Patent Publication Number: US-2012031121-A1

Title: Air conditioner and method for operating an air conditioner

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. DE 102010033518.5, filed Aug. 5, 2010, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The technical field relates to an air conditioner, in particular for a vehicle, further relates to a method for operating such an air conditioner. 
     BACKGROUND 
     A vehicle air conditioner having a cooling circuit is known from DE 603 03 056 T2, which comprises a vaporizer for cooling the vehicle interior, a compressor, and a condenser. Furthermore, expansion valves for relaxing the refrigerant are provided. 
     The HCFCs that were used as refrigerants in earlier years in such air conditioners have been replaced in the meantime by chlorine-free refrigerants because of their ozone degradation potential. However, these alternatively used refrigerants, for example, the refrigerant R134a, which is widely used, are sometimes problematic with respect to their strong action as greenhouse gases. Therefore, alternatives having low greenhouse potential have further been sought, inter alia, the refrigerant R1234yf being promising because of its thermodynamic properties, which are very similar to R134a, and its very low greenhouse potential. 
     However, the more environmentally friendly refrigerants that are currently already partially used and will be used in future are frequently not as high-performance as the HCFCs which were used earlier. An inner heat exchanger is therefore already sometimes used as a type of performance booster, which transmits heat from the high-pressure side to the low-pressure side and thus reduces the temperature of the refrigerant before entry into the expansion valve. 
     It can be problematic that a temperature increase on the low-pressure side before the compressor must be accepted simultaneously. In high load operation of the system, this additional heat introduction can result in critical operating temperatures of the compressor. This reduces the service life of the compressor and possibly further affected components. 
     At least one object is therefore to provide an air conditioner which is high-performance in spite of the use of more environmentally friendly refrigerants and simultaneously has a long service life. In addition, it is at least a further object to specify a method for operating such an air conditioner. 
     SUMMARY 
     An air conditioner is provided having a refrigerant conducted in a cooling circuit is provided, the cooling circuit comprising at least one vaporizer situated on a low-pressure side, a condenser situated on a high pressure side, a compressor connected between the low-pressure side and the high-pressure side, and a thermostatic expansion valve connected between high-pressure side and low-pressure side. Furthermore, an inner heat exchanger is provided for transmitting heat from the refrigerant on the high-pressure side to refrigerant on the low-pressure side. The thermostatic expansion valve has a head having a control filling, the characteristic curve of the control filling having a slope s with approximately 0.75 bar/10° C.≦s≦approximately 1.05 bar/10° C. The specified slope of the characteristic curve relates to an approximation line in the typical operating range of the thermostatic expansion valve in the range of static overheating. 
     Thermostatic expansion valves are used in air conditioners to regulate the quantity of refrigerant entering the vaporizer as a function of the refrigerant temperature in the cooling circuit after the vaporizer in a way known per se. A thermostatic expansion valve has a head having a chamber, which is filled by a fluid used as the control filling. The chamber is delimited by a diaphragm, which absorbs the temperature of the high-pressure-side refrigerant. The temperature-dependent pressure of the control filling, on the one hand, and the high pressure, on the other hand, are applied to the active surface of the expansion valve. 
     A displacement force results from the pressure differential resulting therefrom, which moves the valve element and, as a function of the throttle properties of the valve seat, releases a flow cross-section for the refrigerant flowing to the vaporizer. The valve characteristic is determined, inter alia, from the expansion behavior, i.e., the vapor pressure curve, of the control filling. Control fillings whose characteristic curve has a slope of approximately 0.7 bar/10° C. are typically used. 
     The air conditioner provided operates with a greater slope of the characteristic curve, in contrast, so that the thermostatic expansion valve reacts more sensitively to temperature increases on the low-pressure side. If the temperature rises on the low-pressure side, a greater flow cross-section is therefore opened in the thermostatic expansion valve, so that a larger quantity of refrigerant flows to the vaporizer. In this way, the temperature on the low-pressure side is in turn reduced at uniform load. 
     The air conditioner has the advantage that the refrigerant enters the compressor at a lower temperature, which reduces the operating temperature of the compressor. In this way, the strain of the compressor is also kept low in high load operation and its service life is therefore lengthened. 
     As has been shown, a characteristic curve set to a slope of approximately 0.85 bar/10° C. is very well suitable for relieving the compressor, without disadvantages thus having to be accepted. In one embodiment, the characteristic curve of the control filling has a slope s with approximately 0.8 bar/10° C. K≦s≦approximately 1.0 bar/10° C., preferably approximately 0.8 bar/10° C. ≦s≦approximately 0.9 bar/10° C. 
     In one embodiment, the cooling circuit additionally has a second vaporizer situated on the low-pressure side, the second vaporizer being able to be provided for cooling a battery. This embodiment can be used in hybrid vehicles, for example, in which there is a stronger need for cooling the battery. The second vaporizer can also be used for additional cooling of the vehicle cab. 
     A thermostatic expansion valve can also be situated before the second vaporizer, the thermostatic expansion valve having a head with a control filling, the characteristic curve of the control filling having a slope s with approximately 0.75 bar/10° C.≦s≦approximately 1.05 bar/10° C., preferably approximately 0.8 bar/10° C.≦s≦approximately 1.0 bar/10° C., more preferably approximately 0.8 bar/10° C.≦s≦approximately 0.9 bar/10° C. 
     The slope of the characteristic curve of the second thermostatic expansion valve can be selected to be greater than that of the first, to stabilize the cooling circuit. The cooling circuit therefore comprises one or more thermostatic expansion valves, for the slope s of each individual thermostatic expansion valve in the cooling circuit, 0.75 bar/10° C.≦s≦1.05 bar/10° C. For example, R134a or R1234yf is provided as the refrigerant conducted in the cooling circuit, the refrigerants being named in the typical way according to DIN 8960. 
     A vehicle is also provided having the described air conditioner. A method is provided for operating an air conditioner is provided, in a cooling circuit, a refrigerant being vaporized in the vaporizer while absorbing ambient heat, subsequently being compressed using a compressor and condensed in a condenser. Before the vaporization: heat is withdrawn from the condensed refrigerant, the withdrawn heat being introduced into the cooling circuit between the vaporizer and the compressor, and the refrigerant is subsequently relaxed via a thermostatic expansion valve, a control filling having a characteristic curve, which has a slope s with 0.75 bar/10° C.≦s≦1.05 bar/10° C., being used for the head of the thermostatic expansion valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and: 
         FIG. 1  schematically shows the cooling circuit of an air conditioner according to a first embodiment; 
         FIG. 2  schematically shows the cooling circuit of an air conditioner according to a second embodiment; and 
         FIG. 3  schematically shows a characteristic curve of a control filling of a thermostatic expansion valve according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
     The air conditioner  1  according to the first embodiment, which is shown in  FIG. 1 , has a cooling circuit  2 , through which a refrigerant flows in the flow direction indicated by the arrow  3 . Refrigerant is vaporized in the vaporizer  4  while absorbing ambient heat from a vehicle cab and subsequently compressed by the compressor  5  and condensed in the condenser  6 . 
     A thermostatic expansion valve  9  regulates the quantity of refrigerant flowing into the vaporizer  4  as a function of the refrigerant temperature prevailing in the cooling circuit  2  after the vaporizer  4 . A control line  15  (not shown in detail) is provided for this purpose. Alternatively, the cooling circuit  2  itself can be conducted through the expansion valve  9  after the vaporizer  4 . 
     Furthermore, the cooling circuit  2  has an inner heat exchanger  10 , which can be implemented as a tube-in-tube system, for example, and which transmits heat from the high-pressure side  8  to the low-pressure side  7  and thus reduces the temperature of the refrigerant before entry into the expansion valve  9 . A greater quantity of heat can thus be absorbed in the vaporizer  4 , so that the performance of the air conditioner  1  is amplified. 
       FIG. 2  shows an air conditioner  1  according to a second embodiment of the invention. The air conditioner  1  according to the second embodiment differs from that shown in  FIG. 1  in that it has a branch circuit  11  having a further vaporizer  12 , which can also be used for cooling the vehicle cab or also for cooling a battery or other components in need of cooling. The entry of refrigerant into the further vaporizer  12  is also regulated using a thermostatic expansion valve, which is shown as a further thermostatic expansion valve  13  with the control line  16  in  FIG. 2 . 
     According to the second embodiment, the slope of the characteristic curve of the thermostatic expansion valve  9  is selected to be approximately 0.85 bar/10° C. and the slope of the further thermostatic expansion valve  13  is selected to be approximately 1.05 bar/10° C. In this way, overheating of compressor  5  may be prevented and great stability may also be achieved in the cooling circuit  2 . 
     Both in the air conditioner  1  according to  FIG. 1  and also in that according to  FIG. 2 , a control filling is used in the valve head of the thermostatic expansion valves  9 ,  13 , whose characteristic curve  14  is selected such that a thermal overload of the compressor  5  is avoided even in high-load operation. For this purpose, the composition of the control filling is selected in such a way that, as shown in  FIG. 3 , the slope of the characteristic curve  14  is 0.85 bar/10° C., for example. The slope can also move in a tolerance range of at least +/−0.05 bar/10° C. up to, for example, +/−0.1 bar/10° C., without the functional capability being substantially impaired. 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.