Patent Publication Number: US-6698234-B2

Title: Method for increasing efficiency of a vapor compression system by evaporator heating

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method for increasing the efficiency of a vapor compression system by heating the refrigerant in the evaporator with heat provided by the compressor. 
     Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. “Natural” refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run transcritical, or above the critical point. 
     When a vapor compression system runs transcritical, the high side pressure of the refrigerant is typically high so that the refrigerant does not change phases from vapor to liquid while passing through the heat rejecting heat exchanger. Therefore, the heat rejecting heat exchanger operates as a gas cooler in a transcritical cycle, rather than as a condenser. The pressure of a subcritical fluid is a function of temperature under saturated conditions (where both liquid and vapor are present). However, the pressure of a transcritical fluid is a function of fluid density when the temperature is higher than the critical temperature. 
     In a prior vapor compression system, the heat generated by the compressor motor either is lost by being discharged to the ambient or superheats the suction gas in the compressor. If the heat superheats the suction gas in the compressor, the density and the mass flow rate of the refrigerant decreases, decreasing system efficiency. It would be beneficial to utilize compressor heat to improve system efficiency and reduce system size and cost. 
     SUMMARY OF THE INVENTION 
     The efficiency of a vapor compression system can be increased by coupling the evaporator with the compressor to provide heat from the compressor to the refrigerant in the evaporator. An intercooler of a two-stage vapor compression system or a compressor component can also be coupled to the evaporator to provide the heat to the evaporator refrigerant. Preferably, the compressor component is a compressor oil cooler or a compressor motor. The refrigerant in the evaporator accepts heat from the refrigerant in the intercooler or the compressor component, increasing the temperature of the refrigerant in the evaporator. As pressure is directly related to temperature, the temperature of the refrigerant in the evaporator increases, increasing the low side pressure of the refrigerant exiting the evaporator. As the low side pressure increases, the compressor needs to do less work to bring the refrigerant to the high side pressure, increasing system efficiency and/or capacity. 
     Additionally, as the heat from the refrigerant in the intercooler or the compressor component is rejected to the refrigerant in the evaporator, the refrigerant in the compressor is cooled. By cooling the refrigerant in the compressor, the density and the mass flow rate of the refrigerant in the compressor increases, increasing system efficiency. 
     These and other features of the present invention will be best understood from the following specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
     FIG. 1 illustrates a schematic diagram of a prior art vapor compression system; 
     FIG. 2 illustrates a schematic diagram of the evaporator coupled to the intercooler of a multistage vapor compression system to increase efficiency; 
     FIG. 3 illustrates an alternative coupling of the evaporator to the intercooler; 
     FIG. 4 illustrates a schematic diagram of the evaporator coupled to a compressor component to increase efficiency; and 
     FIG. 5 illustrates an alternative coupling of the evaporator to the compressor component. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a schematic diagram of a prior art vapor compression system  20 . The system  20  includes a compressor  22  with a motor  23 , a first heat exchanger  24 , an expansion device  26 , a second heat exchanger  28 , and a flow reversing device  30  to reverse the flow of refrigerant circulating through the system  20 . When operating in a heating mode, after the refrigerant exits the compressor  22  at high pressure and enthalpy, the refrigerant flows through the first heat exchanger  24 , which acts as a condenser or gas cooler. The refrigerant loses heat, exiting the first heat exchanger  24  at low enthalpy and high pressure. The refrigerant then passes through the expansion device  26 , and the pressure drops. After expansion, the refrigerant flows through the second heat exchanger  28 , which acts as an evaporator, and exits at a high enthalpy and low pressure. The refrigerant passes through the heat pump  30  and then re-enters the compressor  22 , completing the system  20 . The heat pump  30  can reverse the flow of the refrigerant to change the system  20  from the heating mode to a cooling mode. 
     In a preferred embodiment of the invention, carbon dioxide is used as the refrigerant. While carbon dioxide is illustrated, other refrigerants may benefit from this invention. Because carbon dioxide has a low critical point, systems utilizing carbon dioxide as a refrigerant usually require the vapor compression system  20  to run transcritical. This concept can be applied to refrigeration cycles that operate at multiple pressure levels, such that those systems having two or more compressors, gas coolers, expansion devices, or evaporators. Although a transcritical vapor compression system is described, it is to be understood that a convention sub-critical vapor compression system can be employed as well. Additionally, the present invention can also be applied to refrigeration cycles that operate at multiple pressure levels, such as systems having more than one compressors, gas cooler, expander motors, or evaporators. 
     FIG. 2 illustrates a multi-stage compression system  120 . Like numerals are increased by multiples of  100  to indicate like parts. The system  120  includes an expansion device  126 , a second heat exchanger  128  or evaporator, either a single compressor with two stages or two single stage compressors  122   a  and  122   b , an intercooler  124   a  positioned between the two compressors  122   a  and  122   b , and a first heat exchanger or gas cooler  124   b.    
     In the present invention, the evaporator  128  is coupled to the intercooler  124   a . Heat from the refrigerant in the intercooler  124   a  is accepted by the refrigerant passing through the evaporator  128 . Increasing the temperature of the refrigerant in the evaporator  128  increases the performance of the evaporator  128  and the system  120 . As pressure is directly related to temperature, increasing the temperature of the refrigerant exiting the evaporator  128  increases the low side pressure of the refrigerant exiting the evaporator  128 . 
     The work of the compressor  122   a  and  122   b  is a function of the difference between the high side pressure and the low side pressure of the system  120 . As the low side pressure increases, the compressors  122   a  and  122   b  are required to do less work, increasing system  120  efficiency. Additionally, as heat is provided by the refrigerant in the intercooler  128 , the evaporator  128  is required to perform less refrigerant heating, reducing or eliminating the heating function of the evaporator  128 . 
     As heat in the refrigerant in the intercooler  124   a  is rejected into the refrigerant in the evaporator  128 , the temperature of the refrigerant exiting the intercooler  124   a  and entering the second stage compressor  122   b  decreases. This reduces the superheating of the suction gas in the second stage compressor  122   b , increasing the density and the fluid mass of the refrigerant in the second stage compressor  122   b , further increasing system  120  efficiency. The discharge temperature of the second stage compressor  122   b  is also reduced, prolonging compressor  122   b  life. 
     Alternatively, as shown in FIG. 3, the multistage vapor compression system  220  includes two evaporators  228   a  and  228   b . The first evaporator  228   a  is positioned between a first expansion device  226   a  and the first stage compressor  222   a . The second evaporator  228   b  is positioned between a second expansion device  226   b  and the first stage compressor  222   a  and is coupled to the intercooler  224   a.    
     Heat from the refrigerant in the intercooler  224   a  is provided to the refrigerant passing through the second evaporator  228   b  to increase the temperature of the refrigerant exiting the second evaporator  228   b . Additionally, the temperature of the refrigerant in the intercooler  224   b  is reduced, increasing efficiency of the system  220  by increasing the density and the mass flow rate of the suction gas in the second stage compressor  222   b.    
     The first expansion device  226   a  and the second expansion device  226   b  control the flow of the refrigerant through the evaporators  228   a  and  228   b , respectively. By closing the expansion device  226   a , the refrigerant flows through evaporator  228   b  and accepts heat from the refrigerant in the intercooler  224   a . Alternatively, by closing the expansion device  226   b , the refrigerant flows through evaporator  228   a  and does not accept heat from the refrigerant in the intercooler  224   a . Both expansion devices  226   a  and  226   b  can be adjusted to a desired degree to achieve a desired flow of the refrigerant through the evaporators  228   a  and  228   b , respectively. A control  232  monitors the system  220  to determine the optimal distribution of the refrigerant through the evaporators  228   a  and  228   b  and adjusts the expansion devices  226   a  and  226   b  to achieve the optimal distribution. For example, if refrigerant is passing through expansion device  226   a  and the control  232  determines that system  220  efficiency is low, the control  232  will begin to close the expansion device  226   a  and begin to open the expansion device  226   b , increasing system  220  efficiency. Once a desired efficiency is achieved, the expansion devices  226   a  and  226   b  are set to maintain this efficiency. The factors that would be used to determine the optimum pressure are within the skill of a worker in the art. 
     FIG. 4 illustrates a vapor compression system  320  employing an evaporator  328  coupled to a compressor component  325  of a compressor  322 . Preferably, the compressor component  325  is a compressor oil cooler or a compressor motor. The compressor  322  heat is accepted by the refrigerant in the evaporator  328 . As the temperature of the refrigerant in the evaporator  328  increases, the low side pressure of the system  320  increases, decreasing compressor  322  work and increasing system  320  efficiency. As the temperature of the refrigerant in the compressor  322  decreases, system  320  efficiency increases. 
     Alternatively, as shown in FIG. 5, the system  420  includes two evaporators  428   a  and  428   b . The first evaporator  428   a  is positioned between a first expansion device  426   a  and the compressor  422 , and the second evaporator  428   b  is between a second expansion device  426   b  and the compressor  422 . The second evaporator  428   b  is coupled with the compressor component  425  to increase the temperature of the refrigerant in the second evaporator  428   b  and to cool the compressor component  425 . 
     The first expansion device  426   a  and the second expansion device  426   b  control the flow of the refrigerant through the evaporators  428   a  and  428   b , respectively. By closing the expansion device  426   a , the refrigerant flows through evaporator  428   b  and exchanges heat with the refrigerant in the compressor component  425 . Alternatively, by closing the expansion device  426   b , the refrigerant flows through evaporator  428   a  and does not exchange heat with the refrigerant in the compressor component  425 . Both expansion devices  426   a  and  426   b  can be adjusted to a desired degree to achieve a desired flow. A control  432  monitors the system  420  to determine the optimal distribution of the refrigerant through the evaporators  428   a  and  428   b  and adjusts the expansion devices  426   a  and  426   b  to achieve the optimal distribution. For example, if refrigerant is passing through expansion device  426   a  and the control  432  determines that system  420  efficiency is low, the control  432  will begin to close the expansion device  426   a  and begin to open the expansion device  426   b , increasing system  420  efficiency. Once a desired efficiency is achieved, the expansion devices  426   a  and  426   b  are set to maintain this efficiency. The factors that would be used to determine the optimum pressure are within the skill of a worker in the art. 
     Although the intercooler  124   a  and  224   a  and the compressor component  325  and  425  have been described separately, it is to be understood that a vapor compression system could utilize both the intercooler  124   a  and  224   a  and the compressor component  325  and  425  to heat the refrigerant in the evaporator  128 ,  228 ,  328   b , and  428   b . If both the intercooler  124   a  and  224   a  and the compressor component  325  and  425  are employed, they can be applied either in series or parallel. 
     Additionally, although it has been disclosed that the evaporators  128 ,  228   b ,  328  and  428   b  are coupled to the intercoolers and compressor components  124   a ,  224   a ,  325  and  425 , respectively, it is to be understood that the internal heat transfer between these components could occur through a third medium, such as air. 
     The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specially described. For that reason the following claims should be studied to determine the true scope and content of this invention.