeds its critical pressure. Heat is then removed from the refrigerant in a first heat exchanger, e.g., a gas cooler. The pressure of the refrigerant exiting the gas cooler is reduced in an expansion device and the refrigerant then absorbs thermal energy in a second heat exchanger, e.g., an evaporator, before being returned to the compressor. The first heat exchanger of such a system can be used for heating purposes, alternatively, the second heat exchanger can be used for cooling purposes. 
     FIG. 1  illustrates a typical transcritical vapor compression system  10 . In the illustrated example, a two stage compressor is employed having a first compression mechanism  12  and a second compression mechanism  14 . The first compression mechanism compresses the refrigerant from a suction pressure to an intermediate pressure. An intercooler  16  is positioned between the first and second compression mechanisms and cools the intermediate pressure refrigerant. The second compression mechanism then compresses the refrigerant from the intermediate pressure to a discharge pressure that exceeds the critical pressure of the refrigerant. The refrigerant is then cooled in a gas cooler  18 . In the illustrated example, a suction line heat exchanger  20  further cools the high pressure refrigerant before the pressure of the refrigerant is reduced by expansion device  22 . The refrigerant then enters evaporator  24  where it is boiled and cools a secondary medium, such as air, that may be used, for example, to cool a refrigerated cabinet. The refrigerant discharged from the evaporator  24  passes through the suction line heat exchanger  20  where it absorbs thermal energy from the high pressure refrigerant before entering the first compression mechanism  12  to repeat the cycle. 
   The capacity and efficiency of such a transcritical system can be regulated by regulating the pressure of the refrigerant in gas cooler  18 . The pressure of the high side gas cooler may, in turn, be regulated by regulating the mass of refrigerant contained therein which is dependent upon, among other things, the total charge of refrigerant actively circulating through the system. It is known to provide a reservoir in communication with the system for retaining a variable mass of refrigerant. The total charge of refrigerant actively circulating through the system can then be adjusted by changing the mass of refrigerant contained within the reservoir. By regulating the mass of refrigerant actively circulated through the system, the pressure of the refrigerant in the gas cooler can also be regulated. One problem associated with use of such reservoirs to contain a variable mass of refrigerant is that they can increase the cost and complexity of the system. 
   An alternative apparatus and method for adjusting the efficiency and capacity of a transcritical vapor compression system is desirable. 
   SUMMARY OF THE INVENTION 
   The present invention provides a vapor compression system that includes an expansion device in the form of a capillary tube and means for controlling the temperature of the refrigerant within the capillary tube. The temperature of the refrigerant within the capillary tube can be adjusted to control the ratio of refrigerant liquid to refrigerant vapor in the capillary tube and, thus, the density of the refrigerant within the tube. Regulating the temperature, and consequently density, of the refrigerant also regulates the velocity and mass flow rate of refrigerant through the capillary tube which in turn regulates the capacity of the system. 
   The invention comprises, in one form thereof, a transcritical vapor compression system including a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a compressor, a first heat exchanger, a first capillary tube and a second heat exchanger. The compressor compresses the refrigerant from a low pressure to a supercritical pressure. The first heat exchanger is positioned in a high pressure side of the fluid circuit and the second heat exchanger is positioned in a low pressure side of the fluid circuit. The first capillary tube reduces the pressure of the refrigerant from a supercritical pressure to a relatively lower pressure and refrigerant passes through the first capillary tube at a velocity having a maximum value substantially equivalent to the critical velocity of the refrigerant. Means for controlling the temperature of the refrigerant in the first capillary tube is also provided. 
   The present invention comprises, in another form thereof, a transcritical vapor compression system including a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a compressor, a first heat exchanger, a first capillary tube and a second heat exchanger. The compressor compresses the refrigerant from a low pressure to a supercritical pressure. The first heat exchanger is positioned in a high pressure side of the fluid circuit and the second heat exchanger is positioned in a low pressure side of the fluid circuit. The first capillary tube reduces the pressure of the refrigerant from a supercritical pressure to a relatively lower pressure and refrigerant passes through the first capillary tube at a velocity having a maximum value substantially equivalent to the critical velocity of the refrigerant. A device disposed in thermal exchange with the fluid circuit proximate the first capillary tube is also provided whereby the temperature of the refrigerant in the first capillary tube is adjustable with the device. 
   The present invention comprises, in yet another form thereof, a transcritical vapor compression system including a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a compressor, a first heat exchanger, a first capillary tube and a second heat exchanger. The compressor compresses the refrigerant from a low pressure to a supercritical pressure. The first heat exchanger is positioned in a high pressure side of the fluid circuit and the second heat exchanger is positioned in a low pressure side of the fluid circuit. The first capillary tube reduces the pressure of the refrigerant from a supercritical pressure to a relatively lower pressure and the refrigerant passes through the first capillary tube at a velocity having a maximum velocity substantially equivalent to the critical velocity of the refrigerant. An internal heat exchanger exchanges thermal energy between the refrigerant at a first location in the fluid circuit between the first heat exchanger and the first capillary tube and the refrigerant at a second location in the low pressure side of the fluid circuit. 
   The present invention comprises, in a further form thereof, a method of controlling a transcritical vapor compression system, including providing a fluid circuit circulating a refrigerant in a closed loop. The fluid circuit has operably disposed therein, in serial order, a compressor, a first heat exchanger, a first capillary tube and a second heat exchanger. The refrigerant is compressed from a low pressure to a supercritical pressure in the compressor. Thermal energy is removed from the refrigerant in the first heat exchanger. The pressure of the refrigerant is reduced as it is passed through the first capillary tube. Thermal energy is added to the refrigerant in the second heat exchanger. The capacity of the system is regulated by controlling the mass flow rate of the refrigerant through the first capillary tube. Such a method may involve adjusting the temperature of the refrigerant while passing the refrigerant through the first capillary tube at a substantially constant velocity. 
   An advantage of the present invention is that the capacity and efficiency of the system can be regulated with inexpensive non-moving parts. Thus, the system of the present invention is less costly and more reliable than prior art systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic representation of a prior art vapor compression system; 
       FIG. 2  is a schematic view of a vapor compression system in accordance with the present invention; 
       FIG. 3  is a graph illustrating the thermodynamic properties of carbon dioxide; and 
       FIG. 4  is a schematic view of another vapor compression system in accordance with present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates an embodiment of the invention, the embodiment disclosed below is not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise form disclosed. 
   DESCRIPTION OF THE PRESENT INVENTION 
   A vapor compression system  30  in accordance with the present invention is schematically illustrated in  FIG. 2  as including a fluid circuit circulating refrigerant in a closed loop. System  30  has a compression mechanism  32  which may be any suitable type of compression mechanism such as a rotary, reciprocating or scroll-type compressor mechanism. The compression mechanism  32  compresses the refrigerant, e.g., carbon dioxide, from a low pressure to a supercritical pressure. A heat exchanger in the form of a conventional gas cooler  38  cools the refrigerant discharged from compression mechanism  32 . Another heat exchanger in the form of suction line heat exchanger  40  further cools the high pressure refrigerant. The pressure of the refrigerant is reduced from a supercritical pressure to a lower subcritical pressure by an expansion device in the form of a capillary tube  42 . 
   The capillary tube  42  can be a piece of drawn copper tubing, for example. The dimensions of the capillary tube  42  can be approximately the same as the typical dimensions of a conventional capillary tube. For example, the capillary tube  42  can have an inside diameter of approximately between 0.5 mm and 2.0 mm and a length approximately between 1 meter and 6 meters, however, capillary tubes having other dimensions may also be used with the present invention. The inside diameter as well as an equivalent roughness of the capillary tube  42  can be constant along the length of the tube  42 . The refrigerant experiences a substantial pressure drop from the inlet to the outlet of the capillary tube  42 . The magnitude of the pressure drop has an inverse relationship with the inside diameter of the tube  42 . Other parameters, however, such as the pressure of the refrigerant at the inlet of tube  42  may also affect the magnitude of the pressure drop. 
   After the pressure of the refrigerant is reduced by capillary tube  42 , the refrigerant enters another heat exchanger in the form of an evaporator  44  positioned in the low pressure side of the fluid circuit. The refrigerant absorbs thermal energy in the evaporator  44  as the refrigerant is converted from a liquid phase to a vapor phase. The evaporator  44  may be of a conventional construction well known in the art. After exiting evaporator  44 , the low or suction pressure refrigerant passes through heat exchanger  40  to cool the high pressure refrigerant. More particularly, heat exchanger  40  exchanges thermal energy between the relatively warm refrigerant at a first location in the high pressure side of the fluid circuit and the relatively cool refrigerant at a second location in the low pressure side of the fluid circuit. After passing through the heat exchanger  40  on the low pressure side of the fluid circuit, the refrigerant is returned to compression mechanism  32  and the cycle is repeated. 
   Schematically represented fluid lines or conduits  35 ,  37 ,  41 , and  43  provide fluid communication between compression mechanism  32 , gas cooler  38 , capillary tube  42 , evaporator  44  and compression mechanism  32  in serial order. Heat exchanger  40  exchanges thermal energy between different points of the fluid circuit that are located in that portion of the circuit schematically represented by conduits  37  and  43  cooling the high pressure refrigerant conveyed within line  37 . The fluid circuit extending from the outlet of the compression mechanism  32  to the inlet of the compression mechanism  32  has a high pressure side and a low pressure side. The high pressure side extends from the outlet of compression mechanism  32  to capillary tube  42  and includes conduit  35 , gas cooler  38  and conduit  37 . The low pressure side extends from capillary tube  42  to compression mechanism  32  and includes conduit  41 , evaporator  44  and conduit  43 . 
   According to the present invention, the system  30  includes a device for directly or indirectly controlling the temperature of the refrigerant in the capillary tube  42 . Controlling the temperature of the refrigerant in capillary tube  42  provides for the regulation of the pressure of the refrigerant in the gas cooler  38 , and, in turn, the capacity and/or efficiency of the system  30 . For example, the system  30  may include an auxiliary cooling device in the form of a fan  46  for blowing air over the heat exchanger  40 . By controlling the speed of fan  46  the rate of cooling of the refrigerant in the high pressure side of the fluid circuit can be controlled. The speed of fan  46  may be continuously adjustable or have a limited number of different speed settings. It would also be possible to use a single speed fan with a damper or other device for controlling the flow of air over heat exchanger  40 . Moreover, the fan  46  may be disposed proximate or adjacent the capillary tube  42  such that the air flow from the fan  46  may cool the capillary tube  42  and the refrigerant therein more directly. The fan  46  is shown as being oriented to blow air from a low pressure portion  48  to a high pressure portion  50  of the heat exchanger  40 , however, other configurations are also possible. The fan  46  and the heat exchanger  40  form a temperature adjustment device capable of adjusting the temperature of the refrigerant in the capillary tube  42  and, thus, adjusting the capacity of the system as described in greater detail below. 
   In addition to the fan  46 , or in place of the fan  46 , the system  30  may also include a heater/cooler  52  associated with the capillary tube  42 . More particularly, the heating/cooling device  52  may be disposed proximate or adjacent the capillary tube  42  such that device  52  can heat or cool the capillary tube  42  and the refrigerant therein. 
   In operation, the illustrated embodiment of system  30  is a transcritical system utilizing carbon dioxide as the refrigerant wherein the refrigerant is compressed above its critical pressure and returns to a subcritical pressure with each cycle through the vapor compression system. Refrigerant enters the capillary tube  42  at a supercritical pressure and the pressure of the refrigerant is lowered to a subcritical pressure as the refrigerant progresses through the tube  42 . 
   The velocity at which the refrigerant flows through the capillary tube  42  increases with increases in the pressure differential between the inlet and outlet of capillary tube  42  until the refrigerant reaches a critical velocity at which point, further increases in the pressure differential between the inlet and outlet of the capillary tube will not substantially increase the velocity of the refrigerant within the capillary tube. At this critical or choke velocity, the refrigerant inside the capillary tube  42  is moving at approximately the speed of sound. Changes in the temperature, and thus density, of the refrigerant when the refrigerant is flowing through capillary tube  42  at or near its critical velocity, will change the mass flow rate of the refrigerant through the tube. Although changes in the temperature and density of the refrigerant may alter the critical velocity of the refrigerant, the changes in the density of the refrigerant caused by a change in temperature will be of far greater significance than the change in the critical velocity of the refrigerant and, consequently, by controlling the temperature of the refrigerant through capillary tube  42  when the refrigerant is at or near its critical velocity the mass flow rate of the refrigerant through system  30  can be effectively controlled. 
   Capacity control for a transcritical system is typically accomplished by regulating the pressure in the gas cooler while maintaining the mass flow rate of the system substantially constant. However, controlling the mass flow rate while maintaining a substantially constant pressure in the gas cooler can also be used to control the capacity of a transcritical system. 
   As mentioned above, the mass flow rate through expansion device  42  can be controlled by regulating the vapor/liquid ratio of the refrigerant within the expansion device which is, in turn, a function of the temperature of the refrigerant within expansion device  42 . For example, an increase in the temperature of the refrigerant within the expansion device, e.g., capillary tube  42 , results in a decrease in the liquid/vapor ratio, i.e., a decrease in density, of the refrigerant exiting capillary tube  42 . When the velocity of the refrigerant within capillary tube  42  is at the critical or choke velocity and, thus, the velocity of the refrigerant in capillary tube  42  is effectively invariable, a decrease in the density of the refrigerant results in a corresponding decrease in the mass flow rate of the refrigerant through the expansion device. On the other hand, a decrease in the temperature in the expansion device results in an increase in the liquid/vapor ratio, i.e., an increase in density, of the refrigerant exiting capillary tube  42  and an increase in the mass flow rate of the refrigerant through the expansion device. By regulating the temperature of the refrigerant in the capillary tube  42 , the mass flow rate through system  30  can thereby be controlled and, consequently, the capacity of system  30  can also be controlled. 
   The thermodynamic properties of carbon dioxide are shown in the graph of  FIG. 3 . Lines  80  are isotherms and represent the properties of carbon dioxide at a constant temperature. Lines  82  and  84  represent the boundary between two phase conditions and single phase conditions and meet at point  86 , a maximum pressure point of the common line defined by lines  82 ,  84 . Line  82  represents the liquid saturation curve while line  84  represents the vapor saturation curve. 
   The area below lines  82 ,  84  represents the two phase subcritical region where boiling of carbon dioxide takes place at a constant pressure and temperature. The area above point  86  represents the supercritical region where cooling or heating of the carbon dioxide does not change the phase (liquid/vapor) of the carbon dioxide. The phase of a carbon dioxide in the supercritical region is commonly referred to as “gas” instead of liquid or vapor. 
   Point A represents the refrigerant properties as discharged from compression mechanism  32  (and at the inlet of gas cooler  38 ). Point B represents the refrigerant properties at the inlet to capillary tube  42  (if system  30  did not include heat exchanger  40 , point B would also represent the outlet of gas cooler  38 ). Point C represents the refrigerant properties at the inlet of evaporator  44  (or outlet of capillary tube  42 ). Point D represents the refrigerant at the inlet to compression mechanism  32  (if system  30  did not include heat exchanger  40 , point C would also represent the outlet of evaporator  44 ). Movement from point D to point A represents the compression of the refrigerant. As can be seen, compressing the refrigerant both raises its pressure and its temperature. Moving from point A to point B represents the cooling of the high pressure refrigerant at a constant pressure in gas cooler  38  (and heat exchanger  40 ). Movement from point B to point C represents the action of capillary tube  42  which lowers the pressure of the refrigerant to a subcritical pressure. Movement from point C to point D represents the action of evaporator  44  (and heat exchanger  40 ). Since the refrigerant is at a subcritical pressure in evaporator  44 , thermal energy is transferred to the refrigerant to change it from a liquid phase to a vapor phase at a constant temperature and pressure. The capacity of the system (when used as a cooling system) is determined by the mass flow rate through the system and the location of point C and the length of line C-D which in turn is determined by the specific enthalpy of the refrigerant at the evaporator inlet. 
   The lines Q max  and COP max  represent gas cooler discharge values (i.e., the location of point B) for maximizing the capacity and efficiency respectively of the system. The central line positioned therebetween represents values that provide relatively high, although not maximum, capacity and efficiency. By operating the system along the central line between the Q max  and COP max  curves, when the system fails to operate precisely according to the design parameters defined by this central line, the system will suffer a decrease in either the capacity or efficiency and an increase in the other value unless such variances are of such magnitude that they represent a point no longer located between the Q max  and COP max  lines. 
   Thus, while altering the efficiency of the system requires altering the relative position of point B (representing the temperature and pressure of the refrigerant at the inlet to the expansion device) in  FIG. 3 , the capacity of the system can be altered by changing either the relative position of point B, and hence the length of line C-D, or by altering the mass flow rate of the system. 
   In system  30 , the adjustment of the temperature of the refrigerant entering capillary tube  42  adjusts both the mass flow rate of the system and the relative of point B. By increasing the temperature, the density, and thus the mass flow rate, of the refrigerant decreases and point B moves to the right, both of which act to decrease the capacity of the system. By decreasing the temperature of the refrigerant, the density, and mass flow rate, increase and point B moves to the left, both of which act to increase the capacity of the system. Thus, it can be seen that the capacity of the system can be controlled by controlling the temperature of the refrigerant within capillary tube  42 . The movement of point B (i.e., changes in the temperature and pressure of the refrigerant at the inlet to the expansion device as represented by point B in  FIG. 3 ) will also affect the efficiency of the system, however, the adjustment of the system capacity and efficiency effected by the relative repositioning of point B may be relatively insignificant compared to the change in capacity effected by the change in the mass flow rate. 
   The system  30  has been shown herein as including an internal heat exchanger  40 . However, it is to be understood that it is also possible within the scope of the present invention for the vapor compression system to not include an internal heat exchanger  40 . Moreover, regardless of whether a heat exchanger  40  is present, it is possible for an air mover, such as fan  46  to blow air directly on capillary tube  42  or fluid line  37  at a position proximate capillary tube  42  in order to control the temperature of the refrigerant within capillary tube  42 . 
   The system  30  has been described above as including one or both of the fan  46  and the heater/cooler  52  in order to change the temperature and density of the refrigerant within the capillary tube  42 . The present invention is not limited to these exemplary embodiments of a heating or cooling device, however. Rather, the present invention may include any device  52  capable of heating or cooling the refrigerant. For example, device  52  may be a Peltier device. Peltier devices are well known in the art and, with the application of a DC current, move heat from one side of the device to the other side of the device and, thus, could be used for either heating or cooling purposes. Other devices that might be used include electrical resistance heaters and heat pipes. Fans or other air movers could also be used alone to form device  52  or in conjunction with other such devices. Further, the heating/cooling device can be disposed in association with either the capillary tube  42  or some other component of the fluid circuit upstream of capillary tube  42 , such as the heat exchanger  40 , where the heating/cooling device affects the refrigerant temperature more indirectly. 
   A second embodiment  30   a  of a transcritical vapor compression system in accordance with the present invention is schematically represented in  FIG. 4 . System  30   a  is similar to system  30  shown in  FIG. 2  but, in addition to the components of system  30 , system  30   a  also includes a second compressor mechanism  34 , an intermediate cooler  36 , a mass storage tank or flash gas vessel  54 , a second capillary tube  56  and a third capillary tube  58 . System  30   a  also includes additional fluid lines or conduits  31 ,  33 , and  45 . Flash gas vessel  54  stores both liquid phase refrigerant  60  and vapor phase refrigerant  62 . 
   In this embodiment, the first compressor mechanism  32  compresses the refrigerant from a low pressure to an intermediate pressure. Intercooler  36  is positioned between compressor mechanisms  32 ,  34  to cool the intermediate refrigerant. After the fluid line  33  communicates the refrigerant to the second compressor mechanism  34 , the second compressor mechanism  34  compresses the refrigerant from the intermediate pressure to a supercritical pressure. The refrigerant entering second compressor mechanism  34  also includes refrigerant communicated from flash gas vessel  54  through fluid line  45  to fluid line  33 . More particularly, a capillary tube  58  is disposed in the fluid line  45  and reduces the pressure of the refrigerant from flash gas vessel  54  and introduces the reduced pressure refrigerant into fluid line  33 . The introduction of refrigerant from flash gas vessel  54  at a point between first and second compressor mechanisms  32 ,  34  can improve the performance of compressor mechanisms  32 ,  34 . 
   It may be desirable to ensure that the refrigerant exiting flash gas vessel  54  and entering capillary tube  56  includes both liquid and vapor phase refrigerant. For example, it may be desirable that the refrigerant leaving the vessel  54  has the same liquid/vapor ratio as the refrigerant entering vessel  54 . There are several possible methods of controlling the liquid/vapor ratio of the refrigerant exiting vessel  54 . A first of these methods is to constantly stir the liquid/vapor mixture of refrigerant once the refrigerant has entered the vessel  54 . A second method is to heat or cool the vessel  54 . A third method is to provide the vessel  54  with physical characteristics that promote mixing of the liquid and vapor. Such physical characteristics may include the shape of the vessel  54  and the locations of the vessel&#39;s inlet and outlet. 
   Alternatively, the outlet of vessel  54  could be provided with a valve or gate to control the release of refrigerant from vessel  54 . For example, such a gated outlet could be controlled based upon the density of the refrigerant in capillary tube  56 . The density of the refrigerant within the capillary tube could be determined by the use of temperature and pressure sensors, or, the density could be determined by measuring the mass of the refrigerant and tube and subtracting the known mass of the tube. 
   It is also possible to add a filter or filter-drier to the system proximate any of the capillary tubes included in the above embodiments. Such a filter when placed upstream of the capillary tube can prevent contamination in the system, e.g., copper filings, abrasive materials or brazing debris, from collecting in the capillary tube and thereby obstructing the passage of refrigerant. 
   While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.