Patent Publication Number: US-6658888-B2

Title: Method for increasing efficiency of a vapor compression system by compressor cooling

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method for increasing the efficiency of a vapor compression system by removing heat in the compressor from the system with the heat accepted by the heat sink of the heat rejecting heat exchanger. 
     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 is lost to the ambient, it is not transferred usefully, reducing system efficiency. Alternatively, if the heat superheats the suction gas in the compressor, the density and the mass flow rate of the refrigerant decrease, also decreasing system efficiency. 
     Another prior system has employed a tapping circuit which branches off from the heat sink of the heat rejecting heat exchanger to cool the compressor motor. After the cooling fluid in the tapping circuit accepts heat from the compressor motor, the tapping circuit returns to flow of the heat sink of the heat rejecting heat exchanger. A drawback to this system is that the cooling fluid which accepts heat from the compressor motor returns to the heat sink heated, lessening the ability of the cooling fluid to accept additional heat from the heat rejecting heat exchanger. 
     Two-stage compression systems employing an intercooler positioned between the compression stages has also been utilized to increase system efficiency. In a prior system, the refrigerant in the intercooler exchanges heat with the ambient or with a circuit of cooling fluid separate from the circuit of cooling fluid in the heat sink of the heat rejecting heat exchanger. 
     SUMMARY OF THE INVENTION 
     Efficiency of a vapor compression system is increased by usefully transferring heat in the compressor from the system with the heat accepted by the heat sink of the heat rejecting heat exchanger. In one embodiment, a stream of cooling fluid absorbs heat from the compressor motor. Preferably, the cooling fluid is water. The heated stream of cooling fluid merges with the heated fluid medium exiting the heat sink of the gas cooler and exits the system. The efficiency of the system is equal to the useful heat transferred divided by the work put into the cycle. As the heat of the compressor is usefully transferred out of the system rather than being lost to the ambient, system efficiency increases. Additionally, by removing the heat in the compressor motor, superheating of the suction gas in the compressor is reduced, increasing the density and mass flow rate of the refrigerant to further increase efficiency. 
     Alternatively, heat from the compressor motor is transferred to a secondary heat exchange medium, such as oil. The heated oil then transfers heat into the stream of cooling fluid for removal from the system. 
     In another embodiment, an intercooler is employed between compression stages for compressor cooling. After the fluid medium absorbs heat from the refrigerant in the gas cooler, the heated fluid medium travels to the intercooler to accept additional heat from the refrigerant in the intercooler. The heated fluid medium then usefully exits the system. As the heat in the intercooler is usefully transferred out of the system and is not lost, system efficiency is increased. Additionally, as the refrigerant exiting the intercooler is cooled, the mass flow rate and density of the refrigerant in the second stage of compression is increased, also increasing 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 a vapor compression system employing a stream of cooling fluid to cool the compressor; 
     FIG. 3 illustrates a schematic diagram of a vapor compression system employing a secondary stream of cooling fluid to cool the compressor; 
     FIG. 4 illustrates a schematic diagram of a vapor compression system employing a stream of cooling fluid to cool both a gas cooler and an intercooler; and 
     FIG. 5 illustrates a pressure-enthalpy diagram of FIG.  4 . 
    
    
     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 reversing valve  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 gas cooler, and loses heat, exiting the first heat exchanger  24  at low enthalpy and high pressure. A fluid medium  38 , such as water, flows through the heat sink  32  and accepts heat from the refrigerant passing through the first heat exchanger  24 . The cooled fluid medium  38  enters the heat sink  32  at the heat sink inlet or return  34  and flows in a direction opposite to the direction of flow of the refrigerant. After accepting heat from the refrigerant, the heated fluid medium  38  exits at the heat sink outlet or supply  36 . 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 reversing valve  30  and then re-enters the compressor  22 , completing the system  20 . The reversing valve 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. 
     FIG. 2 illustrates a vapor compression system  120  employing a stream of cooling fluid  140  to cool the compressor motor  123 . Like numerals are increased by multiples of 100 to indicate like parts. The stream of cooling fluid  140  flows in or near the compressor motor  123 , accepting heat generated by the compressor motor  123 . Preferably, the stream of cooling fluid  140  is water. After accepting heat from the compressor motor  123 , the stream of cooling fluid  140  merges with the heated fluid medium  138  exiting the heat sink  132  at the heat sink outlet  136 . The merged flows of the heated cooling fluid  140  and the heated fluid medium  138  exit the vapor compression system  120 , removing both the heat generated by the compressor motor  123  and the heat rejected by the refrigerant flowing through the first heat exchanger  124 . The heated merged flows can then be used by the customer. 
     As the heat of the compressor motor  123  is usefully transferred out of the system  120  rather than being lost to the ambient, more useful heat of the system  220  is transferred. The efficiency of the system  120  is equal to the useful heat transferred divided by the work put into the system  120 . As more useful heat is transferred, system  120  efficiency increases. Additionally, by accepting the heat in the compressor motor  123  with the cooling fluid  140 , the superheating of the suction gas in the compressor  122  is reduced, increasing the density and mass flow rate of the refrigerant in the compressor  122 , further increasing efficiency. 
     Alternatively, as shown in FIG. 3, the heat from the compressor motor  123  is transferred to a secondary heat exchange medium  125 , such as oil. The stream of cooling fluid  140  accepts heat from the secondary heat exchange medium  125  and then merges with the heated fluid medium  138  to exit the system  120 . 
     As shown in FIG. 4, system  220  efficiency is also increased by employing a multi-stage compression system  220 . The vapor compression system  220  includes an expansion device  226 , a second heat exchanger  228  or evaporator, either a single compressor with two stages or two single stage compressors  222   a  and  222   b,  an intercooler  224   a  positioned between the two stages of the compressors  222   a  and  222   b,  and a first heat exchanger or gas cooler  224   b.    
     In the present invention, the refrigerant in the intercooler  224   a  exchanges heat with the same fluid medium  238  which flows through the heat sink  232  and exchanges heat with the refrigerant in the gas cooler  224   b.  After the fluid medium  238  accepts heat from the refrigerant in the gas cooler  224   b,  the heated fluid medium flows  238  to the intercooler  224   a  to accept additional heat from the refrigerant in the intercooler  224   a.  The heated fluid medium  238  then exits the system. As heat in the refrigerant in the intercooler  224   a  is usefully transferred to the fluid medium  238  and is not lost to the ambient, more useful heat is transferred from the system  220 . 
     Additionally, as the refrigerant exiting the intercooler  224   a  is cooled, the mass flow rate and density of the refrigerant in the second stage of compression  222   b  is increased, also increasing efficiency. 
     FIG. 5 illustrates a pressure-enthalpy diagram of the vapor compression system  220 . As shown, by employing the intercooler  224   a,  the discharge temperature of the second stage of the compressor  222   b  is lowered, increasing the reliability and life of the compressor  222   b.  For the same conditions, the combined work of the first  222   a  and the second  222   b  stages of compression is lower than it would be for single stage compression. This is shown by the decrease in the slope of entropy with respect to pressure after the refrigerant flows through the intercooler  224   b.    
     Preferably, the volumetric displacement ratio between the first  222   a  and the second stages  222   b  of compression is two or greater. For a transcritical cycle, the efficiency of the system  220  is a function of the high side pressure. At a volumetric displacement ratio of two or greater, the discharge pressure from both stages  222   a  and  222   b  of compression are in the proper range for the optimal coefficient of performance. 
     The fluid medium  238  employed depends on the type of heating. For fan coil heating, the fluid medium is room air. Recirculating water is the fluid medium for hydronic space heating, and tap water is the fluid medium for domestic hot water. 
     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.